RFC 6681
Raptor Forward Error Correction (FEC) Schemes for FECFRAME
Pages: 22
Proposed Standard
Proposed Standard
Internet Engineering Task Force (IETF) M. Watson Request for Comments: 6681 Netflix Category: Standards Track T. Stockhammer ISSN: 2070-1721 Nomor Research M. Luby Qualcomm Incorporated August 2012 Raptor Forward Error Correction (FEC) Schemes for FECFRAMEAbstract
This document describes Fully-Specified Forward Error Correction (FEC) Schemes for the Raptor and RaptorQ codes and their application to reliable delivery of media streams in the context of the FEC Framework. The Raptor and RaptorQ codes are systematic codes, where a number of repair symbols are generated from a set of source symbols and sent in one or more repair flows in addition to the source symbols that are sent to the receiver(s) within a source flow. The Raptor and RaptorQ codes offer close to optimal protection against arbitrary packet losses at a low computational complexity. Six FEC Schemes are defined: two for the protection of arbitrary packet flows, two that are optimized for small source blocks, and two for the protection of a single flow that already contains a sequence number. Repair data may be sent over arbitrary datagram transport (e.g., UDP) or using RTP. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6681.
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Table of Contents
1. Introduction ....................................................4 2. Document Outline ................................................5 3. Requirements Notation ...........................................5 4. Definitions and Abbreviations ...................................5 4.1. Definitions ................................................6 4.2. Abbreviations ..............................................6 5. General Procedures for Raptor FEC Schemes .......................6 6. Raptor FEC Schemes for Arbitrary Packet Flows ...................8 6.1. Introduction ...............................................8 6.2. Formats and Codes ..........................................8 6.2.1. FEC Framework Configuration Information .............8 6.2.2. Source FEC Payload ID ...............................9 6.2.3. Repair FEC Payload ID ..............................10 6.3. Procedures ................................................11 6.3.1. Source Symbol Construction .........................11 6.3.2. Repair Packet Construction .........................12 6.4. FEC Code Specification ....................................12 7. Optimized Raptor FEC Scheme for Arbitrary Packet Flows .........12 7.1. Introduction ..............................................12 7.2. Formats and Codes .........................................13 7.2.1. FEC Framework Configuration Information ............13 7.2.2. Source FEC Payload ID ..............................13 7.2.3. Repair FEC Payload ID ..............................13 7.3. Procedures ................................................13 7.3.1. Source Symbol Construction .........................13 7.3.2. Repair Packet Construction .........................14 7.4. FEC Code Specification ....................................14 8. Raptor FEC Scheme for a Single Sequenced Flow ..................15 8.1. Formats and Codes .........................................15 8.1.1. FEC Framework Configuration Information ............15 8.1.2. Source FEC Payload ID ..............................15 8.1.3. Repair FEC Payload ID ..............................15 8.2. Procedures ................................................16 8.2.1. Source Symbol Construction .........................16 8.2.2. Derivation of Source FEC Packet Identification Information .........................17 8.2.3. Repair Packet Construction .........................18 8.2.4. Procedures for RTP Source Flows ....................18 8.3. FEC Code Specification ....................................18 9. Security Considerations ........................................18 10. Session Description Protocol (SDP) Signaling ..................19 11. Congestion Control Considerations .............................19 12. IANA Considerations ...........................................19 12.1. Registration of FEC Scheme IDs ...........................19 13. Acknowledgements ..............................................20 14. References ....................................................21
1. Introduction
The "Forward Error Correction (FEC) Framework" [RFC6363] describes a general framework for the use of Forward Error Correction in association with arbitrary packet flows. Modeled after the FEC Building Block developed by the IETF Reliable Multicast Transport working group [RFC5052], the FEC Framework defines the concept of FEC Schemes that provide specific Forward Error Correction Schemes. This document describes six FEC Schemes that make use of the Raptor and RaptorQ FEC codes as defined in [RFC5053] and [RFC6330]. The FEC protection mechanism is independent of the type of source data that can be an arbitrary sequence of packets, for example audio or video data. In general, the operation of the protection mechanism is as follows: o The sender determines a set of source packets (a source block) to be protected together based on the FEC Framework Configuration Information. o The sender arranges the source packets into a set of source symbols, each of which is the same size. o The sender applies the Raptor/RaptorQ protection operation on the source symbols to generate the required number of repair symbols. o The sender packetizes the repair symbols and sends the repair packet(s) and the source packets to the receiver(s). Per the FEC Framework requirements, the sender MUST transmit the source and repair packets in different source and repair flows, or in the case Real-time Transport Protocol (RTP) transport is used for repair packets, in different RTP streams. o At the receiver side, if all of the source packets are successfully received, there is no need for FEC recovery and the repair packets are discarded. However, if there are missing source packets, the repair packets can be used to recover the missing information. The operation of the FEC mechanism requires that the receiver is able to identify the relationships between received source packets and repair packets, in particular, which source packets are missing. In many cases, data already exists in the source packets that can be used to refer to source packets and to identify which packets are missing. In this case, we assume it is possible to derive a "sequence number" directly or indirectly from the source packets, and this sequence number can be used within the FEC Scheme. This case is referred to as a "single sequenced flow". In this case, the FEC
Source Payload ID defined in [RFC6363] is empty and the source packets are not modified by the application of FEC, with obvious backwards compatibility advantages. Otherwise, it is necessary to add data to the source packets for FEC purposes in the form of a non-empty FEC Source Payload ID. This is referred to as the "arbitrary packet flow" case. This document defines six FEC Schemes, two for the case of a single sequenced flow and four for the case of arbitrary packet flows.2. Document Outline
This document is organized as follows: o Section 5 defines general procedures applicable to the use of the Raptor and RaptorQ codes in the context of the FEC Framework. o Section 6 defines a FEC Scheme for the case of arbitrary source flows and follows the format defined for FEC Schemes in [RFC6363]. When used with Raptor codes, this scheme is equivalent to that defined in 3GPP TS 26.346, "Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs" [MBMSTS]. o Section 7 defines a FEC Scheme similar to that defined in Section 6 but with optimizations for the case where only limited source block sizes are required. When used with Raptor codes, this scheme is equivalent to that defined in ETSI TS 102.034, "Digital Video Broadcasting (DVB); Transport of MPEG-2 Based DVB Services over IP Based Networks" [DVBTS] for arbitrary packet flows. o Section 8 defines a FEC Scheme for the case of a single flow, which is already provided with a source packet sequence number. When used with Raptor codes, this scheme is equivalent to that defined in [DVBTS] for the case of a single sequenced flow.3. 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 [RFC2119].4. Definitions and Abbreviations
The definitions, notations, and abbreviations commonly used in this document are summarized in this section.
4.1. Definitions
The FEC-specific terminology used in this document is defined in [RFC6363]. In this document, as in [RFC6363], the first letter of each FEC-specific term is capitalized along with the new terms defined here: Symbol: A unit of data. Its size, in octets, is referred to as the symbol size. FEC Framework Configuration Information: Information that controls the operation of the FEC Framework. Each FEC Framework instance has its own configuration information.4.2. Abbreviations
This document uses abbreviations that apply to the FEC Framework in general as defined in [RFC6363]. In addition, this document uses the following abbreviations FSSI: FEC-Scheme-Specific Information. ADU: Application Data Unit ADUI: Application Data Unit Information. SPI: Source Packet Information. MSBL: Maximum Source Block Length5. General Procedures for Raptor FEC Schemes
This section specifies general procedures that apply to all Raptor and RaptorQ FEC Schemes, specifically the construction of source symbols from a set of source transport payloads. For any field defined in this document, the octets are ordered in network byte order. As described in [RFC6363], for each Application Data Unit (ADU) in a source block, the FEC Scheme is provided with: o A description of the source data flow with which the ADU is associated and an integer identifier associated with that flow. o The ADU itself. o The length of the ADU.
For each ADU, we define the Application Data Unit Information (ADUI)
as follows:
Let
o n be the number of ADUs in the source block.
o T be the source symbol size in octets. Note: this information is
provided by the FEC Scheme as defined below.
o i the index to the (i+1)-th ADU to be added to the source block,
0 <= i < n.
o f[i] denote the integer identifier associated with the source data
flow from which the i-th ADU was taken.
o F[i] denote a single octet representing the value of f[i].
o l[i] be a length indication associated with the i-th ADU -- the
nature of the length indication is defined by the FEC Scheme.
o L[i] denote two octets representing the value of l[i] in network
byte order (high order octet first) of the i-th ADU.
o R[i] denote the number of octets in the (i+1)-th ADU.
o s[i] be the smallest integer such that s[i]*T >= (l[i]+3). Note:
s[i] is the length of SPI[i] in units of symbols of size T octets.
o P[i] denote s[i]*T-(l[i]+3) zero octets. Note: P[i] are padding
octets to align the start of each UDP packet with the start of a
symbol.
o ADUI[i] be the concatenation of F[i], L[i], R[i], and P[i].
Then, a source data block is constructed by concatenating ADUI[i] for
i = 0, 1, 2, ... n-1. The source data block size, S, is then given
by sum {s[i]*T, i=0, ..., n-1}. Symbols are allocated integer
encoding symbol IDs (ESI) consecutively starting from zero within the
source block. Each ADU is associated with the ESI of the first
symbol containing SPI for that packet. Thus, the encoding symbol ID
value associated with the j-th source packet, ESI[j], is given by
ESI[j] = 0, for j=0 and ESI[j] = sum{s[i], i=0,...,(j-1)}, for
0 < j < n.
Source blocks are identified by integer Source Block Numbers. This
specification does not specify how Source Block Numbers are allocated
to the source blocks. The Source FEC Packet Identification
Information consists of the identity of the source block and the encoding symbol ID associated with the packet.6. Raptor FEC Schemes for Arbitrary Packet Flows
6.1. Introduction
This section specifies a FEC Scheme for the application of the Raptor and RaptorQ codes to arbitrary packet flows. This scheme is recommended in scenarios where maximal generality is required. When used with the Raptor codes specified in [RFC5053], this scheme is equivalent to that specified in [MBMSTS].6.2. Formats and Codes
6.2.1. FEC Framework Configuration Information
6.2.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the Fully-Specified FEC scheme defined in this section is 1 when [RFC5053] is used and 2 when [RFC6330] is used, as assigned by IANA.6.2.1.2. Scheme-Specific Elements
The scheme-specific elements of the FEC Framework Configuration information for this scheme are as follows: MSBL: The maximum source block length. A non-negative integer less than 8192 for FEC Scheme 1 and less than 56403 for FEC Scheme 2, in units of symbols. The field type is unsigned integer. T: The encoding symbol size. A non-negative integer less than 65536, in units of octets. The field type is unsigned integer. P: The payload ID format indicator. The P bit shall be set to zero to indicate payload ID format A or to one to indicate payload ID format B. The field type is unsigned integer.
An encoding format for the encoding symbol size, MSBL and payload ID format indicator is defined below. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Symbol Size (T) | MSBL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |P| Reserved | +-+-+-+-+-+-+-+-+ Figure 1: FEC-Scheme-Specific Information The P bit shall be set to zero to indicate Payload ID format A or to one to indicate Payload ID format B. The last octet of FEC-Scheme- Specific Information SHOULD be omitted, indicating that Payload ID format A is in use. The payload ID format indicator defines which of the Source FEC Payload ID and Repair FEC Payload ID formats defined below shall be used. Payload ID format B SHALL NOT be used for FEC Scheme 1. The two formats enable different use cases. Format A is appropriate in case the stream has many typically smaller source blocks, and format B is applicable if the stream has fewer large source blocks, each with many encoding symbols.6.2.2. Source FEC Payload ID
This scheme makes use of an Explicit Source FEC Payload ID, which is appended to the end of the source packets. Two formats are defined for the Source FEC Payload ID, format A and format B. The format that is used is signaled as part of the FEC Framework Configuration Information. The Source FEC Payload ID for format A is provided in Figure 2. 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 (SBN) | Encoding Symbol ID (ESI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Source FEC Payload ID - Format A Source Block Number (SBN), (16 bits): Identifier for the source block that the source data within the packet relates. The field type is unsigned integer. Encoding Symbol ID (ESI), (16 bits): The starting symbol index of the source packet in the source block. The field type is unsigned integer.
The Source FEC Payload ID for format B is provided in Figure 3. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SBN | Encoding Symbol ID (ESI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Source FEC Payload ID - Format B Source Block Number (SBN), (8 bits): Identifier for the source block that the source data within the packet relates. The field type is unsigned integer. Encoding Symbol ID (ESI), (24 bits): The starting symbol index of the source packet in the source block. The field type is unsigned integer.6.2.3. Repair FEC Payload ID
Two formats for the Repair FEC Payload ID, format A and format B, are defined below. The Repair FEC Payload ID for format A is provided in Figure 4. 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 (SBN) | Encoding Symbol ID (ESI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Block Length (SBL) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Repair FEC Payload ID - Format A Source Block Number (SBN), (16 bits): Identifier for the source block that the repair symbols within the packet relate. For format A, it is of size 16 bits. The field type is unsigned integer. Encoding Symbol ID (ESI), (16 bits): Identifier for the encoding symbols within the packet. The field type is unsigned integer. Source Block Length (SBL), (16 bits): The number of source symbols in the source block. The field type is unsigned integer.
The Repair FEC Payload ID for format B is provided in Figure 5. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SBN | Encoding Symbol ID (ESI) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Block Length (SBL) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Repair FEC Payload ID - Format B Source Block Number (SBN), (8 bits): Identifier for the source block that the repair symbols within the packet relate. For format B, it is of size 8 bits. The field type is unsigned integer. Encoding Symbol ID (ESI), (24 bits): Identifier for the encoding symbols within the packet. The field type is unsigned integer. Source Block Length (SBL), (16 bits): The number of source symbols in the source block. The field type is unsigned integer. The interpretation of the Source Block Number, encoding symbol ID, and Source Block Length is defined by the FEC Code Specification in [RFC5053] for FEC Scheme 1 and [RFC6330] for FEC Scheme 2.6.3. Procedures
6.3.1. Source Symbol Construction
FEC Scheme 1 and FEC Scheme 2 use the procedures defined in Section 5 to construct a set of source symbols to which the FEC Code can be applied. The sender MUST allocate Source Block Numbers to source blocks sequentially, wrapping around to zero after Source Block Number 65535 (format A) or 255 (format B). During the construction of the source block: o the length indication, l[i], included in the Source Packet Information for each packet shall be the transport payload length, i.e., the length of the ADU. o the value of s[i] in the construction of the Source Packet Information for each packet shall be the smallest integer such that s[i]*T >= (l[i]+3).
6.3.2. Repair Packet Construction
For FEC Scheme 1 [RFC5053], the ESI value placed into a repair packet is calculated as specified in Section 5.3.2 of [RFC5053]. For FEC Scheme 2 [RFC6330], the ESI value placed into a repair packet is calculated as specified in Section 4.4.2 of [RFC6330]. In both cases, K is identical to SBL.6.4. FEC Code Specification
The FEC encoder defined in [RFC5053] SHALL be used for FEC Scheme 1 and the FEC encoder defined in [RFC6330] SHALL be used for FEC Scheme 2. For both FEC Scheme 1 and FEC Scheme 2, the source symbols passed to the FEC encoder SHALL consist of the source symbols constructed according to Section 6.3.1. Thus, the value of the parameter K used by the FEC encoder (equal to the Source Block Length) may vary amongst the blocks of the stream but SHALL NOT exceed the Maximum Source Block Length signaled in the FEC-Scheme-Specific Information. The symbol size, T, to be used for source block construction and the repair symbol construction is equal to the encoding symbol size signaled in the FEC-Scheme-Specific Information.7. Optimized Raptor FEC Scheme for Arbitrary Packet Flows
7.1. Introduction
This section specifies a slightly modified version of the FEC Scheme specified in Section 6 that is applicable to scenarios in which only relatively small block sizes will be used. These modifications admit substantial optimizations to both sender and receiver implementations. In outline, the modifications are: o All source blocks within a stream are encoded using the same source block size. Code shortening is used to encode blocks of different sizes. This is achieved by padding every block to the required size using zero symbols before encoding. The zero symbols are then discarded after decoding. The source block size to be used for a stream is signaled in the Maximum Source Block Length (MSBL) field of the scheme-specific information. The extended source block is constructed by adding zero or more padding symbols such that the total number of symbols, MSBL, is one of the values listed in Section 7.4. Each padding symbol consists of T octets where the value of each octet is zero. MSBL
MUST be selected as the smallest value of the possible values in
Section 7.4 that is greater than or equal to K.
o The possible choices of the MSBL for a stream is restricted to a
small specified set. This allows explicit operation sequences for
encoding and decoding the restricted set of source block lengths
to be pre-calculated and embedded in software or hardware.
When used with the Raptor codes specified in [RFC5053], this scheme
is equivalent to that specified in [DVBTS] for arbitrary packet
flows.
7.2. Formats and Codes
7.2.1. FEC Framework Configuration Information
7.2.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the Fully-Specified FEC scheme
defined in this section is 3 when [RFC5053] is used and 4 when
[RFC6330] is used, as assigned by IANA.
7.2.1.2. FEC-Scheme-Specific Information
The elements for FEC Scheme 3 are the same as specified for FEC
Scheme 1, and the elements specified for FEC Scheme 4 are the same as
specified for FEC 2, as specified in Section 6.2.1.2, except that the
MSBL value is as defined in Section 7.4.
7.2.2. Source FEC Payload ID
The elements for FEC Scheme 3 are the same as specified for FEC
Scheme 1, and the elements specified for FEC Scheme 4 are the same as
specified for FEC 2, as specified in Section 6.2.2.
7.2.3. Repair FEC Payload ID
The elements for FEC Scheme 3 are the same as specified for FEC
Scheme 1, and the elements specified for FEC Scheme 4 are the same as
specified for FEC 2, as specified in Section 6.2.3.
7.3. Procedures
7.3.1. Source Symbol Construction
See Section 6.3.1.
7.3.2. Repair Packet Construction
The number of repair symbols contained within a repair packet is computed from the packet length. The ESI value placed into a repair packet is calculated as X + MSBL - SBL, where X would be the ESI value of the repair packet if the ESI were calculated as specified in Section 5.3.2 of [RFC5053] for FEC Scheme 3 and as specified in Section 4.4.2 of [RFC6330] for FEC Scheme 4, where K=SBL. The value of SBL SHALL be, at most, the value of MSBL.7.4. FEC Code Specification
The FEC encoder defined in [RFC5053] SHALL be used for FEC Scheme 3 and the FEC encoder defined in [RFC6330] SHALL be used for FEC Scheme 4. The source symbols passed to the FEC encoder SHALL consist of the source symbols constructed according to Section 6.3.1 extended with zero or more padding symbols. The extension SHALL be such that the total number of symbols in the source block is equal to the MSBL signaled in the FEC-Scheme-Specific Information. Thus, the value of the parameter K used by the FEC encoder is equal to the MSBL for all blocks of the stream. Padding symbols shall consist entirely of octets set to the value zero. The symbol size, T, to be used for the source block construction and the repair symbol construction, is equal to the encoding symbol size signaled in the FEC-Scheme-Specific Information. For FEC Scheme 3, the parameter T SHALL be set such that the number of source symbols in any source block is, at most, 8192. The MSBL parameter, and hence the number of symbols used in the FEC Encoding and Decoding operations, SHALL be set to one of the following values: 101, 120, 148, 164, 212, 237, 297, 371, 450, 560, 680, 842, 1031, 1139, 1281 For FEC Scheme 4, the parameter T SHALL be set such that the number of source symbols in any source block is less than 56403. The MSBL parameter SHALL be set to one of the supported values for K' defined in Section 5.6 of [RFC6330].
8. Raptor FEC Scheme for a Single Sequenced Flow
8.1. Formats and Codes
8.1.1. FEC Framework Configuration Information
8.1.1.1. FEC Scheme ID
The value of the FEC Scheme ID for the Fully-Specified FEC scheme defined in this section is 5 when [RFC5053] is used and 6 when [RFC6330] is used, as assigned by IANA.8.1.1.2. Scheme-Specific Elements
The elements for FEC Scheme 5 are the same as specified for FEC Scheme 1, and the elements specified for FEC Scheme 6 are the same as specified for FEC 2, as specified in Section 6.2.1.2.8.1.2. Source FEC Payload ID
The Source FEC Payload ID field is not used by this FEC Scheme. Source packets are not modified by this FEC Scheme.8.1.3. Repair FEC Payload ID
Two formats for the Repair FEC Payload ID are defined, format A and format B. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initial Sequence Number | Source Block Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encoding Symbol ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Repair FEC Payload ID - Format A Initial Sequence Number (Flow i ISN), (16 bits): This field specifies the lowest 16 bits of the sequence number of the first packet to be included in this sub-block. If the sequence numbers are shorter than 16 bits, then the received Sequence Number SHALL be logically padded with zero bits to become 16 bits in length, respectively. The field type is unsigned integer. Source Block Length (SBL), (16 bits): This field specifies the length of the source block in symbols. The field type is unsigned integer.
Encoding Symbol ID (ESI), (16 bits): This field indicates which
repair symbols are contained within this repair packet. The ESI
provided is the ESI of the first repair symbol in the packet. The
field type is unsigned integer.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Sequence Number | Source Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Repair FEC Payload ID - Format B
Initial Sequence Number (Flow i ISN), (16 bits): This field specifies
the lowest 16 bits of the sequence number in the first packet to
be included in this sub-block. If the sequence numbers are
shorter than 16 bits, then the received Sequence Number SHALL be
logically padded with zero bits to become 16 bits in length,
respectively. The field type is unsigned integer.
Source Block Length (SBL), (16 bits): This field specifies the length
of the source block in symbols. The field type is unsigned
integer.
Encoding Symbol ID (ESI); (24 bits): This field indicates which
repair symbols are contained within this repair packet. The ESI
provided is the ESI of the first repair symbol in the packet. The
field type is unsigned integer.
8.2. Procedures
8.2.1. Source Symbol Construction
FEC Scheme 5 and FEC Scheme 6 use the procedures defined in Section 5
to construct a set of source symbols to which the FEC code can be
applied.
During the construction of the source block:
o the length indication, l[i], included in the Source Packet
Information for each packet shall be dependent on the protocol
carried within the transport payload. Rules for RTP are specified
below.
o the value of s[i] in the construction of the Source Packet
Information for each packet shall be the smallest integer such
that s[i]*T >= (l[i]+3)
8.2.2. Derivation of Source FEC Packet Identification Information
The Source FEC Packet Identification Information for a source packet
is derived from the sequence number of the packet and information
received in any repair FEC packet belonging to this source block.
Source blocks are identified by the sequence number of the first
source packet in the block. This information is signaled in all
repair FEC packets associated with the source block in the Initial
Sequence Number field.
The length of the Source Packet Information (in octets) for source
packets within a source block is equal to the length of the payload
containing encoding symbols of the repair packets (i.e., not
including the Repair FEC Payload ID) for that block, which MUST be
the same for all repair packets. The Application Data Unit
Information Length (ADUIL) in symbols is equal to this length divided
by the encoding symbol size (which is signaled in the FEC Framework
Configuration Information). The set of source packets included in
the source block is determined by the Initial Sequence Number (ISN)
and Source Block Length (SBL) as follows:
Let,
o I be the Initial Sequence Number of the source block
o LP be the Source Packet Information Length in symbols
o LB be the Source Block Length in symbols
Then, source packets with sequence numbers from I to I +(LB/LP)-1
inclusive are included in the source block. The Source Block Length,
LB, MUST be chosen such that it is at least as large as the largest
Source Packet Information Length LP.
Note that if no FEC repair packets are received, then no FEC decoding
is possible, and it is unnecessary for the receiver to identify the
Source FEC Packet Identification Information for the source packets.
The encoding symbol ID for a packet is derived from the following
information:
o The sequence number, Ns, of the packet
o The Source Packet Information Length for the source block, LP
o The Initial Sequence Number of the source block, I
Then, the encoding symbol ID for the packet with sequence number Ns
is determined by the following formula:
ESI = ( Ns - I ) * LP
Note that all repair packets associated to a given source block MUST
contain the same Source Block Length and Initial Sequence Number.
Note also that the source packet flow processed by the FEC encoder
MUST have consecutive sequence numbers. In case the incoming source
packet flow has a gap in the sequence numbers, then implementors
SHOULD insert an ADU in the source block that complies to the format
of the source packet flow, but is ignored at the application with
high probability. For additional guidelines, refer to [RFC6363],
Section 10.2, paragraph 5.
8.2.3. Repair Packet Construction
See Section 7.3.2
8.2.4. Procedures for RTP Source Flows
In the specific case of RTP source packet flows, the RTP Sequence
Number field SHALL be used as the sequence number in the procedures
described above. The length indication included in the Application
Data Unit Information SHALL be the RTP payload length plus the length
of the contributing sources (CSRCs), if any, the RTP Header
Extension, if present, and the RTP padding octets, if any. Note that
this length is always equal to the UDP payload length of the packet
minus 12.
8.3. FEC Code Specification
The elements for FEC Scheme 5 are the same as specified for FEC
Scheme 3, and the elements specified for FEC Scheme 6 are the same as
specified for FEC 4, as specified in Section 7.4.
9. Security Considerations
For the general security considerations related to the use of FEC,
refer to [RFC6363]. Also consider relevant security considerations
in [RFC5053] and [RFC6330]. No security vulnerabilities specific to
this document have been identified.
10. Session Description Protocol (SDP) Signaling
This section provides an SDP [RFC4566] example. The syntax follows the definition in [RFC6364]. Assume we have one source video stream (mid:S1) and one FEC repair stream (mid:R1). We form one FEC group with the "a=group:FEC-FR S1 R1" line. The source and repair streams are sent to the same port on different multicast groups. The repair window is set to 200 ms. v=0 o=ali 1122334455 1122334466 IN IP4 fec.example.com s=Raptor FEC Example t=0 0 a=group:FEC-FR S1 R1 m=video 30000 RTP/AVP 100 c=IN IP4 233.252.0.1/127 a=rtpmap:100 MP2T/90000 a=fec-source-flow: id=0 a=mid:S1 m=application 30000 UDP/FEC c=IN IP4 233.252.0.2/127 a=fec-repair-flow: encoding-id=6; fssi=Kmax:8192,T:128,P:A a=repair-window:200ms a=mid:R111. Congestion Control Considerations
For the general congestion control considerations related to the use of FEC, refer to [RFC6363].12. IANA Considerations
12.1. Registration of FEC Scheme IDs
The value of FEC Scheme IDs is subject to IANA registration. For general guidelines on IANA considerations as they apply to this document, refer to [RFC6363]. This document registers six values in the "FEC Framework (FECFRAME) FEC Encoding IDs" registry (http://www.iana.org/assignments/ rmt-fec-parameters/) as provided in Table 1. Each value refers to a Fully-Specified FEC scheme.
+----------+---------------------+----------------------------------+ | FEC | FEC Scheme | Reference | | Encoding | Description | | | ID | | | +----------+---------------------+----------------------------------+ | 1 | Raptor FEC Scheme | Section 6 in this document using | | | for Arbitrary | [RFC5053] | | | Packet Flows | | +----------+---------------------+----------------------------------+ | 2 | RaptorQ FEC Scheme | Section 6 in this document using | | | for Arbitrary | [RFC6330]. | | | Packet Flows | | +----------+---------------------+----------------------------------+ | 3 | Raptor FEC Scheme | Section 7 in this document using | | | Optimized for | Raptor [RFC5053]. | | | Arbitrary Packet | | | | Flows | | +----------+---------------------+----------------------------------+ | 4 | RaptorQ FEC Scheme | Section 7 in this document | | | Optimized for | using RaptorQ [RFC6330]. | | | Arbitrary Packet | | | | Flows | | +----------+---------------------+----------------------------------+ | 5 | Raptor FEC Scheme | Section 8 in this document using | | | for a Single | Raptor [RFC5053]. | | | Sequence Flow | | +----------+---------------------+----------------------------------+ | 6 | RaptorQ FEC Scheme | Section 8 in this document using | | | for a Single | RaptorQ [RFC6330]. | | | Sequence Flow | | +----------+---------------------+----------------------------------+ Table 1: FEC Framework (FECFRAME) FEC Encoding IDs13. Acknowledgements
Thanks are due to Ali C. Begen and David Harrington for thorough review of earlier draft versions of this document.
14. References
14.1. Normative References
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error Correction (FEC) Framework", RFC 6363, October 2011. [RFC5053] Luby, M., Shokrollahi, A., Watson, M., and T. Stockhammer, "Raptor Forward Error Correction Scheme for Object Delivery", RFC 5053, October 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC6330] Luby, M., Shokrollahi, A., Watson, M., Stockhammer, T., and L. Minder, "RaptorQ Forward Error Correction Scheme for Object Delivery", RFC 6330, August 2011.14.2. Informative References
[RFC5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, August 2007. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC6364] Begen, A., "Session Description Protocol Elements for the Forward Error Correction (FEC) Framework", RFC 6364, October 2011. [DVBTS] ETSI, "Digital Video Broadcasting (DVB); Transport of MPEG-2 Based DVB Services over IP Based Networks", ETSI TS 102 034, March 2009. [MBMSTS] 3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs", 3GPP TS 26.346, April 2005.
Authors' Addresses
Mark Watson Netflix 100 Winchester Circle Los Gatos, CA 95032 United States EMail: watsonm@netflix.com Thomas Stockhammer Nomor Research Brecherspitzstrasse 8 Munich 81541 Germany EMail: stockhammer@nomor.de Michael Luby Qualcomm Research Berkeley 2030 Addison Street Berkeley, CA 94704 United States EMail: luby@qualcomm.com