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rfc:rfc5740

Network Working Group B. Adamson Request for Comments: 5740 Naval Research Laboratory Obsoletes: 3940 C. Bormann Category: Standards Track Universitaet Bremen TZI

                                                            M. Handley
                                             University College London
                                                             J. Macker
                                             Naval Research Laboratory
                                                         November 2009
     NACK-Oriented Reliable Multicast (NORM) Transport Protocol

Abstract

 This document describes the messages and procedures of the Negative-
 ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.
 This protocol can 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 (forward error correction) repair and other IETF
 Reliable Multicast Transport (RMT) building blocks in its design.
 This document obsoletes RFC 3940.

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

Adamson, et al. Standards Track [Page 1] RFC 5740 NORM Protocol November 2009

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction and Applicability . . . . . . . . . . . . . . . .  4
   1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   1.2.  NORM Data Delivery Service Model . . . . . . . . . . . . .  5
   1.3.  NORM Scalability . . . . . . . . . . . . . . . . . . . . .  7
   1.4.  Environmental Requirements and Considerations  . . . . . .  8
 2.  Architecture Definition  . . . . . . . . . . . . . . . . . . .  8
   2.1.  Protocol Operation Overview  . . . . . . . . . . . . . . . 10
   2.2.  Protocol Building Blocks . . . . . . . . . . . . . . . . . 12
   2.3.  Design Trade-Offs  . . . . . . . . . . . . . . . . . . . . 12
 3.  Conformance Statement  . . . . . . . . . . . . . . . . . . . . 13
 4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 15
   4.1.  NORM Common Message Header and Extensions  . . . . . . . . 15
   4.2.  Sender Messages  . . . . . . . . . . . . . . . . . . . . . 18
     4.2.1.  NORM_DATA Message  . . . . . . . . . . . . . . . . . . 18
     4.2.2.  NORM_INFO Message  . . . . . . . . . . . . . . . . . . 28
     4.2.3.  NORM_CMD Messages  . . . . . . . . . . . . . . . . . . 29
   4.3.  Receiver Messages  . . . . . . . . . . . . . . . . . . . . 47
     4.3.1.  NORM_NACK Message  . . . . . . . . . . . . . . . . . . 47
     4.3.2.  NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53
   4.4.  General Purpose Messages . . . . . . . . . . . . . . . . . 55
     4.4.1.  NORM_REPORT Message  . . . . . . . . . . . . . . . . . 55
 5.  Detailed Protocol Operation  . . . . . . . . . . . . . . . . . 55
   5.1.  Sender Initialization and Transmission . . . . . . . . . . 57
     5.1.1.  Object Segmentation Algorithm  . . . . . . . . . . . . 58

Adamson, et al. Standards Track [Page 2] RFC 5740 NORM Protocol November 2009

   5.2.  Receiver Initialization and Reception  . . . . . . . . . . 59
   5.3.  Receiver NACK Procedure  . . . . . . . . . . . . . . . . . 59
   5.4.  Sender NACK Processing and Response  . . . . . . . . . . . 62
     5.4.1.  Sender Repair State Aggregation  . . . . . . . . . . . 62
     5.4.2.  Sender FEC Repair Transmission Strategy  . . . . . . . 63
     5.4.3.  Sender NORM_CMD(SQUELCH) Generation  . . . . . . . . . 64
     5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
   5.5.  Additional Protocol Mechanisms . . . . . . . . . . . . . . 65
     5.5.1.  Group Round-Trip Time (GRTT) Collection  . . . . . . . 65
     5.5.2.  NORM Congestion Control Operation  . . . . . . . . . . 67
     5.5.3.  NORM Positive Acknowledgment Procedure . . . . . . . . 75
     5.5.4.  Group Size Estimate  . . . . . . . . . . . . . . . . . 77
 6.  Configurable Elements  . . . . . . . . . . . . . . . . . . . . 77
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 80
   7.1.  Baseline Secure NORM Operation . . . . . . . . . . . . . . 82
     7.1.1.  IPsec Approach . . . . . . . . . . . . . . . . . . . . 83
     7.1.2.  IPsec Requirements . . . . . . . . . . . . . . . . . . 85
 8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 86
   8.1.  Explicit IANA Assignment Guidelines  . . . . . . . . . . . 87
     8.1.1.  NORM Header Extension Types  . . . . . . . . . . . . . 87
     8.1.2.  NORM Stream Control Codes  . . . . . . . . . . . . . . 88
     8.1.3.  NORM_CMD Message Sub-Types . . . . . . . . . . . . . . 88
 9.  Suggested Use  . . . . . . . . . . . . . . . . . . . . . . . . 89
 10. Changes from RFC 3940  . . . . . . . . . . . . . . . . . . . . 90
 11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 91
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 91
   12.1. Normative References . . . . . . . . . . . . . . . . . . . 91
   12.2. Informative References . . . . . . . . . . . . . . . . . . 92

Adamson, et al. Standards Track [Page 3] RFC 5740 NORM Protocol November 2009

1. Introduction and Applicability

 The Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM)
 protocol can provide reliable transport of data from one or more
 senders 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 self-adapting to a wide range of dynamic
 network conditions with little or no pre-configuration.  The protocol
 is tolerant of inaccurate timing estimations or lossy conditions that
 can occur in many networks including mobile and wireless.  The
 protocol can also converge and maintain efficient operation even in
 situations of heavy packet loss and large queuing or transmission
 delays.  This document obsoletes the Experimental RFC 3940
 specification.
 This document is a product of the IETF RMT working group and follows
 the guidelines provided in the Author Guidelines for Reliable
 Multicast Transport (RMT) Building Blocks and Protocol Instantiation
 documents [RFC3269].
 Statement of Intent
 This memo contains the definitions necessary to fully specify a
 Reliable Multicast Transport protocol in accordance with the criteria
 of IETF Criteria for Evaluating Reliable Multicast Transport and
 Application Protocols [RFC2357].  The NORM specification described in
 this document was previously published in the Experimental Category
 [RFC3940].  It was the stated intent of the RMT working group to re-
 submit this specifications as an IETF Proposed Standard in due
 course.  This Proposed Standard specification is thus based on RFC
 3940 and has been updated according to accumulated experience and
 growing protocol maturity since the publication of RFC 3940.  Said
 experience applies both to this specification itself and to
 congestion control strategies related to the use of this
 specification.  The differences between RFC 3940 and this document
 are listed in Section 10.

Adamson, et al. Standards Track [Page 4] RFC 5740 NORM Protocol November 2009

1.1. Requirements Language

 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 RFC 2119 [RFC2119].

1.2. NORM Data 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 chosen via means outside the context of
 the given NormSession.  Other existing IETF data format and protocol
 standards MAY be applied to describe and convey the necessary a
 priori information for a specific NormSession (e.g., Session
 Description Protocol (SDP) [RFC4566], Session Announcement Protocol
 (SAP) [RFC2974], 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
 independently of one another and receivers will maintain state as
 necessary for each sender.  In future versions of NORM, it is
 possible some aspects of protocol operation (e.g., round-trip time
 collection) will 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 to allocate for
 received content (i.e., memory or file storage).  Other than that

Adamson, et al. Standards Track [Page 5] RFC 5740 NORM Protocol November 2009

 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
 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 its message headers.  Note the NORM_INFO out-
 of-band data mechanism can be leveraged by the application for this
 purpose if desired, or identification can 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.  Participants, including senders,
 in NORM protocol sessions are also identified with unique identifiers
 (NormNodeIds).  Each sender maintains its NormTransportId assignments
 independently and thus individual NormObjects can be uniquely
 identified during transport by concatenation of the session-unique
 sender identifier (NormNodeId) and the assigned NormTransportId.  The
 NormTransportIds are assigned from a large, but fixed, numeric space
 in increasing order and will be reassigned during long-lived
 sessions.  The NORM protocol provides mechanisms so the sender
 application can terminate transmission of data content and inform the
 group of this in an efficient manner.  Other similar protocol control

Adamson, et al. Standards Track [Page 6] RFC 5740 NORM Protocol November 2009

 mechanisms (e.g., session termination, receiver synchronization,
 etc.) are specified so reliable multicast application variants can
 realize 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.

1.3. NORM Scalability

 Group communication scalability requirements lead to adaptation of
 NACK-based protocol schemes when feedback for reliability is needed
 [RmComparison].  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 [RFC3453].  FEC-based repair can be used to greatly reduce
 the quantity of reliable multicast repair requests and repair
 transmissions [MdpToolkit] 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 [McastFeedback].  NORM dynamically
 measures the group's round-trip timing status to set its suppression
 and other protocol timers.  This allows NORM to scale well while
 maintaining reliable data delivery transport with low latency
 relative to the network topology over which it is operating.
 Feedback messages can be either multicast to the group at large or
 sent via unicast routing to the sender.  In the case of unicast
 feedback, the sender relays the feedback state to the group to
 facilitate feedback suppression.  In typical Internet environments,
 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 [NormFeedback].  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 for NORM to scale
 to larger group sizes.  With respect to computer resource usage, the

Adamson, et al. Standards Track [Page 7] RFC 5740 NORM Protocol November 2009

 NORM protocol does not need state to be kept on all receivers in the
 group.  NORM senders maintain state only for receivers providing
 explicit congestion control feedback.  However, NORM receivers need
 to maintain state for each active sender.  This can constrain the
 number of simultaneous senders in some uses of NORM.

1.4. Environmental Requirements and Considerations

 All of the environmental requirements and considerations that apply
 to the "Multicast Negative-Acknowledgment (NACK) Building Blocks"
 [RFC5401], "Forward Error Correction (FEC) Building Block" [RFC5052],
 and "TCP-Friendly Multicast Congestion Control (TFMCC) Protocol
 Specification" [RFC4654] 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 "Host Extensions for IP Multicasting"
 [RFC1112], but it SHALL also be capable of scalable operation in
 asymmetric topologies such as Source-Specific Multicast (SSM)
 [RFC4607] where only unicast routing service is available from the
 receivers to the sender(s).
 NORM is compatible with IPv4 and IPv6.  Additionally, NORM can be
 used with networks employing Network Address Translation (NAT)
 provided that 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 multiple possible
 senders and to distinguish feedback information from different
 receivers.  There are two reserved NormNodeId values.  A value of
 0x00000000 is considered an invalid NormNodeId (NORM_NODE_NONE), and
 a value of 0xffffffff is a "wild card" NormNodeId (NORM_NODE_ANY).

Adamson, et al. Standards Track [Page 8] RFC 5740 NORM Protocol November 2009

 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
 provided by TCP for unicast data transport.  The format of the stream
 content is application-defined and can 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, a 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 of 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 needs to receive a sufficient number of symbols to
 reconstruct (via FEC decoding) the original user data for the given
 block.
 Transmitted NormObjects are temporarily, yet uniquely, identified
 within the NormSession context using the given sender's NormNodeId,
 NormInstanceId, and a temporary NormTransportId.  Depending upon the

Adamson, et al. Standards Track [Page 9] RFC 5740 NORM Protocol November 2009

 implementation, individual NORM senders can manage their
 NormInstanceIds independently, or a common NormInstanceId could be
 agreed upon for all participating nodes within a session, if needed,
 as a session identifier.  NORM NormTransportId 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 NormTransportId field can wrap and
 previously used identifiers will be re-used.  Note that globally
 unique identification of transported data content is not provided by
 NORM and, if necessary, is expected to be managed by the NORM
 application.  The individual segments or symbols of the NormObject
 are further identified with FEC payload identifiers that 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 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 configuration is sensible for certain network conditions and can
 allow for robust, asymmetric multicast (e.g., unidirectional routing,
 satellite, cable) [FecHybrid] 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.  The NORM_INFO message can
 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

Adamson, et al. Standards Track [Page 10] RFC 5740 NORM Protocol November 2009

 certain protocol operations such as congestion control, end-of-
 transmission flushing, group round-trip time (GRTT) 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: single, best-effort unreliable transmission of
 the command; repeated redundant transmissions of the command; and
 positively acknowledged commands.  The transmission technique used
 for a given command depends upon the function of the command.
 Several core commands are defined for basic protocol operation.
 Additionally, implementations MAY wish to consider providing the
 OPTIONAL application-defined commands that can take advantage of the
 transmission methodologies available for commands.  This allows for
 application-level session management mechanisms that can make use of
 information available to the underlying NORM protocol engine (e.g.,
 round-trip timing, transmission rate, etc.).  A notable distinction
 between NORM_DATA message and some NORM_CMD message transmissions is
 that typically a receiver will need to allocate resources to manage
 reliable reception when NORM_DATA messages are received.  However,
 some NORM_CMD messages are completely atomic and no specific
 reliability (buffering) state needs to be kept.  Thus, for session
 management or other purposes, it is possible that even participants
 acting principally as data receivers MAY transmit NORM_CMD messages.
 However, it is RECOMMENDED that this is not done within the context
 of the NORM multicast session unless congestion control is addressed.
 For example, many receiver nodes transmitting NORM_CMD messages
 simultaneously can cause congestion for the destination(s).
 All sender transmissions are subject to rate control governed by a
 peak transmission rate set for each participant by the application.
 This can be used to limit the quantity of multicast data transmitted
 by the group.  When NORM's congestion control algorithm is enabled,
 the rate for senders is automatically adjusted.  In some networks, it
 is 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.
 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

Adamson, et al. Standards Track [Page 11] RFC 5740 NORM Protocol November 2009

 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 are available for use.

2.2. Protocol Building Blocks

 The operation of the NORM protocol is based primarily upon the
 concepts presented in the Multicast NACK Building Block [RFC5401]
 document.  This includes the basic NORM architecture and the data
 transmission, repair, and feedback strategies discussed in that
 document.  The reliable multicast building block approach, as
 described in "Reliable Multicast Transport Building Blocks for One-
 to-Many Bulk-Data Transfer" [RFC3048], is applied in creating the
 full NORM protocol instantiation.  NORM also makes use of the parity-
 based encoding techniques for repair messaging and added transmission
 robustness as described in "The Use of Forward Error Correction (FEC)
 in Reliable Multicast" [RFC3453].  NORM uses the FEC Payload ID as
 specified by the FEC Building Block document [RFC5052].
 Additionally, for congestion control, this document fully specifies a
 baseline congestion control mechanism (NORM-CC) based on the TCP-
 Friendly Multicast Congestion Control (TFMCC) scheme [TfmccPaper],
 [RFC4654].

2.3. Design Trade-Offs

 While the various features of NORM provide some measure of general
 purpose utility, it is important to emphasize the understanding that
 "no one size fits all" in the reliable multicast transport arena.
 There are numerous engineering trade-offs involved in reliable
 multicast transport design and this necessitates 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

Adamson, et al. Standards Track [Page 12] RFC 5740 NORM Protocol November 2009

 applications.  NORM protocol implementations SHOULD operate with the
 greatest efficiency and robustness possible within application-
 defined buffer constraints.  Buffer requirements for reliability, as
 always, are a function of the delay-bandwidth product of the network
 topology.  NORM performs best when allowed more buffering resources
 than typical point-to-point transport protocols.  This is because
 NORM feedback suppression is based upon randomly delayed
 transmissions from the receiver set, rather than immediately
 transmitted feedback.  There are definitive trade-offs between buffer
 utilization, group size scalability, and efficiency of performance.
 Large buffer sizes allow the NORM protocol to perform most
 efficiently in large delay-bandwidth topologies and allow for longer
 feedback suppression backoff timeouts.  This yields improved group
 size scalability.  NORM can operate with reduced buffering but at a
 cost of decreased efficiency (lower relative goodput) and reduced
 group size scalability.

3. Conformance Statement

 This RMT Protocol Instantiation document, in conjunction with the
 "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401]
 and "Forward Error Correction (FEC) Building Block" [RFC5052]
 Building Blocks, 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
 that 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.                                  |

Adamson, et al. Standards Track [Page 13] RFC 5740 NORM Protocol November 2009

 | 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 is 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 that 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.            |

Adamson, et al. Standards Track [Page 14] RFC 5740 NORM Protocol November 2009

 | 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

 There are two primary classes of NORM messages (see Section 2.1):
 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.  Sender messages SHALL be governed by congestion control
 for Internet use.  For session management or other purposes,
 receivers can also employ NORM_CMD message transmissions.  The
 principal rationale for distinguishing sender and receiver messages
 is that receivers will typically need to allocate resources to
 support reliable reception from sender(s) and NORM sender messages
 are subject to congestion control.  NORM receivers MAY employ the
 NORM_CMD message type for application-defined purposes, but it is
 RECOMMENDED that congestion control and feedback implosion issues be
 addressed.  Additionally, an auxiliary message type of 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
 compatible with the Maximum Transmission Unit (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:

Adamson, et al. Standards Track [Page 15] RFC 5740 NORM Protocol November 2009

    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                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 1: 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 that are 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 the addition of header extensions.  The presence of header
 extensions is 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.  The "sequence" field serves two separate purposes,
 depending upon the message type:
 1.  NORM senders MUST set the "sequence" field of sender messages
     (NORM_INFO, NORM_DATA, and NORM_CMD) so that receivers can
     monitor the "sequence" value to maintain an estimate of packet
     loss that can be used for congestion control purposes (see
     Section 5.5.2 for a detailed description of NORM Congestion
     Control operation).  A monotonically increasing sequence number
     space MUST be maintained to mark NORM sender messages in this
     way.  Note that this "sequence" number is explicitly NOT used in

Adamson, et al. Standards Track [Page 16] RFC 5740 NORM Protocol November 2009

     NORM as part of its reliability procedures.  The NORM object and
     FEC payload identifiers are used to detect missing content for
     reliable transfer purposes.
 2.  NORM receivers SHOULD set the "sequence" field to support
     protection from message replay attacks of NORM_NACK or NORM_NACK
     messages.  Note that, depending upon configuration, NORM feedback
     messages are sent to the session multicast address or the unicast
     address(es) of the active NORM sender(s).  Thus, a separate,
     monotonically increasing sequence number space MUST be maintained
     for each destination address to which the NORM receiver is
     transmitting feedback messages.
 Note that these two separate purposes necessitate the maintenance of
 separate sequence spaces to support the functions described here.
 And, in the case of NORM receivers, additional sequence spaces are
 needed when feedback messages are sent to the sender unicast
 address(es) instead of the session address.
 The "source_id" field is a 32-bit value that uniquely identifies the
 node that sent the message within the context of a single
 NormSession.  This value is termed the NORM node identifier
 (NormNodeId) and unique NormNodeIds MUST be assigned within a single
 NormSession.  In some cases, use of the host IPv4 address or a hash
 of an address can suffice, but alternative methodologies for
 assignment and potential collision resolution of node identifiers
 within a multicast session SHOULD be considered.  For example, the
 techniques for managing the 32-bit "synchronization source" (SSRC)
 identifiers defined in the Real-Time Protocol (RTP) specification
 [RFC3550] are applicable for use with NORM node identifiers when an
 ASM traffic model is observed.  In most deployments of the NORM
 protocol to date, the NormNodeId assignments are administratively
 configured, and this form of NormNodeId assignment is RECOMMENDED for
 most purposes.  NORM sender NormNodeId values MUST be unique within
 an ASM session so that NORM receiver feedback can be properly
 demultiplexed by senders, and NORM receiver NormNodeId values MUST
 also be unique for congestion control operation or when the OPTIONAL
 positive acknowledgment mechanism is used.
 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.

Adamson, et al. Standards Track [Page 17] RFC 5740 NORM Protocol November 2009

 For variable-length extensions, the value of the "hel" (header
 extension length) field is the length of the entire header extension,
 expressed in multiples of 32-bit words.  The "hel" field MUST be
 present for variable-length extensions ("het" between 0 and 127) and
 MUST NOT be present for fixed-length extensions ("het" between 128
 and 255).
 The formats of the variable-length and fixed-length header extensions
 are given, respectively, 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                   |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Figure 2: 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   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 3: NORM Fixed-Length (32-bit) Header Extension Format
 The "Header Extension Content" portion of the header extension is
 defined for each extension type.  Some header extensions are defined
 within this document for NORM baseline FEC and congestion control
 operations.

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 generally 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 contain original or FEC-
 encoded application data content.

Adamson, et al. Standards Track [Page 18] RFC 5740 NORM Protocol November 2009

 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_len*         |       payload_msg_start*      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        payload_offset*                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          payload_data*                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 4: NORM_DATA Message Format
  • IMPORTANT NOTE: The "payload_len", "payload_msg_start" and

"payload_offset" fields are present only for objects of type

 NORM_OBJECT_STREAM.  These fields, as with the entire payload, are
 subject to any FEC encoding used.  Thus, when systematic FEC codes
 are used, these values can be directly interpreted only for packets
 containing source symbols while packets containing FEC parity content
 need decoding before these fields can be interpreted.
 The "version", "type", "hdr_len", "sequence", and "source_id" fields

Adamson, et al. Standards Track [Page 19] RFC 5740 NORM Protocol November 2009

 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 (i.e., four 32-bit words) plus the size of the
 "fec_payload_id" field.  The "fec_payload_id" field size depends upon
 the FEC encoding type referenced by the "fec_id" field.  For example,
 when small block, systematic codes are used, a "fec_id" value of 129
 is indicated, and the size of the "fec_payload_id" is two 32-bit
 words.  In this case the 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, or at least treat this
 "instance" as a new, separate sender.
 The "grtt" field contains a non-linear quantized representation of
 the sender's current estimate of group round-trip time (GRTT_sender)
 (this is also referred to as R_max in [TfmccPaper]).  This value is
 used to control timing of the NACK repair process and other aspects
 of protocol operation as described in this document.  Normally, the
 advertised "grtt" value will correspond to what the sender has
 measured based on feedback from the group, but, at low transmission
 rates, the advertised "grtt" SHALL be set to MAX(grttMeasured,
 NormSegmentSize/senderRate) where the NormSegmentSize is the sender's
 segment size in bytes and the senderRate is the sender's current
 transmission rate in bytes per second.  The algorithm for encoding
 and decoding this field is described in the Multicast NACK Building
 Block [RFC5401] 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
 that are multiplied by GRTT_sender to determine the maximum backoff
 timeout.  The "backoff" field informs the receivers of the sender's
 backoff factor parameter (K_sender).  Recommended values and their
 uses 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 (GSIZE_sender).  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

Adamson, et al. Standards Track [Page 20] RFC 5740 NORM Protocol November 2009

 base-10 exponent (order of magnitude).  For example, a field value of
 "0x0" represents 1.0e+01 (10), a value of "0x8" represents 5.0e+01
 (50), a value of "0x1" represents 1.0e+02 (100), and a value of "0xf"
 represents 5.0e+08.  For NORM feedback suppression purposes, the
 group size does not need to be represented with a high degree of
 precision.  The group size MAY even be estimated somewhat
 conservatively (i.e., overestimated) to maintain low levels of
 feedback traffic.  A default group size estimate of 10,000 ("gsize" =
 0x3) is RECOMMENDED for general purpose reliable multicast
 applications using the NORM protocol.
 The "flags" field contains a number of different binary flags
 providing information and hints for the receiver to appropriately
 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 a 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_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" (not previously transmitted) parity segments as

Adamson, et al. Standards Track [Page 21] RFC 5740 NORM Protocol November 2009

 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.  There are cases where receivers can
 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
 SHALL 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.  The presence of NORM_FLAG_STREAM overrides that
 of NORM_FLAG_FILE with respect to interpretation of object size and
 the format of NORM_DATA messages.
 The "fec_id" field corresponds to the FEC Encoding Identifier
 described in the FEC Building Block document [RFC5052].  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 systematic FEC
 codes are RECOMMENDED for the most efficient performance of
 NORM_OBJECT_STREAM transport.
 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
 will wrap and be repeated, but it is presumed that the 16-bit field
 size provides a sufficient 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.

Adamson, et al. Standards Track [Page 22] RFC 5740 NORM Protocol November 2009

 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 descriptions of specific FEC schemes such as those
 described in the FEC Basic Schemes [RFC5445] specification or in
 other FEC Schemes.  As an example, the format of the "fec_payload_id"
 format for Small Block, Systematic codes ("fec_id" = 129) from the
 FEC Basic Schemes [RFC5445] specification 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       source_block_number                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        source_block_len       |      encoding_symbol_id       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure 5: Example: FEC Payload Id Format for 'fec_id' = 129
 In this example, FEC payload identifier, the "source_block_number",
 "source_block_len", and "encoding_symbol_id" fields correspond to the
 "Source Block Number", "Source Block Length", and "Encoding Symbol
 ID" fields of the FEC Payload ID format for Small Block Systematic
 FEC Schemes identified by a "fec_id" value of 129 as specified by the
 FEC Basic Schemes [RFC5445] specification.  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" will 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.  Applications MAY dynamically
 "shorten" code blocks when the pending information content is not
 predictable (e.g., real-time message streams).  In that case, the
 "source_block_len" value given for an "encoding_symbol_id" that
 contains FEC parity content SHALL take precedence over the
 "source_block_len" value provided for any packets containing source
 symbols.  Also, the "source_block_len" value given for an ordinally
 higher "encoding_symbol_id" SHALL take precedence over the
 "source_block_len" given for prior encoding symbols.  The reason for
 this is that the sender will only know the maximum source block
 length at the time it is transmitting source symbols, but then
 subsequently "shorten" the code and then provide that last source
 symbol and/or encoding symbols with FEC parity content.  The
 "encoding_symbol_id" identifies which specific symbol (segment)
 within the coding block the attached payload conveys.  Depending upon
 the value of the "encoding_symbol_id" and the associated
 "source_block_len" parameters for the block, the symbol (segment)

Adamson, et al. Standards Track [Page 23] RFC 5740 NORM Protocol November 2009

 referenced will be a user data or a 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 (FTI) (as described in
 the FEC Building Block [RFC5052]) document is needed to properly
 receive and decode NORM transport objects.  This information MAY be
 provided as out-of-band session information.  In some cases, it will
 be useful for the sender to include this information "in-band" to
 facilitate receiver operation with minimal pre-configuration.  For
 this purpose, the NORM FEC Object Transmission Information Header
 Extension (EXT_FTI) is defined.  This header extension MAY be applied
 to NORM_DATA and NORM_INFO messages to provide this necessary
 information.  The format of the EXT_FTI consists of two parts, a
 general part that contains the size of the associated transport
 object and a portion that depends upon the FEC scheme being used.
 The "fec_id" field in NORM_DATA and NORM_INFO messages identifies the
 FEC scheme.  The format of the EXT_FTI general part 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_size (msb)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       object_size (lsb)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  FEC scheme-specific content ...              |
       Figure 6: EXT_FTI Header Extension General Portion Format
 The header extension type "het" field value for the EXT_FTI header
 extension is 64.  The header extension length "hel" value depends
 upon the format of the FTI for encoding type identified by the
 "fec_id" field.
 The 48-bit "object_size" field indicates the total length 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 might 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_size"
 field is used by the sender to advertise the size of its stream

Adamson, et al. Standards Track [Page 24] RFC 5740 NORM Protocol November 2009

 buffer to the receiver group.  In turn, the receivers SHOULD use this
 information to allocate a stream buffer for reception of
 corresponding size.
 As noted, the format of the extension depends upon the FEC code in
 use, but in general, it contains any necessary details on the code in
 use (e.g., FEC Instance ID, etc.).  As an example, the format of the
 EXT_FTI for small block systematic codes ("fec_id" = 129) is given
 here:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    het = 64   |    hel = 4    |       object_size (msb)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       object_size (lsb)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       fec_instance_id         |          segment_size         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       fec_max_block_len       |         fec_num_parity        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 7: Example: EXT_FTI Header Extension Format for 'fec_id' = 129
 In this example (for "fec_id" = 129), the "hel" field value is 4.
 The size of the EXT_FTI header extension will possibly be different
 for other FEC schemes.
 The 48-bit "object_size" serves the purpose described previously.
 The "fec_instance_id" corresponds to the "FEC Instance ID" described
 in the FEC Building Block [RFC5052] document.  In this case, the
 "fec_instance_id" is a value corresponding to the particular type of
 Small Block Systematic Code being used (e.g., Reed-Solomon GF(2^8),
 Reed-Solomon GF(2^16), etc).  The standardized assignment of FEC
 Instance ID values is described in RFC 5052.
 The "segment_size" field indicates the sender's current setting for
 maximum message payload content (in bytes).  This allows receivers to
 allocate appropriate buffering resources and to determine other
 information in order to properly process received data messaging.
 Typically, FEC parity symbol segments will be of this size.
 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

Adamson, et al. Standards Track [Page 25] RFC 5740 NORM Protocol November 2009

 symbols that can be generated for any source block" as described in
 FEC Object Transmission Information for Small Block Systematic Codes
 as described in the FEC Building Block [RFC5052] document.  For
 example, Reed-Solomon codes can 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 content of this payload depends
 upon the FEC scheme being employed, and support for streaming using
 the NORM_OBJECT_STREAM type, when applicable, necessitates some
 additional content in the payload.
 The "payload_len", "payload_msg_start", and "payload_offset" fields
 are present only for transport objects of type NORM_OBJECT_STREAM.
 These REQUIRED 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 REQUIRED block partitioning algorithm
 described in the FEC Building Block [RFC5052] document.  When
 systematic FEC codes (e.g., "fec_id" = 129) are used, the
 "payload_len", "payload_msg_start", and "payload_offset" fields
 contain actual payload_data length, message start index (or stream
 control code), and byte offset values for the associated application
 stream data segment (the remainder of the "payload_data" field
 content) for those NORM_DATA messages containing source data symbols.
 In NORM_DATA messages that contain FEC parity content, these fields
 do not contain values that can be directly interpreted, but instead
 are values computed from FEC encoding the "payload_len",
 "payload_msg_start", and "payload_offset" fields for the source data
 segments of the corresponding coding block.  The actual
 "payload_msg_start", "payload_len" and, "payload_offset" values of
 missing data content can be determined upon decoding a FEC coding
 block.  Note that these fields do NOT contribute to the value of the
 NORM_DATA "hdr_len" field.  These fields are present only when the
 "flags" portion of the NORM_DATA message indicate the transport
 object is of type NORM_OBJECT_STREAM.
 The "payload_len" value, when non-zero, indicates the length (in
 bytes) of the source content contained in the associated
 "payload_data" field.  However, when the "payload_len" value is equal
 to ZERO, this indicates that the "payload_msg_start" field be

Adamson, et al. Standards Track [Page 26] RFC 5740 NORM Protocol November 2009

 alternatively interpreted as a "stream_control_code".  The only
 "stream_control_code" value defined is NORM_STREAM_END = 0.  The
 NORM_STREAM_END code indicates that the sender is terminating the
 transmission of stream content at the corresponding position in the
 stream and the receiver MUST NOT expect content (or request repair
 for any content) following that position in the stream.  Additional
 specifications MAY extend the functionality of the NORM stream
 transport mode by defining additional stream control codes.  These
 control codes are delivered to the recipient application reliably,
 in-order with respect to the streamed application data content.
 The "payload_msg_start" field serves one of two exclusive purposes.
 When the "payload_len" value is non-zero, the "payload_msg_start"
 field, when also set to a non-zero value, indicates that the
 associated "payload_data" content contains an application-defined
 message boundary (start-of-message).  When such a message boundary is
 indicated, the first byte of an application-defined message, with
 respect to the "payload_data" field, will be found at an offset of
 "payload_msg_start - 1" bytes.  Thus, if a NORM_DATA payload for a
 NORM_OBJECT_STREAM contains the start of an application message at
 the first byte of the "payload_data" field, the value of the
 "payload_msg_start" field will be '1'.  NORM implementations SHOULD
 provide sender stream applications with a capability to mark message
 boundaries in this manner.  Similarly, the NORM receiver
 implementation SHOULD enable the application to recover such message
 boundary information.  This enables NORM receivers to "synchronize"
 reliable reception of transmitted message stream content in a
 meaningful way (i.e., meaningful to the application) at any time,
 whether joining a session already in progress, or departing the
 session and returning.  Note that if the value of the
 "payload_msg_start" field is ZERO, no message boundary is present.
 The "payload_msg_start" value will always be less than or equal to
 the "payload_len" value except for the special case of "payload_len =
 0", which indicates the "payload_msg_start" field be instead
 interpreted as a "stream_control_code"
 The "payload_offset" field indicates the relative byte position (from
 the sender stream transmission start) of the source content contained
 in the "payload_data" field.  Note that for long-lived streams, the
 "payload_offset" field will 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

Adamson, et al. Standards Track [Page 27] RFC 5740 NORM Protocol November 2009

 padding for data segments with a 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 can possibly vary on a per-object
 basis.  The NormSegmentSize SHOULD be configurable by the sender
 application prior to session participation as needed for network
 topology 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 block partitioning algorithm described in the
 FEC Building Block [RFC5052] document.  For NORM_OBJECT_STREAM
 objects, the length and offset is obtained from the segment's
 corresponding embedded "payload_len" and "payload_offset" fields.

4.2.2. NORM_INFO Message

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

Adamson, et al. Standards Track [Page 28] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=1|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     flags     |     fec_id    |     object_transport_id       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                header_extensions (if applicable)              |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         payload_data                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 8: NORM_INFO Message Format
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM common message header as described in Section 4.1.  The
 value of the "hdr_len" field when no header extensions are present is
 4.
 The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
 "object_transport_id" fields carry the same information and serve the
 same purpose as NORM_DATA messages.  These values allow the receiver
 to prepare appropriate buffering, etc., for further transmissions
 from the sender when NORM_INFO is the first message received.
 As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
 MAY be optionally applied to NORM_INFO messages.  To conserve
 protocol overhead, NORM implementations MAY apply the EXT_FTI when
 used to NORM_INFO messages only and not to NORM_DATA messages.
 The NORM_INFO "payload_data" field contains sender application-
 defined content that can be used by receiver applications for various
 purposes as described above.

4.2.3. NORM_CMD Messages

 NORM_CMD messages are transmitted by senders to perform a number of
 different protocol functions.  This includes functions such as round-
 trip timing collection, congestion control functions, synchronization
 of sender/receiver repair "windows", and notification of sender
 status.  A core set of NORM_CMD messages is enumerated.
 Additionally, a range of command types remain available for potential

Adamson, et al. Standards Track [Page 29] RFC 5740 NORM Protocol November 2009

 application-specific use.  Some NORM_CMD types can have dynamic
 content attached.  Any attached content will be limited to the
 maximum length of the sender NormSegmentSize to retain the atomic
 nature of the commands.  All NORM_CMD messages begin with a common
 set of fields, after the usual NORM message common header.  The
 standard NORM_CMD fields are:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    sub-type   |                                               |
   +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 9: NORM_CMD Standard Fields
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM common message header as described in Section 4.1.  The
 value of the "hdr_len" field for NORM_CMD messages without header
 extensions present depends upon the "sub-type" field.
 The "instance_id", "grtt", "backoff", and "gsize" fields provide the
 same information and serve the same purpose as NORM_DATA and
 NORM_INFO messages.  The "sub-type" field indicates the type of
 command to follow.  The remainder of the NORM_CMD message is
 dependent upon the command sub-type.  NORM command sub-types include:
 +-----------------------+----------+--------------------------------+
 | Command               | Sub-type | Purpose                        |
 +-----------------------+----------+--------------------------------+
 | NORM_CMD(FLUSH)       |     1    | Used to indicate sender        |
 |                       |          | temporary end-of-transmission. |
 |                       |          | (Assists in robustly           |
 |                       |          | initiating outstanding repair  |
 |                       |          | requests from receivers).  May |
 |                       |          | also be optionally used to     |
 |                       |          | collect positive               |
 |                       |          | acknowledgment of reliable     |
 |                       |          | reception from a subset of     |
 |                       |          | receivers.                     |
 | NORM_CMD(EOT)         |     2    | Used to indicate sender        |
 |                       |          | permanent end-of-transmission. |

Adamson, et al. Standards Track [Page 30] RFC 5740 NORM Protocol November 2009

 | NORM_CMD(SQUELCH)     |     3    | Used to advertise sender's     |
 |                       |          | current repair window in       |
 |                       |          | response to out-of-range NACKs |
 |                       |          | from receivers.                |
 | NORM_CMD(CC)          |     4    | Used for GRTT measurement and  |
 |                       |          | collection of congestion       |
 |                       |          | control feedback.              |
 | NORM_CMD(REPAIR_ADV)  |     5    | Used to advertise sender's     |
 |                       |          | aggregated repair/feedback     |
 |                       |          | state for suppression of       |
 |                       |          | unicast feedback from          |
 |                       |          | receivers.                     |
 | NORM_CMD(ACK_REQ)     |     6    | Used to request                |
 |                       |          | application-defined positive   |
 |                       |          | acknowledgment from a list of  |
 |                       |          | receivers (OPTIONAL).          |
 | NORM_CMD(APPLICATION) |     7    | Used for application-defined   |
 |                       |          | purposes that need to          |
 |                       |          | temporarily preempt or         |
 |                       |          | supplement data transmission   |
 |                       |          | (OPTIONAL).                    |
 +-----------------------+----------+--------------------------------+

4.2.3.1. NORM_CMD(FLUSH) Message

 The NORM_CMD(FLUSH) command is sent when the sender reaches the end
 of all data content and pending repairs it has queued for
 transmission.  This can indicate either a temporary or permanent end-
 of-data transmission, but that the sender is still willing to respond
 to repair requests.  This command is repeated once per 2*GRTT_sender
 to excite the receiver set for any outstanding repair requests up to
 and including the transmission point indicated within the
 NORM_CMD(FLUSH) message.  The number of repeats is equal to
 NORM_ROBUST_FACTOR unless a list of receivers from which explicit
 positive acknowledgment is expected ("acking_node_list") is given.
 In that case, the "acking_node_list" is updated as acknowledgments
 are received and the NORM_CMD(FLUSH) is repeated according to the
 mechanism described in Section 5.5.3.  The greater the
 NORM_ROBUST_FACTOR, the greater the probability that all applicable
 receivers will be excited for acknowledgment or repair requests
 (NACKs) AND that the corresponding NACKs are delivered to the sender.
 A default value of NORM_ROBUST_FACTOR equal to 20 is RECOMMENDED.  If
 a NORM_NACK message interrupts the flush process, the sender SHALL
 re-initiate the flush process after any resulting repair
 transmissions are completed.
 Note that receivers also employ a timeout mechanism to self-initiate
 NACKing (if there are outstanding repair needs) when no messages of

Adamson, et al. Standards Track [Page 31] RFC 5740 NORM Protocol November 2009

 any type are received from a sender.  This inactivity timeout is
 related to the NORM_CMD(FLUSH) and NORM_ROBUST_FACTOR and is
 specified in Section 5.3.  Receivers SHALL self-initiate the NACK
 repair process when the inactivity timeout has expired for a specific
 sender and the receiver has pending repairs needed from that sender.
 With a sufficiently large NORM_ROBUST_FACTOR value, data content is
 delivered with a high assurance of reliability.  The penalty of a
 large NORM_ROBUST_FACTOR value is the potential transmission of
 excess NORM_CMD(FLUSH) messages and a longer inactivity timeout for
 receivers to self-initiate a terminal NACK process.
 For finite-sized transport objects such as NORM_OBJECT_DATA and
 NORM_OBJECT_FILE, the flush process (if there are no further pending
 objects) occurs at the end of these objects.  Thus, FEC repair
 information is always available for repairs in response to repair
 requests elicited by the flush command.  However, for
 NORM_OBJECT_STREAM, the flush can occur at any time, including in the
 middle of a FEC coding block if systematic FEC codes are employed.
 In this case, the sender will not yet be able to provide FEC parity
 content for the concurrent coding block and will be limited to
 explicitly repairing the stream with source data content for that
 block.  Applications that anticipate frequent flushing of stream
 content SHOULD be judicious in the selection of the FEC coding block
 size (i.e., do not use a very large coding block size if frequent
 flushing occurs).  For example, a reliable multicast application
 transmitting an ongoing series of intermittent, relatively small
 messages will need to trade-off using the NORM_OBJECT_DATA paradigm
 versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
 block size.  This is analogous to application trade-offs for other
 transport protocols such as the selection of different TCP modes of
 operation such as "no delay", etc.

Adamson, et al. Standards Track [Page 32] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  sub-type = 1 |    fec_id     |      object_transport_id      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         fec_payload_id                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                acking_node_list (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 10: NORM_CMD(FLUSH) Message Format
 The "version", "type", "hdr_len", "sequence", and "source_id" fields
 form the NORM common message header as described in Section 4.1.  In
 addition to the NORM common message header and standard NORM_CMD
 fields, the NORM_CMD(FLUSH) message contains fields to identify the
 current status and logical transmit position of the sender.
 The "fec_id" field indicates the FEC type used for the flushing
 "object_transport_id" and implies the size and format of the
 "fec_payload_id" field.  Note the "hdr_len" value for the
 NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
 field when no header extensions are present.
 The "object_transport_id" and "fec_payload_id" fields indicate the
 sender's current logical "transmit position".  These fields are
 interpreted in the same manner as in the NORM_DATA message type.
 Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
 their completion state THROUGH (including) this transmission
 position.  If receivers have outstanding repair needs in this range,
 they SHALL initiate the NORM NACK Repair Process as described in
 Section 5.3.  If receivers have no outstanding repair needs, no
 response to the NORM_CMD(FLUSH) is generated.
 For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
 MUST request "explicit-only" repair of the identified
 "source_block_number" if the given "encoding_symbol_id" is less than
 the "source_block_len".  This condition indicates the sender has not
 yet completed encoding the corresponding FEC block and parity content
 is not yet available.  An "explicit-only" repair request consists of

Adamson, et al. Standards Track [Page 33] RFC 5740 NORM Protocol November 2009

 NACK content for the applicable "source_block_number" that does not
 include any requests for parity-based repair.  This allows NORM
 sender applications to "flush" an ongoing stream of transmission when
 needed, even if in the middle of a FEC block.  Once the sender
 resumes stream transmission and passes the end of the pending coding
 block, subsequent NACKs from receivers SHALL request parity-based
 repair as usual.  Note that the use of a systematic FEC code is
 assumed here.  Note that a sender has the option of arbitrarily
 shortening a given code block when such an application "flush"
 occurs.  In this case, the receiver will request explicit repair, but
 the sender MAY provide FEC-based repair (parity segments) in
 response.  These parity segments MUST contain the corrected
 "source_block_len" for the shortened block and that
 "source_block_len" associated with segments containing parity content
 SHALL override the previously advertised "source_block_len".
 Similarly, the "source_block_len" associated with the highest ordinal
 "encoding_symbol_id" SHALL take precedence over prior symbols when a
 difference (e.g., due to code shortening at the sender) occurs.
 Normal receiver NACK initiation and construction is discussed in
 detail in Section 5.3.
 The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
 for receivers from which the sender is requesting explicit positive
 acknowledgment of reception up through the transmission point
 identified by the "object_transport_id" and "fec_payload_id" fields.
 The length of the list can be inferred from the length of the
 received NORM_CMD(FLUSH) message.  When the "acking_node_list" is
 present, the lightweight positive acknowledgment process described in
 Section 5.5.3 SHALL be observed.

4.2.3.2. NORM_CMD(EOT) Message

 The NORM_CMD(EOT) command is sent when the sender reaches permanent
 end-of-transmission with respect to the NormSession and will not
 respond to further repair requests.  This allows receivers to
 gracefully reach closure of operation with this sender (without
 requiring any timeout) and free any resources that are no longer
 needed.  The NORM_CMD(EOT) command SHOULD be sent with the same
 robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
 high assurance of reception by the receiver set.

Adamson, et al. Standards Track [Page 34] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  sub-type = 2 |                    reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 11: NORM_CMD(EOT) Message Format
 The value of the "hdr_len" field for NORM_CMD(EOT) messages without
 header extensions present is 4.  The "reserved" field is reserved for
 future use and MUST be set to an all ZERO value.  Receivers MUST
 ignore the "reserved" field.

4.2.3.3. NORM_CMD(SQUELCH) Message

 The NORM_CMD(SQUELCH) command is transmitted in response to outdated
 or invalid NORM_NACK content received by the sender.  Invalid
 NORM_NACK content consists of repair requests for NormObjects for
 which the sender is unable or unwilling to provide repair.  This
 includes repair requests for outdated objects, aborted objects, or
 those objects that the sender previously transmitted marked with the
 NORM_FLAG_UNRELIABLE flag.  This command indicates to receivers what
 content is available for repair, thus serving as a description of the
 sender's current "repair window".  Receivers SHALL NOT generate
 repair requests for content identified as invalid by a
 NORM_CMD(SQUELCH).
 The NORM_CMD(SQUELCH) command is sent once per 2*GRTT_sender at the
 most.  The NORM_CMD(SQUELCH) advertises the current "repair window"
 of the sender by identifying the earliest (lowest) transmission point
 for which it will provide repair, along with an encoded list of
 objects from that point forward that are no longer valid for repair.
 This mechanism allows the sender application to cancel or abort
 transmission and/or repair of specific previously enqueued objects.
 The list also contains the identifiers for any objects within the
 repair window that were sent with the NORM_FLAG_UNRELIABLE flag set.
 In normal conditions, the NORM_CMD(SQUELCH) will be needed
 infrequently, and generally only to provide a reference repair window
 for receivers who have fallen "out-of-sync" with the sender due to
 extremely poor network conditions.
 The starting point of the invalid NormObject list begins with the

Adamson, et al. Standards Track [Page 35] RFC 5740 NORM Protocol November 2009

 lowest invalid NormTransportId greater than the current "repair
 window" start from the invalid NACK(s) that prompted the generation
 of the squelch.  The length of the list is limited by the sender's
 NormSegmentSize.  This allows the receivers to learn the status of
 the sender's applicable object repair window with minimal
 transmission of NORM_CMD(SQUELCH) commands.  The format of the
 NORM_CMD(SQUELCH) message is:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | sub-type = 3  |     fec_id    |      object_transport_id      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         fec_payload_id                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        invalid_object_list                    |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 12: NORM_CMD(SQUELCH) Message Format
 In addition to the NORM common message header and standard NORM_CMD
 fields, the NORM_CMD(SQUELCH) message contains fields to identify the
 earliest logical transmit position of the sender's current repair
 window and an "invalid_object_list" beginning with the index of the
 logically earliest invalid repair request from the offending NACK
 message that initiated the NORM_CMD(SQUELCH) transmission.  The value
 of the "hdr_len" field when no extensions are present is 4 plus the
 size of the "fec_payload_id" field that is dependent upon the FEC
 scheme identified by the "fec_id" field.
 The "object_transport_id" and "fec_payload_id" fields are
 concatenated to indicate the beginning of the sender's current repair
 window (i.e., the logically earliest point in its transmission
 history for which the sender can provide repair).  The "fec_id" field
 implies the size and format of the "fec_payload_id" field.  This
 serves as an advertisement of a "synchronization" point for receivers
 to request repair.  Note, that while an "encoding_symbol_id" MAY be
 included in the "fec_payload_id" field, the sender's repair window
 SHOULD be aligned on FEC coding block boundaries and thus the
 "encoding_symbol_id" SHOULD be zero.

Adamson, et al. Standards Track [Page 36] RFC 5740 NORM Protocol November 2009

 The "invalid_object_list" is a list of 16-bit NormTransportIds that,
 although they are within the range of the sender's current repair
 window, are no longer available for repair from the sender.  For
 example, a sender application MAY dequeue an out-of-date object even
 though it is still within the repair window.  The total size of the
 "invalid_object_list" content can be determined from the packet's
 payload length and is limited to a maximum of the NormSegmentSize of
 the sender.  Thus, for very large repair windows, it is possible that
 a single NORM_CMD(SQUELCH) message cannot include the entire set of
 invalid objects in the repair window.  In this case, the sender SHALL
 ensure that the list begins with a NormTransportId that is greater
 than or equal to the lowest ordinal invalid NormTransportId from the
 NACK message(s) that prompted the NORM_CMD(SQUELCH) generation.  The
 NormTransportId in the "invalid_object_list" MUST be ordinally
 greater than the "object_transport_id" marking the beginning of the
 sender's repair window.  This ensures convergence of the squelch
 process, even if multiple invalid NACK/squelch iterations are
 required.  This explicit description of invalid content within the
 sender's current window allows the sender application (most notably
 for discrete object transport) to arbitrarily invalidate (i.e.,
 dequeue) portions of enqueued content (e.g., certain objects) for
 which it no longer wishes to provide reliable transport.

4.2.3.4. NORM_CMD(CC) Message

 The NORM_CMD(CC) message contains fields to enable sender-to-group
 GRTT measurement and to excite the group for congestion control
 feedback.  A baseline NORM congestion control scheme (NORM-CC), based
 on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of
 RFC 4654 is fully specified in Section 5.5.2 of this document.  The
 NORM_CMD(CC) message is usually transmitted as part of NORM-CC
 operation.  A NORM header extension is defined below to be used with
 the NORM_CMD(CC) message to support NORM-CC operation.  Different
 header extensions MAY be defined for the NORM_CMD(CC) (and/or other
 NORM messages as needed) to support alternative congestion control
 schemes in the future.  If NORM is operated in a network where
 resources are explicitly dedicated to the NORM session and therefore
 congestion control operation is disabled, the NORM_CMD(CC) message is
 then used solely for GRTT measurement and MAY be sent less frequently
 than with congestion control operation.

Adamson, et al. Standards Track [Page 37] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |            sequence           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  sub-type = 4 |    reserved   |          cc_sequence          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         send_time_sec                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        send_time_usec                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               header extensions (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  cc_node_list (if applicable)                 |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 13: NORM_CMD(CC) Message Format
 The NORM common message header and standard NORM_CMD fields serve
 their usual purposes.  The value of the "hdr_len" field when no
 header extensions are present is 6.
 The "reserved" field is for potential future use and MUST be set to
 ZERO in this version of the NORM protocol and its baseline NORM-CC
 congestion control scheme.  It is possible for alternative congestion
 control schemes to use the NORM_CMD(CC) message defined here and
 leverage the "reserved" field for scheme-specific purposes.
 The "cc_sequence" field is a sequence number applied by the sender.
 For NORM-CC operation, it is used to provide functionality equivalent
 to the "feedback round number" (fb_nr) described in RFC 4654.  The
 most recently received "cc_sequence" value is recorded by receivers
 and can be fed back to the sender in congestion control feedback
 generated by the receivers for that sender.  The "cc_sequence" number
 can also be used in NORM implementations to assess how recently a
 receiver has received NORM_CMD(CC) probes from the sender.  This can
 be useful instrumentation for complex or experimental multicast
 routing environments.
 The "send_time" field is a timestamp indicating the time that the
 NORM_CMD(CC) message was transmitted.  This consists of a 64-bit
 field containing 32-bits with the time in seconds ("send_time_sec")

Adamson, et al. Standards Track [Page 38] RFC 5740 NORM Protocol November 2009

 and 32-bits with the time in microseconds ("send_time_usec") since
 some reference time the source maintains (usually 00:00:00, 1 January
 1970).  The byte ordering of the fields is "Big Endian" network
 order.  Receivers use this timestamp adjusted by the amount of delay
 from the time they received the NORM_CMD(CC) message to the time of
 their response as the "grtt_response" portion of NORM_ACK and
 NORM_NACK messages generated.  This allows the sender to evaluate
 round-trip times to different receivers for congestion control and
 other (e.g., GRTT determination) purposes.
 To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
 a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
 group of the sender's current transmission rate.  This is used along
 with the loss detection "sequence" field of all NORM sender messages
 and the NORM_CMD(CC) GRTT collection process to support NORM-CC
 congestion control operation.  The format of this header extension is
 as follows:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    het = 128  |    reserved   |           send_rate           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The "send_rate" field indicates the sender's current transmission
 rate in bytes per second.  The 16-bit "send_rate" field consists of
 12 bits of mantissa in the most significant portion and 4 bits of
 base 10 integer exponent (E) information in the least significant
 portion.  The 12-bit mantissa portion of the field is scaled such
 that a base 10 mantissa (M) floating point value of 0.0 corresponds
 to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of
 the 16-bit "send_rate" field.  Thus:
        send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;
 For example, to represent a transmission rate of 256 kbit/s (3.2e+04
 bytes per second), the lower 4 bits of the 16-bit field contain a
 value of 0x04 to represent the exponent (E) while the upper 12 bits
 contain a value of 0x51f (M) as determined from the equation given
 above:
      send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
                = (0x51f << 4) | 0x4
                = 0x51f4
 To decode the "send_rate" field, the following equation can be used:
 value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f))
 Note the maximum transmission rate that can be represented by this

Adamson, et al. Standards Track [Page 39] RFC 5740 NORM Protocol November 2009

 scheme is approximately 9.99e+15 bytes per second.
 When this extension is present, a "cc_node_list" might be attached as
 the payload of the NORM_CMD(CC) message.  The presence of this header
 extension also implies that NORM receivers MUST respond according to
 the procedures described in Section 5.5.2.
 The "cc_node_list" consists of a list of NormNodeIds and their
 associated congestion control status.  This includes the current
 limiting receiver (CLR) node, any potential limiting receiver (PLR)
 nodes that have been identified, and some number of receivers for
 which congestion control status is being provided, most notably
 including the receivers' current RTT measurement.  The maximum length
 of the "cc_node_list" provides for at least the CLR and one other
 receiver, but can be increased for more timely feedback to the group.
 The list length can be inferred from the length of the NORM_CMD(CC)
 message.
 Each item in the "cc_node_list" is in the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          cc_node_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    cc_flags   |     cc_rtt    |            cc_rate            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The "cc_node_id" is the NormNodeId of the receiver the item
 represents.
 The "cc_flags" field contains flags indicating the congestion control
 status of the indicated receiver.  The following flags are defined:
 +--------------------+-------+--------------------------------------+
 | Flag               | Value | Purpose                              |
 +--------------------+-------+--------------------------------------+
 | NORM_FLAG_CC_CLR   |  0x01 | Receiver is the current limiting     |
 |                    |       | receiver (CLR).                      |
 | NORM_FLAG_CC_PLR   |  0x02 | Receiver is a potential limiting     |
 |                    |       | receiver (PLR).                      |
 | NORM_FLAG_CC_RTT   |  0x04 | Receiver has measured RTT with       |
 |                    |       | respect to sender.                   |

Adamson, et al. Standards Track [Page 40] RFC 5740 NORM Protocol November 2009

 | NORM_FLAG_CC_START |  0x08 | Sender/receiver is in "slow start"   |
 |                    |       | phase of congestion control          |
 |                    |       | operation (i.e., the receiver has    |
 |                    |       | not yet detected any packet loss and |
 |                    |       | the "cc_rate" field is the           |
 |                    |       | receiver's actual measured receive   |
 |                    |       | rate).                               |
 | NORM_FLAG_CC_LEAVE |  0x10 | Receiver is imminently leaving the   |
 |                    |       | session and its feedback SHOULD not  |
 |                    |       | be considered in congestion control  |
 |                    |       | operation.                           |
 +--------------------+-------+--------------------------------------+
 The "cc_rtt" contains a quantized representation of the RTT as
 measured by the sender with respect to the indicated receiver.  This
 field is valid only if the NORM_FLAG_CC_RTT flag is set in the
 "cc_flags" field.  This one-byte field is a quantized representation
 of the RTT using the algorithm described in the Multicast NACK
 Building Block [RFC5401] document.
 The "cc_rate" field contains a representation of the receiver's
 current calculated (during steady-state congestion control operation)
 or twice its measured (during the slow start phase) congestion
 control rate.  This field is encoded and decoded using the same
 technique as described for the NORM_CMD(CC) "send_rate" field.

4.2.3.5. NORM_CMD(REPAIR_ADV) Message

 The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
 its aggregated repair state from NORM_NACK messages accumulated
 during a repair cycle and/or congestion control feedback received.
 This message is sent only when the sender has received NORM_NACK
 and/or NORM_ACK(CC) (when congestion control is enabled) messages via
 unicast transmission instead of multicast.  By relaying this
 information to the receiver set, suppression of feedback can be
 achieved even when receivers are unicasting that feedback instead of
 multicasting it among the group [NormFeedback].

Adamson, et al. Standards Track [Page 41] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | sub-type = 5  |     flags     |            reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               header extensions (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       repair_adv_payload                      |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 14: NORM_CMD(REPAIR_ADV) Message Format
 The "instance_id", "grtt", "backoff", "gsize", and "sub-type" fields
 serve the same purpose as in other NORM_CMD messages.  The value of
 the "hdr_len" field when no extensions are present is 4.
 The "flags" field provides information on the NORM_CMD(REPAIR_ADV)
 content.  There is currently one NORM_CMD(REPAIR_ADV) flag defined:
                   NORM_REPAIR_ADV_FLAG_LIMIT = 0x01
 This flag is set by the sender when it is unable to fit its full
 current repair state into a single NormSegmentSize.  If this flag is
 set, receivers SHALL limit their NACK response to generating NACK
 content only up through the maximum ordinal transmission position
 (objectTransportId::fecPayloadId) included in the
 "repair_adv_content".
 When congestion control operation is enabled, a header extension
 SHOULD be applied to the NORM_CMD(REPAIR_ADV) representing the most
 limiting (in terms of congestion control feedback suppression)
 congestion control response.  This allows the NORM_CMD(REPAIR_ADV)
 message to suppress receiver congestion control responses as well as
 NACK feedback messages.  The field is defined as a header extension
 so that alternative congestion control schemes can be used for NORM
 without revision to this document.  A NORM-CC Feedback Header
 Extension (EXT_CC) is defined to encapsulate congestion control
 feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
 messages.  If another congestion control technique (e.g., Pragmatic
 General Multicast Congestion Control (PGMCC) [PgmccPaper]) is used

Adamson, et al. Standards Track [Page 42] RFC 5740 NORM Protocol November 2009

 within a NORM implementation, an additional header extension MAY need
 to be defined to encapsulate any required feedback content.  The
 NORM-CC Feedback Header Extension format is:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     het = 3   |    hel = 3    |          cc_sequence          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    cc_flags   |     cc_rtt    |            cc_loss            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            cc_rate            |          cc_reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The "cc_sequence" field contains the current greatest "cc_sequence"
 value receivers have received in NORM_CMD(CC) messages from the
 sender.  This information assists the sender in congestion control
 operation by providing an indicator of how current ("fresh") the
 receiver's round-trip measurement reference time is and whether the
 receiver has been successfully receiving recent congestion control
 probes.  For example, if it is apparent the receiver has not been
 receiving recent congestion control probes (and thus possibly other
 messages from the sender), the sender SHOULD choose to take
 congestion avoidance measures.  For NORM_CMD(REPAIR_ADV) messages,
 the sender SHALL set the "cc_sequence" field value to the value set
 in the last NORM_CMD(CC) message sent.
 The "cc_flags" field contains bits representing the receiver's state
 with respect to congestion control operation.  The possible values
 for the "cc_flags" field are those specified for the NORM_CMD(CC)
 message node list item flags.  These fields are used by receivers in
 controlling (suppressing as necessary) their congestion control
 feedback.  For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
 SHALL be set only when all feedback messages received by the sender
 have the flag set.  Similarly, the NORM_FLAG_CC_CLR or
 NORM_FLAG_CC_PLR SHALL be set only when no feedback has been received
 from non-CLR or non-PLR receivers.  And the NORM_FLAG_CC_LEAVE SHALL
 be set only when all feedback messages the sender has received have
 this flag set.  These heuristics for setting the flags in
 NORM_CMD(REPAIR_ADV) ensure the most effective suppression of
 receivers providing unicast feedback messages.
 The "cc_rtt" field SHALL be set to a default maximum value, and the
 NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
 received RTT measurement information.  When a receiver has received
 RTT measurement information, it SHALL set the "cc_rtt" value
 accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
 field.  For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the
 "cc_rtt" field value to the largest non-CLR/non-PLR RTT it has

Adamson, et al. Standards Track [Page 43] RFC 5740 NORM Protocol November 2009

 measured from receivers for the current feedback round.
 The "cc_loss" field represents the receiver's current packet loss
 fraction estimate for the indicated source.  The loss fraction is a
 value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
 packet loss.  The 16-bit "cc_loss" value is calculated by the
 following formula:
           "cc_loss" = floor(decimal_loss_fraction * 65535.0)
 For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
 field value to the largest non-CLR/non-PLR loss estimate it has
 received from receivers for the current feedback round.
 The "cc_rate" field represents the receiver's current local
 congestion control rate.  During "slow start", when the receiver has
 detected no loss, this value is set to twice the actual rate it has
 measured from the corresponding sender and the NORM_FLAG_CC_START is
 set in the "cc_flags" field.  Otherwise, the receiver calculates a
 congestion control rate based on its loss measurement and RTT
 measurement information (even if default) for the "cc_rate" field.
 For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
 field value to the lowest non-CLR/non-PLR "cc_rate" report it has
 received from receivers for the current feedback round.
 The "cc_reserved" field is reserved for future NORM protocol use.
 Currently, senders SHALL set this field to ZERO, and receivers SHALL
 ignore the content of this field.
 The "repair_adv_payload" is in exactly the same form as the
 "nack_content" of NORM_NACK messages and can be processed by
 receivers for suppression purposes in the same manner, with the
 exception of the condition when the NORM_REPAIR_ADV_FLAG_LIMIT is
 set.

4.2.3.6. NORM_CMD(ACK_REQ) Message

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

Adamson, et al. Standards Track [Page 44] RFC 5740 NORM Protocol November 2009

 described in Section 5.5.3.  The format of the NORM_CMD(ACK_REQ)
 message is:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | sub-type = 6  |    reserved   |    ack_type   |    ack_id     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       acking_node_list                        |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 15: NORM_CMD(ACK_REQ) Message Format
 The NORM common message header and standard NORM_CMD fields serve
 their usual purposes.  The value of the "hdr_len" field for
 NORM_CMD(ACK_REQ) messages with no header extension present is 4.
 The "ack_type" field indicates the type of acknowledgment being
 requested and thus implies rules for how the receiver will treat this
 request.  The following "ack_type" values are defined and are also
 used in NORM_ACK messages described later:
 +-----------------------+------------+------------------------------+
 | ACK Type              | Value      | Purpose                      |
 +-----------------------+------------+------------------------------+
 | NORM_ACK(CC)          | 1          | Used to identify NORM_ACK    |
 |                       |            | messages sent in response to |
 |                       |            | NORM_CMD(CC) messages.       |
 | NORM_ACK(FLUSH)       | 2          | Used to identify NORM_ACK    |
 |                       |            | messages sent in response to |
 |                       |            | NORM_CMD(FLUSH) messages.    |
 | NORM_ACK(RESERVED)    | 3-15       | Reserved for possible future |
 |                       |            | NORM protocol use.           |
 | NORM_ACK(APPLICATION) | 16-255     | Used at application's        |
 |                       |            | discretion.                  |
 +-----------------------+------------+------------------------------+
 The NORM_ACK(CC) value is provided for use only in NORM_ACKs
 generated in response to the NORM_CMD(CC) messages used in congestion
 control operation.  Similarly, the NORM_ACK(FLUSH) is provided for
 use only in NORM_ACKs generated in response to applicable
 NORM_CMD(FLUSH) messages.  NORM_CMD(ACK_REQ) messages with "ack_type"

Adamson, et al. Standards Track [Page 45] RFC 5740 NORM Protocol November 2009

 of NORM_ACK(CC) or NORM_ACK(FLUSH) SHALL NOT be generated by the
 sender.
 The NORM_ACK(RESERVED) range of "ack_type" values is provided for
 possible future NORM protocol use.
 The NORM_ACK(APPLICATION) range of "ack_type" values is provided so
 that NORM applications can implement application-defined, positively
 acknowledged commands that are able to leverage internal transmission
 and round-trip timing information available to the NORM protocol
 implementation.
 The "ack_id" provides a sequenced identifier for the given
 NORM_CMD(ACK_REQ) message.  This "ack_id" is returned in NORM_ACK
 messages generated by the receivers so that the sender can associate
 the response with its corresponding request.
 The "reserved" field is reserved for possible future protocol use and
 SHALL be set to ZERO by senders and ignored by receivers.
 The "acking_node_list" field contains the NormNodeIds of the current
 NORM receivers that are desired to provide positive acknowledgment
 (NORM_ACK) to this request.  The packet payload length implies the
 length of the "acking_node_list", and its length is limited to the
 sender NormSegmentSize.  The individual NormNodeId items are listed
 in network (Big Endian) byte order.  If a receiver's NormNodeId is
 included in the "acking_node_list", it SHALL schedule transmission of
 a NORM_ACK message as described in Section 5.5.3.

4.2.3.7. NORM_CMD(APPLICATION) Message

 This command allows the NORM application to robustly transmit
 application-defined commands.  The command message preempts any
 ongoing data transmission and is repeated up to NORM_ROBUST_FACTOR
 times at a rate of once per 2*GRTT_sender.  This rate of repetition
 allows the application to observe any response (if that is the
 application's purpose for the command) before it is repeated.
 Possible responses can include initiation of data transmission, other
 NORM_CMD(APPLICATION) messages, or even application-defined,
 positively acknowledged commands from other NormSession participants.
 The transmission of these commands will preempt data transmission
 when they are scheduled and can be multiplexed with ongoing data
 transmission.  This type of robustly transmitted command allows NORM
 applications to define a complete set of session control mechanisms
 with less state than the transfer of FEC-encoded reliable content
 needs while taking advantage of NORM transmission and round-trip
 timing information.

Adamson, et al. Standards Track [Page 46] RFC 5740 NORM Protocol November 2009

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=3|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          instance_id          |     grtt      |backoff| gsize |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | sub-type = 7  |                    reserved                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Application-Defined Content                 |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 16: NORM_CMD(APPLICATION) Message Format
 The NORM common message header and NORM_CMD fields are interpreted as
 previously described.  The value of the NORM_CMD(APPLICATION)
 "hdr_len" field when no header extensions are present is 4.
 The "Application-Defined Content" area contains information in a
 format at the discretion of the application.  The size of this
 payload SHALL be limited to a maximum of the sender's NormSegmentSize
 setting.  Upon reception, the NORM protocol implementation SHALL
 deliver the content to the receiver application.  Note that any
 detection of duplicate reception of a NORM_CMD(APPLICATION) message
 is the responsibility of the application.

4.3. Receiver Messages

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

4.3.1. NORM_NACK Message

 The principal purpose of NORM_NACK messages is for receivers to
 request repair of sender content via selective, negative
 acknowledgment upon detection of incomplete data.  NORM_NACK messages
 will be transmitted according to the rules of NORM_NACK generation
 and suppression described in Section 5.3.  NORM_NACK messages also
 contain additional fields to provide feedback to the sender(s) for
 purposes of round-trip timing collection and congestion control.
 The payload of NORM_NACK messages contains one or more repair

Adamson, et al. Standards Track [Page 47] RFC 5740 NORM Protocol November 2009

 requests for different objects or portions of those objects.  The
 NORM_NACK message format is as follows:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=4|    hdr_len    |            sequence           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           server_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           instance_id         |            reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       grtt_response_sec                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       grtt_response_usec                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               header extensions (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          nack_payload                         |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 17: NORM_NACK Message Format
 The NORM common message header fields serve their usual purposes.
 The value of the "hdr_len" field for NORM_NACK messages without
 header extensions present is 6.
 The "server_id" field identifies the NORM sender to which the
 NORM_NACK message is destined.
 The "instance_id" field contains the current session identifier given
 by the sender identified by the "server_id" field in its sender
 messages.  The sender SHOULD ignore feedback messages containing an
 invalid "instance_id" value.
 The "grtt_response" fields contain an adjusted version of the
 timestamp from the most recently received NORM_CMD(CC) message for
 the indicated NORM sender.  The format of the "grtt_response" is the
 same as the "send_time" field of the NORM_CMD(CC).  The
 "grtt_response" value is relative to the "send_time" the source
 provided with a corresponding NORM_CMD(CC) command.  The receiver
 adjusts the source's NORM_CMD(CC) "send_time" timestamp by adding the
 time delta from when the receiver received the NORM_CMD(CC) to when
 the NORM_NACK is transmitted in response to calculate the value in
 the "grtt_response" field.  This is the "receive_to_response_delta"

Adamson, et al. Standards Track [Page 48] RFC 5740 NORM Protocol November 2009

 value used in the following formula:
   grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta
 The receiver SHALL set the "grtt_response" to a ZERO value, to
 indicate it has not yet received a NORM_CMD(CC) message from the
 indicated sender, and the sender MUST ignore the "grtt_response" in
 this message.
 For NORM-CC operation, the NORM-CC Feedback Header Extension, as
 described in the NORM_CMD(REPAIR_ADV} message description, is added
 to NORM_NACK messages to provide feedback on the receiver's current
 state with respect to congestion control operation.  Alternative
 header extensions for congestion control feedback MAY be defined for
 alternative congestion control schemes for NORM use in the future.
 The "reserved" field is for potential future NORM use and SHALL be
 set to ZERO for this version of the protocol.
 The "nack_payload" of the NORM_NACK message specifies the repair
 needs of the receiver with respect to the NORM sender indicated by
 the "server_id" field.  The receiver constructs repair requests based
 on the NORM_DATA and/or NORM_INFO segments it needs from the sender
 to complete reliable reception up to the sender's transmission
 position at the moment the receiver initiates the NACK procedure as
 described in Section 5.3.  A single NORM Repair Request consists of a
 list of items, ranges, and/or FEC coding block erasure counts for
 needed NORM_DATA and/or NORM_INFO content.  Multiple repair requests
 can be concatenated within the "nack_payload" field of a NORM_NACK
 message.  A single NORM Repair Request can possibly include multiple
 "items", "ranges", or "erasure_counts".  In turn, the "nack_payload"
 field MAY contain multiple repair requests.  A single NORM Repair
 Request has the following format:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      form     |     flags     |             length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      repair_request_items                     |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 18: NORM Repair Request Format
 The "form" field indicates the type of repair request items given in
 the "repair_request_items" list.  Possible values for the "form"
 field include:

Adamson, et al. Standards Track [Page 49] RFC 5740 NORM Protocol November 2009

                    +--------------------+-------+
                    | Form               | Value |
                    +--------------------+-------+
                    | NORM_NACK_ITEMS    |   1   |
                    | NORM_NACK_RANGES   |   2   |
                    | NORM_NACK_ERASURES |   3   |
                    +--------------------+-------+
 A "form" value of NORM_NACK_ITEMS indicates each repair request item
 in the "repair_request_items" list is to be treated as an individual
 request.  A value of NORM_NACK_RANGES indicates the
 "repair_request_items" list consists of pairs of repair request items
 corresponding to the inclusive ranges of repair needs.  The
 NORM_NACK_ERASURES "form" indicates the repair request items are to
 be treated individually and the "encoding_symbol_id" portion of the
 "fec_payload_id" field of the repair request item (see below) is to
 be interpreted as an erasure count for the FEC coding block
 identified by the repair request item's "source_block_number".
 The "flags" field is currently used to indicate the level of data
 content for which the repair request items apply (i.e., an individual
 segment, entire FEC coding block, or entire transport object).
 Possible flag values include:
 +-------------------+--------+--------------------------------------+
 | Flag              |  Value | Purpose                              |
 +-------------------+--------+--------------------------------------+
 | NORM_NACK_SEGMENT |  0x01  | Indicates the listed segment(s) or   |
 |                   |        | range of segments needed as repair.  |
 | NORM_NACK_BLOCK   |  0x02  | Indicates the listed block(s) or     |
 |                   |        | range of blocks in entirety that are |
 |                   |        | needed as repair.                    |
 | NORM_NACK_INFO    |  0x04  | Indicates NORM_INFO is needed as     |
 |                   |        | repair for the listed object(s).     |
 | NORM_NACK_OBJECT  |  0x08  | Indicates the listed object(s) or    |
 |                   |        | range of objects in entirety are     |
 |                   |        | needed as repair.                    |
 +-------------------+--------+--------------------------------------+
 When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
 "fec_payload_id" fields are used to determine which sets or ranges of
 individual NORM_DATA segments are needed to repair content at the
 receiver.  When the NORM_NACK_BLOCK flag is set, this indicates the
 receiver is completely missing the indicated coding block(s), and
 that transmissions sufficient to repair the indicated block(s) in
 their entirety are needed.  When the NORM_NACK_INFO flag is set, this
 indicates the receiver is missing the NORM_INFO segment for the
 indicated "object_transport_id".  Note the NORM_NACK_INFO can be set

Adamson, et al. Standards Track [Page 50] RFC 5740 NORM Protocol November 2009

 in combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags,
 or can be set alone.  When the NORM_NACK_OBJECT flag is set, this
 indicates the receiver is missing the entire NormTransportObject
 referenced by the "object_transport_id".  This also implicitly
 requests any available NORM_INFO for the NormObject, if applicable.
 The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
 is set.
 The "length" field value is the length in bytes of the
 "repair_request_items" field.
 The "repair_request_items" field consists of a list of individual or
 range pairs of transport data unit identifiers in the following
 format.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     fec_id    |   reserved    |      object_transport_id      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        fec_payload_id                         |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 19: NORM Repair Request Item Format
 The "fec_id" indicates the FEC type and can be used to determine the
 format of the "fec_payload_id" field.  The "reserved" field is kept
 for possible future use and SHALL be set to a ZERO value and ignored
 by NORM nodes processing NACK content.
 The "object_transport_id" corresponds to the NormObject for which
 repair is being requested, and the "fec_payload_id" identifies the
 specific FEC coding block and/or segment being requested.  When the
 NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
 is ignored.  When the NORM_NACK_BLOCK flag is set, only the FEC code
 block identifier portion of the "fec_payload_id" is to be
 interpreted.
 The format of the "fec_payload_id" field depends upon the "fec_id"
 field value.
 When the receiver's repair needs dictate that different forms (mixed
 ranges and/or individual items) or types (mixed specific segments
 and/or blocks or objects in entirety) are needed to complete reliable
 transmission, multiple NORM Repair Requests with different "form" and
 or "flags" values can be concatenated within a single NORM_NACK
 message.  Additionally, NORM receivers SHALL construct NORM_NACK
 messages with their repair requests in ordinal order with respect to

Adamson, et al. Standards Track [Page 51] RFC 5740 NORM Protocol November 2009

 "object_transport_id" and "fec_payload_id" values.  The
 "nack_payload" size SHALL NOT exceed the NormSegmentSize for the
 sender to which the NORM_NACK is destined.
 NORM_NACK Content Examples:
 In these examples, a small block, systematic FEC code ("fec_id" =
 129) is assumed with a user data block length of 32 segments.  In
 Example 1, a list of individual NORM_NACK_ITEMS repair requests is
 given.  In Example 2, a list of NORM_NACK_RANGES requests AND a
 single NORM_NACK_ITEMS request are concatenated to illustrate the
 possible content of a NORM_NACK message.  Note that FEC coding block
 erasure counts could also be provided in each case.  However, the
 erasure counts are not really necessary since the sender can easily
 determine the erasure count while processing the NACK content.
 However, the erasure count option can be useful for operation with
 other FEC codes or for intermediate system purposes.
  Example 1: NORM_NACK "nack_payload" for: Object 12, Coding Block 3,
                         Segments 2, 5, and 8
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   form = 1    | flags = 0x01  |       length  = 36            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 3                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 3                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 5     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 3                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 8     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Adamson, et al. Standards Track [Page 52] RFC 5740 NORM Protocol November 2009

  Example 2: NORM_NACK "nack_payload" for: Object 18, Coding Block 6,
 Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
                             1, Segment 3
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   form = 2    | flags = 0x01  |       length  = 24            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 6                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 5     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 6                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 10    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   form = 1    | flags = 0x05  |       length  = 12            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  fec_id = 129 |   reserved    |    object_transport_id = 19   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    source_block_number = 1                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    source_block_length = 32   |    encoding_symbol_id = 3     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.2. NORM_ACK Message

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

Adamson, et al. Standards Track [Page 53] RFC 5740 NORM Protocol November 2009

 application's discretion.  Implementations making use of application-
 defined positive acknowledgments MAY also make use of the
 "nack_payload" as needed, observing the constraint that the
 "nack_payload" field size be limited to a maximum of the
 NormSegmentSize for the sender to which the NORM_ACK is destined.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |version| type=5|    hdr_len    |          sequence             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           source_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           server_id                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           instance_id         |    ack_type  |     ack_id     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       grtt_response_sec                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       grtt_response_usec                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               header extensions (if applicable)               |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   ack_payload (if applicable)                 |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 20: NORM_ACK Message Format
 The NORM common message header fields serve their usual purposes.
 The value of the "hdr_len" field when no header extensions are
 present is 6.
 The "server_id", "instance_id", and "grtt_response" fields serve the
 same purpose as the corresponding fields in NORM_NACK messages.
 Header extensions can be applied to support congestion control
 feedback or other functions in the same manner.
 The "ack_type" field indicates the nature of the NORM_ACK message.
 This directly corresponds to the "ack_type" field of the
 NORM_CMD(ACK_REQ) message to which this acknowledgment applies.
 The "ack_id" field serves as a sequence number so the sender can
 verify a received NORM_ACK message actually applies to a current
 acknowledgment request.  The "ack_id" field is not used in the case
 of the NORM_ACK(CC) and NORM_ACK(FLUSH) acknowledgment types.
 The "ack_payload" format is a function of the "ack_type".  The

Adamson, et al. Standards Track [Page 54] RFC 5740 NORM Protocol November 2009

 NORM_ACK(CC) message has no attached content.  Only the NORM_ACK
 header applies.  In the case of NORM_ACK(FLUSH), a specific
 "ack_payload" format is defined:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     fec_id    |   reserved    |      object_transport_id      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        fec_payload_id                         |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The "object_transport_id" and "fec_payload_id" are used by the
 receiver to acknowledge applicable NORM_CMD(FLUSH) messages
 transmitted by the sender identified by the "server_id" field.
 The "ack_payload" of NORM_ACK messages for application-defined
 "ack_type" values is specific to the application but is limited in
 size to a maximum of the NormSegmentSize of the sender referenced by
 the "server_id".

4.4. General Purpose Messages

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

4.4.1. NORM_REPORT Message

 This is an OPTIONAL message generated by NORM participants.  This
 message can be used for periodic performance reports from receivers
 in experimental NORM implementations.  The format of this message is
 currently undefined.  Experimental NORM implementations MAY define
 NORM_REPORT formats as needed for test purposes.  These report
 messages SHOULD be disabled for interoperability testing between
 different compliant NORM implementations.

5. Detailed Protocol Operation

 This section describes the detailed interactions of senders and
 receivers participating in a NORM session.  A simple synopsis of the
 protocol operation is given here:
 1.  The sender periodically transmits NORM_CMD(CC) messages as needed
     to initialize and collect round-trip timing and congestion
     control feedback from the receiver set.

Adamson, et al. Standards Track [Page 55] RFC 5740 NORM Protocol November 2009

 2.  The sender transmits an ordinal set of NormObjects segmented in
     the form of NORM_DATA messages labeled with NormTransportIds and
     logically identified with FEC encoding block numbers and symbol
     identifiers.  When applicable, NORM_INFO messages MAY optionally
     precede the transmission of data content for NORM transport
     objects.
 3.  As receivers detect missing content from the sender, they
     initiate repair requests with NORM_NACK messages.  The receivers
     track the sender's most recent objectTransportId::fecPayloadId
     transmit position and NACK only for content that is ordinally
     prior to that current transmit position.  The receivers schedule
     random backoff timeouts before generating NORM_NACK messages and
     wait an appropriate amount of time before repeating the NORM_NACK
     if their repair request is not satisfied.
 4.  The sender aggregates repair requests from the receivers and
     logically "rewinds" its transmit position to send appropriate
     repair messages.  The sender sends repairs for the earliest
     ordinal transmit position first and maintains this ordinal repair
     transmission sequence.  FEC parity content not previously
     transmitted for the applicable FEC coding block is used for
     repair transmissions to the greatest extent possible.  If the
     sender exhausts its available FEC parity content on multiple
     repair cycles for the same coding block, it resorts to an
     explicit repair strategy (possibly using parity content) to
     complete repairs.  (The use of explicit repair is an exception in
     general protocol operation, but the possibility does exist for
     extreme conditions).  The sender immediately assumes transmission
     of new content once it has sent pending repairs.
 5.  The sender transmits NORM_CMD(FLUSH) messages when it reaches the
     end of enqueued transmit content and pending repairs.  Receivers
     respond to the NORM_CMD(FLUSH) messages with NORM_NACK
     transmissions (following the same suppression backoff timeout
     strategy as for data) if they need further repair.
 6.  The sender transmissions are subject to rate control limits
     determined by congestion control mechanisms.  In the baseline
     NORM-CC operation, each sender in a NormSession maintains its own
     independent congestion control state.  Receivers provide
     congestion control feedback in NORM_NACK and NORM_ACK messages.
     NORM_ACK feedback for congestion control purposes is governed
     using a suppression mechanism similar to that for NORM_NACK
     messages.
 While this overall concept is relatively simple, there are details to
 each of these aspects that need to be addressed for successful,

Adamson, et al. Standards Track [Page 56] RFC 5740 NORM Protocol November 2009

 efficient, robust, and scalable NORM protocol operation.

5.1. Sender Initialization and Transmission

 Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
 messages to collect round-trip timing and other information from the
 potential group.  If NORM-CC congestion control operation is enabled,
 the NORM-CC Rate header extension MUST be included in these messages.
 Congestion control operation SHALL be observed at all times when not
 operating using dedicated resources, like in the general Internet.
 Even if congestion control operation is disabled at the sender, it
 can be desirable to use the NORM_CMD(CC) messaging to collect
 feedback from the group using the baseline NORM-CC feedback
 mechanisms.  This proactive feedback collection can be used to
 establish a GRTT estimate prior to data transmission and potential
 NACK operation.
 In some cases, applications might need the sender to also proceed
 with data transmission immediately.  In other cases, the sender might
 wish to defer data transmission until it has received some feedback
 or request from the receiver set indicating receivers are indeed
 present.  Note, in some applications (e.g., web push), this
 indication MAY come out-of-band with respect to the multicast session
 via other means.  As noted, the periodic transmission of NORM_CMD(CC)
 messages MAY precede actual data transmission in order to have an
 initial GRTT estimate.
 With inclusion of the OPTIONAL NORM FEC Object Transmission
 Information Header Extension (EXT_FTI), the NORM protocol sender
 message headers can contain all information necessary to prepare
 receivers for subsequent reliable reception.  This includes FEC
 coding parameters, the sender NormSegmentSize, and other information.
 If this header extension is not used, it is presumed receivers have
 received the FEC Object Transmission Information via other means.
 Additionally, applications MAY leverage the use of NORM_INFO messages
 associated with the session data objects in the session to provide
 application-specific context information for the session and data
 being transmitted.  These mechanisms allow for operation with minimal
 pre-coordination among the senders and receivers.
 The NORM sender begins segmenting application-enqueued data into
 NORM_DATA segments and transmitting it to the group.  For objects of
 type NORM_OBJECT_DATA and NORM_OBJECT_FILE, the segmentation
 algorithm described in FEC Building Block [RFC5052] is RECOMMENDED.
 For objects of type NORM_OBJECT_STREAM, segmentation will typically
 be into uniform FEC coding block sizes, with individual segment sizes
 controlled by the application.  In most cases, the application and
 NORM implementation SHOULD strive to produce full-sized

Adamson, et al. Standards Track [Page 57] RFC 5740 NORM Protocol November 2009

 (NormSegmentSize) segments when possible.  The rate of transmission
 is controlled via congestion control mechanisms or is a fixed rate if
 desired for closed network operations.  The receivers participating
 in the multicast group provide feedback to the sender as needed.
 When the sender reaches the end of data it has enqueued for
 transmission or any pending repairs, it transmits a series of
 NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT_sender.  Similar
 to the end of each transmitted FEC coding block during transmission,
 receivers SHALL respond to these NORM_CMD(FLUSH) messages with
 additional repair requests as needed.  A protocol parameter
 NORM_ROBUST_FACTOR determines the number of flush messages sent.  If
 receivers request repair, the repair is provided, and flushing occurs
 again at the end of repair transmission.  The sender MAY attach an
 OPTIONAL "acking_node_list" to NORM_CMD(FLUSH) containing the
 NormNodeIds for receivers from which it expects explicit positive
 acknowledgment of reception.  The NORM_CMD(FLUSH) message MAY be also
 used for this OPTIONAL purpose any time prior to the end of data
 enqueued for transmission with the NORM_CMD(FLUSH) messages
 multiplexed with ongoing data transmissions.  The OPTIONAL NORM
 positive acknowledgment procedure is described in Section 5.5.3.

5.1.1. Object Segmentation Algorithm

 NORM senders and receivers MUST use a common algorithm for logically
 segmenting transport data into FEC encoding blocks and symbols so
 appropriate NACKs can be constructed to request repair of missing
 data.  NORM FEC coding blocks are comprised of multi-byte symbols
 (segments) transmitted in the payload of NORM_DATA messages.  Each
 NORM_DATA message will contain one or more source or encoding symbols
 identified by the "fec_payload_id" field, and the NormSegmentSize
 sender parameter defines the maximum size (in bytes) of the
 "payload_data" field containing the content (a "segment").  The FEC
 encoding type and associated parameters govern the source block size
 (number of source symbols per coding block, etc.).  NORM senders and
 receivers use these FEC parameters, along with the NormSegmentSize
 and transport object size to compute the source block structure for
 transport objects.  These parameters are provided in the FEC Object
 Transmission Information for each object.  The block partitioning
 algorithm described in the FEC Building Block [RFC5052] document is
 RECOMMENDED for use in computing a source block structure such that
 all source blocks are as close to being equal length as possible.
 This helps avoid the performance disadvantages of "short" FEC blocks.
 Note that this algorithm applies only to the statically sized
 NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
 the object size is fixed and predetermined.  For NORM_OBJECT_STREAM
 objects, the object is segmented according to the maximum source
 block length given in the FEC Transmission Information, unless the
 FEC Payload ID indicates an alternative size for a given block.

Adamson, et al. Standards Track [Page 58] RFC 5740 NORM Protocol November 2009

5.2. Receiver Initialization and Reception

 For typical operation, NORM receivers will join a specified multicast
 group and listen on a specific port number for sender transmissions.
 As the NORM receiver receives NORM_DATA messages, it will establish
 buffering state and provide content to its application as appropriate
 for the given data type.  The NORM protocol allows receivers to join
 and leave the group at will, although some applications might need
 receivers to be members of the group prior to start of data
 transmission.  Thus, different NORM applications MAY use different
 policies to constrain the impact of new receivers joining the group
 in the middle of a session.  For example, a useful implementation
 policy is for new receivers joining the group to limit or avoid
 repair requests for transport objects already in progress.  The NORM
 sender implementation MAY impose additional constraints to limit the
 ability of receivers to disrupt reliable multicast performance by
 joining, leaving, and rejoining the group often.  Different receiver
 "join policies" might be appropriate for different applications
 and/or scenarios.  For general purpose operation, a default policy
 where receivers are allowed to request repair only for coding blocks
 with a NormTransportId and FEC coding block number greater than or
 equal to the first non-repair NORM_DATA or NORM_INFO message received
 upon joining the group is RECOMMENDED.  For objects of type
 NORM_OBJECT_STREAM, it is RECOMMENDED the join policy constrain
 receivers to begin reliable reception at the current FEC coding block
 for which non-repair content is received.
 In some deployments, different multicast receivers might have
 differing quality of network connectivity.  Some receivers may suffer
 significantly poorer performance with very limited goodput due to low
 connection rate or substantial packet loss.  Similar to the "join
 policies" described above, a NORM sender implementation MAY choose to
 enforce different "service policies" to perhaps exclude exceptionally
 poorly performing (or otherwise badly behaving) receivers from the
 group.  The sender implementation could choose to ignore NACKs from
 such receivers and/or force advancement of its logical "repair
 window" (i.e., enforcing a minimal level of service) and use the
 NORM_CMD(SQUELCH) message to advise those poor performers of its
 advance.  Note in some cases, the application may need to support the
 "weakest member" regardless of the time needed to achieve reliable
 delivery.  When implemented, the protocol instantiation SHOULD expose
 controls to the set of "join" and/or "service" policies available to
 support the needs of different applications.

5.3. Receiver NACK Procedure

 When the receiver detects it is missing data from a sender's NORM
 transmissions, it initiates its NACKing procedure.  The NACKing

Adamson, et al. Standards Track [Page 59] RFC 5740 NORM Protocol November 2009

 procedure SHALL be initiated only at FEC coding block boundaries,
 NormObject boundaries, upon receipt of a NORM_CMD(FLUSH) message, or
 upon an "inactivity" timeout when NORM_DATA or NORM_INFO
 transmissions are no longer received from a previously active sender.
 The RECOMMENDED value of such an inactivity timeout is:
          T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTT_sender
 where the GRTT_sender value corresponds to the GRTT estimate
 advertised in the "grtt" field of NORM sender messages.  A minimum
 T_inactivity value of 1 second is RECOMMENDED.  The NORM receiver
 SHOULD reset this inactivity timer and repeat NACK initiation upon
 timeout for up to NORM_ROBUST_FACTOR times or more depending upon the
 application's need for persistence by its receivers.  It is also
 important receivers rescale the T_inactivity timeout as the sender's
 advertised GRTT changes.
 The NACKing procedure begins with a random backoff timeout.  The
 duration of the backoff timeout is chosen using the "RandomBackoff"
 algorithm described in the Multicast NACK Building Block [RFC5401]
 document using (K_sender*GRTT_sender) for the maxTime parameter and
 the sender advertised group size (GSIZE_sender) as the groupSize
 parameter.  NORM senders provide values for GRTT_sender, K_sender and
 GSIZE_sender via the "grtt", "backoff", and "gsize" fields of
 transmitted messages.  The GRTT_sender value is determined by the
 sender based on feedback it has received from the group while the
 K_sender and GSIZE_sender values can be determined by application
 requirements and expectations or ancillary information.  The backoff
 factor K_sender MUST be greater than one to provide for effective
 feedback suppression.  A value of K_sender = 4 is RECOMMENDED for the
 Any Source Multicast (ASM) model, while a value of K_sender = 6 is
 RECOMMENDED for Single Source Multicast (SSM) operation.
 Thus:
     T_backoff = RandomBackoff(K_sender*GRTT_sender, GSIZE_sender)
 To avoid the possibility of NACK implosion in the case of sender or
 network failure during SSM operation, the receiver SHALL
 automatically suppress its NACK and immediately enter the "holdoff"
 period described below when T_backoff is greater than (K_sender-
 1)*GRTT_sender.  Otherwise, the backoff period is entered and the
 receiver MUST accumulate external pending repair state from NORM_NACK
 messages and NORM_CMD(REPAIR_ADV) messages received.  At the end of
 the backoff time, the receiver SHALL generate a NORM_NACK message
 only if the following conditions are met:

Adamson, et al. Standards Track [Page 60] RFC 5740 NORM Protocol November 2009

 1.  The sender's current transmit position (in terms of
     objectTransportId::fecPayloadId) exceeds the earliest repair
     position of the receiver.
 2.  The repair state accumulated from NORM_NACK and
     NORM_CMD(REPAIR_ADV) messages does not equal or supersede the
     receiver's repair needs up to the sender transmission position at
     the time the NACK procedure (backoff timeout) was initiated.
 If these conditions are met, the receiver immediately generates a
 NORM_NACK message when the backoff timeout expires.  Otherwise, the
 receiver's NACK is considered to be "suppressed" and the message is
 not sent.  At this time, the receiver begins a "holdoff" period
 during which it constrains itself to not re-initiate the NACKing
 process.  The purpose of this timeout is to allow the sender worst-
 case time to respond to the repair needs before the receiver requests
 repair again.  The value of this "holdoff" timeout (T_rcvrHoldoff) as
 described in [RFC5401] is:
                T_rcvrHoldoff =(K_sender+2)*GRTT_sender
 The NORM_NACK message contains repair request content beginning with
 the lowest ordinal repair position of the receiver up through the
 coding block prior to the most recently heard ordinal transmission
 position for the sender.  If the size of the NORM_NACK content
 exceeds the sender's NormSegmentSize, the NACK content is truncated
 so the receiver only generates a single NORM_NACK message per NACK
 cycle for a given sender.  In summary, a single NACK message is
 generated containing the receiver's lowest ordinal repair needs.
 For each partially received FEC coding block requiring repair, the
 receiver SHALL, on its FIRST repair attempt for the block, request
 the parity portion of the FEC coding block beginning with the lowest
 ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
 "source_block_len") and request the number of FEC symbols
 corresponding to its data segment erasure count for the block.  On
 subsequent repair cycles for the same coding block, the receiver
 SHALL request only those repair symbols from the first set it has not
 yet received up to the remaining erasure count for that applicable
 coding block.  Note the sender might have transmitted other
 different, additional parity segments for other receivers that could
 also be used to satisfy the local receiver's erasure-filling needs.
 In the case where the erasure count for a partially received FEC
 coding block exceeds the maximum number of parity symbols available
 from the sender for the block (as indicated by the NORM_DATA
 "fec_num_parity" field), the receiver SHALL request all available
 parity segments plus the ordinally highest missing data segments
 needed to satisfy its total erasure needs for the block.  The goal of
 this strategy is for the overall receiver set to request a lowest

Adamson, et al. Standards Track [Page 61] RFC 5740 NORM Protocol November 2009

 common denominator set of repair symbols for a given FEC coding
 block.  This allows the sender to construct the most efficient repair
 transmission segment set and enables effective NACK suppression among
 the receivers even with uncorrelated packet loss.  This approach also
 does not demand synchronization among the receiver set in their
 repair requests for the sender.
 For FEC coding blocks or NormObjects missed in their entirety, the
 NORM receiver constructs repair requests with NORM_NACK_BLOCK or
 NORM_NACK_OBJECT flags set as appropriate.  The request for
 retransmission of NORM_INFO is accomplished by setting the
 NORM_NACK_INFO flag in a corresponding repair request.

5.4. Sender NACK Processing and Response

 The principal goal of the sender is to make forward progress in the
 transmission of data its application has enqueued.  However, the
 sender will need to occasionally "rewind" its logical transmission
 point to satisfy the repair needs of receivers who have NACKed.
 Aggregation of multiple NACKs is used to determine an optimal repair
 strategy when a NACK event occurs.  Since receivers initiate the NACK
 process on coding block or object boundaries, there is some loose
 degree of synchronization of the repair process even when receivers
 experience uncorrelated data loss.

5.4.1. Sender Repair State Aggregation

 When a sender is in its normal state of transmitting new data and
 receives a NACK, it begins a procedure to accumulate NACK repair
 state from NORM_NACK messages before beginning repair transmissions.
 Note that this period of aggregating repair state does NOT interfere
 with its ongoing transmission of new data.
 As described in [RFC5401], the period of time during which the sender
 aggregates NORM_NACK messages is equal to:
             T_sndrAggregate = (K_sender + 1) * GRTT_sender
 where K_sender is the backoff scaling value advertised to the
 receivers, and GRTT_sender is the sender's current estimate of the
 group's greatest round-trip time.  Note, for NORM unicast sessions,
 the T_sndrAggregate time can be set to ZERO since there is only one
 receiver.  Similarly, the K_sender value SHOULD be set to ZERO for
 NORM unicast sessions to minimize repair latency.
 When this period ends, the sender "rewinds" by incorporating the
 accumulated repair state into its pending transmission state and
 begins transmitting repair messages.  After pending repair

Adamson, et al. Standards Track [Page 62] RFC 5740 NORM Protocol November 2009

 transmissions are completed, the sender continues with new
 transmissions of any enqueued data.  Also, at this point in time, the
 sender begins a "holdoff" timeout during which time the sender
 constrains itself from initiating a new repair aggregation cycle,
 even if NORM_NACK messages arrive.  As described in [RFC5401], the
 value of this sender "holdoff" period is:
                   T_sndrHoldoff = (1 * GRTT_sender)
 If additional NORM_NACK messages are received during this sender
 "holdoff" period, the sender will immediately incorporate these late-
 arriving messages into its pending transmission state if, and only
 if, the NACK content is ordinally greater than the sender's current
 transmission position.  This "holdoff" time allows worst-case time
 for the sender to propagate its current transmission sequence
 position to the group, thus avoiding redundant repair transmissions.
 After the holdoff timeout expires, a new NACK accumulation period can
 be started (upon arrival of a NACK) in concert with the pending
 repair and new data transmission.  Recall receivers are not to
 initiate the NACK repair process until the sender's logical
 transmission position exceeds the lowest ordinal position of their
 repair needs.  With the new NACK aggregation period, the sender
 repeats the same process of incorporating accumulated repair state
 into its transmission plan and subsequently "rewinding" to transmit
 the lowest ordinal repair data when the aggregation period expires.
 Again, this is conducted in concert with ongoing new data and/or
 pending repair transmissions.

5.4.2. Sender FEC Repair Transmission Strategy

 The NORM sender SHOULD leverage transmission of FEC parity content
 for repair to the greatest extent possible.  Recall that receivers
 use a strategy to request a lowest common denominator of explicit
 repair (including parity content) in the formation of their NORM_NACK
 messages.  Before falling back to explicitly satisfying different
 receivers' repair needs, the sender can make use of the general
 erasure-filling capability of FEC-generated parity segments.  The
 sender can determine the maximum erasure-filling needs for individual
 FEC coding blocks from the NORM_NACK messages received during the
 repair aggregation period.  Then, if the sender has a sufficient
 number (less than or equal to the maximum erasure count) of
 previously unsent parity segments available for the applicable coding
 blocks, the sender can transmit these in lieu of the specific packets
 the receiver set has requested.  The sender SHOULD NOT resort to
 explicit transmission of the receiver set's repair needs until after
 exhausting its supply of "fresh" (unsent) parity segments for a given
 coding block.  In general, if a sufficiently powerful FEC code is
 used, the need for explicit repair will be an exception, and the

Adamson, et al. Standards Track [Page 63] RFC 5740 NORM Protocol November 2009

 fulfillment of reliable multicast can be accomplished quite
 efficiently.  However, the ability to resort to explicit repair
 allows the protocol to be continue to operate under even very extreme
 circumstances.
 NORM_DATA messages sent as repair transmissions SHALL be flagged with
 the NORM_FLAG_REPAIR flag.  This allows receivers to obey any
 policies limiting new receivers from joining the reliable
 transmission when only repair transmissions have been received.
 Additionally, the sender SHOULD flag NORM_DATA transmissions sent as
 explicit repair with the NORM_FLAG_EXPLICIT flag.
 Although NORM end system receivers do not make use of the
 NORM_FLAG_EXPLICIT flag, this message transmission status could be
 leveraged by intermediate systems wishing to "assist" NORM protocol
 performance.  If such systems are properly positioned with respect to
 reciprocal reverse-path multicast routing, they need to sub-cast only
 a sufficient count of non-explicit parity repairs to satisfy a
 multicast routing sub-tree's erasure-filling needs for a given FEC
 coding block.  When the sender has resorted to explicit repair, then
 the intermediate systems SHOULD sub-cast all of the explicit repair
 packets to those portions of the routing tree still requiring repair
 for a given coding block.  Note the intermediate systems will need to
 conduct repair state accumulation for sub-routes in a manner similar
 to the sender's repair state accumulation in order to have sufficient
 information to perform the sub-casting.  Additionally, the
 intermediate systems could perform NORM_NACK suppression/aggregation
 as it conducts this repair state accumulation for NORM repair cycles.
 The details of this type of operation are beyond the scope of this
 document, but this information is provided for possible future
 consideration.

5.4.3. Sender NORM_CMD(SQUELCH) Generation

 If the sender receives a NORM_NACK message for repair of data it is
 no longer supporting, the sender generates a NORM_CMD(SQUELCH)
 message to advertise its repair window and squelch any receivers from
 additional NACKing of invalid data.  The transmission rate of
 NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT_sender.  The
 "invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
 message SHALL begin with the lowest "object_transport_id" from the
 invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
 transmission.  The list includes as many lower ordinal invalid
 "object_transport_ids" that can fit for the NORM_CMD(SQUELCH) payload
 size to less than or equal to the sender's NormSegmentSize parameter.

Adamson, et al. Standards Track [Page 64] RFC 5740 NORM Protocol November 2009

5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation

 When a NORM sender receives NORM_NACK messages from receivers via
 unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
 advertise its accumulated repair state to the receiver set since the
 receiver set is not directly sharing their repair needs via multicast
 communication.  A NORM sender implementation MAY use a separate port
 number from the NormSession port number as the source port for its
 transmissions.  Thus, NORM receivers can direct any unicast feedback
 messages to this separate sender port number, distinct from the NORM
 session (or destination) port number.  Then, the NORM sender
 implementation can discriminate unicast feedback messages from
 multicast feedback messages when there is a mix of multicast and
 unicast feedback receivers.  The NORM_CMD(REPAIR_ADV) message is
 multicast to the receiver set by the sender.  The payload portion of
 this message has content in the same format as the NORM_NACK receiver
 message payload.  Receivers are then able to perform feedback
 suppression in the same manner as with NORM_NACK messages directly
 received from other receivers.  Note that the sender does not merely
 retransmit NACK content it receives, but instead transmits a
 representation of its aggregated repair state.  The transmission of
 NORM_CMD(REPAIR_ADV) messages is subject to the sender transmit rate
 limit and NormSegmentSize limitation.  When the NORM_CMD(REPAIR_ADV)
 message is of maximum size (as indicated by the flag
 NORM_REPAIR_ADV_FLAG_LIMIT), receivers SHALL consider the maximum
 ordinal transmission position value embedded in the message as the
 senders current transmission position and implicitly suppress
 requests for ordinally higher repair.  For congestion control
 operation, the sender will also need to provide any information
 needed so dynamic congestion control feedback can be suppressed among
 receivers.  This document specifies the NORM-CC Feedback Header
 Extension that is applied for baseline NORM-CC operation.  If other
 congestion control mechanisms are used within a NORM implementation,
 other header extensions MAY be defined.  Whatever content format is
 used for this purpose SHOULD ensure that maximum possible suppression
 state is conveyed to the receiver set.

5.5. Additional Protocol Mechanisms

 In addition to the principal function of data content transmission
 and repair, there are some other protocol mechanisms to help NORM to
 adapt to network conditions and play fairly with other coexistent
 protocols.

5.5.1. Group Round-Trip Time (GRTT) Collection

 For NORM receivers to appropriately scale backoff timeouts and the
 senders to use proper corresponding timeouts, the participants need

Adamson, et al. Standards Track [Page 65] RFC 5740 NORM Protocol November 2009

 to use a common timeout basis.  Each NORM sender monitors the round-
 trip time of active receivers and determines the greatest group
 round-trip time.  The sender advertises this GRTT estimate in every
 message it transmits so receivers have this value available for
 scaling their timers.  To measure the current GRTT, the sender
 periodically sends NORM_CMD(CC) messages containing a locally
 generated timestamp.  Receivers are expected to record this timestamp
 along with the time the NORM_CMD(CC) message is received.  Then, when
 the receivers generate feedback messages to the sender, an adjusted
 version of the sender timestamp is embedded in the feedback message
 (NORM_NACK or NORM_ACK).  The adjustment adds the amount of time the
 receiver held the timestamp before generating its response.  Upon
 receipt of this adjusted timestamp, the sender is able to calculate
 the round-trip time to that receiver.
 The round-trip time for each receiver is fed into an algorithm that
 assigns weights and smoothes the values for a conservative estimate
 of the GRTT.  The algorithm and methodology are described in the
 Multicast NACK Building Block [RFC5401] document in the section
 entitled "One-to-Many Sender GRTT Measurement".  A conservative
 estimate helps guarantee feedback suppression at a small cost in
 overall protocol repair delay.  The sender's current estimate of GRTT
 is advertised in the "grtt" field found in all NORM sender messages.
 The advertised GRTT is also limited to a minimum of the nominal
 inter-packet transmission time given the sender's current
 transmission rate and system clock granularity.  The reason for this
 additional limit is to keep the receiver somewhat event-driven by
 making sure the sender has had adequate time to generate any response
 to repair requests from receivers given transmit rate limitations due
 to congestion control or configuration.
 When the NORM-CC Rate header extension is present in NORM_CMD(CC)
 messages, the receivers respond to NORM_CMD(CC) messages as described
 in Section 5.5.2, "NORM Congestion Control Operation".  The
 NORM_CMD(CC) messages are periodically generated by the sender as
 described for congestion control operation.  This provides for
 proactive, but controlled, feedback from the group in the form of
 NORM_ACK messages.  This provides for GRTT feedback even if no
 NORM_NACK messages are being sent.  If operating without congestion
 control in a closed network, the NORM_CMD(CC) messages MAY be sent
 periodically without the NORM-CC Rate header extension.  In this
 case, receivers will only provide GRTT measurement feedback when
 NORM_NACK messages are generated since no NORM_ACK messages are
 generated.  In this case, the NORM_CMD(CC) messages MAY be sent less
 frequently, perhaps as little as once per minute, to conserve network
 capacity.  Note the NORM-CC Rate header extension MAY also be used to
 proactively solicit RTT feedback from the receiver group per
 congestion control operation even when the sender is not conducting

Adamson, et al. Standards Track [Page 66] RFC 5740 NORM Protocol November 2009

 congestion control rate adjustment.  NORM operation without
 congestion control SHOULD be considered only in closed networks.

5.5.2. NORM Congestion Control Operation

 This section describes baseline congestion control operation for the
 NORM protocol (NORM-CC).  The supporting NORM message formats and
 approach described here are an adaptation of the equation-based TCP-
 Friendly Multicast Congestion Control (TFMCC) approach [RFC4654].
 This congestion control scheme is REQUIRED for operation within the
 general Internet unless the NORM implementation is adapted to use
 another IETF-sanctioned reliable multicast congestion control
 mechanism.  With this TFMCC-based approach, the transmissions of NORM
 senders are controlled in a rate-based manner as opposed to window-
 based congestion control algorithms as in TCP.  However, it is
 possible the NORM protocol message set MAY alternatively be used to
 support a window-based multicast congestion control scheme such as
 PGMCC.  The details of such an alternative MAY be described
 separately or in a future revision of this document.  In either case
 (rate-based TFMCC or window-based PGMCC), successful control of
 sender transmission depends upon collection of sender-to-receiver
 packet loss estimates and RTTs to identify the congestion control
 bottleneck path(s) within the multicast topology and adjust the
 sender rate accordingly.  The receiver with loss and RTT estimates
 corresponding to the lowest resulting calculated transmission rate is
 identified as the "current limiting receiver" (CLR).  In the case of
 a tie (where candidate CLRs are within 10% of the same calculated
 rate), the receiver with the largest RTT value SHOULD be designated
 as the CLR.
 As described in [TcpModel], a steady-state sender transmission rate,
 to be "friendly" with competing TCP flows, can be calculated as:
                                  S
 Rsender = ----------------------------------------------------------
         T_rtt*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))
 where
 S = nominal transmitted packet size.  (In NORM, the "nominal" packet
 size can be determined by the sender as an exponentially weighted
 moving average (EWMA) of transmitted packet sizes to account for
 variable message sizes).
 T_rtt = RTT estimate of the current "current limiting receiver"
 (CLR).
 p = loss event fraction of the CLR.

Adamson, et al. Standards Track [Page 67] RFC 5740 NORM Protocol November 2009

 To support congestion control feedback collection and operation, the
 NORM sender periodically transmits NORM_CMD(CC) command messages.
 NORM_CMD(CC) messages are multiplexed with NORM data and repair
 transmissions and serve several purposes, they:
 1.  Stimulate explicit feedback from the general receiver set to
     collect congestion control information.
 2.  Communicate state to the receiver set on the sender's current
     congestion control status including details of the CLR.
 3.  Initiate rapid (immediate) feedback from the CLR in order to
     closely track the dynamics of congestion control for the current
     worst path in the group multicast topology.
 The format of the NORM_CMD(CC) message is described in Section 4.2.3
 of this document.  The NORM_CMD(CC) message contains information to
 allow measurement of RTTs, to inform the group of the congestion
 control CLR, and to provide feedback of individual RTT measurements
 to the receivers in the group.  The NORM_CMD(CC) also provides for
 exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
 nodes that might be determined administratively or possibly
 algorithmically based upon congestion control feedback.  PLR nodes
 are receivers that have been identified to have potential for
 (perhaps soon) becoming the CLR and thus immediate, up-to-date
 feedback is beneficial for congestion control performance.  The PLR
 list MAY be populated with a small number of receivers the sender
 identifies as approaching the CLR loss and delay conditions based on
 feedback from the group.

5.5.2.1. NORM_CMD(CC) Transmission

 The NORM_CMD(CC) message is transmitted periodically by the sender
 along with its normal data transmission.  Note the repeated
 transmission of NORM_CMD(CC) messages MAY be initiated some time
 before transmission of user data content at session startup.  This
 can be done to collect some estimation of the current state of the
 multicast topology with respect to group and individual RTT and
 congestion control state.
 A NORM_CMD(CC) message is immediately transmitted at sender startup.
 The interval of subsequent NORM_CMD(CC) message transmission is
 determined as follows:
 1.  By default, the interval is set according to the current sender
     GRTT estimate.  A startup initial value of GRTT_sender = 0.5
     seconds is RECOMMENDED when no feedback has yet been received
     from the group.

Adamson, et al. Standards Track [Page 68] RFC 5740 NORM Protocol November 2009

 2.  Until a CLR has been identified (based on previous receiver
     feedback) or when no data transmission is pending, the
     NORM_CMD(CC) interval is doubled up from its current interval to
     a maximum of once per 30 seconds.  This results in a low duty
     cycle for NORM_CMD(CC) probing when no CLR is identified or there
     is no pending data to transmit.
 3.  When a CLR has been identified (based on receiver feedback) and
     data transmission is pending, the probing interval is set to the
     RTT between the sender and the CLR (RTT_clr).
 4.  Additionally, when the data transmission rate is low with respect
     to the RTT_clr interval used for probing, the implementation
     SHOULD ensure no more than one NORM_CMD(CC) message is sent per
     NORM_DATA message when there is data pending transmission.  This
     ensures the transmission of this control message is not done to
     the exclusion of user data transmission.
 The NORM_CMD(CC) "cc_sequence" field is incremented with each
 transmission of a NORM_CMD(CC) command.  The greatest "cc_sequence"
 recently received by receivers is included in their feedback to the
 sender.  This allows the sender to determine the age of feedback to
 assist in congestion avoidance.
 The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
 message and the sender advertises its current transmission rate in
 the "send_rate" field.  The rate information is used by receivers to
 initialize loss estimation during congestion control startup or
 restart.
 The "cc_node_list" contains a list of entries identifying receivers
 and their current congestion control state (status "flags", "rtt",
 and "loss" estimates).  The list will be empty if the sender has not
 yet received any feedback from the group.  If the sender has received
 feedback, the list will minimally contain an entry identifying the
 CLR.  A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
 field to identify the CLR entry.  It is RECOMMENDED the CLR entry be
 the first in the list for implementation efficiency.  Additional
 entries in the list are used to provide sender-measured individual
 RTT estimates to receivers in the group.  The number of additional
 entries in this list is dependent upon the percentage of control
 traffic the sender application is willing to send with respect to
 user data message transmissions.  More entries in the list will allow
 the sender to be more responsive to congestion control dynamics.  The
 length of the list can be dynamically determined according to the
 current transmission rate and scheduling of NORM_CMD(CC) messages.
 The maximum length of the list corresponds to the sender's
 NormSegmentSize parameter for the session.  The inclusion of

Adamson, et al. Standards Track [Page 69] RFC 5740 NORM Protocol November 2009

 additional entries in the list based on receiver feedback is
 prioritized with the following rules:
 1.  Receivers that have not yet been provided an RTT measurement get
     first priority.  Of these, those with the greatest loss fraction
     receive precedence for list inclusion.
 2.  Secondly, receivers that have previously been provided an RTT
     measurement are included with receivers yielding the lowest
     calculated congestion rate getting precedence.
 There are "cc_flag" values in addition to NORM_FLAG_CC_CLR used for
 other congestion control functions.  The NORM_FLAG_CC_PLR flag value
 is used to mark additional receivers from which the sender would like
 to have immediate, non-suppressed feedback.  These can be receivers
 the sender algorithmically identified as potential future CLRs or
 have been pre-configured as potential congestion control points in
 the network.  The NORM_FLAG_CC_RTT indicates the validity of the
 "cc_rtt" field for the associated receiver node.  Normally, this flag
 will be set since the receivers in the list will typically be
 receivers from which the sender has received feedback.  However, in
 the case the NORM sender has been pre-configured with a set of PLR
 nodes, feedback from those receivers might not have yet been
 collected and thus the "cc_rtt" field does not contain a valid value
 when this flag is not set.  Similarly, a value of ZERO for the
 "cc_rate" field here MUST be treated as an invalid value and be
 ignored for the purposes of feedback suppression, etc.

5.5.2.2. NORM_CMD(CC) Feedback Response

 Receivers explicitly respond to NORM_CMD(CC) messages in the form of
 a NORM_ACK(RTT) message.  The goal of the congestion control feedback
 is to determine the receivers with the lowest congestion control
 rates.  Receivers marked as CLR or PLR nodes in the NORM_CMD(CC)
 "cc_node_list" immediately provide feedback in the form of a NORM_ACK
 to this message.  When a NORM_CMD(CC) is received, non-CLR or non-PLR
 nodes initiate random feedback backoff timeouts similar to those used
 when the receiver initiates a repair cycle (see Section 5.3) in
 response to detection of data loss.  The backoff timeout for the
 congestion control response is generated as follows:
    T_backoff = RandomBackoff(K_backoff * GRTT_sender, GSIZE_sender)
 The RandomBackoff() algorithm provides a truncated exponentially
 distributed random number and is described in the Multicast NACK
 Building Block [RFC5401] document.  The same backoff factor,
 K_backoff = K_sender, as used with NORM_NACK suppression is generally
 RECOMMENDED.  However, in cases where the application purposefully

Adamson, et al. Standards Track [Page 70] RFC 5740 NORM Protocol November 2009

 specifies a very small K_sender backoff factor to minimize the NACK
 repair process latency (trading off group size scalability), it is
 RECOMMENDED a larger backoff factor for congestion control feedback
 be maintained, since there can be a larger volume of congestion
 control feedback than NACKs in many cases and some congestion control
 feedback latency might be tolerable where reliable delivery latency
 is not.  As previously noted, a backoff factor value of K_sender = 4
 is generally RECOMMENDED for ASM operation and K_sender = 6 for SSM
 operation.  A receiver SHALL cancel the backoff timeout and thus its
 pending transmission of a NORM_ACK(RTT) message under the following
 conditions:
 1.  The receiver generates another feedback message (NORM_NACK or
     other NORM_ACK) before the congestion control feedback timeout
     expires (these messages will convey the current congestion
     control feedback information).
 2.  A NORM_CMD(CC) or other receiver feedback with an ordinally
     greater "cc_sequence" field value is received before the
     congestion control feedback timeout expires (this is similar to
     the TFMCC feedback round number).
 3.  When the T_backoff is greater than 1*GRTT_sender.  This prevents
     NACK implosion in the event of sender or network failure.
 4.  "Suppressing" congestion control feedback is heard from another
     receiver (in a NORM_ACK or NORM_NACK) or via a
     NORM_CMD(REPAIR_ADV) message from the sender.  The local
     receiver's feedback is "suppressed" if the rate of the competing
     feedback (Rfb) is sufficiently close to or less than the local
     receiver's calculated rate (Rcalc).  The local receiver's
     feedback is canceled when Rcalc > (0.9 * Rfb).  Also, note
     receivers that have not yet received an RTT measurement from the
     sender are suppressed only by other receivers that have not yet
     measured RTT.  Additionally, receivers whose RTT estimate has
     aged considerably (i.e., they haven't been included in the
     NORM_CMD(CC) "cc_node_list" in a long time) might wish to compete
     as a receiver with no prior RTT measurement after some long-term
     expiration period.
 When the backoff timer expires, the receiver SHALL generate a
 NORM_ACK(RTT) message to provide feedback to the sender and group.
 This message MAY be multicast to the group for most effective
 suppression in ASM topologies or unicast to the sender depending upon
 how the NORM protocol is deployed and configured.
 Whenever any feedback is generated (including this NORM_ACK(RTT)
 message), receivers include an adjusted version of the sender

Adamson, et al. Standards Track [Page 71] RFC 5740 NORM Protocol November 2009

 timestamp from the most recently received NORM_CMD(CC) message and
 its "cc_sequence" value in the corresponding NORM_ACK or NORM_NACK
 message fields.  For NORM-CC operation, any generated feedback
 message SHALL also contain the NORM-CC Feedback header extension.
 The receiver provides its current "cc_rate" estimate, "cc_loss"
 estimate, "cc_rtt" if known, and any applicable "cc_flags" via this
 header extension.
 During slow start (when the receiver has not yet detected loss from
 the sender), the receiver uses a value equal to two times its
 measured rate from the sender in the "cc_rate" field.  For steady-
 state congestion control operation, the receiver "cc_rate" value is
 from the equation-based value using its current loss event estimate
 and sender<->receiver RTT information.  (The GRTT_sender is used when
 the receiver has not yet measured its individual RTT.)
 The "cc_loss" field value reflects the receiver's current loss event
 estimate with respect to the sender in question.
 When the receiver has a valid individual RTT measurement, it SHALL
 include this value in the "cc_rtt" field.  The NORM_FLAG_CC_RTT MUST
 be set when the "cc_rtt" field is valid.
 After a congestion control feedback message is generated or when the
 feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
 period during which it will restrain itself from providing congestion
 control feedback, even if NORM_CMD(CC) messages are received from the
 sender (unless the receive becomes marked as a CLR or PLR node).  The
 value of this holdoff timeout (T_ccHoldoff) period is:
                 T_ccHoldoff = (K_sender * GRTT_sender)
 Thus, non-CLR receivers are constrained to providing explicit
 congestion control feedback once per K_sender*GRTT_sender intervals.
 However, as the session progresses, different receivers will be
 responding to different NORM_CMD(CC) messages and there will be
 relatively continuous feedback of congestion control information
 while the sender is active.

5.5.2.3. Congestion Control Rate Adjustment

 During steady-state operation, the sender will directly adjust its
 transmission rate to the rate indicated by the feedback from its
 currently selected CLR.  As noted in [TfmccPaper], the estimation of
 parameters (loss and RTT) for the CLR will generally constrain the
 rate changes possible within acceptable bounds.  For rate increases,
 the sender SHALL observe a maximum rate of increase of one packet per
 RTT at all times during steady-state operation.

Adamson, et al. Standards Track [Page 72] RFC 5740 NORM Protocol November 2009

 The sender processes congestion control feedback from the receivers
 and selects the CLR based on the lowest rate receiver.  Receiver
 rates are determined either directly from the slow start "cc_rate"
 provided by the receiver in the NORM-CC Feedback header extension or
 by performing the equation-based calculation using individual RTT and
 loss estimates ("cc_loss") as feedback is received.
 The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
 using the "grtt_response" timestamp included in feedback messages.
 When the "cc_rtt" value in a response is not valid, the sender simply
 uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
 For non-CLR and non-PLR receivers, the sender SHOULD use the "cc_rtt"
 provided in the NORM-CC Feedback header extension as the receiver's
 previous RTT measurement (RTT_rcvrPrev) averaged with the current
 measurement ("RTT_rcvrNew") as the receiver's RTT value:
           RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
 For CLR receivers where feedback is received more regularly, the
 sender SHOULD maintain a more smoothed RTT estimate upon new feedback
 from the CLR where:
               RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
 RTT_clrNew is the new RTT calculated from the timestamp in the
 feedback message received from the CLR.  The RTT_clr is initialized
 to RTT_clrNew on the first feedback message received.  Note that the
 same procedure is observed by the sender for PLR receivers, and if a
 PLR is "promoted" to CLR status, the smoothed estimate can be
 continued.
 There are some additional periods besides steady-state operation to
 be considered in NORM-CC operation.  These periods are:
 1.  during session startup,
 2.  when no feedback is received from the CLR, and
 3.  when the sender has a break in data transmission.
 During session startup, the congestion control operation SHALL
 observe a "slow-start" procedure to quickly approach its fair
 bandwidth share.  An initial sender startup rate is assumed where:
  Rinit = MIN(NormSegmentSize/GRTT_sender, NormSegmentSize) bytes/sec
 The rate is increased only when feedback is received from the
 receiver set.  The "slow start" phase proceeds until any receiver

Adamson, et al. Standards Track [Page 73] RFC 5740 NORM Protocol November 2009

 provides feedback indicating loss has occurred.  Rate increase during
 slow start is applied as:
                            Rnew = Rrecv_min
 where Rrecv_min is the minimum reported receiver rate in the
 "cc_rate" field of congestion control feedback messages received from
 the group.  Note during slow start, receivers use two times their
 measured rate from the sender in the "cc_rate" field of their
 feedback.  Rate increase adjustment is limited to once per GRTT
 during slow start.
 If the CLR or any receiver intends to leave the group, it will set
 the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
 an indication the sender SHOULD NOT select it as the CLR.  When the
 CLR changes to a lower rate receiver, the sender SHOULD immediately
 adjust to the new lower rate.  The sender is limited to increasing
 its rate at one additional packet per RTT towards any new, higher CLR
 rate.
 The sender SHOULD also track the age of the feedback it has received
 from the CLR by comparing its current "cc_sequence" value
 (Seq_sender) to the last "cc_sequence" value received from the CLR
 (Seq_clr).  As the age of the CLR feedback increases with no new
 feedback, the sender SHALL begin reducing its rate once per RTT_clr
 as a congestion avoidance measure.  The following algorithm is used
 to determine the decrease in sender rate (Rsender bytes/sec) as the
 CLR feedback, unexpectedly, excessively ages:
                 Age = Seq_sender - Seq_clr;
                 if (Age > 4) Rsender = Rsender * 0.5;
 This rate reduction is limited to the lower bound on NORM
 transmission rates.  After NORM_ROBUST_FACTOR consecutive
 NORM_CMD(CC) rounds without any feedback from the CLR, the sender
 SHOULD assume the CLR has left the group and pick the receiver with
 the next lowest rate as the new CLR.  Note this assumes the sender
 does not have explicit knowledge the CLR intentionally left the
 group.  If no receiver feedback is received, the sender MAY wish to
 withhold further transmissions of NORM_DATA segments and maintain
 NORM_CMD(CC) transmissions only until feedback is detected.  After
 such a CLR timeout, the sender will be transmitting with a minimal
 rate and SHOULD return to slow start as described here for a break in
 data transmission.
 When the sender has a break in its data transmission, it can continue
 to probe the group with NORM_CMD(CC) messages to maintain RTT
 collection from the group.  This will enable the sender to quickly
 determine an appropriate CLR upon data transmission restart.

Adamson, et al. Standards Track [Page 74] RFC 5740 NORM Protocol November 2009

 However, the sender SHOULD exponentially reduce its target rate to be
 used for transmission restart as time since the break elapses.  The
 target rate SHOULD be recalculated once per RTT_clr as:
                        Rsender = Rsender * 0.5;
 If the minimum NORM rate is reached, the sender SHOULD set the
 NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
 the group SHOULD observe slow-start congestion control procedures
 until any receiver experiences a new loss event.

5.5.3. NORM Positive Acknowledgment Procedure

 NORM provides options for the source application to request positive
 acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
 messages from members of the group.  There are some specific
 acknowledgment requests defined for the NORM protocol and a range of
 acknowledgment request types left to be defined by the application.
 One predefined acknowledgment type is the NORM_ACK(FLUSH) type.  This
 acknowledgment is used to determine if receivers have achieved
 completion of reliable reception up through a specific logical
 transmission point with respect to the sender's sequence of
 transmission.  The NORM_ACK(FLUSH) acknowledgment MAY be used to
 assist in application flow control when the sender has information on
 a portion of the receiver set.  Another predefined acknowledgment
 type is NORM_ACK(CC) used to explicitly provide congestion control
 feedback in response to NORM_CMD(CC) messages transmitted by the
 sender for NORM-CC operation.  Note the NORM_ACK(CC) response does
 NOT follow the positive acknowledgment procedure described here.  The
 NORM_CMD(ACK_REQ) and NORM_ACK messages contain an "ack_type" field
 to identify the type of acknowledgment requested and provided.  A
 range of "ack_type" values is provided for application-defined use.
 While the application is responsible for initiating the
 acknowledgment request and interprets application-defined "ack_type"
 values, the acknowledgment procedure SHOULD be conducted within the
 protocol implementation to take advantage of timing and transmission
 scheduling information available to the NORM transport.
 The NORM Positive Acknowledgment Procedure uses polling by the sender
 to query the receiver group for response.  Note this polling
 procedure is not intended to scale to very large receiver groups, but
 could be used in a large group setting to query a critical subset of
 the group.  Either the NORM_CMD(ACK_REQ), or when applicable, the
 NORM_CMD(FLUSH) message is used for polling and contains a list of
 NormNodeIds of the receivers expected to respond to the command.  The
 list of receivers providing acknowledgment is determined by the
 source application with a priori knowledge of participating nodes or
 via some other application-level mechanism.

Adamson, et al. Standards Track [Page 75] RFC 5740 NORM Protocol November 2009

 The ACK process is initiated by the sender generating NORM_CMD(FLUSH)
 or NORM_CMD(ACK_REQ) messages in periodic rounds.  For
 NORM_ACK(FLUSH) requests, the NORM_CMD(FLUSH) contains a
 "object_transport_id" and "fec_payload_id" denoting the watermark
 transmission point for which acknowledgment is requested.  This
 watermark transmission point is echoed in the corresponding fields of
 the NORM_ACK(FLUSH) message sent by the receiver in response.
 NORM_CMD(ACK_REQ) messages contain an "ack_id" field that is
 similarly echoed in response so the sender can match the response to
 the appropriate request.
 In response to the NORM_CMD(ACK_REQ), the listed receivers randomly,
 with a uniform distribution, transmit NORM_ACK messages over a time
 window of (1*GRTT_sender).  These NORM_ACK messages are typically
 unicast to the sender.  (Note NORM_ACK(CC) messages SHALL be
 multicast or unicast in the same manner as NORM_NACK messages.)
 The ACK process is self-limiting and avoids ACK implosion because:
 1.  Only a single NORM_CMD(ACK_REQ) message is generated once per
     (2*GRTT_sender), and
 2.  The size of the "acking_node_list" of NormNodeIds from which
     acknowledgment is requested is limited to a maximum of the sender
     NormSegmentSize setting per round of the positive acknowledgment
     process.
 Because the size of the included list is limited to the sender's
 NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds will
 sometimes be necessary to achieve responses from all receivers
 specified.  The content of the attached NormNodeId list will be
 dynamically updated as this process progresses and NORM_ACK responses
 are received from the specified receiver set.  As the sender receives
 valid responses (i.e., matching watermark point or "ack_id") from
 receivers, it SHALL eliminate those receivers from the subsequent
 NORM_CMD(ACK_REQ) message "acking_node_list" and add in any pending
 receiver NormNodeIds while keeping within the NormSegmentSize
 limitation of the list size.  Each receiver is queried a maximum
 number of times (NORM_ROBUST_FACTOR, by default).  Receivers not
 responding within this number of repeated requests are removed from
 the payload list to make room for other potential receivers pending
 acknowledgment.  The transmission of the NORM_CMD(ACK_REQ) is
 repeated until no further responses are needed or until the repeat
 threshold is exceeded for all pending receivers.  The transmission of
 NORM_CMD(ACK_REQ) or NORM_CMD(FLUSH) messages to conduct the positive
 acknowledgment process is multiplexed with ongoing sender data
 transmissions.  However, the NORM_CMD(FLUSH) positive acknowledgment
 process MAY be interrupted in response to negative acknowledgment

Adamson, et al. Standards Track [Page 76] RFC 5740 NORM Protocol November 2009

 repair requests (NACKs) received from receivers during the
 acknowledgment period.  The NORM_CMD(FLUSH) positive acknowledgment
 process is restarted for receivers pending acknowledgment once any
 the repairs have been transmitted.
 In the case of NORM_CMD(FLUSH) commands with an attached
 "acking_node_list", receivers will not ACK until they have received
 complete transmission of all data up to and including the given
 watermark transmission point.  All receivers SHALL interpret the
 watermark point provided in the request NACK for repairs if needed as
 for NORM_CMD(FLUSH) commands with no attached "acking_node_list".

5.5.4. Group Size Estimate

 NORM sender messages contain a "gsize" field that is a representation
 of the group size and that is used in scaling random backoff timer
 ranges.  The use of the group size estimate within the NORM protocol
 does not demand a precise estimation and works reasonably well if the
 estimate is within an order of magnitude of the actual group size.
 By default, the NORM sender group size estimate MAY be
 administratively configured.  Also, given the expected scalability of
 the NORM protocol for general use, a default value of 10,000 is
 RECOMMENDED for use as the group size estimate.  It is also possible
 the group size MAY be algorithmically approximated from the volume of
 congestion control feedback messages based on the exponentially
 weighted random backoff.  However, the specification of such an
 algorithm is currently beyond the scope of this document.

6. Configurable Elements

 The NORM protocol supports a modest number of configurable parameters
 that control operation.  Most of these need only be set at NORM
 sender(s) and the configuration information is communicated to the
 receiver set in NORM header and/or header extension fields.  A
 notable exception to this is the NORM_ROBUST_FACTOR that is presumed
 to be a common value preset among senders and receivers for a given
 NORM session.  The following table summarizes these configurable
 elements:

Adamson, et al. Standards Track [Page 77] RFC 5740 NORM Protocol November 2009

 +--------------------+----------------------------------------------+
 | Configurable       | Purpose                                      |
 | Element            |                                              |
 +--------------------+----------------------------------------------+
 | Sender initial     | Sender's initial estimate of greatest group  |
 | GRTT Estimate      | round-trip time.  Affects timing of feedback |
 | (GRTT_sender)      | suppression and sender command transmissions |
 |                    | at sender startup.                           |
 | Backoff Factor     | Sender's scaling factor used for timer-based |
 | (K_sender)         | feedback suppression.                        |
 | Group Size         | Sender's rough estimate of receiver group    |
 | Estimate           | size used in generation of random feedback   |
 | (GSIZE_sender)     | backoff timeout.                             |
 | NORM_ROBUST_FACTOR | Integer factor determining how persistently  |
 |                    | (i.e., robust) senders transmit repeated     |
 |                    | control messages and receivers self-initiate |
 |                    | timeout-based NACKing in the absence of      |
 |                    | sender activity.                             |
 | FEC Type           | Sender FEC encoding type.                    |
 | ("fec_id")         |                                              |
 | Sender segment     | Maximum size (in bytes) of the payload       |
 | size               | portion of NORM_DATA and other messages.     |
 | (NormSegmentSize)  |                                              |
 | NormNodeId         | Unique identifiers pre-assigned to all NORM  |
 |                    | session participants.                        |
 +--------------------+----------------------------------------------+
 The sender-controlled GRTT estimate (referred to as GRTT_sender in
 this document) is used to set and scale various timers associated
 with NORM protocol operation.  During steady-state operation, the
 sender probes the receiver set, adapts to the group round-trip timing
 state, and advertises its estimate to the receiver set in the "grtt"
 field of relevant NORM protocol messages.  However, an initial value
 must be assumed at sender startup.  A large initial estimate is
 conservative and safer with regard to preventing feedback implosion
 and starting up congestion control operation, but requires the sender
 and receivers to allocate more buffering resources for a given
 transmission rate (i.e., larger effective delay*bandwidth product) to
 maintain efficient operation.  A default initial value of GRTT_sender
 = 0.5 seconds is RECOMMENDED.
 The sender-controlled Backoff Factor (referred to a K_sender in this
 document) is used to scale protocol timers and contributes to the
 generation of the random backoff timeout value that facilitates
 timer-based feedback suppression.  The sender advertises its
 configured Backoff Factor to the receiver set in the "backoff" field
 of applicable NORM messages and thus no receiver configuration is
 necessary.  For ASM operation, a default value of K_sender = 4 is

Adamson, et al. Standards Track [Page 78] RFC 5740 NORM Protocol November 2009

 RECOMMENDED; for SSM operation, a default value of K_sender = 6 is
 RECOMMENDED.
 The sender estimate of session Group Size (referred to as
 GSIZE_sender in this document) also plays a role in the random
 selection of feedback suppression timeout values.  The sender
 advertises its configured Group Size estimate to the receiver set in
 the "gsize" field of applicable NORM messages; thus, no receiver
 configuration is necessary.  Only a rough estimate (i.e., "order-of-
 magnitude") is needed for effective feedback suppression and a
 default value of GSIZE_sender = 10,000 is RECOMMENDED as a
 conservative estimate for most uses.
 The NORM_ROBUST_FACTOR is an integer parameter that determines how
 persistently NORM senders transmit control messages (NORM_CMD
 messages) such as end-of-transmission flushing, OPTIONAL positive
 acknowledgment requests, etc.  Additionally, the receivers use their
 knowledge of NORM_ROBUST_FACTOR to determine when to consider a NORM
 sender inactive and MAY use the factor in determining how
 persistently to self-initiate repeated NACK repair requests upon such
 timeouts.  This parameter is NOT communicated in NORM protocol
 message headers and is presumed to be preset to a consistent value
 among sender and receivers for a given NORM session.  A default value
 of NORM_ROBUST_FACTOR = 20 is RECOMMENDED.
 Another NORM sender configuration element is the FEC type used to
 encode NORM_DATA message content.  The FEC type is communicated from
 the sender to the receiver set in the "fec_id" field of relevant NORM
 message headers.  The "fec_id" value corresponds to an IANA-assigned
 value identifying the FEC encoding type as described in the FEC
 Building Block [RFC5052] document.  Typically, a sender SHOULD use a
 consistent FEC encoding for its participation in a session to
 simplify receiver state allocation and maintenance, but its
 implementations MAY vary the FEC encoding type on a per-object basis
 if necessary.
 The sender NormSegmentSize setting determines the maximum size of the
 payload portion of NORM_DATA and other messages that the sender
 transmits.  Additionally, the payload size of feedback messages from
 receivers to a given sender is limited to that sender's
 NormSegmentSize.  The NormSegmentSize SHOULD be configured to be
 compatible with expected network MTU limitations, given the added
 overhead of NORM, UDP, and IP protocol message headers.
 Additionally, MTU Discovery MAY be employed by the sender to
 determine an appropriate NormSegmentSize.  The NormSegmentSize for a
 given sender can be determined by receivers from the FEC Object
 Transmission Information (FTI) provided either in applied EXT_FTI
 header extensions or pre-configured session information.

Adamson, et al. Standards Track [Page 79] RFC 5740 NORM Protocol November 2009

 Although it is not technically a configurable element, the receivers
 MUST have FEC Object Transmission Information for transmitted
 NormObjects to properly buffer, decode, and reassemble the original
 content.  For loosely organized NORM protocol sessions, the sender
 MAY apply the EXT_FTI Header Extension to NORM_DATA and NORM_INFO (if
 applicable) messages so that receivers can get this information
 without prior coordination.  An implementation MAY also apply the
 EXT_FTI only to NORM_INFO messages for reduced overhead.  Finally,
 applications MAY also provide the FTI out-of-band prior to sender
 transmission.
 Each participant in a NORM protocol session MUST be configured with a
 unique NormNodeId value.  The NormNodeId value is used by receivers
 to identify the sender to which their NACK or other feedback messages
 are addressed, and senders use the NormNodeId to differentiate
 receivers for purposes of congestion control and OPTIONAL positive
 acknowledgment collection.  Assignment of unique NormNodeId values
 can be done via a priori coordination and/or use of a deconfliction
 mechanism external to the NORM protocol itself.  The values of
 NORM_NODE_NONE = 0x00000000 and NORM_NODE_ANY = 0xffffffff are
 reserved and MUST NOT be assigned to NORM participants.

7. Security Considerations

 The same security considerations that apply to the Multicast NACK
 [RFC5401], TFMCC [RFC4654], and FEC [RFC5052] Building Blocks also
 apply to the NORM protocol.  In addition to the vulnerabilities to
 which any IP and IP multicast protocol implementation is subject,
 malicious hosts might engage in excessive NACKing in an attempt to
 prevent the NORM sender(s) from making forward progress in reliable
 transmission.  Receiver "join" and "service" policy enforcement as
 described in Section 5.2 can be applied if such activity is detected.
 The use of cryptographic peer authentication, integrity checks,
 and/or confidentiality mechanisms can be used to provide a more
 effective degree of protection from objectionable transmissions from
 unauthorized hosts.  But in some cases, even with authentication and
 integrity checks, the NACK-based feedback of NORM can be exploited by
 replay attacks forcing the NORM sender to unnecessarily transmit
 repair information.  This MAY be addressed in part with network-layer
 IP security implementations that guard against this potential
 security exploitation or alternatively with a security mechanism
 using the EXT_AUTH header extension for similar purposes.  Such
 security mechanisms SHOULD be deployed and used when available.  Use
 of security mechanisms will impose additional "a priori"
 configuration upon the NORM deployment depending upon the techniques
 used.
 The NORM protocol is compatible with the use of IP security (IPsec)

Adamson, et al. Standards Track [Page 80] RFC 5740 NORM Protocol November 2009

 [RFC4301], and the IPsec Encapsulating Security Payload (ESP)
 protocol or Authentication Header (AH) extension can be used to
 secure IP packets transmitted by NORM participants.  A baseline
 approach to secure NORM operation using IPsec is described below.
 Compliant implementations of this specification are REQUIRED to be
 compatible with IPsec usage as described in Section 7.1.  IPsec can
 be used to provide peer authentication, integrity protection, and/or
 encryption of packets containing NORM messages.
 Additionally, the EXT_AUTH header extension (HET = 1) is reserved for
 use by security mechanisms to provide alternatives to IPsec for the
 security of NORM messages.  The format of this header extension and
 its processing is outside the scope of this document and is to be
 communicated out-of-band as part of the session description.  It is
 possible an EXT_AUTH implementation MAY also provide for encryption
 of NORM message payloads as well as peer authentication and integrity
 protection.  The use of this approach as compared to IPsec can allow
 for header compression techniques to be applied jointly to IP and
 NORM protocol headers.  In cases where security analysis deems
 encryption of NORM protocol header content to be beneficial or
 necessary, the aforementioned use of IPsec ESP might be more
 appropriate.  Additionally, the EXT_AUTH header extension can be
 utilized when NORM is implemented in a network with Network Address
 Translation (NAT) systems that are incompatible with use of the IPsec
 AH extension.  If EXT_AUTH is present, whatever packet authentication
 or integrity checks that can be performed immediately upon reception
 of the packet MUST be performed before accepting the packet and
 performing any congestion-control-related action on it.  Some packet
 authentication schemes impose a delay of several seconds between when
 a packet is received and when the packet can be fully authenticated.
 Any appropriate congestion control related action MUST NOT be
 postponed by any such packet security mechanism (i.e., security
 mechanisms MUST NOT result in poor congestion control behavior).
 Consideration MUST also be given to the potential for replay-attacks
 that would transplant authenticated packets from one NORM session to
 another to disrupt service.  To avoid this potential, unique keys
 SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD
 be configured to use unique "instance_id" identifiers managed as part
 of the security association for the sessions.
 Note NORM implementations can use the "sequence" field from the NORM
 common message header to detect replay attacks.  This can be
 accomplished if the NORM sender maintains state on actively NACKing
 receivers.  A cache of such receiver state can be used to provide
 protection against NACK replay attacks.  NORM receivers MUST also
 maintain similar state for protection against possible replay of
 other receiver messages in ASM operation as well.  For example, a

Adamson, et al. Standards Track [Page 81] RFC 5740 NORM Protocol November 2009

 receiver could be suppressed from providing NACK or congestion
 control feedback by replay of certain receiver messages.  For these
 reasons, authentication of NORM messages (e.g., via IPsec) SHOULD be
 applied for protection against similar attacks that use fabricated
 messages.  Also, encryption of messages to provide confidentiality of
 application data and protect privacy of users MAY also be applied
 using IPsec or similar mechanisms.
 When applicable security measures are used, automated key management
 mechanisms such as those described in the Group Domain of
 Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY)
 [RFC3830], or Group Secure Association Key Management Protocol
 (GSAKMP) [RFC4535] specifications SHOULD be applied.
 While NORM does leverage FEC-based repair for scalability, this alone
 does not guarantee integrity of received data.  Application-level
 integrity-checking of received data content is highly RECOMMENDED.
 This recommendation also applies when the IPsec security approach
 described below is used for added assurance in data content integrity
 given the shared use of IPsec Security Association information among
 the group.

7.1. Baseline Secure NORM Operation

 This section describes a baseline mode of secure NORM protocol
 operation based on application of the IPsec security protocol.  This
 approach is documented here to provide a baseline interoperable
 secure mode of operation.  This particular approach represents one
 possible trade-off in the level of assurance that can be achieved and
 the scalability of multicast group-size given current IPsec
 mechanisms and the state required to support them.  For example, this
 baseline approach specifies the use of a Security Association that is
 shared among the receiver set for feedback messages to the sender.
 This model requires that the receiver membership receiving the
 session keys is trusted and only provides protection from attacks
 that are external to the NORM group membership.  More stateful and
 complex IPsec approaches and key management schemes may be applied
 for higher levels of assurance, but those are beyond the scope of
 this transport protocol specification.  Additional approaches to NORM
 security, including other forms of IPsec application, MAY be
 specified in the future.  For example, the use of the EXT_AUTH header
 extension could enable NORM-specific authentication or security
 encapsulation headers similar to those of IPsec to be specified and
 inserted into the NORM protocol message headers.  This would allow
 header compression techniques to be applied to IP and NORM protocol
 headers when needed in a similar fashion to RTP [RFC3550] and as
 preserved in the specification for Secure Real Time Protocol (SRTP)
 [RFC3711].

Adamson, et al. Standards Track [Page 82] RFC 5740 NORM Protocol November 2009

 The baseline approach described is applicable to NORM operation
 configured for SSM (or SSM-like) operation where there is a single
 sender and the receivers are providing unicast feedback.  This form
 of NORM operation allows for IPsec to be used with a manageable
 number of security associations (SA).

7.1.1. IPsec Approach

 For NORM one-to-many SSM operation with unicast feedback from
 receivers, each node SHALL be configured with two transport mode
 IPsec security associations and corresponding Security Policy
 Database (SPD) entries.  One entry will be used for sender-to-group
 multicast packet authentication and optionally encryption while the
 other entry will be used to provide security for the unicast feedback
 messaging from the receiver(s) to the sender.  Note that this single
 SA for NORM receiver feedback messages is shared to protect traffic
 from possibly multiple receivers to the single sender.
 For each NormSession, the NORM sender SHALL use an IPsec SA
 configured for ESP protocol [RFC4303] operation with the option for
 data origin authentication enabled.  It is also RECOMMENDED this
 IPsec ESP SA be also configured to provide confidentiality protection
 for IP packets containing NORM protocol messages.  This is suggested
 to make the realization of complex replay attacks much more
 difficult.  The encryption key for this SA SHALL be preplaced at the
 sender and receiver(s) prior to NORM protocol operation.  Use of
 automated key management is RECOMMENDED as a rekey SHALL be REQUIRED
 prior to expiration of the sequence space for the SA.  This is
 necessary so receivers can use the built-in IPsec replay attack
 protection possible for an IPsec SA with a single source (the NORM
 sender).  Thus, the receivers SHALL enable replay attack protection
 for this SA used to secure NORM sender traffic.  An IPsec SPD entry
 MUST be configured to process outbound packets to the session
 (destination) address and UDP port number of the applicable
 (NormSession).
 The NORM receiver(s) MUST be configured with the SA and SPD entry to
 properly process the IPsec-secured packets from the sender.  The NORM
 receiver(s) SHALL also use a common, second IPsec SA (common Security
 Parameter Index (SPI) and encryption key) configured for ESP
 operation with the option for data origination authentication
 enabled.  Similar to the NORM sender, is RECOMMENDED this IPsec ESP
 SA be also configured to provide confidentiality protection for IP
 packets containing NORM protocol messages.  The receivers MUST have
 an IPsec SPD entry configured to process outbound NORM/UDP packets
 directed to the NORM sender source address and port number using this
 second SA.  To support NORM unicast feedback, the sender's
 transmission port number SHOULD be selected to be distinct from the

Adamson, et al. Standards Track [Page 83] RFC 5740 NORM Protocol November 2009

 multicast session port number to allow discrimination between unicast
 and multicast feedback messages when access to the IP destination
 address is not possible (e.g., a user-space NORM implementation).
 For processing of packets from receivers, the NORM sender SHALL be
 configured with this common, second SA (and the corresponding SPD
 entry needed) in order to properly process messages from the
 receiver.
 Multiple receivers using a common IPsec SA for traffic directed to
 the NORM sender (i.e., many-to-one) typically prevents the use of
 built-in IPsec replay attack protection by the NORM sender with
 current IPsec implementations.  Thus the built-in IPsec replay attack
 protection for this second SA at the sender MUST be disabled unless
 the particular IPsec implementation manages its replay protection on
 a per-source basis (which is not typical of existing IPsec
 implementations).  So, to support a fully secure mode of operation,
 the NORM sender implementation MUST provide replay attack protection
 based upon the "sequence" field of NORM protocol messages from
 receivers.  This can be accomplished with a high assurance of
 security, even with the limited size (16-bits) of this field,
 because:
 1.  NORM receiver NACK and non-CLR ACK feedback messages are sparse.
 2.  The more frequent NORM_ACK feedback from CLR or PLR nodes is only
     a small set of receivers for which the sender needs to keep more
     persistent replay attack state.
 3.  NORM_NACK feedback messages preceding the sender's current repair
     window do not significantly impact protocol operation (generation
     of NORM_CMD(SQUELCH) is limited) and could be in fact ignored.
     This means the sender can prune any replay attack state that
     precedes the current repair window.
 4.  NORM_ACK messages correspond to either a specific sender
     "ack_id", the sender "cc_sequence" for ACKs sent in response to
     NORM_CMD(CC), or the sender's current repair window in the case
     of ACKs sent in response to NORM_CMD(FLUSH).  Thus, the sender
     can prune any replay attack state for receivers that precede the
     current applicable sequence or repair window space.
 The use of ESP confidentiality for secure NORM protocol operation
 makes it more difficult for adversaries to conduct any form of replay
 attacks.  Additionally, a NORM sender implementation with access to
 the full ESP protocol header could also use the ESP sequence
 information to make replay attack protection even more robust by
 maintaining the per-source ESP sequence state that existing IPsec
 implementations typically do not provide.  The design of this

Adamson, et al. Standards Track [Page 84] RFC 5740 NORM Protocol November 2009

 baseline security approach for NORM intentionally places any more
 complex processing state or processing (e.g., replay attack
 protection given multiple receivers) at the NORM sender since NORM
 receiver implementations might often need to be less complex.
 This baseline approach can be used for NORM protocol sessions with
 multiple senders if the SA pairs described are established for each
 sender.  For small-sized groups, it is even possible many-to-many
 (ASM) IPsec configuration could be achieved where each participant
 uses a unique SA (with a unique SPI).  In this case, the sender(s)
 would maintain an SA for each other participant rather than a single,
 shared SA for receiver feedback messages.  This does not scale to
 larger group sizes given the complex set of SA and SPD entries each
 participant would need to maintain.
 It is anticipated in early deployments of this baseline approach to
 NORM security that key management will be conducted out-of-band with
 respect to NORM protocol operation.  In the case of one-to-many NORM
 operation, it is possible receivers will retrieve keying information
 from a central server as needed or otherwise conduct group key
 updates with a similar centralized approach.  Alternatively, it is
 possible with some key management schemes for rekey messages to be
 transmitted to the group as a message or transport object within the
 NORM reliable transfer session.  Similarly, for group-wise
 communication sessions, it is possible for potential group
 participants to request keying and/or rekeying as part of NORM
 communications.  Additional specification is necessary to define an
 in-band key management scheme for NORM sessions perhaps using the
 mechanisms of the automated group key management specifications cited
 in this document.  Additional specification outside of the scope of
 this document would be needed to provide an interoperable approach
 for key management in-band of a NORM reliable transport session.

7.1.2. IPsec Requirements

 In order to implement this secure mode of NORM protocol operation,
 the following IPsec capabilities are REQUIRED.

7.1.2.1. Selectors

 The implementation MUST be able to use the source address,
 destination address, protocol (UDP), and UDP port numbers as
 selectors in the SPD.

7.1.2.2. Mode

 IPsec in transport mode MUST be supported.  The use of IPsec
 [RFC4301] processing for secure NORM traffic MUST be configured such

Adamson, et al. Standards Track [Page 85] RFC 5740 NORM Protocol November 2009

 that unauthenticated packets are not received by the NORM protocol
 implementation.

7.1.2.3. Key Management

 An automated key management scheme for group key distribution and
 rekeying such as GDOI [RFC3547], GSAKMP [RFC4535], or MIKEY [RFC3830]
 is RECOMMENDED for use.  Note it is possible for key update messages
 (e.g., the GDOI GROUPKEY-PUSH message) to be included as part of the
 NORM application reliable data transmission if appropriate interfaces
 are available between the NORM application and the key management
 daemon.  Relatively short-lived NORM sessions MAY be able to use
 Manual Keying with a single, preplaced key, particularly if Extended
 Sequence Numbering (ESN) [RFC4303] is available in the IPsec
 implementation used.  When manual keys are used, it is important that
 cryptographic algorithms suitable for manual key use are selected.

7.1.2.4. Security Policy

 Receivers MUST accept protocol messages only from the designated,
 authorized sender(s).  Appropriate key management will provide
 authentication, integrity and/or encryption keys only to receivers
 authorized to participate in a designated session.  The approach
 outlined here allows receiver sets to be controlled on a per-sender
 basis.

7.1.2.5. Authentication and Encryption

 Large NORM group sizes will necessitate some form of key management
 that does rely upon shared secrets.  The GDOI and GSAKMP protocols
 mentioned here allow for certificate-based authentication.  It is
 RECOMMENDED these certificates use IP addresses for authentication.

7.1.2.6. Availability

 The IPsec requirements profile outlined here is commonly available on
 many potential NORM hosts.  Configuration and operation of IPsec
 typically requires privileged user authorization.  Automated key
 management implementations are typically configured with the
 privileges necessary to affect system IPsec configuration.

8. IANA Considerations

 Values of NORM Header Extension Types, Stream Control Codes, and
 NORM_CMD message sub-types are subject to IANA registration.  They
 are in the registry named "Reliable Multicast Transport (RMT) NORM
 Protocol Parameters" available from http://www.iana.org.

Adamson, et al. Standards Track [Page 86] RFC 5740 NORM Protocol November 2009

 Note the reliable multicast building block components used by this
 specification also have their respective IANA considerations, and
 those documents SHOULD be consulted accordingly.  In particular, the
 FEC Building Block used by NORM does REQUIRE IANA registration of the
 FEC codecs used.  The registration instructions for FEC codecs are
 provided in RFC 5052.  It is possible additional extensions of the
 NORM protocol might be specified in the future (e.g., additional NORM
 message types) and additional registries be established at that time
 with appropriate IETF standards action.

8.1. Explicit IANA Assignment Guidelines

 This document introduces three registries for the NORM Header
 Extension Types, Stream Control Codes, and NORM_CMD Message sub-
 types.  This section describes explicit IANA assignment guidelines
 for each of these.

8.1.1. NORM Header Extension Types

 This document defines a registry for NORM Header Extensions named
 "NORM Header Extension Types".
 The NORM Header Extension Type field is an 8-bit value.  The values
 of this field identify extended header content allowing the protocol
 functionality to be expanded to include additional features and
 operating modes.  The values that can be assigned within the "NORM
 Header Extensions" registry are numeric indexes in the range {0,
 255}, boundaries included.  Values in the range {0,127} indicate
 variable-length extended header fields while values in the range
 {128,255} indicate extensions of a fixed 4-byte length.  This
 specification registers the following NORM Header Extension Types:
               +-------+----------+--------------------+
               | Value | Name     | Reference          |
               +-------+----------+--------------------+
               | 1     | EXT_AUTH | This specification |
               | 3     | EXT_CC   | This specification |
               | 64    | EXT_FTI  | This specification |
               | 128   | EXT_RATE | This specification |
               +-------+----------+--------------------+
 Requests for assignment of additional NORM Header Extension Type
 values are granted on a "Specification Required" basis as defined by
 IANA Guidelines [RFC5226].  Any such header extension specifications
 MUST include a description of protocol actions to be taken when the
 extension type is encountered by a protocol implementation not
 supporting that specific option.  For example, it is often possible
 for protocol implementations to ignore unknown header extensions.

Adamson, et al. Standards Track [Page 87] RFC 5740 NORM Protocol November 2009

8.1.2. NORM Stream Control Codes

 This document defines a registry for NORM Stream Control Codes named
 "NORM Stream Control Codes".
 NORM Stream Control Codes are 16-bit values that can be inserted
 within a NORM_OBJECT_STREAM delivery object to convey sequenced, out-
 of-band (with respect to the stream data) control signaling
 applicable to the referenced stream object.  These control codes are
 to be delivered to the application or protocol implementation with
 reliable delivery, in-order with respect to the their inserted
 position within the stream.  This specification registers the
 following NORM Stream Control Code:
           +-------+-----------------+--------------------+
           | Value | Name            | Reference          |
           +-------+-----------------+--------------------+
           | 0     | NORM_STREAM_END | This specification |
           +-------+-----------------+--------------------+
 Additional NORM Stream Control Code value assignment requests are
 granted on a "Specification Required" basis as defined by IANA
 Guidelines [RFC5226].  The full 16-bit space outside of the value
 assigned in this specification are available for future assignment.
 In addition to describing the control code's expected interpretation,
 such specifications MUST include a description of protocol actions to
 be taken when the control code is encountered by a protocol
 implementation not supporting that specific option.

8.1.3. NORM_CMD Message Sub-Types

 This document defines a registry for NORM_CMD message sub-types named
 "NORM Command Message Sub-types".
 The NORM_CMD message "sub-type" field is an 8-bit value with valid
 values in the range of 1-255.  Note the value 0 is reserved to
 indicate an invalid NORM_CMD message sub-type.  The current
 specification defines a number of NORM_CMD message sub-types senders
 can use to signal the receivers in various aspects of NORM protocol
 operation.  This specification registers the following NORM_CMD
 Message Sub-types:

Adamson, et al. Standards Track [Page 88] RFC 5740 NORM Protocol November 2009

        +-------+-----------------------+--------------------+
        | Value | Name                  | Reference          |
        +-------+-----------------------+--------------------+
        | 0     | reserved              | This specification |
        | 1     | NORM_CMD(FLUSH)       | This specification |
        | 2     | NORM_CMD(EOT)         | This specification |
        | 3     | NORM_CMD(SQUELCH)     | This specification |
        | 4     | NORM_CMD(CC)          | This specification |
        | 5     | NORM_CMD(REPAIR_ADV)  | This specification |
        | 6     | NORM_CMD(ACK_REQ)     | This specification |
        | 7     | NORM_CMD(APPLICATION) | This specification |
        +-------+-----------------------+--------------------+
 Future specifications extending NORM MAY define additional NORM_CMD
 messages to enhance protocol functionality.  NORM_CMD message sub-
 type value assignment requests are granted on a "Specification
 Required" basis as defined by IANA Guidelines [RFC5226].  In addition
 to describing the command sub-type's expected interpretation,
 specifications MUST include a description of protocol actions to be
 taken when the command is encountered by a protocol implementation
 not supporting that specific option.
 This specification already defines an "application-defined" NORM_CMD
 message sub-type for use at the discretion of individual applications
 using NORM for transport.  These "application-defined" commands are
 suitable for many application-specific purposes and do not involve
 standards action.  In any case, such additional messages SHALL be
 subject to the same congestion control constraints as the existing
 NORM sender message set.

9. Suggested Use

 The present NORM protocol is seen as a useful tool for the reliable
 data transfer over generic IP multicast services.  It is not the
 intention of the authors to suggest it is suitable for supporting all
 envisioned multicast reliability requirements.  NORM provides a
 simple and flexible framework for multicast applications with a
 degree of concern for network traffic implosion and protocol overhead
 efficiency.  NORM-like protocols have been successfully demonstrated
 within the MBone for bulk data dissemination applications, including
 weather satellite compressed imagery updates servicing a large group
 of receivers and a generic web content reliable "push" application.
 In addition, this framework approach has some design features making
 it attractive for bulk transfer in asymmetric and wireless
 internetwork applications.  NORM is capable of successfully operating
 independent of network structure and in environments with high packet
 loss, delay, and out-of-order delivery.  Hybrid proactive/reactive

Adamson, et al. Standards Track [Page 89] RFC 5740 NORM Protocol November 2009

 FEC-based repairing improve protocol performance in some multicast
 scenarios.  A sender-only repair approach often makes additional
 engineering sense in asymmetric networks.  NORM's unicast feedback
 capability is suitable for use in asymmetric networks or in networks
 where only unidirectional multicast routing/delivery service exists.
 Asymmetric architectures supporting multicast delivery are likely to
 make up an important portion of the future Internet structure (e.g.,
 direct broadcast satellite (DBS) or cable and public-switched
 telephone network (PSTN) hybrids, etc.) and efficient, reliable bulk
 data transfer will be an important capability for servicing large
 groups of subscribed receivers.

10. Changes from RFC 3940

 This section lists the changes between the Experimental version of
 this specification, RFC 3940, and this version:
 1.  Removal of the NORM_FLAG_MSG_START for NORM_OBJECT_STREAM,
     replacing it with the "payload_msg_start" field in the FEC-
     encoded preamble of the NORM_OBJECT_STREAM NORM_DATA payload.
 2.  Definition of IANA registry for header extension and other
     assignments.
 3.  Removal of file blocking scheme description now specified in the
     FEC Building Block document [RFC5052].
 4.  Removal of restriction of NORM receiver feedback message rate to
     local NORM sender rate (this caused congestion control failures
     in high speed operation.  The extremely low feedback rate of the
     NORM protocol as compared to TCP avoids any resultant impact to
     the network as shown in [Mdpcc].)
 5.  Correction of errors in some message format descriptions.
 6.  Correction of inconsistency in specification of the inactivity
     timeout.
 7.  Addition of IPsec secure mode description with IPsec
     requirements.
 8.  Addition of the EXT_AUTH header extension definition.
 9.  Clarification of interpretation of "Source Block Length" when FEC
     codes are arbitrarily shortened by the sender.

Adamson, et al. Standards Track [Page 90] RFC 5740 NORM Protocol November 2009

11. Acknowledgments

 (and these are not Negative)
 The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
 Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
 and comments on this document.  The authors would also like to thank
 the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
 their support in development of this specification, and Sally Floyd
 for her early input into this document.

12. References

12.1. Normative References

 [RFC1112]        Deering, S., "Host extensions for IP multicasting",
                  STD 5, RFC 1112, August 1989.
 [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC4301]        Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, December 2005.
 [RFC4303]        Kent, S., "IP Encapsulating Security Payload (ESP)",
                  RFC 4303, December 2005.
 [RFC4607]        Holbrook, H. and B. Cain, "Source-Specific Multicast
                  for IP", RFC 4607, August 2006.
 [RFC4654]        Widmer, J. and M. Handley, "TCP-Friendly Multicast
                  Congestion Control (TFMCC): Protocol Specification",
                  RFC 4654, August 2006.
 [RFC5052]        Watson, M., Luby, M., and L. Vicisano, "Forward
                  Error Correction (FEC) Building Block", RFC 5052,
                  August 2007.
 [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                  Writing an IANA Considerations Section in RFCs",
                  BCP 26, RFC 5226, May 2008.
 [RFC5401]        Adamson, B., Bormann, C., Handley, M., and J.
                  Macker, "Multicast Negative-Acknowledgment (NACK)
                  Building Blocks", RFC 5401, November 2008.

Adamson, et al. Standards Track [Page 91] RFC 5740 NORM Protocol November 2009

12.2. Informative References

 [FecHybrid]      Gossink, D. and J. Macker, "Reliable Multicast and
                  Integrated Parity Retransmission with Channel
                  Estimation", IEEE GLOBECOMM, 1998.
 [McastFeedback]  Nonnenmacher, J. and E. Biersack, "Optimal Multicast
                  Feedback", IEEE INFOCOM, p. 964, March/April 1998.
 [MdpToolkit]     Macker,  J. and B. Adamson, "The Multicast
                  Dissemination Protocol (MDP) Toolkit", Proc.
                  IEEE MILCOM, October 1999.
 [Mdpcc]          Adamson,  B. and J. Macker, "A TCP-Friendly, Rate-
                  based Mechanism for NACK-Oriented Reliable Multicast
                  Congestion Control", Proc. IEEE GLOBECOMM,
                  November 2001.
 [NormFeedback]   Adamson, B. and J. Macker, "Quantitative Prediction
                  of NACK-Oriented Reliable Multicast (NORM)
                  Feedback", IEEE MILCOM, October 2002.
 [PgmccPaper]     Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate
                  Multicast Congestion Control Scheme", ACM SIGCOMM,
                  August 2000.
 [RFC2357]        Mankin, A., Romanov, A., Bradner, S., and V. Paxson,
                  "IETF Criteria for Evaluating Reliable Multicast
                  Transport and Application Protocols", RFC 2357,
                  June 1998.
 [RFC2974]        Handley, M., Perkins, C., and E. Whelan, "Session
                  Announcement Protocol", RFC 2974, October 2000.
 [RFC3048]        Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
                  Floyd, S., and M. Luby, "Reliable Multicast
                  Transport Building Blocks for One-to-Many Bulk-Data
                  Transfer", RFC 3048, January 2001.
 [RFC3269]        Kermode, R. and L. Vicisano, "Author Guidelines for
                  Reliable Multicast Transport (RMT) Building Blocks
                  and Protocol Instantiation documents", RFC 3269,
                  April 2002.
 [RFC3453]        Luby, M., Vicisano, L., Gemmell, J., Rizzo, L.,
                  Handley, M., and J. Crowcroft, "The Use of Forward
                  Error Correction (FEC) in Reliable Multicast",
                  RFC 3453, December 2002.

Adamson, et al. Standards Track [Page 92] RFC 5740 NORM Protocol November 2009

 [RFC3547]        Baugher, M., Weis, B., Hardjono, T., and H. Harney,
                  "The Group Domain of Interpretation", RFC 3547,
                  July 2003.
 [RFC3550]        Schulzrinne, H., Casner, S., Frederick, R., and V.
                  Jacobson, "RTP: A Transport Protocol for Real-Time
                  Applications", STD 64, RFC 3550, July 2003.
 [RFC3711]        Baugher, M., McGrew, D., Naslund, M., Carrara, E.,
                  and K. Norrman, "The Secure Real-time Transport
                  Protocol (SRTP)", RFC 3711, March 2004.
 [RFC3830]        Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,
                  and K. Norrman, "MIKEY: Multimedia Internet KEYing",
                  RFC 3830, August 2004.
 [RFC3940]        Adamson, B., Bormann, C., Handley, M., and J.
                  Macker, "Negative-acknowledgment (NACK)-Oriented
                  Reliable Multicast (NORM) Protocol", RFC 3940,
                  November 2004.
 [RFC4535]        Harney, H., Meth, U., Colegrove, A., and G. Gross,
                  "GSAKMP: Group Secure Association Key Management
                  Protocol", RFC 4535, June 2006.
 [RFC4566]        Handley, M., Jacobson, V., and C. Perkins, "SDP:
                  Session Description Protocol", RFC 4566, July 2006.
 [RFC5445]        Watson, M., "Basic Forward Error Correction (FEC)
                  Schemes", RFC 5445, March 2009.
 [RmComparison]   Pingali, S., Towsley, D., and J. Kurose, "A
                  Comparison of Sender-Initiated and Receiver-
                  Initiated Reliable Multicast Protocols", Proc.
                  INFOCOMM, San Francisco CA, October 1993.
 [TcpModel]       Padhye,  J., Firoiu, V., Towsley, D., and J. Kurose,
                  "Modeling TCP Throughput: A Simple Model and its
                  Empirical Validation", ACM SIGCOMM, 1998.
 [TfmccPaper]     Widmer, J. and M. Handley, "Extending Equation-Based
                  Congestion Control to Multicast Applications",
                  ACM SIGCOMM, August 2001.

Adamson, et al. Standards Track [Page 93] RFC 5740 NORM Protocol November 2009

Authors' Addresses

 Brian Adamson
 Naval Research Laboratory
 Washington, DC  20375
 USA
 EMail: adamson@itd.nrl.navy.mil
 Carsten Bormann
 Universitaet Bremen TZI
 Postfach 330440
 D-28334 Bremen
 Germany
 EMail: cabo@tzi.org
 Mark Handley
 University College London
 Gower Street
 London  WC1E 6BT
 UK
 EMail: M.Handley@cs.ucl.ac.uk
 Joe Macker
 Naval Research Laboratory
 Washington, DC  20375
 USA
 EMail: macker@itd.nrl.navy.mil

Adamson, et al. Standards Track [Page 94]

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