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RTP Payload Format for VP8 Video
draft-ietf-payload-vp8-08

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This is an older version of an Internet-Draft that was ultimately published as RFC 7741.
Authors Patrik Westin , Henrik Lundin , Michael Glover , Justin Uberti , Frank Galligan
Last updated 2013-07-09 (Latest revision 2013-01-18)
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Stream WG state Submitted to IESG for Publication
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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

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   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

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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.

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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].

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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.

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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.

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   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.

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   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.

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   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.

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         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.

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      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.

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   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.

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   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:

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   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.

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   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.

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   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 &
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   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.

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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   |
     +-+-+-+-+-+-+-+-+

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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    |
     :               :
     +-+-+-+-+-+-+-+-+

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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     |
     :               :
     +-+-+-+-+-+-+-+-+

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   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     |
     :               :
     +-+-+-+-+-+-+-+-+

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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   |
     :               :
     +-+-+-+-+-+-+-+-+

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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,

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   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

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   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].

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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.

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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.

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   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

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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

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      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>.

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   [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/>.

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   as read from the payload descriptor.  If the payload descriptor of
   the current frame does not have a PictureID, the receiver MAY send
   the last received PictureID+1 in the SLI message.  The receiver MAY
   set the First parameter to 0, and the Number parameter to the total
   number of macroblocks per frame, even though only parts of the frame
   is corrupted.  When the sender receives an SLI message, it can make
   use of the knowledge from the latest received RPSI message.  Knowing
   that the last golden or altref frame was successfully received, it
   can encode the next frame with reference to that established
   reference.

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)  |                         |             |
   |       |                   |                         |             |

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   | 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

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   the RPSI is lost, the sender will try to update the golden (or
   altref) again after a while, without releasing the established
   reference.  Also, if an SLI is lost, the receiver can keep sending
   SLI messages at any interval, as long as the picture is corrupted.

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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.

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   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

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   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.

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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.

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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.

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9.  IANA Considerations

   The IANA is requested to register the following values:
   - Media type registration as described in Section 6.1.

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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.

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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

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