< draft-ietf-payload-flexible-fec-scheme-13.txt   draft-ietf-payload-flexible-fec-scheme-14.txt >
PAYLOAD M. Zanaty PAYLOAD M. Zanaty
Internet-Draft Cisco Internet-Draft Cisco
Intended status: Standards Track V. Singh Intended status: Standards Track V. Singh
Expires: June 14, 2019 callstats.io Expires: July 7, 2019 callstats.io
A. Begen A. Begen
Networked Media Networked Media
G. Mandyam G. Mandyam
Qualcomm Inc. Qualcomm Inc.
December 11, 2018 January 3, 2019
RTP Payload Format for Flexible Forward Error Correction (FEC) RTP Payload Format for Flexible Forward Error Correction (FEC)
draft-ietf-payload-flexible-fec-scheme-13 draft-ietf-payload-flexible-fec-scheme-14
Abstract Abstract
This document defines new RTP payload formats for the Forward Error This document defines new RTP payload formats for the Forward Error
Correction (FEC) packets that are generated by the non-interleaved Correction (FEC) packets that are generated by the non-interleaved
and interleaved parity codes from source media encapsulated in RTP. and interleaved parity codes from source media encapsulated in RTP.
These parity codes are systematic codes, where a number of FEC repair These parity codes are systematic codes, where a number of FEC repair
packets are generated from a set of source packets from one or more packets are generated from a set of source packets from one or more
source RTP streams. These FEC repair packets are sent in a source RTP streams. These FEC repair packets are sent in a
redundancy RTP stream separate from the source RTP stream(s) that redundancy RTP stream separate from the source RTP stream(s) that
carries the source packets. RTP source packets that were lost in carries the source packets. RTP source packets that were lost in
transmission can be reconstructed using the source and repair packets transmission can be reconstructed using the source and repair packets
that were received. The non-interleaved and interleaved parity codes that were received. The non-interleaved and interleaved parity codes
which are defined in this specification offer a good protection which are defined in this specification offer a good protection
against random and bursty packet losses, respectively, at a cost of against random and bursty packet losses, respectively, at a cost of
decent complexity. The RTP payload formats that are defined in this complexity. The RTP payload formats that are defined in this
document address scalability issues experienced with the earlier document address scalability issues experienced with the earlier
specifications, and offer several improvements. Due to these specifications, and offer several improvements. Due to these
changes, the new payload formats are not backward compatible with changes, the new payload formats are not backward compatible with
earlier specifications, but endpoints that do not implement this earlier specifications, but endpoints that do not implement this
specification can still work by simply ignoring the FEC repair specification can still work by simply ignoring the FEC repair
packets. packets.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 2, line 7 skipping to change at page 2, line 7
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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."
This Internet-Draft will expire on June 14, 2019. This Internet-Draft will expire on July 7, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 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 Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Parity Codes . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Parity Codes . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1. 1-D Non-interleaved (Row) FEC Protection . . . . . . 5 1.1.1. One-Dimensionsal (1-D) Non-interleaved (Row) FEC
1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 5 Protection . . . . . . . . . . . . . . . . . . . . . 6
1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 6 1.1.2. 1-D Interleaved (Column) FEC Protection . . . . . . . 7
1.1.4. 2-D (Row and Column) FEC Protection . . . . . . . . . 8 1.1.3. Use Cases for 1-D FEC Protection . . . . . . . . . . 8
1.1.5. FEC Overhead Considerations . . . . . . . . . . . . . 9 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection 10
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 9 1.1.5. FEC Overhead Considerations . . . . . . . . . . . . . 12
3. Definitions and Notations . . . . . . . . . . . . . . . . . . 10 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 13
3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 10 3. Definitions and Notations . . . . . . . . . . . . . . . . . . 13
3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 13
4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 11 4. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 14
4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 11 4.1. Source Packets . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 12 4.2. FEC Repair Packets . . . . . . . . . . . . . . . . . . . 15
4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 14 4.2.1. RTP Header of FEC Repair Packets . . . . . . . . . . 16
5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 18 4.2.2. FEC Header of FEC Repair Packets . . . . . . . . . . 17
5.1. Media Type Registration - Parity Codes . . . . . . . . . 19 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 25
5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 19 5.1. Media Type Registration - Parity Codes . . . . . . . . . 25
5.1.2. Registration of video/flexfec . . . . . . . . . . . . 20 5.1.1. Registration of audio/flexfec . . . . . . . . . . . . 25
5.1.3. Registration of text/flexfec . . . . . . . . . . . . 22 5.1.2. Registration of video/flexfec . . . . . . . . . . . . 26
5.1.4. Registration of application/flexfec . . . . . . . . . 23 5.1.3. Registration of text/flexfec . . . . . . . . . . . . 28
5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 25 5.1.4. Registration of application/flexfec . . . . . . . . . 29
5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 25 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 31
5.2.2. Declarative Considerations . . . . . . . . . . . . . 26 5.2.1. Offer-Answer Model Considerations . . . . . . . . . . 31
6. Protection and Recovery Procedures - Parity Codes . . . . . . 26 5.2.2. Declarative Considerations . . . . . . . . . . . . . 32
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 26
6.2. Repair Packet Construction . . . . . . . . . . . . . . . 26 6. Protection and Recovery Procedures - Parity Codes . . . . . . 32
6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 28 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 32
6.3.1. Associating the Source and Repair Packets . . . . . . 29 6.2. Repair Packet Construction . . . . . . . . . . . . . . . 33
6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 30 6.3. Source Packet Reconstruction . . . . . . . . . . . . . . 34
6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 31 6.3.1. Associating the Source and Repair Packets . . . . . . 35
6.3.2. Recovering the RTP Header . . . . . . . . . . . . . . 37
6.3.3. Recovering the RTP Payload . . . . . . . . . . . . . 38
6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC 6.3.4. Iterative Decoding Algorithm for the 2-D Parity FEC
Protection . . . . . . . . . . . . . . . . . . . . . 32 Protection . . . . . . . . . . . . . . . . . . . . . 38
7. Signaling Requirements . . . . . . . . . . . . . . . . . . . 34 7. Signaling Requirements . . . . . . . . . . . . . . . . . . . 42
7.1. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 35 7.1. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 43
7.1.1. Example SDP for Flexible FEC Protection with in-band 7.1.1. Example SDP for Flexible FEC Protection with in-band
SSRC mapping . . . . . . . . . . . . . . . . . . . . 35 SSRC mapping . . . . . . . . . . . . . . . . . . . . 43
7.1.2. Example SDP for Flexible FEC Protection with explicit 7.1.2. Example SDP for Flexible FEC Protection with explicit
signalling in the SDP . . . . . . . . . . . . . . . . 36 signalling in the SDP . . . . . . . . . . . . . . . . 44
7.2. On the Use of the RTP Stream Identifier Source 7.2. On the Use of the RTP Stream Identifier Source
Description . . . . . . . . . . . . . . . . . . . . . . . 36 Description . . . . . . . . . . . . . . . . . . . . . . . 44
8. Congestion Control Considerations . . . . . . . . . . . . . . 36 8. Congestion Control Considerations . . . . . . . . . . . . . . 45
9. Security Considerations . . . . . . . . . . . . . . . . . . . 37 9. Security Considerations . . . . . . . . . . . . . . . . . . . 45
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 46
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
12.1. Normative References . . . . . . . . . . . . . . . . . . 38 12.1. Normative References . . . . . . . . . . . . . . . . . . 46
12.2. Informative References . . . . . . . . . . . . . . . . . 39 12.2. Informative References . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 49
1. Introduction 1. Introduction
This document defines new RTP payload formats for the Forward Error This document defines new RTP payload formats for the Forward Error
Correction (FEC) that is generated by the non-interleaved and Correction (FEC) that is generated by the non-interleaved and
interleaved parity codes from a source media encapsulated in RTP interleaved parity codes from a source media encapsulated in RTP
[RFC3550]. The type of the source media protected by these parity [RFC3550]. The type of the source media protected by these parity
codes can be audio, video, text or application. The FEC data are codes can be audio, video, text or application. The FEC data are
generated according to the media type parameters, which are generated according to the media type parameters, which are
communicated out-of-band (e.g., in SDP). Furthermore, the communicated out-of-band (e.g., in SDP). Furthermore, the
associations or relationships between the source and repair RTP associations or relationships between the source and repair RTP
streams may be communicated in-band or out-of-band. The in-band streams may be communicated in-band or out-of-band. The in-band
mechanism is advantageous when the endpoint is adapting the FEC mechanism is advantageous when the endpoint is adapting the FEC
parameters. The out-of-band mechanism may be preferable when the FEC parameters. The out-of-band mechanism may be preferable when the FEC
parameters are fixed. parameters are fixed. While this document fully defines the use of
FEC to protect RTP streams, it also leverages several definitions
along with the basic source/repair header description from [RFC6363]
in their application to the parity codes defined here.
The Redunadncy RTP Stream [RFC7656] repair packets proposed in this The Redundancy RTP Stream [RFC7656] repair packets proposed in this
document protect the Source RTP Stream packets that belong to the document protect the Source RTP Stream packets that belong to the
same RTP session. same RTP session.
The RTP payload formats that are defined in this document address the The RTP payload formats that are defined in this document address the
scalability issues experienced with the formats defined in earlier scalability issues experienced with the formats defined in earlier
specifications including [RFC2733], [RFC5109] and [SMPTE2022-1]. specifications including [RFC2733], [RFC5109] and [SMPTE2022-1].
1.1. Parity Codes 1.1. Parity Codes
Both the non-interleaved and interleaved parity codes use the Both the non-interleaved and interleaved parity codes use the
skipping to change at page 4, line 29 skipping to change at page 5, line 5
repair packets may be sent proactively or on-demand based on RTCP repair packets may be sent proactively or on-demand based on RTCP
feedback messages such as NACK [RFC4585]. feedback messages such as NACK [RFC4585].
At the receiver side, if all of the source packets are successfully At the receiver side, if all of the source packets are successfully
received, there is no need for FEC recovery and the repair packets received, there is no need for FEC recovery and the repair packets
are discarded. However, if there are missing source packets, the are discarded. However, if there are missing source packets, the
repair packets can be used to recover the missing information. repair packets can be used to recover the missing information.
Figure 1 and Figure 2 describe example block diagrams for the Figure 1 and Figure 2 describe example block diagrams for the
systematic parity FEC encoder and decoder, respectively. systematic parity FEC encoder and decoder, respectively.
+------------+ +------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder | | Encoder |
| (Sender) | --> +==+ +==+ | (Sender) | --> +==+ +==+
+------------+ +==+ +==+ +------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+ Source Packet: +--+ Repair Packet: +==+
+--+ +==+ +--+ +==+
Figure 1: Block diagram for systematic parity FEC encoder Figure 1: Block diagram for systematic parity FEC encoder
+------------+ +------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+ +--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder | | Decoder |
+==+ +==+ --> | (Receiver) | +==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+ +==+ +==+ +------------+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+ +--+ +==+
Figure 2: Block diagram for systematic parity FEC decoder Figure 2: Block diagram for systematic parity FEC decoder
In Figure 2, it is clear that the FEC repair packets have to be In Figure 2, it is clear that the FEC repair packets have to be
received by the endpoint within a certain amount of time for the FEC received by the endpoint within a certain amount of time for the FEC
recovery process to be useful. The repair window is defined as the recovery process to be useful. The repair window is defined as the
time that spans a FEC block, which consists of the source packets and time that spans a FEC block, which consists of the source packets and
the corresponding repair packets. At the receiver side, the FEC the corresponding repair packets. At the receiver side, the FEC
decoder SHOULD buffer source and repair packets at least for the decoder SHOULD buffer source and repair packets at least for the
duration of the repair window, to allow all the repair packets to duration of the repair window, to allow all the repair packets to
arrive. The FEC decoder can start decoding the already received arrive. The FEC decoder can start decoding the already received
packets sooner; however, it should not register a FEC decoding packets sooner; however, it should not register a FEC decoding
failure until it waits at least for the duration of the repair failure until it waits at least for the duration of the repair
window. window.
1.1.1. 1-D Non-interleaved (Row) FEC Protection 1.1.1. One-Dimensionsal (1-D) Non-interleaved (Row) FEC Protection
Consider a group of D x L source packets that have sequence numbers Consider 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 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 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 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 protection. As a result of this process, D repair packets are
generated, which are referred to as non-interleaved (or row) FEC generated, which are referred to as non-interleaved (or row) FEC
repair packets. repair packets. In general D and L represent values that describe
how packets are grouped together from a depth and length perspective
(respectively) when interleaving all D x L source packets.
+--------------------------------------------------+ --- +===+ +--------------------------------------------------+ --- +===+
| S_1 S_2 S3 ... S_L | + |XOR| = |R_1| | 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_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| | 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 repair packets Figure 3: Generating non-interleaved (row) FEC repair packets
1.1.2. 1-D Interleaved (Column) FEC Protection 1.1.2. 1-D Interleaved (Column) FEC Protection
If the XOR operation is applied to the group of the source packets If the XOR operation is applied to the group of the source packets
whose sequence numbers are L apart from each other, as sketched in whose sequence numbers are L apart from each other, as sketched in
Figure 4. In this case the endpoint generates L repair packets. Figure 4. In this case the endpoint generates L repair packets.
This process is referred to as 1-D interleaved FEC protection, and This process is referred to as 1-D interleaved FEC protection, and
skipping to change at page 7, line 29 skipping to change at page 10, line 6
The sender may generate interleaved FEC repair packets to combat with The sender may generate interleaved FEC repair packets to combat with
the bursty packet losses. However, two or more random packet losses the bursty packet losses. However, two or more random packet losses
may hit the source and repair packets in the same column. In that may hit the source and repair packets in the same column. In that
case, the repair operation fails as well. This is illustrated in case, the repair operation fails as well. This is illustrated in
Figure 6. Note that it is possible that two burst losses may occur Figure 6. Note that it is possible that two burst losses may occur
back-to-back, in which case interleaved FEC repair packets may still back-to-back, in which case interleaved FEC repair packets may still
fail to recover the lost data. fail to recover the lost data.
+---+ +---+ +---+ +---+ +---+ +---+
| 1 | X | 3 | | 4 | | 1 | X | 3 | | 4 |
+---+ +---+ +---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+ +---+ +---+
| 5 | X | 7 | | 8 | | 5 | X | 7 | | 8 |
+---+ +---+ +---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12| | 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+ +===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4| |C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+ +===+ +===+ +===+ +===+
Figure 6: Example scenario where 1-D interleaved FEC protection fails Figure 6: Example scenario where 1-D interleaved FEC protection fails
error recovery (Periodic Loss) error recovery (Periodic Loss)
1.1.4. 2-D (Row and Column) FEC Protection 1.1.4. Two-Dimensional (2-D) (Row and Column) FEC Protection
In networks where the source packets are lost both randomly and in In networks where the source packets are lost both randomly and in
bursts, the sender ought to generate both non-interleaved and bursts, the sender ought to generate both non-interleaved and
interleaved FEC repair packets. This type of FEC protection is known interleaved FEC repair packets. This type of FEC protection is known
as 2-D parity FEC protection. At the expense of generating more FEC as 2-D parity FEC protection. At the expense of generating more FEC
repair packets, thus increasing the FEC overhead, 2-D FEC provides repair packets, thus increasing the FEC overhead, 2-D FEC provides
superior protection against mixed loss patterns. However, it is superior protection against mixed loss patterns. However, it is
still possible for 2-D parity FEC protection to fail to recover all still possible for 2-D parity FEC protection to fail to recover all
of the lost source packets if a particular loss pattern occurs. An of the lost source packets if a particular loss pattern occurs. An
example scenario is illustrated in Figure 7. example scenario is illustrated in Figure 7.
skipping to change at page 9, line 6 skipping to change at page 12, line 6
2-D parity FEC protection also fails when at least two rows are 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 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 (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 loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC
protection cannot repair all missing source packets when at least two protection cannot repair all missing source packets when at least two
columns are missing a source and the FEC packet and the missing 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. source packets (in at least two columns) are aligned in the same row.
+---+ +---+ +---+ +---+ +---+ +---+
| 1 | | 2 | X | 4 | X | 1 | | 2 | X | 4 | X
+---+ +---+ +---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+ +===+ +---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2| | 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+ +---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +---+ +---+
| 9 | | 10| X | 12| X | 9 | | 10| X | 12| X
+---+ +---+ +---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+ +===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4| |C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+ +===+ +===+ +===+ +===+
Figure 8: Example scenario #2 where 2-D parity FEC protection fails Figure 8: Example scenario #2 where 2-D parity FEC protection fails
error recovery error recovery
1.1.5. FEC Overhead Considerations 1.1.5. FEC Overhead Considerations
The overhead is defined as the ratio of the number of bytes belonging 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 to the repair packets to the number of bytes belonging to the
protected source packets. protected source packets.
Generally, repair packets are larger in size compared to the source Generally, repair packets are larger in size compared to the source
packets. Also, not all the source packets are necessarily equal in packets. Also, not all the source packets are necessarily equal in
size. However, assuming that each repair packet carries an equal size. However, assuming that each repair packet carries an equal
number of bytes carried by a source packet, the overhead for number of bytes as carried by a source packet, the overhead for
different FEC protection methods can be computed as follows: different FEC protection methods can be computed as follows:
o 1-D Non-interleaved FEC Protection: Overhead = 1/L o 1-D Non-interleaved FEC Protection: Overhead = 1/L
o 1-D Interleaved FEC Protection: Overhead = 1/D o 1-D Interleaved FEC Protection: Overhead = 1/D
o 2-D Parity FEC Protection: Overhead = 1/L + 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, where L and D are the number of columns and rows in the source block,
respectively. respectively.
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", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Definitions and Notations 3. Definitions and Notations
3.1. Definitions 3.1. Definitions
This document uses a number of definitions from [RFC6363]. This document uses a number of definitions from [RFC6363].
1-D Non-interleaved Row FEC: A protection scheme that operates on 1-D Non-interleaved Row FEC: A protection scheme that operates on
consecutive source packets in the source block, able to recover a consecutive source packets in the source block, able to recover a
single lost source packet per row of the source block. single lost source packet per row of the source block.
skipping to change at page 11, line 37 skipping to change at page 15, line 7
missing source packets, making this scheme a systematic code. missing source packets, making this scheme a systematic code.
The source packets are full RTP packets with optional CSRC list, RTP The source packets are full RTP packets with optional CSRC list, RTP
header extension, and padding. If any of these optional elements are header extension, and padding. If any of these optional elements are
present in the source RTP packet, and that source packet is lost, present in the source RTP packet, and that source packet is lost,
they are recovered by the FEC repair operation, which recovers the they are recovered by the FEC repair operation, which recovers the
full source RTP packet including these optional elements. full source RTP packet including these optional elements.
4.2. FEC Repair Packets 4.2. FEC Repair Packets
The FEC repair packets MUST contain information that identifies the The FEC repair packets will contain information that identifies the
source block they pertain to and the relationship between the source block they pertain to and the relationship between the
contained repair packets and the original source block. For this contained repair packets and the original source block. For this
purpose, the RTP header of the repair packets is used, as well as purpose, the RTP header of the repair packets is used, as well as
another header within the RTP payload, called the FEC header, as another header within the RTP payload, called the FEC header, as
shown in Figure 9. shown in Figure 9.
Note that all the source stream packets that are protected by a Note that all the source stream packets that are protected by a
particular FEC packet need to be in the same RTP session. particular FEC packet need to be in the same RTP session.
+------------------------------+ +------------------------------+
skipping to change at page 12, line 6 skipping to change at page 15, line 18
source block they pertain to and the relationship between the source block they pertain to and the relationship between the
contained repair packets and the original source block. For this contained repair packets and the original source block. For this
purpose, the RTP header of the repair packets is used, as well as purpose, the RTP header of the repair packets is used, as well as
another header within the RTP payload, called the FEC header, as another header within the RTP payload, called the FEC header, as
shown in Figure 9. shown in Figure 9.
Note that all the source stream packets that are protected by a Note that all the source stream packets that are protected by a
particular FEC packet need to be in the same RTP session. particular FEC packet need to be in the same RTP session.
+------------------------------+ +------------------------------+
| IP Header | | IP Header |
+------------------------------+ +------------------------------+
| Transport Header | | Transport Header |
+------------------------------+ +------------------------------+
| RTP Header | | RTP Header |
+------------------------------+ ---+ +------------------------------+ ---+
| FEC Header | | | FEC Header | |
+------------------------------+ | RTP Payload +------------------------------+ | RTP Payload
| Repair "Payload" | | | Repair "Payload" | |
+------------------------------+ ---+ +------------------------------+ ---+
Figure 9: Format of FEC repair packets Figure 9: Format of FEC repair packets
Repair "Payload", which follows the FEC Header, includes repair of The Repair "Payload", which follows the FEC Header, includes repair
everything following the fixed 12-byte RTP header of the source of everything following the fixed 12-byte RTP header of each source
packet, including any CSRC list and header extensions if present. packet, including any CSRC identifier list and header extensions if
present.
4.2.1. RTP Header of FEC Repair Packets 4.2.1. RTP Header of FEC 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:
Version (V) 2 bits: This MUST be set to 2 (binary 10), as this Version (V) 2 bits: This MUST be set to 2 (binary 10), as this
specification requires all source RTP packets and all FEC repair specification requires all source RTP packets and all FEC repair
packets to use RTP version 2. The reason for this restriction is packets to use RTP version 2. The reason for this restriction is
the first 2 bits of the FEC header contain other information (R the first 2 bits of the FEC header contain other information (R
and F bits) rather than recovering the RTP version field. and F bits) rather than recovering the RTP version field.
Padding (P) bit: Source packets can have optional RTP padding, Padding (P) bit: Source packets can have optional RTP padding,
which can be recovered. FEC repaire packets can have optional RTP which can be recovered. FEC repair packets can have optional RTP
padding, which is independent of the RTP padding of the source padding, which is independent of the RTP padding of the source
pakcets. pakcets.
Extension (X) bit: Source packets can have optional RTP header Extension (X) bit: Source packets can have optional RTP header
extensions, which can be recovered. FEC repair packets can have extensions, which can be recovered. FEC repair packets can have
optional RTP header extensions, which are independent of the RTP optional RTP header extensions, which are independent of the RTP
header extensions of the source packets. header extensions of the source packets.
CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each: CSRC Count (CC) 4 bits, and CSRC List (CSRC_i) 32 bits each:
Source packets can have an optional CSRC list and count, which can Source packets can have an optional CSRC list and count, which can
skipping to change at page 13, line 14 skipping to change at page 16, line 44
Payload Type: The (dynamic) payload type for the FEC repair Payload Type: The (dynamic) payload type for the FEC repair
packets is determined through out-of-band means. Note that this packets is determined through out-of-band means. Note that this
document registers new payload formats for the repair packets document registers new payload formats for the repair packets
(Refer to Section 5 for details). According to [RFC3550], an RTP (Refer to Section 5 for details). According to [RFC3550], an RTP
receiver that cannot recognize a payload type must discard it. receiver that cannot recognize a payload type must discard it.
This provides backward compatibility. If a non-FEC-capable This provides backward compatibility. If a non-FEC-capable
receiver receives a repair packet, it will not recognize the receiver receives a repair packet, it will not recognize the
payload type, and hence, will discard the repair packet. payload type, and hence, will discard the repair packet.
Sequence Number (SN): The sequence number has the standard Sequence Number (SN): The sequence number follows the standard
definition. It MUST be one higher than the sequence number in the definition provided in [RFC3550]. definition. Therefore it must
previously transmitted repair packet. The initial value of the be one higher than the sequence number in the previously
sequence number SHOULD be random (unpredictable, based on transmitted repair packet, and the initial value of the sequence
[RFC3550]). number should be random (i.e. unpredictable).
Timestamp (TS): The timestamp SHALL be set to a time corresponding Timestamp (TS): The timestamp SHALL be set to a time corresponding
to the repair packet's transmission time. Note that the timestamp to the repair packet's transmission time. Note that the timestamp
value has no use in the actual FEC protection process and is value has no use in the actual FEC protection process and is
usually useful for jitter calculations. usually useful for jitter calculations.
Synchronization Source (SSRC): The SSRC value for each repair Synchronization Source (SSRC): The SSRC value for each repair
stream SHALL be randomly assigned as suggested by [RFC3550]. This stream SHALL be randomly assigned as per the guidelines provided
allows the sender to multiplex the source and repair RTP streams in Section 8 of [RFC3550]. This allows the sender to multiplex
in the same RTP session, or multiplex multiple repair streams in the source and repair RTP streams in the same RTP session, or
an RTP session. The repair streams' SSRC's CNAME SHOULD be multiplex multiple repair streams in an RTP session. The repair
identical to the CNAME of the source RTP stream(s) that this streams' SSRC's CNAME MUST be identical to the CNAME of the source
repair stream protects. In cases when the repair stream covers RTP stream(s) that this repair stream protects. In cases when the
packets from multiple source RTP streams with different CNAME repair stream covers packets from multiple source RTP streams with
values, any of these CNAME values MAY be used. different CNAME values, any of these CNAME values MAY be used.
In some networks, the RTP Source, which produces the source In some networks, the RTP Source, which produces the source
packets and the FEC Source, which generates the repair packets packets and the FEC Source, which generates the repair packets
from the source packets may not be the same host. In such from the source packets may not be the same host. In such
scenarios, using the same CNAME for the source and repair RTP scenarios, using the same CNAME for the source and repair RTP
streams means that the RTP Source and the FEC Source MUST share streams means that the RTP Source and the FEC Source will share
the same CNAME (for this specific source-repair stream the same CNAME (for this specific source-repair stream
association). A common CNAME may be produced based on an association). A common CNAME may be produced based on an
algorithm that is known both to the RTP and FEC Source [RFC7022]. algorithm that is known both to the RTP and FEC Source [RFC7022].
This usage is compliant with [RFC3550]. This usage is compliant with [RFC3550].
Note that due to the randomness of the SSRC assignments, there is Note that due to the randomness of the SSRC assignments, there is
a possibility of SSRC collision. In such cases, the collisions a possibility of SSRC collision. In such cases, the collisions
MUST be resolved as described in [RFC3550]. must be resolved as described in [RFC3550].
4.2.2. FEC Header of FEC Repair Packets 4.2.2. FEC Header of FEC Repair Packets
The format of the FEC header has 3 variants, depending on the values The format of the FEC header has 3 variants, depending on the values
in the first 2 bits (R and F bits) as shown in Figure 10. in the first 2 bits (R and F bits) as shown in Figure 10. Two of
these variants are meant to describe different methods for deriving
the source data from a source packet for a repair packet. This
allows for customizing the FEC method to allow for robustness against
different levels of burst errors and random packet losses. The third
variant is for a straight retransmission of the source packet.
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... | |R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| ...varies depending on R/F... | | ...varies depending on R/F... |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Repair "Payload" follows FEC Header : : Repair "Payload" follows FEC Header :
: : : :
Figure 10: FEC Header Figure 10: FEC Header
Repair "Payload", which follows the FEC Header, includes repair of The Repair "Payload", which follows the FEC Header, includes repair
everything following the fixed 12-byte RTP header of the source of everything following the fixed 12-byte RTP header of each source
packet, including any CSRC list and header extensions if present. packet, including any CSRC identifier list and header extensions if
present.
+---+---+----------------------------------------------------------+ +---+---+----------------------------------------------------------+
| R | F | FEC Header variant | | R | F | FEC Header variant |
+---+---+----------------------------------------------------------+ +---+---+----------------------------------------------------------+
| 0 | 0 | Flexible FEC Mask fields indicate source packets | | 0 | 0 | Flexible FEC Mask fields indicate source packets |
| 0 | 1 | Fixed FEC L/D (cols/rows) fields indicate source packets | | 0 | 1 | Fixed FEC L/D (cols/rows) fields indicate source packets |
| 1 | 0 | Retransmission of a single source packet | | 1 | 0 | Retransmission of a single source packet |
| 1 | 1 | Invalid, MUST NOT send, MUST ignore if received | | 1 | 1 | Invalid, MUST NOT send, MUST ignore if received |
+---+---+----------------------------------------------------------+ +---+---+----------------------------------------------------------+
Figure 11: R and F bit values for FEC Header variants Figure 11: R and F bit values for FEC Header variants
The first variant, when R=0 and F=0, has a mask to signal protected The first variant, when R=0 and F=0, has a mask to signal protected
source packets, as shown in Figure 12. source packets, as shown in Figure 12.
The second variant, when R=0 and F=1, has a number of columns (L) and The second variant, when R=0 and F=1, has a number of columns (L) and
rows (D) to signal protected source packets, as shown in Figure 13. rows (D) to signal protected source packets, as shown in Figure 13.
skipping to change at page 16, line 29 skipping to change at page 21, line 35
o The Mask fields indicate a bitmask of which source packets are o The Mask fields indicate a bitmask of which source packets are
protected by this FEC repair packet, where bit j of the mask set protected by this FEC repair packet, where bit j of the mask set
to 1 indicates that the source packet with sequence number (SN to 1 indicates that the source packet with sequence number (SN
base_i + j) is protected by this FEC repair packet, where j=0 is base_i + j) is protected by this FEC repair packet, where j=0 is
the most significant bit in the mask. the most significant bit in the mask.
o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110 o The k-bit in the bitmasks indicates if the mask is 15, 46, or 110
bits. k=1 denotes that another mask follows, and k=0 denotes that bits. k=1 denotes that another mask follows, and k=0 denotes that
it is the last block of mask. it is the last block of mask.
o Repair "Payload", which follows the FEC Header, includes repair of o The Repair "Payload", which follows the FEC Header, includes
everything following the fixed 12-byte RTP header of the source repair of everything following the fixed 12-byte RTP header of
packet, including any CSRC list and header extensions if present. each source packet, including any CSRC identifier list and header
extensions if present.
4.2.2.2. FEC Header with Fixed L Columns and D Rows 4.2.2.2. FEC Header with Fixed L Columns and D Rows
When R=0 and F=1, the FEC Header includes L and D fields for fixed When R=0 and F=1, the FEC Header includes L and D fields for fixed
columns and rows. The other fields are the same as the prior columns and rows. The other fields are the same as the prior
section. As in the previous section, the CSRC_i (32 bits) field in section. As in the previous section, the CSRC_i (32 bits) field in
the RTP Header (not FEC Header) describes the SSRC of the source the RTP Header (not FEC Header) describes the SSRC of the source
packets protected by this particular FEC packet. If there are packets protected by this particular FEC packet. If there are
multiple SSRC's protected by the FEC packet, then there will be multiple SSRC's protected by the FEC packet, then there will be
multiple blocks of data containing an SN base along with L and D multiple blocks of data containing an SN base along with L and D
skipping to change at page 20, line 19 skipping to change at page 26, line 28
Published specification: [RFCXXXX]. Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant want to improve resiliency against packet loss by sending redundant
data in addition to the source media. data in addition to the source media.
Fragment identifier considerations: None. Fragment identifier considerations: None.
Additional information: None. Additional information: None.
Person & email address to contact for further information: Varun Person & email address to contact for further information: IESG
Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads <varun@callstats.io> and IETF Audio/Video Transport Payloads Working
Working Group. Group (or it's successor as delegated by the IESG).
Intended usage: COMMON. Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550]. hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>. Author: IESG <iesg@ietf.org>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
Provisional registration? (standards tree only): Yes. Change controller: IETF Audio/Video Transport Payloads Working Group
delegated from the IESG (or it's successor as delegated by the IESG).
5.1.2. Registration of video/flexfec 5.1.2. Registration of video/flexfec
Type name: video Type name: video
Subtype name: flexfec Subtype name: flexfec
Required parameters: Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
skipping to change at page 21, line 40 skipping to change at page 27, line 49
Published specification: [RFCXXXX]. Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant want to improve resiliency against packet loss by sending redundant
data in addition to the source media. data in addition to the source media.
Fragment identifier considerations: None. Fragment identifier considerations: None.
Additional information: None. Additional information: None.
Person & email address to contact for further information: Varun Person & email address to contact for further information: IESG
Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads <iesg@ietf.org> and IETF Audio/Video Transport Payloads Working Group
Working Group. (or it's successor as delegated by the IESG).
Intended usage: COMMON. Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550]. hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>. Author: IESG <iesg@ietf.org>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
Provisional registration? (standards tree only): Yes. Change controller: IETF Audio/Video Transport Payloads Working Group
delegated from the IESG (or it's successor as delegated by the IESG).
5.1.3. Registration of text/flexfec 5.1.3. Registration of text/flexfec
Type name: text Type name: text
Subtype name: flexfec Subtype name: flexfec
Required parameters: Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
skipping to change at page 23, line 13 skipping to change at page 29, line 22
Published specification: [RFCXXXX]. Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant want to improve resiliency against packet loss by sending redundant
data in addition to the source media. data in addition to the source media.
Fragment identifier considerations: None. Fragment identifier considerations: None.
Additional information: None. Additional information: None.
Person & email address to contact for further information: Varun Person & email address to contact for further information: IESG
Singh <vvarun@callstats.io> and IETF Audio/Video Transport Payloads <viesg@ietf.org> and IETF Audio/Video Transport Payloads Working
Working Group. Group (or it's successor as delegated by the IESG).
Intended usage: COMMON. Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550]. hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>. Author: IESG <iesg@ietf.org>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
Provisional registration? (standards tree only): Yes. Change controller: IETF Audio/Video Transport Payloads Working Group
delegated from the IESG (or it's successor as delegated by the IESG).
5.1.4. Registration of application/flexfec 5.1.4. Registration of application/flexfec
Type name: application Type name: application
Subtype name: flexfec Subtype name: flexfec
Required parameters: Required parameters:
o rate: The RTP timestamp (clock) rate. The rate SHALL be larger o rate: The RTP timestamp (clock) rate. The rate SHALL be larger
skipping to change at page 24, line 36 skipping to change at page 30, line 42
Published specification: [RFCXXXX]. Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that Applications that use this media type: Multimedia applications that
want to improve resiliency against packet loss by sending redundant want to improve resiliency against packet loss by sending redundant
data in addition to the source media. data in addition to the source media.
Fragment identifier considerations: None. Fragment identifier considerations: None.
Additional information: None. Additional information: None.
Person & email address to contact for further information: Varun Person & email address to contact for further information: IESG
Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads <iesg@ietf.org> and IETF Audio/Video Transport Payloads Working Group
Working Group. (or it's successor as delegated by the IESG).
Intended usage: COMMON. Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP [RFC3550]. hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>. Author: IESG <iesg@ietf.org>.
Change controller: IETF Audio/Video Transport Working Group delegated
from the IESG.
Provisional registration? (standards tree only): Yes. Change controller: IETF Audio/Video Transport Payloads Working Group
delegated from the IESG (or it's successor as delegated by the IESG).
5.2. Mapping to SDP Parameters 5.2. Mapping to SDP Parameters
Applications that are using RTP transport commonly use Session Applications that use the RTP transport commonly use Session
Description Protocol (SDP) [RFC4566] to describe their RTP sessions. Description Protocol (SDP) [RFC4566] to describe their RTP sessions.
The information that is used to specify the media types in an RTP The information that is used to specify the media types in an RTP
session has specific mappings to the fields in an SDP description. session has specific mappings to the fields in an SDP description.
This section provides these mappings for the media subtypes This section provides these mappings for the media subtypes
registered by this document. Note that if an application does not registered by this document. Note that if an application does not
use SDP to describe the RTP sessions, an appropriate mapping must be use SDP to describe the RTP sessions, an appropriate mapping must be
defined and used to specify the media types and their parameters for defined and used to specify the media types and their parameters for
the control/description protocol employed by the application. the control/description protocol employed by the application.
The mapping of the media type specification for "non-interleaved- The mapping of the media type specification for "non-interleaved-
skipping to change at page 25, line 43 skipping to change at page 31, line 46
5.2.1. Offer-Answer Model Considerations 5.2.1. Offer-Answer Model Considerations
When offering 1-D interleaved parity FEC over RTP using SDP in an When offering 1-D interleaved parity FEC over RTP using SDP in an
Offer/Answer model [RFC3264], the following considerations apply: Offer/Answer model [RFC3264], the following considerations apply:
o Each combination of the L and D parameters produces a different o Each combination of the L and D parameters produces a different
FEC data and is not compatible with any other combination. A FEC data and is not compatible with any other combination. A
sender application may desire to offer multiple offers with sender application may desire to offer multiple offers with
different sets of L and D values as long as the parameter values different sets of L and D values as long as the parameter values
are valid. The receiver SHOULD normally choose the offer that has are valid. The receiver SHOULD choose the offer that has a
a sufficient amount of interleaving. If multiple such offers sufficient amount of interleaving. If multiple such offers exist,
exist, the receiver may choose the offer that has the lowest the receiver may choose the offer that has the lowest overhead or
overhead or the one that requires the smallest amount of the one that requires the smallest amount of buffering. The
buffering. The selection depends on the application requirements. selection depends on the application requirements.
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 the maximum delay between the the repair-window is related to the maximum delay between the
transmission of a source packet and the associated repair packet. transmission of a source packet and the associated repair packet.
This directly impacts the buffering requirement on the receiver This directly impacts the buffering requirement on the receiver
side and the receiver must consider this when choosing an offer. side and the receiver must consider this when choosing an offer.
o Any unknown option in the offer MUST be ignored and deleted from o 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, see [RFC2326] and [RFC7826]) or the Session Announcement (RTSP, for RTSP 1.0 see [RFC2326] and for RTSP 2.0 see [RFC7826]) or
Protocol (SAP) [RFC2974], the following considerations apply: the Session Announcement Protocol (SAP) [RFC2974], the following
considerations apply:
o The payload format configuration parameters are all declarative o The payload format configuration parameters are all declarative
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 stream that is best for them. should choose the repair stream that is best for them.
6. Protection and Recovery Procedures - Parity Codes 6. Protection and Recovery Procedures - Parity Codes
skipping to change at page 29, line 27 skipping to change at page 35, line 33
To use flexible bitmasks, the first two FEC header bits MUST have R=0 To use flexible bitmasks, the first two FEC header bits MUST have R=0
and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source and F=0. A 15-bit, 46-bit, or 110-bit mask indicates which source
packets are protected by a FEC repair packet. If the bit i in the packets are protected by a FEC repair packet. If the bit i in the
mask is set to 1, the source packet number N + i is protected by this mask is set to 1, the source packet number N + i is protected by this
FEC repair packet, where N is the sequence number base indicated in FEC repair packet, where N is the sequence number base indicated in
the FEC header. The most significant bit of the mask corresponds to the FEC header. The most significant bit of the mask corresponds to
i=0. The least signficant bit of the mask corresponds to i=14 in the i=0. The least signficant bit of the mask corresponds to i=14 in the
15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit mask. 15-bit mask, i=45 in the 46-bit mask, or i=109 in the 110-bit mask.
The bitmasks are able to represent arbitrary protection patterns, for The bitmasks are able to represent arbitrary protection patterns, for
example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase. example, 1-D interleaved, 1-D non-interleaved, 2-D.
6.3.1.2. Using L and D Offsets 6.3.1.2. Using L and D Offsets
Denote the set of the source packets associated with repair packet p* 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 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 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 the FEC protection applied over a column, and L source packets plus
one repair packet for the FEC protection applied over a row. Recall one repair packet for the FEC protection applied over a row. Recall
that 1-D interleaved and non-interleaved FEC protection can fully that 1-D interleaved and non-interleaved FEC protection can fully
recover the missing information if there is only one source packet recover the missing information if there is only one source packet
skipping to change at page 32, line 9 skipping to change at page 38, line 32
recovery of the RTP "payload" is as follows, where "payload" refers recovery of the RTP "payload" is as follows, where "payload" refers
to everything following the fixed 12-byte RTP header, including to everything following the fixed 12-byte RTP header, including
extensions, CSRC list, true payload and padding. extensions, CSRC list, true payload and padding.
1. Append Y bytes to the new packet. 1. Append Y bytes to the new packet.
2. For each of the source packets that are successfully received in 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 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 the 13th octet of the packet. If any of the bit strings
generated from the source packets has a length shorter than Y, 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 pad them to that length. The zero-padding octets MUST be added
the end of the bit string. Note that the information of the at the end of the bit string. Note that the information of the
first 8 octets are protected by the FEC header. first 8 octets are protected by the FEC header.
3. For the repair packet in T, compute the FEC bit string from the 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 repair packet payload, i.e., the Y octets of data following the
FEC header. Note that the FEC header may be different sizes FEC header. Note that the FEC header may be different sizes
depending on the variant and bitmask size. depending on the variant and bitmask size.
4. Calculate the recovered bit string as the XOR of the bit strings 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 all source packets in T and the FEC bit string
generated from the repair packet in T. generated from the repair packet in T.
skipping to change at page 36, line 15 skipping to change at page 44, line 15
7.1.2. Example SDP for Flexible FEC Protection with explicit signalling 7.1.2. Example SDP for Flexible FEC Protection with explicit signalling
in the SDP in the SDP
This example shows one source video stream (ssrc:1234) and one FEC This example shows one source video stream (ssrc:1234) and one FEC
repair streams (ssrc:2345). One FEC group is formed with the repair streams (ssrc:2345). One FEC group is formed with the
"a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams "a=ssrc-group:FEC-FR 1234 2345" line. The source and repair streams
are multiplexed on different SSRCs. The repair window is set to 200 are multiplexed on different SSRCs. The repair window is set to 200
ms. ms.
v=0 v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=2-D Parity FEC with no in band signalling Example s=2-D Parity FEC with no in band signalling Example
t=0 0 t=0 0
m=video 30000 RTP/AVP 100 110 m=video 30000 RTP/AVP 100 110
c=IN IP4 233.252.0.1/127
c=IN IP4 192.0.2.0/24
a=rtpmap:100 MP2T/90000 a=rtpmap:100 MP2T/90000
a=rtpmap:110 flexfec/90000 a=rtpmap:110 flexfec/90000
a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000 a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
a=ssrc:1234 a=ssrc:1234
a=ssrc:2345 a=ssrc:2345
a=ssrc-group:FEC-FR 1234 2345 a=ssrc-group:FEC-FR 1234 2345
7.2. On the Use of the RTP Stream Identifier Source Description 7.2. On the Use of the RTP Stream Identifier Source Description
The RTP Stream Identifier Source Description [I-D.ietf-avtext-rid] is The RTP Stream Identifier Source Description [I-D.ietf-avtext-rid] is
a format that can be used to identify a single RTP source stream a format that can be used to identify a single RTP source stream
along with an associated repair stream. However, this specification along with an associated repair stream. However, this specification
already defines a method of source and repair stream identification already defines a method of source and repair stream identification
that can enable protection of multiple source streams with a single that can enable protection of multiple source streams with a single
repair stream. Therefore the RTP Stream Idenfifer Source Description repair stream. Therefore the RTP Stream Idenfifer Source Description
SHOULD NOT be used for the Flexible FEC payload format SHOULD NOT be used for the Flexible FEC payload format
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 MUST be considered carefully before injecting the repair using FEC should be considered carefully before injecting the repair
streams into the network. In particular, in bandwidth-limited streams into the network. In particular, in bandwidth-limited
networks, FEC repair streams may consume a significant part of the networks, FEC repair streams may consume a significant part of the
available bandwidth and consequently may congest the network. In available bandwidth and consequently may congest the network. In
such cases, the applications MUST NOT arbitrarily increase the amount such cases, the applications MUST NOT arbitrarily increase the amount
of FEC protection since doing so may lead to a congestion collapse. of FEC protection since doing so may lead to a congestion collapse.
If desired, stronger FEC protection MAY be applied only after the If desired, stronger FEC protection MAY be applied only after the
source rate has been reduced. source rate has been reduced.
In a network-friendly implementation, an application SHOULD NOT send/ In a network-friendly implementation, an application should avoid
receive FEC repair streams if it knows that sending/receiving those sending/receiving FEC repair streams if it knows that sending/
FEC repair streams would not help at all in recovering the missing receiving those FEC repair streams would not help at all in
packets. It is RECOMMENDED that the amount and type (row, column, or recovering the missing packets. It is RECOMMENDED that the amount
both) of FEC protection is adjusted dynamically based on the packet and type (row, column, or both) of FEC protection is adjusted
loss rate and burst loss length observed by the applications. dynamically based on the packet loss rate and burst loss length
observed by the applications.
In multicast scenarios, it may be difficult to optimize the FEC In multicast scenarios, it may be difficult to optimize the FEC
protection per receiver. If there is a large variation among the protection per receiver. If there is a large variation among the
levels of FEC protection needed by different receivers, it is levels of FEC protection needed by different receivers, it is
RECOMMENDED that the sender offers multiple repair streams with RECOMMENDED that the sender offers multiple repair streams with
different levels of FEC protection and the receivers join the different levels of FEC protection and the receivers join the
corresponding multicast sessions to receive the repair stream(s) that corresponding multicast sessions to receive the repair stream(s) that
is best for them. is best for them.
9. Security Considerations 9. Security Considerations
skipping to change at page 37, line 47 skipping to change at page 46, line 13
payloads following this memo may vary. It is dependent on the payloads following this memo may vary. It is dependent on the
application, transport and signaling protocol employed. Therefore, a application, transport and signaling protocol employed. Therefore, a
single mechanism is not sufficient, although if suitable, using the single mechanism is not sufficient, although if suitable, using the
Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended.
Other mechanisms that may be used are IPsec [RFC4301] and Transport Other mechanisms that may be used are IPsec [RFC4301] and Transport
Layer Security (TLS, see [RFC8446]) (RTP over TCP); other Layer Security (TLS, see [RFC8446]) (RTP over TCP); other
alternatives may exist. alternatives may exist.
Given that FLEX FEC enables the protection of multiple source Given that FLEX FEC enables the protection of multiple source
streams, there exists the possibility that multiple source buffers streams, there exists the possibility that multiple source buffers
may be created that may not be used. In addition, the interaction may be created that may not be used. An attacker could leverage
between a FLEX FEC implementation and higher-layer applications may unused source buffers to as a means of occupying memory in a FLEX FEC
be affected by non-uniform processing requirements of the FEC scheme. endpoint. Moreover the application source data may not be perfectly
matched with FLEX FEC source partitioning. If this is the case,
there is a possibility for unprotected source data if, for instance,
the FLEX FEC implementation discards data that does not fit perfectly
into its source processing requirements.
10. IANA Considerations 10. IANA Considerations
New media subtypes are subject to IANA registration. For the New media subtypes are subject to IANA registration. For the
registration of the payload formats and their parameters introduced registration of the payload formats and their parameters introduced
in this document, refer to Section 5. in this document, refer to Section 5.
11. Acknowledgments 11. Acknowledgments
Some parts of this document are borrowed from [RFC5109]. Thus, the Some parts of this document are borrowed from [RFC5109]. Thus, the
skipping to change at page 39, line 25 skipping to change at page 47, line 43
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013, RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>. <https://www.rfc-editor.org/info/rfc6838>.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP) "Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
September 2013, <https://www.rfc-editor.org/info/rfc7022>. September 2013, <https://www.rfc-editor.org/info/rfc7022>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informative References 12.2. Informative References
[I-D.ietf-avtext-rid] [I-D.ietf-avtext-rid]
Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream Roach, A., Nandakumar, S., and P. Thatcher, "RTP Stream
Identifier Source Description (SDES)", draft-ietf-avtext- Identifier Source Description (SDES)", draft-ietf-avtext-
rid-09 (work in progress), October 2016. rid-09 (work in progress), October 2016.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, Streaming Protocol (RTSP)", RFC 2326,
DOI 10.17487/RFC2326, April 1998, DOI 10.17487/RFC2326, April 1998,
 End of changes. 127 change blocks. 
143 lines changed or deleted 237 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/