NACK-Oriented Reliable Multicast

52:(UDP), and ensures reliable communication based upon a negative acknowledgement (NACK), selective Automatic Repeat Request (ARQ) mechanism, as opposed to the positive acknowledgement (ACK) approach that the standard Transmission Control Protocol (TCP) uses. In other words, receivers using NORM only send feedback when they do not receive a packet, as opposed to the TCP model where receivers regularly acknowledge packet receipt as part of it protocol operation. This allows NORM to support large-scale receiver groups. 457:

period, it resends the associated packet. The sender will continue doing this until it receives an ACK (although beyond a certain point the sender will assume the connection has been lost and stop the session). This is loosely analogous to a human listener nodding his or her head and saying "uh-huh" in a one-on-one conversation between persons. (TCP's sister protocol, UDP, does not do this. UDP simply sends data packets over the network in a

2564:. With this option the sender requests positive acknowledgement from a specific set of receivers regarding the successful reception of a designated point, or watermark, in the current transmission. If all the receivers acknowledge successful receipt of the watermark, the sender can continue sending new data. 2609:

NACK-oriented multicast communications are susceptible to NACK implosions. If a large number of receivers NACK simultaneously, this could overwhelm the sender as well as the entire network. NORM uses a NACK suppression mechanism, discussed in RFC 5740, Section 5.3, Receiver NACK Procedure, to prevent

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The receiver enters a holdoff period based on a random backoff algorithm. The duration of this timeout is governed by the "RandomBackoff" algorithm described in the Multicast NACK Building Block , and is based on the information the sender has been transmitting in the "grtt", "backoff", and "gsize"

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NORM_CMD messages, defined in RFC 5740 Section 4.2.3, are used to manage NORM sessions. These messages serve to collect round-trip timing, gather and send data related to congestion control, synchronize repair windows, and make notifications of a sender's status. There is a core set of specified and

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Protocol instantiations were defined as “specifications that define the necessary gluing logic and minimal additional functionality required to realize a working protocol from one or more building blocks.” Those specifications would also include an abstract API that defined the interface between the

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The Multicast Dissemination Protocol (MDP) was an initial attempt to ensure reliability and to address the problem of ACK implosions through the use of NACKs. MDP used selective negative acknowledgement (NACK) to support reliability. Additionally, MDP implemented probabilistic techniques to suppress

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An early problem noted with TCP's ACK mechanism was it did not work well in large multicast group communications. In multicast group communications, data packets are transmitted to a group of receivers simultaneously. In large multicast groups, the use of ACKs can create “ACK implosions,” in which a

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The TFMCC congestion control approach is an equation-based approach. Sender transmission rates rely on the collection of packet loss estimates and round trip times that are collected with NORM_CMD(CC) messages. This information identifies bottlenecks and adjusts the sender rate to accommodate them.

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2. Implicit watermark. This option is based on a lack of negative acknowledgement (NACK) repair requests from the receiver set. The sender uses the NORM_CMD(FLUSH) to alert receivers to NACK for any repairs needed through the indicated watermark. The sender then waits until the flushing has fully

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1. Following the establishment of a NORM session, a sender segments a NORM object into an ordinally-numbered sequence. These segments are transmitted as NORM_DATA messages. In addition to data content, NORM_DATA messages are labelled with unique identifiers and FEC identifiers. The sender may also

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As with TCP, the NORM congestion control scheme assumes packet loss is due to buffer overflows, and the sender consequently backs off it transmission rate when packet loss reaches a specified level. This can lead to limitations in wireless networks, where packet loss is often due to bit errors or

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Optional header extensions, defined in RFC 5740 Section 4.1, follow the common header and the message-specific header, and they precede the payload (if there is one). Header extensions typically carry information related to baseline FEC, congestion control operations, and other session management

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refers to an individual node in the context of a continuous segment of a NORM session. When a node joins a NORM session, it has a unique node identification as well as an instance identification. If the node leaves the session for any reason, and later rejoins the session, the node identification

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MDP also used packet-level forward error correction coding concepts as a repair mechanism. Encoded parity repair packets were generally sent in response to repair requests by receivers. In this way no additional protocol overhead was added above selective retransmission methods. Additionally, MDP

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Unlike the connection-oriented TCP protocol, NORM does not establish a session with a setup mechanism (i.e., a three-way handshake). Rather, NORM uses predesignated port numbers and addresses that senders and receivers must know. This allows receivers to join ongoing NORM sessions, and it allows

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All NORM messages consist of a mandatory common header, a message type header, and a payload (data) section. An optional extension field that specifies the error correction encoding being used, the congestion control algorithm, or other session management information, can be inserted between the

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The NORM_OBJECT_STREAM refers to streams (non-finite) of continuous data content. NORM supports reliable transport of streaming data similar to that of TCP, with an exception being that NORM receivers are able to join in the reception of stream content without regard for the point of time in the

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Building blocks were defined as “a set of easily-separable coarse-grained modular components that are common to multiple protocols along with abstract APIs that define a building block's access methods and their arguments.” Initial building blocks included negative acknowledgments, forward error

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MDP was a direct predecessor to NORM, with an initial IETF draft published in November 1996. The National Aeronautics and Space Administration (NASA) adopted MDP for reliable file transfers during space missions., and the U.S. Army used in for tactical group messaging in its Force Battle Command

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The NORM_INFO content must fit into the payload portion of a single NORM message. Thus, it is considered atomic. An example of NORM_INFO use would be to send MIME-type information for the associated NORM data content. This would allow receivers to make decisions about their participation in the

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To support further scalability, NORM also employs packet erasure coding using forward error correction (FEC) codes coupled with suppression of redundant NACK feedback from the receiver group. Additionally, NORM can be configured to operate with “silent receivers” relying upon its packet erasure

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NORM uses a TCP-Friendly congestion control scheme that enables it to fairly share available bandwidth with TCP and other transport protocols. Specifically, NORM's congestion control scheme uses a rate control procedure, and it is an adaptation of the TCP-Friendly Multicast Congestion Control

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NORM_OBJECT_DATA refers to static computer memory data content, while NORM_OBJECT_FILE refers to computer storage files. Both message types provide reliable transport to finite blocks of content, but a distinction is made to allow receivers to determine what type of storage to allocate for the

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With the ACK mechanism, data packets are sequentially numbered, and the sender does not send a packet in a sequence until it receives an acknowledgement (ACK) from the receiver that the previous packet has been successfully received. If the sender does not receive an ACK after a specified time

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3. Soft, timer-based flow control. This option holds transmit data and limits repair window advancement based on group round-trip timing (GRTT) and NACK activity. A minimum, adaptable time limit is set, after which the sender can continue sending new data. This time limit is based on the

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2. When receivers detect missing content from a sender, they may attempt repair using FEC mechanisms. Barring successful repairs, the receivers will send NORM_NACK messages to the sender. Using the ordinally-numbered sequence information, the receivers specify missing content.

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The RMTWG pursued the strategy of developing building blocks and protocol instantiations. This strategy avoided a "one size fits all" protocol, which in turn could accommodate the large number of applications and types of applications that reliable multicast could support.

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NORM_NACK messages, defined in RFC 5740 Section 4.3.1, are used primarily for receivers to request repairs of sender content. Additionally, these messages contain fields to provide information to the sender(s) related to round-trip timing collection and congestion control.

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coding for high assurance delivery, thus operating as a broadcast-only protocol. The FEC can be configured to be used either reactively (with NACKing receivers) or proactively (silent receivers), or in a hybrid manner that allows tradeoffs in latency and network overhead.

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NORM has two general message classes, sender and receiver messages, that are defined in RFC 5740 Section 4, NORM Message Formats. NORM sender message types are: NORM_DATA, NORM_INFO, and NORM_CMD. NORM receiver message types are: NORM_NACK, NORM_ACK, and NORM_REPORT.

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In the TCP/IP network model, the transport layer of the network (Layer 4) is responsible for the reliable transport of data. The TCP protocol is the principal means of ensuring reliable unicast (point-to-point) transport. TCP does this through an ACK mechanism.

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The number of 32-bit words that constitute the given message's header portion. This is used to identify the addition of header extensions. If the "hdr_len" value is greater than the base value for the given message type, it implies the presence of a header

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to be sent in association with the data content objects. This allows receivers to determine the nature of the corresponding content being transmitted, which in turn allows application-level control of the receiver node's participation in the session.

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send out-of-band NORM_INFO messages associated with the NORM_DATA content that allow receivers to determine the nature of the corresponding content, which in turn allows application-level control of the receiver node's participation in the session.

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A fundamental assumption about a NORM session is that it will consist of a single transmitter communicating with a group of receivers. Multiple senders may operate in a given NORM session, although each receiver must maintain state for each sender.

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Both FEC coding schemes can be used in either a reactive or proactive manner. In both cases, FEC-based repair can be affected, which allows receivers to repair packets and thereby reduces the level of repair requests and repair transmissions.

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Several other approaches to reliable multicast were being developed at approximately the same time, and in April 1999, the IETF chartered the Reliable Multicast Transport Working Group (RMTWG) to standardize reliable multicast transport.

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The NORM_DATA message, defined in RFC 5740 Section 4.2.1, is the most common message type transmitted by NORM senders. NORM_DATA messages carry segmented data content for NORM_OBJECT_DATA, NORM_OBJECT_FILE, and NORM_OBJECT_STREAM types.

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In July 2005 the NACK-based protocol building blocks and protocol instantiation were submitted as “Experimental” in RFC 3940, and in November 2009 “NACK-Oriented Reliable Multicast (NORM) Transport Protocol” was approved in RFC 5740.

67:. Unlike TCP, which uses the ACK mechanism for congestion control and flow control, NORM uses separate mechanisms for each. This allows for a wide variety of configurations to meet different application data delivery needs. 2123:

A sequence number so the sender can verify a received NORM_ACK message actually applies to a current acknowledgment request. The "ack_id" field is not used in the case of the NORM_ACK(CC) and NORM_ACK(FLUSH) acknowledgment

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Because NORM's rate control mechanism is separate from its other components, e.g., reliability mechanism, flow control mechanism, it can be replaced with an alternative algorithm and appropriate header extensions.

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control, application-controlled positive acknowledgement, and other functions towards building complete point-to-point and group network communications applications that are highly robust and efficient.

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If the NORM session is not taking place in a multicast routing environment, the NACK is transmitted to the sender, and the sender immediately re-sends it to all the other nodes in the network.

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large number of simultaneous ACKs can overwhelm the sender. This is loosely analogous to a large roomful of human listeners nodding their heads and saying "uh-huh" while a speaker is talking.

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At the same time, the sender periodically transmits NORM_CMD(CC) messages as needed to collect feedback information necessary for congestion control and other session management activities.

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4. Purposeful disabling. This option deliberately disables flow control mechanisms and allows the application to push transmission forward with newly enqueued data in a best effort manner.

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S. Floyd, V. Jacobson, S. McCanne, C. Liu, and L. Zhang. “A Reliable Multicast Framework for Lightweight Sessions and Application Level Framing”, Proc. ACMSIGCOMM, August 1995.

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A value assigned by the sender to NORM objects being transmitted that receivers use for transmissions and repair requests. It is increased in a monotonically incremental manner.

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A value assigned by the sender to NORM objects being transmitted that receivers use for transmissions and repair requests. It is increased in a monotonically incremental manner.

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FLUSH : Indicates sender's temporary end-of-transmission. May also be optionally used to collect positive acknowledgment of reliable reception from a subset of receivers.

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A value assigned by the sender to NormObjects being transmitted that receivers use for transmissions and repair requests. It is increased in a monotonically incremental manner.

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S. Pingali, D. Towsley, J. Kurose. “A Comparison of Sender-Initiated and Receiver-Initiated Reliable Multicast Protocols”. Proc. INFOCOM, San Francisco, CA, October 1993.

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NORM has four options for flow control, which allows the NORM sender to manage the rate of transmission to NORM receivers to ensure the receivers are not overwhelmed.

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Within the context of general operations, NORM has specific mechanisms to address session control, congestion control, flow control, reliability, and NACK management.

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refers to an individual node taking part in a NORM session. Each node has a unique identifier. When a node transmits a NORM message, this identifier is noted as the

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NORM's NACK suppression mechanism, combined with its FEC mechanism, allows NORM multicast groups to scale to very large group sizes while maintaining reliability.

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4. When the sender reaches the end of a collection of repair requests, it transmits a NORM_CMD(FLUSH) message, indicating the temporary end of transmission.

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If the NORM session is taking place in a multicast routing environment, the NACK is transmitted to the sender as well as all the other nodes in the network.

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

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3. As the sender receives NACKs, it aggregates repair requests and sends appropriate repairs associated with the ordinally-numbered sequence information.

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A. Mankin, A. Romanow, S. Bradner, V. Paxson, “IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols”, RFC 2357, June 1998.

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B. Whetton and J. Conlan, "A Rate Based Congestion Control Scheme for Reliable Multicast", Technical Report, GlobalCast Communication, October 1998.

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S. Floyd and V. Jacobson, "Random Early Detection Gateways for Congestion Avoidance", IEEE/ACM Transactions on Networking, V.1 N.4, August 1993.

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NORM_ACK messages, defined in RFC 5740 4.3.2, are used primarily to support congestion control operations and round-trip timing measurements.

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There is an assumption that one or more other receivers have missed that same data, and that a NACK may have already been sent to the sender.

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refers to the exchange of information among two or more network hosts using NORM. Typically a NORM session takes place using pre-determined

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Protocol Specification approach described in RFC4654. This approach has been demonstrated to work well with TCP and multicast data flows.

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D. Gossink, J. Macker, "Reliable Multicast and Integrated Parity Retransmission with Channel Estimation", IEEE GLOBECOM 98, October 1998.

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During the holdoff period the receiver continues to receive and evaluate repair messages from the sender, and it suppresses its own NACK.

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A value used by receivers to determine the maximum backoff timer value when using timer-based NORM NACK feedback suppression mechanisms.

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A value used by receivers to determine the maximum backoff timer value when using timer-based NORM NACK feedback suppression mechanisms.

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A value used by receivers to determine the maximum backoff timer value when using timer-based NORM NACK feedback suppression mechanisms.

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All NORM messages begin with the following common header, defined in RFC 5740, Section 4.1, NORM Common Message Header and Extension.

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APPLICATION : Used for application-defined purposes that need to temporarily preempt or supplement data transmission (OPTIONAL).

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J. Macker, "Reliable Multicast Transport and Integrated Erasure-based Forward Error Correction", Proc. IEEE MILCOM 97, October 1997.

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D. Clark, D. Tennenhouse, “Architectural Considerations for a New Generation of Protocols”. Proc. ACM SIGCOMM, September 1990.

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At the same time, NORM senders and receivers maintain state to ensure reliable transfer, hence NORM is not totally connection-less.

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A range of NORM_CMD message sub-types is reserved to support session control protocols that may be developed in the future.

580:(SAP) . Although NORM was specifically designed for multicast group communication, it supports unicast communication, also. 2694:- Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents, April 2002 1693:

REPAIR_ADV : Advertises sender's aggregated repair/feedback state for suppression of unicast feedback from receivers.

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allowed the protocol to be optionally configured to send proactive repair packets in the original data transmission block.

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completed. If there was no NACK activity, the sender assumes the “watermark” completed and can continue sending new data.

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If, at the end of the holdoff period, the receiver has not received sufficient repair information, it initiates its NACK.

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The NORM_REPORT message, discussed in RFC 5740 Section 4.4.1, is an optional message. The format is currently undefined.

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An adjusted version of the timestamp from the most recently received NORM_CMD(CC) message for the indicated NORM sender.

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An adjusted version of the timestamp from the most recently received NORM_CMD(CC) message for the indicated NORM sender.

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An adjusted version of the timestamp from the most recently received NORM_CMD(CC) message for the indicated NORM sender.

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An adjusted version of the timestamp from the most recently received NORM_CMD(CC) message for the indicated NORM sender.

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The NORM message type (i.e., 1 = NORM_INFO, 2 = NORM_DATA, 3 = NORM_CMD, 4 = NORM_NACK, 5 = NORM_ACK, 6 = NORM_REPORT).

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is the fundamental NORM architectural construct. A NORM message is contained in the data field of a UDP datagram.

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Use of Nack Oriented Reliable Multicast (NORM) protocol for transport of spacecraft telemetry in ground networks

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When a receiver detects it is missing data from a sender's NORM transmissions, it initiates its NACK procedure:

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The following general NORM protocol operation is described in RFC 5740 Section 5, Detailed Protocol Operation.

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that are determined prior to the session. These addresses may be determined using other protocols, such as the

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The header extension type. For variable-length header extensions, a value between 0 and 127 (inclusive).

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ACK_REQ : Requests application-defined positive acknowledgment from a list of receivers (OPTIONAL).

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The header extension type. For fixed-length header extensions, a value between 128 and 255 (inclusive).

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The unique identify of the node that originated the message within the context of a given NORM session.

3260:, "Congestion Control Performance of a Reliable Multicast Protocol", Proc. IEEE ICNP 98, August 1998. 1445:

enumerated NORM_CMD messages, as well as a range other available types for application-specific use.

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The repair needs of the receiver with respect to the NORM sender indicated by the "server_id" field.

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The following architectural constructs are defined in RFC 5740 Section 2, Architecture Definition.

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Although NORM was developed primarily to support multicast group communication, it also supports

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Binary flags providing information to assist the receiver in appropriately handling the payload.

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Binary flags providing information to assist the receiver in appropriately handling the payload.

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Binary flags providing information to assist the receiver in appropriately handling the payload.

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remains the same, but the instance identification changes. The current instance is noted as the

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protocol, like TCP, it does have both connection-oriented and connection-less characteristics.

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and non-systematic codes. These are discussed in RFC 5740 Section 2, Architecture Definition:

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payload_length, payload_msg, and payload_offset only pertain to NORM_OBJECT_STREAM content.

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With non-systematic codes, the sender encodes the content with FEC prior to transmission.

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SQUELCH : Indicates current repair window of sender in response to out-of-range NACKs

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With systematic codes, the sender transmits blocks of content followed by blocks of FEC.

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information. There are two types of header extensions: variable-length and fixed-length.

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J.P. Macker; P.B. Adamson (1999). "The multicast dissemination protocol (MDP) toolkit".

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Allows receivers to maintain an estimate of packet loss to facilitate congestion control

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correction, a generic signaling mechanism for router assist, and transport protection

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MILCOM 1999. IEEE Military Communications. Conference Proceedings (Cat. No.99CH36341)

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