RTP Payload Format for VP8 Video
draft-ietf-payload-vp8-08
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7741.
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Authors | Patrik Westin , Henrik Lundin , Michael Glover , Justin Uberti , Frank Galligan | ||
Last updated | 2013-07-09 (Latest revision 2013-01-18) | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
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Additional resources | Mailing list discussion | ||
Stream | WG state | Submitted to IESG for Publication | |
Document shepherd | Ali C. Begen | ||
Shepherd write-up | Show Last changed 2013-06-04 | ||
IESG | IESG state | Became RFC 7741 (Proposed Standard) | |
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Responsible AD | Richard Barnes | ||
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Send notices to | payload-chairs@tools.ietf.org, draft-ietf-payload-vp8@tools.ietf.org | ||
IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-payload-vp8-08
Payload Working Group P. Westin Internet-Draft H. Lundin Intended status: Standards Track M. Glover Expires: July 20, 2013 J. Uberti F. Galligan Google January 16, 2013 RTP Payload Format for VP8 Video draft-ietf-payload-vp8-08 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 July 20, 2013. Copyright Notice Copyright (c) 2013 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 July 20, 2013 [Page 1] Internet-Draft RTP Payload Format for VP8 January 2013 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 Acronyms . . . . . . . . . . . . 4 3. Media Format Description . . . . . . . . . . . . . . . . . . . 5 4. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. RTP Header Usage . . . . . . . . . . . . . . . . . . . . . 6 4.2. VP8 Payload Descriptor . . . . . . . . . . . . . . . . . . 7 4.3. VP8 Payload Header . . . . . . . . . . . . . . . . . . . . 10 4.4. Aggregated and Fragmented Payloads . . . . . . . . . . . . 11 4.5. Examples of VP8 RTP Stream . . . . . . . . . . . . . . . . 13 4.5.1. Key frame in a single RTP packet . . . . . . . . . . . 13 4.5.2. Non-discardable VP8 interframe in a single RTP packet; no PictureID . . . . . . . . . . . . . . . . . 13 4.5.3. VP8 partitions in separate RTP packets . . . . . . . . 14 4.5.4. VP8 frame fragmented across RTP packets . . . . . . . 15 4.5.5. VP8 frame with long PictureID . . . . . . . . . . . . 17 5. Using VP8 with RPSI and SLI Feedback . . . . . . . . . . . . . 18 5.1. RPSI . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2. SLI . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 19 6. Payload Format Parameters . . . . . . . . . . . . . . . . . . 22 6.1. Media Type Definition . . . . . . . . . . . . . . . . . . 22 6.2. SDP Parameters . . . . . . . . . . . . . . . . . . . . . . 23 6.2.1. Mapping of MIME Parameters to SDP . . . . . . . . . . 23 6.2.2. Offer/Answer Considerations . . . . . . . . . . . . . 23 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25 8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 26 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 Westin, et al. Expires July 20, 2013 [Page 2] Internet-Draft RTP Payload Format for VP8 January 2013 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 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. Westin, et al. Expires July 20, 2013 [Page 3] Internet-Draft RTP Payload Format for VP8 January 2013 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]. Westin, et al. Expires July 20, 2013 [Page 4] Internet-Draft RTP Payload Format for VP8 January 2013 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. 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.) 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 "PartID", "1st partition size" and "PictureID" fields. The ability to encode a temporally scalable stream motivates the "TL0PICIDX" and "TID" fields. Westin, et al. Expires July 20, 2013 [Page 5] Internet-Draft RTP Payload Format for VP8 January 2013 4. Payload Format This section describes how the encoded VP8 bitstream is encapsulated in RTP. Usage of RTP/AVPF [RFC4585] is recommended. 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 #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): 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. Westin, et al. Expires July 20, 2013 [Page 6] Internet-Draft RTP Payload Format for VP8 January 2013 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]. 4.2. VP8 Payload Descriptor The first octets after the RTP header are the VP8 payload descriptor, with the following structure. 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |X|R|N|S|PartID | (REQUIRED) +-+-+-+-+-+-+-+-+ X: |I|L|T|K| RSV | (OPTIONAL) +-+-+-+-+-+-+-+-+ I: |M| PictureID | (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. Westin, et al. Expires July 20, 2013 [Page 7] Internet-Draft RTP Payload Format for VP8 January 2013 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. PartID: Partition index. Denotes which VP8 partition the first payload octet of the packet belongs to. The first VP8 partition (containing modes and motion vectors) MUST be labeled with PartID = 0. PartID SHOULD be incremented for each subsequent partition, but MAY be kept at 0 for all packets. PartID MUST NOT be larger than 8. If more than one packet in an encoded frame contains the same PartID, the S bit MUST NOT be set for any other packet than the first packet with that PartID. When the X bit is set to 1 in the first octet, the OPTIONAL extension bit field MUST be present in the second octet. If the X bit is 0, the extension bit field MUST NOT be present, and all bits below MUST be implicitly interpreted as 0. I: PictureID present. When set to one, the OPTIONAL 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 OPTIONAL 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 OPTIONAL TID/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/ KEYIDX octet MUST be present, but the TID|Y field MUST be ignored. If neither T nor K is set to one, the TID/KEYIDX octet MUST NOT be present. K: KEYIDX present. When set to one, the OPTIONAL TID/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/KEYIDX octet MUST be present, but the KEYIDX field MUST be ignored. If neither T nor K is set to one, the TID/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. Westin, et al. Expires July 20, 2013 [Page 8] Internet-Draft RTP Payload Format for VP8 January 2013 M: The most significant bit of the first octet is an extension flag. The field MUST be present if the I bit is equal to one. If set the PictureID field MUST contain 16 bits else it MUST contain 8 bits including this MSB, see PictureID. PictureID: 8 or 16 bits including the M bit. This is a running index of the frames. The field MUST be present if the I bit is equal to one. The 7 following bits carry (parts of) the PictureID. If the extension flag is one, the PictureID continues in the next octet forming a 15 bit index, where the 8 bits in the second octet are the least significant bits of the PictureID. If the extension flag is zero, there is no extension, and the PictureID is the 7 remaining bits of the first (and only) octet. The sender may choose 7 or 15 bits index. The PictureID SHOULD start on a random number, and MUST wrap after reaching the maximum ID. TL0PICIDX: 8 bits temporal level zero index. The field MUST be present if the L bit is equal to 1, and MUST NOT be present otherwise. 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. TID: 2 bits temporal layer index. The TID/KEYIDX octet MUST be present when either the T bit or the K bit or both are equal to 1, and MUST NOT be present otherwise. 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 TID/KEYIDX octet MUST be present when either the T bit or the K bit or both are equal to 1, and MUST NOT be present otherwise. 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 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 an change in the KEYIDX field. Additionally this frame MUST NOT depend on any of the three codec buffers (as defined by rfc 6386) that have been updated since the last time the KEYIDX field was changed. Westin, et al. Expires July 20, 2013 [Page 9] Internet-Draft RTP Payload Format for VP8 January 2013 Informative note: This document does not describe how to determine the dependence 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 TID/KEYIDX octet MUST be present when either the T bit or the K bit or both are equal to 1, and MUST NOT be present otherwise. 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 SHOULD 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. 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. 4.3. VP8 Payload Header The beginning of an encoded VP8 frame is referred to as an "uncompressed data chunk" in [RFC6386], and co-serve as 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 PartID equal to zero in the payload descriptor. Subsequent packets for the same frame do not carry the payload header. Westin, et al. Expires July 20, 2013 [Page 10] Internet-Draft RTP Payload Format for VP8 January 2013 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 size in bytes is calculated from the 19 bits in Size0, Size1, and Size2 as 1stPartitionSize = Size0 + 8 * Size1 + 2048 * Size2. [RFC6386]. 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 PartID fields described in Section 4.2 should be used to indicate what the packet contains. The PartID 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 Westin, et al. Expires July 20, 2013 [Page 11]quot; as defined in [RFC8288]. These terms respectively correspond with "Context IRI" and "Target IRI" as used in [RFC5988]. Although defined as IRIs, in common scenarios they are also URIs. Additionally, this specification uses the following terms: o "access URI": A URI at which a user agent accesses a web resource. o "reference URI": A URI, other than the access URI, that should preferentially be used for referencing. By interacting with the access URI, the user agent may discover typed links. For such links, the access URI is the link context. 3. Scenarios 3.1. Persistent Identifiers Despite sound advice regarding the design of Cool URIs [CoolURIs], link rot ("HTTP 404 Not Found") is a common phenomena when following links on the web. Certain communities of practice have introduced solutions to combat this problem that typically consist of: o Accepting the reality that the web location of a resource - the access URI - may change over time. o Minting an additional URI for the resource - the reference URI - that is specifically intended to remain persistent over time. o Redirecting (typically "HTTP 301 Moved Permanently", "HTTP 302 Found", or "HTTP 303 See Other") from the reference URI to the access URI. o As a community, committing to adjust that redirection whenever the access URI changes over time. Van de Sompel, et al. Expires December 27, 2018 [Page 3] Internet-Draft cite-as relation June 2018 This approach is, for example, used by: o Scholarly publishers that use DOIs [DOIs] to identify articles and DOI URLs [DOI-URLs] as a means to keep cross-publisher article-to- article links operational, even when the journals in which the articles are published change hands from one publisher to another, for example, as a result of an acquisition. o Authors of controlled vocabularies that use PURLs [PURLs] for vocabulary terms to ensure that the term URIs remain stable even if management of the vocabulary is transfered to a new custodian. o A variety of organizations, including libraries, archives, and museums that assign ARK URLs [draft-kunze-ark-18] to information objects in order to support long-term access. In order for the investments in infrastructure involved in these approaches to pay off, and hence for links to effectively remain operational as intended, it is crucial that a resource be referenced by means of its reference URI. However, the access URI is where a user agent actually accesses the resource (e.g., it is the URI in the browser's address bar). As such, there is a considerable risk that the access URI instead of the reference URI is used for referencing [PIDs-must-be-used]. The link relation type defined in this specification allows to convey to user agents that the reference URI is the preferred URI for referencing. 3.2. Version Identifiers Resource versioning systems often use a naming approach whereby: o The most recent version of a resource is at any time available at the same, generic URI. o Each version of the resource - including the most recent one - has a distinct version URI. For example, Wikipedia uses generic URIs of the form http://en.wikipedia.org/wiki/John_Doe and version URIs of the form https://en.wikipedia.org/w/index.php?title=John_Doe&oldid=776253882. While the current version of a resource is accessed at the generic URI, some versioning systems adhere to a policy that favors linking and referencing a specific version URI. To express this using the terminology of Section 2, these policies intend that the generic URI is the access URI, and that the version URI is the reference URI. Van de Sompel, et al. Expires December 27, 2018 [Page 4] Internet-Draft cite-as relation June 2018 These policies are informed by the understanding that the content at the generic URI is likely to evolve over time, and that accurate links or references should lead to the content as it was at the time of referencing. To that end, Wikipedia's "Permanent link" and "Cite this page" functionalities promote the version URI, not the generic URI. The link relation type defined in this specification allows to convey to user agents that the version URI is preferred over the generic URI for referencing. 3.3. Preferred Social Identifier A web user commonly has multiple profiles on the web, for example, one per social network, a personal homepage, a professional homepage, a FOAF profile [FOAF], etc. Each of these profiles is accessible at a distinct URI. But the user may have a preference for one of those profiles, for example, because it is most complete, kept up-to-date, or expected to be long-lived. As an example, the first author of this document has, among others, the following profile URIs: o https://hvdsomp.info o http://public.lanl.gov/herbertv/ o https://www.linkedin.com/in/herbertvandesompel/ o https://orcid.org/0000-0002-0715-6126 Of these, from the perspective of the person described by these profiles, the first URI may be the preferred profile URI for the purpose of referencing because the domain is not under the custodianship of a third party. When an agent accesses another profile URI, such as http://public.lanl.gov/herbertv/, this preference for referencing by means of the first URI could be expressed. The link relation type defined in this specification allows to convey to user agents that a profile URI - the reference URI - other than the one the agent is accessing - the access URI - is preferred for referencing. 3.4. Multi-Resource Publications When publishing on the web, it is not uncommon to make distinct components of a publication available as different web resources, each with their own URI. For example: Van de Sompel, et al. Expires December 27, 2018 [Page 5] Internet-Draft cite-as relation June 2018 o Contemporary scholarly publications routinely consists of a traditional article as well as additional materials that are considered an integral part of the publication such as supplementary information, high-resolution images, a video recording of an experiment. o Scientific or governmental open data sets frequently consist of multiple files. o Online books typically consist of multiple chapters. While each of these components are accessible at their distinct URI - the access URI - they often also share a URI assigned to the intellectual publication of which they are components - the reference URI. The link relation type defined in this specification allows to convey to user agents that, for the purpose of referencing, the reference URI of the intellectual publication is preferred over an access URI of a component of the publication. 4. The "cite-as" Relation Type for Expressing a Preferred URI for the Purpose of Referencing A link with the "cite-as" relation type indicates that, for referencing the link context, use of the URI of the link target is preferred over use of the URI of the link context. It allows the resource identified by the access URI (link context) to unambiguously link to its corresponding reference URI (link target), thereby expressing that the link target is preferred over the link context for the purpose of permanent citation. The link target of a "cite-as" link SHOULD support protocol-based access as a means to ensure that applications that store them can effectively re-use them for access. The link target of a "cite-as" link SHOULD provide the ability for a user agent to follow its nose back to the context of the link, e.g. by following redirects and/or links. This helps a user agent to establish trust in the target URI. Because a link with the "cite-as" relation type expresses a preferred URI for the purpose of referencing, the access URI SHOULD only provide one link with that relation type. If more than one "cite-as" link is provided, the user agent may decide to select one (e.g. an HTTP URI over a mailto URI), for example, based on the purpose that the reference URI will serve. Van de Sompel, et al. Expires December 27, 2018 [Page 6] Internet-Draft cite-as relation June 2018 Providing a link with the "cite-as" relation type does not prevent using the access URI for the purpose of referencing if such specificity is needed for the application at hand. For example, in the case of scenario Section 3.4 the access URI is likely required for the purpose of annotating a specific component of an intellectual publication. Yet, the annotation application may also want to appropriately include the reference URI in the annotation. Applications can leverage the information provided by a "cite-as" link in a variety of ways, for example: o Bookmarking tools and citation managers can take this preference into account when recording a URI. o Webometrics applications that trace URIs can trace both the access URI and the reference URI. o Discovery tools can support look-up by means of both the access and the reference URI. This includes web archives that typically make archived versions of web resources discoverable by means of the original access URI of the archived resource; they can additionally make these archived resources discoverable by means of the associated reference URI. 5. Distinction with Other Relation Types Some existing IANA-registered relationships intuitively resemble the relationship that "cite-as" is intended to convey. But a closer inspection of these candidates provided in the blog posts [identifier-blog], [canonical-blog], and [bookmark-blog] shows that they are not appropriate for various reasons and that a new relation type is required. The remainder of this section provides a summary of the detailed explanations provided in the referenced blog posts. It can readily be seen that the following relation types are not fit for purpose: o "alternate" [RFC4287]: The link target provides an alternate version of the content at the link context. These are typically variants according to dimensions that are subject to content negotiation, for example the same content with varying Content- Type (e.g., application/pdf vs. text/html) and/or Content-Language (e.g., en vs. fr). The representations provided by the context URIs and target URIs in the scenarios of Section 3.1 through Section 3.4 are not variants in the sense intended by [RFC4287], and, as such, the use of "alternate" is not appropriate. Van de Sompel, et al. Expires December 27, 2018 [Page 7] Internet-Draft cite-as relation June 2018 o "duplicate" [RFC6249]: The link target is a resource whose available representations are byte-for-byte identical with the corresponding representations of the link context, for example, an identical file on a mirror site. In none of the above scenarios do the link context and the link target provide identical content. As such, the use of & Internet-Draft RTP Payload Format for VP8 January 2013 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 PartID 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. Westin, et al. Expires July 20, 2013 [Page 12] Internet-Draft RTP Payload Format for VP8 January 2013 4.5. Examples of VP8 RTP Stream A few examples of how the VP8 RTP payload can be used are included below. 4.5.1. 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; PartID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ |Size0|1| VER |0| P = 0 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | VP8 payload | +-+-+-+-+-+-+-+-+ 4.5.2. 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; PartID = 0 +-+-+-+-+-+-+-+-+ |Size0|1| VER |1| P = 1 +-+-+-+-+-+-+-+-+ | Size1 | +-+-+-+-+-+-+-+-+ | Size2 | +-+-+-+-+-+-+-+-+ | VP8 payload | +-+-+-+-+-+-+-+-+ Westin, et al. Expires July 20, 2013 [Page 13] Internet-Draft RTP Payload Format for VP8 January 2013 4.5.3. 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; PartID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 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; PartID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | Remaining VP8 | | partitions | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires July 20, 2013 [Page 14] Internet-Draft RTP Payload Format for VP8 January 2013 4.5.4. 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; PartID = 0 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 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; PartID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | First fragment| | of second | | partition | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires July 20, 2013 [Page 15] Internet-Draft RTP Payload Format for VP8 January 2013 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; PartID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 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; PartID = 1 +-+-+-+-+-+-+-+-+ |1|0|0|0|0 0 0 0| I = 1 +-+-+-+-+-+-+-+-+ |0 0 0 0 1 0 0 1| PictureID = 17 +-+-+-+-+-+-+-+-+ | Last fragment | | of second | | partition | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires July 20, 2013 [Page 16] Internet-Draft RTP Payload Format for VP8 January 2013 4.5.5. 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; PartID = 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 | : : +-+-+-+-+-+-+-+-+ Westin, et al. Expires July 20, 2013 [Page 17] Internet-Draft RTP Payload Format for VP8 January 2013 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 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, Westin, et al. Expires July 20, 2013 [Page 18] Internet-Draft RTP Payload Format for VP8 January 2013 quot;duplicate" is not appropriate. o "related" [RFC4287]: The link target is a resource that is related to the link context. While "related" could be used in all of the above scenarios, its semantics are too vague to convey the specific semantics intended by "cite-as". Two existing IANA-registered relationships deserve closer attention and are discussed in the remainder of this section. 5.1. bookmark "bookmark" [W3C.REC-html5-20151028]: The link target provides a URI for the purpose of bookmarking the link context. The intent of "bookmark" is closest to that of "cite-as" in that the link target is intended to be a permalink for the link context, for bookmarking purposes. The relation type dates back to the earliest days of news syndication, before blogs and news feeds had permalinks to identify individual resources that were aggregated into a single page. As such, its intent is to provide permalinks for different sections of an HTML document. It was originally used with HTML elements such as <div>, <h1>, <h2>, etc. and, more recently, HTML5 revised it to be exclusively used with the <article> element. Moreover, it is explictly excluded from use in the <link> element in HTML <head>, and, as a consequence, in the HTTP Link header that is semantically equivalent. For these technical and semantic reasons, the use of "bookmark" to convey the relationship intented by "cite- as" is not appropriate. A more detailed justification regarding the inappropriatenss of "bookmark", including a thorough overview of its turbulent history, is provided in [bookmark-blog]. 5.2. canonical "canonical" [RFC6596]: The meaning of "canonical" is commonly misunderstood on the basis of its brief definition as being "the preferred version of a resource." The description in the abstract of [RFC6596] is more helpful and states that "canonical" is intended to link to a resource that is preferred over resources with duplicative content. A more detailed reading of [RFC6596] clarifies that the intended meaning is preferred for the purpose of content indexing. A Van de Sompel, et al. Expires December 27, 2018 [Page 8] Internet-Draft cite-as relation June 2018 typical use case is linking from each page in a multi-page magazine article to a single page version of the article provided for indexing by search engines: the former pages provide content that is duplicative to the superset content that is available at the latter page. The semantics intended by "canonical" as preferred for the purpose of content indexing differ from the semantics intended by "cite-as" as preferred for the purpose of referencing. A further exploration of the various scenarios shows that the use of "canonical" is not appropriate to convey the semantics intended by "cite-as": o Scenario of Section 3.1: The reference URI that is intended to be persistent over time does not serve content that needs to be indexed, it merely redirects to the access URI. Since the meaning intended by "canonical" is "preferred for the purpose of content indexing", it is not appropriate to point at the reference URI (persistent identifier) using the "canonical" relation type. Moreover, Section 6.1 shows that scholarly publishers that assign persistent identifiers, already use the "canonical" relation type for search engine optimization, and how that use contrasts with the intended use of "cite-as". o Scenario of Section 3.2: In most common cases, custodians of resource versioning systems want search engines to index the most recent version of a page and hence would use a "canonical" link to point from version URIs of a resource to the associated generic URI. Wikipedia effectively does this. However, for some resource versioning systems, including Wikipedia, for the purpose of referencing, version URIs are preferred. As such, a "cite-as" link would point from the generic URI to the most recent version URI. That is, in the opposite direction of the "canonical" link. o Scenario of Section 3.3: The content at the link target and the link context are different profiles for a same person. Each profile, not just a preferred one, should be indexed. But a single one could be preferred for referencing. o Scenario of Section 3.4: The content at the link target, if any, would typically be a landing page that includes descriptive metadata pertaining to the multi-resource publication and links to its component resources. Each component resource provides content that is different, not duplicative, to the landing page. A more detailed justification regarding the inappropriatenss of "canonical", including examples, is provided in [canonical-blog]. Van de Sompel, et al. Expires December 27, 2018 [Page 9] Internet-Draft cite-as relation June 2018 6. Examples Sections Section 6.1 through Section 6.4 show examples of the use of links with the "cite-as" relation type. They illustrate how the typed links can be used in a response header and/or response body. 6.1. Persistent HTTP URI PLOS ONE is one of many scholarly publishers that assigns DOIs to the articles it publishes. For example, https://doi.org/10.1371/ journal.pone.0171057 is the persistent identifier for such an article. Via the DOI resolver, this persistent identifier redirects to http://journals.plos.org/plosone/doi?id=10.1371/ journal.pone.0171057 in the plos.org domain. This URI itself redirects to http://journals.plos.org/plosone/article?id=10.1371/ journal.pone.0171057, which delivers the actual article in HTML. The HTML article contains a <link> element with the "canonical" relation type pointing at itself, http://journals.plos.org/plosone/article?id=10.1371/ journal.pone.0167475. As per Section 5.2, this indicates that the article content at that URI should be indexed by search engines. PLOS ONE can additionally provide a link with the "cite-as" relation type pointing at the persistent identifier to indicate it is the preferred URI for permanent citation of the article. Figure 1 shows the addition of the "cite-as" link both in the HTTP header and the HTML that results from an HTTP GET on the article URI http://journals.plos.org/plosone/article?id=10.1371/ journal.pone.0167475. Van de Sompel, et al. Expires December 27, 2018 [Page 10] Internet-Draft cite-as relation June 2018 HTTP/1.1 200 OK Link: <https://doi.org/10.1371/journal.pone.0171057> ; rel="cite-as" Content-Type: text/html;charset=utf-8 <html> <head> ... <link rel="cite-as" href="https://doi.org/10.1371/journal.pone.0171057" /> <link rel="canonical" href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0167475" /> ... </head> <body> ... </body> </html> Figure 1: Response to HTTP GET on the URI of a scholarly article 6.2. Version URIs The preprint server arXiv.org has a versioning approach like the one described in Section 3.2: o The most recent version of a preprint is at any time available at the same, generic URI. Consider the preprint with generic URI https://arxiv.org/abs/1711.03787. o Each version of the preprint - including the most recent one - has a distinct version URI. The considered preprint has two versions with respective version URIs https://arxiv.org/abs/1711.03787v1 (published 10 November 2017) and https://arxiv.org/ abs/1711.03787v2 (published 24 January 2018). A reader who accessed https://arxiv.org/abs/1711.03787 between 10 November 2017 and 23 January 2018, obtained the first version of the preprint. Starting 24 January 2018, the second version was served at that URI. In order to support accurate referencing, arXiv.org could implement the "cite-as" link to point from the generic URI to the most recent version URI. In doing so, assuming the existence of reference manager tools that consume "cite-as" links: o The reader who accesses https://arxiv.org/abs/1711.03787 between 10 November 2017 and 23 January 2018 would reference https://arxiv.org/abs/1711.03787v1. Van de Sompel, et al. Expires December 27, 2018 [Page 11] Internet-Draft cite-as relation June 2018 o The reader who accesses https://arxiv.org/abs/1711.03787 starting 24 January 2018 would reference https://arxiv.org/ abs/1711.03787v2. Figure 2 shows the header that arXiv.org would have returned in the first case, in response to a HTTP HEAD on the generic URI https://arxiv.org/abs/1711.03787. HTTP/1.1 200 OK Date: Sun, 24 Dec 2017 16:12:43 GMT Content-Type: text/html; charset=utf-8 Link: <https://arxiv.org/abs/1711.03787v1> ; rel="cite-as" Vary: Accept-Encoding,User-Agent Figure 2: Response to HTTP HEAD on the generic URI of the landing page of an arXiv.org preprint 6.3. Preferred Profile URI If the access URI is the home page of John Doe, John can add a link with the "cite-as" relation type to it, as a means to convey that he would preferably be referenced by means of the URI of his FOAF profile. Figure 3 shows the response to an HTTP GET on the URI of John's home page. HTTP/1.1 200 OK Content-Type: text/html;charset=utf-8 <html> <head> ... <link rel="cite-as" href="http://johndoe.example.com/foaf" type="text/ttl"/> ... </head> <body> ... </body> </html> Figure 3: Response to HTTP GET on the URI of John Doe's home page Van de Sompel, et al. Expires December 27, 2018 [Page 12] Internet-Draft cite-as relation June 2018 6.4. Multi-Resource Publication The Dryad Digital Repository at datadryad.org specializes in hosting and preserving scientific datasets. Each dataset typically consists of multiple resources. For example, the dataset "Data from: Climate, demography, and lek stability in an Amazonian bird" consists of an Excel spreadsheet, a csv file, and a zip file. Each of these resources have different content and are accessible at their respective URIs. In addition, the dataset has a landing page at https://datadryad.org/resource/doi:10.5061/dryad.5d23f. Each of these resources should be permanently cited by means of the persistent identifier that was assigned to the entire dataset as an intellectual publication, i.e. https://doi.org/10.5061/dryad.5d23f. To that end, the Dryad Digital Repository can add "cite-as" links pointing from the URIs of each of these resources to https://doi.org/10.5061/dryad.5d23f. This is shown in Figure 4 for the csv file that is a component resource of the dataset, through use of the HTTP Link header. HTTP/1.1 200 OK Date: Tue, 12 Jun 2018 19:19:22 GMT Last-Modified: Wed, 17 Feb 2016 18:37:02 GMT Content-Type: text/csv;charset=ISO-8859-1 Content-Length: 25414 Link: <https://doi.org/10.5061/dryad.5d23f> ; rel="cite-as" DATE,Year,PLOT/TRAIL,LOCATION,SPECIES CODE,BAND NUM,COLOR,SEX,AGE,TAIL,WING, TARSUS,NARES,DEPTH,WIDTH,WEIGHT 6/26/02,2002,DANTA,325,PIPFIL,969,B/O,M,AHY,80,63,16,7.3,3.9,4.1,14.4 ... 2/3/13,2013,LAGO,,PIPFIL,BR-5095,O/YPI,M,SCB,78,65.5,14.2,7.5,3.8,3.7,14.3 Figure 4: Response to HTTP GET on the URI of a csv file that is a component of a scientfic dataset 7. IANA Considerations 7.1. Link Relation Type: cite-as The link relation type below has been registered by IANA per Section 2.1.1 of [RFC8288]: Relation Name: cite-as Van de Sompel, et al. Expires December 27, 2018 [Page 13] Internet-Draft cite-as relation June 2018 Description: A link with the "cite-as" relation type indicates that the link target is preferred over the link context for the purpose of permanent citation. Reference: [[ This document ]] 8. Security Considerations In cases where there is no way for the agent to automatically verify the correctness of the reference URI (cf. Section 4), out-of-band mechanisms might be required to establish trust. If a trusted site is compromised, the "cite-as" link relation could be used with malicious intent to supply misleading URIs for referencing. Use of these links might direct user agents to an attacker's site, break the referencing record they are intended to support, or corrupt algorithmic interpretation of referencing data. 9. References 9.1. 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, <https://www.rfc-editor.org/info/rfc2119>. [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication Format", RFC 4287, DOI 10.17487/RFC4287, December 2005, <https://www.rfc-editor.org/info/rfc4287>. [RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010, <https://www.rfc-editor.org/info/rfc5988>. [RFC6249] Bryan, A., McNab, N., Tsujikawa, T., Poeml, P., and H. Nordstrom, "Metalink/HTTP: Mirrors and Hashes", RFC 6249, DOI 10.17487/RFC6249, June 2011, <https://www.rfc-editor.org/info/rfc6249>. [RFC6596] Ohye, M. and J. Kupke, "The Canonical Link Relation", RFC 6596, DOI 10.17487/RFC6596, April 2012, <https://www.rfc-editor.org/info/rfc6596>. [RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017, <https://www.rfc-editor.org/info/rfc8288>. Van de Sompel, et al. Expires December 27, 2018 [Page 14] Internet-Draft cite-as relation June 2018 [W3C.REC-html5-20151028] Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Doyle Navara, E., O'Connor, E., and S. Pfeiffer, "HTML5", World Wide Web Consortium Recommendation REC-HTML5-20141028, October 2014, <https://www.w3.org/TR/2014/REC-html5-20141028/>. 9.2. Informative References [bookmark-blog] Nelson, M. and H. Van de Sompel, "rel=bookmark also does not mean what you think it means", August 2017, <http://ws-dl.blogspot.com/2017/08/2017-08-26-relbookmark- also-does-not.html>. [canonical-blog] Nelson, M. and H. Van de Sompel, "rel=canonical does not mean what you think it means", August 2017, <http://ws- dl.blogspot.nl/2017/08/2017-08-07-relcanonical-does-not- mean.html>. [CoolURIs] Berners-Lee, T., "Cool URIs don't change", World Wide Web Consortium Style, 1998, <https://www.w3.org/Provider/Style/URI.html>. [DOI-URLs] Hendricks, G., "Display guidelines for Crossref DOIs", June 2017, <https://blog.crossref.org/display-guidelines/>. [DOIs] "Information and documentation - Digital object identifier system", ISO 26324:2012(en), 2012, <https://www.iso.org/obp/ ui/#iso:std:iso:26324:ed-1:v1:en>. [draft-kunze-ark-18] Kunze, J. and R. Rodgers, "The ARK Identifier Scheme", Internet Draft draft-kunze-ark-18, April 2013, <https://datatracker.ietf.org/doc/html/draft-kunze-ark>. [FOAF] Brickley, D. and L. Miller, "FOAF Vocabulary Specification 0.99", January 2014, <http://xmlns.com/foaf/spec/>. Van de Sompel, et al. Expires December 27, 2018 [Page 15] Internet-Draft cite-as relation June 2018 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 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 | | 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) | | | | | | | | Westin, et al. Expires July 20, 2013 [Page 19] Internet-Draft RTP Payload Format for VP8 January 2013 | 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: Exemple signaling between sender and receiver Note that the scheme is robust to loss of the feedback messages. If Westin, et al. Expires July 20, 2013 [Page 20] Internet-Draft RTP Payload Format for VP8 January 2013 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. Westin, et al. Expires July 20, 2013 [Page 21] Internet-Draft RTP Payload Format for VP8 January 2013 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 [RFC4288] and following [RFC4855]. Type name: video Subtype name: VP8 Required parameters: none Optional parameters: max-fr, max-fs These parameters MAY be used to signal the capabilities of a receiver implementation. 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. Encoding considerations: This media type is framed in RTP and contains binary data; see Section 4.8 of [RFC4288]. 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. Westin, et al. Expires July 20, 2013 [Page 22] Internet-Draft RTP Payload Format for VP8 January 2013 Additional information: None. Person & email address to contact for further information: Patrik Westin, patrik.westin@gmail.com Intended usage: COMMON Restrictions on usage: 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 parameter unspecified in this memo. 6.2.1. Mapping of MIME Parameters to SDP The MIME media type video/VP8 string is mapped to fields in the Session Description Protocol (SDP) [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 MIME subtype). o The clock rate in the "a=rtpmap" line MUST be 90000. o The OPTIONAL parameters "max-fs", and "max-fr", when present, MUST be included in the "a=fmtp" line of SDP. These parameters are expressed as a MIME media type 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 6.2.2. Offer/Answer Considerations The VP8 codec offers a decode complexity that is roughly linear with the number of pixels encoded. In some practical applications, there Westin, et al. Expires July 20, 2013 [Page 23] Internet-Draft RTP Payload Format for VP8 January 2013 will be a need for negotiating frame rate and resolution, provided by the OPTIONAL parameters "max-fs" and "max-fr", in addition to these parameters, many practical applications will need a mean to communicate the max bitrate. The SDP endpoints MAY negotiate a method to communicate the maximum media bitrate, such as TMMBR in [RFC5104], therefore VP8 does not add any new mechanisms for this negotiation. The parameter "max-fr" and "max-fs" are defined in Section 6.1, where the macroblock size is 16x16 pixels as defined in [RFC6386]. In many practical applications, the max frame size and max frame rate are known from other information; if they are not constrained by other means, the max-fs and max-fr parameters MUST be used to establish these limits. Westin, et al. Expires July 20, 2013 [Page 24] Internet-Draft RTP Payload Format for VP8 January 2013 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. Westin, et al. Expires July 20, 2013 [Page 25] Internet-Draft RTP Payload Format for VP8 January 2013 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. Westin, et al. Expires July 20, 2013 [Page 26] Internet-Draft RTP Payload Format for VP8 January 2013 9. IANA Considerations The IANA is requested to register the following values: - Media type registration as described in Section 6.1. Westin, et al. Expires July 20, 2013 [Page 27] Internet-Draft RTP Payload Format for VP8 January 2013 10. 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. [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. [RFC3984] Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund, M., and D. Singer, "RTP Payload Format for H.264 Video", RFC 3984, February 2005. [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005. [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. [RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, February 2008. [RFC6386] Bankoski, J., Koleszar, J., Quillio, L., Salonen, J., Wilkins, P., and Y. Xu, "VP8 Data Format and Decoding Guide", RFC 6386, November 2011. Westin, et al. Expires July 20, 2013 [Page 28] Internet-Draft RTP Payload Format for VP8 January 2013 Authors' Addresses Patrik Westin Google, Inc. 1600 Amphitheatre Parkway Mountain View, CA 94043 USA Email: patrik.westin@gmail.com 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 July 20, 2013 [Page 29]