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RFC 5740

 
 
 

NACK-Oriented Reliable Multicast (NORM) Transport Protocol

Part 2 of 4, p. 18 to 47
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4.2.  Sender Messages

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

4.2.1.  NORM_DATA Message

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

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   The format of NORM_DATA messages is comprised of three logical
   portions: 1) a fixed-format NORM_DATA header portion, 2) a FEC
   Payload ID portion with a format dependent upon the FEC encoding
   used, and 3) a payload portion containing source or encoded
   application data content.  Note for objects of type
   NORM_OBJECT_STREAM, the payload portion contains additional fields
   used to appropriately recover stream content.  NORM implementations
   MAY also extend the NORM_DATA header to include a FEC Object
   Transmission Information (EXT_FTI) header extension.  This allows
   NORM receivers to automatically allocate resources and properly
   perform FEC decoding without the need for pre-configuration or out-
   of-band information.
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |version| type=2|    hdr_len    |          sequence             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           source_id                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          instance_id          |     grtt      |backoff| gsize |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     flags     |    fec_id     |     object_transport_id       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         fec_payload_id                        |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                header_extensions (if applicable)              |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          payload_len*         |       payload_msg_start*      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        payload_offset*                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          payload_data*                        |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: NORM_DATA Message Format

   *IMPORTANT NOTE: The "payload_len", "payload_msg_start" and
   "payload_offset" fields are present only for objects of type
   NORM_OBJECT_STREAM.  These fields, as with the entire payload, are
   subject to any FEC encoding used.  Thus, when systematic FEC codes
   are used, these values can be directly interpreted only for packets
   containing source symbols while packets containing FEC parity content
   need decoding before these fields can be interpreted.

   The "version", "type", "hdr_len", "sequence", and "source_id" fields

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   form the NORM common message header as described in Section 4.1.  The
   value of the NORM_DATA "type" field is 2.  The NORM_DATA base
   "hdr_len" value is 4 (i.e., four 32-bit words) plus the size of the
   "fec_payload_id" field.  The "fec_payload_id" field size depends upon
   the FEC encoding type referenced by the "fec_id" field.  For example,
   when small block, systematic codes are used, a "fec_id" value of 129
   is indicated, and the size of the "fec_payload_id" is two 32-bit
   words.  In this case the NORM_DATA base "hdr_len" value is 6.  The
   cumulative size of any header extensions applied is added into the
   "hdr_len" field.

   The "instance_id" field contains a value generated by the sender to
   uniquely identify its current instance of participation in the
   NormSession.  This allows receivers to detect when senders have
   perhaps left and rejoined a session in progress.  When a sender
   (identified by its "source_id") is detected to have a new
   "instance_id", the NORM receivers SHOULD drop their previous state on
   the sender and begin reception anew, or at least treat this
   "instance" as a new, separate sender.

   The "grtt" field contains a non-linear quantized representation of
   the sender's current estimate of group round-trip time (GRTT_sender)
   (this is also referred to as R_max in [TfmccPaper]).  This value is
   used to control timing of the NACK repair process and other aspects
   of protocol operation as described in this document.  Normally, the
   advertised "grtt" value will correspond to what the sender has
   measured based on feedback from the group, but, at low transmission
   rates, the advertised "grtt" SHALL be set to MAX(grttMeasured,
   NormSegmentSize/senderRate) where the NormSegmentSize is the sender's
   segment size in bytes and the senderRate is the sender's current
   transmission rate in bytes per second.  The algorithm for encoding
   and decoding this field is described in the Multicast NACK Building
   Block [RFC5401] document.

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

   The "gsize" field contains a representation of the sender's current
   estimate of group size (GSIZE_sender).  This 4-bit field can roughly
   represent values from ten to 500 million where the most significant
   bit value of 0 or 1 represents a mantissa of 1 or 5, respectively,
   and the three least significant bits incremented by one represent a

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

   The "flags" field contains a number of different binary flags
   providing information and hints for the receiver to appropriately
   handle the identified object.  Defined flags in this field include:

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

   NORM_FLAG_REPAIR is set when the associated message is a repair
   transmission.  This information can be used by receivers to help
   observe a join policy where it is desired that newly joining
   receivers only begin participating in the NACK process upon receipt
   of new (non-repair) data content.  NORM_FLAG_EXPLICIT is used to mark
   repair messages sent when the data sender has exhausted its ability
   to provide "fresh" (not previously transmitted) parity segments as

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   repair.  This flag could possibly be used by intermediate systems
   implementing functionality to control sub-casting of repair content
   to different legs of a reliable multicast topology with disparate
   repair needs.  NORM_FLAG_INFO is set only when OPTIONAL NORM_INFO
   content is actually available for the associated object.  Thus,
   receivers will NACK for retransmission of NORM_INFO only when it is
   available for a given object.  NORM_FLAG_UNRELIABLE is set when the
   sender wishes to transmit an object with only "best effort" delivery
   and will not supply repair transmissions for the object.  NORM
   receivers SHOULD NOT execute repair requests for objects marked with
   the NORM_FLAG_UNRELIABLE flag.  There are cases where receivers can
   inadvertently request repair of such objects when all segments (or
   info content) for those objects are not received (i.e., a gap in the
   "object_transport_id" sequence is noted).  In this case, the sender
   SHALL invoke the NORM_CMD(SQUELCH) process as described in
   Section 4.2.3.

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

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

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

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

        Figure 5: Example: FEC Payload Id Format for 'fec_id' = 129

   In this example, FEC payload identifier, the "source_block_number",
   "source_block_len", and "encoding_symbol_id" fields correspond to the
   "Source Block Number", "Source Block Length", and "Encoding Symbol
   ID" fields of the FEC Payload ID format for Small Block Systematic
   FEC Schemes identified by a "fec_id" value of 129 as specified by the
   FEC Basic Schemes [RFC5445] specification.  The "source_block_number"
   identifies the coding block's relative position with a NormObject.
   Note that, for NormObjects of type NORM_OBJECT_STREAM, the
   "source_block_number" will wrap for very long-lived sessions.  The
   "source_block_len" indicates the number of user data segments in the
   identified coding block.  Given the "source_block_len" information of
   how many symbols of application data are contained in the block, the
   receiver can determine whether the attached segment is data or parity
   content and treat it appropriately.  Applications MAY dynamically
   "shorten" code blocks when the pending information content is not
   predictable (e.g., real-time message streams).  In that case, the
   "source_block_len" value given for an "encoding_symbol_id" that
   contains FEC parity content SHALL take precedence over the
   "source_block_len" value provided for any packets containing source
   symbols.  Also, the "source_block_len" value given for an ordinally
   higher "encoding_symbol_id" SHALL take precedence over the
   "source_block_len" given for prior encoding symbols.  The reason for
   this is that the sender will only know the maximum source block
   length at the time it is transmitting source symbols, but then
   subsequently "shorten" the code and then provide that last source
   symbol and/or encoding symbols with FEC parity content.  The
   "encoding_symbol_id" identifies which specific symbol (segment)
   within the coding block the attached payload conveys.  Depending upon
   the value of the "encoding_symbol_id" and the associated
   "source_block_len" parameters for the block, the symbol (segment)

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   referenced will be a user data or a FEC parity segment.  For
   systematic codes, encoding symbols numbered less than the
   source_block_len contain original application data while segments
   greater than or equal to source_block_len contain parity symbols
   calculated for the block.  The concatenation of object_transport_id::
   fec_payload_id can be viewed as a unique transport protocol data unit
   identifier for the attached segment with respect to the NORM sender's
   instance within a session.

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    het = 64   |    hel = 4    |       object_size (msb)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       object_size (lsb)                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  FEC scheme-specific content ...              |

         Figure 6: EXT_FTI Header Extension General Portion Format

   The header extension type "het" field value for the EXT_FTI header
   extension is 64.  The header extension length "hel" value depends
   upon the format of the FTI for encoding type identified by the
   "fec_id" field.

   The 48-bit "object_size" field indicates the total length of the
   object (in bytes) for the static object types of NORM_OBJECT_FILE and
   NORM_OBJECT_DATA.  This information is used by receivers to determine
   storage requirements and/or allocate storage for the received object.
   Receivers with insufficient storage capability might wish to forego
   reliable reception (i.e., not NACK for) of the indicated object.  In
   the case of objects of type NORM_OBJECT_STREAM, the "object_size"
   field is used by the sender to advertise the size of its stream

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   buffer to the receiver group.  In turn, the receivers SHOULD use this
   information to allocate a stream buffer for reception of
   corresponding size.

   As noted, the format of the extension depends upon the FEC code in
   use, but in general, it contains any necessary details on the code in
   use (e.g., FEC Instance ID, etc.).  As an example, the format of the
   EXT_FTI for small block systematic codes ("fec_id" = 129) is given
   here:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    het = 64   |    hel = 4    |       object_size (msb)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       object_size (lsb)                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       fec_instance_id         |          segment_size         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       fec_max_block_len       |         fec_num_parity        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Example: EXT_FTI Header Extension Format for 'fec_id' = 129

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

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

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

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

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

   The "fec_num_parity" corresponds to the "maximum number of encoding

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

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

   The "payload_len", "payload_msg_start", and "payload_offset" fields
   are present only for transport objects of type NORM_OBJECT_STREAM.
   These REQUIRED fields allow senders to arbitrarily vary the size of
   NORM_DATA payload segments for streams.  This allows applications to
   flush transmitted streams as needed to meet unique streaming
   requirements.  For objects of types NORM_OBJECT_FILE and
   NORM_OBJECT_DATA, these fields are unnecessary since the receiver can
   calculate the payload length and offset information from the
   "fec_payload_id" using the REQUIRED block partitioning algorithm
   described in the FEC Building Block [RFC5052] document.  When
   systematic FEC codes (e.g., "fec_id" = 129) are used, the
   "payload_len", "payload_msg_start", and "payload_offset" fields
   contain actual payload_data length, message start index (or stream
   control code), and byte offset values for the associated application
   stream data segment (the remainder of the "payload_data" field
   content) for those NORM_DATA messages containing source data symbols.
   In NORM_DATA messages that contain FEC parity content, these fields
   do not contain values that can be directly interpreted, but instead
   are values computed from FEC encoding the "payload_len",
   "payload_msg_start", and "payload_offset" fields for the source data
   segments of the corresponding coding block.  The actual
   "payload_msg_start", "payload_len" and, "payload_offset" values of
   missing data content can be determined upon decoding a FEC coding
   block.  Note that these fields do NOT contribute to the value of the
   NORM_DATA "hdr_len" field.  These fields are present only when the
   "flags" portion of the NORM_DATA message indicate the transport
   object is of type NORM_OBJECT_STREAM.

   The "payload_len" value, when non-zero, indicates the length (in
   bytes) of the source content contained in the associated
   "payload_data" field.  However, when the "payload_len" value is equal
   to ZERO, this indicates that the "payload_msg_start" field be

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   alternatively interpreted as a "stream_control_code".  The only
   "stream_control_code" value defined is NORM_STREAM_END = 0.  The
   NORM_STREAM_END code indicates that the sender is terminating the
   transmission of stream content at the corresponding position in the
   stream and the receiver MUST NOT expect content (or request repair
   for any content) following that position in the stream.  Additional
   specifications MAY extend the functionality of the NORM stream
   transport mode by defining additional stream control codes.  These
   control codes are delivered to the recipient application reliably,
   in-order with respect to the streamed application data content.

   The "payload_msg_start" field serves one of two exclusive purposes.
   When the "payload_len" value is non-zero, the "payload_msg_start"
   field, when also set to a non-zero value, indicates that the
   associated "payload_data" content contains an application-defined
   message boundary (start-of-message).  When such a message boundary is
   indicated, the first byte of an application-defined message, with
   respect to the "payload_data" field, will be found at an offset of
   "payload_msg_start - 1" bytes.  Thus, if a NORM_DATA payload for a
   NORM_OBJECT_STREAM contains the start of an application message at
   the first byte of the "payload_data" field, the value of the
   "payload_msg_start" field will be '1'.  NORM implementations SHOULD
   provide sender stream applications with a capability to mark message
   boundaries in this manner.  Similarly, the NORM receiver
   implementation SHOULD enable the application to recover such message
   boundary information.  This enables NORM receivers to "synchronize"
   reliable reception of transmitted message stream content in a
   meaningful way (i.e., meaningful to the application) at any time,
   whether joining a session already in progress, or departing the
   session and returning.  Note that if the value of the
   "payload_msg_start" field is ZERO, no message boundary is present.
   The "payload_msg_start" value will always be less than or equal to
   the "payload_len" value except for the special case of "payload_len =
   0", which indicates the "payload_msg_start" field be instead
   interpreted as a "stream_control_code"

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

   The "payload_data" field contains the original application source or
   parity content for the symbol identified by the "fec_payload_id".
   The length of this field SHALL be limited to a maximum of the
   sender's NormSegmentSize bytes as given in the FTI for the object.
   Note the length of this field for messages containing parity content
   will always be of length NormSegmentSize.  When encoding data
   segments of varying sizes, the FEC encoder SHALL assume ZERO value

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   padding for data segments with a length less than the
   NormSegmentSize.  It is RECOMMENDED that a sender's NormSegmentSize
   generally be constant for the duration of a given sender's term of
   participation in the session, but can possibly vary on a per-object
   basis.  The NormSegmentSize SHOULD be configurable by the sender
   application prior to session participation as needed for network
   topology MTU considerations.  For IPv6, MTU discovery MAY be possibly
   leveraged at session startup to perform this configuration.  The
   "payload_data" content MAY be delivered directly to the application
   for source symbols (when systematic FEC encoding is used) or upon
   decoding of the FEC block.  For NORM_OBJECT_FILE and
   NORM_OBJECT_STREAM objects, the data segment length and offset can be
   calculated using the block partitioning algorithm described in the
   FEC Building Block [RFC5052] document.  For NORM_OBJECT_STREAM
   objects, the length and offset is obtained from the segment's
   corresponding embedded "payload_len" and "payload_offset" fields.

4.2.2.  NORM_INFO Message

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

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

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

                    Figure 8: NORM_INFO Message Format

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

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

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

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

4.2.3.  NORM_CMD Messages

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

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

                    Figure 9: NORM_CMD Standard Fields

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM common message header as described in Section 4.1.  The
   value of the "hdr_len" field for NORM_CMD messages without header
   extensions present depends upon the "sub-type" field.

   The "instance_id", "grtt", "backoff", and "gsize" fields provide the
   same information and serve the same purpose as NORM_DATA and
   NORM_INFO messages.  The "sub-type" field indicates the type of
   command to follow.  The remainder of the NORM_CMD message is
   dependent upon the command sub-type.  NORM command sub-types include:

   +-----------------------+----------+--------------------------------+
   | Command               | Sub-type | Purpose                        |
   +-----------------------+----------+--------------------------------+
   | NORM_CMD(FLUSH)       |     1    | Used to indicate sender        |
   |                       |          | temporary end-of-transmission. |
   |                       |          | (Assists in robustly           |
   |                       |          | initiating outstanding repair  |
   |                       |          | requests from receivers).  May |
   |                       |          | also be optionally used to     |
   |                       |          | collect positive               |
   |                       |          | acknowledgment of reliable     |
   |                       |          | reception from a subset of     |
   |                       |          | receivers.                     |
   | NORM_CMD(EOT)         |     2    | Used to indicate sender        |
   |                       |          | permanent end-of-transmission. |

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

4.2.3.1.  NORM_CMD(FLUSH) Message

   The NORM_CMD(FLUSH) command is sent when the sender reaches the end
   of all data content and pending repairs it has queued for
   transmission.  This can indicate either a temporary or permanent end-
   of-data transmission, but that the sender is still willing to respond
   to repair requests.  This command is repeated once per 2*GRTT_sender
   to excite the receiver set for any outstanding repair requests up to
   and including the transmission point indicated within the
   NORM_CMD(FLUSH) message.  The number of repeats is equal to
   NORM_ROBUST_FACTOR unless a list of receivers from which explicit
   positive acknowledgment is expected ("acking_node_list") is given.
   In that case, the "acking_node_list" is updated as acknowledgments
   are received and the NORM_CMD(FLUSH) is repeated according to the
   mechanism described in Section 5.5.3.  The greater the
   NORM_ROBUST_FACTOR, the greater the probability that all applicable
   receivers will be excited for acknowledgment or repair requests
   (NACKs) AND that the corresponding NACKs are delivered to the sender.
   A default value of NORM_ROBUST_FACTOR equal to 20 is RECOMMENDED.  If
   a NORM_NACK message interrupts the flush process, the sender SHALL
   re-initiate the flush process after any resulting repair
   transmissions are completed.

   Note that receivers also employ a timeout mechanism to self-initiate
   NACKing (if there are outstanding repair needs) when no messages of

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   any type are received from a sender.  This inactivity timeout is
   related to the NORM_CMD(FLUSH) and NORM_ROBUST_FACTOR and is
   specified in Section 5.3.  Receivers SHALL self-initiate the NACK
   repair process when the inactivity timeout has expired for a specific
   sender and the receiver has pending repairs needed from that sender.
   With a sufficiently large NORM_ROBUST_FACTOR value, data content is
   delivered with a high assurance of reliability.  The penalty of a
   large NORM_ROBUST_FACTOR value is the potential transmission of
   excess NORM_CMD(FLUSH) messages and a longer inactivity timeout for
   receivers to self-initiate a terminal NACK process.

   For finite-sized transport objects such as NORM_OBJECT_DATA and
   NORM_OBJECT_FILE, the flush process (if there are no further pending
   objects) occurs at the end of these objects.  Thus, FEC repair
   information is always available for repairs in response to repair
   requests elicited by the flush command.  However, for
   NORM_OBJECT_STREAM, the flush can occur at any time, including in the
   middle of a FEC coding block if systematic FEC codes are employed.
   In this case, the sender will not yet be able to provide FEC parity
   content for the concurrent coding block and will be limited to
   explicitly repairing the stream with source data content for that
   block.  Applications that anticipate frequent flushing of stream
   content SHOULD be judicious in the selection of the FEC coding block
   size (i.e., do not use a very large coding block size if frequent
   flushing occurs).  For example, a reliable multicast application
   transmitting an ongoing series of intermittent, relatively small
   messages will need to trade-off using the NORM_OBJECT_DATA paradigm
   versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
   block size.  This is analogous to application trade-offs for other
   transport protocols such as the selection of different TCP modes of
   operation such as "no delay", etc.

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

                 Figure 10: NORM_CMD(FLUSH) Message Format

   The "version", "type", "hdr_len", "sequence", and "source_id" fields
   form the NORM common message header as described in Section 4.1.  In
   addition to the NORM common message header and standard NORM_CMD
   fields, the NORM_CMD(FLUSH) message contains fields to identify the
   current status and logical transmit position of the sender.

   The "fec_id" field indicates the FEC type used for the flushing
   "object_transport_id" and implies the size and format of the
   "fec_payload_id" field.  Note the "hdr_len" value for the
   NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
   field when no header extensions are present.

   The "object_transport_id" and "fec_payload_id" fields indicate the
   sender's current logical "transmit position".  These fields are
   interpreted in the same manner as in the NORM_DATA message type.
   Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
   their completion state THROUGH (including) this transmission
   position.  If receivers have outstanding repair needs in this range,
   they SHALL initiate the NORM NACK Repair Process as described in
   Section 5.3.  If receivers have no outstanding repair needs, no
   response to the NORM_CMD(FLUSH) is generated.

   For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
   MUST request "explicit-only" repair of the identified
   "source_block_number" if the given "encoding_symbol_id" is less than
   the "source_block_len".  This condition indicates the sender has not
   yet completed encoding the corresponding FEC block and parity content
   is not yet available.  An "explicit-only" repair request consists of

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

   The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
   for receivers from which the sender is requesting explicit positive
   acknowledgment of reception up through the transmission point
   identified by the "object_transport_id" and "fec_payload_id" fields.
   The length of the list can be inferred from the length of the
   received NORM_CMD(FLUSH) message.  When the "acking_node_list" is
   present, the lightweight positive acknowledgment process described in
   Section 5.5.3 SHALL be observed.

4.2.3.2.  NORM_CMD(EOT) Message

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

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

                  Figure 11: NORM_CMD(EOT) Message Format

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

4.2.3.3.  NORM_CMD(SQUELCH) Message

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

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

   The starting point of the invalid NormObject list begins with the

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

                Figure 12: NORM_CMD(SQUELCH) Message Format

   In addition to the NORM common message header and standard NORM_CMD
   fields, the NORM_CMD(SQUELCH) message contains fields to identify the
   earliest logical transmit position of the sender's current repair
   window and an "invalid_object_list" beginning with the index of the
   logically earliest invalid repair request from the offending NACK
   message that initiated the NORM_CMD(SQUELCH) transmission.  The value
   of the "hdr_len" field when no extensions are present is 4 plus the
   size of the "fec_payload_id" field that is dependent upon the FEC
   scheme identified by the "fec_id" field.

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

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

4.2.3.4.  NORM_CMD(CC) Message

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

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

                  Figure 13: NORM_CMD(CC) Message Format

   The NORM common message header and standard NORM_CMD fields serve
   their usual purposes.  The value of the "hdr_len" field when no
   header extensions are present is 6.

   The "reserved" field is for potential future use and MUST be set to
   ZERO in this version of the NORM protocol and its baseline NORM-CC
   congestion control scheme.  It is possible for alternative congestion
   control schemes to use the NORM_CMD(CC) message defined here and
   leverage the "reserved" field for scheme-specific purposes.

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

   The "send_time" field is a timestamp indicating the time that the
   NORM_CMD(CC) message was transmitted.  This consists of a 64-bit
   field containing 32-bits with the time in seconds ("send_time_sec")

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

   To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
   a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
   group of the sender's current transmission rate.  This is used along
   with the loss detection "sequence" field of all NORM sender messages
   and the NORM_CMD(CC) GRTT collection process to support NORM-CC
   congestion control operation.  The format of this header extension is
   as follows:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    het = 128  |    reserved   |           send_rate           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The "send_rate" field indicates the sender's current transmission
   rate in bytes per second.  The 16-bit "send_rate" field consists of
   12 bits of mantissa in the most significant portion and 4 bits of
   base 10 integer exponent (E) information in the least significant
   portion.  The 12-bit mantissa portion of the field is scaled such
   that a base 10 mantissa (M) floating point value of 0.0 corresponds
   to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of
   the 16-bit "send_rate" field.  Thus:

          send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;

   For example, to represent a transmission rate of 256 kbit/s (3.2e+04
   bytes per second), the lower 4 bits of the 16-bit field contain a
   value of 0x04 to represent the exponent (E) while the upper 12 bits
   contain a value of 0x51f (M) as determined from the equation given
   above:
        send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
                  = (0x51f << 4) | 0x4
                  = 0x51f4

   To decode the "send_rate" field, the following equation can be used:

   value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f))

   Note the maximum transmission rate that can be represented by this

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   scheme is approximately 9.99e+15 bytes per second.

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

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

   Each item in the "cc_node_list" is in the following format:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          cc_node_id                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    cc_flags   |     cc_rtt    |            cc_rate            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The "cc_node_id" is the NormNodeId of the receiver the item
   represents.

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

   +--------------------+-------+--------------------------------------+
   | Flag               | Value | Purpose                              |
   +--------------------+-------+--------------------------------------+
   | NORM_FLAG_CC_CLR   |  0x01 | Receiver is the current limiting     |
   |                    |       | receiver (CLR).                      |
   | NORM_FLAG_CC_PLR   |  0x02 | Receiver is a potential limiting     |
   |                    |       | receiver (PLR).                      |
   | NORM_FLAG_CC_RTT   |  0x04 | Receiver has measured RTT with       |
   |                    |       | respect to sender.                   |

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   | NORM_FLAG_CC_START |  0x08 | Sender/receiver is in "slow start"   |
   |                    |       | phase of congestion control          |
   |                    |       | operation (i.e., the receiver has    |
   |                    |       | not yet detected any packet loss and |
   |                    |       | the "cc_rate" field is the           |
   |                    |       | receiver's actual measured receive   |
   |                    |       | rate).                               |
   | NORM_FLAG_CC_LEAVE |  0x10 | Receiver is imminently leaving the   |
   |                    |       | session and its feedback SHOULD not  |
   |                    |       | be considered in congestion control  |
   |                    |       | operation.                           |
   +--------------------+-------+--------------------------------------+

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

   The "cc_rate" field contains a representation of the receiver's
   current calculated (during steady-state congestion control operation)
   or twice its measured (during the slow start phase) congestion
   control rate.  This field is encoded and decoded using the same
   technique as described for the NORM_CMD(CC) "send_rate" field.

4.2.3.5.  NORM_CMD(REPAIR_ADV) Message

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

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |version| type=3|    hdr_len    |          sequence             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           source_id                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          instance_id          |     grtt      |backoff| gsize |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | sub-type = 5  |     flags     |            reserved           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               header extensions (if applicable)               |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       repair_adv_payload                      |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 14: NORM_CMD(REPAIR_ADV) Message Format

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

   The "flags" field provides information on the NORM_CMD(REPAIR_ADV)
   content.  There is currently one NORM_CMD(REPAIR_ADV) flag defined:

                     NORM_REPAIR_ADV_FLAG_LIMIT = 0x01

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

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

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   within a NORM implementation, an additional header extension MAY need
   to be defined to encapsulate any required feedback content.  The
   NORM-CC Feedback Header Extension format is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     het = 3   |    hel = 3    |          cc_sequence          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    cc_flags   |     cc_rtt    |            cc_loss            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            cc_rate            |          cc_reserved          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The "cc_sequence" field contains the current greatest "cc_sequence"
   value receivers have received in NORM_CMD(CC) messages from the
   sender.  This information assists the sender in congestion control
   operation by providing an indicator of how current ("fresh") the
   receiver's round-trip measurement reference time is and whether the
   receiver has been successfully receiving recent congestion control
   probes.  For example, if it is apparent the receiver has not been
   receiving recent congestion control probes (and thus possibly other
   messages from the sender), the sender SHOULD choose to take
   congestion avoidance measures.  For NORM_CMD(REPAIR_ADV) messages,
   the sender SHALL set the "cc_sequence" field value to the value set
   in the last NORM_CMD(CC) message sent.

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

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

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   measured from receivers for the current feedback round.

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

             "cc_loss" = floor(decimal_loss_fraction * 65535.0)

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

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

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

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

4.2.3.6.  NORM_CMD(ACK_REQ) Message

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

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   described in Section 5.5.3.  The format of the NORM_CMD(ACK_REQ)
   message is:
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |version| type=3|    hdr_len    |          sequence             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           source_id                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          instance_id          |     grtt      |backoff| gsize |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | sub-type = 6  |    reserved   |    ack_type   |    ack_id     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       acking_node_list                        |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 15: NORM_CMD(ACK_REQ) Message Format

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

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

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

   The NORM_ACK(CC) value is provided for use only in NORM_ACKs
   generated in response to the NORM_CMD(CC) messages used in congestion
   control operation.  Similarly, the NORM_ACK(FLUSH) is provided for
   use only in NORM_ACKs generated in response to applicable
   NORM_CMD(FLUSH) messages.  NORM_CMD(ACK_REQ) messages with "ack_type"

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   of NORM_ACK(CC) or NORM_ACK(FLUSH) SHALL NOT be generated by the
   sender.

   The NORM_ACK(RESERVED) range of "ack_type" values is provided for
   possible future NORM protocol use.

   The NORM_ACK(APPLICATION) range of "ack_type" values is provided so
   that NORM applications can implement application-defined, positively
   acknowledged commands that are able to leverage internal transmission
   and round-trip timing information available to the NORM protocol
   implementation.

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

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

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

4.2.3.7.  NORM_CMD(APPLICATION) Message

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

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |version| type=3|    hdr_len    |          sequence             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           source_id                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          instance_id          |     grtt      |backoff| gsize |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | sub-type = 7  |                    reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Application-Defined Content                 |
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 16: NORM_CMD(APPLICATION) Message Format

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

   The "Application-Defined Content" area contains information in a
   format at the discretion of the application.  The size of this
   payload SHALL be limited to a maximum of the sender's NormSegmentSize
   setting.  Upon reception, the NORM protocol implementation SHALL
   deliver the content to the receiver application.  Note that any
   detection of duplicate reception of a NORM_CMD(APPLICATION) message
   is the responsibility of the application.



(page 47 continued on part 3)

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