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Differences from draft-ietf-rmt-pi-norm-07.txt

RMT Working Group B. Adamson/NRL
INTERNET-DRAFT C. Bormann/Tellique
draft-ietf-rmt-pi-norm-08 M. Handley/ACIRI
Expires: April 2004 J. Macker/NRL
October 2003

NACK-Oriented Reliable Multicast Protocol (NORM)

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference mate-
rial or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

This document describes the messages and procedures of the Negative-
acknowledgement (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 acknowledgement
mechanism for transport reliability and offers additional protocol
mechanisms to conduct reliable multicast sessions with limited "a
priori" coordination among senders and receivers. A congestion
control scheme is specified to allow the NORM protocol 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) from the
senders to receivers. The protocol offers a number of features to

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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.

1.0 Introduction and Applicability

The Negative-acknowledgement (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 queueing or transmission delays.

This document is a product of the IETF RMT WG and follows the
guidelines provided in RFC 3269 [1]. 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 BCP 14, RFC 2119 [2].

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1.1 NORM Delivery Service Model

A NORM protocol instance (NormSession) 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
NormNodes will communicate using a common IP multicast group address
and port number that has been chosen via means outside the context of
the given NormSession. Other IETF data format and protocol standards
exist that may be applied to describe and convey the required "a
priori" information for a specific NormSession (e.g., Session
Description Protocol (SDP) [5], Session Announcement Protocol (SAP)
[6], etc).

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 NormSession. 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.

NORM provides for three types of bulk data content objects
(NormObjects) to be reliably transported. These types include:

1) static computer memory data content (NORM_OBJECT_DATA type),

2) computer storage files (NORM_OBJECT_FILE type), and

3) non-finite streams of continuous data content
(NORM_OBJECT_STREAM type).

The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
simply to provide a "hint" to receivers in NormSessions 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

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of static data/file delivery services might make use of these
transport object types, too. The use of the NORM_OBJECT_STREAM 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
NORM_OBJECT_STREAM 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.

The NORM protocol also allows for a small amount of "out-of-band" data
(sent as NORM_INFO 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 NORM_INFO content is
designed to be atomic in that its size MUST fit into the payload
portion of a single NORM message.

NORM does _not_ provide for global or application-level identification
of data content within in its message headers. Note the NORM_INFO
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 (NormObjects) with transport identifiers that are applicable
only while the sender is transmitting and/or repairing the given
object. These transport data content identifiers (NormTransportIds)
are assigned in a monotonically increasing fashion by each NORM sender
during the course of a NormSession. Each sender maintains its
NormTransportId assignments independently so that individual
NormObjects may be uniquely identified during transport with the
concatenation of the sender session-unique identifier (NormNodeId) and
the assigned NormTransportId. The NormTransportIds 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.

To summarize, the NORM protocol provides reliable transport of
different types of data content (including potentially mixed types).

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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.

1.2 NORM Scalability

Group communication scalability requirements lead to adaptation of
negative acknowledgement (NACK) based protocol schemes when feedback
for reliability is required [7]. 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[8]. FEC-based repair can be used to greatly
reduce the quantity of reliable multicast repair requests and repair
transmissions[9]. 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 [10]. NORM dynamically
measures the group's roundtrip 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.

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 "advertises" 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 [11].
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. NORM
receivers must maintain state for for each active sender. This may
constrain the number of simultaneous senders in some uses of NORM.

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1.3 NORM Environmental Requirements and Considerations

All of the environmental requirements and considerations that apply to
the RMT FEC Building Block and the the RMT TCP-Friendly Multicast
Congestion Control (TFMCC) Building Block [17] also apply to NORM.

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. The
NORM protocol SHOULD be compatible with techniques like GRA for
performance benefits when applicable. While the techniques utilized
in NORM are principally applicable to "flat" end-to-end IP multicast
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 RFC 1112 [12], but SHALL also be capable of scalable
operation in asymmetric topologies such as Source Specific Multicast
(SSM) [13] where there may only be unicast routing service from the
receivers to the sender(s).

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).

2.0 NORM Architecture Definition

A NormSession is comprised of participants (NormNodes) acting as
senders and/or receivers. NORM senders transmit data content in the
form of NormObjects to the session destination address and the NORM
receivers attempt to reliably receive the transmitted content using
negative acknowledgments to request repair. Each NormNode within a
NormSession is assumed to have a preselected unique 32-bit identifier
(NormNodeId). NormNodes MUST have uniquely assigned identifiers
within a single NormSession to distinquish between possible multiple
senders and to distinguish feedback information from different
receivers. There are two reserved NormNodeId values. A value of
0x00000000 is considered an invalid NormNodeId value and a value of
0xffffffff is a "wildcard" NormNodeId. 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.

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As previusly noted, NORM allows for reliable transmission of three
different basic types of data content. The first type is
NORM_OBJECT_DATA, which is used for static, persistent blocks of data
content maintained in the sender's application memory storage. The
second type is NORM_OBJECT_FILE, which corresponds to data stored in
the sender's non-volatile file system. The NORM_OBJECT_DATA and
NORM_OBJECT_FILE types both represent "NormObjects" of finite but
potentially very large size. The third type of data content is
NORM_OBJECT_STREAM, 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. All NormObjects are logically segmented into FEC coding
blocks and symbols for transmission by the sender.

This document generally refers to FEC encoding symbols as "segments".
In the case of systematic FEC codes, the symbols for the portion of
FEC blocks containing data content _are_ actually "segments" of the
NormObject data content being transported while the remaining symbols
for the block contain FEC parity content. However, for non-systematic
encoding schemes, all of the "segments" transmitted are actually
symbols containing FEC parity content. In either case, the terms
"symbol" and "segment" are used interchangeably throughout this
document.

Transmitted NormObjects are temporarily yet uniquely identified within
the NormSession context using the given sender's NormNodeId and a
temporary NormObjectTransportId. These data content identifiers are
sender-assigned and applicable and valid only during a NormObject's
actual _transport_ (i.e. for as long as the sender is transmitting and
providing repair of the indicated NormObject). For a long-lived
session, the NormObjectTransportId 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 NormObject are further identified with FEC payload
identifiers which include coding block and symbol identifiers. These
are discussed in detail later in this document.

2.1 NORM Protocol Operation Overview

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A NORM sender primarily generates messages of type NORM_DATA. These
messages carry original data segments or FEC symbols and repair
segments/symbols for the bulk data/file or stream NormObjects 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) [18] with reduced receiver feedback, or, in some
cases, no feedback.

A sender message of type NORM_INFO is also defined and is used to
carry OPTIONAL "out-of-band" context information for a given transport
object. A single NORM_INFO message can be associated with a
NormObject. Because of its atomic nature, missing NORM_INFO messages
can be NACKed and repaired with a slightly lower delay process than
NORM's general FEC-encoded data content. NORM_INFO 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 NORM_INFO will be described later. When the NORM_INFO
message type is used, its transmission should precede transmisson of
any NORM_DATA message for the associated NormObject.

The sender also generates messages of type NORM_CMD to assist in
certain protocol operations such as congestion control, end-of-
transmission flushing, round trip time estimation, receiver
synchronization, and optional positive acknowledgement requests or
application defined commands. The transmission of NORM_CMD messages
from the sender is accomplished by one of three different processes.
These 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).

NORM receivers generate messages of type NORM_NACK or NORM_ACK in
response to transmissions of data and commands from a sender. The

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NORM_NACK 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. NORM_ACK messages are generated in response
to certain commands transmitted by the sender. In the general (and
most scalable) protocol mode, NORM_ACK messages are sent only in
response to congestion control commands from the sender. The feedback
volume of these congestion control NORM_ACK messages is controlled
using the same timer-based probabilistic suppression techniques as for
NORM_NACK messages to avoid feedback implosion. In order to meet
potential application requirements for positive acknowledgement from
receivers, other NORM_ACK messages are defined and available for use.
All sender and receiver 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 which is
compatible with coexistent TCP flows.

2.2 NORM Protocol Building Blocks

The operation of the NORM protocol is based upon the concepts
presented in the Nack-Oriented Reliable Multicast (NORM) Building
Block document[14]. This includes the basic NORM architecture and the
data transmission, repair, and feedback strategies discussed in that
document. NORM also makes use of Forward Error Correction encoding
techiques for repair messaging and optional transmission robustness as
described in [15]. NORM uses the FEC Payload ID as specified by the
FEC Building Block Document[16]. Additionally, for congestion
control, this document includes a baseline congestion control
mechanism (NORM-CC) based on the TCP-Friendly Multicast Congestion
Control (TFMCC) Building Block described in [17].

2.3 NORM Design Tradeoffs

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 tradeoffs 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

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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.

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 tradeoffs 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.

3.0 Conformance Statement

This Protocol Instantiation document, in conjunction with the Building
Block documents identified in [14], [15], [16], and [17] completely
specifies a working reliable multicast transport protocol that
conforms to the requirements described in RFC 2357 [3].

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This document specifies the following message types and mechanisms
which are REQUIRED in complying NORM protocol implementations:

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This document also describes the following message types and
associated mechanisms which are OPTIONAL for complying NORM protocol
implementations:

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4.0 NORM Message Formats

As mentioned in Section 2.1, there are two primary classes of NORM
messages: sender messages and receiver messages. NORM_CMD, NORM_INFO,
and NORM_DATA message types are generated by senders of data content,
and NORM_NACK and NORM_ACK messages generated by receivers within a
NormSession. An auxillary message type of NORM_REPORT is also
provided for experimental purposes. This section described 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 Section 5.0. 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.

4.1 NORM Common Message Header and Extensions

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:

NORM Common Message Header Format

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.

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The message "type" field is a 4-bit value indicating the NORM protocol
message type. These types are defined as follows:

Message Value

NORM_INFO 1
NORM_DATA 2
NORM_CMD 3
NORM_NACK 4
NORM_ACK 5
NORM_REPORT 6

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".

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 to a given destination address. A "sequence"
field number space SHOULD be maintained for messages sent to the
NormSession group address. This value can be monitored by receiving
nodes to detect packet losses in the transmission from a sender and
used in estimating 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. With message
authentication, the "sequence" field may also be leveraged for
protection from message "replay" attacks, particularly of NORM_NACK or
other feedback messages. In this case, the receiver node should
maintain a monotonically increasing "sequence" field space for each
destination to which it transmits (This may be multiple destinations
when unicast feedback is used). The size of this 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.

The "source_id" field is a 32-bit value identifying the node that sent
the message. A participant's NORM node identifier (NormNodeId) can be
set according to application needs but unique identifiers must be
assigned within a single NormSession. 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 [19] may be applicable to use for NORM node identifiers.

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At this point in time, the protocol makes no assumptions about how
these unique identifiers are actually assigned.

NORM Header Extensions

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:

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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM Variable Length Header Extension Format

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ext_type >=128| ext_len | Header Extension Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NORM Fixed Length (32-bit) Header Extension Format

The "Header Extension Content" portion of these header extension
format is defined for each header extension type defined for NORM
messages. Some header extensions are defined within this document for
NORM baseline FEC and congestion control operations.

4.2 NORM Sender Messages

NORM sender messages include the NORM_DATA type, the NORM_INFO type,
and the NORM_CMD type. NORM_DATA and NORM_INFO messages contain
application data content while NORM_CMD messages are used for various
protocol control functions.

4.2.1 NORM_DATA Message

The NORM_DATA message is expected to be the predominant type

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transmitted by NORM senders. These messages are used to encapsulate
segmented data content for objects of type NORM_OBJECT_DATA,
NORM_OBJECT_FILE, and NORM_OBJECT_STREAM. NORM_DATA messages may
contain original or FEC-encoded application data content.

The format of NORM_DATA messages is comprised of three logical
portions: 1) a fixed-format NORM_DATA header portion, 2) an FEC
Payload ID portion with a format dependent upon the FEC encoding used,
and 3) a payload portion that includes length and offset fields as
well as application data content. Additionally, NORM implementations
MAY extend the NORM_DATA header to include an 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.

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_offset (msb)* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset (lsb)* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM_DATA Message Format

*NOTE: The "payload_len" and "payload_offset" fields in the payload
portion of NORM_DATA messages are present only when systematic FEC
codes (e.g., "fec_id" = 129) are used. For such FEC codes, these
fields contain actual length and offset values for the encapsulated
application data segment for NORM_DATA messages containing original
data information. However, in NORM_DATA messages containing parity

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information for a coding block, these fields are not actual length or
offset values but are instead values computed from FEC encoding of the
"payload_len" and "payload_offset" fields of the _data_ segments of
the applicable coding block. For systematic FEC codes, parity
segments can be identified as segments where "encoding_symbol_id >=
source_block_len", while data segments are those where
"encoding_symbol_id < source_block_len".

The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the NORM_DATA "type" field is 2. The NORM_DATA _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 used for the referenced NormObject. 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 NORM_DATA base "hdr_len" value is 6. The cumulative
size of any header extensions applied is added into the "hdr_len"
field.

The "instance_id" field contains a value generated by the sender to
uniquely identify its current instance of participation in the
NormSession. 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 SHALL drop their previous state on
the sender and begin reception anew.

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 R_max in the TFMCC Building Block [17]). This
value is used to control timing of the NACK repair process and other
aspects of protocol operation as described in this document. The
algorithm for encoding and decoding this field is described in the RMT
NORM Building Block document[14].

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 use of these values in the NORM receiver NACK procedure
is described in detail in Section 5.3.

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

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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" = 0x4) is recommended for
general purpose reliable multicast applications using the NORM
protocol.

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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:

The NORM_FLAG_REPAIR flag 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. The NORM_FLAG_EXPLICIT flag is used to
mark repair messages sent when the data sender has exhausted its
ability to provide "fresh" (previously untransmitted) parity segments
as repair. This flag may be used by intermediate systems implementing
Generic Router Assist (GRA) functionality to control subcasting of
repair content to different legs of a reliable multicast topology with
disparate repair needs. The NORM_FLAG_INFO flag is set only when

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optional NORM_INFO content is actually available for the associated
object. Thus, receivers will NACK for retransmission of NORM_INFO
only when it is available for a given object. The
NORM_FLAG_UNRELIABLE flag 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 NORM_FLAG_UNRELIABLE 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 NORM_CMD(SQUELCH) process as
described in Section 4.2.3.

The NORM_FLAG_FILE flag can be set as a "hint" from the sender that
the associated object should be stored in non-volatile storage. The
NORM_FLAG_STREAM flag is set when the identified object is of type
NORM_OBJECT_STREAM. When the NORM_FLAG_STREAM flag is set, the
NORM_FLAG_MSG_START can be optionally used to mark the first data
segments of application-layer messages transported within the NORM
stream. This allows NORM receiver applications to "synchronize" with
NORM senders and to be able to properly interpret application layer
data when joining a NORM session already in progress. In practice,
the NORM implementation MAY set this flag for the segment transmitted
following an explicit "flush" of the stream by the application.

The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [16]. 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 NORM_OBJECT_STREAM requires
systematic FEC codes for most efficient performance.

The "object_transport_id" field is a monotonically and incrementally
increasing value assigned by the sender to NormObjects 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 NORM_INFO messages
associated with the identified object carry the same
"object_transport_id" value.

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The "fec_payload_id" identifies the attached NORM_DATA "payload"
content. The size and format of of the "fec_payload_id" field depends
upon the FEC type indicated by the "fec_id" field. These formats are
given in the FEC Building Block document [16] and any subsequent
extensions of that document. As an example, the format of the
"fec_payload_id" format small block, systematic codes ("fec_id" = 129)
given here:

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Small Block, Systematic Code ("fec_id" = 129) "fec_payload_id" Format

The FEC payload identifier "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 IETF FEC Building Block
document[16]. The "source_block_number" identifies the coding block's
relative position with a NormObject. Note that, for NormObjects of
type NORM_OBJECT_STREAM, 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.
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
source_block_len contain original application data while segments
greater than or equal to source_block_len contain parity symbols
calculated for the block. The concatenation of
object_transport_id::fec_payload_id can be viewed as a unique
transport data unit (TPDU) identifier for the attached segment with
respect to the NORM sender.

Additional FEC Object Transmission Information (as described in the
FEC Building Block document[16]) 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

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facilitate receiver operation with minimal preconfiguration. For this
purpose, the NORM FEC Object Transmission Information Header Extension
(EXT_FTI) is defined. This header extension MAY be applied to
NORM_DATA and NORM_INFO messages to provide this necessary
information. The exact 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) and the byte size of
the associated NormObject (For the NORM_OBJECT_STREAM type, this size
corresponds to the stream buffer size maintained by the NORM sender).
As an example, the format of the EXT_FTI for small block systematic
codes ("fec_id" = 129) is given here:

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_length (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_length (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | fec_max_block_len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_num_parity | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

FEC Object Transmission Information Header Extension (EXT_FTI) for Small Block Systematic Codes ("fec_id" = 129)

The header extension type "het" field value for this header extension
is 64. The header extension length "hel" depends upon the format of
the FTI for FEC code type identifed by the "fec_id" field. In this
example (for "fec_id" = 129), the "hel" field value is 4.

The 48-bit "object_length" field indicates the total size of the
object (in bytes) for the static object types of NORM_OBJECT_FILE and
NORM_OBJECT_DATA. 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 NORM_OBJECT_STREAM, the "object_length"
field is used by the sender to indicate 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.

The "fec_instance_id" corresponds to the "FEC Instance ID" described
in the FEC Building Block document[16]. In this case, the
"fec_instance_id" SHALL be 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

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FEC Instance ID values is described in [16].

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.

The "fec_num_parity" corresponds to the "maximum number of 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 [16]. 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.

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.

The payload portion of NORM_DATA messages includes the data or FEC
payload and additional fields indicating the payload content "length"
and "offset" in the case that payload content is recovered using FEC
decoding.

The "payload_len" and "payload_offset" fields are used to specify the
size and relative position (within the NormObject) of the application
content included in the message payload. For senders employing
systematic FEC encoding, these fields correspond to actual length and
offset values for the payload of messages which contain original data
content. For NORM_DATA messages containing calculated parity content,
these fields will actually contain values computed by FEC encoding of
the "payload_len" and "payload_offset" values of the NORM_DATA data
segments of the corresponding FEC coding block. Thus, the
"payload_len" and "payload_offset" values of missing data content can
be determined when decoding an FEC coding block. Note that these
fields are present in NORM_DATA messages only when small block,
systematic FEC encoding is used. Also, these fields do _not_
contribute to the value of the NORM_DATA "hdr_len" field.

The "payload_data" field contains the original application data or
computed parity content associated with the segment. The maximum
length of this field SHALL be limited to a maximum of the sender's
NormSegmentSize as given in the FTI for the object. The length of
this field for messages containing parity content will always be of

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length NormSegmentSize. When encoding data segments of varying sizes,
the FEC encoder SHALL assume ZERO value padding for data segments with
length less than the NormSegmentSize. When applicable, the receiver
MAY use the "payload_len" information to properly retrieve received
data content and deliver it to the application. A sender's
NormSegmentSize SHOULD 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 NormSegmentSize 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.

4.2.2 NORM_INFO Message

The NORM_INFO message is used to convey OPTIONAL, application-defined,
"out-of-band" context information for transmitted NormObjects. An
example NORM_INFO use for bulk file transfer is to place MIME type
information for the associated file, data, or stream object into the
NORM_INFO payload. Receivers may use the NORM_INFO content to make a
decision as whether to participate in reliable reception of the
associated object. Each NormObject can have an independent unit of
NORM_INFO associated with it. NORM_DATA messages contain a flag to
indicate the availability of NORM_INFO for a given NormObject. NORM
receivers may NACK for retransmission of NORM_INFO when they have not
received it for a given NormObject. The size of the NORM_INFO content
is limited to that of a single NormSegmentSize for the given sender.
This atomic nature allows the NORM_INFO to be rapidly and efficiently
repaired within the NORM reliable transmission process.

When NORM_INFO content is available for a NormObject, the
NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
corresponding "object_transport_id" and the NORM_INFO message shall be
transmitted as the first message for the NormObject.

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NORM_INFO Message Format

The "version", "type","hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of "hdr_len" field when no header extensions are present is 4.

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 NORM_DATA messages. These values allow the
receiver to prepare appropriate buffering, etc, for further
transmissions from the sender when NORM_INFO is the first message
received.

As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
may be optionally applied to NORM_INFO messages. To conserve protocol
overhead, some NORM implementations may wish to apply the EXT_FTI when
used to NORM_INFO messages only and not to NORM_DATA messages.

The NORM_INFO "payload_data" field contains sender application-defined
content which can be used by receiver applications for various
purposes as described above.

4.2.3 NORM_CMD Message

NORM_CMD 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 NORM_CMD messages is enumerated. Additionally,
a range of command types remain available for potential application-

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specific use. Some NORM_CMD types may have dynamic content attached.
Any attached content will be limited to maximum length of the sender
NormSegmentSize to retain the atomic nature of commands. All NORM_CMD
messages begin with a common set of fields, after the usual NORM
message common header. The standard NORM_CMD fields are:

NORM_CMD Standard Fields

The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM Common Message Header as described in Section 4.1. The
value of the "hdr_len" field for NORM_CMD messages without header
extensions present depends upon the "flavor" field.

The "instance_id", "grtt", "backoff", and "gsize" fields provide the
same information and serve the same purpose as with NORM_DATA and
NORM_INFO messages. The "flavor" field indicates the type of command
to follow. The remainder of the NORM_CMD message is dependent upon
the command type ("flavor"). NORM command flavors include:

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NORM_CMD(FLUSH) Message

The NORM_CMD(FLUSH) 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 2*GRTT to excite the receiver set for any

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outstanding repair requests up to and including the transmission point
indicated within the NORM_CMD(FLUSH) message. The number of repeats
is equal to NORM_ROBUST_FACTOR unless a list of receivers from which
explicit positive acknowledgement ("acking_node_list") is given. In
that case, the "acking_node_list" is updated as acknowledgements are
received and the NORM_CMD(FLUSH) is repeated according to the
mechanism described in Section 5.5.3. The greater the
NORM_ROBUST_FACTOR, the greater the probability that all applicable
receivers will be excited for acknowledgement or repair requests
(NACKs) _and_ that the corresponding NACKs are delivered to the
sender. If a NORM_NACK message interrupts the flush process, the
sender will re-initiate the flush process after any resulting repair
transmissions are completed.

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 2*GRTT*NORM_ROBUST_FACTOR and will be discussed more later.
With a sufficient NORM_ROBUST_FACTOR value, data content is delivered
with a high assurance of reliability. The penalty of a large
NORM_ROBUST_FACTOR value is potentially excess sender NORM_CMD(FLUSH)
transmissions and a longer timeout for receivers to self-initiate the
terminal NACK process.

For finite-size transport objects such as NORM_OBJECT_DATA and
NORM_OBJECT_FILE, 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
NORM_OBJECT_STREAM, 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 as repair for the concurrent coding block and will be limited
to explicitly repairing stream 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 messaging content
will need to trade-off using the NORM_OBJECT_DATA paradigm versus the
NORM_OBJECT_STREAM 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.

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NORM_CMD(FLUSH) Message Format

In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(FLUSH) message contains fields to identify the
current status and logical transmit position of the sender.

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_is" field. Note the "hdr_len" value for the
NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
field when no header extensions are present.

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 NORM_DATA message type. Upon
receipt of the the NORM_CMD(FLUSH), 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
Section 5.3. If receivers have no outstanding repair needs, no
response to the NORM_CMD(FLUSH) is generated.

For NORM_OBJECT_STREAM 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

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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.
Normal receiver NACK inititation and construction is discussed in
detail in Section 5.3.

The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
for receivers from which the sender is requesting explicit positive
acknowledgement 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
NORM_CMD(FLUSH) message. When the "acking_node_list" is present, the
lightweight positive acknowledgement process described in Section
5.5.3 SHALL be observed.

NORM_CMD(EOT) Message

The NORM_CMD(EOT) command is sent when the sender reaches permanent
end-of-transmission with respect to the NormSession 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.

NORM_CMD(EOT) Message Format

The value of the "hdr_len" field for NORM_CMD(EOT) 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.

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NORM_CMD(SQUELCH) Message

The NORM_CMD(SQUELCH) command is transmitted in response to outdated
or invalid NORM_NACK content received by the sender. Invalid
NORM_NACK content consists of repair requests for NormObjects 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
NORM_FLAG_UNRELIABLE 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 NORM_CMD(SQUELCH).

The NORM_CMD(SQUELCH) command is sent once per 2*GRTT at the most.
The NORM_CMD(SQUELCH) 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 NORM_FLAG_UNRELIABLE flag set.
In normal conditions, it is expected the NORM_CMD(SQUELCH) 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.

The starting point of the invalid NormObject list begins with the
lowest invalid NormTransportId 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
NormSegmentSize. This allows the receivers to learn the status of the
sender's applicable object repair window with minimal transmission of
NORM_CMD(SQUELCH) commands. The format of the NORM_CMD(SQUELCH)
message is:

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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 | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flavor = 3 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM_CMD(SQUELCH) Message Format

In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(SQUELCH) 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 squelch transmission.

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.

The "invalid_object_list" is a list of 16-bit NormTransportIds 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 NormSegmentSize of
the sender. Thus, for very large repair windows, it is possible that
a single NORM_CMD(SQUELCH) message may not be capable of listing the
entire set of invalid objects in the repair window. In this case, the

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sender SHALL ensure that the list begins with a NormObjectId that is
greater than or equal to the lowest ordinal invalid NormObjectId from
the NACK message(s) that prompted the NORM_CMD(SQUELCH) generation.
The NormObjectIds in the "invalid_object_list" MUST be 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"
based 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.

NORM_CMD(CC) Message

The NORM_CMD(CC) messages contains fields to enable sender->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) Building Block [17] is described in Section
5.5.2 of this document. The NORM_CMD(CC) message is usually
transmitted as part of NORM-CC congestion control operation. A NORM
header extension is defined below to be used with the NORM_CMD(CC)
message to support NORM-CC operation. Different header extensions may
be defined for the NORM_CMD(CC) (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 NORM_CMD(CC) message is then used for GRTT
measurement only and may optionally be sent less frequently than with
congestion control operation.

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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) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM_CMD(CC) Message Format

The NORM common message header and standard NORM_CMD fields serve
their usual purposes.

The "reserved" field is for potential future use and should be set to
ZERO in this version of the NORM protocol.

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" (fb_nr)described in the TFMCC Building
Block document [17]. 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
NORM_CMD(CC) probes from the sender. This can be useful
instrumentation for complex or experimental multicast routing
environments.

The "send_time" field is a timestamp indicating the time that the
NORM_CMD(CC) message was transmitted. This consists of a 64-bit field
containing 32-bits with the time in seconds ("sent_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.

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Receivers use this timestamp adjusted by the amount of delay from the
time they received the NORM_CMD(CC) message to the time of their
response as the "grtt_response" portion of NORM_ACK and NORM_NACK
messages generated. This allows the sender to evaluate round-trip
times to different receivers for congestion control and other (e.g.,
GRTT determination) purposes.

To facilitate the baseline NORM-CC scheme described in Section 5.2.2,
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 NORM_CMD(CC) GRTT collection process to support NORM-CC
congestion control operation. The format of this header extension is
as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ext_type = 128| reserved | send_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM-CC Rate Header Extension Format (EXT_RATE)

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
exponent (order of magnitude) information in the least significant
portion. The 12-bit mantissa portion of the field is scaled such that
a floating point value of 0.0 corresponds to 0 and a floating point
value of 10.0 corresponds to 4096. Thus:

send_rate = (((int)(Value_mantissa * 4096.0 / 10.0 + 0.5)) << 4) | Value_exponent;

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 while the upper 12 bits
contain a value of 0x51f as determined from the equation given above:

send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;

= (0x51f << 4) | 0x4

= 0x51f4

To decode the "send_rate" field, the following equation can be used:

value = (send_rate >> 4) * 10.0 / 4096.0 * power(10.0, (send_rate & x000f))

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Note the maximum transmission rate representable by this scheme is
approximately 9.99e+15 bytes per second.

When this extension is present, a "cc_node_list" may be attached as
the payload of the NORM_CMD(CC) message. The presence of this header
extension also implies that NORM receivers should respond according to
the procedures described in Section 5.2.2.

The "cc_node_list" consists of a list of NormNodeIds 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
NORM_CMD(CC) message.

Each item in the "cc_node_list" is in the following format:

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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Congestion Control Node List Item Format

The "cc_node_id" is the NormNodeId of the receiver which the item
represents.

The "cc_flags" field contains flags indicating the congestion control
status of the indicated receiver. The following flags are defined:

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The "cc_rtt" contains a quantized representation of the receiver's
individual sender<->receiver RTT as measured by the sender. This
field is valid only if the NORM_FLAG_CC_RTT 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 NORM Building Block
document [14].

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 "slow start" phase) congestion
control rate. This field is encoded and decoded using the same
technique as described for the NORM_CMD(CC) "send_rate" field.

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NORM_CMD(REPAIR_ADV) Message

The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
its aggregated repair state from NORM_NACK messages accumulated during
a repair cycle and/or congestion control feedback received. This
message is sent only when the sender has received NORM_NACK and/or
NORM_ACK(CC) (when congestion control is enabled) messages via unicast
transmission instead of multicast. By "echoing" 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[11].

NORM_CMD(REPAIR_ADV) Message Format

The "instance_id", "grtt", "backoff", "gsize", and "flavor" fields
serve the same purpose as in other NORM_CMD messages. The value of
the "hdr_len" field when no extensions are present is 4.

The "flags" field provide information on the NORM_CMD(REPAIR_ADV)
content. There is currently one NORM_CMD(REPAIR_ADV) flag defined:

NORM_REPAIR_ADV_FLAG_LIMIT = 0x01

This flag is set by the sender when it is unable to fit its full
current repair state into a single NormSegmentSize. If this flag is
set, receivers should limit their NACK response to generating NACK
content only up through the maximum ordinal transmission position
(objectId::fecPayloadId) included in the "repair_adv_content".

When congestion control operation is enabled, a header extension may

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be applied to the NORM_CMD(REPAIR_ADV) representing the most limiting
(in terms of congestion control feedback suppression) congestion
control response. This allows the NORM_CMD(REPAIR_ADV) 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
NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV) messages. If another
congestion control technique (e.g., Pragmatic General Multicast
Congestion Control (PGMCC) [21]) 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:

NORM-CC Feedback Header Extension (EXT_CC) Format

The "cc_sequence" field contains the current greatest "cc_sequence"
value receivers have received in NORM_CMD(CC) 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 NORM_CMD(REPAIR_ADV) messages, the sender
SHALL set the "cc_sequence" field value to the value set in the last
NORM_CMD(CC) message sent.

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 NORM_CMD(CC) message
node list item flags. These fields are used by receivers in
controlling (suppressing as necessary) their congestion control
feedback. For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
should be set only when all feedback messages received by the sender
have the flag set. Similarly, the NORM_FLAG_CC_CLR or
NORM_FLAG_CC_PLR should be set only when no feedback has been received
from non-CLR or non-PLR receivers. And the NORM_FLAG_CC_LEAVE should

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be set only when all feedback messages the sender has received have
this flag set. These heuristics for setting the flags in
NORM_CMD(REPAIR_ADV) ensure the most effective suppression of
receivers providing unicast feedback messages.

The "cc_rtt" field SHALL be set to a default maximum value and the
NORM_FLAG_CC_RTT 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 NORM_FLAG_CC_RTT flag in the "cc_flags" field.
For NORM_CMD(REPAIR_ADV) 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.

The "cc_loss" field reperesents 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:

"cc_loss" = decimal_loss_fraction * 65535.0

For NORM_CMD(REPAIR_ADV) 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.

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 NORM_FLAG_CC_START 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
NORM_CMD(REPAIR_ADV) 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.

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.

The "repair_adv_payload" is in exactly the same form as the
"nack_content" of NORM_NACK messages and can be processed by receivers
for suppression purposes in the same manner, with the exception of the
condition when the NORM_REPAIR_ADV_FLAG_LIMIT is set.

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NORM_CMD(ACK_REQ) Message

The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgement from a specified list of receivers. This message is
used in providing a lightweight positive acknowledgement mechanism
that is OPTIONAL for use by the reliable multicast application. A
range of acknowledgement 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 acknowledgement process including transmission of the
NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are
described in Section 5.5.3. The format of the NORM_CMD(ACK_REQ)
message is:

NORM_CMD(ACK_REQ) Message Format

The NORM common message header and standard NORM_CMD fields serve
their usual purposes. The value of the "hdr_len" field for
NORM_CMD(ACK_REQ) messages with no header extension present is 4.

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The "ack_type" field indicates the type of acknowledgement 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 NORM_ACK messages described later:

The NORM_ACK_CC value is provided for use only in NORM_ACKs generated
in response to the NORM_CMD(CC) messages used in congestion control
operation. Similarly, the NORM_ACK_FLUSH is provided for use only in
NORM_ACKs generated in response to applicable NORM_CMD(FLUSH)
messages. NORM_CMD(ACK_REQ) messages with "ack_type" of NORM_ACK_CC
or NORM_ACK_FLUSH SHALL NOT be generated by the sender.

The NORM_ACK_RESERVED range of "ack_type" values is provided for
possible future NORM protocol use.

The NORM_ACK_APPLICATION 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.

The "ack_id" provides a sequenced identifier for the given
NORM_CMD(ACK_REQ) message. This "ack_id" is returned in NORM_ACK
messages generated by the receivers so that the sender may associate
the response with its corresponding request.

The "reserved" field is reserved for possible future protocol use and
SHALL be set to ZERO by senders and ignored by receivers.

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The "acking_node_list" field contains the NormNodeIds of the current
NORM receivers that are desired to provide positive acknowledge
(NORM_ACK) to this request. The packet payload length implies the
length of the "acking_node_list" and its length is limited to the
sender NormSegmentSize. The individual NormNodeId items are listed in
network (Big Endian) byte order. If a receiver's NormNodeId is
included in the "acking_node_list", it SHALL schedule transmission of
a NORM_ACK message as described in Section 5.5.3.

NORM_CMD(APPLICATION) Message

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 NORM_ROBUST_FACTOR
times at a rate of once per 2*GRTT. 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
NORM_CMD(APPLICATION) messages, or even application-defined,
positively-acknowledge commands from other NormSession 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.

NORM_CMD(APPLICATION) Message Format

The NORM common message header and NORM_CMD fields are interpreted as
previously described. The value of the NORM_CMD(APPLICATION)
"hdr_len" field when no header extensions are present is 4.

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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 NormSegmentSize setting.

4.3 Receiver Messages

The NORM message types generated by pariticipating receivers consist
of NORM_NACK and NORM_ACK message types. NORM_NACK messages are sent
to request repair of missing data content from sender transmission and
NORM_ACK messages are generated in response to certain sender commands
including NORM_CMD(CC) and NORM_CMD(ACK_REQ).

4.3.1 NORM_NACK Message

The principal purpose of NORM_NACK messages is for receivers to
request repair of sender content via selective, negative
acknowledgement upon detection of incomplete data. NORM_NACK messages
will be transmitted according to the rules of NORM_NACK generation and
suppression described in Section 5.3. The content of these messages
is in a format that could potentially be used by compatible
intermediate systems to provide assistance (e.g. Generic Router
Assist) in promoting protocol scalability and efficiency when
available. NORM_NACK messages also contain additional fields to
provide feedback to the sender(s) for purposes of round-trip timing
collection and congestion control.

The payload of NORM_NACK messages contains one or more repair requests
for different objects or portions of those objects. The NORM_NACK
message format is as follows:

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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM_NACK Message Format

The NORM common message header fields serve their usual purposes. The
value of the "hdr_len" field for NORM_NACK messages without header
extensions present is 6.

The "server_id" field identifies the NORM sender to which the
NORM_NACK message is destined.

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.

The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received NORM_CMD(CC) message for the
indicated NORM sender. The format of the "grtt_response" is the same
as the "send_time" field of the NORM_CMD(CC). The "grtt_response"
value is _relative_ to the "send_time" the source provided with a
corresponding NORM_CMD(CC) command. The receiver adjusts the source's
NORM_CMD(CC) "send_time" timestamp by adding the time differential
from when the receiver received the NORM_CMD(CC) to when the
NORM_NACK is transmitted to calculate the value in the "grtt_response"
field. This is the "receive_to_response_differential" value used in
the following formula:

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"grtt_response" = NORM_CMD(CC) "send_time" + receive_to_response_differential

The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate that it has not yet received a NORM_CMD(CC) message from the
indicated sender and that the sender should ignore the "grtt_response"
in this message.

For NORM-CC operation, the NORM-CC Feedback Header Extension, as
described in the NORM_CMD(REPAIR_ADV} message description, is added to
NORM_NACK 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.

The "reserved" field is for potential future NORM use and SHALL be
set to ZERO for this version of the protocol.

The "nack_content" of the NORM_NACK 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 NORM_DATA and/or NORM_INFO 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 Section 5.3. A single NORM Repair Request consists of a
list of items, ranges, and/or FEC coding block erasure counts for
needed NORM_DATA and/or NORM_INFO content. Multiple repair requests
may be concatenated within the "nack_payload" field of a NORM_NACK
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:

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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM Repair Request Format

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The "form" field indicates the type of repair request items given in
the "repair_request_items" list. Possible values for the "form" field
include:

Form Value
NORM_NACK_ITEMS 1
NORM_NACK_RANGES 2
NORM_NACK_ERASURES 3

A "form" value of NORM_NACK_ITEMS indicates each repair request item
in the "repair_request_items" list is to be treated as an individual
request. A value of NORM_NACK_RANGES indicates that the
"repair_request_items" list consists of pairs of repair request items
that correspond to inclusive ranges of repair needs. And the
NORM_NACK_ERASURES "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".

The "flags" field is currently used to indicate the level of data
content for which the repair request items apply (i.e. a individual
segment, entire FEC coding block, or entire transport object).
Possible flag values include:

When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
"fec_payload_id" fields are used to determine which sets or ranges of

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individual NORM_DATA segments are needed to repair content at the
receiver. When the NORM_NACK_BLOCK 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 NORM_NACK_INFO flag is set, this indicates
the receiver is missing the NORM_INFO segment for the indicated
"object_transport_id". Note the NORM_NACK_INFO may be set in
combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags, or
may be set alone. When the NORM_NACK_OBJECT flag is set, this
indicates the receiver is missing the entire NormTransportObject
referenced by the "object_transport_id". This also implicitly
requests any available NORM_INFO for the NormObject, if applicable.
The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
is set.

The "length" field value is the length in bytes of the
"repair_request_items" field.

The "repair_request_items" field consists of a list of individual or
range pairs of transport data unit identifiers in the following
format.

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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM Repair Request Item Format

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.

The "object_transport_id" corresponds to the NormObject for which
repair is being requested and the "fec_payload_id" identifies the
specific FEC coding block and/or segment being requested. When the
NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
is ignored. When the NORM_NACK_BLOCK flag is set, only the FEC code
block identifier portion of the "fec_payload_id" is to be interpreted.

The format of the "fec_payload_id" field depends upon the "fec_id"
field value.

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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
NORM_NACK message. Additionally, NORM receivers SHALL construct
NORM_NACK 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 NormSegmentSize for the
sender to which the NORM_NACK is destined.

NORM_NACK Content Examples:

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 NORM_NACK_ITEMS repair requests is given. In
Example 2, a list of NORM_NACK_RANGES requests _and_ a single
NORM_NACK_ITEMS request are concatenated to illustrate the possible
content of a NORM_NACK 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 Generic Router Assist (GRA) purposes.

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Example 1: NORM_NACK "nack_payload" for: Object 12, Coding Block 3, Segments 2,5,8

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Example 2: NORM_NACK "nack_payload" for: Object 18 Coding Block 6,
Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
1, segment 3

4.3.2 NORM_ACK Message

The NORM_ACK message is intended to be used primarily as part of NORM
congestion control operation and round-trip timing measurement. As
mentioned in the NORM_CMD(ACK_REQ) message description, the
acknowledgement type NORM_ACK_CC is provided for this purpose. The
generation of NORM_ACK(CC) messages for round-trip timing estimation
and congestion-control operation is described in Sections 5.5.1 and
5.5.2, respectively. However, some multicast applications may benefit
from some limited form of positive acknowledgement for certain
functions. A simple, scalable positive acknowledgement scheme is
defined in Section 5.5.3 that can be leveraged by protocol
implementations when appropriate. The NORM_CMD(FLUSH) may be used for
OPTIONAL collection of positive acknowledgement of reliable reception
to a certain "watermark" transmission point from specific receivers
using this mechanism. The NORM_ACK type NORM_ACK_FLUSH is provided
for this purpose and the format of the "nack_payload" for this
acknowledgement type is given below. Beyond that, a range of

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application-defined "ack_type" values is provided for use at the NORM
application's discretion. Implementations making use of application-
defined positive acknowledgements may also make use the "nack_payload"
as needed, observing the constraint that the "nack_payload" field size
be limited to a maximum of theNormSegmentSize for the sender to which
the NORM_ACK is destined.

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) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NORM_ACK Message Format

The NORM common message header fields serve their usual purposes.

The "server_id", "instance_id", and "grtt_response" fields serve the
same purpose as the corresponding fields in NORM_NACK messages. And
header extensions may be applied to support congestion control
feedback or other functions in the same manner.

The "ack_type" field indicates the nature of the NORM_ACK message.
This directly corresponds to the "ack_type" field of the
NORM_CMD(ACK_REQ) message to which this acknowledgement applies.

The "ack_id" field serves as a sequence number so that the sender can
verify that a NORM_ACK message received actually applies to a current
acknowledgement request. The "ack_id" field is not used in the case
of the NORM_ACK_CC and NORM_ACK_FLUSH acknowledgement types.

The "ack_payload" format is a function of the "ack_type". The

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NORM_ACK_CC message has no attached content. Only the NORM_ACK header
applies. In the case of NORM_ACK_FLUSH, a specific "ack_payload"
format is defined:

NORM_ACK_FLUSH "ack_payload" Format

The "object_transport_id" and "fec_payload_id" are used by the
receiver to acknowledge applicable NORM_CMD(FLUSH) messages
transmitted by the sender identified by the "server_id" field.

The "ack_payload" of NORM_ACK messages for application-defined
"ack_type" values is specific to the application but is limited in
size to a maximum the NormSegmentSize of the sender referenced by the
"server_id".

4.4 General Messages

4.4.1 NORM_REPORT

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
NORM_REPORT formats as needed for test purposes. These report
messages SHOULD be disabled for interoperability testing between
different NORM implementations.

5.0 Functionality Definition

This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of
protocol operation is given in the following:

1) The sender periodically transmits NORM_CMD(CC) messages as
needed to initialize and collect roundtrip timing and
congestion control feedback from the receiver set.

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2) The sender transmits an ordinal set of NormObjects segmented
in the form of NORM_DATA messages labeled with
NormTransportIds and logically identified with FEC encoding
block numbers and symbol identifiers. NORM_INFO messages
may optionally precede the transmission of data content for
NORM transport objects.

3) As receivers detect missing content from the sender, they
initiate repair requests with NORM_NACK messages. Note the
receivers track the sender's most recent
objectId::fecPayloadId transmit position and NACK _only_ for
content ordinally prior to that transmit position. The
receivers schedule random backoff timeouts before generating
NORM_NACK messages and wait an appropriate amount of time
before repeating the NORM_NACK if their repair request is
not satisified.

4) 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. Previously
untransmitted FEC parity content 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.

5) The sender transmits NORM_CMD(FLUSH) messages when it
reaches the end of enqueued transmit content. Receivers
respond to the NORM_CMD(FLUSH) messages with NORM_NACK
transmissions (following the same suppression backoff
timeout strategy as for data) if they require further
repair.

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6) The sender transmissions are subject to rate control limits
determined by congestion control. In the baseline NORM-CC
operation, each sender in a NormSession maintains its own
independent congestion control state. Receivers provide
congestion control feedback in NORM_NACK and NORM_ACK
messages. NORM_ACK feedback for congestion control purposes
is governed using a suppression mechanism similar to that
for NORM_NACK messages.

While the overall concept of the NORM protocol is relatively simple,
there are details to each of these aspects that need to be addressed
for successful, efficient, robust, and scalable operation.

5.1 NORM Sender Initialization and Transmission

Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
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
NORM_CMD(CC) messaging to collect feedback from the group using the
baseline NORM-CC feedback mechanisms. This proactive feedback
collection can be used to establish GRTT measurement prior to data
transmission and potential NACK operation.

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 NORM_CMD(CC) messages
may precede actual data transmission in order to have initial GRTT
measurement.

With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension, 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 NormSegmentSize, and other information. If this header
extension is not used, it is presumed that receivers have been
properly pre-configured via other means. Additionally, applications
may leverage the use of NORM_INFO messages associated with the session
data objects in the session to provide application-specific context
information for the session and data being transmitted. These

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mechanisms allow for operation with minimal pre-coordination among the
senders and receivers.

The NORM sender begins segmenting application-enqueued data into
NORM_DATA segments and transmitting it to the group. The rate of
transmission is controlled via congestion control mechanisms or at 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
NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT. Receivers may
respond to these NORM_CMD(FLUSH) messages with additional repair
requests. A protocol parameter "NORM_ROBUST_FACTOR" 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
NORM_CMD(FLUSH) containing the NormNodeIds for receivers from which it
expects explicit positive acknowledgement of reception. The
NORM_CMD(FLUSH) message may be also used for this optional function
any time prior to the end of data enqueued for transmission with the
NORM_CMD(FLUSH) messages multiplexed with ongoing data transmissions.
The OPTIONAL NORM positive acknowledgement procedure is described in
Section 5.5.3.

5.2 NORM Receiver Initialization and Reception

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 restrain requesting repair of transport
objects 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, default policy where receivers are allowed to
request repair only for coding blocks with a NormTransportId and FEC
coding block number greater than or equal to the first non-repair
NORM_DATA or NORM_INFO message received upon joining the group is
RECOMMENDED. For objects of type NORM_OBJECT_STREAM 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.

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5.3 NORM Receiver NACK Procedure

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,
NormObject boundaries, and upon receipt of a NORM_CMD(FLUSH) message.

The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the NORM Building Block document [14] using
(Ksender*GRTTsender) for the "maxTime" parameter and the sender
advertised group size (GSIZEsender) as the "groupSize" parameter.
NORM senders provide values for GRTTsender, Ksender and GSIZEsender
via the "grtt", "backoff", and "gsize" fields of transmitted messages.
The GRTTsender value is determined by the sender based on feedback it
has received from the group while the Ksender and GSIZEsender values
may determined by application requirements and expectations or
ancillary information. The backoff factor "Ksender" MUST be greater
than one to provide for effective feedback suppression. A value of K
= 4 is RECOMMENDED for the Any Source Multicast (ASM) model while a
value of K = 6 is RECOMMENDED for Single Source Multicast (SSM)
operation.

Thus:

T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)

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 T_backoff is greater than (Ksender-1)*GRTTsender.
Otherwise, the backoff period is entered and the receiver MUST
accumulate external pending repair state from NORM_NACK messages and
NORM_CMD(REPAIR_ADV) messages received. At the end of the backoff
time, the receiver SHALL generate a NORM_NACK message only if the
following conditions are met:

1) The sender's current transmit position (in terms of
objectId::fecPayloadId) exceeds the earliest repair position
of the receiver.

2) The repair state accumulated from NORM_NACK and
NORM_CMD(REPAIR_ADV) 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.

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If these conditions are met, the receiver immediately generates a
NORM_NACK 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 reinitiate 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 (T_rcvrHoldoff) as described in
[14] is:

T_rcvrHoldoff =(Ksender+2)*GRTTsender

The NORM_NACK 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 NORM_NACK content exceeds the
sender's NormSegmentSize, the NACK content is truncated so that the
receiver only generates a single NORM_NACK message per NACK cycle for
a given sender. In summary, a single NACK message is generated
containing the receiver's lowest ordinal repair needs.

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 NORM_DATA "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.

For FEC coding blocks or NormObjects missed in their entirety, the

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NORM receiver constructs repair requests with NORM_NACK_BLOCK or
NORM_NACK_OBJECT flags set as appropriate. The request for
retransmission of NORM_INFO is accomplished by setting the
NORM_NACK_INFO flag in a corresponding repair request.

5.4 NORM Sender NACK Processing and Repair Transmission

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.

5.4.1 NORM Sender Repair State Aggregation

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 NORM_NACK messages before beginning repair transmissions. Note
that this period of aggregating repair state does _not_ interfere with
its ongoing transmission of new data.

As described in [14], the period of time during which the sender
aggregates NORM_NACK messages is equal to:

T_sndrAggregate = (Ksender+1)*GRTT

where "Ksender" is the same backoff scaling value used by the
receivers, and "GRTT" is the sender's current estimate of the group's
greatest round-trip time.

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 NORM_NACK messages arrive. As described in [14], the value of this
sender "holdoff" period is:

T_sndrHoldoff = (1*GRTT)

If additional NORM_NACK messages are received during this sender
"holdoff" period, the sender will immediately incorporate these "late

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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 convert with ongoing new data
and/or pending repair transmissions.

5.4.2 NORM Sender FEC Repair Transmission Strategy

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 NORM_NACK
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 NORM_NACK 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.

NORM_DATA messages sent as repair transmissions are flagged with the
NORM_FLAG_REPAIR flag. This allows receivers to obey any policies
that limit new receivers from joining the reliable transmission when
only repair transmissions have been received.

To facilitate operation with Generic Router Assist (GRA), the sender
can additionally flag NORM_DATA transmissions sent as explicit repair
with the NORM_FLAG_EXPLICIT flag. The GRA router needs to only

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subcast a sufficient count of non-explicit parity repairs to satisfy
the sub-tree's erasure filling needs for a given FEC coding block.
When the sender has resorted to explicit repair, the GRA router will
subcast all of the explicit repair packets to those portions of the
routing tree still requiring repair for a given coding block. (Note
the GRA router 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
subcasting. Additionally, the GRA router can perform additional
NORM_NACK suppression/aggregation as it conducts this repair state
accumulation for NORM repair cycles).

5.4.3 NORM Sender NORM_CMD(SQUELCH) Generation

If the sender receives a NORM_NACK message for repair of data it is no
longer supporting, the sender generates a NORM_CMD(SQUELCH) message to
advertise its repair window and squelch any receivers from additional
NACKing of invalid data. The transmission rate of NORM_CMD(SQUELCH)
messages is limited to once per 2*GRTT. The "invalid_object_list" (if
applicable) of the NORM_CMD(SQUELCH) message SHALL begin with the
lowest "object_transport_id" from the invalid NORM_NACK messages
received since the last NORM_CMD(SQUELCH) transmission. Lower ordinal
invalid "object_transport_ids" should be included only while the
NORM_CMD(SQUELCH) payload is less than the sender's NormSegmentSize
parameter.

5.4.4 NORM Sender NORM_CMD(REPAIR_ADV) Generation

When a NORM sender receives NORM_NACK messages from receivers via
unicast transmission, it uses NORM_CMD(REPAIR_ADV) 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. The NORM_CMD(REPAIR_ADV) message is multicast to the
receiver set by the sender. The payload portion of this message has
content in the same format as the NORM_NACK receiver message payload.
Receivers are then able to perform feedback suppression in the same
manner as with NORM_NACK 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 NORM_CMD(REPAIR_ADV) messages are
subject to the sender transmit rate limit and NormSegmentSize
limitation. When the NORM_CMD(REPAIR_ADV) 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 the receivers. This document specificies

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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.

5.5 Additional NORM Protocol Mechanisms

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.

5.5.1 NORM Greatest Round-trip Time (GRTT) Collection

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 NORM_CMD(CC) messages that contain a locally
generated timestamp. Receivers are expected to record this timestamp
along with the time the NORM_CMD(CC) 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
(NORM_NACK or NORM_ACK). 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.

The round-trip time for each receiver is fed into an algorithm that
weights and smooths the values for a conservative estimate of the
GRTT. The algorithm and methodology are described in the NORM
Building Block document [11] in the section entitled "One-to-Many
Sender GRTT Measurement". A conservative estimate helps 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.

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When the NORM-CC Rate header extension is present in NORM_CMD(CC)
messages, the receivers respond to NORM_CMD(CC) messages as described
in Section 5.5.2, "NORM Congestion Control Operation". The
NORM_CMD(CC) 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
NORM_ACK messages. This provides for GRTT feedback even if no
NORM_NACK messages are being sent. If operating without congestion
control in a closed network, the NORM_CMD(CC) messages may be sent
periodically without the NORM-CC Rate header extension. In this case,
receivers will only provide GRTT measurement feedback when NORM_NACK
messages are generated since no NORM_ACK messages are generatedR. In
this case, the NORM_CMD(CC) 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
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.

5.5.2 NORM Congestion Control Operation (NORM-CC)

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
[17] and [20]. 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 [21]). 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->receiver packet loss estimates and
sender<->receiver RTT 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 result transmission rate is identified as the "current
limiting receiver" (CLR).

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As described in [23], a steady-state sender transmission rate, to be
"friendly" with competing TCP flows can be calculated as:

S
Rsender = ---------------------------------------------------------------
tRTT * (sqrt((2/3)*p) + 12 * sqrt((3/8)*p) * p * (1 + 32*(p^2)))

where

S = 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).

tRTT = The RTT estimate of the current "current limiting receiver"
(CLR).

p = The loss event fraction of the CLR.

To support congestion control feedback collection and operation, the
NORM sender periodically transmits NORM_CMD(CC) command messages.
NORM_CMD(CC) messages are multiplexed with NORM data and repair
transmissions and serve several purposes:

1) Stimulate explicit feedback from the general receiver set to
collect congestion control information.

2) Communicate state to the receiver set on the sender's
current congestion control status including details of the
CLR.

3) Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for that
"worst path" in the sender->receiver multicast topology.

The format of the NORM_CMD(CC) message is describe in Section 4.2.3 of
this document. The NORM_CMD(CC) message contains information to allow
determination of sender<->receiver RTTs, to inform the group of the
congestion control CLR, and to provide feedback of individual RTT
information to the receivers in the group. The NORM_CMD(CC) 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

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(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.

5.5.2.1 NORM_CMD(CC) Transmission

The NORM_CMD(CC) message is transmitted periodically by the sender
along with its normal data transmission. Note that the repeated
transmission of NORM_CMD(CC) 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.

A NORM_CMD(CC) message is immediately transmitted at sender startup.
The interval of subsequent NORM_CMD(CC) message transmission is
determined as follows:

1) 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.

2) If a CLR has been identified (based on previous receiver
feedback), the interval is the sender<->receiver RTT for the
CLR.

3) Additionally, if the interval of nominal data message
transmission is greater than the GRTT or RTT_clr interval,
the NORM_CMD(CC) interval is set to this greater value.
This ensures that the transmission of this control message
is not done to the exclusion of user data transmission.

The NORM_CMD(CC) "cc_sequence" field is incremented with each
transmission of a NORM_CMD(CC) 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.

The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
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.

The "cc_node_list" contains a list of entries identifying receivers

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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 NORM_FLAG_CC_CLR 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 NORM_CMD(CC) messages. The
maximum length of the list corresponds to the sender's NormSegmentSize
parameter for the session. The inclusion of additional entries in the
list based on receiver feedback are prioritized with following rules:

1) Receivers that have not yet been provided RTT feedback get
first priority. Of these, those with the greatest loss
fraction receive precedence for list inclusion.

2) Secondly, receivers that have previously been provided RTT
are included with receivers yielding the lowest calculated
congestion rate getting precedence.

There are "cc_flag" values in addition to NORM_FLAG_CC_CLR that are
used for other congestion control functions. The NORM_FLAG_CC_PLR
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 NORM_FLAG_CC_RTT 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" and "cc_rate" fields do not
contain valid values when this flag is not set.

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5.5.2.2 NORM_CMD(CC) Feedback Response

Receivers explicitly respond to NORM_CMD(CC) messages in the form of a
NORM_ACK(RTT) 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 NORM_CMD(CC)
"cc_node_list" immediately provide feedback in the form of a NORM_ACK
to this message. When a NORM_CMD(CC) 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 Section 5.3) in
response to detection of data loss. The backoff timeout for the
congestion control response is generated as follows:

T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)

The "RandomBackoff()" algorithm provides a truncated exponentially
distributed random number and is described in the NORM Building Block
document [11]. The same backoff factor K = Ksender MAY be used as
with NORM_NACK suppression. However, in cases where the application
purposefully specifies a very small Ksender backoff factor to minimize
the NACK repair process latency (trading off group size scalability),
it may still be desirable to maintain a larger backoff factor for
congestion control feedback, since there may often be a larger volume
of congestion control feedback than NACKs in many cases and congestion
control feedback latency may be tolerable where reliable delivery
latency is not. As previously noted, a backoff factor value of K = 4
is generally recommended for ASM operation and K = 6 for SSM
operation. A receiver SHALL cancel the backoff timeout and thus its
pending transmission of a NORM_ACK(RTT) message under the following
conditions:

1) The receiver generates another feedback message (NORM_NACK
or other NORM_ACK) before the congestion control feedback
timeout expires,

2) A NORM_CMD(CC) 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),

3) When the T_backoff is greater than 1*GRTT. This prevents
NACK implosion in the event of sender or network failure.

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4) "Suppressing" congestion control feedback is heard from
another receiver (in a NORM_ACK or NORM_NACK) or via a
NORM_CMD(REPAIR_ADV) message from the sender. The local
receiver's feedback is "suppressed" if the rate of the
competing feedback (Rfb) is sufficiently close to or less
than the local receiver's calculated rate (Rcalc). The
local receiver's feedback is canceled when:

Rcalc > (0.9 * Rfb)

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 NORM_CMD(CC)
"cc_node_list" in a long time) may wish to compete as a
receiver with no prior RTT measurement after some expiration
period.

When the backoff timer expires, the receiver SHALL generate a
NORM_ACK(RTT) 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.

Whenever any feedback is generated (including this NORM_ACK(RTT)
message), receivers include an adjusted version of the sender
timestamp from the most recently received NORM_CMD(CC) message and the
"cc_sequence" value from that command in the applicable NORM_ACK or
NORM_NACK 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.

During slow start (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).

The "cc_loss" field value reflects the receiver's current loss event
estimate with respect to the sender in question.

When the receiver has a valid individual RTT measurement, it SHALL

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include this value in the "cc_rtt" field. The NORM_FLAG_CC_RTT MUST
be set when the "cc_rtt" field is valid.

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 NORM_CMD(CC) messages are received from the
sender (unless the receive becomes marked as a CLR or PLR node). The
value of this holdoff timeout (T_ccHoldoff) period is:

T_ccHoldoff = (K*GRTT)

Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per K*GRTT intervals. Note, however,
that as the session progresses, different receivers will be responding
to different NORM_CMD(CC) messages and there will be relatively
continuous feedback of congestion control information while the sender
is active.

5.5.2.3 Congestion Control Rate Adjustment

During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR according to any limitations described in [17].
As noted there, 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.

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 slow start "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.

The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
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 RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
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 (RTT_rcvrPrev) to smooth according
to:

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RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew

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:

RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew

"RTT_clrNew" is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The RTT_clr is initialized to
RTT_clrNew 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.

There are some additional periods besides steady-state operation that
need to be considered in NORM-CC operation. These periods are:

1) during session startup,
2) when no feedback is received from the CLR, and
3) when the sender has a break in data transmission.

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:

Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.

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 slow
start is applied as:

Rnew = Rrecv_min

where "Rrecv_min" is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received from
the group. Note that during "slow start", 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.

If the CLR or any receiver intends to leave the group, it will set the
NORM_FLAG_CC_LEAVE 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

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rate.

The sender should also track the "age" of the feedback it has received
from the CLR by comparing its current "cc_sequence" value (Seq_sender)
to the last "cc_sequence" value received from the CLR (Seq_clr). As
the "age" of the CLR feedback increases with no new feedback, the
sender SHALL begin reducing its rate once per RTT_clr 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:

Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;

This rate reduction is limited to the lower bound on NORM transmission
rate. After NORM_ROBUST_FACTOR consecutive NORM_CMD(CC) 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 withold further
transmissions of NORM_DATA segements and maintain NORM_CMD(CC)
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.

When the sender has a break in its data transmission, it can continue
to probe the group with NORM_CMD(CC) 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 RTT_clr as:

Rsender = Rsender * 0.5;

If the minimum NORM rate is reached, the sender should set the
NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and the
group should observer "slow start" congestion control procedures until
any receiver experiences a new loss event.

5.5.3 NORM Positive Acknowledgment Procedure

NORM provides options for the source application to request positive
acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ) messages
from members of the group. There are some specific acknowledgement
requests defined for the NORM protocol and a range of acknowledgment

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request types that are left to be defined by the application. One
predefined acknowledgement type is the NORM_ACK_FLUSH type. This
acknowledgement 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 NORM_ACK_FLUSH acknowledgement may be used to
assist in application flow control when the sender has information on
a portion of the receiver set. Another predefined acknowledgement
type is NORM_ACK(CC), which is used to explicitly provide congestion
control feedback in response to NORM_CMD(CC) messages transmitted by
the sender for NORM-CC operation. Note the NORM_ACK(CC) response does
NOT follow the positive acknowledgement procedure described here. The
NORM_CMD(ACK_REQ) and NORM_ACK messages contain an "ack_type" field to
identify the type of acknowledgement requested and provided. A range
of "ack_type" values is provided for application-defined use. While
the application is responsible for initiating the acknowledgement
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.

The NORM positive acknowledgement 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 NORM_CMD(ACK_REQ), or when applicable, the NORM_CMD(FLUSH)
message is used for polling and contains a list of NormNodeIds for
receivers that should respond to the command. The list of receivers
providing acknowledgement is determined by the source application with
"a priori" knowledge of participating nodes or via some other
application-level mechanism.

The ACK process is initiated by the sender that generates
NORM_CMD(FLUSH) or NORM_CMD(ACK_REQ) messages in periodic "rounds".
For NORM_ACK_FLUSH requests, the NORM_CMD(FLUSH) contain a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgement is requested. This
watermark transmission point is "echoed" in the corresponding fields
of the NORM_ACK(FLUSH) message sent by the receiver in response.
NORM_CMD(ACK_REQ) messages contain an "ack_id" field which is
similarly "echoed" in response so that the sender may match the
response to the appropriate request.

In response to the NORM_CMD(ACK_REQ), the listed receivers randomly
spread NORM_ACK messages uniformly in time over a window of (1*GRTT).
These NORM_ACK messages are typically unicast to the sender. (Note
that NORM_ACK(CC) messages SHALL be multicast or unicast in the same

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manner as NORM_NACK messages).

The ACK process is self-limiting and avoids ACK implosion in that:

1) Only a single NORM_CMD(ACK_REQ) message is generated once
per (2*GRTT), and,

2) The size of the "acking_node_list" of NormNodeIds from which
acknowledgment is requested is limited to a maximum of the
sender NormSegmentSize setting per round of the positive
acknowledgement process.

Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds may be
required to achieve responses from all receivers specified. The
content of the attached NormNodeId list will be dynamically updated as
this process progresses and NORM_ACK 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 NORM_CMD(ACK_REQ)
message "acking_node_list" and add in any pending receiver NormNodeIds
while keeping within the NormSegmentSize limitation of the list size.
Each receiver is queried a maximum number of times
(NORM_ROBUST_FACTOR, 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 acknowledgement. The
transmission of the NORM_CMD(ACK_REQ) is repeated until no further
responses are required or until the repeat threshold is exceeded for
all pending receivers. The transmission of NORM_CMD(ACK_REQ) or
NORM_CMD(FLUSH) messages to conduct the positive acknowledgment
process is multiplexed with ongoing sender data transmissions.
However, the NORM_CMD(FLUSH) positive acknowledgment process may be
interrupted in response to negative acknowledgement repair requests
(NACKs) received from receivers during the acknowledgment period. The
NORM_CMD(FLUSH) positive acknowledgment process is restarted for
receivers pending acknowledgement once any the repairs have been
transmitted.

In the case of NORM_CMD(FLUSH) 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 NORM_CMD(FLUSH) commands with no attached "acking_node_list".

5.5.4 Group Size Estimation

NORM sender messages contain a "gsize" field that is a representation

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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.

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.

5.5.5 Operation with Generic Router Assist (GRA)

NORM packet formats will be extended to allow for operation with GRA
reliable multicast functions. Additional NACK suppression and
selective sub-casting of repair transmissions in the network will be
possible with GRA. (Section 5.4.2 discusses some NORM mechanisms
related to this). Additional details will be provide in future
versions of this document as GRA specifications mature.

6.0 Security Considerations

The same security considerations that apply to the NORM, FEC, and
TFMCC 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. It is RECOMMENDED that such IP
security mechanisms be used when available. Another possible approach
is for NORM senders to use the "sequence" field from the NORM Common
Message Header to detect replay attacks. This can be accomplished if
the sender is willing to maintain state on receivers which are
NACKing. A cache of receiver state may provide some protection
against replay attacks. Note that the "sequence" field should be
incremented with independent values for "sender" messages versus
"receiver" messages so that the congestion control loss estimation
function of the "sequence" field can be preserved for sender messages
when receiver messages are unicast to the sender.

While NORM does leverage FEC-based repair for scalability, this does
not alone guarantee integrity of received data. Application-level
integrity-checking of data content is highly RECOMMENDED.

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The NORM protocol is compatible with the use of the IP security
(IPSEC) architecture described in [22].

7.0 Suggested Use

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.

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 misordering. 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.

8.0 Acknowledgements (and these are not Negative)

The authors would like to thank Rick Jones, Vincent Roca, and Joerg
Widmer for their valuable 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.

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9.0 References

[1] Kermode, R., Vicisano, L., "Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol
Instantiation documents", RFC 3269, April 2002.

[2] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[3] Mankin, A., Romanow, A., Bradner, S. and V. Paxson, "IETF
Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.

[4] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd
S. and Luby, M., "Reliable Multicast Transport Building
Blocks for One-to-Many Bulk-Data Transfer", RFC 3048,
January 2001.

[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.

[6] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.

[7] S. Pingali, D. Towsley, J. Kurose, "A Comparison of Sender-
Initiated and Receiver-Initiated Reliable Multicast
Protocols", In Proc. INFOCOM, San Francisco CA, October
1993.

[8] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M.
and J. Crowcroft, "The Use of Forward Error Correction (FEC)
in Reliable Multicast", RFC 3453, December 2002.

[9] J. Macker, R. Adamson, "The Multicast Dissemination Protocol
(MDP) Toolkit", Proc. IEEE MILCOM 99, October 1999.

[10] J. Nonnenmacher and E. Biersack, "Optimal Multicast
Feedback", Proc. IEEE INFOCOMM, p. 964, March/April 1998.

[11] J. Macker, R. Adamson, "Quantitative Prediction of Nack
Oriented Reliable Multicast (NORM) Feedback", Proc. IEEE
MILCOM 2002, October 2002.

[12] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989.

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[13] Holbrook, H. W., "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer
Science, Stanford, California, August 2001.

[14] B. Adamson, C. Bormann, M. Handley, and J. Macker, "NACK-
Oriented Reliable Multicast (NORM) Protocol Building
Blocks", Internet Draft draft-ietf-rmt-bb-norm-05.txt, March
2003, work in progress. Citation for informational purposes
only.

[15] M. Luby, L. Vicisano, J. Gemmell, L. Rizzo, M. Handley, and
J. Crowcroft, "The Use of Forward Error Correction (FEC) in
Reliable Multicast", RFC 3453, December 2002.

[16] M. Luby, L. Vicisano, J. Gemmell, L. Rizzo, M. Handley, and
J. Crowcroft, "Forward Error Correction (FEC) Building
BLock", RFC 3452, December 2002.

[17] J. Widmer, M. Handley, "TCP-Friendly Multicast Congestion
Control (TFMCC) Protocol Specification", Internet Draft
draft-ietf-rmt-bb-tfmcc-01.txt, November 2002, work in
progress. Citation for informational purposes only.

[18] D. Gossink, J. Macker, "Reliable Multicast and Integrated
Parity Retransmission with Channel Estimation", IEEE
GLOBECOMM 98', September 1998.

[19] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
January 1996.

[20] J. Widmer and M. Handley, "Extending Equation-Based
Congestion Control to Multicast Applications", Proc ACM
SIGCOMM 2001, San Diego, August 2001.

[21] L. Rizzo, "pgmcc: A TCP-Friendly Single-Rate Multicast
Congestion Control Scheme", Proc ACM SIGCOMM 2000,
Stockholm, August 2000.

[22] S. Kent and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[23] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, "Modelling
TCP Throughput: A Simple Model and its Empirical
Validation", Proc ACM SIGCOMM 1998.

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10.0 Authors' Addresses

Brian Adamson
adamson@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC, USA, 20375

Carsten Bormann
cabo@tellique.de
Tellique Kommunikationstechnik GmbH
Gustav-Meyer-Allee 25 Geb ude 12
D-13355 Berlin, Germany

Mark Handley
mjh@aciri.org
1947 Center Street, Suite 600
Berkeley, CA 94704

Joe Macker
macker@itd.nrl.navy.mil
Naval Research Laboratory
Washington, DC, USA, 20375

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