RMT Working Group B. Adamson/Newlink INTERNET-DRAFT C. Bormann/Tellique draft-ietf-rmt-pi-norm-02.txt M. Handley/ACIRI Expires: January 2002 J. Macker/NRL July 2001 NACK-Oriented Reliable Multicast Protocol (NORM) Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other docu- ments at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract This document describes the messages and procedures of the Nega- tive-acknowledgement (NACK) oriented reliable multicast (NORM). This revision of the document represents an initial outline of the protocol description. The document requires refinement in a number of areas to be considered complete. At this time, the document describes the high level details of the reliable multicast bulk transfer service model this protocol hopes to fulfill and the gen- eral message types and mechanisms which will be used to accomplish those goals. Adamson, Borman, et al. Expires January 2002 [Page 1]
Internet Draft NORM Protocol July 2001 1.0 Protocol Design Goals NORM is designed to provide end-to-end reliable transport of data from sender(s) to a group of receivers over a multicast-capable network. The primary design goal of NORM is to provide for effi- cient, scalable, and robust bulk data (e.g. computer files, trans- mission of persistent data) transfer adaptable (preferably in an automated fashion) across heterogeneous networks and topologies. The protocol is capable of operating in an end-to-end fashion with no assistance from intermediate systems beyond basic IP multicast group management and forwarding services. However, an additional design goal will be compatibility with other reliable multicast "building blocks" [REF RMT Building Block Guidelines] to take advantage of additional network capabilities when available. Thus, while the techniques utilized in NORM are principally applicable to "flat" network distribution, they might also be applied to a given level of a hierarchical (e.g. tree-based) multicast distribution system if so desired. NORM can make use of reciprocal (among senders and receivers) multicast routing when available but will also be capable of efficient operation in asymmetric multicast topologies [REF single source multicast, etc]. Group communication scalability requirements leads to adaptation of negative acknowledgement (NACK) based protocol schemes [REF.]. NORM is a protocol centered around the use of selective NACKs to request repairs of missing data. NORM also uses NACK suppression methods and dynamic event timers to reduce retransmission requests and avoid congestion within the network. When used in pure multi- cast session operation, both NACKs and repair transmissions are multicast to the group to aid in feedback and control message sup- pression. This feature and additional message aggregation func- tionality reduce the likelihood of multicast control message implo- sion. NORM also dynamically measures the greatest group roundtrip time (GRTT) between sources and the set of multicast receivers to further improve the efficiency of the protocol state timers and probabilistic backoff algorithms. This allows NORM to scale well while maintaining reliable data delivery transport with low latency relative to the network topology over which it is operating. NORM also provides for the use of packet-level forward error correction (FEC) techniques for efficient multicast repair and optional proac- tive transmission robustness. Another aspect of the NORM protocol design is providing support for distributed multicast session participation with minimal coordina- tion among sources and receivers. The protocol allows sources and receivers to dynamically join and leave multicast sessions at will with minimal overhead for control information and timing synchro- nization among participants. To accommodate this capability, NORM Adamson, Borman, et al. Expires January 2002 [Page 2]
Internet Draft NORM Protocol July 2001 protocol message headers contain some common information allowing receivers to easily synchronize to sources throughout the lifetime of a defined session. These common headers also include support for collection of transmission timing information (e.g., round trip delays) that allows NORM to adapt itself to a wide range of dynamic network conditions with little or no pre-configuration. The proto- col is purposely designed to be tolerant of inaccurate timing esti- mations or lossy conditions which may occur many networks includin mobile and wireless. The protocol is also designed to exhibit con- vergence even under cases of heavy packet loss and large queueing or transmission delays. While the various features of NORM are designed to provide some measure of general purpose utility, it is important to emphasize the understanding that "no one size fits all" in the reliable mul- ticast transport arena. There are numerous engineering tradeoffs involved in reliable multicast transport design and this requires an increased awareness of application and network architecture con- siderations. Performance requirements affecting design can include: group size, heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data ordering, delivery delay, group dynamics, mobility, congestion control, and transport across low capacity connections. NORM contains various protocol parameters to accommo- date many of these differing requirements, but there is an assumed model of bulk transfer transport service that drives the trade-offs resulting in the protocol described here. 1.1 NORM Transport Service Model An instance of the NORM protocol (NormSession) is defined within the context of one or more senders and receivers mutually communi- cating with prdefined IP addresses and host port(s). While point- to-point (unicast) NormSessions may be established between a pair of protocol participants (NormNodes), it is anticipated the proto- col will be used for multicast data distribution and that partici- pating nodes will communicate on a common IP multicast group address and port number which has been chosen via other means (e.g. MBONE session directory tools, administrative coordination, SIP signalling, etc). Note that the protocol provides for an optional mechanism for receiver nodes to use unicast addressing to provide feedback to senders in networks where this is required (e.g. Single Source Multicast Routing, asymmetric topologies, etc). The protocol design is principally driven with the assumption of a single sender transmitting bulk data content to a group of receivers. However, the protocol does provide for multiple senders to coexist within the context of a NormSession. In initial imple- mentations of this protocol, it is anticipated that multiple Adamson, Borman, et al. Expires January 2002 [Page 3]
Internet Draft NORM Protocol July 2001 senders will transmit independently of one another and receivers will maintain state as necessary for each independent sender. In future iterations of this document, it is possible that some aspects of protocol operation (e.g. round-trip time collection) will provide for alternate modes allowing more efficient perfor- mance for applications requiring multiple senders. NORM provides for three types of bulk data content objects (NormOb- jects) to be reliably transported. These types include static com- puter memory data content (NORM_OBJECT_DATA), computer storage files (NORM_OBJECT_FILE), and non-finite streams of continuous data content (NORM_OBJECT_STREAM). The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is simply to provide a "hint" to receivers in NormSessions serving multiple types of content as to what type of storage should be allocated for received content (i.e. memory or file storage). Other than that distinction, the two are identical, providing for reliable transport of finite units of content. The use of the NORM_OBJECT_STREAM type is at the application's discretion and conceivably be used to carry static data or file content also. Reliable stream service also opens up other possibilities such as reliable messaging or other unbounded, perhaps dynamically produced content. The NORM_OBJECT_STREAM pro- vides for reliable transport analogous to that of the Transmission Control Protocol (TCP) although NORM receivers will be able to begin receiving stream content at any point in time (The applica- bility of this feature will depend upon the application). The static data and file services are anticpated to be useful for mul- ticast-based cache applications with the ability to reliably pro- vide transmission/repair of a large set of static data. Other types of static data/file "casting" services might make use of these transport object types, too. The NORM protocol allows for a small amount of "out-of-band" data (NORM_INFO) to be attached to the data content objects transmitted by the sender. This readily- available "out-of-band" data allows multicast receivers to quickly and efficiently determine the nature of the bulk content (data, file, or stream) being transmitted to allow application-level con- trol of the receiver node's participation in the current transport activity. This allows the protocol to be flexible with minimal pre-coordination among senders and receivers. NORM does _not_ provide for global or application-level identifica- tion of data content within in its message headers (It should be noted that the NORM_INFO out-of-band data mechanism can be lever- aged by the application for this purpose if desired, or identifica- tion could alternatively be embedded within the data content). NORM identifies data content objects (NormObjects) with transport identifiers which are applicable while the sender is transmitting and/or repairing the given object. These transport data content Adamson, Borman, et al. Expires January 2002 [Page 4]
Internet Draft NORM Protocol July 2001 identifiers are assigned in a montonically increasing fashion by each NORM sender during the course of a NormSession. Each sender maintains its transport identifier assignments independently so NormObjects are uniquely identified during transport by the con- catenation of the sender's session-unique identifier (NormNodeId) and the assigned NormObject transport identifier (NormTransportId). The NormTransportIds are assigned from a large (32 bit?) numeric space in increasing order and may be reassigned for long-lived ses- sions. The NORM protocol provides mechanisms so that the sender application may terminate transmission of data content and inform the group of this in an efficient manner. Other similar protocol control mechanisms (e.g. session termination, receiver synchroniza- tion, etc) are specified so that reliable multicast application variants may construct different, complete bulk transfer communica- tion models to meet their goals. In summary, the NORM protocol's goal is to provide reliable trans- port of data content objects (including a potentially mixed set of types) to the receiver set from one or more senders. The senders will queue and transmit content in the form of static data or files and/or non-finite, ongoing stream types. The sender will provide for repair transmission of this content in response to NACK mes- sages received from the receiver group. Mechanisms for "out-of- band" information and other session management mechanisms are also specified for use by applications to form a complete reliable mul- ticast transport solutions for a range different purposes. 2.0 Protocol Definition 2.1 Assumptions A NORM protocol instantiation (NormSession) is defined by the con- text of participants communicating connectionless (e.g. User Data- gram Protocol (UDP)) packets over an Internet Protocol (IP) network on a common, pre-determined network address and host port number. Generally, the participants exchange packets on an IP multicast group address, but unicast transport may also be established or applied as an adjunct to multicast delivery. Currently the protocol uses a single multicast address for transmissions associated with a given NORM session. However, in the future, it is possible that multiple multicast addresses might be employed to segregate sepa- rate degrees of repair information to different groups of receivers experiencing different packet loss characteristics with respect to a given sender. This capability is under ongoing investigation in the research community [REF]. For multicast operation, the NORM protocol assumes basic IP multicast forwarding service is available at least from the sender(s) to the receiver set. However, the Adamson, Borman, et al. Expires January 2002 [Page 5]
Internet Draft NORM Protocol July 2001 protocol also supports asymmetry where receiver participants may transmit back to sender participants via unicast routing instead of broadcasting to the session multicast address. Each participant (NormNode) within an NormSession is assumed to have an preselected unique XX-bit (TBD) identifier (NormNodeId). NormNodes MUST have uniquely assigned identifiers within a single NormSession to distinquish between possible multiple senders and to distinguish feedback information from different receivers. The protocol does not preclude multiple sender nodes actively transmit- ting within the context of a single NORM session (i.e. many- to- many operation), but any type of interactive coordination among these senders is assumed to be controlled by a higher protocol layer (perhaps using some of the optional NORM mechanisms later specified to perform this coordination). Unique data content transmitted within a NormSession uses sender- assigned identifiers (NormObjectTransportIds) which are valid and applicable only during the actual _transport_ of the particular portion of data content (i.e. for as long as the sender is trans- mitting and providing repair of the indicated data content). Any globally unique identification of transported data content must be assigned and processed by the higher level application using the NORM transport service. 2.2 General Protocol Operation A NORM sender primarily generates messages of type NORM_DATA which carry the data content and related FEC parity-based repair informa- tion for the bulk data/file or stream objects being transferred. Parity content is by default sent only on in response to receiver repair requests (NACKs) and thus normally imposes no additional protocol overhead. However, the transport of an object can be optionally configured to proactively transmit some amount of parity packets along with the original data content to potentially enhance performance (e.g., improved delay) at the cost of additional over- head with initial data transmission. This configuration may be sensible for certain network conditions and can allow for robust, asymmetric multicast (e.g., unidirectional routing, satellite, cable) [REF] with minimal receiver feedback, or, in some cases, none. A sender message of type NORM_INFO is also defined and is used to carry any optional "out-of-band" context information for a given transport object. Because of its simple, nature content of NORM_INFO messages can be NACKed and repaired with a slightly lower delay process than NORM's general FEC-encoded data content. NORM_INFO may serve special purposes for some buld transfer, Adamson, Borman, et al. Expires January 2002 [Page 6]
Internet Draft NORM Protocol July 2001 reliable multicast applications where receivers join the group mid- stream and need to ascertain contextual information on the current content being transmitted. The NACK process for NORM_INFO will be described later. The sender also generates messages of type NORM_CMD to perform cer- tain protocol operations such as congestion control, end-of-trans- mission flushing, round trip time estimation, receiver synchroniza- tion, and optional positive acknowledgement requests or application defined commands. The transmission of NORM_CMD messages from the sender is accomplished by one of three different processes. These include single, best effort unreliable transmission of the command, repeated redundant transmission of the command, and positively acknowledged commands. The transmission technique used for a given command depends upon the function of the command. Several core commands are defined for basic protocol operation. Additionally, implementations may wish to consider providing the option of appli- cation-defined commands which can take advantage of these transmis- sion methodologies available for command. These transmission methodologies make use of information available to the protocol engine (e.g. round-trip timing, transmission rate, etc) to perform efficiently. An NORM receiver generates messages of type NORM_NACK or NORM_ACK in response to transmissions of data and commands from a sender. The NORM_NACK messages are generated to request repair of detected data transmission losses. Receivers generally detect losses by tracking the sequence of transmission from a sender. Sequencing information is embedded in the transmitted data packets and end-of- transmission commands from the sender. NORM_ACK messages are gen- erated in response to certain commands transmitted by the sender. In the general (and most scalable) protocol mode, receivers do not transmit any NORM_ACK messages. However, in order to meet poten- tial user requirements for positive data acknowledgement, and to collect more detailed information for potential multicast conges- tion control algorithms, NORM_ACK messages are defined and poten- tially used. NORM_ACK messages are also generated by a small sub- set of receivers when NORM dynamic end-to-end congestion control is in operation. NORM allows for reliable transfer of three different types of data content. These include the type NORM_OBJECT_DATA which are static, persistent blocks of data content maintained in the sender's appli- cation memory storage and the type NORM_OBJECT_FILE which corre- sponds to data stored in the sender's non-volatile file system. Both of these types represent "NormObjects" of finite size which are encapsulated for transmission and are temporarily yet uniquely identified with the given sender's NormNodeId and a temporarily Adamson, Borman, et al. Expires January 2002 [Page 7]
Internet Draft NORM Protocol July 2001 unique NormObjectTransportId. The third type of All transmissions by individual senders and receivers are subject to rate control governed by a peak transmission rate set for each participant by the application. This can be used to limit the quantity of multicast data transmitted by the group. When NORM's congestion control algorithm is enabled the rate for senders is automatically adjusted. And even when congestion control is enabled, it may be desirable in some cases to establish minimum and maximum bounds for the rate adjustment depending upon the applica- tion. 2.3 Message Type and Header Definitions As mentioned previously, there are two primary classes of NORM mes- sages: messages generated by the sender of reliable multicast traf- fic and messages generated by receivers. 2.3.1 NORM Common Message Header There are some common message fields contained in all NORM message types. All NORM protocol messages begin with a common header with information fields as follows: NORM Common Packet Header 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 | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "version" field is a (TBD)-bit value indicating the protocol version number. Currently, NORM implementations SHOULD ignore received messages with a different protocol version number. This number is intended to indicate and distinguish upgrades of the pro- tocol which may be non-interoperable. The message "type" field is a (TBD)-bit value indicating the NORM protocol message type. These types are defined as follows: Adamson, Borman, et al. Expires January 2002 [Page 8]
Internet Draft NORM Protocol July 2001 Message Value NORM_INFO 1 NORM_DATA 2 NORM_CMD 3 NORM_NACK 4 NORM_ACK 5 NORM_REPORT 6 The "sequence" field is a 16-bit value which is set by the message originator as a monotonically increasing number incremented with each NORM message transmitted. This value can be monitored by receiving nodes to detect packet losses in the transmission. Note that this value is NOT used to detect missing reliable data con- tent, but is intended for use in an algorithm estimating raw packet loss for congestion control purposes. The size of this field is intended to be sufficient to allow detection of a reasonable range of packet loss within the expected delay-bandwidth product of expected network connections. The "source_id" field is a 32-bit value identifying the node which sent the message. A participant's NORM node identifier (NormNodeId) can be set according to the application needs but unique identifiers must be assigned within a single NormSession. In some cases, use of the host IP address or a hash of it can suf- fice, but alternative methodologies for assignment and potential collision resolution of node identifiers within a multicast session need to be considered. For example, the "source identifier" mecha- nism defined in the RTPv2 specification [REF RTP] may be applicable to use for NORM node identifiers. At this point in time, the pro- tocol makes no assumptions about how these unique identifiers are actually assigned. NORM Message Types Sender Messages: NORM_DATA This is expected to be the predominant message type transmitted by NORM senders. These messages will contain data content for objects of type NORM_OBJECT_DATA, NORM_OBJECT_FILE, and NORM_OBJECT_STREAM. A goal of the protocol design is to provide for parallel transmis- sion of different streams and data/file sets. NORM_DATA messages will generally consist of original data content of the application data being transmitted. The content size of these messages will a Adamson, Borman, et al. Expires January 2002 [Page 9]
Internet Draft NORM Protocol July 2001 maximum of NormSegmentSize which is constant for the duration of a given sender's term of participation in the session. Senders advertise their NormSegmentSize in applicable messages which they transmit. This allows receivers to allocate appropriate buffering resources and to determine other information in order to properly process received data messaging. Note this message type will also be used to convey FEC parity repair content for NormObjects sent. NORM_DATA Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | grtt | segment_size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size_lsb | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size_msb | reserved | fec_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_encoding_name | fec_num_parity | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_block_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_symbol_id | fec_num_data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | offset_lsb* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | offset_msb* | payload_len* | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_data* | * Note the "offset" and "payload_len" fields for NORM_DATA messages containing parity information are actually values computed from FEC encoding of the "offset" and "payload_len" fields of the data seg- ments of the applicable coding block. Thus, for parity packets, these do _not_ represent these values directly. The "flags" field contains a number of different binary flags pro- viding information and hints regarding how the receiver should han- dle the identified object. Defined flags in this field include: Adamson, Borman, et al. Expires January 2002 [Page 10]
Internet Draft NORM Protocol July 2001 +---------------------+-------+------------------------------------------+ | Flag | Value | Purpose | +---------------------+-------+------------------------------------------+ |NORM_FLAG_REPAIR | 0x01 | Indicates message is a repair transmis- | | | | sion | +---------------------+-------+------------------------------------------+ |NORM_FLAG_INFO | 0x02 | Indicates availability of NORM_INFO for | | | | object, | +---------------------+-------+------------------------------------------+ |NORM_FLAG_UNRELIABLE | 0x04 | Indicates that repair transmissions for | | | | the specified object will be unavail- | | | | able. (One-shot, best effort transmis- | | | | sion) | +---------------------+-------+------------------------------------------+ |NORM_FLAG_FILE | 0x08 | Indicates object is "file-based" data | | | | (hint to use disk storage for reception) | +---------------------+-------+------------------------------------------+ |NORM_FLAG_STREAM | 0x10 | Indicates object is of type | | | | NORM_OBJECT_STREAM. | +---------------------+-------+------------------------------------------+ The NORM_FLAG_REPAIR flag is set when the associated transmission is a repair transmission. This information can be used by receivers to facilitate a join policy where it is desired that newly joining receivers only begin participating in the NACK pro- cess upon receipt of new "fresh" data. The NORM_FLAG_INFO flag is se only when there optional NORM_INFO content is available for the associated object. Thus, receivers will NACK for retransmission of NORM_INFO only when it is available. The NORM_FLAG_UNRELIABLE flag is set when the sender wishes to transmit and object with "best effort" delivery only and will not supply repair transmissions for the object. The NORM_FLAG_FILE flag can be set as a "hint" from the sender that the associated object should be stored in non- volatile storage. The NORM_FLAG_STREAM flag is set when the iden- tified object is of type NORM_OBJECT_STREAM. Note that the "object_size" field is no longer applicable (Another use for this field for "stream" objects may be determined as this capability is designed). The "grtt" field contains a non-linear quantized representation of the sender's current estimate of group round-trip time (GRTT). This value is used to control timing of the NACK repair process and other aspects of protocol operation as described in this document. The "segment_size" field indicates the sender's current setting for maximum message payload content (in bytes). The "fec_type" field describes the error correction coding technique and parameters the sender is using to calculate parity repair segments. Knowledge of Adamson, Borman, et al. Expires January 2002 [Page 11]
Internet Draft NORM Protocol July 2001 these fiels allows a NORM receiver to allocate appropriate buffer- ing and FEC decoding resources. The "object_transport_id" field is a monotonically and incremen- tally increasing value assigned by a sender to the object being transmitted. Transmissions and repair requests related to that object use the same "object_id" value. For sessions of very long or indefinite duration, the "object_id" field may be repeated, but it is presumed that the 32-bit field size provides an adequate enough sequence space to prevent temporary 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 "node_id" and the given "object_transport_id". Note that NORM_INFO messages associated with the identified object carry the same "object_trans- port_id" value. The "object_size" fields indicate the total size of the object (in bytes). This information is used by receivers to determine storage requirements and/or allocate storage for the received object. Receivers with insufficient storage capability may wish to forego reception (i.e. not NACK for) of the indicated object. The "object_size" fields are not applicable for objects of type NORM_OBJECT_STREAM. (Note: The "object_size" fields _may_ be defined to serve an alternative use in this case). The "fec_id" field corresponds to the FEC Encoding Identifier described in the FEC Building Block document (currently "draft- ietf-rmt-bb-fec-03.txt"). The packet format illustrated above assumes "Small Block Systematic Codes" which correspond to an FEC Encoding Identifier equal to 129. The "fec_encoding_name" and "fec_num_parity" fields correspond to the "FEC Encoding Name" and "Number of redundant symbols" fields of the FEC Object Transmission Information format given by the FEC Building Block document. The "fec_encoding_name" shall be 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 these values are TBD. The "fec_num_parity" field corresponds to a parameter for generating specific FEC encoding/decoding algorithms for the named code. For example, Reed-Solomon codes may be arbitrarily shortened to create different code variations for a given block length. In this case, the "fec_num_parity" value indicates the maximum number of avail- able parity segments for the coding block from the sender. This Adamson, Borman, et al. Expires January 2002 [Page 12]
Internet Draft NORM Protocol July 2001 field _may_ be interpreted differently for other systematic codes as they are defined. The "fec_block_id", "fec_symbol_id", "fec_num_data" fields directly correspond to the "encoding block number", "encoding symbol id", and "Source block length" fields of the FEC Payload ID format given by the FEC Building Block document. The "fec_block_id" identifies the coding block while the "fec_symbol_id" identifies which spe- cific symbol (segment) within the coding block the attached packet conveys. Given the "fec_num_data" (Source block length) informa- tion of how many symbols of application data is contained in the block, the receiver can determine whether the attached symbol is data or parity content and treat it appropriately. (For systematic codes, symbols numbered 0 through (fec_num_data-1) contain applica- tion data while symbols numbered (fec_num_data) through (fec_num_data+fec_numparity-1) contain the parity content calcu- lated for the block). The "offset" and "payload_len" fields are used to identify the position and quantity of the content of the packet payload. For senders employing systematic FEC encoding, these fields will corre- spond to the actual values for NORM_DATA messages which contain original data content. For NORM_DATA messages containing calcu- lated parity content, these fields will actually contain the values computed by FEC encoding of the "offset" and "length" values of the data segments of the corresponding FEC coding block. This allows the "offset" and "length" values of missing data content to be determined when decoding an FEC coding block. The "payload_data" field contains original data or computed parity content for the identified segment. The maximum length of this field corresponds to the sender's NormSegmentSize. The length of this field for messages containing parity content will always be of the length NormSegmentSize. When encoding a block of data segments of varying sizes, the FEC encoder SHALL assume zero value padding for data segments less than the NormSegmentSize. The receiver will use the "length" information to properly retrieve receive data con- tent and deliver it to the application. NORM_INFO The NORM_INFO message is used to convey _optional_ "out-of-band" context information for objects transmitted. An example may be MIME type information for the associated file, data, or stream object. Receivers might use this information to make a decision as whether to participate in reliable reception of the associated Adamson, Borman, et al. Expires January 2002 [Page 13]
Internet Draft NORM Protocol July 2001 object. Each NormObject may have an independent unit of NORM_INFO associated with it. NORM_DATA messages contain a flag to indicate the availability of NORM_INFO for a given NormObject. NORM receivers may NACK for retransmission of NORM_INFO when they have not received it for a given NormObject. The size of the NORM_INFO content is limited to that of a single NormSegmentSize for the given sender. This atomic nature allows the NORM_INFO to be rapidly and efficiently repaired within the NORM transmission pro- cess. NORM_INFO Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | grtt | segment_size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size_lsb | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_size_msb | reserved | fec_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_encoding_name | fec_num_parity | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | payload_data | The "flags", "grtt", "segment_size", "object_transport_id", "object_size", "fec_id", "fec_encoding_name", and "fec_num_parity" fields carry the same information and serve the same purpose as with NORM_DATA messages. The "payload_data" field contains the application-defined content which can be used by the receiver application for various purposes. NORM_CMD NORM_CMD messages are transmitted by senders to perform a number of different protocol functions. This includes round-trip timing col- lection, potential congestion control functions, synchronization of receiver NACKing "windows", notification of sender status and other core protocol functions. A core set of NORM_CMD messages will be enumerated. A range of command types will remain undefined for potential application-specific usage. Some NORM_CMD types (possi- bly including application-defined commands) may have some dynamic content attached. This content will be limited to a single Norm- SegmentSize to retain the atomic nature of commands. The NORM_CMD message begins with a common header, following the usual NORM Adamson, Borman, et al. Expires January 2002 [Page 14]
Internet Draft NORM Protocol July 2001 message common header. The header format is defined as: NORM_CMD Common Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt | flavor | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The "grtt" field provides the same information and serves the same purpose as with NORM_DATA and NORM_INFO messages. The "flavor" field indicates the type of command to follow. The command flavors (types) include: +------------------+--------------+----------------------------------+ | Command | Flavor Value | Purpose | +------------------+--------------+----------------------------------+ |NORM_CMD_FLUSH | 1 | Indicates sender temporary or | | | | permanent end-of-transmission. | | | | (Assists in robustly initiating | | | | outstanding repair requests from | | | | receivers). | +------------------+--------------+----------------------------------+ |NORM_CMD_SQUELCH | 2 | Advertisement of current repair | | | | window in response to out-of- | | | | range NACKs. | +------------------+--------------+----------------------------------+ |NORM_CMD_ACK_REQ | 3 | Requests positive acknowledge- | | | | ment of a watermark point from a | | | | specific list of receivers. | +------------------+--------------+----------------------------------+ |NORM_CMD_GRTT_REQ | 4 | Probe used in collection of | | | | sender's group GRTT estimate and | | | | possibly congestion control | | | | feedback. | +------------------+--------------+----------------------------------+ NORM_CMD(FLUSH) The NORM_CMD_FLUSH command is sent when the sender reaches the end of any data content and pending repairs it has queued for transmis- sion. This command is repeated once per 2*GRTT to excite the receiver set for any outstanding repair requests for data up to and including the point indicated by the FLUSH message. The number of repeats is equal to ROBUST_FACTOR. The greater this number, the higher the probability that all applicable receivers will be Adamson, Borman, et al. Expires January 2002 [Page 15]
Internet Draft NORM Protocol July 2001 excited for repair requests (NACKs) _and_ the corresponding NACKs are delivered to the sender. If a NACK message interrupts the flush process, the sender will re-initiate the flush process when repair transmissions are completed. Note that receivers also employ a timeout mechanism to self-initiate NACKing when a sender is determined to have gone "idle". This inactivity timeout is related to 2*GRTT*ROBUST_FACTOR and will be discussed more later. With a sufficient ROBUST_FACTOR value, data content is delivered with a high assurance of reliability. The penalty of a large ROBUST_FACTOR value is potentially excess sender NORM_CMD_FLUSH transmissions and a longer timeout for receivers to self-initiate a terminal NACK process. The format of the NORM_CMD_FLUSH message (in addition to the NORM message common header) is: NORM_CMD(FLUSH) Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt | flavor = 1 | fec_symbol_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_block_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ In addition to the common NORM_CMD "grtt" and "flavor" fields, the NORM_CMD(FLUSH) message contains fields to identify the current logical transmit position of the sender. These fields consist of "fec_symbol_id", "fec_block_id", and "object_transport_id". These fields are interpreted in the same manner as the fields of the same names in the NORM_DATA message type. Receivers are expected to check their completion state and initiate the NACK repair process if they have outstanding repair needs up to this transmission point. If the receivers have no outstanding repair needs, no response is generated. NORM_CMD(SQUELCH) The NORM_CMD_SQUELCH command is multicast to the receiver set in response to invalid NACKs received by the sender. The NORM_CMD_SQUELCH command is limited to be sent at once per 2*GRTT at the most. The NORM_CMD_SQUELCH advertises the "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 which are still valid for repair. This mechanism allows the sender application to abort Adamson, Borman, et al. Expires January 2002 [Page 16]
Internet Draft NORM Protocol July 2001 intermediate objects still in repair transmission. For example, an object pending transmission/repair may for some reason may have become obsolete. The receiver set learns from the NORM_CMD_SQUELCH the set of data for which it is valid to request repair. In normal conditions, it is expected the NORM_CMD_SQUELCH will be used infre- quently, and is generally anticipated to provide a reference for receivers who have fallen "out-of-sync" with the sender. The NORM_CMD_SQUELCH contains the identity of the earliest transmission point and includes a set of NormTransportIds which are valid. The starting point of the included set begins at the greatest (latest) of the sender's earliest transmission point or the lowest invalid NormTransportId in the invalid NACK(s) which prompted the genera- tion of the NORM_CMD_SQUELCH. The length of the list in the NORM_CMD_SQUELCH 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 (in addition to the NORM message common header) is: NORM_CMD(SQUELCH) Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt | flavor = 2 | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_block_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | invalid_object_list ... | In addition to the common NORM_CMD "grtt" and "flavor" fields, the NORM_CMD(SQUELCH) message contains fields to identify the earliest logical transmit position of the sender's current repair window and a "repair window description" beginning with the index of the logi- cally earliest invalid repair request from the offending NACK mes- sage which initiated the SQUELCH response. The "fec_block_id", and "object_transport_id" fields are concate- nated to indicate the beginning of the sender's current repair win- dow (i.e. the logically earliest point in its transmission history for which the sender can provide repair). This serves as an adver- tisement of a "synchronization point" for receivers to request repair. The "invalid_object_list" is a list of 32-bit object_transport_ids which, although they are within the sender's current repair window, Adamson, Borman, et al. Expires January 2002 [Page 17]
Internet Draft NORM Protocol July 2001 are no longer available for repair from the sender. The total size of the "invalid_object_list" content is limited by the NormSegment- Size of the sender. Thus, it is possible that in some cases a sin- gle SQUELCH message may not be capable of completely listing the entire set. In these cases, the sender should ensure that the list begins with a "object_transport_id" (providing it is greater than the "synchronization point" from a NACK message for which the SQUELCH is being generated. This insures convergence of the SQUELCH process, even if multiple invalid NACK/ SQUELCH response iterations are required. This explicit description of invalid con- tent within the sender's current window, allows the sender applica- tion (most notably for discrete "object" based transport) to arbi- trarily invalidate (i.e. dequeue) portions of enqueued content (e.g. certain objects) for which it no longer wishes to provide reliable transport. (TBD) Provide example SQUELCH messages. NORM_CMD_ACK_REQ The NORM_CMD_ACK_REQ message is used by the sender to request acknowledgement from a specified list of receivers. This message serves in a lightweight positive acknowledgement mechanism which can be optionally employed by the reliable multicast application to deterministically determine that watermark points in the reliable transmission have been achieved by specific receivers. er's appli- cable object repair window with minimal transmission of NORM_CMD_SQUELCH commands. The format of the NORM_CMD_ACK_REQ mes- sage (in addition to the NORM message common header and the NORM_CMD common header) is: +-------------+---------------+----------------------------------+ | Field | Length (bits) | Purpose | +-------------+---------------+----------------------------------+ | object_id | 32 | NormObjectTransportId of water- | | | | mark NormObject for which the | | | | sender supports acknowledgement. | +-------------+---------------+----------------------------------+ | segment_id | (TBD) | Segment identifier of watermark | | | | point within the identified | | | | object. | +-------------+---------------+----------------------------------+ | data | -- | List of NormNodeIds to which the | | | | request applies. | +-------------+---------------+----------------------------------+ Adamson, Borman, et al. Expires January 2002 [Page 18]
Internet Draft NORM Protocol July 2001 NORM_CMD(ACK_REQ) Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt | flavor = 1 | fec_symbol_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | fec_block_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object_transport_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | acking_node_list ... | The "fec_symbol_id", "fec_block_id", and "object_transport_id" are used to identify the watermark point for which the positive acknowledgement request applies. This watermark point is similar to that used in MDP_CMD(FLUSH) message. It should be noted that all receivers are expected to treat the ACK_REQ command equiva- lently to a FLUSH command and appropriately initiate NACK repair cycles in response to any detected missing data up to the watermark point. The "acking_node_list" field contains the current list of receiver NormNodeIds which should reply with postive acknowledgement to this request. The NormNodeIds are listed in network (Big Endian) order. The indicated receivers SHALL send a NORM_ACK message in response to this request IF they have no outstanding repair needs up to and including the watermark point. Note this does _not_ necessarily mean the receivers actually received all of the data, but simply that, for whatever reason (including the fact they may have already received the data or if the receiving application simply chose _not_ to receive the indicated data), they have no outstanding repair needs prior to the watermark point. Verification of actual received data content must be accomplished by another means outside of this transport layer protocol. Receivers SHALL randomly spread their response to this request using a uniform distribution over 1 GRTT of time. Note the size of the included list is limited to the sender's NormSegmentSize setting. Thus, multiple NORM_CMD_ACK_REQ cycles may be required to achieve responses from all receivers specified. Also, the number of attempts to excite a response from a given receive SHALL be limited to ROBUST_FACTOR. The NORM_CMD_ACK_REQ is repeated at a rate of once per 2*GRTT. Note that the content of the attached NormNodeId list will be dynami- cally updated as this process progresses and ACKs are received from the specified receiver set. The process SHALL terminate when all desired receivers have responded or the maximum number of attempts has been achieved. Note that repair requests can interrupt the positive acknowledgement process and the positive acknowledgment Adamson, Borman, et al. Expires January 2002 [Page 19]
Internet Draft NORM Protocol July 2001 process will resume only when there are no pending repair transmis- sions up to the specified watermark point. NORM_CMD_RTT_REQ The NORM_CMD_RTT_REQ is periodically transmitted by the sender to provide a reference point (a timestamp) so that receivers can cal- culate appropriate response content in NORM_NACK and NORM_NACK mes- sages from which the sender can monitor and estimate the current GRTT. Currently, this reference is sent separately from other sender message and not included in every message because of the excessive overhead it may impose on data transmission. Generally, the GRTT is not expected to be so dynamic as to require rapid update. However, a technique is being investigated by the author to provide a low overhead reference which could be attached to every sender transmission and used for the receiver response gener- ation [REF]. This command may also potentially be leveraged serve as part of NORM congestion control to periodically provide updated congestion control information and/or probing to the group. There is expected to be sufficient content in this message that it merits a separate message rather than be periodically included in the overhead of other sender transmissions. This command may also be extended to assume some responsibility in initializing and updating a group size estimator used to appropri- ately scale NACK suppression back-off timing, etc. For now, a min- imal format is defined as a placeholder for this message. The for- mat of the NORM_CMD_RTT_REQ message (in addition to the NORM mes- sage common header and the NORM_CMD common header) is: +----------+---------------+----------------------------------+ | Field | Length (bits) | Purpose | +----------+---------------+----------------------------------+ | req_seq | 8 | NORM_CMD_RTT_REQ sequence number | | | | T} send_time 64 T{ Timestamp | | | | referenced to sender clock for | | | | when the command was generated. | +----------+---------------+----------------------------------+ The "req_seq" field is an 8-bit, monotonically increasing sequence number which is incremented with each NORM_CMD_RTT_REQ command gen- erated by the sender. Responses to the NORM_CMD_RTT_REQ embedded in receiver NORM_NACK and NORM_ACK messages will identify the sequence number of the NORM_CMD_RTT_REQ which they have most recently received. The "send_time" field is a precision timestamp indicating the time Adamson, Borman, et al. Expires January 2002 [Page 20]
Internet Draft NORM Protocol July 2001 that the NORM_CMD_GRTT_REQ message was transmitted. This consists of a 64-bit field containing 32-bits with the time in seconds and 32-bits with the time in microseconds since some reference time the source maintains (usually 00:00:00, 1 January 1970). The ordering of the fields in Big Endian network order. Other candidate fields for this message include: "flags" to mark different forms/purposes of the request. (e.g. a WILDCARD flag to prompt an explicit response from the entire group) "hold_time" field to provide a random back-off scaling factor when an entire group response is expected. Congestion control parameters such as "tx_rate", "rtt", "loss", etc for assisting a congestion control mechanism appropiate for NORM. Techniques are under investigation. Receiver Messages: NORM_NACK The principal purpose of NORM_NACK messages will be for receivers to request repair of content via negative acknowledgement upon detection of incomplete data. NORM_NACKs will be transmitted according to the rules of NACK generation and suppression of the NORM NACK process. A goal for the content of these messages is to use a format which can be potentially used by compatible intermedi- ate systems [REF Generic Router Assist Building Block] to provide assistance in promoting protocol scalability and efficiency when available. NORM_NACK messages generated will also contain addi- tional content to provide feedback to sender(s) for purposes of round-trip timing collection, congestion control, etc. NORM_NACK messages are transmitted by NORM receivers in response to the detection of missing data in the sequence of transmissions received from a particular source. The specific times and condi- tions under which receivers will generate and transmit these NORM_NACK messages are governed by the processes described in detail later in this document. The payload of NORM_NACK messages contains a list of "ObjectNACKs" for different objects and portions of a those objects. In addition to the common message header the NORM_NACK messages contain the following fields: Adamson, Borman, et al. Expires January 2002 [Page 21]
Internet Draft NORM Protocol July 2001 NORM_NACK Header 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | server_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_msb | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | grtt_response_lsb | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | loss_estimate |grtt_req_sequence| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data ... | The "server_id" field identifies the source to which the NORM_NACK message is destined. Other sources should ignore this message. (Note that this another reason why multiple potential sources within an NORM session MUST have unique NormNodeIds). The "grtt_response" field contains a timestamp indicating the time at which the NORM_NACK was transmitted. The format of this times- tamp is the same as the "send_time" field of the NORM_CMD_GRTT_REQ. However, note that the "grtt_response" timestamp is _relative_ to the "send_time" the source provided with the corresponding NORM_CMD_GRTT_REQ command. The receiver adjusts the source's NORM_CMD_GRTT_REQ "send_time" timestamp by the time differential from when the receiver received the NORM_CMD_GRTT_REQ to when the NORM_NACK was transmitted to calculate the value in the "grtt_response" field. The following formula applies: "grtt_response" = request "send_time" + request_to_response_differential If the "grtt_response" has ZERO value, that is an indication that the receiver has not yet received a NORM_CMD_GRTT_REQ command from the source and the source should ignore this portion of the response. The "loss_estimate" field is 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 per- cent packet loss. The 16-bit "loss_estimate" value is calculated by the following formula: "loss_estimate" = decimal_loss_fraction * 65535.0 The "grtt_req_sequence" field contains the sequence number identi- fier of the received NORM_CMD_GRTT_REQ to which the response Adamson, Borman, et al. Expires January 2002 [Page 22]
Internet Draft NORM Protocol July 2001 information in this NORM_NACK applies. The "data" field of the NORM_NACK message specifies the repair needs of this client pertaining to the indicated "server_id". These repair needs are in the format described by the "General Pur- pose NACK Content Encapsulation Format" in Section 3.2.3 of the NORM Building Block specification (currently "draft-ietf-rmt-bb- norm-02.txt". Note that the NACK content for multiple objects or ranges of stream data may be present in one NORM_NACK message and that each ObjectNACK consists of a hierarchical set of indicators and bit masks depending upon what data the receiver has detected is missing. (TBD) Example NACK messages. (See NORM Building Block document for now) NORM_ACK The basic operation of NORM transport will _not_ rely on the use NORM_ACK (positive acknowledgement) messages. However, some appli- cations may benefit from some limited form of positive acknowledge- ment for certain functions. A simple, scalable positive acknowl- edgement scheme is defined which can be leveraged by protocol implementations when appropriate. (TBD) Detailed description. General Messages: NORM_REPORT This is an optional message generated by NORM participants. This message may include periodic performance reports from receivers. Additionally, this message type may be potentially used by applica- tions to perform other session management functions such as period- ically advertising the full identity of a participant or the gen- eral context (more general than NORM_INFO messages which are asso- ciated with specific data content objects) of the content being transmitted to the group by a sender. 3.0 Detailed Protocol Operation (TBD) This section describes the detailed interactions of senders and receivers participating in a NORM session. Candidate subsec- tions: 3.1 Sender Initialization and Transmission Adamson, Borman, et al. Expires January 2002 [Page 23]
Internet Draft NORM Protocol July 2001 (TBD) Describes how a sender becomes active within the group, transmits data content and how it may potentially go inactive or leave the group. 3.2 Receiver Initialization and Reception (TBD) Describes how a receiver joins the group, begins receiving data content and any requirements on dynamically leaving and poten- tially rejoining the group. 3.3 Receiver NACK Process (TBD) Describes receiver criteria by which/when it chooses to transmit NACK-based repair requests and the content of the these messages. 3.3.1 NACK initiation 3.3.2 NACK content 3.4 Sender NACK Processing and Repair Transmission (TBD) Describes how the sender accumulates NACK repair requests and transmits repair information in response to these requests. 3.5 Additional Protocol Mechanisms (TBD) Describes any other protocol mechanisms running periperally or embedded as part of other protocol messaging. 3.5.1 Round-trip time collection 3.5.2 Congestion control operation 3.5.3 Other (e.g. optional scalable, positive acknowledgements, asymmet- ric feedback, performance reporting, etc) 4.0 Security Considerations (TBD) 5.0 Suggested Use (TBD) Adamson, Borman, et al. Expires January 2002 [Page 24]
Internet Draft NORM Protocol July 2001 6.0 References (TBD) 7.0 Authors' Addresses Brian Adamson adamson@itd.nrl.navy.mil Newlink Global Engineering Corporation 8580 Cinder Bed Road, Suite 1000 Newington, VA, USA, 22122 Carsten Bormann cabo@tellique.de Tellique Kommunikationstechnik GmbH Gustav-Meyer-Allee 25 Geb ude 12 D-13355 Berlin, Germany Mark Handley mjh@aciri.org 1947 Center Street, Suite 600 Berkeley, CA 94704 Joe Macker macker@itd.nrl.navy.mil Naval Research Laboratory Washington, DC, USA, 20375 Adamson, Borman, et al. Expires January 2002 [Page 25]
