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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 3940

RMT Working Group                                         B. Adamson/NRL
INTERNET-DRAFT                                       C. Bormann/Tellique
draft-ietf-rmt-pi-norm-10                               M. Handley/ACIRI
Expires: January 2005                                      J. Macker/NRL
                                                               July 2004


            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.  By submitting this Internet-Draft,
we certify that any applicable patent or other IPR claims of which we
are aware have been disclosed, and any of which we become aware will be
disclosed, in accordance with RFC 3668.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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Copyright Notice

Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract

This document describes the messages and procedures of the Negative-
acknowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.  This
protocol is designed to provide end-to-end reliable transport of bulk
data objects or streams over generic IP multicast routing and forwarding
services.  NORM uses a selective, negative acknowledgment mechanism for
transport reliability and offers additional protocol mechanisms to allow
for operation with minimal "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) between the senders and receivers.  The protocol offers a number
of features to allow different types of applications or possibly other
higher level transport protocols to utilize its service in different



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 The protocol leverages the use of FEC-based repair and other IETF
reliable multicast transport (RMT) building blocks in its design.


                           Table of Contents


1. Introduction and Applicability. . . . . . . . . . . . . . . . . .   3
 1.1. NORM Delivery Service Model. . . . . . . . . . . . . . . . . .   3
 1.2. NORM Scalability . . . . . . . . . . . . . . . . . . . . . . .   5
 1.3. Environmental Requirements and Considerations. . . . . . . . .   6
2. Architecture Definition . . . . . . . . . . . . . . . . . . . . .   6
 2.1. Protocol Operation Overview. . . . . . . . . . . . . . . . . .   8
 2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . . .   9
 2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . . . . .   9
3. Conformance Statement . . . . . . . . . . . . . . . . . . . . . .  10
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . . .  12
 4.1. NORM Common Message Header and Extensions. . . . . . . . . . .  13
 4.2. Sender Messages. . . . . . . . . . . . . . . . . . . . . . . .  15
  4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . . . .  15
  4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . . . .  22
  4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . . . .  24
 4.3. Receiver Messages. . . . . . . . . . . . . . . . . . . . . . .  39
  4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . . . .  39
  4.3.2. NORM_ACK Message. . . . . . . . . . . . . . . . . . . . . .  45
 4.4. General Purpose Messages . . . . . . . . . . . . . . . . . . .  46
  4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . . . .  46
5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . . .  47
 5.1. Sender Initialization and Transmission . . . . . . . . . . . .  48
  5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . . . .  49
 5.2. Receiver Initialization and Reception. . . . . . . . . . . . .  51
 5.3. Receiver NACK Procedure. . . . . . . . . . . . . . . . . . . .  51
 5.4. Sender NACK Processing and Response. . . . . . . . . . . . . .  53
  5.4.1. Sender Repair State Aggregation . . . . . . . . . . . . . .  53
  5.4.2. Sender FEC Repair Transmission Strategy . . . . . . . . . .  54
  5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . . . . . .  55
  5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation. . . . . . . . . . .  55
 5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . . . .  56
  5.5.1. Greatest Round-trip Time Collection . . . . . . . . . . . .  56
  5.5.2. NORM Congestion Control Operation . . . . . . . . . . . . .  57
  5.5.3. NORM Positive Acknowledgment Procedure. . . . . . . . . . .  64
  5.5.4. Group Size Estimate . . . . . . . . . . . . . . . . . . . .  66
6. Security Considerations . . . . . . . . . . . . . . . . . . . . .  66
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  67
8. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . . .  67
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  68
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  68
 10.1. Normative References. . . . . . . . . . . . . . . . . . . . .  68
 10.2. Informative References. . . . . . . . . . . . . . . . . . . .  68
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  69
12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  70





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1.  Introduction and Applicability

The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
protocol is designed to provide reliable transport of data from one or
more sender(s) to a group of receivers over an IP multicast network.
The primary design goals of NORM are to provide efficient, scalable, and
robust bulk data (e.g., computer files, transmission of persistent data)
transfer across possibly heterogeneous IP networks and topologies.  The
NORM protocol design provides support for distributed multicast session
participation with minimal coordination among senders and receivers.
NORM allows senders and receivers to dynamically join and leave
multicast sessions at will with minimal overhead for control information
and timing synchronization among participants.  To accommodate this
capability, NORM protocol message headers contain some common
information allowing receivers to easily synchronize to senders
throughout the lifetime of a reliable multicast session.  NORM is
designed to be self-adapting to a wide range of dynamic network
conditions with little or no pre-configuration.  The protocol is
purposely designed to be tolerant of inaccurate timing estimations or
lossy conditions that may occur in many networks including mobile and
wireless.  The protocol is also designed to exhibit convergence and
efficient operation even in situations of heavy packet loss and large
queuing or transmission delays.

This document 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].

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) [7],  Session Announcement Protocol (SAP) [8],  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



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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 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



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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).  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 acknowledgment (NACK) based protocol schemes when feedback for
reliability is required [9].   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 [10].     FEC-based repair can be used to
greatly reduce the quantity of reliable multicast repair requests and
repair transmissions [11]  in a NACK-oriented protocol.  The principal
factor in NORM scalability is the volume of feedback traffic generated
by the receiver set to facilitate reliability and congestion control.
NORM uses probabilistic suppression of redundant feedback based on
exponentially distributed random backoff timers.  The performance of
this type of suppression relative to other techniques is described in
[12].   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



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of thousands of receivers.  A study of the quantity of feedback for this
type of protocol is described in [13].   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 each active sender.
This may constrain the number of simultaneous senders in some uses of
NORM.

1.3.  Environmental Requirements and Considerations

All of the environmental requirements and considerations that apply to
the RMT NORM Building Block [4]  and the RMT FEC Building Block [5]
also apply to the NORM protocol.

The NORM protocol SHALL be capable of operating in an end-to-end fashion
with no assistance from intermediate systems beyond basic IP multicast
group management, routing, and forwarding services.  While the
techniques utilized in NORM are principally applicable to "flat" end-to-
end IP multicast topologies, they could also be applied in the sub-
levels of hierarchical (e.g., tree-based) multicast distribution if so
desired.  NORM can make use of reciprocal (among senders and receivers)
multicast communication under the Any-Source Multicast (ASM) model
defined in RFC 1112 [3],  but SHALL also be capable of scalable
operation in asymmetric topologies such as Source Specific Multicast
(SSM) [14]  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.  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 distinguish  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



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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.

As previously 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.  In NORM, an FEC encoding symbol
directly corresponds to the payload of NORM_DATA messages or "segment".
Note that when systematic FEC codes are used, the payload of NORM_DATA
messages sent for the first portion of a FEC encoding block are source
symbols (actual segments of original user data), while the remaining
symbols for the block consist of parity symbols generated by FEC
encoding.  These parity symbols are generally sent in response to repair
requests, but some number may be sent proactively at the end each
encoding block to increase the robustness of transmission.  When non-
systematic FEC codes are used, all symbols sent consist of FEC encoding
parity content.  In this case, the receiver must receive a sufficient
number of symbols to reconstruct (via FEC decoding) the original user
data for the given block.  In this document, the terms "symbol" and
"segment" are used interchangeably.

Transmitted NormObjects are temporarily yet uniquely identified within
the NormSession context using the given sender's NormNodeId,
NormInstanceId, and a temporary NormObjectTransportId.  Depending upon
the implementation, individual NORM senders may manage their
NormInstanceIds independently, or a common NormInstanceId may be agreed
upon for all participating nodes within a session if needed as a session
identifier.  NORM NormObjectTransportId 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



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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.  Protocol Operation Overview

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)
[15]  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 transmission 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 acknowledgment requests or application defined
commands.  The transmission of NORM_CMD messages from the sender is
accomplished by one of three different procedures.  These procedures
are: single, best effort unreliable transmission of the command;
repeated redundant transmissions of the command; and positively-
acknowledged commands.  The transmission technique used for a given
command depends upon the function of the command.  Several core commands
are defined for basic protocol operation.  Additionally, implementations
MAY wish to consider providing the OPTIONAL application-defined commands
that can take advantage of the transmission methodologies available for
commands.  This allows for application-level session management
mechanisms that can make use of information available to the underlying
NORM protocol engine (e.g., round-trip timing, transmission rate, etc).

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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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 acknowledgment 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.  Protocol Building Blocks

The operation of the NORM protocol is based primarily upon the concepts
presented in the Nack-Oriented Reliable Multicast (NORM) Building Block
document [4].   This includes the basic NORM architecture and the data
transmission, repair, and feedback strategies discussed in that
document.  Additional reliable multicast building blocks are applied in
creating the full NORM protocol instantiation [16].   NORM also makes
use of Forward Error Correction encoding techniques for repair messaging
and optional transmission robustness as described in [10].   NORM uses
the FEC Payload ID as specified by the FEC Building Block Document [5].
Additionally, for congestion control, this document includes a baseline
congestion control mechanism (NORM-CC) based on the TCP-Friendly
Multicast Congestion Control (TFMCC) scheme described in [19].

2.3.  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
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



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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.  Conformance Statement

This Protocol Instantiation document, in conjunction with the RMT
Building Block documents of [4]  and [5],  completely specifies a
working reliable multicast transport protocol that conforms to the
requirements described in RFC 2357 [17].





























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


 +---------------------+-----------------------------------------------+
 |    Message Type     |                    Purpose                    |
 +---------------------+-----------------------------------------------+
 |NORM_DATA            | Sender message for application data           |
 |                     | transmission.  Implementations must support   |
 |                     | at least one of the NORM_OBJECT_DATA,         |
 |                     | NORM_OBJECT_FILE, or NORM_OBJECT_STREAM       |
 |                     | delivery services.  The use of the NORM FEC   |
 |                     | Object Transmission Information header        |
 |                     | extension is OPTIONAL with NORM_DATA          |
 |                     | messages.                                     |
 +---------------------+-----------------------------------------------+
 |NORM_CMD(FLUSH)      | Sender command to excite receivers for repair |
 |                     | requests in lieu of ongoing NORM_DATA         |
 |                     | transmissions.  Note the use of the           |
 |                     | NORM_CMD(FLUSH) for positive acknowledgment   |
 |                     | of data receipt is OPTIONAL.                  |
 +---------------------+-----------------------------------------------+
 |NORM_CMD(SQUELCH)    | Sender command to advertise its current valid |
 |                     | repair window in response to invalid requests |
 |                     | for repair.                                   |
 +---------------------+-----------------------------------------------+
 |NORM_CMD(REPAIR_ADV) | Sender command to advertise current repair    |
 |                     | (and congestion control state) to group when  |
 |                     | unicast feedback messages are detected.  Used |
 |                     | to control/suppress excessive receiver        |
 |                     | feedback in asymmetric multicast topologies.  |
 +---------------------+-----------------------------------------------+
 |NORM_CMD(CC)         | Sender command used in collection of round    |
 |                     | trip timing and congestion control status     |
 |                     | from group (This may be OPTIONAL if           |
 |                     | alternative congestion control mechanism and  |
 |                     | round trip timing collection is used).        |
 +---------------------+-----------------------------------------------+
 |NORM_NACK            | Receiver message used to request repair of    |
 |                     | missing transmitted content.                  |
 +---------------------+-----------------------------------------------+
 |NORM_ACK             | Receiver message used to proactively provide  |
 |                     | feedback for congestion control purposes.     |
 |                     | Also used with the OPTIONAL NORM Positive     |
 |                     | Acknowledgment Process.                       |
 +---------------------+-----------------------------------------------+











Adamson, Bormann, et al.  Expires January 2005                  [Page 11]

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


+-----------------------+-----------------------------------------------+
|     Message Type      |                    Purpose                    |
+-----------------------+-----------------------------------------------+
|NORM_INFO              | Sender message for providing ancillary        |
|                       | context information associated with NORM      |
|                       | transport objects.  The use of the NORM FEC   |
|                       | Object Transmission Information header        |
|                       | extension is OPTIONAL with NORM_INFO          |
|                       | messages.                                     |
+-----------------------+-----------------------------------------------+
|NORM_CMD(EOT)          | Sender command to indicate it has reach end-  |
|                       | of-transmission and will no longer respond to |
|                       | repair requests.                              |
+-----------------------+-----------------------------------------------+
|NORM_CMD(ACK_REQ)      | Sender command to support application-        |
|                       | defined, positively acknowledged commands     |
|                       | sent outside of the context of the bulk data  |
|                       | content being transmitted.  The NORM Positive |
|                       | Acknowledgment Procedure associated with this |
|                       | message type is OPTIONAL.                     |
+-----------------------+-----------------------------------------------+
|NORM_CMD(APPLICATION)  | Sender command containing application-defined |
|                       | commands sent outside of the context of the   |
|                       | bulk data content being transmitted.          |
+-----------------------+-----------------------------------------------+
|NORM_REPORT            | Optional message type reserved for            |
|                       | experimental implementations of the NORM      |
|                       | protocol.                                     |
+-----------------------+-----------------------------------------------+

4.  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 auxiliary message type of NORM_REPORT is also provided
for experimental purposes.  This section describes the message formats
used by the NORM protocol.  These messages and their fields are
referenced in the detailed functional description of the NORM protocol
given in Section 5.   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.






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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:

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version|  type |    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   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.

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



Adamson, Bormann, et al.  Expires January 2005                  [Page 13]

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 [18]  may be applicable to use for NORM node identifiers.
At this point in time, the protocol makes no assumptions about how these
unique identifiers are actually assigned.

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



Adamson, Bormann, et al.  Expires January 2005                  [Page 14]

FEC and congestion control operations.

4.2.  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 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) a FEC Payload ID portion
with a format dependent upon the FEC encoding used, and 3) a payload
portion containing source or encoded application data content.  Note for
objects of type NORM_OBJECT_STREAM, the payload portion contains
additional fields used to appropriately recover stream content.  NORM
implementations MAY also extend the NORM_DATA header to include a FEC
Object Transmission Information (EXT_FTI) header extension.  This allows
NORM receivers to automatically allocate resources and properly perform
FEC decoding without the need for pre-configuration or out-of-band
information.




























Adamson, Bormann, et al.  Expires January 2005                  [Page 15]

      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_reserved*       |          payload_len*         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        payload_offset*                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          payload_data*                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        NORM_DATA Message Format

*NOTE:  The "payload_reserved", "payload_len" and "payload_offset"
fields are present only for objects of type NORM_OBJECT_STREAM.  The
"payload_len" and "payload_offset" fields allow senders to arbitrarily
vary the size of NORM_DATA payload segments for streams.  This allows
applications to flush transmitted streams as needed to meet unique
streaming requirements.  For objects of types NORM_OBJECT_FILE and
NORM_OBJECT_DATA, these fields are unnecessary since the receiver can
calculate the payload length and offset information from the
"fec_payload_id" using the algorithm described in Section 5.1.1.   The
"payload_reserved" field is kept for anticipated future NORM stream
control functions.  When systematic FEC codes (e.g., "fec_id" = 129) are
used, the "payload_len" and "payload_offset" fields contain actual
length and offset values for the encapsulated application data segment
for those NORM_DATA messages containing source data symbols.  In
NORM_DATA messages that contain parity information, these fields are not
actual length or offset values, but instead are values computed from FEC
encoding the "payload_len" and "payload_offset" fields of the _source_
data symbols of the corresponding applicable coding block.

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



Adamson, Bormann, et al.  Expires January 2005                  [Page 16]

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 SHOULD 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 [19]).   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 [4].

The "backoff" field value is used by receivers to determine the maximum
backoff timer value used in the timer-based NORM NACK feedback
suppression.  This 4-bit field supports values from 0-15 which is
multiplied by the sender GRTT to determine the maximum backoff timeout.
The "backoff" field informs the receiver set of the sender's backoff
factor parameter "Ksender".  Recommended values and their use are
described in the NORM receiver NACK procedure description 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
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.
















Adamson, Bormann, et al.  Expires January 2005                  [Page 17]

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:


+---------------------+-------+------------------------------------------+
|        Flag         | Value |                 Purpose                  |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_REPAIR     | 0x01  | Indicates message is a repair            |
|                     |       | transmission                             |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_EXPLICIT   | 0x02  | Indicates a repair segment intended to   |
|                     |       | meet a specific receiver erasure, as     |
|                     |       | compared to parity segments provided by  |
|                     |       | the sender for general purpose (with     |
|                     |       | respect to an FEC coding block) erasure  |
|                     |       | filling.                                 |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_INFO       | 0x04  | Indicates availability of NORM_INFO for  |
|                     |       | object.                                  |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_UNRELIABLE | 0x08  | Indicates that repair transmissions for  |
|                     |       | the specified object will be unavailable |
|                     |       | (One-shot, best effort transmission).    |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_FILE       | 0x10  | Indicates object is "file-based" data    |
|                     |       | (hint to use disk storage for            |
|                     |       | reception).                              |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_STREAM     | 0x20  | Indicates object is of type              |
|                     |       | NORM_OBJECT_STREAM.                      |
+---------------------+-------+------------------------------------------+
|NORM_FLAG_MSG_START  | 0x40  | Marks the first segment of application   |
|                     |       | messages embedded in                     |
|                     |       | NORM_OBJECT_STREAMs.                     |
+---------------------+-------+------------------------------------------+

NORM_FLAG_REPAIR 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.  NORM_FLAG_EXPLICIT 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 could
possibly be used by intermediate systems implementing functionality to
control sub-casting of repair content to different legs of a reliable
multicast topology with disparate repair needs.  NORM_FLAG_INFO is set
only when 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.
NORM_FLAG_UNRELIABLE 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



Adamson, Bormann, et al.  Expires January 2005                  [Page 18]

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.  NORM_FLAG_FILE can be set as a "hint" from the sender
that the associated object should be stored in non-volatile storage.
NORM_FLAG_STREAM is set when the identified object is of type
NORM_OBJECT_STREAM.  When NORM_FLAG_STREAM 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 [5].   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.

The "fec_payload_id" identifies the attached NORM_DATA "payload"
content.  The size and format of the "fec_payload_id" field depends upon
the FEC type indicated by the "fec_id" field.  These formats are given
in the FEC Building Block document [5]  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:











Adamson, Bormann, et al.  Expires January 2005                  [Page 19]

      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 [5].   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 protocol data unit identifier for the attached segment
with respect to the NORM sender's instance within a session.

Additional FEC Object Transmission Information (as described in the FEC
Building Block document [5])  is required to properly receive and decode
NORM transport objects.  This information MAY be provided as out-of-band
session information.  However, in some cases, it may be useful for the
sender to include this information "in band" to facilitate receiver
operation with minimal 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:








Adamson, Bormann, et al.  Expires January 2005                  [Page 20]

      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         |          segment_size         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       fec_max_block_len       |         fec_num_parity        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 identified 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 [5].   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 FEC
Instance ID values is described in [5].  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 "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 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 [5].   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



Adamson, Bormann, et al.  Expires January 2005                  [Page 21]

differently for other systematic codes as they are defined.

The payload portion of NORM_DATA messages includes source data or FEC
encoded application content.

The "payload_reserved", "payload_len" and "payload_offset" fields are
present ONLY for transport objects of type NORM_OBJECT_STREAM.  These
fields indicated the size and relative position (within the stream) of
the application content represented by the message payload.  For senders
employing systematic FEC encoding, these fields contain _actual_ length
and offset values (in bytes) for the payload of messages which contain
original data source symbols.  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 upon decoding a FEC coding block.  Note
that these fields do NOT contribute to the value of the NORM_DATA
"hdr_len" field.  These fields are NOT present when the "flags" portion
of the NORM_DATA| message indicate the transport object if of type
NORM_OBJECT_FILE or NORM_OBJECT_DATA.  In this case, the length and
offset information can be calculated from the "fec_payload_id" using the
methodology described in Section 5.1.1.   Note that for long-lived
streams, the "payload_offset" field can wrap.

The "payload_data" field contains the original application source  or
parity content for the symbol identified by the "fec_payload_id".  The
length of this field SHALL be limited to a maximum of the sender's
NormSegmentSize bytes as given in the FTI for the object.  Note the
length of this field for messages containing parity content will always
be of length 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.  It is RECOMMENDED that a
sender's NormSegmentSize 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.  The "payload_data"
content may be delivered directly to the application for source symbols
(when systematic FEC encoding is used) or upon decoding of the FEC
block.  For NORM_OBJECT_FILE and NORM_OBJECT_STREAM objects, the data
segment length and offset can be calculated using the algorithm
described in Section 5.1.1.   For NORM_OBJECT_STREAM objects, the length
and offset is obtained from the segment's corresponding "payload_len"
and "payload_offset" fields.

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



Adamson, Bormann, et al.  Expires January 2005                  [Page 22]

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.

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=1|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     flags     |     fec_id    |     object_transport_id       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                header_extensions (if applicable)              |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         payload_data                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        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.




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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 Messages

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-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:

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     flavor    |                                               |
   +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        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:













Adamson, Bormann, et al.  Expires January 2005                  [Page 24]

+----------------------+--------------+----------------------------------+
|       Command        | Flavor Value |            Purpose               |
+----------------------+--------------+----------------------------------+
|NORM_CMD(FLUSH)       |      1       | Used to indicate sender          |
|                      |              | temporary end-of-transmission.   |
|                      |              | (Assists in robustly initiating  |
|                      |              | outstanding repair requests from |
|                      |              | receivers).  May also be         |
|                      |              | optionally used to collect       |
|                      |              | positive acknowledgment of       |
|                      |              | reliable reception from subset   |
|                      |              | of receivers.                    |
+----------------------+--------------+----------------------------------+
|NORM_CMD(EOT)         |      2       | Used to indicate sender          |
|                      |              | permanent end-of-transmission.   |
+----------------------+--------------+----------------------------------+
|NORM_CMD(SQUELCH)     |      3       | Used to advertise sender's       |
|                      |              | current repair window in         |
|                      |              | response to out-of-range NACKs   |
|                      |              | from receivers.                  |
+----------------------+--------------+----------------------------------+
|NORM_CMD(CC)          |      4       | Used for GRTT measurement and    |
|                      |              | collection of congestion control |
|                      |              | feedback.                        |
+----------------------+--------------+----------------------------------+
|NORM_CMD(REPAIR_ADV)  |      5       | Used to advertise sender's       |
|                      |              | aggregated repair/feedback state |
|                      |              | for suppression of unicast       |
|                      |              | feedback from receivers.         |
+----------------------+--------------+----------------------------------+
|NORM_CMD(ACK_REQ)     |      6       | Used to request application-     |
|                      |              | defined positive acknowledgment  |
|                      |              | from a list of receivers         |
|                      |              | (OPTIONAL).                      |
+----------------------+--------------+----------------------------------+
|NORM_CMD(APPLICATION) |      7       | Used for application-defined     |
|                      |              | purposes which may need to       |
|                      |              | temporarily preempt data         |
|                      |              | transmission (OPTIONAL).         |
+----------------------+--------------+----------------------------------+


4.2.3.1.  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
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 acknowledgment ("acking_node_list") is given.  In that
case, the "acking_node_list" is updated as acknowledgments are received



Adamson, Bormann, et al.  Expires January 2005                  [Page 25]

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
acknowledgment 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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      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   flavor = 1  |    fec_id     |      object_transport_id      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         fec_payload_id                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                acking_node_list (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     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_id" 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 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
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 initiation and construction is discussed in detail in Section 5.3.



Adamson, Bormann, et al.  Expires January 2005                  [Page 27]

OPTIONAL "acking_node_list" field contains a list of NormNodeIds for
receivers from which the sender is requesting explicit positive
acknowledgment of reception up through the transmission point identified
by the "object_transport_id" and "fec_payload_id" fields.  The length of
the list can be inferred from the length of the received NORM_CMD(FLUSH)
message.  When the "acking_node_list" is present, the lightweight
positive acknowledgment process described in Section 5.5.3  SHALL be
observed.


4.2.3.2.  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.  The NORM_CMD(EOT)
command SHOULD be sent with the same robust mechanism as used for
NORM_CMD(FLUSH) commands to provide a high assurance of reception by the
receiver set.

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   flavor = 2  |                    reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      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.


4.2.3.3.  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).



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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:

      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



Adamson, Bormann, et al.  Expires January 2005                  [Page 29]

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
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.


4.2.3.4.  NORM_CMD(CC) Message

The NORM_CMD(CC) messages contains fields to enable sender-to-receiver
group greatest round-trip time (GRTT) measurement and to excite the
group for congestion control feedback.  A baseline NORM congestion
control scheme (NORM-CC), based on the TCP-Friendly Multicast Congestion
Control (TFMCC) scheme of [19]  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 [19].   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 ("send_time_sec") and
32-bits with the time in microseconds ("send_time_usec") since some
reference time the source maintains (usually 00:00:00, 1 January 1970).
The byte ordering of the fields is "Big Endian" network order.
Receivers use this timestamp adjusted by the amount of delay from the
time they received the 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 5.5.2,  a



Adamson, Bormann, et al.  Expires January 2005                  [Page 31]

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))

Note the maximum transmission rate that can be represented 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 5.5.2.  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.



Adamson, Bormann, et al.  Expires January 2005                  [Page 32]

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:


+-------------------+-------+------------------------------------------+
|       Flag        | Value |                 Purpose                  |
+-------------------+-------+------------------------------------------+
|NORM_FLAG_CC_CLR   | 0x01  | Receiver is the current limiting         |
|                   |       | receiver (CLR).                          |
+-------------------+-------+------------------------------------------+
|NORM_FLAG_CC_PLR   | 0x02  | Receiver is a potential limiting         |
|                   |       | receiver (PLR).                          |
+-------------------+-------+------------------------------------------+
|NORM_FLAG_CC_RTT   | 0x04  | Receiver has measured RTT with respect   |
|                   |       | to sender.                               |
+-------------------+-------+------------------------------------------+
|NORM_FLAG_CC_START | 0x08  | Sender/receiver is in "slow start" phase |
|                   |       | of congestion control operation (i.e.    |
|                   |       | The receiver has not yet detected any    |
|                   |       | packet loss and the "cc_rate" field is   |
|                   |       | the receiver's actual measured receive   |
|                   |       | rate).                                   |
+-------------------+-------+------------------------------------------+
|NORM_FLAG_CC_LEAVE | 0x10  | Receiver is imminently leaving the       |
|                   |       | session and its feedback should not be   |
|                   |       | considered in congestion control         |
|                   |       | operation.                               |
+-------------------+-------+------------------------------------------+

The "cc_rtt" contains a quantized representation of the RTT as measured
by the sender with respect to the indicated receiver.  This field is
valid only if the 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 [4].  The
"cc_rate" field contains a representation of the receiver's current
calculated (during steady-state congestion control operation) or twice



Adamson, Bormann, et al.  Expires January 2005                  [Page 33]

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.


4.2.3.5.  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
[13].
      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  flavor = 5   |     flags     |            reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               header extensions (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       repair_adv_payload                      |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  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 be
applied to the NORM_CMD(REPAIR_ADV) representing the most limiting (in



Adamson, Bormann, et al.  Expires January 2005                  [Page 34]

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) [20])  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:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ext_type = 3 |  ext_len = 3  |          cc_sequence          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    cc_flags   |     cc_rtt    |            cc_loss            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            cc_rate            |          cc_reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           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 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



Adamson, Bormann, et al.  Expires January 2005                  [Page 35]

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 represents the receiver's current packet loss
fraction estimate for the indicated source.  The loss fraction is a
value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
packet loss. The 16-bit "cc_loss" value is calculated by the following
formula:

              "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.


4.2.3.6.  NORM_CMD(ACK_REQ) Message

The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgment from a specified list of receivers.  This message is used
in providing a lightweight positive acknowledgment mechanism that is
OPTIONAL for use by the reliable multicast application.  A range of
acknowledgment request types is provided for use at the application's
discretion.  Provision for application-defined, positively-acknowledged
commands allows the application to automatically take advantage of
transmission and round-trip timing information available to the NORM
protocol.  The details of the NORM positive acknowledgment process
including transmission of the 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:




Adamson, Bormann, et al.  Expires January 2005                  [Page 36]

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  flavor = 6   |    reserved   |    ack_type   |    ack_id     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       acking_node_list                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    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.

The "ack_type" field indicates the type of acknowledgment being
requested and thus implies rules for how the receiver will treat this
request.  The following "ack_type" values are defined and are also used
in NORM_ACK messages described later:


   +---------------------+--------+----------------------------------+
   |      ACK Type       | Value  |            Purpose               |
   +---------------------+--------+----------------------------------+
   |NORM_ACK_CC          |      1 | Used to identify NORM_ACK        |
   |                     |        | messages sent in response to     |
   |                     |        | NORM_CMD(CC) messages.           |
   +---------------------+--------+----------------------------------+
   |NORM_ACK_FLUSH       |      2 | Used to identify NORM_ACK        |
   |                     |        | messages sent in response to     |
   |                     |        | NORM_CMD(FLUSH) messages.        |
   +---------------------+--------+----------------------------------+
   |NORM_ACK_RESERVED    |   3-15 | Reserved for possible future     |
   |                     |        | NORM protocol use.               |
   +---------------------+--------+----------------------------------+
   |NORM_ACK_APPLICATION | 16-255 | Used at application's            |
   |                     |        | discretion.                      |
   +---------------------+--------+----------------------------------+

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.



Adamson, Bormann, et al.  Expires January 2005                  [Page 37]

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.

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.

4.2.3.7.  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.

















Adamson, Bormann, et al.  Expires January 2005                  [Page 38]

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  flavor = 7   |                    reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Application-Defined Content                 |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  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.

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 participating 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 acknowledgment 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.   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:











Adamson, Bormann, et al.  Expires January 2005                  [Page 39]

      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:

"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.




Adamson, Bormann, et al.  Expires January 2005                  [Page 40]

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

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



Adamson, Bormann, et al.  Expires January 2005                  [Page 41]

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., an individual
segment, entire FEC coding block, or entire transport object).  Possible
flag values include:


 +------------------+-------+------------------------------------------+
 |      Flag        | Value |                 Purpose                  |
 +------------------+-------+------------------------------------------+
 |NORM_NACK_SEGMENT | 0x01  | Indicates the listed segment(s) or range |
 |                  |       | of segments are required as repair.      |
 +------------------+-------+------------------------------------------+
 |NORM_NACK_BLOCK   | 0x02  | Indicates the listed block(s) or range   |
 |                  |       | of blocks in entirety are required as    |
 |                  |       | repair.                                  |
 +------------------+-------+------------------------------------------+
 |NORM_NACK_INFO    | 0x04  | Indicates that NORM_INFO is required as  |
 |                  |       | repair for the listed object(s).         |
 +------------------+-------+------------------------------------------+
 |NORM_NACK_OBJECT  | 0x08  | Indicates the listed object(s) or range  |
 |                  |       | of objects in entirety are required as   |
 |                  |       | repair.                                  |
 +------------------+-------+------------------------------------------+

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
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.







Adamson, Bormann, et al.  Expires January 2005                  [Page 42]

      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.

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
intermediate system purposes.







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Example 1:  NORM_NACK "nack_payload" for: Object 12, Coding Block 3, Segments 2,5,8
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   form = 1    | flags = 0x01  |       length  = 36            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 12   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 3                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 2     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 12   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 3                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 5     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 12   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 3                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 8     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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
 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   form = 2    | flags = 0x01  |       length  = 24            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 18   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 6                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 5     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 18   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 6                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 10    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   form = 1    | flags = 0x05  |       length  = 12            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  fec_id = 129 |   reserved    |    object_transport_id = 19   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    source_block_number = 1                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    source_block_length = 32   |    encoding_symbol_id = 3     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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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
acknowledgment 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 acknowledgment for certain functions.  A
simple, scalable positive acknowledgment 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 acknowledgment 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 acknowledgment type is given below.
Beyond that, a range of application-defined "ack_type" values is
provided for use at the NORM application's discretion.  Implementations
making use of application-defined positive acknowledgments may also make
use the "nack_payload" as needed, observing the constraint that the
"nack_payload" field size be limited to a maximum of the NormSegmentSize
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



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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 acknowledgment 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
acknowledgment request.  The "ack_id" field is not used in the case of
the NORM_ACK_CC and NORM_ACK_FLUSH acknowledgment types.

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


      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_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 Purpose Messages

Some additional message formats are defined for general purpose in NORM
multicast sessions whether the participant is acting as a sender and/or
receiver within the group.

4.4.1.  NORM_REPORT Message

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.




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5.  Detailed Protocol Operation

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


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.

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 satisfied.

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

5.1.  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 a GRTT estimate 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 an initial GRTT estimate.

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 received the FEC Object
Transmission Information 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
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 segmentation
algorithm is described in Section 5.1.1.   The rate of transmission is



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via congestion control mechanisms or is a fixed rate if desired for
closed network operations.  The receivers participating in the multicast
group provide feedback to the sender as needed.  When the sender reaches
the end of data it has enqueued for transmission or any pending repairs,
it transmits a series of 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 acknowledgment 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 acknowledgment procedure
is described in Section 5.5.3.

5.1.1.  Object Segmentation Algorithm

NORM senders and receivers must use a common algorithm for logically
segmenting transport data into FEC encoding blocks and symbols so that
appropriate NACKs can be constructed to request repair of missing data.
NORM FEC coding blocks are comprised of multi-byte symbols which are
transmitted in the payload of NORM_DATA messages.  Each NORM_DATA
message contains one source or encoding symbol and the NormSegmentSize
sender parameter defines the maximum symbol size in bytes.  The FEC
encoding type and associated parameters govern the source block size
(number of source symbols per coding block).  NORM senders and receivers
use these FEC parameters, along with the NormSegmentSize and transport
object size to compute the source block structure for transport objects.
These parameters are provided in the FEC Transmission Information for
each object.  The algorithm given below is used to compute a source
block structure such that all source blocks are as close to being equal
length as possible.  This helps avoid the performance disadvantages of
"short" FEC blocks.  Note this algorithm applies only to the statically-
sized NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
the object size is fixed and predetermined.  For NORM_OBJECT_STREAM
objects, the object is segmented according to the maximum source block
length  given in the FEC Transmission Information, unless the FEC
Payload ID indicates an alternative size for a given block.

The NORM block segmentation algorithm is defined as follows.  For a
transport object of a given length (L_obj) in bytes , a first number of
FEC source blocks (N_large) is delineated of a larger block size
(B_large), and a second number of source blocks (N_small) is delineated
of a smaller block size (B_small).  Given the maximum FEC source block
size (B_max) and the sender's NormSegmentSize, the block segmentation
for a given NORM transport object is determined as follows:

Inputs:





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   B_max = Maximum source block length (i.e.,  maximum number of source
           symbols per source block)

   L_sym = Encoding symbol length in bytes (i.e., NormSegmentSize)

   L_obj = Object length in bytes


Outputs:

   N_total = The total number of source blocks into which the transport
             object is partitioned.

   N_large = Number of larger source blocks (first set of blocks)

   B_large = Size (in encoding symbols) of the larger source blocks

   N_small = Number of smaller source blocks (second set of blocks)

   B_small = Size (in encoding symbols) of the smaller source blocks

   L_final = Length (in bytes) of the last source symbol of the last
             source block (All other symbols are of length L_sym).


Algorithm:

1)   The total number of source symbols in the transport object is computed as:
     S_total = L_obj/L_sym [rounded up to the nearest integer]

2)   The transport object is partitioned into N_total source blocks, where:
     N_total = S_total/B_max [rounded up to the nearest integer]

3)   The average length of a source block is computed as:
     B_ave = S_total/N_total (this may be non-integer)

4)   The size of the first set of larger blocks is computed as:
     B_large = B_ave [rounded up to the nearest integer]
     (Note it will always be the case that B_large <= B_max)

5)   The size of the second set of smaller blocks is computed as:
     B_small = B_ave [rounded down to the nearest integer]
     (Note if B_ave is an integer B_small = B_large; otherwise B_small = B_large
     - 1)

6)   The fractional part of B_ave is computed as:
     B_fraction = B_ave - B_small

7)   The number of larger source blocks is computed as:
     N_large = B_fraction * N_total
     (Note N_large is an integer in the range 0 through N_total - 1)

8)   The number of smaller source blocks is computed as:




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     N_small = N_total - N_large

9)   Each of the first N_large source blocks consists of B_large source symbols.
     Each of the remaining N_small source blocks consists of B_small source
     symbols.  All symbols are L_sym bytes in length except for the final source
     symbol of the final source block which is of length (in bytes):
     L_final = L_obj - (N_large*B_large + N_small*B_small - 1) * L_sym


5.2.  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 limit or avoid repair requests for
transport objects already in progress.  The NORM sender implementation
may wish to impose additional constraints to limit the ability of
receivers to disrupt reliable multicast performance by joining, leaving,
and rejoining the group often.  Different receiver "join policies" may
be appropriate for different applications and/or scenarios.  For general
purpose operation, 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.

5.3.  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 [4]  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:



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       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.


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 [4]  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



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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 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.  Sender NACK Processing and Response

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.  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 [4],  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



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repair aggregation cycle, even if NORM_NACK messages arrive.  As
described in [4],  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
messages" into its pending transmission state ONLY if the NACK content
is ordinally greater than the sender's current transmission position.
This "holdoff" time allows worst case time for the sender to propagate
its current transmission sequence position to the group, thus avoiding
redundant repair transmissions.  After the holdoff timeout expires, a
new NACK accumulation period can be begun (upon arrival of a NACK) in
concert with the pending repair and new data transmission.  Recall that
receivers are not to initiate the NACK repair process until the sender's
logical transmission position exceeds the lowest ordinal position of
their repair needs.  With the new NACK aggregation period, the sender
repeats the same process of incorporating accumulated repair state into
its transmission plan and subsequently "rewinding" to transmit the
lowest ordinal repair data when the aggregation period expires.  Again,
this is conducted in concert with ongoing new data and/or pending repair
transmissions.

5.4.2.  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 SHALL be 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.  Additionally, the sender
SHOULD additionally flag NORM_DATA transmissions sent as explicit repair
with the NORM_FLAG_EXPLICIT flag.




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Although NORM end system receivers do not make use of the
NORM_FLAG_EXPLICIT flag, this message transmission status could be
leveraged by intermediate systems wishing to "assist" NORM protocol
performance.  If such systems are properly positioned with respect to
reciprocal reverse-path multicast routing, they need to sub-cast only a
sufficient count of non-explicit parity repairs to satisfy a multicast
routing sub-tree's erasure filling needs for a given FEC coding block.
When the sender has resorted to explicit repair, then the intermediate
systems should sub-cast all of the explicit repair packets to those
portions of the routing tree still requiring repair for a given coding
block.  Note the intermediate systems will be required to conduct repair
state accumulation for sub-routes in a manner similar to the sender's
repair state accumulation in order to have sufficient information to
perform the sub-casting.  Additionally, the intermediate systems could
perform additional NORM_NACK suppression/aggregation as it conducts this
repair state accumulation for NORM repair cycles.  The detail of this
type of operation are beyond the scope of this document, but this
information is provided for possible future consideration.

5.4.3.  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.  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



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receivers.  This document specifies the NORM-CC Feedback Header
Extension that is applied for baseline NORM-CC operation.  If other
congestion control mechanisms are used within a NORM implementation,
other header extensions may be defined.  Whatever content format is used
for this purpose should ensure that maximum possible suppression state
is conveyed to the receiver set.

5.5.  Additional 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.  Greatest Round-trip Time 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 smoothes the values for a conservative estimate of the GRTT.
The algorithm and methodology are described in the NORM Building Block
document [4]  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.

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.



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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 generated.  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

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
[19].   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 [20]).   With this TFMCC-based approach, the transmissions
of NORM senders are controlled in a rate-based manner as opposed to
window-based congestion control algorithms as in TCP.  However, it is
possible that the NORM protocol message set may alternatively be used to
support a window-based multicast congestion control scheme such as
PGMCC.  The details of that alternative may be described separately or
in a future revision of this document.  In either case (rate-based TFMCC
or window-based PGMCC), successful control of sender transmission
depends upon collection of sender-to-receiver packet loss estimates and
RTTs to identify the congestion control bottleneck path(s) within the
multicast topology and adjust the sender rate accordingly.  The receiver
with loss and RTT estimates that correspond to the lowest result
transmission rate is identified as the "current limiting receiver"
(CLR).

As described in [21],  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)))












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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
   current "worst path" in the group 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
measurement of RTTs, to inform the group of the congestion control CLR,
and to provide feedback of individual RTT measurements to the receivers
in the group.  The 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 (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.



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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 RTT between the sender and
   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 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:






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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.


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 [4].   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:



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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,

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



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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
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.  As noted in [19],  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:

           RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew




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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 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



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 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 withhold further transmissions of
NORM_DATA segments 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 acknowledgment
requests defined for the NORM protocol and a range of acknowledgment
request types that are left to be defined by the application.  One
predefined acknowledgment type is the NORM_ACK_FLUSH type.  This
acknowledgment is used to determine if receivers have achieved
completion of reliable reception up through a specific logical
transmission point with respect to the sender's sequence of
transmission.  The NORM_ACK_FLUSH acknowledgment may be used to assist
in application flow control when the sender has information on a portion
of the receiver set.  Another predefined acknowledgment type is
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 acknowledgment procedure described here.  The
NORM_CMD(ACK_REQ) and NORM_ACK messages contain an "ack_type" field to



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the type of acknowledgment requested and provided.  A range of
"ack_type" values is provided for application-defined use.  While the
application is responsible for initiating the acknowledgment request and
interprets application-defined "ack_type" values, the acknowledgment
procedure SHOULD be conducted within the protocol implementation to take
advantage of timing and transmission scheduling information available to
the NORM transport.

The NORM positive acknowledgment procedure uses polling by the sender to
query the receiver group for response.  Note this polling procedure is
not intended to scale to very large receiver groups, but could be used
in large group setting to query a critical subset of the group.  Either
the 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
acknowledgment 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 acknowledgment 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 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
          acknowledgment 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.



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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 acknowledgment.  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
acknowledgment repair requests (NACKs) received from receivers during
the acknowledgment period.  The NORM_CMD(FLUSH) positive acknowledgment
process is restarted for receivers pending acknowledgment 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 Estimate

NORM sender messages contain a "gsize" field that is a representation of
the group size and is used in scaling random backoff timer ranges.  The
use of the group size estimate within the NORM protocol does not require
a precise estimation and works reasonably well if the estimate is within
an order of magnitude of the actual group size.  By default, the NORM
sender group size estimate may be administratively configured.  Also,
given the expected scalability of the NORM protocol for general use, a
default value of 10,000 is recommended for use as the group size
estimate.

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.

6.  Security Considerations

The same security considerations that apply to the NORM, and FEC
Building Blocks also apply to the NORM protocol.  In addition to
vulnerabilities that any IP and IP multicast protocol implementation may
be generally subject to, the NACK-based feedback of NORM may be
exploited by replay attacks which force the NORM sender to unnecessarily
transmit repair information.  This MAY be addressed by network layer IP
security implementations that guard against this potential security
exploitation.  It is RECOMMENDED that such IP security mechanisms be



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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 NORM packets are
cryptographically protected and 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
of NORM messages should be incremented with independent values for
different destinations (e.g., group-addressed versus unicast-addressed
messages versus "receiver" messages).  Thus, the congestion control loss
estimation function of the "sequence" field can be preserved for sender
messages when receiver messages are unicast to the sender.  The NORM
protocol is compatible with the use of the IP security (IPsec)
architecture described in [22]  It is important to note that 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.

7.  IANA Considerations

No information in this specification is currently subject to IANA
registration.  However, several Header Extensions are defined within
this document.  If/when additional Header Extensions are developed, the
first RFC MUST establish an IANA registry for them, with a
"Specification Required" policy [6]  and all Header Extensions,
including those in the present document, MUST be registered thereafter.
Additionally, building blocks components used by NORM may introduce
additional IANA considerations.  In particular, the FEC Building Block
used by NORM does require IANA registration of the FEC codecs used.  The
registration instructions for FEC codecs are provided in [5].

8.  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



Adamson, Bormann, et al.  Expires January 2005                  [Page 67]

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.

9.  Acknowledgments (and these are not Negative)

The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input and
comments on this document.  The authors would also like to thank the RMT
working group chairs, Roger Kermode and Lorenzo Vicisano, for their
support in development of this specification, and Sally Floyd for her
early input into this document.

10.  References

10.1.  Normative References

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

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

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

[4] B. Adamson, C. Bormann, M. Handley, and J. Macker, "NACK-Oriented
Reliable Multicast (NORM) Protocol Building Blocks", draft-ietf-rmt-bb-
norm-09, July 2004.

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

[6] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 14, RFC 2434, October 1998.

10.2.  Informative References

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

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

[9] 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.






Adamson, Bormann, et al.  Expires January 2005                  [Page 68]

[10] 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.

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

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

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

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

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

[16] 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.

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

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

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

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

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

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











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11.  Authors' Addresses

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

Carsten Bormann
cabo@tzi.org
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, 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

12.  Full Copyright Statement

Copyright (C) The Internet Society (2004).  This document is subject to
the rights, licenses and restrictions contained in BCP 78 and except as
set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.




















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