One document matched: draft-ietf-payload-flexible-fec-scheme-00.xml
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<rfc category="std" docName="draft-ietf-payload-flexible-fec-scheme-00"
ipr="trust200902">
<front>
<title abbrev="RTP Payload Format for Parity FEC">RTP Payload Format for
Non-Interleaved and Interleaved Parity Forward Error Correction
(FEC)</title>
<author fullname="Varun Singh" initials="V." surname="Singh">
<organization>Aalto University</organization>
<address>
<postal>
<street/>
<city>Espoo</city>
<region>FIN</region>
<code/>
<country>Finland</country>
</postal>
<phone/>
<email>varun@comnet.tkk.fi</email>
</address>
</author>
<author fullname="Ali Begen" initials="A." surname="Begen">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>181 Bay Street</street>
<city>Toronto</city>
<region>ON</region>
<code>M5J 2T3</code>
<country>Canada</country>
</postal>
<email>abegen@cisco.com</email>
</address>
</author>
<author fullname="Mo Zanaty" initials="M." surname="Zanaty">
<organization>Cisco</organization>
<address>
<postal>
<street/>
<city>Raleigh</city>
<region>NC</region>
<code/>
<country>USA</country>
</postal>
<phone/>
<email>mzanaty@cisco.com</email>
</address>
</author>
<date year="2015"/>
<workgroup>PAYLOAD</workgroup>
<abstract>
<t>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 a source media encapsulated in RTP. These
parity codes are systematic codes, where a number of repair symbols are
generated from a set of source symbols. These repair symbols are sent in
a repair flow separate from the source flow that carries the source
symbols. The non-interleaved and interleaved parity codes offer a good
protection against random and bursty packet losses, respectively, at a
cost of decent complexity. The RTP payload formats that are defined in
this document address the scalability issues experienced with the
earlier specifications including RFC 2733, RFC 5109 and SMPTE 2022-1,
and offer several improvements. Due to these changes, the new payload
formats are not backward compatible with the earlier specifications, but
endpoints that do not implement the scheme can still work by simply
ignoring the FEC packets.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>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 <xref
target="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 flows may be communicated
in-band or out-of-band. Situtations where adaptivitiy of FEC parameters
is desired, the endpoint can use the in-band mechanism, whereas when the
FEC parameters are fixed, the endpoint may prefer to negotiate them
out-of-band.</t>
<t>Both the non-interleaved and interleaved parity codes use the
eXclusive OR (XOR) operation to generate the repair symbols. In a
nutshell, the following steps take place:</t>
<t><list style="numbers">
<t>The sender determines a set of source packets to be protected by
FEC based on the media type parameters.</t>
<t>The sender applies the XOR operation on the source symbols to
generate the required number of repair symbols.</t>
<t>The sender packetizes the repair symbols and sends the repair
packet(s) along with the source packets to the receiver(s) (in
different flows). The repair packets may be sent proactively or
on-demand.</t>
</list></t>
<t>Note that the source and repair packets belong to different source
and repair flows, and the sender must provide a way for the receivers to
demultiplex them, even in the case they are sent in the same 5-tuple
(i.e., same source/destination address/port with UDP). This is required
to offer backward compatibility for endpoints that do not understand the
FEC packets (See <xref target="sec_formats"/>). 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. <xref target="fig_encoder"/> and <xref
target="fig_decoder"/> describe example block diagrams for the
systematic parity FEC encoder and decoder, respectively.</t>
<t><figure anchor="fig_encoder"
title="Block diagram for systematic parity FEC encoder">
<preamble/>
<artwork align="center"><![CDATA[ +------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder |
| (Sender) | --> +==+ +==+
+------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+
+--+ +==+]]></artwork>
</figure></t>
<t><figure anchor="fig_decoder"
title="Block diagram for systematic parity FEC decoder">
<preamble/>
<artwork align="center"><![CDATA[ +------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder |
+==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+]]></artwork>
</figure></t>
<t>In <xref target="fig_decoder"/>, it is clear that the FEC packets
have to be received by the endpoint within a certain amount of time for
the FEC recovery process to be useful. In this document, we refer to the
time that spans a FEC block, which consists of the source packets and
the corresponding repair packets, as the repair window. At the receiver
side, the FEC decoder should wait at least for the duration of the
repair window after getting the first packet in a FEC block, to allow
all the repair packets to arrive. (The waiting time can be adjusted if
there are missing packets at the beginning of the FEC block.) 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.</t>
<t>Suppose that we have a group of D x L source packets that have
sequence numbers starting from 1 running to D x L, and a repair packet
is generated by applying the XOR operation to every L consecutive
packets as sketched in <xref target="fig_fecblock_row"/>. This process
is referred to as 1-D non-interleaved FEC protection. As a result of
this process, D repair packets are generated, which we refer to as
non-interleaved (or row) FEC packets.</t>
<t><figure anchor="fig_fecblock_row"
title="Generating non-interleaved (row) FEC packets">
<preamble/>
<artwork align="center"><![CDATA[+--------------------------------------------------+ --- +===+
| 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|
+--------------------------------------------------+ --- +===+]]></artwork>
</figure></t>
<t>If we apply the XOR operation to the group of the source packets
whose sequence numbers are L apart from each other, as sketched in <xref
target="fig_fecblock_column"/>. 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 packets.</t>
<t><figure anchor="fig_fecblock_column"
title="Generating interleaved (column) FEC packets">
<preamble/>
<artwork align="center"><![CDATA[+-------------+ +-------------+ +-------------+ +-------+
| 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|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
<!-- VS: confirm it is non-interleaved OR interleaved, not both. Both
would imply 2-D FEC! -->
<section title="Use Cases for 1-D FEC Protection">
<t>We 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 packets
and a burst loss hits the source packets, the repair operation fails.
This is illustrated in <xref target="fig_1d_a"/>.</t>
<t><figure anchor="fig_1d_a"
title="Example scenario where 1-D non-interleaved FEC protection fails error recovery (Burst Loss)">
<artwork align="center"><![CDATA[+---+ +---+ +===+
| 1 | X X | 4 | |R_1|
+---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+ +---+ +===+
| 9 | | 10| | 11| | 12| |R_3|
+---+ +---+ +---+ +---+ +===+
]]></artwork>
</figure></t>
<t>The sender may generate interleaved FEC packets to combat with the
bursty packet losses. However, two or more random packet losses may
hit the source and repair packets in the same column. In that case,
the repair operation fails as well. This is illustrated in <xref
target="fig_1d_b"/>. Note that it is possible that two burst losses
may occur back-to-back, in which case interleaved FEC packets may
still fail to recover the lost data.</t>
<t><figure anchor="fig_1d_b"
title="Example scenario where 1-D interleaved FEC protection fails error recovery (Periodic Loss)">
<artwork align="center"><![CDATA[+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
</section>
<section anchor="sec_2d" title="Use Cases for 2-D Parity FEC Protection">
<t>In networks where the source packets are lost both randomly and in
bursts, the sender ought to generate both non-interleaved and
interleaved FEC packets. This type of FEC protection is known as 2-D
parity FEC protection. At the expense of generating more FEC packets,
thus increasing the FEC overhead, 2-D FEC provides 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 <xref target="fig_2d1"/>.</t>
<t><figure anchor="fig_2d1"
title="Example scenario #1 where 2-D parity FEC protection fails error recovery">
<artwork align="center"><![CDATA[+---+ +---+ +===+
| 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|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
<t>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 <xref target="fig_2d2"/>. 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.</t>
<t><figure anchor="fig_2d2"
title="Example scenario #2 where 2-D parity FEC protection fails error recovery">
<artwork align="center"><![CDATA[+---+ +---+ +---+
| 1 | | 2 | X | 4 | X
+---+ +---+ +---+
+---+ +---+ +---+ +---+ +===+
| 5 | | 6 | | 7 | | 8 | |R_2|
+---+ +---+ +---+ +---+ +===+
+---+ +---+ +---+
| 9 | | 10| X | 12| X
+---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
</section>
<section title="Overhead Computation">
<t>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.</t>
<t>Generally, repair packets are larger in size compared to the source
packets. Also, not all the source packets are necessarily equal in
size. However, if we assume that each repair packet carries an equal
number of bytes carried by a source packet, we can compute the
overhead for different FEC protection methods as follows:</t>
<t><list style="symbols">
<t>1-D Non-interleaved FEC Protection: Overhead = 1/L</t>
<t>1-D Interleaved FEC Protection: Overhead = 1/D</t>
<t>2-D Parity FEC Protection: Overhead = 1/L + 1/D</t>
</list>where L and D are the number of columns and rows in the
source block, respectively.</t>
</section>
</section>
<section title="Requirements Notation">
<t>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 <xref
target="RFC2119"/>.</t>
</section>
<section title="Definitions and Notations">
<t/>
<section title="Definitions">
<t>This document uses a number of definitions from <xref
target="RFC6363"/>.</t>
</section>
<section title="Notations">
<t><list style="symbols">
<t>L: Number of columns of the source block.</t>
<t>D: Number of rows of the source block.</t>
<!-- <t>ToP: Type of protection.</t> -->
<t>bitmask: Run-length encoding of packets protected by a FEC
packet. If the bit i in the mask is set to 1, the source packet
number N + i is protected by this FEC packet. Here, N is the
sequence number base, which is indicated in the FEC packet as
well.</t>
</list></t>
</section>
</section>
<section anchor="sec_formats" title="Packet Formats">
<t>This section defines the formats of the source and repair
packets.</t>
<section title="Source Packets">
<t>The source packets MUST contain the information that identifies the
source block and the position within the source block occupied by the
packet. Since the source packets that are carried within an RTP stream
already contain unique sequence numbers in their RTP headers <xref
target="RFC3550"/>, we can identify the source packets in a
straightforward manner and there is no need to append additional
field(s). The primary advantage of not modifying the source packets in
any way is that it provides backward compatibility for the receivers
that do not support FEC at all. In multicast scenarios, this backward
compatibility becomes quite useful as it allows the non-FEC-capable
and FEC-capable receivers to receive and interpret the same source
packets sent in the same multicast session.</t>
</section>
<section anchor="sec_repair_fec_payload_id" title="Repair Packets">
<t>The repair packets MUST contain information that identifies the
source block they pertain to and the relationship between the
contained repair symbols and the original source block. For this
purpose, we use the RTP header of the repair packets as well as
another header within the RTP payload, which we refer to as the FEC
header, as shown in <xref target="fig_repairpacket"/>.</t>
<t><figure anchor="fig_repairpacket" title="Format of repair packets">
<preamble/>
<artwork align="center"><![CDATA[+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header | __
+------------------------------+ |
| FEC Header | \
+------------------------------+ > RTP Payload
| Repair Symbols | /
+------------------------------+ __|]]></artwork>
<postamble/>
</figure></t>
<t>The RTP header is formatted according to <xref target="RFC3550"/>
with some further clarifications listed below:</t>
<t><list style="symbols">
<t>Marker (M) Bit: This bit is not used for this payload type, and
SHALL be set to 0.</t>
<t>Payload Type: The (dynamic) payload type for the repair packets
is determined through out-of-band means. Note that this document
registers new payload formats for the repair packets (Refer to
<xref target="sec_parameters"/> for details). According to <xref
target="RFC3550"/>, an RTP receiver that cannot recognize a
payload type must discard it. This provides backward
compatibility. If a non-FEC-capable receiver receives a repair
packet, it will not recognize the payload type, and hence, will
discard the repair packet.</t>
<!---->
<t>Sequence Number (SN): The sequence number has the standard
definition. It MUST be one higher than the sequence number in the
previously transmitted repair packet. The initial value of the
sequence number SHOULD be random (unpredictable, based on <xref
target="RFC3550"/>).</t>
<t>Timestamp (TS): The timestamp SHALL be set to a time
corresponding to the repair packet's transmission time. Note that
the timestamp value has no use in the actual FEC protection
process and is usually useful for jitter calculations.</t>
<t>Synchronization Source (SSRC): The SSRC value SHALL be randomly
assigned as suggested by <xref target="RFC3550"/>. This allows the
sender to multiplex the source and repair flows on the same port,
or multiplex multiple repair flows on a single port. The repair
flows SHOULD use the RTCP CNAME field to associate themselves with
the source flow. <vspace blankLines="1"/>In some networks, the RTP
Source, which produces the source packets and the FEC Source,
which generates the repair packets from the source packets may not
be the same host. In such scenarios, using the same CNAME for the
source and repair flows means that the RTP Source and the FEC
Source MUST share the same CNAME (for this specific source-repair
flow association). A common CNAME may be produced based on an
algorithm that is known both to the RTP and FEC Source <xref
target="RFC7022"/>. This usage is compliant with <xref
target="RFC3550"/>. <vspace blankLines="1"/>Note that due to the
randomness of the SSRC assignments, there is a possibility of SSRC
collision. In such cases, the collisions MUST be resolved as
described in <xref target="RFC3550"/>.</t>
</list></t>
<t>The format of the FEC header is shown in <xref
target="fig_repairfecpayloadid2"/>.</t>
<t><figure anchor="fig_repairfecpayloadid2"
title="Format of the FEC header">
<preamble/>
<artwork align="center"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MSK|P|X| CC |M| PT recovery | SN base |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length recovery |M or Mask[8-15]| N or Mask[0-7]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mask [16-47] (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Mask [48-111] (optional) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<!--
OLD FORMAT:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|I|P|X| CC |M| PT recovery | SN base |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length recovery | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<t>The E bit is the extension flag reserved to indicate any future
extension to this specification.</t>
<t>The I bit is used to indicate the length of padding in the FEC
header. The padding length SHOULD be selected based on the
platform architecture and the impact of header length on the
header processing performance.</t>
<t>The Padding field is used to pad the FEC header to 12 bytes
(integer multiples of 32 bits).</t>
-->
<t>The FEC header consists of the following fields:</t>
<t><list style="symbols">
<t>The MSK field (2 bits) indicates the type of the mask. Namely:
<figure anchor="table-fec-msk-bits" title="MSK bit values">
<artwork><![CDATA[
+---------------+-------------------------------------+
| MSK bits | Use |
+---------------+-------------------------------------+
| 00 | 16-bit mask |
| 01 | 48-bit mask |
| 10 | 112-bit mask |
| 11 | packets indicated by offset M and N |
+---------------+-------------------------------------+
]]></artwork>
</figure></t>
<!-- VS: I changed L and D in this section to M and N, to avoid
confusion with the use of L and D with regular patterns. -->
<t>The P, X, CC, M and PT recovery fields are used to determine
the corresponding fields of the recovered packets.</t>
<t>The SN base field is used to indicate the lowest sequence
number, taking wrap around into account, of those source packets
protected by this repair packet.</t>
<t>The TS recovery field is used to determine the timestamp of the
recovered packets.</t>
<t>The Length recovery field is used to determine the length of
the recovered packets.</t>
<t>Mask is a run-length encoding of packets protected by the FEC
packet. Where a bit i set to 1 indicates that the source packet
with sequence number (SN base + i) is protected by this FEC
packet.</t>
<t>If the the MSK field is set to 11, it indicates the offset of
packets protected by this FEC packet. Consequently, the following
conditions may occur: <figure
anchor="table-fec-ld-field"
title="Interpreting the M and N field values">
<artwork><![CDATA[
If M=0, N=0, regular protection pattern code with the values of
L and D are indicared in the SDP description.
If M>0, N=0, indicates a non-interleaved (row) FEC of M packets
starting at SN base.
Hence, FEC = SN, SN+1, SN+2, ... , SN+(M-1), SN+M.
If M>0, N>0, indicates interleaved (column) FEC of every M packet
in a group of N packets starting at SN base.
Hence, FEC = SN+(Mx0), SN+(Mx1), ... , SN+(MxN).
]]></artwork>
</figure></t>
</list></t>
<t>The details on setting the fields in the FEC header are provided in
<xref target="sec_repair_packet_construction"/>.</t>
<t>
It should be noted that a mask-based approach (similar to the ones
specified in <xref target="RFC2733"/> and <xref target="RFC5109"/>)
may not be very efficient to indicate which source packets in the
current source block are associated with a given repair packet. In
particular, for the applications that would like to use large source
block sizes, the size of the mask that is required to describe the
source-repair packet associations may be prohibitively large. The
8-bit fields proposed in <xref target="SMPTE2022-1"/> indicate a
systematized approach. Instead the approach in this document uses
the 8-bit fields to indicate packet offsets protected by the FEC
packet. The approach in <xref target="SMPTE2022-1"/> is inherently
more efficient for regular patterns, it does not provide flexibility
to represent other protection patterns (e.g., staircase).
</t>
<t>
<!-- Yet, <xref target="SMPTE2022-1"/> carries the values of D and L in
8-bit fields. While this approach can support larger blocks compared
to the mask-based approaches, 8-bit fields may still be limiting when
a high-bitrate source flow (e.g., a flow carrying ultra high-
definition video) is to be protected or when network outages/lossy
periods span more than 255 packets. -->
</t>
</section>
</section>
<section anchor="sec_parameters" title="Payload Format Parameters">
<t>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.</t>
<section title="Media Type Registration">
<t>This registration is done using the template defined in <xref
target="RFC6838"/> and following the guidance provided in <xref
target="RFC3555"/>.</t>
<t>Note to the RFC Editor: In the following sections, please replace
"XXXX" with the number of this document prior to publication as an
RFC.</t>
<section title="Registration of audio/non-interleaved-parityfec">
<t>Type name: audio</t>
<t>Subtype name: non-interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of video/non-interleaved-parityfec">
<t>Type name: video</t>
<t>Subtype name: non-interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of text/non-interleaved-parityfec">
<t>Type name: text</t>
<t>Subtype name: non-interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of application/non-interleaved-parityfec">
<t>Type name: application</t>
<t>Subtype name: non-interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of audio/interleaved-parityfec">
<t>Type name: audio</t>
<t>Subtype name: interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of video/interleaved-parityfec">
<t>Type name: video</t>
<t>Subtype name: interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations" /> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of text/interleaved-parityfec">
<t>Type name: text</t>
<t>Subtype name: interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838" />) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations"/> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550"/>.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
<section title="Registration of application/interleaved-parityfec">
<t>Type name: application</t>
<t>Subtype name: interleaved-parityfec</t>
<t>Required parameters:</t>
<t><list style="symbols">
<t>rate: The RTP timestamp (clock) rate. The rate SHALL be
larger than 1000 Hz to provide sufficient resolution to RTCP
operations. However, it is RECOMMENDED to select the rate that
matches the rate of the protected source RTP stream.</t>
<t>L: Number of columns of the source block. L is a positive
integer.</t>
<t>D: Number of rows of the source block. D is a positive
integer.</t>
<t>ToP: Type of the protection applied by the sender: 0 for 1-D
interleaved FEC protection, 1 for 1-D non-interleaved FEC
protection, and 2 for 2-D parity FEC protection. The ToP value
of 3 is reserved for future uses.</t>
<t>repair-window: The time that spans the source packets and the
corresponding repair packets. The size of the repair window is
specified in microseconds.</t>
</list>Optional parameters: None.</t>
<t>Encoding considerations: This media type is framed (See Section
4.8 in the template document <xref target="RFC6838"/>) and
contains binary data.</t>
<t>Security considerations: See <xref
target="sec_security_considerations"/> of [RFCXXXX].</t>
<t>Interoperability considerations: None.</t>
<t>Published specification: [RFCXXXX].</t>
<t>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.</t>
<t>Fragment identifier considerations: None.</t>
<t>Additional information: None.</t>
<t>Person & email address to contact for further information:
Varun Singh <varun.singh@iki.fi> and IETF Audio/Video Transport
Payloads Working Group.</t>
<t>Intended usage: COMMON.</t>
<t>Restriction on usage: This media type depends on RTP framing, and
hence, is only defined for transport via RTP <xref
target="RFC3550" />.</t>
<t>Author: Varun Singh <varun.singh@iki.fi>.</t>
<t>Change controller: IETF Audio/Video Transport Working Group
delegated from the IESG.</t>
<t>Provisional registration? (standards tree only): Yes.</t>
</section>
</section>
<section title="Mapping to SDP Parameters">
<t>Applications that are using RTP transport commonly use Session
Description Protocol (SDP) <xref target="RFC4566"/> to describe their
RTP sessions. The information that is used to specify the media types
in an RTP session has specific mappings to the fields in an SDP
description. In this section, we provide these mappings for the media
subtypes registered by this document. Note that if an application does
not use SDP to describe the RTP sessions, an appropriate mapping must
be defined and used to specify the media types and their parameters
for the control/description protocol employed by the application.</t>
<t>The mapping of the media type specification for
"non-interleaved-parityfec" and "interleaved-parityfec" and their
parameters in SDP is as follows:</t>
<t><list style="symbols">
<t>The media type (e.g., "application") goes into the "m=" line as
the media name.</t>
<t>The media subtype goes into the "a=rtpmap" line as the encoding
name. The RTP clock rate parameter ("rate") also goes into the
"a=rtpmap" line as the clock rate.</t>
<t>The remaining required payload-format-specific parameters go
into the "a=fmtp" line by copying them directly from the media
type string as a semicolon-separated list of parameter=value
pairs.</t>
</list>SDP examples are provided in <xref target="sec_sdp"/>.</t>
<section title="Offer-Answer Model Considerations">
<t>When offering 1-D interleaved parity FEC over RTP using SDP in an
Offer/Answer model <xref target="RFC3264"/>, the following
considerations apply:</t>
<t><list style="symbols">
<t>Each combination of the L and D parameters produces a
different FEC data and is not compatible with any other
combination. A sender application may desire to offer multiple
offers with different sets of L and D values as long as the
parameter values are valid. The receiver SHOULD normally choose
the offer that has a sufficient amount of interleaving. If
multiple such offers exist, the receiver may choose the offer
that has the lowest overhead or the one that requires the
smallest amount of buffering. The selection depends on the
application requirements.</t>
<t>The value for the repair-window parameter depends on the L
and D values and cannot be chosen arbitrarily. More
specifically, L and D values determine the lower limit for the
repair-window size. The upper limit of the repair-window size
does not depend on the L and D values.</t>
<t>Although combinations with the same L and D values but with
different repair-window sizes produce the same FEC data, such
combinations are still considered different offers. The size of
the repair-window is related to the maximum delay between the
transmission of a source packet and the associated repair
packet. This directly impacts the buffering requirement on the
receiver side and the receiver must consider this when choosing
an offer.</t>
<t>There are no optional format parameters defined for this
payload. Any unknown option in the offer MUST be ignored and
deleted from the answer. If FEC is not desired by the receiver,
it can be deleted from the answer.</t>
</list></t>
</section>
<section title="Declarative Considerations">
<t>In declarative usage, like SDP in the Real-time Streaming
Protocol (RTSP) <xref target="RFC2326"/> or the Session Announcement
Protocol (SAP) <xref target="RFC2974"/>, the following
considerations apply:</t>
<t><list style="symbols">
<t>The payload format configuration parameters are all
declarative and a participant MUST use the configuration that is
provided for the session.</t>
<t>More than one configuration may be provided (if desired) by
declaring multiple RTP payload types. In that case, the
receivers should choose the repair flow that is best for
them.</t>
</list></t>
</section>
</section>
</section>
<section title="Protection and Recovery Procedures">
<t>This section provides a complete specification of the 1-D and 2-D
parity codes and their RTP payload formats.</t>
<section title="Overview">
<t>The following sections specify the steps involved in generating the
repair packets and reconstructing the missing source packets from the
repair packets.</t>
</section>
<section anchor="sec_repair_packet_construction"
title="Repair Packet Construction">
<t>The RTP header of a repair packet is formed based on the guidelines
given in <xref target="sec_repair_fec_payload_id"/>.</t>
<t>The FEC header includes 12 octets (or upto 28 octets when the longer
optional masks are used). It is constructed by applying the XOR operation
on the bit strings that are generated from the individual source packets
protected by this particular repair packet. The set of the source
packets that are associated with a given repair packet can be computed
by the formula given in <xref target="sec_associating_source_repair"/>.</t>
<t>The bit string is formed for each source packet by concatenating
the following fields together in the order specified:</t>
<t><list style="symbols">
<t>The first 64 bits of the RTP header (64 bits).</t>
<t>Unsigned network-ordered 16-bit representation of the source
packet length in bytes minus 12 (for the fixed RTP header), i.e.,
the sum of the lengths of all the following if present: the CSRC
list, extension header, RTP payload and RTP padding (16 bits).</t>
</list>
By applying the parity operation on the bit strings produced from
the source packets, we generate the FEC bit string. The FEC header is
generated from the FEC bit string as follows: </t>
<t><list style="symbols">
<t>The first (most significant) 2 bits in the FEC bit string are
skipped. The MSK bits in the FEC header are set to the appropriate
value, i.e., it depends on the chosen bitmask length.</t>
<t>The next bit in the FEC bit string is written into the P
recovery bit in the FEC header.</t>
<t>The next bit in the FEC bit string is written into the X
recovery bit in the FEC header.</t>
<t>The next 4 bits of the FEC bit string are written into the CC
recovery field in the FEC header.</t>
<t>The next bit is written into the M recovery bit in the FEC
header.</t>
<t>The next 7 bits of the FEC bit string are written into the PT
recovery field in the FEC header.</t>
<t>The next 16 bits are skipped.</t>
<t>The next 32 bits of the FEC bit string are written into the TS
recovery field in the FEC header.</t>
<t>The next 16 bits are written into the length recovery field in
the FEC header.</t>
<t>Depending on the chosen MSK value, the bit mask of appropriate
length will be set to the appropriate values.</t>
</list>
As described in <xref target="sec_repair_fec_payload_id"/>, the SN
base field of the FEC header MUST be set to the lowest sequence number
of the source packets protected by this repair packet. When MSK
represents a bitmask (MSK=00,01,10), the SN base field corresponds to
the lowest sequence number indicated in the bitmask. When MSK=11, the
following considerations apply: 1) for the interleaved FEC packets,
this corresponds to the lowest sequence number of the source packets
that forms the column, 2) for the non-interleaved FEC packets, the SN
base field MUST be set to the lowest sequence number of the source
packets that forms the row.</t>
<t>The repair packet payload consists of the bits that are generated
by applying the XOR operation on the payloads of the source RTP
packets. If the payload lengths of the source packets are not equal,
each shorter packet MUST be padded to the length of the longest packet
by adding octet 0's at the end.</t>
<t>Due to this possible padding and mandatory FEC header, a repair
packet has a larger size than the source packets it protects. This may
cause problems if the resulting repair packet size exceeds the Maximum
Transmission Unit (MTU) size of the path over which the repair flow is
sent.</t>
</section>
<section title="Source Packet Reconstruction">
<t>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.</t>
<t>In the following, we describe the RECOMMENDED algorithms for the
first and second steps. Based on the implementation, different
algorithms MAY be adopted. However, the end result MUST be identical
to the one produced by the algorithms described below.</t>
<t>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 <xref
target="sec_iterative_decoding"/>.</t>
<section anchor="sec_associating_source_repair"
title="Associating the Source and Repair Packets">
<t>
We denote the set of the source packets associated with repair
packet p* by set T(p*). Note that in a source block whose size is L
columns by D rows, set T includes D source packets plus one repair
packet for the FEC protection applied over a column, and L source
packets plus one repair packet for the FEC protection applied over a
row. Recall that 1-D interleaved and non-interleaved FEC protection
can fully recover the missing information if there is only one
source packet missing in set T. If there are more than one source
packets missing in set T, 1-D FEC protection will not work.
</t>
<section anchor="sec_repair_sdp" title="Signaled in SDP">
<t>
The first step is associating the source and repair packets. If
the endpoint relies entirely on out-of-band signaling (MSK=11, and
M=N=0), then this information may be inferred from the media type
parameters specified in the SDP description. Furtheremore, the
payload type field in the RTP header, assists the receiver
distinguish an interleaved or non-interleaved FEC packet.
</t>
<t>Mathematically, for any received repair packet, p*, we can
determine the sequence numbers of the source packets that are
protected by this repair packet as follows:</t>
<t> <figure> <preamble/> <artwork align="center">
<![CDATA[p*_snb + i * X_1 (modulo 65536)]]>
</artwork> </figure> </t>
<t>where p*_snb denotes the value in the SN base field of p*'s FEC
header, X_1 is set to L and 1 for the interleaved and
non-interleaved FEC packets, respectively, and</t>
<t> <figure> <preamble/> <artwork align="center">
<![CDATA[0 <= i < X_2]]>
</artwork> </figure> </t>
<t>where X_2 is set to D and L for the interleaved and
non-interleaved FEC packets, respectively.</t>
</section>
<section anchor="sec_repair_bitmask" title="Using bitmasks">
<t>
When using fixed size bitmasks (16-, 48-, 112-bits), the SN base
field in the FEC header indicates the lowest sequence number of
the source packets that forms the FEC packet. Finally, the bits
maked by "1" in the bitmask are offsets from the SN base and
make up the rest of the packets protected by the FEC packet.
The bitmasks are able to represent arbitrary protection patterns,
for example, 1-D interleaved, 1-D non-interleaved, 2-D, staircase.
</t>
</section>
<section anchor="sec_repair_offset" title="Using M and N Offsets">
<t>
When value of M is non-zero, the 8-bit fields indicate the
offset of packets protected by an interleaved (N>0) or
non-interleaved (N=0) FEC packet. Using a combination of
interleaved and non-interleaved FEC packets can form
2-D protection patterns.
</t>
<t>Mathematically, for any received repair packet, p*,
we can determine the sequence numbers of the source
packets that are protected by this repair packet are as
follows:</t>
<t> <figure> <preamble/> <artwork align="center">
<![CDATA[When N = 0:
p*_snb, p*_snb+1,..., p*_snb+(M-1), p*_snb+M
When N > 0:
p*_snb, p*_snb+(Mx1), p*_snb+(Mx2),..., p*_snb+(Mx(N-1)), p*_snb+(MxN)]]>
</artwork> </figure> </t>
</section>
</section>
<section anchor="sec_recovering_rtp_header"
title="Recovering the RTP Header">
<t>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:</t>
<t><list style="numbers">
<t>For each of the source packets that are successfully received
in T, compute the 80-bit string by concatenating the first 64
bits of their RTP header and the unsigned network-ordered 16-bit
representation of their length in bytes minus 12.</t>
<t>For the repair packet in T, compute the FEC bit string from
the first 80 bits of the FEC header.</t>
<t>Calculate the recovered bit string as the XOR of the bit
strings generated from all source packets in T and the FEC bit
string generated from the repair packet in T.</t>
<t>Create a new packet with the standard 12-byte RTP header and
no payload.</t>
<t>Set the version of the new packet to 2. Skip the first 2 bits
in the recovered bit string.</t>
<t>Set the Padding bit in the new packet to the next bit in the
recovered bit string.</t>
<t>Set the Extension bit in the new packet to the next bit in
the recovered bit string.</t>
<t>Set the CC field to the next 4 bits in the recovered bit
string.</t>
<t>Set the Marker bit in the new packet to the next bit in the
recovered bit string.</t>
<t>Set the Payload type in the new packet to the next 7 bits in
the recovered bit string.</t>
<t>Set the SN field in the new packet to SEQNUM. Skip the next
16 bits in the recovered bit string.</t>
<t>Set the TS field in the new packet to the next 32 bits in the
recovered bit string.</t>
<t>Take the next 16 bits of the recovered bit string and set the
new variable Y to whatever unsigned integer this represents
(assuming network order). Convert Y to host order. Y represents
the length of the new packet in bytes minus 12 (for the fixed
RTP header), i.e., the sum of the lengths of all the following
if present: the CSRC list, header extension, RTP payload and RTP
padding.</t>
<t>Set the SSRC of the new packet to the SSRC of the source RTP
stream.</t>
</list>This procedure recovers the header of an RTP packet up to
(and including) the SSRC field.</t>
</section>
<section anchor="sec_recovering_rtp_payload"
title="Recovering the RTP Payload">
<t>Following the recovery of the RTP header, the procedure for the
recovery of the RTP payload is as follows:</t>
<t><list style="numbers">
<t>Append Y bytes to the new packet.</t>
<t>For each of the source packets that are successfully received
in T, compute the bit string from the Y octets of data starting
with the 13th octet of the packet. If any of the bit strings
generated from the source packets has a length shorter than Y,
pad them to that length. The padding of octet 0 MUST be added at
the end of the bit string. Note that the information of the
first 8 octets are protected by the FEC header.</t>
<t>For the repair packet in T, compute the FEC bit string from
the repair packet payload, i.e., the Y octets of data following
the FEC header. Note that the FEC header may be 12, 16, 32
octets depending on the length of the bitmask.</t>
<t>Calculate the recovered bit string as the XOR of the bit
strings generated from all source packets in T and the FEC bit
string generated from the repair packet in T.</t>
<t>Append the recovered bit string (Y octets) to the new packet
generated in <xref target="sec_recovering_rtp_header"/>.</t>
</list></t>
</section>
<section anchor="sec_iterative_decoding"
title="Iterative Decoding Algorithm for the 2-D Parity FEC Protection">
<!-- VS: FIXME: multiple passes over source and repair streams -->
<t>In 2-D parity FEC protection, the sender generates both
non-interleaved and interleaved FEC packets to combat with the mixed
loss patterns (random and bursty). At the receiver side, these FEC
packets are used iteratively to overcome the shortcomings of the 1-D
non-interleaved/interleaved FEC protection and improve the chances
of full error recovery.</t>
<t>The iterative decoding algorithm runs as follows:</t>
<t><list style="numbers">
<t>Set num_recovered_until_this_iteration to zero</t>
<t>Set num_recovered_so_far to zero</t>
<t>Recover as many source packets as possible by using the
non-interleaved FEC packets as outlined in <xref
target="sec_recovering_rtp_header"/> and <xref
target="sec_recovering_rtp_payload"/>, and increase the value of
num_recovered_so_far by the number of recovered source
packets.</t>
<t>Recover as many source packets as possible by using the
interleaved FEC packets as outlined in <xref
target="sec_recovering_rtp_header"/> and <xref
target="sec_recovering_rtp_payload"/>, and increase the value of
num_recovered_so_far by the number of recovered source
packets.</t>
<t>If num_recovered_so_far >
num_recovered_until_this_iteration<vspace
blankLines="0"/>---num_recovered_until_this_iteration =
num_recovered_so_far<vspace blankLines="0"/>---Go to step
3<vspace blankLines="0"/>Else<vspace
blankLines="0"/>---Terminate</t>
</list></t>
<t>The algorithm terminates either when all missing source packets
are fully recovered or when there are still remaining missing source
packets but the FEC packets are not able to recover any more source
packets. For the example scenarios when the 2-D parity FEC
protection fails full recovery, refer to <xref target="sec_2d"/>.
Upon termination, variable num_recovered_so_far has a value equal to
the total number of recovered source packets.</t>
<t>Example:</t>
<t>Suppose that the receiver experienced the loss pattern sketched
in <xref target="fig_ite1"/>.</t>
<t><figure anchor="fig_ite1"
title="Example loss pattern for the iterative decoding algorithm">
<artwork align="center"><![CDATA[ +---+ +---+ +===+
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|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
<t>The receiver executes the iterative decoding algorithm and
recovers source packets #1 and #11 in the first iteration. The
resulting pattern is sketched in <xref target="fig_ite2"/>.</t>
<t><figure anchor="fig_ite2"
title="The resulting pattern after the first iteration">
<artwork align="center"><![CDATA[+---+ +---+ +---+ +===+
| 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|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
<t>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 <xref
target="fig_ite3"/>.</t>
<t><figure anchor="fig_ite3"
title="The resulting pattern after the second iteration">
<artwork align="center"><![CDATA[+---+ +---+ +---+ +---+ +===+
| 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|
+===+ +===+ +===+ +===+]]></artwork>
</figure></t>
</section>
</section>
</section>
<section anchor="sec_sdp" title="SDP Examples">
<t>This section provides two SDP <xref target="RFC4566"/> examples. The
examples use the FEC grouping semantics defined in <xref
target="RFC4756"/>.</t>
<section title="Example SDP for 1-D Parity FEC Protection">
<t>In this example, we have one source video stream (ssrc:1234) and one
FEC repair stream (ssrc:2345). We form one FEC group 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.</t>
<t><figure>
<preamble/>
<artwork><![CDATA[
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=1-D Interleaved Parity FEC 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 interleaved-parityfec/90000
a=fmtp:110 L:5; D:10; ToP:0; repair-window:200000
a=ssrc:1234
a=ssrc:2345
a=ssrc-group:FEC-FR 1234 2345
]]></artwork>
</figure></t>
</section>
<section title="Example SDP for 2-D Parity FEC Protection">
<t>In this example, we have one source video stream (ssrc:1234) and two
FEC repair streams (ssrc:2345 and ssrc:3456). We form one FEC group with the
"a=ssrc-group:FEC-FR 1234 2345 3456" line. The source and repair streams are
multiplexed on different SSRCs. The repair window is set to 200 ms.</t>
<t><figure>
<preamble/>
<artwork><![CDATA[
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=2-D Parity FEC Example
t=0 0
m=video 30000 RTP/AVP 100 110 111
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=rtpmap:110 interleaved-parityfec/90000
a=fmtp:110 L:5; D:10; ToP:2; repair-window:200000
a=rtpmap:111 non-interleaved-parityfec/90000
a=fmtp:111 L:5; D:10; ToP:2; repair-window:200000
a=ssrc:1234
a=ssrc:2345
a=ssrc:3456
a=ssrc-group:FEC-FR 1234 2345 3456
]]></artwork>
<postamble/>
</figure></t>
<t>Note that the sender might be generating two repair flows carrying
non-interleaved and interleaved FEC packets, however the receiver
might be interested only in the interleaved FEC packets. The receiver
can identify the repair flow carrying the desired repair data by
checking the payload types associated with each repair flow described
in the SDP description.</t>
</section>
</section>
<section title="Congestion Control Considerations">
<t>FEC is an effective approach to provide applications resiliency
against packet losses. However, in networks where the congestion is a
major contributor to the packet loss, the potential impacts of using FEC
SHOULD be considered carefully before injecting the repair flows into
the network. In particular, in bandwidth-limited networks, FEC repair
flows may consume most or all of the available bandwidth and
consequently may congest the network. In such cases, the applications
MUST NOT arbitrarily increase the amount of FEC protection since doing
so may lead to a congestion collapse. If desired, stronger FEC
protection MAY be applied only after the source rate has been
reduced <xref target="I-D.singh-rmcat-adaptive-fec" />.</t>
<t>In a network-friendly implementation, an application SHOULD NOT
send/receive FEC repair flows if it knows that sending/receiving those
FEC repair flows would not help at all in recovering the missing
packets. However, it MAY still continue to use FEC if considered for
bandwidth estimation instead of speculatively probe for additional
capacity <xref target="Holmer13" /><xref target="Nagy14" />.
It is RECOMMENDED that the amount of FEC protection is adjusted
dynamically based on the packet loss rate observed by the applications.</t>
<t>In multicast scenarios, it may be difficult to optimize the FEC
protection per receiver. If there is a large variation among the levels
of FEC protection needed by different receivers, it is RECOMMENDED that
the sender offers multiple repair flows with different levels of FEC
protection and the receivers join the corresponding multicast sessions
to receive the repair flow(s) that is best for them.</t>
<t>Editor's note: Additional congestion control considerations regarding
the use of 2-D parity codes should be added here.</t>
</section>
<section anchor="sec_security_considerations"
title="Security Considerations">
<t>RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification <xref target="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.</t>
<t>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) <xref target="RFC3711"/> is
recommended. Other mechanisms that may be used are IPsec <xref
target="RFC4301"/> and Transport Layer Security (TLS) <xref
target="RFC5246"/> (RTP over TCP); other alternatives may exist.</t>
</section>
<section anchor="sec_iana_considerations" title="IANA Considerations">
<t>New media subtypes are subject to IANA registration. For the
registration of the payload formats and their parameters introduced in
this document, refer to <xref target="sec_parameters"/>.</t>
</section>
<section title="Acknowledgments">
<t>Some parts of this document are borrowed from <xref
target="RFC5109"/>. Thus, the author would like to thank the editor of
<xref target="RFC5109"/> and those who contributed to <xref
target="RFC5109"/>.</t>
</section>
<section title="Change Log">
<t> Note to the RFC-Editor: please remove this section prior to
publication as an RFC.</t>
<section title="draft-ietf-payload-flexible-fec-scheme-00">
<t>Initial WG version, based on draft-singh-payload-1d2d-parity-scheme-00.</t>
</section>
<section title="draft-singh-payload-1d2d-parity-scheme-00">
<t>This is the initial version, which is based on
draft-ietf-fecframe-1d2d-parity-scheme-00. The following are the major
changes compared to that document:</t>
<t><list style="symbols">
<t>Updated packet format with 16-, 48-, 112- bitmask.</t>
<t>Updated the sections on: repair packet construction,
source packet construction.</t>
<t>Updated the media type registration and aligned to RFC6838.</t>
</list></t>
</section>
<section title="draft-ietf-fecframe-1d2d-parity-scheme-00">
<t><list style="symbols">
<t>Some details were added regarding the use of CNAME field.</t>
<t>Offer-Answer and Declarative Considerations sections have been
completed.</t>
<t>Security Considerations section has been completed.</t>
<t>The timestamp field definition has changed.</t>
</list></t>
</section>
</section>
</middle>
<back>
<references title="Normative References">
&__reference.RFC.2119;
&__reference.RFC.3550;
&__reference.RFC.4566;
<!-- &__reference.RFC.4288; updated by RFC6838-->
&__reference.RFC.3555;
&__reference.RFC.4756;
&__reference.RFC.3264;
&__reference.RFC.6363;
&__reference.RFC.7022;
&__reference.RFC.6838;
</references>
<references title="Informative References">
&__reference.RFC.2733;
&__reference.RFC.5109;
<reference anchor="SMPTE2022-1">
<front>
<title>Forward Error Correction for Real-Time Video/Audio Transport
over IP Networks</title>
<author fullname="" surname="SMPTE 2022-1-2007">
<organization/>
</author>
<date year="2007"/>
</front>
</reference>
&__reference.RFC.2326;
&__reference.RFC.2974;
&__reference.RFC.3711;
&__reference.RFC.4301;
&__reference.RFC.5246;
&__reference.adaptive-fec;
<reference anchor="Nagy14">
<front>
<title>Congestion Control using FEC for Conversational Multimedia Communication</title>
<author initials="M" surname="Nagy"></author>
<author initials="V" surname="Singh"></author>
<author initials="J" surname="Ott"></author>
<author initials="L" surname="Eggert"></author>
<date month="3" year="2014" />
</front>
<seriesInfo name="Proc. of 5th ACM Internation Conference on Multimedia Systems (MMSys 2014)" value="" />
</reference>
<reference anchor="Holmer13">
<front>
<title>Handling Packet Loss in WebRTC</title>
<author initials="S" surname="Holmer"></author>
<author initials="M" surname="Shemer"></author>
<author initials="M" surname="Paniconi"></author>
<date month="9" year="2013" />
</front>
<seriesInfo name="Proc. of IEEE International Conference on Image Processing (ICIP 2013)" value="" />
</reference>
</references>
</back>
</rfc>
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