Internet DRAFT - draft-ietf-fecframe-raptor
draft-ietf-fecframe-raptor
FEC Framework M. Watson
Internet-Draft Netflix
Intended status: Standards Track T. Stockhammer
Expires: November 11, 2012 Nomor Research
M. Luby
Qualcomm Incorporated
May 10, 2012
Raptor FEC Schemes for FECFRAME
draft-ietf-fecframe-raptor-11
Abstract
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 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 protection of arbitrary packet flows,
two that are optimised for small source blocks and another two for
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 11, 2012.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Document Outline . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements Notation . . . . . . . . . . . . . . . . . . . . 6
4. Definitions and Abbreviations . . . . . . . . . . . . . . . . 6
4.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 7
5. General procedures for Raptor FEC Schemes . . . . . . . . . . 7
6. Raptor FEC Schemes for arbitrary packet flows . . . . . . . . 9
6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 9
6.2.1. FEC Framework Configuration Information . . . . . . . 9
6.2.2. Source FEC Payload ID . . . . . . . . . . . . . . . . 10
6.2.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 11
6.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3.1. Source symbol construction . . . . . . . . . . . . . . 13
6.3.2. Repair packet construction . . . . . . . . . . . . . . 13
6.4. FEC Code Specification . . . . . . . . . . . . . . . . . . 13
7. Optimised Raptor FEC Scheme for arbitrary packet flows . . . . 13
7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Formats and Codes . . . . . . . . . . . . . . . . . . . . 14
7.2.1. FEC Framework Configuration Information . . . . . . . 14
7.2.2. Source FEC Payload ID . . . . . . . . . . . . . . . . 15
7.2.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 15
7.3. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3.1. Source symbol construction . . . . . . . . . . . . . . 15
7.3.2. Repair packet construction . . . . . . . . . . . . . . 15
7.4. FEC Code Specification . . . . . . . . . . . . . . . . . . 15
8. Raptor FEC Scheme for a single sequenced flow . . . . . . . . 16
8.1. Formats and codes . . . . . . . . . . . . . . . . . . . . 16
8.1.1. FEC Framework Configuration Information . . . . . . . 16
8.1.2. Source FEC Payload ID . . . . . . . . . . . . . . . . 16
8.1.3. Repair FEC Payload ID . . . . . . . . . . . . . . . . 16
8.2. Procedures . . . . . . . . . . . . . . . . . . . . . . . . 18
8.2.1. Source symbol construction . . . . . . . . . . . . . . 18
8.2.2. Derivation of Source FEC Packet Identification
Information . . . . . . . . . . . . . . . . . . . . . 18
8.2.3. Repair packet construction . . . . . . . . . . . . . . 19
8.2.4. Procedures for RTP source flows . . . . . . . . . . . 19
8.3. FEC Code Specification . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Session Description Protocol (SDP) Signaling . . . . . . . . . 20
11. Congestion Control Considerations . . . . . . . . . . . . . . 21
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12.1. Registration of FEC Scheme IDs . . . . . . . . . . . . . . 21
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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14.1. Normative References . . . . . . . . . . . . . . . . . . . 22
14.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 which provide specific Forward Error Correction schemes.
This document describes six FEC Schemes which 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 the source
data, which 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 can
identify the relationships between received source packets and repair
packets and in particular which source packets are missing. In many
cases, data already exists in the source packets which 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
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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 case
is referred to as the "arbitrary packet flow" case. Accordingly,
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 organised 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 an 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 an FEC Scheme similar to that defined in
Section 6 but with optimisations 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 an 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
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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 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 FEC Framework in
general as defined in [RFC6363]. In addition, this document uses the
following abbreviations
FSSI: FEC-Scheme-Specific Information.
SS-FSSI: Sender-Side FEC-Scheme-Specific Information.
RS-FSSI: Receiver-Side FEC-Scheme-Specific Information.
ADU: Application Data Unit
ADUI: Application Data Unit Information.
SPI: Source Packet Information.
MSBL: Maximum Source Block Length
5. General procedures for Raptor FEC Schemes
This section specifies general procedures which 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:
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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 consecutively starting from zero within the
source block. Each ADU is associated with the Encoding Symbol ID of
the first symbol containing SPI for that packet. Thus, the Encoding
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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 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 an 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 XXX1 when [RFC5053] is used and XXX2 when
[RFC6330] is used, as assigned by IANA.
NOTE: To the RFC Editor: please change these XXX notations once
assigned, and remove this NOTE.
6.2.1.2. Scheme-Specific Elements
The scheme-specific elements of the FEC Framework Configuration
information for this scheme are as follows:
MSBL Value range: An non-negative integer less than 8192 for FEC
Scheme XXX1 and less than 56403 for FEC Scheme XXX2, in units of
symbols. The field type is unsigned integer.
Encoding Symbol Size Name: "T", Value range: A non-negative
integer less than 65536, in units of octets. The field type is
unsigned integer.
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Payload ID Format Name: "P", Value range: "A" or "B". The P bit
shall be set to zero to indicate Payload ID Format A or to one to
indicate Payload ID Format B.
An encoding format for the MSBL and Encoding Symbol Size 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 identifier 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 XXX1. 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.
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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.
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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
Identifier and Source Block Length is defined by the FEC Code
Specification in [RFC5053] for FEC Scheme XXX1 and [RFC6330] for FEC
Scheme XXX2.
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6.3. Procedures
6.3.1. Source symbol construction
FEC Scheme XXX1 and FEC Scheme XXX2 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 XXX1, the ESI value placed into a repair packet is
calculated as specified in Section 5.3.2 of [RFC5053].
For FEC Scheme XXX2 [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 FEC Scheme XXX1
and the FEC encoder defined in [RFC6330] SHALL be used for FEC Scheme
XXX2. For both FEC Scheme XXX1 and FEC Scheme XXX2, 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. Optimised Raptor FEC Scheme for arbitrary packet flows
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7.1. Introduction
This section specifies a slightly modified version of the FEC Scheme
specified in Section 6 which is applicable to scenarios in which only
relatively small block sizes will be used. These modifications admit
substantial optimisations 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 XXX3 when [RFC5053] is used and XXX4 when
[RFC6330] is used, as assigned by IANA.
NOTE: To the RFC Editor: please change these XXX notations once
assigned, and remove this NOTE.
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7.2.1.2. FEC Scheme specific information
The same as specified for FEC Scheme XXX1 for FEC Scheme XXX3, and
the same as specified for FEC Scheme XXX2 for FEC Scheme XXX4, 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 same as specified for FEC Scheme XXX1 for FEC Scheme XXX3, and
the same as specified for FEC Scheme XXX2 for FEC Scheme XXX4, as
specified in Section 6.2.2.
7.2.3. Repair FEC Payload ID
The same as specified for FEC Scheme XXX1 for FEC Scheme XXX3, and
the same as specified for FEC Scheme XXX2 for FEC Scheme XXX4, 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 XXX3 and as specified in
Section 4.4.2 of [RFC6330] for FEC Scheme XXX4, 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
XXX3 and the FEC encoder defined in [RFC6330] SHALL be used for FEC
Scheme XXX4. 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 encoded 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
source block construction and the repair symbol construction is equal
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to the Encoding Symbol Size signaled in the FEC Scheme Specific
Information.
For FEC Scheme XXX3, 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 XXX4, 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 XXX5 when [RFC5053] is used and XXX6 when
[RFC6330] is used, as assigned by IANA.
NOTE: To the RFC Editor: please change these XXX notations once
assigned, and remove this NOTE.
8.1.1.2. Scheme-specific elements
The same as specified for FEC Scheme XXX1 for FEC Scheme XXX5, and
the same as specified for FEC Scheme XXX2 for FEC Scheme XXX6, 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.
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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 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.
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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 XXX5 and FEC Scheme XXX6 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 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 which are included in the
source block is determined from the Initial Sequence Number (ISN) and
Source Block Length (SBL) as follows:
Let,
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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 packet with sequence number Ns is
determined by the following formula:
ESI = ( Ns - I ) * LP
Note that all repair packet 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, then the RTP
Sequence Number field SHALL be used as the sequence number in the
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procedures described above. The length indication included in the
Application Data Unit Information SHALL be the RTP payload length
plus the length of the 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 same as specified for FEC Scheme XXX3 for FEC Scheme XXX5, and
the same as specified for FEC Scheme XXX4 for FEC Scheme XXX6, 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:R1
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11. 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/rmt-fec-parameters.xml#fecframe-fec-encoding-ids)
as provided in Table 1. Each value refers to a fully-specified FEC
scheme.
NOTE: To the RFC Editor: please change these XXX notations once
assigned, and remove this NOTE.
+----------+---------------------+----------------------------------+
| FEC | FEC Scheme | Reference |
| Encoding | Description | |
| ID | | |
+----------+---------------------+----------------------------------+
| XXX1 | Raptor FEC Scheme | Section 6 in this document using |
| | for Arbitrary | [RFC5053] |
| | Packet Flows | |
+----------+---------------------+----------------------------------+
| XXX2 | RaptorQ FEC Scheme | Section 6 in this document using |
| | for Arbitrary | [RFC6330]. |
| | Packet Flows | |
+----------+---------------------+----------------------------------+
| XXX3 | Raptor FEC Scheme | Section 7 in this document using |
| | Optimised for | Raptor [RFC5053]. |
| | Arbitrary Packet | |
| | Flows | |
+----------+---------------------+----------------------------------+
| XXX4 | RaptorQ FEC Scheme | XXX4 for the Optimised RaptorQ |
| | Optimised for | FEC Scheme for Arbitrary Packet |
| | Arbitrary Packet | Flows (Section 7) using RaptorQ |
| | Flows | [RFC6330]. |
+----------+---------------------+----------------------------------+
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+----------+---------------------+----------------------------------+
| XXX5 | Raptor FEC Scheme | XXX5 for the Raptor FEC Scheme |
| | for a single | for a single sequence flow |
| | sequence flow | (Section 8) using Raptor |
| | | [RFC5053]. |
+----------+---------------------+----------------------------------+
| XXX6 | RaptorQ FEC Scheme | XXX6 for the RaptorQ FEC Scheme |
| | for a single | for a single sequence flow |
| | sequence flow | (Section 8) using RaptorQ |
| | | [RFC6330]. |
+----------+---------------------+----------------------------------+
Table 1: FEC Framework (FECFRAME) FEC Encoding IDs
13. 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
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Forward Error Correction (FEC) Framework", RFC 6364,
October 2011.
[dvbts] "ETSI TS 102 034 - Digital Video Broadcasting (DVB);
Transport of MPEG-2 Based DVB Services over IP Based
Networks", March 2005.
[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
U.S.A.
Email: watsonm@netflix.com
Thomas Stockhammer
Nomor Research
Brecherspitzstrasse 8
Munich 81541
Germany
Email: stockhammer@nomor.de
Michael Luby
Qualcomm Incorporated
3165 Kifer Road
Santa Clara, CA 95051
U.S.A.
Email: luby@qualcomm.com
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