INTERNET-DRAFT J. Ott/C. Perkins
Expires: November 2003 TZI/ISI
15 May 2003
SDPng Transition
draft-ietf-mmusic-sdpng-trans-04.txt
Status of this memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
Distribution of this document is unlimited.
This document is a product of the Multiparty Multimedia Session
Control (MMUSIC) working group of the Internet Engineering Task
Force. Comments are solicited and should be addressed to the working
group's mailing list at mmusic@ietf.org and/or the authors.
Abstract
The Session Description Protocol (SDP) is today widely used in the
Internet to announce as well as negotiate multimedia sessions and
exchange capabilities. Having originally been designed for session
announcements only, as opposed to announcements and capabilities
negotiation announcements, native SDP lacks numerous features to be
applicable in many session scenarios. Numerous extensions have been
developed to circumvent SDP's shortcomings -- but they have also
repeatedly shown its inherent limitations. A successor protocol --
termed "SDPng" for the time being -- is developed to address the
aforementioned needs of Internet applications in a more structured
manner. With the huge installed base of SDP-based applications, a
migration path needs to be developed to move from SDP to SDPng over
time. This document outlines how this migration can be achieved: in
general as well as for the various IETF control protocols that
Ott/Perkins [Page 1]
INTERNET-DRAFT SDPng Transition 15 May 2003
potentially make use of SDP and SDPng.
1. Introduction
SDP is now widely used within the Internet community to describe
media sessions and, in a limited fashion, system capabilities
relating to (multi)media sessions, for a variety of application
scenarios: session announcements, interactive session setup,
capability assessment and remote control of media streams. All but
the first of these are rather different from what SDP was originally
designed for -- but all of them share the idea of setting up and
configuring media streams. Over time, its wide range of uses has
revealed numerous shortcomings -- most of which stem from the fact
that SDP has been used for lack of a better alternative and its
semantics have been re-interpreted to make it fit the respective
scenarios' needs. In many cases, workrounds (typically called
"extensions") for those shortcomings could be found which are often
rather cumbersome. While this practice has extended SDP's lifetime
and provided at least a suitable basis for numerous applications, in
parallel, a successor protocol -- currently referred to as "SDPng"
-- has been developed.
It is worthwhile noting that the aforementioned applications'
needs are sufficiently similar for a single description protocol
to take care of them if it was designed for this purpose from
the beginning. As a lesson learned from SDP, any further
expansion in scope should be avoided where no clear fit can be
seen -- and specific (different) solutions should be developed
instead.
The design of SDPng takes into account the requirements arising from
the above application scenarios and puts particular emphasis on
protocol extensibility and modularization of extensions, at the same
time keeping the core description format simple. SDPng uses a
different (more expressive) syntax than SDP does and hence is not
backward compatible at the syntax level. Nevertheless, the concepts
of SDPng take into account the migration issues from SDP to SDPng by
providing straightforward mappings between the two formats where
possible and try to maximize compatibility from a semantics
perspective.
The current revisions of SDP and SDPng are documented in
o draft-ietf-mmusic-sdp-new-12.txt [1] and
o draft-ietf-mmusic-sdpng-06.txt [9].
For SDP, a number of additional features are defined in the following
documents that need to be taken into account:
o the offer/answer scheme of interpreting and matching media
session descriptions to negotiate media sessions to be used in
Ott/Perkins [Page 2]
INTERNET-DRAFT SDPng Transition 15 May 2003
call or conference in a single round-trip (RFC 3264 [6]);
o full support for IPv6 as network layer protocol (RFC 3266 [2]);
o SDP extensions to allow selecting ATM virtual circuits as
additional network protocol and specifying ATM-specific
parameters (RFC 3108 [3]);
o a general extension to deal with connection-oriented transport
protocols such as TCP [12];
o an extension to support SCTP as media transport protocol in
addition to UDP and TCP [17];
o an SDP extension to explicitly specify RTCP port number to
enable the necessary expressiveness for NAT traversal [8];
o a mechanism (media identification, "mid") for naming and
grouping ("group") SDP media lines according to one or more
criteria, e.g. for the purpose of lip-synchronization or for
identifying media sessions carrying the same content (RFC 3388
[4], [10]);
o the capability to indicate which media sessions shall be mapped
into the same resource reservation context [13];
o an extension to allow expression of simultanous capabilities
across media sessions and formats (RFC 3407 [5])
o attributes for passing parameters of a keying protocol (such as
MIKEY) as part of a session description [11];
o support for conveying cryptographic parameters as part of a
session description [15];
o a mechanism to explicitly specify the sources allowed to provide
input to media sessions [16]; and
o a simple language to provide instructions to media mixers on
which incoming media streams shall be combined to produce which
outgoing ones (and possbily how they shall be combined) [14].
This document outlines a migration path from SDP to SDPng, starting
from a short overview of the current application scenarios. In the
next step, we highlight which design decisions taken for SDPng should
simplify a smooth migration and describe how mappings between the two
description formats can be performed at an abstract level. We then
address procedural issues for integrating SDP and SDPng into the
various protocols relying on those media description formats.
Finally, we summarize work items on the agenda for SDPng.
Ott/Perkins [Page 3]
INTERNET-DRAFT SDPng Transition 15 May 2003
2. Application Scenarios
The following session control protocols that make use of SDP have
been standardized in the IETF so far:
1. SDP was originally developed to announce (Mbone-based)
multimedia sessions via session directories using the Session
Announcement Protocol (SAP) -- but other mechanisms for
disseminating the session descriptions (such as HTTP, SMTP,
NNTP, etc.) are conceivable as well.
The major property of this application scenario is that the
creator of the session description defines a (set of) fixed
choice(s) for all media types in a conference and the conference
partipants have no way to influence these. If they support at
least one of the codecs for a particular media type they can
participate in this media session, otherwise they cannot. There
is no interaction between sender(s) and receiver(s) to negotiate
the media stream codecs and parameters.
This scenario is referred to as "announcement".
2. Another use of SDP is in conjunction with the Real-Time
Streaming Protocol (RTSP). In RTSP, SDP is used to convey
descriptions of a media stream interactively requested to be
played from a server (or recorded by a server). SDP itself is
not used for capability negotiation, not even for the addresses
to be used; those are negotiated within RTSP and may override
the addresses specified as part of SDP.
This scenario is referred to as "retrieval".
3. With SIP, SDP is used to propose media stream configurations and
choose out of these (i.e. enable a subset of these). By
proposing and accepting media stream configurations, endpoints
use SDP to implicitly describe their capabilities and carry out
a negotiation procedure on the media streams to use.
In the context of SIP, specific meanings (including required
extensions) have been defined for use of SDP with unicast
addresses, for connection-oriented transports, and for certain
media level attributes (such as the direction attribute send-
only, receive-only, and inactive).
Numerous extensions have been proposed to extend SDP to better
suit SIP's needs. Besides a description of the offer/answer
model, these extensions particularly include the ability to
describe simultaneous capabilites and to group media stream
semantically.
This scenario is referred to as "offer/answer".
4. SDP is used to convey the capability descriptions of a MEGACO
Ott/Perkins [Page 4]
INTERNET-DRAFT SDPng Transition 15 May 2003
media gateway (MG) to its media gateway controller (MGC) as well
as for the MGC to instruct the MG where to send media streams to
and from where to receive media streams, including codec and
parameter choice.
For this purpose, SDP has been modified/extended to some degree
to fit the MEGACO needs.
This scenario is referred to as "gateway control".
It should noted that the original SDP concept already provided an
extension mechanism to cover other network types than IPv4 and IPv6;
however, specific extensions have only been defined recently for ATM
and are now under discussion for TDM. Extensions to other transport
(including radio interfaces or next generation wireless networks) as
well as to new types of session descriptions (e.g. electronic program
guides) are conceivable.
3. Mapping SDP to SDPng
On a transition path from SDP to SDPng, allowing for a somewhat
straightforward mapping of (parts of) one description format onto the
other is of crucial importance. SDPng has been designed in way that
allows many of the session description features of SDP to be easily
mapped onto the SDPng format and vice versa -- except that SDPng is
more expressive than SDP and hence information loss is not unlikely
to occur when doing the reverse mapping. The final mapping rules
between SDP and SDPng to be drawn up shall ensure that when mapping
SDP to SDPng and then back to SDP will produce an SDP that is
functionally identical to the one originally fed into the mapping
process. Note that the use of a number of SDP extensions (FID,
SIMCAP) may be implied in this mapping process, depending on the use
of SDP. The mapping rules will ensure that no information loss will
occur when translating from SDP to SDPng.
The SDPng design uses a structure of four sections: definitions,
potential or actual configurations, constraints and session
attributes. Of these, the "Configurations" and "Session
Attributes" sections map well onto the current SDP. The
"Definitions" and "Constraints" sections provide additional
structure which is not directly expressible in SDP.
o At the media description level, the Potential and Actual
Configurations specified in the "Configurations" section maps
well to media descriptions ("m=", possibly "c=", and
associated attributes ("a=") lines).
o At the session description level, the SDP session parameters are
largely reflected in the "Session Attributes" section of
SDPng. The attributes proven suitable for session announcements
have been used as the basis when defining SDPng.
In SDPng, media descriptions are explicitly tagged with identifiers
Ott/Perkins [Page 5]
INTERNET-DRAFT SDPng Transition 15 May 2003
and thus are easily referenced for semantically grouping media
streams (e.g. to describe alternative audio in different languages,
media streams to be synchronized, or media streams to carry the same
information simultaneously but with different encodings) -- as has
been defined for SDP in a limited fashion by the "fid" attribute
set. SDPng allows even to more formally describe the syntax of
individual or compound media streams in the "Session Attributes"
section. Furthermore, SDPng supports a superset of additional
constraints that may be realized by the "simcap" extensions for SDP
in the "Constraints" section.
Additional address families such as ATM or TDM bearers, next
generation wireless network bearers, DVB channels, etc. can be
incorporated into SDPng by defining the appropriate extensions for
the SDPng transports.
Similarly, new codecs can be added by just defining new codec
specifications or defining entire new classes of applications to be
described as new content types ("codec") to be carried in a media
session (including e.g. text, fax, slide presentations, shared
editors, etc). If necessary, sophisticated parameter structures can
be supported (even though the authors believe that simplicity is key
to interoperability here). This is similar to, but more structured
than, the definition of new codec attributes in MIME registrations
for SDP.
It is expected that the MIME namespace for codecs will be mapped
into the SDPng namespace, and a consistent mapping from SDP
"a=fmtp:" attributes to SDPng codec parameters will be
defined.
By means of its conceptual differentiation into Potential and Actual
Configurations, SDPng supports both indicating a system's
capabilities (without specifying transport addresses) separately from
the instantiation of a particular media stream as well as conveying
capability descriptions and instantiation proposals at the same time
-- thereby providing a good fit for all the above session control
scenarios: the "announcement" and "retrieval" scenarios will just
use rather fixed Actual Configurations. The "offer/answer" model
will use use Actual Configuration but use them to negotiate media
strams in a two-way handshake but may in addition use Potential
Configurations to indicate capabilities that shall not be used
immediately. The "gateway control" scenario will use both:
Potential Configurations to describe an MG's capabilities and Actual
Configurations for setting up media sessions at MGs as well as
retrieving information about currently active media sessions. This
differentiation is not directly expressible in SDP, although various
extensions can be used to overload SDP semantics to achieve at least
part of this effect.
Finally, while the short-term SDPng specification aims at supporting
only the widespread offer/answer model for capability negotiation, a
future extension will also allow for content-independent negotiation
Ott/Perkins [Page 6]
INTERNET-DRAFT SDPng Transition 15 May 2003
of session parameters by defining collapsing/intersection rules. In
particular, SDPng will take the need for multicast-based distributed
calculation of joint capabilities into account for those rules (but
note that it is *not* intended as a generic format for describing
conference state information). Such functionality is not covered by
current SDP -- but there is also no perceived urgent demand so that
this sophisticated functional component of SDPng is left to a future
protocol extension. The base SDPng protocol will provide the
necessary flexibility, however.
4. Integration with Session Control Protocols
This section outlines for each of the session control protocols
described above how SDP and SDPng can be used in parallel and
indicates how a suitable transition could be achieved.
4.1. Session Announcement Protocol (SAP)
There are two revisions of SAP specified, version 0 which is
implemented in a number of experimental tools, and version 1 which is
defined in RFC 2972.
SAPv0:SAPv0 does not support a mechanism to identify the content type
of a session announcement but implicitly assumes SDP. Proper
parsers will note that the contents of the SAPv0 message does
not begin with a "v=" line and hence will ignore the entire
announcement. SDPng contents MAY be identified by a different
character sequence in the beginning of the announcement body,
but this is not recommended. Instead, SAPv1 should be used,
since it contains an explicit payload identifier.
SAPv1:In SAPv1, an explicit payload type field (containing a MIME
type) is available and should be used to differentiate between
SDP anf SDPng contents. Two approaches are conceivable: Either
multipart MIME message is used with two parts containing the
same session descriptions -- one expressing it in SDP and the
other in SDPng. Alternatively, two alternate session
announcements may be used (being properly distinguished by the
SDP "o=" field and the SDPng equivalent).
It is recommended that implementations recognize the MIME
multipart/alternative type in SAPv1 announcements, allowing for
a simple transition to SDPng.
It should be noted that current session directory implementations
only support SDP. Nevertheless, using the SAP Message Identifier
Hash and the source address, they should be able to perform session
deletions and modifications properly -- even without understanding
the format contained in the SAP message body.
For the introduction of SDPng, session announcements should be made
"bi-lingual", i.e. in SDP and SDPng. If a SAP announcer for some
reason knows that all its potential audience will support SDPng, the
Ott/Perkins [Page 7]
INTERNET-DRAFT SDPng Transition 15 May 2003
SDP announcement should be omitted.
It should be noted that, for IPv4-based multicast sessions, session
directories still may rely on parsing the session specifications to
avoid clashes in the multicast address space. Introducing a new
session description language will prevent older implementations from
continuing this practice successfully -- assuming that only SDPng
announcements are used and/or that old implementations do not support
MIME multipart/alternative message bodies. This use of SAP is
deprecated, however.
4.2. Real-Time Streaming Protocol (RTSP)
RTSP uses SDP to provide presentation descriptions (with a
presentation comprising one or more media sessions), typically
communicated from the server to the client (for playing) and in the
opposite direction for recording. The presentation description may
also include initialization data for the various media streams and
URLs to be used for controlling the entire presentation as well as
the individual media sessions. Transport parameters -- such as IP
addresses, port numbers, etc. -- are conveyed as part of RTSP header
fields.
RTSP uses the Content-Type: header field to indicate the format of
the enclosed entity. This provides a straightforward means for
distinguishing SDP and SDPng-based presentation descriptions. In
addition, the Accept: header should be used by the client to indicate
which content types it supports. If the client specifies both SDP
and SDPng as acceptable, the server should provide only the SDPng-
based presentation description.
If the client does not indicate a particular Content-Type: the server
can, theoretically, use MIME multipart bodies (e.g.
"multipart/alternative") to convey both description types
simultaneously. However, it is generally not expected that today's
RTSP clients and servers will be able handle multipart bodies.
Hence, if no hint is provided by the client by means of the Accept:
header, the server must provide only an SDP description.
In general, it would be preferrable to have the servers migrate to
always supporting both description formats, thus enabling the clients
to choose. The servers should indicate SDPng support by means of
suitable header fields whenever possible.
Finally, RTSP makes special provision to interwork with firewalls by
including the crucial transport parameters in a separate RTSP header
field _in_addition_ to the presentation description. This practice,
in principle, allows to change the presentation description format
without having to worry about the operation of firewalls and similar
devices.
Ott/Perkins [Page 8]
INTERNET-DRAFT SDPng Transition 15 May 2003
4.3. Session Initiation Protocol (SIP)
The use of SDP with SIP follows the offer/anser model and is
described in [6]. It is key to the (efficiency of the) offer/answer
model that a complete capability exchange and media stream
instantiation be carried out in one round-trip -- which is supported
by SDP. While SDPng allows to separate capability exchange from
media sesssion instantiation, those two pieces are also easily
integrated in a single step.
SIP also uses a Content-Type: header to indicate the nature of data
carried in its message body; and SIP explicitly calls for supporting
MIME multipart message bodies. While, again, the use of MIME
multipart/alternative would in principle be possible (from a
theoretical perspective), issues regarding the actual implementation
of multipart/alternative in SIP entities have been raised. As
backward compatibility has to be achieved, a different approach is
suggested:
A SIP UAC MAY use an SDPng message body in a SIP INVITE (or other)
message. If the SIP UAS does not support SDPng, it will return a
"415 Unsupported Media Type" response to the UAC and indicate
acceptable content types in the Accept: header (probably including
"application/sdp"). The SIP UAC must then retry INVITE (or other)
message using the indicated session description language. The SIP
UAC should cache knowledge about which peers did not understand SDPng
as session description formats for a limited amount of time (e.g.
several days) so that extra round-trips for session setup are only
incurred infrequently. Whenever a peer has sent an SDPng description
(or it is known from other means that the peer supports SDPng), this
information should also be cached.
The SIP Accept: header can be exploited to determine the capability
of a peer to understand SDPng in addition (or instead) plain SDP.
Methods such as OPTIONS MAY be used to determine a peer's support for
SDPng. However, a peer's capabilities may not be known when the
first message is sent which may introduce an extra round-trip if
including SDP and SDPng in the initial INVITE message is not an
option. Further approaches to make a UA's support for SDPng known
ahead of time should be explored.
A number of SDP extensions have been motivated by SIP-based
applications and these need to be accommodated in SDPng as well.
Features such as "simcap" and "FID" are inherently supported by
SDPng; proper definitions for connection-oriented media need to be
fully understood and then incorporated. Key management attributes as
defined in [11] need to be included (not just for SIP) and so may
need to be general mechanisms to signal security capabilities [11]
[15] and indicate their optional or mandatory use. The same applies
to quality of service parameters [13] (which are largely also
motivated by SIP but are also useful with control protocols).
Numerous extensions to SDP have been developed for the purpose of
Ott/Perkins [Page 9]
INTERNET-DRAFT SDPng Transition 15 May 2003
supporting certain SIP requirements -- actually most of those listed
in section 1. fall into this category. The following paragraphs
address how those are handled by and mapped to SDPng.
IPv6 is natively supported by SDPng. For other network protocols --
such as ATM and TDM, which have only come up in the context of
MEGACO, see below -- similar SDPng packages need to be defined that
provide the same information as the corresponding SDP extensions.
Support for connection-oriented media in general will be supported in
SDPng using a similar parameterization. Support for SCTP will be
equivalent to the approach taken for SDP as the parameters are
comparable.
SDP's explicit RTCP port number parameter (that helps with NAT
traversal) is inherently available in the RTP transport specification
of SDPng.
Media session identification is also provided by the SDPng spec by
means of naming attributes in the potential as well as actual
configurations. The "Session Attributes" section of SDPng is meant
to provide meta-information about the media sessions such as grouping
of lip synchronization, indicating streams semantics, etc. This
section is also the place to express media "mixing" attributes as
discussed in [14]. QoS parameterizations for SDPng are developed
separately as package enhancements and are still under discussion.
Simultaneous capabilities are dealt with by the Constraints section
of SDPng where restrictions across several components as well as
within a single component can be expressed.
Security parameters have not yet been developed for the SDPng
specification. The intention is to define a separate security
package (similar to codec and transport definitions). Security
parameters may be provided in the definition section for later
reference from within the component specification or may be specified
inline in a component.
Indicating permissible sources for unicast and particularly multicast
media sessions is already covered in the basic SDPng transport
specification.
In summary, most of the newly developed SDP attributes and their
usages have either been considered in the SDPng base specification
and the transport packagaes or will be added additional attributes or
as separate packages.
Note that the above discussions are not just applicable to SIP but
may be used in a broader scope (e.g. with RTSP or MEGACO).
4.4. Media Gateway Control Protocol (MEGACOP)
The MEGACO specification already supports two different encodings for
Ott/Perkins [Page 10]
INTERNET-DRAFT SDPng Transition 15 May 2003
capability and media stream descriptions: a text-based variant based
upon (a modified) SDP and a binary representation of the same
information set. MGCs are required to implement both encodings while
MGs have the choice to pick either or both. Differentiation between
the protocol encoding variants is done using different port numbers:
2944 for the text-based and 2945 for the binary encoding.
Unfortunately, within the text-based encoding, there is no means to
differentiate several description formats. SDP messages are carried
as an "octet string" without any type identifier. Defining a third
port number for this further differentiation does not seem to be
appropriate, particularly since the message encoding is still a text
format.
The remaining means for distinction is that an SDP specification
would start with a "v=0" line while an SDPng document would begin
with a different character sequence.
Note that MEGACOP also supports a binary encoding for SDP
messages; current practice seems to favor the text encoding for
SDP and hence we will not address a binary encoding for SDPng.
Within the context of MEGACO, various extensions to SDP have been
defined, addressing its use for capability description and also
defining support for further network types (presently, ATM and TDM).
Capability descriptions are inherently supported by SDPng. To add
support for further networks, the respective parameters need to be
defined as network-specific SDNng packages.
5. SDPng and Middleboxes
Middleboxes (e.g. firewalls, NATs, NAPTs) are of significant
importance for many deployment scenarios for SDP-based signaling
protocols. The SDP description typically carries addressing
paramaters for media sessions which may be invalidated by
middleboxes: by firewalls because they block packet destined at the
respective addresses and by NA(P)Ts because they change the addresses
that must actually be used.
A number of approaches have been devised to deal with currently
deployed and, eventually, future middleboxes: 1) co-locating a proxy
for the respective signaling protocol(s) with a middlebox, 2) using
extra protocols to determine the presence and the mode of operation
of a NA(P)T (which does not work for firewalls), and 3) the
definition of a control protocol for middleboxes. While approach 1)
is entirely up to the manufacturers of middleboxes, 2) and 3) are
subject to IETF standardization in the MIDCOM WG.
1) Proxies that are incorporated in middleboxes to parse and (in
case of NATs) possibly alter the contents of session
descriptions exchanged in the signaling path will need to
implement SDPng in the future. For a (potentially long) interim
period, both SDP and SDPng need to be supported by such devices.
Ott/Perkins [Page 11]
INTERNET-DRAFT SDPng Transition 15 May 2003
2) If entities involved in the respective signaling path use
protocols such as STUN to determine the presence of a NAT and
its mode of operation, it is up to these entities to include the
correct addressing information in the SDP or SDPng session
descriptions. NATs continue to operate as before and do not
require any changes because of a migration from SDP to SDPng.
3) If local signaling servers or other entities use a MIDCOM
protocol to configure a firewall or NAT to allow certain media
streams to pass through, again, no changes need to be made to
MIDCOM-enabled firewalls. The migration from SDP to SDPng is
transparent to them; only the involved signaling component need
to support -- but they would need to do so anyway.
6. Directing the Evolution of SDP
With the transition from SDP to SDPng, there is the question of the
evolution of SDP, and legacy systems which use it.
The SDP specification [1] is stable, and mostly corrects errors in
the original specification, with the addition of very few new
features. The revision is expected to be published as a proposed
standard RFC shortly, obsoleting RFC 2327.
A number of extensions to SDP for use in offer/answer scenarios are
also available. These include grouping [4] and capability negotiation
[5], which have recently been approved as proposed standard RFCs,
bringing minimal capability/alternative descriptions to SDP.
Related is the SDP offer/answer model for SDP, published as RFC3264
[6]. This defines the model used to complete the steps of a
negotiation using SDP. A similar mode of operation will be provided
for SDPng for baseline operation with SIP (and possibly RTSP).
All these are subsumed into SDPng, so there should be no further need
for development in these areas; applications with requirements that
are not met by these specifications should use SDPng.
There have recently been proposals to add quality of service
negotiation for SDP and, similarly, we expect other extensions to be
proposed over time. Due to the well-known limitations of SDP, we do
not believe it appropriate to continue development of more elaborate
extensions: for negotiation, for QoS, for security, and for other
general-purpose or application-specific needs.
The exceptions to this rule are clearly the addition of security
features to SDP (which are required for many current SDP deployment
scenarios) as well as minor extensions for media session attributes
(e.g. indicating the use of joint vs. separate resource reservations
as documented in [7]).
Overall, new work should be done in the framework of SDPng where
applications and their requirements for (new) expressiveness in end-
Ott/Perkins [Page 12]
INTERNET-DRAFT SDPng Transition 15 May 2003
to-end exchanges to negotiate and configure media sessions will
hopefully act as a driver for that process.
7. IANA Considerations
The transition from SDP to SDPng will require IANA to define new
parameter registries, which will be created and populated as SDPng
evolves. This memo does not, in itself, require any action by IANA.
8. Security Considerations
Since SDPng performs largely the same role as SDP+extensions, it is
not expected that there will be significant new security
considerations as a result of the transition. The security
considerations section of [9] provides further details.
During the transition process it is likely that dual descriptions
will be common. There is a potential for inconsistancy between
definitions, which may have unintended consequences if one part of
the system, for example a middlebox, interprets the SDP format whilst
another interprets the SDPng format definition.
9. Author's Addresses
Joerg Ott <jo@tzi.org>
Universitaet Bremen
MZH 5180
Bibliothekstr. 1
D-28359 Bremen
Germany
tel:+49-421-201-7028
sip:jo@tzi.org
Colin Perkins <csp@csperkins.org>
USC Information Sciences Institute
3811 North Fairfax Drive, Suite 200
Arlington, VA 22203
USA
tel:+1-703-812-3705
10. Normative References
[1] Mark Handley, Van Jacobson, Colin Perkins, "SDP: Session
Description Protocol," Internet Draft draft-ietf-mmusic-sdp-
new-11.txt, Work in Progress, November 2002.
[2] Sean Olson, Gonzalo Camarillo, Adam Roach, "Support for IPv6 in
Session Description Protocol (SDP)", RFC 3266, June 2002.
Ott/Perkins [Page 13]
INTERNET-DRAFT SDPng Transition 15 May 2003
[3] 3108 R. Kumar and M. Mostafa, "Conventions for the use of the
Session Description Protocol (SDP) for ATM Bearer Connections,"
RFC 3108, May 2001.
[4] Gonzalo Camarillo, Jan Holler, Goran AP Eriksson, Henning
Schulzrinne, "Grouping of media lines in SDP," RFC 3388,
December 2002.
[5] Flemming Andreasen, "SDP Simple Capability Declaration," RFC
3407, October 2002.
[6] Jonathan Rosenberg and Henning Schulzrinne, "An Offer/Answer
Model with SDP," RFC 3264, June 2002.
[7] G. Camarillo and A. Monrad, "Mapping of Media Streams to
Resource Reservation Flows", RFC 3524, April 2003.
[8] Christian Huitema, "RTCP attribute in SDP," Internet Draft
draft-ietf-mmusic-sdp4nat-03.txt, Work in Progress, September
2002.
10.1. Informative References
[9] Dirk Kutscher, Joerg Ott, Carsten Bormann, "Session Description
and Capability Negotiation", Internet Draft draft-ietf-mmusic-
sdpng-06.txt, Work in Progress, March 2003.
[10] Gonzalo Camarillo and Jonathan Rosenberg, "The Alternative
Semantics for the Session Description Protocol Grouping
Framework," Internet Draft draft-camarillo-mmusic-alt-00.txt,
Work in Progress, February 2003.
[11] Jari Arkko, Elisabetta Carrarra, Fredrik Lindholm, Mats Naslund,
and Karl Norrman, "Key Management Extensions for SDP and
RTSP," Internet Draft draft-ietf-mmusic-kmgmt-ext-06.txt, Work
in Progress, February 2003.
[12] David Yon, "Connection-Oriented Media Transport in SDP,"
Internet Draft draft-ietf-mmusic-sdp-comedia-05.txt, Work in
Progress, March 2003.
[13] G. Camarillo (ed), W. Marshall (ed), J. Rosenberg, "Integration
of Resource Management and SIP", Internet Draft draft-ietf-sip-
manyfolks-resource-03.txt, Work in Progress, November 2001.
[14] G. Camarillo, H. Schulzrinne, and E. Burger, "The source and
sink attributes for the Session Description Protocol", Internet
Draft draft-camarillo-mmusic-source-sink-00.txt, Work in
Progress, September 2002.
[15] Mark Baugher, "SDP Security Descriptions for Media Streams",
Internet Draft draft-ietf-mmusic-sdescriptions-00.txt, Work in
Ott/Perkins [Page 14]
INTERNET-DRAFT SDPng Transition 15 May 2003
Progress, February 2003.
[16] B. Quinn and R. Finlayson, "SDP Source-Filters", Internet
Draft draft-ietf-mmusic-sdp-srcfilter-04.txt, Work in Progress,
April 2003.
[17] Robert Fairlie-Cuninghame, "Guidelines for specifying SCTP-
based media transport using SDP," Internet Draft draft-fairlie-
mmusic-sdp-sctp-00.txt, Work in Progress, May 2001.
11. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Ott/Perkins [Page 15]
