One document matched: draft-ietf-rmt-pi-norm-revised-07.xml
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<rfc category="std" docName="draft-ietf-rmt-pi-norm-revised-07" ipr="full3978">
<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="October" year="2008" />
<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.</t>
</abstract>
<note 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"></xref>.</t>
</note>
</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.</t>
<t>This document is a product of the IETF RMT WG and follows the
guidelines provided in <xref target="RFC3269"></xref>.</t>
<t>Statement of Intent</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"></xref>. A prior document, <xref
target="RFC3940"></xref>, contained a previous description of the NORM
Protocol specification described in this document. RFC3940 was published
in the "Experimental" category. It was the stated intent of the RMT
working group to re-submit this specifications as an IETF Proposed
Standard in due course.</t>
<t>This Proposed Standard specification is thus based on <xref
target="RFC3940"></xref> and has been updated according to accumulated
experience and growing protocol maturity since the publication of
RFC3940. Said experience applies both to this specification itself and
to congestion control strategies related to the use of this
specification.</t>
<t>The differences between <xref target="RFC3940"></xref> and this
document are listed in <xref target="ProtocolChanges"></xref>.</t>
<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 RMT <xref target="I-D.ietf-rmt-bb-norm-revised">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"></xref>, but SHALL also
be capable of scalable operation in asymmetric topologies such as
Source-Specific Multicast (SSM) <xref target="RFC4607"></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="I-D.ietf-rmt-bb-norm-revised">Multicast NACK Building Block
document</xref>. This includes the basic NORM architecture and the
data transmission, repair, and feedback strategies discussed in that
document. Additional reliable multicast building blocks are applied in
creating the full NORM protocol instantiation as described in <xref
target="RFC3048"></xref>. NORM also makes use of Forward Error
Correction encoding techniques for repair messaging and optional
transmission robustness as described in <xref
target="RFC3453"></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 <xref
target="TfmccPaper">TCP-Friendly Multicast Congestion Control (TFMCC)
scheme of </xref> and <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 Protocol Instantiation document, in conjunction with the RMT
Building Block documents of <xref
target="I-D.ietf-rmt-bb-norm-revised"></xref> and <xref
target="RFC5052"></xref>, completely specifies a working reliable
multicast transport protocol that conforms to the requirements described
in RFC 2357 <xref target="RFC2357"></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 as a monotonically increasing number incremented with each
NORM message transmitted. Note that two independent "sequence" spaces
MUST be maintained. One sequence space SHALL be kept for NORM sender
messages (<spanx style="verb">NORM_INFO</spanx>, <spanx
style="verb">NORM_DATA</spanx>, and <spanx
style="verb">NORM_CMD</spanx>) generated, and a separate, independent
"sequence" space SHALL be kept for NORM receiver messages (<spanx
style="verb">NORM_NACK</spanx> and <spanx
style="verb">NORM_NACK</spanx>). The sender message "sequence" value
can be monitored by receiving nodes to detect packet losses in the
transmissions from a sender and used to estimate raw packet loss for
congestion control purposes. Note that this value is NOT used in the
NORM protocol to detect missing reliable data content and does NOT
identify the application data or FEC payload that may be attached. The
"sequence" field may also be leveraged for protection from message
replay attacks, particularly of <spanx style="verb">NORM_NACK</spanx>
or other feedback messages. For this reason, NORM receiver messages
are also sequence numbered. An independent sequence space MUST be used
for receiver messages because when receivers generate unicast <spanx
style="verb">NORM_NACK</spanx> or <spanx style="verb">NORM_ACK</spanx>
messages, those messages will not be visible to the group at large
that may be performing loss estimation. Also, NORM congestion control
is applied only to sender messages. The size of the "sequence" field
is intended to be sufficient to allow detection of a reasonable range
of packet loss within the delay-bandwidth product of expected network
connections.</t>
<t>The "source_id" field is a 32-bit value identifying the node that
sent the message. A participant's NORM node identifier (<spanx
style="emph">NormNodeId</spanx>) can be set according to application
needs but unique 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 need to be considered. For example, the
"source identifier" mechanism defined in the Real-Time Protocol (RTP)
specification <xref target="RFC3550"></xref> may be applicable to use
for NORM node identifiers. At this point in time, the protocol makes
no assumptions about how these unique identifiers are actually
assigned.</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 (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 . The "fec_id" field is used to indicate the FEC coding type.
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
RMT <xref target="I-D.ietf-rmt-bb-norm-revised">Multicast NACK
Building Block document</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 as described in the IETF FEC
Basic Schemes document <xref
target="I-D.ietf-rmt-bb-fec-basic-schemes-revised">I-d</xref> or
additional FEC Scheme documents that may be defined. As an example,
the format of the "fec_payload_id" format for Small Block,
Systematic codes ("fec_id" = 129) 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
given by the FEC Basic Schemes document <xref
target="I-D.ietf-rmt-bb-fec-basic-schemes-revised"></xref> for Small
Block Systematic FEC Schemes identified by a "fec_id" value of 129.
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 FEC Building Block document <xref
target="RFC5052"></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 FEC Building Block document <xref
target="RFC5052"></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 target="RFC5052"></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 FEC Building Block document <xref
target="RFC5052"></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 FEC Building Block document
<xref target="RFC5052"></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. 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 currently 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.
Future versions of this specification may define additional stream
control codes if necessary. Values of "stream_control_code" that are
not understood SHOULD be ignored.</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 FEC Building Block document <xref
target="RFC5052"></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 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 target="RFC4654"></xref> is described 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 private network
with congestion control operation disabled, the <spanx
style="verb">NORM_CMD(CC)</spanx> message is then used for GRTT
measurement only 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
target="RFC4654"></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="I-D.ietf-rmt-bb-norm-revised">Multicast NACK
Building Block document</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" = 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" title="">
<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 could 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 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 operating 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 document</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 document</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="I-D.ietf-rmt-bb-norm-revised">Multicast NACK Building Block
document</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="I-D.ietf-rmt-bb-norm-revised"></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="I-D.ietf-rmt-bb-norm-revised"></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="I-D.ietf-rmt-bb-norm-revised"></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="I-D.ietf-rmt-bb-norm-revised">Multicast NACK Building Block
document</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 described
in <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 (e.g., PGMCC <xref
target="PgmccPaper"></xref>). 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 describe 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 details of PLR selection are not
discussed in this document.</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="I-D.ietf-rmt-bb-norm-revised">Multicast NACK Building
Block document</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 NORM, TFMCC, and
FEC Building Blocks also apply to the NORM protocol. In addition to
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. The NORM protocol is compatible with the use of the IP
security (IPsec) architecture described in <xref
target="RFC4301"></xref> and the IPsec Encapsulating Security Payload
(ESP) protocol or Authentication Header (AF) extension MAY be used to
secure IP packets transmitted by NORM participants.</t>
<t>Alternatively, a header extension may be applied to the NORM protocol
to provide authentication of NORM messages. For this purpose the <spanx
style="verb">EXT_AUTH</spanx> header extension (HET = 1) is defined. 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 SHOULD 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 is
fully authenticated. Any congestion control related action that is
appropriate MUST NOT be postponed by any such full packet
authentication. Consideration SHOULD 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 used on a per-session basis or NORM sender nodes SHOULD
use unique "instance_id" identifiers that are managed as part of the
security association for the sessions.</t>
<t>It is RECOMMENDED that such security mechanisms be used when
available. It should be noted that NORM participants can use the
"sequence" field from the NORM Common Message Header to detect replay
attacks. This can be accomplished if the NORM sender is willing to
maintain state on receivers which are NACKing. A cache of such receiver
state can be used to provide protection against NACK replay attacks.
NORM receivers SHOULD 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) is
RECOMMENDED for protection against similar attacks that might 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. When any such
cryptographic measures are used, it is RECOMMENDED that an approach such
as described in the Group Domain of Interpretation (GDOI) <xref
target="RFC3547"></xref>, Multimedia Internet KEYing (MIKEY) <xref
target="RFC3830"></xref> or Group Secure Association Key Management
Protocol (GSAKMP) <xref target="RFC4535"></xref> specifications for
automated key management is 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 data content is
highly RECOMMENDED.</t>
<section 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 RTP <xref target="RFC1889"></xref> and
as preserved in the specification for Secure Real Time Protocol (SRTP)
<xref target="RFC3711"></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 ESP protocol
<xref target="RFC4303"></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 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. Note that built-in IPsec replay attack protection for this
second SA at the sender MUST be disabled.</t>
<t>Multiple receivers using a common IPsec SA for traffic directed
to the NORM sender (i.e., many-to-one) prevents the use of built-in
IPsec replay attack protection by the NORM sender with current IPsec
implementations. 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, as RECOMMENDED, for secure
NORM protocol operation makes it more difficult for adversaries to
conduct effective 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
this form of replay attack protection even more robust. 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 IPsec
<xref target="RFC4301"></xref> processing for secure NORM traffic
SHOULD also be REQUIRED 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 GDOI <xref target="RFC3547"></xref>, GSAKMP
<xref target="RFC4535"></xref>, or MIKEY <xref
target="RFC3830"></xref> SHOULD be used. Relatively short-lived
NORM sessions MAY be able to use Manual Keying with a single,
preplaced key, particularly if Extended Sequence Numbering (ESN)
<xref target="RFC4303"></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 in the NORM application reliable data
transmission if appropriate interfaces were available between the
NORM application and the key management daemon.</t>
</section>
<section title="Security Policy">
<t>Receivers SHOULD accept connections 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. These certificates SHOULD 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>Header extension identifiers for the NORM protocol are subject to
IANA registration. Additionally, building blocks components used by this
NORM Protocol specification may introduce additional IANA
considerations. 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
target="RFC5052"></xref>.</t>
<section title="Explicit IANA Assignment Guidelines">
<t>This document defines a name-space for NORM Header Extensions
named:</t>
<t><spanx style="verb">ietf:rmt:norm:extensions</spanx></t>
<t>These values represent extended header fields that allow the
protocol functionality to be expanded to include additional optional
features and operating modes. The values that can be assigned within
the <spanx style="verb">ietf:rmt:norm:extension</spanx> name-space 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 extension of a fixed
4-byte length. NORM header extension identifier value assignment
requests are granted on a "Specification Required" basis as defined in
<xref target="RFC2434"></xref>. Additional header extension
specifications MUST include a description of protocol actions to be
taken when the extended header 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>
<t>This specification registers the following NORM Header Extension
types in namespace <spanx
style="verb">ietf:rmt:norm:extensions</spanx>:</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>
</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 target="RFC3940"></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 FEC Building Block document <xref
target="RFC5052"></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>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.I-D.ietf-rmt-bb-norm-revised"?>
<?rfc include="reference.RFC.5052"?>
<?rfc include="reference.RFC.4607"?>
<?rfc include="reference.RFC.4654"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.3269"?>
<?rfc include="reference.RFC.4303"?>
<?rfc include="reference.RFC.2434"?>
<?rfc include="reference.RFC.4301"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.3940"?>
<?rfc include="reference.RFC.4566"?>
<?rfc include="reference.RFC.2974"?>
<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.1889"?>
<?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>
<?rfc include="reference.I-D.ietf-rmt-bb-fec-basic-schemes-revised"?>
<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>
<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="D. Towsley" initials="D." surname="Towsley"></author>
<author fullname="J. Kurose" initials="J." surname="Kurose"></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>| PAFTECH AB 2003-2026 | 2026-04-23 10:11:36 |