AVT D. Singer
Internet-Draft Apple Computer Inc.
Expires: June 18, 2007 H. Desineni
Qualcomm
December 15, 2006
A general mechanism for RTP Header Extensions
draft-ietf-avt-rtp-hdrext-08.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
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
unregistered. The actual extensions in use in a session are signaled
in the setup information for that session.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 6
5. SDP Signalling Design . . . . . . . . . . . . . . . . . . . . 8
6. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9.1. New space for IANA to manage . . . . . . . . . . . . . . . 15
9.2. Registration of the SDP extmap attribute . . . . . . . . . 15
10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 16
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
12. Normative References . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
In the RTP specification [RFC3550] there is provision for header
extensions in section 5.3.1.
It permits at most one extension in a given packet; the extension has
a length in 32-bit words, and there is a 16-bit identifier 'defined
by profile' to identify the extension in use.
This mechanism has two conspicuous drawbacks: only one extension is
possible, and there is no documentation of how the 16-bit identifiers
are allocated.
This header extension value applies to the RTP/AVP profile and its
extensions.
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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].
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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 is required to
extract the metadata. Also, depending on the media format, the
metadata may need to be added at the time of encoding the media so
that the bit-rate required for the metadata is taken into account.
But the metadata may not be known at that time. Inserting metadata
at a later time can require a decode and re-encode to meet bit-rate
requirements.
In some cases a more appropriate, higher level mechanism may be
available, and if so, it should 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 meta-data be tied to a specific form
of media data.
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4. Packet Design
The following design is fit into the "header extension" of the RTP
extension, as described above. The presence and format of this
header extension is negotiated or defined out-of-band, such as
through signaling (see below for SDP signaling), and therefore the
"identifier" used above is only defined here for diagnostic and
cross-check purposes (e.g. by network analyzers); it is the
negotiation/definition which is the definitive indication that this
header extension is present. For this specification the 16-bit value
required by the RTP specification for a header extension, labelled in
the RTP specification as "defined by profile", takes the fixed bit
pattern 0xBEDE (the first draft of this specification was written on
the feast day of the Venerable Bede).
The RTP specification states that the header extension "is designed
so that the header extension may be ignored". This specification
therefore inherits this requirement. To be specific, header
extensions using this specification MUST only be used for data that
can safely be ignored by the recipient without affecting
interoperability. Examples might include meta-data that is
additional to the usual RTP information.
The header extension is formed of a set of extension elements. Each
extension element has a local identifier and a length. Since it is
expected that (a) the number of extensions in any given RTP session
is small and (b) the extensions themselves are small, only 4 bits are
allocated to each of these. The local identifiers may be mapped to a
larger namespace in the negotiation (e.g. session signaling).
The form of the header extension block is as 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len | extension element bytes... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .... |
The 4-bit ID is the local identifier of this element in the range
1-14 inclusive. The values present in the stream MUST have been
negotiated or defined out-of-band. There are no static allocations
of identifiers. Each distinct extension MUST have a unique ID.
The value 0 is reserved for padding and MUST NOT be used as an
identifier. The value 15 is reserved for future extension and MUST
NOT be used as an identifier. If the value 15 is encountered,
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processing of the extension should terminate at that point, and only
the extension elements present prior to the ID=15 considered.
The 4-bit length is the length minus one of the data bytes of this
header extension element (excluding this one-byte header). Therefore
the 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.
There are as many extension elements as fit into the length as
indicated in the RTP header-extension length. Since the RTP header
extension length is signaled in full 32-bit words, padding bytes are
placed after the last extension element to pad to a 32-bit boundary.
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, as needed to pad to full 32-bit words. A padding
byte is not interpreted as the ID of an ID/length pair, and no length
byte follows it. When a padding byte is found it is ignored and the
parser moves on to interpreting the next byte.
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 may be present in some packets,
not that they are in fact present in all (or indeed, any) packets.
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5. SDP Signalling Design
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 range 1-14, and each ID in this
range MUST be used only once for each media (or only once if the
mappings are session level). Mappings which 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
will be necessary before they can be applied.
Each extension is named by a URI. That URI MUST be absolute, and
should precisely identify the format and meaning of the extension.
In general, the URI SHOULD also be de-referencable by any system that
sees or receives the SDP containing it. URIs that contain a domain
name SHOULD also contain a month-date in the form mmyyyy. That date
MUST be near the time of the definition of the extension, and it MUST
be true that the extension was defined in a way authorized by the
owner of the domain name at 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).
An extension URI 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 may, of course, be used
for several streams.)
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. These URNs are managed by IANA. 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
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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.
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 range 1-14 inclusive (0
and 15 are reserved, as noted above), and <direction> is one of
"sendonly", "recvonly", "sendrecv", "inactive" (without the quotes).
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) which is diagnostic that this style of
header extensions is used, not the magic number indicated above.
Rationale: the use of URIs provides for a large, unallocated space,
gives documentation on the extension. The URIs are not required 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.
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6. Offer/Answer
The simple signaling described above 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;
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 may be one of "sendonly",
"recvonly", "sendrecv", "inactive". A "sendonly" direction indicates
an ability to send; a "recvonly" direction indicates a desire to
receive; a "sendrecv" direction indicates both. An "inactive"
direction indicates neither, but later re-negotiation may make an
extension active.
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).
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 which 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 which has no desire to, or is unable to, send
the extension SHOULD remove it from the SDP answer.
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Identifiers in the range 1-14 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 range 1-14 MUST NOT be
altered (remapped).
Note that, under this rule, the same 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 then 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 range 1-14, for that extension to be usable.
Similarly, if more than 14 extensions are offered, 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 range 1-14.
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 range 1-14). Extensions with an identifier outside the range
1-14 cannot, of course, be used. If required, 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 (e.g. by using a full byte for an ID).
Note that 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 with an
identifier in the range 1-14, and with a declaration, can be sent.
Example (port numbers, RTP profiles, payload IDs and rtpmaps etc. all
omitted for brevity):
The offer:
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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. They are not interested in
transmission offsets on audio, and do not understand the URI-obscure
extension. They therefore move the extensions from session level to
media level, and adjust 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
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7. BNF Syntax
The syntax element 'URI-reference' is as defined in [RFC3986], except
that only absolute URIs are permitted here. The syntax element
'extmap' is an attribute as defined in [RFC4566].
Extensionattributes are not defined here, but by the specification
that defines a specific extension name; there may be several,
separated by spaces.
space = " "
extensionname = URI-reference
direction = "sendonly" | "recvonly" | "sendrecv" | "inactive"
mapentry = "extmap" ":" integer ("/" direction)
mapattrs = (space extensionattributes)
extmap = mapentry space extensionname mapattrs
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8. Security Considerations
This defines only a place to transmit information; the security
implications of the extensions must be discussed with those
extensions.
Care should be taken when defining extensions. Clearly, they should
be solely informative, but even when the information is extracted,
should not cause security concerns.
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9. IANA Considerations
9.1. New space for IANA to manage
The rtp-hdrext namespace under urn:ietf:params: needs to be created
for management, referenced to RFCxxxx.
Note: Names drawn from other spaces than the IETF are managed outside
both the IETF and IANA, and the handling of registration and
documentation is the responsibility of the owner of the internet
domain name as of the date specified in the registration; no IANA
action is required for these names.
9.2. Registration of the SDP extmap attribute
This section contains the information required by [RFC4566] for an
SDP attribute.
o contact name, email address and telephone number: D. Singer,
singer@apple.com, +1 408-974-3162
o attribute-name (as it will appear in SDP): extmap
o long-form attribute name in English: generic header extension map
definition
o type of attribute (session level, media level, or both): both
o whether the attribute value is subject to the charset attribute:
not subject to the charset attribute
o a one paragraph explanation of the purpose of the attribute: This
attribute defines the mapping from the extension numbers used in
packet headers into extension names as documented in
specifications and appropriately registered.
o a specification of appropriate attribute values for this
attribute: see RFCxxxx.
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10. RFC Editor Considerations
RFCxxxx in the IANA considerations needs to be replaced with the RFC
number (two places).
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11. Acknowledgments
Both Brian Link and John Lazzaro provided helpful comments on an
initial draft. 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.
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, STD 0064, July 2003.
[RFC3986] Berners-Lee, MT., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
January 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
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Authors' Addresses
David Singer
Apple Computer Inc.
1 Infinite Loop
Cupertino, CA 95014
US
Phone: +1 408 996 1010
Email: singer@apple.com
URI: http://www.apple.com/quicktime
Harikishan Desineni
Qualcomm
5775 Morehouse Drive
San Diego, CA 92126
USA
Phone: +1 858 845 8996
Email: hd@qualcomm.com
URI: http://www.qualcomm.com
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