Network Working Group                                         B. Adamson
Internet-Draft                                 Naval Research Laboratory
Obsoletes: 3940 (if approved)                                 C. Bormann
Intended status: Standards Track                 Universitaet Bremen TZI
Expires: November 29, December 5, 2009                                     M. Handley
                                               University College London
                                                               J. Macker
                                               Naval Research Laboratory
                                                            May 28,
                                                            June 3, 2009

          NACK-Oriented Reliable Multicast Transport Protocol

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

Table of Contents

   1.  Introduction and Applicability . . . . . . . . . . . . . . . .  5
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
     1.2.  NORM Data Delivery Service Model . . . . . . . . . . . . .  6
     1.3.  NORM Scalability . . . . . . . . . . . . . . . . . . . . .  8
     1.4.  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  . . . . . . . . . . . . . . . . . . . . 47 48
       4.3.1.  NORM_NACK Message  . . . . . . . . . . . . . . . . . . 47 48
       4.3.2.  NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53 54
     4.4.  General Purpose Messages . . . . . . . . . . . . . . . . . 55 56
       4.4.1.  NORM_REPORT Message  . . . . . . . . . . . . . . . . . 55 56
   5.  Detailed Protocol Operation  . . . . . . . . . . . . . . . . . 55 56
     5.1.  Sender Initialization and Transmission . . . . . . . . . . 57 58
       5.1.1.  Object Segmentation Algorithm  . . . . . . . . . . . . 58 59
     5.2.  Receiver Initialization and Reception  . . . . . . . . . . 59 60
     5.3.  Receiver NACK Procedure  . . . . . . . . . . . . . . . . . 59 61
     5.4.  Sender NACK Processing and Response  . . . . . . . . . . . 61 63
       5.4.1.  Sender Repair State Aggregation  . . . . . . . . . . . 62 63
       5.4.2.  Sender FEC Repair Transmission Strategy  . . . . . . . 63 64
       5.4.3.  Sender NORM_CMD(SQUELCH) Generation  . . . . . . . . . 64 65
       5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 64 66
     5.5.  Additional Protocol Mechanisms . . . . . . . . . . . . . . 65 66
       5.5.1.  Greatest  Group Round-trip Time (GRTT) Collection  . . . . . . . . . 65 66
       5.5.2.  NORM Congestion Control Operation  . . . . . . . . . . 66 68
       5.5.3.  NORM Positive Acknowledgment Procedure . . . . . . . . 74 76
       5.5.4.  Group Size Estimate  . . . . . . . . . . . . . . . . . 76 78
   6.  Configurable Elements  . . . . . . . . . . . . . . . . . . . . 78
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 76
     6.1. 81
     7.1.  Baseline Secure NORM Operation . . . . . . . . . . . . . . 78
       6.1.1. 83
       7.1.1.  IPsec Approach . . . . . . . . . . . . . . . . . . . . 78
       6.1.2. 83
       7.1.2.  IPsec Requirements . . . . . . . . . . . . . . . . . . 81
   7. 85
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 82
     7.1. 86
     8.1.  Explicit IANA Assignment Guidelines  . . . . . . . . . . . 82
       7.1.1. 87
       8.1.1.  NORM Header Extension Types  . . . . . . . . . . . . . 82
       7.1.2. 87
       8.1.2.  NORM Stream Control Codes  . . . . . . . . . . . . . . 83
       7.1.3. 88
       8.1.3.  NORM_CMD Message Sub-types . . . . . . . . . . . . . . 84
   8. 88
   9.  Suggested Use  . . . . . . . . . . . . . . . . . . . . . . . . 84
   9. 89
   10. Changes from RFC3940 . . . . . . . . . . . . . . . . . . . . . 85
   10. 90
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 86
   11. 91
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 86
     11.1. 91
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 86
     11.2. 91
     12.2. Informative References . . . . . . . . . . . . . . . . . . 87 92
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 89 93

1.  Introduction and Applicability

   The Negative-acknowledgment (NACK) Oriented Reliable Multicast (NORM)
   protocol is designed to can 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. pre-configuration.  The protocol
   is purposely designed to be tolerant of inaccurate timing estimations or lossy conditions that
   can occur in many networks including mobile and wireless.  The
   protocol is can also
   designed to exhibit convergence converge and maintain efficient operation even in
   situations of heavy packet loss and large queuing or transmission
   delays.  This document obsoletes the Experimental RFC 3940

   This document is a product of the IETF RMT working group and follows
   the guidelines provided in the Author Guidelines for Reliable
   Multicast Transport (RMT) Building Blocks and Protocol Instantiation
   documents [RFC3269].

   Statement of Intent

   This memo contains the definitions necessary to fully specify a
   Reliable Multicast Transport protocol in accordance with the criteria
   of IETF Criteria for Evaluating Reliable Multicast Transport and
   Application Protocols [RFC2357].  The NORM specification described in
   this document was previously published in the "Experimental Category"
   [RFC3940].  It was the stated intent of the RMT working group to re-
   submit this specifications as an IETF Proposed Standard in due
   course.  This Proposed Standard specification is thus based on RFC
   3940 and has been updated according to accumulated experience and
   growing protocol maturity since the publication of RFC 3940.  Said
   experience applies both to this specification itself and to
   congestion control strategies related to the use of this
   specification.  The differences between RFC 3940 and this document
   are listed in Section 9. 10.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  NORM Data Delivery Service Model

   A NORM protocol instance (NormSession) is defined within the context
   of participants communicating connectionless (e.g., Internet Protocol
   (IP) or User Datagram Protocol (UDP)) packets over a network using
   pre-determined addresses and host port numbers.  Generally, the
   participants exchange packets using an IP multicast group address,
   but unicast transport MAY also be established or applied as an
   adjunct to multicast delivery.  In the case of multicast, the
   participating NormNodes will communicate using a common IP multicast
   group address and port number that has been chosen via means outside the context of
   the given NormSession.  Other existing IETF data format and protocol
   standards exist that MAY be applied to describe and convey the necessary a
   priori information for a specific NormSession (e.g., Session
   Description Protocol (SDP) [RFC4566], Session Announcement Protocol
   (SAP) [RFC2974], etc.).

   The NORM protocol design is principally driven by the assumption of a
   single sender transmitting bulk data content to a group of receivers.
   However, the protocol MAY operate with multiple senders within the
   context of a single NormSession.  In initial implementations of this
   protocol, it is anticipated that multiple senders will transmit
   independent of one another and receivers will maintain state as
   necessary for each sender.  In future versions of NORM, it is
   possible that some aspects of protocol operation (e.g., round-trip time
   collection) will provide for alternate modes allowing more efficient
   performance for applications requiring multiple senders.

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

   1.  static computer memory data content ("NORM_OBJECT_DATA" type),

   2.  computer storage files ("NORM_OBJECT_FILE" type), and

   3.  non-finite streams of continuous data content
       ("NORM_OBJECT_STREAM" type).

   The distinction between "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE" is
   simply to provide a hint to receivers in NormSessions serving
   multiple types of content as to what type of storage to allocate for
   received content (i.e., memory or file storage).  Other than that
   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 can be leveraged by the application for
   this purpose if desired, or identification could can alternatively be
   embedded within the data content.  NORM does identify transmitted
   content (NormObjects) with transport identifiers that are applicable
   only while the sender is transmitting and/or repairing the given
   object.  These transport data content identifiers (NormTransportIds)
   are assigned in a monotonically increasing fashion by each NORM
   sender during the course of a NormSession.  Participants, including
   senders, in NORM protocol sessions are also identified with unique
   identifiers (NormNodeIds).  Each sender maintains its NormTransportId
   assignments independently so that and thus individual NormObjects can be
   uniquely identified during transport with the by concatenation of the session-
   unique sender session-unique identifier (NormNodeId) and the assigned
   NormTransportId.  The NormTransportIds are assigned from a large, but
   fixed, numeric space in increasing order and will be reassigned
   during long-lived sessions.  The NORM protocol provides mechanisms so that
   the sender application can 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 can realize different, complete bulk transfer
   communication models to meet their goals.

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

1.3.  NORM Scalability

   Group communication scalability requirements lead to adaptation of
   negative acknowledgment (NACK) based protocol schemes when feedback
   for reliability is needed [RmComparison].  NORM is a protocol
   centered around the use of selective NACKs to request repairs of
   missing data.  NORM provides for the use of packet-level forward
   error correction (FEC) techniques for efficient multicast repair and
   OPTIONAL proactive transmission robustness [RFC3453].  FEC-based
   repair can be used to greatly reduce the quantity of reliable
   multicast repair requests and repair transmissions [MdpToolkit] in a
   NACK-oriented protocol.  The principal factor in NORM scalability is
   the volume of feedback traffic generated by the receiver set to
   facilitate reliability and congestion control.  NORM uses
   probabilistic suppression of redundant feedback based on
   exponentially distributed random backoff timers.  The performance of
   this type of suppression relative to other techniques is described in
   [McastFeedback].  NORM dynamically measures the group's round-trip
   timing status to set its suppression and other protocol timers.  This
   allows NORM to scale well while maintaining reliable data delivery
   transport with low latency relative to the network topology over
   which it is operating.

   Feedback messages can be either multicast to the group at large or
   sent via unicast routing to the sender.  In the case of unicast
   feedback, the sender relays the feedback state to the group to
   facilitate feedback suppression.  In typical Internet environments,
   the NORM protocol will readily scale to group sizes on the order of
   tens of thousands of receivers.  A study of the quantity of feedback
   for this type of protocol is described in [NormFeedback].  NORM is
   able to operate with a smaller amount of feedback than a single TCP
   connection, even with relatively large numbers of receivers.  Thus,
   depending upon the network topology, it is possible for NORM to scale
   to larger group sizes.  With respect to computer resource usage, the
   NORM protocol does not need state to be kept on all receivers in the
   group.  NORM senders maintain state only for receivers providing
   explicit congestion control feedback.  However, NORM receivers need
   to maintain state for each active sender.  This can constrain the
   number of simultaneous senders in some uses of NORM.

1.4.  Environmental Requirements and Considerations

   All of the environmental requirements and considerations that apply
   to the Multicast NACK Building Block [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 Host Extensions for IP Multicasting [RFC1112],
   but SHALL also be capable of scalable operation in asymmetric
   topologies such as Source-Specific Multicast (SSM) [RFC4607] where
   only unicast routing service is available from the receivers to the

   NORM is compatible with IPv4 and IPv6.  Additionally, NORM can 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 ("NORM_NODE_NONE")
   and a value of "0xffffffff" is a "wild card" NormNodeId. NormNodeId
   ("NORM_NODE_ANY").  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 that 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 that 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 that 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 can be
   "byte" or "message" oriented.  The NORM protocol provides for
   "flushing" of the stream to expedite delivery or possibly enforce
   application message boundaries.  NORM protocol implementations MAY
   offer either (or both) in-order delivery of the stream data to the
   receive application or out-of-order (more immediate) delivery of
   received segments of the stream to the receiver application.  In
   either case, NORM sender and receiver implementations provide
   buffering to facilitate repair of the stream as it is transported.

   All NormObjects are logically segmented into FEC coding blocks and
   symbols for transmission by the sender.  In NORM, 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 needs to receive a sufficient number of symbols to
   reconstruct (via FEC decoding) the original user data for the given

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

2.1.  Protocol Operation Overview

   A NORM sender primarily generates messages of type "NORM_DATA".
   These messages carry original data segments or FEC symbols and repair
   segments/symbols for the bulk data/file or stream NormObjects being
   transferred.  By default, redundant FEC symbols are sent only in
   response to receiver repair requests (NACKs) and thus normally little
   or no additional transmission overhead is imposed due to FEC
   encoding.  However, the NORM implementation MAY be configured to
   proactively transmit some amount of redundant FEC symbols along with
   the original content to potentially enhance performance (e.g.,
   improved delay) at the cost of additional transmission overhead.
   This configuration is sensible for certain network conditions and can
   allow for robust, asymmetric multicast (e.g., unidirectional routing,
   satellite, cable) [FecHybrid] with reduced receiver feedback, or, in
   some cases, no feedback.

   A sender message of type "NORM_INFO" is also defined and is used to
   carry OPTIONAL out-of-band context information for a given transport
   object.  A single "NORM_INFO" message can be associated with a
   NormObject.  Because of its atomic nature, missing "NORM_INFO"
   messages can be NACKed and repaired with a slightly lower delay
   process than NORM's general FEC-encoded data content.  The
   "NORM_INFO" message can serve special purposes for some bulk
   transfer, reliable multicast applications where receivers join the
   group mid-stream and need to ascertain contextual information on the
   current content being transmitted.  The NACK process for "NORM_INFO"
   will be described later.  When the "NORM_INFO" message type is used,
   its transmission SHOULD precede transmission of any "NORM_DATA"
   message for the associated NormObject.

   The sender also generates messages of type "NORM_CMD" to assist in
   certain protocol operations such as congestion control, end-of-
   transmission flushing, group round trip time (GRTT) estimation,
   receiver synchronization, and OPTIONAL positive acknowledgment
   requests or application defined commands.  The transmission of
   "NORM_CMD" messages from the sender is accomplished by one of three
   different procedures.  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 are completely atomic and no specific reliability
   (buffering) state needs to be kept.  Thus, for session management or
   other purposes it is possible that even participants acting
   principally as data receivers MAY transmit "NORM_CMD" messages.
   However, it is RECOMMENDED that this is not done within the context
   of the NORM multicast session unless congestion control is addressed.
   For example, many receiver nodes transmitting "NORM_CMD" messages
   simultaneously can cause congestion for the destination(s).

   All sender transmissions are subject to rate control governed by a
   peak transmission rate set for each participant by the application.
   This can be used to limit the quantity of multicast data transmitted
   by the group.  When NORM's congestion control algorithm is enabled
   the rate for senders is automatically adjusted.  In some networks, it
   is desirable to establish minimum and maximum bounds for the rate
   adjustment depending upon the application even when dynamic
   congestion control is enabled.  However, in the case of the general
   Internet, congestion control policy SHALL be observed that is
   compatible with coexistent TCP flows.

   NORM receivers generate messages of type "NORM_NACK" or "NORM_ACK" in
   response to transmissions of data and commands from a sender.  The
   "NORM_NACK" messages are generated to request repair of detected data
   transmission losses.  Receivers generally detect losses by tracking
   the sequence of transmission from a sender.  Sequencing information
   is embedded in the transmitted data packets and end-of-transmission
   commands from the sender.  "NORM_ACK" messages are generated in
   response to certain commands transmitted by the sender.  In the
   general (and most scalable) protocol mode, "NORM_ACK" messages are
   sent 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 [RFC5401]
   document.  This includes the basic NORM architecture and the data
   transmission, repair, and feedback strategies discussed in that
   document.  The reliable multicast building block approach, as
   described in Reliable Multicast Transport Building Blocks for One-to-
   Many Bulk-Data Transfer [RFC3048], is applied in creating the full
   NORM protocol instantiation.  NORM also makes use of the parity-based
   encoding techniques for repair messaging and added transmission
   robustness as described in The Use of Forward Error Correction (FEC)
   in Reliable Multicast [RFC3453].  NORM uses the FEC Payload ID as
   specified by the FEC Building Block document [RFC5052].
   Additionally, for congestion control, this document fully specifies a
   baseline congestion control mechanism (NORM-CC) based on the TCP-
   Friendly Multicast Congestion Control (TFMCC) scheme[TfmccPaper],

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 necessitates an increased
   awareness of application and network architecture considerations.
   Performance requirements affecting design can include: group size,
   heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
   data ordering, delivery delay, group dynamics, mobility, congestion
   control, and transport across low capacity connections.  NORM
   contains various parameters to accommodate many of these differing
   requirements.  The NORM protocol and its mechanisms MAY be applied in
   multicast applications outside of bulk data transfer, but there is an
   assumed model of bulk transfer transport service that drives the
   trade-offs that determine the scalability and performance described
   in this document.

   The ability of NORM to provide reliable data delivery is also
   governed by any buffer constraints of the sender and receiver
   applications.  NORM protocol implementations SHOULD be designed to operate with the
   greatest efficiency and robustness possible within
   application-defined 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 randomly-delayed
   transmissions from the receiver set, rather than immediately
   transmitted feedback.  There are definitive trade-offs between buffer
   utilization, group size scalability, and efficiency of performance.
   Large buffer sizes allow the NORM protocol to perform most
   efficiently in large delay-bandwidth topologies and allow for longer
   feedback suppression backoff timeouts.  This yields improved group
   size scalability.  NORM can operate with reduced buffering but at a
   cost of decreased efficiency (lower relative goodput) and reduced
   group size scalability.

3.  Conformance Statement

   This RMT Protocol Instantiation document, in conjunction with the
   Multicast Negative-Acknowledgment (NACK) [RFC5401] and Forward Error
   Correction (FEC) [RFC5052] Building Blocks, completely specifies a
   working reliable multicast transport protocol that conforms to the
   requirements described in RFC 2357.

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

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

   This document also describes the following message types and
   associated mechanisms which that 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

   There are two primary classes of NORM messages (see Section 2.1):
   sender messages and receiver messages.  "NORM_CMD", "NORM_INFO", and
   "NORM_DATA" message types are generated by senders of data content,
   and "NORM_NACK" and "NORM_ACK" messages generated by receivers within
   a NormSession.  Sender messages SHALL be governed by congestion
   control for Internet use.  For session management or other purposes,
   receivers can also employ "NORM_CMD" message transmissions.  The
   principal rationale for distinguishing sender and receiver messages
   is that receivers will typically need to allocate resources to
   support reliable reception from sender(s) and NORM sender messages
   are subject to congestion control.  NORM receivers MAY employ the
   "NORM_CMD" message type for application-defined purposes but it is
   RECOMMENDED that congestion control and feedback implosion issues be
   addressed.  Additionally, an auxiliary message type of "NORM_REPORT"
   is also provided for experimental purposes.  This section describes
   the message formats used by the NORM protocol.  These messages and
   their fields are referenced in the detailed functional description of
   the NORM protocol given in Section 5.  Individual NORM messages are
   designed to be
   compatible with the MTU Maximum Transmission Unit (MTU) limitations of
   encapsulating Internet protocols including IPv4, IPv6, and UDP.  The
   current NORM protocol specification assumes UDP encapsulation and
   leverages the transport features of UDP.  The NORM messages are
   independent of network addresses and can be used in IPv4 and IPv6

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 that are non-
   interoperable.  The NORM version number for this specification is 1.

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


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

   The 8-bit "hdr_len" field indicates the number of 32-bit words that
   comprise the given message's header portion.  This is used to
   facilitate addition of header extensions.  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.  The "sequence" field serves two separate purposes,
   depending upon the message type:

   1.  NORM senders MUST set the "sequence" field of sender messages
       ("NORM_INFO", "NORM_DATA", and "NORM_CMD") so that receivers can
       monitor the "sequence" value to maintain an estimate of packet
       loss that can be used for congestion control purposes (See
       Section 5.5.2 for a detailed description of NORM Congestion
       Control operation).  A monotonically-increasing sequence number
       space MUST be maintained to mark NORM sender messages in this
       way.  Note that this "sequence" number is explicitly NOT used in
       NORM as part of its reliability procedures.  The NORM object and
       FEC payload identifiers are used to detect missing content for
       reliable transfer purposes.

   2.  NORM receivers SHOULD set the "sequence" field to support
       protection from message replay attacks of "NORM_NACK" or
       "NORM_NACK" messages.  Note that, depending upon configuration,
       NORM feedback messages are sent to the session multicast address
       or the unicast address[es] of the active NORM sender[s].  Thus, a
       separate, monotonically-increasing sequence number space MUST be
       maintained for each destination address to which the NORM
       receiver is transmitting feedback messages.

   Note that these two separate purposes necessitate the maintenance of
   separate sequence spaces to support the functions described here.
   And, in the case of NORM receivers, additional sequence spaces are
   needed when feedback messages are sent to the sender unicast
   address[es] instead of the session address.

   The "source_id" field is a 32-bit value that uniquely identifies the
   node that sent the message within the context of a single
   NormSession.  This value is termed the NORM node identifier
   (NormNodeId) and unique NormNodeId identifiers MUST be assigned
   within a single NormSession.  In some cases, use of the host IP IPv4
   address or a hash of it an address can suffice, but alternative
   methodologies for assignment and potential collision resolution of
   node identifiers within a multicast session SHOULD be considered.
   For example, the techniques for managing the 32-bit "synchronization
   source" (SSRC) identifiers defined in the Real-Time Protocol (RTP)
   specification [RFC3550] are applicable for use with NORM node
   identifiers.  In most deployments of the NORM protocol to date, the
   NormNodeId assignments are administratively configured.

   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

   For variable-length extensions, the value of the "hel" field is the
   length of the entire header extension, expressed in multiples of 32-
   bit words.  The "hel" field MUST be present for variable-length
   extensions ("het" between 0 and 127) and MUST NOT be present for
   fixed-length extensions ("het" between 128 and 255).

   The formats of the variable-length and fixed-length header extensions
   are given given, respectively, here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   het <=127   |      hel      |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                    Header Extension Content                   |
     |                              ...                              |

               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 generally the predominant type transmitted
   by NORM senders.  These messages are used to encapsulate segmented
   data content for objects of type "NORM_OBJECT_DATA",
   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 only for objects of type
   "NORM_OBJECT_STREAM".  These fields, as with the entire payload, are
   subject to any FEC encoding used.  Thus, when systematic FEC codes
   are used, these values can be directly interpreted only for packets
   containing source symbols only while packets containing FEC parity content
   need 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 (i.e. 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 "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)
   ("GRTT_sender") (this is also referred to as "R_max" in
   [TfmccPaper]).  This value is used to control timing of the NACK
   repair process and other aspects of protocol operation as described
   in this document.  Normally, the advertised "grtt" value will
   correspond to what the sender has measured based on feedback from the
   group, but, at low transmission rates, the advertised "grtt" SHALL be
   set to "MAX(grttMeasured, NormSegmentSize/senderRate)" where the
   "NormSegmentSize" is sender's segment size in bytes and the
   "senderRate" is the sender's current transmission rate in bytes per
   second.  The algorithm for encoding and decoding this field is
   described in the Multicast NACK Building Block [RFC5401].

   The "backoff" field value is used by receivers to determine the
   maximum backoff timer value used in the timer-based NORM NACK
   feedback suppression.  This 4-bit field supports values from 0-15
   that is multiplied by the sender GRTT "GRTT_sender" to determine the maximum backoff
   timeout.  The "backoff" field informs the receivers of the sender's
   backoff factor parameter "Ksender". ("K_sender").  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. size ("GSIZE_sender").  This 4-bit field can
   roughly represent values from ten to 500 million where the most
   significant bit value of 0 or 1 represents a mantissa of 1 or 5,
   respectively and the three least significant bits incremented by one
   represent a 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 for the receiver to appropriately
   handle the identified object.  Defined flags in this field include:

   | Flag                   | Value | Purpose                          |
   | "NORM_FLAG_REPAIR"     |  0x01 | Indicates message is a repair    |
   |                        |       | transmission                     |
   | "NORM_FLAG_EXPLICIT"   |  0x02 | Indicates a repair segment       |
   |                        |       | intended to meet a specific      |
   |                        |       | receiver erasure, as compared to |
   |                        |       | parity segments provided by the  |
   |                        |       | sender for general purpose (with |
   |                        |       | respect to 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.  There are cases where
   receivers can inadvertently request repair of such objects when all
   segments (or info content) for those objects are not received (i.e.,
   a gap in the "object_transport_id" sequence is noted).  In this case,
   the sender SHALL invoke the "NORM_CMD(SQUELCH)" process as described
   in Section 4.2.3.

   "NORM_FLAG_FILE" can be set as a hint from the sender that the
   associated object SHOULD be stored in non-volatile storage.
   "NORM_FLAG_STREAM" is set when the identified object is of type
   "NORM_OBJECT_STREAM".  The presence of "NORM_FLAG_STREAM" overrides
   that of "NORM_FLAG_FILE" with respect to interpretation of object
   size and the format of "NORM_DATA" messages.

   The "fec_id" field corresponds to the FEC Encoding Identifier
   described in the FEC Building Block document [RFC5052].  The "fec_id"
   value implies the format of the "fec_payload_id" field and, coupled
   with FEC Object Transmission Information, the procedures to decode
   FEC encoded content.  Small block, systematic codes ("fec_id" = 129)
   are expected to be used for most NORM purposes and systematic FEC
   codes are RECOMMENDED for most efficient performance of
   "NORM_OBJECT_STREAM" transport.

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

   The "fec_payload_id" identifies the attached "NORM_DATA" "payload"
   content.  The size and format of the "fec_payload_id" field depends
   upon the FEC type indicated by the "fec_id" field.  These formats are
   given in the descriptions of specific FEC schemes such as those
   described in the FEC Basic Schemes [RFC5445] specification or in
   other FEC Schemes.  As an example, the format of the "fec_payload_id"
   format for Small Block, Systematic codes ("fec_id" = 129) from theFEC
   Basic Schemes [RFC5445] specification is given here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                       source_block_number                     |
     |        source_block_len       |      encoding_symbol_id       |

             Example: FEC Payload Id Format for 'fec_id' = 129

   In this example FEC payload identifier, the "source_block_number",
   "source_block_len", and "encoding_symbol_id" fields correspond to the
   "Source Block Number", "Source Block Length, and "Encoding Symbol ID"
   fields of the FEC Payload ID format for Small Block Systematic FEC
   Schemes identified by a "fec_id" value of 129 as specified by the FEC
   Basic Schemes [RFC5445] specification.  The "source_block_number"
   identifies the coding block's relative position with a NormObject.
   Note that, for NormObjects of type "NORM_OBJECT_STREAM", the
   "source_block_number" will wrap for very long lived sessions.  The
   "source_block_len" indicates the number of user data segments in the
   identified coding block.  Given the "source_block_len" information of
   how many symbols of application data are contained in the block, the
   receiver can determine whether the attached segment is data or parity
   content and treat it appropriately.  Applications MAY dynamically
   "shorten" code blocks when the pending information content is not
   predictable (e.g. real-time message streams).  In that case, the
   "source_block_len" value given for an "encoding_symbol_id" that
   contains FEC parity content SHALL take precedence over the
   "source_block_len" value provided for any packets containing source
   symbols.  Also, the "source_block_len" value given for an ordinally
   higher "encoding_symbol_id" SHALL take precedence over the
   "source_block_len" given for prior encoding symbols.  The reason for
   this is that the sender will only know the maximum source block
   length at the time 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 will be a user data or an FEC parity segment.  For
   systematic codes, encoding symbols numbered less than the
   "source_block_len" contain original application data while segments
   greater than or equal to "source_block_len" contain parity symbols
   calculated for the block.  The concatenation of
   "object_transport_id::fec_payload_id" can be viewed as a unique
   transport protocol data unit identifier for the attached segment with
   respect to the NORM sender's instance within a session.

   Additional FEC Object Transmission Information (FTI) (as described in
   the FEC Building Block [RFC5052]) is needed to properly receive and
   decode NORM transport objects.  This information MAY be provided as
   out-of-band session information.  In some cases, it will be useful
   for the sender to include this information "in-band" to facilitate
   receiver operation with minimal pre-configuration.  For this purpose,
   the NORM FEC Object Transmission Information Header Extension
   (EXT_FTI) is defined.  This header extension MAY be applied to
   "NORM_DATA" and "NORM_INFO" messages to provide this necessary
   information.  The format of the EXT_FTI consists of two parts, a
   general part that contains the size of the associated transport
   object and a portion that depends upon the FEC scheme being used.
   The "fec_id" field in "NORM_DATA" and "NORM_INFO" messages identifies
   the FEC scheme.  The format of the EXT_FTI general part is given

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

   As noted, the format of the extension depends upon the FEC code in
   use, but in general it contains any necessary details on the code in
   use (e.g., FEC Instance ID, etc.).  As an example, the format of the
   EXT_FTI for small block systematic codes ("fec_id" = 129) is given
      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 'fec_id' = 129

   In this example (for "fec_id" = 129), the "hel" field value is 4.
   The size of the EXT_FTI header extension will possibly be different
   for other FEC schemes.

   The 48-bit "object_size" serves the purpose described previously.

   The "fec_instance_id" corresponds to the "FEC Instance ID" described
   in the FEC Building Block [RFC5052].  In this case, the
   "fec_instance_id" is a value corresponding to the particular type of
   Small Block Systematic Code being used (e.g., Reed-Solomon GF(2^8),
   Reed-Solomon GF(2^16), etc).  The standardized assignment of FEC
   Instance ID values is described in RFC 5052.

   The "segment_size" field indicates the sender's current setting for
   maximum message payload content (in bytes).  This allows receivers to
   allocate appropriate buffering resources and to determine other
   information in order to properly process received data messaging.
   Typically, FEC parity symbol segments will be of this size.

   The "fec_max_block_len" indicates the current maximum number of user
   data segments per FEC coding block to be used by the sender during
   the session.  This allows receivers to allocate appropriate buffer
   space for buffering blocks transmitted by the sender.

   The "fec_num_parity" corresponds to the "maximum number of encoding
   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 [RFC5052].  For example, Reed-Solomon
   codes can be arbitrarily shortened to create different code
   variations for a given block length.  In the case of Reed-Solomon
   (GF(2^8) and GF(2^16)) codes, this value indicates the maximum number
   of parity segments available from the sender for the coding blocks.
   This field MAY be interpreted differently for other systematic codes
   as they are defined.

   The payload portion of "NORM_DATA" messages includes source data or
   FEC encoded application content.  The content of this payload depends
   upon the FEC scheme being employed, and support for streaming using
   the "NORM_OBJECT_STREAM" type, when applicable, necessitates some
   additional content in the payload.

   The "payload_len", "payload_msg_start", and "payload_offset" fields
   are present ONLY only for transport objects of type "NORM_OBJECT_STREAM".
   These REQUIRED fields allow senders to arbitrarily vary the size of
   "NORM_DATA" payload segments for streams.  This allows applications
   to flush transmitted streams as needed to meet unique streaming
   requirements.  For objects of types "NORM_OBJECT_FILE" and
   "NORM_OBJECT_DATA", these fields are unnecessary since the receiver
   can calculate the payload length and offset information from the
   "fec_payload_id" using the REQUIRED block partitioning algorithm
   described in the FEC Building Block [RFC5052].  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

   The "payload_len" value, when non-zero, indicates the length (in
   bytes) of the source content contained in the associated
   "payload_data" field.  However, when the "payload_len" value is equal
   to ZERO, "ZERO", this indicates that the "payload_msg_start" field be
   alternatively interpreted as a "stream_control_code".  The only
   "stream_control_code" value defined is "NORM_STREAM_END = 0".  The
   "NORM_STREAM_END" code indicates that the sender is terminating
   transmission of stream content at the corresponding position in the
   stream and the receiver MUST not NOT expect content (or request repair
   for any content) following that position in the stream.  Additional
   specifications MAY extend the functionality of the NORM stream
   transport mode by defining additional stream control codes.  These
   control codes are delivered to the recipient application reliably,
   in-order with respect to the streamed application data content.

   The "payload_msg_start" field serves one of two exclusive purposes.
   When the "payload_len" value is non-zero, the "payload_msg_start"
   field, when also set to a non-zero value, indicates that the
   associated "payload_data" content contains an application-defined
   message boundary (start-of-message).  When such a message boundary is
   indicated, the first byte of an application-defined message, with
   respect to the "payload_data" field, will be found at an offset of
   "payload_msg_start - 1" bytes.  Thus, if a "NORM_DATA" payload for a
   "NORM_OBJECT_STREAM" contains the start of an application message at
   the first byte of the "payload_data" field, the value of the
   "payload_msg_start" field will be '1'.  NORM implementations SHOULD
   provide sender stream applications with a capability to mark message
   boundaries in this manner.  Similarly, the NORM receiver
   implementation SHOULD enable the application to recover such message
   boundary information.  This enables NORM receivers to "synchronize"
   reliable reception of transmitted message stream content in a
   meaningful way (i.e., meaningful to the application) at any time,
   whether joining a session already in progress, or departing the
   session and returning.  Note that if the value of the
   "payload_msg_start" field is ZERO, "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 be instead
   interpreted as a "stream_control_code"

   The "payload_offset" field indicates the relative byte position (from
   the sender stream transmission start) of the source content contained
   in the "payload_data" field.  Note that for long-lived streams, the
   "payload_offset" field will wrap.

   The "payload_data" field contains the original application source or
   parity content for the symbol identified by the "fec_payload_id".
   The length of this field SHALL be limited to a maximum of the
   sender's NormSegmentSize bytes as given in the FTI for the object.
   Note the length of this field for messages containing parity content
   will always be of length NormSegmentSize.  When encoding data
   segments of varying sizes, the FEC encoder SHALL assume ZERO "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 can possibly vary on a per-object basis.  The
   NormSegmentSize SHOULD be configurable by the sender application
   prior to session participation as needed for network topology maximum
   transmission unit (MTU) 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
   objects, the data segment length and offset can be calculated using
   the block partitioning algorithm described in the FEC Building Block
   [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 could then 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 will NACK for retransmission
   of "NORM_INFO" when they have not received it for a given NormObject.
   The size of the "NORM_INFO" content is limited to that of a single
   NormSegmentSize for the given sender.  This atomic nature allows the
   "NORM_INFO" to be rapidly and efficiently repaired within the NORM
   reliable transmission process.

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

      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, NORM implementations MAY apply the EXT_FTI when
   used to "NORM_INFO" messages only and not to "NORM_DATA" messages.

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

4.2.3.  NORM_CMD Messages

   "NORM_CMD" messages are transmitted by senders to perform a number of
   different protocol functions.  This includes functions such as round-
   trip timing collection, congestion control functions, synchronization
   of sender/receiver repair "windows", and notification of sender
   status.  A core set of "NORM_CMD" messages is enumerated.
   Additionally, a range of command types remain available for potential
   application-specific use.  Some "NORM_CMD" types can 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    sub-type   |                                               |
     +-+-+-+-+-+-+-+-+        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" "sub-type" 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" "sub-type" field indicates the type of
   command to follow.  The remainder of the "NORM_CMD" message is
   dependent upon the command type ("flavor"). sub-type.  NORM command flavors sub-types include:


   | Command                 | Flavor Sub-type | Purpose                      |
   | "NORM_CMD(FLUSH)"       |     1    | Used to indicate sender      |
   |                         |          | temporary                    |
   |                         |          | end-of-transmission.         |
   |                         |          | (Assists in robustly         |
   |                         |          | initiating outstanding repair       |
   |                         |          | repair requests from receivers).  May         |
   |                         |          | receivers).  May also be optionally used to     |
   |                         |          | optionally used to collect positive   |
   |                         |          | positive acknowledgment of reliable   |
   |                         |          | reliable reception from subset of      |
   |                         |          | 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     |
   |                         |          | NACKs from receivers.        |
   | "NORM_CMD(CC)"          |     4    | Used for GRTT measurement and    |
   |                         |          | 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   |
   |                         |          | of receivers (OPTIONAL).     |
   | "NORM_CMD(APPLICATION)" |     7    | Used for application-defined |
   |                         |          | purposes which that need to        |
   |                         |          | temporarily preempt or       |
   |                         |          | supplement 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 can indicate either 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" "2*GRTT_sender"
   to excite the receiver set for any outstanding repair requests up to
   and including the transmission point indicated within the
   "NORM_CMD(FLUSH)" message.  The number of repeats is equal to
   "NORM_ROBUST_FACTOR" unless a list of receivers from which explicit
   positive acknowledgment is expected ("acking_node_list") is given.
   In that case, the "acking_node_list" is updated as acknowledgments
   are received and the "NORM_CMD(FLUSH)" is repeated according to the
   mechanism described in Section 5.5.3.  The greater the
   "NORM_ROBUST_FACTOR", the greater the probability that all applicable
   receivers will be excited for acknowledgment or repair requests
   (NACKs) AND that the corresponding NACKs are delivered to the sender.
   A default value of "NORM_ROBUST_FACTOR" equal to 20 is RECOMMENDED.
   If a "NORM_NACK" message interrupts the flush process, the sender
   SHALL re-initiate the flush process after any resulting repair
   transmissions are completed.

   Note that receivers also employ a timeout mechanism to self-initiate
   NACKing (if there are outstanding repair needs) when no messages of
   any type are received from a sender.  This inactivity timeout is
   related to the "NORM_CMD(FLUSH)" and "NORM_ROBUST_FACTOR" and is
   specified in Section 5.3.  Receivers SHALL self-initiate the NACK
   repair process when the inactivity timeout has expired for a specific
   sender and the receiver has pending repairs needed from that sender.
   With a sufficiently large "NORM_ROBUST_FACTOR" value, data content is
   delivered with a high assurance of reliability.  The penalty of a
   large "NORM_ROBUST_FACTOR" value is the potential transmission of
   excess "NORM_CMD(FLUSH)" messages and a longer inactivity timeout for
   receivers to self-initiate a terminal NACK process.

   For finite-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 can 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  sub-type = 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 that does not
   include any requests for parity-based repair.  This allows NORM
   sender applications to "flush" an ongoing stream of transmission when
   needed, even if in the middle of 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.  Note that a sender has the option of arbitrarily
   shortening a given code block when such an application "flush"
   occurs.  In this case, the receiver will request explicit repair, but
   the sender MAY provide FEC-based repair (parity segments) in
   response.  These parity segments MUST contain the corrected
   "source_block_len" for the shortened block and that
   "source_block_len" associated with segments containing parity content
   SHALL override the previously advertised "source_block_len".
   Similarly, the "source_block_len" associated with the highest ordinal
   "encoding_symbol_id" SHALL take precedence over prior symbols when a
   difference (e.g., due to code shortening at the sender) occurs.
   Normal receiver NACK initiation and construction is discussed in
   detail in Section 5.3.

   The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
   for receivers from which the sender is requesting explicit positive
   acknowledgment of reception up through the transmission point
   identified by the "object_transport_id" and "fec_payload_id" fields.
   The length of the list can be inferred from the length of the
   received "NORM_CMD(FLUSH)" message.  When the "acking_node_list" is
   present, the lightweight positive acknowledgment process described in
   Section 5.5.3 SHALL be observed.  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  sub-type = 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 "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 that the sender previously transmitted
   marked with the "NORM_FLAG_UNRELIABLE" flag.  This command indicates
   to receivers what content is available for repair, thus serving as a
   description of the sender's current "repair window".  Receivers SHALL
   NOT generate repair requests for content identified as invalid by a

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

   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 sub-type = 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 that initiated the "NORM_CMD(SQUELCH)" transmission.  The
   value of the "hdr_len" field when no extensions are present is 4 plus
   the size of the "fec_payload_id" field that is dependent upon the FEC
   scheme identified by the "fec_id" field.

   The "object_transport_id" and "fec_payload_id" fields are
   concatenated to indicate the beginning of the sender's current repair
   window (i.e., the logically earliest point in its transmission
   history for which the sender can provide repair).  The "fec_id" field
   implies the size and format of the "fec_payload_id" field.  This
   serves as an advertisement of a "synchronization" point for receivers
   to request repair.  Note, that while an "encoding_symbol_id" MAY be
   included in the "fec_payload_id" field, the sender's repair window
   SHOULD be aligned on FEC coding block boundaries and thus the
   "encoding_symbol_id" SHOULD be zero.

   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 cannot include the entire set of
   invalid objects in the repair window.  In this case, the sender SHALL
   ensure that the list begins with a 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) sender-to-group
   GRTT measurement and to excite the group for congestion control
   feedback.  A baseline NORM congestion control scheme (NORM-CC), based
   on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of
   RFC 4654 is fully specified in Section 5.5.2 of this document.  The
   "NORM_CMD(CC)" message is usually transmitted as part of NORM-CC
   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 network where resources are explicitly dedicated to the
   NORM session and therefore congestion control operation is disabled,
   the "NORM_CMD(CC)" message is then used soley solely for GRTT measurement
   and MAY be sent less frequently than with congestion control

      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  sub-type = 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 is possible for alternative congestion
   control schemes to use the "NORM_CMD(CC)" message defined here and
   leverage the "reserved" field for scheme-specific purposes.

   The "cc_sequence" field is a sequence number applied by the sender.
   For NORM-CC operation, it is used to provide functionality equivalent
   to the "feedback round number" ("fb_nr") described in RFC 4654.  The
   most recently received "cc_sequence" value is recorded by receivers
   and can be fed back to the sender in congestion control feedback
   generated by the receivers for that sender.  The "cc_sequence" number
   can also be used in NORM implementations to assess how recently a
   receiver has received "NORM_CMD(CC)" probes from the sender.  This
   can be useful instrumentation for complex or experimental multicast
   routing environments.

   The "send_time" field is a timestamp indicating the time that the
   "NORM_CMD(CC)" message was transmitted.  This consists of a 64-bit
   field containing 32-bits with the time in seconds ("send_time_sec")
   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" might be attached as
   the payload of the "NORM_CMD(CC)" message.  The presence of this
   header extension also implies that NORM receivers MUST respond
   according to the procedures described in Section 5.5.2.

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

   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 [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 sub-type = 5  |     flags     |            reserved           |
     |               header extensions (if applicable)               |
     |                              ...                              |
     |                       repair_adv_payload                      |
     |                              ...                              |
                    NORM_CMD(REPAIR_ADV) Message Format

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

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

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

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

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

             "cc_loss" = floor(decimal_loss_fraction * 65535.0)

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

   The "cc_rate" field represents the 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, "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 sub-type = 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   |
   |                        |            | to "NORM_CMD(CC)" messages. |
   | "NORM_ACK_FLUSH"       | 2          | Used to identify "NORM_ACK" |
   |                        |            | messages sent in response to   |
   |                        |            | to "NORM_CMD(FLUSH)"        |
   |                        |            | messages.                   |
   | "NORM_ACK_RESERVED"    | 3-15       | Reserved for possible future       |
   |                        |            | 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 can 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 can associate
   the response with its corresponding request.

   The "reserved" field is reserved for possible future protocol use and
   SHALL be set to ZERO "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". "2*GRTT_sender".  This rate of repetition
   allows the application to observe any response (if that is the
   application's purpose for the command) before it is repeated.
   Possible responses can include initiation of data transmission, other
   "NORM_CMD(APPLICATION)" messages, or even application-defined,
   positively-acknowledge commands from other NormSession participants.
   The transmission of these commands will preempt data transmission
   when they are scheduled and can be multiplexed with ongoing data
   transmission.  This type of robustly transmitted command allows NORM
   applications to define a complete set of session control mechanisms
   with less state than the transfer of FEC encoded reliable content
   needs while taking advantage of NORM transmission and round-trip
   timing information.
      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 sub-type = 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 containing an
   invalid "instance_id" value.

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

   For NORM-CC operation, the NORM-CC Feedback Header Extension, as
   described in the "NORM_CMD(REPAIR_ADV}" message description, is added
   to "NORM_NACK" messages to provide feedback on the receivers current
   state with respect to congestion control operation.  Note that
   alternative  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 "ZERO" for this version of the protocol.

   The "nack_payload" of the "NORM_NACK" message specifies the repair
   needs of the receiver with respect to the NORM sender indicated by
   the "server_id" field.  The receiver constructs repair requests based
   on the "NORM_DATA" and/or "NORM_INFO" segments it needs from the
   sender to complete reliable reception up to the sender's transmission
   position at the moment the receiver initiates the NACK Procedure as
   described in Section 5.3.  A single NORM Repair Request consists of a
   list of items, ranges, and/or FEC coding block erasure counts for
   needed "NORM_DATA" and/or "NORM_INFO" content.  Multiple repair
   requests can be concatenated within the "nack_payload" field of a
   "NORM_NACK" message.  Note that a  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
   corresponding to the 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 needed as repair. |
   | "NORM_NACK_BLOCK"   |  0x02 | Indicates the listed block(s) or    |
   |                     |       | range of blocks in entirety are     |
   |                     |       | needed as repair.                   |
   | "NORM_NACK_INFO"    |  0x04 | Indicates that "NORM_INFO" is needed as  |
   |                     |       | needed as repair for the listed     |
   |                     |       | object(s).    |
   | "NORM_NACK_OBJECT"  |  0x08 | Indicates the listed object(s) or   |
   |                     |       | range of objects in entirety are    |
   |                     |       | needed as repair.                   |

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

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

   The "repair_request_items" field consists of a list of individual or
   range pairs of transport data unit identifiers in the following
      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 "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 needed to complete reliable
   transmission, multiple NORM Repair Requests with different "form" and
   or "flags" values can be concatenated within a single "NORM_NACK"
   message.  Additionally, NORM receivers SHALL construct "NORM_NACK"
   messages with their repair requests in ordinal order with respect to
   "object_transport_id" and "fec_payload_id" values.  The
   "nack_payload" size SHALL NOT exceed the NormSegmentSize for the
   sender to which the "NORM_NACK" is destined.

   NORM_NACK Content Examples:

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

   Example 1: "NORM_NACK" "nack_payload" for: Object 12, Coding Block 3,
                            Segments 2,5,and 8
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   form = 1    | flags = 0x01  |       length  = 36            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 2     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 5     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 12   |
     |                    source_block_number = 3                    |
     |    source_block_length = 32   |    encoding_symbol_id = 8     |
   Example 2: "NORM_NACK" "nack_payload" for: Object 18, Coding Block 6,
     Segments 5, 6, 7, 8, 9, 10; and Object 19 "NORM_INFO" and Coding
                            Block 1, segment 3
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |   form = 2    | flags = 0x01  |       length  = 24            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
     |                    source_block_number = 6                    |
     |    source_block_length = 32   |    encoding_symbol_id = 5     |
     |  fec_id = 129 |   reserved    |    object_transport_id = 18   |
     |                    source_block_number = 6                    |
     |    source_block_length = 32   |    encoding_symbol_id = 10    |
     |   form = 1    | flags = 0x05  |       length  = 12            |
     |  fec_id = 129 |   reserved    |    object_transport_id = 19   |
     |                    source_block_number = 1                    |
     |    source_block_length = 32   |    encoding_symbol_id = 3     |

4.3.2.  NORM_ACK Message

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

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

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

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

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

5.  Detailed Protocol Operation

   This section describes the detailed interactions of senders and
   receivers participating in a NORM session.  A simple synopsis of
   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.  "When applicable, NORM_INFO" messages MAY
       optionally precede the transmission of data content for NORM
       transport objects.

   3.  As receivers detect missing content from the sender, they
       initiate repair requests with "NORM_NACK" messages.  Note the  The
       receivers track the sender's most recent objectId::fecPayloadId
       transmit position and NACK ONLY only for content that is ordinally
       prior to that current transmit position.  The receivers schedule
       random backoff timeouts before generating "NORM_NACK" messages
       and wait an appropriate amount of time before repeating the
       "NORM_NACK" if their repair request is not satisfied.

   4.  The sender aggregates repair requests from the receivers and
       logically "rewinds" its transmit position to send appropriate
       repair messages.  The sender sends repairs for the earliest
       ordinal transmit position first and maintains this ordinal repair
       transmission sequence.  FEC parity content not previously
       transmitted for the applicable FEC coding block is used for
       repair transmissions to the greatest extent possible.  If the
       sender exhausts its available FEC parity content on multiple
       repair cycles for the same coding block, it resorts to an
       explicit repair strategy (possibly using parity content) to
       complete repairs.  (The use of explicit repair is an exception in
       general protocol operation, but the possibility does exist for
       extreme conditions).  The sender immediately assumes transmission
       of new content once it has sent pending repairs.

   5.  The sender transmits "NORM_CMD(FLUSH)" messages when it reaches
       the end of enqueued transmit content and pending repairs.
       Receivers respond to the "NORM_CMD(FLUSH)" messages with
       "NORM_NACK" transmissions (following the same suppression backoff
       timeout strategy as for data) if they need further repair.

   6.  The sender transmissions are subject to rate control limits
       determined by congestion control mechanisms.  In the baseline
       NORM-CC operation, each sender in a NormSession maintains its own
       independent congestion control state.  Receivers provide
       congestion control feedback in "NORM_NACK" and "NORM_ACK"
       messages.  "NORM_ACK" feedback for congestion control purposes is
       governed using a suppression mechanism similar to that for
       "NORM_NACK" messages.

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

5.1.  Sender Initialization and Transmission

   Upon startup, the NORM sender immediately begins sending
   "NORM_CMD(CC)" messages to collect round trip timing and other
   information from the potential group.  If NORM-CC congestion control
   operation is enabled, the NORM-CC Rate header extension MUST be
   included in these messages.  Congestion control operation SHALL be
   observed at all times when not operating using dedicated resources,
   like in the general Internet.  Even if congestion control operation
   is disabled at the sender, it can be desirable to use the
   "NORM_CMD(CC)" messaging to collect feedback from the group using the
   baseline NORM-CC feedback mechanisms.  This proactive feedback
   collection can be used to establish a GRTT estimate prior to data
   transmission and potential NACK operation.

   In some cases, applications might need the sender to also proceed
   with data transmission immediately.  In other cases, the sender might
   wish to defer data transmission until it has received some feedback
   or request from the receiver set indicating 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 receivers.

   The NORM sender begins segmenting application-enqueued data into
   "NORM_DATA" segments and transmitting it to the group.  For objects
   of type "NORM_OBJECT_DATA" and "NORM_OBJECT_FILE", the segmentation
   algorithm described in FEC Building Block [RFC5052] is RECOMMENDED.
   For objects of type "NORM_OBJECT_STREAM", segmentation will typically
   be into uniform FEC coding block sizes, with individual segment sizes
   controlled by the application.  In most cases, the application and
   NORM implementation SHOULD strive to produce full-sized
   ("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". "2*GRTT_sender".
   Similar to end of each transmitted FEC coding block during
   transmission, receivers SHALL respond to these "NORM_CMD(FLUSH)"
   messages with additional repair requests as needed.  A protocol
   parameter ""NORM_ROBUST_FACTOR"" determines the number of flush
   messages sent.  If receivers request repair, the repair is provided
   and flushing occurs again at the end of repair transmission.  The
   sender MAY attach an OPTIONAL "acking_node_list" to "NORM_CMD(FLUSH)"
   containing the NormNodeIds for receivers from which it expects
   explicit positive acknowledgment of reception.  The "NORM_CMD(FLUSH)"
   message MAY be also used for this OPTIONAL purpose any time prior to
   the end of data enqueued for transmission with the "NORM_CMD(FLUSH)"
   messages multiplexed with ongoing data transmissions.  The OPTIONAL
   NORM positive acknowledgment procedure is described in Section 5.5.3.

5.1.1.  Object Segmentation Algorithm

   NORM senders and receivers MUST use a common algorithm for logically
   segmenting transport data into FEC encoding blocks and symbols so
   appropriate NACKs can be constructed to request repair of missing
   data.  NORM FEC coding blocks are comprised of multi-byte symbols
   (segments) 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 [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

   For typical operation, NORM receivers will join a specified multicast
   group and listen on an specific port number for sender transmissions.
   As the NORM receiver receives "NORM_DATA" messages it will establish
   buffering state and provide content to its application as appropriate
   for the given data type.  The NORM protocol is designed such that allows receivers can to join
   and leave the group at will.  However, will although some applications might be constrained
   such that receivers need
   receivers to be members of the group prior to start of data
   transmission.  Thus, different NORM applications MAY use different
   policies to constrain the impact of new receivers joining the group
   in the middle of a session.  For example, a useful implementation
   policy is for new receivers joining the group to limit or avoid
   repair requests for transport objects already in progress.  The NORM
   sender implementation MAY impose additional constraints to limit the
   ability of receivers to disrupt reliable multicast performance by
   joining, leaving, and rejoining the group often.  Different receiver
   "join policies" might be appropriate for different applications
   and/or scenarios.  For general purpose operation, a default policy
   where receivers are allowed to request repair only for coding blocks
   with a NormTransportId and FEC coding block number greater than or
   equal to the first non-repair "NORM_DATA" or "NORM_INFO" message
   received upon joining the group is RECOMMENDED.  For objects of type
   "NORM_OBJECT_STREAM" it is RECOMMENDED 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, NORM receivers will join a specified

   In some deployments, different 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 receivers might have
   differing quality of network connectivity.  Some receivers may suffer
   significantly poorer performance with very limited goodput due to low
   connection rate or substantial packet loss.  Similar to its application as appropriate.

5.3.  Receiver NACK Procedure

   When the receiver detects it is missing data from "join
   policies" described above, a sender's NORM sender implementation MAY choose to
   enforce different "service policies" to perhaps exclude exceptionally
   poor-performing (or otherwise badly-behaving) receivers from the
   group.  The sender implementation could choose to ignore NACKs from
   such receivers and/or force advancement of its logical "repair
   window" (i.e. enforcing a minimal level of service) and use the
   "NORM_CMD(SQUELCH)" message to advise those poor performers of its
   advance.  Note in some cases, the application may need to support the
   "weakest member" regardless of the time needed to achieve reliable
   delivery.  When implemented, the protocol instantiation SHOULD expose
   controls to the set of "join" and/or "service" policies available to
   support the needs of different applications.

5.3.  Receiver NACK Procedure

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

   where the ""GRTTsender"" ""GRTT_sender"" value corresponds to the GRTT estimate
   advertised in the "grtt" field of NORM sender messages.  A minimum
   ""T_inactivity"" value of 1 second is RECOMMENDED.  The NORM receiver
   SHOULD reset this inactivity timer and repeat NACK initiation upon
   timeout for up to "NORM_ROBUST_FACTOR" times or more depending upon
   the application's need for persistence by its receivers.  It is also
   important 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 [RFC5401]
   using ("Ksender*GRTTsender") ("K_sender*GRTT_sender") for the "maxTime" parameter and the
   sender advertised group size ("GSIZEsender") as the "groupSize"
   parameter.  NORM senders provide values for "GRTTsender", "Ksender" "GRTT_sender", "K_sender"
   and "GSIZEsender" "GSIZE_sender" via the "grtt", "backoff", and "gsize" fields of
   transmitted messages.  The "GRTTsender" "GRTT_sender" value is determined by the
   sender based on feedback it has received from the group while the
   "K_sender" and "GSIZEsender" "GSIZE_sender" values can be determined by application
   requirements and expectations or ancillary information.  The backoff
   factor ""Ksender"" ""K_sender"" MUST be greater than "one" to provide for
   effective feedback suppression.  A value of "K "K_sender = 4" is
   RECOMMENDED for the Any Source Multicast (ASM) model while a value of "K
   "K_sender = 6" is RECOMMENDED for Single Source Multicast (SSM)

       T_backoff = RandomBackoff(Ksender*GRTTsender, GSIZEsender) RandomBackoff(K_sender*GRTT_sender, GSIZE_sender)

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

   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 [RFC5401] is:
                  T_rcvrHoldoff =(Ksender+2)*GRTTsender =(K_sender+2)*GRTT_sender

   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 might have transmitted other
   different, additional parity segments for other receivers that could
   also be used to satisfy the local receiver's erasure-filling needs.
   In the case where the erasure count for a partially-received FEC
   coding block exceeds the maximum number of parity symbols available
   from the sender for the block (as indicated by the "NORM_DATA"
   "fec_num_parity" field), the receiver SHALL request all available
   parity segments plus the ordinally highest missing data segments
   needed to satisfy its total erasure needs for the block.  The goal of
   this strategy is for the overall receiver set to request a lowest
   common denominator set of repair symbols for a given FEC coding
   block.  This allows the sender to construct the most efficient repair
   transmission segment set and enables effective NACK suppression among
   the receivers even with uncorrelated packet loss.  This approach also
   does not demand synchronization among the receiver set in their
   repair requests for the sender.

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

5.4.  Sender NACK Processing and Response

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

5.4.1.  Sender Repair State Aggregation

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

   As described in [RFC5401], the period of time during which the sender
   aggregates "NORM_NACK" messages is equal to:
               T_sndrAggregate = (Ksender+1)*GRTT (K_sender + 1) * GRTT_sender

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

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

5.4.2.  Sender FEC Repair Transmission Strategy

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

   "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 limiting 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 need to
   conduct repair state accumulation for sub-routes in a manner similar
   to the sender's repair state accumulation in order to have sufficient
   information to perform the sub-casting.  Additionally, the
   intermediate systems could perform 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". "2*GRTT_sender".
   The "invalid_object_list" (if applicable) of the "NORM_CMD(SQUELCH)"
   message SHALL begin with the lowest "object_transport_id" from the
   invalid "NORM_NACK" messages received since the last
   "NORM_CMD(SQUELCH)" transmission.  The list includes as many lower
   ordinal invalid "object_transport_ids" that can fit for the
   "NORM_CMD(SQUELCH)" payload size to less than or equal to the
   sender's NormSegmentSize parameter.

5.4.4.  Sender NORM_CMD(REPAIR_ADV) Generation

   When a NORM sender receives "NORM_NACK" messages from receivers via
   unicast transmission, it uses "NORM_CMD(REPAIR_ADV)" messages to
   advertise its accumulated repair state to the receiver set since the
   receiver set is not directly sharing their repair needs via multicast
   communication.  A NORM sender implementation MAY use a separate port
   number from the NormSession port number as the source port for its
   transmissions.  Thus NORM receivers can direct any unicast feedback
   messages to this separate sender port number that is number, distinct from the NORM
   session (or destination) port number.  Then, the NORM sender
   implementation can discriminate unicast feedback messages from
   multicast feedback messages when there is a mix of multicast and
   unicast feedback receivers.  The "NORM_CMD(REPAIR_ADV)" message is
   multicast to the receiver set by the sender.  The payload portion of
   this message has content in the same format as the "NORM_NACK"
   receiver message payload.  Receivers are then able to perform
   feedback suppression in the same manner as with "NORM_NACK" messages
   directly received from other receivers.  Note 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 will also need to provide
   any information needed so that dynamic congestion control feedback can be
   suppressed among receivers.  This document specifies the NORM-CC
   Feedback Header Extension that is applied for baseline NORM-CC
   operation.  If other congestion control mechanisms are used within a
   NORM implementation, other header extensions MAY be defined.
   Whatever content format is used for this purpose SHOULD ensure that
   maximum possible suppression state is conveyed to the receiver set.

5.5.  Additional Protocol Mechanisms

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

5.5.1.  Greatest  Group Round-trip Time (GRTT) Collection

   For NORM receivers to appropriately scale backoff timeouts and the
   senders to use proper corresponding timeouts, the participants need
   to use a common timeout basis.  Each NORM sender monitors the round-
   trip time of active receivers and determines the group greatest group
   round-trip time (GRTT). time.  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 containing a locally
   generated timestamp.  Receivers are expected to record this timestamp
   along with the time the "NORM_CMD(CC)" message is received.  Then,
   when the receivers generate feedback messages to the sender, an
   adjusted version of the sender timestamp is embedded in the feedback
   message ("NORM_NACK" or "NORM_ACK").  The adjustment adds the amount
   of time the receiver held the timestamp before generating its
   response.  Upon receipt of this adjusted timestamp, the sender is
   able to calculate the round-trip time to that receiver.

   The round-trip time for each receiver is fed into an algorithm that
   weights and smoothes the values for a conservative estimate of the
   GRTT.  The algorithm and methodology are described in the Multicast
   NACK Building Block [RFC5401] in the section entitled "One-to-Many
   Sender GRTT Measurement".  A conservative estimate helps guarantee
   feedback suppression at a small cost in overall protocol repair
   delay.  The sender's current estimate of GRTT is advertised in the
   "grtt" field found in all NORM sender messages.  The advertised GRTT
   is also limited to a minimum of the nominal inter-packet transmission
   time given the sender's current transmission rate and system clock
   granularity.  The reason for this additional limit is to keep the
   receiver somewhat event-driven by making sure the sender has had
   adequate time to generate any response to repair requests from
   receivers given transmit rate limitations due to congestion control
   or configuration.

   When the NORM-CC Rate header extension is present in "NORM_CMD(CC)"
   messages, the receivers respond to "NORM_CMD(CC)" messages as
   described in Section 5.5.2, "NORM Congestion Control Operation".  The
   "NORM_CMD(CC)" messages are periodically generated by the sender as
   described for congestion control operation.  This provides for
   proactive, but controlled, feedback from the group in the form of
   "NORM_ACK" messages.  This provides for GRTT feedback even if no
   "NORM_NACK" messages are being sent.  If operating without congestion
   control in a closed network, the "NORM_CMD(CC)" messages MAY be sent
   periodically without the NORM-CC Rate header extension.  In this
   case, receivers will only provide GRTT measurement feedback when
   "NORM_NACK" messages are generated since no "NORM_ACK" messages are
   generated.  In this case, the "NORM_CMD(CC)" messages MAY be sent
   less frequently, perhaps as little as once per minute, to conserve
   network capacity.  Note 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 when the sender is not conducting
   congestion control rate adjustment.  NORM operation without
   congestion control SHOULD be considered only in closed networks.

5.5.2.  NORM Congestion Control Operation

   This section describes baseline congestion control operation for the
   NORM protocol (NORM-CC).  The supporting NORM message formats and
   approach described here are an adaptation of the equation-based TCP-
   Friendly Multicast Congestion Control (TFMCC) approach[RFC4654].
   This congestion control scheme is REQUIRED for operation within the
   general Internet unless the NORM implementation is adapted to use
   another IETF-sanctioned reliable multicast congestion control
   mechanism.  With this TFMCC-based approach, the transmissions of NORM
   senders are controlled in a rate-based manner as opposed to window-
   based congestion control algorithms as in TCP.  However, it is
   possible 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 such an alternative MAY be described
   separately or in a future revision of this document.  In either case
   (rate-based TFMCC or window-based PGMCC), successful control of
   sender transmission depends upon collection of sender-to-receiver
   packet loss estimates and RTTs to identify the congestion control
   bottleneck path(s) within the multicast topology and adjust the
   sender rate accordingly.  The receiver with loss and RTT estimates that
   corresponding to the lowest resulting calculated transmission rate is
   identified as the "current limiting receiver" (CLR).  In the case of
   a tie (where candidate CLRs are within 10% of the same calculated
   rate), the receiver with the largest RTT value SHOULD be designated
   as the CLR.

   As described in [TcpModel], a steady-state sender transmission rate,
   to be "friendly" with competing TCP flows can be calculated as:
   Rsender = ----------------------------------------------------------
           T_rtt*(sqrt((2/3)*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).


   "T_rtt" = 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 the current
       worst path in the group multicast topology.

   The format of the "NORM_CMD(CC)" message is described in
   Section 4.2.3 of this document.  The "NORM_CMD(CC)" message contains
   information to allow measurement of RTTs, to inform the group of the
   congestion control CLR, and to provide feedback of individual RTT
   measurements to the receivers in the group.  The "NORM_CMD(CC)" also
   provides for exciting feedback from OPTIONAL "potential limiting
   receiver" (PLR) nodes that might be determined administratively or
   possibly algorithmically based upon congestion control feedback.  PLR
   nodes are receivers that have been identified to have potential for
   (perhaps soon) becoming the CLR and thus immediate, up-to-date
   feedback is beneficial for congestion control performance.  The PLR
   list MAY be populated with a small number of receivers the sender
   identifies as approaching the CLR loss and delay conditions based on
   feedback from the group.  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
   can be done to collect some estimation of the current state of the
   multicast topology with respect to group and individual RTT and
   congestion control state.

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

   1.  By default, the interval is set according to the current sender
       GRTT estimate.  A startup GRTT initial value of 0.5 "GRTT_sender = 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 will be empty if the sender has not yet
   received any feedback from the group.  If the sender has received
   feedback, the list will minimally contain an entry identifying the
   CLR.  A "NORM_FLAG_CC_CLR" flag value is provided for the "cc_flags"
   field to identify the CLR entry.  It is RECOMMENDED 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 will allow
   the sender to be more responsive to congestion control dynamics.  The
   length of the list can be dynamically determined according to the
   current transmission rate and scheduling of "NORM_CMD(CC)" messages.
   The maximum length of the list corresponds to the sender's
   NormSegmentSize parameter for the session.  The inclusion of
   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 which the sender
   would like to have immediate, non-suppressed feedback.  These can 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 might not have yet been
   collected and thus the "cc_rtt" field does not contain a valid value
   when this flag is not set.  Similarly, a value of ZERO "ZERO" for the
   "cc_rate" field here MUST 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) RandomBackoff(K_backoff * GRTT_sender, GSIZE_sender)

   The ""RandomBackoff()"" algorithm provides a truncated exponentially
   distributed random number and is described in the Multicast NACK
   Building Block [RFC5401].  The same backoff factor "K factor, "K_backoff = Ksender" MAY
   be used
   K_sender", as used with "NORM_NACK" suppression. " NORM_NACK" suppression is generally
   RECOMMENDED.  However, in cases where the application purposefully
   specifies a very small "Ksender" "K_sender" 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
   be maintained, since there can often be a larger volume of congestion
   control feedback than NACKs in many cases and some congestion control
   feedback latency might be tolerable where reliable delivery latency
   is not.  As previously noted, a backoff factor value of "K "K_sender =
   4" is generally RECOMMENDED for ASM operation and "K "K_sender = 6" for
   SSM operation.  A receiver SHALL cancel the backoff timeout and thus
   its pending transmission of a "NORM_ACK(RTT)" message under the
   following conditions:

   1.  The receiver generates another feedback message ("NORM_NACK" or
       other "NORM_ACK") before the congestion control feedback timeout
       expires (these messages will convey the current congestion
       control feedback information),

   2.  A "NORM_CMD(CC)" or other receiver feedback with an ordinally
       greater "cc_sequence" field value is received before the
       congestion control feedback timeout expires (this is similar to
       the TFMCC feedback round number),

   3.  When the "T_backoff" is greater than "1*GRTTsender". "1*GRTT_sender".  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) might wish to
       compete as a receiver with no prior RTT measurement after some
       long term expiration period.

   When the backoff timer expires, the receiver SHALL generate a
   "NORM_ACK(RTT)" message to provide feedback to the sender and group.
   This message MAY be multicast to the group for most effective
   suppression in ASM topologies or unicast to the sender depending upon
   how the NORM protocol is deployed and configured.

   Whenever any feedback is generated (including this "NORM_ACK(RTT)"
   message), receivers include an adjusted version of the sender
   timestamp from the most recently received "NORM_CMD(CC)" message and
   its "cc_sequence" value from that command in the applicable corresponding "NORM_ACK" or
   "NORM_NACK" message fields.  For NORM-CC operation, any generated
   feedback message SHALL also contain the NORM-CC Feedback header
   extension.  The receiver provides its current "cc_rate" estimate,
   "cc_loss" estimate, "cc_rtt" if known, and any applicable "cc_flags"
   via this header extension.

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

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

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

   After a congestion control feedback message is generated or when the
   feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
   period during which it will restrain itself from providing congestion
   control feedback, even if "NORM_CMD(CC)" messages are received from
   the sender (unless the receive becomes marked as a CLR or PLR node).
   The value of this holdoff timeout ("T_ccHoldoff") period is:
                   T_ccHoldoff = (K*GRTT) (K_sender * GRTT_sender)

   Thus, non-CLR receivers are constrained to providing explicit
   congestion control feedback once per "K*GRTT" "K_sender*GRTT_sender"
   intervals.  Note,
   however, that  However, as the session progresses, different receivers
   will be responding to different "NORM_CMD(CC)" messages and there
   will be relatively continuous feedback of congestion control
   information while the sender is active.  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 receivers, and
   that if a
   PLR is "promoted" to CLR status, the smoothed estimate can be

   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:
    Rinit = MIN(NormSegmentSize / GRTT, MIN(NormSegmentSize/GRTT_sender, NormSegmentSize) bytes/second. bytes/sec

   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 observe slow start congestion control procedures
   until any receiver experiences a new loss event.

5.5.3.  NORM Positive Acknowledgment Procedure

   NORM provides options for the source application to request positive
   acknowledgment (ACK) of "NORM_CMD(FLUSH)" and "NORM_CMD(ACK_REQ)"
   messages from members of the group.  There are some specific
   acknowledgment requests defined for the NORM protocol and a range of
   acknowledgment request types 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 "NORM_ACK(CC)" used to explicitly provide congestion control
   feedback in response to "NORM_CMD(CC)" messages transmitted by the
   sender for NORM-CC operation.  Note the "NORM_ACK(CC)" response does
   NOT follow the positive acknowledgment procedure described here.  The
   "NORM_CMD(ACK_REQ)" and "NORM_ACK" messages contain an "ack_type"
   field to identify the type of acknowledgment requested and provided.
   A range of "ack_type" values is provided for application-defined use.
   While the application is responsible for initiating the
   acknowledgment request and interprets application-defined "ack_type"
   values, the acknowledgment procedure SHOULD be conducted within the
   protocol implementation to take advantage of timing and transmission
   scheduling information available to the NORM transport.

   The NORM positive acknowledgment procedure uses polling by the sender
   to query the receiver group for response.  Note this polling
   procedure is not intended to scale to very large receiver groups, but
   could be used in large group setting to query a critical subset of
   the group.  Either the "NORM_CMD(ACK_REQ)", or when applicable, the
   "NORM_CMD(FLUSH)" message is used for polling and contains a list of
   NormNodeIds of the receivers expected to respond to the command.  The
   list of receivers providing acknowledgment is determined by the
   source application with a priori knowledge of participating nodes or
   via some other application-level mechanism.

   The ACK process is initiated by the sender that generates generating
   "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 that is
   similarly echoed in response so that the sender can match the response to
   the appropriate request.

   In response to the "NORM_CMD(ACK_REQ)", the listed receivers randomly
   randomly, with a uniform distribution, transmit "NORM_ACK" messages uniformly in time
   over a time window of
   (1*GRTT). ("1*GRTT_sender").  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"

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

   1.  Only a single "NORM_CMD(ACK_REQ)" message is generated once per
       ("2*GRTT_sender"), and,

   2.  The size of the "acking_node_list" of NormNodeIds from which
       acknowledgment is requested is limited to a maximum of the sender
       NormSegmentSize setting per round of the positive acknowledgment

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

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

5.5.4.  Group Size Estimate

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

6.  Security Considerations  Configurable Elements

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

   | Configurable Element | Purpose                                    |
   | Sender Initial GRTT  | Sender's Initial estimate of greatest      |
   | Estimate             | group round trip time.  Affects timing of  |
   | ("GRTT_sender")      | feedback suppression and sender command    |
   |                      | transmissions at sender startup.           |
   | Backoff Factor       | Sender's scaling factor used for           |
   | ("K_sender")         | timer-based feedback suppression.          |
   | Group Size Estimate  | Sender's rough estimate of receiver group  |
   | ("GSIZE_sender")     | size used in generation of random feedback |
   |                      | backoff timeout.                           |
   | "NORM_ROBUST_FACTOR" | Integer factor determining how             |
   |                      | persistently (i.e. robust) senders         |
   |                      | transmit repeated control messages and     |
   |                      | receivers self-initiate timeout-based      |
   |                      | NACKing in absence of sender activity.     |
   | FEC Type ("fec_id")  | Sender FEC encoding type.                  |
   | Sender segment size  | Maximum size (in bytes) of the payload     |
   | ("NormSegmentSize")  | portion of "NORM_DATA" and other messages. |
   | NormNodeId           | Unique identifiers pre-assigned to all     |
   |                      | NORM session participants.                 |

   The sender-controlled GRTT estimate (referred to as "GRTT_sender" in
   this document) is used to set and scale various timers associated
   with NORM protocol operation.  During steady-state operation, the
   sender probes the receiver set, adapts to the group round trip timing
   state, and advertises its estimate to the receiver set in "grtt"
   field of relevant NORM protocol messages.  However, an initial value
   must be assumed at sender startup.  A large initial estimate is
   conservative and safer with regards to preventing feedback implosion
   and starting up congestion control operation, but requires the sender
   and receivers to allocate more buffering resources for a given
   transmission rate (i.e. larger effective delay*bandwidth product) to
   maintain efficient operation.  A default initial value of
   "GRTT_sender = 0.5" seconds is RECOMMENDED.

   The sender-controlled Backoff Factor (referred to a "K_sender" in
   this document) is used to scale protocol timers and contributes to
   the generation of the random backoff timeout value that facilitates
   timer-based feedback suppression.  The sender advertises its
   configured Backoff Factor to the receiver set in the "backoff" field
   of applicable NORM messages and thus no receiver configuration is
   necessary.  For ASM operation a default value of "K_sender = 4" is
   RECOMMENDED while for SSM operation a default value of "K_sender = 6"

   The sender estimate of session Group Size (referred to as
   "GSIZE_sender" in this document) also plays a role in the random
   selection of feedback suppression timeout values.  The sender
   advertises its configured Group Size estimate to the receiver set in
   the "gsize" field of applicable NORM messages and thus no receiver
   configuration is necessary.  Only a rough estimate (i.e. "order-of-
   magnitude") is needed for effective feedback suppression and a
   default value of "GSIZE_sender = 10,000" is RECOMMENDED as a
   conservative estimate for most uses.

   The "NORM_ROBUST_FACTOR" is an integer parameter that determines how
   persistently NORM senders transmit control message ("NORM_CMD"
   messages) such as end-of-transmission flushing, OPTIONAL positive
   acknowledgement requests, etc.  Additionally, the receivers use their
   knowledge of "NORM_ROBUST_FACTOR" to determine when to consider a
   NORM sender inactive and MAY use the factor in determining how
   persistently to self-initiate repeated NACK repair requests upon such
   timeouts.  This parameter is NOT communication in NORM protocol
   message headers and is presumed to be preset to a consistent value
   among sender and receivers for a given NORM session.  A default value

   Another NORM sender configuration element is the FEC Type used to
   encode "NORM_DATA" message content.  The FEC type is communicated
   from the sender to the receiver set in the "fec_id" field of relevant
   NORM message headers.  The "fec_id" value corresponds to an IANA-
   assigned value identifying the FEC encoding type as described in the
   FEC Building Block [RFC5052].  Typically, a sender SHOULD use a
   consistent FEC encoding for its participation in a session to simply
   receiver state allocation and maintenance, but it implementations MAY
   vary the FEC encoding type on a per-object basis if necessary.

   The sender NormSegmentSize setting determines the maximum size of the
   payload portion of "NORM_DATA" and other messages that the sender
   transmits.  Additionally the payload size of feedback messages from
   receivers to a given sender is limited to that sender's
   NormSegmentSize.  The NormSegmentSize SHOULD be configured to be
   compatible with expected network MTU limitations, given the added
   overhead of NORM, UDP, and IP protocol message headers.
   Additionally, MTU Discovery MAY be employed by the sender to
   determine an appropriate NormSegmentSize.  The NormSegmentSize for a
   given sender can be determined by receivers from the FEC Object
   Transmission Information (FTI) provided either in applied EXT_FTI
   header extensions or pre-configured session information.

   Although it is not technically a configurable element, the receivers
   MUST have FEC Object Transmission Information for transmitted
   NormObjects to properly buffer, decode, and reassemble the original
   content.  For loosely organized NORM protocol sessions, the sender
   MAY apply the "EXT_FTI" Header Extension to "NORM_DATA" and
   "NORM_INFO" (if applicable) messages so that receivers can get this
   information without prior coordination.  An implementation MAY also
   apply the "EXT_FTI" only to "NORM_INFO" messages for reduced
   overhead.  Or, finally, applications MAY also provide the FTI out-of-
   band prior to sender transmission.

   Each participant in a NORM protocol session MUST be configured with a
   unique NormNodeId value.  The NormNodeId value is used by receivers
   to identify the sender to which their NACK or other feedback messages
   are addressed and senders use the NormNodeId to differentiate
   receivers for purposes of congestion control and OPTIONAL positive
   acknowledgement collection.  Assignment of unique NormNodeId values
   can be done via a priori coordination and/or use of a deconfliction
   mechanism external to the NORM protocol itself.  The values of
   "NORM_NODE_NONE = 0x00000000" and "NORM_NODE_ANY = 0xffffffff" are
   reserved and MUST NOT be assigned to NORM participants.

7.  Security Considerations

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

   The NORM protocol is compatible with the use of IP security (IPsec)
   [RFC4301] and the IPsec Encapsulating Security Payload (ESP) protocol
   or Authentication Header (AF) extension can be used to secure IP
   packets transmitted by NORM participants.  A baseline approach to
   secure NORM operation using IPsec is described below.  Compliant
   implementations of this specification are REQUIRED to be compatible
   with IPsec usage as described in Section 6.1. 7.1.

   Additionally, the "EXT_AUTH" header extension (HET = 1) is defined reserved
   for use by security mechanisms to provide an alternative form of
   authentication and/or encryption of NORM messages.  The format of
   this header extension and its processing is outside the scope of this
   document and is to be communicated out-of-band as part of the session
   description.  It is possible that an EXT_AUTH "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
   encryption of NORM protocol header content is beneficial or
   necessary, the aforementioned use of IPsec ESP might be more
   appropriate.  If EXT_AUTH "EXT_AUTH" is present, whatever packet
   authentication checks that can be performed immediately upon
   reception of the packet MUST be performed before accepting the packet
   and performing any congestion control-related action on it.  Some
   packet authentication schemes impose a delay of several seconds
   between when a packet is received and when the packet can be fully
   authenticated.  Any appropriate congestion control related action that is appropriate
   MUST NOT be postponed by any such full packet authentication. authentication (i.e.
   authentication mechanisms MUST NOT result in poor congestion control

   Consideration MUST also be given to the potential for replay-attacks
   that would transplant authenticated packets from one NORM session to
   another to disrupt service.  To avoid this potential, unique keys
   SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD
   be configured to use unique "instance_id" identifiers that are managed as part
   of the security association for the sessions.

   Note that NORM implementations can use the "sequence" field from the NORM
   Common Message Header to detect replay attacks.  This can be
   accomplished if the NORM sender maintains state on receivers which
   are NACKing. actively NACKing
   receivers.  A cache of such receiver state can be used to provide
   protection against NACK replay attacks.  NORM receivers MUST also
   maintain similar state for protection against possible replay of
   other receiver messages in ASM operation as well.  For example, a
   receiver could be suppressed from providing NACK or congestion
   control feedback by replay of certain receiver messages.  For these
   reasons, authentication of NORM messages (e.g., via IPsec) SHOULD be
   applied for protection against similar attacks that use fabricated
   messages.  Also, encryption of messages to provide confidentiality of
   application data and protect privacy of users MAY also be applied
   using IPsec or similar mechanisms.

   When applicable security measures are used, automated key management
   mechanisms such as those described in the Group Domain of
   Interpretation (GDOI) [RFC3547], Multimedia Internet KEYing (MIKEY)
   [RFC3830] or Group Secure Association Key Management Protocol
   (GSAKMP) [RFC4535] specifications SHOULD be applied.

   While NORM does leverage FEC-based repair for scalability, this alone
   does not guarantee integrity of received data.  Application-level
   integrity-checking of received data content is highly RECOMMENDED.


7.1.  Baseline Secure NORM Operation

   This section describes a baseline mode of secure NORM protocol
   operation based on application of the IPsec security protocol.  This
   approach is documented here to provide a reference, interoperable
   secure mode of operation.  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 [RFC3550] 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).


7.1.1.  IPsec Approach

   For NORM one-to-many SSM operation with unicast feedback from
   receivers, each node SHALL be configured with two transport mode
   IPsec security associations and corresponding Security Policy
   Database (SPD) entries.  One entry will be used for sender-to-group
   multicast packet authentication and optionally encryption while the
   other entry will be used to provide security for the unicast feedback
   messaging from the receiver(s) to the sender.

   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
   receivers can use the built-in IPsec replay attack protection
   possible for an IPsec SA with a single source (the NORM sender).
   Thus the receivers SHALL enable replay attack protection for this SA
   used to secure NORM sender traffic.  An IPsec SPD entry MUST be
   configured to process outbound packets to the session (destination)
   address and UDP port number of the applicable (NormSession).

   The NORM receiver(s) MUST be configured with the SA and SPD entry to
   properly process the IPsec-secured packets from the sender.  The NORM
   receiver(s) SHALL also use a common, second IPsec SA (common Security
   Parameter Index (SPI) and encryption key) configured for ESP
   operation with the option for data origination authentication
   enabled.  Similar to the NORM sender, is RECOMMENDED this IPsec ESP
   SA be also configured to provide confidentiality protection for IP
   packets containing NORM protocol messages.  The receivers MUST have
   an IPsec SPD entry configured to process outbound NORM/UDP packets
   directed to the NORM sender source address and port number using this
   second SA.  To support NORM unicast feedback, the sender's
   transmission port number SHOULD be selected to be distinct from the
   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

   Multiple receivers using a common IPsec SA for traffic directed to
   the NORM sender (i.e., many-to-one) typically prevents the use of
   built-in IPsec replay attack protection by the NORM sender with
   current IPsec implementations.  Thus the built-in IPsec replay attack
   protection for this second SA at the sender MUST be disabled unless
   the particular IPsec implementation manages its replay protection on
   a per-source basis.  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 needs to keep
       more persistent replay attack state.

   3.  "NORM_NACK" feedback messages that precede preceding the sender's current
       repair window do not significantly impact protocol operation
       (generation of "NORM_CMD(SQUELCH)" is limited) and could be in
       fact ignored.  This means the sender can prune any replay attack
       state that precedes the current repair window.

   4.  "NORM_ACK" messages correspond to either a specific sender
       "ack_id", the sender "cc_sequence" for ACKs sent in response to
       "NORM_CMD(CC)", or the sender's current repair window in the case
       of ACKs sent in response to "NORM_CMD(FLUSH)".  Thus, the sender
       can prune any replay attack state for receivers that precede the
       current applicable sequence or repair window space.

   The use of ESP confidentiality for secure NORM protocol operation
   makes it more difficult for adversaries to conduct any form of replay
   attacks.  Additionally, a NORM sender implementation with access to
   the full ESP protocol header could also use the ESP sequence
   information to make replay attack protection even more robust by
   maintaining per-source sequence state.  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 might often need to be less complex.

   This baseline approach can be used for NORM protocol sessions with
   multiple senders if the SA pairs described are established for each
   sender.  For small-sized groups, it is even possible that many-to-
   many 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 will retrieve keying information
   from a central server as needed or otherwise conduct group key
   updates with a similar centralized approach.  Alternatively, it is
   possible with some key management schemes for rekey messages to be
   transmitted to the group as a message or transport object within the
   NORM reliable transfer session.  Similarly, for group-wise
   communication sessions it is possible that for potential group
   participants to request keying and/or rekeying as part of NORM
   communications.  Additional specification is necessary to define an
   in-band key management scheme for NORM sessions perhaps using the
   mechanisms of the automated group key management specifications cited
   in this document.


7.1.2.  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 MUST be configured 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]
   is RECOMMENDED for use.  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.  Note it is possible for key update
   messages (e.g., the GDOI GROUPKEY-PUSH message) to be included as
   part of the NORM application reliable data transmission if
   appropriate interfaces are available between the NORM application and
   the key management daemon.  Security Policy

   Receivers MUST accept protocol messages only from the designated,
   authorized sender(s).  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.  It is
   RECOMMENDED these certificates use IP addresses for authentication.  Availability

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


8.  IANA Considerations

   Values of NORM Header Extension Types, Stream Control Codes, and
   "NORM_CMD" message sub-types are subject to IANA registration.  They
   are in the registry named "Reliable Multicast Transport (RMT) NORM
   Protocol Parameters" located at time of publication at:

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


8.1.  Explicit IANA Assignment Guidelines

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


8.1.1.  NORM Header Extension Types

   This document defines a registry for NORM Header Extensions named
   "NORM Header Extension Types".

   The NORM Header Extension Type field is an 8-bit value.  The values
   of this field identify extended header content that allows allowing the protocol
   functionality to be expanded to include additional features and
   operating modes.  The values that can be assigned within the "NORM
   Header Extensions" registry are numeric indexes in the range {0,
   255}, boundaries included.  Values in the range {0,127} indicate
   variable length extended header fields while values in the range
   {128,255} indicate extensions of a fixed 4-byte length.  This
   specification registers the following NORM Header Extension Types:

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

   Requests for assignment of additional NORM Header Extension Type
   values are granted on a "Specification Required" basis as defined by
   IANA Guidelines [RFC5226].  Any such header extension specifications
   MUST include a description of protocol actions to be taken when the
   extension type is encountered by a protocol implementation not
   supporting that specific option.  For example, it is often possible
   for protocol implementations to ignore unknown header extensions.


8.1.2.  NORM Stream Control Codes

   This document defines a registry for NORM Stream Control Codes named
   "NORM Stream Control Codes".

   NORM Stream Control Codes are 16-bit values that can be inserted
   within a "NORM_OBJECT_STREAM" delivery object to convey sequenced,
   out-of-band (with respect to the stream data) control signaling
   applicable to the referenced stream object.  These control codes are
   to be delivered to the application or protocol implementation with
   reliable delivery, in-order with respect to the their inserted
   position within the stream.  This specification registers the
   following NORM Stream Control Code:

            | Value | Name              | Reference          |
            | 0     | "NORM_STREAM_END" | This specification |

   Additional NORM Stream Control Code value assignment requests are
   granted on a "Specification Required" basis as defined by IANA
   Guidelines [RFC5226].  The full 16-bit space outside of the value
   assigned in this specification are available for future assignment.
   Note that in
   In addition to describing the control code's expected interpretation,
   such specifications MUST include a description of protocol actions to
   be taken when the control code is encountered by a protocol
   implementation not supporting that specific option.


8.1.3.  NORM_CMD Message Sub-types

   This document defines a registry for "NORM_CMD" message sub-types
   named "NORM Command Message Sub-types".

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

         | Value | Name                    | Reference          |
         | 0     | reserved                | This specification |
         | 1     | "NORM_CMD(FLUSH)"       | This specification |
         | 2     | "NORM_CMD(EOT)"         | This specification |
         | 3     | "NORM_CMD(SQUELCH)"     | This specification |
         | 4     | "NORM_CMD(CC)"          | This specification |
         | 5     | "NORM_CMD(REPAIR_ADV)"  | This specification |
         | 6     | "NORM_CMD(ACK_REQ)"     | This specification |
         | 7     | "NORM_CMD(APPLICATION)" | This specification |

   Future specifications extending NORM MAY define additional "NORM_CMD"
   messages to enhance protocol functionality.  "NORM_CMD" message sub-
   type value assignment requests are granted on a "Specification
   Required" basis as defined by IANA Guidelines [RFC5226].  Note that
   in  In addition
   to describing the command sub-type's expected interpretation,
   specifications MUST include a description of protocol actions to be
   taken when the command is encountered by a protocol implementation
   not supporting that specific option.

   Note that this

   This specification already provides for defines an "application-
   defined" "application-defined"
   "NORM_CMD" message sub-type that can be used for use at the discretion of individual
   applications using NORM for transport.  These "application-defined"
   commands are suitable for many application-specific purposes and do
   not involve standards action.  In any case, such additional messages
   SHALL be subject to the same congestion control constraints as the
   existing NORM sender message set.


9.  Suggested Use

   The present NORM protocol is seen as 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 is suitable for use in asymmetric networks or in networks
   where only unidirectional multicast routing/delivery service exists.
   Asymmetric architectures supporting multicast delivery are likely to
   make up an important portion of the future Internet structure (e.g.,
   DBS/cable/PSTN hybrids)
   direct broadcast satellite (DBS) or cable and public-switched
   telephone network (PSTN) hybrids, etc) and efficient, reliable bulk
   data transfer will be an important capability for servicing large
   groups of subscribed receivers.


10.  Changes from RFC3940

   This section lists the changes between the Experimental version of
   this specification, RFC 3940, and this version:

   1.  Removal of the "NORM_FLAG_MSG_START" for "NORM_OBJECT_STREAM",
       replacing it with the "payload_msg_start" field in the FEC-
       encoded preamble of the "NORM_OBJECT_STREAM NORM_DATA" payload,

   2.  Definition of IANA namespace registry for header extension assignment, and other

   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.  Addition of the EXT_AUTH header extension definition.

   9.  Clarification of interpretation of "Source Block Length" when FEC
       codes are arbitrarily shortened by the sender.


11.  Acknowledgments

   (and these are not Negative)

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


12.  References


12.1.  Normative References

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

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

   [RFC5052]  Watson, M., Luby, M., and L. Vicisano, "Forward Error
              Correction (FEC) Building Block", RFC 5052, August 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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


12.2.  Informative References

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

              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.

   [RFC2357]  Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
              Criteria for Evaluating Reliable Multicast Transport and
              Application Protocols", RFC 2357, June 1998.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC3048]  Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
              Floyd, S., and M. Luby, "Reliable Multicast Transport
              Building Blocks for One-to-Many Bulk-Data Transfer",
              RFC 3048, January 2001.

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

   [RFC3453]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
              M., and J. Crowcroft, "The Use of Forward Error Correction
              (FEC) in Reliable Multicast", RFC 3453, December 2002.

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC3940]  Adamson, B., Bormann, C., Handley, M., and J. Macker,
              "Negative-acknowledgment (NACK)-Oriented Reliable
              Multicast (NORM) Protocol", RFC 3940, November 2004.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5445]  Watson, M., "Basic Forward Error Correction (FEC)
              Schemes", RFC 5445, March 2009.

              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