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Versions: 00 draft-ietf-fecframe-interleaved-fec-scheme
FEC Framework A. Begen
Internet-Draft Cisco Systems
Intended status: Standards Track July 7, 2008
Expires: January 8, 2009
1-D Interleaved Parity FEC Scheme for FEC Framework
draft-begen-fecframe-interleaved-fec-scheme-00
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes a Fully-Specified Forward Error Correction
(FEC) Scheme for the one-dimensional (1-D) interleaved parity code
and its application to reliable delivery of media streams in the
context of FEC Framework. The 1-D interleaved parity code is a
systematic code, 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 1-D interleaved parity code offers a good
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protection against bursty packet losses at a cost of decent
complexity. This document extends the FEC header defined in RFC 2733
and registers a new RTP payload format for the FEC that is generated
by the 1-D interleaved parity code from a source media encapsulated
in RTP. This new payload format is compatible with and used as a
part of the DVB Application-layer FEC Specification.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Overhead Computation . . . . . . . . . . . . . . . . . . . 8
1.3. Relation to Existing Specifications . . . . . . . . . . . 8
1.3.1. RFC 2733 and RFC 3009 . . . . . . . . . . . . . . . . 8
1.3.2. SMPTE 2022-1 . . . . . . . . . . . . . . . . . . . . . 8
1.3.3. ETSI TS 102 034 . . . . . . . . . . . . . . . . . . . 9
1.4. Document Outline . . . . . . . . . . . . . . . . . . . . . 9
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 10
3. Definitions, Notations and Abbreviations . . . . . . . . . . . 10
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 11
4. Formats and Codes . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Source FEC Payload ID . . . . . . . . . . . . . . . . . . 11
4.2. Repair FEC Payload ID . . . . . . . . . . . . . . . . . . 12
4.3. FEC Framework Configuration Information . . . . . . . . . 15
4.3.1. Mandatory Elements . . . . . . . . . . . . . . . . . . 16
4.3.2. Scheme-Specific Elements . . . . . . . . . . . . . . . 16
4.3.3. Encoding Format . . . . . . . . . . . . . . . . . . . 16
5. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Configuration Information Signaling Procedures . . . . . . 17
5.2. Content Delivery Protocol Requirements . . . . . . . . . . 17
5.3. Determination of Source Block Size and Repair Window . . . 17
6. 1-D Interleaved Parity FEC Code Specification . . . . . . . . 17
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Repair Packet Construction . . . . . . . . . . . . . . . . 18
6.3. Source Packet Reconstruction . . . . . . . . . . . . . . . 20
6.3.1. Associating the Source and Repair Packets . . . . . . 20
6.3.2. Recovering the RTP Header and Payload . . . . . . . . 21
7. Session Description Protocol (SDP) Signaling . . . . . . . . . 22
8. Congestion Control Considerations . . . . . . . . . . . . . . 23
9. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
10.1. Registration of FEC Encoding ID . . . . . . . . . . . . . 23
10.2. Registration of audio/1d-interleaved-parityfec . . . . . . 24
10.3. Registration of video/1d-interleaved-parityfec . . . . . . 24
10.4. Registration of text/1d-interleaved-parityfec . . . . . . 24
10.5. Registration of application/1d-interleaved-parityfec . . . 24
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . . 24
12.2. Informative References . . . . . . . . . . . . . . . . . . 25
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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1. Introduction
This document extends the FEC header defined in [RFC2733] and
registers a new RTP payload format for the FEC that is generated by
the 1-D interleaved parity code from a source media encapsulated in
RTP [RFC3550]. The type of the protected source media can be audio,
video, text or application. The FEC data is generated by an instance
of the FEC Framework, which is configured by the FEC Framework
Configuration Information. This configuration information, which is
communicated through out-of-band means, plus the information
contained in the payload format let the receiver(s) know the exact
associations/relationships between the source and repair packets.
The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation
to generate the repair symbols. In a nutshell, the following steps
take place:
o The sender determines a set of source packets to be protected
together based on the FEC Framework Configuration Information.
o The sender applies the XOR 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) along with 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,
respectively. 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. Block diagrams for the systematic parity FEC encoder
and decoder are sketched in Figure 1 and Figure 2, respectively.
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+------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder |
| (Sender) | --> +==+ +==+
+------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+
+--+ +==+
Figure 1: Block diagram for systematic parity FEC encoder
+------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder |
+==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+
Figure 2: Block diagram for systematic parity FEC decoder
Suppose that we have a group of D x L source packets that have
sequence numbers starting from 1 running to D x L. If we apply the
XOR operation to the group of the source packets whose sequence
numbers are L apart from each other as sketched in Figure 3, we
generate L repair packets. This process is referred to as 1-D
interleaved FEC protection, and the resulting L repair packets are
referred to as interleaved (or column) FEC packets.
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+-------------+ +-------------+ +-------------+ +-------+
| S_1 | | S_2 | | S3 | ... | S_L |
| S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL |
| . | | . | | | | |
| . | | . | | | | |
| . | | . | | | | |
| S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL |
+-------------+ +-------------+ +-------------+ +-------+
+ + + +
------------- ------------- ------------- -------
| XOR | | XOR | | XOR | ... | XOR |
------------- ------------- ------------- -------
= = = =
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| ... |C_L|
+===+ +===+ +===+ +===+
Figure 3: Generating interleaved (column) FEC packets
In Figure 3, S_n and C_m denote the source packet with a sequence
number n and the interleaved (column) FEC packet with a sequence
number m, respectively.
1.1. Use Cases
We generate one interleaved repair packet out of D non-consecutive
source packets. This repair packet can provide a full recovery of
the missing information if there is only one packet missing among the
corresponding source packets. This implies that 1-D interleaved FEC
protection performs well under bursty loss conditions provided that L
is chosen large enough, i.e., L-packet duration SHOULD NOT be shorter
than the duration of the burst that is intended to be repaired.
For example, consider the scenario depicted in Figure 4 where the
sender generates interleaved FEC packets and a bursty loss hits the
source packets. Since the number of columns is larger than the
number of packets lost due to the bursty loss, the repair operation
succeeds.
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+---+
| 1 | X X X
+---+
+---+ +---+ +---+ +---+
| 5 | | 6 | | 7 | | 8 |
+---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 4: Example scenario where 1-D interleaved FEC protection
succeeds error recovery
The sender may generate interleaved FEC packets to combat with the
bursty packet losses. However, two or more random packet losses may
hit the source and repair packets in the same column. In that case,
the repair operation fails. This is illustrated in Figure 5. Note
that it is possible that two or more bursty losses may occur in the
same source block, in which case interleaved FEC packets may still
fail to recover the lost data.
+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 5: Example scenario where 1-D interleaved FEC protection fails
error recovery
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1.2. Overhead Computation
The overhead is defined as the ratio of the number of bytes belonging
to the repair packets to the number of bytes belonging to the
protected source packets.
Assuming that each repair packet carries an equal number of bytes
carried by a source packet, we can compute the overhead as follows:
Overhead = 1/D
where D is the number of rows in the source block.
1.3. Relation to Existing Specifications
This section discusses the relation of the current specification to
other existing specifications.
1.3.1. RFC 2733 and RFC 3009
The current specification extends the FEC header defined in [RFC2733]
and registers a new RTP payload format. This new payload format is
not backward compatible with the payload format that was registered
by [RFC3009].
1.3.2. SMPTE 2022-1
In 2007, the Society of Motion Picture and Television Engineers
(SMPTE) - Technology Committee N26 on File Management and Networking
Technology - decided to revise the Pro-MPEG Code of Practice (CoP) #3
Release 2 specification, which (was initially produced by the Pro-
MPEG Forum in 2004) discussed the several aspects of the transmission
of MPEG-2 transport streams over IP networks. The new SMPTE
specification is referred to as [SMPTE2022-1].
The Pro-MPEG CoP #3 r2 document was originally based on [RFC2733].
SMPTE revised the document by extending the FEC header (by setting
the E bit) proposed in [RFC2733]. This extended header offers some
improvements.
For example, instead of utilizing the bitmap field used in [RFC2733],
[SMPTE2022-1] introduces separate fields to convey the number of rows
(D) and columns (L) of the source block as well as the type of the
repair packet (i.e., whether the repair packet is an interleaved FEC
packet computed over a column or a non-interleaved FEC packet
computed over a row). These fields plus the base sequence number
allow the receiver side to establish the associations between the
source and repair packets. Note that although the bitmap field is
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not utilized, the FEC header of [SMPTE2022-1] inherently carries over
the bitmap field from [RFC2733].
On the other hand, some parts of [SMPTE2022-1] are not in compliant
with RTP [RFC3550]. For example, [SMPTE2022-1] sets the SSRC field
to zero and does not use the timestamp field in the RTP headers of
the repair packets (Receivers ignore the timestamps of the repair
packets). Furthermore, [SMPTE2022-1] also sets the CC field in the
RTP header to zero and does not allow any Contributing Source (CSRC)
entry in the RTP header.
The current document adopts the extended FEC header of [SMPTE2022-1]
and registers a new RTP payload format. At the same time, this
document fixes the parts of [SMPTE2022-1] that are not in compliant
with RTP [RFC3550].
1.3.3. ETSI TS 102 034
In 2007, the Digital Video Broadcasting (DVB) consortium published a
technical specification [DVB-AL-FEC] through European
Telecommunications Standards Institute (ETSI). This specification
covers several areas related to the transmission of MPEG-2 transport
stream-based services over IP networks.
The Annex E of [DVB-AL-FEC] defines an optional protocol for
Application-layer FEC (AL-FEC) protection of streaming media for
DVB-IP services carried over RTP [RFC3550] transport. AL-FEC
protocol uses two layers for protection: a base layer that is
produced by a packet-based interleaved parity code, and an
enhancement layer that is produced by a Raptor code. While the use
of the enhancement layer is optional, the use of the base layer is
mandatory wherever AL-FEC is used.
The interleaved parity code that is used in the base layer is a
subset of [SMPTE2022-1]. In particular, AL-FEC base layer uses the
1-D interleaved FEC protection only from [SMPTE2022-1]. The new RTP
payload format that is defined and registered in this document is
compatible with the AL-FEC base layer.
1.4. Document Outline
This FEC scheme specification follows the document structure defined
in [I-D.ietf-fecframe-framework].
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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 [RFC2119].
3. Definitions, Notations and Abbreviations
The definitions, notations and abbreviations commonly used in this
document are summarized in this section.
3.1. Definitions
This document uses the following definitions. For further
definitions that apply to FEC Framework in general, see
[I-D.ietf-fecframe-framework].
Source Flow: The packet flow(s) carrying the source data and to
which FEC protection is to be applied.
Repair Flow: The packet flow(s) carrying the repair data.
Symbol: A unit of data. Its size, in bytes, is referred to as the
symbol size.
Source Symbol: The smallest unit of data used during the encoding
process.
Repair Symbol: Repair symbols are generated from the source symbols.
Source Packet: Data packets that contain only source symbols.
Repair Packet: Data packets that contain only repair symbols.
Source Block: A block of source symbols that are considered together
in the encoding process.
FEC Framework Configuration Information: Information that controls
the operation of the FEC Framework. Each FEC Framework instance has
its own configuration information.
FEC Payload ID: Information that identifies the contents of a packet
with respect to the FEC scheme.
Source FEC Payload ID: An FEC Payload ID specifically used with
source packets.
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Repair FEC Payload ID: An FEC Payload ID specifically used with
repair packets.
3.2. Notations
o L: Number of columns of the source block.
o D: Number of rows of the source block.
3.3. Abbreviations
o XOR: Bitwise exclusive OR operation.
0 XOR 0 = 0
0 XOR 1 = 1
1 XOR 0 = 1
1 XOR 1 = 0
o FSSI: FEC-Scheme-Specific Information.
o SS-FSSI: Sender-Side FEC-Scheme-Specific Information.
o RS-FSSI: Receiver-Side FEC-Scheme-Specific Information.
4. Formats and Codes
This section defines the formats of the source and repair packets as
well as the configuration information for the FEC scheme.
4.1. Source FEC Payload ID
The source packets MUST contain the information that identifies the
source block and the position within the source block occupied by the
packet. This information is referred to as the Source FEC Payload
ID. In some cases, Source FEC Payload ID may be inferred from the
fields already existing in the packet. In other cases, however, the
required information is explicitly encoded into a specific field
called Explicit Source FEC Payload ID, which is appended to the end
of the source packets [I-D.ietf-fecframe-framework].
Since the source packets that are carried within an RTP stream
already contain unique sequence numbers in their RTP headers
[RFC3550], the Source FEC Payload ID can be derived in a
straightforward manner. Thus, there is no need to use the Explicit
Source FEC Payload ID field. The primary advantage of this approach
is that the source packets are not modified in anyway. This provides
backward compatibility for the receivers that do not support FEC at
all. In multicast scenarios, this backward compatibility becomes
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quite useful as it allows the non-FEC-capable receivers to receive
and interpret the source packets.
The derivation of the Source FEC Payload ID from the RTP sequence
number is discussed in Section 5.
Editor's note: This section should specify the additional
requirements (if any) that are relevant to grouping multiple source
flows together before applying FEC protection.
4.2. Repair FEC Payload ID
The repair packets MUST contain information that identifies the
source block they pertain to and the relationship between the
contained repair symbols and the original source block. This
information is referred to as the Repair FEC Payload ID. This
information MUST be encoded into a specific field between the
transport header and the repair symbols within a repair packet, as
shown in Figure 7 [I-D.ietf-fecframe-framework].
+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| Repair FEC Payload ID |
+------------------------------+
| Repair Symbols |
+------------------------------+
Figure 7: Format of repair packets
Since the repair packets are carried within an RTP stream, the Repair
FEC Payload ID consists of an RTP header and an FEC header. This is
shown in Figure 8.
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+------------------------------+
| IP Header |
+------------------------------+
| Transport Header | ,--+------------------------------+
+------------------------------+-' | RTP Header |
| Repair FEC Payload ID | +------------------------------+
+------------------------------+-. | FEC Header |
| Repair Symbols | `--+------------------------------+
+------------------------------+
Figure 8: Format of Repair FEC Payload ID
The RTP header is formatted according to [RFC3550] with some further
clarifications listed below:
o Version: The version field is set to 2.
o Padding (P) Bit: This bit is obtained by applying protection to
the corresponding P bits from the RTP headers of the source
packets protected by this repair packet.
o Extension (X) Bit: This bit is obtained by applying protection to
the corresponding X bits from the RTP headers of the source
packets protected by this repair packet. However, an RTP header
extension is never present in a repair packet, independent of the
value of the X bit.
o CSRC Count (CC): This field is obtained by applying protection to
the corresponding CC values from the RTP headers of the source
packets protected by this repair packet. However, a CSRC list is
never present in a repair packet, independent of the value of the
CC field.
o Marker (M) Bit: This bit is obtained by applying protection to
the corresponding M bits from the RTP headers of the source
packets protected by this repair packet..
o Payload Type: The payload type for the repair packets is
determined through the payload format specified in the FEC
Framework Configuration Information. Note that this document
registers a new payload format for the repair packets (Refer to
Section 10 for details). According to [RFC3550], an RTP receiver
that cannot recognize a payload type must discard it. This
provides backward compatibility. The FEC mechanisms can then be
used in a multicast group with mixed FEC-capable and non-FEC-
capable receivers. If a non-FEC-capable receiver receives a
repair packet, it will not recognize the payload type, and hence,
discards the repair packet.
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o Sequence Number (SN): The sequence number has the standard
definition. It MUST be one higher than the sequence number in the
previously transmitted repair packet.
o Timestamp (TS): The timestamp MUST be set to the timestamp of the
source packet whose sequence number is the lowest among the source
packets protected by this repair packet.
o Synchronization Source (SSRC): The SSRC value SHALL be randomly
assigned as suggested by [RFC3550]. This allows the sender to
multiplex the source and repair flows on the same port, or
multiplex multiple repair flows on a single port. The repair
flows SHOULD use the RTCP CNAME field to associate themselves with
the source flow. Note that due to the randomness of the SSRC
assignments, there is a possibility of SSRC collision. In such
cases, the collisions MUST be resolved as described in [RFC3550].
Note that the P bit, X bit, CC field and M bit of the source packets
are protected by the corresponding bits/fields in the RTP header of
the repair packet. On the other hand, the payload of a repair packet
protects the concatenation of the CSRC list, RTP extension, payload
and padding of the source RTP packets associated with this repair
packet.
The FEC header is 16 octets. The format of the FEC header is shown
in Figure 9.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base low | Length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| PT recovery | Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|D|Type |Index| Offset | NA | SN base ext |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of the FEC header
The FEC header consists of the following fields:
o The SN base low field is used to indicate the lowest sequence
number, taking wrap around into account, of those source packets
protected by this repair packet.
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o The Length recovery field is used to determine the length of any
recovered packets.
o The E bit is the extension flag introduced in [RFC2733] and used
to extend the [RFC2733] FEC header.
o The PT recovery field is used to determine the payload type of the
recovered packets.
o The Mask field is not used.
o The TS recovery field is used to determine the timestamp of the
recovered packets.
o The N bit is the extension flag that is reserved for future uses.
o The D bit is not used.
o The Type field indicates the type of the error-correcting code
used. This document defines only one error-correcting code.
o The Index field is not used.
o The Offset and NA fields are used to indicate the number of
columns (L) and rows (D) of the source block, respectively.
o The SN base ext field is not used.
The details on setting the fields in the FEC header are provided in
Section 6.2.
It should be noted that a mask-based approach (similar to the one
specified in [RFC2733]) may not be very efficient to indicate which
source packets in the current source block are associated with a
given repair packet. In particular, for the applications that would
like to use large source block sizes, the size of the mask that is
required to describe the source-repair packet associations may be
prohibitively large. Instead, a systematic approach is inherently
more efficient.
4.3. FEC Framework Configuration Information
The FEC Framework defines a minimum set of information that MUST be
communicated between the sender and receiver(s) for a proper
operation of the FEC scheme. This information is called the FEC
Framework Configuration Information. This information specifies how
the sender applies protection to the source flow(s) and how the
repair flow(s) can be used to recover the lost data. In other words,
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this information specifies the relationship(s) between the source and
repair flows. The FEC Framework requires every FEC Framework
instance to provide its own configuration information.
From the FEC scheme point of view, the FEC Framework Configuration
Information consists of mandatory and scheme-specific elements. We
describe these elements below.
4.3.1. Mandatory Elements
o FEC Encoding ID: The value of the FEC Encoding ID for the fully-
specified FEC scheme defined in this document MUST be TBD as
assigned by IANA. Refer to Section 10.
4.3.2. Scheme-Specific Elements
FEC-Scheme-Specific Information (FSSI) includes the information that
is specific to the FEC scheme used by the Content Delivery Protocol.
FSSI is used to communicate the information that cannot be adequately
represented otherwise and is essential for proper FEC encoding and
decoding operations.
The FSSI is carried in two opaque containers. The first container
contains the FSSI required only by the sender. This information is
referred to as the Sender-Side FEC-Scheme-Specific Information (SS-
FSSI). Rest of the FSSI is referred to as the Receiver-Side FEC-
Scheme-Specific Information (RS-FSSI) and carried in the second
container.
The following parameters are carried in the FEC Scheme-Specific
Information element:
o L: Number of columns of the source block. L is a positive
integer.
o D: Number of rows of the source block. D is a positive integer.
All of the parameters listed above MUST be included in the FSSI. The
parameters L and D are carried within the SS-FSSI container.
4.3.3. Encoding Format
TBC.
5. Procedures
This section describes the procedures that are specific to the 1-D
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interleaved parity FEC scheme.
5.1. Configuration Information Signaling Procedures
This specification makes use of the signaling protocol to signal the
FEC Framework Configuration Information between the sender and
receiver(s). This enables the sender and receiver(s) to be in sync
with respect to the information needed for the operation of FEC
Framework.
5.2. Content Delivery Protocol Requirements
Content Delivery Protocol (CDP) is a complete application-protocol
specification that provides FEC capabilities by making use of the FEC
Schemes through the use of FEC Framework defined in
[I-D.ietf-fecframe-framework].
The parity FEC encoder and decoder require the following from the
CDP:
o The size of the source block, namely the number of columns (L) and
the number of rows (D).
This information is transmitted to the receiver side by the CDP
through the FEC Framework Configuration Information. The parity
encoder additionally requires:
o The data to be protected.
The parity encoder provides the following information to the CDP:
o An interleaved (column) FEC packet that is generated by applying
protection over each column in the current source block.
The source packets as well as the repair packets are then transmitted
to the receiver(s) by the transport protocol chosen by the CDP.
5.3. Determination of Source Block Size and Repair Window
TBC.
Editor's note: This section should discuss the derivation of the
Source FEC Payload ID from the RTP sequence number.
6. 1-D Interleaved Parity FEC Code Specification
This section provides a complete specification of the 1-D interleaved
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parity FEC scheme.
6.1. Overview
The following sections specify the steps involved in generating the
repair packets and reconstructing the missing source packets from the
repair packets.
6.2. Repair Packet Construction
The Repair FEC Payload ID consists of an RTP header and an FEC
header. The RTP header of an repair packet is formed based on the
guidelines given in Section 4.2.
The FEC header includes 16 octets. It is constructed by applying the
XOR operation on the bit strings that are generated from the
individual source packets protected by this particular repair packet.
The set of the source packets that are associated with a given repair
packet can be computed by the formula given in Section 6.3.1.
The bit string is formed for each source packet by concatenating the
following fields together in the order specified:
o Padding bit (1 bit) (This is the most significant bit of the bit
string)
o Extension bit (1 bit)
o CC field (4 bits)
o Marker bit (1 bit)
o PT field (7 bits)
o Timestamp (32 bits)
o Unsigned network-ordered 16-bit representation of the source
packet length in bytes minus 12 (for the fixed RTP header), i.e.,
the sum of the lengths of all the following if present: the CSRC
list, header extension, RTP payload, and RTP padding (16 bits)
o If CC is nonzero, the CSRC list (variable length)
o If X is 1, the header extension (variable length)
o Payload (variable length)
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o Padding, if present (variable length)
Note that if the payload lengths of the source packets are not equal,
each shorter packet MUST be padded to the length of the longest
packet by adding octet 0's at the end. Due to this possible padding
and mandatory FEC header, a repair packet usually has a larger size
than the source packets it protects. This may cause problems if the
resulting repair packet size exceeds the Maximum Transmission Unit
(MTU) size of the path over which the repair flow is sent.
By applying the parity operation on the bit strings produced from the
source packets, we generate the FEC bit string. Some parts of the
RTP header and the FEC header of the repair packet are generated from
the FEC bit string as follows:
o The first (most significant) bit in the FEC bit string is written
into the Padding bit in the RTP header of the repair packet.
o The next bit in the FEC bit string is written into the Extension
bit in the RTP header of the repair packet.
o The next 4 bits of the FEC bit string are written into the CC
field in the RTP header of the repair packet.
o The next bit of the FEC bit string is written into the Marker bit
in the RTP header of the repair packet.
o The next 7 bits of the FEC bit string are written into the PT
recovery field in the FEC header.
o The next 32 bits of the FEC bit string are written into the TS
recovery field in the FEC header.
o The next 16 bits are written into the Length recovery field in the
FEC header. This allows the FEC procedure to be applied even when
the lengths of the protected source packets are not identical.
o The remaining bits are set to be the payload of the repair packet.
The remaining parts of the FEC header are set as follows:
o The SN base low field MUST be set to the lowest sequence number,
taking wrap around into account, of those source packets protected
by this repair packet.
o The E bit MUST be set to 1 to extend the [RFC2733] FEC header.
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o The Mask field SHALL be set to 0 and ignored by the receiver.
o The N bit SHALL be set to 0 and ignored by the receiver.
o The D bit SHALL be set to 0 and ignored by the receiver.
o The Type field MUST be set to 0.
o The Index field SHALL be set to 0 and ignored by the receiver.
o The Offset field MUST be set to the number of columns of the
source block (L).
o The NA field MUST be set to the number of rows of the source block
(D).
o The SN base ext field SHALL be set to 0 and ignored by the
receiver.
6.3. Source Packet Reconstruction
This section describes the recovery procedures that are required to
reconstruct the missing packets. The recovery process has two steps.
In the first step, the FEC decoder determines which source and repair
packets should be used in order to recover a missing packet. In the
second step, the decoder recovers the missing packet, which consists
of an RTP header and RTP payload.
In the following, we describe the RECOMMENDED algorithms for the
first and second steps. Based on the implementation, different
algorithms MAY be adopted. However, the end result MUST be identical
to the one produced by the following algorithms.
6.3.1. Associating the Source and Repair Packets
The first step is associating the source and repair packets. The SN
base low field in the FEC header shows the lowest sequence number of
the source packets that form the particular column. In addition, the
information of how many source packets are available in each column
and row is available from the FEC Framework Configuration
Information. This set of information uniquely identifies all of the
source packets associated with a given repair packet.
Mathematically, for any received repair packet, p*, we can determine
the sequence numbers of the source packets that are protected by this
repair packet as follows:
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p*_snb + i * L
where p*_snb denotes the value in the SN base low field of p*'s FEC
header, L is the number of columns of the source block (conveyed in
the Offset field) and
0 <= i < D
where D is the number of rows of the source block (conveyed in the NA
field).
We denote the set of the source packets associated with repair packet
p* by set T(p*). Note that in a source block whose size is L columns
by D rows, set T includes D source packets. Recall that 1-D
interleaved FEC protection can fully recover the missing information
if there is only source packet is missing in set T. If the repair
packet that protects the source packets in set T is missing, or the
repair packet is available but two or more source packets are
missing, then missing source packets in set T cannot be recovered by
1-D interleaved FEC protection.
6.3.2. Recovering the RTP Header and Payload
For a given set T, the procedure for the recovery of the RTP header
of the missing packet, whose sequence number is denoted by SEQNUM, is
as follows:
1. For each of the source packets that are successfully received in
set T, compute the bit string as described in Section 6.2.
2. For the repair packet associated with set T, compute the bit
string in the same fashion except use the PT recovery field
instead of the PT field and TS recovery field instead of the
Timestamp field, and set the CSRC list, header extension, and
padding to null regardless of the values of the X bit and CC
field.
3. If any of the bit strings generated from the source packets are
shorter than the bit string generated from the repair packet,
pad them to be the same length as the bit string generated from
the repair packet. For padding, the padding of octet 0 MUST be
added at the end of the bit string.
4. Calculate the recovered bit string as the XOR of the bit strings
generated from all source packets in set T and the FEC bit
string generated from the repair packet associated with set T.
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5. Create a new packet with the standard 12-byte RTP header and no
payload.
6. Set the version of the new packet to 2.
7. Set the Padding bit in the new packet to the first bit in the
recovered bit string.
8. Set the Extension bit in the new packet to the next bit in the
recovered bit string.
9. Set the CC field to the next 4 bits in the recovered bit string.
10. Set the Marker bit in the new packet to the next bit in the
recovered bit string.
11. Set the Payload type in the new packet to the next 7 bits in the
recovered bit string.
12. Set the SN field in the new packet to SEQNUM.
13. Set the TS field in the new packet to the next 32 bits in the
recovered bit string.
14. Take the next 16 bits of the recovered bit string and set Y to
whatever unsigned integer this represents (assuming network-
order). Take Y bytes from the recovered bit string and append
them to the new packet. Y represents the length of the new
packet in bytes minus 12 (for the fixed RTP header), i.e., the
sum of the lengths of all the following if present: the CSRC
list, header extension, RTP payload and RTP padding.
15. Set the SSRC of the new packet to the SSRC of the source RTP
stream.
This procedure completely recovers both the header and payload of an
RTP packet.
7. Session Description Protocol (SDP) Signaling
This section provides an SDP [RFC4566] example. The following
example uses the SDP elements for FEC Framework, which were
introduced in [I-D.ietf-fecframe-sdp-elements], and the FEC grouping
semantics [RFC4756].
Editor's note: No FEC Encoding ID has been registered with IANA for
the FEC scheme proposed in this document. In the example below, an
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FEC Encoding ID of zero will be used.
In this example, 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
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.rocks.com
s=Interleaved Parity FEC Example
t=0 0
a=group:FEC S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127
a=rtpmap:100 MP2T/90000
a=fec-source-flow: id=0
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fec-repair-flow: encoding-id=0; ss-fssi=L:5 D:10
a=repair-window: 200
a=mid:R1
8. Congestion Control Considerations
For the general congestion control considerations related to the use
of FEC, refer to [I-D.ietf-fecframe-framework].
9. Security Considerations
For the general security considerations related to the use of FEC,
refer to [I-D.ietf-fecframe-framework].
10. IANA Considerations
10.1. Registration of FEC Encoding ID
The value of FEC Encoding ID is subject to IANA registration. For
general guidelines on IANA considerations as they apply to this
document, refer to [I-D.ietf-fecframe-framework].
This document assigns the Fully-Specified FEC Encoding ID TBD under
the ietf:fecframe:fec:encoding name-space to "1-D Interleaved Parity
FEC Code."
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10.2. Registration of audio/1d-interleaved-parityfec
TBC.
10.3. Registration of video/1d-interleaved-parityfec
TBC.
10.4. Registration of text/1d-interleaved-parityfec
TBC.
10.5. Registration of application/1d-interleaved-parityfec
TBC.
11. Acknowledgments
A major part of this document is borrowed from [RFC2733] and
[SMPTE2022-1]. Thus, the author would like to thank the authors and
editors of these earlier specifications.
12. References
12.1. Normative References
[I-D.ietf-fecframe-framework]
Watson, M., "Forward Error Correction (FEC) Framework",
draft-ietf-fecframe-framework-01 (work in progress),
November 2007.
[I-D.ietf-fecframe-sdp-elements]
Begen, A., "SDP Elements for FEC Framework",
draft-ietf-fecframe-sdp-elements-00 (work in progress),
February 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
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[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
12.2. Informative References
[RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
for Generic Forward Error Correction", RFC 2733,
December 1999.
[RFC3009] Rosenberg, J. and H. Schulzrinne, "Registration of
parityfec MIME types", RFC 3009, November 2000.
[DVB-AL-FEC]
DVB Document A086 Rev. 4 (ETSI TS 102 034 V1.3.1),
"Transport of MPEG 2 Transport Stream (TS) Based DVB
Services over IP Based Networks", March 2007.
[SMPTE2022-1]
SMPTE 2022-1-2007, "Forward Error Correction for Real-Time
Video/Audio Transport over IP Networks", 2007.
Author's Address
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
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