draft-ietf-mmusic-sdp-comedia-09.txt   draft-ietf-mmusic-sdp-comedia-10.txt 
MMUSIC Working Group D. Yon MMUSIC Working Group D. Yon
Internet-Draft Tactical Software, LLC Internet-Draft Tactical Software, LLC
Expires: March 30, 2005 G. Camarillo Expires: May 27, 2005 G. Camarillo
Ericsson Ericsson
September 29, 2004 November 26, 2004
Connection-Oriented Media Transport in the Session Description TCP-Based Media Transport in the Session Description Protocol (SDP)
Protocol (SDP) draft-ietf-mmusic-sdp-comedia-10.txt
draft-ietf-mmusic-sdp-comedia-09.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with which he or she become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
skipping to change at page 1, line 38 skipping to change at page 1, line 36
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 30, 2005. This Internet-Draft will expire on May 27, 2005.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2004).
Abstract Abstract
This document describes how to express media transport over This document describes how to express media transport over TCP using
connection-oriented protocols using the Session Description Protocol the Session Description Protocol (SDP). It defines the SDP 'TCP'
(SDP). It defines the SDP TCP protocol identifier, the SDP setup protocol identifier, the SDP 'setup' attribute, which describes the
attribute, which describes the connection setup procedure, and the connection setup procedure, and the SDP 'connection' attribute, which
SDP connection attribute, which handles connection reestablishment. handles connection reestablishment.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3
4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4 4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4 4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4
5. The Connection Attribute . . . . . . . . . . . . . . . . . . . 5 5. The Connection Attribute . . . . . . . . . . . . . . . . . . . 5
5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6 5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7 5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7
6. Connection Management . . . . . . . . . . . . . . . . . . . . 7 6. Connection Management . . . . . . . . . . . . . . . . . . . . 8
6.1 Connection Establishment . . . . . . . . . . . . . . . . . 7 6.1 Connection Establishment . . . . . . . . . . . . . . . . . 8
6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8 6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8
6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8 6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 9
7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 9 7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 10
7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10 7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 8. Other Connection-Oriented Transport Protocols . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 12 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.2 Informative References . . . . . . . . . . . . . . . . . . . 12 12.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13 12.2 Informative References . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
1. Introduction 1. Introduction
The Session Description Protocol [4] provides a general-purpose The Session Description Protocol [4] provides a general-purpose
format for describing multimedia sessions in announcements or format for describing multimedia sessions in announcements or
invitations. SDP uses an entirely textual data format (the US-ASCII invitations. SDP uses an entirely textual data format (the US-ASCII
subset of UTF-8 [11]) to maximize portability among transports. SDP subset of UTF-8 [11]) to maximize portability among transports. SDP
does not define a protocol, but only the syntax to describe a does not define a protocol, but only the syntax to describe a
multimedia session with sufficient information to participate in that multimedia session with sufficient information to participate in that
session. Session descriptions may be sent using arbitrary existing session. Session descriptions may be sent using arbitrary existing
application protocols for transport (e.g., SAP [9], SIP [10], RTSP application protocols for transport (e.g., SAP [9], SIP [10], RTSP
[6], email, HTTP [8], etc.). [6], email, HTTP [8], etc.).
SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of
which represent unreliable connectionless protocols. While these which represent unreliable connectionless protocols. While these
transports are appropriate choices for multimedia streams, there are transports are appropriate choices for multimedia streams, there are
applications for which connection-oriented transports, such as TCP, applications for which TCP is more appropriate. This document
are more appropriate. This document defines a new protocol defines a new protocol identifier, 'TCP', to describe TCP connetions
identifier, TCP, to describe TCP connetions in SDP. in SDP.
Connection-oriented protocols introduce two new factor when TCP introduces two new factors when describing a session: how and
describing a session: how and when should endpoints perform the when should endpoints perform the TCP connection setup procedure.
connection setup procedure. This document defines two new attributes This document defines two new attributes to describe TCP connection
to describe connection setups: setup and connection. setups: 'setup' and 'connection'.
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [3] and indicate requirement levels for described in BCP 14, RFC 2119 [3] and indicate requirement levels for
compliant implementations. compliant implementations.
3. Protocol Identifier 3. Protocol Identifier
The following is the ABNF for an m= line, as specified by RFC 2327 The following is the ABNF for an 'm' line, as specified by RFC 2327
[4]. [4].
media-field = "m=" media space port ["/" integer] media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF space proto 1*(space fmt) CRLF
This document defines a new value for the proto field: TCP. This document defines a new value for the proto field: '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
qualify the application-layer protocol using an fmt identifier.
Media described using an m= lines containing the TCP protocol
identifier are carried using TCP [1].
It is RECOMMENDED that documents defining new SDP protocol
identifiers that involve extra protocol layers between TCP and the
media itself (e.g., TLS [7] over TCP) start with the string "TCP/"
(e.g., TCP/TLS).
The following sections define the setup and the connection The 'TCP' protocol identifier is similar to the 'UDP' protocol
attributes. While both attributes are applicable to m= lines that identifier in that it only describes the transport protocol, and not
use the TCP protocol identifier, they are not limited to them. These the upper-layer protocol. An 'm' line that specifies 'TCP' MUST
attributes MAY be used in conjunction with any m= line which uses a further qualify the application-layer protocol using an fmt
connection- oriented transport protocol, even if the protocol identifier. Media described using an 'm' line containing the 'TCP'
identifier of the m= line is not TCP. protocol identifier are carried using TCP [1].
4. Setup Attribute 4. Setup Attribute
The setup attribute indicates which of the end points should initiate The 'setup' attribute indicates which of the end points should
the connection establishment (e.g., send the initial TCP SYN). The initiate the TCP connection establishment (i.e., send the initial TCP
setup attribute is charset-independent and can be a session-level or SYN). The 'setup' attribute is charset-independent and can be a
a media-level attribute. The following is the ABNF of the setup session-level or a media-level attribute. The following is the ABNF
attribute: of the 'setup' attribute:
setup-attr = "a=setup:" role setup-attr = "a=setup:" role
role = "active" / "passive" / "actpass" role = "active" / "passive" / "actpass"
/ "holdconn" / "holdconn"
Active: The endpoint will initiate an outgoing connection. 'active': The endpoint will initiate an outgoing connection.
Passive: The endpoint will accept an incoming connection. 'passive': The endpoint will accept an incoming connection.
ActPass: The endpoint is willing to accept an incoming connection 'actpass': The endpoint is willing to accept an incoming
or to initiate an outgoing connection. connection or to initiate an outgoing connection.
Holdconn: The endpoint does not want the connection to be 'holdconn': The endpoint does not want the connection to be
established for the time being. established for the time being.
4.1 The Setup Attribute in the Offer/answer Model 4.1 The Setup Attribute in the Offer/answer Model
The offer/answer model, defined in RFC 3264 [5], provides endpoints The offer/answer model, defined in RFC 3264 [5], provides endpoints
with a means to obtain shared view of a session. Some session with a means to obtain shared view of a session. Some session
parameters are negotiated (e.g., codecs to use), while others are parameters are negotiated (e.g., codecs to use), while others are
simply communicated from one endpoint to the other (e.g., IP simply communicated from one endpoint to the other (e.g., IP
addresses). The value of the setup attribute falls into the first addresses). The value of the 'setup' attribute falls into the first
category. That is, both endpoints negotiate its value using the category. That is, both endpoints negotiate its value using the
offer/answer model. offer/answer model.
The negotiation of the value of the setup attribute takes places as The negotiation of the value of the 'setup' attribute takes places as
follows. The offerer states which role or roles it is willing to follows. The offerer states which role or roles it is willing to
perform and the answerer, taking the offerer's willingness into perform and the answerer, taking the offerer's willingness into
consideration, chooses which roles both endpoints will actually consideration, chooses which roles both endpoints will actually
perform during connection establishment. The following are the perform during connection establishment. The following are the
values that the setup attribute can take in an offer/answer exchange: values that the 'setup' attribute can take in an offer/answer
exchange:
Offer Answer Offer Answer
________________ ________________
active passive / holdconn active passive / holdconn
passive active / holdconn passive active / holdconn
actpass active / passive / holdconn actpass active / passive / holdconn
holdconn holdconn holdconn holdconn
The active endpoint SHOULD initiate a connection to the port number The active endpoint SHOULD initiate a connection to the port number
on the m= line of the other endpoint. The port number on its own m= on the 'm' line of the other endpoint. The port number on its own
line is irrelevant, and the opposite endpoint MUST NOT attempt to 'm' line is irrelevant, and the opposite endpoint MUST NOT attempt to
initiate a connection to the port number specified there. initiate a connection to the port number specified there.
Nevertheless, since the m= line must contain a valid port number, the Nevertheless, since the 'm' line must contain a valid port number,
endpoint specifying using the value active SHOULD specify a port 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 number of 9 (the discard port) on its 'm' line. The endpoint MUST
specify a port number of zero, except to denote an m= line that has NOT specify a port number of zero, except to denote an 'm' line that
been or is being refused. has been or is being refused.
The passive endpoint SHOULD be ready to accept a connection on the The passive endpoint SHOULD be ready to accept a connection on the
port number specified in the m= line. port number specified in the 'm' line.
A value of actpass indicates that the offerer can either initiate a A value of 'actpass' indicates that the offerer can either initiate a
connection to the port number on the m= line in the answer or accept connection to the port number on the 'm' line in the answer or accept
a connection on the port number specified in the m= line in the a connection on the port number specified in the 'm' line in the
offer. That is, the offerer has no preference as to whether it offer. That is, the offerer has no preference as to whether it
accepts or initiates the connection and, so, is letting the answerer accepts or initiates the connection and, so, is letting the answerer
choose. choose.
A value of holdconn indicates that the connection should not be A value of 'holdconn' indicates that the connection should not be
established for the time being. established for the time being.
The default value of the setup attribute in an offer/answer exchange The default value of the setup attribute in an offer/answer exchange
is active in the offer and passive in the answer. is 'active' in the offer and 'passive' in the answer.
5. The Connection Attribute 5. The Connection Attribute
The preceding description of the setup attribute has been in the The preceding description of the 'setup' attribute is placed in the
context of using SDP to initiate a session. Still, SDP may be context of using SDP to initiate a session. Still, SDP may be
exchanged between endpoints at various stages of a session to exchanged between endpoints at various stages of a session to
accomplish tasks such as terminating a session, redirecting media to accomplish tasks such as terminating a session, redirecting media to
a new endpoint, or renegotiating the media parameters for a session. a new endpoint, or renegotiating the media parameters for a session.
After the initial session has been established, it may be ambiguous After the initial session has been established, it may be ambiguous
as to whether subsequent SDP exchange represents a confirmation that as to whether subsequent SDP exchange represents a confirmation that
the endpoint is to continue using the current media connection the endpoint is to continue using the current TCP connection
unchanged, or is a request to make a new media connection. The unchanged, or is a request to make a new TCP connection. The
media-level connection attribute, which is charset-independent, is media-level 'connection' attribute, which is charset-independent, is
used to disambiguate these two scenarios. The following is the ABNF used to disambiguate these two scenarios. The following is the ABNF
of the connection attribute: of the connection attribute:
connection-attr = "a=connection:" conn-value connection-attr = "a=connection:" conn-value
conn-value = "new" / "existing" conn-value = "new" / "existing"
5.1 Offerer Behaviour 5.1 Offerer Behaviour
Offerers and answerers use the connection attribute to decide whether Offerers and answerers use the 'connection' attribute to decide
a new transport connection needs to be established or, on the other whether a new transport connection needs to be established or, on the
hand, the existing transport connection should still be used. The other hand, the existing TCP connection should still be used. When
connection value resulting from an offer/answer exchange is the an offerer generates an 'm' line which uses TCP, it SHOULD provide a
connection value in the answer. If the connection value in the connection attribute for the 'm' line unless the application using
answer is "new", the end-points SHOULD establish a new connection. the 'm' line has other means to deal with connection reestablishment.
If the connection value in the answer is "existing", the end-points
SHOULD continue using the exiting connection.
When an offerer generates an m= line which uses a connection-oriented
transport, it SHOULD provide a connection attribute for the m= line
unless the application using the m= line has other means to deal with
connection reestablishment. The connection attribute in an initial
offer (i.e., no transport connection has been established yet) takes
the value of "new".
After the initial offer/answer exchange, any of the endpoints can After the initial offer/answer exchange, any of the endpoints can
generate a new offer to change some characteristics of the session generate a new offer to change some characteristics of the session
(e.g., the direction attribute). If such an offerer wants to (e.g., the direction attribute). If such an offerer wants to
continue using the previously-established transport-layer connection continue using the previously-established transport-layer connection
for the m= line, the offerer MUST use use a connection value of for the 'm' line, the offerer MUST use a connection value of
"existing" for the m= line. If, on the other hand, the offerer wants 'existing' for the 'm' line. If, on the other hand, the offerer
to establish a new transport-layer connection for the m= line, it wants to establish a new transport-layer connection for the 'm' line,
MUST use a connection value of "new". it MUST use a connection value of 'new'.
Note that, according to the rules in this section, an offer that Note that, according to the rules in this section, an offer that
changes the transport address (IP address or port number) of an m= changes the transport address (IP address or port number) of an
line will have a connection value of "new". 'm' line will have a connection value of 'new'. The same way, the
'connection' attribute in an initial offer (i.e., no transport
connection has been established yet) takes the value of 'new'.
The default value of the connection attribute in an offer/answer The 'connection' value resulting from an offer/answer exchange is the
exchange is "new". 'connection' value in the answer. If the 'connection' value in the
answer is 'new', the end-points SHOULD establish a new connection.
If the connection value in the answer is 'existing', the end-points
SHOULD continue using the exiting connection.
Taking into consideration the rules in Section 5.2, the following are
the values that the 'connetion' attribute can take in an offer/answer
exchange:
Offer Answer
________________
new new
existing existing / new
If the connection value resulting from an offer/answer exchange is
'existing', the end-points continue using the existing connection.
Consequently, the port numbers, IP addresses, and 'setup' attributes
negotiated in the offer/answer exchange are ignored because there is
no need to establish a new connection.
The previous rule implies that an offerer generating an offer with a
connection value of 'existing' and a setup value of 'passive' needs
to be ready (i.e., needs to allocate resources) to receive a
connection request from the answerer just in case the answerer
chooses a connection value of 'new' for the answer. However, if the
answerer uses a connection value of 'existing' in the answer, the
offerer would need to deallocate the previously allocated resources
which were never used because no connection request was received.
To avoid allocating resources unnecessary, offerers using a
connection value of 'existing' in their offers may choose to use a
setup value of 'holdconn'. Nevertheless, offerers using this
strategy should be aware that in the case the answerer chooses a
connection value of 'new', a new offer/answer exchange (typically
initiated by the previous offerer) with setup value different than
'holdconn' will be needed to establish the new connection. This may,
of course, cause delays in the application using the TCP connection.
The default value of the connection attribute in both offers and
answers is 'new'.
5.2 Answerer Behaviour 5.2 Answerer Behaviour
The connection value for an m= line is negotiated using the offer/ The connection value for an 'm' line is negotiated using the offer/
answer model. The resulting connection value after an offer/answer answer model. The resulting connection value after an offer/answer
exchange is the connection value in the answer. If the connection exchange is the connection value in the answer. If the connection
value in the offer is "new", the answerer MUST also use a value of value in the offer is 'new', the answerer MUST also use a value of
"new" in the answer. If the connection value in the offer is 'new' in the answer. If the connection value in the offer is
"existing", the answerer uses a value of "existing" in the answer if 'existing', the answerer uses a value of 'existing' in the answer if
it wishes to continue using the existing connection and a value of it wishes to continue using the existing connection and a value of
"new" if it wants a new connection to be established. 'new' if it wants a new connection to be established.
In some scenarios where third party call control [12] is used, an In some scenarios where third party call control [12] is used, an
endpoint may receive an initial offer with a connection value of endpoint may receive an initial offer with a connection value of
"existing". Following the previous rules, such an answerer would 'existing'. Following the previous rules, such an answerer would
use a connection value of "new" in the answer. use a connection value of 'new' in the answer.
If the connection value for an m= line resulting from an offer/answer If the connection value for an 'm' line resulting from an offer/
exchange is "new", the endpoints SHOULD establish a new answer exchange is 'new', the endpoints SHOULD establish a new TCP
transport-layer connection as indicated by the setup attribute. If a connection as indicated by the 'setup' attribute. If a previous TCP
previous connection is still up, the endpoints SHOULD close it as connection is still up, the endpoints SHOULD close it as soon as the
soon as the offer/answer exchange is completed. It is up to the offer/answer exchange is completed. It is up to the application to
application to ensure proper data synchornization between the two ensure proper data synchornization between the two TCP connections.
connections.
If the connection value for an m= line resulting from an offer/answer If the connection value for an 'm' line resulting from an offer/
exchange is "existing", the endpoints SHOULD continue using the answer exchange is 'existing', the endpoints SHOULD continue using
existing connection. the existing TCP connection.
6. Connection Management 6. Connection Management
This section addresses connection establishment, connection This section addresses connection establishment, connection
reestablishment, and connection termination. reestablishment, and connection termination.
6.1 Connection Establishment 6.1 Connection Establishment
An endpoint that according to an offer/answer exchange is supposed to An endpoint that according to an offer/answer exchange is supposed to
initiate a new connection SHOULD initiate it as soon as it is able initiate a new TCP connection SHOULD initiate it as soon as it is
to, even if the endpoint does not intend to immediately begin sending able to, even if the endpoint does not intend to immediately begin
media to the remote endpoint. This allows media to flow from the sending media to the remote endpoint. This allows media to flow from
remote endpoint if needed. the remote endpoint if needed.
Note that some endpoints need to wait for some event to happen Note that some endpoints need to wait for some event to happen
before being able to establish the connection. For example, a before being able to establish the connection. For example, a
wireless terminal may need to set up a radio bearer before being wireless terminal may need to set up a radio bearer before being
able to initiate a connection. able to initiate a TCP connection.
6.2 Connection Reestablishment 6.2 Connection Reestablishment
If an endpoint determines that the transport-connection for an m= If an endpoint determines that the TCP for an 'm' line has been
line has been closed and it should be reestablished, it SHOULD closed and it should be reestablished, it SHOULD perform a new offer/
perform a new offer/answer exchange using a connection value of "new" answer exchange using a connection value of 'new' for this 'm' line.
for this m= line.
Note that the SDP direction attribute (e.g., a=sendonly) deals Note that the SDP direction attribute (e.g., 'a=sendonly') deals
with the media sent over the transport-connection, but has no with the media sent over the TCP connection, but has no impact on
impact on the transport-connection itself. the TCP connection itself.
6.3 Connection Termination 6.3 Connection Termination
Typically, endpoints do not close the connection until the session Typically, endpoints do not close the TCP connection until the
has expired, been explicitly terminated, or a new connection value session has expired, been explicitly terminated, or a new connection
has been provided for the m= line. Additionaly, specific value has been provided for the 'm' line. Additionaly, specific
applications can describe further scenarios where an end-point may applications can describe further scenarios where an end-point may
close a given connection. As soon as an end-point notices that it close a given TCP connection (e.g., whenever a connection is in the
needs to terminate a connection, it SHOULD do so. half-close state). As soon as an end-point notices that it needs to
terminate a TCP connection, it SHOULD do so.
While in TCP both end-points need to close a connection, other
connection-oriented transport protocols may not have the concept of
half-close connections. In this case, a connection would be
terminated as soon as one of the end-points closed it, making it
unnecessary for the other end-point to perform any further action to
terminate the connection.
In any case, individual applications may provide further In any case, individual applications may provide further
considerations on how to achieve a graceful connection termination. considerations on how to achieve a graceful connection termination.
For example, a file application using TCP receiving a FIN from the For example, a file application using TCP receiving a FIN from the
remote endpoint may need to finish the ongoing transmission of a file remote endpoint may need to finish the ongoing transmission of a file
before sending its own FIN. before sending its own FIN.
7. Examples 7. Examples
The following examples show the most common usage of the setup The following examples show the most common usage of the 'setup'
attribute combined with TCP-based media descriptions. For the attribute combined with TCP-based media descriptions. For the
purpose of brevity, the main portion of the session description is purpose of brevity, the main portion of the session description is
omitted in the examples, which only show m= lines and their omitted in the examples, which only show 'm' lines and their
attributes (including c= lines). attributes (including 'c' lines).
7.1 Passive/Active 7.1 Passive/Active
An offerer at 192.0.2.2 signals its availability for a T.38 fax An offerer at 192.0.2.2 signals its availability for a T.38 fax
session at port 54111: session at port 54111:
m=image 54111 TCP t38 m=image 54111 TCP t38
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
a=setup:passive a=setup:passive
a=connection:new a=connection:new
skipping to change at page 10, line 18 skipping to change at page 10, line 28
The endpoint at 192.0.2.2 also wishes to use the existing connection The endpoint at 192.0.2.2 also wishes to use the existing connection
and responds with the following description: and responds with the following description:
m=image 9 TCP t38 m=image 9 TCP t38
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
a=setup:active a=setup:active
a=connection:existing a=connection:existing
The existing connection from 192.0.2.2 to 192.0.2.1 will be reused. The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.
Note that the endpoint at 192.0.2.2 uses setup:active in response Note that the endpoint at 192.0.2.2 uses 'setup:active' in
to the offer of setup:passive, and uses port 9 because it is response to the offer of 'setup:passive', and uses port 9 because
active. it is active.
7.4 Existing Connection Refusal 7.4 Existing Connection Refusal
Subsequent to the exchange in Section 7.3, another offer/answer Subsequent to the exchange in Section 7.3, another offer/answer
exchange is initiated by the endpoint at 192.0.2.2, again wishing to exchange is initiated by the endpoint at 192.0.2.2, again wishing to
reuse the existing connection: reuse the existing connection:
m=image 54111 TCP t38 m=image 54111 TCP t38
c=IN IP4 192.0.2.2 c=IN IP4 192.0.2.2
a=setup:actpass a=setup:passive
a=connection:existing a=connection:existing
However, this time the answerer is unaware of the old connection and However, this time the answerer is unaware of the old connection and
so wishes to establish a new one. (This could be the result of a so wishes to establish a new one. (This could be the result of a
transfer via 3pcc.) It is unable to act in the passive mode so transfer via third-party call control.) It is unable to act in the
responds as active: 'passive' mode so responds as 'active':
m=image 9 TCP t38 m=image 9 TCP t38
c=IN IP4 192.0.2.3 c=IN IP4 192.0.2.3
a=setup:active a=setup:active
a=connection:new a=connection:new
The endpoint at 192.0.2.3 then initiates the TCP connection to port The endpoint at 192.0.2.3 then initiates the TCP connection to port
54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old 54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old
connection. connection.
Note that the endpoint at 192.0.2.2, while specifying connection: Note that the endpoint at 192.0.2.2, while using a connection
existing has reverted to setup:actpass and its real port number, value of 'existing' has used a setup value of 'passive'. Had it
rather than repeating setup:active and port 9 from the previous not done this and used a setup value of 'holdconn' instead
cycle. Had it not done this, this negotiation would have failed. (probably to avoid allocating resources as described in Section
5.1), a new offer/answer exchange would have been needed in order
to establish the new connection.
8. Security Considerations 8. Other Connection-Oriented Transport Protocols
This document specifies how to describe TCP-based media streams using
SDP. Still, some of the attributes defined here could possibly be
used to describe media streams based on other connection-oriented
transport protocols as well. This section provides advice to authors
of specifications of SDP extensions which deal with
connetion-oriented transport protocols other than TCP.
It is recommended that documents defining new SDP protocol
identifiers that involve extra protocol layers between TCP and the
media itself (e.g., TLS [7] over TCP) start with the string 'TCP/'
(e.g., 'TCP/TLS').
The 'setup' and the 'connection' attributes are specified in Section
4 and Section 5 respectively. While both attributes are applicable
to 'm' lines that use the 'TCP' protocol identifier, they are general
enough to be reused in 'm' lines with other connection-oriented
transport protocols. Therefore, it is recommended that the 'setup'
and 'connection' attributes are reused, as long as it is possible,
for new proto values associated with connection-oriented transport
protocols.
Section 6 deals with TCP connection management. It should be noted
that while in TCP both end-points need to close a connection, other
connection-oriented transport protocols may not have the concept of
half-close connections. In such a case, a connection would be
terminated as soon as one of the end-points closed it, making it
unnecessary for the other end-point to perform any further action to
terminate the connection. So, specifications dealing with such
transport protocols may need to specify slightly different procedures
regarding connection termination.
9. Security Considerations
See RFC 2327 [4] for security and other considerations specific to See RFC 2327 [4] for security and other considerations specific to
the Session Description Protocol in general. the Session Description Protocol in general.
An attacker may attempt to modify the values of the connection and An attacker may attempt to modify the values of the connection and
setup attributes to have endpoints reestablish connections setup attributes to have endpoints reestablish connections
unnecesaryly or to keep them from establishing a connection. So, it unnecesaryly or to keep them from establishing a connection. So, it
is STRONGLY RECOMMENDED that integrity protection be applied to the is strongly RECOMMENDED that integrity protection be applied to the
SDP session descriptions. For session descriptions carried in SIP SDP session descriptions. For session descriptions carried in SIP
[10], S/MIME is the natural choice to provide such end-to-end [10], S/MIME is the natural choice to provide such end-to-end
integrity protection, as described in RFC 3261 [10]. Other integrity protection, as described in RFC 3261 [10]. Other
applications MAY use a different form of integrity protection. applications MAY use a different form of integrity protection.
9. IANA Considerations 10. IANA Considerations
This document defines two session and media level SDP attributes: This document defines two session and media level SDP attributes:
setup and connection. Their formats are defined in Section 4 and setup and connection. Their formats are defined in Section 4 and
Section 5 respectively. These two attributes should be registered by Section 5 respectively. These two attributes should be registered by
the IANA on the IANA under "Session Description Protocol (SDP) Parameters" under
"att-field (both session and media level)".
http://www.iana.org/assignments/sdp-parameters
under "att-field (both session and media level)".
This document defines a proto value: TCP. Its format is defined in This document defines a proto value: TCP. Its format is defined in
Section 3. This proto value should be registered by the IANA on Section 3. This proto value should be registered by the IANA under
"Session Description Protocol (SDP) Parameters" under "proto".
http://www.iana.org/assignments/sdp-parameters
under "proto".
Specifications defining new proto values, like this one, must define The SDP specification, RFC2327, states that specifications defining
the rules by which their media format (fmt) namespace is managed. new proto values, like the TCP proto value defined in this RFC, must
For the TCP protocol, new formats SHOULD have an associated MIME define the rules by which their media format (fmt) namespace is
registration. Use of an existing MIME subtype for the format is managed. For the TCP protocol, new formats SHOULD have an associated
MIME registration. Use of an existing MIME subtype for the format is
encouraged. If no MIME subtype exists, it is RECOMMENDED that a encouraged. If no MIME subtype exists, it is RECOMMENDED that a
suitable one is registered through the IETF process [2] by production suitable one is registered through the IETF process [2] by production
of, or reference to, a standards-track RFC that defines the transport of, or reference to, a standards-track RFC that defines the transport
protocol for the format. protocol for the format.
10. Acknowledgements 11. Acknowledgements
Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
Holmberg provided valuable insights and contributions. Holmberg provided valuable insights and contributions.
11. References 12. References
11.1 Normative References 12.1 Normative References
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
[2] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet [2] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996. 13, RFC 2048, November 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[4] Handley, M. and V. Jacobson, "SDP: Session Description [4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998. Protocol", RFC 2327, April 1998.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002. Session Description Protocol (SDP)", RFC 3264, June 2002.
11.2 Informative References 12.2 Informative References
[6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming [6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998. Protocol (RTSP)", RFC 2326, April 1998.
[7] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC [7] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999. 2246, January 1999.
[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., [8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999. HTTP/1.1", RFC 2616, June 1999.
 End of changes. 

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