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INTERNET-DRAFT D. Yon
Document: draft-ietf-mmusic-sdp-comedia-05.txt Dialout.Net
Expires September 2003 March 2003
Connection-Oriented Media Transport in SDP
<draft-ietf-mmusic-sdp-comedia-05.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright (C) The Internet Society (2002). 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 TLS. It
also defines the syntax and semantics for an SDP "direction"
attribute that describes the connection setup procedure.
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1 Introduction
The Session Description Protocol [SDP] 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]) to maximize portability among transports. SDP
does not define a protocol, but only the syntax to describe a
multimedia session with sufficient information to discover and
participate in that session. Session descriptions may be sent using
arbitrary existing application protocols for transport (e.g., SAP,
SIP, RTSP, email, HTTP, etc.).
[SDP] describes two protocol identifiers: RTP/AVP and UDP, both of
which are unreliable, connectionless protocols, an appropriate
choice for multimedia streams. There are, however, applications for
which the connection-oriented transports such as TCP are more
appropriate, but [SDP] provides no way to describe a session that
uses protocols other than RTP or UDP.
Connection-oriented protocols introduce a new factor when describing
a session: not only must it be possible to express that a protocol
will be based on this protocol, but it must also describe the
connection setup procedure. This memo defines two new protocol
identifiers, TCP and TLS, along with the syntax and semantics of the
a=direction and a=reconnect attributes.
2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
and indicate requirement levels for compliant implementations.
3 Protocol Identifiers
The m= line in [SDP] is where an endpoint specifies the protocol
used for the media in the session. See the "Media Announcements"
section of [SDP] for a discussion on protocol identifiers.
3.1 TCP
The TCP protocol identifier is similar to the UDP protocol
identifier in that it only describes the transport protocol without
any connotation as to the upper-layer protocol. An m= line that
specifies "TCP" MUST further qualify the protocol using a fmt
identifier (see [SDP] Appendix B).
3.2 TLS
The TLS protocol identifier specifies that the session will use the
Transport Layer Security protocol [TLS] with an implied transport
protocol of TCP. To describe a media session that uses TLS over
TCP, the protocol identifier "TLS" must be specified in the m= line.
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An m= line that specifies TLS MUST further qualify the protocol
using a fmt identifier.
4 Direction Attribute
An important attribute of connection-oriented protocols is the setup
procedure. One endpoint needs to initiate the connection and the
other endpoint needs to accept the connection. The direction
attribute is used to describe these roles, and the syntax is as
follows:
a=direction:<role>
The <role> is one of the following:
passive: The endpoint will accept an incoming connection.
active: The endpoint will initiate an outgoing connection.
both: The endpoint will both accept an incoming connection
and will initiate an outgoing connection.
4.1 Semantics of direction:passive
By specifying direction:passive, the endpoint indicates that the
port number specified in the m= line is available to accept a
connection from the other endpoint.
4.2 Semantics of direction:active
By specifying direction:active, the endpoint indicates that it will
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
direction: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 following
SDP fragment shows an example of direction:active:
c=IN IP4 10.1.1.1
m=image 9 TCP t38
a=direction:active IN IP4
4.3 Semantics of direction:both
By specifying direction:both, the endpoint indicates that it will
both accept a TCP connection on the port number of its own m= line,
and that it will also initiate a connection to the port number on
the m= line of the other endpoint.
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Since this attribute describes behavior that is similar to
connectionless media descriptions in [SDP], it is the default value
for the direction attribute and is therefore optional.
Endpoints may choose to specify direction:both for one or more of
the following reasons:
1) The endpoint has no preference as to whether it accepts or
initiates the connection, and therefore is offering the remote
endpoint a choice of connection setup procedures.
2) The endpoints intend to use a single connection to transport
the media, but it is not known whether firewall issues will
prevent either endpoint from initiating or accepting the
connection. Therefore both endpoints will attempt to initiate
a connection in hopes that at least one will succeed.
If one endpoint specifies either direction:active or
direction:passive and the other specifies direction:both, both
endpoints MUST behave as if the latter had specified the inverse
direction of the former. For example, specifying direction:both
when the other endpoint specifies direction:active SHALL cause both
endpoints to behave as if the former had specified
direction:passive. Conversely, specifying direction:both when the
other endpoint specifies direction:passive SHALL cause both
endpoints to behave as if the former had specified direction:active.
If both endpoints specify direction:both then each endpoint MUST
initiate a connection to the port number specified on the m= line of
the opposite endpoint. There is one exception to this requirement:
if an endpoint receives the incoming connection from the opposite
endpoint prior to initiating its own outbound connection, then that
endpoint MAY use that connection rather than attempt to make an
outbound connection to the opposite endpoint.
If only one connection succeeds, then that connection will be used
to carry the media. Once it has transmitted data on this
connection, the initiating endpoint MUST NOT perform another
connection attempt to the accepting endpoint. This allows the
accepting endpoint to release or recycle the listening port for
another session once it has received data from the initiating
endpoint.
If both connections succeed, the following rules SHALL apply:
a) Each endpoint MUST accept data from either connection.
b) Once an endpoint has transmitted data to one of the connections,
it MUST use that connection exclusively for transmission.
c) Once an endpoint has transmitted AND received data, if one of the
connections is determined to be idle, the endpoint SHOULD close
the idle connection.
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4.4 Optimizing direction:both
As discussed in the previous section, there is the possibility that
two connections will be created when only one is needed. While
rules in the previous section accommodate the closing of an idle
connection, they do not prevent a race condition where the endpoints
simultaneously start sending data on opposite connections thereby
causing two connections to be used where one would have sufficed.
While it is not possible to entirely eliminate this race condition,
it is in the endpoints' interest to minimize its occurrence.
Therefore, when a session is negotiated through interactive exchange
of SDP between endpoints (as in the case of SIP) AND the result of
the negotiation is that each endpoint specifies direction:both, it
is RECOMMENDED that the endpoints use the following guidelines:
a) There comes a point during the exchange of SDP where one endpoint
is prepared to send the final message that will complete the
negotiation and allow the session to begin. For the purposes of
this discussion, the endpoint that will send this final message
will be called the Initiator, and the endpoint that will receive
this message will be called the Acceptor.
b) The Initiator, upon receiving sufficient information to initiate a
connection, MUST attempt to connect to the Acceptor as soon as
possible.
c) In order to lower the likelihood that the Acceptor will also
attempt to initiate a connection, the Initiator SHOULD incorporate
a short delay between initiating the connection and sending the
final SDP to the Acceptor.
d) The delay time chosen by the Initiator MUST NOT introduce an
unacceptable session setup delay should the connection to the
Acceptor not succeed.
4.5 Bidirectional versus Unidirectional Media
In traditional SDP transport types the flow is unidirectional. If
the intent is for media to flow in both directions, both endpoints
must specify SDP that describes where to deliver the media and what
media type(s) to use. For example, if only Endpoint A presents SDP
then media can only flow towards Endpoint A, as Endpoint B has not
specified where and how to send media to it.
Because most connection-oriented media is inherently bi-directional,
endpoints may encounter a situation where only one side presented
SDP yet there is now a network path that can carry media in either
direction. In keeping with traditional SDP semantics, an endpoint
MUST NOT send data to the other endpoint unless it has specified SDP
information describing the type of media it can accept.
It is, however, perfectly acceptable for an endpoint to transmit
data on the same connection it is using to receive data, so long as
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the other endpoint has advertised its willingness to accept data.
Likewise, it is perfectly acceptable for an endpoint to receive data
on the same connection it is using to transmit data to the
corresponding remote endpoint. In other words, for a bi-directional
application-level session, a connection may be used to send data in
both directions (contingent to rules outlined in Section 2.3) as
long as one side of the connection is attached to either of the
advertised SDP transport addresses.
4.6 Treating UDP and RTP/AVP like Connection Oriented Media
Endpoints MAY specify a direction attribute for UDP or RTP/AVP
media. This indicates that the endpoint would like to treat this
media as a type of connection-oriented media. (The endpoint may do
this to facilitate NAT traversal for example.) Note that for
backwards compatibility, an endpoint which can specify
direction:active MUST include valid addresses and ports in the SDP
as always. If the peer's SDP does not include a direction
attribute, it knows that the peer does not support connection-
oriented media, and media exchange will proceed normally, as if
connection-oriented media were not offered.
Endpoints that specify direction:passive MUST NOT send any media,
any packets whatsoever (including control packets such as RTCP),
from their passive ports until they receive a packet on these ports
and record the source address and port of the sender. The passive
endpoint then assumes that the first packet received corresponds to
its active peer. From this point onward, passive endpoints MUST
send UDP or RTP media from the same port as the port indicated in
the m= line. Passive endpoints MUST send RTCP media (if any) from
the port on which they expect to receive it (typically the RTP port
number plus 1).
Endpoints that specify direction:active MUST be prepared to receive
on the ports from which they send. Once they learn the IP address
and port of their peer from the peer's SDP, they SHOULD immediately
send some kind of media (even if just comfort noise) to each of
these ports. This is so the peer can learn their IP address and
port, in order to send media back without additional delay.
Effectively, the exchange of the first media packet completes a bi-
directional handshake between the active and passive peer.
5 Reconnect Attribute
The preceding description of the a=direction attribute has been in
the context of using SDP to initiate a session. However, 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, renegotiating the media parameters for a session,
etc. 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
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connection. The reconnect attribute is used to disambiguate these
two scenarios, and the syntax is as follows:
a=reconnect
SDP containing a=reconnect signals that the specified session does
NOT refer to an existing connection between the two endpoints. If
the endpoints agree to continue the session, the endpoints MUST
close the existing connection for the currently negotiated session,
and MUST create a new connection according to the a=direction
attribute in the SDP. If an endpoint receives SDP that contains
a=reconnect, the endpoint's response MUST also contain a=reconnect.
Endpoints MUST NOT include a=reconnect in SDP that negotiates the
start of a session.
See section 6, "Connection and Listener Lifetime Considerations" for
more information on scenarios that are relevant to the a=reconnect
attribute.
6 Connection and Listener Lifetime Considerations
6.1 Listener Lifetime
An endpoint that has specified direction:both or direction:passive
MUST be ready to accept a connection on the appropriate address and
port during the time slot(s) advertised for that session. The
endpoint MUST keep the address and port available for incoming
connections until either:
a) The time window for the session has expired, or
b) The endpoint has received the expected number of incoming
connections on that address and port, or
c) Subsequent exchanges have superceded the SDP that originally
advertised the availability of the address and port.
Once the endpoint has determined that a listener is no longer needed
on a specific address and port, it SHOULD terminate the listener.
The endpoint is then free to re-use the address and port for
subsequent session advertisements.
6.2 Connection Lifetime
An endpoint that intends to initiate the connection MUST 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 MUST NOT close the connection until the session has
expired, been explicitly terminated, or the media stream is
redirected to a different address or port.
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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/or context dependant. If the endpoint opts to
re-establish the connection, it MUST NOT assume that the original
address and port advertised by the remote endpoint is still valid.
Instead, the endpoint MUST renegotiate the session parameters by
exchanging new SDP.
6.3 Session Renegotiation and Connection Lifetime
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 direction attribute which can be
construed as a request to create a new connection, as opposed to a
request to maintain steady state. To avoid this ambiguity, the
following rule SHALL apply to subsequent exchanges of SDP:
If the transport section (the c= and m= lines)
combined with the direction attribute of an SDP
message describes an existing connection between two
endpoints, AND the SDP does not contain a=reconnect,
then the endpoints MUST use that connection to carry
the media described in the remainder of the message.
The endpoints MUST NOT attempt to set up a new
connection, regardless of what is specified in the
direction attribute.
This disambiguates most session renegotiation scenarios, with the
exception of muting. Muting a media stream is accomplished by
sending the original session SDP but with an "a=inactive" or
"a=sendonly/recvonly" attribute. This is still valid for connection
oriented media, with the additional caveat that the endpoints MUST
NOT close the connection described by that SDP.
7 Examples
What follows are a number of examples that show the most common
usage of the direction 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:
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v=0
o=me 2890844526 2890842807 IN IP4 10.1.1.2
s=Call me using TCP
t=3034423619 3042462419
7.1 Example: simple passive/active
An endpoint at 10.1.1.2 signals the availability of a T.38 fax
session at port 54111:
c=IN IP4 10.1.1.2
m=image 54111 TCP t38
a=direction:passive
An endpoint at 10.1.1.1 receiving this description responds with the
following:
c=IN IP4 10.1.1.1
m=image 9 TCP t38
a=direction:active
The endpoint at 10.1.1.1 then initiates the TCP connection to port
54111 at 10.1.1.2.
7.2 Example: simple 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 10.1.1.2 signals this
intent with the following SDP:
c=IN IP4 10.1.1.2
m=image 54111 TCP t38
a=direction:passive
a=reconnect
The a=reconnect attribute informs the endpoint at 10.1.1.1 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 10.1.1.1 may not yet be aware that the TCP
connection has failed, this eliminates any ambiguity. If 10.1.1.1
agrees to continue the session using a new connection, it responds
with:
c=IN IP4 10.1.1.1
m=image 9 TCP t38
a=direction:active IN IP4
a=reconnect
7.3 Example: agnostic both
An endpoint at 10.1.1.2 signals the availability of a T.38 fax
session at TCP port 54111, but is also willing to set up the media
stream by initiating the TCP connection:
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c=IN IP4 10.1.1.2
m=image 54111 TCP t38
a=direction:both
The endpoint at 10.1.1.1 has three choices:
1) It can respond with either of the two direction:active
descriptions listed in the previous example. In this case the
endpoint at 10.1.1.1 must initiate a connection to port 54111
at 10.1.1.2.
2) It can respond with a description similar to the following:
c=IN IP4 10.1.1.1
m=image 54321 TCP t38
a=direction:passive
In this case the endpoint at 10.1.1.2 must initiate a
connection to port 54321 at 10.1.1.1.
3) It can respond with a description that specifies
direction:both, which is covered in the next example.
7.4 Example: redundant both
An endpoint at 10.1.1.2 uses the same description as the previous
example:
c=IN IP4 10.1.1.2
m=image 54111 TCP t38
a=direction:both
Unlike the previous example, the endpoint at 10.1.1.1 responds with
the following description:
c=IN IP4 10.1.1.1
m=image 54321 TCP t38
a=direction:both
This will cause the endpoint at 10.1.1.2 to initiate a connection to
port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a
connection to port 54111 at 10.1.1.2. Whichever TCP connection
succeeds will be used. If both succeed, one of the connections may
be closed as an optimization, using the rules in section 3.3.
In order to minimize the chance that two connections are created,
the endpoint at 10.1.1.1 may opt to use the recommendation in
section 3.4, which would result in events occurring in the following
sequence:
1) The endpoint at 10.1.1.2 sends SDP as listed above. The
endpoint MUST enable a listener on port 54111 at this time,
but is not able to initiate a TCP connection due to the fact
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that it does not have sufficient information from the endpoint
at 10.1.1.1.
2) The endpoint at 10.1.1.1, upon receiving the SDP, immediately
initiates a TCP connection to 10.1.1.2:54111.
3) In order to minimize the chance of a duplicate connection, the
endpoint at 10.1.1.1 pauses for a short time to allow the
endpoint at 10.1.1.2 to receive the TCP connection initiation.
4) After the short pause, the endpoint at 10.1.1.1 sends the SDP
response as listed above.
The pause in #3 gives the first TCP connection attempt a chance to
succeed, since withholding the SDP response deprives the endpoint at
10.1.1.2 of the information it needs to attempt its own TCP
connection.
7.5 Example: "Bidirectional" RTP and RTCP
An endpoint at 10.1.1.2 is behind a NAT and does not know its own
public address.
c=IN IP4 10.1.1.2
m=audio 9 RTP/AVP 0
a=direction:active
A peer with a public IP address responds as follows and waits to
receive RTP and RTCP packets from its active peer.
c=IN IP4 1.2.3.4
m=audio 18240 RTP/AVP 0
a=direction:passive
The endpoint at 10.1.1.2 immediately sends RTP from port 9012 to
1.2.3.4 port 18240. A NAT translates the source address to 5.6.7.8
port 1542. The passive endpoint receives this RTP packet and stores
this source address. When the passive endpoint wants to send RTP
media it sends it back to 5.6.7.8 port 1542. The NAT translates this
destination address back to 10.1.1.2 port 9012 and delivers it to
the active endpoint.
Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port
9013 to 1.2.3.4:18241. The NAT translates this to 5.6.7.8:1984. The
passive endpoint receives the RTCP packet and stores the source
address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which
is translated back to 10.1.1.2:9013 and delivered to the active
endpoint.
8 Security Considerations
See [SDP] for security and other considerations specific to the
Session Description Protocol in general.
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9 IANA Considerations
As recommended by [SDP] Appendix B, the direction and reconnect
attributes described in this document should be registered with
IANA, as should the "TCP" and "TLS" protocol identifiers.
Acknowledgements
The author would like to thank Jonathan Rosenberg, Rohan Mahy,
Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie-
Cuninghame for their valuable insights and contributions.
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Appendix A: Direction Attribute Syntax
This appendix provides an Augmented BNF [ABNF] grammar for
expressing the direction attribute for connection setup. It is
intended as an extension to the grammar for the Session Description
Protocol, as defined in [SDP]. Specifically, it describes the
syntax for the new "connection-setup" attribute field, which MAY be
either a session-level or media-level attribute.
connection-setup = "direction" ":" direction-spec
direction-spec = "both" / "active" / "passive"
reconnect-attribute = "reconnect"
References
[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF," RFC 2234, November 1997
[SDP] M. Handley, V. Jacobson, "SDP: Session Description
Protocol," RFC 2327, April 1998
[T38] International Telecommunication Union, "Procedures for
Real-Time Group 3 Facsimile Communications over IP
Networks," Recommendation T.38, June 1998
[TLS] T. Dierks, C. Allen, "The TLS Protocol," RFC 2246,
January 1999
[UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode
and ISO 10646," RFC 2044, October 1996
Author's Address
David Yon
Dialout.Net, Inc.
One Indian Head Plaza
Nashua, NH 03060
Phone: (603) 324-4100
EMail: yon@dialout.net
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