Network Working Group                                         B. Adamson
Internet-Draft                                 Naval Research Laboratory
Intended status: Standards Track                              C. Bormann
Expires: April 27, June 20, 2009                           Universitaet Bremen TZI
                                                              M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                        October 24,
                                                       December 17, 2008

               NACK-Oriented Reliable Multicast Protocol

Status of this Memo

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   This document describes the messages and procedures of the Negative-
   ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) Protocol.
   This protocol is designed to provide end-to-end reliable transport of
   bulk data objects or streams over generic IP multicast routing and
   forwarding services.  NORM uses a selective, negative acknowledgment
   mechanism for transport reliability and offers additional protocol
   mechanisms to allow for operation with minimal a priori coordination
   among senders and receivers.  A congestion control scheme is
   specified to allow the NORM protocol to fairly share available
   network bandwidth with other transport protocols such as Transmission
   Control Protocol (TCP).  It is capable of operating with both
   reciprocal multicast routing among senders and receivers and with
   asymmetric connectivity (possibly a unicast return path) between the
   senders and receivers.  The protocol offers a number of features to
   allow different types of applications or possibly other higher level
   transport protocols to utilize its service in different ways.  The
   protocol leverages the use of FEC-based repair and other IETF
   reliable multicast transport (RMT) building blocks in its design.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   this document are to be interpreted as described in [RFC2119].

Table of Contents

   1.  Introduction and Applicability . . . . . . . . . . . . . . . .  5
     1.1.  NORM Data Delivery Service Model . . . . . . . . . . . . .  6
     1.2.  NORM Scalability . . . . . . . . . . . . . . . . . . . . .  8
     1.3.  Environmental Requirements and Considerations  . . . . . .  9
   2.  Architecture Definition  . . . . . . . . . . . . . . . . . . .  9
     2.1.  Protocol Operation Overview  . . . . . . . . . . . . . . . 11
     2.2.  Protocol Building Blocks . . . . . . . . . . . . . . . . . 13
     2.3.  Design Tradeoffs . . . . . . . . . . . . . . . . . . . . . 13
   3.  Conformance Statement  . . . . . . . . . . . . . . . . . . . . 14
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  NORM Common Message Header and Extensions  . . . . . . . . 16
     4.2.  Sender Messages  . . . . . . . . . . . . . . . . . . . . . 19
       4.2.1.  NORM_DATA Message  . . . . . . . . . . . . . . . . . . 19
       4.2.2.  NORM_INFO Message  . . . . . . . . . . . . . . . . . . 29
       4.2.3.  NORM_CMD Messages  . . . . . . . . . . . . . . . . . . 30
     4.3.  Receiver Messages  . . . . . . . . . . . . . . . . . . . . 48
       4.3.1.  NORM_NACK Message  . . . . . . . . . . . . . . . . . . 48
       4.3.2.  NORM_ACK Message . . . . . . . . . . . . . . . . . . . 54
     4.4.  General Purpose Messages . . . . . . . . . . . . . . . . . 56
       4.4.1.  NORM_REPORT Message  . . . . . . . . . . . . . . . . . 56
   5.  Detailed Protocol Operation  . . . . . . . . . . . . . . . . . 56
     5.1.  Sender Initialization and Transmission . . . . . . . . . . 58
       5.1.1.  Object Segmentation Algorithm  . . . . . . . . . . . . 59
     5.2.  Receiver Initialization and Reception  . . . . . . . . . . 60
     5.3.  Receiver NACK Procedure  . . . . . . . . . . . . . . . . . 60
     5.4.  Sender NACK Processing and Response  . . . . . . . . . . . 62
       5.4.1.  Sender Repair State Aggregation  . . . . . . . . . . . 63
       5.4.2.  Sender FEC Repair Transmission Strategy  . . . . . . . 64
       5.4.3.  Sender NORM_CMD(SQUELCH) Generation  . . . . . . . . . 65
       5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
     5.5.  Additional Protocol Mechanisms . . . . . . . . . . . . . . 66
       5.5.1.  Greatest Round-trip Time Collection  . . . . . . . . . 66
       5.5.2.  NORM Congestion Control Operation  . . . . . . . . . . 67
       5.5.3.  NORM Positive Acknowledgment Procedure . . . . . . . . 75
       5.5.4.  Group Size Estimate  . . . . . . . . . . . . . . . . . 77
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 78 77
     6.1.  Baseline Secure NORM Operation . . . . . . . . . . . . . . 79
       6.1.1.  IPsec Approach . . . . . . . . . . . . . . . . . . . . 80 79
       6.1.2.  IPsec Requirements . . . . . . . . . . . . . . . . . . 82
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 83
     7.1.  Explicit IANA Assignment Guidelines  . . . . . . . . . . . 83
   8.  Suggested Use  . . . . . . . . . . . . . . . . . . . . . . . . 84
   9.  Changes from RFC3940 . . . . . . . . . . . . . . . . . . . . . 85 84
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 85
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 86 85
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 86 85
     11.2. Informative References . . . . . . . . . . . . . . . . . . 86
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 88
   Intellectual Property and Copyright Statements . . . . . . . . . . 90

1.  Introduction and Applicability

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

   This document is a product of the IETF RMT WG and follows the
   guidelines provided in [RFC3269].

   Statement of Intent

   This memo contains the definitions necessary to fully specify a
   Reliable Multicast Transport protocol in accordance with the criteria
   of [RFC2357].  A prior document, [RFC3940], contained a previous
   description of the NORM Protocol specification described in this
   document.  RFC3940 was published in the "Experimental" category.  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 [RFC3940] and
   has been updated according to accumulated experience and growing
   protocol maturity since the publication of RFC3940.  Said experience
   applies both to this specification itself and to congestion control
   strategies related to the use of this specification.

   The differences between [RFC3940] and this document are listed in
   Section 9.

1.1.  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 that has been chosen via means outside
   the context of the given NormSession.  Other IETF data format and
   protocol standards exist that may be applied to describe and convey
   the required a priori information for a specific NormSession (e.g.,
   Session Description Protocol (SDP) [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
   independent of one another and receivers will maintain state as
   necessary for each sender.  However, in future versions of NORM, it
   is possible that some aspects of protocol operation (e.g., round-trip
   time collection) may provide for alternate modes allowing more
   efficient performance for applications requiring multiple senders.

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

   1.  static computer memory data content ("NORM_OBJECT_DATA" type),
   2.  computer storage files ("NORM_OBJECT_FILE" type), and
   3.  non-finite streams of continuous data content
       ("NORM_OBJECT_STREAM" type).

   The distinction between "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" is
   simply to provide a hint to receivers in NormSessions serving
   multiple types of content as to what type of storage should be
   allocated for received content (i.e., memory or file storage).  Other
   than that distinction, the two are identical, providing for reliable
   transport of finite (but potentially very large) units of content.
   These static data and file services are anticipated to be useful for
   multicast-based cache applications with the ability to reliably
   provide transmission of large quantities of static data.  Other types
   of static data/file delivery services might make use of these
   transport object types, too.  The use of the "NORM_OBJECT_STREAM"
   type is at the application's discretion and could be used to carry
   static data or file content also.  The NORM reliable stream service
   opens up additional possibilities such as serialized reliable
   messaging or other unbounded, perhaps dynamically produced content.
   The "NORM_OBJECT_STREAM" provides for reliable transport analogous to
   that of the Transmission Control Protocol (TCP), although NORM
   receivers will be able to begin receiving stream content at any point
   in time.  The applicability of this feature will depend upon the

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

   NORM does NOT provide for global or application-level identification
   of data content within in its message headers.  Note the "NORM_INFO"
   out-of-band data mechanism could be leveraged by the application for
   this purpose if desired, or identification could alternatively be
   embedded within the data content.  NORM does identify transmitted
   content (NormObjects) with transport identifiers that are applicable
   only while the sender is transmitting and/or repairing the given
   object.  These transport data content identifiers (NormTransportIds)
   are assigned in a monotonically increasing fashion by each NORM
   sender during the course of a NormSession.  Each sender maintains its
   NormTransportId assignments independently so that individual
   NormObjects may be uniquely identified during transport with the
   concatenation of the sender session-unique identifier (NormNodeId)
   and the assigned NormTransportId.  The NormTransportIds are assigned
   from a large, but fixed, numeric space in increasing order and may be
   reassigned during long-lived sessions.  The NORM protocol provides
   mechanisms so that the sender application may terminate transmission
   of data content and inform the group of this in an efficient manner.
   Other similar protocol control mechanisms (e.g., session termination,
   receiver synchronization, etc.) are specified so that reliable
   multicast application variants may construct different, complete bulk
   transfer communication models to meet their goals.

   To summarize, the NORM protocol provides reliable transport of
   different types of data content (including potentially mixed types).
   The senders enqueue and transmit bulk content in the form of static
   data or files and/or non-finite, ongoing stream types.  NORM senders
   provide for repair transmission of data and/or FEC content in
   response to NACK messages received from the receiver group.
   Mechanisms for out-of-band information and other transport control
   mechanisms are specified for use by applications to form complete
   reliable multicast solutions for different purposes.

1.2.  NORM Scalability

   Group communication scalability requirements lead to adaptation of
   negative acknowledgment (NACK) based protocol schemes when feedback
   for reliability is required [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,
   it is expected that the NORM protocol will readily scale to group
   sizes on the order of tens of thousands of receivers.  A study of the
   quantity of feedback for this type of protocol is described in
   [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 that NORM may scale to larger group sizes.  With respect
   to computer resource usage, the NORM protocol does NOT require that
   state be kept on all receivers in the group.  NORM senders maintain
   state only for receivers providing explicit congestion control
   feedback.  However, NORM receivers must maintain state for each
   active sender.  This may constrain the number of simultaneous senders
   in some uses of NORM.

1.3.  Environmental Requirements and Considerations

   All of the environmental requirements and considerations that apply
   to the RMT Multicast NACK Building Block
   [I-D.ietf-rmt-bb-norm-revised], [RFC5401], FEC Building
   Block [RFC5052], and TCP-Friendly Multicast Congestion Control
   (TFMCC) Building Block [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 [RFC1112], but SHALL also be capable of
   scalable operation in asymmetric topologies such as Source-Specific
   Multicast (SSM) [RFC4607] where there may only be unicast routing
   service from the receivers to the sender(s).

   NORM is compatible with IPv4 and IPv6.  Additionally, NORM may be
   used with networks employing Network Address Translation (NAT)
   providing the NAT device supports IP multicast and/or can cache UDP
   traffic source port numbers for remapping feedback traffic from
   receivers to the sender(s).

2.  Architecture Definition

   A NormSession is comprised of participants (NormNodes) acting as
   senders and/or receivers.  NORM senders transmit data content in the
   form of NormObjects to the session destination address and the NORM
   receivers attempt to reliably receive the transmitted content using
   negative acknowledgments to request repair.  Each NormNode within a
   NormSession is assumed to have a preselected unique 32-bit identifier
   (NormNodeId).  NormNodes MUST have uniquely assigned identifiers
   within a single NormSession to distinguish between possible multiple
   senders and to distinguish feedback information from different
   receivers.  There are two reserved NormNodeId values.  A value of
   "0x00000000" is considered an invalid NormNodeId value and a value of
   "0xffffffff" is a "wild card" NormNodeId.  While the protocol does
   not preclude multiple sender nodes concurrently transmitting within
   the context of a single NORM session (i.e., many- to-many operation),
   any type of interactive coordination among NORM senders is assumed to
   be controlled by the application or higher protocol layer.  There are
   some optional mechanisms specified in this document that can be
   leveraged for such application layer coordination.

   As previously noted, NORM allows for reliable transmission of three
   different basic types of data content.  The first type is
   "NORM_OBJECT_DATA", which is used for static, persistent blocks of
   data content maintained in the sender's application memory storage.
   The second type is "NORM_OBJECT_FILE", which corresponds to data
   stored in the sender's non-volatile file system.  The
   "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" types both represent
   NormObjects of finite but potentially very large size.  The third
   type of data content is "NORM_OBJECT_STREAM", which corresponds to an
   ongoing transmission of undefined length.  This is analogous to the
   reliable stream service provide by TCP for unicast data transport.
   The format of the stream content is application-defined and may be
   byte or message oriented.  The NORM protocol provides for "flushing"
   of the stream to expedite delivery or possibly enforce application
   message boundaries.  NORM protocol implementations may offer either
   (or both) in-order delivery of the stream data to the receive
   application or out-of-order (more immediate) delivery of received
   segments of the stream to the receiver application.  In either case,
   NORM sender and receiver implementations provide buffering to
   facilitate repair of the stream as it is transported.

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

   Transmitted NormObjects are temporarily yet uniquely identified
   within the NormSession context using the given sender's NormNodeId,
   NormInstanceId, and a temporary NormObjectTransportId.  Depending
   upon the implementation, individual NORM senders may manage their
   NormInstanceIds independently, or a common NormInstanceId may be
   agreed upon for all participating nodes within a session if needed as
   a session identifier.  NORM NormObjectTransportId data content
   identifiers are sender-assigned and applicable and valid only during
   a NormObject's actual transport (i.e., for as long as the sender is
   transmitting and providing repair of the indicated NormObject).  For
   a long-lived session, the NormObjectTransportId field can wrap and
   previously-used identifiers may be re-used.  Note that globally
   unique identification of transported data content is not provided by
   NORM and, if required, must be managed by the NORM application.  The
   individual segments or symbols of the NormObject are further
   identified with FEC payload identifiers which include coding block
   and symbol identifiers.  These are discussed in detail later in this

2.1.  Protocol Operation Overview

   A NORM sender primarily generates messages of type "NORM_DATA".
   These messages carry original data segments or FEC symbols and repair
   segments/symbols for the bulk data/file or stream NormObjects being
   transferred.  By default, redundant FEC symbols are sent only in
   response to receiver repair requests (NACKs) and thus normally little
   or no additional transmission overhead is imposed due to FEC
   encoding.  However, the NORM implementation MAY be optionally
   configured to proactively transmit some amount of redundant FEC
   symbols along with the original content to potentially enhance
   performance (e.g., improved delay) at the cost of additional
   transmission overhead.  This option may be sensible for certain
   network conditions and can allow for robust, asymmetric multicast
   (e.g., unidirectional routing, satellite, cable) [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.  "NORM_INFO"
   may serve special purposes for some bulk transfer, reliable multicast
   applications where receivers join the group mid-stream and need to
   ascertain contextual information on the current content being
   transmitted.  The NACK process for "NORM_INFO" will be described
   later.  When the "NORM_INFO" message type is used, its transmission
   should precede transmission of any "NORM_DATA" message for the
   associated NormObject.

   The sender also generates messages of type "NORM_CMD" to assist in
   certain protocol operations such as congestion control, end-of-
   transmission flushing, round trip time estimation, receiver
   synchronization, and optional positive acknowledgment requests or
   application defined commands.  The transmission of "NORM_CMD"
   messages from the sender is accomplished by one of three different
   procedures.  These procedures are: single, best effort unreliable
   transmission of the command; repeated redundant transmissions of the
   command; and positively-acknowledged commands.  The transmission
   technique used for a given command depends upon the function of the
   command.  Several core commands are defined for basic protocol
   operation.  Additionally, implementations MAY wish to consider
   providing the OPTIONAL application-defined commands that can take
   advantage of the transmission methodologies available for commands.
   This allows for application-level session management mechanisms that
   can make use of information available to the underlying NORM protocol
   engine (e.g., round-trip timing, transmission rate, etc.).  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 may be completely atomic
   and no specific state may need 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

   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
   may be desirable to establish minimum and maximum bounds for the rate
   adjustment depending upon the application even when dynamic
   congestion control is enabled.  However, in the case of the general
   Internet, congestion control policy SHALL be observed that is
   compatible with coexistent TCP flows.

   NORM receivers generate messages of type "NORM_NACK" or "NORM_ACK" in
   response to transmissions of data and commands from a sender.  The
   "NORM_NACK" messages are generated to request repair of detected data
   transmission losses.  Receivers generally detect losses by tracking
   the sequence of transmission from a sender.  Sequencing information
   is embedded in the transmitted data packets and end-of-transmission
   commands from the sender.  "NORM_ACK" messages are generated in
   response to certain commands transmitted by the sender.  In the
   general (and most scalable) protocol mode, "NORM_ACK" messages are
   sent only in response to congestion control commands from the sender.
   The feedback volume of these congestion control "NORM_ACK" messages
   is controlled using the same timer-based probabilistic suppression
   techniques as for "NORM_NACK" messages to avoid feedback implosion.
   In order to meet potential application requirements for positive
   acknowledgment from receivers, other "NORM_ACK" messages are defined
   and available for use.

2.2.  Protocol Building Blocks

   The operation of the NORM protocol is based primarily upon the
   concepts presented in the Multicast NACK Building Block document
   [RFC5401].  This includes the basic NORM architecture and the data
   transmission, repair, and feedback strategies discussed in that
   document.  Additional reliable multicast building blocks blocks, as
   described in [RFC3048], are applied in creating the full NORM
   protocol instantiation as described in [RFC3048]. .  NORM also makes use of Forward Error
   Correction encoding techniques for repair messaging and optional
   transmission robustness as described in [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 TCP-
   Friendly Multicast Congestion Control (TFMCC) scheme of [TfmccPaper]
   and [RFC4654].

2.3.  Design Tradeoffs

   While the various features of NORM are designed to provide some
   measure of general purpose utility, it is important to emphasize the
   understanding that "no one size fits all" in the reliable multicast
   transport arena.  There are numerous engineering trade-offs involved
   in reliable multicast transport design and this requires an increased
   awareness of application and network architecture considerations.
   Performance requirements affecting design can include: group size,
   heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
   data ordering, delivery delay, group dynamics, mobility, congestion
   control, and transport across low capacity connections.  NORM
   contains various parameters to accommodate many of these differing
   requirements.  The NORM protocol and its mechanisms MAY be applied in
   multicast applications outside of bulk data transfer, but there is an
   assumed model of bulk transfer transport service that drives the
   trade-offs that determine the scalability and performance described
   in this document.

   The ability of NORM to provide reliable data delivery is also
   governed by any buffer constraints of the sender and receiver
   applications.  NORM protocol implementations SHOULD be designed to
   operate with the greatest efficiency and robustness possible within
   application-defined buffer constraints.  Buffer requirements for
   reliability, as always, are a function of the delay-bandwidth product
   of the network topology.  NORM performs best when allowed more
   buffering resources than typical point-to-point transport protocols.
   This is because NORM feedback suppression is based upon randomly-
   delayed transmissions from the receiver set, rather than immediately
   transmitted feedback.  There are definitive 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 Protocol Instantiation document, in conjunction with the RMT
   Building Block documents of [I-D.ietf-rmt-bb-norm-revised] [RFC5401] and [RFC5052], completely
   specifies a working reliable multicast transport protocol that
   conforms to the requirements described in RFC 2357 [RFC2357].

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

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

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

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

4.  Message Formats

   As mentioned in Section 2.1, there are two primary classes of NORM
   messages: sender messages and receiver messages.  "NORM_CMD",
   "NORM_INFO", and "NORM_DATA" message types are generated by senders
   of data content, and "NORM_NACK" and "NORM_ACK" messages generated by
   receivers within a NormSession.  Sender messages SHOULD be governed
   by congestion control for Internet use.  For session management or
   other purposes, receivers may wish to 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 designed to be compatible with the MTU
   limitations of encapsulating Internet protocols including IPv4, IPv6,
   and UDP.  The current NORM protocol specification assumes UDP
   encapsulation and leverages the transport features of UDP.  The NORM
   messages are independent of network addresses and can be used in IPv4
   and IPv6 networks.

4.1.  NORM Common Message Header and Extensions

   There are some common message fields contained in all NORM message
   types.  Additionally, a header extension mechanism is defined to
   expand the functionality of the NORM protocol without revision to
   this document.  All NORM protocol messages begin with a common header
   with information fields as follows:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version|  type |    hdr_len    |          sequence             |
     |                           source_id                           |

                     NORM Common Message Header Format

   The "version" field is a 4-bit value indicating the protocol version
   number.  NORM implementations SHOULD ignore received messages with
   version numbers different from their own.  This number is intended to
   indicate and distinguish upgrades of the protocol which may be non-
   interoperable.  The NORM version number for this specification is 1.

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

                         | Message       | Value |
                         | "NORM_INFO"   | 1     |
                         | "NORM_DATA"   | 2     |
                         | "NORM_CMD"    | 3     |
                         | "NORM_NACK"   | 4     |
                         | "NORM_ACK"    | 5     |
                         | "NORM_REPORT" | 6     |

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

   The "sequence" field is a 16-bit value that is set by the message
   originator as a monotonically increasing number incremented with each
   NORM message transmitted.  Note that two independent "sequence"
   spaces MUST be maintained.  One sequence space SHALL be kept for NORM
   sender messages ("NORM_INFO", "NORM_DATA", and "NORM_CMD") generated,
   and a separate, independent "sequence" space SHALL be kept for NORM
   receiver messages ("NORM_NACK" and "NORM_NACK").  The sender message
   "sequence" value can be monitored by receiving nodes to detect packet
   losses in the transmissions from a sender and used to estimate raw
   packet loss for congestion control purposes.  Note that this value is
   NOT used in the NORM protocol to detect missing reliable data content
   and does NOT identify the application data or FEC payload that may be
   attached.  The "sequence" field may also be leveraged for protection
   from message replay attacks, particularly of "NORM_NACK" or other
   feedback messages.  For this reason, NORM receiver messages are also
   sequence numbered.  An independent sequence space MUST be used for
   receiver messages because when receivers generate unicast "NORM_NACK"
   or "NORM_ACK" messages, those messages will not be visible to the
   group at large that may be performing loss estimation.  Also, NORM
   congestion control is applied only to sender messages.  The size of
   the "sequence" field is intended to be sufficient to allow detection
   of a reasonable range of packet loss within the delay-bandwidth
   product of expected network connections.

   The "source_id" field is a 32-bit value identifying the node that
   sent the message.  A participant's NORM node identifier (NormNodeId)
   can be set according to application needs but unique identifiers must
   be assigned within a single NormSession.  In some cases, use of the
   host IP address or a hash of it can suffice, but alternative
   methodologies for assignment and potential collision resolution of
   node identifiers within a multicast session need to be considered.
   For example, the "source identifier" mechanism defined in the Real-
   Time Protocol (RTP) specification [RFC3550] may be applicable to use
   for NORM node identifiers.  At this point in time, the protocol makes
   no assumptions about how these unique identifiers are actually

   NORM Header Extensions

   When header extensions are applied, they follow the message type's
   base header and precede any payload portion.  There are two formats
   for header extensions, both of which begin with an 8-bit "het"
   (header extension type) field.  One format is provided for variable-
   length extensions with "het" values in the range from 0 through 127.
   The other format is for fixed length (one 32-bit word) extensions
   with "het" values in the range from 128 through 255.  These formats
   are given here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het <=127   |      hel      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                    Header Extension Content                   |
     |                              ...                              |

               NORM Variable Length Header Extension Format

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het >=128   |    reserved   |    Header Extension Content   |

            NORM Fixed Length (32-bit) Header Extension Format

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

4.2.  Sender Messages

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

4.2.1.  NORM_DATA Message

   The "NORM_DATA" message is expected to be the predominant type
   transmitted by NORM senders.  These messages are used to encapsulate
   segmented data content for objects of type "NORM_OBJECT_DATA",
   may contain original or FEC-encoded application data content.

   The format of "NORM_DATA" messages is comprised of three logical
   portions: 1) a fixed-format "NORM_DATA" header portion, 2) a FEC
   Payload ID portion with a format dependent upon the FEC encoding
   used, and 3) a payload portion containing source or encoded
   application data content.  Note for objects of type
   "NORM_OBJECT_STREAM", the payload portion contains additional fields
   used to appropriately recover stream content.  NORM implementations
   MAY also extend the "NORM_DATA" header to include a FEC Object
   Transmission Information (EXT_FTI) header extension.  This allows
   NORM receivers to automatically allocate resources and properly
   perform FEC decoding without the need for pre-configuration or out-
   of-band information.

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

                         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 may be directly interpreted for packets
   containing source symbols only while packets containing FEC parity
   content require decoding before these fields can be interpreted.

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM Common Message Header as described in Section 4.1.  The
   value of the "NORM_DATA" "type" field is 2.  The "NORM_DATA" base
   "hdr_len" value is 4 (32-bit words) plus the size of the
   "fec_payload_id" field.  The "fec_payload_id" field size depends upon
   the FEC encoding 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 .  The "fec_id" field is used to indicate
   the FEC coding type.  For example, when small block, systematic codes
   are used, a "fec_id" value of 129 is indicated and the size of the
   "fec_payload_id" is two 32-bit words.  In this case the "NORM_DATA"
   base "hdr_len" value is 6.  The cumulative size of any header
   extensions applied is added into the "hdr_len" field.

   The "instance_id" field contains a value generated by the sender to
   uniquely identify its current instance of participation in the
   NormSession.  This allows receivers to detect when senders have
   perhaps left and rejoined a session in progress.  When a sender
   (identified by its "source_id") is detected to have a new
   "instance_id", the NORM receivers SHOULD drop their previous state on
   the sender and begin reception anew, 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) (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 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 RMT Multicast NACK
   Building Block document [I-D.ietf-rmt-bb-norm-revised]. [RFC5401].

   The "backoff" field value is used by receivers to determine the
   maximum backoff timer value used in the timer-based NORM NACK
   feedback suppression.  This 4-bit field supports values from 0-15
   which is multiplied by the sender GRTT to determine the maximum
   backoff timeout.  The "backoff" field informs the receivers of the
   sender's backoff factor parameter "Ksender".  Recommended values and
   their use are described in the NORM receiver NACK procedure
   description in Section 5.3.

   The "gsize" field contains a representation of the sender's current
   estimate of group size.  This 4-bit field can roughly represent
   values from ten to 500 million where the most significant bit value
   of 0 or 1 represents a mantissa of 1 or 5, respectively and the three
   least significant bits incremented by one represent a base 10
   exponent (order of magnitude).  For examples, a field value of "0x0"
   represents 1.0e+01 (10), a value of "0x8" represents 5.0e+01 (50), a
   value of "0x1" represents 1.0e+02 (100), and a value of "0xf"
   represents 5.0e+08.  For NORM feedback suppression purposes, the
   group size does not need to be represented with a high degree of
   precision.  The group size may even be estimated somewhat
   conservatively (i.e., overestimated) to maintain low levels of
   feedback traffic.  A default group size estimate of 10,000 ("gsize" =
   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 regarding how the receiver should
   handle the identified object.  Defined flags in this field include:

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

   "NORM_FLAG_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 repair.  This flag could possibly be used by intermediate
   systems implementing functionality to control sub-casting of repair
   content to different legs of a reliable multicast topology with
   disparate repair needs.  "NORM_FLAG_INFO" is set only when optional
   "NORM_INFO" content is actually available for the associated object.
   Thus, receivers will NACK for retransmission of "NORM_INFO" only when
   it is available for a given object.  "NORM_FLAG_UNRELIABLE" is set
   when the sender wishes to transmit an object with only "best effort"
   delivery and will not supply repair transmissions for the object.

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

   "NORM_FLAG_FILE" can be set as a hint from the sender that the
   associated object should be stored in non-volatile storage.
   "NORM_FLAG_STREAM" is set when the identified object is of type
   "NORM_OBJECT_STREAM".  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 the
   "NORM_OBJECT_STREAM" requires systematic FEC codes for most efficient

   The "object_transport_id" field is a monotonically and incrementally
   increasing value assigned by the sender to NormObjects being
   transmitted.  Transmissions and repair requests related to that
   object use the same "object_transport_id" value.  For sessions of
   very long or indefinite duration, the "object_transport_id" field may
   be repeated, but it is presumed that the 16-bit field size provides
   an adequate enough sequence space to avoid object confusion amongst
   receivers and sources (i.e., receivers SHOULD re-synchronize with a
   server when receiving object sequence identifiers sufficiently out-
   of-range with the current state kept for a given source).  During the
   course of its transmission within a NORM session, an object is
   uniquely identified by the concatenation of the sender "source_id"
   and the given "object_transport_id".  Note that "NORM_INFO" messages
   associated with the identified object carry the same
   "object_transport_id" value.

   The "fec_payload_id" identifies the attached "NORM_DATA" "payload"
   content.  The size and format of the "fec_payload_id" field depends
   upon the FEC type indicated by the "fec_id" field.  These formats are
   given in the descriptions of specific FEC schemes such as those
   described in the IETF FEC Basic Schemes document I-d
   [I-D.ietf-rmt-bb-fec-basic-schemes-revised] specification or in
   additional FEC Scheme documents that may be defined.  As an example,
   the format of the "fec_payload_id" format for Small Block, Systematic
   codes ("fec_id" = 129) from the FEC Basic Schemes
   [I-D.ietf-rmt-bb-fec-basic-schemes-revised] specification is given
      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       |

        Example: FEC Payload Id Format for &quot;fec_id&quot; = 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 given by the FEC Basic Schemes
   document [I-D.ietf-rmt-bb-fec-basic-schemes-revised] for Small Block Systematic FEC
   Schemes identified by a "fec_id" value of 129. 129 as specified by the FEC
   Basic Schemes [I-D.ietf-rmt-bb-fec-basic-schemes-revised] document .
   The "source_block_number" identifies the coding block's relative
   position with a NormObject.  Note that, for NormObjects of type
   "NORM_OBJECT_STREAM", the "source_block_number" may wrap for very
   long lived sessions.  The "source_block_len" indicates the number of
   user data segments in the identified coding block.  Given the
   "source_block_len" information of how many symbols of application
   data are contained in the block, the receiver can determine whether
   the attached segment is data or parity content and treat it
   appropriately.  Some 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 may only know the maximum source block length
   at the time 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) referenced may be a user data or
   an FEC parity segment.  For systematic codes, encoding symbols
   numbered less than the "source_block_len" contain original
   application data while segments greater than or equal to
   "source_block_len" contain parity symbols calculated for the block.

   The concatenation of "object_transport_id::fec_payload_id" can be
   viewed as a unique transport protocol data unit identifier for the
   attached segment with respect to the NORM sender's instance within a

   Additional FEC Object Transmission Information (FTI) (as described in
   the FEC Building Block document [RFC5052]) is required to properly
   receive and decode NORM transport objects.  This information MAY be
   provided as out-of-band session information.  However, in some cases,
   it may be useful for the sender to include this information "in-band"
   to facilitate receiver operation with minimal 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

      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 ...              |

              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 may
   wish to forego reliable reception (i.e., not NACK for) of the
   indicated object.  In the case of objects of type
   "NORM_OBJECT_STREAM", the "object_size" field is used by the sender
   to advertise the size of its stream buffer to the receiver group.  In
   turn, the receivers SHOULD use this information to allocate a stream
   buffer for reception of corresponding size.

   As noted, the format of the extension depends upon the FEC code in
   use, but in general it SHOULD contain any required details on the FEC
   code in use (e.g., FEC Instance ID, etc.).  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        |

   Example: EXT_FTI Header Extension Format for &quot;fec_id&quot; = 129

   In this example (for "fec_id" = 129), the "hel" field value is 4.
   The size of the EXT_FTI header extension may 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 document [RFC5052].  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 [RFC5052].

   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
   symbols that can be generated for any source block" as described in
   for FEC Object Transmission Information for Small Block Systematic
   Codes in the FEC Building Block document [RFC5052].  For example,
   Reed-Solomon codes may be arbitrarily shortened to create different
   code variations for a given block length.  In the case of Reed-
   Solomon (GF(2^8) and GF(2^16)) codes, this value indicates the
   maximum number of parity segments available from the sender for the
   coding blocks.  This field MAY be interpreted differently for other
   systematic codes as they are defined.

   The payload portion of "NORM_DATA" messages includes source data or
   FEC encoded application content.  The 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 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 document [RFC5052].  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.  When the "payload_len" value is equal to ZERO,
   this indicates that the "payload_msg_start" field should be
   alternatively interpreted as a "stream_control_code".  The only
   "stream_control_code" value currently defined is "NORM_STREAM_END =
   0".  The "NORM_STREAM_END" code indicates that the sender is
   terminating transmission of stream content at the corresponding
   position in the stream and the receiver should not expect content (or
   NACK for any content) following that position in the stream.  Future
   versions of this specification may define additional stream control
   codes if necessary.  Values of "stream_control_code" that are not
   understood SHOULD be ignored.

   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", that indicates the "payload_msg_start" field should instead be
   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 may wrap.

   The "payload_data" field contains the original application source or
   parity content for the symbol identified by the "fec_payload_id".
   The length of this field SHALL be limited to a maximum of the
   sender's NormSegmentSize bytes as given in the FTI for the object.
   Note the length of this field for messages containing parity content
   will always be of length NormSegmentSize.  When encoding data
   segments of varying sizes, the FEC encoder SHALL assume ZERO value
   padding for data segments with length less than the NormSegmentSize.
   It is RECOMMENDED that a sender's NormSegmentSize generally be
   constant for the duration of a given sender's term of participation
   in the session, but may possibly vary on a per-object basis.  The
   NormSegmentSize is expected to be configurable by the sender
   application prior to session participation as needed for network
   topology maximum transmission unit (MTU) considerations.  For IPv6,
   MTU discovery may be possibly leveraged at session startup to perform
   this configuration.  The "payload_data" content may be delivered
   directly to the application for source symbols (when systematic FEC
   encoding is used) or upon decoding of the FEC block.  For
   "NORM_OBJECT_FILE" and "NORM_OBJECT_STREAM" objects, the data segment
   length and offset can be calculated using the block partitioning
   algorithm described in the FEC Building Block document [RFC5052].
   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 may use the "NORM_INFO" content
   to make a decision as whether to participate in reliable reception of
   the associated object.  Each NormObject can have an independent unit
   of "NORM_INFO" associated with it.  "NORM_DATA" messages contain a
   flag to indicate the availability of "NORM_INFO" for a given
   NormObject.  NORM receivers may NACK for retransmission of
   "NORM_INFO" when they have not received it for a given NormObject.
   The size of the "NORM_INFO" content is limited to that of a single
   NormSegmentSize for the given sender.  This atomic nature allows the
   "NORM_INFO" to be rapidly and efficiently repaired within the NORM
   reliable transmission process.

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

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

                         NORM_INFO Message Format

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

   The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
   "object_transport_id" fields carry the same information and serve the
   same purpose as with "NORM_DATA" messages.  These values allow the
   receiver to prepare appropriate buffering, etc, for further
   transmissions from the sender when "NORM_INFO" is the first message

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

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

4.2.3.  NORM_CMD Messages

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

   Additionally, a range of command types remain available for potential
   application-specific use.  Some "NORM_CMD" types may have dynamic
   content attached.  Any attached content will be limited to maximum
   length of the sender NormSegmentSize to retain the atomic nature of
   commands.  All "NORM_CMD" messages begin with a common set of fields,
   after the usual NORM message common header.  The standard "NORM_CMD"
   fields are:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |     flavor    |                                               |
     +-+-+-+-+-+-+-+-+        NORM_CMD Content                       +
     |                              ...                              |

                         NORM_CMD Standard Fields

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

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

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

   The "NORM_CMD(FLUSH)" command is sent when the sender reaches the end
   of all data content and pending repairs it has queued for
   transmission.  This may indicate a temporary or permanent end of data
   transmission, but the sender is still willing to respond to repair
   requests.  This command is repeated once per "2*GRTT" to excite the
   receiver set for any outstanding repair requests up to and including
   the transmission point indicated within the "NORM_CMD(FLUSH)"
   message.  The number of repeats is equal to "NORM_ROBUST_FACTOR"
   unless a list of receivers from which explicit positive
   acknowledgment 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
   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 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-size transport objects such as "NORM_OBJECT_DATA" and
   "NORM_OBJECT_FILE", the flush process (if there are no further
   pending objects) occurs at the end of these objects.  Thus, FEC
   repair information is always available for repairs in response to
   repair requests elicited by the flush command.  However, for
   "NORM_OBJECT_STREAM", the flush may occur at any time, including in
   the middle of an FEC coding block if systematic FEC codes are
   employed.  In this case, the sender will not yet be able to provide
   FEC parity content 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 on-going 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",

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

                      NORM_CMD(FLUSH) Message Format

   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
   NACK content for the applicable "source_block_number" which does not
   include any requests for parity-based repair.  This allows NORM
   sender applications to "flush" an ongoing stream of transmission when
   needed, even if in the middle of an FEC block.  Once the sender
   resumes stream transmission and passes the end of the pending coding
   block, subsequent NACKs from receivers SHALL request parity-based
   repair as usual.  Note that the use of a systematic FEC code is
   assumed here.  It should also be noted 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.  NORM_CMD(EOT) Message

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

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

                       NORM_CMD(EOT) Message Format

   The value of the "hdr_len" field for "NORM_CMD(EOT)" messages without
   header extensions present is 4.  The "reserved" field is reserved for
   future use and MUST be set to an all ZERO value.  Receivers MUST
   ignore the "reserved" field.  NORM_CMD(SQUELCH) Message

   The "NORM_CMD(SQUELCH)" command is transmitted in response to
   outdated or invalid "NORM_NACK" content received by the sender.
   Invalid "NORM_NACK" content consists of repair requests for
   NormObjects for which the sender is unable or unwilling to provide
   repair.  This includes repair requests for outdated objects, aborted
   objects, or those objects which the sender previously transmitted
   marked with the "NORM_FLAG_UNRELIABLE" flag.  This command indicates
   to receivers what content is available for repair, thus serving as a
   description of the sender's current "repair window".  Receivers SHALL
   not generate repair requests for content identified as invalid by a

   The "NORM_CMD(SQUELCH)" command is sent once per "2*GRTT" at the
   most.  The "NORM_CMD(SQUELCH)" advertises the current "repair window"
   of the sender by identifying the earliest (lowest) transmission point
   for which it will provide repair, along with an encoded list of
   objects from that point forward that are no longer valid for repair.
   This mechanism allows the sender application to cancel or abort
   transmission and/or repair of specific previously enqueued objects.
   The list also contains the identifiers for any objects within the
   repair window that were sent with the "NORM_FLAG_UNRELIABLE" flag
   set.  In normal conditions, it is expected the "NORM_CMD(SQUELCH)"
   will be needed infrequently, and generally only to provide a
   reference repair window for receivers who have fallen "out-of-sync"
   with the sender due to extremely poor network conditions.

   The starting point of the invalid NormObject list begins with the
   lowest invalid NormTransportId greater than the current "repair
   window" start from the invalid NACK(s) that prompted the generation
   of the squelch.  The length of the list is limited by the sender's
   NormSegmentSize.  This allows the receivers to learn the status of
   the sender's applicable object repair window with minimal
   transmission of "NORM_CMD(SQUELCH)" commands.  The format of the
   "NORM_CMD(SQUELCH)" message is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  flavor = 3   |     fec_id    |      object_transport_id      |
     |                         fec_payload_id                        |
     |                              ...                              |
     |                        invalid_object_list                    |
     |                              ...                              |

                     NORM_CMD(SQUELCH) Message Format

   In addition to the NORM common message header and standard "NORM_CMD"
   fields, the "NORM_CMD(SQUELCH)" message contains fields to identify
   the earliest logical transmit position of the sender's current repair
   window and an "invalid_object_list" beginning with the index of the
   logically earliest invalid repair request from the offending NACK
   message which initiated the "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.

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

   The "NORM_CMD(CC)" messages contains fields to enable sender-to-
   receiver group greatest round-trip time (GRTT) measurement and to
   excite the group for congestion control feedback.  A baseline NORM
   congestion control scheme (NORM-CC), based on the TCP-Friendly
   Multicast Congestion Control (TFMCC) scheme of [RFC4654] is described
   in Section 5.5.2 of this document.  The "NORM_CMD(CC)" message is
   usually transmitted as part of NORM-CC congestion control operation.
   A NORM header extension is defined below to be used with the
   "NORM_CMD(CC)" message to support NORM-CC operation.  Different
   header extensions may be defined for the "NORM_CMD(CC)" (and/or other
   NORM messages as needed) to support alternative congestion control
   schemes in the future.  If NORM is operated in a private network with
   congestion control operation disabled, the "NORM_CMD(CC)" message is
   then used for GRTT measurement only and may optionally be sent less
   frequently than with congestion control operation.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |            sequence           |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |   flavor = 4  |    reserved   |          cc_sequence          |
     |                         send_time_sec                         |
     |                        send_time_usec                         |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                  cc_node_list (if applicable)                 |
     |                              ...                              |

                        NORM_CMD(CC) Message Format

   The NORM common message header and standard "NORM_CMD" fields serve
   their usual purposes.  The 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 may be possible that alternative
   congestion control schemes may 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 [RFC4654].  The
   most recently received "cc_sequence" value is recorded by receivers
   and can be fed back to the sender in congestion control feedback
   generated by the receivers for that sender.  The "cc_sequence" number
   can also be used in NORM implementations to assess how recently a
   receiver has received "NORM_CMD(CC)" probes from the sender.  This
   can be useful instrumentation for complex or experimental multicast
   routing environments.

   The "send_time" field is a timestamp indicating the time that the
   "NORM_CMD(CC)" message was transmitted.  This consists of a 64-bit
   field containing 32-bits with the time in seconds ("send_time_sec")
   and 32-bits with the time in microseconds ("send_time_usec") since
   some reference time the source maintains (usually 00:00:00, 1 January
   1970).  The byte ordering of the fields is "Big Endian" network
   order.  Receivers use this timestamp adjusted by the amount of delay
   from the time they received the "NORM_CMD(CC)" message to the time of
   their response as the "grtt_response" portion of "NORM_ACK" and
   "NORM_NACK" messages generated.  This allows the sender to evaluate
   round-trip times to different receivers for congestion control and
   other (e.g., GRTT determination) purposes.

   To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
   a 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 256kbps (3.2e+04
   bytes per second), the lower 4 bits of the 16-bit field contain a
   value of 0x04 to represent the exponent (E) while the upper 12 bits
   contain a value of 0x51f (M) as determined from the equation given
        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
   scheme is approximately 9.99e+15 bytes per second.

   When this extension is present, a "cc_node_list" may be attached as
   the payload of the "NORM_CMD(CC)" message.  The presence of this
   header extension also implies that NORM receivers should respond
   according to the procedures described in Section 5.5.2.

   The "cc_node_list" consists of a list of NormNodeIds and their
   associated congestion control status.  This includes the current
   limiting receiver (CLR) node, any potential limiting receiver (PLR)
   nodes that have been identified, and some number of receivers for
   which congestion control status is being provided, most notably
   including the receivers' current RTT measurement.  The maximum length
   of the "cc_node_list" provides for at least the CLR and one other
   receiver, but may be configurable for more timely feedback to the
   group.  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 which the item

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

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

   The "cc_rtt" contains a quantized representation of the RTT as
   measured by the sender with respect to the indicated receiver.  This
   field is valid only if the "NORM_FLAG_CC_RTT" flag is set in the
   "cc_flags" field.  This one byte field is a quantized representation
   of the RTT using the algorithm described in the Multicast NACK
   Building Block document [I-D.ietf-rmt-bb-norm-revised]. [RFC5401].

   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.  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].
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  flavor = 5   |     flags     |            reserved           |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                       repair_adv_payload                      |
     |                              ...                              |
                    NORM_CMD(REPAIR_ADV) Message Format

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

   The "flags" field provide information on the "NORM_CMD(REPAIR_ADV)"
   content.  There is currently one "NORM_CMD(REPAIR_ADV)" flag defined:
                     NORM_REPAIR_ADV_FLAG_LIMIT = 0x01

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

   When congestion control operation is enabled, a header extension may
   be applied to the "NORM_CMD(REPAIR_ADV)" representing the most
   limiting (in terms of congestion control feedback suppression)
   congestion control response.  This allows the "NORM_CMD(REPAIR_ADV)"
   message to suppress receiver congestion control responses as well as
   NACK feedback messages.  The field is defined as a header extension
   so that alternative congestion control schemes may be used with NORM
   without revision to this document.  A NORM-CC Feedback Header
   Extension (EXT_CC) is defined to encapsulate congestion control
   feedback within "NORM_NACK", "NORM_ACK", and "NORM_CMD(REPAIR_ADV)"
   messages.  If another congestion control technique (e.g., Pragmatic
   General Multicast Congestion Control (PGMCC) [PgmccPaper]) is used
   within a NORM implementation, an additional header extension MAY need
   to be defined encapsulate any required feedback content.  The NORM-CC
   Feedback Header Extension format is:
      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 may choose to take congestion
   avoidance measures.  For "NORM_CMD(REPAIR_ADV)" messages, the sender
   SHALL set the "cc_sequence" field value to the value set in the last
   "NORM_CMD(CC)" message sent.

   The "cc_flags" field contains bits representing the receiver's state
   with respect to congestion control operation.  The possible values
   for the "cc_flags" field are those specified for the "NORM_CMD(CC)"
   message node list item flags.  These fields are used by receivers in
   controlling (suppressing as necessary) their congestion control
   feedback.  For "NORM_CMD(REPAIR_ADV)" messages, the
   "NORM_FLAG_CC_RTT" should be set only when all feedback messages
   received by the sender have the flag set.  Similarly, the
   "NORM_FLAG_CC_CLR" or "NORM_FLAG_CC_PLR" should be set only when no
   feedback has been received from non-CLR or non-PLR receivers.  And
   the "NORM_FLAG_CC_LEAVE" should be set only when all feedback
   messages the sender has received have this flag set.  These
   heuristics for setting the flags in "NORM_CMD(REPAIR_ADV)" ensure the
   most effective suppression of receivers providing unicast feedback

   The "cc_rtt" field SHALL be set to a default maximum value and the
   "NORM_FLAG_CC_RTT" flag SHALL be cleared when no receiver has yet
   received RTT measurement information.  When a receiver has received
   RTT measurement information, it shall set the "cc_rtt" value
   accordingly and set the "NORM_FLAG_CC_RTT" flag in the "cc_flags"
   field.  For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the
   "cc_rtt" field value to the largest non-CLR/non-PLR RTT it has
   measured from receivers for the current feedback round.

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

                "cc_loss" = decimal_loss_fraction * 65535.0

   For "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the
   "cc_loss" field value to the largest non-CLR/non-PLR loss estimate it
   has received from receivers for the current feedback round.

   The "cc_rate" field represents the receivers current local congestion
   control rate.  During "slow start", when the receiver has detected no
   loss, this value is set to twice the actual rate it has measured from
   the corresponding sender and the "NORM_FLAG_CC_START" is set in the
   "cc_flags' field.  Otherwise, the receiver calculates a congestion
   control rate based on its loss measurement and RTT measurement
   information (even if default) for the "cc_rate" field.  For
   "NORM_CMD(REPAIR_ADV)" messages, the sender SHALL set the "cc_loss"
   field value to the lowest non-CLR/non-PLR "cc_rate" report it has
   received from receivers for the current feedback round.

   The "cc_reserved" field is reserved for future NORM protocol use.
   Currently, senders SHALL set this field to ZERO, and receivers SHALL
   ignore the content of this field.

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

   The "NORM_CMD(ACK_REQ)" message is used by the sender to request
   acknowledgment from a specified list of receivers.  This message is
   used in providing a lightweight positive acknowledgment mechanism
   that is OPTIONAL for use by the reliable multicast application.  A
   range of acknowledgment request types is provided for use at the
   application's discretion.  Provision for application-defined,
   positively-acknowledged commands allows the application to
   automatically take advantage of transmission and round-trip timing
   information available to the NORM protocol.  The details of the NORM
   positive acknowledgment process including transmission of the
   "NORM_CMD(ACK_REQ)" messages and the receiver response ("NORM_ACK")
   are described in Section 5.5.3.  The format of the
   "NORM_CMD(ACK_REQ)" message is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  flavor = 6   |    reserved   |    ack_type   |    ack_id     |
     |                       acking_node_list                        |
     |                              ...                              |

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

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

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

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

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

   The "NORM_ACK_APPLICATION" range of "ack_type" values is provided so
   that NORM applications may implement application-defined, positively-
   acknowledged commands that are able to leverage internal transmission
   and round-trip timing information available to the NORM protocol

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

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

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

   This command allows the NORM application to robustly transmit
   application-defined commands.  The command message preempts any
   ongoing data transmission and is repeated up to "NORM_ROBUST_FACTOR"
   times at a rate of once per "2*GRTT".  This rate of repetition allows
   the application to observe any response (if that is the application's
   purpose for the command) before it is repeated.  Possible responses
   may include initiation of data transmission, other
   "NORM_CMD(APPLICATION)" messages, or even application-defined,
   positively-acknowledge commands from other NormSession participants.
   The transmission of these commands will preempt data transmission
   when they are scheduled and may be multiplexed with ongoing data
   transmission.  This type of robustly transmitted command allows NORM
   applications to define a complete set of session control mechanisms
   with less state than the transfer of FEC encoded reliable content
   requires while taking advantage of NORM transmission and round-trip
   timing information.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=3|    hdr_len    |          sequence             |
     |                           source_id                           |
     |          instance_id          |     grtt      |backoff| gsize |
     |  flavor = 7   |                    reserved                   |
     |                   Application-Defined Content                 |
     |                              ...                              |

                   NORM_CMD(APPLICATION) Message Format

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

   The "Application-Defined Content" area contains information in a
   format at the discretion of the application.  The size of this
   payload SHALL be limited to a maximum of the sender's NormSegmentSize
   setting.  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

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

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=4|    hdr_len    |            sequence           |
     |                           source_id                           |
     |                           server_id                           |
     |           instance_id         |            reserved           |
     |                       grtt_response_sec                       |
     |                       grtt_response_usec                      |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                          nack_payload                         |
     |                              ...                              |

                         NORM_NACK Message Format

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

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

   The "instance_id" field contains the current session identifier given
   by the sender identified by the "server_id" field in its sender
   messages.  The sender SHOULD ignore feedback messages which contain
   an invalid "instance_id" value.

   The "grtt_response" fields contain an adjusted version of the
   timestamp from the most recently received "NORM_CMD(CC)" message for
   the indicated NORM sender.  The format of the "grtt_response" is the
   same as the "send_time" field of the "NORM_CMD(CC)".  The
   "grtt_response" value is relative to the "send_time" the source
   provided with a corresponding "NORM_CMD(CC)" command.  The receiver
   adjusts the source's "NORM_CMD(CC)" "send_time" timestamp by adding
   the time 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" 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 that it has not yet received a "NORM_CMD(CC)" message from
   the indicated sender and that the sender should ignore the
   "grtt_response" in this message.

   For NORM-CC operation, the NORM-CC Feedback Header Extension, as
   described in the "NORM_CMD(REPAIR_ADV}" message description, is added
   to "NORM_NACK" messages to provide feedback on the receivers current
   state with respect to congestion control operation.  Note that
   alternative header extensions for congestion control feedback may be
   defined for alternative congestion control schemes for NORM use in
   the future.

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

   The "nack_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 requires from the
   sender in order to complete reliable reception up to the sender's
   transmission position at the moment the receiver initiates the NACK
   Procedure as described in Section 5.3.  A single NORM Repair Request
   consists of a list of items, ranges, and/or FEC coding block erasure
   counts for needed "NORM_DATA" and/or "NORM_INFO" content.  Multiple
   repair requests may be concatenated within the "nack_payload" field
   of a "NORM_NACK" message.  Note that a single NORM Repair Request can
   possibly include multiple "items", "ranges", or "erasure_counts".  In
   turn, the "nack_payload" field MAY contain multiple repair requests.
   A single NORM Repair Request has the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |      form     |     flags     |             length            |
     |                      repair_request_items                     |
     |                             ...                               |

                        NORM Repair Request Format

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

                     | Form                 | Value |
                     | "NORM_NACK_ITEMS"    |   1   |
                     | "NORM_NACK_RANGES"   |   2   |
                     | "NORM_NACK_ERASURES" |   3   |

   A "form" value of "NORM_NACK_ITEMS" indicates each repair request
   item in the "repair_request_items" list is to be treated as an
   individual request.  A value of "NORM_NACK_RANGES" indicates that the
   "repair_request_items" list consists of pairs of repair request items
   that correspond to inclusive ranges of repair needs.  And the
   "NORM_NACK_ERASURES" "form" indicates that the repair request items
   are to be treated individually and that the "encoding_symbol_id"
   portion of the "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 are required as   |
   |                     |       | repair.                             |
   | "NORM_NACK_BLOCK"   |  0x02 | Indicates the listed block(s) or    |
   |                     |       | range of blocks in entirety are     |
   |                     |       | required as repair.                 |
   | "NORM_NACK_INFO"    |  0x04 | Indicates that "NORM_INFO" is       |
   |                     |       | required as repair for the listed   |
   |                     |       | object(s).                          |
   | "NORM_NACK_OBJECT"  |  0x08 | Indicates the listed object(s) or   |
   |                     |       | range of objects in entirety are    |
   |                     |       | required as repair.                 |

   When the "NORM_NACK_SEGMENT" flag is set, the "object_transport_id"
   and "fec_payload_id" fields are used to determine which sets or
   ranges of individual "NORM_DATA" segments are needed to repair
   content at the receiver.  When the "NORM_NACK_BLOCK" flag is set,
   this indicates the receiver is completely missing the indicated
   coding block(s) and requires transmissions sufficient to repair the
   indicated block(s) in their entirety.  When the "NORM_NACK_INFO" flag
   is set, this indicates the receiver is missing the "NORM_INFO"
   segment for the indicated "object_transport_id".  Note the
   "NORM_NACK_INFO" may be set in combination with the "NORM_NACK_BLOCK"
   or "NORM_NACK_SEGMENT" flags, or may be set alone.  When the
   "NORM_NACK_OBJECT" flag is set, this indicates the receiver is
   missing the entire NormTransportObject referenced by the
   "object_transport_id".  This also implicitly requests any available
   "NORM_INFO" for the NormObject, if applicable.  The "fec_payload_id"
   field is ignored when the flag "NORM_NACK_OBJECT" is set.

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

   The "repair_request_items" field consists of a list of individual or
   range pairs of transport data unit identifiers in the following
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     fec_id    |   reserved    |      object_transport_id      |
     |                        fec_payload_id                         |
     |                              ...                              |

                      NORM Repair Request Item Format

   The "fec_id" indicates the FEC type and can be used to determine the
   format of the "fec_payload_id" field.  The "reserved" field is kept
   for possible future use and SHALL be set to a ZERO value and ignored
   by NORM nodes processing NACK content.

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

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

   When the receiver's repair needs dictate that different forms (mixed
   ranges and/or individual items) or types (mixed specific segments
   and/or blocks or objects in entirety) are required to complete
   reliable transmission, multiple NORM Repair Requests with different
   "form" and or "flags" values can be concatenated within a single
   "NORM_NACK" message.  Additionally, NORM receivers SHALL construct
   "NORM_NACK" messages with their repair requests in ordinal order with
   respect to "object_transport_id" and "fec_payload_id" values.  The
   "nack_payload" size SHALL NOT exceed the NormSegmentSize for the
   sender to which the "NORM_NACK" is destined.

   NORM_NACK Content Examples:

   In these examples, a small block, systematic FEC code ("fec_id" =
   129) is assumed with a user data block length of 32 segments.  In
   Example 1, a list of individual "NORM_NACK_ITEMS" repair requests is
   given.  In Example 2, a list of "NORM_NACK_RANGES" requests AND a
   single "NORM_NACK_ITEMS" request are concatenated to illustrate the
   possible content of a "NORM_NACK" message.  Note that FEC coding
   block erasure counts could also be provided in each case.  However,
   the erasure counts are not really necessary since the sender can
   easily determine the erasure count while processing the NACK content.
   However, the erasure count option may be useful for operation with
   other FEC codes or for intermediate system purposes.

   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     |
   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.
   As mentioned in the "NORM_CMD(ACK_REQ)" message description, the
   acknowledgment type "NORM_ACK_CC" is provided for this purpose.  The
   generation of "NORM_ACK(CC)" messages for round-trip timing
   estimation and congestion-control operation is described in
   Section 5.5.1 and Section 5.5.2, respectively.  However, some
   multicast applications may benefit from some limited form of positive
   acknowledgment for certain functions.  A simple, scalable positive
   acknowledgment scheme is defined in Section 5.5.3 that can be
   leveraged by protocol implementations when appropriate.  The
   "NORM_CMD(FLUSH)" may be used for OPTIONAL collection of positive
   acknowledgment of reliable reception to a certain "watermark"
   transmission point from specific receivers using this mechanism.  The
   "NORM_ACK" type "NORM_ACK_FLUSH" is provided for this purpose and the
   format of the "nack_payload" for this acknowledgment type is given
   below.  Beyond that, a range of application-defined "ack_type" values
   is provided for use at the NORM application's discretion.
   Implementations making use of application-defined positive
   acknowledgments may also make use the "nack_payload" as needed,
   observing the constraint that the "nack_payload" field size be
   limited to a maximum of the NormSegmentSize for the sender to which
   the "NORM_ACK" is destined.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |version| type=5|    hdr_len    |          sequence             |
     |                           source_id                           |
     |                           server_id                           |
     |           instance_id         |    ack_type  |     ack_id     |
     |                       grtt_response_sec                       |
     |                       grtt_response_usec                      |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                   ack_payload (if applicable)                 |
     |                              ...                              |

                          NORM_ACK Message Format

   The NORM common message header fields serve their usual purposes.
   The 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.
   And header extensions may be applied to support congestion control
   feedback or other functions in the same manner.

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

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

   The "ack_payload" format is a function of the "ack_type".  The
   "NORM_ACK_CC" message has no attached content.  Only the "NORM_ACK"
   header applies.  In the case of "NORM_ACK_FLUSH", a specific
   "ack_payload" format is defined:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     fec_id    |   reserved    |      object_transport_id      |
     |                        fec_payload_id                         |
     |                              ...                              |

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

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

4.4.  General Purpose Messages

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

4.4.1.  NORM_REPORT Message

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

5.  Detailed Protocol Operation

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

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

   2.  The sender transmits an ordinal set of NormObjects segmented in
       the form of "NORM_DATA" messages labeled with NormTransportIds
       and logically identified with FEC encoding block numbers and
       symbol identifiers.  "NORM_INFO" messages may optionally precede
       the transmission of data content for NORM transport objects.
   3.  As receivers detect missing content from the sender, they
       initiate repair requests with "NORM_NACK" messages.  Note the
       receivers track the sender's most recent objectId::fecPayloadId
       transmit position and NACK ONLY for content ordinally prior to
       that transmit position.  The receivers schedule random backoff
       timeouts before generating "NORM_NACK" messages and wait an
       appropriate amount of time before repeating the "NORM_NACK" if
       their repair request is not satisfied.
   4.  The sender aggregates repair requests from the receivers and
       logically "rewinds" its transmit position to send appropriate
       repair messages.  The sender sends repairs for the earliest
       ordinal transmit position first and maintains this ordinal repair
       transmission sequence.  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 expected to be
       an exception in general protocol operation, but the possibility
       does exist for extreme conditions).  The sender immediately
       assumes transmission of new content once it has sent pending
   5.  The sender transmits "NORM_CMD(FLUSH)" messages when it reaches
       the end of enqueued transmit content and pending repairs.
       Receivers respond to the "NORM_CMD(FLUSH)" messages with
       "NORM_NACK" transmissions (following the same suppression backoff
       timeout strategy as for data) if they require further repair.
   6.  The sender transmissions are subject to rate control limits
       determined by congestion control mechanisms.  In the baseline
       NORM-CC operation, each sender in a NormSession maintains its own
       independent congestion control state.  Receivers provide
       congestion control feedback in "NORM_NACK" and "NORM_ACK"
       messages.  "NORM_ACK" feedback for congestion control purposes is
       governed using a suppression mechanism similar to that for
       "NORM_NACK" messages.

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

5.1.  Sender Initialization and Transmission

   Upon startup, the NORM sender immediately begins sending
   "NORM_CMD(CC)" messages to collect round trip timing and other
   information from the potential group.  If NORM-CC congestion control
   operation is enabled, the NORM-CC Rate header extension MUST be
   included in these messages.  Congestion control operation SHALL be
   observed at all times when operating in the general Internet.  Even
   if congestion control operation is disabled at the sender, it may be
   desirable to use the "NORM_CMD(CC)" messaging to collect feedback
   from the group using the baseline NORM-CC feedback mechanisms.  This
   proactive feedback collection can be used to establish a GRTT
   estimate prior to data transmission and potential NACK operation.

   In some cases, applications may wish for the sender to also proceed
   with data transmission immediately.  In other cases, the sender may
   wish to defer data transmission until it has received some feedback
   or request from the receiver set indicating that receivers are indeed
   present.  Note, in some applications (e.g., web push), this
   indication may come out-of-band with respect to the multicast session
   via other means.  As noted, the periodic transmission of
   "NORM_CMD(CC)" messages may precede actual data transmission in order
   to have an initial GRTT estimate.

   With inclusion of the OPTIONAL NORM FEC Object Transmission
   Information Header Extension (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 that receivers
   have received the FEC Object Transmission Information via other
   means.  Additionally, applications may leverage the use of
   "NORM_INFO" messages associated with the session data objects in the
   session to provide application-specific context information for the
   session and data being transmitted.  These mechanisms allow for
   operation with minimal pre-coordination among the senders and

   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 document [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 ("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".
   Receivers may respond to these "NORM_CMD(FLUSH)" messages with
   additional repair requests.  A protocol parameter
   ""NORM_ROBUST_FACTOR"" determines the number of flush messages sent.
   If receivers request repair, the repair is provided and flushing
   occurs again at the end of repair transmission.  The sender may
   attach an OPTIONAL "acking_node_list" to "NORM_CMD(FLUSH)" containing
   the NormNodeIds for receivers from which it expects explicit positive
   acknowledgment of reception.  The "NORM_CMD(FLUSH)" message may be
   also used for this optional function any time prior to the end of
   data enqueued for transmission with the "NORM_CMD(FLUSH)" messages
   multiplexed with ongoing data transmissions.  The OPTIONAL NORM
   positive acknowledgment procedure is described in Section 5.5.3.

5.1.1.  Object Segmentation Algorithm

   NORM senders and receivers MUST use a common algorithm for logically
   segmenting transport data into FEC encoding blocks and symbols so
   that appropriate NACKs can be constructed to request repair of
   missing data.  NORM FEC coding blocks are comprised of multi-byte
   symbols (segments) that are transmitted in the payload of "NORM_DATA"
   messages.  Each "NORM_DATA" message will contain one or more source
   or encoding symbol(s) 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
   document [RFC5052] is RECOMMENDED for use to compute a source block
   structure such that all source blocks are as close to being equal
   length as possible.  This helps avoid the performance disadvantages
   of "short" FEC blocks.  Note this algorithm applies only to the
   statically-sized "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" transport
   object types where the object size is fixed and predetermined.  For
   "NORM_OBJECT_STREAM" objects, the object is segmented according to
   the maximum source block length given in the FEC Transmission
   Information, unless the FEC Payload ID indicates an alternative size
   for a given block.

5.2.  Receiver Initialization and Reception

   The NORM protocol is designed such that receivers may join and leave
   the group at will.  However, some applications may be constrained
   such that receivers need to be members of the group prior to start of
   data transmission.  NORM applications may use different policies to
   constrain the impact of new receivers joining the group in the middle
   of a session.  For example, a useful implementation policy is for new
   receivers joining the group to limit or avoid repair requests for
   transport objects already in progress.  The NORM sender
   implementation may wish to impose additional constraints to limit the
   ability of receivers to disrupt reliable multicast performance by
   joining, leaving, and rejoining the group often.  Different receiver
   "join policies" may be appropriate for different applications and/or
   scenarios.  For general purpose operation, 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 that the join policy constrain
   receivers to start reliable reception at the current FEC coding block
   for which non-repair content is received.

   For typical operation, it is expected that NORM receivers will join a
   specified multicast group and/or listen on an specific port number
   for sender transmissions.  As the NORM receiver receives "NORM_DATA"
   messages it will provide content to its application as appropriate.

5.3.  Receiver NACK Procedure

   When the receiver detects it is missing data from a sender's NORM
   transmissions, it initiates its NACKing procedure.  The NACKing
   procedure SHALL be initiated ONLY at FEC coding block boundaries,
   NormObject boundaries, 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 * GRTTSender

   where the ""GRTTsender"" 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 that 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 document
   [RFC5401] using ("Ksender*GRTTsender") for the "maxTime" parameter
   and the sender advertised group size ("GSIZEsender") as the
   "groupSize" parameter.  NORM senders provide values for "GRTTsender",
   "Ksender" and "GSIZEsender" via the "grtt", "backoff", and "gsize"
   fields of transmitted messages.  The "GRTTsender" value is determined
   by the sender based on feedback it has received from the group while
   the "Ksender" and "GSIZEsender" values may determined by application
   requirements and expectations or ancillary information.  The backoff
   factor ""Ksender"" MUST be greater than "one" to provide for
   effective feedback suppression.  A value of "K = 4" is RECOMMENDED
   for the Any Source Multicast (ASM) model while a value of "K = 6" is
   RECOMMENDED for Single Source Multicast (SSM) operation.

         T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender)

   To avoid the possibility of NACK implosion in the case of sender or
   network failure during SSM operation, the receiver SHALL
   automatically suppress its NACK and immediately enter the "holdoff"
   period described below when "T_backoff" is greater than
   "(Ksender-1)*GRTTsender".  Otherwise, the backoff period is entered
   and the receiver MUST accumulate external pending repair state from
   "NORM_NACK" messages and "NORM_CMD(REPAIR_ADV)" messages received.
   At the end of the backoff time, the receiver SHALL generate a
   "NORM_NACK" message only if the following conditions are met:

   1.  The sender's current transmit position (in terms of objectId::
       fecPayloadId) exceeds the earliest repair position of the
   2.  The repair state accumulated from "NORM_NACK" and
       "NORM_CMD(REPAIR_ADV)" messages do not equal or supersede the
       receiver's repair needs up to the sender transmission position at
       the time the NACK procedure (backoff timeout) was initiated.

   If these conditions are met, the receiver immediately generates a
   "NORM_NACK" message when the backoff timeout expires.  Otherwise, the
   receiver's NACK is considered to be "suppressed" and the message is
   not sent.  At this time, the receiver begins a "holdoff" period
   during which it constrains itself to not 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 [I-D.ietf-rmt-bb-norm-revised] [RFC5401] is:
                   T_rcvrHoldoff =(Ksender+2)*GRTTsender
   The "NORM_NACK" message contains repair request content beginning
   with lowest ordinal repair position of the receiver up through the
   coding block prior to the most recently heard ordinal transmission
   position for the sender.  If the size of the "NORM_NACK" content
   exceeds the sender's NormSegmentSize, the NACK content is truncated
   so that the receiver only generates a single "NORM_NACK" message per
   NACK cycle for a given sender.  In summary, a single NACK message is
   generated containing the receiver's lowest ordinal repair needs.

   For each partially-received FEC coding block requiring repair, the
   receiver SHALL, on its FIRST repair attempt for the block, request
   the parity portion of the FEC coding block beginning with the lowest
   ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
   "source_block_len") and request the number of FEC symbols
   corresponding to its data segment erasure count for the block.  On
   subsequent repair cycles for the same coding block, the receiver
   SHALL request only those repair symbols from the first set it has not
   yet received up to the remaining erasure count for that applicable
   coding block.  Note that the sender may have provided other
   different, additional parity segments for other receivers that could
   also be used to satisfy the local receiver's erasure-filling needs.
   In the case where the erasure count for a partially-received FEC
   coding block exceeds the maximum number of parity symbols available
   from the sender for the block (as indicated by the "NORM_DATA"
   "fec_num_parity" field), the receiver SHALL request all available
   parity segments plus the ordinally highest missing data segments
   required to satisfy its total erasure needs for the block.  The goal
   of this strategy is for the overall receiver set to request a lowest
   common denominator set of repair symbols for a given FEC coding
   block.  This allows the sender to construct the most efficient repair
   transmission segment set and enables effective NACK suppression among
   the receivers even with uncorrelated packet loss.  This approach also
   requires no synchronization among the receiver set in their repair
   requests for the sender.

   For FEC coding blocks or NormObjects missed in their entirety, the
   NORM receiver constructs repair requests with "NORM_NACK_BLOCK" or
   "NORM_NACK_OBJECT" flags set as appropriate.  The request for
   retransmission of "NORM_INFO" is accomplished by setting the
   "NORM_NACK_INFO" flag in a corresponding repair request.

5.4.  Sender NACK Processing and Response

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

5.4.1.  Sender Repair State Aggregation

   When a sender is in its normal state of transmitting new data and
   receives a NACK, it begins a procedure to accumulate NACK repair
   state from "NORM_NACK" messages before beginning repair
   transmissions.  Note that this period of aggregating repair state
   does NOT interfere with its ongoing transmission of new data.

   As described in [I-D.ietf-rmt-bb-norm-revised], [RFC5401], the period of time during which the sender
   aggregates "NORM_NACK" messages is equal to:
                     T_sndrAggregate = (Ksender+1)*GRTT

   where ""Ksender"" is the same backoff scaling value used by the
   receivers, and "GRTT" is the sender's current estimate of the group's
   greatest round-trip time.  Note that for NORM unicast sessions the
   ""T_sndrAggregate"" time can be set to ZERO since there is only one
   receiver.  Similarly, the ""Ksender"" 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
   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
   [I-D.ietf-rmt-bb-norm-revised], [RFC5401], the
   value of this sender "holdoff" period is:
                          T_sndrHoldoff = (1*GRTT)

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

5.4.2.  Sender FEC Repair Transmission Strategy

   The NORM sender should leverage transmission of FEC parity content
   for repair to the greatest extent possible.  Recall that the
   receivers use a strategy to request a lowest common denominator of
   explicit repair (including parity content) in the formation of their
   "NORM_NACK" messages.  Before falling back to explicitly satisfying
   different receivers' repair needs, the sender can make use of the
   general erasure-filling capability of FEC-generated parity segments.
   The sender can determine the maximum erasure filling needs for
   individual FEC coding blocks from the "NORM_NACK" messages received
   during the repair aggregation period.  Then, if the sender has a
   sufficient number (less than or equal to the maximum erasure count)
   of previously unsent parity segments available for the applicable
   coding blocks, the sender can transmit these in lieu of the specific
   packets the receiver set has requested.  Only after exhausting its
   supply of "fresh" (unsent) parity segments for a given coding block
   should the sender resort to explicit transmission of the receiver
   set's repair needs.  In general, if a sufficiently powerful FEC code
   is used, the need for explicit repair will be an exception, and the
   fulfillment of reliable multicast can be accomplished quite
   efficiently.  However, the ability to resort to explicit repair
   allows the protocol to be reliable under even very extreme

   "NORM_DATA" messages sent as repair transmissions SHALL be flagged
   with the "NORM_FLAG_REPAIR" flag.  This allows receivers to obey any
   policies that limit new receivers from joining the reliable
   transmission when only repair transmissions have been received.
   Additionally, the sender SHOULD additionally flag "NORM_DATA"
   transmissions sent as explicit repair with the "NORM_FLAG_EXPLICIT"

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

5.4.3.  Sender NORM_CMD(SQUELCH) Generation

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

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 sender port number that is 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 the sender does not
   merely retransmit NACK content it receives, but instead transmits a
   representation of its aggregated repair state.  The transmission of
   "NORM_CMD(REPAIR_ADV)" messages are subject to the sender transmit
   rate limit and NormSegmentSize limitation.  When the
   "NORM_CMD(REPAIR_ADV)" message is of maximum size, receivers SHALL
   consider the maximum ordinal transmission position value embedded in
   the message as the senders current transmission position and
   implicitly suppress requests for ordinally higher repair.  For
   congestion control operation, the sender may also need to provide
   information so that dynamic congestion control feedback can be
   suppressed as needed among receivers.  This document specifies the
   NORM-CC Feedback Header Extension that is applied for baseline
   NORM-CC operation.  If other congestion control mechanisms are used
   within a NORM implementation, other header extensions may be defined.
   Whatever content format is used for this purpose should ensure that
   maximum possible suppression state is conveyed to the receiver set.

5.5.  Additional Protocol Mechanisms

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

5.5.1.  Greatest Round-trip Time Collection

   For NORM receivers to appropriately scale backoff timeouts and the
   senders to use proper corresponding timeouts, the participants must
   agree on a common timeout basis.  Each NORM sender monitors the
   round-trip time of active receivers and determines the group greatest
   round-trip time (GRTT).  The sender advertises this GRTT estimate in
   every message it transmits so that receivers have this value
   available for scaling their timers.  To measure the current GRTT, the
   sender periodically sends "NORM_CMD(CC)" messages that contain a
   locally generated timestamp.  Receivers are expected to record this
   timestamp along with the time the "NORM_CMD(CC)" message is received.
   Then, when the receivers generate feedback messages to the sender, an
   adjusted version of the sender timestamp is embedded in the feedback
   message ("NORM_NACK" or "NORM_ACK").  The adjustment adds the amount
   of time the receiver held the timestamp before generating its
   response.  Upon receipt of this adjusted timestamp, the sender is
   able to calculate the round-trip time to that receiver.

   The round-trip time for each receiver is fed into an algorithm that
   weights and smoothes the values for a conservative estimate of the
   GRTT.  The algorithm and methodology are described in the Multicast
   NACK Building Block document [I-D.ietf-rmt-bb-norm-revised] [RFC5401] in the section entitled "One-to-Many "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 that the NORM-CC Rate header extension may
   also be used to proactively solicit RTT feedback from the receiver
   group per congestion control operation even though the sender may not
   be conducting congestion control rate adjustment.  NORM operation
   without congestion control should be considered only in closed

5.5.2.  NORM Congestion Control Operation

   This section describes baseline congestion control operation for the
   NORM protocol (NORM-CC).  The supporting NORM message formats and
   approach described here are an adaptation of the equation-based TCP-
   Friendly Multicast Congestion Control (TFMCC) approach described in
   [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 (e.g., PGMCC [PgmccPaper]).  With this TFMCC-based
   approach, the transmissions of NORM senders are controlled in a rate-
   based manner as opposed to window-based congestion control algorithms
   as in TCP.  However, it is possible that the NORM protocol message
   set may alternatively be used to support a window-based multicast
   congestion control scheme such as PGMCC.  The details of that
   alternative may be described separately or in a future revision of
   this document.  In either case (rate-based TFMCC or window-based
   PGMCC), successful control of sender transmission depends upon
   collection of sender-to-receiver packet loss estimates and RTTs to
   identify the congestion control bottleneck path(s) within the
   multicast topology and adjust the sender rate accordingly.  The
   receiver with loss and RTT estimates that correspond to the lowest
   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:
   Rsender = -------------------------------------------------------------- ----------------------------------------------------------
            tRTT*(sqrt((2/3)*p) + 12 * sqrt((3/8)*p) 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))


   "S" = nominal transmitted packet size.  (In NORM, the "nominal"
   packet size can be determined by the sender as an exponentially
   weighted moving average (EWMA) of transmitted packet sizes to account
   for variable message sizes).

   "tRTT" = RTT estimate of the current "current limiting receiver"

   "p" = loss event fraction of the CLR.

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

   1.  Stimulate explicit feedback from the general receiver set to
       collect congestion control information.
   2.  Communicate state to the receiver set on the sender's current
       congestion control status including details of the CLR.
   3.  Initiate rapid (immediate) feedback from the CLR in order to
       closely track the dynamics of congestion control for that current
       worst path in the group multicast topology.

   The format of the "NORM_CMD(CC)" message is describe in Section 4.2.3
   of this document.  The "NORM_CMD(CC)" message contains information to
   allow measurement of RTTs, to inform the group of the congestion
   control CLR, and to provide feedback of individual RTT measurements
   to the receivers in the group.  The "NORM_CMD(CC)" also provides for
   exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
   nodes that may be determined administratively or possibly
   algorithmically based on congestion control feedback.  PLR nodes are
   receivers that have been identified to have potential for (perhaps
   soon) becoming the CLR and thus immediate, up-to-date feedback is
   beneficial for congestion control performance.  The details of PLR
   selection are not discussed in this document.  NORM_CMD(CC) Transmission

   The "NORM_CMD(CC)" message is transmitted periodically by the sender
   along with its normal data transmission.  Note that the repeated
   transmission of "NORM_CMD(CC)" messages may be initiated some time
   before transmission of user data content at session startup.  This
   may be done to collect some estimation of the current state of the
   multicast topology with respect to group and individual RTT and
   congestion control state.

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

   1.  By default, the interval is set according to the current sender
       GRTT estimate.  A startup GRTT of 0.5 seconds is recommended when
       no feedback has yet been received from the group.
   2.  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 that no more than one "NORM_CMD(CC)" message is
       sent per "NORM_DATA" message when there is data pending
       transmission.  This ensures that the transmission of this control
       message is not done to the exclusion of user data transmission.

   The "NORM_CMD(CC)" "cc_sequence" field is incremented with each
   transmission of a "NORM_CMD(CC)" command.  The greatest "cc_sequence"
   recently received by receivers is included in their feedback to the
   sender.  This allows the sender to determine the age of feedback to
   assist in congestion avoidance.

   The NORM-CC Rate Header Extension is applied to the "NORM_CMD(CC)"
   message and the sender advertises its current transmission rate in
   the "send_rate" field.  The rate information is used by receivers to
   initialize loss estimation during congestion control startup or

   The "cc_node_list" contains a list of entries identifying receivers
   and their current congestion control state (status "flags", "rtt" and
   "loss" estimates).  The list may be empty if the sender has not yet
   received any feedback from the group.  If the sender has received
   feedback, the list will minimally contain an entry identifying the
   CLR.  A "NORM_FLAG_CC_CLR" flag value is provided for the "cc_flags"
   field to identify the CLR entry.  It is RECOMMENDED that the CLR
   entry be the first in the list for implementation efficiency.
   Additional entries in the list are used to provide sender-measured
   individual RTT estimates to receivers in the group.  The number of
   additional entries in this list is dependent upon the percentage of
   control traffic the sender application is willing to send with
   respect to user data message transmissions.  More entries in the list
   may allow the sender to be more responsive to congestion control
   dynamics.  The length of the list may be dynamically determined
   according to the current transmission rate and scheduling of
   "NORM_CMD(CC)" messages.  The maximum length of the list corresponds
   to the sender's NormSegmentSize parameter for the session.  The
   inclusion of additional entries in the list based on receiver
   feedback are prioritized with following rules:

   1.  Receivers that have not yet been provided a 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 a 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" that are
   used for other congestion control functions.  The "NORM_FLAG_CC_PLR"
   flag value is used to mark additional receivers from that the sender
   would like to have immediate, non-suppressed feedback.  These may be
   receivers that the sender algorithmically identified as potential
   future CLRs or that have been pre-configured as potential congestion
   control points in the network.  The "NORM_FLAG_CC_RTT" indicates the
   validity of the "cc_rtt" field for the associated receiver node.
   Normally, this flag will be set since the receivers in the list will
   typically be receivers from which the sender has received feedback.
   However, in the case that the NORM sender has been pre-configured
   with a set of PLR nodes, feedback from those receivers may not yet
   have been collected and thus the "cc_rtt" field does not contain a
   valid value when this flag is not set.  Similarly, a value of ZERO
   for the "cc_rate" field here should be treated as an invalid value
   and be ignored for the purposes of feedback suppression, etc.  NORM_CMD(CC) Feedback Response

   Receivers explicitly respond to "NORM_CMD(CC)" messages in the form
   of a "NORM_ACK(RTT)" message.  The goal of the congestion control
   feedback is to determine the receivers with the lowest congestion
   control rates.  Receivers that are marked as CLR or PLR nodes in the
   "NORM_CMD(CC)" "cc_node_list" immediately provide feedback in the
   form of a "NORM_ACK" to this message.  When a "NORM_CMD(CC)" is
   received, non-CLR or non-PLR nodes initiate random feedback backoff
   timeouts similar to that used when the receiver initiates a repair
   cycle (see Section 5.3) in response to detection of data loss.  The
   backoff timeout for the congestion control response is generated as
            T_backoff = RandomBackoff(K*GRTTsender, GSIZEsender)

   The ""RandomBackoff()"" algorithm provides a truncated exponentially
   distributed random number and is described in the Multicast NACK
   Building Block document [I-D.ietf-rmt-bb-norm-revised]. [RFC5401].  The same backoff factor "K =
   Ksender" MAY be used as with "NORM_NACK" suppression.  However, in
   cases where the application purposefully specifies a very small
   "Ksender" backoff factor to minimize the NACK repair process latency
   (trading off group size scalability), it is RECOMMENDED that a larger
   backoff factor for congestion control feedback is maintained, since
   there may often be a larger volume of congestion control feedback
   than NACKs in many cases and some congestion control feedback latency
   may be tolerable where reliable delivery latency is not.  As
   previously noted, a backoff factor value of "K = 4" is generally
   recommended for ASM operation and "K = 6" for SSM operation.  A
   receiver SHALL cancel the backoff timeout and thus its pending
   transmission of a "NORM_ACK(RTT)" message under the following

   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*GRTTsender".  This
       prevents NACK implosion in the event of sender or network
   4.  "Suppressing" congestion control feedback is heard from another
       receiver (in a "NORM_ACK" or "NORM_NACK") or via a
       "NORM_CMD(REPAIR_ADV)" message from the sender.  The local
       receiver's feedback is "suppressed" if the rate of the competing
       feedback ("Rfb") is sufficiently close to or less than the local
       receiver's calculated rate ("Rcalc").  The local receiver's
       feedback is canceled when "Rcalc > (0.9 * Rfb)".  Also note
       receivers that have not yet received an RTT measurement from the
       sender are suppressed only by other receivers that have not yet
       measured RTT.  Additionally, receivers whose RTT estimate has
       aged considerably (i.e., they haven't been included in the
       "NORM_CMD(CC)" "cc_node_list" in a long time) may wish to compete
       as a receiver with no prior RTT measurement after some 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
   timestamp from the most recently received "NORM_CMD(CC)" message and
   the "cc_sequence" value from that command in the applicable
   "NORM_ACK" or "NORM_NACK" message fields.  For NORM-CC operation, any
   generated feedback message SHALL also contain the NORM-CC Feedback
   header extension.  The receiver provides its current "cc_rate"
   estimate, "cc_loss" estimate, "cc_rtt" if known, and any applicable
   "cc_flags" via this header extension.

   During slow start (when the receiver has not yet detected loss from
   the sender), the receiver uses a value equal to two times its
   measured rate from the sender in the "cc_rate" field.  For steady-
   state congestion control operation, the receiver "cc_rate" value is
   from the equation-based value using its current loss event estimate
   and sender<->receiver RTT information.  (The GRTT is used when the
   receiver has not yet measured its individual RTT).

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

   When the receiver has a valid individual RTT measurement, it SHALL
   include this value in the "cc_rtt" field.  The "NORM_FLAG_CC_RTT"
   MUST be set when the "cc_rtt" field is valid.

   After a congestion control feedback message is generated or when the
   feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
   period during which it will restrain itself from providing congestion
   control feedback, even if "NORM_CMD(CC)" messages are received from
   the sender (unless the receive becomes marked as a CLR or PLR node).
   The value of this holdoff timeout ("T_ccHoldoff") period is:
                           T_ccHoldoff = (K*GRTT)
   Thus, non-CLR receivers are constrained to providing explicit
   congestion control feedback once per "K*GRTT" intervals.  Note,
   however, that as the session progresses, different receivers will be
   responding to different "NORM_CMD(CC)" messages and there will be
   relatively continuous feedback of congestion control information
   while the sender is active.  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.

   The sender processes congestion control feedback from the receivers
   and selects the CLR based on the lowest rate receiver.  Receiver
   rates are either determined directly from the slow start "cc_rate"
   provided by the receiver in the NORM-CC Feedback header extension or
   by performing the equation-based calculation using individual RTT and
   loss estimates ("cc_loss") as feedback is received.

   The sender can calculate a current RTT for a receiver ("RTT_rcvrNew")
   using the "grtt_response" timestamp included in feedback messages.
   When the "cc_rtt" value in a response is not valid, the sender simply
   uses this "RTT_rcvrNew" value as the receiver's current RTT
   ("RTT_rcvr").  For non-CLR and non-PLR receivers, the sender can use
   the "cc_rtt" value provided in the NORM-CC Feedback header extension
   as the receiver's previous RTT measurement ("RTT_rcvrPrev") to smooth
   according to:
             RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew

   For CLR receivers where feedback is received more regularly, the
   sender SHOULD maintain a more smoothed RTT estimate upon new feedback
   from the CLR where:
                 RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew

   ""RTT_clrNew"" is the new RTT calculated from the timestamp in the
   feedback message received from the CLR.  The "RTT_clr" is initialized
   to "RTT_clrNew" on the first feedback message received.  Note that
   the same procedure is observed by the sender for PLR receivers and
   that if a PLR is "promoted" to CLR status, the smoothed estimate can
   be continued.

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

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

   During session startup, the congestion control operation SHALL
   observe a "slow start" procedure to quickly approach its fair
   bandwidth share.  An initial sender startup rate is assumed where:
   Rinitial = MIN(NormSegmentSize / GRTT, NormSegmentSize) bytes/second.

   The rate is increased only when feedback is received from the
   receiver set.  The "slow start" phase proceeds until any receiver
   provides feedback indicating that loss has occurred.  Rate increase
   during slow start is applied as:
                              Rnew = Rrecv_min

   where "Rrecv_min" is the minimum reported receiver rate in the
   "cc_rate" field of congestion control feedback messages received from
   the group.  Note that during slow start, receivers use two times
   their measured rate from the sender in the "cc_rate" field of their
   feedback.  Rate increase adjustment is limited to once per GRTT
   during slow start.

   If the CLR or any receiver intends to leave the group, it will set
   the "NORM_FLAG_CC_LEAVE" in its congestion control feedback message
   as an indication that the sender should not select it as the CLR.
   When the CLR changes to a lower rate receiver, the sender should
   immediately adjust to the new lower rate.  The sender is limited to
   increasing its rate at one additional packet per RTT towards any new,
   higher CLR rate.

   The sender should also track the age of the feedback it has received
   from the CLR by comparing its current "cc_sequence" value
   ("Seq_sender") to the last "cc_sequence" value received from the CLR
   ("Seq_clr").  As the age of the CLR feedback increases with no new
   feedback, the sender SHALL begin reducing its rate once per "RTT_clr"
   as a congestion avoidance measure.  The following algorithm is used
   to determine the decrease in sender rate (Rsender bytes/sec) as the
   CLR feedback, unexpectedly, excessively ages:
                   Age = Seq_sender - Seq_clr;
                   if (Age > 4) Rsender = Rsender * 0.5;

   This rate reduction is limited to the lower bound on NORM
   transmission rate.  After "NORM_ROBUST_FACTOR" consecutive
   "NORM_CMD(CC)" rounds without any feedback from the CLR, the sender
   SHOULD assume the CLR has left the group and pick the receiver with
   the next lowest rate as the new CLR.  Note this assumes that the
   sender does not have explicit knowledge that the CLR intentionally
   left the group.  If no receiver feedback is received, the sender MAY
   wish to withhold further transmissions of "NORM_DATA" segments and
   maintain "NORM_CMD(CC)" transmissions only until feedback is
   detected.  After such a CLR timeout, the sender will be transmitting
   with a minimal rate and should return to slow start as described here
   for a break in data transmission.

   When the sender has a break in its data transmission, it can continue
   to probe the group with "NORM_CMD(CC)" messages to maintain RTT
   collection from the group.  This will enable the sender to quickly
   determine an appropriate CLR upon data transmission restart.
   However, the sender should exponentially reduce its target rate to be
   used for transmission restart as time since the break elapses.  The
   target rate SHOULD be recalculated once per "RTT_clr" as:
                          Rsender = Rsender * 0.5;

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

5.5.3.  NORM Positive Acknowledgment Procedure

   NORM provides options for the source application to request positive
   acknowledgment (ACK) of "NORM_CMD(FLUSH)" and "NORM_CMD(ACK_REQ)"
   messages from members of the group.  There are some specific
   acknowledgment requests defined for the NORM protocol and a range of
   acknowledgment request types that are left to be defined by the
   application.  One predefined acknowledgment type is the
   "NORM_ACK_FLUSH" type.  This acknowledgment is used to determine if
   receivers have achieved completion of reliable reception up through a
   specific logical transmission point with respect to the sender's
   sequence of transmission.  The "NORM_ACK_FLUSH" acknowledgment may be
   used to assist in application flow control when the sender has
   information on a portion of the receiver set.  Another predefined
   acknowledgment type is "NORM_ACK(CC)", which is used to explicitly
   provide congestion control feedback in response to "NORM_CMD(CC)"
   messages transmitted by the sender for NORM-CC operation.  Note the
   "NORM_ACK(CC)" response does NOT follow the positive acknowledgment
   procedure described here.  The "NORM_CMD(ACK_REQ)" and "NORM_ACK"
   messages contain an "ack_type" field to 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 large group setting to query a critical subset of
   the group.  Either the "NORM_CMD(ACK_REQ)", or when applicable, the
   "NORM_CMD(FLUSH)" message is used for polling and contains a list of
   NormNodeIds for receivers that should respond to the command.  The
   list of receivers providing acknowledgment is determined by the
   source application with a priori knowledge of participating nodes or
   via some other application-level mechanism.

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

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

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

   1.  Only a single "NORM_CMD(ACK_REQ)" message is generated once per
       (2*GRTT), and,
   2.  The size of the "acking_node_list" of NormNodeIds from which
       acknowledgment is requested is limited to a maximum of the sender
       NormSegmentSize setting per round of the positive acknowledgment

   Because the size of the included list is limited to the sender's
   NormSegmentSize setting, multiple "NORM_CMD(ACK_REQ)" rounds may be
   required to achieve responses from all receivers specified.  The
   content of the attached NormNodeId list will be dynamically updated
   as this process progresses and "NORM_ACK" responses are received from
   the specified receiver set.  As the sender receives valid responses
   (i.e., matching watermark point or "ack_id") from receivers, it SHALL
   eliminate those receivers from the subsequent "NORM_CMD(ACK_REQ)"
   message "acking_node_list" and add in any pending receiver
   NormNodeIds while keeping within the NormSegmentSize limitation of
   the list size.  Each receiver is queried a maximum number of times
   ("NORM_ROBUST_FACTOR", by default).  Receivers not responding within
   this number of repeated requests are removed from the payload list to
   make room for other potential receivers pending acknowledgment.  The
   transmission of the "NORM_CMD(ACK_REQ)" is repeated until no further
   responses are required or until the repeat threshold is exceeded for
   all pending receivers.  The transmission of "NORM_CMD(ACK_REQ)" or
   "NORM_CMD(FLUSH)" messages to conduct the positive acknowledgment
   process is multiplexed with ongoing sender data transmissions.
   However, the "NORM_CMD(FLUSH)" positive acknowledgment process may be
   interrupted in response to negative acknowledgment repair requests
   (NACKs) received from receivers during the acknowledgment period.
   The "NORM_CMD(FLUSH)" positive acknowledgment process is restarted
   for receivers pending acknowledgment once any the repairs have been

   In the case of "NORM_CMD(FLUSH)" commands with an attached
   "acking_node_list", receivers will not ACK until they have received
   complete transmission of all data up to and including the given
   watermark transmission point.  All receivers SHALL interpret the
   watermark point provided in the request NACK for repairs if needed as
   for "NORM_CMD(FLUSH)" commands with no attached "acking_node_list".

5.5.4.  Group Size Estimate

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

   It is possible that group size may be algorithmically approximated
   from the volume of congestion control feedback messages which follow
   the exponentially weighted random backoff.  However, the
   specification of such an algorithm is currently beyond the scope of
   this document.

6.  Security Considerations

   The same security considerations that apply to the NORM, TFMCC, and
   FEC Building Blocks also apply to the NORM protocol.  In addition to
   vulnerabilities that any IP and IP multicast protocol implementation
   may be generally subject to, the NACK-based feedback of NORM may be
   exploited by replay attacks which force the NORM sender to
   unnecessarily transmit repair information.  This MAY be addressed by
   network layer IP security implementations that guard against this
   potential security exploitation.  The NORM protocol is compatible
   with the use of the IP security (IPsec) architecture described in
   [RFC4301] and the IPsec Encapsulating Security Payload (ESP) protocol
   or Authentication Header (AF) extension MAY be used to secure IP
   packets transmitted by NORM participants.

   Alternatively, a header extension may be applied to the NORM protocol
   to provide authentication of NORM messages.  For this purpose the
   "EXT_AUTH" header extension (HET = 1) is defined.  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 that an EXT_AUTH implementation of MAY
   also provide for encryption of NORM message payloads as well as
   authentication.  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
   that encryption of NORM protocol header content is beneficial or
   necessary, the aforementioned use of IPsec ESP may be more
   appropriate.  If EXT_AUTH is present, whatever packet authentication
   checks that can be performed immediately upon reception of the packet
   SHOULD 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 is fully authenticated.  Any congestion
   control related action that is appropriate MUST NOT be postponed by
   any such full packet authentication.  Consideration SHOULD 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 used on a
   per-session basis or NORM sender nodes SHOULD use unique
   "instance_id" identifiers that are managed as part of the security
   association for the sessions.

   It is RECOMMENDED that such security mechanisms be used when
   available.  It should be noted that NORM participants can use the
   "sequence" field from the NORM Common Message Header to detect replay
   attacks.  This can be accomplished if the NORM sender is willing to
   maintain state on receivers which are NACKing.  A cache of such
   receiver state can be used to provide protection against NACK replay
   attacks.  NORM receivers SHOULD also maintain similar state for
   protection against possible replay of other receiver messages in ASM
   operation as well.  For example, a 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) is RECOMMENDED for protection against
   similar attacks that might 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 any such cryptographic measures are used, it is
   RECOMMENDED that an approach such as described in the Group Domain of
   Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY)
   [RFC3830] or Group Secure Association Key Management Protocol
   (GSAKMP) [RFC4535] specifications for automated key management is

   It is also important to note that while NORM does leverage FEC-based
   repair for scalability, this alone does not guarantee integrity of
   received data.  Application-level integrity-checking of data content
   is highly RECOMMENDED.

6.1.  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 reference, interoperable
   secure mode of operation.  However, 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 that of RTP [RFC1889] and
   as preserved in the specification for Secure Real Time Protocol
   (SRTP) [RFC3711].

   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).

6.1.1.  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.

   The NORM sender SHALL use an IPsec SA configured for ESP protocol
   [RFC4303] operation with the option for data origination
   authentication enabled.  It is also RECOMMENDED that 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
   that receivers may 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.  As noted for NORM unicast feedback, the sender's
   transmission port number SHOULD be distinct from the 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.  Note that built-in IPsec replay attack protection for this
   second SA at the sender MUST be disabled.

   Multiple receivers using a common IPsec SA for traffic directed to
   the NORM sender (i.e., many-to-one) prevents the use of built-in
   IPsec replay attack protection by the NORM sender with current 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 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 are
       only a small set of receivers for which the sender must keep more
       persistent replay attack state.
   3.  "NORM_NACK" feedback messages that precede 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 for receivers that precede 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.

   Note that use of ESP confidentiality, as RECOMMENDED, for secure NORM
   protocol operation makes it more difficult for adversaries to conduct
   effective replay attacks.  Additionally, it should be noted that a
   NORM sender implementation with access to the full ESP protocol
   header could also use the ESP sequence information to make this form
   of replay attack protection even more robust.  The design of this
   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 may need to have a more light-weight realization in
   many cases.

   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 that many-to-
   many (ASM) IPsec configuration could be achieved where each
   participant uses a unique SA (with a unique SPI).  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 that receivers may retrieve keying
   information from a central server as needed or otherwise conduct
   group key updates with a similar centralized approach.  However, it
   may be 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 that potential group
   participants may 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.

6.1.2.  IPsec Requirements

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

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

   IPsec in transport mode MUST be supported.  The use of IPsec
   [RFC4301] processing for secure NORM traffic SHOULD also be REQUIRED
   such that unauthenticated packets are not received by the NORM
   protocol implementation .  Key Management

   An automated key management scheme for group key distribution and
   rekeying such as GDOI [RFC3547], GSAKMP [RFC4535], or MIKEY [RFC3830]
   SHOULD be used.  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.  It should also be noted that it may be possible
   for key update messages (e.g., the GDOI GROUPKEY-PUSH message) to be
   included in the NORM application reliable data transmission if
   appropriate interfaces were available between the NORM application
   and the key management daemon.  Security Policy

   Receivers SHOULD accept connections only from the designated,
   authorized sender(s).  It is expected that appropriate key management
   will provide 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.  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.  These
   certificates SHOULD use IP addresses for authentication although it
   may alternatively possible to have authentication associated with
   pre-assigned NormNodeId values.  However, it is likely that available
   group key management implementations will not be NORM-specific.  Availability

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

7.  IANA Considerations

   Header extension identifiers for the NORM protocol are subject to
   IANA registration.  Additionally, building blocks components used by
   this NORM Protocol specification may introduce additional IANA
   considerations.  In particular, the FEC Building Block used by NORM
   does require IANA registration of the FEC codecs used.  The
   registration instructions for FEC codecs are provided in [RFC5052].

7.1.  Explicit IANA Assignment Guidelines

   This document defines a name-space for NORM Header Extensions named:


   These values represent extended header fields that allow the protocol
   functionality to be expanded to include additional optional features
   and operating modes.  The values that can be assigned within the
   "ietf:rmt:norm:extension" name-space 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 extension of a fixed 4-byte length.  NORM header
   extension identifier value assignment requests are granted on a
   "Specification Required" basis as defined in [RFC2434].  Additional
   header extension specifications MUST include a description of
   protocol actions to be taken when the extended header is encountered
   by a protocol implementation not supporting that specific option.
   For example, it may be possible for protocol implementations to
   ignore unknown header extensions in many cases.

   This specification registers the following NORM Header Extension
   types in namespace "ietf:rmt:norm:extensions":

                | Value | Name       | Reference          |
                | 1     | "EXT_AUTH" | This specification |
                | 3     | "EXT_CC"   | This specification |
                | 64    | "EXT_FTI"  | This specification |
                | 128   | "EXT_RATE" | This specification |

8.  Suggested Use

   The present NORM protocol is seen as useful tool for the reliable
   data transfer over generic IP multicast services.  It is not the
   intention of the authors to suggest it is suitable for supporting all
   envisioned multicast reliability requirements.  NORM provides a
   simple and flexible framework for multicast applications with a
   degree of concern for network traffic implosion and protocol overhead
   efficiency.  NORM-like protocols have been successfully demonstrated
   within the MBone for bulk data dissemination applications, including
   weather satellite compressed imagery updates servicing a large group
   of receivers and a generic web content reliable "push" application.

   In addition, this framework approach has some design features making
   it attractive for bulk transfer in asymmetric and wireless
   internetwork applications.  NORM is capable of successfully operating
   independent of network structure and in environments with high packet
   loss, delay, and out-of-order delivery.  Hybrid proactive/reactive
   FEC-based repairing improve protocol performance in some multicast
   scenarios.  A sender-only repair approach often makes additional
   engineering sense in asymmetric networks.  NORM's unicast feedback
   capability may be suitable for use in asymmetric networks or in
   networks where only unidirectional multicast routing/delivery service
   exists.  Asymmetric architectures supporting multicast delivery are
   likely to make up an important portion of the future Internet
   structure (e.g., DBS/cable/PSTN hybrids) and efficient, reliable bulk
   data transfer will be an important capability for servicing large
   groups of subscribed receivers.

9.  Changes from RFC3940

   This section lists the changes between the Experimental version of
   this specification, [RFC3940], 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 namespace for header extension assignment,
   3.  Removal of file blocking scheme description that is 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, and
   6.  Correction of inconsistency in specification of the inactivity
   7.  Addition of IPsec secure mode description with IPsec
   8.  Clarification of interpretation of "Source Block Length" when FEC
       codes are arbitrarily shortened by the sender.

10.  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.

11.  References

11.1.  Normative References

              Adamson, B., Bormann, C., London, U., and J. Macker,
              "Multicast Negative-Acknowledgment (NACK) Building
              Blocks", draft-ietf-rmt-bb-norm-revised-07 (work in
              progress), September 2008.

   [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.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC3269]  Kermode, R. and L. Vicisano, "Author Guidelines for
              Reliable Multicast Transport (RMT) Building Blocks and
              Protocol Instantiation documents", RFC 3269, April 2002.

   [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.

   [RFC5401]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "Multicast Negative-Acknowledgment (NACK) Building
              Blocks", RFC 5401, November 2008.

11.2.  Informative References

              Gossink, D. and J. Macker, "Reliable Multicast and
              Integrated Parity Retransmission with Channel Estimation",
              IEEE Globecomm  , 1998.

              Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", draft-ietf-rmt-bb-fec-basic-schemes-revised-05 draft-ietf-rmt-bb-fec-basic-schemes-revised-06
              (work in progress), July October 2008.

              Nonnenmacher, J. and E. Biersack, "Optimal Multicast
              Feedback", IEEE INFOCOM,  p. 964, March/April 1998.

              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.

              Adamson, B. and J. Macker, "Quantitative Prediction of
              NACK-Oriented Reliable Multicast (NORM) Feedback", IEEE
              MILCOM , October 2002.

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

   [RFC1889]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 1889, January 1996.

   [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.

   [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.

   [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.

   [RFC4654]  Widmer, J. and M. Handley, "TCP-Friendly Multicast
              Congestion Control (TFMCC): Protocol Specification",
              RFC 4654, August 2006.

              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.

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

              Widmer, J. and M. Handley, "Extending Equation-Based
              Congestion Control to Multicast Applications", ACM
              SIGCOMM , August 2001.

Authors' Addresses

   Brian Adamson
   Naval Research Laboratory
   Washington, DC  20375


   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28334 Bremen


   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT

   Joe Macker
   Naval Research Laboratory
   Washington, DC  20375


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