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RFC 3940

Negative-acknowledgment (NACK)-Oriented Reliable Multicast (NORM) Protocol

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Network Working Group                                         B. Adamson
Request for Comments: 3940                                           NRL
Category: Experimental                                        C. Bormann
                                                 Universitaet Bremen TZI
                                                              M. Handley
                                                               J. Macker
                                                           November 2004

                Negative-acknowledgment (NACK)-Oriented
                   Reliable Multicast (NORM) Protocol

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).


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 to fairly share available network bandwidth with other transport protocols such as Transmission Control Protocol (TCP). It is capable of operating with both reciprocal multicast routing among senders and receivers and with asymmetric connectivity (possibly a unicast return path) between the senders and receivers. The protocol offers a number of features to allow different types of applications or possibly other higher level transport protocols to utilize its service in different ways. The protocol leverages the use of FEC-based repair and other IETF reliable multicast transport (RMT) building blocks in its design.
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Table of Contents

1. Introduction and Applicability. . . . . . . . . . . . . . . . 3 1.1. NORM Delivery Service Model. . . . . . . . . . . . . . . 4 1.2. NORM Scalability . . . . . . . . . . . . . . . . . . . . 6 1.3. Environmental Requirements and Considerations. . . . . . 7 2. Architecture Definition . . . . . . . . . . . . . . . . . . . 7 2.1. Protocol Operation Overview. . . . . . . . . . . . . . . 9 2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . 10 2.3. Design Tradeoffs . . . . . . . . . . . . . . . . . . . . 11 3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 12 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. NORM Common Message Header and Extensions. . . . . . . . 14 4.2. Sender Messages. . . . . . . . . . . . . . . . . . . . . 16 4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . 16 4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . 24 4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . 26 4.3. Receiver Messages. . . . . . . . . . . . . . . . . . . . 43 4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . 43 4.3.2. NORM_ACK Message. . . . . . . . . . . . . . . . . 50 4.4. General Purpose Messages . . . . . . . . . . . . . . . . 52 4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . 52 5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 52 5.1. Sender Initialization and Transmission . . . . . . . . . 54 5.1.1. Object Segmentation Algorithm . . . . . . . . . . 55 5.2. Receiver Initialization and Reception. . . . . . . . . . 57 5.3. Receiver NACK Procedure. . . . . . . . . . . . . . . . . 57 5.4. Sender NACK Processing and Response. . . . . . . . . . . 59 5.4.1. Sender Repair State Aggregation . . . . . . . . . 60 5.4.2. Sender FEC Repair Transmission Strategy . . . . . 61 5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . 62 5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation. . . . . . 62 5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . 63 5.5.1. Greatest Round-trip Time Collection . . . . . . . 63 5.5.2. NORM Congestion Control Operation . . . . . . . . 64 5.5.3. NORM Positive Acknowledgment Procedure. . . . . . 72 5.5.4. Group Size Estimate . . . . . . . . . . . . . . . 74 6. Security Considerations . . . . . . . . . . . . . . . . . . . 75 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 75 8. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 75 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 76 10. References. . . . . . . . . . . . . . . . . . . . . . . . . . 76 10.1. Normative References. . . . . . . . . . . . . . . . . . 76 10.2. Informative References. . . . . . . . . . . . . . . . . 77 11. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 79 Full Copyright Statement. . . . . . . . . . . . . . . . . . . 80
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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]. Statement of Intent This memo contains part of the definitions necessary to fully specify a Reliable Multicast Transport protocol in accordance with RFC 2357. As per RFC 2357, the use of any reliable multicast protocol in the Internet requires an adequate congestion control scheme. While waiting for such a scheme to be available, or for an existing scheme to be proven adequate, the Reliable Multicast Transport working group (RMT) publishes this Request for Comments in the "Experimental" category. It is the intent of RMT to re-submit this specification as an IETF Proposed Standard as soon as the above condition is met.
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1.1. NORM Delivery Service Model

A NORM protocol instance (NormSession) is defined within the context of participants communicating connectionless (e.g., Internet Protocol (IP) or User Datagram Protocol (UDP)) packets over a network using pre-determined addresses and host port numbers. Generally, the participants exchange packets using an IP multicast group address, but unicast transport may also be established or applied as an adjunct to multicast delivery. In the case of multicast, the participating NormNodes will communicate using a common IP multicast group address and port number that has been chosen via means outside the context of the given NormSession. Other IETF data format and protocol standards exist that may be applied to describe and convey the required "a priori" information for a specific NormSession (e.g., Session Description Protocol (SDP) [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 possible that some aspects of protocol operation (e.g., round-trip time collection) may provide for alternate modes allowing more efficient performance for applications requiring multiple senders. NORM provides for three types of bulk data content objects (NormObjects) to be reliably transported. These types include: 1) static computer memory data content (NORM_OBJECT_DATA type), 2) computer storage files (NORM_OBJECT_FILE type), and 3) non-finite streams of continuous data content (NORM_OBJECT_STREAM type). The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is simply to provide a "hint" to receivers in NormSessions serving multiple types of content as to what type of storage should be allocated for received content (i.e., memory or file storage). Other than that distinction, the two are identical, providing for reliable transport of finite (but potentially very large) units of content. These static data and file services are anticipated to be useful for multicast-based cache applications with the ability to reliably provide transmission of large quantities of static data. Other types of static data/file delivery services might make use of these
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   transport object types, too.  The use of the NORM_OBJECT_STREAM type
   is at the application's discretion and could be used to carry static
   data or file content also.  The NORM reliable stream service opens up
   additional possibilities such as serialized reliable messaging or
   other unbounded, perhaps dynamically produced content.  The
   NORM_OBJECT_STREAM provides for reliable transport analogous to that
   of the Transmission Control Protocol (TCP), although NORM receivers
   will be able to begin receiving stream content at any point in time.
   The applicability of this feature will depend upon the application.

   The NORM protocol also allows for a small amount of "out-of-band"
   data (sent as NORM_INFO messages) to be attached to the data content
   objects transmitted by the sender.  This readily-available "out-of-
   band" data allows multicast receivers to quickly and efficiently
   determine the nature of the corresponding data, file, or stream bulk
   content being transmitted.  This allows application-level control of
   the receiver node's participation in the current transport activity.
   This also allows the protocol to be flexible with minimal pre-
   coordination among senders and receivers.  The NORM_INFO content is
   designed to be atomic in that its size MUST fit into the payload
   portion of a single NORM message.

   NORM does _not_ provide for global or application-level
   identification of data content within in its message headers.  Note
   the NORM_INFO out-of-band data mechanism could be leveraged by the
   application for this purpose if desired, or identification could
   alternatively be embedded within the data content.  NORM does
   identify transmitted content (NormObjects) with transport identifiers
   that are applicable only while the sender is transmitting and/or
   repairing the given object.  These transport data content identifiers
   (NormTransportIds) are assigned in a monotonically increasing fashion
   by each NORM sender during the course of a NormSession.  Each sender
   maintains its NormTransportId assignments independently so that
   individual NormObjects may be uniquely identified during transport
   with the concatenation of the sender session-unique identifier
   (NormNodeId) and the assigned NormTransportId.  The NormTransportIds
   are assigned from a large, but fixed, numeric space in increasing
   order and may be reassigned during long-lived sessions.  The NORM
   protocol provides mechanisms so that the sender application may
   terminate transmission of data content and inform the group of this
   in an efficient manner.  Other similar protocol control mechanisms
   (e.g., session termination, receiver synchronization, etc.) are
   specified so that reliable multicast application variants may
   construct different, complete bulk transfer communication models to
   meet their goals.
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   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 tens 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.
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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 among NORM senders is assumed to be controlled by the application or higher protocol layer. There are some optional mechanisms specified in this document that can be leveraged for such application layer coordination.
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   As 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

   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
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   a long-lived session, the NormObjectTransportId field can wrap and
   previously-used identifiers may be re-used.  Note that globally
   unique identification of transported data content is not provided by
   NORM and, if required, must be managed by the NORM application.  The
   individual segments or symbols of the NormObject are further
   identified with FEC payload identifiers which include coding block
   and symbol identifiers.  These are discussed in detail later in this

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
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   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
   NORM_NACK messages are generated to request repair of detected data
   transmission losses.  Receivers generally detect losses by tracking
   the sequence of transmission from a sender.  Sequencing information
   is embedded in the transmitted data packets and end-of-transmission
   commands from the sender.  NORM_ACK messages are generated in
   response to certain commands transmitted by the sender.  In the
   general (and most scalable) protocol mode, NORM_ACK messages are sent
   only in response to congestion control commands from the sender.  The
   feedback volume of these congestion control NORM_ACK messages is
   controlled using the same timer-based probabilistic suppression
   techniques as for NORM_NACK messages to avoid feedback implosion.  In
   order to meet potential application requirements for positive
   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 that 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
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   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 model of bulk transfer transport service that drives the trade-offs that determine the scalability and performance described in this document. The ability of NORM to provide reliable data delivery is also governed by any buffer constraints of the sender and receiver applications. NORM protocol implementations SHOULD be designed to operate with the greatest efficiency and robustness possible within application-defined buffer constraints. Buffer requirements for reliability, as always, are a function of the delay-bandwidth product of the network topology. NORM performs best when allowed more buffering resources than typical point-to-point transport protocols. This is because NORM feedback suppression is based upon randomly- delayed transmissions from the receiver set, rather than immediately transmitted feedback. There are definitive tradeoffs between buffer utilization, group size scalability, and efficiency of performance. Large buffer sizes allow the NORM protocol to perform most efficiently in large delay-bandwidth topologies and allow for longer feedback suppression backoff timeouts. This yields improved group size scalability. NORM can operate with reduced buffering but at a cost of decreased efficiency (lower relative goodput) and reduced group size scalability.
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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]. 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. | +--------------------+-----------------------------------------------+
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|NORM_ACK            | Receiver message used to proactively provide  |
|                    | feedback for congestion control purposes.     |
|                    | Also used with the OPTIONAL NORM Positive     |
|                    | Acknowledgment Process.                       |

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

|     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 reached    |
|                      | 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
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   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

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
ToP   noToC   RFC3940 - Page 15
   The 8-bit "hdr_len" field indicates the number of 32-bit words that
   comprise the given message's header portion.  This is used to
   facilitate header extensions that may be applied.  The presence of
   header extensions are implied when the "hdr_len" value is greater
   than the base value for the given message "type".

   The "sequence" field is a 16-bit value that is set by the message
   originator as a monotonically increasing number incremented with each
   NORM message transmitted to a given destination address.  A
   "sequence" field number space SHOULD be maintained for messages sent
   to the NormSession group address.  This value can be monitored by
   receiving nodes to detect packet losses in the transmission from a
   sender and used in estimating raw packet loss for congestion control
   purposes.  Note that this value is NOT used in the NORM protocol to
   detect missing reliable data content and does NOT identify the
   application data or FEC payload that may be attached.  With message
   authentication, the "sequence" field may also be leveraged for
   protection from message "replay" attacks, particularly of NORM_NACK
   or other feedback messages.  In this case, the receiver node should
   maintain a monotonically increasing "sequence" field space for each
   destination to which it transmits (this may be multiple destinations
   when unicast feedback is used).  The size of this field is intended
   to be sufficient to allow detection of a reasonable range of packet
   loss within the delay-bandwidth product of expected network

   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:
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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
   |   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
   |   het >=128   |   reserved    |    Header Extension Content   |
           NORM Fixed Length (32-bit) Header Extension Format

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

4.2. 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
ToP   noToC   RFC3940 - Page 17
   Transmission Information (EXT_FTI) header extension.  This allows
   NORM receivers to automatically allocate resources and properly
   perform FEC decoding without the need for pre-configuration or out-
   of-band information.

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |version| type=2|    hdr_len    |          sequence             |
   |                           source_id                           |
   |          instance_id          |     grtt      |backoff| gsize |
   |     flags     |    fec_id     |     object_transport_id       |
   |                         fec_payload_id                        |
   |                              ...                              |
   |                header_extensions (if applicable)              |
   |                              ...                              |
   |       payload_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
ToP   noToC   RFC3940 - Page 18
   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 size of the "fec_payload_id" is two 32-bit words.
   In this case the NORM_DATA base "hdr_len" value is 6.  The cumulative
   size of any header extensions applied is added into the "hdr_len"

   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
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   (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.

   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
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   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 that receivers may inadvertently
   request repair of such objects when all segments (or info content)
   for those objects are not received (i.e., a gap in the
   "object_transport_id" sequence is noted).  In this case, the sender
   should invoke the NORM_CMD(SQUELCH) process as described in Section
   4.2.3.  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_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
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   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:

      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
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   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:

      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"
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   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 interpreted 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 indicate 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
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   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.

(page 24 continued on part 2)

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