Internet Engineering Task Force (IETF) P. Westin Request for Comments: 7741 H. Lundin Category: Standards Track Google ISSN: 2070-1721 M. Glover Twitter J. Uberti F. Galligan Google March 2016 RTP Payload Format for VP8 Video 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-bitrate peer-to-peer usage to high-bitrate video conferences. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7741. Copyright Notice Copyright (c) 2016 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents 1. Introduction ....................................................3 2. Conventions, Definitions, and Abbreviations .....................3 3. Media Format Description ........................................4 4. Payload Format ..................................................5 4.1. RTP Header Usage ...........................................6 4.2. VP8 Payload Descriptor .....................................7 4.3. VP8 Payload Header ........................................11 4.4. Aggregated and Fragmented Payloads ........................12 4.5. Example Algorithms ........................................13 4.5.1. Frame Reconstruction Algorithm .....................13 4.5.2. Partition Reconstruction Algorithm .................13 4.6. Examples of VP8 RTP Stream ................................14 4.6.1. Key Frame in a Single RTP Packet ...................14 4.6.2. Non-discardable VP8 Interframe in a Single RTP Packet; No PictureID ...........................14 4.6.3. VP8 Partitions in Separate RTP Packets .............15 4.6.4. VP8 Frame Fragmented across RTP Packets ............16 4.6.5. VP8 Frame with Long PictureID ......................18 5. Using VP8 with RPSI and SLI Feedback ...........................18 5.1. RPSI ......................................................18 5.2. SLI .......................................................19 5.3. Example ...................................................19 6. Payload Format Parameters ......................................21 6.1. Media Type Definition .....................................21 6.2. SDP Parameters ............................................23 6.2.1. Mapping of Media Subtype Parameters to SDP .........23 6.2.2. Offer/Answer Considerations ........................23 7. Security Considerations ........................................24 8. Congestion Control .............................................24 9. IANA Considerations ............................................24 10. References ....................................................25 10.1. Normative References .....................................25 10.2. Informative References ...................................26 Authors' Addresses ................................................28
1. Introduction This memo describes an RTP payload specification applicable to the transmission of video streams encoded using the VP8 video codec [RFC6386]. The format described in this document can be used both in peer-to-peer and video-conferencing applications. VP8 is based on the decomposition of frames into square sub-blocks of pixels known as "macroblocks" (see Section 2 of [RFC6386]). Prediction of such sub-blocks using previously constructed blocks, and adjustment of such predictions (as well as synthesis of unpredicted blocks) is done using a discrete cosine transform (hereafter abbreviated as DCT). In one special case, however, VP8 uses a "Walsh-Hadamard" transform (hereafter abbreviated as WHT) instead of a DCT. An encoded VP8 frame is divided into two or more partitions, as described in [RFC6386]. The first partition (prediction or mode) contains prediction mode parameters and motion vectors for all macroblocks. The remaining partitions all contain the quantized DCT/WHT coefficients for the residuals. There can be 1, 2, 4, or 8 DCT/WHT partitions per frame, depending on encoder settings. In summary, the payload format described in this document enables a number of features in VP8, including: o Taking partition boundaries into consideration, to improve loss robustness and facilitate efficient packet-loss concealment at the decoder. o Temporal scalability. o Advanced use of reference frames to enable efficient error recovery. o Marking of frames that have no impact on the decoding of any other frame, so that these non-reference frames can be discarded in a server or media-aware network element if needed. 2. Conventions, Definitions, and Abbreviations The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
This document uses the definitions of [RFC6386]. In particular, the following terms are used. Key frames: Frames that are decoded without reference to any other frame in a sequence (also called intraframes and I-frames). Interframes: Frames that are encoded with reference to prior frames, specifically all prior frames up to and including the most recent key frame (also called prediction frames and P-frames). Golden and altref frames: alternate prediction frames. Blocks in an interframe may be predicted using blocks in the immediately previous frame as well as the most recent golden frame or altref frame. Every key frame is automatically golden and altref, and any interframe may optionally replace the most recent golden or altref frame. Macroblock: a square array of pixels whose Y (luminance) dimensions are 16x16 pixels and whose U and V (chrominance) dimensions are 8x8 pixels. Two definitions from [RFC4585] are also used in this document. RPSI: Reference picture selection indication. A feedback message to let the encoder know that the decoder has correctly decoded a certain frame. SLI: Slice loss indication. A feedback message to let a decoder inform an encoder that it has detected the loss or corruption of one or several macroblocks. 3. Media Format Description The VP8 codec uses three different reference frames for interframe prediction: the previous frame, the golden frame, and the altref frame. Blocks in an interframe may be predicted using blocks in the immediately previous frame as well as the most recent golden frame or altref frame. Every key frame is automatically golden and altref, and any interframe may optionally replace the most recent golden or altref frame. Golden frames and altref frames may also be used to increase the tolerance to dropped frames. The payload specification in this memo has elements that enable advanced use of the reference frames, e.g., for improved loss robustness. One specific use case of the three reference frame types is temporal scalability. By setting up the reference hierarchy in the appropriate way, up to five temporal layers can be encoded. (How to set up the reference hierarchy for temporal scalability is not within
the scope of this memo.) Support for temporal scalability is provided by the optional TL0PICIDX and TID/Y/KEYIDX fields described in Section 4.2. For a general description of temporal scalability for video coding, see [Sch07]. Another property of the VP8 codec is that it applies data partitioning to the encoded data. Thus, an encoded VP8 frame can be divided into two or more partitions, as described in "VP8 Data Format and Decoding Guide" [RFC6386]. The first partition (prediction or mode) contains prediction mode parameters and motion vectors for all macroblocks. The remaining partitions all contain the transform 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. Accordingly, this document RECOMMENDS that the frame be packetized by the sender with each data partition in a separate packet or packets. This may be beneficial for decoder-side error concealment, and the payload format described in Section 4 provides fields that allow the partitions to be identified even if the first partition is not available. The sender can, alternatively, aggregate the data partitions into a single data stream and, optionally, split it into several packets without consideration of the partition boundaries. The receiver can use the length information in the first partition to identify the partitions during decoding. The format specification is described in Section 4. In Section 5, a method to acknowledge receipt of reference frames using RTCP techniques is described. The payload partitioning and the acknowledging method both serve as motivation for three of the fields included in the payload format: the "PID", "1st partition size", and "PictureID" fields. The ability to encode a temporally scalable stream motivates the "TL0PICIDX" and "TID" fields. 4. Payload Format This section describes how the encoded VP8 bitstream is encapsulated in RTP. To handle network losses, usage of RTP/AVPF [RFC4585] is RECOMMENDED. All integer fields in the specifications are encoded as unsigned integers in network octet order.
4.1. RTP Header Usage 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 #octets) | : : | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : VP8 payload header (3 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | VP8 pyld hdr : | +-+-+-+-+-+-+-+-+ | : Octets 4..N of VP8 payload : | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | : OPTIONAL RTP padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The VP8 payload descriptor and VP8 payload header will be described in Sections 4.2 and 4.3. OPTIONAL RTP padding MUST NOT be included unless the P bit is set. The figure specifically shows the format for the first packet in a frame. Subsequent packets will not contain the VP8 payload header and will have later octets in the frame payload. Figure 1 Marker bit (M): MUST be set for the very last packet of each encoded frame in line with the normal use of the M bit in video formats. This enables a decoder to finish decoding the picture, where it otherwise may need to wait for the next packet to explicitly know that the frame is complete. Payload type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document and will not be specified here.
Timestamp: The RTP timestamp indicates the time when the frame was sampled. The granularity of the clock is 90 kHz, so a delta of 1 represents 1/90,000 of a second. The remaining RTP Fixed Header Fields (V, P, X, CC, sequence number, SSRC, and CSRC identifiers) are used as specified in Section 5.1 of [RFC3550]. 4.2. VP8 Payload Descriptor The first octets after the RTP header are the VP8 payload descriptor, with the following structure. The single-octet version of the PictureID is illustrated to the left (M bit set to 0), while the dual-octet version (M bit set to 1) is shown to the right. 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |X|R|N|S|R| PID | (REQUIRED) |X|R|N|S|R| PID | (REQUIRED) +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ X: |I|L|T|K| RSV | (OPTIONAL) X: |I|L|T|K| RSV | (OPTIONAL) +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ I: |M| PictureID | (OPTIONAL) I: |M| PictureID | (OPTIONAL) +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ L: | TL0PICIDX | (OPTIONAL) | PictureID | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ T/K: |TID|Y| KEYIDX | (OPTIONAL) L: | TL0PICIDX | (OPTIONAL) +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ T/K: |TID|Y| KEYIDX | (OPTIONAL) +-+-+-+-+-+-+-+-+ Figure 2 X: Extended control bits present. When set to 1, the extension octet MUST be provided immediately after the mandatory first octet. If the bit is zero, all optional fields MUST be omitted. Note: this X bit is not to be confused with the X bit in the RTP header. R: Bit reserved for future use. MUST be set to 0 and MUST be ignored by the receiver. N: Non-reference frame. When set to 1, the frame can be discarded without affecting any other future or past frames. If the reference status of the frame is unknown, this bit SHOULD be set to 0 to avoid discarding frames needed for reference. Informative note: This document does not describe how to determine if an encoded frame is non-reference. The reference status of an encoded frame is preferably provided from the encoder implementation.
S: Start of VP8 partition. SHOULD be set to 1 when the first payload octet of the RTP packet is the beginning of a new VP8 partition, and MUST NOT be 1 otherwise. The S bit MUST be set to 1 for the first packet of each encoded frame. PID: Partition index. Denotes to which VP8 partition the first payload octet of the packet belongs. The first VP8 partition (containing modes and motion vectors) MUST be labeled with PID = 0. PID SHOULD be incremented by 1 for each subsequent partition, but it MAY be kept at 0 for all packets. PID cannot be larger than 7. If more than one packet in an encoded frame contains the same PID, the S bit MUST NOT be set for any packet other than the first packet with that PID. When the X bit is set to 1 in the first octet, the Extended Control Bits field octet MUST be provided as the second octet. If the X bit is 0, the Extended Control Bits field octet MUST NOT be present, and no extensions (I, L, T, or K) are permitted. I: PictureID present. When set to 1, the PictureID MUST be present after the extension bit field and specified as below. Otherwise, PictureID MUST NOT be present. L: TL0PICIDX present. When set to 1, the TL0PICIDX MUST be present and specified as below, and the T bit MUST be set to 1. Otherwise, TL0PICIDX MUST NOT be present. T: TID present. When set to 1, the TID/Y/KEYIDX octet MUST be present. The TID|Y part of the octet MUST be specified as below. If K (below) is set to 1 but T is set to 0, the TID/Y/KEYIDX octet MUST be present, but the TID field MUST be ignored. If neither T nor K is set to 1, the TID/Y/KEYIDX octet MUST NOT be present. K: KEYIDX present. When set to 1, the TID/Y/KEYIDX octet MUST be present. The KEYIDX part of the octet MUST be specified as below. If T (above) is set to 1 but K is set to 0, the TID/Y/KEYIDX octet MUST be present, but the KEYIDX field MUST be ignored. If neither T nor K is set to 1, the TID/Y/KEYIDX octet MUST NOT be present. RSV: Bits reserved for future use. MUST be set to 0 and MUST be ignored by the receiver.
After the extension bit field follow the extension data fields that are enabled. The PictureID extension: If the I bit is set to 1, the PictureID extension field MUST be present, and it MUST NOT be present otherwise. The field consists of two parts: M: The most significant bit of the first octet is an extension flag. If M is set, the remainder of the PictureID field MUST contain 15 bits, else it MUST contain 7 bits. Note: this M bit is not to be confused with the M bit in the RTP header. PictureID: 7 or 15 bits (shown left and right, respectively, in Figure 2) not including the M bit. This is a running index of the frames, which MAY start at a random value, MUST increase by 1 for each subsequent frame, and MUST wrap to 0 after reaching the maximum ID (all bits set). The 7 or 15 bits of the PictureID go from most significant to least significant, beginning with the first bit after the M bit. The sender chooses a 7- or 15-bit index and sets the M bit accordingly. The receiver MUST NOT assume that the number of bits in PictureID stays the same through the session. Having sent a 7-bit PictureID with all bits set to 1, the sender may either wrap the PictureID to 0 or extend to 15 bits and continue incrementing. The TL0PICIDX extension: If the L bit is set to 1, the TL0PICIDX extension field MUST be present, and it MUST NOT be present otherwise. The field consists of one part: TL0PICIDX: 8 bits temporal level zero index. TL0PICIDX is a running index for the temporal base layer frames, i.e., the frames with TID set to 0. If TID is larger than 0, TL0PICIDX indicates on which base-layer frame the current image depends. TL0PICIDX MUST be incremented when TID is 0. The index MAY start at a random value, and it MUST wrap to 0 after reaching the maximum number 255. Use of TL0PICIDX depends on the presence of TID. Therefore, it is RECOMMENDED that the TID be used whenever TL0PICIDX is. The TID/Y/KEYIDX extension: If either of the T or K bits are set to 1, the TID/Y/KEYIDX extension field MUST be present. It MUST NOT be present if both T and K are zero. The field consists of three parts: TID: 2 bits temporal-layer index. The TID field MUST be ignored by the receiver when the T bit is set equal to 0. The TID field indicates which temporal layer the packet represents.
The lowest layer, i.e., the base layer, MUST have the TID set to 0. Higher layers SHOULD increment the TID according to their position in the layer hierarchy. Y: 1 layer sync bit. The Y bit SHOULD be set to 1 if the current frame depends only on the base layer (TID = 0) frame with TL0PICIDX equal to that of the current frame. The Y bit MUST be set to 0 if the current frame depends on any other frame than the base layer (TID = 0) frame with TL0PICIDX equal to that of the current frame. Additionally, the Y bit MUST be set to 0 if any frame following the current frame depends on a non- base-layer frame older than the base-layer frame with TL0PICIDX equal to that of the current frame. If the Y bit is set when the T bit is equal to 0, the current frame MUST only depend on a past base-layer (TID=0) key frame as signaled by a change in the KEYIDX field. Additionally, this frame MUST NOT depend on any of the three codec buffers (as defined by [RFC6386]) that have been updated since the last time the KEYIDX field was changed. Informative note: This document does not describe how to determine the dependency status for a frame; this information is preferably provided from the encoder implementation. In the case of unknown status, the Y bit can safely be set to 0. KEYIDX: 5 bits temporal key frame index. The KEYIDX field MUST be ignored by the receiver when the K bit is set equal to 0. The KEYIDX field is a running index for key frames. KEYIDX MAY start at a random value, and it MUST wrap to 0 after reaching the maximum number 31. When in use, the KEYIDX SHOULD be present for both key frames and interframes. The sender MUST increment KEYIDX for key frames that convey parameter updates critical to the interpretation of subsequent frames, and it SHOULD leave the KEYIDX unchanged for key frames that do not contain these critical updates. If the KEYIDX is present, a receiver SHOULD NOT decode an interframe if it has not received and decoded a key frame with the same KEYIDX after the last KEYIDX wraparound. Informative note: This document does not describe how to determine if a key frame updates critical parameters; this information is preferably provided from the encoder implementation. A sender that does not have this information may either omit the KEYIDX field (set K equal to 0) or increment the KEYIDX on every key frame. The benefit with the latter is that any key-frame loss will be detected by the receiver, which can signal for re-transmission or request a new key frame.
Informative note: Implementations doing splicing of VP8 streams will have to make sure the rules for incrementing TL0PICIDX and KEYIDX are obeyed across the splice. This will likely require rewriting values of TL0PICIDX and KEYIDX after the splice. 4.3. VP8 Payload Header The beginning of an encoded VP8 frame is referred to as an "uncompressed data chunk" in Section 9.1 of [RFC6386], and it also serves as a payload header in this RTP format. The codec bitstream format specifies two different variants of the uncompressed data chunk: a 3-octet version for interframes and a 10-octet version for key frames. The first 3 octets are common to both variants. In the case of a key frame, the remaining 7 octets are considered to be part of the remaining payload in this RTP format. Note that the header is present only in packets that have the S bit equal to one and the PID equal to zero in the payload descriptor. Subsequent packets for the same frame do not carry the payload header. The length of the first partition can always be obtained from the first partition-size parameter in the VP8 payload header. The VP8 bitstream format [RFC6386] specifies that if multiple DCT/WHT partitions are produced, the location of each partition start is found at the end of the first (prediction or mode) partition. In this RTP payload specification, the location offsets are considered to be part of the first partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |Size0|H| VER |P| +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | Octets 4..N of| | VP8 payload | : : +-+-+-+-+-+-+-+-+ | OPTIONAL RTP | | padding | : : +-+-+-+-+-+-+-+-+ Figure 3
A packetizer needs access to the P bit. The other fields are defined in [RFC6386], Section 9.1, and their meanings do not influence the packetization process. None of these fields are modified by the packetization process. 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 [RFC6386] 4.4. Aggregated and Fragmented Payloads An encoded VP8 frame can be divided into two or more partitions, as described in Section 1. It is OPTIONAL for a packetizer implementing this RTP specification to pay attention to the partition boundaries within an encoded frame. If packetization of a frame is done without considering the partition boundaries, the PID field MAY be set to 0 for all packets and the S bit MUST NOT be set to 1 for any other packet than the first. If the preferred usage suggested in Section 3 is followed, with each packet carrying data from exactly one partition, the S bit and PID fields described in Section 4.2 SHOULD be used to indicate what the packet contains. The PID field should indicate to which partition the first octet of the payload belongs and the S bit indicates that the packet starts on a new partition. If the packetizer does not pay attention to the partition boundaries, one packet can contain a fragment of a partition, a complete partition, or an aggregate of fragments and partitions. There is no explicit signaling of partition boundaries in the payload, and the partition lengths at the end of the first partition have to be used to identify the boundaries. Partitions MUST be aggregated in decoding order. Two fragments from different partitions MAY be aggregated into the same packet along with one or more complete partitions. In all cases, the payload of a packet MUST contain data from only one video frame. Consequently, the set of packets carrying the data from a particular frame will contain exactly one VP8 Payload Header (see Section 4.3) carried in the first packet of the frame. The last, or only, packet carrying data for the frame MUST have the M bit set in the RTP header.
4.5. Example Algorithms 4.5.1. Frame Reconstruction Algorithm Example of frame reconstruction algorithm. 1: Collect all packets with a given RTP timestamp. 2: Go through packets in order, sorted by sequence numbers, if packets are missing, send NACK as defined in [RFC4585] or decode with missing partitions, see Section 4.5.2 below. 3: A frame is complete if the frame has no missing sequence numbers, the first packet in the frame contains S=1 with partId=0 and the last packet in the frame has the marker bit set. 4.5.2. Partition Reconstruction Algorithm Example of partition reconstruction algorithm. The algorithm only applies for the RECOMMENDED use case with partitions in separate packets. 1: Scan for the start of a new partition; S=1. 2: Continue scan to detect end of partition; hence, a new S=1 (previous packet was the end of the partition) is found or the marker bit is set. If a loss is detected before the end of the partition, abandon all packets in this partition and continue the scan repeating from step 1. 3: Store the packets in the complete partition, continue the scan repeating from step 1 until end of frame is reached. 4: Send all complete partitions to the decoder. If no complete partition is found discard the whole frame.
4.6. Examples of VP8 RTP Stream A few examples of how the VP8 RTP payload can be used are included below. 4.6.1. Key Frame in a Single RTP Packet 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 1 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ |Size0|1| VER |0| P = 0 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | VP8 payload | +-+-+-+-+-+-+-+-+ 4.6.2. Non-discardable VP8 Interframe in a Single RTP Packet; No PictureID 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 1 | +-+-+-+-+-+-+-+-+ |0|0|0|1|0|0 0 0| X = 0; S = 1; PID = 0 +-+-+-+-+-+-+-+-+ |Size0|1| VER |1| P = 1 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | VP8 payload | +-+-+-+-+-+-+-+-+
4.6.3. VP8 Partitions in Separate RTP Packets First RTP packet; complete first partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 0 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ |Size0|1| VER |1| P = 1 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | Octets 4..L of| | first VP8 | | partition | : : +-+-+-+-+-+-+-+-+ Second RTP packet; complete second partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 1 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 1| X = 1; S = 1; PID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | Remaining VP8 | | partitions | : : +-+-+-+-+-+-+-+-+
4.6.4. VP8 Frame Fragmented across RTP Packets First RTP packet; complete first partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 0 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ |Size0|1| VER |1| P = 1 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | Complete | | first | | partition | : : +-+-+-+-+-+-+-+-+ Second RTP packet; first fragment of second partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 0 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 1| X = 1; S = 1; PID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | First fragment| | of second | | partition | : : +-+-+-+-+-+-+-+-+
Third RTP packet; second fragment of second partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 0 | +-+-+-+-+-+-+-+-+ |1|0|0|0|0|0 0 1| X = 1; S = 0; PID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | Mid fragment | | of second | | partition | : : +-+-+-+-+-+-+-+-+ Fourth RTP packet; last fragment of second partition. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 1 | +-+-+-+-+-+-+-+-+ |1|0|0|0|0|0 0 1| X = 1; S = 0; PID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 1 0 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | Last fragment | | of second | | partition | : : +-+-+-+-+-+-+-+-+
4.6.5. VP8 Frame with Long PictureID PictureID = 4711 = 001001001100111 binary (first 7 bits: 0010010, last 8 bits: 01100111). 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | RTP header | | M = 1 | +-+-+-+-+-+-+-+-+ |1|0|0|1|0|0 0 0| X = 1; S = 1; PID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1; +-+-+-+-+-+-+-+-+ |1 0 0 1 0 0 1 0| Long PictureID flag = 1 |0 1 1 0 0 1 1 1| PictureID = 4711 +-+-+-+-+-+-+-+-+ |Size0|1| VER |1| +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | Octets 4..N of| | VP8 payload | : : +-+-+-+-+-+-+-+-+ 5. Using VP8 with RPSI and SLI Feedback The VP8 payload descriptor defined in Section 4.2 contains an optional PictureID parameter. This parameter is included mainly to enable use of reference picture selection indication (RPSI) and slice loss indication (SLI), both defined in [RFC4585]. 5.1. RPSI The RPSI 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 for point-to-point (unicast) communication. 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 has 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 [RFC4585]) is simply the PictureID of the received frame. 5.2. SLI The SLI 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 [RFC4585]: 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, as defined in [RFC4585]. 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, 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 part 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. 5.3. Example The use of RPSI 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 acknowledged, 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 | | Send RPSI(0) | | | | | | | | 1002 | Receive RPSI(0) | | golden | | | | | | | ... | (sending regular | | | | | frames) | | | | | | | | | 1100 | Send altref frame | | | | | PictureID = 100 | | | | | | | | | | | Altref corrupted or | golden | | | | lost | | | | | | | | 1101 | | Send SLI(100) | golden | | | | | | | 1102 | Receive SLI(100) | | | | | | | | | 1103 | Send frame with | | | | | reference to | | | | | golden | | | | | | | | | | | Receive and decode | golden | | | | frame (decoder state | | | | | restored) | | | | | | | | ... | (sending regular | | | | | frames) | | | | | | | | | 1200 | Send altref frame | | | | | PictureID = 200 | | | | | | | | | | | Receive and decode | golden | | | | altref frame | | | | | | | | 1201 | | Send RPSI(200) | | | | | | | | 1202 | Receive RPSI(200) | | altref | | | | | |
| ... | (sending regular | | | | | frames) | | | | | | | | | 1300 | Send golden frame | | | | | PictureID = 300 | | | | | | | | | | | Receive and decode | altref | | | | golden frame | | | | | | | | 1301 | | Send RPSI(300) | altref | | | | | | | 1302 | RPSI lost | | | | | | | | | 1400 | Send golden frame | | | | | PictureID = 400 | | | | | | | | | | | Receive and decode | altref | | | | golden frame | | | | | | | | 1401 | | Send RPSI(400) | | | | | | | | 1402 | Receive RPSI(400) | | golden | +-------+-------------------+-------------------------+-------------+ Table 1: Example Signaling between Sender and Receiver 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 allowed by the RTCP sending timing restrictions as specified in [RFC4585], as long as the picture is corrupted. 6. Payload Format Parameters This payload format has two optional parameters. 6.1. Media Type Definition This registration is done using the template defined in [RFC6838] and following [RFC4855]. Type name: video Subtype name: VP8 Required parameters: None.
Optional parameters: These parameters are used to signal the capabilities of a receiver implementation. If the implementation is willing to receive media, both parameters MUST be provided. These parameters MUST NOT be used for any other purpose. max-fr: The value of max-fr is an integer indicating the maximum frame rate in units of frames per second that the decoder is capable of decoding. max-fs: The value of max-fs is an integer indicating the maximum frame size in units of macroblocks that the decoder is capable of decoding. The decoder is capable of decoding this frame size as long as the width and height of the frame in macroblocks are less than int(sqrt(max-fs * 8)). For instance, a max-fs of 1200 (capable of supporting 640x480 resolution) will support widths and heights up to 1552 pixels (97 macroblocks). Encoding considerations: This media type is framed in RTP and contains binary data; see Section 4.8 of [RFC6838]. Security considerations: See Section 7 of RFC 7741. Interoperability considerations: None. Published specification: VP8 bitstream format [RFC6386] and RFC 7741. Applications that use this media type: For example: Video over IP, video conferencing. Fragment identifier considerations: N/A. Additional information: None. Person & email address to contact for further information: Patrik Westin, firstname.lastname@example.org Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence it is only defined for transfer via RTP [RFC3550].
Author: Patrik Westin, email@example.com Change controller: IETF Payload Working Group delegated from the IESG. 6.2. SDP Parameters The receiver MUST ignore any fmtp parameter unspecified in this memo. 6.2.1. Mapping of Media Subtype Parameters to SDP The media type video/VP8 string is mapped to fields in the Session Description Protocol (SDP) [RFC4566] 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 media subtype). o The clock rate in the "a=rtpmap" line MUST be 90000. o The parameters "max-fs" and "max-fr" MUST be included in the "a=fmtp" line if the SDP is used to declare receiver capabilities. These parameters are expressed as a media subtype string, in the form of a semicolon-separated list of parameter=value pairs. 220.127.116.11. Example An example of media representation in SDP is as follows: m=video 49170 RTP/AVPF 98 a=rtpmap:98 VP8/90000 a=fmtp:98 max-fr=30; max-fs=3600; 6.2.2. Offer/Answer Considerations The VP8 codec offers a decode complexity that is roughly linear with the number of pixels encoded. The parameters "max-fr" and "max-fs" are defined in Section 6.1, where the macroblock size is 16x16 pixels as defined in [RFC6386], the max-fs and max-fr parameters MUST be used to establish these limits.
7. Security Considerations RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550], and in any applicable RTP profile such as RTP/AVP [RFC3551], RTP/AVPF [RFC4585], RTP/SAVP [RFC3711], or RTP/ SAVPF [RFC5124]. However, as "Securing the RTP Protocol Framework: Why RTP Does Not Mandate a Single Media Security Solution" [RFC7202] discusses, it is not an RTP payload format's responsibility to discuss or mandate what solutions are used to meet the basic security goals like confidentiality, integrity, and source authenticity for RTP in general. This responsibility lays on anyone using RTP in an application. They can find guidance on available security mechanisms and important considerations in "Options for Securing RTP Sessions" [RFC7201]. Applications SHOULD use one or more appropriate strong security mechanisms. The rest of this security consideration section discusses the security impacting properties of the payload format itself. This RTP payload format and its media decoder do not exhibit any significant difference in the receiver-side computational complexity for packet processing and, thus, are unlikely to pose a denial-of- service threat due to the receipt of pathological data. Nor does the RTP payload format contain any active content. 8. Congestion Control Congestion control for RTP SHALL be used in accordance with RFC 3550 [RFC3550] and with any applicable RTP profile; e.g., RFC 3551 [RFC3551]. The congestion control mechanism can, in a real-time encoding scenario, adapt the transmission rate by instructing the encoder to encode at a certain target rate. Media-aware network elements MAY use the information in the VP8 payload descriptor in Section 4.2 to identify non-reference frames and discard them in order to reduce network congestion. Note that discarding of non- reference frames cannot be done if the stream is encrypted (because the non-reference marker is encrypted). 9. IANA Considerations The IANA has registered a media type as described in Section 6.1.
10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, <http://www.rfc-editor.org/info/rfc3550>. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, DOI 10.17487/RFC3551, July 2003, <http://www.rfc-editor.org/info/rfc3551>. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006, <http://www.rfc-editor.org/info/rfc4566>. [RFC4585] 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, DOI 10.17487/RFC4585, July 2006, <http://www.rfc-editor.org/info/rfc4585>. [RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, <http://www.rfc-editor.org/info/rfc4855>. [RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J., Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding Guide", RFC 6386, DOI 10.17487/RFC6386, November 2011, <http://www.rfc-editor.org/info/rfc6386>. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, <http://www.rfc-editor.org/info/rfc6838>.
10.2. Informative References [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004, <http://www.rfc-editor.org/info/rfc3711>. [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February 2008, <http://www.rfc-editor.org/info/rfc5124>. [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, <http://www.rfc-editor.org/info/rfc7201>. [RFC7202] Perkins, C. and M. Westerlund, "Securing the RTP Framework: Why RTP Does Not Mandate a Single Media Security Solution", RFC 7202, DOI 10.17487/RFC7202, April 2014, <http://www.rfc-editor.org/info/rfc7202>. [Sch07] Schwarz, H., Marpe, D., and T. Wiegand, "Overview of the Scalable Video Coding Extension of the H.264/AVC Standard", IEEE Transactions on Circuits and Systems for Video Technology, Volume 17: Issue 9, DOI 10.1109/TCSVT.2007.905532, September 2007, <http://dx.doi.org/10.1109/TCSVT.2007.905532>.
Authors' Addresses Patrik Westin Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 United States Email: firstname.lastname@example.org Henrik F Lundin Google, Inc. Kungsbron 2 Stockholm 11122 Sweden Email: email@example.com Michael Glover Twitter Boston 10 Hemlock Way Durham, NH 03824 United States Email: firstname.lastname@example.org Justin Uberti Google, Inc. 747 6th Street South Kirkland, WA 98033 United States Email: email@example.com Frank Galligan Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 United States Email: firstname.lastname@example.org