Internet Engineering Task Force (IETF) M. Westerlund
Request for Comments: 6064 P. Frojdh
Category: Informational Ericsson
ISSN: 2070-1721 January 2011
SDP and RTSP Extensions Defined for 3GPP Packet-Switched Streaming
Service and Multimedia Broadcast/Multicast Service
Abstract
The Packet-switched Streaming Service (PSS) and the Multimedia
Broadcast/Multicast Service (MBMS) defined by 3GPP use the Session
Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)
with some extensions. This document provides information about these
extensions and registers the RTSP and SDP extensions with IANA.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6064.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. PSS SDP Extensions . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Video Buffering Attributes . . . . . . . . . . . . . . . . 5
4.2. Video Frame Size Attribute . . . . . . . . . . . . . . . . 6
4.3. Integrity-Protection Configuration Attributes . . . . . . 6
4.4. The Alternative Attributes . . . . . . . . . . . . . . . . 7
4.5. Adaptation Attribute . . . . . . . . . . . . . . . . . . . 7
4.6. Quality of Experience Attribute . . . . . . . . . . . . . 7
4.7. Asset Information Attribute . . . . . . . . . . . . . . . 8
5. MBMS SDP Extensions . . . . . . . . . . . . . . . . . . . . . 8
5.1. MBMS Bearer Mode Declaration Attribute . . . . . . . . . . 8
5.2. FEC Flow ID Attribute . . . . . . . . . . . . . . . . . . 8
5.3. MBMS Repair Attribute . . . . . . . . . . . . . . . . . . 8
5.4. SDP Protocol Identifiers for FEC . . . . . . . . . . . . . 8
5.4.1. RTP Protocol Identifiers . . . . . . . . . . . . . . . 9
5.4.2. FEC Repair Data Identifier . . . . . . . . . . . . . . 9
5.5. Video Buffering Attribute . . . . . . . . . . . . . . . . 9
6. SDP Offer/Answer Consideration . . . . . . . . . . . . . . . . 9
7. PSS RTSP Extensions . . . . . . . . . . . . . . . . . . . . . 10
7.1. 3GPP-Link-Char Header . . . . . . . . . . . . . . . . . . 10
7.2. 3GPP-Adaptation Header . . . . . . . . . . . . . . . . . . 10
7.3. 3GPP-QoE-Metrics Header . . . . . . . . . . . . . . . . . 10
7.4. 3GPP-QoE-Feedback Header . . . . . . . . . . . . . . . . . 11
7.5. Video Buffer Headers . . . . . . . . . . . . . . . . . . . 11
7.6. Integrity Protection . . . . . . . . . . . . . . . . . . . 11
7.7. RTSP URI Extension . . . . . . . . . . . . . . . . . . . . 11
7.8. Fast Start-Up and Content Switching . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8.1. SDP Registrations . . . . . . . . . . . . . . . . . . . . 12
8.2. RTSP Registrations . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
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1. Introduction
3GPP has specified the Packet-switched Streaming Service (PSS) that
uses both RTSP [RFC2326] and SDP [RFC4566]. The service is specified
in technical specifications TS 26.233 [PSS-233] and TS 26.234
[PSS-234] in Release 4 and subsequent releases. The basic service
defined in Release 4 is enhanced in Release 5 with capability
exchange, and in Release 6 with a number of features, such as
adaptation, digital rights management (DRM), progressive download, as
well as a streaming server file format defined in [PSS-3GP]. Fast
start-up and content switching are addressed in Release 7.
3GPP has also specified the Multimedia Broadcast/Multicast Service
(MBMS) that uses SDP. The IP-layer protocols used by this service
are specified in technical specification TS 26.346 Release 6 [MBMS].
Release 7 extends the MBMS User Service to also work with unicast
bearers for interactive and streaming traffic classes.
In the process of defining these services, there has occasionally
been a need to extend both SDP and RTSP functionalities. These
extensions have mostly been in the form of SDP attributes and RTSP
headers and option tags. 3GPP uses the name "feature tags" (like RTSP
2.0 for what RTSP 1.0 calls "option tags"); "option tag" is the name
that will be used in this document. The purpose of this
informational document is to register these SDP and RTSP extensions,
in order to avoid future conflicts, and also to raise the awareness
of their existence within IETF.
In Section 5.4, this document defines three SDP protocol identifiers
used in MBMS to enable the usage of block-based FEC. The SDP
protocol identifiers require an RFC to be defined and registered. As
this is an RFC from the IETF stream, any semantic change will require
a new IETF-approved RFC. The other SDP and RTSP extensions
registered by this document are not normatively defined in this
document. Instead, the normative definitions are referenced by the
registrations. 3GPP can update the normative definition in future
versions of their specifications. However, to ensure that such a
change is visible in the IETF, at minimum, IANA should be notified
and the reference to the 3GPP specification updated, and preferably
an updated version of this RFC published.
The document begins with two sections presenting the SDP extensions
for PSS and MBMS, respectively. They are followed by a section
noting that offer/answer considerations are not applicable here. The
subsequent section presents the extensions of RTSP for PSS. The IANA
registration of SDP attributes and protocol identifiers is given in
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Section 8.1, and the RTSP headers and option tags in Section 8.2.
For normative descriptions of all SDP and RTSP extensions, we refer
to TS 26.234 [PSS-234] and TS 26.346 [MBMS].
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Glossary
3GP: 3GPP file format, a multimedia file format based on the ISO
base media file format, existing in different profiles
intended for multimedia messages, direct playback on
clients, progressive download, usage on servers to deliver
on-demand multimedia sessions in PSS, or servers sending
MBMS sessions.
3GPP: Third Generation Partnership Project; see
http://www.3gpp.org for more information about this
organization.
FEC: Forward Error Correction
MBMS: Multimedia Broadcast/Multicast Service, a service defined by
3GPP that utilizes broadcast or multicast technology in
combination with unicast for delivery of a wide range of
content to mobile terminals.
PSS: Packet-switched Streaming Service, a unicast-based streaming
service for delivery of on-demand or live streaming
multimedia content to mobile terminals.
RTSP: Real Time Streaming Protocol; see [RFC2326].
SDP: Session Description Protocol; see [RFC4566].
SRTP: Secure Real-time Transport Protocol; see [RFC3711].
QoE: Quality of Experience, the quality level of the user
experience of a service. In PSS, this is estimated by a
combination of application-level metrics.
QoS: Quality of Service, the quality (properties) that the
network provides toward the upper-layer service.
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3. Applicability Statement
This document describes 3GPP-defined extensions to SDP [RFC4566] and
RTSP [RFC2326] and registers attributes that are normatively defined
in 3GPP technical specifications 26.234, 26.244, and 26.346, up to
the referenced versions of the respective documents.
The SDP and RTSP extensions have only been defined for usage with the
3GPP service in mind. The applicability for usage outside of these
services has not been considered nor addressed. Usage of these
attributes in other contexts may require further definitions or
clarifications. For example, all SDP attributes lack offer/answer
usage rules [RFC3264], which currently makes it impossible to use
them with offer/answer. Please note that change control of these SDP
and RTSP extensions belongs to 3GPP.
4. PSS SDP Extensions
The PSS specification [PSS-234] defines a number of different SDP
attributes for different purposes. They are listed below, grouped by
their purpose. The text is intentionally not specific enough to
allow implementation from this document. The normative definition is
in the 3GPP technical specification cited.
4.1. Video Buffering Attributes
The following attributes are used to provide parameters for the video
buffer model provided in Annex G and Section 5.3.3.2 of [PSS-234].
The attributes were defined in Release 5 as "X-" attributes and, at
the time, were not considered for registration. In hindsight,
however, they should not have been "X-" attributes, and they should
have been registered, as the registration rules of SDP [RFC4566]
point out. Changing their names today is impossible due to the
deployed base of millions of mobile handsets supporting PSS, and
therefore they are registered in their current form.
All attributes are defined at media level.
o The "a=X-predecbufsize" attribute provides the size of the pre-
decoder buffer in bytes.
o The "a=X-initpredecbufperiod" attribute provides the time during
which a receiver should initially buffer, in 90 kHz ticks, before
starting to consume the data in the buffer in order to ensure that
underflow does not occur, assuming correct data delivery.
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o The "a=X-initpostdecbufperiod" attribute provides the initial
buffering period, in 90 kHz ticks, for the post-decoder buffer
present in H.263 and MPEG-4 Visual.
o The "a=X-decbyterate" attribute indicates the maximum peak byte-
decoding rate used in the verification of the Annex G buffer model
expressed in bytes per second.
o The "a=3gpp-videopostdecbufsize" attribute is used to indicate the
value used in determining the H.264 video post-decoder buffer
size.
Note that complete descriptions of these attributes can be found in
Section 5.3.3.2 of [PSS-234].
4.2. Video Frame Size Attribute
This media-level attribute provides the receiver with the largest
picture size that a specific H.263 payload type will carry within the
session. The attribute has the following form (see Section 5.3.3.2
of [PSS-234]):
"a=framesize: -"
4.3. Integrity-Protection Configuration Attributes
These attributes are all used to configure the integrity-protection
mechanism defined in Annex K (Sections K.2.2.1, K.2.2.2, and K.2.2.3)
of [PSS-234].
o The session-level attribute "a=3GPP-Integrity-Key" carries the
integrity key used to derive SRTP master keys for integrity
protection. The key is protected in different ways depending on a
method identifier. When using Open Mobile Alliance (OMA) DRM key
management, the key is encrypted using AES [AES] before it is
base64 encoded [RFC4648].
o The media-level attribute "a=3GPP-SRTP-Config" is used to
configure SRTP for integrity protection and contains an integrity
nonce, a key salt used in deriving the SRTP master key from the
integrity key, and any SRTP configuration parameters, such as the
integrity tag length.
o The session-level attribute "a=3GPP-SDP-Auth" is used to carry an
authentication tag calculated over certain parts of the SDP to
prevent manipulation of the security attributes.
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4.4. The Alternative Attributes
Two media-level and one session-level attributes are used in a
mechanism for providing alternative SDP lines. One or more SDP lines
at media level can be replaced, if desired, by alternatives. The
mechanism is backwards compatible in the way that a receiver that
does not support the attributes will get the default configuration.
The different alternatives can be grouped using different attributes
that can be specified hierarchically with a top and a lower level.
3GPP Release 6 supports grouping based on bit-rate, according to the
SDP bandwidth modifiers AS [RFC4566] and TIAS [RFC3890], and
language.
The SDP attributes (see Sections 5.3.3.3 and 5.3.3.4 of [PSS-234])
are:
o The media-level attribute "a=alt::" carries any SDP
line and an alternative identifier.
o The media-level attribute "a=alt-default-id:" identifies the
default configuration to be used in groupings.
o The session-level attribute "a=alt-group" is used to group
different recommended media alternatives. This allows providing
aggregated properties for the whole group according to the
grouping type. Language and bit-rate are two defined grouping
types.
4.5. Adaptation Attribute
The media-level SDP attribute "a=3GPP-Adaptation-Support" (see
Section 5.3.3.5 in [PSS-234]) is defined as part of the negotiation
procedure of the PSS adaptation mechanism. The attribute carries a
single value indicating how often the RTCP "Next Application Data
Unit" (NADU) APP packet shall be included in sent RTCP compound
packets. The adaptation mechanism allows the client to provide the
server with information on the available transmission bit-rate and
receiver buffer status.
4.6. Quality of Experience Attribute
The session- and media-level attribute "a=3GPP-QoE-Metrics" (see
Section 5.3.3.6 of [PSS-234]) is used to negotiate the usage of the
QoE metrics. The included parameters indicate which metrics should
be used, over which duration there should be measurements, and how
often reports should be sent.
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4.7. Asset Information Attribute
The session- and media-level attribute "a=3GPP-Asset-Information"
(see Section 5.3.3.7 of [PSS-234]) can exist in multiple instances in
a description and describes different types of asset information.
The different asset classes defined in Release 6 are Title,
Description, Copyright, Performer, Author, Genre, Rating,
Classification, Keywords, Location, Album, and Recording Year. The
different assets are described with a base64-encoded asset box from
the 3GP file format [PSS-3GP].
5. MBMS SDP Extensions
The MBMS specification [MBMS] defines a number of different SDP
attributes for different purposes. They are informatively listed
below.
5.1. MBMS Bearer Mode Declaration Attribute
The session- and media-level attribute "a=mbms-mode" (see Section
7.3.2.7 of [MBMS]) is used to describe MBMS broadcast mode media.
The attribute may be used at the session level to set the default for
all media and at the media level to specify differences between
media. However, the attribute is never used at the session level
when the session includes MBMS multicast mode media, nor at the media
level to describe MBMS multicast mode media.
5.2. FEC Flow ID Attribute
The media-level attribute "a=mbms-flowid" (see Section 8.3.1.9 of
[MBMS]) maps one or more FEC source block flow IDs to their
corresponding destination IP addresses and UDP port numbers. It is
present in each SDP media block for repair packet streams.
5.3. MBMS Repair Attribute
The session- and media-level attribute "a=mbms-repair" (see Section
8.3.1.8 of [MBMS]) is used to provide FEC repair packets with non-FEC
specific parameters. For Release 6, one such parameter is defined to
specify the required minimum receiver buffer time.
5.4. SDP Protocol Identifiers for FEC
MBMS defines a mechanism to provide block-based FEC for UDP-based
traffic. This solution uses the SDP protocol "proto" identifier to
identify the media streams that use the FEC shim layer. The media
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streams may be either source streams or repair streams. As required
by SDP [RFC4566], these protocol identifiers are normatively defined
in this document in accordance with their usage specified by 3GPP.
5.4.1. RTP Protocol Identifiers
For FEC-protected RTP streams, the following two "proto" identifiers
are defined:
o UDP/MBMS-FEC/RTP/AVP
o UDP/MBMS-FEC/RTP/SAVP
They indicate the usage of UDP [RFC0768] with MBMS FEC source packet
formats, as defined in Section 8.2.2.4 of [MBMS], that transport RTP
packets in accordance with the AVP [RFC3551] or SAVP (Secure RTP)
[RFC3711] profiles, respectively. These protocol identifiers SHALL
use the media formats ("fmt") namespace rules that are used for RTP/
AVP and RTP/SAVP, respectively.
5.4.2. FEC Repair Data Identifier
A media stream carrying MBMS FEC repair information over UDP requires
its own "proto" identifier. Protocol identifier "UDP/MBMS-REPAIR"
identifies the FEC repair packet containing the protocol combination
of UDP [RFC0768], FEC repair payload ID, and repair symbols as
specified in Section 8.2.2.5 of [MBMS]. The "fmt" namespace is not
used and SHALL be set to "*".
5.5. Video Buffering Attribute
The PSS media-level buffer attribute "a=X-initpredecbufperiod" (see
Section 4.1) that specifies an initial buffering time is also used
for MBMS in Release 7. It is mainly intended for video streams, but
may be used for other media types as well (see Section 8.3.1.1 of
[MBMS]).
6. SDP Offer/Answer Consideration
The usage of the SDP attributes in an offer/answer [RFC3264] context
is not defined. These SDP attributes are defined for use in a
declarative context, and for PSS specifically in the RTSP [RFC2326]
context.
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7. PSS RTSP Extensions
The RTSP extensions for PSS consist of a number of new RTSP headers
and option tags and a narrowing of URI usage in regards to 3GP files.
The headers and option tags are informatively described here; see
[PSS-234] for the normative declaration.
7.1. 3GPP-Link-Char Header
The "3GPP-Link-Char" header (see Section 5.3.2.1 of [PSS-234]) is
used by clients to provide the server with QoS information about the
wireless link it is currently using. The header can be used to
provide the server with three different QoS parameters:
o Guaranteed Bandwidth
o Maximum Bandwidth
o Maximum Transfer Delay
The header may be included in RTSP requests using either of the
methods SETUP, PLAY, OPTIONS, or SET_PARAMETER.
7.2. 3GPP-Adaptation Header
The "3GPP-Adaptation" header (see Section 5.3.2.2 of [PSS-234]) is
used by the client to provide the server with adaptation-related
parameters and to indicate support of the adaptation function. The
header carries the resource identification as a URI, the client's
buffer size, and the desired target time.
The header may be included in requests using the methods SETUP, PLAY,
OPTIONS, and SET_PARAMETER. The response to a request using this
method shall include this header.
7.3. 3GPP-QoE-Metrics Header
The "3GPP-QoE-Metrics" header (see Section 5.3.2.3.1 of [PSS-234]) is
used to negotiate the usage of the QoE metrics (see Section 11 of
[PSS-234]).
The header may be included in requests and responses using the SETUP,
SET_PARAMETER, OPTIONS, or PLAY method.
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7.4. 3GPP-QoE-Feedback Header
The "3GPP-QoE-Feedback" header (see Section 5.3.2.3.2 of [PSS-234])
is used to carry QoE metrics from the client to the server when it
reports, which happens either during or at the end of the media
delivery.
The header may be included in requests using the SET_PARAMETER,
PAUSE, or TEARDOWN method.
7.5. Video Buffer Headers
PSS uses several headers to provide the client with the different
buffer parameters. They provide the buffer status at the point of a
stream from which a PLAY request plays. These headers may only be
used in PLAY responses. See Section 5.3.2.4 and Annex G of [PSS-234]
for normative definitions.
The three "x-" headers were defined in 3GPP Release 5. When it was
realized that they should not have been given "x-" names, it was too
late to rename them due to deployment.
The RTSP headers are:
o x-predecbufsize
o x-initpredecbufperiod
o x-initpostdecbufperiod
o 3gpp-videopostdecbufsize
7.6. Integrity Protection
The integrity-protection mechanism defined in PSS Annex K uses the
"3GPP-Freshness-Token" RTSP header (see Section K.2.2.4 of [PSS-234])
to carry a freshness token in DESCRIBE requests.
7.7. RTSP URI Extension
The PSS specification also defines syntax for referencing tracks
within the 3GP file format [PSS-3GP]. The 3GP format is based on the
ISO base media file format and is defined in several different
profiles, including a streaming-server profile, in Release 6.
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This syntax is fully contained within the generic URI syntax defined
for RTSP URIs. It is only a syntax restriction that server
manufacturers follow to allow clients or proxies to understand what
encodes the track number in the URI. This is provided for
information only.
To identify a track within a 3GP file, the last URI segment has to
contain a structure that is =
RFC, FYI, BCP