Network Working Group B. Adamson Request for Comments: 3940 NRL Category: Experimental C. Bormann Universitaet Bremen TZI M. Handley UCL J. Macker NRL 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). 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 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.
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
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 . 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 . 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.
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) , Session Announcement Protocol (SAP) , 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
transport object types, too. The use of the NORM_OBJECT_STREAM type is at the application's discretion and could be used to carry static data or file content also. The NORM reliable stream service opens up additional possibilities such as serialized reliable messaging or other unbounded, perhaps dynamically produced content. The NORM_OBJECT_STREAM provides for reliable transport analogous to that of the Transmission Control Protocol (TCP), although NORM receivers will be able to begin receiving stream content at any point in time. The applicability of this feature will depend upon the application. The NORM protocol also allows for a small amount of "out-of-band" data (sent as NORM_INFO messages) to be attached to the data content objects transmitted by the sender. This readily-available "out-of- band" data allows multicast receivers to quickly and efficiently determine the nature of the corresponding data, file, or stream bulk content being transmitted. This allows application-level control of the receiver node's participation in the current transport activity. This also allows the protocol to be flexible with minimal pre- coordination among senders and receivers. The NORM_INFO content is designed to be atomic in that its size MUST fit into the payload portion of a single NORM message. NORM does _not_ provide for global or application-level identification of data content within in its message headers. Note the NORM_INFO out-of-band data mechanism could be leveraged by the application for this purpose if desired, or identification could alternatively be embedded within the data content. NORM does identify transmitted content (NormObjects) with transport identifiers that are applicable only while the sender is transmitting and/or repairing the given object. These transport data content identifiers (NormTransportIds) are assigned in a monotonically increasing fashion by each NORM sender during the course of a NormSession. Each sender maintains its NormTransportId assignments independently so that individual NormObjects may be uniquely identified during transport with the concatenation of the sender session-unique identifier (NormNodeId) and the assigned NormTransportId. The NormTransportIds are assigned from a large, but fixed, numeric space in increasing order and may be reassigned during long-lived sessions. The NORM protocol provides mechanisms so that the sender application may terminate transmission of data content and inform the group of this in an efficient manner. Other similar protocol control mechanisms (e.g., session termination, receiver synchronization, etc.) are specified so that reliable multicast application variants may construct different, complete bulk transfer communication models to meet their goals.
To summarize, the NORM protocol provides reliable transport of different types of data content (including potentially mixed types). 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 . 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 . FEC-based repair can be used to greatly reduce the quantity of reliable multicast repair requests and repair transmissions  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 . 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 . 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  and the RMT FEC Building Block  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 , but SHALL also be capable of scalable operation in asymmetric topologies such as Source Specific Multicast (SSM)  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.
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 be managed by the NORM application. The individual segments or symbols of the NormObject are further identified with FEC payload identifiers which include coding block and symbol identifiers. These are discussed in detail later in this document. 2.1. 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)  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 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 . 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 . NORM also makes use of Forward Error Correction encoding techniques for repair messaging and optional transmission robustness as described in . NORM uses the FEC Payload ID as
specified by the FEC Building Block Document . 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 . 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.
3. Conformance Statement This Protocol Instantiation document, in conjunction with the RMT Building Block documents of  and , completely specifies a working reliable multicast transport protocol that conforms to the requirements described in RFC 2357 . 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. | +--------------------+-----------------------------------------------+ 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 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
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. 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 payload that may be attached. With message authentication, the "sequence" field may also be leveraged for protection from message "replay" attacks, particularly of NORM_NACK or other feedback messages. In this case, the receiver node should maintain a monotonically increasing "sequence" field space for each destination to which it transmits (this may be multiple destinations when unicast feedback is used). The size of this field is intended to be sufficient to allow detection of a reasonable range of packet loss within the delay-bandwidth product of expected network connections. The "source_id" field is a 32-bit value identifying the node that sent the message. A participant's NORM node identifier (NormNodeId) can be set according to application needs but unique identifiers must be assigned within a single NormSession. In some cases, use of the host IP address or a hash of it can suffice, but alternative methodologies for assignment and potential collision resolution of node identifiers within a multicast session need to be considered. For example, the "source identifier" mechanism defined in the Real- Time Protocol (RTP) specification  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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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
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
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" 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 ). 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 . 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. 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 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_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 . 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  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 . 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 ) 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"
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 . 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 . 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 . 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
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.