One document matched: draft-ietf-rmt-pi-norm-revised-11.xml
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<rfc category="std" docName="draft-ietf-rmt-pi-norm-revised-11"
ipr="pre5378Trust200902" obsoletes="3940">
<front>
<title abbrev="NORM Protocol">NACK-Oriented Reliable Multicast
Protocol</title>
<author fullname="Brian Adamson" initials="B." surname="Adamson">
<organization>Naval Research Laboratory</organization>
<address>
<postal>
<street></street>
<city>Washington</city>
<region>DC</region>
<code>20375</code>
<country>USA</country>
</postal>
<email>adamson@itd.nrl.navy.mil</email>
</address>
</author>
<author fullname="Carsten Bormann" initials="C." surname="Bormann">
<organization>Universitaet Bremen TZI</organization>
<address>
<postal>
<street>Postfach 330440</street>
<city>D-28334 Bremen</city>
<country>Germany</country>
</postal>
<email>cabo@tzi.org</email>
</address>
</author>
<author fullname="Mark Handley" initials="M." surname="Handley">
<organization>University College London</organization>
<address>
<postal>
<street>Gower Street</street>
<city>London</city>
<code>WC1E 6BT</code>
<country>UK</country>
</postal>
<email>M.Handley@cs.ucl.ac.uk</email>
</address>
</author>
<author fullname="Joe Macker" initials="J." surname="Macker">
<organization>Naval Research Laboratory</organization>
<address>
<postal>
<street></street>
<city>Washington</city>
<region>DC</region>
<code>20375</code>
<country>USA</country>
</postal>
<email>macker@itd.nrl.navy.mil</email>
</address>
</author>
<date day="24" month="April" year="2009" />
<abstract>
<t>This document describes the messages and procedures of the
Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM)
Protocol. This protocol is designed to provide end-to-end reliable
transport of bulk data objects or streams over generic IP multicast
routing and forwarding services. NORM uses a selective, negative
acknowledgment mechanism for transport reliability and offers additional
protocol mechanisms to allow for operation with minimal <spanx
style="emph">a priori</spanx> coordination among senders and receivers.
A congestion control scheme is specified to allow the NORM protocol to
fairly share available network bandwidth with other transport protocols
such as Transmission Control Protocol (TCP). It is capable of operating
with both reciprocal multicast routing among senders and receivers and
with asymmetric connectivity (possibly a unicast return path) between
the senders and receivers. The protocol offers a number of features to
allow different types of applications or possibly other higher level
transport protocols to utilize its service in different ways. The
protocol leverages the use of FEC-based repair and other IETF Reliable
Multicast Transport (RMT) building blocks in its design. This document
obsoletes <xref format="none" target="RFC3940">RFC 3940</xref>.</t>
</abstract>
</front>
<middle>
<section title="Introduction and Applicability">
<t>The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
protocol is designed to provide reliable transport of data from one or
more sender(s) to a group of receivers over an IP multicast network. The
primary design goals of NORM are to provide efficient, scalable, and
robust bulk data (e.g., computer files, transmission of persistent data)
transfer across possibly heterogeneous IP networks and topologies. The
NORM protocol design provides support for distributed multicast session
participation with minimal coordination among senders and receivers.
NORM allows senders and receivers to dynamically join and leave
multicast sessions at will with minimal overhead for control information
and timing synchronization among participants. To accommodate this
capability, NORM protocol message headers contain some common
information allowing receivers to easily synchronize to senders
throughout the lifetime of a reliable multicast session. NORM is
designed to be self-adapting to a wide range of dynamic network
conditions with little or no pre-configuration. The protocol is
purposely designed to be tolerant of inaccurate timing estimations or
lossy conditions that may occur in many networks including mobile and
wireless. The protocol is also designed to exhibit convergence and
efficient operation even in situations of heavy packet loss and large
queuing or transmission delays. This document obsoletes the Experimental
<xref format="none" target="RFC3940">RFC 3940</xref> specification.</t>
<t>This document is a product of the IETF RMT working group and follows
the guidelines provided in the <xref target="RFC3269">Author Guidelines
for Reliable Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents</xref>.</t>
<t><spanx style="strong">Statement of Intent</spanx></t>
<t>This memo contains the definitions necessary to fully specify a
Reliable Multicast Transport protocol in accordance with the criteria of
<xref target="RFC2357">IETF Criteria for Evaluating Reliable Multicast
Transport and Application Protocols</xref>. The NORM specification
described in this document was previously published in the "Experimental
Category" <xref target="RFC3940"></xref>. It was the stated intent of
the RMT working group to re-submit this specifications as an IETF
Proposed Standard in due course. This Proposed Standard specification is
thus based on RFC 3940 and has been updated according to accumulated
experience and growing protocol maturity since the publication of RFC
3940. Said experience applies both to this specification itself and to
congestion control strategies related to the use of this specification.
The differences between RFC 3940 and this document are listed in <xref
target="ProtocolChanges"></xref>.</t>
<section title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
</section>
<section title="NORM Data Delivery Service Model">
<t>A NORM protocol instance (<spanx style="emph">NormSession</spanx>)
is defined within the context of participants communicating
connectionless (e.g., Internet Protocol (IP) or User Datagram Protocol
(UDP)) packets over a network using pre-determined addresses and host
port numbers. Generally, the participants exchange packets using an IP
multicast group address, but unicast transport may also be established
or applied as an adjunct to multicast delivery. In the case of
multicast, the participating <spanx style="emph">NormNodes</spanx>
will communicate using a common IP multicast group address and port
number that has been chosen via means outside the context of the given
<spanx style="emph">NormSession</spanx>. Other IETF data format and
protocol standards exist that may be applied to describe and convey
the required <spanx style="emph">a priori</spanx> information for a
specific <spanx style="emph">NormSession</spanx> (e.g., <xref
target="RFC4566">Session Description Protocol (SDP) </xref>, <xref
target="RFC2974">Session Announcement Protocol (SAP)</xref>,
etc.).</t>
<t>The NORM protocol design is principally driven by the assumption of
a single sender transmitting bulk data content to a group of
receivers. However, the protocol MAY operate with multiple senders
within the context of a single <spanx style="emph">NormSession</spanx>.
In initial implementations of this protocol, it is anticipated that
multiple senders will transmit independent of one another and
receivers will maintain state as necessary for each sender. However,
in future versions of NORM, it is possible that some aspects of
protocol operation (e.g., round-trip time collection) may provide for
alternate modes allowing more efficient performance for applications
requiring multiple senders.</t>
<t>NORM provides for three types of bulk data content objects (<spanx
style="emph">NormObjects</spanx>) to be reliably transported. These
types include:</t>
<t><list style="numbers">
<t>static computer memory data content (<spanx style="verb">NORM_OBJECT_DATA</spanx>
type),</t>
<t>computer storage files (<spanx style="verb">NORM_OBJECT_FILE</spanx>
type), and</t>
<t>non-finite streams of continuous data content (<spanx
style="verb">NORM_OBJECT_STREAM</spanx> type).</t>
</list></t>
<t>The distinction between <spanx style="verb">NORM_OBJECT_DATA</spanx>
and <spanx style="verb">NORM_OBJECT_FILE</spanx> is simply to provide
a hint to receivers in <spanx style="emph">NormSessions</spanx>
serving multiple types of content as to what type of storage should be
allocated for received content (i.e., memory or file storage). Other
than that distinction, the two are identical, providing for reliable
transport of finite (but potentially very large) units of content.
These static data and file services are anticipated to be useful for
multicast-based cache applications with the ability to reliably
provide transmission of large quantities of static data. Other types
of static data/file delivery services might make use of these
transport object types, too. The use of the <spanx style="verb">NORM_OBJECT_STREAM</spanx>
type is at the application's discretion and could be used to carry
static data or file content also. The NORM reliable stream service
opens up additional possibilities such as serialized reliable
messaging or other unbounded, perhaps dynamically produced content.
The <spanx style="verb">NORM_OBJECT_STREAM</spanx> provides for
reliable transport analogous to that of the Transmission Control
Protocol (TCP), although NORM receivers will be able to begin
receiving stream content at any point in time. The applicability of
this feature will depend upon the application.</t>
<t>The NORM protocol also allows for a small amount of out-of-band
data (sent as <spanx style="verb">NORM_INFO</spanx> messages) to be
attached to the data content objects transmitted by the sender. This
readily-available out-of-band data allows multicast receivers to
quickly and efficiently determine the nature of the corresponding
data, file, or stream bulk content being transmitted. This allows
application-level control of the receiver node's participation in the
current transport activity. This also allows the protocol to be
flexible with minimal pre-coordination among senders and receivers.
The <spanx style="verb">NORM_INFO</spanx> content is designed to be
atomic in that its size MUST fit into the payload portion of a single
NORM message.</t>
<t>NORM does NOT provide for global or application-level
identification of data content within in its message headers. Note the
<spanx style="verb">NORM_INFO</spanx> out-of-band data mechanism could
be leveraged by the application for this purpose if desired, or
identification could alternatively be embedded within the data
content. NORM does identify transmitted content (<spanx style="emph">NormObjects</spanx>)
with transport identifiers that are applicable only while the sender
is transmitting and/or repairing the given object. These transport
data content identifiers (<spanx style="emph">NormTransportIds</spanx>)
are assigned in a monotonically increasing fashion by each NORM sender
during the course of a <spanx style="emph">NormSession</spanx>. Each
sender maintains its <spanx style="emph">NormTransportId</spanx>
assignments independently so that individual <spanx style="emph">NormObjects</spanx>
may be uniquely identified during transport with the concatenation of
the sender session-unique identifier (<spanx style="emph">NormNodeId</spanx>)
and the assigned <spanx style="emph">NormTransportId</spanx>. The
<spanx style="emph">NormTransportId</spanx>s are assigned from a
large, but fixed, numeric space in increasing order and may be
reassigned during long-lived sessions. The NORM protocol provides
mechanisms so that the sender application may terminate transmission
of data content and inform the group of this in an efficient manner.
Other similar protocol control mechanisms (e.g., session termination,
receiver synchronization, etc.) are specified so that reliable
multicast application variants may construct different, complete bulk
transfer communication models to meet their goals.</t>
<t>To summarize, the NORM protocol provides reliable transport of
different types of data content (including potentially mixed types).
The senders enqueue and transmit bulk content in the form of static
data or files and/or non-finite, ongoing stream types. NORM senders
provide for repair transmission of data and/or FEC content in response
to NACK messages received from the receiver group. Mechanisms for
out-of-band information and other transport control mechanisms are
specified for use by applications to form complete reliable multicast
solutions for different purposes.</t>
</section>
<section title="NORM Scalability">
<t>Group communication scalability requirements lead to adaptation of
negative acknowledgment (NACK) based protocol schemes when feedback
for reliability is required <xref target="RmComparison"></xref>. NORM
is a protocol centered around the use of selective NACKs to request
repairs of missing data. NORM provides for the use of packet-level
forward error correction (FEC) techniques for efficient multicast
repair and optional proactive transmission robustness <xref
target="RFC3453"></xref>. FEC-based repair can be used to greatly
reduce the quantity of reliable multicast repair requests and repair
transmissions <xref target="MdpToolkit"></xref> in a NACK-oriented
protocol. The principal factor in NORM scalability is the volume of
feedback traffic generated by the receiver set to facilitate
reliability and congestion control. NORM uses probabilistic
suppression of redundant feedback based on exponentially distributed
random backoff timers. The performance of this type of suppression
relative to other techniques is described in <xref
target="McastFeedback"></xref>. NORM dynamically measures the group's
round-trip timing status to set its suppression and other protocol
timers. This allows NORM to scale well while maintaining reliable data
delivery transport with low latency relative to the network topology
over which it is operating.</t>
<t>Feedback messages can be either multicast to the group at large or
sent via unicast routing to the sender. In the case of unicast
feedback, the sender relays the feedback state to the group to
facilitate feedback suppression. In typical Internet environments, it
is expected that the NORM protocol will readily scale to group sizes
on the order of tens of thousands of receivers. A study of the
quantity of feedback for this type of protocol is described in <xref
target="NormFeedback"></xref>. NORM is able to operate with a smaller
amount of feedback than a single TCP connection, even with relatively
large numbers of receivers. Thus, depending upon the network topology,
it is possible that NORM may scale to larger group sizes. With respect
to computer resource usage, the NORM protocol does NOT require that
state be kept on all receivers in the group. NORM senders maintain
state only for receivers providing explicit congestion control
feedback. However, NORM receivers must maintain state for each active
sender. This may constrain the number of simultaneous senders in some
uses of NORM.</t>
</section>
<section title="Environmental Requirements and Considerations">
<t>All of the environmental requirements and considerations that apply
to the <xref target="RFC5401">Multicast NACK Building Block</xref>,
<xref target="RFC5052">FEC Building Block</xref>, and <xref
target="RFC4654">TCP-Friendly Multicast Congestion Control (TFMCC)
Building Block</xref> also apply to the NORM protocol.</t>
<t>The NORM protocol SHALL be capable of operating in an end-to-end
fashion with no assistance from intermediate systems beyond basic IP
multicast group management, routing, and forwarding services. While
the techniques utilized in NORM are principally applicable to flat,
end-to-end IP multicast topologies, they could also be applied in the
sub-levels of hierarchical (e.g., tree-based) multicast distribution
if so desired. NORM can make use of reciprocal (among senders and
receivers) multicast communication under the Any-Source Multicast
(ASM) model defined in <xref target="RFC1112">Host Extensions for IP
Multicasting</xref>, but SHALL also be capable of scalable operation
in asymmetric topologies such as <xref
target="RFC4607">Source-Specific Multicast (SSM)</xref> where there
may only be unicast routing service from the receivers to the
sender(s).</t>
<t>NORM is compatible with IPv4 and IPv6. Additionally, NORM may be
used with networks employing Network Address Translation (NAT)
providing the NAT device supports IP multicast and/or can cache UDP
traffic source port numbers for remapping feedback traffic from
receivers to the sender(s).</t>
</section>
</section>
<section title="Architecture Definition">
<t>A <spanx style="emph">NormSession</spanx> is comprised of
participants (<spanx style="emph">NormNodes</spanx>) acting as senders
and/or receivers. NORM senders transmit data content in the form of
<spanx style="emph">NormObjects</spanx> to the session destination
address and the NORM receivers attempt to reliably receive the
transmitted content using negative acknowledgments to request repair.
Each <spanx style="emph">NormNode</spanx> within a <spanx style="emph">NormSession</spanx>
is assumed to have a preselected unique 32-bit identifier (<spanx
style="emph">NormNodeId</spanx>). <spanx style="emph">NormNodes</spanx>
MUST have uniquely assigned identifiers within a single <spanx
style="emph">NormSession</spanx> to distinguish between possible
multiple senders and to distinguish feedback information from different
receivers. There are two reserved <spanx style="emph">NormNodeId</spanx>
values. A value of <spanx style="verb">0x00000000</spanx> is considered
an invalid <spanx style="emph">NormNodeId</spanx> value and a value of
<spanx style="verb">0xffffffff</spanx> is a "wild card" <spanx
style="emph">NormNodeId</spanx>. While the protocol does not preclude
multiple sender nodes concurrently transmitting within the context of a
single NORM session (i.e., many- to-many operation), any type of
interactive coordination among NORM senders is assumed to be controlled
by the application or higher protocol layer. There are some optional
mechanisms specified in this document that can be leveraged for such
application layer coordination.</t>
<t>As previously noted, NORM allows for reliable transmission of three
different basic types of data content. The first type is <spanx
style="verb">NORM_OBJECT_DATA</spanx>, which is used for static,
persistent blocks of data content maintained in the sender's application
memory storage. The second type is <spanx style="verb">NORM_OBJECT_FILE</spanx>,
which corresponds to data stored in the sender's non-volatile file
system. The <spanx style="verb">NORM_OBJECT_DATA</spanx> and <spanx
style="verb">NORM_OBJECT_FILE</spanx> types both represent <spanx
style="emph">NormObjects</spanx> of finite but potentially very large
size. The third type of data content is <spanx style="verb">NORM_OBJECT_STREAM</spanx>,
which corresponds to an ongoing transmission of undefined length. This
is analogous to the reliable stream service provide by TCP for unicast
data transport. The format of the stream content is application-defined
and may be byte or message oriented. The NORM protocol provides for
"flushing" of the stream to expedite delivery or possibly enforce
application message boundaries. NORM protocol implementations may offer
either (or both) in-order delivery of the stream data to the receive
application or out-of-order (more immediate) delivery of received
segments of the stream to the receiver application. In either case, NORM
sender and receiver implementations provide buffering to facilitate
repair of the stream as it is transported.</t>
<t>All <spanx style="emph">NormObjects</spanx> are logically segmented
into FEC coding blocks and symbols for transmission by the sender. In
NORM, an FEC encoding symbol directly corresponds to the payload of
<spanx style="verb">NORM_DATA</spanx> messages or "segment". Note that
when systematic FEC codes are used, the payload of <spanx style="verb">NORM_DATA</spanx>
messages sent for the first portion of a FEC encoding block are source
symbols (actual segments of original user data), while the remaining
symbols for the block consist of parity symbols generated by FEC
encoding. These parity symbols are generally sent in response to repair
requests, but some number may be sent proactively at the end each
encoding block to increase the robustness of transmission. When
non-systematic FEC codes are used, all symbols sent consist of FEC
encoding parity content. In this case, the receiver must receive a
sufficient number of symbols to reconstruct (via FEC decoding) the
original user data for the given block.</t>
<t>Transmitted <spanx style="emph">NormObjects</spanx> are temporarily
yet uniquely identified within the <spanx style="emph">NormSession</spanx>
context using the given sender's <spanx style="emph">NormNodeId</spanx>,
<spanx style="emph">NormInstanceId</spanx>, and a temporary <spanx
style="emph">NormObjectTransportId</spanx>. Depending upon the
implementation, individual NORM senders may manage their <spanx
style="emph">NormInstanceIds</spanx> independently, or a common <spanx
style="emph">NormInstanceId</spanx> may be agreed upon for all
participating nodes within a session if needed as a session identifier.
NORM <spanx style="emph">NormObjectTransportId</spanx> data content
identifiers are sender-assigned and applicable and valid only during a
<spanx style="emph">NormObject</spanx>'s actual transport (i.e., for as
long as the sender is transmitting and providing repair of the indicated
<spanx style="emph">NormObject</spanx>). For a long-lived session, the
<spanx style="emph">NormObjectTransportId</spanx> field can wrap and
previously-used identifiers may be re-used. Note that globally unique
identification of transported data content is not provided by NORM and,
if required, must be managed by the NORM application. The individual
segments or symbols of the <spanx style="emph">NormObject</spanx> are
further identified with FEC payload identifiers which include coding
block and symbol identifiers. These are discussed in detail later in
this document.</t>
<section anchor="ProtocolOverview" title="Protocol Operation Overview">
<t>A NORM sender primarily generates messages of type <spanx
style="verb">NORM_DATA</spanx>. These messages carry original data
segments or FEC symbols and repair segments/symbols for the bulk
data/file or stream <spanx style="emph">NormObjects</spanx> being
transferred. By default, redundant FEC symbols are sent only in
response to receiver repair requests (NACKs) and thus normally little
or no additional transmission overhead is imposed due to FEC encoding.
However, the NORM implementation MAY be optionally configured to
proactively transmit some amount of redundant FEC symbols along with
the original content to potentially enhance performance (e.g.,
improved delay) at the cost of additional transmission overhead. This
option may be sensible for certain network conditions and can allow
for robust, asymmetric multicast (e.g., unidirectional routing,
satellite, cable) <xref target="FecHybrid"></xref> with reduced
receiver feedback, or, in some cases, no feedback.</t>
<t>A sender message of type <spanx style="verb">NORM_INFO</spanx> is
also defined and is used to carry OPTIONAL out-of-band context
information for a given transport object. A single <spanx
style="verb">NORM_INFO</spanx> message can be associated with a <spanx
style="emph">NormObject</spanx>. Because of its atomic nature, missing
<spanx style="verb">NORM_INFO</spanx> messages can be NACKed and
repaired with a slightly lower delay process than NORM's general
FEC-encoded data content. <spanx style="verb">NORM_INFO</spanx> may
serve special purposes for some bulk transfer, reliable multicast
applications where receivers join the group mid-stream and need to
ascertain contextual information on the current content being
transmitted. The NACK process for <spanx style="verb">NORM_INFO</spanx>
will be described later. When the <spanx style="verb">NORM_INFO</spanx>
message type is used, its transmission should precede transmission of
any <spanx style="verb">NORM_DATA</spanx> message for the associated
<spanx style="emph">NormObject</spanx>.</t>
<t>The sender also generates messages of type <spanx style="verb">NORM_CMD</spanx>
to assist in certain protocol operations such as congestion control,
end-of-transmission flushing, round trip time estimation, receiver
synchronization, and optional positive acknowledgment requests or
application defined commands. The transmission of <spanx style="verb">NORM_CMD</spanx>
messages from the sender is accomplished by one of three different
procedures. These procedures are: single, best effort unreliable
transmission of the command; repeated redundant transmissions of the
command; and positively-acknowledged commands. The transmission
technique used for a given command depends upon the function of the
command. Several core commands are defined for basic protocol
operation. Additionally, implementations MAY wish to consider
providing the OPTIONAL application-defined commands that can take
advantage of the transmission methodologies available for commands.
This allows for application-level session management mechanisms that
can make use of information available to the underlying NORM protocol
engine (e.g., round-trip timing, transmission rate, etc.). A notable
distinction between <spanx style="verb">NORM_DATA</spanx> message and
some <spanx style="verb">NORM_CMD</spanx> message transmissions is
that typically a receiver will need to allocate resources to manage
reliable reception when <spanx style="verb">NORM_DATA</spanx> messages
are received. However some <spanx style="verb">NORM_CMD</spanx>
messages may be completely atomic and no specific state may need to be
kept. Thus, for session management or other purposes it is possible
that even participants acting principally as data receivers MAY
transmit <spanx style="verb">NORM_CMD</spanx> messages. However, it is
RECOMMENDED that this is not done within the context of the NORM
multicast session unless congestion control is addressed. For example,
many receiver nodes transmitting <spanx style="verb">NORM_CMD</spanx>
messages simultaneously can cause congestion for the
destination(s).</t>
<t>All sender transmissions are subject to rate control governed by a
peak transmission rate set for each participant by the application.
This can be used to limit the quantity of multicast data transmitted
by the group. When NORM's congestion control algorithm is enabled the
rate for senders is automatically adjusted. In some networks, it may
be desirable to establish minimum and maximum bounds for the rate
adjustment depending upon the application even when dynamic congestion
control is enabled. However, in the case of the general Internet,
congestion control policy SHALL be observed that is compatible with
coexistent TCP flows.</t>
<t>NORM receivers generate messages of type <spanx style="verb">NORM_NACK</spanx>
or <spanx style="verb">NORM_ACK</spanx> in response to transmissions
of data and commands from a sender. The <spanx style="verb">NORM_NACK</spanx>
messages are generated to request repair of detected data transmission
losses. Receivers generally detect losses by tracking the sequence of
transmission from a sender. Sequencing information is embedded in the
transmitted data packets and end-of-transmission commands from the
sender. <spanx style="verb">NORM_ACK</spanx> messages are generated in
response to certain commands transmitted by the sender. In the general
(and most scalable) protocol mode, <spanx style="verb">NORM_ACK</spanx>
messages are sent only in response to congestion control commands from
the sender. The feedback volume of these congestion control <spanx
style="verb">NORM_ACK</spanx> messages is controlled using the same
timer-based probabilistic suppression techniques as for <spanx
style="verb">NORM_NACK</spanx> messages to avoid feedback implosion.
In order to meet potential application requirements for positive
acknowledgment from receivers, other <spanx style="verb">NORM_ACK</spanx>
messages are defined and available for use.</t>
</section>
<section title="Protocol Building Blocks">
<t>The operation of the NORM protocol is based primarily upon the
concepts presented in the <xref target="RFC5401">Multicast NACK
Building Block</xref> document. This includes the basic NORM
architecture and the data transmission, repair, and feedback
strategies discussed in that document. The reliable multicast building
block approach, as described in <xref target="RFC3048">Reliable
Multicast Transport Building Blocks for One-to-Many Bulk-Data
Transfer</xref>, is applied in creating the full NORM protocol
instantiation. NORM also makes use of the parity-based encoding
techniques for repair messaging and optional transmission robustness
as described in <xref target="RFC3453">The Use of Forward Error
Correction (FEC) in Reliable Multicast</xref>. NORM uses the FEC
Payload ID as specified by the <xref target="RFC5052">FEC Building
Block document </xref>. Additionally, for congestion control, this
document fully specifies a baseline congestion control mechanism
(NORM-CC) based on the TCP-Friendly Multicast Congestion Control
(TFMCC) scheme<xref target="TfmccPaper"></xref>, <xref
target="RFC4654"></xref>.</t>
</section>
<section title="Design Tradeoffs">
<t>While the various features of NORM are designed to provide some
measure of general purpose utility, it is important to emphasize the
understanding that "no one size fits all" in the reliable multicast
transport arena. There are numerous engineering trade-offs involved in
reliable multicast transport design and this requires an increased
awareness of application and network architecture considerations.
Performance requirements affecting design can include: group size,
heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data
ordering, delivery delay, group dynamics, mobility, congestion
control, and transport across low capacity connections. NORM contains
various parameters to accommodate many of these differing
requirements. The NORM protocol and its mechanisms MAY be applied in
multicast applications outside of bulk data transfer, but there is an
assumed model of bulk transfer transport service that drives the
trade-offs that determine the scalability and performance described in
this document.</t>
<t>The ability of NORM to provide reliable data delivery is also
governed by any buffer constraints of the sender and receiver
applications. NORM protocol implementations SHOULD be designed to
operate with the greatest efficiency and robustness possible within
application-defined buffer constraints. Buffer requirements for
reliability, as always, are a function of the delay-bandwidth product
of the network topology. NORM performs best when allowed more
buffering resources than typical point-to-point transport protocols.
This is because NORM feedback suppression is based upon
randomly-delayed transmissions from the receiver set, rather than
immediately transmitted feedback. There are definitive trade-offs
between buffer utilization, group size scalability, and efficiency of
performance. Large buffer sizes allow the NORM protocol to perform
most efficiently in large delay-bandwidth topologies and allow for
longer feedback suppression backoff timeouts. This yields improved
group size scalability. NORM can operate with reduced buffering but at
a cost of decreased efficiency (lower relative goodput) and reduced
group size scalability.</t>
</section>
</section>
<section title="Conformance Statement">
<t>This RMT Protocol Instantiation document, in conjunction with the
<xref target="RFC5401">Multicast Negative-Acknowledgment (NACK)</xref>
and <xref target="RFC5052">Forward Error Correction (FEC)</xref>
Building Blocks, completely specifies a working reliable multicast
transport protocol that conforms to the requirements described in <xref
format="none" target="RFC2357">RFC 2357</xref>.</t>
<t>This document specifies the following message types and mechanisms
which are REQUIRED in complying NORM protocol implementations:</t>
<texttable>
<ttcol>Message Type</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_DATA</spanx></c>
<c>Sender message for application data transmission. Implementations
must support at least one of the <spanx style="verb">NORM_OBJECT_DATA</spanx>,
<spanx style="verb">NORM_OBJECT_FILE</spanx>, or <spanx style="verb">NORM_OBJECT_STREAM</spanx>
delivery services. The use of the NORM FEC Object Transmission
Information header extension is OPTIONAL with <spanx style="verb">NORM_DATA</spanx>
messages.</c>
<c><spanx style="verb">NORM_CMD(FLUSH)</spanx></c>
<c>Sender command to excite receivers for repair requests in lieu of
ongoing <spanx style="verb">NORM_DATA</spanx> transmissions. Note the
use of the <spanx style="verb">NORM_CMD(FLUSH)</spanx> for positive
acknowledgment of data receipt is OPTIONAL.</c>
<c><spanx style="verb">NORM_CMD(SQUELCH)</spanx></c>
<c>Sender command to advertise its current valid repair window in
response to invalid requests for repair.</c>
<c><spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx></c>
<c>Sender command to advertise current repair (and congestion control
state) to group when unicast feedback messages are detected. Used to
control/suppress excessive receiver feedback in asymmetric multicast
topologies.</c>
<c><spanx style="verb">NORM_CMD(CC)</spanx></c>
<c>Sender command used in collection of round trip timing and
congestion control status from group (this may be OPTIONAL if
alternative congestion control mechanism and round trip timing
collection is used).</c>
<c><spanx style="verb">NORM_NACK</spanx></c>
<c>Receiver message used to request repair of missing transmitted
content.</c>
<c><spanx style="verb">NORM_ACK</spanx></c>
<c>Receiver message used to proactively provide feedback for
congestion control purposes. Also used with the OPTIONAL NORM Positive
Acknowledgment Process.</c>
</texttable>
<t>This document also describes the following message types and
associated mechanisms which are OPTIONAL for complying NORM protocol
implementations:</t>
<texttable>
<ttcol>Message Type</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_INFO</spanx></c>
<c>Sender message for providing ancillary context information
associated with NORM transport objects. The use of the NORM FEC Object
Transmission Information header extension is OPTIONAL with <spanx
style="verb">NORM_INFO</spanx> messages.</c>
<c><spanx style="verb">NORM_CMD(EOT)</spanx></c>
<c>Sender command to indicate it has reached end-of-transmission and
will no longer respond to repair requests.</c>
<c><spanx style="verb">NORM_CMD(ACK_REQ)</spanx></c>
<c>Sender command to support application-defined, positively
acknowledged commands sent outside of the context of the bulk data
content being transmitted. The NORM Positive Acknowledgment Procedure
associated with this message type is OPTIONAL.</c>
<c><spanx style="verb">NORM_CMD(APPLICATION)</spanx></c>
<c>Sender command containing application-defined commands sent outside
of the context of the bulk data content being transmitted.</c>
<c><spanx style="verb">NORM_REPORT</spanx></c>
<c>Optional message type reserved for experimental implementations of
the NORM protocol.</c>
</texttable>
</section>
<section title="Message Formats">
<t>As mentioned in <xref target="ProtocolOverview"></xref>, there are
two primary classes of NORM messages: sender messages and receiver
messages. <spanx style="verb">NORM_CMD</spanx>, <spanx style="verb">NORM_INFO</spanx>,
and <spanx style="verb">NORM_DATA</spanx> message types are generated by
senders of data content, and <spanx style="verb">NORM_NACK</spanx> and
<spanx style="verb">NORM_ACK</spanx> messages generated by receivers
within a <spanx style="emph">NormSession</spanx>. Sender messages SHOULD
be governed by congestion control for Internet use. For session
management or other purposes, receivers may wish to employ <spanx
style="verb">NORM_CMD</spanx> message transmissions. The principal
rationale for distinguishing sender and receiver messages is that
receivers will typically need to allocate resources to support reliable
reception from sender(s) and NORM sender messages are subject to
congestion control. NORM receivers MAY employ the <spanx style="verb">NORM_CMD</spanx>
message type for application-defined purposes but it is RECOMMENDED that
congestion control and feedback implosion issues be addressed.
Additionally, an auxiliary message type of <spanx style="verb">NORM_REPORT</spanx>
is also provided for experimental purposes. This section describes the
message formats used by the NORM protocol. These messages and their
fields are referenced in the detailed functional description of the NORM
protocol given in <xref target="ProtocolDetails"></xref>. Individual
NORM messages are designed to be compatible with the MTU limitations of
encapsulating Internet protocols including IPv4, IPv6, and UDP. The
current NORM protocol specification assumes UDP encapsulation and
leverages the transport features of UDP. The NORM messages are
independent of network addresses and can be used in IPv4 and IPv6
networks.</t>
<section anchor="CommonHeader"
title="NORM Common Message Header and Extensions">
<t>There are some common message fields contained in all NORM message
types. Additionally, a header extension mechanism is defined to expand
the functionality of the NORM protocol without revision to this
document. All NORM protocol messages begin with a common header with
information fields as follows:</t>
<figure align="center" title="NORM Common Message Header Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type | hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "version" field is a 4-bit value indicating the protocol
version number. NORM implementations SHOULD ignore received messages
with version numbers different from their own. This number is intended
to indicate and distinguish upgrades of the protocol which may be
non-interoperable. The NORM version number for this specification is
1.</t>
<t>The message "type" field is a 4-bit value indicating the NORM
protocol message type. These types are defined as follows:</t>
<texttable>
<ttcol>Message</ttcol>
<ttcol>Value</ttcol>
<c><spanx style="verb">NORM_INFO</spanx></c>
<c>1</c>
<c><spanx style="verb">NORM_DATA</spanx></c>
<c>2</c>
<c><spanx style="verb">NORM_CMD</spanx></c>
<c>3</c>
<c><spanx style="verb">NORM_NACK</spanx></c>
<c>4</c>
<c><spanx style="verb">NORM_ACK</spanx></c>
<c>5</c>
<c><spanx style="verb">NORM_REPORT</spanx></c>
<c>6</c>
</texttable>
<t>The 8-bit "hdr_len" field indicates the number of 32-bit words that
comprise the given message's header portion. This is used to
facilitate header extensions that may be applied. The presence of
header extensions are implied when the "hdr_len" value is greater than
the base value for the given message "type".</t>
<t>The "sequence" field is a 16-bit value that is set by the message
originator. The "sequence" field serves two separate purposes,
depending upon the message type:<list style="numbers">
<t>NORM senders MUST set the "sequence" field of sender messages
(<spanx style="verb">NORM_INFO</spanx>, <spanx style="verb">NORM_DATA</spanx>,
and <spanx style="verb">NORM_CMD</spanx>) so that receivers can
monitor the "sequence" value to maintain an estimate of packet
loss that can be used for congestion control purposes (See <xref
target="CongestionControl"></xref> for a detailed description of
NORM Congestion Control operation). A monotonically-increasing
sequence number space MUST be maintained to mark NORM sender
messages in this way. Note that this "sequence" number is
explicitly NOT used in NORM as part of its reliability procedures.
The NORM object and FEC payload identifiers are used to detect
missing content for reliable transfer purposes.</t>
<t>NORM receivers SHOULD set the "sequence" field to support
protection from message replay attacks of <spanx style="verb">NORM_NACK</spanx>
or <spanx style="verb">NORM_NACK</spanx> messages. Note that,
depending upon configuration, NORM feedback messages may be sent
to the session multicast address or unicast address[es] of the
active NORM sender[s]. Thus, a separate, monotonically-increasing
sequence number space MUST be maintained for each destination
address to which the NORM receiver is transmitting feedback
messages.</t>
</list></t>
<t>Note that these two separate purposes necessitate the maintenance
of separate sequence spaces to support the functions described here.
And, in the case of NORM receivers, additional sequence spaces are
needed when feedback messages are sent to the sender unicast
address[es] instead of the session address.</t>
<t>The "source_id" field is a 32-bit value that uniquely identifies
the node that sent the message within the context of a single <spanx
style="emph">NormSession</spanx>. This value is termed the NORM node
identifier (<spanx style="emph">NormNodeId</spanx>) and unique <spanx
style="emph">NormNodeId</spanx> identifiers MUST be assigned within a
single <spanx style="emph">NormSession</spanx>. In some cases, use of
the host IP address or a hash of it can suffice, but alternative
methodologies for assignment and potential collision resolution of
node identifiers within a multicast session SHOULD be considered. For
example, the techniques for managing the 32-bit "synchronization
source" (SSRC) identifiers defined in the Real-Time Protocol (RTP)
specification <xref target="RFC3550"></xref> are applicable for use
with NORM node identifiers. In most deployments of the NORM protocol
to date, the <spanx style="emph">NormNodeId</spanx> assignments are
administratively configured.</t>
<t><spanx style="strong">NORM Header Extensions</spanx></t>
<t>When header extensions are applied, they follow the message type's
base header and precede any payload portion. There are two formats for
header extensions, both of which begin with an 8-bit "het" (header
extension type) field. One format is provided for variable-length
extensions with "het" values in the range from 0 through 127. The
other format is for fixed length (one 32-bit word) extensions with
"het" values in the range from 128 through 255. These formats are
given here:</t>
<figure align="center"
title="NORM Variable Length Header Extension Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het <=127 | hel | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Header Extension Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<figure align="center"
title="NORM Fixed Length (32-bit) Header Extension Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het >=128 | reserved | Header Extension Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "Header Extension Content" portion of the header extension is
defined for each extension type. Some header extensions are defined
within this document for NORM baseline FEC and congestion control
operations.</t>
</section>
<section title="Sender Messages">
<t>NORM sender messages include the <spanx style="verb">NORM_DATA</spanx>
type, the <spanx style="verb">NORM_INFO</spanx> type, and the <spanx
style="verb">NORM_CMD</spanx> type. <spanx style="verb">NORM_DATA</spanx>
and <spanx style="verb">NORM_INFO</spanx> messages contain application
data content while <spanx style="verb">NORM_CMD</spanx> messages are
used for various protocol control functions.</t>
<section title="NORM_DATA Message">
<t>The <spanx style="verb">NORM_DATA</spanx> message is expected to
be the predominant type transmitted by NORM senders. These messages
are used to encapsulate segmented data content for objects of type
<spanx style="verb">NORM_OBJECT_DATA</spanx>, <spanx style="verb">NORM_OBJECT_FILE</spanx>,
and <spanx style="verb">NORM_OBJECT_STREAM</spanx>. <spanx
style="verb">NORM_DATA</spanx> messages may contain original or
FEC-encoded application data content.</t>
<t>The format of <spanx style="verb">NORM_DATA</spanx> messages is
comprised of three logical portions: 1) a fixed-format <spanx
style="verb">NORM_DATA</spanx> header portion, 2) a FEC Payload ID
portion with a format dependent upon the FEC encoding used, and 3) a
payload portion containing source or encoded application data
content. Note for objects of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>,
the payload portion contains additional fields used to appropriately
recover stream content. NORM implementations MAY also extend the
<spanx style="verb">NORM_DATA</spanx> header to include a FEC Object
Transmission Information (EXT_FTI) header extension. This allows
NORM receivers to automatically allocate resources and properly
perform FEC decoding without the need for pre-configuration or
out-of-band information.</t>
<figure align="center" title="NORM_DATA Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=2| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_len* | payload_msg_start* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>*IMPORTANT NOTE: The "payload_len", "payload_msg_start" and
"payload_offset" fields are present ONLY for objects of type <spanx
style="verb">NORM_OBJECT_STREAM</spanx>. These fields, as with the
entire payload, are subject to any FEC encoding used. Thus, when
systematic FEC codes are used, these values may be directly
interpreted for packets containing source symbols only while packets
containing FEC parity content require decoding before these fields
can be interpreted.</t>
<t>The "version", "type", "hdr_len", "sequence", and "source_id"
fields form the NORM Common Message Header as described in <xref
target="CommonHeader"></xref>. The value of the <spanx style="verb">NORM_DATA</spanx>
"type" field is 2. The <spanx style="verb">NORM_DATA</spanx> base
"hdr_len" value is 4 (i.e. 4 32-bit words) plus the size of the
"fec_payload_id" field. The "fec_payload_id" field size depends upon
the FEC encoding type referenced by the "fec_id" field. For example,
when small block, systematic codes are used, a "fec_id" value of 129
is indicated and the size of the "fec_payload_id" is two 32-bit
words. In this case the <spanx style="verb">NORM_DATA</spanx> base
"hdr_len" value is 6. The cumulative size of any header extensions
applied is added into the "hdr_len" field.</t>
<t>The "instance_id" field contains a value generated by the sender
to uniquely identify its current instance of participation in the
<spanx style="emph">NormSession</spanx>. This allows receivers to
detect when senders have perhaps left and rejoined a session in
progress. When a sender (identified by its "source_id") is detected
to have a new "instance_id", the NORM receivers SHOULD drop their
previous state on the sender and begin reception anew, or at least
treat this "instance" as a new, separate sender.</t>
<t>The "grtt" field contains a non-linear quantized representation
of the sender's current estimate of group round-trip time (GRTT)
(this is also referred to as <spanx style="verb">R_max</spanx> in
<xref target="TfmccPaper"></xref>). This value is used to control
timing of the NACK repair process and other aspects of protocol
operation as described in this document. Normally, the advertised
"grtt" value will correspond to what the sender has measured based
on feedback from the group, but, at low transmission rates, the
advertised "grtt" SHALL be set to <spanx style="verb">MAX(grttMeasured, NormSegmentSize/senderRate)</spanx>
where the <spanx style="verb">NormSegmentSize</spanx> is sender's
segment size in bytes and the <spanx style="verb">senderRate</spanx>
is the sender's current transmission rate in bytes per second. The
algorithm for encoding and decoding this field is described in the
<xref target="RFC5401">Multicast NACK Building Block</xref>.</t>
<t>The "backoff" field value is used by receivers to determine the
maximum backoff timer value used in the timer-based NORM NACK
feedback suppression. This 4-bit field supports values from 0-15
which is multiplied by the sender GRTT to determine the maximum
backoff timeout. The "backoff" field informs the receivers of the
sender's backoff factor parameter <spanx style="verb">Ksender</spanx>.
Recommended values and their use are described in the NORM receiver
NACK procedure description in <xref
target="NackProcedure"></xref>.</t>
<t>The "gsize" field contains a representation of the sender's
current estimate of group size. This 4-bit field can roughly
represent values from ten to 500 million where the most significant
bit value of 0 or 1 represents a mantissa of 1 or 5, respectively
and the three least significant bits incremented by one represent a
base 10 exponent (order of magnitude). For examples, a field value
of "0x0" represents 1.0e+01 (10), a value of "0x8" represents
5.0e+01 (50), a value of "0x1" represents 1.0e+02 (100), and a value
of "0xf" represents 5.0e+08. For NORM feedback suppression purposes,
the group size does not need to be represented with a high degree of
precision. The group size may even be estimated somewhat
conservatively (i.e., overestimated) to maintain low levels of
feedback traffic. A default group size estimate of 10,000 ("gsize" =
0x3) is recommended for general purpose reliable multicast
applications using the NORM protocol.</t>
<t>The "flags" field contains a number of different binary flags
providing information and hints regarding how the receiver should
handle the identified object. Defined flags in this field
include:</t>
<texttable align="center">
<ttcol>Flag</ttcol>
<ttcol>Value</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_FLAG_REPAIR</spanx></c>
<c>0x01</c>
<c>Indicates message is a repair transmission</c>
<c><spanx style="verb">NORM_FLAG_EXPLICIT</spanx></c>
<c>0x02</c>
<c>Indicates a repair segment intended to meet a specific receiver
erasure, as compared to parity segments provided by the sender for
general purpose (with respect to an FEC coding block) erasure
filling.</c>
<c><spanx style="verb">NORM_FLAG_INFO</spanx></c>
<c>0x04</c>
<c>Indicates availability of <spanx style="verb">NORM_INFO</spanx>
for object.</c>
<c><spanx style="verb">NORM_FLAG_UNRELIABLE</spanx></c>
<c>0x08</c>
<c>Indicates that repair transmissions for the specified object
will be unavailable (One-shot, best effort transmission).</c>
<c><spanx style="verb">NORM_FLAG_FILE</spanx></c>
<c>0x10</c>
<c>Indicates object is file-based data (hint to use disk storage
for reception).</c>
<c><spanx style="verb">NORM_FLAG_STREAM</spanx></c>
<c>0x20</c>
<c>Indicates object is of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>.</c>
</texttable>
<t><spanx style="verb">NORM_FLAG_REPAIR</spanx> is set when the
associated message is a repair transmission. This information can be
used by receivers to help observe a join policy where it is desired
that newly joining receivers only begin participating in the NACK
process upon receipt of new (non-repair) data content. <spanx
style="verb">NORM_FLAG_EXPLICIT</spanx> is used to mark repair
messages sent when the data sender has exhausted its ability to
provide "fresh" (not previously transmitted) parity segments as
repair. This flag could possibly be used by intermediate systems
implementing functionality to control sub-casting of repair content
to different legs of a reliable multicast topology with disparate
repair needs. <spanx style="verb">NORM_FLAG_INFO</spanx> is set only
when optional <spanx style="verb">NORM_INFO</spanx> content is
actually available for the associated object. Thus, receivers will
NACK for retransmission of <spanx style="verb">NORM_INFO</spanx>
only when it is available for a given object. <spanx style="verb">NORM_FLAG_UNRELIABLE</spanx>
is set when the sender wishes to transmit an object with only "best
effort" delivery and will not supply repair transmissions for the
object. NORM receivers SHOULD NOT execute repair requests for
objects marked with the <spanx style="verb">NORM_FLAG_UNRELIABLE</spanx>
flag. Note that receivers may inadvertently request repair of such
objects when all segments (or info content) for those objects are
not received (i.e., a gap in the "object_transport_id" sequence is
noted). In this case, the sender should invoke the <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> process as described in <xref
target="NORM_CMD"></xref>.</t>
<t><spanx style="verb">NORM_FLAG_FILE</spanx> can be set as a hint
from the sender that the associated object should be stored in
non-volatile storage. <spanx style="verb">NORM_FLAG_STREAM</spanx>
is set when the identified object is of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>.
The presence of <spanx style="verb">NORM_FLAG_STREAM</spanx>
overrides that of <spanx style="verb">NORM_FLAG_FILE</spanx> with
respect to interpretation of object size and the format of <spanx
style="verb">NORM_DATA</spanx> messages.</t>
<t>The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document <xref
target="RFC5052"></xref>. The "fec_id" value implies the format of
the "fec_payload_id" field and, coupled with FEC Object Transmission
Information, the procedures to decode FEC encoded content. Small
block, systematic codes ("fec_id" = 129) are expected to be used for
most NORM purposes and the <spanx style="verb">NORM_OBJECT_STREAM</spanx>
requires systematic FEC codes for most efficient performance.</t>
<t>The "object_transport_id" field is a monotonically and
incrementally increasing value assigned by the sender to <spanx
style="emph">NormObjects</spanx> being transmitted. Transmissions
and repair requests related to that object use the same
"object_transport_id" value. For sessions of very long or indefinite
duration, the "object_transport_id" field may be repeated, but it is
presumed that the 16-bit field size provides an adequate enough
sequence space to avoid object confusion amongst receivers and
sources (i.e., receivers SHOULD re-synchronize with a server when
receiving object sequence identifiers sufficiently out-of-range with
the current state kept for a given source). During the course of its
transmission within a NORM session, an object is uniquely identified
by the concatenation of the sender "source_id" and the given
"object_transport_id". Note that <spanx style="verb">NORM_INFO</spanx>
messages associated with the identified object carry the same
"object_transport_id" value.</t>
<t>The "fec_payload_id" identifies the attached <spanx style="verb">NORM_DATA</spanx>
"payload" content. The size and format of the "fec_payload_id" field
depends upon the FEC type indicated by the "fec_id" field. These
formats are given in the descriptions of specific FEC schemes such
as those described in the <xref target="RFC5445">FEC Basic
Schemes</xref> specification or in other FEC Schemes. As an example,
the format of the "fec_payload_id" format for Small Block,
Systematic codes ("fec_id" = 129) from the<xref target="RFC5445">
FEC Basic Schemes</xref> specification is given here:</t>
<figure align="center"
title="Example: FEC Payload Id Format for 'fec_id' = 129">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_len | encoding_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>In this example FEC payload identifier, the
"source_block_number", "source_block_len", and "encoding_symbol_id"
fields correspond to the "Source Block Number", "Source Block
Length, and "Encoding Symbol ID" fields of the FEC Payload ID format
for Small Block Systematic FEC Schemes identified by a "fec_id"
value of 129 as specified by the <xref target="RFC5445">FEC Basic
Schemes</xref> specification. The "source_block_number" identifies
the coding block's relative position with a <spanx style="emph">NormObject</spanx>.
Note that, for <spanx style="emph">NormObjects</spanx> of type
<spanx style="verb">NORM_OBJECT_STREAM</spanx>, the
"source_block_number" may wrap for very long lived sessions. The
"source_block_len" indicates the number of user data segments in the
identified coding block. Given the "source_block_len" information of
how many symbols of application data are contained in the block, the
receiver can determine whether the attached segment is data or
parity content and treat it appropriately. Some applications may
dynamically "shorten" code blocks when the pending information
content is not predictable (e.g. real-time message streams). In that
case, the "source_block_len" value given for an "encoding_symbol_id"
that contains FEC parity content SHALL take precedence over the
"source_block_len" value provided for any packets containing source
symbols. Also, the "source_block_len" value given for an ordinally
higher "encoding_symbol_id" SHALL take precedence over the
"source_block_len" given for prior encoding symbols. The reason for
this is that the sender may only know the maximum source block
length at the time is transmitting source symbols, but then
subsequently "shorten" the code and then provide that last source
symbol and/or encoding symbols with FEC parity content. The
"encoding_symbol_id" identifies which specific symbol (segment)
within the coding block the attached payload conveys. Depending upon
the value of the "encoding_symbol_id" and the associated
"source_block_len" parameters for the block, the symbol (segment)
referenced may be a user data or an FEC parity segment. For
systematic codes, encoding symbols numbered less than the <spanx
style="verb">source_block_len</spanx> contain original application
data while segments greater than or equal to <spanx style="verb">source_block_len</spanx>
contain parity symbols calculated for the block. The concatenation
of <spanx style="verb">object_transport_id::fec_payload_id</spanx>
can be viewed as a unique transport protocol data unit identifier
for the attached segment with respect to the NORM sender's instance
within a session.</t>
<t>Additional FEC Object Transmission Information (FTI) (as
described in the <xref target="RFC5052">FEC Building Block</xref>)
is required to properly receive and decode NORM transport objects.
This information MAY be provided as out-of-band session information.
However, in some cases, it may be useful for the sender to include
this information "in-band" to facilitate receiver operation with
minimal pre-configuration. For this purpose, the NORM FEC Object
Transmission Information Header Extension (EXT_FTI) is defined. This
header extension MAY be applied to <spanx style="verb">NORM_DATA</spanx>
and <spanx style="verb">NORM_INFO</spanx> messages to provide this
necessary information. The format of the EXT_FTI consists of two
parts, a general part that contains the size of the associated
transport object and a portion that depends upon the FEC scheme
being used. The "fec_id" field in <spanx style="verb">NORM_DATA</spanx>
and <spanx style="verb">NORM_INFO</spanx> messages identifies the
FEC scheme. The format of the EXT_FTI general part is given
here.</t>
<figure align="center"
title="EXT_FTI Header Extension General Portion Format">
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Scheme specific content ... |]]></artwork>
</figure>
<t>The header extension type "het" field value for the EXT_FTI
header extension is 64. The header extension length "hel" value
depends upon the format of the FTI for encoding type identified by
the "fec_id" field.</t>
<t>The 48-bit "object_size" field indicates the total length of the
object (in bytes) for the static object types of <spanx
style="verb">NORM_OBJECT_FILE</spanx> and <spanx style="verb">NORM_OBJECT_DATA</spanx>.
This information is used by receivers to determine storage
requirements and/or allocate storage for the received object.
Receivers with insufficient storage capability may wish to forego
reliable reception (i.e., not NACK for) of the indicated object. In
the case of objects of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>,
the "object_size" field is used by the sender to advertise the size
of its stream buffer to the receiver group. In turn, the receivers
SHOULD use this information to allocate a stream buffer for
reception of corresponding size.</t>
<t>As noted, the format of the extension depends upon the FEC code
in use, but in general it SHOULD contain any required details on the
FEC code in use (e.g., FEC Instance ID, etc.). As an example, the
format of the EXT_FTI for small block systematic codes ("fec_id" =
129) is given here:</t>
<figure align="center"
title="Example: EXT_FTI Header Extension Format for 'fec_id' = 129">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>In this example (for "fec_id" = 129), the "hel" field value is 4.
The size of the EXT_FTI header extension may be different for other
FEC schemes.</t>
<t>The 48-bit "object_size" serves the purpose described
previously.</t>
<t>The "fec_instance_id" corresponds to the "FEC Instance ID"
described in the <xref target="RFC5052">FEC Building Block</xref>.
In this case, the "fec_instance_id" is a value corresponding to the
particular type of Small Block Systematic Code being used (e.g.,
Reed-Solomon GF(2^8), Reed-Solomon GF(2^16), etc). The standardized
assignment of FEC Instance ID values is described in <xref
format="none" target="RFC5052">RFC 5052</xref>.</t>
<t>The "segment_size" field indicates the sender's current setting
for maximum message payload content (in bytes). This allows
receivers to allocate appropriate buffering resources and to
determine other information in order to properly process received
data messaging. Typically, FEC parity symbol segments will be of
this size.</t>
<t>The "fec_max_block_len" indicates the current maximum number of
user data segments per FEC coding block to be used by the sender
during the session. This allows receivers to allocate appropriate
buffer space for buffering blocks transmitted by the sender.</t>
<t>The "fec_num_parity" corresponds to the "maximum number of
encoding symbols that can be generated for any source block" as
described in for FEC Object Transmission Information for Small Block
Systematic Codes in the <xref target="RFC5052">FEC Building
Block</xref>. For example, Reed-Solomon codes may be arbitrarily
shortened to create different code variations for a given block
length. In the case of Reed-Solomon (GF(2^8) and GF(2^16)) codes,
this value indicates the maximum number of parity segments available
from the sender for the coding blocks. This field MAY be interpreted
differently for other systematic codes as they are defined.</t>
<t>The payload portion of <spanx style="verb">NORM_DATA</spanx>
messages includes source data or FEC encoded application content.
The content of this payload depends upon the FEC scheme being
employed, and support for streaming using the <spanx style="verb">NORM_OBJECT_STREAM</spanx>
type, when applicable, necessitates some additional content in the
payload.</t>
<t>The "payload_len", "payload_msg_start", and "payload_offset"
fields are present ONLY for transport objects of type <spanx
style="verb">NORM_OBJECT_STREAM</spanx>. These fields allow senders
to arbitrarily vary the size of <spanx style="verb">NORM_DATA</spanx>
payload segments for streams. This allows applications to flush
transmitted streams as needed to meet unique streaming requirements.
For objects of types <spanx style="verb">NORM_OBJECT_FILE</spanx>
and <spanx style="verb">NORM_OBJECT_DATA</spanx>, these fields are
unnecessary since the receiver can calculate the payload length and
offset information from the "fec_payload_id" using the REQUIRED
block partitioning algorithm described in the <xref
target="RFC5052">FEC Building Block</xref>. When systematic FEC
codes (e.g., "fec_id" = 129) are used, the "payload_len",
"payload_msg_start", and "payload_offset" fields contain actual
payload_data length, message start index (or stream control code),
and byte offset values for the associated application stream data
segment (the remainder of the "payload_data" field content) for
those <spanx style="verb">NORM_DATA</spanx> messages containing
source data symbols. In <spanx style="verb">NORM_DATA</spanx>
messages that contain FEC parity content, these fields do not
contain values that can be directly interpreted, but instead are
values computed from FEC encoding the "payload_len",
"payload_msg_start", and "payload_offset" fields for the source data
segments of the corresponding coding block. The actual
"payload_msg_start", "payload_len" and "payload_offset" values of
missing data content can be determined upon decoding a FEC coding
block. Note that these fields do NOT contribute to the value of the
<spanx style="verb">NORM_DATA</spanx> "hdr_len" field. These fields
are present only when the "flags" portion of the <spanx
style="verb">NORM_DATA</spanx> message indicate the transport object
is of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>.</t>
<t>The "payload_len" value, when non-zero, indicates the length (in
bytes) of the source content contained in the associated
"payload_data" field. However, when the "payload_len" value is equal
to ZERO, this indicates that the "payload_msg_start" field should be
alternatively interpreted as a "stream_control_code". The only
"stream_control_code" value defined is <spanx style="verb">NORM_STREAM_END = 0</spanx>.
The <spanx style="verb">NORM_STREAM_END</spanx> code indicates that
the sender is terminating transmission of stream content at the
corresponding position in the stream and the receiver should not
expect content (or NACK for any content) following that position in
the stream. It is expected that additional specifications may extend
the functionality of the NORM stream transport mode by defining
additional stream control codes. These control codes are delivered
to the recipient application reliably, in-order with respect to the
streamed application data content.</t>
<t>The "payload_msg_start" field serves one of two exclusive
purposes. When the "payload_len" value is non-zero, the
"payload_msg_start" field, when also set to a non-zero value,
indicates that the associated "payload_data" content contains an
application-defined message boundary (start-of-message). When such a
message boundary is indicated, the first byte of an
application-defined message, with respect to the "payload_data"
field, will be found at an offset of "payload_msg_start - 1" bytes.
Thus, if a <spanx style="verb">NORM_DATA</spanx> payload for a
<spanx style="verb">NORM_OBJECT_STREAM</spanx> contains the start of
an application message at the first byte of the "payload_data"
field, the value of the "payload_msg_start" field will be '1'. NORM
implementations SHOULD provide sender stream applications with a
capability to mark message boundaries in this manner. Similarly, the
NORM receiver implementation SHOULD enable the application to
recover such message boundary information. This enables NORM
receivers to "synchronize" reliable reception of transmitted message
stream content in a meaningful way (i.e., meaningful to the
application) at any time, whether joining a session already in
progress, or departing the session and returning. Note that if the
value of the "payload_msg_start" field is ZERO, no message boundary
is present. The "payload_msg_start" value will always be less than
or equal to the "payload_len" value except for the special case of
"payload_len = 0", that indicates the "payload_msg_start" field
should instead be interpreted as a "stream_control_code"</t>
<t>The "payload_offset" field indicates the relative byte position
(from the sender stream transmission start) of the source content
contained in the "payload_data" field. Note that for long-lived
streams, the "payload_offset" field may wrap.</t>
<t>The "payload_data" field contains the original application source
or parity content for the symbol identified by the "fec_payload_id".
The length of this field SHALL be limited to a maximum of the
sender's <spanx style="emph">NormSegmentSize</spanx> bytes as given
in the FTI for the object. Note the length of this field for
messages containing parity content will always be of length <spanx
style="emph">NormSegmentSize</spanx>. When encoding data segments of
varying sizes, the FEC encoder SHALL assume ZERO value padding for
data segments with length less than the <spanx style="emph">NormSegmentSize</spanx>.
It is RECOMMENDED that a sender's <spanx style="emph">NormSegmentSize</spanx>
generally be constant for the duration of a given sender's term of
participation in the session, but may possibly vary on a per-object
basis. The <spanx style="emph">NormSegmentSize</spanx> is expected
to be configurable by the sender application prior to session
participation as needed for network topology maximum transmission
unit (MTU) considerations. For IPv6, MTU discovery may be possibly
leveraged at session startup to perform this configuration. The
"payload_data" content may be delivered directly to the application
for source symbols (when systematic FEC encoding is used) or upon
decoding of the FEC block. For <spanx style="verb">NORM_OBJECT_FILE</spanx>
and <spanx style="verb">NORM_OBJECT_STREAM</spanx> objects, the data
segment length and offset can be calculated using the block
partitioning algorithm described in the <xref target="RFC5052">FEC
Building Block</xref>. For <spanx style="verb">NORM_OBJECT_STREAM</spanx>
objects, the length and offset is obtained from the segment's
corresponding embedded "payload_len" and "payload_offset"
fields.</t>
</section>
<section title="NORM_INFO Message">
<t>The <spanx style="verb">NORM_INFO</spanx> message is used to
convey OPTIONAL, application-defined, out-of-band context
information for transmitted <spanx style="emph">NormObjects</spanx>.
An example <spanx style="verb">NORM_INFO</spanx> use for bulk file
transfer is to place MIME type information for the associated file,
data, or stream object into the <spanx style="verb">NORM_INFO</spanx>
payload. Receivers may use the <spanx style="verb">NORM_INFO</spanx>
content to make a decision as whether to participate in reliable
reception of the associated object. Each <spanx style="emph">NormObject</spanx>
can have an independent unit of <spanx style="verb">NORM_INFO</spanx>
associated with it. <spanx style="verb">NORM_DATA</spanx> messages
contain a flag to indicate the availability of <spanx style="verb">NORM_INFO</spanx>
for a given <spanx style="emph">NormObject</spanx>. NORM receivers
may NACK for retransmission of <spanx style="verb">NORM_INFO</spanx>
when they have not received it for a given <spanx style="emph">NormObject</spanx>.
The size of the <spanx style="verb">NORM_INFO</spanx> content is
limited to that of a single <spanx style="emph">NormSegmentSize</spanx>
for the given sender. This atomic nature allows the <spanx
style="verb">NORM_INFO</spanx> to be rapidly and efficiently
repaired within the NORM reliable transmission process.</t>
<t>When <spanx style="verb">NORM_INFO</spanx> content is available
for a <spanx style="emph">NormObject</spanx>, the NORM_FLAG_INFO
flag SHALL be set in <spanx style="verb">NORM_DATA</spanx> messages
for the corresponding "object_transport_id" and the <spanx
style="verb">NORM_INFO</spanx> message shall be transmitted as the
first message for the <spanx style="emph">NormObject</spanx>.</t>
<figure align="center" title="NORM_INFO Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=1| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "version", "type", "hdr_len", "sequence", and "source_id"
fields form the NORM Common Message Header as described in <xref
target="CommonHeader"></xref>. The value of "hdr_len" field when no
header extensions are present is 4.</t>
<t>The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id",
and "object_transport_id" fields carry the same information and
serve the same purpose as with <spanx style="verb">NORM_DATA</spanx>
messages. These values allow the receiver to prepare appropriate
buffering, etc, for further transmissions from the sender when
<spanx style="verb">NORM_INFO</spanx> is the first message
received.</t>
<t>As with <spanx style="verb">NORM_DATA</spanx> messages, the NORM
FTI Header Extension (EXT_FTI) may be optionally applied to <spanx
style="verb">NORM_INFO</spanx> messages. To conserve protocol
overhead, some NORM implementations may wish to apply the EXT_FTI
when used to <spanx style="verb">NORM_INFO</spanx> messages only and
not to <spanx style="verb">NORM_DATA</spanx> messages.</t>
<t>The <spanx style="verb">NORM_INFO</spanx> "payload_data" field
contains sender application-defined content which can be used by
receiver applications for various purposes as described above.</t>
</section>
<section anchor="NORM_CMD" title="NORM_CMD Messages">
<t><spanx style="verb">NORM_CMD</spanx> messages are transmitted by
senders to perform a number of different protocol functions. This
includes functions such as round-trip timing collection, congestion
control functions, synchronization of sender/receiver repair
"windows", and notification of sender status. A core set of <spanx
style="verb">NORM_CMD</spanx> messages is enumerated. Additionally,
a range of command types remain available for potential
application-specific use. Some <spanx style="verb">NORM_CMD</spanx>
types may have dynamic content attached. Any attached content will
be limited to maximum length of the sender <spanx style="emph">NormSegmentSize</spanx>
to retain the atomic nature of commands. All <spanx style="verb">NORM_CMD</spanx>
messages begin with a common set of fields, after the usual NORM
message common header. The standard <spanx style="verb">NORM_CMD</spanx>
fields are:</t>
<figure align="center" title="NORM_CMD Standard Fields">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor | |
+-+-+-+-+-+-+-+-+ NORM_CMD Content +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "version", "type", "hdr_len", "sequence", and "source_id"
fields form the NORM Common Message Header as described in <xref
target="CommonHeader"></xref>. The value of the "hdr_len" field for
<spanx style="verb">NORM_CMD</spanx> messages without header
extensions present depends upon the "flavor" field.</t>
<t>The "instance_id", "grtt", "backoff", and "gsize" fields provide
the same information and serve the same purpose as with <spanx
style="verb">NORM_DATA</spanx> and <spanx style="verb">NORM_INFO</spanx>
messages. The "flavor" field indicates the type of command to
follow. The remainder of the <spanx style="verb">NORM_CMD</spanx>
message is dependent upon the command type ("flavor"). NORM command
flavors include:</t>
<texttable>
<ttcol>Command</ttcol>
<ttcol>Flavor</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_CMD(FLUSH)</spanx></c>
<c>1</c>
<c>Used to indicate sender temporary end-of-transmission. (Assists
in robustly initiating outstanding repair requests from
receivers). May also be optionally used to collect positive
acknowledgment of reliable reception from subset of receivers.</c>
<c><spanx style="verb">NORM_CMD(EOT)</spanx></c>
<c>2</c>
<c>Used to indicate sender permanent end-of-transmission.</c>
<c><spanx style="verb">NORM_CMD(SQUELCH)</spanx></c>
<c>3</c>
<c>Used to advertise sender's current repair window in response to
out-of-range NACKs from receivers.</c>
<c><spanx style="verb">NORM_CMD(CC)</spanx></c>
<c>4</c>
<c>Used for GRTT measurement and collection of congestion control
feedback.</c>
<c><spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx></c>
<c>5</c>
<c>Used to advertise sender's aggregated repair/feedback state for
suppression of unicast feedback from receivers.</c>
<c><spanx style="verb">NORM_CMD(ACK_REQ)</spanx></c>
<c>6</c>
<c>Used to request application-defined positive acknowledgment
from a list of receivers (OPTIONAL).</c>
<c><spanx style="verb">NORM_CMD(APPLICATION)</spanx></c>
<c>7</c>
<c>Used for application-defined purposes which may need to
temporarily preempt data transmission (OPTIONAL).</c>
</texttable>
<section title="NORM_CMD(FLUSH) Message">
<t>The <spanx style="verb">NORM_CMD(FLUSH)</spanx> command is sent
when the sender reaches the end of all data content and pending
repairs it has queued for transmission. This may indicate a
temporary or permanent end of data transmission, but the sender is
still willing to respond to repair requests. This command is
repeated once per <spanx style="verb">2*GRTT</spanx> to excite the
receiver set for any outstanding repair requests up to and
including the transmission point indicated within the <spanx
style="verb">NORM_CMD(FLUSH)</spanx> message. The number of
repeats is equal to <spanx style="verb">NORM_ROBUST_FACTOR</spanx>
unless a list of receivers from which explicit positive
acknowledgment is expected ("acking_node_list") is given. In that
case, the "acking_node_list" is updated as acknowledgments are
received and the <spanx style="verb">NORM_CMD(FLUSH)</spanx> is
repeated according to the mechanism described in <xref
target="PositiveAcknowledgment"></xref>. The greater the <spanx
style="verb">NORM_ROBUST_FACTOR</spanx>, the greater the
probability that all applicable receivers will be excited for
acknowledgment or repair requests (NACKs) AND that the
corresponding NACKs are delivered to the sender. A default value
of <spanx style="verb">NORM_ROBUST_FACTOR</spanx> equal to 20 is
RECOMMENDED. If a <spanx style="verb">NORM_NACK</spanx> message
interrupts the flush process, the sender SHALL re-initiate the
flush process after any resulting repair transmissions are
completed.</t>
<t>Note that receivers also employ a timeout mechanism to
self-initiate NACKing (if there are outstanding repair needs) when
no messages of any type are received from a sender. This
inactivity timeout is related to the <spanx style="verb">NORM_CMD(FLUSH)</spanx>
and <spanx style="verb">NORM_ROBUST_FACTOR</spanx> and is
specified in <xref target="NackProcedure"></xref>. Receivers SHALL
self-initiate the NACK repair process when the inactivity timeout
has expired for a specific sender and the receiver has pending
repairs needed from that sender. With a sufficiently large <spanx
style="verb">NORM_ROBUST_FACTOR</spanx> value, data content is
delivered with a high assurance of reliability. The penalty of a
large <spanx style="verb">NORM_ROBUST_FACTOR</spanx> value is the
potential transmission of excess <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages and a longer inactivity timeout for receivers to
self-initiate a terminal NACK process.</t>
<t>For finite-size transport objects such as <spanx style="verb">NORM_OBJECT_DATA</spanx>
and <spanx style="verb">NORM_OBJECT_FILE</spanx>, the flush
process (if there are no further pending objects) occurs at the
end of these objects. Thus, FEC repair information is always
available for repairs in response to repair requests elicited by
the flush command. However, for <spanx style="verb">NORM_OBJECT_STREAM</spanx>,
the flush may occur at any time, including in the middle of an FEC
coding block if systematic FEC codes are employed. In this case,
the sender will not yet be able to provide FEC parity content for
the concurrent coding block and will be limited to explicitly
repairing the stream with source data content for that block.
Applications that anticipate frequent flushing of stream content
SHOULD be judicious in the selection of the FEC coding block size
(i.e., do not use a very large coding block size if frequent
flushing occurs). For example, a reliable multicast application
transmitting an on-going series of intermittent, relatively small
messages will need to trade-off using the <spanx style="verb">NORM_OBJECT_DATA</spanx>
paradigm versus the <spanx style="verb">NORM_OBJECT_STREAM</spanx>
paradigm with an appropriate FEC coding block size. This is
analogous to application trade-offs for other transport protocols
such as the selection of different TCP modes of operation such as
"no delay", etc.</t>
<figure align="center" title="NORM_CMD(FLUSH) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 1 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "version", "type", "hdr_len", "sequence", and "source_id"
fields form the NORM Common Message Header as described in <xref
target="CommonHeader"></xref>. In addition to the NORM common
message header and standard <spanx style="verb">NORM_CMD</spanx>
fields, the <spanx style="verb">NORM_CMD(FLUSH)</spanx> message
contains fields to identify the current status and logical
transmit position of the sender.</t>
<t>The "fec_id" field indicates the FEC type used for the flushing
"object_transport_id" and implies the size and format of the
"fec_payload_id" field. Note the "hdr_len" value for the <spanx
style="verb">NORM_CMD(FLUSH)</spanx> message is 4 plus the size of
the "fec_payload_id" field when no header extensions are
present.</t>
<t>The "object_transport_id" and "fec_payload_id" fields indicate
the sender's current logical "transmit position". These fields are
interpreted in the same manner as in the <spanx style="verb">NORM_DATA</spanx>
message type. Upon receipt of the <spanx style="verb">NORM_CMD(FLUSH)</spanx>,
receivers are expected to check their completion state THROUGH
(including) this transmission position. If receivers have
outstanding repair needs in this range, they SHALL initiate the
NORM NACK Repair Process as described in <xref
target="NackProcedure"></xref>. If receivers have no outstanding
repair needs, no response to the <spanx style="verb">NORM_CMD(FLUSH)</spanx>
is generated.</t>
<t>For <spanx style="verb">NORM_OBJECT_STREAM</spanx> objects
using systematic FEC codes, receivers MUST request "explicit-only"
repair of the identified "source_block_number" if the given
"encoding_symbol_id" is less than the "source_block_len". This
condition indicates the sender has not yet completed encoding the
corresponding FEC block and parity content is not yet available.
An "explicit-only" repair request consists of NACK content for the
applicable "source_block_number" which does not include any
requests for parity-based repair. This allows NORM sender
applications to "flush" an ongoing stream of transmission when
needed, even if in the middle of an FEC block. Once the sender
resumes stream transmission and passes the end of the pending
coding block, subsequent NACKs from receivers SHALL request
parity-based repair as usual. Note that the use of a systematic
FEC code is assumed here. It should also be noted that a sender
has the option of arbitrarily shortening a given code block when
such an application "flush" occurs. In this case, the receiver
will request explicit repair, but the sender MAY provide FEC-based
repair (parity segments) in response. These parity segments MUST
contain the corrected "source_block_len" for the shortened block
and that "source_block_len" associated with segments containing
parity content SHALL override the previously advertised
"source_block_len". Similarly, the "source_block_len" associated
with the highest ordinal "encoding_symbol_id" shall take
precedence over prior symbols when a difference (e.g., due to code
shortening at the sender) occurs. Normal receiver NACK initiation
and construction is discussed in detail in <xref
target="NackProcedure"></xref>.</t>
<t>The OPTIONAL "acking_node_list" field contains a list of <spanx
style="emph">NormNodeIds</spanx> for receivers from which the
sender is requesting explicit positive acknowledgment of reception
up through the transmission point identified by the
"object_transport_id" and "fec_payload_id" fields. The length of
the list can be inferred from the length of the received <spanx
style="verb">NORM_CMD(FLUSH)</spanx> message. When the
"acking_node_list" is present, the lightweight positive
acknowledgment process described in <xref
target="PositiveAcknowledgment"></xref> SHALL be observed.</t>
</section>
<section title="NORM_CMD(EOT) Message">
<t>The <spanx style="verb">NORM_CMD(EOT)</spanx> command is sent
when the sender reaches permanent end-of-transmission with respect
to the <spanx style="emph">NormSession</spanx> and will not
respond to further repair requests. This allows receivers to
gracefully reach closure of operation with this sender (without
requiring any timeout) and free any resources that are no longer
needed. The <spanx style="verb">NORM_CMD(EOT)</spanx> command
SHOULD be sent with the same robust mechanism as used for <spanx
style="verb">NORM_CMD(FLUSH)</spanx> commands to provide a high
assurance of reception by the receiver set.</t>
<figure align="center" title="NORM_CMD(EOT) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The value of the "hdr_len" field for <spanx style="verb">NORM_CMD(EOT)</spanx>
messages without header extensions present is 4. The "reserved"
field is reserved for future use and MUST be set to an all ZERO
value. Receivers MUST ignore the "reserved" field.</t>
</section>
<section title="NORM_CMD(SQUELCH) Message">
<t>The <spanx style="verb">NORM_CMD(SQUELCH)</spanx> command is
transmitted in response to outdated or invalid <spanx
style="verb">NORM_NACK</spanx> content received by the sender.
Invalid <spanx style="verb">NORM_NACK</spanx> content consists of
repair requests for <spanx style="emph">NormObjects</spanx> for
which the sender is unable or unwilling to provide repair. This
includes repair requests for outdated objects, aborted objects, or
those objects which the sender previously transmitted marked with
the <spanx style="verb">NORM_FLAG_UNRELIABLE</spanx> flag. This
command indicates to receivers what content is available for
repair, thus serving as a description of the sender's current
"repair window". Receivers SHALL NOT generate repair requests for
content identified as invalid by a <spanx style="verb">NORM_CMD(SQUELCH)</spanx>.</t>
<t>The <spanx style="verb">NORM_CMD(SQUELCH)</spanx> command is
sent once per <spanx style="verb">2*GRTT</spanx> at the most. The
<spanx style="verb">NORM_CMD(SQUELCH)</spanx> advertises the
current "repair window" of the sender by identifying the earliest
(lowest) transmission point for which it will provide repair,
along with an encoded list of objects from that point forward that
are no longer valid for repair. This mechanism allows the sender
application to cancel or abort transmission and/or repair of
specific previously enqueued objects. The list also contains the
identifiers for any objects within the repair window that were
sent with the <spanx style="verb">NORM_FLAG_UNRELIABLE</spanx>
flag set. In normal conditions, it is expected the <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> will be needed
infrequently, and generally only to provide a reference repair
window for receivers who have fallen "out-of-sync" with the sender
due to extremely poor network conditions.</t>
<t>The starting point of the invalid <spanx style="emph">NormObject</spanx>
list begins with the lowest invalid <spanx style="emph">NormTransportId</spanx>
greater than the current "repair window" start from the invalid
NACK(s) that prompted the generation of the squelch. The length of
the list is limited by the sender's <spanx style="emph">NormSegmentSize</spanx>.
This allows the receivers to learn the status of the sender's
applicable object repair window with minimal transmission of
<spanx style="verb">NORM_CMD(SQUELCH)</spanx> commands. The format
of the <spanx style="verb">NORM_CMD(SQUELCH)</spanx> message
is:</t>
<figure align="center" title="NORM_CMD(SQUELCH) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 3 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>In addition to the NORM common message header and standard
<spanx style="verb">NORM_CMD</spanx> fields, the <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> message contains fields to
identify the earliest logical transmit position of the sender's
current repair window and an "invalid_object_list" beginning with
the index of the logically earliest invalid repair request from
the offending NACK message which initiated the <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> transmission. The value of
the "hdr_len" field when no extensions are present is 4 plus the
size of the "fec_payload_id" field that is dependent upon the FEC
scheme identified by the "fec_id" field.</t>
<t>The "object_transport_id" and "fec_payload_id" fields are
concatenated to indicate the beginning of the sender's current
repair window (i.e., the logically earliest point in its
transmission history for which the sender can provide repair). The
"fec_id" field implies the size and format of the "fec_payload_id"
field. This serves as an advertisement of a "synchronization"
point for receivers to request repair. Note, that while an
"encoding_symbol_id" may be included in the "fec_payload_id"
field, the sender's repair window SHOULD be aligned on FEC coding
block boundaries and thus the "encoding_symbol_id" SHOULD be
zero.</t>
<t>The "invalid_object_list" is a list of 16-bit <spanx
style="emph">NormTransportIds</spanx> that, although they are
within the range of the sender's current repair window, are no
longer available for repair from the sender. For example, a sender
application may dequeue an out-of-date object even though it is
still within the repair window. The total size of the
"invalid_object_list" content is can be determined from the
packet's payload length and is limited to a maximum of the <spanx
style="emph">NormSegmentSize</spanx> of the sender. Thus, for very
large repair windows, it is possible that a single <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> message may not be capable
of listing the entire set of invalid objects in the repair window.
In this case, the sender SHALL ensure that the list begins with a
<spanx style="emph">NormObjectId</spanx> that is greater than or
equal to the lowest ordinal invalid <spanx style="emph">NormObjectId</spanx>
from the NACK message(s) that prompted the <spanx style="verb">NORM_CMD(SQUELCH)</spanx>
generation. The <spanx style="emph">NormObjectIds</spanx> in the
"invalid_object_list" MUST be ordinally greater than the
"object_transport_id" marking the beginning of the sender's repair
window. This insures convergence of the squelch process, even if
multiple invalid NACK/ squelch iterations are required. This
explicit description of invalid content within the sender's
current window allows the sender application (most notably for
discrete object transport) to arbitrarily invalidate (i.e.,
dequeue) portions of enqueued content (e.g., certain objects) for
which it no longer wishes to provide reliable transport.</t>
</section>
<section title="NORM_CMD(CC) Message">
<t>The <spanx style="verb">NORM_CMD(CC)</spanx> messages contains
fields to enable sender-to-receiver group greatest round-trip time
(GRTT) measurement and to excite the group for congestion control
feedback. A baseline NORM congestion control scheme (NORM-CC),
based on the TCP-Friendly Multicast Congestion Control (TFMCC)
scheme of <xref format="none" target="RFC4654">RFC 4654</xref> is
fully specified in <xref target="CongestionControl"></xref> of
this document. The <spanx style="verb">NORM_CMD(CC)</spanx>
message is usually transmitted as part of NORM-CC congestion
control operation. A NORM header extension is defined below to be
used with the <spanx style="verb">NORM_CMD(CC)</spanx> message to
support NORM-CC operation. Different header extensions may be
defined for the <spanx style="verb">NORM_CMD(CC)</spanx> (and/or
other NORM messages as needed) to support alternative congestion
control schemes in the future. If NORM is operated in a network
where resources are explicitly dedicated to the NORM session and
therefore congestion control operation is disabled, the <spanx
style="verb">NORM_CMD(CC)</spanx> message is then used soley for
GRTT measurement and may optionally be sent less frequently than
with congestion control operation.</t>
<figure align="center" title="NORM_CMD(CC) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 4 | reserved | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The NORM common message header and standard <spanx
style="verb">NORM_CMD</spanx> fields serve their usual purposes.
The value of the "hdr_len" field when no header extensions are
present is 6.</t>
<t>The "reserved" field is for potential future use and MUST be
set to ZERO in this version of the NORM protocol and its baseline
NORM-CC congestion control scheme. It may be possible that
alternative congestion control schemes may use the <spanx
style="verb">NORM_CMD(CC)</spanx> message defined here and
leverage the "reserved" field for scheme-specific purposes.</t>
<t>The "cc_sequence" field is a sequence number applied by the
sender. For NORM-CC operation, it is used to provide functionality
equivalent to the "feedback round number" (<spanx style="verb">fb_nr</spanx>)
described in <xref format="none" target="RFC4654">RFC 4654</xref>.
The most recently received "cc_sequence" value is recorded by
receivers and can be fed back to the sender in congestion control
feedback generated by the receivers for that sender. The
"cc_sequence" number can also be used in NORM implementations to
assess how recently a receiver has received <spanx style="verb">NORM_CMD(CC)</spanx>
probes from the sender. This can be useful instrumentation for
complex or experimental multicast routing environments.</t>
<t>The "send_time" field is a timestamp indicating the time that
the <spanx style="verb">NORM_CMD(CC)</spanx> message was
transmitted. This consists of a 64-bit field containing 32-bits
with the time in seconds ("send_time_sec") and 32-bits with the
time in microseconds ("send_time_usec") since some reference time
the source maintains (usually 00:00:00, 1 January 1970). The byte
ordering of the fields is "Big Endian" network order. Receivers
use this timestamp adjusted by the amount of delay from the time
they received the <spanx style="verb">NORM_CMD(CC)</spanx> message
to the time of their response as the "grtt_response" portion of
<spanx style="verb">NORM_ACK</spanx> and <spanx style="verb">NORM_NACK</spanx>
messages generated. This allows the sender to evaluate round-trip
times to different receivers for congestion control and other
(e.g., GRTT determination) purposes.</t>
<t>To facilitate the baseline NORM-CC scheme described in <xref
target="CongestionControl"></xref>, a NORM-CC Rate header
extension (EXT_RATE) is defined to inform the group of the
sender's current transmission rate. This is used along with the
loss detection "sequence" field of all NORM sender messages and
the <spanx style="verb">NORM_CMD(CC)</spanx> GRTT collection
process to support NORM-CC congestion control operation. The
format of this header extension is as follows:</t>
<figure align="center">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 128 | reserved | send_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "send_rate" field indicates the sender's current
transmission rate in bytes per second. The 16-bit "send_rate"
field consists of 12 bits of mantissa in the most significant
portion and 4 bits of base 10 integer exponent (E) information in
the least significant portion. The 12-bit mantissa portion of the
field is scaled such that a base 10 mantissa (M) floating point
value of 0.0 corresponds to 0 and a value of 10.0 corresponds to
4096 in the upper 12 bits of the 16-bit "send_rate" field.
Thus:</t>
<figure align="center">
<artwork align="center"><![CDATA[
send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;]]></artwork>
</figure>
<t>For example, to represent a transmission rate of 256kbps
(3.2e+04 bytes per second), the lower 4 bits of the 16-bit field
contain a value of 0x04 to represent the exponent (E) while the
upper 12 bits contain a value of 0x51f (M) as determined from the
equation given above:</t>
<figure align="center">
<artwork align="center"><![CDATA[send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
= (0x51f << 4) | 0x4
= 0x51f4]]></artwork>
</figure>
<t>To decode the "send_rate" field, the following equation can be
used:</t>
<figure align="center">
<artwork align="center"><![CDATA[value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f))]]></artwork>
</figure>
<t>Note the maximum transmission rate that can be represented by
this scheme is approximately 9.99e+15 bytes per second.</t>
<t>When this extension is present, a "cc_node_list" may be
attached as the payload of the <spanx style="verb">NORM_CMD(CC)</spanx>
message. The presence of this header extension also implies that
NORM receivers should respond according to the procedures
described in <xref target="CongestionControl"></xref>.</t>
<t>The "cc_node_list" consists of a list of <spanx style="emph">NormNodeIds</spanx>
and their associated congestion control status. This includes the
current limiting receiver (CLR) node, any potential limiting
receiver (PLR) nodes that have been identified, and some number of
receivers for which congestion control status is being provided,
most notably including the receivers' current RTT measurement. The
maximum length of the "cc_node_list" provides for at least the CLR
and one other receiver, but may be configurable for more timely
feedback to the group. The list length can be inferred from the
length of the <spanx style="verb">NORM_CMD(CC)</spanx>
message.</t>
<t>Each item in the "cc_node_list" is in the following format:</t>
<figure align="center">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "cc_node_id" is the <spanx style="emph">NormNodeId</spanx>
of the receiver which the item represents.</t>
<t>The "cc_flags" field contains flags indicating the congestion
control status of the indicated receiver. The following flags are
defined:</t>
<texttable>
<ttcol>Flag</ttcol>
<ttcol>Value</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_FLAG_CC_CLR</spanx></c>
<c>0x01</c>
<c>Receiver is the current limiting receiver (CLR).</c>
<c><spanx style="verb">NORM_FLAG_CC_PLR</spanx></c>
<c>0x02</c>
<c>Receiver is a potential limiting receiver (PLR).</c>
<c><spanx style="verb">NORM_FLAG_CC_RTT</spanx></c>
<c>0x04</c>
<c>Receiver has measured RTT with respect to sender.</c>
<c><spanx style="verb">NORM_FLAG_CC_START</spanx></c>
<c>0x08</c>
<c>Sender/receiver is in "slow start" phase of congestion
control operation (i.e., The receiver has not yet detected any
packet loss and the "cc_rate" field is the receiver's actual
measured receive rate).</c>
<c><spanx style="verb">NORM_FLAG_CC_LEAVE</spanx></c>
<c>0x10</c>
<c>Receiver is imminently leaving the session and its feedback
should not be considered in congestion control operation.</c>
</texttable>
<t>The "cc_rtt" contains a quantized representation of the RTT as
measured by the sender with respect to the indicated receiver.
This field is valid only if the <spanx style="verb">NORM_FLAG_CC_RTT</spanx>
flag is set in the "cc_flags" field. This one byte field is a
quantized representation of the RTT using the algorithm described
in the <xref target="RFC5401">Multicast NACK Building
Block</xref>.</t>
<t>The "cc_rate" field contains a representation of the receiver's
current calculated (during steady-state congestion control
operation) or twice its measured (during the <spanx style="emph">slow start</spanx>
phase) congestion control rate. This field is encoded and decoded
using the same technique as described for the <spanx style="verb">NORM_CMD(CC)</spanx>
"send_rate" field.</t>
</section>
<section title="NORM_CMD(REPAIR_ADV) Message">
<t>The <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> message is
used by the sender to "advertise" its aggregated repair state from
<spanx style="verb">NORM_NACK</spanx> messages accumulated during
a repair cycle and/or congestion control feedback received. This
message is sent only when the sender has received <spanx
style="verb">NORM_NACK</spanx> and/or <spanx style="verb">NORM_ACK(CC)</spanx>
(when congestion control is enabled) messages via unicast
transmission instead of multicast. By relaying this information to
the receiver set, suppression of feedback can be achieved even
when receivers are unicasting that feedback instead of
multicasting it among the group <xref
target="NormFeedback"></xref>.</t>
<figure align="center" title="NORM_CMD(REPAIR_ADV) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 5 | flags | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_adv_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "instance_id", "grtt", "backoff", "gsize", and "flavor"
fields serve the same purpose as in other <spanx style="verb">NORM_CMD</spanx>
messages. The value of the "hdr_len" field when no extensions are
present is 4.</t>
<t>The "flags" field provide information on the <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> content. There is
currently one <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
flag defined:</t>
<figure align="center">
<artwork align="center"><![CDATA[NORM_REPAIR_ADV_FLAG_LIMIT = 0x01]]></artwork>
</figure>
<t>This flag is set by the sender when it is unable to fit its
full current repair state into a single <spanx style="emph">NormSegmentSize</spanx>.
If this flag is set, receivers should limit their NACK response to
generating NACK content only up through the maximum ordinal
transmission position <spanx style="emph">(objectId::fecPayloadId)</spanx>
included in the "repair_adv_content".</t>
<t>When congestion control operation is enabled, a header
extension may be applied to the <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
representing the most limiting (in terms of congestion control
feedback suppression) congestion control response. This allows the
<spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> message to
suppress receiver congestion control responses as well as NACK
feedback messages. The field is defined as a header extension so
that alternative congestion control schemes may be used with NORM
without revision to this document. A NORM-CC Feedback Header
Extension (EXT_CC) is defined to encapsulate congestion control
feedback within <spanx style="verb">NORM_NACK</spanx>, <spanx
style="verb">NORM_ACK</spanx>, and <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
messages. If another congestion control technique (e.g., Pragmatic
General Multicast Congestion Control (PGMCC) <xref
target="PgmccPaper"></xref>) is used within a NORM implementation,
an additional header extension MAY need to be defined encapsulate
any required feedback content. The NORM-CC Feedback Header
Extension format is:</t>
<figure align="center">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 3 | hel = 3 | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "cc_sequence" field contains the current greatest
"cc_sequence" value receivers have received in <spanx
style="verb">NORM_CMD(CC)</spanx> messages from the sender. This
information assists the sender in congestion control operation by
providing an indicator of how current ("fresh") the receiver's
round-trip measurement reference time is and whether the receiver
has been successfully receiving recent congestion control probes.
For example, if it is apparent the receiver has not been receiving
recent congestion control probes (and thus possibly other messages
from the sender), the sender may choose to take congestion
avoidance measures. For <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
messages, the sender SHALL set the "cc_sequence" field value to
the value set in the last <spanx style="verb">NORM_CMD(CC)</spanx>
message sent.</t>
<t>The "cc_flags" field contains bits representing the receiver's
state with respect to congestion control operation. The possible
values for the "cc_flags" field are those specified for the <spanx
style="verb">NORM_CMD(CC)</spanx> message node list item flags.
These fields are used by receivers in controlling (suppressing as
necessary) their congestion control feedback. For <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages, the <spanx
style="verb">NORM_FLAG_CC_RTT</spanx> should be set <spanx
style="emph">only</spanx> when <spanx style="emph">all</spanx>
feedback messages received by the sender have the flag set.
Similarly, the <spanx style="verb">NORM_FLAG_CC_CLR</spanx> or
<spanx style="verb">NORM_FLAG_CC_PLR</spanx> should be set only
when <spanx style="emph">no</spanx> feedback has been received
from non-CLR or non-PLR receivers. And the <spanx style="verb">NORM_FLAG_CC_LEAVE</spanx>
should be set only when all feedback messages the sender has
received have this flag set. These heuristics for setting the
flags in <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> ensure
the most effective suppression of receivers providing unicast
feedback messages.</t>
<t>The "cc_rtt" field SHALL be set to a default maximum value and
the <spanx style="verb">NORM_FLAG_CC_RTT</spanx> flag SHALL be
cleared when no receiver has yet received RTT measurement
information. When a receiver has received RTT measurement
information, it shall set the "cc_rtt" value accordingly and set
the <spanx style="verb">NORM_FLAG_CC_RTT</spanx> flag in the
"cc_flags" field. For <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
messages, the sender SHALL set the "cc_rtt" field value to the
largest non-CLR/non-PLR RTT it has measured from receivers for the
current feedback round.</t>
<t>The "cc_loss" field represents the receiver's current packet
loss fraction estimate for the indicated source. The loss fraction
is a value from 0.0 to 1.0 corresponding to a range of zero to 100
percent packet loss. The 16-bit "cc_loss" value is calculated by
the following formula:</t>
<figure align="center">
<artwork align="center"><![CDATA[
"cc_loss" = floor(decimal_loss_fraction * 65535.0)]]></artwork>
</figure>
<t>For <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages,
the sender SHALL set the "cc_loss" field value to the largest
non-CLR/non-PLR loss estimate it has received from receivers for
the current feedback round.</t>
<t>The "cc_rate" field represents the receivers current local
congestion control rate. During "slow start", when the receiver
has detected no loss, this value is set to twice the actual rate
it has measured from the corresponding sender and the <spanx
style="verb">NORM_FLAG_CC_START</spanx> is set in the "cc_flags'
field. Otherwise, the receiver calculates a congestion control
rate based on its loss measurement and RTT measurement information
(even if default) for the "cc_rate" field. For <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages, the sender
SHALL set the "cc_loss" field value to the lowest non-CLR/non-PLR
"cc_rate" report it has received from receivers for the current
feedback round.</t>
<t>The "cc_reserved" field is reserved for future NORM protocol
use. Currently, senders SHALL set this field to ZERO, and
receivers SHALL ignore the content of this field.</t>
<t>The "repair_adv_payload" is in exactly the same form as the
"nack_content" of <spanx style="verb">NORM_NACK</spanx> messages
and can be processed by receivers for suppression purposes in the
same manner, with the exception of the condition when the <spanx
style="verb">NORM_REPAIR_ADV_FLAG_LIMIT</spanx> is set.</t>
</section>
<section title="NORM_CMD(ACK_REQ) Message">
<t>The <spanx style="verb">NORM_CMD(ACK_REQ)</spanx> message is
used by the sender to request acknowledgment from a specified list
of receivers. This message is used in providing a lightweight
positive acknowledgment mechanism that is OPTIONAL for use by the
reliable multicast application. A range of acknowledgment request
types is provided for use at the application's discretion.
Provision for application-defined, positively-acknowledged
commands allows the application to automatically take advantage of
transmission and round-trip timing information available to the
NORM protocol. The details of the NORM positive acknowledgment
process including transmission of the <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
messages and the receiver response (<spanx style="verb">NORM_ACK</spanx>)
are described in <xref target="PositiveAcknowledgment"></xref>.
The format of the <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
message is:</t>
<figure align="center" title="NORM_CMD(ACK_REQ) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 6 | reserved | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The NORM common message header and standard <spanx
style="verb">NORM_CMD</spanx> fields serve their usual purposes.
The value of the "hdr_len" field for <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
messages with no header extension present is 4.</t>
<t>The "ack_type" field indicates the type of acknowledgment being
requested and thus implies rules for how the receiver will treat
this request. The following "ack_type" values are defined and are
also used in <spanx style="verb">NORM_ACK</spanx> messages
described later:</t>
<texttable>
<ttcol>ACK Type</ttcol>
<ttcol>Value</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_ACK_CC</spanx></c>
<c>1</c>
<c>Used to identify <spanx style="verb">NORM_ACK</spanx>
messages sent in response to <spanx style="verb">NORM_CMD(CC)</spanx>
messages.</c>
<c><spanx style="verb">NORM_ACK_FLUSH</spanx></c>
<c>2</c>
<c>Used to identify <spanx style="verb">NORM_ACK</spanx>
messages sent in response to <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages.</c>
<c><spanx style="verb">NORM_ACK_RESERVED</spanx></c>
<c>3-15</c>
<c>Reserved for possible future NORM protocol use.</c>
<c><spanx style="verb">NORM_ACK_APPLICATION</spanx></c>
<c>16-255</c>
<c>Used at application's discretion.</c>
</texttable>
<t>The <spanx style="verb">NORM_ACK_CC</spanx> value is provided
for use only in <spanx style="verb">NORM_ACKs</spanx> generated in
response to the <spanx style="verb">NORM_CMD(CC)</spanx> messages
used in congestion control operation. Similarly, the <spanx
style="verb">NORM_ACK_FLUSH</spanx> is provided for use only in
<spanx style="verb">NORM_ACKs</spanx> generated in response to
applicable <spanx style="verb">NORM_CMD(FLUSH)</spanx> messages.
<spanx style="verb">NORM_CMD</spanx>(ACK_REQ) messages with
"ack_type" of <spanx style="verb">NORM_ACK_CC</spanx> or <spanx
style="verb">NORM_ACK_FLUSH</spanx> SHALL NOT be generated by the
sender.</t>
<t>The <spanx style="verb">NORM_ACK_RESERVED</spanx> range of
"ack_type" values is provided for possible future NORM protocol
use.</t>
<t>The <spanx style="verb">NORM_ACK_APPLICATION</spanx> range of
"ack_type" values is provided so that NORM applications may
implement application-defined, positively-acknowledged commands
that are able to leverage internal transmission and round-trip
timing information available to the NORM protocol
implementation.</t>
<t>The "ack_id" provides a sequenced identifier for the given
<spanx style="verb">NORM_CMD(ACK_REQ)</spanx> message. This
"ack_id" is returned in <spanx style="verb">NORM_ACK</spanx>
messages generated by the receivers so that the sender may
associate the response with its corresponding request.</t>
<t>The "reserved" field is reserved for possible future protocol
use and SHALL be set to ZERO by senders and ignored by
receivers.</t>
<t>The "acking_node_list" field contains the <spanx style="emph">NormNodeIds</spanx>
of the current NORM receivers that are desired to provide positive
acknowledge (<spanx style="verb">NORM_ACK</spanx>) to this
request. The packet payload length implies the length of the
"acking_node_list" and its length is limited to the sender <spanx
style="emph">NormSegmentSize</spanx>. The individual <spanx
style="emph">NormNodeId</spanx> items are listed in network (Big
Endian) byte order. If a receiver's <spanx style="emph">NormNodeId</spanx>
is included in the "acking_node_list", it SHALL schedule
transmission of a <spanx style="verb">NORM_ACK</spanx> message as
described in <xref target="PositiveAcknowledgment"></xref>.</t>
</section>
<section title="NORM_CMD(APPLICATION) Message">
<t>This command allows the NORM application to robustly transmit
application-defined commands. The command message preempts any
ongoing data transmission and is repeated up to <spanx
style="verb">NORM_ROBUST_FACTOR</spanx> times at a rate of once
per <spanx style="verb">2*GRTT</spanx>. This rate of repetition
allows the application to observe any response (if that is the
application's purpose for the command) before it is repeated.
Possible responses may include initiation of data transmission,
other <spanx style="verb">NORM_CMD(APPLICATION)</spanx> messages,
or even application-defined, positively-acknowledge commands from
other <spanx style="emph">NormSession</spanx> participants. The
transmission of these commands will preempt data transmission when
they are scheduled and may be multiplexed with ongoing data
transmission. This type of robustly transmitted command allows
NORM applications to define a complete set of session control
mechanisms with less state than the transfer of FEC encoded
reliable content requires while taking advantage of NORM
transmission and round-trip timing information.</t>
<figure align="center"
title="NORM_CMD(APPLICATION) Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 7 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application-Defined Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The NORM common message header and <spanx style="verb">NORM_CMD</spanx>
fields are interpreted as previously described. The value of the
<spanx style="verb">NORM_CMD(APPLICATION)</spanx> "hdr_len" field
when no header extensions are present is 4.</t>
<t>The "Application-Defined Content" area contains information in
a format at the discretion of the application. The size of this
payload SHALL be limited to a maximum of the sender's <spanx
style="emph">NormSegmentSize</spanx> setting. Upon reception, the
NORM protocol implementation SHALL deliver the content to the
receiver application. Note that any detection of duplicate
reception of a <spanx style="verb">NORM_CMD(APPLICATION)</spanx>
message is the responsibility of the application.</t>
</section>
</section>
</section>
<section title="Receiver Messages">
<t>The NORM message types generated by participating receivers consist
of the <spanx style="verb">NORM_NACK</spanx> and <spanx style="verb">NORM_ACK</spanx>
message types. <spanx style="verb">NORM_NACK</spanx> messages are sent
to request repair of missing data content from sender transmission and
<spanx style="verb">NORM_ACK</spanx> messages are generated in
response to certain sender commands including <spanx style="verb">NORM_CMD(CC)</spanx>
and <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>.</t>
<section title="NORM_NACK Message">
<t>The principal purpose of <spanx style="verb">NORM_NACK</spanx>
messages is for receivers to request repair of sender content via
selective, negative acknowledgment upon detection of incomplete
data. <spanx style="verb">NORM_NACK</spanx> messages will be
transmitted according to the rules of <spanx style="verb">NORM_NACK</spanx>
generation and suppression described in <xref
target="NackProcedure"></xref>. <spanx style="verb">NORM_NACK</spanx>
messages also contain additional fields to provide feedback to the
sender(s) for purposes of round-trip timing collection and
congestion control.</t>
<t>The payload of <spanx style="verb">NORM_NACK</spanx> messages
contains one or more repair requests for different objects or
portions of those objects. The <spanx style="verb">NORM_NACK</spanx>
message format is as follows:</t>
<figure align="center" title="NORM_NACK Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=4| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nack_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field for <spanx style="verb">NORM_NACK</spanx>
messages without header extensions present is 6.</t>
<t>The "server_id" field identifies the NORM sender to which the
<spanx style="verb">NORM_NACK</spanx> message is destined.</t>
<t>The "instance_id" field contains the current session identifier
given by the sender identified by the "server_id" field in its
sender messages. The sender SHOULD ignore feedback messages which
contain an invalid "instance_id" value.</t>
<t>The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received <spanx style="verb">NORM_CMD(CC)</spanx>
message for the indicated NORM sender. The format of the
"grtt_response" is the same as the "send_time" field of the <spanx
style="verb">NORM_CMD(CC)</spanx>. The "grtt_response" value is
relative to the "send_time" the source provided with a corresponding
<spanx style="verb">NORM_CMD(CC)</spanx> command. The receiver
adjusts the source's <spanx style="verb">NORM_CMD(CC)</spanx>
"send_time" timestamp by adding the time delta from when the
receiver received the <spanx style="verb">NORM_CMD(CC)</spanx> to
when the <spanx style="verb">NORM_NACK</spanx> is transmitted in
response to calculate the value in the "grtt_response" field. This
is the "receive_to_response_delta" value used in the following
formula:</t>
<figure align="center">
<artwork align="center"><![CDATA[grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta]]></artwork>
</figure>
<t>The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate that it has not yet received a <spanx style="verb">NORM_CMD(CC)</spanx>
message from the indicated sender and that the sender should ignore
the "grtt_response" in this message.</t>
<t>For NORM-CC operation, the NORM-CC Feedback Header Extension, as
described in the <spanx style="verb">NORM_CMD(REPAIR_ADV}</spanx>
message description, is added to <spanx style="verb">NORM_NACK</spanx>
messages to provide feedback on the receivers current state with
respect to congestion control operation. Note that alternative
header extensions for congestion control feedback may be defined for
alternative congestion control schemes for NORM use in the
future.</t>
<t>The "reserved" field is for potential future NORM use and SHALL
be set to ZERO for this version of the protocol.</t>
<t>The "nack_payload" of the <spanx style="verb">NORM_NACK</spanx>
message specifies the repair needs of the receiver with respect to
the NORM sender indicated by the "server_id" field. The receiver
constructs repair requests based on the <spanx style="verb">NORM_DATA</spanx>
and/or <spanx style="verb">NORM_INFO</spanx> segments it requires
from the sender in order to complete reliable reception up to the
sender's transmission position at the moment the receiver initiates
the NACK Procedure as described in <xref
target="NackProcedure"></xref>. A single NORM Repair Request
consists of a list of items, ranges, and/or FEC coding block erasure
counts for needed <spanx style="verb">NORM_DATA</spanx> and/or
<spanx style="verb">NORM_INFO</spanx> content. Multiple repair
requests may be concatenated within the "nack_payload" field of a
<spanx style="verb">NORM_NACK</spanx> message. Note that a single
NORM Repair Request can possibly include multiple "items", "ranges",
or "erasure_counts". In turn, the "nack_payload" field MAY contain
multiple repair requests. A single NORM Repair Request has the
following format:</t>
<figure align="center" title="NORM Repair Request Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form | flags | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_request_items |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The "form" field indicates the type of repair request items given
in the "repair_request_items" list. Possible values for the "form"
field include:</t>
<texttable align="center">
<ttcol>Form</ttcol>
<ttcol align="center">Value</ttcol>
<c><spanx style="verb">NORM_NACK_ITEMS</spanx></c>
<c>1</c>
<c><spanx style="verb">NORM_NACK_RANGES</spanx></c>
<c>2</c>
<c><spanx style="verb">NORM_NACK_ERASURES</spanx></c>
<c>3</c>
</texttable>
<t>A "form" value of <spanx style="verb">NORM_NACK_ITEMS</spanx>
indicates each repair request item in the "repair_request_items"
list is to be treated as an individual request. A value of <spanx
style="verb">NORM_NACK_RANGES</spanx> indicates that the
"repair_request_items" list consists of <spanx style="emph">pairs</spanx>
of repair request items that correspond to inclusive ranges of
repair needs. And the <spanx style="verb">NORM_NACK_ERASURES</spanx>
"form" indicates that the repair request items are to be treated
individually and that the "encoding_symbol_id" portion of the
"fec_payload_id" field of the repair request item (see below) is to
be interpreted as an erasure count for the FEC coding block
identified by the repair request item's "source_block_number".</t>
<t>The "flags" field is currently used to indicate the level of data
content for which the repair request items apply (i.e., an
individual segment, entire FEC coding block, or entire transport
object). Possible flag values include:</t>
<texttable>
<ttcol>Flag</ttcol>
<ttcol align="center">Value</ttcol>
<ttcol>Purpose</ttcol>
<c><spanx style="verb">NORM_NACK_SEGMENT</spanx></c>
<c>0x01</c>
<c>Indicates the listed segment(s) or range of segments are
required as repair.</c>
<c><spanx style="verb">NORM_NACK_BLOCK</spanx></c>
<c>0x02</c>
<c>Indicates the listed block(s) or range of blocks in entirety
are required as repair.</c>
<c><spanx style="verb">NORM_NACK_INFO</spanx></c>
<c>0x04</c>
<c>Indicates that <spanx style="verb">NORM_INFO</spanx> is
required as repair for the listed object(s).</c>
<c><spanx style="verb">NORM_NACK_OBJECT</spanx></c>
<c>0x08</c>
<c>Indicates the listed object(s) or range of objects in entirety
are required as repair.</c>
</texttable>
<t>When the <spanx style="verb">NORM_NACK_SEGMENT</spanx> flag is
set, the "object_transport_id" and "fec_payload_id" fields are used
to determine which sets or ranges of individual <spanx style="verb">NORM_DATA</spanx>
segments are needed to repair content at the receiver. When the
<spanx style="verb">NORM_NACK_BLOCK</spanx> flag is set, this
indicates the receiver is completely missing the indicated coding
block(s) and requires transmissions sufficient to repair the
indicated block(s) in their entirety. When the <spanx style="verb">NORM_NACK_INFO</spanx>
flag is set, this indicates the receiver is missing the <spanx
style="verb">NORM_INFO</spanx> segment for the indicated
"object_transport_id". Note the <spanx style="verb">NORM_NACK_INFO</spanx>
may be set in combination with the <spanx style="verb">NORM_NACK_BLOCK</spanx>
or <spanx style="verb">NORM_NACK_SEGMENT</spanx> flags, or may be
set alone. When the <spanx style="verb">NORM_NACK_OBJECT</spanx>
flag is set, this indicates the receiver is missing the entire
<spanx style="emph">NormTransportObject</spanx> referenced by the
"object_transport_id". This also implicitly requests any available
<spanx style="verb">NORM_INFO</spanx> for the <spanx style="emph">NormObject</spanx>,
if applicable. The "fec_payload_id" field is ignored when the flag
<spanx style="verb">NORM_NACK_OBJECT</spanx> is set.</t>
<t>The "length" field value is the length in bytes of the
"repair_request_items" field.</t>
<t>The "repair_request_items" field consists of a list of individual
or range pairs of transport data unit identifiers in the following
format.</t>
<figure align="center" title="NORM Repair Request Item Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "fec_id" indicates the FEC type and can be used to determine
the format of the "fec_payload_id" field. The "reserved" field is
kept for possible future use and SHALL be set to a ZERO value and
ignored by NORM nodes processing NACK content.</t>
<t>The "object_transport_id" corresponds to the <spanx style="emph">NormObject</spanx>
for which repair is being requested and the "fec_payload_id"
identifies the specific FEC coding block and/or segment being
requested. When the <spanx style="verb">NORM_NACK_OBJECT</spanx>
flag is set, the value of the "fec_payload_id" field is ignored.
When the <spanx style="verb">NORM_NACK_BLOCK</spanx> flag is set,
only the FEC code block identifier portion of the "fec_payload_id"
is to be interpreted.</t>
<t>The format of the "fec_payload_id" field depends upon the
"fec_id" field value.</t>
<t>When the receiver's repair needs dictate that different forms
(mixed ranges and/or individual items) or types (mixed specific
segments and/or blocks or objects in entirety) are required to
complete reliable transmission, multiple NORM Repair Requests with
different "form" and or "flags" values can be concatenated within a
single <spanx style="verb">NORM_NACK</spanx> message. Additionally,
NORM receivers SHALL construct <spanx style="verb">NORM_NACK</spanx>
messages with their repair requests in ordinal order with respect to
"object_transport_id" and "fec_payload_id" values. The
"nack_payload" size SHALL NOT exceed the <spanx style="emph">NormSegmentSize</spanx>
for the sender to which the <spanx style="verb">NORM_NACK</spanx> is
destined.</t>
<t><spanx style="strong">NORM_NACK Content Examples:</spanx></t>
<t>In these examples, a small block, systematic FEC code ("fec_id" =
129) is assumed with a user data block length of 32 segments. In
Example 1, a list of individual <spanx style="verb">NORM_NACK_ITEMS</spanx>
repair requests is given. In Example 2, a list of <spanx
style="verb">NORM_NACK_RANGES</spanx> requests AND a single <spanx
style="verb">NORM_NACK_ITEMS</spanx> request are concatenated to
illustrate the possible content of a <spanx style="verb">NORM_NACK</spanx>
message. Note that FEC coding block erasure counts could also be
provided in each case. However, the erasure counts are not really
necessary since the sender can easily determine the erasure count
while processing the NACK content. However, the erasure count option
may be useful for operation with other FEC codes or for intermediate
system purposes.</t>
<figure align="center">
<preamble>Example 1: <spanx style="verb">NORM_NACK</spanx>
"nack_payload" for: Object 12, Coding Block 3, Segments 2,5,and
8</preamble>
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x01 | length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<figure align="center">
<preamble>Example 2: <spanx style="verb">NORM_NACK</spanx>
"nack_payload" for: Object 18, Coding Block 6, Segments 5, 6, 7,
8, 9, 10; and Object 19 <spanx style="verb">NORM_INFO</spanx> and
Coding Block 1, segment 3</preamble>
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 2 | flags = 0x01 | length = 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x05 | length = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 19 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
</section>
<section title="NORM_ACK Message">
<t>The <spanx style="verb">NORM_ACK</spanx> message is intended to
be used primarily as part of NORM congestion control operation and
round-trip timing measurement. As mentioned in the <spanx
style="verb">NORM_CMD(ACK_REQ)</spanx> message description, the
acknowledgment type <spanx style="verb">NORM_ACK_CC</spanx> is
provided for this purpose. The generation of <spanx style="verb">NORM_ACK(CC)</spanx>
messages for round-trip timing estimation and congestion-control
operation is described in <xref target="GrttCollection"></xref> and
<xref target="CongestionControl"></xref>, respectively. However,
some multicast applications may benefit from some limited form of
positive acknowledgment for certain functions. A simple, scalable
positive acknowledgment scheme is defined in <xref
target="PositiveAcknowledgment"></xref> that can be leveraged by
protocol implementations when appropriate. The <spanx style="verb">NORM_CMD(FLUSH)</spanx>
may be used for OPTIONAL collection of positive acknowledgment of
reliable reception to a certain "watermark" transmission point from
specific receivers using this mechanism. The <spanx style="verb">NORM_ACK</spanx>
type <spanx style="verb">NORM_ACK_FLUSH</spanx> is provided for this
purpose and the format of the "nack_payload" for this acknowledgment
type is given below. Beyond that, a range of application-defined
"ack_type" values is provided for use at the NORM application's
discretion. Implementations making use of application-defined
positive acknowledgments may also make use the "nack_payload" as
needed, observing the constraint that the "nack_payload" field size
be limited to a maximum of the <spanx style="emph">NormSegmentSize</spanx>
for the sender to which the <spanx style="verb">NORM_ACK</spanx> is
destined.</t>
<figure align="center" title="NORM_ACK Message Format">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=5| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_payload (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field when no header extensions are
present is 6.</t>
<t>The "server_id", "instance_id", and "grtt_response" fields serve
the same purpose as the corresponding fields in <spanx style="verb">NORM_NACK</spanx>
messages. And header extensions may be applied to support congestion
control feedback or other functions in the same manner.</t>
<t>The "ack_type" field indicates the nature of the <spanx
style="verb">NORM_ACK</spanx> message. This directly corresponds to
the "ack_type" field of the <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
message to which this acknowledgment applies.</t>
<t>The "ack_id" field serves as a sequence number so that the sender
can verify that a <spanx style="verb">NORM_ACK</spanx> message
received actually applies to a current acknowledgment request. The
"ack_id" field is not used in the case of the <spanx style="verb">NORM_ACK_CC</spanx>
and <spanx style="verb">NORM_ACK_FLUSH</spanx> acknowledgment
types.</t>
<t>The "ack_payload" format is a function of the "ack_type". The
<spanx style="verb">NORM_ACK_CC</spanx> message has no attached
content. Only the <spanx style="verb">NORM_ACK</spanx> header
applies. In the case of <spanx style="verb">NORM_ACK_FLUSH</spanx>,
a specific "ack_payload" format is defined:</t>
<figure align="center">
<artwork align="center"><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t>The "object_transport_id" and "fec_payload_id" are used by the
receiver to acknowledge applicable <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages transmitted by the sender identified by the "server_id"
field.</t>
<t>The "ack_payload" of <spanx style="verb">NORM_ACK</spanx>
messages for application-defined "ack_type" values is specific to
the application but is limited in size to a maximum the <spanx
style="emph">NormSegmentSize</spanx> of the sender referenced by the
"server_id".</t>
</section>
</section>
<section title="General Purpose Messages">
<t>Some additional message formats are defined for general purpose in
NORM multicast sessions whether the participant is acting as a sender
and/or receiver within the group.</t>
<section title="NORM_REPORT Message">
<t>This is an OPTIONAL message generated by NORM participants. This
message may be used for periodic performance reports from receivers
in experimental NORM implementations. The format of this message is
currently undefined. Experimental NORM implementations may define
<spanx style="verb">NORM_REPORT</spanx> formats as needed for test
purposes. These report messages SHOULD be disabled for
interoperability testing between different compliant NORM
implementations.</t>
</section>
</section>
</section>
<section anchor="ProtocolDetails" title="Detailed Protocol Operation">
<t>This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of protocol
operation is given here:</t>
<t><list style="numbers">
<t>The sender periodically transmits <spanx style="verb">NORM_CMD(CC)</spanx>
messages as needed to initialize and collect round-trip timing and
congestion control feedback from the receiver set.</t>
<t>The sender transmits an ordinal set of <spanx style="emph">NormObjects</spanx>
segmented in the form of <spanx style="verb">NORM_DATA</spanx>
messages labeled with <spanx style="emph">NormTransportIds</spanx>
and logically identified with FEC encoding block numbers and symbol
identifiers. <spanx style="verb">NORM_INFO</spanx> messages may
optionally precede the transmission of data content for NORM
transport objects.</t>
<t>As receivers detect missing content from the sender, they
initiate repair requests with <spanx style="verb">NORM_NACK</spanx>
messages. Note the receivers track the sender's most recent <spanx
style="emph">objectId::fecPayloadId</spanx> transmit position and
NACK ONLY for content ordinally prior to that transmit position. The
receivers schedule random backoff timeouts before generating <spanx
style="verb">NORM_NACK</spanx> messages and wait an appropriate
amount of time before repeating the <spanx style="verb">NORM_NACK</spanx>
if their repair request is not satisfied.</t>
<t>The sender aggregates repair requests from the receivers and
logically "rewinds" its transmit position to send appropriate repair
messages. The sender sends repairs for the earliest ordinal transmit
position first and maintains this ordinal repair transmission
sequence. FEC parity content not previously transmitted for the
applicable FEC coding block is used for repair transmissions to the
greatest extent possible. If the sender exhausts its available FEC
parity content on multiple repair cycles for the same coding block,
it resorts to an explicit repair strategy (possibly using parity
content) to complete repairs. (The use of explicit repair is
expected to be an exception in general protocol operation, but the
possibility does exist for extreme conditions). The sender
immediately assumes transmission of new content once it has sent
pending repairs.</t>
<t>The sender transmits <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages when it reaches the end of enqueued transmit content and
pending repairs. Receivers respond to the <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages with <spanx style="verb">NORM_NACK</spanx> transmissions
(following the same suppression backoff timeout strategy as for
data) if they require further repair.</t>
<t>The sender transmissions are subject to rate control limits
determined by congestion control mechanisms. In the baseline NORM-CC
operation, each sender in a <spanx style="emph">NormSession</spanx>
maintains its own independent congestion control state. Receivers
provide congestion control feedback in <spanx style="verb">NORM_NACK</spanx>
and <spanx style="verb">NORM_ACK</spanx> messages. <spanx
style="verb">NORM_ACK</spanx> feedback for congestion control
purposes is governed using a suppression mechanism similar to that
for <spanx style="verb">NORM_NACK</spanx> messages.</t>
</list></t>
<t>While this overall concept is relatively simple, there are details to
each of these aspects that need to be addressed for successful,
efficient, robust, and scalable NORM protocol operation.</t>
<section title="Sender Initialization and Transmission">
<t>Upon startup, the NORM sender immediately begins sending <spanx
style="verb">NORM_CMD(CC)</spanx> messages to collect round trip
timing and other information from the potential group. If NORM-CC
congestion control operation is enabled, the NORM-CC Rate header
extension MUST be included in these messages. Congestion control
operation SHALL be observed at all times when not operating using
dedicated resources, like in the general Internet. Even if congestion
control operation is disabled at the sender, it may be desirable to
use the <spanx style="verb">NORM_CMD(CC)</spanx> messaging to collect
feedback from the group using the baseline NORM-CC feedback
mechanisms. This proactive feedback collection can be used to
establish a GRTT estimate prior to data transmission and potential
NACK operation.</t>
<t>In some cases, applications may wish for the sender to also proceed
with data transmission immediately. In other cases, the sender may
wish to defer data transmission until it has received some feedback or
request from the receiver set indicating that receivers are indeed
present. Note, in some applications (e.g., web push), this indication
may come out-of-band with respect to the multicast session via other
means. As noted, the periodic transmission of <spanx style="verb">NORM_CMD(CC)</spanx>
messages may precede actual data transmission in order to have an
initial GRTT estimate.</t>
<t>With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension (EXT_FTI), the NORM protocol sender
message headers can contain all information necessary to prepare
receivers for subsequent reliable reception. This includes FEC coding
parameters, the sender <spanx style="emph">NormSegmentSize</spanx>,
and other information. If this header extension is not used, it is
presumed that receivers have received the FEC Object Transmission
Information via other means. Additionally, applications may leverage
the use of <spanx style="verb">NORM_INFO</spanx> messages associated
with the session data objects in the session to provide
application-specific context information for the session and data
being transmitted. These mechanisms allow for operation with minimal
pre-coordination among the senders and receivers.</t>
<t>The NORM sender begins segmenting application-enqueued data into
<spanx style="verb">NORM_DATA</spanx> segments and transmitting it to
the group. For objects of type <spanx style="verb">NORM_OBJECT_DATA</spanx>
and <spanx style="verb">NORM_OBJECT_FILE</spanx>, the segmentation
algorithm described in <xref target="RFC5052">FEC Building
Block</xref> is RECOMMENDED. For objects of type <spanx style="verb">NORM_OBJECT_STREAM</spanx>,
segmentation will typically be into uniform FEC coding block sizes,
with individual segment sizes controlled by the application. In most
cases, the application and NORM implementation SHOULD strive to
produce full-sized (<spanx style="verb">NormSegmentSize</spanx>)
segments when possible. The rate of transmission is controlled via
congestion control mechanisms or is a fixed rate if desired for closed
network operations. The receivers participating in the multicast group
provide feedback to the sender as needed. When the sender reaches the
end of data it has enqueued for transmission or any pending repairs,
it transmits a series of <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages at a rate of one per <spanx style="verb">2*GRTT</spanx>.
Receivers may respond to these <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages with additional repair requests. A protocol parameter "<spanx
style="verb">NORM_ROBUST_FACTOR</spanx>" determines the number of
flush messages sent. If receivers request repair, the repair is
provided and flushing occurs again at the end of repair transmission.
The sender may attach an OPTIONAL "acking_node_list" to <spanx
style="verb">NORM_CMD(FLUSH)</spanx> containing the <spanx
style="emph">NormNodeIds</spanx> for receivers from which it expects
explicit positive acknowledgment of reception. The <spanx
style="verb">NORM_CMD(FLUSH)</spanx> message may be also used for this
optional function any time prior to the end of data enqueued for
transmission with the <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages multiplexed with ongoing data transmissions. The OPTIONAL
NORM positive acknowledgment procedure is described in <xref
target="PositiveAcknowledgment"></xref>.</t>
<section anchor="SegmentationAlgorithm"
title="Object Segmentation Algorithm">
<t>NORM senders and receivers MUST use a common algorithm for
logically segmenting transport data into FEC encoding blocks and
symbols so that appropriate NACKs can be constructed to request
repair of missing data. NORM FEC coding blocks are comprised of
multi-byte symbols (segments) that are transmitted in the payload of
<spanx style="verb">NORM_DATA</spanx> messages. Each <spanx
style="verb">NORM_DATA</spanx> message will contain one or more
source or encoding symbol(s) identified by the "fec_payload_id"
field and the <spanx style="emph">NormSegmentSize</spanx> sender
parameter defines the maximum size (in bytes) of the "payload_data"
field containing the content (a "segment"). The FEC encoding type
and associated parameters govern the source block size (number of
source symbols per coding block, etc.). NORM senders and receivers
use these FEC parameters, along with the <spanx style="emph">NormSegmentSize</spanx>
and transport object size to compute the source block structure for
transport objects. These parameters are provided in the FEC Object
Transmission Information for each object. The block partitioning
algorithm described in the <xref target="RFC5052">FEC Building
Block</xref> is RECOMMENDED for use to compute a source block
structure such that all source blocks are as close to being equal
length as possible. This helps avoid the performance disadvantages
of "short" FEC blocks. Note this algorithm applies only to the
statically-sized <spanx style="verb">NORM_OBJECT_DATA</spanx> and
<spanx style="verb">NORM_OBJECT_FILE</spanx> transport object types
where the object size is fixed and predetermined. For <spanx
style="verb">NORM_OBJECT_STREAM</spanx> objects, the object is
segmented according to the maximum source block length given in the
FEC Transmission Information, unless the FEC Payload ID indicates an
alternative size for a given block.</t>
</section>
</section>
<section title="Receiver Initialization and Reception">
<t>The NORM protocol is designed such that receivers may join and
leave the group at will. However, some applications may be constrained
such that receivers need to be members of the group prior to start of
data transmission. NORM applications may use different policies to
constrain the impact of new receivers joining the group in the middle
of a session. For example, a useful implementation policy is for new
receivers joining the group to limit or avoid repair requests for
transport objects already in progress. The NORM sender implementation
may wish to impose additional constraints to limit the ability of
receivers to disrupt reliable multicast performance by joining,
leaving, and rejoining the group often. Different receiver "join
policies" may be appropriate for different applications and/or
scenarios. For general purpose operation, a default policy where
receivers are allowed to request repair only for coding blocks with a
<spanx style="emph">NormTransportId</spanx> and FEC coding block
number greater than or equal to the first non-repair <spanx
style="verb">NORM_DATA</spanx> or <spanx style="verb">NORM_INFO</spanx>
message received upon joining the group is RECOMMENDED. For objects of
type <spanx style="verb">NORM_OBJECT_STREAM</spanx> it is RECOMMENDED
that the join policy constrain receivers to start reliable reception
at the current FEC coding block for which non-repair content is
received.</t>
<t>For typical operation, it is expected that NORM receivers will join
a specified multicast group and/or listen on an specific port number
for sender transmissions. As the NORM receiver receives <spanx
style="verb">NORM_DATA</spanx> messages it will provide content to its
application as appropriate.</t>
</section>
<section anchor="NackProcedure" title="Receiver NACK Procedure">
<t>When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
procedure SHALL be initiated ONLY at FEC coding block boundaries,
<spanx style="emph">NormObject</spanx> boundaries, upon receipt of a
<spanx style="verb">NORM_CMD(FLUSH)</spanx> message, or upon an
"inactivity" timeout when <spanx style="verb">NORM_DATA</spanx> or
<spanx style="verb">NORM_INFO</spanx> transmissions are no longer
received from a previously active sender. The RECOMMENDED value of
such an inactivity timeout is:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTTSender]]></artwork>
</figure>
<t>where the "<spanx style="verb">GRTTsender</spanx>" value
corresponds to the GRTT estimate advertised in the "grtt" field of
NORM sender messages. A minimum "<spanx style="verb">T_inactivity</spanx>"
value of 1 second is RECOMMENDED. The NORM receiver SHOULD reset this
inactivity timer and repeat NACK initiation upon timeout for up to
<spanx style="verb">NORM_ROBUST_FACTOR</spanx> times or more depending
upon the application's need for persistence by its receivers. It is
also important that receivers rescale the "<spanx style="verb">T_inactivity</spanx>"
timeout as the sender's advertised GRTT changes.</t>
<t>The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the <xref target="RFC5401">Multicast NACK
Building Block</xref> using (<spanx style="verb">Ksender*GRTTsender</spanx>)
for the <spanx style="verb">maxTime</spanx> parameter and the sender
advertised group size (<spanx style="verb">GSIZEsender</spanx>) as the
<spanx style="verb">groupSize</spanx> parameter. NORM senders provide
values for <spanx style="verb">GRTTsender</spanx>, <spanx
style="verb">Ksender</spanx> and <spanx style="verb">GSIZEsender</spanx>
via the "grtt", "backoff", and "gsize" fields of transmitted messages.
The <spanx style="verb">GRTTsender</spanx> value is determined by the
sender based on feedback it has received from the group while the
<spanx style="verb">Ksender</spanx> and <spanx style="verb">GSIZEsender</spanx>
values may determined by application requirements and expectations or
ancillary information. The backoff factor "<spanx style="verb">Ksender</spanx>"
MUST be greater than <spanx style="verb">one</spanx> to provide for
effective feedback suppression. A value of <spanx style="verb">K = 4</spanx>
is RECOMMENDED for the Any Source Multicast (ASM) model while a value
of <spanx style="verb">K = 6</spanx> is RECOMMENDED for Single Source
Multicast (SSM) operation.</t>
<t>Thus:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)]]></artwork>
</figure>
<t>To avoid the possibility of NACK implosion in the case of sender or
network failure during SSM operation, the receiver SHALL automatically
suppress its NACK and immediately enter the "holdoff" period described
below when <spanx style="verb">T_backoff</spanx> is greater than
<spanx style="verb">(Ksender-1)*GRTTsender</spanx>. Otherwise, the
backoff period is entered and the receiver MUST accumulate external
pending repair state from <spanx style="verb">NORM_NACK</spanx>
messages and <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages
received. At the end of the backoff time, the receiver SHALL generate
a <spanx style="verb">NORM_NACK</spanx> message only if the following
conditions are met:</t>
<t><list style="numbers">
<t>The sender's current transmit position (in terms of <spanx
style="emph">objectId::fecPayloadId</spanx>) exceeds the earliest
repair position of the receiver.</t>
<t>The repair state accumulated from <spanx style="verb">NORM_NACK</spanx>
and <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages do
not equal or supersede the receiver's repair needs up to the
sender transmission position at the time the NACK procedure
(backoff timeout) was initiated.</t>
</list></t>
<t>If these conditions are met, the receiver immediately generates a
<spanx style="verb">NORM_NACK</spanx> message when the backoff timeout
expires. Otherwise, the receiver's NACK is considered to be
"suppressed" and the message is not sent. At this time, the receiver
begins a "holdoff" period during which it constrains itself to not
re-initiate the NACKing process. The purpose of this timeout is to
allow the sender worst-case time to respond to the repair needs before
the receiver requests repair again. The value of this "holdoff"
timeout (<spanx style="verb">T_rcvrHoldoff</spanx>) as described in
<xref target="RFC5401"></xref> is:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_rcvrHoldoff =(Ksender+2)*GRTTsender]]></artwork>
</figure>
<t>The <spanx style="verb">NORM_NACK</spanx> message contains repair
request content beginning with lowest ordinal repair position of the
receiver up through the coding block prior to the most recently heard
ordinal transmission position for the sender. If the size of the
<spanx style="verb">NORM_NACK</spanx> content exceeds the sender's
<spanx style="emph">NormSegmentSize</spanx>, the NACK content is
truncated so that the receiver only generates a single <spanx
style="verb">NORM_NACK</spanx> message per NACK cycle for a given
sender. In summary, a single NACK message is generated containing the
receiver's lowest ordinal repair needs.</t>
<t>For each partially-received FEC coding block requiring repair, the
receiver SHALL, on its FIRST repair attempt for the block, request the
parity portion of the FEC coding block beginning with the lowest
ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
"source_block_len") and request the number of FEC symbols
corresponding to its data segment erasure count for the block. On
subsequent repair cycles for the same coding block, the receiver SHALL
request only those repair symbols from the first set it has not yet
received up to the remaining erasure count for that applicable coding
block. Note that the sender may have provided other different,
additional parity segments for other receivers that could also be used
to satisfy the local receiver's erasure-filling needs. In the case
where the erasure count for a partially-received FEC coding block
exceeds the maximum number of parity symbols available from the sender
for the block (as indicated by the <spanx style="verb">NORM_DATA</spanx>
"fec_num_parity" field), the receiver SHALL request all available
parity segments plus the ordinally highest missing data segments
required to satisfy its total erasure needs for the block. The goal of
this strategy is for the overall receiver set to request a lowest
common denominator set of repair symbols for a given FEC coding block.
This allows the sender to construct the most efficient repair
transmission segment set and enables effective NACK suppression among
the receivers even with uncorrelated packet loss. This approach also
requires no synchronization among the receiver set in their repair
requests for the sender.</t>
<t>For FEC coding blocks or <spanx style="emph">NormObjects</spanx>
missed in their entirety, the NORM receiver constructs repair requests
with <spanx style="verb">NORM_NACK_BLOCK</spanx> or <spanx
style="verb">NORM_NACK_OBJECT</spanx> flags set as appropriate. The
request for retransmission of <spanx style="verb">NORM_INFO</spanx> is
accomplished by setting the <spanx style="verb">NORM_NACK_INFO</spanx>
flag in a corresponding repair request.</t>
</section>
<section title="Sender NACK Processing and Response">
<t>The principle goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the sender
must occasionally "rewind" its logical transmission point to satisfy
the repair needs of receivers who have NACKed. Aggregation of multiple
NACKs is used to determine an optimal repair strategy when a NACK
event occurs. Since receivers initiate the NACK process on coding
block or object boundaries, there is some loose degree of
synchronization of the repair process even when receivers experience
uncorrelated data loss.</t>
<section title="Sender Repair State Aggregation">
<t>When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair
state from <spanx style="verb">NORM_NACK</spanx> messages before
beginning repair transmissions. Note that this period of aggregating
repair state does NOT interfere with its ongoing transmission of new
data.</t>
<t>As described in <xref target="RFC5401"></xref>, the period of
time during which the sender aggregates <spanx style="verb">NORM_NACK</spanx>
messages is equal to:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_sndrAggregate = (Ksender+1)*GRTT]]></artwork>
</figure>
<t>where "<spanx style="verb">Ksender</spanx>" is the same backoff
scaling value used by the receivers, and <spanx style="verb">GRTT</spanx>
is the sender's current estimate of the group's greatest round-trip
time. Note that for NORM unicast sessions the "<spanx style="verb">T_sndrAggregate</spanx>"
time can be set to ZERO since there is only one receiver. Similarly,
the "<spanx style="verb">Ksender</spanx>" value should be set to
ZERO for NORM unicast sessions to minimize repair latency.</t>
<t>When this period ends, the sender "rewinds" by incorporating the
accumulated repair state into its pending transmission state and
begins transmitting repair messages. After pending repair
transmissions are completed, the sender continues with new
transmissions of any enqueued data. Also, at this point in time, the
sender begins a "holdoff" timeout during which time the sender
constrains itself from initiating a new repair aggregation cycle,
even if <spanx style="verb">NORM_NACK</spanx> messages arrive. As
described in <xref target="RFC5401"></xref>, the value of this
sender "holdoff" period is:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_sndrHoldoff = (1*GRTT)]]></artwork>
</figure>
<t>If additional <spanx style="verb">NORM_NACK</spanx> messages are
received during this sender "holdoff" period, the sender will
immediately incorporate these late-arriving messages into its
pending transmission state ONLY if the NACK content is ordinally
greater than the sender's current transmission position. This
"holdoff" time allows worst case time for the sender to propagate
its current transmission sequence position to the group, thus
avoiding redundant repair transmissions. After the holdoff timeout
expires, a new NACK accumulation period can be begun (upon arrival
of a NACK) in concert with the pending repair and new data
transmission. Recall that receivers are not to initiate the NACK
repair process until the sender's logical transmission position
exceeds the lowest ordinal position of their repair needs. With the
new NACK aggregation period, the sender repeats the same process of
incorporating accumulated repair state into its transmission plan
and subsequently "rewinding" to transmit the lowest ordinal repair
data when the aggregation period expires. Again, this is conducted
in concert with ongoing new data and/or pending repair
transmissions.</t>
</section>
<section title="Sender FEC Repair Transmission Strategy">
<t>The NORM sender should leverage transmission of FEC parity
content for repair to the greatest extent possible. Recall that the
receivers use a strategy to request a lowest common denominator of
explicit repair (including parity content) in the formation of their
<spanx style="verb">NORM_NACK</spanx> messages. Before falling back
to explicitly satisfying different receivers' repair needs, the
sender can make use of the general erasure-filling capability of
FEC-generated parity segments. The sender can determine the maximum
erasure filling needs for individual FEC coding blocks from the
<spanx style="verb">NORM_NACK</spanx> messages received during the
repair aggregation period. Then, if the sender has a sufficient
number (less than or equal to the maximum erasure count) of
previously unsent parity segments available for the applicable
coding blocks, the sender can transmit these in lieu of the specific
packets the receiver set has requested. Only after exhausting its
supply of "fresh" (unsent) parity segments for a given coding block
should the sender resort to explicit transmission of the receiver
set's repair needs. In general, if a sufficiently powerful FEC code
is used, the need for explicit repair will be an exception, and the
fulfillment of reliable multicast can be accomplished quite
efficiently. However, the ability to resort to explicit repair
allows the protocol to be reliable under even very extreme
circumstances.</t>
<t><spanx style="verb">NORM_DATA</spanx> messages sent as repair
transmissions SHALL be flagged with the <spanx style="verb">NORM_FLAG_REPAIR</spanx>
flag. This allows receivers to obey any policies that limit new
receivers from joining the reliable transmission when only repair
transmissions have been received. Additionally, the sender SHOULD
additionally flag <spanx style="verb">NORM_DATA</spanx>
transmissions sent as explicit repair with the <spanx style="verb">NORM_FLAG_EXPLICIT</spanx>
flag.</t>
<t>Although NORM end system receivers do not make use of the <spanx
style="verb">NORM_FLAG_EXPLICIT</spanx> flag, this message
transmission status could be leveraged by intermediate systems
wishing to "assist" NORM protocol performance. If such systems are
properly positioned with respect to reciprocal reverse-path
multicast routing, they need to sub-cast only a sufficient count of
non-explicit parity repairs to satisfy a multicast routing
sub-tree's erasure filling needs for a given FEC coding block. When
the sender has resorted to explicit repair, then the intermediate
systems should sub-cast all of the explicit repair packets to those
portions of the routing tree still requiring repair for a given
coding block. Note the intermediate systems will be required to
conduct repair state accumulation for sub-routes in a manner similar
to the sender's repair state accumulation in order to have
sufficient information to perform the sub-casting. Additionally, the
intermediate systems could perform additional <spanx style="verb">NORM_NACK</spanx>
suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles. The detail of this type of
operation are beyond the scope of this document, but this
information is provided for possible future consideration.</t>
</section>
<section title="Sender NORM_CMD(SQUELCH) Generation">
<t>If the sender receives a <spanx style="verb">NORM_NACK</spanx>
message for repair of data it is no longer supporting, the sender
generates a <spanx style="verb">NORM_CMD(SQUELCH)</spanx> message to
advertise its repair window and squelch any receivers from
additional NACKing of invalid data. The transmission rate of <spanx
style="verb">NORM_CMD(SQUELCH)</spanx> messages is limited to once
per <spanx style="verb">2*GRTT</spanx>. The "invalid_object_list"
(if applicable) of the <spanx style="verb">NORM_CMD(SQUELCH)</spanx>
message SHALL begin with the lowest "object_transport_id" from the
invalid <spanx style="verb">NORM_NACK</spanx> messages received
since the last <spanx style="verb">NORM_CMD(SQUELCH)</spanx>
transmission. Lower ordinal invalid "object_transport_ids" should be
included only while the <spanx style="verb">NORM_CMD(SQUELCH)</spanx>
payload is less than the sender's <spanx style="emph">NormSegmentSize</spanx>
parameter.</t>
</section>
<section title="Sender NORM_CMD(REPAIR_ADV) Generation">
<t>When a NORM sender receives <spanx style="verb">NORM_NACK</spanx>
messages from receivers via unicast transmission, it uses <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> messages to advertise its
accumulated repair state to the receiver set since the receiver set
is not directly sharing their repair needs via multicast
communication. A NORM sender implementation MAY use a separate port
number from the <spanx style="emph">NormSession</spanx> port number
as the source port for its transmissions. Thus NORM receivers can
direct any unicast feedback messages to this sender port number that
is distinct from the NORM session (or destination) port number.
Then, the NORM sender implementation can discriminate unicast
feedback messages from multicast feedback messages when there is a
mix of multicast and unicast feedback receivers. The <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> message is multicast to
the receiver set by the sender. The payload portion of this message
has content in the same format as the <spanx style="verb">NORM_NACK</spanx>
receiver message payload. Receivers are then able to perform
feedback suppression in the same manner as with <spanx style="verb">NORM_NACK</spanx>
messages directly received from other receivers. Note the sender
does not merely retransmit NACK content it receives, but instead
transmits a representation of its aggregated repair state. The
transmission of <spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx>
messages are subject to the sender transmit rate limit and <spanx
style="emph">NormSegmentSize</spanx> limitation. When the <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> message is of maximum
size, receivers SHALL consider the maximum ordinal transmission
position value embedded in the message as the senders current
transmission position and implicitly suppress requests for ordinally
higher repair. For congestion control operation, the sender may also
need to provide information so that dynamic congestion control
feedback can be suppressed as needed among receivers. This document
specifies the NORM-CC Feedback Header Extension that is applied for
baseline NORM-CC operation. If other congestion control mechanisms
are used within a NORM implementation, other header extensions may
be defined. Whatever content format is used for this purpose should
ensure that maximum possible suppression state is conveyed to the
receiver set.</t>
</section>
</section>
<section title="Additional Protocol Mechanisms">
<t>In addition to the principal function of data content transmission
and repair, there are some other protocol mechanisms that help NORM to
adapt to network conditions and play fairly with other coexistent
protocols.</t>
<section anchor="GrttCollection"
title="Greatest Round-trip Time Collection">
<t>For NORM receivers to appropriately scale backoff timeouts and
the senders to use proper corresponding timeouts, the participants
must agree on a common timeout basis. Each NORM sender monitors the
round-trip time of active receivers and determines the group
greatest round-trip time (GRTT). The sender advertises this GRTT
estimate in every message it transmits so that receivers have this
value available for scaling their timers. To measure the current
GRTT, the sender periodically sends <spanx style="verb">NORM_CMD(CC)</spanx>
messages that contain a locally generated timestamp. Receivers are
expected to record this timestamp along with the time the <spanx
style="verb">NORM_CMD(CC)</spanx> message is received. Then, when
the receivers generate feedback messages to the sender, an adjusted
version of the sender timestamp is embedded in the feedback message
(<spanx style="verb">NORM_NACK</spanx> or <spanx style="verb">NORM_ACK</spanx>).
The adjustment adds the amount of time the receiver held the
timestamp before generating its response. Upon receipt of this
adjusted timestamp, the sender is able to calculate the round-trip
time to that receiver.</t>
<t>The round-trip time for each receiver is fed into an algorithm
that weights and smoothes the values for a conservative estimate of
the GRTT. The algorithm and methodology are described in the <xref
target="RFC5401">Multicast NACK Building Block</xref> in the section
entitled "One-to-Many Sender GRTT Measurement". A conservative
estimate helps guarantee feedback suppression at a small cost in
overall protocol repair delay. The sender's current estimate of GRTT
is advertised in the "grtt" field found in all NORM sender messages.
The advertised GRTT is also limited to a minimum of the nominal
inter-packet transmission time given the sender's current
transmission rate and system clock granularity. The reason for this
additional limit is to keep the receiver somewhat event-driven by
making sure the sender has had adequate time to generate any
response to repair requests from receivers given transmit rate
limitations due to congestion control or configuration.</t>
<t>When the NORM-CC Rate header extension is present in <spanx
style="verb">NORM_CMD(CC)</spanx> messages, the receivers respond to
<spanx style="verb">NORM_CMD(CC)</spanx> messages as described in
<xref target="CongestionControl"></xref>, "NORM Congestion Control
Operation". The <spanx style="verb">NORM_CMD(CC)</spanx> messages
are periodically generated by the sender as described for congestion
control operation. This provides for proactive, but controlled,
feedback from the group in the form of <spanx style="verb">NORM_ACK</spanx>
messages. This provides for GRTT feedback even if no <spanx
style="verb">NORM_NACK</spanx> messages are being sent. If operating
without congestion control in a closed network, the <spanx
style="verb">NORM_CMD(CC)</spanx> messages may be sent periodically
without the NORM-CC Rate header extension. In this case, receivers
will only provide GRTT measurement feedback when <spanx
style="verb">NORM_NACK</spanx> messages are generated since no
<spanx style="verb">NORM_ACK</spanx> messages are generated. In this
case, the <spanx style="verb">NORM_CMD(CC)</spanx> messages may be
sent less frequently, perhaps as little as once per minute, to
conserve network capacity. Note that the NORM-CC Rate header
extension may also be used to proactively solicit RTT feedback from
the receiver group per congestion control operation even though the
sender may not be conducting congestion control rate adjustment.
NORM operation without congestion control should be considered only
in closed networks.</t>
</section>
<section anchor="CongestionControl"
title="NORM Congestion Control Operation">
<t>This section describes baseline congestion control operation for
the NORM protocol (NORM-CC). The supporting NORM message formats and
approach described here are an adaptation of the equation-based
TCP-Friendly Multicast Congestion Control (TFMCC) approach<xref
target="RFC4654"></xref>. This congestion control scheme is REQUIRED
for operation within the general Internet unless the NORM
implementation is adapted to use another IETF-sanctioned reliable
multicast congestion control mechanism. With this TFMCC-based
approach, the transmissions of NORM senders are controlled in a
rate-based manner as opposed to window-based congestion control
algorithms as in TCP. However, it is possible that the NORM protocol
message set may alternatively be used to support a window-based
multicast congestion control scheme such as PGMCC. The details of
that alternative may be described separately or in a future revision
of this document. In either case (rate-based TFMCC or window-based
PGMCC), successful control of sender transmission depends upon
collection of sender-to-receiver packet loss estimates and RTTs to
identify the congestion control bottleneck path(s) within the
multicast topology and adjust the sender rate accordingly. The
receiver with loss and RTT estimates that correspond to the lowest
resulting calculated transmission rate is identified as the "current
limiting receiver" (CLR). In the case of a tie (where candidate CLRs
are within 10% of the same calculated rate), the receiver with the
largest RTT value SHOULD be designated as the CLR.</t>
<t>As described in <xref target="TcpModel"></xref>, a steady-state
sender transmission rate, to be "friendly" with competing TCP flows
can be calculated as:</t>
<figure align="center">
<artwork align="center"><![CDATA[ S
Rsender = ----------------------------------------------------------
tRTT*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))]]></artwork>
</figure>
<t>where</t>
<t><spanx style="verb">S</spanx> = nominal transmitted packet size.
(In NORM, the "nominal" packet size can be determined by the sender
as an exponentially weighted moving average (EWMA) of transmitted
packet sizes to account for variable message sizes).</t>
<t><spanx style="verb">tRTT</spanx> = RTT estimate of the current
"current limiting receiver" (CLR).</t>
<t><spanx style="verb">p</spanx> = loss event fraction of the
CLR.</t>
<t>To support congestion control feedback collection and operation,
the NORM sender periodically transmits <spanx style="verb">NORM_CMD(CC)</spanx>
command messages. <spanx style="verb">NORM_CMD(CC)</spanx> messages
are multiplexed with NORM data and repair transmissions and serve
several purposes:</t>
<t><list style="numbers">
<t>Stimulate explicit feedback from the general receiver set to
collect congestion control information.</t>
<t>Communicate state to the receiver set on the sender's current
congestion control status including details of the CLR.</t>
<t>Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that
current worst path in the group multicast topology.</t>
</list></t>
<t>The format of the <spanx style="verb">NORM_CMD(CC)</spanx>
message is described in <xref target="NORM_CMD"></xref> of this
document. The <spanx style="verb">NORM_CMD(CC)</spanx> message
contains information to allow measurement of RTTs, to inform the
group of the congestion control CLR, and to provide feedback of
individual RTT measurements to the receivers in the group. The
<spanx style="verb">NORM_CMD(CC)</spanx> also provides for exciting
feedback from OPTIONAL "potential limiting receiver" (PLR) nodes
that may be determined administratively or possibly algorithmically
based on congestion control feedback. PLR nodes are receivers that
have been identified to have potential for (perhaps soon) becoming
the CLR and thus immediate, up-to-date feedback is beneficial for
congestion control performance. The PLR list may be populated with a
small number of receivers the sender identifies as approaching the
CLR loss and delay conditions based on feedback from the group.</t>
<section title="NORM_CMD(CC) Transmission">
<t>The <spanx style="verb">NORM_CMD(CC)</spanx> message is
transmitted periodically by the sender along with its normal data
transmission. Note that the repeated transmission of <spanx
style="verb">NORM_CMD(CC)</spanx> messages may be initiated some
time before transmission of user data content at session startup.
This may be done to collect some estimation of the current state
of the multicast topology with respect to group and individual RTT
and congestion control state.</t>
<t>A <spanx style="verb">NORM_CMD(CC)</spanx> message is
immediately transmitted at sender startup. The interval of
subsequent <spanx style="verb">NORM_CMD(CC)</spanx> message
transmission is determined as follows:</t>
<t><list style="numbers">
<t>By default, the interval is set according to the current
sender GRTT estimate. A startup GRTT of 0.5 seconds is
recommended when no feedback has yet been received from the
group.</t>
<t>Until a CLR has been identified (based on previous receiver
feedback) or when no data transmission is pending, the <spanx
style="verb">NORM_CMD(CC)</spanx> interval is doubled up from
its current interval to a maximum of once per 30 seconds. This
results in a low duty cycle for <spanx style="verb">NORM_CMD(CC)</spanx>
probing when no CLR is identified or there is no pending data
to transmit.</t>
<t>When a CLR has been identified (based on receiver feedback)
and data transmission is pending, the probing interval is set
to the RTT between the sender and the CLR (<spanx
style="verb">RTT_clr</spanx>).</t>
<t>Additionally, when the data transmission rate is low with
respect to the <spanx style="verb">RTT_clr</spanx> interval
used for probing, the implementation should ensure that no
more than one <spanx style="verb">NORM_CMD(CC)</spanx> message
is sent per <spanx style="verb">NORM_DATA</spanx> message when
there is data pending transmission. This ensures that the
transmission of this control message is not done to the
exclusion of user data transmission.</t>
</list></t>
<t>The <spanx style="verb">NORM_CMD(CC)</spanx> "cc_sequence"
field is incremented with each transmission of a <spanx
style="verb">NORM_CMD(CC)</spanx> command. The greatest
"cc_sequence" recently received by receivers is included in their
feedback to the sender. This allows the sender to determine the
age of feedback to assist in congestion avoidance.</t>
<t>The NORM-CC Rate Header Extension is applied to the <spanx
style="verb">NORM_CMD(CC)</spanx> message and the sender
advertises its current transmission rate in the "send_rate" field.
The rate information is used by receivers to initialize loss
estimation during congestion control startup or restart.</t>
<t>The "cc_node_list" contains a list of entries identifying
receivers and their current congestion control state (status
"flags", "rtt" and "loss" estimates). The list may be empty if the
sender has not yet received any feedback from the group. If the
sender has received feedback, the list will minimally contain an
entry identifying the CLR. A <spanx style="verb">NORM_FLAG_CC_CLR</spanx>
flag value is provided for the "cc_flags" field to identify the
CLR entry. It is RECOMMENDED that the CLR entry be the first in
the list for implementation efficiency. Additional entries in the
list are used to provide sender-measured individual RTT estimates
to receivers in the group. The number of additional entries in
this list is dependent upon the percentage of control traffic the
sender application is willing to send with respect to user data
message transmissions. More entries in the list may allow the
sender to be more responsive to congestion control dynamics. The
length of the list may be dynamically determined according to the
current transmission rate and scheduling of <spanx style="verb">NORM_CMD(CC)</spanx>
messages. The maximum length of the list corresponds to the
sender's <spanx style="emph">NormSegmentSize</spanx> parameter for
the session. The inclusion of additional entries in the list based
on receiver feedback are prioritized with following rules:</t>
<t><list style="numbers">
<t>Receivers that have not yet been provided a RTT measurement
get first priority. Of these, those with the greatest loss
fraction receive precedence for list inclusion.</t>
<t>Secondly, receivers that have previously been provided a
RTT measurement are included with receivers yielding the
lowest calculated congestion rate getting precedence.</t>
</list></t>
<t>There are "cc_flag" values in addition to <spanx style="verb">NORM_FLAG_CC_CLR</spanx>
that are used for other congestion control functions. The <spanx
style="verb">NORM_FLAG_CC_PLR</spanx> flag value is used to mark
additional receivers from that the sender would like to have
immediate, non-suppressed feedback. These may be receivers that
the sender algorithmically identified as potential future CLRs or
that have been pre-configured as potential congestion control
points in the network. The <spanx style="verb">NORM_FLAG_CC_RTT</spanx>
indicates the validity of the "cc_rtt" field for the associated
receiver node. Normally, this flag will be set since the receivers
in the list will typically be receivers from which the sender has
received feedback. However, in the case that the NORM sender has
been pre-configured with a set of PLR nodes, feedback from those
receivers may not yet have been collected and thus the "cc_rtt"
field does not contain a valid value when this flag is not set.
Similarly, a value of ZERO for the "cc_rate" field here should be
treated as an invalid value and be ignored for the purposes of
feedback suppression, etc.</t>
</section>
<section title="NORM_CMD(CC) Feedback Response">
<t>Receivers explicitly respond to <spanx style="verb">NORM_CMD(CC)</spanx>
messages in the form of a <spanx style="verb">NORM_ACK(RTT)</spanx>
message. The goal of the congestion control feedback is to
determine the receivers with the lowest congestion control rates.
Receivers that are marked as CLR or PLR nodes in the <spanx
style="verb">NORM_CMD(CC)</spanx> "cc_node_list" immediately
provide feedback in the form of a <spanx style="verb">NORM_ACK</spanx>
to this message. When a <spanx style="verb">NORM_CMD(CC)</spanx>
is received, non-CLR or non-PLR nodes initiate random feedback
backoff timeouts similar to that used when the receiver initiates
a repair cycle (see <xref target="NackProcedure"></xref>) in
response to detection of data loss. The backoff timeout for the
congestion control response is generated as follows:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)]]></artwork>
</figure>
<t>The "<spanx style="verb">RandomBackoff()</spanx>" algorithm
provides a truncated exponentially distributed random number and
is described in the <xref target="RFC5401">Multicast NACK Building
Block</xref>. The same backoff factor <spanx style="verb">K = Ksender</spanx>
MAY be used as with <spanx style="verb">NORM_NACK</spanx>
suppression. However, in cases where the application purposefully
specifies a very small <spanx style="verb">Ksender</spanx> backoff
factor to minimize the NACK repair process latency (trading off
group size scalability), it is RECOMMENDED that a larger backoff
factor for congestion control feedback is maintained, since there
may often be a larger volume of congestion control feedback than
NACKs in many cases and some congestion control feedback latency
may be tolerable where reliable delivery latency is not. As
previously noted, a backoff factor value of <spanx style="verb">K = 4</spanx>
is generally recommended for ASM operation and <spanx
style="verb">K = 6</spanx> for SSM operation. A receiver SHALL
cancel the backoff timeout and thus its pending transmission of a
<spanx style="verb">NORM_ACK(RTT)</spanx> message under the
following conditions:</t>
<t><list style="numbers">
<t>The receiver generates another feedback message (<spanx
style="verb">NORM_NACK</spanx> or other <spanx style="verb">NORM_ACK</spanx>)
before the congestion control feedback timeout expires (these
messages will convey the current congestion control feedback
information),</t>
<t>A <spanx style="verb">NORM_CMD(CC)</spanx> or other
receiver feedback with an ordinally greater "cc_sequence"
field value is received before the congestion control feedback
timeout expires (this is similar to the TFMCC feedback round
number),</t>
<t>When the <spanx style="verb">T_backoff</spanx> is greater
than <spanx style="verb">1*GRTTsender</spanx>. This prevents
NACK implosion in the event of sender or network failure,</t>
<t>"Suppressing" congestion control feedback is heard from
another receiver (in a <spanx style="verb">NORM_ACK</spanx> or
<spanx style="verb">NORM_NACK</spanx>) or via a <spanx
style="verb">NORM_CMD(REPAIR_ADV)</spanx> message from the
sender. The local receiver's feedback is "suppressed" if the
rate of the competing feedback (<spanx style="verb">Rfb</spanx>)
is sufficiently close to or less than the local receiver's
calculated rate (<spanx style="verb">Rcalc</spanx>). The local
receiver's feedback is canceled when <spanx style="verb">Rcalc > (0.9 * Rfb)</spanx>.
Also note receivers that have not yet received an RTT
measurement from the sender are suppressed only by other
receivers that have not yet measured RTT. Additionally,
receivers whose RTT estimate has aged considerably (i.e., they
haven't been included in the <spanx style="verb">NORM_CMD(CC)</spanx>
"cc_node_list" in a long time) may wish to compete as a
receiver with no prior RTT measurement after some long term
expiration period.</t>
</list></t>
<t>When the backoff timer expires, the receiver SHALL generate a
<spanx style="verb">NORM_ACK(RTT)</spanx> message to provide
feedback to the sender and group. This message may be multicast to
the group for most effective suppression in ASM topologies or
unicast to the sender depending upon how the NORM protocol is
deployed and configured.</t>
<t>Whenever any feedback is generated (including this <spanx
style="verb">NORM_ACK(RTT)</spanx> message), receivers include an
adjusted version of the sender timestamp from the most recently
received <spanx style="verb">NORM_CMD(CC)</spanx> message and the
"cc_sequence" value from that command in the applicable <spanx
style="verb">NORM_ACK</spanx> or <spanx style="verb">NORM_NACK</spanx>
message fields. For NORM-CC operation, any generated feedback
message SHALL also contain the NORM-CC Feedback header extension.
The receiver provides its current "cc_rate" estimate, "cc_loss"
estimate, "cc_rtt" if known, and any applicable "cc_flags" via
this header extension.</t>
<t>During <spanx style="emph">slow start</spanx> (when the
receiver has not yet detected loss from the sender), the receiver
uses a value equal to two times its measured rate from the sender
in the "cc_rate" field. For steady-state congestion control
operation, the receiver "cc_rate" value is from the equation-based
value using its current loss event estimate and
sender<->receiver RTT information. (The GRTT is used when
the receiver has not yet measured its individual RTT).</t>
<t>The "cc_loss" field value reflects the receiver's current loss
event estimate with respect to the sender in question.</t>
<t>When the receiver has a valid individual RTT measurement, it
SHALL include this value in the "cc_rtt" field. The <spanx
style="verb">NORM_FLAG_CC_RTT</spanx> MUST be set when the
"cc_rtt" field is valid.</t>
<t>After a congestion control feedback message is generated or
when the feedback is suppressed, a non-CLR receiver begins a
"holdoff" timeout period during which it will restrain itself from
providing congestion control feedback, even if <spanx
style="verb">NORM_CMD(CC)</spanx> messages are received from the
sender (unless the receive becomes marked as a CLR or PLR node).
The value of this holdoff timeout (<spanx style="verb">T_ccHoldoff</spanx>)
period is:</t>
<figure align="center">
<artwork align="center"><![CDATA[T_ccHoldoff = (K*GRTT)]]></artwork>
</figure>
<t>Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per <spanx style="verb">K*GRTT</spanx>
intervals. Note, however, that as the session progresses,
different receivers will be responding to different <spanx
style="verb">NORM_CMD(CC)</spanx> messages and there will be
relatively continuous feedback of congestion control information
while the sender is active.</t>
</section>
<section title="Congestion Control Rate Adjustment">
<t>During steady-state operation, the sender will directly adjust
its transmission rate to the rate indicated by the feedback from
its currently selected CLR. As noted in <xref
target="TfmccPaper"></xref>, the estimation of parameters (loss
and RTT) for the CLR will generally constrain the rate changes
possible within acceptable bounds. For rate increases, the sender
SHALL observe a maximum rate of increase of one packet per RTT at
all times during steady-state operation.</t>
<t>The sender processes congestion control feedback from the
receivers and selects the CLR based on the lowest rate receiver.
Receiver rates are either determined directly from the <spanx
style="emph">slow start</spanx> "cc_rate" provided by the receiver
in the NORM-CC Feedback header extension or by performing the
equation-based calculation using individual RTT and loss estimates
("cc_loss") as feedback is received.</t>
<t>The sender can calculate a current RTT for a receiver (<spanx
style="verb">RTT_rcvrNew</spanx>) using the "grtt_response"
timestamp included in feedback messages. When the "cc_rtt" value
in a response is not valid, the sender simply uses this <spanx
style="verb">RTT_rcvrNew</spanx> value as the receiver's current
RTT (<spanx style="verb">RTT_rcvr</spanx>). For non-CLR and
non-PLR receivers, the sender can use the "cc_rtt" value provided
in the NORM-CC Feedback header extension as the receiver's
previous RTT measurement (<spanx style="verb">RTT_rcvrPrev</spanx>)
to smooth according to:</t>
<figure align="center">
<artwork align="center"><![CDATA[RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew]]></artwork>
</figure>
<t>For CLR receivers where feedback is received more regularly,
the sender SHOULD maintain a more smoothed RTT estimate upon new
feedback from the CLR where:</t>
<figure align="center">
<artwork align="center"><![CDATA[RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew]]></artwork>
</figure>
<t>"<spanx style="verb">RTT_clrNew</spanx>" is the new RTT
calculated from the timestamp in the feedback message received
from the CLR. The <spanx style="verb">RTT_clr</spanx> is
initialized to <spanx style="verb">RTT_clrNew</spanx> on the first
feedback message received. Note that the same procedure is
observed by the sender for PLR receivers and that if a PLR is
"promoted" to CLR status, the smoothed estimate can be
continued.</t>
<t>There are some additional periods besides steady-state
operation that need to be considered in NORM-CC operation. These
periods are:</t>
<t><list style="numbers">
<t>during session startup,</t>
<t>when no feedback is received from the CLR, and</t>
<t>when the sender has a break in data transmission.</t>
</list></t>
<t>During session startup, the congestion control operation SHALL
observe a "slow start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed
where:</t>
<figure align="center">
<artwork align="center"><![CDATA[Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.]]></artwork>
</figure>
<t>The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
provides feedback indicating that loss has occurred. Rate increase
during <spanx style="emph">slow start</spanx> is applied as:</t>
<figure align="center">
<artwork align="center"><![CDATA[Rnew = Rrecv_min]]></artwork>
</figure>
<t>where <spanx style="verb">Rrecv_min</spanx> is the minimum
reported receiver rate in the "cc_rate" field of congestion
control feedback messages received from the group. Note that
during <spanx style="emph">slow start</spanx>, receivers use two
times their measured rate from the sender in the "cc_rate" field
of their feedback. Rate increase adjustment is limited to once per
GRTT during slow start.</t>
<t>If the CLR or any receiver intends to leave the group, it will
set the <spanx style="verb">NORM_FLAG_CC_LEAVE</spanx> in its
congestion control feedback message as an indication that the
sender should not select it as the CLR. When the CLR changes to a
lower rate receiver, the sender should immediately adjust to the
new lower rate. The sender is limited to increasing its rate at
one additional packet per RTT towards any new, higher CLR
rate.</t>
<t>The sender should also track the age of the feedback it has
received from the CLR by comparing its current "cc_sequence" value
(<spanx style="verb">Seq_sender</spanx>) to the last "cc_sequence"
value received from the CLR (<spanx style="verb">Seq_clr</spanx>).
As the age of the CLR feedback increases with no new feedback, the
sender SHALL begin reducing its rate once per <spanx style="verb">RTT_clr</spanx>
as a congestion avoidance measure. The following algorithm is used
to determine the decrease in sender rate (Rsender bytes/sec) as
the CLR feedback, unexpectedly, excessively ages:</t>
<figure align="center">
<artwork align="center"><![CDATA[Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;]]></artwork>
</figure>
<t>This rate reduction is limited to the lower bound on NORM
transmission rate. After <spanx style="verb">NORM_ROBUST_FACTOR</spanx>
consecutive <spanx style="verb">NORM_CMD(CC)</spanx> rounds
without any feedback from the CLR, the sender SHOULD assume the
CLR has left the group and pick the receiver with the next lowest
rate as the new CLR. Note this assumes that the sender does not
have explicit knowledge that the CLR intentionally left the group.
If no receiver feedback is received, the sender MAY wish to
withhold further transmissions of <spanx style="verb">NORM_DATA</spanx>
segments and maintain <spanx style="verb">NORM_CMD(CC)</spanx>
transmissions only until feedback is detected. After such a CLR
timeout, the sender will be transmitting with a minimal rate and
should return to slow start as described here for a break in data
transmission.</t>
<t>When the sender has a break in its data transmission, it can
continue to probe the group with <spanx style="verb">NORM_CMD(CC)</spanx>
messages to maintain RTT collection from the group. This will
enable the sender to quickly determine an appropriate CLR upon
data transmission restart. However, the sender should
exponentially reduce its target rate to be used for transmission
restart as time since the break elapses. The target rate SHOULD be
recalculated once per <spanx style="verb">RTT_clr</spanx> as:</t>
<figure align="center">
<artwork align="center"><![CDATA[Rsender = Rsender * 0.5;]]></artwork>
</figure>
<t>If the minimum NORM rate is reached, the sender should set the
<spanx style="verb">NORM_FLAG_START</spanx> flag in its <spanx
style="verb">NORM_CMD(CC)</spanx> messages upon restart and the
group should observer <spanx style="emph">slow start</spanx>
congestion control procedures until any receiver experiences a new
loss event.</t>
</section>
</section>
<section anchor="PositiveAcknowledgment"
title="NORM Positive Acknowledgment Procedure">
<t>NORM provides options for the source application to request
positive acknowledgment (ACK) of <spanx style="verb">NORM_CMD(FLUSH)</spanx>
and <spanx style="verb">NORM_CMD(ACK_REQ)</spanx> messages from
members of the group. There are some specific acknowledgment
requests defined for the NORM protocol and a range of acknowledgment
request types that are left to be defined by the application. One
predefined acknowledgment type is the <spanx style="verb">NORM_ACK_FLUSH</spanx>
type. This acknowledgment is used to determine if receivers have
achieved completion of reliable reception up through a specific
logical transmission point with respect to the sender's sequence of
transmission. The <spanx style="verb">NORM_ACK_FLUSH</spanx>
acknowledgment may be used to assist in application flow control
when the sender has information on a portion of the receiver set.
Another predefined acknowledgment type is <spanx style="verb">NORM_ACK(CC)</spanx>,
which is used to explicitly provide congestion control feedback in
response to <spanx style="verb">NORM_CMD(CC)</spanx> messages
transmitted by the sender for NORM-CC operation. Note the <spanx
style="verb">NORM_ACK(CC)</spanx> response does NOT follow the
positive acknowledgment procedure described here. The <spanx
style="verb">NORM_CMD(ACK_REQ)</spanx> and <spanx style="verb">NORM_ACK</spanx>
messages contain an "ack_type" field to identify the type of
acknowledgment requested and provided. A range of "ack_type" values
is provided for application-defined use. While the application is
responsible for initiating the acknowledgment request and interprets
application-defined "ack_type" values, the acknowledgment procedure
SHOULD be conducted within the protocol implementation to take
advantage of timing and transmission scheduling information
available to the NORM transport.</t>
<t>The NORM positive acknowledgment procedure uses polling by the
sender to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups,
but could be used in large group setting to query a critical subset
of the group. Either the <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>,
or when applicable, the <spanx style="verb">NORM_CMD(FLUSH)</spanx>
message is used for polling and contains a list of <spanx
style="emph">NormNodeIds</spanx> for receivers that should respond
to the command. The list of receivers providing acknowledgment is
determined by the source application with <spanx style="emph">a
priori</spanx> knowledge of participating nodes or via some other
application-level mechanism.</t>
<t>The ACK process is initiated by the sender that generates <spanx
style="verb">NORM_CMD(FLUSH)</spanx> or <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
messages in periodic rounds. For <spanx style="verb">NORM_ACK_FLUSH</spanx>
requests, the <spanx style="verb">NORM_CMD(FLUSH)</spanx> contain a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgment is requested. This
watermark transmission point is echoed in the corresponding fields
of the <spanx style="verb">NORM_ACK(FLUSH)</spanx> message sent by
the receiver in response. <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
messages contain an "ack_id" field which is similarly echoed in
response so that the sender may match the response to the
appropriate request.</t>
<t>In response to the <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>,
the listed receivers randomly spread <spanx style="verb">NORM_ACK</spanx>
messages uniformly in time over a window of (1*GRTT). These <spanx
style="verb">NORM_ACK</spanx> messages are typically unicast to the
sender. (Note that <spanx style="verb">NORM_ACK(CC)</spanx> messages
SHALL be multicast or unicast in the same manner as <spanx
style="verb">NORM_NACK</spanx> messages).</t>
<t>The ACK process is self-limiting and avoids ACK implosion in
that:</t>
<t><list style="numbers">
<t>Only a single <spanx style="verb">NORM_CMD(ACK_REQ)</spanx>
message is generated once per (2*GRTT), and,</t>
<t>The size of the "acking_node_list" of <spanx style="emph">NormNodeIds</spanx>
from which acknowledgment is requested is limited to a maximum
of the sender <spanx style="emph">NormSegmentSize</spanx>
setting per round of the positive acknowledgment process.</t>
</list></t>
<t>Because the size of the included list is limited to the sender's
<spanx style="emph">NormSegmentSize</spanx> setting, multiple <spanx
style="verb">NORM_CMD(ACK_REQ)</spanx> rounds may be required to
achieve responses from all receivers specified. The content of the
attached <spanx style="emph">NormNodeId</spanx> list will be
dynamically updated as this process progresses and <spanx
style="verb">NORM_ACK</spanx> responses are received from the
specified receiver set. As the sender receives valid responses
(i.e., matching watermark point or "ack_id") from receivers, it
SHALL eliminate those receivers from the subsequent <spanx
style="verb">NORM_CMD(ACK_REQ)</spanx> message "acking_node_list"
and add in any pending receiver <spanx style="emph">NormNodeIds</spanx>
while keeping within the <spanx style="emph">NormSegmentSize</spanx>
limitation of the list size. Each receiver is queried a maximum
number of times (<spanx style="verb">NORM_ROBUST_FACTOR</spanx>, by
default). Receivers not responding within this number of repeated
requests are removed from the payload list to make room for other
potential receivers pending acknowledgment. The transmission of the
<spanx style="verb">NORM_CMD(ACK_REQ)</spanx> is repeated until no
further responses are required or until the repeat threshold is
exceeded for all pending receivers. The transmission of <spanx
style="verb">NORM_CMD(ACK_REQ)</spanx> or <spanx style="verb">NORM_CMD(FLUSH)</spanx>
messages to conduct the positive acknowledgment process is
multiplexed with ongoing sender data transmissions. However, the
<spanx style="verb">NORM_CMD(FLUSH)</spanx> positive acknowledgment
process may be interrupted in response to negative acknowledgment
repair requests (NACKs) received from receivers during the
acknowledgment period. The <spanx style="verb">NORM_CMD(FLUSH)</spanx>
positive acknowledgment process is restarted for receivers pending
acknowledgment once any the repairs have been transmitted.</t>
<t>In the case of <spanx style="verb">NORM_CMD(FLUSH)</spanx>
commands with an attached "acking_node_list", receivers will not ACK
until they have received complete transmission of all data up to and
including the given watermark transmission point. All receivers
SHALL interpret the watermark point provided in the request NACK for
repairs if needed as for <spanx style="verb">NORM_CMD(FLUSH)</spanx>
commands with no attached "acking_node_list".</t>
</section>
<section title="Group Size Estimate">
<t>NORM sender messages contain a "gsize" field that is a
representation of the group size and is used in scaling random
backoff timer ranges. The use of the group size estimate within the
NORM protocol does not require a precise estimation and works
reasonably well if the estimate is within an order of magnitude of
the actual group size. By default, the NORM sender group size
estimate may be administratively configured. Also, given the
expected scalability of the NORM protocol for general use, a default
value of 10,000 is RECOMMENDED for use as the group size
estimate.</t>
<t>It is possible that group size may be algorithmically
approximated from the volume of congestion control feedback messages
which follow the exponentially weighted random backoff. However, the
specification of such an algorithm is currently beyond the scope of
this document.</t>
</section>
</section>
</section>
<section title="Security Considerations">
<t>The same security considerations that apply to the <xref
target="RFC5401">Multicast NACK</xref>, <xref
target="RFC4654">TFMCC</xref>, and <xref target="RFC5052">FEC</xref>
Building Blocks also apply to the NORM protocol. In addition to the
vulnerabilities that any IP and IP multicast protocol implementation may
be generally subject to, the NACK-based feedback of NORM may be
exploited by replay attacks which force the NORM sender to unnecessarily
transmit repair information. This MAY be addressed by network layer IP
security implementations that guard against this potential security
exploitation or alternatively with a security mechanism that uses the
<spanx style="verb">EXT_AUTH</spanx> header extension for similar
purposes. Such security mechanisms SHOULD be deployed and used when
available.</t>
<t>The NORM protocol is compatible with the use of <xref
target="RFC4301">IP security (IPsec)</xref> and the IPsec Encapsulating
Security Payload (ESP) protocol or Authentication Header (AF) extension
can be used to secure IP packets transmitted by NORM participants. A
baseline approach to secure NORM operation using IPsec is described
below. Compliant implementations of this specification are REQUIRED to
be compatible with IPsec usage as described in <xref
target="NormIPsec"></xref>.</t>
<t>Additionally, the <spanx style="verb">EXT_AUTH</spanx> header
extension (HET = 1) is defined for use by security mechanisms to provide
an alternative form of authentication and/or encryption of NORM
messages. The format of this header extension and its processing is
outside the scope of this document and is to be communicated out-of-band
as part of the session description. It is possible that an EXT_AUTH
implementation of MAY also provide for encryption of NORM message
payloads as well as authentication. The use of this approach as compared
to IPsec can allow for header compression techniques to be applied
jointly to IP and NORM protocol headers. In cases where security
analysis deems that encryption of NORM protocol header content is
beneficial or necessary, the aforementioned use of IPsec ESP may be more
appropriate. If EXT_AUTH is present, whatever packet authentication
checks that can be performed immediately upon reception of the packet
MUST be performed before accepting the packet and performing any
congestion control-related action on it. Some packet authentication
schemes impose a delay of several seconds between when a packet is
received and when the packet can be fully authenticated. Any congestion
control related action that is appropriate MUST NOT be postponed by any
such full packet authentication.</t>
<t>Consideration MUST also be given to the potential for replay-attacks
that would transplant authenticated packets from one NORM session to
another to disrupt service. To avoid this potential, unique keys SHOULD
be assigned on a per-session basis or NORM sender nodes SHOULD be
configured to use unique "instance_id" identifiers that are managed as
part of the security association for the sessions.</t>
<t>It should be noted that NORM implementations can use the "sequence"
field from the NORM Common Message Header to detect replay attacks. This
can be accomplished if the NORM sender maintains state on receivers
which are NACKing. A cache of such receiver state can be used to provide
protection against NACK replay attacks. NORM receivers MUST also
maintain similar state for protection against possible replay of other
receiver messages in ASM operation as well. For example, a receiver
could be suppressed from providing NACK or congestion control feedback
by replay of certain receiver messages. For these reasons,
authentication of NORM messages (e.g., via IPsec) SHOULD be applied for
protection against similar attacks that use fabricated messages. Also,
encryption of messages to provide confidentiality of application data
and protect privacy of users MAY also be applied using IPsec or similar
mechanisms.</t>
<t>When applicable security measures are used, automated key management
mechanisms such as those described in the <xref target="RFC3547">Group
Domain of Interpretation (GDOI)</xref>, <xref
target="RFC3830">Multimedia Internet KEYing (MIKEY) </xref> or <xref
target="RFC4535">Group Secure Association Key Management Protocol
(GSAKMP)</xref> specifications SHOULD be applied.</t>
<t>It is also important to note that while NORM does leverage FEC-based
repair for scalability, this alone does not guarantee integrity of
received data. Application-level integrity-checking of received data
content is highly RECOMMENDED.</t>
<section anchor="NormIPsec" title="Baseline Secure NORM Operation">
<t>This section describes a baseline mode of secure NORM protocol
operation based on application of the IPsec security protocol. This
approach is documented here to provide a reference, interoperable
secure mode of operation. However, additional approaches to NORM
security, including other forms of IPsec application, MAY be specified
in the future. For example, the use of the EXT_AUTH header extension
could enable NORM-specific authentication or security encapsulation
headers similar to those of IPsec to be specified and inserted into
the NORM protocol message headers. This would allow header compression
techniques to be applied to IP and NORM protocol headers when needed
in a similar fashion to that of <xref target="RFC3550">RTP</xref> and
as preserved in the specification for <xref target="RFC3711">Secure
Real Time Protocol (SRTP)</xref>.</t>
<t>The baseline approach described is applicable to NORM operation
configured for SSM (or SSM-like) operation where there is a single
sender and the receivers are providing unicast feedback. This form of
NORM operation allows for IPsec to be used with a manageable number of
security associations (SA).</t>
<section title="IPsec Approach">
<t>For NORM one-to-many SSM operation with unicast feedback from
receivers, each node SHALL be configured with two transport mode
IPsec security associations and corresponding Security Policy
Database (SPD) entries. One entry will be used for sender-to-group
multicast packet authentication and optionally encryption while the
other entry will be used to provide security for the unicast
feedback messaging from the receiver(s) to the sender.</t>
<t>The NORM sender SHALL use an IPsec SA configured for <xref
target="RFC4303">ESP protocol</xref> operation with the option for
data origination authentication enabled. It is also RECOMMENDED that
this IPsec ESP SA be also configured to provide confidentiality
protection for IP packets containing NORM protocol messages. This is
suggested to make the realization of complex replay attacks much
more difficult. The encryption key for this SA SHALL be preplaced at
the sender and receiver(s) prior to NORM protocol operation. Use of
automated key management is RECOMMENDED as a rekey SHALL be required
prior to expiration of the sequence space for the SA. This is
necessary so that receivers may use the built-in IPsec replay attack
protection possible for an IPsec SA with a single source (the NORM
sender). Thus the receivers SHALL enable replay attack protection
for this SA used to secure NORM sender traffic. An IPsec SPD entry
MUST be configured to process outbound packets to the session
(destination) address and UDP port number of the applicable (<spanx
style="emph">NormSession</spanx>).</t>
<t>The NORM receiver(s) MUST be configured with the SA and SPD entry
to properly process the IPsec-secured packets from the sender. The
NORM receiver(s) SHALL also use a common, second IPsec SA (common
Security Parameter Index (SPI) and encryption key) configured for
ESP operation with the option for data origination authentication
enabled. Similar to the NORM sender, is RECOMMENDED this IPsec ESP
SA be also configured to provide confidentiality protection for IP
packets containing NORM protocol messages. The receivers MUST have
an IPsec SPD entry configured to process outbound NORM/UDP packets
directed to the NORM sender source address and port number using
this second SA. As noted for NORM unicast feedback, the sender's
transmission port number SHOULD be selected to be distinct from the
multicast session port number to allow discrimination between
unicast and multicast feedback messages when access to the IP
destination address is not possible (e.g., a user-space NORM
implementation). For processing of packets from receivers, the NORM
sender SHALL be configured with this common, second SA (and the
corresponding SPD entry needed) in order to properly process
messages from the receiver.</t>
<t>Multiple receivers using a common IPsec SA for traffic directed
to the NORM sender (i.e., many-to-one) typically prevents the use of
built-in IPsec replay attack protection by the NORM sender with
current IPsec implementations. Thus the built-in IPsec replay attack
protection for this second SA at the sender MUST be disabled unless
the particular IPsec implementation manages its replay protection on
a per-source basis. So, to support a fully secure mode of operation,
the NORM sender implementation MUST provide replay attack protection
based upon the "sequence" field of NORM protocol messages from
receivers. This can be accomplished with high assurance of security,
even with the limited size (16-bits) of this field, because</t>
<t><list style="numbers">
<t>NORM receiver NACK and non-CLR ACK feedback messages are
sparse.</t>
<t>The more frequent <spanx style="verb">NORM_ACK</spanx>
feedback from CLR or PLR nodes are only a small set of receivers
for which the sender must keep more persistent replay attack
state.</t>
<t><spanx style="verb">NORM_NACK</spanx> feedback messages that
precede the sender's current repair window do not significantly
impact protocol operation (generation of <spanx style="verb">NORM_CMD(SQUELCH)</spanx>
is limited) and could be in fact ignored. This means the sender
can prune any replay attack state for receivers that precede the
current repair window.</t>
<t><spanx style="verb">NORM_ACK</spanx> messages correspond to
either a specific sender "ack_id", the sender "cc_sequence" for
ACKs sent in response to <spanx style="verb">NORM_CMD(CC)</spanx>,
or the sender's current repair window in the case of ACKs sent
in response to <spanx style="verb">NORM_CMD(FLUSH)</spanx>.
Thus, the sender can prune any replay attack state for receivers
that precede the current applicable sequence or repair window
space.</t>
</list></t>
<t>Note that use of ESP confidentiality for secure NORM protocol
operation makes it more difficult for adversaries to conduct any
form of replay attacks. Additionally, it should be noted that a NORM
sender implementation with access to the full ESP protocol header
could also use the ESP sequence information to make replay attack
protection even more robust, by maintaining per-source sequence
state. The design of this baseline security approach for NORM
intentionally places any more complex processing state or processing
(e.g. replay attack protection given multiple receivers) at the NORM
sender since NORM receiver implementations may need to have a more
light-weight realization in many cases.</t>
<t>This baseline approach can be used for NORM protocol sessions
with multiple senders if the SA pairs described are established for
each sender. For small-sized groups, it is even possible that
many-to-many (ASM) IPsec configuration could be achieved where each
participant uses a unique SA (with a unique SPI). This does not
scale to larger group sizes given the complex set of SA and SPD
entries each participant would need to maintain.</t>
<t>It is anticipated in early deployments of this baseline approach
to NORM security that key management will be conducted out-of-band
with respect to NORM protocol operation. In the case of one-to-many
NORM operation, it is possible that receivers may retrieve keying
information from a central server as needed or otherwise conduct
group key updates with a similar centralized approach. However, it
may be possible with some key management schemes for rekey messages
to be transmitted to the group as a message or transport object
within the NORM reliable transfer session. Similarly, for group-wise
communication sessions it is possible that potential group
participants may request keying and/or rekeying as part of NORM
communications. Additional specification is necessary to define an
in-band key management scheme for NORM sessions perhaps using the
mechanisms of the automated group key management specifications
cited in this document.</t>
</section>
<section title="IPsec Requirements">
<t>In order to implement this secure mode of NORM protocol
operation, the following IPsec capabilities are required.</t>
<section title="Selectors">
<t>The implementation MUST be able to use the source address,
destination address, protocol (UDP), and UDP port numbers as
selectors in the SPD.</t>
</section>
<section title="Mode">
<t>IPsec in transport mode MUST be supported. The use of <xref
target="RFC4301">IPsec</xref> processing for secure NORM traffic
MUST be configured such that unauthenticated packets are not
received by the NORM protocol implementation.</t>
</section>
<section title="Key Management">
<t>An automated key management scheme for group key distribution
and rekeying such as <xref target="RFC3547">GDOI</xref>, <xref
target="RFC4535">GSAKMP</xref>, or <xref
target="RFC3830">MIKEY</xref> is RECOMMENDED for use. Relatively
short-lived NORM sessions MAY be able to use Manual Keying with a
single, preplaced key, particularly if <xref
target="RFC4303">Extended Sequence Numbering (ESN)</xref> is
available in the IPsec implementation used. It should also be
noted that it may be possible for key update messages (e.g., the
GDOI GROUPKEY-PUSH message) to be included as part of the NORM
application reliable data transmission if appropriate interfaces
are available between the NORM application and the key management
daemon.</t>
</section>
<section title="Security Policy">
<t>Receivers MUST accept protocol messages only from the
designated, authorized sender(s). It is expected that appropriate
key management will provide encryption keys only to receivers
authorized to participate in a designated session. The approach
outlined here allows receiver sets to be controlled on a
per-sender basis.</t>
</section>
<section title="Authentication and Encryption">
<t>Large NORM group sizes will necessitate some form of key
management that does rely upon shared secrets. The GDOI and GSAKMP
protocols mentioned here allow for certificate-based
authentication. It is RECOMMENDED these certificates use IP
addresses for authentication although it may alternatively
possible to have authentication associated with pre-assigned
NormNodeId values. However, it is likely that available group key
management implementations will not be NORM-specific.</t>
</section>
<section title="Availability">
<t>The IPsec requirements profile outlined here is commonly
available on many potential NORM hosts. The principal issue is
that configuration and operation of IPsec typically requires
privileged user authorization. Automated key management
implementations are typically configured with the privileges
necessary to effect system IPsec configuration needed.</t>
</section>
</section>
</section>
</section>
<section title="IANA Considerations">
<t>Values of NORM Header Extension Types, Stream Control Codes, and
<spanx style="verb">NORM_CMD</spanx> message sub-types are subject to
IANA registration. They are in the registry named "Reliable Multicast
Transport (RMT) NORM Protocol Parameters" located at time of publication
at:</t>
<t><eref
target="http:///www.iana.org/assignments/norm-parameters">http:///www.iana.org/assignments/norm-parameters</eref></t>
<t>It should be also noted that reliable multicast building block
components used by this specification also have their respective IANA
considerations and those documents should be consulted accordingly. In
particular, the FEC Building Block used by NORM does require IANA
registration of the FEC codecs used. The registration instructions for
FEC codecs are provided in <xref format="none" target="RFC5052">RFC
5052</xref>.</t>
<section title="Explicit IANA Assignment Guidelines">
<t>This document introduces three namespaces that are registered for
the NORM Header Extension Types, Stream Control Codes and <spanx
style="verb">NORM_CMD</spanx> Message Sub-types. This section
describes explicit IANA assignment guidelines for each of these.</t>
<section title="NORM Header Extension Types">
<t>This document defines a namespace for NORM Header Extension Types
named:</t>
<t><spanx style="verb">ietf:rmt:norm:extension</spanx></t>
<t>The NORM Header Extension Type field is an 8-bit value. The
values of this field identify extended header content that allows
the protocol functionality to be expanded to include additional
features and operating modes. The values that can be assigned within
the <spanx style="verb">ietf:rmt:norm:extension</spanx> namespace
are numeric indexes in the range {0, 255}, boundaries included.
Values in the range {0,127} indicate variable length extended header
fields while values in the range {128,255} indicate extensions of a
fixed 4-byte length. This specification registers the following NORM
Header Extension Types:</t>
<texttable>
<ttcol>Value</ttcol>
<ttcol>Name</ttcol>
<ttcol>Reference</ttcol>
<c>1</c>
<c><spanx style="verb">EXT_AUTH</spanx></c>
<c>This specification</c>
<c>3</c>
<c><spanx style="verb">EXT_CC</spanx></c>
<c>This specification</c>
<c>64</c>
<c><spanx style="verb">EXT_FTI</spanx></c>
<c>This specification</c>
<c>128</c>
<c><spanx style="verb">EXT_RATE</spanx></c>
<c>This specification</c>
</texttable>
<t>Requests for assignment of additional NORM Header Extension Type
values are granted on a "Specification Required" basis as defined by
<xref target="RFC5226">IANA Guidelines</xref>. Any such header
extension specifications MUST include a description of protocol
actions to be taken when the extension type is encountered by a
protocol implementation not supporting that specific option. For
example, it may be possible for protocol implementations to ignore
unknown header extensions in many cases.</t>
</section>
<section title="NORM Stream Control Codes">
<t>This document defines a namespace for NORM Stream Control Codes
named:</t>
<t><spanx style="verb">ietf:rmt:norm:streamControlCode</spanx></t>
<t>NORM Stream Control Codes are 16-bit values that may be inserted
within a <spanx style="verb">NORM_OBJECT_STREAM</spanx> delivery
object to convey sequenced, out-of-band (with respect to the stream
data) control signaling applicable to the referenced stream object.
These control codes are to be delivered to the application or
protocol implementation with reliable delivery, in-order with
respect to the their inserted position within the stream. This
specification registers the following NORM Stream Control Code:</t>
<texttable>
<ttcol>Value</ttcol>
<ttcol>Name</ttcol>
<ttcol>Reference</ttcol>
<c>0</c>
<c><spanx style="verb">NORM_STREAM_END</spanx></c>
<c>This specification</c>
</texttable>
<t>Additional NORM Stream Control Code value assignment requests are
granted on a "Specification Required" basis as defined by <xref
target="RFC5226">IANA Guidelines</xref>. The full 16-bit space
outside of the value assigned in this specification are available
for future assignment. Note that in addition to describing the
control code's expected interpretation, such specifications MUST
include a description of protocol actions to be taken when the
control code is encountered by a protocol implementation not
supporting that specific option.</t>
</section>
<section title="NORM_CMD Message Sub-types">
<t>This document defines a namespace for <spanx style="verb">NORM_CMD</spanx>
Message Sub-types named:</t>
<t><spanx style="verb">ietf:rmt:norm:command</spanx></t>
<t>The <spanx style="verb">NORM_CMD</spanx> sub-type field is an
8-bit value with valid values in the range of 1-255. Note the value
0 is reserved to indicate an invalid <spanx style="verb">NORM_CMD</spanx>
message sub-type. The current specification defines a number of
<spanx style="verb">NORM_CMD</spanx> message sub-types that senders
can use to signal the receivers in various aspects of NORM protocol
operation. This specification registers the following <spanx
style="verb">NORM_CMD</spanx> Message Sub-types:</t>
<texttable>
<ttcol>Value</ttcol>
<ttcol>Name</ttcol>
<ttcol>Reference</ttcol>
<c>0</c>
<c>reserved</c>
<c>This specification</c>
<c>1</c>
<c><spanx style="verb">NORM_CMD(FLUSH)</spanx></c>
<c>This specification</c>
<c>2</c>
<c><spanx style="verb">NORM_CMD(EOT)</spanx></c>
<c>This specification</c>
<c>3</c>
<c><spanx style="verb">NORM_CMD(SQUELCH)</spanx></c>
<c>This specification</c>
<c>4</c>
<c><spanx style="verb">NORM_CMD(CC)</spanx></c>
<c>This specification</c>
<c>5</c>
<c><spanx style="verb">NORM_CMD(REPAIR_ADV)</spanx></c>
<c>This specification</c>
<c>6</c>
<c><spanx style="verb">NORM_CMD(ACK_REQ)</spanx></c>
<c>This specification</c>
<c>7</c>
<c><spanx style="verb">NORM_CMD(APPLICATION)</spanx></c>
<c>This specification</c>
</texttable>
<t>Future specifications extending NORM may wish to define
additional <spanx style="verb">NORM_CMD</spanx> messages to enhance
protocol functionality. <spanx style="verb">NORM_CMD</spanx> message
sub-type value assignment requests are granted on a "Specification
Required" basis as defined by <xref target="RFC5226">IANA
Guidelines</xref>. Note that in addition to describing the command
sub-type's expected interpretation, specifications MUST include a
description of protocol actions to be taken when the command is
encountered by a protocol implementation not supporting that
specific option.</t>
<t>Note that this specification already provides for an
"application-defined" <spanx style="verb">NORM_CMD</spanx> message
sub-type that may be used at the discretion of individual
applications using NORM for transport. These "application-defined"
commands may be suitable for many application-specific purposes and
do not require standards action. In any case, such additional
messages SHALL be subject to the same congestion control constraints
as the existing NORM sender message set.</t>
</section>
</section>
</section>
<section title="Suggested Use">
<t>The present NORM protocol is seen as useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting all
envisioned multicast reliability requirements. NORM provides a simple
and flexible framework for multicast applications with a degree of
concern for network traffic implosion and protocol overhead efficiency.
NORM-like protocols have been successfully demonstrated within the MBone
for bulk data dissemination applications, including weather satellite
compressed imagery updates servicing a large group of receivers and a
generic web content reliable "push" application.</t>
<t>In addition, this framework approach has some design features making
it attractive for bulk transfer in asymmetric and wireless internetwork
applications. NORM is capable of successfully operating independent of
network structure and in environments with high packet loss, delay, and
out-of-order delivery. Hybrid proactive/reactive FEC-based repairing
improve protocol performance in some multicast scenarios. A sender-only
repair approach often makes additional engineering sense in asymmetric
networks. NORM's unicast feedback capability may be suitable for use in
asymmetric networks or in networks where only unidirectional multicast
routing/delivery service exists. Asymmetric architectures supporting
multicast delivery are likely to make up an important portion of the
future Internet structure (e.g., DBS/cable/PSTN hybrids) and efficient,
reliable bulk data transfer will be an important capability for
servicing large groups of subscribed receivers.</t>
</section>
<section anchor="ProtocolChanges" title="Changes from RFC3940">
<t>This section lists the changes between the Experimental version of
this specification, <xref format="none" target="RFC3940">RFC
3940</xref>, and this version:</t>
<t><list style="numbers">
<t>Removal of the <spanx style="verb">NORM_FLAG_MSG_START</spanx>
for <spanx style="verb">NORM_OBJECT_STREAM</spanx>, replacing it
with the "payload_msg_start" field in the FEC-encoded preamble of
the <spanx style="verb">NORM_OBJECT_STREAM NORM_DATA</spanx>
payload,</t>
<t>Definition of IANA namespace for header extension assignment,</t>
<t>Removal of file blocking scheme description that is now specified
in the <xref target="RFC5052">FEC Building Block
document</xref>,</t>
<t>Removal of restriction of NORM receiver feedback message rate to
local NORM sender rate (This caused congestion control failures in
high speed operation. The extremely low feedback rate of the NORM
protocol as compared to TCP avoids any resultant impact to the
network as shown in <xref target="Mdpcc"></xref>),</t>
<t>Correction of errors in some message format descriptions, and</t>
<t>Correction of inconsistency in specification of the inactivity
timeout.</t>
<t>Addition of IPsec secure mode description with IPsec
requirements.</t>
<t>Addition of the EXT_AUTH header extension definition.</t>
<t>Clarification of interpretation of "Source Block Length" when FEC
codes are arbitrarily shortened by the sender.</t>
</list></t>
</section>
<section title="Acknowledgments">
<t>(and these are not Negative)</t>
<t>The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input and
comments on this document. The authors would also like to thank the RMT
working group chairs, Roger Kermode and Lorenzo Vicisano, for their
support in development of this specification, and Sally Floyd for her
early input into this document.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.1112"?>
<?rfc include="reference.RFC.5401"?>
<?rfc include="reference.RFC.5052"?>
<?rfc include="reference.RFC.4607"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.4303"?>
<?rfc include="reference.RFC.5226"?>
<?rfc include="reference.RFC.4301"?>
<?rfc include="reference.RFC.4654"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.3269"?>
<?rfc include="reference.RFC.3940"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.2974"?>
<?rfc include="reference.RFC.5445"?>
<reference anchor="RmComparison">
<front>
<title>A Comparison of Sender-Initiated and Receiver-Initiated
Reliable Multicast Protocols</title>
<author fullname="S. Pingali" initials="S." surname="Pingali">
<organization></organization>
</author>
<author fullname="Don Towsley" initials="D." surname="Towsley">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="Jim Kurose" initials="J." surname="Kurose">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="October" year="1993" />
</front>
<seriesInfo name="Proc. INFOCOMM," value="San Francisco CA" />
</reference>
<?rfc include="reference.RFC.3453"?>
<reference anchor="MdpToolkit">
<front>
<title>The Multicast Dissemination Protocol (MDP) Toolkit</title>
<author fullname="Joseph Macker" initials=" J." surname="Macker">
<organization></organization>
</author>
<author fullname="Brian Adamson" initials="B." surname="Adamson">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="October" year="1999" />
</front>
<seriesInfo name="Proc. IEEE MILCOM" value="" />
</reference>
<reference anchor="McastFeedback">
<front>
<title>Optimal Multicast Feedback</title>
<author fullname="J. Nonnenmacher" initials="J."
surname="Nonnenmacher">
<organization></organization>
</author>
<author fullname="E. Biersack" initials="E." surname="Biersack">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="March/April" year="1998" />
</front>
<seriesInfo name="IEEE INFOCOM, " value="p. 964" />
</reference>
<reference anchor="NormFeedback">
<front>
<title>Quantitative Prediction of NACK-Oriented Reliable Multicast
(NORM) Feedback</title>
<author fullname="Brian Adamson" initials="B." surname="Adamson">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<author fullname="Joseph Macker" initials=" J." surname="Macker">
<organization></organization>
</author>
<date month="October" year="2002" />
</front>
<seriesInfo name="IEEE MILCOM" value="" />
</reference>
<reference anchor="FecHybrid">
<front>
<title>Reliable Multicast and Integrated Parity Retransmission with
Channel Estimation</title>
<author fullname="Don Gossink" initials="D." surname="Gossink">
<organization></organization>
</author>
<author fullname="Joseph Macker" initials="J." surname="Macker">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="" year="1998" />
</front>
<seriesInfo name="IEEE Globecomm " value="" />
</reference>
<?rfc include="reference.RFC.3048"?>
<?rfc include="reference.RFC.3550"?>
<?rfc include="reference.RFC.2357"?>
<?rfc include="reference.RFC.3711"?>
<reference anchor="TfmccPaper">
<front>
<title>Extending Equation-Based Congestion Control to Multicast
Applications</title>
<author fullname="Joerg Widmer" initials="J." surname="Widmer">
<organization>Joerg</organization>
</author>
<author fullname="Mark Handley" initials="M." surname="Handley">
<organization></organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="August" year="2001" />
</front>
<seriesInfo name="ACM SIGCOMM" value="" />
</reference>
<reference anchor="PgmccPaper">
<front>
<title>pgmcc: A TCP-Friendly Single-Rate Multicast Congestion
Control Scheme</title>
<author fullname="Luigi Rizzo" initials="L." surname="Rizzo">
<organization></organization>
</author>
<date month="August" year="2000" />
</front>
<seriesInfo name="ACM SIGCOMM" value="" />
</reference>
<reference anchor="TcpModel">
<front>
<title>Modeling TCP Throughput: A Simple Model and its Empirical
Validation</title>
<author fullname="J. Padhye" initials=" J." surname="Padhye">
<organization></organization>
</author>
<author fullname="V. Firoiu" initials="V." surname="Firoiu">
<organization>Firoiu</organization>
</author>
<author fullname="D. Towsley" initials="D." surname="Towsley">
<organization></organization>
</author>
<author fullname="J. Kurose" initials="J." surname="Kurose">
<organization></organization>
</author>
<date year="1998" />
</front>
<seriesInfo name="ACM SIGCOMM" value="" />
</reference>
<?rfc include="reference.RFC.3547"?>
<?rfc include="reference.RFC.3830"?>
<?rfc include="reference.RFC.4535"?>
<reference anchor="Mdpcc">
<front>
<title>A TCP-Friendly, Rate-based Mechanism for NACK-Oriented
Reliable Multicast Congestion Control</title>
<author fullname="Brian Adamson" initials=" B." surname="Adamson">
<organization></organization>
</author>
<author fullname="Joseph Macker" initials="J." surname="Macker">
<organization>J</organization>
<address>
<postal>
<street></street>
<city></city>
<region></region>
<code></code>
<country></country>
</postal>
<phone></phone>
<facsimile></facsimile>
<email></email>
<uri></uri>
</address>
</author>
<date month="November" year="2001" />
</front>
<seriesInfo name="Proc. IEEE GLOBECOMM" value="" />
</reference>
</references>
</back>
</rfc>
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