FEC Framework A. Begen
Internet-Draft Cisco Systems
Intended status: Standards Track May 24, 2009
Expires: November 25, 2009
RTP Payload Format for Non-Interleaved and Interleaved Parity FEC
draft-ietf-fecframe-1d2d-parity-scheme-01
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Abstract
This document defines new RTP payload formats for the Forward Error
Correction (FEC) that is generated by the non-interleaved and
interleaved parity codes from a source media encapsulated in RTP.
These parity codes are systematic codes, where a number of repair
symbols are generated from a set of source symbols and sent in a
repair flow separate from the source flow that carries the source
symbols. The non-interleaved and interleaved parity codes offer a
good protection against random and bursty packet losses,
respectively, at a cost of decent complexity. The RTP payload
formats that are defined in this document address the scalability
issues experienced with the earlier specifications including RFC
2733, RFC 5109 and SMPTE 2022-1, and offer several improvements. Due
to these changes, the new payload formats are not backward compatible
with the earlier specifications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Use Cases for 1-D FEC Protection . . . . . . . . . . . . . 7
1.2. Use Cases for 2-D Parity FEC Protection . . . . . . . . . 9
1.3. Overhead Computation . . . . . . . . . . . . . . . . . . . 11
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 11
3. Definitions, Notations and Abbreviations . . . . . . . . . . . 11
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Repair Packets . . . . . . . . . . . . . . . . . . . . . . 12
5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 15
5.1. Media Type Registration . . . . . . . . . . . . . . . . . 15
5.1.1. Registration of audio/non-interleaved-parityfec . . . 16
5.1.2. Registration of video/non-interleaved-parityfec . . . 17
5.1.3. Registration of text/non-interleaved-parityfec . . . . 18
5.1.4. Registration of
application/non-interleaved-parityfec . . . . . . . . 19
5.1.5. Registration of audio/interleaved-parityfec . . . . . 20
5.1.6. Registration of video/interleaved-parityfec . . . . . 21
5.1.7. Registration of text/interleaved-parityfec . . . . . . 23
5.1.8. Registration of application/interleaved-parityfec . . 24
5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 25
5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 26
5.2.2. Declarative Considerations . . . . . . . . . . . . . . 26
6. Protection and Recovery Procedures . . . . . . . . . . . . . . 27
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.2. Repair Packet Construction . . . . . . . . . . . . . . . . 27
6.3. Source Packet Reconstruction . . . . . . . . . . . . . . . 29
6.3.1. Associating the Source and Repair Packets . . . . . . 29
6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 30
6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . . 31
6.3.4. Iterative Decoding Algorithm for the 2-D Parity
FEC Protection . . . . . . . . . . . . . . . . . . . . 32
7. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.1. Example SDP for 1-D Parity FEC Protection . . . . . . . . 34
7.2. Example SDP for 2-D Parity FEC Protection . . . . . . . . 35
8. Congestion Control Considerations . . . . . . . . . . . . . . 35
9. Security Considerations . . . . . . . . . . . . . . . . . . . 36
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37
12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 37
12.1. draft-ietf-fecframe-1d2d-parity-scheme-01 . . . . . . . . 37
12.2. draft-ietf-fecframe-1d2d-parity-scheme-00 . . . . . . . . 37
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1. Normative References . . . . . . . . . . . . . . . . . . . 37
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13.2. Informative References . . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39
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1. Introduction
This document defines new RTP payload formats for the FEC that is
generated by the non-interleaved and interleaved parity codes from a
source media encapsulated in RTP [RFC3550]. The type of the source
media protected by these parity codes can be audio, video, text or
application. The FEC data are generated according to the media type
parameters that are communicated through out-of-band means. The
associations/relationships between the source and repair flows are
also communicated through out-of-band means.
Both the non-interleaved and interleaved parity codes use the
exclusive OR (XOR) operation to generate the repair symbols. In a
nutshell, the following steps take place:
1. The sender determines a set of source packets to be protected
together based on the media type parameters.
2. The sender applies the XOR operation on the source symbols to
generate the required number of repair symbols.
3. The sender packetizes the repair symbols and sends the repair
packet(s) along with the source packets to the receiver(s) (in
different flows). The repair packets MAY be sent proactively or
on-demand.
Note that the source and repair packets belong to different source
and repair flows, and the sender MUST provide a way for the receivers
to demultiplex them, even in the case they are sent in the same
transport flow (i.e., same source/destination address/port with UDP).
This is required to offer backward compatibility (See Section 4). 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
In Figure 2, it is clear that the FEC packets have to be received by
the receiver within a certain time to be useful in the FEC recovery
process. In this document, we refer to the time that spans the
source packets and the corresponding repair packets as the repair
window. Assuming that there is no issue of delay variation, the FEC
decoder SHOULD NOT wait longer than the repair window since
additional waiting would not help the recovery process. The size of
the repair window depends on the source block size and the regime
adopted for sending the repair packets.
Suppose that we have a group of D x L source packets that have
sequence numbers starting from 1 running to D x L, and a repair
packet is generated by applying the XOR operation to every L
consecutive packets as sketched in Figure 3. This process is
referred to as 1-D non-interleaved FEC protection. As a result of
this process, D repair packets are generated, which we refer to as
non-interleaved (or row) FEC packets.
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+--------------------------------------------------+ --- +===+
| S_1 S_2 S3 ... S_L | + |XOR| = |R_1|
+--------------------------------------------------+ --- +===+
+--------------------------------------------------+ --- +===+
| S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2|
+--------------------------------------------------+ --- +===+
. . . . . .
. . . . . .
. . . . . .
+--------------------------------------------------+ --- +===+
| S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D|
+--------------------------------------------------+ --- +===+
Figure 3: Generating non-interleaved (row) FEC packets
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 4, 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.
+-------------+ +-------------+ +-------------+ +-------+
| 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 4: Generating interleaved (column) FEC packets
1.1. Use Cases for 1-D FEC Protection
We generate one non-interleaved repair packet out of L consecutive
source packets and one interleaved repair packet out of D non-
consecutive source packets. Regardless of whether the repair packet
is a non-interleaved or an interleaved one, it can provide a full
recovery of the missing information if there is only one packet
missing among the corresponding source packets. This implies that
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1-D non-interleaved FEC protection performs better when the source
packets are randomly lost. However, if the packet losses occur in
bursts, 1-D interleaved FEC protection performs better provided that
L is chosen large enough, i.e., L-packet duration SHOULD NOT be
shorter than the observed burst duration. The sender SHOULD monitor
the occurrences of the loss events on the source packets and generate
non-interleaved and interleaved FEC packets when the losses occur
randomly and in bursts, respectively.
If the sender generates non-interleaved FEC packets and a burst loss
hits the source packets, the repair operation fails. This is
illustrated in Figure 5.
+---+ +---+ +===+
| 1 | X X | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 9 | | 10| | 11| | 12| |R_3|
+---+ +---+ +---+ +---+ +===+
Figure 5: Example scenario where 1-D non-interleaved FEC protection
fails 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 6. Note
that it is possible that two burst losses may occur back-to-back, in
which case interleaved FEC packets may still fail to recover the lost
data.
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+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 6: Example scenario where 1-D interleaved FEC protection fails
error recovery
1.2. Use Cases for 2-D Parity FEC Protection
In networks where the source packets are lost both randomly and in
bursts, the sender may generate both non-interleaved and interleaved
FEC packets. This type of FEC protection is known as 2-D parity FEC
protection. At the expense of generating more FEC packets, thus
increasing the FEC overhead, 2-D FEC provides a superior protection
against mixed loss patterns. However, 2-D parity FEC protection is
still not hitless and may fail to recover all of the lost source
packets if a particular loss pattern hits the source packets. An
example scenario is illustrated in Figure 7.
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+---+ +---+ +===+
| 1 | X X | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +===+
| 9 | X X | 12| |R_3|
+---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 7: Example scenario #1 where 2-D parity FEC protection fails
error recovery
2-D parity FEC protection also fails when at least two rows are
missing a source and the FEC packet and the missing source packets
(in at least two rows) are aligned in the same column. An example
loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC
protection cannot repair all missing source packets when at least two
columns are missing a source and the FEC packet and the missing
source packets (in at least two columns) are aligned in the same row.
+---+ +---+ +---+
| 1 | | 2 | X | 4 | X
+---+ +---+ +---+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+
| 9 | | 10| X | 12| X
+---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 8: Example scenario #2 where 2-D parity FEC protection fails
error recovery
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1.3. 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.
Generally, repair packets are larger in size compared to the source
packets. Also, not all the source packets are necessarily equal in
size. However, if we assume that each repair packet carries an equal
number of bytes carried by a source packet, we can compute the
overhead for different FEC protection methods as follows:
o 1-D Non-interleaved FEC Protection: Overhead = 1/L
o 1-D Interleaved FEC Protection: Overhead = 1/D
o 2-D Parity FEC Protection: Overhead = 1/L + 1/D
where L and D are the number of columns and rows in the source block,
respectively.
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 and notations commonly used in this document are
summarized in this section.
3.1. Definitions
This document uses the following definitions:
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.
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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.
3.2. Notations
o L: Number of columns of the source block.
o D: Number of rows of the source block.
o ToP: Type of protection.
4. Packet Formats
This section defines the formats of the source and repair packets.
4.1. Source Packets
The source packets MUST contain the information that identifies the
source block and the position within the source block occupied by the
packet. Since the source packets that are carried within an RTP
stream already contain unique sequence numbers in their RTP headers
[RFC3550], we can identify the source packets in a straightforward
manner and there is no need to append additional field(s). The
primary advantage of not modifying the source packets in any way is
that it provides backward compatibility for the receivers that do not
support FEC at all. In multicast scenarios, this backward
compatibility becomes quite useful as it allows the non-FEC-capable
and FEC-capable receivers to receive and interpret the same source
packets sent in the same multicast session.
4.2. Repair Packets
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. For this
purpose, we use the RTP header of the repair packets as well as
another header within the RTP payload, which we refer to as the FEC
header, as shown in Figure 9.
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+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header | __
+------------------------------+ |
| FEC Header | \
+------------------------------+ > RTP Payload
| Repair Symbols | /
+------------------------------+ __|
Figure 9: Format of repair packets
The RTP header is formatted according to [RFC3550] with some further
clarifications listed below:
o Marker (M) Bit: This bit is not used for this payload type, and
SHALL be set to 0.
o Payload Type: The (dynamic) payload type for the repair packets
is determined through out-of-band means. Note that this document
registers new payload formats for the repair packets (Refer to
Section 5 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,
will discard the repair packet.
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. The initial value of the
sequence number SHOULD be random (unpredictable) [RFC3550].
o Timestamp (TS): The timestamp SHALL be set to a time
corresponding to the repair packet's transmission time. Note that
the timestamp value has no use in the actual FEC protection
process and is usually useful for jitter calculations.
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.
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In some networks, the RTP Source, which produces the source
packets and the FEC Source, which generates the repair packets
from the source packets may not be the same host. In such
scenarios, using the same CNAME for the source and repair flows
means that the RTP Source and the FEC Source MUST share the same
CNAME (for this specific source-repair flow association). A
common CNAME may be produced based on an algorithm that is known
both to the RTP and FEC Source. This usage is compliant with
[RFC3550].
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].
The FEC header is 12 octets (or 16 octets when the optional padding
is used). The format of the FEC header is shown in Figure 10.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|I|P|X| CC |M| PT recovery | SN base |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length recovery | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Format of the FEC header
The FEC header consists of the following fields:
o The E bit is the extension flag reserved to indicate any future
extension to this specification.
o The I bit is used to indicate the length of padding in the FEC
header. The padding length SHOULD be selected based on the
platform architecture and the impact of header length on the
header processing performance.
o The P, X, CC, M and PT recovery fields are used to determine the
corresponding fields of the recovered packets.
o The SN base 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 TS recovery field is used to determine the timestamp of the
recovered packets.
o The Length recovery field is used to determine the length of the
recovered packets.
o The Padding field is used to pad the FEC header to 12 bytes
(integer multiples of 32 bits).
o The second (optional) Padding field is used to pad the FEC header
to 16 bytes (integer multiples of 64 bits).
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 ones
specified in [RFC2733] and [RFC5109]) 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
systematized approach similar to the one proposed in [SMPTE2022-1] is
inherently more efficient. Yet, [SMPTE2022-1] carries the values of
D and L in 8-bit fields. While this approach can support larger
blocks compared to the mask-based approaches, 8-bit fields may still
be limiting when a high-bitrate source flow (e.g., a flow carrying
Ultra HD video) is to be protected or when network outages/lossy
periods span more than 255 packets.
5. Payload Format Parameters
This section provides the media subtype registration for the non-
interleaved and interleaved parity FEC. The parameters that are
required to configure the FEC encoding and decoding operations are
also defined in this section.
5.1. Media Type Registration
This registration is done using the template defined in [RFC4288] and
following the guidance provided in [RFC3555].
Note to the RFC Editor: In the following sections, please replace
"XXXX" with the number of this document prior to publication as an
RFC.
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5.1.1. Registration of audio/non-interleaved-parityfec
Type name: audio
Subtype name: non-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
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Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.2. Registration of video/non-interleaved-parityfec
Type name: video
Subtype name: non-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
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data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.3. Registration of text/non-interleaved-parityfec
Type name: text
Subtype name: non-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
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Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.4. Registration of application/non-interleaved-parityfec
Type name: application
Subtype name: non-interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
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o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.5. Registration of audio/interleaved-parityfec
Type name: audio
Subtype name: interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.6. Registration of video/interleaved-parityfec
Type name: video
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Subtype name: interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
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Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.7. Registration of text/interleaved-parityfec
Type name: text
Subtype name: interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
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Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.1.8. Registration of application/interleaved-parityfec
Type name: application
Subtype name: interleaved-parityfec
Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
than 1000 Hz to provide sufficient resolution to RTCP operations.
However, it is RECOMMENDED to select the rate that matches the
rate of the protected source RTP stream.
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.
o ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value of
3 is reserved for future uses.
o repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.
Optional parameters: None.
Encoding considerations: This media type is framed (See Section 4.8
in the template document [RFC4288]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
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Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant
data in addition to the source media.
Additional information: None.
Person & email address to contact for further information: Ali Begen
<abegen@cisco.com> and IETF Audio/Video Transport Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550].
Author: Ali Begen <abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.
5.2. Mapping to SDP Parameters
Applications that are using RTP transport commonly use Session
Description Protocol (SDP) [RFC4566] to describe their RTP sessions.
The information that is used to specify the media types in an RTP
session has specific mappings to the fields in an SDP description.
In this section, we provide these mappings for the media subtypes
registered by this document. Note that if an application does not
use SDP to describe the RTP sessions, an appropriate mapping must be
defined and used to specify the media types and their parameters for
the control/description protocol employed by the application.
The mapping of the media type specification for "non-interleaved-
parityfec" and "interleaved-parityfec" and their parameters in SDP is
as follows:
o The media type (e.g., "application") goes into the "m=" line as
the media name.
o The media subtype goes into the "a=rtpmap" line as the encoding
name. The RTP clock rate parameter ("rate") also goes into the
"a=rtpmap" line as the clock rate.
o The remaining required payload-format-specific parameters go into
the "a=fmtp" line by copying them directly from the media type
string as a semicolon-separated list of parameter=value pairs.
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SDP examples are provided in Section 7.
5.2.1. Offer-Answer Model Considerations
When offering 1-D interleaved parity FEC over RTP using SDP in an
Offer/Answer model [RFC3264], the following considerations apply:
o Each combination of the L and D parameters produces a different
FEC data and is not compatible with any other combination. A
sender application may desire to offer multiple offers with
different sets of L and D values as long as the parameter values
are valid. The receiver SHOULD normally choose the offer that has
a sufficient amount of interleaving. If multiple such offers
exist, the receiver may choose the offer that has the lowest
overhead or the one that requires the smallest amount of
buffering. The selection depends on the application requirements.
o The value for the repair-window parameter depends on the L and D
values and cannot be chosen arbitrarily. More specifically, L and
D values determine the lower limit for the repair-window size.
The upper limit of the repair-window size does not depend on the L
and D values.
o Although combinations with the same L and D values but with
different repair-window sizes produce the same FEC data, such
combinations are still considered different offers. The size of
the repair-window is related to the maximum delay between the
transmission of a source packet and the associated repair packet.
This directly impacts the buffering requirement on the receiver
side and the receiver must consider this when choosing an offer.
o There are no optional format parameters defined for this payload.
Any unknown option in the offer MUST be ignored and deleted from
the answer. If FEC is not desired by the receiver, it can be
deleted from the answer.
5.2.2. Declarative Considerations
In declarative usage, like SDP in the Real-time Streaming Protocol
(RTSP) [RFC2326] or the Session Announcement Protocol (SAP)
[RFC2974], the following considerations apply:
o The payload format configuration parameters are all declarative
and a participant MUST use the configuration that is provided for
the session.
o More than one configuration may be provided (if desired) by
declaring multiple RTP payload types. In that case, the receivers
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should choose the repair flow that is best for them.
6. Protection and Recovery Procedures
This section provides a complete specification of the 1-D and 2-D
parity codes and their RTP payload formats.
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 RTP header of a repair packet is formed based on the guidelines
given in Section 4.2.
The FEC header includes 12 octets (or 16 octets when the optional
padding is used). 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 The first 64 bits of the RTP header (64 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, extension header, RTP payload and RTP padding (16 bits).
By applying the parity operation on the bit strings produced from the
source packets, we generate the FEC bit string. The FEC header is
generated from the FEC bit string as follows:
o The first (most significant) 2 bits in the FEC bit string are
skipped. The E bit in the FEC header is set to 0. The I bit in
the FEC header is set to 0 if only 2-byte padding is used, or to 1
if 6-byte padding is used.
o The next bit in the FEC bit string is written into the P recovery
bit in the FEC header.
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o The next bit in the FEC bit string is written into the X recovery
bit in the FEC header.
o The next 4 bits of the FEC bit string are written into the CC
recovery field in the FEC header.
o The next bit is written into the M recovery bit in the FEC header.
o The next 7 bits of the FEC bit string are written into the PT
recovery field in the FEC header.
o The next 16 bits are skipped.
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.
o The 2-byte padding field of the FEC header SHALL be set to 0.
o If the I bit is set to 1, indicating that 6-byte padding is used,
four more bytes SHALL be added to the FEC header and these bytes
SHALL be set to 0.
As described in Section 4.2, the SN base field of the FEC header MUST
be set to the lowest sequence number of the source packets protected
by this repair packet. For the interleaved FEC packets, this
corresponds to the lowest sequence number of the source packets that
form the column. For the non-interleaved FEC packets, the SN base
field MUST be set to the lowest sequence number of the source packets
that form the row.
The repair packet payload consists of the bits that are generated by
applying the XOR operation on the payloads of the source RTP packets.
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 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.
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6.3. Source Packet Reconstruction
This section describes the recovery procedures that are required to
reconstruct the missing source 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 algorithms described below.
Note that the same algorithms are used by the 1-D parity codes,
regardless of whether the FEC protection is applied over a column or
a row. The 2-D parity codes, on the other hand, usually require
multiple iterations of the procedures described here. This iterative
decoding algorithm is further explained in Section 6.3.4.
6.3.1. Associating the Source and Repair Packets
The first step is associating the source and repair packets. By
virtue of the payload type field in the RTP header, each repair
packet is indicated whether it is an interleaved or non-interleaved
FEC packet. In addition, the SN base field in the FEC header shows
the lowest sequence number of the source packets that form the
particular column or row. Finally, the information of how many
source packets are included in each column or row is available from
the media type parameters specified in the SDP description. 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:
p*_snb + i * X_1 (modulo 65536)
where p*_snb denotes the value in the SN base field of p*'s FEC
header, X_1 is set to L and 1 for the interleaved and non-interleaved
FEC packets, respectively, and
0 <= i < X_2
where X_2 is set to D and L for the interleaved and non-interleaved
FEC packets, respectively.
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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 plus one repair packet for
the FEC protection applied over a column, and L source packets plus
one repair packet for the FEC protection applied over a row. Recall
that 1-D interleaved and non-interleaved FEC protection can fully
recover the missing information if there is only one source packet
missing in set T. If there are more than one source packets missing
in set T, 1-D FEC protection will not work.
6.3.2. Recovering the RTP Header
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
T, compute the 80-bit string by concatenating the first 64 bits
of their RTP header and the unsigned network-ordered 16-bit
representation of their length in bytes minus 12.
2. For the repair packet in T, compute the FEC bit string from the
first 80 bits of the FEC header.
3. Calculate the recovered bit string as the XOR of the bit strings
generated from all source packets in T and the FEC bit string
generated from the repair packet in T.
4. Create a new packet with the standard 12-byte RTP header and no
payload.
5. Set the version of the new packet to 2. Skip the first 2 bits
in the recovered bit string.
6. Set the Padding bit in the new packet to the next bit in the
recovered bit string.
7. Set the Extension bit in the new packet to the next bit in the
recovered bit string.
8. Set the CC field to the next 4 bits in the recovered bit string.
9. Set the Marker bit in the new packet to the next bit in the
recovered bit string.
10. Set the Payload type in the new packet to the next 7 bits in the
recovered bit string.
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11. Set the SN field in the new packet to SEQNUM. Skip the next 16
bits in the recovered bit string.
12. Set the TS field in the new packet to the next 32 bits in the
recovered bit string.
13. Take the next 16 bits of the recovered bit string and set the
new variable Y to whatever unsigned integer this represents
(assuming network order). Convert Y to host order. 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.
14. Set the SSRC of the new packet to the SSRC of the source RTP
stream.
This procedure recovers the header of an RTP packet up to (and
including) the SSRC field.
6.3.3. Recovering the RTP Payload
Following the recovery of the RTP header, the procedure for the
recovery of the RTP payload is as follows:
1. Append Y bytes to the new packet.
2. For each of the source packets that are successfully received in
T, compute the bit string from the Y octets of data starting with
the 13th octet of the packet. If any of the bit strings
generated from the source packets has a length shorter than Y,
pad them to that length. The padding of octet 0 MUST be added at
the end of the bit string. Note that the information of the
first 8 octets are protected by the FEC header.
3. For the repair packet in T, compute the FEC bit string from the
repair packet payload, i.e., the Y octets of data following the
FEC header. Note that the FEC header may be 12 octets or 16
octets depending on whether the optional padding is used or not.
4. Calculate the recovered bit string as the XOR of the bit strings
generated from all source packets in T and the FEC bit string
generated from the repair packet in T.
5. Append the recovered bit string (Y octets) to the new packet
generated in Section 6.3.2.
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6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC Protection
In 2-D parity FEC protection, the sender generates both non-
interleaved and interleaved FEC packets to combat with the mixed loss
patterns (random and bursty). At the receiver side, these FEC
packets are used iteratively to overcome the shortcomings of the 1-D
non-interleaved/interleaved FEC protection and improve the chances of
full error recovery.
The iterative decoding algorithm runs as follows:
1. Set num_recovered_until_this_iteration to zero
2. Set num_recovered_so_far to zero
3. Recover as many source packets as possible by using the non-
interleaved FEC packets as outlined in Section 6.3.2 and
Section 6.3.3, and increase the value of num_recovered_so_far by
the number of recovered source packets.
4. Recover as many source packets as possible by using the
interleaved FEC packets as outlined in Section 6.3.2 and
Section 6.3.3, and increase the value of num_recovered_so_far by
the number of recovered source packets.
5. If num_recovered_so_far > num_recovered_until_this_iteration
---num_recovered_until_this_iteration = num_recovered_so_far
---Go to step 3
Else
---Terminate
The algorithm terminates either when all missing source packets are
fully recovered or when there are still remaining missing source
packets but the FEC packets are not able to recover any more source
packets. For the example scenarios when the 2-D parity FEC
protection fails full recovery, refer to Section 1.2. Upon
termination, variable num_recovered_so_far has a value equal to the
total number of recovered source packets.
Example:
Suppose that the receiver experienced the loss pattern sketched in
Figure 11.
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+---+ +---+ +===+
X X | 3 | | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +===+
| 9 | X X | 12| |R_3|
+---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 11: Example loss pattern for the iterative decoding algorithm
The receiver executes the iterative decoding algorithm and recovers
source packets #1 and #11 in the first iteration. The resulting
pattern is sketched in Figure 12.
+---+ +---+ +---+ +===+
| 1 | X | 3 | | 4 | |R_1|
+---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +===+
| 9 | X | 11| | 12| |R_3|
+---+ +---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 12: The resulting pattern after the first iteration
Since the if condition holds true, the receiver runs a new iteration.
In the second iteration, source packets #2 and #10 are recovered,
resulting in a full recovery as sketched in Figure 13.
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+---+ +---+ +---+ +---+ +===+
| 1 | | 2 | | 3 | | 4 | |R_1|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 9 | | 10| | 11| | 12| |R_3|
+---+ +---+ +---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 13: The resulting pattern after the second iteration
7. SDP Examples
This section provides two SDP [RFC4566] examples. The examples use
the FEC grouping semantics defined in [RFC4756].
7.1. Example SDP for 1-D Parity FEC Protection
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.example.com
s=1-D Interleaved Parity FEC Example
t=0 0
a=group:FEC S1 R1
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 233.252.0.2/127
a=rtpmap:110 interleaved-parityfec/90000
a=fmtp:110 L:5; D:10; ToP:0; repair-window:200000
a=mid:R1
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7.2. Example SDP for 2-D Parity FEC Protection
In this example, we have one source video stream (mid:S1) and two FEC
repair streams (mid:R1 and mid:R2). We form one FEC group with the
"a=group:FEC S1 R1 R2" 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=2-D Parity FEC Example
t=0 0
a=group:FEC S1 R1 R2
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 110
c=IN IP4 233.252.0.2/127
a=rtpmap:110 interleaved-parityfec/90000
a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
a=mid:R1
m=application 30000 RTP/AVP 111
c=IN IP4 233.252.0.3/127
a=rtpmap:111 non-interleaved-parityfec/90000
a=fmtp:111 L:5; D:10; ToP:2; repair-window:200000
a=mid:R2
Note that the sender might be generating two repair flows carrying
non-interleaved and interleaved FEC packets, however the receiver
might be interested only in the interleaved FEC packets. The
receiver can identify the repair flow carrying the desired repair
data by checking the payload types associated with each repair flow
described in the SDP description.
8. Congestion Control Considerations
FEC is an effective approach to provide applications resiliency
against packet losses. However, in networks where the congestion is
a major contributor to the packet loss, the potential impacts of
using FEC SHOULD be considered carefully before injecting the repair
flows into the network. In particular, in bandwidth-limited
networks, FEC repair flows may consume most or all of the available
bandwidth and consequently may congest the network. In such cases,
the applications MUST NOT arbitrarily increase the amount of FEC
protection since doing so may lead to a congestion collapse. If
desired, stronger FEC protection MAY be applied only after the source
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rate has been reduced.
In a network-friendly implementation, an application SHOULD NOT send/
receive FEC repair flows if it knows that sending/receiving those FEC
repair flows would not help at all in recovering the missing packets.
Such a practice helps reduce the amount of wasted bandwidth. It is
RECOMMENDED that the amount of FEC protection is adjusted dynamically
based on the packet loss rate observed by the applications.
In multicast scenarios, it may be difficult to optimize the FEC
protection per receiver. If there is a large variation among the
levels of FEC protection needed by different receivers, it is
RECOMMENDED that the sender offers multiple repair flows with
different levels of FEC protection and the receivers join the
corresponding multicast sessions to receive the repair flow(s) that
is best for them.
Editor's note: Additional congestion control considerations
regarding the use of 2-D parity codes should be added here.
9. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550] and in any applicable RTP profile. The main
security considerations for the RTP packet carrying the RTP payload
format defined within this memo are confidentiality, integrity and
source authenticity. Confidentiality is achieved by encrypting the
RTP payload. Integrity of the RTP packets is achieved through a
suitable cryptographic integrity protection mechanism. Such a
cryptographic system may also allow the authentication of the source
of the payload. A suitable security mechanism for this RTP payload
format should provide confidentiality, integrity protection, and at
least source authentication capable of determining if an RTP packet
is from a member of the RTP session.
Note that the appropriate mechanism to provide security to RTP and
payloads following this memo may vary. It is dependent on the
application, transport and signaling protocol employed. Therefore, a
single mechanism is not sufficient, although if suitable, using the
Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended.
Other mechanisms that may be used are IPsec [RFC4301] and Transport
Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may
exist.
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10. IANA Considerations
New media subtypes are subject to IANA registration. For the
registration of the payload formats and their parameters introduced
in this document, refer to Section 5.
11. Acknowledgments
Some parts of this document are borrowed from [RFC5109]. Thus, the
author would like to thank the editor of [RFC5109] and those who
contributed to [RFC5109].
The author would also like to thank the FEC Framework Design Team for
their inputs, suggestions and contributions.
12. Change Log
12.1. draft-ietf-fecframe-1d2d-parity-scheme-01
The following are the major changes compared to version 00:
o Some details were added regarding the use of CNAME field.
o Offer-Answer and Declarative Considerations sections have been
completed.
o Security Considerations section has been completed.
12.2. draft-ietf-fecframe-1d2d-parity-scheme-00
This is the initial version, which is based on an earlier individual
submission. The following are the major changes compared to that
document:
o The timestamp field definition has changed.
13. References
13.1. Normative References
[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
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Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP
Payload Formats", RFC 3555, July 2003.
[RFC4756] Li, A., "Forward Error Correction Grouping Semantics in
Session Description Protocol", RFC 4756, November 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
13.2. Informative References
[RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format
for Generic Forward Error Correction", RFC 2733,
December 1999.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[SMPTE2022-1]
SMPTE 2022-1-2007, "Forward Error Correction for Real-Time
Video/Audio Transport over IP Networks", 2007.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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Author's Address
Ali Begen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: abegen@cisco.com
Begen Expires November 25, 2009 [Page 39]
