One document matched: draft-ietf-mmusic-sdp-comedia-06.txt
Differences from draft-ietf-mmusic-sdp-comedia-05.txt
MMUSIC Working Group D. Yon
Internet-Draft Dialout.Net, Inc
Expires: November 12, 2004 G. Camarillo
Ericsson
May 14, 2004
Connection-Oriented Media Transport in the Session Description
Protocol (SDP)
draft-ietf-mmusic-sdp-comedia-06.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes how to express media transport over
connection-oriented protocols using the Session Description Protocol
(SDP). It defines two new protocol identifiers: TCP and TCP/TLS. It
also defines the SDP setup attribute, which describes the connection
setup procedure, and the SDP reconnect attribute.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 3
3.1 TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 TCP/TLS . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4
4.2 Multiple-Connection Avoidance when Using Actpass . . . . . 5
5. The Reconnect Attribute . . . . . . . . . . . . . . . . . . . 6
6. Connection Lifetime . . . . . . . . . . . . . . . . . . . . . 7
6.1 Session Renegotiation . . . . . . . . . . . . . . . . . . 7
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
7.2 Passive/Active with Reconnect . . . . . . . . . . . . . . 9
7.3 Actpass . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
11.2 Informational References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
The Session Description Protocol [4] provides a general-purpose
format for describing multimedia sessions in announcements or
invitations. SDP uses an entirely textual data format (the US-ASCII
subset of UTF-8 [6]) to maximize portability among transports. SDP
does not define a protocol, but only the syntax to describe a
multimedia session with sufficient information to participate in that
session. Session descriptions may be sent using arbitrary existing
application protocols for transport (e.g., SAP [9], SIP [10], RTSP
[7], email, HTTP [8], etc.).
SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of
which represent unreliable connectionless protocols. While these
transports are appropriate choices for multimedia streams, there are
applications for which connection-oriented transports such as TCP are
more appropriate. We define two new protocol identifiers: TCP and
TCP/TLS. Both represent connection-oriented reliable transports.
Connection-oriented protocols introduce a new factor when describing
a session: how should end points perform the connection setup
procedure. We define two new attributes to describe connection setup:
setup and reconnect.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [2] and indicate requirement levels for
compliant implementations.
3. Protocol Identifiers
The following is the ABNF for an m= line, as specified by RFC 2327
[4].
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
We define two new values for the proto field: TCP and TCP/TLS.
3.1 TCP
The TCP protocol identifier is similar to the UDP protocol identifier
in that it only describes the transport protocol, and not the
upper-layer protocol. An m= line that specifies "TCP" MUST further
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qualify the application-layer protocol using an fmt identifier.
Media lines with the TCP protocol identifier are carried using TCP
[1].
3.2 TCP/TLS
The TCP/TLS protocol identifier specifies that the session will use
the Transport Layer Security (TLS) protocol [3] on top on a TCP [1]
connection.
An m= line that contain the TCP/TLS protocol identifier MUST further
qualify the protocol using a fmt identifier.
4. Setup Attribute
The setup attribute indicates which of the end points should initiate
the connection establishment (e.g., send the initial TCP SYN). The
setup attribute is charset-independent and can be a session-level or
a media-level attribute. The following is the ABNF of the setup
attribute:
setup-attr = "a=setup:" role
role = "active" / "passive" / "actpass"
Active: The endpoint will initiate an outgoing connection.
Passive: The endpoint will accept an incoming connection.
ActPass: The endpoint will both accept an incoming connection and
will initiate an outgoing connection.
The default value of the setup attribute is actpass. That is, an m=
line without an associated setup line is considered to be actpass.
4.1 The Setup Attribute in the Offer/answer Model
The offer/answer model, defined in RFC 3264 [5], provides endpoints
with a means to obtain shared view of a session. Some session
parameters are negotiated (e.g., codecs to use), while others are
simply communicated from one endpoint to the other (e.g., IP
addresses). The value of the setup attribute falls into the first
category. That is, both endpoints negotiate its value using the
offer/answer model.
The negotiation of the value of the setup attribute takes places as
follows. The offerer states which role or roles is willing to perform
and the answerer, taking the offerer's willingness into
consideration, chooses which roles both endpoints will actually
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perform during connection establishment. The following are the values
that the setup attribute can take in an offer/answer exchange:
Offer Answer
_______________
active passive
passive active
actpass active / passive / actpass
The value active indicates that the endpoint SHOULD initiate a
connection to the port number on the m= line of the other endpoint.
The port number on its own m= line is irrelevant, and the opposite
endpoint MUST NOT attempt to initiate a connection to the port number
specified there. Nevertheless, since the m= line must contain a valid
port number, the endpoint specifying using the value active SHOULD
specify a port number of 9 (the discard port) on its m= line. The
endpoint MUST NOT specify a port number of zero, as that carries
other semantics in SDP.
The value passive indicates that the endpoint SHOULD be ready to
accept a connection on the port number specified in the m= line.
The value actpass indicates that the endpoint SHOULD initiate a
connection to the port number on the m= line of the other endpoint
and that the endpoint SHOULD be ready to accept a connection on the
port number specified in the m= line. It is RECOMMENDED that, if
possible, endpoints set the port number on their m= line to the
source port number which they will use to establish the connection
towards the remote endpoint. This way, the transport-layer protocol
(e.g., TCP) can take care of simultaneous opens.
Endpoints typically use the actpass value for the following reasons:
1. The offerer has no preference as to whether it accepts or
initiates the connection and, so, is letting the answerer choose.
2. The endpoints intend to use a single connection to transport the
media, but it is not known whether NAT (Network Address
Translator) issues will prevent either endpoint from initiating
or accepting the connection. So, both endpoints will attempt to
initiate a connection hoping that at least one will succeed.
4.2 Multiple-Connection Avoidance when Using Actpass
When an offer/answer exchange results in actpass, each endpoint
attempts to establish a transport connection towards the other
endpoint. If only one of the connections succeeds, this connection is
used to transfer media. Nevertheless, if both connections succeed,
one of them needs to be terminated so that both endpoints exchange
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data over a single connection. In this section, we provide rules to
choose which of the two connections should be terminated (or not even
initiated).
First of all, if the endpoints follow the recommendation of setting
the port number in their m= line to the source port number which they
will use to establish the connection towards the remote endpoint, the
transport layer should take care of simultaneous opens (at least if
TCP is the transport protocol). If, for some reason, any of the
endpoints does not follow this recommendation, both endpoints should
follow the rules below.
If an endpoint is notified about a connection establishment attempt
from the other endpoint before performing its own connection attempt,
it SHOULD behave as a passive endpoint and SHOULD NOT attempt to
establish any other connection.
In case two connections are established, if an endpoint receives data
(i.e., media) over one of the connections before having sent any data
on any of the connections, the endpoint SHOULD terminate the
connection that has not carried any data.
When two connections are established and both endpoints start sending
data before receiving anything from the other endpoint, it may happen
that each of the endpoints choose a different connection to send
data. If the answerer receives data over a connection after having
sent data on the other connection, it SHOULD continue sending data on
the other connection until an application-layer data boundary. At
that point, the answerer SHOULD terminate this connection and start
using the connection on which the offerer was sending data.
Note that different applications may define application-layer
boundaries in different ways. A typical suitable point for the
answerer to change connections is the end of an application-layer
message and the beginning of the next one.
5. The Reconnect Attribute
The preceding description of the setup attribute has been in the
context of using SDP to initiate a session. Still, SDP may be
exchanged between endpoints at various stages of a session to
accomplish tasks such as terminating a session, redirecting media to
a new endpoint, or renegotiating the media parameters for a session.
After the initial session has been established, it may be ambiguous
as to whether subsequent SDP exchange represents a confirmation that
the endpoint is to continue using the current media connection
unchanged, or is a request to make a new media connection. The
reconnect attribute, which is charset-independent and can be a
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session-level or a media-level attribute, is used to disambiguate
these two scenarios. The following is the ABNF of the reconnect
attribute:
reconnect-attr = "a=reconnect"
On reception of an m= line with a reconnect attribute, the endpoints
SHOULD close the existing connection, in case it was still up, and
SHOULD establish a new connection according to the setup attribute in
the m= line.
Either the offerer or the answerer can include a reconnect attribute
in an m= line. In any event, if the offer contained this attribute,
the answer MUST contain it as well.
6. Connection Lifetime
An endpoint that intends to initiate the connection SHOULD initiate
the connection immediately after it has sufficient information to do
so, even if it does not intend to immediately begin sending media to
the remote endpoint. This allows media to flow from the remote
endpoint. An endpoint SHOULD NOT close the connection until the
session has expired, been explicitly terminated, or the media stream
is redirected to a different address or port.
If the endpoint determines that the connection has been closed, it
MAY attempt to re-establish the connection. The decision to do so is
application and context dependant.
6.1 Session Renegotiation
There are scenarios where SDP is sent by an endpoint in order to
renegotiate an existing session. These include muting/unmuting a
session, renegotiating the attributes of the media used by the
session, or extending the length of a session about to expire.
Connection-oriented media introduces some ambiguities into session
renegotiation as to when the direction attribute must be obeyed and
when it is ignored.
The scenario of extending the duration of an existing session is a
good example: in order to extend an existing session, endpoints will
typically resend the original SDP with updated time information. In
connectionless media the result is no change to the existing media
streams. The problem with connection oriented media is that the
original SDP will contain a setup attribute which can be considered
as a request to create a new connection, as opposed to a request to
maintain steady state. The following rule help avoid this ambiguity:
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If the transport section (the c= and m= lines) of an SDP
description describes an existing connection between two endpoints
and the m= line does not contain a reconnect attribute, the
endpoints SHOULD use that connection to carry the media described
in the remainder of the message. The endpoints SHOULD NOT attempt
to set up a new connection, regardless of what is specified in the
setup attribute.
Note that if the port number in the m= line changes, there is no
need to use the reconnect attribute because the new port will
trigger the establishment of a new connection anyway.
7. Examples
What follows are a number of examples that show the most common usage
of the setup attribute combined with TCP-based media descriptions.
For the purpose of brevity, the main portion of the session
description is omitted in the examples and is assumed to be the
following:
v=0
o=me 2890844526 2890842807 IN IP4 10.1.1.2
s=Call me using TCP
t=3034423619 3042462419
7.1 Passive/Active
An offerer at 192.0.2.2 signals its availability for a T.38 fax
session at port 54111:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38
a=setup:passive
An answerer at 192.0.2.1 receiving this offer responds with the
following answer:
c=IN IP4 192.0.2.1
m=image 9 TCP t38
a=setup:active
The endpoint at 192.0.2.1 then initiates the TCP connection to port
54111 at 192.0.2.2.
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7.2 Passive/Active with Reconnect
Continuing the preceding example, consider the scenario where the TCP
connection fails and the endpoints wish to reestablish the connection
for the session. The endpoint at 192.0.2.2 signals this intent with
the following SDP:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38
a=setup:passive
a=reconnect
The reconnect attribute informs the endpoint at 192.0.2.1 that this
SDP represents the intent to establish a new connection for media
transport, rather than continuing with the original connection.
Because the endpoint at 192.0.2.1 may not yet be aware that the TCP
connection has failed, this eliminates any ambiguity. If 192.0.2.1
agrees to continue the session using a new connection, it responds
with:
c=IN IP4 192.0.2.1
m=image 9 TCP t38
a=setup:active IN IP4
a=reconnect
7.3 Actpass
An offerer at 192.0.2.2 signals its availability for a T.38 fax
session at TCP port 54111. Additionally, this offerer is also willing
to set up the media stream by initiating the TCP connection:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38
a=setup:actpass
The endpoint at 192.0.2.1 responds with the following description:
c=IN IP4 192.0.2.1
m=image 54321 TCP t38
a=setup:actpass
This will cause the offerer (at 192.0.2.2) to initiate a connection
to port 54321 at 192.0.2.1 and the answerer (at 192.0.2.1) to
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initiate a connection to port 54111 at 192.0.2.2. Ideally, the
offerer would use 192.0.2.2:5411 as the source of its connection
attempt and the answerer would use 192.0.2.1:54321 as its.
8. Security Considerations
See RFC 2327 [4] for security and other considerations specific to
the Session Description Protocol in general.
An attacker may attempt to substitute TCP/TLS with only TCP in a
session description. So, it is STRONGLY RECOMMENDED that integrity
protection be applied to the SDP session descriptions. For session
descriptions carried in SIP [10], S/MIME is the natural choice to
provide such end-to-end integrity protection, as described in RFC
3261 [10]. Other applications MAY use a different form of integrity
protection.
This document touches upon NAT traversal. Implementers should be
aware of some issues that relate to the use of private IP addresses
within the offer/answer model (i.e., they are not specific to this
document).
When an endpoint receives a session description with a private IP
address that belongs to a different address space, in most of the
cases, the endpoint will not be able to reach such an address.
Nevertheless, if this particular address also exists in the
endpoint's address space, the endpoint may end up reaching a
different peer than the one that generated the session description.
It is RECOMMENDED that endpoints authenticate their peer somehow
(e.g., using TLS [3]) or that they encrypt their media.
9. IANA Considerations
This document defines two session and media level SDP attributes:
setup and reconnect. Their formats are defined in Section 4 and
Section 5 respectively. These two attributes should be registered by
the IANA on http://www.iana.org/assignments/sdp-parameters under
"att-field (both session and media level)".
This document defines two proto values: TCP and TCP/TLS. Their
formats are defined in Section 3.1 and Section 3.2 respectively.
These two proto values should be registered by the IANA on http://
www.iana.org/assignments/sdp-parameters under "proto".
10. Acknowledgements
The authors would like to thank Jonathan Rosenberg, Rohan Mahy,
Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert
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Fairlie-Cuninghame, and Colin Perkins for their valuable insights and
contributions.
11. References
11.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
11.2 Informational References
[7] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
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Authors' Addresses
David Yon
Dialout.Net, Inc
One Indian Head Plaza
Nashua, NH 03060
USA
EMail: yon@dialout.net
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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