Payload Working Group P. Westin Internet-Draft H. Lundin Intended status: Standards Track M. Glover Expires: December 7, 2015 J. Uberti F. Galligan Google June 5, 2015 RTP Payload Format for VP8 Video draft-ietf-payload-vp8-16 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 December 7, 2015. Copyright Notice Copyright (c) 2015 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 Westin, et al. Expires December 7, 2015 [Page 1] Internet-Draft RTP Payload Format for VP8 June 2015 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions, Definitions and Acronyms . . . . . . . . . . . . 3 3. Media Format Description . . . . . . . . . . . . . . . . . . 3 4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . 4 4.2. VP8 Payload Descriptor . . . . . . . . . . . . . . . . . 6 4.3. VP8 Payload Header . . . . . . . . . . . . . . . . . . . 10 4.4. Aggregated and Fragmented Payloads . . . . . . . . . . . 11 4.5. Frame reconstruction algorithm . . . . . . . . . . . . . 11 4.5.1. Partition reconstruction algorithm . . . . . . . . . 12 4.6. Examples of VP8 RTP Stream . . . . . . . . . . . . . . . 12 4.6.1. Key frame in a single RTP packet . . . . . . . . . . 12 4.6.2. Non-discardable VP8 interframe in a single RTP packet; no PictureID . . . . . . . . . . . . . . . . 13 4.6.3. VP8 partitions in separate RTP packets . . . . . . . 14 4.6.4. VP8 frame fragmented across RTP packets . . . . . . . 15 4.6.5. VP8 frame with long PictureID . . . . . . . . . . . . 16 5. Using VP8 with RPSI and SLI Feedback . . . . . . . . . . . . 17 5.1. RPSI . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.2. SLI . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 18 6. Payload Format Parameters . . . . . . . . . . . . . . . . . . 20 6.1. Media Type Definition . . . . . . . . . . . . . . . . . . 20 6.2. SDP Parameters . . . . . . . . . . . . . . . . . . . . . 22 6.2.1. Mapping of Media Subtype Parameters to SDP . . . . . 22 6.2.2. Offer/Answer Considerations . . . . . . . . . . . . . 22 7. Security Considerations . . . . . . . . . . . . . . . . . . . 23 8. Congestion Control . . . . . . . . . . . . . . . . . . . . . 23 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23 10.1. Normative References . . . . . . . . . . . . . . . . . . 24 10.2. Informative References . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 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 decomposition of frames into square sub-blocks of pixels, prediction of such sub-blocks using previously constructed Westin, et al. Expires December 7, 2015 [Page 2] Internet-Draft RTP Payload Format for VP8 June 2015 blocks, and adjustment of such predictions (as well as synthesis of unpredicted blocks) using a discrete cosine transform (hereafter abbreviated as DCT). In one special case, however, VP8 uses a "Walsh-Hadamard" (hereafter abbreviated as WHT) transform 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 Acronyms 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]. 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 Westin, et al. Expires December 7, 2015 [Page 3] Internet-Draft RTP Payload Format for VP8 June 2015 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.) 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. This memo allows the partitions to be sent separately or in the same RTP packet. It may be beneficial for decoder error-concealment to send the partitions in different packets, even though it is not mandatory according to this specification. 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 Westin, et al. Expires December 7, 2015 [Page 4] Internet-Draft RTP Payload Format for VP8 June 2015 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 P bit is set. 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. Timestamp: The RTP timestamp indicates the time when the frame was sampled at a clock rate of 90 kHz. Sequence number: The sequence numbers are monotonically increasing and set as packets are sent. The remaining RTP header fields are used as specified in [RFC3550]. Westin, et al. Expires December 7, 2015 [Page 5] Internet-Draft RTP Payload Format for VP8 June 2015 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 zero), while the dual-octet version (M bit set to one) is show 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 one, the extension octet MUST be provided immediately after the mandatory first octet. If the bit is zero, all optional fields MUST be omitted. R: Bit reserved for future use. MUST be set to zero and MUST be ignored by the receiver. N: Non-reference frame. When set to one, 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 zero 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 which VP8 partition the first payload octet of the packet belongs to. The first VP8 partition Westin, et al. Expires December 7, 2015 [Page 6] Internet-Draft RTP Payload Format for VP8 June 2015 (containing modes and motion vectors) MUST be labeled with PID = 0. PID SHOULD be incremented for each subsequent partition, but MAY be kept at 0 for all packets. PID MUST NOT 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 other packet than the first packet with that PID. When the X bit is set to 1 in the first octet, the extension bit field octet MUST be provided as the second octet. If the X bit is 0, the extension bit field octet MUST NOT be present, and no extensions (I, L, T, or K) are permitted. I: PictureID present. When set to one, 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 one, 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 one, 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 one but T is set to zero, the TID/Y/KEYIDX octet MUST be present, but the TID field MUST be ignored. If neither T nor K is set to one, the TID/Y/KEYIDX octet MUST NOT be present. K: KEYIDX present. When set to one, 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 one but K is set to zero, the TID/Y/KEYIDX octet MUST be present, but the KEYIDX field MUST be ignored. If neither T nor K is set to one, the TID/Y/KEYIDX octet MUST NOT be present. RSV: Bits reserved for future use. MUST be set to zero 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 one, the PictureID extension field MUST be present, and 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. Westin, et al. Expires December 7, 2015 [Page 7] Internet-Draft RTP Payload Format for VP8 June 2015 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 SHOULD begin as a random number, 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 stay 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 one, the TL0PICIDX extension field MUST be present, and 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 which base layer frame the current image depends on. TL0PICIDX MUST be incremented when TID is 0. The index SHOULD start on a random number, and MUST restart at 0 after reaching the maximum number 255. The TID/Y/KEYIDX extension: If the any of the T or K bits are set to one, 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 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. 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 Westin, et al. Expires December 7, 2015 [Page 8] Internet-Draft RTP Payload Format for VP8 June 2015 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 on a random number, and MUST restart at 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 which convey parameter updates critical to the interpretation of subsequent frames, and 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 wrap-around. 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. Westin, et al. Expires December 7, 2015 [Page 9] Internet-Draft RTP Payload Format for VP8 June 2015 4.3. VP8 Payload Header The beginning of an encoded VP8 frame is referred to as an "uncompressed data chunk" in [RFC6386], and 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 which 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. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |Size0|H| VER |P| +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | Bytes 4..N of | | VP8 payload | : : +-+-+-+-+-+-+-+-+ | OPTIONAL RTP | | padding | : : +-+-+-+-+-+-+-+-+ Figure 3 H: Show frame bit as defined in [RFC6386]. VER: A version number as defined in [RFC6386]. 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] SizeN: The size of the first partition in bytes is calculated from the 19 bits in Size0, Size1, and Size2 as 1stPartitionSize = Size0 + 8 * Size1 + 2048 * Size2. [RFC6386]. Westin, et al. Expires December 7, 2015 [Page 10] Internet-Draft RTP Payload Format for VP8 June 2015 4.4. Aggregated and Fragmented Payloads An encoded VP8 frame can be divided into two or more partitions, as described in Section 1. One packet can contain a fragment of a partition, a complete partition, or an aggregate of fragments and partitions. In the preferred use case, 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 which partition the first octet of the payload belongs to, and the S bit indicates that the packet starts on a new partition. Aggregation of encoded partitions is done without explicit signaling. Partitions MUST be aggregated in decoding order. Two fragments from different partitions MAY be aggregated into the same packet. 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 beginning of 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. 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/mode) partition. In this RTP payload specification, the location offsets are considered to be part of the first partition. 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 zero for all packets, and the S bit MUST NOT be set to one for any other packet than the first. 4.5. 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 4.5.1 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. Westin, et al. Expires December 7, 2015 [Page 11] Internet-Draft RTP Payload Format for VP8 June 2015 4.5.1. Partition reconstruction algorithm Example of partition reconstruction algorithm. 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 | +-+-+-+-+-+-+-+-+ Westin, et al. Expires December 7, 2015 [Page 12] Internet-Draft RTP Payload Format for VP8 June 2015 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 | +-+-+-+-+-+-+-+-+ Westin, et al. Expires December 7, 2015 [Page 13] Internet-Draft RTP Payload Format for VP8 June 2015 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 | +-+-+-+-+-+-+-+-+ | Bytes 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 | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires December 7, 2015 [Page 14] Internet-Draft RTP Payload Format for VP8 June 2015 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 | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires December 7, 2015 [Page 15] Internet-Draft RTP Payload Format for VP8 June 2015 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 Westin, et al. Expires December 7, 2015 [Page 16] Internet-Draft RTP Payload Format for VP8 June 2015 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 | +-+-+-+-+-+-+-+-+ | Bytes 4..N of | | VP8 payload | : : +-+-+-+-+-+-+-+-+ 5. Using VP8 with RPSI and SLI Feedback The VP8 payload descriptor defined in Section 4.2 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 [RFC4585]. 5.1. 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 Westin, et al. Expires December 7, 2015 [Page 17] Internet-Draft RTP Payload Format for VP8 June 2015 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 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 [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. 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 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. 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 | Westin, et al. Expires December 7, 2015 [Page 18] Internet-Draft RTP Payload Format for VP8 June 2015 | | | | 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) | | | Westin, et al. Expires December 7, 2015 [Page 19] Internet-Draft RTP Payload Format for VP8 June 2015 | | | | | | 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: Exemple 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 required 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: Westin, et al. Expires December 7, 2015 [Page 20] Internet-Draft RTP Payload Format for VP8 June 2015 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 xxxx. [RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.] Interoperability considerations: None. Published specification: VP8 bitstream format [RFC6386] and RFC XXXX. [RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.] Applications which 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, patrik.westin@gmail.com Intended usage: COMMON Restrictions on usage: Westin, et al. Expires December 7, 2015 [Page 21] Internet-Draft RTP Payload Format for VP8 June 2015 This media type depends on RTP framing, and hence is only defined for transfer via RTP [RFC3550]. Author: Patrik Westin, patrik.westin@gmail.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. 6.2.1.1. 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. Westin, et al. Expires December 7, 2015 [Page 22] Internet-Draft RTP Payload Format for VP8 June 2015 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. 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 encryption of the RTP payload. Integrity of the RTP packets through suitable cryptographic integrity protection mechanism. 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 or not. Note that the appropriate mechanism to provide security to RTP and payloads following this memo may vary. It is dependent on the application, the transport, and the signaling protocol employed. Therefore a single mechanism is not sufficient, although if suitable the usage of SRTP [RFC3711] is recommended. This RTP payload format and its media decoder do not exhibit any significant non-uniformity 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 is requested to register the following values: - Media type registration as described in Section 6.1. 10. References Westin, et al. Expires December 7, 2015 [Page 23] Internet-Draft RTP Payload Format for VP8 June 2015 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [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, July 2006. [RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, February 2007. [RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J., Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding Guide", RFC 6386, November 2011. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013. 10.2. Informative References [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. Authors' Addresses Patrik Westin Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 USA Email: patrik.westin@gmail.com Westin, et al. Expires December 7, 2015 [Page 24] Internet-Draft RTP Payload Format for VP8 June 2015 Henrik F Lundin Google, Inc. Kungsbron 2 Stockholm 11122 Sweden Email: hlundin@google.com Michael Glover Google, Inc. 5 Cambridge Center Cambridge, MA 02142 USA Justin Uberti Google, Inc. 747 6th Street South Kirkland, WA 98033 USA Frank Galligan Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 USA Westin, et al. Expires December 7, 2015 [Page 25]