wdiff rfc8285.original rfc8285.txt

AVTCore
Internet Engineering Task Force (IETF) D. Singer
Internet-Draft
Request for Comments: 8285 Apple, Inc.
Obsoletes: 5285 (if approved) H. Desineni
Intended status:
Category: Standards Track Qualcomm
Expires: February 3, 2018
ISSN: 2070-1721 R. Even, Ed.
Huawei Technologies
August 2,
October 2017

A General Mechanism for RTP Header Extensions
draft-ietf-avtcore-rfc5285-bis-14.txt

Abstract

This document provides a general mechanism to use the header
extension feature of RTP (the Real-Time 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. decentralized. The actual extensions in use in a
session are signaled in the setup information for that session. This
document obsoletes RFC5285. RFC 5285.

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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 February 3, 2018.
https://www.rfc-editor.org/info/rfc8285.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 ....................................................3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3 ...........................................3
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 4 ...................................................4
4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 4 ....................................................4
4.1.1. Transmission Considerations . . . . . . . . . . . . . 5 .........................5
4.1.2. Header Extension Type Considerations . . . . . . . . 6 ................6
4.2. One-Byte Header . . . . . . . . . . . . . . . . . . . . . 7 ............................................8
4.3. Two-Byte Header . . . . . . . . . . . . . . . . . . . . . 9 ............................................9
5. SDP Signaling Design . . . . . . . . . . . . . . . . . . . . 10 ...........................................10
6. SDP Signaling for support Support of mixed one byte Mixed One-Byte and two bytes
header extensions. . . . . . . . . . . . . . . . . . . . . . 12 Two-Byte
Header Extensions ..........................................12
7. SDP Offer/Answer . . . . . . . . . . . . . . . . . . . . . . 13 ...............................................13
8. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 16 .....................................................17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17 ........................................17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 ...........................................18
10.1. Identifier Space for IANA to Manage . . . . . . . . . . 17 ......................18
10.2. Registration of the SDP extmap "extmap" Attribute . . . . . . . . 19 ...............20
10.3. Registration of the SDP extmap-allow-mixed "extmap-allow-mixed" Attribute . . 19 ...20
11. Changes from RFC5285 . . . . . . . . . . . . . . . . . . . . 20 RFC 5285 .........................................21
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. ....................................................21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. .....................................21
12.2. Informative References . . . . . . . . . . . . . . . . . 22 ...................................23
Acknowledgments ...................................................24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 ................................................25

1. Introduction

The RTP specification [RFC3550] provides a capability to extend the
RTP header. It Section 5.3.1 of [RFC3550] defines the header extension
format and rules for its
use in Section 5.3.1. use. 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.

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 providing 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
RFC 5285 that did not allow sending both one-byte and two-byte header
extensions in the same RTP stream.

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in [RFC2119].
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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. collisions.

This mechanism provides an alternative to the practice of burying
associated metadata into the media format bit stream. bitstream. 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, appropriate and higher-level mechanism may 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 to have the metadata be tied to a specific form of
media data.

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, out of band, such as through signaling (see
below for SDP signaling). The 16-bit identifier for the two forms of
the RTP extension defined here is only an architectural constant
(e.g., for use by network analyzers); it is the negotiation/definition negotiation/
definition (e.g., in SDP) that is the definitive indication that this
header extension is present.

The RTP specification [RFC3550] states that RTP "is designed so that
the header extension may be ignored by other interoperating
implementations that have not been extended". extended." The intent of this
restriction is that RTP header extensions MUST NOT be used to extend
RTP itself in a manner that is backwards backward incompatible with non-
extended
non-extended implementations. For example, a header extension is not
allowed to change the meaning or interpretation of the standard RTP
header fields, fields or of the RTCP RTP Control Protocol (RTCP). Header
extensions MAY carry metadata in addition to the usual RTP header
information, provided the RTP layer can function if that metadata is
missing. For example, RTP header extensions can be used to carry
data that's also sent in RTCP, as an optimisation optimization to lower latency,
since they'll fall back to the original, non-optimised, behaviour original non-optimized behavior if the
header extension is not present. The use of header extensions to
convey information that will, if missing, disrupt the behaviour behavior of a
higher layer
higher-layer application that builds on top of RTP is only acceptable
if this doesn't affect interoperability at the RTP layer. For
example, applications that use the SDP BUNDLE extension with the MID
Media Identification (MID) RTP header extension [I-D.ietf-mmusic-sdp-bundle-negotiation] [SDP-BUNDLE] to
correlate RTP streams with SDP m= "m=" lines likely won't work with full
functionality if the MID is missing, but the operation of the RTP
layer of those applications will be unaffected. Support for RTP
header extensions based on this memo is negotiated using, for
example, SDP Offer/Answer [RFC3264]; intermediaries aware of the RTP
header extensions are advised to be cautious when removing or
generating RTP header extensions see section extensions. See Section 4.7 of [RFC7667].

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 Considerations

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.

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

1. The probability for packet loss and burst loss determine determines 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 losing 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 header extension item information is needed, from
the first received RTP data or only after some set of packets are
received, can guide if whether the header extension(s) should be
(1) in all of the first N packets or be (2) included only once per
set of
packets, packets -- for example example, once per video frame.

4. The use of RTP level RTP-level robustness mechanisms, such as RTP
retransmission [RFC4588], [RFC4588] or Forward Error Correction, e.g.,
[RFC5109] 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 delivery, taking the receiver
population size into account. For the very basic case, N repetitions
of the header extensions should be sufficient, 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 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 Extended Report (XR) Loss RLE report block Report Block [RFC3611], which will
indicate successful delivery of particular packets. If the RTP/AVPF Transport
Layer Feedback Messages
transport-layer feedback messages for generic NACK [RFC4585] is are
used, it they 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 packets were sent, sent does not necessarily mean that the RTP
header extension packets themselves were lost.

4.1.2. Header Extension Type Considerations

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. out of band. There are no static allocations
of local identifiers. Each distinct extension MUST have a unique ID.
The ID value 0 is reserved for padding and MUST NOT be used as a
local identifier.

An extension element with an ID value equal to 0 MUST NOT have len an
associated length field greater than 0. If such an extension element
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 0 and len a length field
greater than 0 SHOULD be considered.

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 headers or only
two-byte headers unless it is known that all recipients support
mixing, either by either SDP Offer/Answer [RFC3264] negotiation (see section
Section 6) or by out-
of-band out-of-band knowledge. Each RTP packet with an RTP
header extension following this specification will indicate if whether
it contains one one-byte or two
byte two-byte header extensions through the use of
the "defined by profile" field. Extension element types that do not
match the header extension format, i.e. one- i.e., one-byte or two-byte,
MUST NOT be used in that RTP packet. Transmitters SHOULD NOT use the
two-byte header form when all extensions are small enough for the
one-byte header form. Transmitters that intend to send the two-byte
form SHOULD negotiate the use of IDs above 14 if they want to let the Receivers
receivers know that they intend to use the two-byte form, for example 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 case, the transmitter SHOULD use the 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 will fit into in the length RTP header extension, as
indicated in by 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 byte by byte to find
each extension element (no alignment is needed), and parsing stops
(1) at the earlier of
the end of the entire header extension, extension or (2) in the "one-byte
headers only" case, on encountering an identifier with the reserved
value of 15. 15 -- whichever happens earlier.

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 does it supply the length
field. When a padding byte is found, it is ignored ignored, and the parser
moves on to interpreting the next byte.

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 that one 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, 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 are 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 pattern was picked for the trivial reason that
the first version of this specification was written on May 25th --
the feast day of the Venerable Bede).

Each extension element MUST start 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 a 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 number, minus one one, of data bytes of this
header extension element following the one-byte header. Therefore,
the value zero (0) 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 -- values, which would often be padded anyway.)
An example header extension, with three extension elements, elements and some
padding
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 application dependent and MAY be
defined to be any value or meaning, and are meaning; this topic is 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, (256), the appbits field
SHOULD be set to all 0s (zeros) 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:

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, elements and the value 256 referring to the 4-bit appbits
field
'appbits' (above). Note that there is one ID space for both the one-byte
form and the 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 with
the same meanings.

The 8-bit length field is the length of extension data in bytes bytes, not
including the ID and length fields. The value zero (0) indicates
that there is no data following. subsequent data.

An example header extension, with three extension elements, elements and some
padding
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, out of band -- for example, as part of an SDP Offer/
Answer
Offer/Answer [RFC3264]. 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 section (or only once if
the mappings are session level). Mappings that do not conform to
these rules MAY be presented, for instance, during SDP Offer/Answer
[RFC3264] negotiation as described in the next section, but remapping
to conformant values is necessary before they can be applied.

Each extension is named by a URI. That URI MUST be absolute, and absolute; it
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 The use of URIs provides for a large, unallocated space, space
and gives documentation on the extension. The URIs do not have to be
de-referencable,
dereferencable, in order to permit confidential or experimental use,
and
or 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, streams and can appear with
different extensionattributes <extensionattributes> for the same stream.)

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

The registration requirements are detailed in the IANA Considerations
section, below. Section 10 ("IANA
Considerations").

An example (this is only an example), where 'avt-example-metadata' "avt-example-metadata" is the hypothetical name of a
header extension, 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 in each media section of the SDP, SDP or
unique in the session for session-level SDP declarations.

Each local identifier potentially used in the stream is mapped to an
extension identified by a URI using an attribute of the form:

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

where <URI> is a URI, as above,

o <value> is the local identifier (ID) of this extension and is an
integer in the valid range (0 is reserved for padding in both
forms, and 15 is reserved in the one-byte header form, as noted above), and
above).

o <direction> is one of "sendonly", "recvonly", "sendrecv", or
"inactive" (without the quotes) with relation to the device being
configured.

o <URI> is a URI, as above.

The formal BNF syntax is presented in a later section Section 8 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' "extmap" attribute(s) that is diagnostic that this style of
header extensions is used, not the magic number ("BEDE" or "100")
indicated above.

6. SDP Signaling for support Support of mixed one byte Mixed One-Byte and two bytes header
extensions. Two-Byte Header
Extensions

In order to allow for backward interoperability with systems that
do not support the mixing of one byte one-byte and two bytes two-byte header extensions 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 level or the media levels. level.
A participant that proposes the use of this mode SHALL itself support
the reception of mixed one byte one-byte and two bytes two-byte header extensions.

If SDP Offer/Answer [RFC3264] is supported and used,the used, the negotiation
for mixed one byte one-byte and two bytes extension two-byte extensions MUST be negotiated using
SDP Offer/Answer per [RFC3264]. In the absence of negotiations using
SDP
Offer/Answer, Offer/Answer -- for example example, when declarative SDP is used, used --
mixed headers MUST NOT occur unless the transmitter has some (out of band)
(out-of-band) knowledge that all potential recipients support
this mode.

The formal definition of this attribute is:

Name: extmap-allow-mixed

Value: none None

Usage Level: session, media

Charset Dependent: no No

Example:

a=extmap-allow-mixed

When doing SDP Offer/Answer [RFC3264] [RFC3264], an offering client that wishes
to use both one one-byte and two bytes two-byte extensions MUST include the
attribute
"a= extmap-allow-mixed " "a=extmap-allow-mixed" in the SDP offer. If "a= extmap-allow-mixed
"
"a=extmap-allow-mixed" is present in the offer SDP, SDP offer, the answerer that
supports this mode and wishes to use it SHALL include the "a=extmap-allow-mixed "
"a=extmap-allow-mixed" attribute in the answer. In the cases where
the attribute has been excluded, both clients SHALL NOT use mixed one bytes
one-byte and two bytes two-byte extensions in the same RTP stream but MAY use
the one-byte or two-bytes two-byte form exclusively (see section Section 4.1.2).

When used in [I-D.ietf-mmusic-sdp-bundle-negotiation] per [SDP-BUNDLE], this attribute is specified as identical category for the
[I-D.ietf-mmusic-sdp-mux-attributes]. This allows
IDENTICAL category [SDP-MUX].

7. SDP Offer/Answer

The simple signaling described above for only a subset
of the m-lines "extmap" attribute MAY
be enhanced in the bundle group to offer extmap-allow-mixed. When an answerer supporting the extmap-allow-mix attribute receives an
offer where only some of the m-lines in the bundle group include the
extmap-allow-mixed attribute, the answerer MUST receive this offer
and support mixed one-byte and two-bytes only for those m-lines.
Transmitters MUST only send RTP header extensions using mixed on
those RTP streams originating from those media sources (m=) blocks
that includes extmap-allow-mixed, and are RECOMMENDED to support
receiving mixed on all RTP streams being received in an RTP session
where at least one bundled m= block is indicating extmap-allow-mixed.

7. SDP Offer/Answer

The simple signaling described above for the extmap attribute MAY be
enhanced in an SDP Offer/Answer [RFC3264] context, to permit:

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

o SDP Offer/Answer [RFC3264] 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; "extmap"; without it, the
direction implicitly inherits, of course, from the stream direction, 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] [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), 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.)

If an extension map is offered as "sendrecv", explicitly or
implicitly, and asymmetric behavior is desired, the SDP answer MAY be
changed 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. An answerer MAY want to respond that he supports the
extension and does not want to receive it at the moment moment, but he may offer
indicate a desire to receive it in a future offer, offer and will mark the
extension as
"inactive" "inactive".

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. An answerer MAY want
to respond that he support supports this extension yet but has no intention of
sending it now but now; he may offer indicate a desire to send it in a future offer
by marking the extension as "inactive" "inactive".

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). The local identifiers MUST be unique in an
RTP session session, and the same identifier MUST be used for the same
offered extension in the answer. A session update MAY change the
direction qualifiers of extensions under use. being used. A session update MAY
add or remove extension(s). Identifiers Identifier 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 "sendrecv" (since re-numbering renumbering is not permitted
either).

If a party wishes to offer mutually exclusive alternatives, then
multiple extensions with the same identifier in the extended range
4096-4351 MAY be offered; the offered. The answerer SHOULD select select, at most most, one
of the offered extensions with the same identifier, identifier and remap it to a
free identifier in the valid range, 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, desired and remap them to a
free identifier in the valid range.

An answerer may copy an extmap "extmap" for an identifier in the extended
range into the answer to indicate to the offerer that it supports
that extension. Of course, such an extension cannot be used, since
there is no way to specify them it in an extension header. If needed, the
offerer or answerer can update the session to assign a valid
identifier to that extension URI.

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

Either party MAY include extensions in the stream other than those
negotiated, or those negotiated as "inactive", for "inactive" (for example, for the
benefit of intermediate nodes. 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 IDs, 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,
video but cannot send GPS at all. It is not interested in
transmission offsets on audio, audio and does not understand the URI-
obscure URI-obscure
extension. It therefore moves the extensions from session level to
media level, 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

When using [I-D.ietf-mmusic-sdp-bundle-negotiation] [SDP-BUNDLE] to bundle multiple m-lines "m=" lines, the extmap "extmap"
attribute falls under the special SPECIAL category of [I-D.ietf-mmusic-sdp-mux-attributes]. [SDP-MUX]. All the m-lines
"m=" lines in a bundle BUNDLE group are considered to be part of the same
local identifier (ID) space. If an RTP header extension, i.e. i.e., a
particular extension URI and configuration using
<extensionattributes>, is offered in multiple m-lines "m=" lines that are
part of the same bundle group BUNDLE group, it MUST use the same ID in all of
these m-lines. "m=" lines. Each m-line "m=" line in a bundle BUNDLE group can include
different RTP header extensions allowing allowing, for example example, audio and
video sources to use different sets of RTP header extensions. It SHALL be assumed that for any RTP header extension, A
difference in configuration using any of the <extensionattributes> is
important and need to
important. Unless an RTP header extension explicitly states
otherwise, any such difference SHALL be preserved communicated to any receiver, thus requiring all receivers
and SHALL cause assignment of different IDs. Any An RTP header extension
that does not
match follow this assumption rule MUST explicitly provide rules for define what are would
constitute compatible configurations that can be sent with the
same ID. The directionality of the RTP header extensions in each m-line
"m=" line of the
bundle BUNDLE group are is handled as in the same way as handling
for non-bundled case. "m=" lines. This allows for specifying different
directionality for each of the repeated extension URI URIs in bundled a BUNDLE
group.

8. BNF Syntax

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

Name: extmap

Value: extmap-value

Syntax:

extmap-value = mapentry SP extensionname
[SP extensionattributes]

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

extensionname = URI

extensionattributes = byte-string

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

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

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

Header extensions have the same security coverage as the RTP header
itself. When the 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 DoS
attacks (for example, by changing the header extension, extension) integrity
protection SHOULD be used. Lower layer Lower-layer security protection like DTLS[RFC6347] such as
Datagram Transport Layer Security (DTLS) [RFC6347] MAY be used. RTP
header extensions can carry sensitive information for which
participants in multimedia sessions want confidentiality.
RFC6904 RFC 6904
[RFC6904] provides a mechanism, extending mechanism that extends the mechanisms of
SRTP, SRTP to
selectively encrypt RTP header extensions in SRTP.

The RTP application designer needs to consider their security needs,
that includes cipher strength for SRTP packets in general and what
that means for the integrity and confidentiality of the RTP header
extensions. As defined by RFC6904 [RFC6904] RFC 6904 [RFC6904], the encryption stream
cipher for the header extension is dependent on the chosen SRTP
cipher.

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

10. IANA Considerations

This document updates the references in three IANA consideration registries to
point to reference this document and instead of RFC 5285, and updates and adds a new
SDP attribute attributes in section 10.3

Note to IANA : change RFCxxxx to this RFC number Sections 10.2 and remove the note. 10.3, respectively.

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 [RFC8126].

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
sub-namespace specification [RFC3553].

o Registry name: RTP Compact Header Extensions

o Specification: RFC 5285 and RFCs updating RFC 5285. 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 Review is REQUIRED. The expert review reviewer
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;

4. that the extension does not duplicate functionality in
existing IETF specifications (including RTP itself), 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, applicable -- for example point-
to-multipoint safe, 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.
unique; and

8. That that for [I-D.ietf-mmusic-sdp-bundle-negotiation] multiplexed
m-lines, "m=" lines [SDP-BUNDLE], any RTP header
extension with difference differences in configurations of
<extensionattributes> that do not require assignment of
different IDs, IDs MUST explicitly indicate this and provide rules
for what are would constitute compatible configurations that can
be sent with the same ID.

o Size and format of entries: a 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. None

10.2. Registration of the SDP extmap "extmap" Attribute

IANA is requested to update has updated the registration of the extmap "extmap" SDP attribute
[RFC4566] attribute. in the "att-field (both session and media level)"
subregistry of the "Session Description Protocol (SDP) Parameters"
registry.

o Contact Name and email address: IETF, contacted via
mmusic@ietf.org, or
<mmusic@ietf.org> (or a successor address designated by IESG the IESG)

o Attribute Name: extmap

o Attribute Syntax: See section Section 8 of [RFCXXXX]. RFC 8285.

o Attribute Semantics: The details of appropriate values are given
in [RFC XXXX]. RFC 8285.

o Usage Level: Media or session level. level

o Charset Dependent: No. No

o Purpose: defines Defines the mapping from the extension numbers used in
packet headers into extension names.

o O/A Offer/Answer (O/A) Procedures: See section Section 7 of [RFCXXXX]. RFC 8285.

o Mux MUX Category: Special. SPECIAL

o Reference: [RFCXXXX] RFC 8285

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

The

IANA is requested to register has registered one new SDP attribute: attribute in the "att-field (both
session and media level)" subregistry of the "Session Description
Protocol (SDP) Parameters" registry:

o Contact Name and email address: IETF, contacted via
mmusic@ietf.org, or
<mmusic@ietf.org> (or a successor address designated by IESG. the IESG)

o Attribute Name: extmap-allow-mixed. extmap-allow-mixed

o Attribute Syntax: See section Section 6 of [RFCXXXX]. RFC 8285.

o Attribute Semantics: See section Section 6 of [RFCXXXX]. RFC 8285.

o Attribute Value: None. None

o Usage Level: Media or session level. level
o Charset Dependent: no. No

o Purpose: Negotiate the use of One one byte and Two two bytes in the same
RTP stream.

o O/A Procedures: See section Section 6 of [RFCXXXX]. RFC 8285.

o Mux MUX Category: Identical IDENTICAL

o Reference: [RFCXXXX] RFC 8285

11. Changes from RFC5285 RFC 5285

The major motivation for updating [RFC5285] was to allow having one
byte
one-byte and two bytes two-byte 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-allow-mixed" attribute, "extmap-allow-mixed", specified
in this document.

The other major change is to update the requirement from the RTP
specification
specifications [RFC3550] and [RFC5285] that the header extension "is
designed so that the header extension MAY may be ignored". ignored." This is
described in section Section 4.1.

The transmission consideration section (4.1.1) adds more

More text was added to Section 4.1.1 ("Transmission Considerations")
to clarify when and how many times to send the RTP header extension
to provide a higher probability of delivery

>The security delivery.

The Security Considerations section was expanded 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.

13. References

13.1.

12.1. 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>.
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508,
DOI 10.17487/RFC2508, February 1999,
<http://www.rfc-editor.org/info/rfc2508>.
<https://www.rfc-editor.org/info/rfc2508>.

[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, DOI 10.17487/RFC3095,
July 2001, <http://www.rfc-editor.org/info/rfc3095>. <https://www.rfc-editor.org/info/rfc3095>.

[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>.
<https://www.rfc-editor.org/info/rfc3264>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
<https://www.rfc-editor.org/info/rfc3711>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
<https://www.rfc-editor.org/info/rfc3986>.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>. <https://www.rfc-editor.org/info/rfc4566>.

[RFC5234] Crocker, D., Ed. Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
<https://www.rfc-editor.org/info/rfc5234>.

[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.
<https://www.rfc-editor.org/info/rfc6904>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174,
DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

12.2. 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-38 (work in progress), April 2017.

[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. <https://www.rfc-editor.org/info/rfc3550>.

[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
IETF URN Sub-namespace for Registered Protocol
Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553,
June 2003, <http://www.rfc-editor.org/info/rfc3553>. <https://www.rfc-editor.org/info/rfc3553>.

[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>.
<https://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>.
<https://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>.
<https://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>. <https://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>. <https://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>. <https://www.rfc-editor.org/info/rfc6347>.

[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>.
<https://www.rfc-editor.org/info/rfc7201>.

[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<http://www.rfc-editor.org/info/rfc7667>.
<https://www.rfc-editor.org/info/rfc7667>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<http://www.rfc-editor.org/info/rfc8126>.
<https://www.rfc-editor.org/info/rfc8126>.

[SDP-BUNDLE]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", Work in Progress,
draft-ietf-mmusic-sdp-bundle-negotiation-39, August 2017.

[SDP-MUX] Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", Work in Progress, draft-ietf-mmusic-sdp-
mux-attributes-16, December 2016.

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.

Authors' Addresses

David Singer
Apple, Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
United States of America

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

Harikishan Desineni
Qualcomm
10001 Pacific Heights Blvd Blvd.
San Diego, CA 92121
USA
United States of America

Phone: +1 858 845 8996
Email: hdesinen@quicinc.com h3dnvb@gmail.com

Roni Even (editor)
Huawei Technologies
Tel Aviv
Israel

Email: Roni.even@huawei.com