Reliable Multicast Transport M. Watson Internet-Draft Digital Fountain Intended status: Standards Track February 22, 2007 Expires: August 26, 2007 Basic Forward Error Correction (FEC) Schemes draft-ietf-rmt-bb-fec-basic-schemes-revised-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 documents 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. This Internet-Draft will expire on August 26, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Watson Expires August 26, 2007 [Page 1] Internet-Draft Basic FEC Schemes February 2007 Abstract This document provides FEC Scheme specifications according to the RMT FEC Building Block for the Compact No-Code FEC Scheme, the Small Block, Large Block and Expandable FEC Scheme, the Small Block Systematic FEC Scheme and the Compact FEC Scheme. Watson Expires August 26, 2007 [Page 2] Internet-Draft Basic FEC Schemes February 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5 3. Compact No-Code FEC Scheme . . . . . . . . . . . . . . . . . . 6 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 6 3.2.1. FEC Payload ID(s) . . . . . . . . . . . . . . . . . . 6 3.2.2. FEC Object Transmission Information . . . . . . . . . 7 3.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 8 3.4. FEC code specification . . . . . . . . . . . . . . . . . . 9 3.4.1. Source Block Logistics . . . . . . . . . . . . . . . . 9 3.4.2. Sending and Receiving a Source Block . . . . . . . . . 10 4. Small Block, Large Block and Expandable FEC Scheme . . . . . . 12 4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 12 4.2.1. FEC Payload ID(s) . . . . . . . . . . . . . . . . . . 12 4.2.2. FEC Object Transmission Information . . . . . . . . . 12 4.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 13 4.4. FEC Code Specification . . . . . . . . . . . . . . . . . . 14 5. Small Block Systematic FEC Scheme . . . . . . . . . . . . . . 15 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 15 5.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 15 5.2.1. FEC Payload ID(s) . . . . . . . . . . . . . . . . . . 15 5.2.2. FEC Object Transmission Information . . . . . . . . . 16 5.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 17 5.4. FEC Code Specification . . . . . . . . . . . . . . . . . . 17 6. Compact FEC Scheme . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 18 6.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 18 6.2.1. FEC Payload ID(s) . . . . . . . . . . . . . . . . . . 18 6.2.2. FEC Object Transmission Information . . . . . . . . . 18 6.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 18 6.4. FEC code specification . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 10. Changes from schemes defined in RFC3452 and RFC3695 . . . . . 22 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 11.1. Normative References . . . . . . . . . . . . . . . . . . . 23 11.2. Informative References . . . . . . . . . . . . . . . . . . 23 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24 Intellectual Property and Copyright Statements . . . . . . . . . . 25 Watson Expires August 26, 2007 [Page 3] Internet-Draft Basic FEC Schemes February 2007 1. Introduction The document specifies the following FEC Schemes according to the specification requirements of the FEC Building Block [2]: o Compact No-Code FEC Scheme o Small Block, Large Block and Expandable FEC Scheme o Small Block Systematic FEC Scheme o Compact FEC Scheme This document inherits the context, language, declarations and restrictions of the FEC building block [2]. This document also uses the terminology of the companion document [4] which describes the use of FEC codes within the context of reliable IP multicast transport and provides an introduction to some commonly used FEC codes. Building blocks are defined in RFC 3048 [9]. This document is a product of the IETF RMT WG and follows the general guidelines provided in RFC 3269 [5]. RFC3452 [3] and RFC3695 [10] contained a previous versions of the FEC Schemes defined in this specification. These RFCs were published in the "Experimental" category. It was the stated intent of the RMT working group to re-submit these specifications as an IETF Proposed Standard in due course. This Proposed Standard specification is thus based on and backwards compatible with the FEC Schemes defined in RFC3452 [3] and RFC3695 [10] updated according to accumulated experience and growing protocol maturity since their original publication. Said experience applies both to this specification itself and to congestion control strategies related to the use of this specification. The differences between the FEC Scheme specifications in RFC3452 [3] and RFC3695 [10] and this document listed in Section 10 Watson Expires August 26, 2007 [Page 4] Internet-Draft Basic FEC Schemes February 2007 2. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [1]. Watson Expires August 26, 2007 [Page 5] Internet-Draft Basic FEC Schemes February 2007 3. Compact No-Code FEC Scheme 3.1. Introduction The Compact No-code FEC Scheme is a Fully-Specified FEC Scheme. The scheme requires no FEC coding and is specified primarily to allow simple interoperability testing between different implementations of protocol instantiations that use the FEC building block. 3.2. Formats and Codes 3.2.1. FEC Payload ID(s) The FEC Payload ID for the Compact No-Code FEC Scheme is composed of a Source Block Number and an Encoding Symbol ID as shown in Figure 1. 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 | Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: FEC Payload ID format for Compact No-Code FEC Scheme The 16-bit Source Block Number is used to identify from which source block of the object the encoding symbol in the payload of the packet is generated. There are two possible modes: In the unique SBN mode each source block within the object has a unique Source Block Number associated with it, and in the non-unique SBN mode the same Source Block Number may be used for more than one source block within the object. Which mode is being used for an object is outside the scope of this document and MUST be communicated, either explicitly or implicitly, out-of-band to receivers. If the unique SBN mode is used then successive Source Block Numbers are associated with consecutive source blocks of the object starting with Source Block Number 0 for the first source block of the object. In this case, there are at most 2^^16 source blocks in the object. If the non-unique SBN mode is used then the mapping from source blocks to Source Block Numbers MUST be communicated out-of-band to receivers, and how this is done is outside the scope of this document. This mapping could be implicit, for example determined by the transmission order of the source blocks. In non-unique SBN mode, packets for two different source blocks mapped to the same Source Block Number SHOULD NOT be sent within an interval of time that is shorter than the transport time of a source block. The transport time of a source block includes the amount of time the source block Watson Expires August 26, 2007 [Page 6] Internet-Draft Basic FEC Schemes February 2007 is processed at the transport layer by the sender, the network transit time for packets, and the amount of time the source block is processed at the transport layer by a receiver. This allows the receiver to clearly decide which packets belong to which source block. The 16-bit Encoding Symbol ID identifies which specific encoding symbol generated from the source block is carried in the packet payload. The exact details of the correspondence between Encoding Symbol IDs and the encoding symbols in the packet payload are specified in Section 3.4. 3.2.2. FEC Object Transmission Information 3.2.2.1. Mandatory The mandatory FEC Object Transmission Information element for the Compact No-Code FEC Scheme is: o FEC Encoding ID: zero (0) 3.2.2.2. Common The common FEC Object Transmission Information elements and their value ranges for the Compact No-code FEC Scheme are: Transfer-Length: a non-negative integer less than 2^^48. Encoding-Symbol-Length: a non-negative integer less than 2^^16. Maximum-Source-Block-Length: a non-negative integer less than 2^^32. Note that the semantics for the above elements are defined in [2] and are not duplicated here. The encoded Common FEC Object Transmission information is defined in Figure 2. Watson Expires August 26, 2007 [Page 7] Internet-Draft Basic FEC Schemes February 2007 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transfer Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoding Symbol Length | Max. Source Block Length (MSB)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Max. Source Block Length (LSB)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Encoded Common FEC OTI for Compact No-Code FEC Scheme All Encoding Symbols of a transport object MUST have length equal to the length specified in the Encoding Symbol Length element, with the optional exception of the last source symbol of the last source block (so that redundant padding is not mandatory in this last symbol). This last source symbol MUST be logically padded out with zeroes when another Encoding Symbol is computed based on this source symbol to ensure the same interpretation of this Encoding Symbol value by the sender and receiver. However, this padding does not actually need to be sent with the data of the last source symbol. The "Reserved" field in the Encoded FEC Object Transmission Information SHOULD be set to zero by senders and its value SHOULD be ignored by receivers. Note: this FEC Scheme was first defined in [10] which did not require that the Encoding Symbol Length should be the same for every source block. This document introduces a general requirement that the Encoding Symbol Length be the same across source blocks. Since no protocols were defined which support variation in the Encoding Symbol Length between source blocks this can be done without introducing backwards compatibility issues. 3.2.2.3. Scheme-Specific No Scheme-Specific FEC Object Transmission Information elements are defined by this FEC Scheme. 3.3. Procedures The algorithm defined in Section 9.1. of [2] MUST be used to partition the file into source blocks. Watson Expires August 26, 2007 [Page 8] Internet-Draft Basic FEC Schemes February 2007 3.4. FEC code specification The Compact No-Code FEC scheme does not require FEC encoding or decoding. Instead, each encoding symbol consists of consecutive bytes of a source block of the object. The following two subsections describe the details of how the Compact No-Code FEC scheme operates for each source block of an object. 3.4.1. Source Block Logistics Let X > 0 be the length of a source block in bytes. Let L > 0 be the length of the encoding symbol contained in the payload of each packet. The value of X and L are part of the FEC Object Transmission Information, and how this information is communicated to a receiver is outside the scope of this document. For a given source block X bytes in length with Source Block Number I, let N = X/L rounded up to the nearest integer. The encoding symbol carried in the payload of a packet consists of a consecutive portion of the source block. The source block is logically partitioned into N encoding symbols, each L bytes in length, and the corresponding Encoding Symbol IDs range from 0 through N-1 starting at the beginning of the source block and proceeding to the end. Thus, the encoding symbol with Encoding Symbol ID Y consists of bytes L*Y through L*(Y+1)-1 of the source block, where the bytes of the source block are numbered from 0 through X-1. If X/L is not integral then the last encoding symbol with Encoding Symbol ID = N-1 consists of bytes L*(N-1) through the last byte X-1 of the source block, and the remaining L*N - X bytes of the encoding symbol can by padded out with zeroes. As an example, suppose that the source block length X = 20,400 and encoding symbol length L = 1,000. The encoding symbol with Encoding Symbol ID = 10 contains bytes 10,000 through 10,999 of the source block, and the encoding symbol with Encoding Symbol ID = 20 contains bytes 20,000 through the last byte 20,399 of the source block and the remaining 600 bytes of the encoding symbol can be padded with zeroes. There are no restrictions beyond the rules stated above on how a sender generates encoding symbols to send from a source block. However, it is recommended that an implementor of refer to the companion document [3] for general advice. In the next subsection a procedure is recommended for sending and receiving source blocks. Watson Expires August 26, 2007 [Page 9] Internet-Draft Basic FEC Schemes February 2007 3.4.2. Sending and Receiving a Source Block The following carousel procedure is RECOMMENDED for a sender to generate packets containing FEC Payload IDs and corresponding encoding symbols for a source block with Source Block Number I. Set the length in bytes of an encoding symbol to a fixed value L which is reasonable for a packet payload (e.g., ensure that the total packet size does not exceed the MTU) and that is smaller than the source block length X, e.g., L = 1,000 for X >= 1,000. Initialize Y to a value randomly chosen in the interval [0..N-1]. Repeat the following for each packet of the source block to be sent. o If Y <= N-1 then generate the encoding symbol Y. o Within the FEC Payload ID, set the Source Block Length to X, set the Source Block Number = I, set the Encoding Symbol ID = Y, place the FEC Payload ID and the encoding symbol into the packet to send. o In preparation for the generation of the next packet: if Y < N-1 then increment Y by one else if Y = N-1 then reset Y to zero. The following procedure is RECOMMENDED for a receiver to recover the source block based on receiving packets for the source block from a sender that is using the carousel procedure described above. The receiver can determine from which source block a received packet was generated by the Source Block Number carried in the FEC Payload ID. Upon receipt of the first FEC Payload ID for a source block, the receiver uses the source block length received out-of-band as part of the FEC Object Transmission Information to determine the length X in bytes of the source block, and allocates space for the X bytes that the source block requires. The receiver also computes the length L of the encoding symbol in the payload of the packet by substracting the packet header length from the total length of the received packet (and the receiver checks that this length is the same in each subsequent received packet from the same source block). After calculating N = X/L rounded up to the nearest integer, the receiver allocates a boolean array RECEIVED[0..N-1] with all N entries initialized to false to track received encoding symbols. The receiver keeps receiving packets for the source block as long as there is at least one entry in RECEIVED still set to false or until the application decides to give up on this source block and move on to other source blocks. For each received packet for the source block (including the first packet) the steps to be taken to help recover the source block are as follows. Let Y be the value of the Encoding Symbol ID within FEC Payload ID of the packet. If Y <= N-1 then the receiver copies the encoding symbol into the appropriate place within the space reserved for the source block and sets Watson Expires August 26, 2007 [Page 10] Internet-Draft Basic FEC Schemes February 2007 RECEIVED[Y] = true. If all N entries of RECEIVED are true then the receiver has recovered the entire source block. Watson Expires August 26, 2007 [Page 11] Internet-Draft Basic FEC Schemes February 2007 4. Small Block, Large Block and Expandable FEC Scheme 4.1. Introduction This section defines an Under-Specified FEC Scheme for Small Block FEC codes, Large Block FEC codes and Expandable FEC codes as described in [4]. 4.2. Formats and Codes 4.2.1. FEC Payload ID(s) The FEC Payload ID is composed of a Source Block Number and an Encoding Symbol ID structured as shown in Figure 3. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Block Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: FEC Payload ID format for Small Block, Large Block and Expandable FEC Codes The Source Block Number identifies from which source block of the object the encoding symbol(s) in the payload are generated. These blocks are numbered consecutively from 0 to N-1, where N is the number of source blocks in the object. The Encoding Symbol ID identifies which specific encoding symbol(s) generated from the source block are carried in the packet payload. The exact details of the correspondence between Encoding Symbol IDs and the encoding symbol(s) in the packet payload are dependent on the particular FEC Scheme instance used as identified by the FEC Encoding ID and by the FEC Instance ID, and these details may be proprietary. 4.2.2. FEC Object Transmission Information 4.2.2.1. Mandatory The mandatory FEC Object Transmission Information element for the Small Block, Large Block and Expandable FEC Scheme are: o FEC Encoding ID: 128 Watson Expires August 26, 2007 [Page 12] Internet-Draft Basic FEC Schemes February 2007 4.2.2.2. Common The common FEC Object Transmission Information elements and their value ranges for the Small Block, Large Block and Expandable FEC Scheme are: FEC Instance ID: a non-negative integer less than 2^^16. Transfer-Length: a non-negative integer less than 2^^48. Encoding-Symbol-Length: a non-negative integer less than 2^^16. Maximum-Source-Block-Length: a non-negative integer less than 2^^32. Note that the semantics for the above elements are defined in [2] and are not duplicated here. The encoded Common FEC Object Transmission information is defined in Section 4.2.2.2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transfer Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | FEC Instance ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoding Symbol Length | Max. Source Block Length (MSB)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Max. Source Block Length (LSB)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Encoded Common FEC OTI for Small Block, Large Block and Expandable FEC Scheme 4.2.2.3. Scheme-Specific No Scheme-Specific FEC Object Transmission Information elements are defined by this FEC Scheme. 4.3. Procedures The algorithm defined in Section 9.1. of [2] MUST be used to partition the file into source blocks. Watson Expires August 26, 2007 [Page 13] Internet-Draft Basic FEC Schemes February 2007 4.4. FEC Code Specification The FEC code specification and the correspondance of Encoding Symbols IDs to encoding symbols are defined by specific instances of this scheme and so are out of scope of this document. Watson Expires August 26, 2007 [Page 14] Internet-Draft Basic FEC Schemes February 2007 5. Small Block Systematic FEC Scheme 5.1. Introduction This section defines an Under-Specified FEC Scheme for Small Block Systematic FEC codes as described in [4]. For Small Block Systematic FEC codes, each source block is of length at most 65536 source symbols. Although these codes can generally be accommodated by the FEC Encoding ID described in Section 4, a specific FEC Encoding ID is defined for Small Block Systematic FEC codes to allow more flexibility and to retain header compactness. The small source block length and small expansion factor that often characterize systematic codes may require the data source to frequently change the source block length. To allow the dynamic variation of the source block length and to communicate it to the receivers with low overhead, the block length is included in the FEC Payload ID. 5.2. Formats and Codes 5.2.1. FEC Payload ID(s) The FEC Payload ID is composed of the Source Block Number, Source Block Length and the Encoding Symbol ID structured as shown in Figure 5. 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 Length | Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: FEC Payload ID format for Small Block Systematic FEC scheme The Source Block Number identifies from which source block of the object the encoding symbol(s) in the payload are generated. These blocks are numbered consecutively from 0 to N-1, where N is the number of source blocks in the object. The Source Block Length is the length in units of source symbols of the source block identified by the Source Block Number. The Encoding Symbol ID identifies which specific encoding symbol(s) generated from the source block are carried in the packet payload. Each encoding symbol is either an original source symbol or a Watson Expires August 26, 2007 [Page 15] Internet-Draft Basic FEC Schemes February 2007 redundant symbol generated by the encoder. The exact details of the correspondence between Encoding Symbol IDs and the encoding symbol(s) in the packet payload are dependent on the particular FEC scheme instance used as identified by the FEC Instance ID, and these details may be proprietary. 5.2.2. FEC Object Transmission Information 5.2.2.1. Mandatory The mandatory FEC Object Transmission Information element for the Small Block Systematic FEC Scheme is: o FEC Encoding ID: 129 5.2.2.2. Common The common FEC Object Transmission Information elements and their value ranges for the Small Block Systematic FEC Scheme are: FEC Instance ID: a non-negative integer less than 2^^16. Transfer-Length: a non-negative integer less than 2^^48. Encoding-Symbol-Length: a non-negative integer less than 2^^16. Maximum-Source-Block-Length: a non-negative integer less than 2^^16. Max-Number-of-Encoding-Symbols: a non-negative integer less than 2^^16 Note that the semantics for the above elements are defined in [2] and are not duplicated here. The encoded Common FEC Object Transmission information is defined in Figure 6. Watson Expires August 26, 2007 [Page 16] Internet-Draft Basic FEC Schemes February 2007 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Transfer Length | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | FEC Instance ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoding Symbol Length | Maximum Source Block Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Max. Num. of Encoding Symbols | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: FEC OTI format for Small Block Systematic FEC Scheme All Encoding Symbols of a transport object MUST have length equal to the length specified in the Encoding Symbol Length field, with the optional exception of the last source symbol of the last source block (so that redundant padding is not mandatory in this last symbol). This last source symbol MUST be logically padded out with zeroes when another Encoding Symbol is computed based on this source symbol to ensure the same interpretation of this Encoding Symbol value by the sender and receiver. However, this padding need not be actually sent with the data of the last source symbol. Note: this FEC Scheme was first defined in [3] which did not require that the Encoding Symbol Length should be the same for every source block. However, no protocols have been defined which support variation in the Encoding Symbol Length between source blocks and thus introduction of a general requirement that the Encoding Symbol Length be the same across source blocks (as defined here) should not cause backwards compatibility issues and will aid interoperability. 5.2.2.3. Scheme-Specific No Scheme-Specific FEC Object Transmission Information elements are defined by this FEC Scheme. 5.3. Procedures The algorithm defined in Section 9.1. of [2] MAY be used to partition the file into source blocks. 5.4. FEC Code Specification The FEC code specification and the correspondance of Encoding Symbols IDs to encoding symbols are defined by specific instances of this scheme and so are out of scope of this document. Watson Expires August 26, 2007 [Page 17] Internet-Draft Basic FEC Schemes February 2007 6. Compact FEC Scheme 6.1. Introduction The Compact FEC Scheme is an Under-Specified FEC scheme. This FEC scheme is similar in spirit to the Compact No-Code FEC scheme, except that a non-trivial FEC encoding (that is Under-Specified) may be used to generate encoding symbol(s) placed in the payload of each packet and a corresponding FEC decoder may be used to produce the source block from received packets. 6.2. Formats and Codes 6.2.1. FEC Payload ID(s) The FEC Payload ID format defined in Section 3.2.1 SHALL be used. 6.2.2. FEC Object Transmission Information 6.2.2.1. Mandatory The mandatory FEC Object Transmission Information element for the Compact No-Code FEC Scheme is: o FEC Encoding ID: 130 6.2.2.2. Common The common FEC Object Transmission Information elements and their encoding are the same as defined for the Small Block, Large Block and Expandable FEC Scheme in Figure 4. 6.2.2.3. Scheme-Specific No Scheme-Specific FEC Object Transmission Information elements are defined by this FEC Scheme. 6.3. Procedures The algorithm defined in Section 9.1. of [2] MUST be used to partition the file into source blocks. 6.4. FEC code specification The FEC code specification and the correspondance of Encoding Symbols IDs to encoding symbols are defined by specific instances of this scheme and so are out of scope of this document. Watson Expires August 26, 2007 [Page 18] Internet-Draft Basic FEC Schemes February 2007 7. Security Considerations This specification does not introduce any further security considerations beyond those described in [2]. Watson Expires August 26, 2007 [Page 19] Internet-Draft Basic FEC Schemes February 2007 8. Acknowledgments This document is substantially based on [10] by Michael Luby and Lorenzo Vicisano and [3] by Michael Luby, Lorenzo Vicisano, Jim Gemmell, Luigi Rizzo, Mark Handley and Jon Crowcroft. Watson Expires August 26, 2007 [Page 20] Internet-Draft Basic FEC Schemes February 2007 9. IANA Considerations FEC Encoding IDs 0 and 130 were first defined and registered in the ietf:rmt:fec:encoding namespace by [10]. This document updates and obsoletes the definitions from that specification. FEC Encoding IDs 128 and 129 were first defined and registered in the ietf:rmt:fec:encoding namespace by [3]. This document updates and obsoletes the definitions from that specification. Watson Expires August 26, 2007 [Page 21] Internet-Draft Basic FEC Schemes February 2007 10. Changes from schemes defined in RFC3452 and RFC3695 This section describes the changes between the Exprimental versions of these FEC Scheme specifictions contained in RFC3452 [3] and RFC3695 [10] and those defined in this specification: o Scheme definitions have been updated to meet the requirements of [2]. o Complete encoding formats for the FEC Object Trasmission Information for each scheme are defined here, instead of within content delivery protocol specifications, since th exact format depends on the FEC Scheme. o The previous specifications for the Compact No-Code and Small Block Systematic FEC Schemes sis not require that all encoding symbols of the object should have the same length. This requirement is introduced in this specification. Since no protocols have been defined which support variation of the encoding symbol length within an object this should not cause backwards compatibility issues. Watson Expires August 26, 2007 [Page 22] Internet-Draft Basic FEC Schemes February 2007 11. References 11.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Watson, M., "Forward Error Correction (FEC) Building Block", draft-ietf-rmt-fec-bb-revised-04 (work in progress), September 2006. 11.2. Informative References [3] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and J. Crowcroft, "Forward Error Correction (FEC) Building Block", RFC 3452, December 2002. [4] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and J. Crowcroft, "The Use of Forward Error Correction (FEC) in Reliable Multicast", RFC 3453, December 2002. [5] Kermode, R. and L. Vicisano, "Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents", RFC 3269, April 2002. [6] Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols", RFC 2357, June 1998. [7] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [9] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S., and M. Luby, "Reliable Multicast Transport Building Blocks for One-to-Many Bulk-Data Transfer", RFC 3048, January 2001. [10] Luby, M. and L. Vicisano, "Compact Forward Error Correction (FEC) Schemes", RFC 3695, February 2004. Watson Expires August 26, 2007 [Page 23] Internet-Draft Basic FEC Schemes February 2007 Author's Address Mark Watson Digital Fountain 39141 Civic Center Drive Suite 300 Fremont, CA 94538 U.S.A. Email: mark@digitalfountain.com Watson Expires August 26, 2007 [Page 24] Internet-Draft Basic FEC Schemes February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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