A General Mechanism for RTP Header Extensions
draft-ietf-avtcore-rfc5285-bis-05

AVTCore                                                     R. Even, Ed.
Internet-Draft                                       Huawei Technologies
Obsoletes: RFC5285 (if approved)                               D. Singer
Intended status: Standards Track                             Apple, Inc.
Expires: May 18, 2017                                        H. Desineni
                                                       November 14, 2016

             A General Mechanism for RTP Header Extensions
                 draft-ietf-avtcore-rfc5285-bis-05.txt

Abstract

   This document provides a general mechanism to use the header
   extension feature of RTP (the Real-Time Transport Protocol).  It
   provides the option to use a small number of small extensions in each
   RTP packet, where the universe of possible extensions is large and
   registration is de-centralized.  The actual extensions in use in a
   session are signaled in the setup information for that session.  The
   document obsoletes RFC5285

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
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   This Internet-Draft will expire on May 18, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (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

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   3
   3.  Design Goals  . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Packet Design . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   4
       4.1.1.  transmission considertions  . . . . . . . . . . . . .   4
       4.1.2.  Header Extension type consideration . . . . . . . . .   6
     4.2.  One-Byte Header . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Two-Byte Header . . . . . . . . . . . . . . . . . . . . .   8
   5.  SDP Signaling Design  . . . . . . . . . . . . . . . . . . . .   9
   6.  SDP Signaling for support of mixed one byte and two bytes
       header extensions.  . . . . . . . . . . . . . . . . . . . . .  11
   7.  Offer/Answer  . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  BNF Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Identifier Space for IANA to Manage  . . . . . . . . . .  16
     10.2.  Registration of the SDP extmap Attribute . . . . . . . .  17
     10.3.  Registration of the SDP extmap-allow-mixed Attribute . .  17
   11. Changes from RFC5285  . . . . . . . . . . . . . . . . . . . .  18
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The RTP specification [RFC3550] provides a capability to extend the
   RTP header.  It defines the header extension format and rules for its
   use in Section 5.3.1.  The existing header extension method permits
   at most one extension per RTP packet, identified by a 16-bit
   identifier and a 16-bit length field specifying the length of the
   header extension in 32-bit words.

   This mechanism has two conspicuous drawbacks.  First, it permits only
   one header extension in a single RTP packet.  Second, the
   specification gives no guidance as to how the 16-bit header extension
   identifiers are allocated to avoid collisions.

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   This specification removes the first drawback by defining a backward-
   compatible and extensible means to carry multiple header extension
   elements in a single RTP packet.  It removes the second drawback by
   defining that these extension elements are named by URIs, defining an
   IANA registry for extension elements defined in IETF specifications,
   and a Session Description Protocol (SDP) method for mapping between
   the naming URIs and the identifier values carried in the RTP packets.

   This header extension applies to RTP/AVP (the Audio/Visual Profile)
   and its extensions.

   This document obsoletes [RFC5285] and removes a limitation from
   RFC5285 that did not allow sending both one byte and two bytes header
   extensions in the same RTP stream

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Design Goals

   The goal of this design is to provide a simple mechanism whereby
   multiple identified extensions can be used in RTP packets, without
   the need for formal registration of those extensions but nonetheless
   avoiding collision.

   This mechanism provides an alternative to the practice of burying
   associated metadata into the media format bit stream.  This has often
   been done in media data sent over fixed-bandwidth channels.  Once
   this is done, a decoder for the specific media format needs to
   extract the metadata.  Also, depending on the media format, the
   metadata can be added at the time of encoding the media so that the
   bit-rate used for the metadata is taken into account.  But the
   metadata can be unknown at that time.  Inserting metadata at a later
   time can cause a decode and re-encode to meet bit-rate requirements.

   In some cases, a more appropriate, higher-level mechanism can be
   available, and if so, it can be used.  For cases where a higher-level
   mechanism is not available, it is better to provide a mechanism at
   the RTP level than have the metadata be tied to a specific form of
   media data.

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4.  Packet Design

4.1.  General

   The following design is fit into the "header extension" of the RTP
   extension, as described above.

   The presence and format of this header extension and its contents are
   negotiated or defined out-of-band, such as through signaling (see
   below for SDP signaling).  The value defined for an RTP extension
   (defined below for the one-byte and two-byte header forms) is only an
   architectural constant (e.g., for use by network analyzers); it is
   the negotiation/definition (e.g., in SDP) that is the definitive
   indication that this header extension is present.

   This specification updates the requirement from the RTP specification
   that the header extension "is designed so that the header extension
   MAY be ignored".  To be specific, header extensions using this
   specification SHOULD be used for data that can safely be ignored by
   the recipient without affecting interoperability, there can be
   essential header extensions for interoperability and intermediaries
   SHOULD NOT remove such header extensions.  Note that the support of
   header extension as specified in this recommendation is negotiated.
   RTP Header extensions MUST NOT be used when the presence of the
   extension has changed the form or nature of the rest of the packet in
   a way that is not compatible with the way the stream is signaled
   (e.g., as defined by the payload type).  Valid examples might include
   metadata that is additional to the usual RTP information, e.g.  Audio
   level from Client to mixer [RFC6464].  Note that some header
   extensions, for example MID [I-D.ietf-mmusic-sdp-bundle-negotiation]
   might, if removed, disrupt the behaviour of the higher-level
   application that builds on RTP, but are acceptable since they do not
   affect interoperability of the RTP stack itself.

   The RTP header extension is formed as a sequence of extension
   elements, with possible padding.  Each extension element has a local
   identifier and a length.  The local identifiers MAY be mapped to a
   larger namespace in the negotiation (e.g., session signaling).

4.1.1.  transmission considertions

   As is good network practice, data SHOULD only be transmitted when
   needed.  The RTP header extension SHOULD only be present in a packet
   if that packet also contains one or more extension elements, as
   defined here.  An extension element SHOULD only be present in a
   packet when needed; the signaling setup of extension elements
   indicates only that those elements can be present in some packets,
   not that they are in fact present in all (or indeed, any) packets.

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   Some general considerations for getting the header extensions
   delivered to the receiver:

   1.  The probability for packet loss and burst loss determine how many
       repetitions of the header extensions will be required to reach a
       targeted delivery probability, and if burst loss is likely, what
       distribution would be needed to avoid getting all repetitions of
       the header extensions lost in a single burst.

   2.  If a set of packets are all needed to enable decoding, there is
       commonly no reason for including the header extension in all of
       these packets, as they share fate.  Instead, at most one instance
       of the header extension per independently decodable set of media
       data would be a more efficient use of the bandwidth.

   3.  How early the Header Extension item information is needed, from
       the first received RTP data or only after some set of packets are
       received, can guide if the header extension(s) should be in all
       of the first N packets or be included only once per set of
       packets, for example once per video frame.

   4.  The use of RTP level robustness mechanisms, such as RTP
       retransmission [RFC4588], or Forward Error Correction, e.g.,
       [RFC5109] may treat packets differently from a robustness
       perspective, and header extensions should be added to packets
       that get a treatment corresponding to the relative importance of
       receiving the information.

   As a summary, the number of header extension transmissions should be
   tailored to a desired probability of delivery taking the receiver
   population size into account.  For the very basic case, N repetitions
   of the header extensions should be sufficient, but may not be
   optimal.  N is selected so that the header extension target delivery
   probability reaches 1-P^N, where P is the probability of packet loss.
   For point to point or small receiver populations, it might also be
   possible to use feedback, such as RTCP, to determine when the
   information in the header extensions has reached all receivers and
   stop further repetitions.  Feedback that can be used includes the
   RTCP XR Loss RLE report block [RFC3611], which will indicate
   successful delivery of particular packets.  If the RTP/AVPF Transport
   Layer Feedback Messages for generic NACK [RFC4585] is used, it can
   indicate the failure to deliver an RTP packet with the header
   extension, thus indicating the need for further repetitions.  The
   normal RTCP report blocks can also provide an indicator of successful
   delivery, if no losses are indicated for a reporting interval
   covering the RTP packets with the header extension.  Note that loss
   of an RTCP packet reporting on an interval where RTP header extension

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   packets were sent, does not necessarily mean that the RTP header
   extension packets themselves were lost.

4.1.2.  Header Extension type consideration

   Each extension element in a packet has a local identifier (ID) and a
   length.  The local identifiers present in the stream MUST have been
   negotiated or defined out-of-band.  There are no static allocations
   of local identifiers.  Each distinct extension MUST have a unique ID.
   The value 0 is reserved for padding and MUST NOT be used as a local
   identifier.

   There are two variants of the extension: one-byte and two-byte
   headers.  Since it is expected that (a) the number of extensions in
   any given RTP session is small and (b) the extensions themselves are
   small, the one-byte header form is preferred and MUST be supported by
   all receivers.  A stream MUST contain only one-byte or two-byte
   headers unless it is known that all recipients support mixing, either
   by offer/answer negotiation (see section 6) or by out-of-band
   knowledge.  Each RTP packet with an RTP header extension following
   this specification will indicate if it contains one or two byte
   header extensions through the use of the "defined by profile" field.
   Only the extension element types that match the header extension
   format, i.e. one- or two-byte, MUST be used in that RTP packet.
   Transmitters SHOULD NOT use the two-byte form when all extensions are
   small enough for the one-byte header form.  Transmitters that intend
   to send the two-byte form SHOULD use IDs above 14 if they want to let
   the Receivers know that they intend to use two-byte form, for example
   if the RTP header extension is longer than 16 bytes.  A transmitter
   MAY be aware that an intermediary may add RTP header extensions in
   this case, the transmitter SHOULD use two-byte form.

   A sequence of extension elements, possibly with padding, forms the
   header extension defined in the RTP specification.  There are as many
   extension elements as fit into the length as indicated in the RTP
   header extension length.  Since this length is signaled in full
   32-bit words, padding bytes are used to pad to a 32-bit boundary.
   The entire extension is parsed byte-by-byte to find each extension
   element (no alignment is needed), and parsing stops at the earlier of
   the end of the entire header extension, or in one-byte headers only
   case, on encountering an identifier with the reserved value of 15.

   In both forms, padding bytes have the value of 0 (zero).  They MAY be
   placed between extension elements, if desired for alignment, or after
   the last extension element, if needed for padding.  A padding byte
   does not supply the ID of an element, nor the length field.  When a
   padding byte is found, it is ignored and the parser moves on to
   interpreting the next byte.

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   Note carefully that the one-byte header form allows for data lengths
   between 1 and 16 bytes, by adding 1 to the signaled length value
   (thus, 0 in the length field indicates 1 byte of data follows).  This
   allows for the important case of 16-byte payloads.  This addition is
   not performed for the two-byte headers, where the length field
   signals data lengths between 0 and 255 bytes.

   Use of RTP header extensions will reduce the efficiency of RTP header
   compression, since the header extension will be sent uncompressed
   unless the RTP header compression module is updated to recognize the
   extension header.  If header extensions are present in some packets,
   but not in others, this can also reduce compression efficiency by
   requiring an update to the fixed header to be conveyed when header
   extensions start or stop being sent.  The interactions of the RTP
   header extension and header compression is explored further in
   [RFC2508] and [RFC3095].

4.2.  One-Byte Header

   In the one-byte header form of extensions, the 16-bit value REQUIRED
   by the RTP specification for a header extension, labeled in the RTP
   specification as "defined by profile", MUST have the fixed bit
   pattern 0xBEDE (the first version of this specification was written
   on the feast day of the Venerable Bede).

   Each extension element MUST starts with a byte containing an ID and a
   length:

       0
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |  ID   |  len  |
      +-+-+-+-+-+-+-+-+

   The 4-bit ID is the local identifier of this element in the range
   1-14 inclusive.  In the signaling section, this is referred to as the
   valid range.

   The local identifier value 15 is reserved for future extension and
   MUST NOT be used as an identifier.  If the ID value 15 is
   encountered, its length field MUST be ignored, processing of the
   entire extension MUST terminate at that point, and only the extension
   elements present prior to the element with ID 15 SHOULD be
   considered.

   The 4-bit length is the number minus one of data bytes of this header
   extension element following the one-byte header.  Therefore, the

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   value zero in this field indicates that one byte of data follows, and
   a value of 15 (the maximum) indicates element data of 16 bytes.
   (This permits carriage of 16-byte values, which is a common length of
   labels and identifiers, while losing the possibility of zero-length
   values -- which would often be padded anyway.)

   An example header extension, with three extension elements, and some
   padding follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       0xBE    |    0xDE       |           length=3            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  ID   | L=0   |     data      |  ID   |  L=1  |   data...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...data   |    0 (pad)    |    0 (pad)    |  ID   | L=3   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          data                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3.  Two-Byte Header

   In the two-byte header form, the 16-bit value defined by the RTP
   specification for a header extension, labeled in the RTP
   specification as "defined by profile", is defined as shown below.

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         0x100         |appbits|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The appbits field is 4 bits that are application-dependent and MAY be
   defined to be any value or meaning, and are outside the scope of this
   specification.  For the purposes of signaling, this field is treated
   as a special extension value assigned to the local identifier 256.
   If no extension has been specified through configuration or signaling
   for this local identifier value 256, the appbits field SHOULD be set
   to all 0s by the sender and MUST be ignored by the receiver.

   Each extension element starts with a byte containing an ID and a byte
   containing a length:

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       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       ID      |     length    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 8-bit ID is the local identifier of this element in the range
   1-255 inclusive.  In the signaling section, the range 1-256 is
   referred to as the valid range, with the values 1-255 referring to
   extension elements, and the value 256 referring to the 4-bit field
   'appbits' (above).  Note that there is one ID space for both one-byte
   and two-byte form this means that the lower values (1-14) can be used
   in the 4-bit ID field in the one-byte header format as well.

   The 8-bit length field is the length of extension data in bytes not
   including the ID and length fields.  The value zero indicates there
   is no data following.

   An example header extension, with three extension elements, and some
   padding follows:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       0x10    |    0x00       |           length=3            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      ID       |     L=0       |     ID        |     L=1       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       data    |    0 (pad)    |       ID      |      L=4      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          data                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.  SDP Signaling Design

   The indication of the presence of this extension, and the mapping of
   local identifiers used in the header extension to a larger namespace,
   MUST be performed out-of-band, for example, as part of a SIP offer/
   answer exchange using SDP.  This section defines such signaling in
   SDP.

   A usable mapping MUST use IDs in the valid range, and each ID in this
   range MUST be used only once for each media (or only once if the
   mappings are session level).  Mappings that do not conform to these
   rules MAY be presented, for instance, during offer/answer negotiation
   as described in the next section, but remapping to conformant values
   is necessary before they can be applied.

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   Each extension is named by a URI.  That URI MUST be absolute, and
   precisely identifies the format and meaning of the extension.  URIs
   that contain a domain name SHOULD also contain a month-date in the
   form mmyyyy.  The definition of the element and assignment of the URI
   MUST have been authorized by the owner of the domain name on or very
   close to that date.  (This avoids problems when domain names change
   ownership.)  If the resource or document defines several extensions,
   then the URI MUST identify the actual extension in use, e.g., using a
   fragment or query identifier (characters after a '#' or '?' in the
   URI).

   Rationale: the use of URIs provides for a large, unallocated space,
   and gives documentation on the extension.  The URIs do not have to be
   de-referencable, in order to permit confidential or experimental use,
   and to cover the case when extensions continue to be used after the
   organization that defined them ceases to exist.

   An extension URI with the same attributes MUST NOT appear more than
   once applying to the same stream, i.e., at session level or in the
   declarations for a single stream at media level.  (The same extension
   can, of course, be used for several streams, and can appear
   differently parameterized for the same stream.)

   For extensions defined in RFCs, the URI used SHOULD be a URN starting
   "urn:ietf:params:rtp-hdrext:" and followed by a registered,
   descriptive name.

   The registration requirements are detailed in the IANA Considerations
   section, below.

   An example (this is only an example), where 'avt-example-metadata' is
   the hypothetical name of a header extension, might be:

      urn:ietf:params:rtp-hdrext:avt-example-metadata

   An example name not from the IETF (this is only an example) might be:

      http://example.com/082005/ext.htm#example-metadata

   The mapping MAY be provided per media stream (in the media-level
   section(s) of SDP, i.e., after an "m=" line) or globally for all
   streams (i.e., before the first "m=" line, at session level).  The
   definitions MUST be either all session level or all media level; it
   is not permitted to mix the two styles.  In addition, as noted above,
   the IDs used MUST be unique for each stream type for a given media,
   or for the session for session-level declarations.

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   Each local identifier potentially used in the stream is mapped to a
   string using an attribute of the form:

      a=extmap:<value>["/"<direction>] <URI> <extensionattributes>

   where <URI> is a URI, as above, <value> is the local identifier (ID)
   of this extension and is an integer in the valid range inclusive (0
   is reserved for padding in both forms, and 15 is reserved in the one-
   byte header form, as noted above), and <direction> is one of
   "sendonly", "recvonly", "sendrecv", or "inactive" (without the
   quotes) with relation to the device being configured.

   The formal BNF syntax is presented in a later section of this
   specification.

   Example:

      a=extmap:1 http://example.com/082005/ext.htm#ttime

      a=extmap:2/sendrecv http://example.com/082005/ext.htm#xmeta short

   When SDP signaling is used for the RTP session, it is the presence of
   the 'extmap' attribute(s) that is diagnostic that this style of
   header extensions is used, not the magic number indicated above.

6.  SDP Signaling for support of mixed one byte and two bytes header
    extensions.

   In order to allow for backward interoperability with systems that do
   not support mixing of one byte and two bytes header extensions this
   document defines the "a=extmap-allow-mixed" Session Description
   Protocol (SDP) [RFC4566] attribute to indicate if the participant is
   capable of supporting this new mode.  The attribute takes no value.
   This attribute can be used at the session or media levels.  A
   participant that proposes the use of this mode SHALL itself support
   the reception of mixed one byte and two bytes header extensions.

   The negotiation for mixed one byte and two bytes extension MUST be
   negotiated in offer/answer [RFC3264].  In the absence of negotiation
   using offer/answer, mixed headers MUST NOT occur unless the
   transmitter has some (out of band) knowledge that all potential
   recipients support this mode.

   The formal definition of this attribute is:

      Name: extmap-allow-mixed

      Value:

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      Usage Level: session, media

      Charset Dependent: no

      Example:

      a=extmap-allow-mixed

   When doing SDP Offer/Answer [RFC3264] an offering client that wishes
   to use both one and two bytes extensions MUST include the attribute
   "a= extmap-allow-mixed " in the SDP offer.  If "a= extmap-allow-mixed
   " is present in the offer SDP, the answerer that supports this mode
   and wishes to use it SHALL include the "a=extmap-allow-mixed "
   attribute in the answer.  In cases the answer has been excluded,
   neither clients SHALL use mixed one bytes and two bytes extensions in
   the same RTP stream but MAY use one-byte or two-bytes form (see
   section 4.1.2).

7.  Offer/Answer

   The simple signaling described above for the extmap attribute MAY be
   enhanced in an offer/answer context, to permit:

   o  asymmetric behavior (extensions sent in only one direction),

   o  the offer of mutually exclusive alternatives, or

   o  the offer of more extensions than can be sent in a single session.

   A direction attribute MAY be included in an extmap; without it, the
   direction implicitly inherits, of course, from the stream direction,
   or is "sendrecv" for session-level attributes or extensions of
   "inactive" streams.  The direction MUST be one of "sendonly",
   "recvonly", "sendrecv", or "inactive" as specified in [RFC3264]

   Extensions, with their directions, MAY be signaled for an "inactive"
   stream.  It is an error to use an extension direction incompatible
   with the stream direction (e.g., a "sendonly" attribute for a
   "recvonly" stream).

   If an offer or answer contains session-level mappings (and hence no
   media-level mappings), and different behavior is desired for each
   stream, then the entire set of extension map declarations MAY be
   moved into the media-level section(s) of the SDP.  (Note that this
   specification does not permit mixing global and local declarations,
   to make identifier management easier.)

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   If an extension map is offered as "sendrecv", explicitly or
   implicitly, and asymmetric behavior is desired, the SDP MAY be
   modified to modify or add direction qualifiers for that extension.

   If an extension is marked as "sendonly" and the answerer desires to
   receive it, the extension MUST be marked as "recvonly" in the SDP
   answer.  An answerer that has no desire to receive the extension or
   does not understand the extension SHOULD remove it from the SDP
   answer.

   If an extension is marked as "recvonly" and the answerer desires to
   send it, the extension MUST be marked as "sendonly" in the SDP
   answer.  An answerer that has no desire to, or is unable to, send the
   extension SHOULD remove it from the SDP answer.

   Local identifiers in the valid range inclusive in an offer or answer
   MUST NOT be used more than once per media section (including the
   session-level section).  A session update MAY change the direction
   qualifiers of extensions under use.  A session update MAY add or
   remove extension(s).  Identifiers values in the valid range MUST NOT
   be altered (remapped).

   Note that, under this rule, the same local identifier cannot be used
   for two extensions for the same media, even when one is "sendonly"
   and the other "recvonly", as it would then be impossible to make
   either of them sendrecv (since re-numbering is not permitted either).

   If a party wishes to offer mutually exclusive alternatives, then
   multiple extensions with the same identifier in the (unusable) range
   4096-4351 MAY be offered; the answerer SHOULD select at most one of
   the offered extensions with the same identifier, and remap it to a
   free identifier in the valid range, for that extension to be usable.

   Similarly, if more extensions are offered than can be fit in the
   valid range, identifiers in the range 4096-4351 MAY be offered; the
   answerer SHOULD choose those that are desired, and remap them to a
   free identifier in the valid range.

   It is always allowed to place the offered identifier value "as is" in
   the SDP answer (for example, due to lack of a free identifier value
   in the valid range).  Extensions with an identifier outside the valid
   range MUST NOT, of course, be used.  If needed, the offerer or
   answerer can update the session to make space for such an extension.

   Rationale: the range 4096-4351 for these negotiation identifiers is
   deliberately restricted to allow expansion of the range of valid
   identifiers in future.

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   Either party MAY include extensions in the stream other than those
   negotiated, or those negotiated as "inactive", for example, for the
   benefit of intermediate nodes.  Only extensions that appeared with an
   identifier in the valid range in SDP originated by the sender can be
   sent.

   Example (port numbers, RTP profiles, payload IDs and rtpmaps, etc.
   all omitted for brevity):

   The offer:

   a=extmap:1 URI-toffset
   a=extmap:14 URI-obscure
   a=extmap:4096 URI-gps-string
   a=extmap:4096 URI-gps-binary
   a=extmap:4097 URI-frametype
   m=video
   a=sendrecv
   m=audio
   a=sendrecv

   The answerer is interested in receiving GPS in string format only on
   video, but cannot send GPS at all.  It is not interested in
   transmission offsets on audio, and does not understand the URI-
   obscure extension.  It therefore moves the extensions from session
   level to media level, and adjusts the declarations:

   m=video
   a=sendrecv
   a=extmap:1 URI-toffset
   a=extmap:2/recvonly URI-gps-string
   a=extmap:3 URI-frametype
   m=audio
   a=sendrecv
   a=extmap:1/sendonly URI-toffset

8.  BNF Syntax

   The syntax definition below uses ABNF according to [RFC5234].  The
   syntax element 'URI' is defined in [RFC3986] (only absolute URIs are
   permitted here).  The syntax element 'extmap' is an attribute as
   defined in [RFC4566], i.e., "a=" precedes the extmap definition.
   Specific extensionattributes are defined by the specification that
   defines a specific extension name; there can be several.

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        extmap = mapentry SP extensionname [SP extensionattributes]

        extensionname = URI

        direction = "sendonly" / "recvonly" / "sendrecv" / "inactive"

        mapentry = "extmap:" 1*5DIGIT ["/" direction]

        extensionattributes = byte-string

        URI = <Defined in RFC 3986>

        byte-string = <Defined in RFC 4566>

        SP = <Defined in RFC 5234>

        DIGIT = <Defined in RFC 5234>

9.  Security Considerations

   This document defines only a place to transmit information; the
   security implications of each of the extensions MUST be discussed
   with those extensions.

   Extensions usage is negotiated using [RFC3264] so integrity
   protection and end-to-end authentication MUST be used.  The security
   considerations of [RFC3264] MUST be followed, to prevent, for
   example, extension usage blocking.

   Header extensions have the same security coverage as the RTP header
   itself.  When Secure Real-time Transport Protocol (SRTP) [RFC3711] is
   used to protect RTP sessions, the RTP payload can be both encrypted
   and integrity protected, while the RTP header is either unprotected
   or integrity protected.  In order to prevent DOS attacks, for
   example, by changing the header extension integrity protection SHOULD
   be used.  Lower layer security protection like DTLS[RFC6347] MAY be
   used.  RTP header extensions can carry sensitive information for
   which participants in multimedia sessions want confidentiality.
   RFC6904 [RFC6904]  provides a mechanism, extending the mechanisms of
   SRTP, to selectively encrypt RTP header extensions in SRTP.

   Other security options for securing RTP are discussed in [RFC7201].

10.  IANA Considerations

   This document updates the IANA consideration to reference this
   document and adds a new SDP attribute in section 10.3

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   Note to IANA : change RFCxxxx to this RFC number and remove the note.

10.1.  Identifier Space for IANA to Manage

   The mapping from the naming URI form to a reference to a
   specification is managed by IANA.  Insertion into this registry is
   under the requirements of "Expert Review" as defined in [RFC5226].

   The IANA will also maintain a server that contains all of the
   registered elements in a publicly accessible space.

   Here is the formal declaration to comply with the IETF URN Sub-
   namespace specification [RFC3553].

   o  Registry name: RTP Compact Header Extensions

   o  Specification: RFC 5285 and RFCs updating RFC 5285.

   o  Information required:

      A.  The desired extension naming URI

      B.  A formal reference to the publicly available specification

      C.  A short phrase describing the function of the extension

      D.  Contact information for the organization or person making the
          registration

      For extensions defined in RFCs, the URI SHOULD be of the form
      urn:ietf:params:rtp-hdrext:, and the formal reference is the RFC
      number of the RFC documenting the extension.

   o  Review process: Expert review is REQUIRED.  The expert review
      SHOULD check the following requirements:

      1.  that the specification is publicly available;

      2.  that the extension complies with the requirements of RTP, and
          this specification, for header extensions (specifically, that
          the header extension can be ignored or discarded without
          breaking the RTP layer);

      3.  that the extension specification is technically consistent (in
          itself and with RTP), complete, and comprehensible;

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      4.  that the extension does not duplicate functionality in
          existing IETF specifications (including RTP itself), or other
          extensions already registered;

      5.  that the specification contains a security analysis regarding
          the content of the header extension;

      6.  that the extension is generally applicable, for example point-
          to-multipoint safe, and the specification correctly describes
          limitations if they exist; and

      7.  that the suggested naming URI form is appropriately chosen and
          unique.

   o  Size and format of entries: a mapping from a naming URI string to
      a formal reference to a publicly available specification, with a
      descriptive phrase and contact information.

   o  Initial assignments: none.

10.2.  Registration of the SDP extmap Attribute

   IANA is requested to register the extmap SDP [RFC4566] attribute.

         SDP Attribute ("att-field"):
         Attribute name:     extmap
         Long form:          generic header extension map definition
         Type of name:       att-field
         Type of attribute:  Media or session level
         Subject to charset: No
         Purpose:            defines the mapping from the extension numbers used in
                                    packet headers into extension names.
         Reference:          [RFCXXXX]
         Values:             See [RFCXXXX]

10.3.  Registration of the SDP extmap-allow-mixed Attribute

   The IANA is requested to register one new SDP attribute:

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            SDP Attribute ("att-field"):
            Attribute name:     extmap-allow-mixed
            Long form:          One and Two bytes mixed mode
            Type of name:       att-field
            Type of attribute:  Media or session level
            Subject to charset: No
            Purpose:            Negotiate the use of One and Two bytes
                                in the same RTP stream.
            Reference:          [RFCXXXX]
            Values:             None

11.  Changes from RFC5285

   The major motivation for updating [RFC5285] was to allow having one
   byte and two bytes RTP header extensions in the same RTP stream (but
   not in the same RTP packet).  The support for this case is negotiated
   using a new SDP attribute "extmap-allowed-mixed" specified in this
   document.

   The other major change is to update the requirement from the RTP
   specification and[RFC5285] that the header extension "is designed so
   that the header extension MAY be ignored".  This is described in
   section 4.1.

   The transmission consideration section (4.1.1) adds more text to
   clarify when and how many times to send the RTP header extension to
   provide higher probability of delivery

   >The security section was expanded

   The rest of the changes are editorial.

12.  Acknowledgments

   Both Brian Link and John Lazzaro provided helpful comments on an
   initial draft of this document.  Colin Perkins was helpful in
   reviewing and dealing with the details.  The use of URNs for IETF-
   defined extensions was suggested by Jonathan Lennox, and Pete Cordell
   was instrumental in improving the padding wording.  Dave Oran
   provided feedback and text in the review.  Mike Dolan contributed the
   two-byte header form.  Magnus Westerlund and Tom Taylor were
   instrumental in managing the registration text.

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13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2508]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
              Headers for Low-Speed Serial Links", RFC 2508, February
              1999.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, June 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,
              <http://www.rfc-editor.org/info/rfc6904>.

13.2.  Informative References

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-34 (work in progress), October 2016.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <http://www.rfc-editor.org/info/rfc3611>.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <http://www.rfc-editor.org/info/rfc4588>.

   [RFC5109]  Li, A., Ed., "RTP Payload Format for Generic Forward Error
              Correction", RFC 5109, DOI 10.17487/RFC5109, December
              2007, <http://www.rfc-editor.org/info/rfc5109>.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <http://www.rfc-editor.org/info/rfc5285>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

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   [RFC6464]  Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
              Transport Protocol (RTP) Header Extension for Client-to-
              Mixer Audio Level Indication", RFC 6464,
              DOI 10.17487/RFC6464, December 2011,
              <http://www.rfc-editor.org/info/rfc6464>.

   [RFC7201]  Westerlund, M. and C. Perkins, "Options for Securing RTP
              Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
              <http://www.rfc-editor.org/info/rfc7201>.

Authors' Addresses

   Roni Even (editor)
   Huawei Technologies
   Shabazi 12A
   Tel Aviv
   Israel

   Email: Roni.even@mail01.huawei.com

   David Singer
   Apple, Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   USA

   Phone: +1 408 996 1010
   Email: singer@apple.com
   URI:   http://www.apple.com/quicktime

   Harikishan Desineni
   10001 Pacific Heights Blvd
   San Diego, CA  92121
   USA

   Phone: +1 858 845 8996
   Email: hdesinen@quicinc.com

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