AVT D. Singer
Internet-Draft Apple Computer Inc.
Expires: December 17, 2006 June 15, 2006
A general mechanism for RTP Header Extensions
draft-ietf-avt-rtp-hdrext-03.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 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 . . . . . . . . . . . . . . . . . . . . 5
3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 7
5. SDP Signalling Design . . . . . . . . . . . . . . . . . . . . 8
6. Transmission offset . . . . . . . . . . . . . . . . . . . . . 9
7. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9.1. New spaces for IANA to manage . . . . . . . . . . . . . . 12
9.2. Registration of the transmit-time offset . . . . . . . . . 12
9.3. Registration of the SDP extmap attribute . . . . . . . . . 12
10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
In the RTP specification [RFC3550] there is provision for header
extensions. The following text is drawn from that specification for
the convenience of readers.
* * * * * * * * * * * * * *
5.3.1 RTP Header Extension
An extension mechanism is provided to allow individual
implementations to experiment with new payload-format-independent
functions that require additional information to be carried in the
RTP data packet header. This mechanism is designed so that the
header extension may be ignored by other interoperating
implementations that have not been extended.
Note that this header extension is intended only for limited use.
Most potential uses of this mechanism would be better done another
way, using the methods described in the previous section. For
example, a profile-specific extension to the fixed header is less
expensive to process because it is not conditional nor in a variable
location. Additional information required for a particular payload
format SHOULD NOT use this header extension, but SHOULD be carried in
the payload section of the packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| defined by profile | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extension |
| .... |
If the X bit in the RTP header is one, a variable-length header
extension MUST be appended to the RTP header, following the CSRC list
if present. The header extension contains a 16-bit length field that
counts the number of 32-bit words in the extension, excluding the
four-octet extension header (therefore zero is a valid length). Only
a single extension can be appended to the RTP data header. To allow
multiple interoperating implementations to each experiment
independently with different header extensions, or to allow a
particular implementation to experiment with more than one type of
header extension, the first 16 bits of the header extension are left
open for distinguishing identifiers or parameters. The format of
these 16 bits is to be defined by the profile specification under
which the implementations are operating. This RTP specification does
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not define any header extensions itself.
* * * * * * * * * * * * * *
This mechanism has two conspicuous drawbacks: only one extension is
possible, and there is no documentation of how the 16-bit identifiers
are allocated.
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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 shown above. The presence and format of this header
extension is negotiated 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 which is the definitive
indication that this header extension is present. The 16-bit
identifier takes the fixed bit pattern 0xBEDE (the first draft of
this specification was written on the feast day of the Venerable
Bede).
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 negotation (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; the values 0
and 15 are reserved and MUST NOT be used. The remaining values
present in the stream MUST have been negotiated out-of-band. There
are no static allocations of identifiers.
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 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. These
padding bytes MUST have the value zero (0).
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5. SDP Signalling Design
The map of the local identifiers used in the packets to a larger un-
allocated namespace may be performed in the session signaling. This
section defines such signaling in SDP.
The universe of possible names is identified by reversed domain-names
(as used in Java, for example), combined with 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 at that date. (This
avoids problems when domain names change ownership).
An example name (this is only an example) might be
"org.example.avt.rtp.transmittime/082005".
The mapping may be provided per media-stream (in the media-specific
section of SDP, i.e. after an "m=" line) or globally for all streams
(i.e. before the first "m=" line). The definitions MUST be either
all global or all local; it is not permitted to mix the two styles.
Each local identifier potentially used in the stream is mapped to a
string using an attribute of the form:
a=extmap:<value> <name>/<month>
where <name> is a reversed domain name, <value> is an integer in the
range 1-14 inclusive (0 and 15 are reserved, as noted above), and
<month> takes the form of a 6-digit number mmyyyy.
Example:
a=extmap:1 org.example.avt.rtp.toffset/082005
a=extmap:2 org.ietf.avt.smpte12M/022006
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 reversed domain names provides for a large,
unallocated space, while still giving some clue as to what
organization or body is responsible for the definition of the
extension.
Extension names starting with "org.ietf" and "arpa." are reserved to
the IETF and MUST be documented in RFCs, and registered with IANA.
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6. Transmission offset
This specification defines only one extension element. The name is
"org.ietf.avt.rtp.toffset/082005". The value is a fixed 24-bit
signed integer. In the case when the sender knows that it is not
transmitting the RTP packets strictly according to their relative RTP
timestamps, this extension element may be used to indicate the
"effective" RTP transmission time of the packet, on the RTP
timescale. The value is added to the RTP timestamp of the packet to
give the RTP time of transmission.
There are no extension attributes defined for this name.
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7. BNF Syntax
The syntax element 'hostname' is as defined in [RFC2396], except that
(a) numeric hostnames are not permitted and (b) the name is in
reverse order (e.g. "com.example.multimedia.tag"). The syntax
element 'extmap' is an attribute as defined in [RFC2327].
Extensionattributes are defined by the specification that defines a
specific extension name; there may be several, separated by spaces.
digit = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
month = "01"|"02"|"03"|"04"|"05|"06"|"07"|"08"|"09"|"10|"11"|"12"
year = digit digit digit digit
date = month year
integer = digit *(digit)
space = " "
extensionname = hostname "/" date
mapentry = "extmap" ":" integer
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.
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9. IANA Considerations
9.1. New spaces for IANA to manage
IANA needs to manage the extension names that start with "org.ietf"
or "arpa". Any use of a name with these prefixes MUST be documented
in a standards-track RFC which defines both the name and the format
of the data. IANA manages the registration by documenting the
association of name to RFC number.
Names drawn from other spaces 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.
9.2. Registration of the transmit-time offset
The first registration within the IETF/IANA space is as documented
above. The registered name is "org.ietf.avt.rtp.toffset/082005",
associated with this specification.
9.3. Registration of the SDP extmap attribute
This section contains the information required by [RFC2327] (which is
identical to that required by draft-ietf-mmusic-sdp-new-26.txt) for
an SDP attribute.
o contact name, email address and telephone number: are as indicated
for this document
o attribute-name (as it will appear in SDP): is 'extmap' (without
the quotation marks)
o long-form attribute name in English: is "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:
no, it is not
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: is as defined in this document
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10. RFC Editor Considerations
None.
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Appendix A. 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.
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Appendix B. Change History
o August 2005: 00 Draft-avt-rtp-hdrext made from
draft-singer-rtp-hdrext; added requirement for dates in DNS names
o January 2006: 01 Updated to XML2RFC, inserted BNF, other cleanup;
IANA considerations cleaned
o June 2006: 02 Made the signaling optional, and indicate only that
the presence of the extensions must be negotiated somehow; more
rationale
o June 2006: 03 Fixed small error in the BNF
11. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC2396] Berners-Lee, MT., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, STD 0064, July 2003.
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Author's Address
David Singer
Apple Computer Inc.
1 Infinite Loop
Cupertino, CA 95014
US
Phone: +1 408 996 1010
Email: singer@apple.com
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