Network Working Group P. Westin
Internet-Draft H. Lundin
Intended status: Experimental M. Glover
Expires: August 12, 2011 J. Uberti
F. Galligan
Google
February 8, 2011
Proposal for the IETF on "RTP Payload Format for VP8 Video"
draft-westin-payload-vp8-00
Abstract
This memo describes an RTP Payload format for the VP8 video codec.
The payload format has wide applicability, as it supports
applications from low bit-rate peer-to-peer usage, to high bit-rate
Video conferences.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 12, 2011.
Copyright Notice
Copyright (c) 2011 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
(http://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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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1. Introduction
An encoded VP8 frame can be divided into two or more partitions, as
described in [1]. The first partition (prediction or mode) contains
prediction mode parameters and motion vectors for all macroblocks.
The remaining partitions all contain the DCT/WHT coefficients for the
residuals. The first partition is decodable without the remaining
residual partitions. The subsequent partitions may be useful even if
some part of the frame is lost. The format specification is
described in Section 2. Section 3 illustrates how VP8 can be
combined with uneven level FEC protection. Section 4 describes a
method to acknowledge receipt of reference frames using RTCP
techniques is described. Both these examples serve as motivation for
two of the fields included in the payload format: the "1st partition
size" and "PictureID" fields.
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2. Payload Format
The general RTP payload format for VP8 is depicted below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| VP8 payload descriptor (integer #bytes) |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : VP8 payload header (3 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VP8 pyld hdr : |
+-+-+-+-+-+-+-+-+ |
: Bytes 4..N of VP8 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The VP8 payload descriptor and VP8 payload header will be described
in the sequel. OPTIONAL RTP padding MUST NOT be included unless the
marker bit is set.
Figure 1
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2.1. VP8 Payload Descriptor
The first bytes after the RTP header are the VP8 payload descriptor,
with the following structure.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RSV |I|N|FI |B| PictureID (integer #bytes) |
+-+-+-+-+-+-+-+-+ |
: :
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : (VP8 data or VP8 payload header; byte aligned)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
RSV: Bits reserved for future use. MUST be equal to zero and MUST
be ignored by the receiver.
I: PictureID present. When set to one, a PictureID is provided
after the first byte of the payload descriptor. When set to zero,
the PictureID is omitted, and the one-byte payload descriptor is
immediately followed by the VP8 payload.
N: Non-reference frame. When set to one, the frame can be
discarded without affecting any other future or past frames.
FI: Fragmentation information field. This field contains
information about the fragmentation of VP8 payloads carried in the
RTP packet. The four different values are listed below.
* 00 The RTP packet contains no fragmented VP8 partitions. The
payload is one or several complete partitions.
* 01 The RTP packet contains the first part of a fragmented
partition. The fragment must be placed in its own RTP packet.
* 10 The RTP packet contains a fragment that is neither the first
nor the last part of a fragmented partition. The fragment must
be placed in its own RTP packet.
* 11 The RTP packet contains the last part of a fragmented
partition. The fragment must be placed in its own RTP packet.
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B: Beginning VP8 frame. When set to 1 this signals that a new VP8
frame starts in this RTP packet.
PictureID: Multiple of 8 bits. This is a running index of the
frames. The field is present only if the I bit is equal to one.
The most significant bit of each byte is an extension flag. The 7
following bits carry (parts of) the PictureID. If the extension
flag is one, the PictureID continues in the next byte. If the
extension flag is zero, the 7 remaining bits are the last (and
least significant) bits in the PictureID. The sender may choose
any number of bytes, smaller or equal to 9 bytes as the maximum
PictureID. The PictureID SHALL start on a random number, and MUST
wrap after reaching the maximum ID. Leading zero bytes MUST be
supressed.
2.2. VP8 Payload Header
The first three bytes of an encoded VP8 frame are uncompressed, and
co-serve as payload header in this RTP format. Note that the header
is present only in packets which have the B bit equal to one in the
payload descriptor. Subsequent packets for the same frame do not
carry the payload header.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|P| VER |H| 1st partition size | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Bytes 4..N of VP8 payload :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3
P: Inverse key frame flag. When set to 0 the current frame is a
key frame. When set to 1 the current frame is an interframe.
Defined in [1]
VER: A version number as defined in [1].
H: Show frame bit as defined in [1].
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1st partition size: 19 bits. A field containing the size of the
first data partition in bytes, as defined in [1].
2.3. Aggregated and Fragmented Payloads
An encoded VP8 frame can be divided into two or more partitions, as
described in Section 1. The fragmentation information described in
Section 2.1 MUST be used to signal if any fragmentation is applied.
Aggregation of encoded partitions is done without explicit signaling.
Partitions MUST be aggregated in decoding order. An aggregation MUST
have exactly one payload descriptor. Aggregated partitions MUST
represent parts of one and the same video frame. Consequently, an
aggregated packet will have one or no payload header, depending on
whether the aggregate contains the first partition of a frame or not,
respectively. Note that the length of the first partition can always
be obtained from the first partition size parameter in the VP8
payload header. Fragments of encoded partitions MUST NOT be
aggregated.
2.4. Examples of VP8 RTP Stream
A few examples of how the VP8 RTP payload can be used are included
below.
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2.4.1. Key frame in a single RTP packet
Marker bit = 1. I = 1. B = 1. PictureID = 17 = 0001001 binary. P
= 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|0: VER :1: 1st partition |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| size = L | |
+-+-+-+-+-+-+-+-+ |
| |
: Bytes 4..L of first VP8 partition :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Remaining VP8 partitions :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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2.4.2. VP8 interframe in a single RTP packet; no PictureID
Marker bit = 1. I = 0. B = 1. P = 1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 1|1: VER :1: 1st partition size = L |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Bytes 4..L of first VP8 partition :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Remaining VP8 partitions :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.4.3. VP8 partitions in separate RTP packets
First RTP packet; marker bit = 0. I = 1. B = 1. PictureID = 17.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|1: VER :1: 1st partition |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| size = L | |
+-+-+-+-+-+-+-+-+ |
| |
: Bytes 4..L of first VP8 partition :
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Second RTP packet; marker bit = 1. B = 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 0 0:0 0 0 0 1 0 0 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Remaining VP8 partitions :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.4.4. VP8 frame fragmented across RTP packets
First RTP packet; marker bit = 0. I = 1. FI = 00. B = 1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 0 1:0 0 0 0 1 0 0 1|1: VER :1: 1st partition |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| size = L | |
+-+-+-+-+-+-+-+-+ |
| |
: Bytes 4..L of first VP8 partition :
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Second RTP packet; marker bit = 0. FI = 01. B = 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 1 0:0 0 0 0 1 0 0 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: First fragment of second VP8 partition :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Third RTP packet; marker bit = 0. FI = 10. B = 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 1 0 0:0 0 0 0 1 0 0 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Middle fragment of second VP8 partition :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Last RTP packet; marker bit = 1. FI = 11. B = 0.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 1 1 0:0 0 0 0 1 0 0 1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Last fragment of second VP8 partition :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| : OPTIONAL RTP padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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2.4.5. VP8 frame with long PictureID
PictureID = 4711 = 01001001100111 binary (first 7 bits: 0100100, last
7 bits: 1100111).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header M=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1 0 0 0 1:1 0 1 0 0 1 0 0 0 1 1 0 0 1 1 1|1: VER :1: 1st |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| partition size = L | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
: Bytes 4..N of first VP8 frame :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3. Using VP8 with Uneven Level FEC Protection
RFC 5109 [2] specifies a payload format for generic forward error
correction (FEC) for RTP packets. (An errata with critical changes
was also published.) One salient feature of RFC 5109 is that it
provides "uneven level protection", ULP, which enables FEC protection
of parts of an RTP packet. Specifically, the first part of an RTP
packet can be given a stronger protection than the remaining part.
The special case is where only the first part of the RTP packet is
protected. The length of the protected part (actually the length of
each protection level) is chosen and changed dynamically during a
session. The concept of ULP FEC fits well with the VP8 video format.
The first partition of an encoded VP8 frame consists of context
variables and prediction parameters (mode and vectors), while the
subsequent partitions contain encoded residual information. For a
decoder, the residual information is not useful without the first
partition. However, a decoder could successfully use the information
in the first partition to provide good packet loss concealment, even
if the subsequent partitions are lost. The conclusion is that the
first partition deserves a higher protection factor than the
remaining data. By including the "first partition size" parameter in
the VP8 payload header(Section 2.2), the application, or even a media
aware network element, can apply the ULP FEC to the VP8 payloads,
since it can readily identify and obtain the length of the first
partition to which (a stronger) protection should be granted. RFC
5109 suggests two methods for multiplexing the media data and the FEC
data: using the RED payload type (RFC 2198, [3]) and using separate
sessions. The example below is based on the RED method, although the
above RTP format for VP8 does not preclude any method. Since we are
only interested in protecting the first partition of the VP8 payload,
it is in some cases possible to obtain increased robustness for that
part even without FEC signaling. If the first partition is isolated
in a separate RTP packet, this packet can simply be sent twice (exact
replica of RTP header and payload).
3.1. Example: Using VP8 with Uneven Level FEC Protection
The following terminology is used in the examples below:
o RTP: RTP header, 12 octets.
(Special rules apply if the header is extended; see [2].)
o RED: Redundancy payload header, 1 or 4 octets.
o VP8pd: VP8 payload descriptor, 1 or more octets (see Section 2.1).
o VP8ph: VP8 payload header, 3 octets (see Section 2.2)).
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o VP8first: First partition of a VP8 frame, length given in VP8ph.
o VP8second: Second (or later) partition of a VP8 frame.
o FEC: FEC header for FEC packets, 10 octets (see [2]).
o ULP: FEC level header for FEC packets, 2 or 4 octets (see [2]).
3.1.1. VP8 first partition isolated
The first partition of the VP8 frame is sent in its own RTP packet,
which can easily be duplicated for increased robustness. This case
provides a 50% protection factor (ratio of number of FEC packet to
the sum of FEC and protected packets; sending two duplicates provides
a 67% protection, and so on).
+-----+-------+------------------+
Packet A: | RTP | VP8pd | VP8ph : VP8first |
+-----+-------+------------------+
+-----+-------+------------------------+
Packet B: | RTP | VP8pd | VP8second |
+-----+-------+------------------------+
+-----+-------+------------------+
Duplicate A: | RTP | VP8pd | VP8ph : VP8first |
+-----+-------+------------------+
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3.1.2. VP8 first partition split
The first partition of the VP8 frame can be split across two (or
more) RTP packets. It must be done if the first partition is larger
than the MTU, but can also be done to facilitate protection factors
lower than 50%. The FEC header is calculated from the RTP headers of
packets A and B, while the FEC payload FEC(A,B) is constructed from
the VP8 payload descriptor, payload header and first partition, as
indicated in the figure below.
+-----+-----+-------+------------------+
A: | RTP | RED | VP8pd | VP8ph : VP8first |
+-----+-----+-------+------------------+
[*****] [**************************]
+-----+-----+-------+------------------+
B: | RTP | RED | VP8pd | VP8first cont'd |
+-----+-----+-------+------------------+
[*****] [**************************]
\ \
---------- ---------
\ \
[*****] [**************************]
+-----+-----+-----+-----+--------------------------+
FEC1: | RTP | RED | FEC | ULP | FEC(A,B) |
+-----+-----+-----+-----+--------------------------+
+-----+-----+-------+---------------------------+
C: | RTP | RED | VP8pd | VP8 second |
+-----+-----+-------+---------------------------+
The split of the first partition is preferably done such that the
payloads used to calculate FEC(A,B) are close to equal size.
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3.1.3. VP8 partitions aggregated
In the case when the first partition is sent in the same packet as
one or more subsequent partitions, the level protection can be
applied to facilitate a bit- conservative protection for only the
first partition.
+-----+-----+-------+------------------+--------------------+
A: | RTP | RED | VP8pd | VP8ph : VP8first | VP8second |
+-----+-----+-------+------------------+--------------------+
[*****] [**************************]
\ \
---------- ---------
\ \
[*****] [**************************]
+-----+-----+-----+-----+--------------------------+
FEC1: | RTP | RED | FEC | ULP | Duplicate |
+-----+-----+-----+-----+--------------------------+
+-----+-----+-------+---------------------------+
C: | RTP | RED | VP8pd | VP8third |
+-----+-----+-------+---------------------------+
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4. Using VP8 with RPSI and SLI Feedback
The VP8 payload descriptor defined in Section 2.1 above contains an
optional PictureID parameter. This parameter is included mainly to
enable use of reference picture selection index (RPSI) and slice loss
indication (SLI), both defined in RFC 4585 [4].
4.1. RPSI
The reference picture selection index is a payload-specific feedback
message defined within the RTCP-based feedback format. The RPSI
message is generated by a receiver and can be used in two ways.
Either it can signal a preferred reference picture when a loss has
been detected by the decoder -- preferably then a reference that the
decoder knows is perfect -- or, it can be used as positive feedback
information to acknowledge correct decoding of certain reference
pictures. The positive feedback method is useful for VP8 used as
unicast. The use of RPSI for VP8 is preferably combined with a
special update pattern of the codec's two special reference frames --
the golden frame and the altref frame -- in which they are updated in
an alternating leapfrog fashion. When a receiver has received and
correctly decoded a golden or altref frame, and that frame had a
PictureID in the payload descriptor, the receiver can acknowledge
this simply by sending an RPSI message back to the sender. The
message body (i.e., the "native RPSI bit string" in RFC 4585 [4]) is
simply the PictureID of the received frame.
4.2. SLI
The slice loss indication is another payload-specific feedback
message defined within the RTCP-based feedback format. The SLI
message is generated by the receiver when a loss or corruption is
detected in a frame. The format of the SLI message is as follows
[4]:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First | Number | PictureID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4
Here, First is the macroblock address (in scan order) of the first
lost block and Number is the number of lost blocks. PictureID is the
six least significant bits of the codec-specific picture identifier
in which the loss or corruption has occurred. For VP8, this codec-
specific identifier is naturally the PictureID of the current frame,
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as read from the payload descriptor. If the payload descriptor of
the current frame does not have a PictureID, the receiver MAY send
the last received PictureID+1 in the SLI message. The receiver MAY
set the First parameter to 0, and the Number parameter to the total
number of macroblocks per frame, even though only parts of the frame
is corrupted. When the sender receives an SLI message, it can make
use of the knowledge from the latest received RPSI message. Knowing
that the last golden or altref frame was successfully received, it
can encode the next frame with reference to that established
reference.
4.3. Example
The use of RSPI and SLI is best illustrated in an example. In this
example, the encoder may not update the altref frame until the last
sent golden frame has been acknowledged with an RPSI message. If an
update is not received within some time, a new golden frame update is
sent instead. Once the new golden frame is established and
acknowledge, the same rule applies when updating the altref frame.
Event Sender Receiver Established
reference
+----+--------------------+--------------------------+------------+
1000 Send golden frame
PictureID = 0
Receive and decode
golden frame
1001 Receive RPSI(0)
Send RPSI(0) golden
... (sending regular frames)
1100 Send altref frame
PictureID = 100
Altref corrupted or lost golden
1101 Receive SLI(100)
Send SLI(100) golden
1102 Send frame with
reference to golden
Receive and decode frame
(decoder state restored) golden
... (sending regular frames)
1200 Send altref frame
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PictureID = 200
Receive and decode
altref frame golden
1201 Receive RPSI(200)
Send RPSI(200) altref
... (sending regular frames)
1300 Send golden frame
PictureID = 300
Receive and decode
golden frame altref
1301 RPSI lost
Send RPSI(300) altref
1400 Send golden frame
PictureID = 400
Receive and decode
golden frame altref
1401 Receive RPSI(400)
Send RPSI(400) golden
+----+--------------------+--------------------------+------------+
Note that the scheme is robust to loss of the feedback messages. If
the RPSI is lost, the sender will try to update the golden (or
altref) again after a while, without releasing the established
reference. Also, if an SLI is lost, the receiver can keep sending
SLI messages at any interval, as long as the picture is corrupted.
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5. Payload Format Parameters
This section specifies the parameters that MAY be used to select
optional features of the payload format and certain features of the
bitstream.
5.1. MIME Registration
The receiver MUST ignore any unspecified parameter.
Media Type name: video
Media subtype name: VP8
Required parameters: none
Security considerations:
- See Section 6 of RFC xxxx.
5.2. SDP Parameters
The receiver MUST ignore any parameter unspecified in this memo.
5.2.1. Mapping of MIME Parameters to SDP
The MIME media type video/VP8 string is mapped to fields in the
Session Description Protocol (SDP) [7] as follows:
o The media name in the "m=" line of SDP MUST be video.
o The encoding name in the "a=rtpmap" line of SDP MUST be VP8 (the
MIME subtype).
o The clock rate in the "a=rtpmap" line MUST be 90000.
o The OPTIONAL parameter "version", if included, MUST be in the
a=fmtp SDP field. This parameter matches the VP8 bitstream
version.
5.3. Example
An example of media representation in SDP is as follows:
m=video 49170 RTP/AVP 98
a=rtpmap:98 VP8/90000
a=fmtp:98 version=0
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6. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [5]. This implies that confidentiality of the media
streams is achieved by encryption; one example of this is SRTP [6].
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7. IANA Considerations
The IANA is requested to register the following values:
- MIME registration as described in Section 5.1.
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8. References
[1] Google, Inc., "VP8 Data Format and Decoding Guide", July 2010,
<http://www.webmproject.org>.
[2] Perkins, C., Kouvelas, U., Hodson, O., Hardman, V., Handley, M.,
Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload
Format for Generic Forward Error Correction", RFC 5109, STD 1,
December 2007.
[3] Li, A., "RTP Payload for Redundant Audio Data", RFC 2198, STD 1,
September 1997.
[4] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control Protocol
(RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, STD 1, July 2006.
[5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, STD 64, July 2003.
[6] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, STD 1, March 2004.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, STD 1, April 1998.
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Authors' Addresses
Patrik Westin
Google, Inc.
Kungsbron 2
Stockholm, 11122
Sweden
Email: patrik.westin@gmail.com
Henrik Lundin
Google, Inc.
Michael Glover
Google, Inc.
Justin Uberti
Google, Inc.
Frank Galligan
Google, Inc.
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