draft-ietf-mmusic-sdp-comedia-10.txt   rfc4145.txt 
MMUSIC Working Group D. Yon
Internet-Draft Tactical Software, LLC Network Working Group D. Yon
Expires: May 27, 2005 G. Camarillo Request for Comments: 4145 Tactical Software, LLC
Category: Standards Track G. Camarillo
Ericsson Ericsson
November 26, 2004 September 2005
TCP-Based Media Transport in the Session Description Protocol (SDP) TCP-Based Media Transport in the Session Description Protocol (SDP)
draft-ietf-mmusic-sdp-comedia-10.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is subject to all provisions This document specifies an Internet standards track protocol for the
of section 3 of RFC 3667. By submitting this Internet-Draft, each Internet community, and requests discussion and suggestions for
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which he or she is aware have been or will be disclosed, and any of Official Protocol Standards" (STD 1) for the standardization state
which he or she become aware will be disclosed, in accordance with and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). Copyright (C) The Internet Society (2005).
Abstract Abstract
This document describes how to express media transport over TCP using This document describes how to express media transport over TCP using
the Session Description Protocol (SDP). It defines the SDP 'TCP' the Session Description Protocol (SDP). It defines the SDP 'TCP'
protocol identifier, the SDP 'setup' attribute, which describes the protocol identifier, the SDP 'setup' attribute, which describes the
connection setup procedure, and the SDP 'connection' attribute, which connection setup procedure, and the 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 . . . . . . . . . . . . . . . . . . . . 8 6. Connection Management . . . . . . . . . . . . . . . . . . . . 8
6.1 Connection Establishment . . . . . . . . . . . . . . . . . 8 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Passive/Active . . . . . . . . . . . . . . . . . . . . . 9
7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 7.2. Actpass/Passive. . . . . . . . . . . . . . . . . . . . . 9
7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 10 7.3. Existing Connection Reuse. . . . . . . . . . . . . . . . 10
7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10 7.4. Existing Connection Refusal. . . . . . . . . . . . . . . 10
8. Other Connection-Oriented Transport Protocols . . . . . . . . 11 8. Other Connection-Oriented Transport Protocols. . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1 Normative References . . . . . . . . . . . . . . . . . . . . 12 12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2 Informative References . . . . . . . . . . . . . . . . . . . 13 12.2. Informative References . . . . . . . . . . . . . . . . . 13
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; it defines 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 TCP is more appropriate. This document applications for which TCP is more appropriate. This document
defines a new protocol identifier, 'TCP', to describe TCP connetions defines a new protocol identifier, 'TCP', to describe TCP connections
in SDP. in SDP.
TCP introduces two new factors when describing a session: how and TCP introduces two new factors when describing a session: how and
when should endpoints perform the TCP connection setup procedure. when should endpoints perform the TCP connection setup procedure.
This document defines two new attributes to describe TCP connection This document defines two new attributes to describe TCP 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 they indicate requirement
compliant implementations. levels for 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'.
skipping to change at page 4, line 28 skipping to change at page 4, line 30
'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 'actpass': The endpoint is willing to accept an incoming
connection 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 values that the 'setup' attribute can take in an offer/answer
exchange: 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 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 '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, Nevertheless, since the 'm' line must contain a valid port number,
the endpoint specifying using the value active SHOULD specify a port the endpoint using the value 'active' SHOULD specify a port number of
number of 9 (the discard port) on its 'm' line. The endpoint MUST 9 (the discard port) on its 'm' line. The endpoint MUST NOT specify
NOT specify a port number of zero, except to denote an 'm' line that a port number of zero, except to denote an 'm' line that has been or
has been or is being refused. 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
a connection on the port number specified in the 'm' line in the accept a connection on the port number specified in the 'm' line in
offer. That is, the offerer has no preference as to whether it the 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 is placed 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 whether a subsequent SDP exchange represents a confirmation that the
the endpoint is to continue using the current TCP connection endpoint is to continue using the current TCP connection unchanged,
unchanged, or is a request to make a new TCP connection. The or is a request to make a new TCP connection. The media-level
media-level 'connection' attribute, which is charset-independent, is 'connection' attribute, which is charset-independent, is used to
used to disambiguate these two scenarios. The following is the ABNF disambiguate these two scenarios. The following is the ABNF of the
of the connection attribute: 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 Offerers and answerers use the 'connection' attribute to decide
whether a new transport connection needs to be established or, on the whether a new transport connection needs to be established or, on the
other hand, the existing TCP connection should still be used. When other hand, the existing TCP connection should still be used. When
an offerer generates an 'm' line which uses TCP, it SHOULD provide a an offerer generates an 'm' line that uses TCP, it SHOULD provide a
connection attribute for the 'm' line unless the application using connection attribute for the 'm' line unless the application using
the 'm' line has other means to deal with connection reestablishment. the 'm' line has other means to deal with connection reestablishment.
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 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 'existing' for the 'm' line. If, on the other hand, the offerer
wants to establish a new transport-layer connection for the 'm' line, wants to establish a new transport-layer connection for the 'm' line,
it 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 changes the transport address (IP address or port number) of an
'm' line will have a connection value of 'new'. The same way, the 'm' line will have a connection value of 'new'. Similarly, the
'connection' attribute in an initial offer (i.e., no transport 'connection' attribute in an initial offer (i.e., no transport
connection has been established yet) takes the value of 'new'. connection has been established yet) takes the value of 'new'.
The 'connection' value resulting from an offer/answer exchange is the The 'connection' value resulting from an offer/answer exchange is the
'connection' value in the answer. If the 'connection' value in the 'connection' value in the answer. If the 'connection' value in the
answer is 'new', the end-points SHOULD establish a new connection. answer is 'new', the end-points SHOULD establish a new connection.
If the connection value in the answer is 'existing', the end-points If the connection value in the answer is 'existing', the end-points
SHOULD continue using the exiting connection. SHOULD continue using the exiting connection.
Taking into consideration the rules in Section 5.2, the following are Taking into consideration the rules in Section 5.2, the following are
the values that the 'connetion' attribute can take in an offer/answer the values that the 'connection' attribute can take in an
exchange: offer/answer exchange:
Offer Answer Offer Answer
________________ ________________
new new new new
existing existing / new existing existing / new
If the connection value resulting from an offer/answer exchange is If the connection value resulting from an offer/answer exchange is
'existing', the end-points continue using the existing connection. 'existing', the end-points continue using the existing connection.
Consequently, the port numbers, IP addresses, and 'setup' attributes Consequently, the port numbers, IP addresses, and 'setup' attributes
negotiated in the offer/answer exchange are ignored because there is negotiated in the offer/answer exchange are ignored because there is
no need to establish a new connection. no need to establish a new connection.
The previous rule implies that an offerer generating an offer with a The previous rule implies that an offerer generating an offer with a
connection value of 'existing' and a setup value of 'passive' needs connection value of 'existing' and a setup value of 'passive' needs
to be ready (i.e., needs to allocate resources) to receive a to be ready (i.e., needs to allocate resources) to receive a
connection request from the answerer just in case the answerer connection request from the answerer just in case the answerer
chooses a connection value of 'new' for the answer. However, if the chooses a connection value of 'new' for the answer. However, if the
answerer uses a connection value of 'existing' in the answer, the answerer uses a connection value of 'existing' in the answer, the
offerer would need to deallocate the previously allocated resources offerer would need to deallocate the previously allocated resources
which were never used because no connection request was received. that were never used because no connection request was received.
To avoid allocating resources unnecessary, offerers using a To avoid allocating resources unnecessarily, offerers using a
connection value of 'existing' in their offers may choose to use a connection value of 'existing' in their offers may choose to use a
setup value of 'holdconn'. Nevertheless, offerers using this setup value of 'holdconn'. Nevertheless, offerers using this
strategy should be aware that in the case the answerer chooses a strategy should be aware that if the answerer chooses a connection
connection value of 'new', a new offer/answer exchange (typically value of 'new', a new offer/answer exchange (typically initiated by
initiated by the previous offerer) with setup value different than the previous offerer) with setup value different than 'holdconn' will
'holdconn' will be needed to establish the new connection. This may, be needed to establish the new connection. This may, of course,
of course, cause delays in the application using the TCP connection. cause delays in the application using the TCP connection.
The default value of the connection attribute in both offers and The default value of the connection attribute in both offers and
answers is 'new'. 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/ If the connection value for an 'm' line resulting from an offer/
answer exchange is 'new', the endpoints SHOULD establish a new TCP answer exchange is 'new', the endpoints SHOULD establish a new TCP
connection as indicated by the 'setup' attribute. If a previous TCP connection as indicated by the 'setup' attribute. If a previous TCP
connection is still up, the endpoints SHOULD close it as soon as the connection is still up, the endpoints SHOULD close it as soon as the
offer/answer exchange is completed. It is up to the application to offer/answer exchange is completed. It is up to the application to
ensure proper data synchornization between the two TCP connections. ensure proper data synchronization between the two TCP connections.
If the connection value for an 'm' line resulting from an offer/ If the connection value for an 'm' line resulting from an offer/
answer exchange is 'existing', the endpoints SHOULD continue using answer exchange is 'existing', the endpoints SHOULD continue using
the existing TCP 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 TCP connection SHOULD initiate it as soon as it is initiate a new TCP connection SHOULD initiate it as soon as it is
able to, even if the endpoint does not intend to immediately begin able to, even if the endpoint does not intend to immediately begin
sending media to the remote endpoint. This allows media to flow from sending media to the remote endpoint. This allows media to flow from
the 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 TCP connection. able to initiate a TCP connection.
6.2 Connection Reestablishment 6.2. Connection Reestablishment
If an endpoint determines that the TCP for an 'm' line has been If an endpoint determines that the TCP for an 'm' line has been
closed and it should be reestablished, it SHOULD perform a new offer/ closed and should be reestablished, it SHOULD perform a new offer/
answer exchange using a connection value of 'new' for this 'm' line. answer exchange using a connection value of 'new' 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 TCP connection, but has no impact on with the media sent over the TCP connection, but has no impact on
the TCP connection itself. the TCP connection itself.
6.3 Connection Termination 6.3. Connection Termination
Typically, endpoints do not close the TCP connection until the Typically, endpoints do not close the TCP connection until the
session has expired, been explicitly terminated, or a new connection session has expired, been explicitly terminated, or a new connection
value has been provided for the 'm' line. Additionaly, specific value has been provided for the 'm' line. Additionally, specific
applications can describe further scenarios where an end-point may applications can describe further scenarios where an end-point may
close a given TCP connection (e.g., whenever a connection is in the close a given TCP connection (e.g., whenever a connection is in the
half-close state). As soon as an end-point notices that it needs to half-close state). As soon as an end-point notices that it needs to
terminate a TCP connection, it SHOULD do so. terminate a TCP connection, it SHOULD do so.
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 to receive 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
An answerer at 192.0.2.1 receiving this offer responds with the An answerer at 192.0.2.1 receiving this offer responds with the
following answer: following answer:
m=image 9 TCP t38 m=image 9 TCP t38
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=setup:active a=setup:active
a=connection:new a=connection:new
The endpoint at 192.0.2.1 then initiates the TCP connection to port The endpoint at 192.0.2.1 then initiates the TCP connection to port
54111 at 192.0.2.2. 54111 at 192.0.2.2.
7.2 Actpass/Passive 7.2. Actpass/Passive
In another example, an offerer at 192.0.2.2 signals its availability In another example, an offerer at 192.0.2.2 signals its availability
for a T.38 fax session at TCP port 54111. Additionally, this offerer 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 is also willing to set up the media stream by initiating the TCP
connection: 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:actpass
a=connection:new a=connection:new
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The endpoint at 192.0.2.1 responds with the following description: The endpoint at 192.0.2.1 responds with the following description:
m=image 54321 TCP t38 m=image 54321 TCP t38
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=setup:passive a=setup:passive
a=connection:new a=connection:new
This will cause the offerer (at 192.0.2.2) to initiate a connection This will cause the offerer (at 192.0.2.2) to initiate a connection
to port 54321 at 192.0.2.1. to port 54321 at 192.0.2.1.
7.3 Existing Connection Reuse 7.3. Existing Connection Reuse
Subsequent to the exchange in Section 7.2, another offer/answer Subsequent to the exchange in Section 7.2, another offer/answer
exchange is initiated in the opposite direction. The endpoint at exchange is initiated in the opposite direction. The endpoint at
192.0.2.1 wishes to continue using the existing connection: 192.0.2.1 wishes to continue using the existing connection:
m=image 54321 TCP t38 m=image 54321 TCP t38
c=IN IP4 192.0.2.1 c=IN IP4 192.0.2.1
a=setup:passive a=setup:passive
a=connection:existing a=connection:existing
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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 Note that the endpoint at 192.0.2.2 uses 'setup:active' in
response to the offer of 'setup:passive', and uses port 9 because response to the offer of 'setup:passive', and uses port 9 because
it is 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:passive 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 thus wishes to establish a new one. (This could be the result of a
transfer via third-party call control.) It is unable to act in the transfer via third-party call control.) It is unable to act in the
'passive' mode so responds as 'active': 'passive' mode and thus 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 using a connection Note that the endpoint at 192.0.2.2, while using a connection
value of 'existing' has used a setup value of 'passive'. Had it value of 'existing', has used a setup value of 'passive'. Had it
not done this and used a setup value of 'holdconn' instead not done this and instead used a setup value of 'holdconn'
(probably to avoid allocating resources as described in Section (probably to avoid allocating resources as described in
5.1), a new offer/answer exchange would have been needed in order Section 5.1), a new offer/answer exchange would have been needed
to establish the new connection. in order to establish the new connection.
8. Other Connection-Oriented Transport Protocols 8. Other Connection-Oriented Transport Protocols
This document specifies how to describe TCP-based media streams using This document specifies how to describe TCP-based media streams using
SDP. Still, some of the attributes defined here could possibly be SDP. Still, some of the attributes defined here could possibly be
used to describe media streams based on other connection-oriented used to describe media streams based on other connection-oriented
transport protocols as well. This section provides advice to authors transport protocols as well. This section provides advice to authors
of specifications of SDP extensions which deal with of specifications of SDP extensions that deal with connection-
connetion-oriented transport protocols other than TCP. oriented transport protocols other than TCP.
It is recommended that documents defining new SDP protocol It is recommended that documents defining new SDP protocol
identifiers that involve extra protocol layers between TCP and the identifiers that involve extra protocol layers between TCP and the
media itself (e.g., TLS [7] over TCP) start with the string 'TCP/' media itself (e.g., TLS [7] over TCP) start with the string 'TCP/'
(e.g., 'TCP/TLS'). (e.g., 'TCP/TLS').
The 'setup' and the 'connection' attributes are specified in Section The 'setup' and the 'connection' attributes are specified in
4 and Section 5 respectively. While both attributes are applicable Section 4 and Section 5 respectively. While both attributes are
to 'm' lines that use the 'TCP' protocol identifier, they are general applicable to 'm' lines that use the 'TCP' protocol identifier, they
enough to be reused in 'm' lines with other connection-oriented are general enough to be reused in 'm' lines with other connection-
transport protocols. Therefore, it is recommended that the 'setup' oriented transport protocols. Therefore, it is recommended that the
and 'connection' attributes are reused, as long as it is possible, 'setup' and 'connection' attributes are reused, as long as it is
for new proto values associated with connection-oriented transport possible, for new proto values associated with connection-oriented
protocols. transport protocols.
Section 6 deals with TCP connection management. It should be noted Section 6 deals with TCP connection management. It should be noted
that while in TCP both end-points need to close a connection, other that while in TCP both end-points need to close a connection, other
connection-oriented transport protocols may not have the concept of connection-oriented transport protocols may not have the concept of
half-close connections. In such a case, a connection would be half-close connections. In such a case, a connection would be
terminated as soon as one of the end-points closed it, making it 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 unnecessary for the other end-point to perform any further action to
terminate the connection. So, specifications dealing with such terminate the connection. So, specifications dealing with such
transport protocols may need to specify slightly different procedures transport protocols may need to specify slightly different procedures
regarding connection termination. regarding connection termination.
9. Security Considerations 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 in order to have endpoints reestablish connections
unnecesaryly or to keep them from establishing a connection. So, it unnecessarily 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.
10. 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
the IANA under "Session Description Protocol (SDP) Parameters" under by the IANA under "Session Description Protocol (SDP) Parameters"
"att-field (both session and media level)". 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 under Section 3. This proto value should be registered by the IANA under
"Session Description Protocol (SDP) Parameters" under "proto". "Session Description Protocol (SDP) Parameters" under "proto".
The SDP specification, RFC2327, states that specifications defining The SDP specification, RFC2327, states that specifications defining
new proto values, like the TCP proto value defined in this RFC, must new proto values, like the TCP proto value defined in this RFC, must
define the rules by which their media format (fmt) namespace is define the rules by which their media format (fmt) namespace is
managed. For the TCP protocol, new formats SHOULD have an associated managed. For the TCP protocol, new formats SHOULD have an associated
MIME registration. Use of an existing MIME subtype for the format is MIME registration. Use of an existing MIME subtype for the format is
skipping to change at page 12, line 49 skipping to change at page 13, line 7
protocol for the format. protocol for the format.
11. 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.
12. References 12. References
12.1 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",
13, RFC 2048, November 1996. BCP 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.
12.2 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",
2246, January 1999. RFC 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.
[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement [9] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000. Protocol", RFC 2974, October 2000.
[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD [11] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
63, RFC 3629, November 2003. STD 63, RFC 3629, November 2003.
[12] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, [12] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in "Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
2004. April 2004.
Authors' Addresses Authors' Addresses
David Yon David Yon
Tactical Software, LLC Tactical Software, LLC
670 N Commercial St 1750 Elm St., Suite 803
Manchester, NH 03101 Manchester, NH 03104
USA USA
EMail: yon-comedia@rfdsoftware.com EMail: yon-comedia@rfdsoftware.com
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Hirsalantie 11 Hirsalantie 11
Jorvas 02420 Jorvas 02420
Finland Finland
EMail: Gonzalo.Camarillo@ericsson.com EMail: Gonzalo.Camarillo@ericsson.com
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ENGINEERING TASK FORCE DISCLAIM 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
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Internet Society. Internet Society.
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