draft-ietf-mmusic-sdp-comedia-06.txt   draft-ietf-mmusic-sdp-comedia-07.txt 
MMUSIC Working Group D. Yon MMUSIC Working Group D. Yon
Internet-Draft Dialout.Net, Inc Internet-Draft Dialout.Net, Inc
Expires: November 12, 2004 G. Camarillo Expires: December 10, 2004 G. Camarillo
Ericsson Ericsson
May 14, 2004 June 11, 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-06.txt draft-ietf-mmusic-sdp-comedia-07.txt
Status of this Memo Status of this Memo
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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). All Rights Reserved.
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 two new protocol identifiers: TCP and TCP/TLS. It (SDP). It defines the SDP TCP protocol identifier, the SDP setup
also defines the SDP setup attribute, which describes the connection attribute, which describes the connection setup procedure, and the
setup procedure, and the SDP reconnect attribute. SDP connid attribute, which provides a connection identifier.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3
3.1 TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2 TCP/TLS . . . . . . . . . . . . . . . . . . . . . . . . . 4
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
4.2 Multiple-Connection Avoidance when Using Actpass . . . . . 5 5. The Connid Attribute . . . . . . . . . . . . . . . . . . . . . 5
5. The Reconnect Attribute . . . . . . . . . . . . . . . . . . . 6 5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
6. Connection Lifetime . . . . . . . . . . . . . . . . . . . . . 7 5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
6.1 Session Renegotiation . . . . . . . . . . . . . . . . . . 7 6. Connection Management . . . . . . . . . . . . . . . . . . . . 7
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8 7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
7.2 Passive/Active with Reconnect . . . . . . . . . . . . . . 9 7.2 Passive/Active with Connection Reestablishment . . . . . . 8
7.3 Actpass . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.3 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 11 11.1 Normative References . . . . . . . . . . . . . . . . . . . . 10
11.2 Informational References . . . . . . . . . . . . . . . . . . 11 11.2 Informational References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . 12
1. Introduction 1. Introduction
The Session Description Protocol [4] provides a general-purpose The Session Description Protocol [3] 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 [6]) to maximize portability among transports. SDP subset of UTF-8 [5]) 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
[7], email, HTTP [8], etc.). [6], email, HTTP [8], etc.).
SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of SDP [3] 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 are applications for which connection-oriented transports, such as TCP,
more appropriate. We define two new protocol identifiers: TCP and are more appropriate. We define a new protocol identifier, TCP, to
TCP/TLS. Both represent connection-oriented reliable transports. describe TCP connetions in SDP.
Connection-oriented protocols introduce a new factor when describing Connection-oriented protocols introduce two new factor when
a session: how should end points perform the connection setup describing a session: how and when should end points perform the
procedure. We define two new attributes to describe connection setup: connection setup procedure. We define two new attributes to describe
setup and reconnect. connection setups: setup and connid.
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 [2] and indicate requirement levels for
compliant implementations. compliant implementations.
3. Protocol Identifiers 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]. [3].
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
We define two new values for the proto field: TCP and TCP/TLS. We define a new values for the proto field: TCP.
3.1 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. qualify the application-layer protocol using an fmt identifier. Media
lines with the TCP protocol identifier are carried using TCP [1].
Media lines with the TCP protocol identifier are carried using TCP
[1].
3.2 TCP/TLS
The TCP/TLS protocol identifier specifies that the session will use It is RECOMMENDED that documents defining new SDP protocol
the Transport Layer Security (TLS) protocol [3] on top on a TCP [1] identifiers that involve extra protocol layers between TCP and the
connection. media itself (e.g., TLS [7] over TCP) start with the string "TCP/"
(e.g., TCP/TLS).
An m= line that contain the TCP/TLS protocol identifier MUST further The following sections define the setup and the connid attributes.
qualify the protocol using a fmt identifier. While they are applicable to m= lines that use the TCP protocol
identifier, they are not limited to them. These attributes SHOULD be
used in any m= line which uses a 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
role = "active" / "passive" / "actpass" role = "active" / "passive" / "actpass"
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 will both accept an incoming connection and ActPass: The endpoint is willing to accept an incoming connection
will initiate an outgoing connection. or to initiate an outgoing connection.
The default value of the setup attribute is actpass. That is, an m=
line without an associated setup line is considered to be actpass.
4.1 The Setup Attribute in the Offer/answer Model 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 [4], 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 is willing to perform follows. The offerer states which role or roles is willing to perform
and the answerer, taking the offerer's willingness into 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 values
that the setup attribute can take in an offer/answer exchange: that the setup attribute can take in an offer/answer exchange:
Offer Answer Offer Answer
_______________ _______________
active passive active passive
passive active passive active
actpass active / passive / actpass actpass active / passive
The value active indicates that the endpoint SHOULD initiate a The value active indicates that the endpoint SHOULD initiate a
connection to the port number on the m= line of the other endpoint. connection to the port number on the m= line of the other endpoint.
The port number on its own m= line is irrelevant, and the opposite The port number on its own m= line is irrelevant, and the opposite
endpoint MUST NOT attempt to initiate a connection to the port number endpoint MUST NOT attempt to initiate a connection to the port number
specified there. Nevertheless, since the m= line must contain a valid specified there. Nevertheless, since the m= line must contain a valid
port number, the endpoint specifying using the value active SHOULD port number, the endpoint specifying using the value active SHOULD
specify a port number of 9 (the discard port) on its m= line. The specify a port number of 9 (the discard port) on its m= line. The
endpoint MUST NOT specify a port number of zero, as that carries endpoint MUST NOT specify a port number of zero, except to denote an
other semantics in SDP. m= line that has been or is being refused.
The value passive indicates that the endpoint SHOULD be ready to The value passive indicates that the endpoint SHOULD be ready to
accept a connection on the port number specified in the m= line. accept a connection on the port number specified in the m= line.
The value actpass indicates that the endpoint SHOULD initiate a The value actpass indicates that the offerer can either initiate a
connection to the port number on the m= line of the other endpoint connection to the port number on the m= line in the answer or accept
and that the endpoint SHOULD be ready to accept a connection on the a connection on the port number specified in the m= line in the
port number specified in the m= line. It is RECOMMENDED that, if offer. That is, the offerer has no preference as to whether it
possible, endpoints set the port number on their m= line to the accepts or initiates the connection and, so, is letting the answerer
source port number which they will use to establish the connection choose.
towards the remote endpoint. This way, the transport-layer protocol
(e.g., TCP) can take care of simultaneous opens.
Endpoints typically use the actpass value for the following reasons:
1. The offerer has no preference as to whether it accepts or
initiates the connection and, so, is letting the answerer choose.
2. The endpoints intend to use a single connection to transport the
media, but it is not known whether NAT (Network Address
Translator) issues will prevent either endpoint from initiating
or accepting the connection. So, both endpoints will attempt to
initiate a connection hoping that at least one will succeed.
4.2 Multiple-Connection Avoidance when Using Actpass
When an offer/answer exchange results in actpass, each endpoint
attempts to establish a transport connection towards the other
endpoint. If only one of the connections succeeds, this connection is
used to transfer media. Nevertheless, if both connections succeed,
one of them needs to be terminated so that both endpoints exchange
data over a single connection. In this section, we provide rules to
choose which of the two connections should be terminated (or not even
initiated).
First of all, if the endpoints follow the recommendation of setting
the port number in their m= line to the source port number which they
will use to establish the connection towards the remote endpoint, the
transport layer should take care of simultaneous opens (at least if
TCP is the transport protocol). If, for some reason, any of the
endpoints does not follow this recommendation, both endpoints should
follow the rules below.
If an endpoint is notified about a connection establishment attempt
from the other endpoint before performing its own connection attempt,
it SHOULD behave as a passive endpoint and SHOULD NOT attempt to
establish any other connection.
In case two connections are established, if an endpoint receives data
(i.e., media) over one of the connections before having sent any data
on any of the connections, the endpoint SHOULD terminate the
connection that has not carried any data.
When two connections are established and both endpoints start sending
data before receiving anything from the other endpoint, it may happen
that each of the endpoints choose a different connection to send
data. If the answerer receives data over a connection after having
sent data on the other connection, it SHOULD continue sending data on
the other connection until an application-layer data boundary. At
that point, the answerer SHOULD terminate this connection and start
using the connection on which the offerer was sending data.
Note that different applications may define application-layer The default value of the setup attribute in an offer/answer exchange
boundaries in different ways. A typical suitable point for the is active in the offer and passive in the answer.
answerer to change connections is the end of an application-layer
message and the beginning of the next one.
5. The Reconnect Attribute 5. The Connid 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
reconnect attribute, which is charset-independent and can be a media-level connid attribute, which is charset-independent, is used
session-level or a media-level attribute, is used to disambiguate to disambiguate these two scenarios. The following is the ABNF of the
these two scenarios. The following is the ABNF of the reconnect connid attribute:
attribute:
reconnect-attr = "a=reconnect" connid = "a=connid:" connection-identifier
connection-identifier = token
On reception of an m= line with a reconnect attribute, the endpoints The connid attribute provides an identifier for the transport-layer
SHOULD close the existing connection, in case it was still up, and connection used by the m= line. Connid values are meaningful in the
SHOULD establish a new connection according to the setup attribute in context of a particular m= line. So, different m= lines in the same
the m= line. session description MAY have the same connid value.
Either the offerer or the answerer can include a reconnect attribute 5.1 Offerer Behaviour
in an m= line. In any event, if the offer contained this attribute,
the answer MUST contain it as well.
6. Connection Lifetime Offerers and answerers use the connid attribute to decide whether a
new transport connection needs to be established or, on the other
hand, the existing transport connection should still be used.
An endpoint that intends to initiate the connection SHOULD initiate When an offerer generates an m= line which uses a connection-oriented
the connection immediately after it has sufficient information to do transport, it SHOULD provide such an m= line with a connection
so, even if it does not intend to immediately begin sending media to identifier using a connid attribute, unless the application using the
the remote endpoint. This allows media to flow from the remote m= line has other means to deal with connection reestablishment. The
endpoint. An endpoint SHOULD NOT close the connection until the connid attribute in an initial offer (i.e., no transport connection
session has expired, been explicitly terminated, or the media stream has been established yet) can take any value. This value identifies
is redirected to a different address or port. the initial connection that the endpoints will attempt to establish.
If the endpoint determines that the connection has been closed, it After the initial offer/answer exchange, any of the endpoints can
MAY attempt to re-establish the connection. The decision to do so is generate a new offer to change some characteristics of the session
application and context dependant. (e.g., the direction attribute). If such an offerer wants to continue
using the previously-established transport-layer connection for the
m= line, the offerer MUST use the same connid value for the m= line.
If, on the other hand, the offerer wants to establish a new
transport-layer connection for the m= line, it MUST use a new connid
value. This new connid value MUST be different from the current
connid value in use and SHOULD be different than any connid value
used previously in the same m= line.
6.1 Session Renegotiation 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, we recommend 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
changes the transport address (IP address plus port number) of an
m= line will have a new connid value for this m=line.
There are scenarios where SDP is sent by an endpoint in order to 5.2 Answerer Behaviour
renegotiate an existing session. These include muting/unmuting a
session, renegotiating the attributes of the media used by the
session, or extending the length of a session about to expire.
Connection-oriented media introduces some ambiguities into session
renegotiation as to when the direction attribute must be obeyed and
when it is ignored.
The scenario of extending the duration of an existing session is a The connid value for an m= line is negotiated using the offer/answer
good example: in order to extend an existing session, endpoints will model. The resulting connid value after an offer/answer exchange is
typically resend the original SDP with updated time information. In the connid value in the answer.
connectionless media the result is no change to the existing media
streams. The problem with connection oriented media is that the
original SDP will contain a setup attribute which can be considered
as a request to create a new connection, as opposed to a request to
maintain steady state. The following rule help avoid this ambiguity:
If the transport section (the c= and m= lines) of an SDP For an m= line, if the offer contains a new connid value (i.e.,
description describes an existing connection between two endpoints different from the one in use) the answerer MUST use this value in
and the m= line does not contain a reconnect attribute, the the answer. If the offer contains the connid value in use and the
endpoints SHOULD use that connection to carry the media described answerer wishes to continue using the existing transport-layer
in the remainder of the message. The endpoints SHOULD NOT attempt connection, the answerer MUST use this connid value in the answer. If
to set up a new connection, regardless of what is specified in the the offer contains the connid value in use but the answerer wishes to
setup attribute. establish a new transport-layer connection, the answerer MUST use a
Note that if the port number in the m= line changes, there is no new connid value in the answer.
need to use the reconnect attribute because the new port will
trigger the establishment of a new connection anyway.
7. Examples If the connid value for an m= line resulting from an offer/answer
exchange is different than the connid in use so far, the endpoints
SHOULD establish a new transport-layer connection as indicated by the
setup attribute. If a previous connection is still up, the endpoint
responsible for establishing the new connection performing the active
role SHOULD close it as soon as the offer/answer exchange is
completed.
What follows are a number of examples that show the most common usage If the connid value for an m= line resulting from an offer/answer
of the setup attribute combined with TCP-based media descriptions. exchange is the same as the connid in use so far, the endpoints
For the purpose of brevity, the main portion of the session SHOULD continue using the existing connection.
description is omitted in the examples and is assumed to be the
following:
v=0 In the past, it was proposed to use the presence of a media-level
o=me 2890844526 2890842807 IN IP4 10.1.1.2 SDP attribute as a flag to indicate that a new connection needed
s=Call me using TCP to be established. We chose not to follow the flag approach
t=3034423619 3042462419 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
An endpoint that according to an offer/answer exchange is supposed to
initiate a new connection SHOULD initiate it as soon as the offer/
answer exchange is completed, even if the endpoint does not intend to
immediately begin sending media to the remote endpoint. This allows
media to flow from the remote endpoint if needed.
Typically, endpoints do not close the connection until the session
has expired, been explicitly terminated, or a new connid value has
been provided for the m= line. Additionaly, specific applications can
describe further scenarios where an end-point may close a given
connection. In case the session is explicitly terminated by one of
the endpoints (e.g., the endpoint sends a SIP [10] BYE), the end
point terminating the session is responsible for closing the
transport-connection.
If an endpoint determines that the transport-connection for an m=
line has been closed and it should be reestablished, it SHOULD
perform a new offer/answer exchange using a new connid value for this
m= line.
Note that the SDP direction attribute (e.g., a=sendonly) deals
with the media sent over the transport-connection, but has no
impact on the transport-connection itself.
7. Examples
The following examples show the most common usage of the setup
attribute combined with TCP-based media descriptions. For the purpose
of brevity, the main portion of the session description is omitted in
the examples, which only show m= lines and their 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:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38 m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive a=setup:passive
a=connid:1
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 c=IN IP4 192.0.2.1
m=image 9 TCP t38 m=image 9 TCP t38
a=setup:active a=setup:active
a=connid:1
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 Reconnect 7.2 Passive/Active with Connection Reestablishment
Continuing the preceding example, consider the scenario where the TCP Continuing the preceding example, consider the scenario where the TCP
connection fails and the endpoints wish to reestablish the connection connection fails and the endpoints wish to reestablish the connection
for the session. The endpoint at 192.0.2.2 signals this intent with for the session. The endpoint at 192.0.2.2 signals this intent with
the following SDP: the following SDP:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38 m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive a=setup:passive
a=reconnect a=connid:2
The new connid value informs the endpoint at 192.0.2.1 that this SDP
The reconnect attribute informs the endpoint at 192.0.2.1 that this represents the intent to establish a new connection for media
SDP represents the intent to establish a new connection for media transport, rather than continuing with the original connection. If
transport, rather than continuing with the original connection. 192.0.2.1 agrees to continue the session using a new connection, it
Because the endpoint at 192.0.2.1 may not yet be aware that the TCP responds with:
connection has failed, this eliminates any ambiguity. If 192.0.2.1
agrees to continue the session using a new connection, it responds
with:
c=IN IP4 192.0.2.1
m=image 9 TCP t38 m=image 9 TCP t38
a=setup:active IN IP4 c=IN IP4 192.0.2.1
a=reconnect a=setup:active
a=connid:2
7.3 Actpass 7.3 Actpass/Passive
An offerer at 192.0.2.2 signals its availability for a T.38 fax In another example, an offerer at 192.0.2.2 signals its availability
session at TCP port 54111. Additionally, this offerer is also willing for a T.38 fax session at TCP port 54111. Additionally, this offerer
to set up the media stream by initiating the TCP connection: is also willing to set up the media stream by initiating the TCP
connection:
c=IN IP4 192.0.2.2
m=image 54111 TCP t38 m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass a=setup:actpass
a=connid:3
The endpoint at 192.0.2.1 responds with the following description: The endpoint at 192.0.2.1 responds with the following description:
c=IN IP4 192.0.2.1
m=image 54321 TCP t38 m=image 54321 TCP t38
a=setup:actpass c=IN IP4 192.0.2.1
a=setup:passive
a=connid:3
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 and the answerer (at 192.0.2.1) to to port 54321 at 192.0.2.1.
initiate a connection to port 54111 at 192.0.2.2. Ideally, the
offerer would use 192.0.2.2:5411 as the source of its connection
attempt and the answerer would use 192.0.2.1:54321 as its.
8. Security Considerations 8. Security Considerations
See RFC 2327 [4] for security and other considerations specific to See RFC 2327 [3] for security and other considerations specific to
the Session Description Protocol in general. the Session Description Protocol in general.
An attacker may attempt to substitute TCP/TLS with only TCP in a An attacker may attempt to modify the values of the connid attributes
session description. So, it is STRONGLY RECOMMENDED that integrity to have endpoints reestablish connections unnecesaryly. So, it is
protection be applied to the SDP session descriptions. For session STRONGLY RECOMMENDED that integrity protection be applied to the SDP
descriptions carried in SIP [10], S/MIME is the natural choice to session descriptions. For session descriptions carried in SIP [10],
provide such end-to-end integrity protection, as described in RFC S/MIME is the natural choice to provide such end-to-end integrity
3261 [10]. Other applications MAY use a different form of integrity protection, as described in RFC 3261 [10]. Other applications MAY use
protection. a different form of integrity protection.
This document touches upon NAT traversal. Implementers should be
aware of some issues that relate to the use of private IP addresses
within the offer/answer model (i.e., they are not specific to this
document).
When an endpoint receives a session description with a private IP
address that belongs to a different address space, in most of the
cases, the endpoint will not be able to reach such an address.
Nevertheless, if this particular address also exists in the
endpoint's address space, the endpoint may end up reaching a
different peer than the one that generated the session description.
It is RECOMMENDED that endpoints authenticate their peer somehow
(e.g., using TLS [3]) or that they encrypt their media.
9. IANA Considerations 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 reconnect. Their formats are defined in Section 4 and setup and connid. Their formats are defined in Section 4 and Section
Section 5 respectively. These two attributes should be registered by 5 respectively. These two attributes should be registered by the IANA
the IANA on http://www.iana.org/assignments/sdp-parameters under on
"att-field (both session and media level)".
This document defines two proto values: TCP and TCP/TLS. Their http://www.iana.org/assignments/sdp-parameters
formats are defined in Section 3.1 and Section 3.2 respectively.
These two proto values should be registered by the IANA on http:// under "att-field (both session and media level)".
www.iana.org/assignments/sdp-parameters under "proto".
This document defines a proto values: TCP. Its format is defined in
Section 3. This proto value should be registered by the IANA on
http://www.iana.org/assignments/sdp-parameters
under "proto".
10. Acknowledgements 10. Acknowledgements
The authors would like to thank Jonathan Rosenberg, Rohan Mahy, Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
Fairlie-Cuninghame, and Colin Perkins for their valuable insights and Holmberg provided valuable insights and contributions.
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] 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] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC [3] Handley, M. and V. Jacobson, "SDP: Session Description
2246, January 1999.
[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 [4] 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.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD [5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003. 63, RFC 3629, November 2003.
11.2 Informational References 11.2 Informational References
[7] 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
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:
 End of changes. 

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