| PAYLOAD | M. Zanaty |
| Internet-Draft | Cisco |
| Intended status: Standards Track | V. Singh |
| Expires: January 17, 2019 | callstats.io |
| A. Begen | |
| Networked Media | |
| G. Mandyam | |
| Qualcomm Innovation Center | |
| July 16, 2018 |
RTP Payload Format for Flexible Forward Error Correction (FEC)
draft-ietf-payload-flexible-fec-scheme-08
This document defines new RTP payload formats for the Forward Error Correction (FEC) packets that are generated by the non-interleaved and interleaved parity codes from source media encapsulated in RTP. 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 source RTP streams. These FEC repair packets are sent in a redundancy RTP stream separate from the source RTP stream(s) that carries the source packets. RTP source packets that were lost in transmission can be reconstructed using the source and repair packets that were received. The non-interleaved and interleaved parity codes which are defined in this specification offer a good protection against random and bursty packet losses, respectively, at a cost of decent complexity. The RTP payload formats that are defined in this document address the scalability issues experienced with the earlier specifications including RFC 2733, RFC 5109 and SMPTE 2022-1, and offer several improvements. Due to these changes, the new payload formats are not backward compatible with the earlier specifications, but endpoints that do not implement this specification can still work by simply ignoring the FEC repair packets.
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This Internet-Draft will expire on January 17, 2019.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document defines new RTP payload formats for the Forward Error Correction (FEC) that is generated by the non-interleaved and interleaved parity codes from a source media encapsulated in RTP [RFC3550]. The type of the source media protected by these parity codes can be audio, video, text or application. The FEC data are generated according to the media type parameters, which are communicated out-of-band (e.g., in SDP). Furthermore, the associations or relationships between the source and repair RTP streams may be communicated in-band or out-of-band. The in-band mechanism is advantageous when the endpoint is adapting the FEC parameters. The out-of-band mechanism may be preferable when the FEC parameters are fixed.
The Redunadncy RTP Stream [RFC7656] repair packets proposed in this document protect the Source RTP Stream packets that belong to the same RTP session.
Both the non-interleaved and interleaved parity codes use the eXclusive OR (XOR) operation to generate the repair packets. The following steps take place:
At the receiver side, if all of the source packets are successfully received, there is no need for FEC recovery and the repair packets are discarded. However, if there are missing source packets, the repair packets can be used to recover the missing information. Figure 1 and Figure 2 describe example block diagrams for the systematic parity FEC encoder and decoder, respectively.
+------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder |
| (Sender) | --> +==+ +==+
+------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+
+--+ +==+
Figure 1: Block diagram for systematic parity FEC encoder
+------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder |
+==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+
Figure 2: Block diagram for systematic parity FEC decoder
In Figure 2, it is clear that the FEC repair packets have to be 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 time that spans a FEC block, which consists of the source packets and the corresponding repair packets. At the receiver side, the FEC decoder SHOULD buffer source and repair packets at least for the duration of the repair window, to allow all the repair packets to arrive. The FEC decoder can start decoding the already received packets sooner; however, it should not register a FEC decoding failure until it waits at least for the duration of the repair window.
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 applying the XOR operation to every L consecutive packets as sketched in Figure 3. This process is referred to as 1-D non-interleaved FEC protection. As a result of this process, D repair packets are generated, which are referred to as non-interleaved (or row) FEC repair packets.
+--------------------------------------------------+ --- +===+
| S_1 S_2 S3 ... S_L | + |XOR| = |R_1|
+--------------------------------------------------+ --- +===+
+--------------------------------------------------+ --- +===+
| S_L+1 S_L+2 S_L+3 ... S_2xL | + |XOR| = |R_2|
+--------------------------------------------------+ --- +===+
. . . . . .
. . . . . .
. . . . . .
+--------------------------------------------------+ --- +===+
| S_(D-1)xL+1 S_(D-1)xL+2 S_(D-1)xL+3 ... S_DxL | + |XOR| = |R_D|
+--------------------------------------------------+ --- +===+
Figure 3: Generating non-interleaved (row) FEC repair 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 Figure 4. In this case the endpoint generates L repair packets. This process is referred to as 1-D interleaved FEC protection, and the resulting L repair packets are referred to as interleaved (or column) FEC repair packets.
+-------------+ +-------------+ +-------------+ +-------+
| S_1 | | S_2 | | S3 | ... | S_L |
| S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL |
| . | | . | | | | |
| . | | . | | | | |
| . | | . | | | | |
| S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL |
+-------------+ +-------------+ +-------------+ +-------+
+ + + +
------------- ------------- ------------- -------
| XOR | | XOR | | XOR | ... | XOR |
------------- ------------- ------------- -------
= = = =
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| ... |C_L|
+===+ +===+ +===+ +===+
Figure 4: Generating interleaved (column) FEC repair packets
A sender may generate one non-interleaved repair packet out of L consecutive source packets or one interleaved repair packet out of D non-consecutive source packets. Regardless of whether the repair packet is a non-interleaved or an interleaved one, it can provide a full recovery of the missing information if there is only one packet missing among the corresponding source packets. This implies that 1-D non-interleaved FEC protection performs better when the source packets are randomly lost. However, if the packet losses occur in bursts, 1-D interleaved FEC protection performs better provided that L is chosen large enough, i.e., L-packet duration is not shorter than the observed burst duration. If the sender generates non-interleaved FEC repair packets and a burst loss hits the source packets, the repair operation fails. This is illustrated in Figure 5.
+---+ +---+ +===+
| 1 | X X | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 9 | | 10| | 11| | 12| |R_3|
+---+ +---+ +---+ +---+ +===+
Figure 5: Example scenario where 1-D non-interleaved FEC protection fails error recovery (Burst Loss)
The sender may generate interleaved FEC repair packets to combat with the bursty packet losses. However, two or more random packet losses may hit the source and repair packets in the same column. In that case, the repair operation fails as well. This is illustrated in Figure 6. Note that it is possible that two burst losses may occur back-to-back, in which case interleaved FEC repair packets may still fail to recover the lost data.
+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 6: Example scenario where 1-D interleaved FEC protection fails error recovery (Periodic Loss)
In networks where the source packets are lost both randomly and in bursts, the sender ought to generate both non-interleaved and interleaved FEC repair packets. This type of FEC protection is known as 2-D parity FEC protection. At the expense of generating more FEC repair packets, thus increasing the FEC overhead, 2-D FEC provides superior protection against mixed loss patterns. However, it is 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 example scenario is illustrated in Figure 7.
+---+ +---+ +===+
| 1 | X X | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +===+
| 9 | X X | 12| |R_3|
+---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 7: Example scenario #1 where 2-D parity FEC protection fails error recovery
2-D parity FEC protection also fails when at least two rows are missing a source and the FEC packet and the missing source packets (in at least two rows) are aligned in the same column. An example loss pattern is sketched in Figure 8. Similarly, 2-D parity FEC protection cannot repair all missing source packets when at least two columns are missing a source and the FEC packet and the missing source packets (in at least two columns) are aligned in the same row.
+---+ +---+ +---+
| 1 | | 2 | X | 4 | X
+---+ +---+ +---+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+
| 9 | | 10| X | 12| X
+---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 8: Example scenario #2 where 2-D parity FEC protection fails error recovery
The overhead is defined as the ratio of the number of bytes belonging to the repair packets to the number of bytes belonging to the protected source packets.
Generally, repair packets are larger in size compared to the source packets. Also, not all the source packets are necessarily equal in size. However, assuming that each repair packet carries an equal number of bytes carried by a source packet, the overhead for different FEC protection methods can be computed as follows:
where L and D are the number of columns and rows in the source block, respectively.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document uses a number of definitions from [RFC6363].
This section describes the formats of the source packets and defines the formats of the FEC repair packets.
The source packets contain the information that identifies the source block and the position within the source block occupied by the packet. Since the source packets that are carried within an RTP stream already contain unique sequence numbers in their RTP headers [RFC3550], the source packets can be identified in a straightforward manner and there is no need to append additional field(s). The primary advantage of not modifying the source packets in any way is that it provides backward compatibility for the receivers that do not support FEC at all. In multicast scenarios, this backward compatibility becomes quite useful as it allows the non-FEC-capable and FEC-capable receivers to receive and interpret the same source packets sent in the same multicast session.
The source packets are transmitted as usual without altering them. They are used along with the FEC repair packets to recover any missing source packets, making this scheme a systematic code.
The source packets are full RTP packets with optional CSRC list, RTP header extension, and padding. If any of these optional elements are present in the source RTP packet, and that source packet is lost, they are recovered by the FEC repair operation, which recovers the full source RTP packet including these optional elements.
The FEC repair packets MUST contain information that identifies the source block they pertain to and the relationship between the contained repair packets and the original source block. For this purpose, the RTP header of the repair packets is used, as well as another header within the RTP payload, called the FEC header, as shown in Figure 9.
Note that all the source stream packets that are protected by a particular FEC packet need to be in the same RTP session.
+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header |
+------------------------------+ ---+
| FEC Header | |
+------------------------------+ | RTP Payload
| Repair "Payload" | |
+------------------------------+ ---+
Figure 9: Format of FEC repair packets
Repair "Payload", which follows the FEC Header, includes repair of everything following the fixed 12-byte RTP header of the source packet, including any CSRC list and header extensions if present.
The RTP header is formatted according to [RFC3550] with some further clarifications listed below:
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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|F|P|X| CC |M| PT recovery | ...varies depending on R/F... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| ...varies depending on R/F... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Repair "Payload" follows FEC Header :
: :
Figure 10: FEC Header
Repair "Payload", which follows the FEC Header, includes repair of everything following the fixed 12-byte RTP header of the source packet, including any CSRC list and header extensions if present.
+---+---+----------------------------------------------------------+
| R | F | FEC Header variant |
+---+---+----------------------------------------------------------+
| 0 | 0 | Flexible FEC Mask fields indicate source packets |
| 0 | 1 | Fixed FEC M/N (cols/rows) fields indicate source packets |
| 1 | 0 | Retransmission of a single source packet |
| 1 | 1 | Invalid, MUST NOT send, MUST ignore if received |
+---+---+----------------------------------------------------------+
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 source packets, as shown in Figure 12.
The second variant, when R=0 and F=1, has a number of columns (M) and rows (N) to signal protected source packets, as shown in Figure 13.
The final variant, when R=1 and F=0, is a retransmission format as shown in Figure 15.
No variant uses R=1 and F=1, which is invalid, and MUST NOT be sent by senders, and MUST be ignored by receivers.
The FEC header for all variants consists of the following common fields:
When R=0 and F=0, the FEC Header includes flexible mask fields.
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|0|P|X| CC |M| PT recovery | length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base_i |k| Mask [0-14] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|k| Mask [15-45] (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mask [46-109] (optional) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... next SN base and Mask for CSRC_i in CSRC list ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Repair "Payload" follows FEC Header :
: :
Figure 12: FEC Header for F=0
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 section.
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|P|X| CC |M| PT recovery | length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base_i | L (columns) | D (rows) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... next SN base and M/N for CSRC_i in CSRC list ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Repair "Payload" follows FEC Header :
: :
Figure 13: FEC Header for F=1
If L=0, D=0, use the optional payload format parameters for L and D.
If L>0, D=0, indicates Row FEC, and no column FEC will follow.
Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L.
If L>0, D=1, indicates Row FEC, and column FEC will follow.
Hence, FEC = SN, SN+1, SN+2, ... , SN+(L-1), SN+L.
And more FEC to come.
If L>0, D>1, indicates column FEC of every L packet
in a group of D packets starting at SN base.
Hence, FEC = SN+(Lx0), SN+(Lx1), ... , SN+(LxD).
Figure 14: Interpreting the L and D field values
Consequently, the following conditions occur for L and D values:
It should be noted that the flexible mask-based approach may be inefficient for protecting a large number of source packets, or impossible to signal if larger than the largest mask size. In such cases, the fixed columns and rows variant may be more useful.
When R=1 and F=0, the FEC packet is a retransmission of a single source packet.
Note that the layout of this retransmission packet is different from other FEC repair packets. The sequence number (SN base_i) replaces the length recovery in the FEC header, since the length is already known for a single packet. There are no L, D or Mask fields, since only a single packet is retransmitted, identified by the sequence number in the FEC header. The source packet SSRC is included in the FEC header for retransmissions, not in the RTP header CSRC list as in the FEC header variants with R=0.
This FEC header layout is identical to the source RTP (version 2) packet, starting with its RTP header, where the retransmission "payload" is everything following the fixed 12-byte RTP header of the source packet, including CSRC list and extensions if present. Therefore, the only operation needed for sending retransmissions is to prepend a new RTP header to the source packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| P|X| CC |M| Payload Type| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Retransmission "Payload" follows FEC Header :
: :
Figure 15: FEC Header for Retransmission
This section provides the media subtype registration for the non-interleaved and interleaved parity FEC. The parameters that are required to configure the FEC encoding and decoding operations are also defined in this section. If no specific FEC code is specified in the subtype, then the FEC code defaults to the parity code defined in this specification.
This registration is done using the template defined in [RFC6838] and following the guidance provided in [RFC3555].
Note to the RFC Editor: In the following sections, please replace "XXXX" with the number of this document prior to publication as an RFC.
Type name: audio
Subtype name: flexfec
Required parameters:
Optional parameters:
Encoding considerations: This media type is framed (See Section 4.8 in the template document [RFC6838]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
Fragment identifier considerations: None.
Additional information: None.
Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
Provisional registration? (standards tree only): Yes.
Type name: video
Subtype name: flexfec
Required parameters:
Optional parameters:
Encoding considerations: This media type is framed (See Section 4.8 in the template document [RFC6838]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
Fragment identifier considerations: None.
Additional information: None.
Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
Provisional registration? (standards tree only): Yes.
Type name: text
Subtype name: flexfec
Required parameters:
Optional parameters:
Encoding considerations: This media type is framed (See Section 4.8 in the template document [RFC6838]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
Fragment identifier considerations: None.
Additional information: None.
Person & email address to contact for further information: Varun Singh <vvarun@callstats.io> and IETF Audio/Video Transport Payloads Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
Provisional registration? (standards tree only): Yes.
Type name: application
Subtype name: flexfec
Required parameters:
Optional parameters:
Encoding considerations: This media type is framed (See Section 4.8 in the template document [RFC6838]) and contains binary data.
Security considerations: See Section 9 of [RFCXXXX].
Interoperability considerations: None.
Published specification: [RFCXXXX].
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
Fragment identifier considerations: None.
Additional information: None.
Person & email address to contact for further information: Varun Singh <varun@callstats.io> and IETF Audio/Video Transport Payloads Working Group.
Intended usage: COMMON.
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
Author: Varun Singh <varun@callstats.io>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
Provisional registration? (standards tree only): Yes.
Applications that are using RTP transport commonly use Session Description Protocol (SDP) [RFC4566] to describe their RTP sessions. The information that is used to specify the media types in an RTP session has specific mappings to the fields in an SDP description. This section provides these mappings for the media subtypes registered by this document. Note that if an application does not use SDP to describe the RTP sessions, an appropriate mapping must be defined and used to specify the media types and their parameters for the control/description protocol employed by the application.
The mapping of the media type specification for "non-interleaved-parityfec" and "interleaved-parityfec" and their parameters in SDP is as follows:
SDP examples are provided in
When offering 1-D interleaved parity FEC over RTP using SDP in an Offer/Answer model [RFC3264], the following considerations apply:
In declarative usage, like SDP in the Real-time Streaming Protocol (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) [RFC2974], the following considerations apply:
This section provides a complete specification of the 1-D and 2-D parity codes and their RTP payload formats. It does not apply to the single packet retransmission format (R=1 in the FEC Header).
The following sections specify the steps involved in generating the repair packets and reconstructing the missing source packets from the repair packets.
The RTP Header of a repair packet is formed based on the guidelines given in Section 4.2.
The FEC Header and Repair "Payload" of repair packets are formed by applying the XOR operation on the bit strings that are generated from the individual source packets protected by this particular repair packet. The set of the source packets that are associated with a given repair packet can be computed by the formula given in Section 6.3.1.
The bit string is formed for each source packet by concatenating the following fields together in the order specified:
The FEC bit string is generated by applying the parity operation on the bit strings produced from the source packets. The FEC header is generated from the FEC bit string as follows:
If the lengths of the source packets are not equal, each shorter packet MUST be padded to the length of the longest packet by adding octet 0's at the end.
Due to this possible padding and mandatory FEC header, a repair packet has a larger size than the source packets it protects. This may cause problems if the resulting repair packet size exceeds the Maximum Transmission Unit (MTU) size of the path over which the repair stream is sent.
This section describes the recovery procedures that are required to reconstruct the missing source packets. The recovery process has two steps. In the first step, the FEC decoder determines which source and repair packets should be used in order to recover a missing packet. In the second step, the decoder recovers the missing packet, which consists of an RTP header and RTP payload.
The following describes the RECOMMENDED algorithms for the first and second steps. Based on the implementation, different algorithms MAY be adopted. However, the end result MUST be identical to the one produced by the algorithms described below.
Note that the same algorithms are used by the 1-D parity codes, regardless of whether the FEC protection is applied over a column or a row. The 2-D parity codes, on the other hand, usually require multiple iterations of the procedures described here. This iterative decoding algorithm is further explained in Section 6.3.4.
The first step is associating the source and repair packets. This association can be via flexible bitmasks, or fixed L and D offsets which can be in the FEC header or signaled in SDP in optional payload format parameters when L=D=0 in the FEC header.
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 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 FEC repair packet, where N is the sequence number base indicated in 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 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 example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase.
Denote the set of the source packets associated with repair packet p* by set T(p*). Note that in a source block whose size is L columns by D rows, set T includes D source packets plus one repair packet for the FEC protection applied over a column, and L source packets plus one repair packet for the FEC protection applied over a row. Recall that 1-D interleaved and non-interleaved FEC protection can fully recover the missing information if there is only one source packet missing per column or row in set T. If there are more than one source packets missing per column or row in set T, 1-D FEC protection may fail to recover all the missing information.
When value of L is non-zero, the 8-bit fields indicate the offset of packets protected by an interleaved (D>0) or non-interleaved (D=0) FEC packet. Using a combination of interleaved and non-interleaved FEC repair packets can form 2-D protection patterns.
Mathematically, for any received repair packet, p*, the sequence numbers of the source packets that are protected by this repair packet are determined as follows, where p*_snb is the sequence number base in the FEC header:
When D = 0:
p*_snb, p*_snb+1,..., p*_snb+L
When D > 0:
p*_snb, p*_snb+(Lx1), p*_snb+(Lx2),..., p*_snb+(LxD)
If the endpoint relies entirely on out-of-band signaling (R=0, F=1, L=0, D=0 in the FEC header), then this information may be inferred from the media type parameters specified in the SDP description. Furthermore, the payload type field in the RTP header assists the receiver to distinguish an interleaved or non-interleaved FEC packet.
Mathematically, for any received repair packet, p*, the sequence numbers of the source packets that are protected by this repair packet are determined as follows:
p*_snb + i * X_1 (modulo 65536)
where p*_snb denotes the value in the SN base field of p*'s FEC header, X_1 is set to L and 1 for the interleaved and non-interleaved FEC repair packets, respectively, and
0 <= i < X_2
where X_2 is set to D and L for the interleaved and non-interleaved FEC repair packets, respectively.
For a given set T, the procedure for the recovery of the RTP header of the missing packet, whose sequence number is denoted by SEQNUM, is as follows:
This procedure recovers the header of an RTP packet up to (and including) the SSRC field.
Following the recovery of the RTP header, the procedure for the recovery of the RTP "payload" is as follows, where "payload" refers to everything following the fixed 12-byte RTP header, including extensions, CSRC list, true payload and padding.
In 2-D parity FEC protection, the sender generates both non-interleaved and interleaved FEC repair packets to combat with the mixed loss patterns (random and bursty). At the receiver side, these FEC packets are used iteratively to overcome the shortcomings of the 1-D non-interleaved/interleaved FEC protection and improve the chances of full error recovery.
The iterative decoding algorithm runs as follows:
The algorithm terminates either when all missing source packets are fully recovered or when there are still remaining missing source packets but the FEC repair packets are not able to recover any more source packets. For the example scenarios when the 2-D parity FEC protection fails full recovery, refer to Section 1.1.4. Upon termination, variable num_recovered_so_far has a value equal to the total number of recovered source packets.
Example:
Suppose that the receiver experienced the loss pattern sketched in Figure 16.
+---+ +---+ +===+
X X | 3 | | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +===+
| 9 | X X | 12| |R_3|
+---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 16: Example loss pattern for the iterative decoding algorithm
The receiver executes the iterative decoding algorithm and recovers source packets #1 and #11 in the first iteration. The resulting pattern is sketched in Figure 17.
+---+ +---+ +---+ +===+
| 1 | X | 3 | | 4 | |R_1|
+---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +===+
| 9 | X | 11| | 12| |R_3|
+---+ +---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 17: The resulting pattern after the first iteration
Since the if condition holds true, the receiver runs a new iteration. In the second iteration, source packets #2 and #10 are recovered, resulting in a full recovery as sketched in Figure 18.
+---+ +---+ +---+ +---+ +===+
| 1 | | 2 | | 3 | | 4 | |R_1|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 9 | | 10| | 11| | 12| |R_3|
+---+ +---+ +---+ +---+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 18: The resulting pattern after the second iteration
This section provides two SDP [RFC4566] examples. The examples use the FEC grouping semantics defined in [RFC5956].
In this example, we have one source video stream and one FEC repair stream. The source and repair streams are multiplexed on different SSRCs. The repair window is set to 200 ms.
v=0
o=mo 1122334455 1122334466 IN IP4 fec.example.com
s=FlexFEC minimal SDP signalling Example
t=0 0
m=video 30000 RTP/AVP 96 98
c=IN IP4 143.163.151.157
a=rtpmap:96 VP8/90000
a=rtpmap:98 flexfec/90000
a=fmtp:98; repair-window=200ms
This example shows one source video stream (ssrc:1234) and one FEC repair streams (ssrc:2345). One FEC group is formed with the "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 ms.
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=2-D Parity FEC with no in band signalling Example
t=0 0
m=video 30000 RTP/AVP 100 110
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=rtpmap:110 flexfec/90000
a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
a=ssrc:1234
a=ssrc:2345
a=ssrc-group:FEC-FR 1234 2345
FEC is an effective approach to provide applications resiliency against packet losses. However, in networks where the congestion is a major contributor to the packet loss, the potential impacts of using FEC MUST be considered carefully before injecting the repair streams into the network. In particular, in bandwidth-limited networks, FEC repair streams may consume a significant part of the available bandwidth and consequently may congest the network. In such cases, the applications MUST NOT arbitrarily increase the amount of FEC protection since doing so may lead to a congestion collapse. If desired, stronger FEC protection MAY be applied only after the source rate has been reduced.
In a network-friendly implementation, an application SHOULD NOT send/receive FEC repair streams if it knows that sending/receiving those FEC repair streams would not help at all in recovering the missing packets. It is RECOMMENDED that the amount and type (row, column, or both) of FEC protection is adjusted 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 protection per receiver. If there is a large variation among the levels of FEC protection needed by different receivers, it is RECOMMENDED that the sender offers multiple repair streams with different levels of FEC protection and the receivers join the corresponding multicast sessions to receive the repair stream(s) that is best for them.
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550] and in any applicable RTP profile. The main security considerations for the RTP packet carrying the RTP payload format defined within this memo are confidentiality, integrity and source authenticity. Confidentiality is achieved by encrypting the RTP payload. Integrity of the RTP packets is achieved through a suitable cryptographic integrity protection mechanism. Such a cryptographic system may also allow the authentication of the source of the payload. A suitable security mechanism for this RTP payload format should provide confidentiality, integrity protection, and at least source authentication capable of determining if an RTP packet is from a member of the RTP session.
Note that the appropriate mechanism to provide security to RTP and payloads following this memo may vary. It is dependent on the application, transport and signaling protocol employed. Therefore, a single mechanism is not sufficient, although if suitable, using the Secure Real-time Transport Protocol (SRTP) [RFC3711] is recommended. Other mechanisms that may be used are IPsec [RFC4301] and Transport Layer Security (TLS) [RFC5246] (RTP over TCP); other alternatives may exist.
New media subtypes are subject to IANA registration. For the registration of the payload formats and their parameters introduced in this document, refer to Section 5.
Some parts of this document are borrowed from [RFC5109]. Thus, the author would like to thank the editor of [RFC5109] and those who contributed to [RFC5109].
Thanks to Stephen Botzko , Bernard Aboba , Rasmus Brandt , Brian Baldino , Roni Even , Stefan Holmer , Jonathan Lennox , and Magnus Westerlund for providing valuable feedback on earlier versions of this draft.