draft-ietf-mmusic-sdp-comedia-08.txt   draft-ietf-mmusic-sdp-comedia-09.txt 
MMUSIC Working Group D. Yon MMUSIC Working Group D. Yon
Internet-Draft Dialout.Net, Inc Internet-Draft Tactical Software, LLC
Expires: January 14, 2005 G. Camarillo Expires: March 30, 2005 G. Camarillo
Ericsson Ericsson
July 16, 2004 September 29, 2004
Connection-Oriented Media Transport in the Session Description Connection-Oriented Media Transport in the Session Description
Protocol (SDP) Protocol (SDP)
draft-ietf-mmusic-sdp-comedia-08.txt draft-ietf-mmusic-sdp-comedia-09.txt
Status of this Memo Status of this Memo
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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
RFC 3668. RFC 3668.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. 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
connection-oriented protocols using the Session Description Protocol connection-oriented protocols using the Session Description Protocol
(SDP). It defines the SDP TCP protocol identifier, the SDP setup (SDP). It defines the SDP TCP protocol identifier, the SDP setup
attribute, which describes the connection setup procedure, and the attribute, which describes the connection setup procedure, and the
SDP connid attribute, which provides a connection identifier. SDP connection attribute, which 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 Connid 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 . . . . . . . . . . . . . . . . . . . . 7
6.1 Connection Establishment . . . . . . . . . . . . . . . . . 7
6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8
6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
7.2 Passive/Active with Connection Reestablishment . . . . . . 9 7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
7.3 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9 7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 10 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.2 Informative References . . . . . . . . . . . . . . . . . . . 11 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 11.2 Informative References . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
1. Introduction 1. Introduction
The Session Description Protocol [3] 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 [10]) 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 [8], SIP [9], RTSP application protocols for transport (e.g., SAP [9], SIP [10], RTSP
[5], email, HTTP [7], etc.). [6], email, HTTP [8], etc.).
SDP [3] 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 connection-oriented transports, such as TCP,
are more appropriate. This document defines a new protocol are more appropriate. This document defines a new protocol
identifier, TCP, to describe TCP connetions in SDP. identifier, TCP, to describe TCP connetions in SDP.
Connection-oriented protocols introduce two new factor when Connection-oriented protocols introduce two new factor when
describing a session: how and when should end points perform the describing a session: how and when should end points perform the
connection setup procedure. This document defines two new attributes connection setup procedure. This document defines two new attributes
to describe connection setups: setup and connid. to describe 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 [2] 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
[3]. [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 The TCP protocol identifier is similar to the UDP protocol identifier
in that it only describes the transport protocol, and not the in that it only describes the transport protocol, and not the
upper-layer protocol. An m= line that specifies "TCP" MUST further upper-layer protocol. An m= line that specifies "TCP" MUST further
qualify the application-layer protocol using an fmt identifier. Media qualify the application-layer protocol using an fmt identifier.
lines with the TCP protocol identifier are carried using TCP [1]. 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 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 [6] 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 following sections define the setup and the connid attributes. The following sections define the setup and the connection
While both attributes are applicable to m= lines that use the TCP attributes. While both attributes are applicable to m= lines that
protocol identifier, they are not limited to them. These attributes use the TCP protocol identifier, they are not limited to them. These
MAY be used in any m= line which uses a connection-oriented transport attributes MAY be used in conjunction with any m= line which uses a
protocol, even if the protocol identifier of the m= line is not TCP. connection- oriented transport protocol, even if the protocol
identifier of the m= line is not TCP.
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 initiate
the connection establishment (e.g., send the initial TCP SYN). The the connection establishment (e.g., send the initial TCP SYN). The
setup attribute is charset-independent and can be a session-level or setup attribute is charset-independent and can be a session-level or
a media-level attribute. The following is the ABNF of the setup a media-level attribute. The following is the ABNF of the setup
attribute: attribute:
setup-attr = "a=setup:" role setup-attr = "a=setup:" role
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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 connection
or to initiate an outgoing 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 [4], 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 values perform during connection establishment. The following are the
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 m=
skipping to change at page 5, line 40 skipping to change at page 5, line 43
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 Connid Attribute 5. The Connection Attribute
The preceding description of the setup attribute has been in the The preceding description of the setup attribute has been 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 media connection
unchanged, or is a request to make a new media connection. The unchanged, or is a request to make a new media connection. The
media-level connid attribute, which is charset-independent, is used media-level connection attribute, which is charset-independent, is
to disambiguate these two scenarios. The following is the ABNF of the used to disambiguate these two scenarios. The following is the ABNF
connid attribute: of the connection attribute:
connid = "a=connid:" connection-identifier
connection-identifier = token
The connid attribute provides an identifier for the transport-layer connection-attr = "a=connection:" conn-value
connection used by the m= line. Connid values are meaningful in the conn-value = "new" / "existing"
context of a particular m= line. So, different m= lines in the same
session description MAY have the same connid value.
5.1 Offerer Behaviour 5.1 Offerer Behaviour
Offerers and answerers use the connid attribute to decide whether a Offerers and answerers use the connection attribute to decide whether
new transport connection needs to be established or, on the other a new transport connection needs to be established or, on the other
hand, the existing transport connection should still be used. hand, the existing transport connection should still be used. The
connection value resulting from an offer/answer exchange is the
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.
When an offerer generates an m= line which uses a connection-oriented When an offerer generates an m= line which uses a connection-oriented
transport, it SHOULD provide such an m= line with a connection transport, it SHOULD provide a connection attribute for the m= line
identifier using a connid attribute, unless the application using the unless the application using the m= line has other means to deal with
m= line has other means to deal with connection reestablishment. The connection reestablishment. The connection attribute in an initial
connid attribute in an initial offer (i.e., no transport connection offer (i.e., no transport connection has been established yet) takes
has been established yet) can take any value. This value identifies the value of "new".
the initial connection that the endpoints will attempt to establish.
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 continue (e.g., the direction attribute). If such an offerer wants to
using the previously-established transport-layer connection for the continue using the previously-established transport-layer connection
m= line, the offerer MUST use the same connid value for the m= line. for the m= line, the offerer MUST use use a connection value of
If, on the other hand, the offerer wants to establish a new "existing" for the m= line. If, on the other hand, the offerer wants
transport-layer connection for the m= line, it MUST use a new connid to establish a new transport-layer connection for the m= line, it
value. This new connid value MUST be different from the current MUST use a connection value of "new".
connid value in use and SHOULD be different than any connid value
used previously in the same m= line.
The connid value in an offer is only compared with the connid
value currently in use. So, having a connid value different than
the one in use is enough to trigger the establishment of a new
connection. Still, it is recommended to use a value different than
all the previous ones used in the m= line to make debugging
easier.
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 plus port number) of an changes the transport address (IP address or port number) of an m=
m= line will have a new connid value for this m= line. line will have a connection value of "new".
5.2 Answerer Behaviour The default value of the connection attribute in an offer/answer
exchange is "new".
The connid value for an m= line is negotiated using the offer/answer 5.2 Answerer Behaviour
model. The resulting connid value after an offer/answer exchange is
the connid value in the answer.
For an m= line, if the offer contains a new connid value (i.e., The connection value for an m= line is negotiated using the offer/
different from the one in use) the answerer MUST use this value in answer model. The resulting connection value after an offer/answer
the answer. If the offer contains the connid value in use and the exchange is the connection value in the answer. If the connection
answerer wishes to continue using the existing transport-layer value in the offer is "new", the answerer MUST also use a value of
connection, the answerer MUST use this connid value in the answer. If "new" in the answer. If the connection value in the offer is
the offer contains the connid value in use but the answerer wishes to "existing", the answerer uses a value of "existing" in the answer if
establish a new transport-layer connection, the answerer MUST use a it wishes to continue using the existing connection and a value of
new connid value in the answer. "new" if it wants a new connection to be established.
If the connid value for an m= line resulting from an offer/answer In some scenarios where third party call control [12] is used, an
exchange is different than the connid in use so far, the endpoints endpoint may receive an initial offer with a connection value of
SHOULD establish a new transport-layer connection as indicated by the "existing". Following the previous rules, such an answerer would
setup attribute. If a previous connection is still up, the endpoint use a connection value of "new" in the answer.
responsible for establishing the new connection performing the active
role SHOULD close it as soon as the offer/answer exchange is
completed. It is up to the application to ensure proper data
synchornization between the two connections.
If the connid value for an m= line resulting from an offer/answer If the connection value for an m= line resulting from an offer/answer
exchange is the same as the connid in use so far, the endpoints exchange is "new", the endpoints SHOULD establish a new
SHOULD continue using the existing connection. transport-layer connection as indicated by the setup attribute. If a
previous 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 ensure proper data synchornization between the two
connections.
In the past, it was proposed to use the presence of a media-level If the connection value for an m= line resulting from an offer/answer
SDP attribute as a flag to indicate that a new connection needed exchange is "existing", the endpoints SHOULD continue using the
to be established. It was decided not to follow the flag approach existing connection.
because an offerer whose intent was to signal "no changes" in a
session would need to issue a different offer than the previous
one (i.e., it would need to remove the flag from the m= line). By
using the connid attribute instead, an offerer signals "no
changes" in a session by issuing an identical offer to the one in
use.
6. Connection Management 6. Connection Management
This section addresses connection establishment, connection
reestablishment, and connection termination.
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 the offer/ initiate a new connection SHOULD initiate it as soon as it is able
answer exchange is completed, even if the endpoint does not intend to to, even if the endpoint does not intend to immediately begin sending
immediately begin sending media to the remote endpoint. This allows media to the remote endpoint. This allows media to flow from the
media to flow from the remote endpoint if needed. remote endpoint if needed.
Typically, endpoints do not close the connection until the session Note that some endpoints need to wait for some event to happen
has expired, been explicitly terminated, or a new connid value has before being able to establish the connection. For example, a
been provided for the m= line. Additionaly, specific applications can wireless terminal may need to set up a radio bearer before being
describe further scenarios where an end-point may close a given able to initiate a connection.
connection. In case the session is explicitly terminated by one of
the endpoints (e.g., the endpoint sends a SIP [9] BYE), the end point 6.2 Connection Reestablishment
terminating the session is responsible for closing the
transport-connection.
If an endpoint determines that the transport-connection for an m= If an endpoint determines that the transport-connection for an m=
line has been closed and it should be reestablished, it SHOULD line has been closed and it should be reestablished, it SHOULD
perform a new offer/answer exchange using a new connid value for this perform a new offer/answer exchange using a connection value of "new"
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 transport-connection, but has no
impact on the transport-connection itself. impact on the transport-connection itself.
6.3 Connection Termination
Typically, endpoints do not close the connection until the session
has expired, been explicitly terminated, or a new connection value
has been provided for the m= line. Additionaly, specific
applications can describe further scenarios where an end-point may
close a given connection. As soon as an end-point notices that it
needs to terminate a 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
considerations on how to achieve a graceful connection termination.
For example, a file application using TCP receiving a FIN from the
remote endpoint may need to finish the ongoing transmission of a file
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 purpose attribute combined with TCP-based media descriptions. For the
of brevity, the main portion of the session description is omitted in purpose of brevity, the main portion of the session description is
the examples, which only show m= lines and their attributes omitted in the examples, which only show m= lines and their
(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=connid:1 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:
c=IN IP4 192.0.2.1
m=image 9 TCP t38 m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active a=setup:active
a=connid:1 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 Passive/Active with Connection Reestablishment 7.2 Actpass/Passive
Continuing the preceding example, consider the scenario where the TCP
connection fails and the endpoints wish to reestablish the connection
for the session. The endpoint at 192.0.2.2 signals this intent with
the following SDP:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connid:2
The new connid value informs the endpoint at 192.0.2.1 that this SDP
represents the intent to establish a new connection for media
transport, rather than continuing with the original connection. If
192.0.2.1 agrees to continue the session using a new connection, it
responds with:
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connid:2
7.3 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=connid:3 a=connection:new
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=connid:3 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
Subsequent to the exchange in Section 7.2, another offer/answer
exchange is initiated in the opposite direction. The endpoint at
192.0.2.1 wishes to continue using the existing connection:
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:existing
The endpoint at 192.0.2.2 also wishes to use the existing connection
and responds with the following description:
m=image 9 TCP t38
c=IN IP4 192.0.2.2
a=setup:active
a=connection:existing
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
to the offer of setup:passive, and uses port 9 because it is
active.
7.4 Existing Connection Refusal
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
reuse the existing connection:
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:existing
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
transfer via 3pcc.) It is unable to act in the passive mode so
responds as active:
m=image 9 TCP t38
c=IN IP4 192.0.2.3
a=setup:active
a=connection:new
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
connection.
Note that the endpoint at 192.0.2.2, while specifying connection:
existing has reverted to setup:actpass and its real port number,
rather than repeating setup:active and port 9 from the previous
cycle. Had it not done this, this negotiation would have failed.
8. Security Considerations 8. Security Considerations
See RFC 2327 [3] 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 connid and setup An attacker may attempt to modify the values of the connection and
attributes to have endpoints reestablish connections unnecesaryly or setup attributes to have endpoints reestablish connections
to keep them from establishing a connection. So, it is STRONGLY unnecesaryly or to keep them from establishing a connection. So, it
RECOMMENDED that integrity protection be applied to the SDP session is STRONGLY RECOMMENDED that integrity protection be applied to the
descriptions. For session descriptions carried in SIP [9], S/MIME is SDP session descriptions. For session descriptions carried in SIP
the natural choice to provide such end-to-end integrity protection, [10], S/MIME is the natural choice to provide such end-to-end
as described in RFC 3261 [9]. Other applications MAY use a different integrity protection, as described in RFC 3261 [10]. Other
form of integrity protection. applications MAY use a different form of integrity protection.
9. IANA Considerations 9. IANA Considerations
This document defines two session and media level SDP attributes: This document defines two session and media level SDP attributes:
setup and connid. Their formats are defined in Section 4 and Section setup and connection. Their formats are defined in Section 4 and
5 respectively. These two attributes should be registered by the IANA Section 5 respectively. These two attributes should be registered by
on the IANA on
http://www.iana.org/assignments/sdp-parameters http://www.iana.org/assignments/sdp-parameters
under "att-field (both session and media level)". under "att-field (both session and media level)".
This document defines a proto values: 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 on
http://www.iana.org/assignments/sdp-parameters http://www.iana.org/assignments/sdp-parameters
under "proto". under "proto".
Specifications defining new proto values, like this one, must define
the rules by which their media format (fmt) namespace 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
suitable one is registered through the IETF process [2] by production
of, or reference to, a standards-track RFC that defines the transport
protocol for the format.
10. Acknowledgements 10. 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 11. References
11.1 Normative References 11.1 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] Bradner, S., "Key words for use in RFCs to Indicate Requirement [2] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", BCP
13, RFC 2048, November 1996.
[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.
[3] 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.
[4] 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 11.2 Informative References
[5] 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.
[6] 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.
[7] 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.
[8] 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.
[9] 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.
[10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD [11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003. 63, RFC 3629, November 2003.
[12] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
Authors' Addresses Authors' Addresses
David Yon David Yon
Dialout.Net, Inc Tactical Software, LLC
One Indian Head Plaza 670 N Commercial St
Nashua, NH 03060 Manchester, NH 03101
USA USA
EMail: yon@dialout.net 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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