draft-ietf-fecframe-interleaved-fec-scheme-04.txt   draft-ietf-fecframe-interleaved-fec-scheme-05.txt 
FEC Framework A. Begen FEC Framework A. Begen
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended status: Standards Track April 29, 2009 Intended status: Standards Track May 5, 2009
Expires: October 31, 2009 Expires: November 6, 2009
RTP Payload Format for 1-D Interleaved Parity FEC RTP Payload Format for 1-D Interleaved Parity FEC
draft-ietf-fecframe-interleaved-fec-scheme-04 draft-ietf-fecframe-interleaved-fec-scheme-05
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 32 skipping to change at page 1, line 32
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on October 31, 2009. This Internet-Draft will expire on November 6, 2009.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Overhead Computation . . . . . . . . . . . . . . . . . . . 8 1.2. Overhead Computation . . . . . . . . . . . . . . . . . . . 8
1.3. Relation to Existing Specifications . . . . . . . . . . . 8 1.3. Relation to Existing Specifications . . . . . . . . . . . 8
1.3.1. RFC 2733 and RFC 3009 . . . . . . . . . . . . . . . . 8 1.3.1. RFC 2733 and RFC 3009 . . . . . . . . . . . . . . . . 8
1.3.2. SMPTE 2022-1 . . . . . . . . . . . . . . . . . . . . . 8 1.3.2. SMPTE 2022-1 . . . . . . . . . . . . . . . . . . . . . 8
1.3.3. ETSI TS 102 034 . . . . . . . . . . . . . . . . . . . 9 1.3.3. ETSI TS 102 034 . . . . . . . . . . . . . . . . . . . 9
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 10 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 10
3. Definitions, Notations and Abbreviations . . . . . . . . . . . 10 3. Definitions, Notations and Abbreviations . . . . . . . . . . . 10
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 11
4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 11 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Repair Packets . . . . . . . . . . . . . . . . . . . . . . 11 4.2. Repair Packets . . . . . . . . . . . . . . . . . . . . . . 11
5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 15 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 14
5.1. Media Type Registration . . . . . . . . . . . . . . . . . 15 5.1. Media Type Registration . . . . . . . . . . . . . . . . . 15
5.1.1. Registration of audio/1d-interleaved-parityfec . . . . 15 5.1.1. Registration of audio/1d-interleaved-parityfec . . . . 15
5.1.2. Registration of video/1d-interleaved-parityfec . . . . 16 5.1.2. Registration of video/1d-interleaved-parityfec . . . . 16
5.1.3. Registration of text/1d-interleaved-parityfec . . . . 18 5.1.3. Registration of text/1d-interleaved-parityfec . . . . 17
5.1.4. Registration of 5.1.4. Registration of
application/1d-interleaved-parityfec . . . . . . . . . 19 application/1d-interleaved-parityfec . . . . . . . . . 18
5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 20 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 20
5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 21 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 20
5.2.2. Declarative Considerations . . . . . . . . . . . . . . 21 5.2.2. Declarative Considerations . . . . . . . . . . . . . . 21
6. Protection and Recovery Procedures . . . . . . . . . . . . . . 22 6. Protection and Recovery Procedures . . . . . . . . . . . . . . 21
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Repair Packet Construction . . . . . . . . . . . . . . . . 22 6.2. Repair Packet Construction . . . . . . . . . . . . . . . . 22
6.3. Source Packet Reconstruction . . . . . . . . . . . . . . . 24 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . . 24
6.3.1. Associating the Source and Repair Packets . . . . . . 24 6.3.1. Associating the Source and Repair Packets . . . . . . 24
6.3.2. Recovering the RTP Header and Payload . . . . . . . . 25 6.3.2. Recovering the RTP Header and Payload . . . . . . . . 25
7. Session Description Protocol (SDP) Signaling . . . . . . . . . 26 7. Session Description Protocol (SDP) Signaling . . . . . . . . . 26
8. Congestion Control Considerations . . . . . . . . . . . . . . 27 8. Congestion Control Considerations . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28 9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 29 12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 28
12.1. draft-ietf-fecframe-interleaved-fec-scheme-04 . . . . . . 29 12.1. draft-ietf-fecframe-interleaved-fec-scheme-05 . . . . . . 28
12.2. draft-ietf-fecframe-interleaved-fec-scheme-03 . . . . . . 29 12.2. draft-ietf-fecframe-interleaved-fec-scheme-04 . . . . . . 29
12.3. draft-ietf-fecframe-interleaved-fec-scheme-02 . . . . . . 29 12.3. draft-ietf-fecframe-interleaved-fec-scheme-03 . . . . . . 29
12.4. draft-ietf-fecframe-interleaved-fec-scheme-01 . . . . . . 29 12.4. draft-ietf-fecframe-interleaved-fec-scheme-02 . . . . . . 29
12.5. draft-ietf-fecframe-interleaved-fec-scheme-00 . . . . . . 29 12.5. draft-ietf-fecframe-interleaved-fec-scheme-01 . . . . . . 29
12.6. draft-ietf-fecframe-interleaved-fec-scheme-00 . . . . . . 29
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . . 30 13.1. Normative References . . . . . . . . . . . . . . . . . . . 30
13.2. Informative References . . . . . . . . . . . . . . . . . . 30 13.2. Informative References . . . . . . . . . . . . . . . . . . 30
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction 1. Introduction
This document extends the Forward Error Correction (FEC) header This document extends the Forward Error Correction (FEC) header
defined in [RFC2733] and uses this new FEC header for the FEC that is defined in [RFC2733] and uses this new FEC header for the FEC that is
generated by the 1-D interleaved parity code from a source media generated by the 1-D interleaved parity code from a source media
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together based on the media type parameters. together based on the media type parameters.
2. The sender applies the XOR operation on the source symbols to 2. The sender applies the XOR operation on the source symbols to
generate the required number of repair symbols. generate the required number of repair symbols.
3. The sender packetizes the repair symbols and sends the repair 3. The sender packetizes the repair symbols and sends the repair
packet(s) along with the source packets to the receiver(s) (in packet(s) along with the source packets to the receiver(s) (in
different flows). The repair packets MAY be sent proactively or different flows). The repair packets MAY be sent proactively or
on-demand. on-demand.
Note that the sender MUST transmit the source and repair packets in Note that the source and repair packets belong to different source
different source and repair flows, respectively to offer backward and repair flows, and the sender MUST provide a way for the receivers
compatibility (See Section 4). At the receiver side, if all of the to demultiplex them, even in the case they are sent in the same
source packets are successfully received, there is no need for FEC transport flow (i.e., same source/destination address/port with UDP).
recovery and the repair packets are discarded. However, if there are This is required to offer backward compatibility (See Section 4). At
missing source packets, the repair packets can be used to recover the the receiver side, if all of the source packets are successfully
missing information. Block diagrams for the systematic parity FEC received, there is no need for FEC recovery and the repair packets
encoder and decoder are sketched in Figure 1 and Figure 2, are discarded. However, if there are missing source packets, the
respectively. 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.
+------------+ +------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder | | Encoder |
| (Sender) | --> +==+ +==+ | (Sender) | --> +==+ +==+
+------------+ +==+ +==+ +------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+ Source Packet: +--+ Repair Packet: +==+
+--+ +==+ +--+ +==+
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the AL-FEC base layer. the AL-FEC base layer.
2. Requirements Notation 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Definitions, Notations and Abbreviations 3. Definitions, Notations and Abbreviations
The definitions, notations and abbreviations commonly used in this The definitions and notations commonly used in this document are
document are summarized in this section. summarized in this section.
3.1. Definitions 3.1. Definitions
This document uses the following definitions: This document uses the following definitions:
Source Flow: The packet flow(s) carrying the source data and to Source Flow: The packet flow(s) carrying the source data and to
which FEC protection is to be applied. which FEC protection is to be applied.
Repair Flow: The packet flow(s) carrying the repair data. Repair Flow: The packet flow(s) carrying the repair data.
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Source Block: A block of source symbols that are considered together Source Block: A block of source symbols that are considered together
in the encoding process. in the encoding process.
3.2. Notations 3.2. Notations
o L: Number of columns of the source block. o L: Number of columns of the source block.
o D: Number of rows 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
4. Packet Formats 4. Packet Formats
This section defines the formats of the source and repair packets. This section defines the formats of the source and repair packets.
4.1. Source Packets 4.1. Source Packets
The source packets MUST contain the information that identifies the The source packets MUST contain the information that identifies the
source block and the position within the source block occupied by the source block and the position within the source block occupied by the
packet. Since the source packets that are carried within an RTP packet. Since the source packets that are carried within an RTP
stream already contain unique sequence numbers in their RTP headers stream already contain unique sequence numbers in their RTP headers
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| Repair Symbols | / | Repair Symbols | /
+------------------------------+ __| +------------------------------+ __|
Figure 6: Format of repair packets Figure 6: Format of repair packets
The RTP header is formatted according to [RFC3550] with some further The RTP header is formatted according to [RFC3550] with some further
clarifications listed below: clarifications listed below:
o Version: The version field is set to 2. o Version: The version field is set to 2.
o Padding (P) Bit: This bit is obtained by applying protection to o Padding (P) Bit: This bit is equal to the XOR sum of the
the corresponding P bits from the RTP headers of the source corresponding P bits from the RTP headers of the source packets
packets protected by this repair packet. However, padding octets protected by this repair packet. However, padding octets are
are never present in a repair packet, independent of the value of never present in a repair packet, independent of the value of the
the P bit. P bit.
o Extension (X) Bit: This bit is obtained by applying protection to o Extension (X) Bit: This bit is equal to the XOR sum of the
the corresponding X bits from the RTP headers of the source corresponding X bits from the RTP headers of the source packets
packets protected by this repair packet. However, an RTP header protected by this repair packet. However, an RTP header extension
extension is never present in a repair packet, independent of the is never present in a repair packet, independent of the value of
value of the X bit. the X bit.
o CSRC Count (CC): This field is obtained by applying protection to o CSRC Count (CC): This field is equal to the XOR sum of the
the corresponding CC values from the RTP headers of the source corresponding CC values from the RTP headers of the source packets
packets protected by this repair packet. However, a CSRC list is protected by this repair packet. However, a CSRC list is never
never present in a repair packet, independent of the value of the present in a repair packet, independent of the value of the CC
CC field. field.
o Marker (M) Bit: This bit is obtained by applying protection to o Marker (M) Bit: This bit is equal to the XOR sum of the
the corresponding M bits from the RTP headers of the source corresponding M bits from the RTP headers of the source packets
packets protected by this repair packet. protected by this repair packet.
o Payload Type: The (dynamic) payload type for the repair packets o Payload Type: The (dynamic) payload type for the repair packets
is determined through out-of-band means. Note that this document is determined through out-of-band means. Note that this document
registers a new payload format for the repair packets (Refer to registers a new payload format for the repair packets (Refer to
Section 5 for details). According to [RFC3550], an RTP receiver Section 5 for details). According to [RFC3550], an RTP receiver
that cannot recognize a payload type must discard it. This that cannot recognize a payload type must discard it. This
provides backward compatibility. The FEC mechanisms can then be provides backward compatibility. The FEC mechanisms can then be
used in a multicast group with mixed FEC-capable and non-FEC- used in a multicast group with mixed FEC-capable and non-FEC-
capable receivers. If a non-FEC-capable receiver receives a capable receivers. If a non-FEC-capable receiver receives a
repair packet, it will not recognize the payload type, and hence, repair packet, it will not recognize the payload type, and hence,
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The details on setting the fields in the FEC header are provided in The details on setting the fields in the FEC header are provided in
Section 6.2. Section 6.2.
It should be noted that a mask-based approach (similar to the one It should be noted that a mask-based approach (similar to the one
specified in [RFC2733]) may not be very efficient to indicate which specified in [RFC2733]) may not be very efficient to indicate which
source packets in the current source block are associated with a source packets in the current source block are associated with a
given repair packet. In particular, for the applications that would given repair packet. In particular, for the applications that would
like to use large source block sizes, the size of the mask that is like to use large source block sizes, the size of the mask that is
required to describe the source-repair packet associations may be required to describe the source-repair packet associations may be
prohibitively large. Instead, a systematic approach is inherently prohibitively large. Instead, a systematized approach is inherently
more efficient. more efficient.
5. Payload Format Parameters 5. Payload Format Parameters
This section provides the media subtype registration for the 1-D This section provides the media subtype registration for the 1-D
interleaved parity FEC. The parameters that are required to interleaved parity FEC. The parameters that are required to
configure the FEC encoding and decoding operations are also defined configure the FEC encoding and decoding operations are also defined
in this section. in this section.
5.1. Media Type Registration 5.1. Media Type Registration
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o The value for the repair-window parameter depends on the L and D o The value for the repair-window parameter depends on the L and D
values and cannot be chosen arbitrarily. More specifically, L and values and cannot be chosen arbitrarily. More specifically, L and
D values determine the lower limit for the repair-window size. 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 The upper limit of the repair-window size does not depend on the L
and D values. and D values.
o Although combinations with the same L and D values but with o Although combinations with the same L and D values but with
different repair-window sizes produce the same FEC data, such different repair-window sizes produce the same FEC data, such
combinations are still considered different offers. The size of combinations are still considered different offers. The size of
the repair-window is related to how fast the sender will send the the repair-window is related to the maximum delay between the
repair packets. This directly impacts the buffering requirement transmission of a source packet and the associated repair packet.
on the receiver side and the receiver must consider this when This directly impacts the buffering requirement on the receiver
choosing an offer. side and the receiver must consider this when choosing an offer.
o There are no optional format parameters defined for this payload. o There are no optional format parameters defined for this payload.
Any unknown option in the offer MUST be ignored and deleted from 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 the answer. If FEC is not desired by the receiver, it can be
deleted from the answer. deleted from the answer.
5.2.2. Declarative Considerations 5.2.2. Declarative Considerations
In declarative usage, like SDP in the Real-time Streaming Protocol In declarative usage, like SDP in the Real-time Streaming Protocol
(RTSP) [RFC2326] or the Session Announcement Protocol (SAP) (RTSP) [RFC2326] or the Session Announcement Protocol (SAP)
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and a participant MUST use the configuration that is provided for and a participant MUST use the configuration that is provided for
the session. the session.
o More than one configuration may be provided (if desired) by o More than one configuration may be provided (if desired) by
declaring multiple RTP payload types. In that case, the receivers declaring multiple RTP payload types. In that case, the receivers
should choose the repair flow that is best for them. should choose the repair flow that is best for them.
6. Protection and Recovery Procedures 6. Protection and Recovery Procedures
This section provides a complete specification of the 1-D interleaved This section provides a complete specification of the 1-D interleaved
parity code. parity code and its RTP payload format.
6.1. Overview 6.1. Overview
The following sections specify the steps involved in generating the The following sections specify the steps involved in generating the
repair packets and reconstructing the missing source packets from the repair packets and reconstructing the missing source packets from the
repair packets. repair packets.
6.2. Repair Packet Construction 6.2. Repair Packet Construction
The RTP header of a repair packet is formed based on the guidelines The RTP header of a repair packet is formed based on the guidelines
skipping to change at page 23, line 17 skipping to change at page 22, line 46
list, header extension, RTP payload and RTP padding (16 bits) list, header extension, RTP payload and RTP padding (16 bits)
o If CC is nonzero, the CSRC list (variable length) o If CC is nonzero, the CSRC list (variable length)
o If X is 1, the header extension (variable length) o If X is 1, the header extension (variable length)
o Payload (variable length) o Payload (variable length)
o Padding, if present (variable length) o Padding, if present (variable length)
Note that if the payload lengths of the source packets are not equal, Note that if the lengths of the source packets are not equal, each
each shorter packet MUST be padded to the length of the longest shorter packet MUST be padded to the length of the longest packet by
packet by adding octet 0's at the end. Due to this possible padding adding octet 0's at the end. Due to this possible padding and
and mandatory FEC header, a repair packet usually has a larger size mandatory FEC header, a repair packet has a larger size than the
than the source packets it protects. This may cause problems if the source packets it protects. This may cause problems if the resulting
resulting repair packet size exceeds the Maximum Transmission Unit repair packet size exceeds the Maximum Transmission Unit (MTU) size
(MTU) size of the path over which the repair flow is sent. of the path over which the repair flow is sent.
By applying the parity operation on the bit strings produced from the By applying the parity operation on the bit strings produced from the
source packets, we generate the FEC bit string. Some parts of 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 RTP header and the FEC header of the repair packet are generated from
the FEC bit string as follows: the FEC bit string as follows:
o The first (most significant) bit in the FEC bit string is written 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. 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 o The next bit in the FEC bit string is written into the Extension
skipping to change at page 24, line 37 skipping to change at page 24, line 19
o The NA field MUST be set to the number of rows of the source block o The NA field MUST be set to the number of rows of the source block
(D). (D).
o The SN base ext field SHALL be set to 0 and ignored by the o The SN base ext field SHALL be set to 0 and ignored by the
receiver. receiver.
6.3. Source Packet Reconstruction 6.3. Source Packet Reconstruction
This section describes the recovery procedures that are required to This section describes the recovery procedures that are required to
reconstruct the missing packets. The recovery process has two steps. reconstruct the missing source packets. The recovery process has two
In the first step, the FEC decoder determines which source and repair steps. In the first step, the FEC decoder determines which source
packets should be used in order to recover a missing packet. In the and repair packets should be used in order to recover a missing
second step, the decoder recovers the missing packet, which consists packet. In the second step, the decoder recovers the missing packet,
of an RTP header and RTP payload. which consists of an RTP header and RTP payload.
In the following, we describe the RECOMMENDED algorithms for the In the following, we describe the RECOMMENDED algorithms for the
first and second steps. Based on the implementation, different first and second steps. Based on the implementation, different
algorithms MAY be adopted. However, the end result MUST be identical algorithms MAY be adopted. However, the end result MUST be identical
to the one produced by the algorithms described below. to the one produced by the algorithms described below.
6.3.1. Associating the Source and Repair Packets 6.3.1. Associating the Source and Repair Packets
The first step is to associate the source and repair packets. The SN The first step is to associate the source and repair packets. The SN
base low field in the FEC header shows the lowest sequence number of base low field in the FEC header shows the lowest sequence number of
the source packets that form the particular column. In addition, the the source packets that form the particular column. In addition, the
information of how many source packets are available in each column information of how many source packets are available in each column
and row is available from the media type parameters specified in the and row is available from the media type parameters specified in the
SDP description. This set of information uniquely identifies all of SDP description. This set of information uniquely identifies all of
the source packets associated with a given repair packet. the source packets associated with a given repair packet.
Mathematically, for any received repair packet, p*, we can determine Mathematically, for any received repair packet, p*, we can determine
the sequence numbers of the source packets that are protected by this the sequence numbers of the source packets that are protected by this
repair packet as follows: repair packet as follows:
p*_snb + i * L p*_snb + i * L (modulo 65536)
where p*_snb denotes the value in the SN base low field of p*'s FEC 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 and header, L is the number of columns of the source block and
0 <= i < D 0 <= i < D
where D is the number of rows of the source block. where D is the number of rows of the source block.
We denote the set of the source packets associated with repair packet 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 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 by D rows, set T includes D source packets. Recall that 1-D
interleaved FEC protection can fully recover the missing information interleaved FEC protection can fully recover the missing information
if there is only one source packet is missing in set T. If the repair if there is only one source packet missing in set T. If the repair
packet that protects the source packets in set T is missing, or the packet that protects the source packets in set T is missing, or the
repair packet is available but two or more source packets are repair packet is available but two or more source packets are
missing, then missing source packets in set T cannot be recovered by missing, then missing source packets in set T cannot be recovered by
1-D interleaved FEC protection. 1-D interleaved FEC protection.
6.3.2. Recovering the RTP Header and Payload 6.3.2. Recovering the RTP Header and Payload
For a given set T, the procedure for the recovery of 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 of the missing packet, whose sequence number is denoted by SEQNUM, is
as follows: as follows:
skipping to change at page 26, line 34 skipping to change at page 26, line 18
recovered bit string. recovered bit string.
11. Set the Payload type in the new packet to the next 7 bits in the 11. Set the Payload type in the new packet to the next 7 bits in the
recovered bit string. recovered bit string.
12. Set the SN field in the new packet to SEQNUM. 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 13. Set the TS field in the new packet to the next 32 bits in the
recovered bit string. recovered bit string.
14. Take the next 16 bits of the recovered bit string and set Y to 14. Take the next 16 bits of the recovered bit string and set the
whatever unsigned integer this represents (assuming network- new variable Y to whatever unsigned integer this represents
order). Take Y bytes from the recovered bit string and append (assuming network order). Convert Y to host order and then take
them to the new packet. Y represents the length of the new Y bytes from the recovered bit string and append them to the new
packet in bytes minus 12 (for the fixed RTP header), i.e., the packet. Y represents the length of the new packet in bytes
sum of the lengths of all the following if present: the CSRC minus 12 (for the fixed RTP header), i.e., the sum of the
list, header extension, RTP payload and RTP padding. 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 15. Set the SSRC of the new packet to the SSRC of the source RTP
stream. stream.
This procedure completely recovers both the header and payload of an This procedure completely recovers both the header and payload of an
RTP packet. RTP packet.
7. Session Description Protocol (SDP) Signaling 7. Session Description Protocol (SDP) Signaling
This section provides an SDP [RFC4566] example. The following This section provides an SDP [RFC4566] example. The following
skipping to change at page 27, line 17 skipping to change at page 27, line 11
repair stream (mid:R1). We form one FEC group with the "a=group: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 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. on different multicast groups. The repair window is set to 200 ms.
v=0 v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=Interleaved Parity FEC Example s=Interleaved Parity FEC Example
t=0 0 t=0 0
a=group:FEC S1 R1 a=group:FEC S1 R1
m=video 30000 RTP/AVP 100 m=video 30000 RTP/AVP 100
c=IN IP4 224.1.1.1/127 c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000 a=rtpmap:100 MP2T/90000
a=mid:S1 a=mid:S1
m=application 30000 RTP/AVP 110 m=application 30000 RTP/AVP 110
c=IN IP4 224.1.2.1/127 c=IN IP4 233.252.0.2/127
a=rtpmap:110 1d-interleaved-parityfec/90000 a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L:5; D:10; repair-window: 200000 a=fmtp:110 L:5; D:10; repair-window: 200000
a=mid:R1 a=mid:R1
8. Congestion Control Considerations 8. Congestion Control Considerations
FEC is an effective approach to provide applications resiliency FEC is an effective approach to provide applications resiliency
against packet losses. However, in networks where the congestion is against packet losses. However, in networks where the congestion is
a major contributor to the packet loss, the potential impacts of a major contributor to the packet loss, the potential impacts of
using FEC SHOULD be considered carefully before injecting the repair using FEC SHOULD be considered carefully before injecting the repair
skipping to change at page 29, line 7 skipping to change at page 28, line 45
11. Acknowledgments 11. Acknowledgments
A major part of this document is borrowed from [RFC2733] and A major part of this document is borrowed from [RFC2733] and
[SMPTE2022-1]. Thus, the author would like to thank the authors and [SMPTE2022-1]. Thus, the author would like to thank the authors and
editors of these earlier specifications. The author also thanks editors of these earlier specifications. The author also thanks
Colin Perkins for his constructive suggestions for this document. Colin Perkins for his constructive suggestions for this document.
12. Change Log 12. Change Log
12.1. draft-ietf-fecframe-interleaved-fec-scheme-04 12.1. draft-ietf-fecframe-interleaved-fec-scheme-05
The following are the major changes compared to version 04:
o Comments from Vincent Roca have been addressed.
12.2. draft-ietf-fecframe-interleaved-fec-scheme-04
The following are the major changes compared to version 03: The following are the major changes compared to version 03:
o Further comments from AVT WG have been addressed. o Further comments from AVT WG have been addressed.
12.2. draft-ietf-fecframe-interleaved-fec-scheme-03 12.3. draft-ietf-fecframe-interleaved-fec-scheme-03
The following are the major changes compared to version 02: The following are the major changes compared to version 02:
o Comments from WGLC have been addressed. o Comments from WGLC have been addressed.
12.3. draft-ietf-fecframe-interleaved-fec-scheme-02 12.4. draft-ietf-fecframe-interleaved-fec-scheme-02
The following are the major changes compared to version 01: The following are the major changes compared to version 01:
o Some details were added regarding the use of CNAME field. o Some details were added regarding the use of CNAME field.
o Offer-Answer and Declarative Considerations sections have been o Offer-Answer and Declarative Considerations sections have been
completed. completed.
o Security Considerations section has been completed. o Security Considerations section has been completed.
12.4. draft-ietf-fecframe-interleaved-fec-scheme-01 12.5. draft-ietf-fecframe-interleaved-fec-scheme-01
The following are the major changes compared to version 00: The following are the major changes compared to version 00:
o The timestamp field definition has changed. o The timestamp field definition has changed.
12.5. draft-ietf-fecframe-interleaved-fec-scheme-00 12.6. draft-ietf-fecframe-interleaved-fec-scheme-00
This is the initial version, which is based on an earlier individual This is the initial version, which is based on an earlier individual
submission. The following are the major changes compared to that submission. The following are the major changes compared to that
document: document:
o Per the discussion in the WG, references to the FEC Framework have o Per the discussion in the WG, references to the FEC Framework have
been removed and the document has been turned into a pure RTP been removed and the document has been turned into a pure RTP
payload format specification. payload format specification.
o A new section is added for congestion control considerations. o A new section is added for congestion control considerations.
 End of changes. 32 change blocks. 
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