RFC 5740
NACK-Oriented Reliable Multicast (NORM) Transport Protocol
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.
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
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
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
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.
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)
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
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
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
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
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.
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
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. |
| 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
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.
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
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.
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
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.
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.
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")
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
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. |
| 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].
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
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
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
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"
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.
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.
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