draft-ietf-mmusic-sdp-comedia-01.txt   draft-ietf-mmusic-sdp-comedia-02.txt 
INTERNET-DRAFT D. Yon INTERNET-DRAFT D. Yon
Document: draft-ietf-mmusic-sdp-comedia-01.txt Dialout.Net Document: draft-ietf-mmusic-sdp-comedia-02.txt Dialout.Net
Expires April 2002 October 2001 Expires October 2002 April 2002
Connection-Oriented Media Transport in SDP Connection-Oriented Media Transport in SDP
<draft-ietf-mmusic-sdp-comedia-01.txt> <draft-ietf-mmusic-sdp-comedia-02.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at line 30 skipping to change at line 30
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at any time. It is inappropriate to use Internet-Drafts as reference at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at: The list of current Internet-Drafts can be accessed at:
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Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2002). 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 TLS. It (SDP). It defines two new protocol identifiers: TCP and TLS. It
also defines the syntax and semantics for an SDP "direction" also defines the syntax and semantics for an SDP "direction"
attribute that describes the connection setup procedure. attribute that describes the connection setup procedure.
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Introduction 1 Introduction
The Session Description Protocol [SDP] provides a general-purpose The Session Description Protocol [SDP] 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]) to maximize portability among transports. SDP subset of [UTF-8]) 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 discover and multimedia session with sufficient information to discover and
participate in that session. Session descriptions may be sent using participate in that session. Session descriptions may be sent using
any number of existing application protocols for transport (e.g., any number of existing application protocols for transport (e.g.,
SAP, SIP, RTSP, email, HTTP, etc.). SAP, SIP, RTSP, email, HTTP, etc.).
Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
and indicate requirement levels for compliant implementations.
Motivation
[SDP] describes two protocol identifiers: RTP/AVP and UDP, both of [SDP] describes two protocol identifiers: RTP/AVP and UDP, both of
which are unreliable, connectionless protocols, an appropriate which are unreliable, connectionless protocols, an appropriate
choice for multimedia streams. There are, however, applications for choice for multimedia streams. There are, however, applications for
which the connection-oriented transports such as TCP are more which the connection-oriented transports such as TCP are more
appropriate, but [SDP] provides no way to describe a session that appropriate, but [SDP] provides no way to describe a session that
uses protocols other than RTP or UDP. uses protocols other than RTP or UDP.
Connection-oriented protocols introduce a new factor when describing Connection-oriented protocols introduce a new factor when describing
a session: not only must it be possible to express that a protocol a session: not only must it be possible to express that a protocol
will be based on this protocol, but it must also describe the will be based on this protocol, but it must also describe the
connection setup procedure. connection setup procedure. This memo defines two new protocol
identifiers, TCP and TLS, along with the syntax and semantics of the
a=direction attribute.
1 Protocol Identifiers Terminology
1.1 TCP In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
and indicate requirement levels for compliant implementations.
2 Protocol Identifiers
The m= line in [SDP] is where an endpoint specifies the protocol
used for the media in the session. See the "Media Announcements"
section of [SDP] for a discussion on protocol identifiers.
2.1 TCP
The TCP protocol identifier is similar to the UDP protocol The TCP protocol identifier is similar to the UDP protocol
identifier in that it only describes the transport protocol without identifier in that it only describes the transport protocol without
any connotation as to the upper-layer protocol. An m= line that any connotation as to the upper-layer protocol. An m= line that
specifies "TCP" MUST further qualify the protocol using a fmt specifies "TCP" MUST further qualify the protocol using a fmt
identifier (see [SDP] Appendix B). identifier (see [SDP] Appendix B).
1.2 TLS 2.2 TLS
The TLS protocol identifier specifies that the session will use the The TLS protocol identifier specifies that the session will use the
Transport Layer Security protocol [TLS] with an implied transport Transport Layer Security protocol [TLS] with an implied transport
protocol of TCP. To describe a media session that uses TLS over protocol of TCP. To describe a media session that uses TLS over
TCP, the protocol identifier "TLS" must be specified in the m= line. TCP, the protocol identifier "TLS" must be specified in the m= line.
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An m= line that specifies TLS MUST further qualify the protocol An m= line that specifies TLS MUST further qualify the protocol
using a fmt identifier. using a fmt identifier.
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2 Direction Attribute
An important attribute of connection-oriented protocols is the setup An important attribute of connection-oriented protocols is the setup
procedure. One endpoint needs to initiate the connection and the procedure. One endpoint needs to initiate the connection and the
other endpoint needs to accept the connection. The direction other endpoint needs to accept the connection. The direction
attribute is used to describe these roles, and the syntax is as attribute is used to describe these roles, and the syntax is as
follows: follows:
a=direction:<role> [<source-address>] a=direction:<role> [<source-address>]
The <role> is one of the following: The <role> is one of the following:
passive: The endpoint will accept an incoming connection. passive: The endpoint will accept an incoming connection.
active: The endpoint will initiate an outgoing connection. active: The endpoint will initiate an outgoing connection.
both: The endpoint will both accept an incoming connection both: The endpoint will both accept an incoming connection
and will initiate an outgoing connection. and will initiate an outgoing connection.
reuse: The endpoint will use the connection that has already
been established with the opposite endpoint.
The <source-address> is a sequence of values that describe the The <source-address> is a sequence of values that describe the
address and port number from where the connection will originate, address and port number from where the connection will originate,
and consists of the following values: and consists of the following values:
nettype addrtype unicast-address [port] nettype addrtype unicast-address [port]
The <source-address> is an optional value that may be specified with The <source-address> is an optional value that may be specified with
direction:active, direction:both, or direction:reuse. Within the direction:active or direction:both. Within the <source-address>,
<source-address>, the source port number is RECOMMENDED but may be the source port number is RECOMMENDED but may be omitted.
omitted.
2.1 Semantics of direction:passive 3.1 Semantics of direction:passive
By specifying direction:passive, the endpoint indicates that the By specifying direction:passive, the endpoint indicates that the
port number specified in the m= line is available to accept a port number specified in the m= line is available to accept a
connection from the other endpoint. The endpoint MUST NOT specify a connection from the other endpoint. The endpoint MUST NOT specify a
<source-address> after direction:passive. <source-address> after direction:passive.
2.2 Semantics of direction:active 3.2 Semantics of direction:active
By specifying direction:active, the endpoint indicates that it will By specifying direction:active, the endpoint indicates that it will
initiate a connection to the port number on the m= line of the other initiate a connection to the port number on the m= line of the other
endpoint. The port number on its own m= line is irrelevant, and the endpoint. The port number on its own m= line is irrelevant, and the
opposite endpoint MUST NOT attempt to initiate a connection to the opposite endpoint MUST NOT attempt to initiate a connection to the
port number specified there. Nevertheless, since the m= line must port number specified there. Nevertheless, since the m= line must
contain a valid port number, the endpoint specifying contain a valid port number, the endpoint specifying
direction:active SHOULD specify a port number of 9 (the discard direction:active SHOULD specify a port number of 9 (the discard
port) on its m= line. The endpoint MUST NOT specify a port number port) on its m= line. The endpoint MUST NOT specify a port number
of zero, as that carries other semantics in [SDP]. of zero, as that carries other semantics in [SDP].
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The endpoint SHOULD specify the address and port number from which The endpoint SHOULD specify the address and port number from which
it will initiate the connection in the <source-address> position on it will initiate the connection in the <source-address> position on
the a= line.
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the a=direction line. The following SDP fragment shows an example
of direction:active:
c=IN IP4 10.1.1.1
m=image 9 TCP t38
a=direction:active IN IP4 10.1.1.1 1892
3.3 Semantics of direction:both
By specifying direction:both, the endpoint indicates that it will By specifying direction:both, the endpoint indicates that it will
both accept a TCP connection on the port number of its own m= line, both accept a TCP connection on the port number of its own m= line,
and that it will also initiate a connection to the port number on and that it will also initiate a connection to the port number on
the m= line of the other endpoint. the m= line of the other endpoint.
As with direction:active, the endpoint SHOULD specify the address As with direction:active, the endpoint SHOULD specify the address
and port number from which it will initiate the connection in the and port number from which it will initiate the connection in the
<source-address> position on the a= line. <source-address> position on the a=direction line.
Since this attribute describes behavior that is similar to Since this attribute describes behavior that is similar to
connectionless media descriptions in [SDP], it is the default value connectionless media descriptions in [SDP], it is the default value
for the direction attribute and is therefore optional. for the direction attribute and is therefore optional.
Endpoints may choose to specify direction:both for one or more of Endpoints may choose to specify direction:both for one or more of
the following reasons: the following reasons:
1) The endpoint has no preference as to whether it accepts or 1) The endpoint has no preference as to whether it accepts or
initiates the connection, and therefore is offering the remote initiates the connection, and therefore is offering the remote
endpoint a choice of connection setup procedures. endpoint a choice of connection setup procedures.
2) The endpoints intend to use a single connection to transport 2) The endpoints intend to use a single connection to transport
the media, but it is not known whether firewall issues will the media, but it is not known whether firewall issues will
prevent either endpoint from initiating or accepting the prevent either endpoint from initiating or accepting the
connection. Therefore both endpoints will attempt to initiate connection. Therefore both endpoints will attempt to initiate
a connection in hopes that at least one will succeed. a connection in hopes that at least one will succeed.
3) The endpoints intend to use two connections to transport the
media, and one must be initiated by the remote endpoint and
the other must be initiated by the local endpoint.
If one endpoint specifies either direction:active or If one endpoint specifies either direction:active or
direction:passive and the other specifies direction:both, both direction:passive and the other specifies direction:both, both
endpoints MUST behave as if the latter had specified the inverse endpoints MUST behave as if the latter had specified the inverse
direction of the former. For example, specifying direction:both direction of the former. For example, specifying direction:both
when the other endpoint specifies direction:active SHALL cause both when the other endpoint specifies direction:active SHALL cause both
endpoints to behave as if the former had specified endpoints to behave as if the former had specified
direction:passive. Conversely, specifying direction:both when the direction:passive. Conversely, specifying direction:both when the
other endpoint specifies direction:passive SHALL cause both other endpoint specifies direction:passive SHALL cause both
endpoints to behave as if the former had specified direction:active. endpoints to behave as if the former had specified direction:active.
If both endpoints specify direction:both then each endpoint MUST If both endpoints specify direction:both then each endpoint MUST
initiate a connection to the port number specified on the m= line of initiate a connection to the port number specified on the m= line of
the opposite endpoint. If a single connection is needed (case #1 or the opposite endpoint. There is one exception to this requirement:
#2 above), there is one exception to this requirement: if an if an endpoint receives the incoming connection from the opposite
endpoint receives the incoming connection from the opposite endpoint endpoint prior to initiating its own outbound connection, then that
prior to initiating its own outbound connection, then that endpoint endpoint MAY use that connection rather than attempt to make an
MAY use that connection rather than attempt to make an outbound outbound connection to the opposite endpoint.
connection to the opposite endpoint.
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If only one connection succeeds, then that connection will be used If only one connection succeeds, then that connection will be used
to carry the media. Once it has transmitted data on this to carry the media. Once it has transmitted data on this
connection, the initiating endpoint MUST NOT perform another connection, the initiating endpoint MUST NOT perform another
connection attempt to the accepting endpoint. This allows the connection attempt to the accepting endpoint. This allows the
accepting endpoint to release or recycle the listening port for accepting endpoint to release or recycle the listening port for
another session once it has received data from the initiating another session once it has received data from the initiating
endpoint. endpoint.
If both connections succeed but only one was needed (case #2 above), If both connections succeed, the following rules SHALL apply:
the following rules SHALL apply:
a) Each endpoint MUST accept data from either connection. a) Each endpoint MUST accept data from either connection.
b) Once an endpoint has transmitted data to one of the b) Once an endpoint has transmitted data to one of the connections,
connections, it MUST use that connection exclusively for it MUST use that connection exclusively for transmission.
transmission.
c) Once an endpoint has transmitted AND received data, if one of c) Once an endpoint has transmitted AND received data, if one of the
the connections is determined to be idle, the endpoint MAY connections is determined to be idle, the endpoint SHOULD close
close the idle connection. the idle connection.
2.4 Semantics of direction:reuse 3.4 Optimizing direction:both
By specifying direction:reuse, the endpoint indicates that it is As discussed in the previous section, there is the possibility that
changing the parameter(s) of an existing session on a previously two connections will be created when only one is needed. While
established connection with the opposite endpoint. Therefore no new rules in the previous section accommodate the closing of an idle
connections are to be created. This is intended for cases where connection, they do not prevent a race condition where the endpoints
media types are added, removed, or changed during a session. For simultaneously start sending data on opposite connections thereby
example, an endpoint adding a video stream to an existing audio causing two connections to be used where one would have sufficed.
session may elect to multiplex the new stream over the same While it is not possible to entirely eliminate this race condition,
connection that is currently transporting the audio stream. it is in the endpoints interest to minimize its occurrence.
Therefore, when a session is negotiated through interactive exchange
of SDP between endpoints (as in the case of SIP) AND the result of
the negotiation is that each endpoint specifies direction:both, it
is RECOMMENDED that the endpoints use the following guidelines:
2.5 Bidirectional versus Unidirectional Media a) There comes a point during the exchange of SDP where one endpoint
is prepared to send the final message that will complete the
negotiation and allow the session to begin. For the purposes of
this discussion, the endpoint that will send this final message
will be called the Initiator, and the endpoint that will receive
this message will be called the Acceptor.
b) The Initiator, upon receiving sufficient information to initiate a
connection, MUST attempt to connect to the Acceptor as soon as
possible.
c) In order to lower the likelihood that the Acceptor will also
attempt to initiate a connection, the Initiator SHOULD incorporate
a short delay between initiating the connection and sending the
final SDP to the Acceptor.
d) The delay time chosen by the Initiator MUST NOT introduce an
unacceptable session setup delay should the connection to the
Acceptor not succeed.
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3.5 Bidirectional versus Unidirectional Media
In traditional SDP transport types the flow is unidirectional. If In traditional SDP transport types the flow is unidirectional. If
the intent is for media to flow in both directions, both endpoints the intent is for media to flow in both directions, both endpoints
must specify SDP that describes where to deliver the media and what must specify SDP that describes where to deliver the media and what
media type(s) to use. For example, if only Endpoint A presents SDP media type(s) to use. For example, if only Endpoint A presents SDP
then media can only flow towards Endpoint A, as Endpoint B has not then media can only flow towards Endpoint A, as Endpoint B has not
specified where and how to send media to it. specified where and how to send media to it.
Because most connection-oriented media is inherently bi-directional, Because most connection-oriented media is inherently bi-directional,
endpoints may encounter a situation where only one side presented endpoints may encounter a situation where only one side presented
SDP yet there is now a network path that can carry media in either SDP yet there is now a network path that can carry media in either
direction. In keeping with traditional SDP semantics, an endpoint direction. In keeping with traditional SDP semantics, an endpoint
MUST NOT send data to the other endpoint unless it has specified SDP MUST NOT send data to the other endpoint unless it has specified SDP
information describing the type of media it can accept. information describing the type of media it can accept.
It is, however, perfectly acceptable for an endpoint to transmit It is, however, perfectly acceptable for an endpoint to transmit
data on the same connection it is using to receive data, so long as data on the same connection it is using to receive data, so long as
the other endpoint has advertised its willingness to accept data. the other endpoint has advertised its willingness to accept data.
Likewise, it is perfectly acceptable for an endpoint to receive data Likewise, it is perfectly acceptable for an endpoint to receive data
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on the same connection it is using to transmit data to the on the same connection it is using to transmit data to the
corresponding remote endpoint. In other words, for a bi-directional corresponding remote endpoint. In other words, for a bi-directional
application-level session, a connection may be used to send data in application-level session, a connection may be used to send data in
both directions (contingent to rules outlined in Section 2.3) as both directions (contingent to rules outlined in Section 2.3) as
long as one side of the connection is attached to either of the long as one side of the connection is attached to either of the
advertised SDP transport addresses. advertised SDP transport addresses.
3 Source-Address Considerations 3.6 Treating UDP and RTP/AVP like Connection Oriented Media
Endpoints MAY specify a direction attribute for UDP or RTP/AVP
media. This indicates that the endpoint would like to treat this
media as a type of connection oriented media. (The endpoint may do
this to facilitate NAT traversal for example.) Note that for
backwards compatibility, an endpoint which can specify
direction:active MUST include valid addresses and ports in the SDP
as always. If the peer's SDP does not include a direction
attribute, it knows that the peer does not support connection-
oriented media, and media exchange will proceed normally, as if
connection-oriented media were not offered.
Endpoints that specify direction:passive MUST NOT send any media,
any packets whatsoever (including control packets such as RTCP),
from their passive ports until they receive a packet on these ports
and record the source address and port of the sender. The passive
endpoint then assumes that the first packet received corresponds to
its active peer. From this point onward, passive endpoints MUST
send UDP or RTP media from the same port as the port indicated in
the m= line. Passive endpoints MUST send RTCP media (if any) from
the port on which they expect to receive it (typically the RTP port
number plus 1).
Endpoints that specify direction:active MUST be prepared to receive
on the ports from which they send. Once they learn the IP address
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and port of their peer from the peer's SDP, they SHOULD immediately
send some kind of media (even if just comfort noise) to each of
these ports. This is so the peer can learn their IP address and
port, in order to send media back without additional delay.
Effectively, the exchange of the first media packet completes a bi-
directional handshake between the active and passive peer.
4 Source-Address Considerations
In the cases where the endpoint is initiating the connection, it is In the cases where the endpoint is initiating the connection, it is
RECOMMENDED that a source address be specified on the a= line by RECOMMENDED that a source address be specified on the a=direction
that endpoint. It is also RECOMMENDED that the source port be line by that endpoint. It is also RECOMMENDED that the source port
included in the source address. In most environments, the source be included in the source address. In most environments, the source
port number can be determined by binding the socket before port number can be determined by binding the socket before
initiating the connect, as shown in the sample C code below: initiating the connect, as shown in the sample C code below:
{ {
SOCKET s_id SOCKET s_id
SOCKADDR_IN cli_sin; SOCKADDR_IN cli_sin;
int namelen; int namelen;
// Create the socket // Create the socket
s_id = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP); s_id = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
skipping to change at line 306 skipping to change at line 364
} }
If the source address is omitted, the receiver of the SDP packet If the source address is omitted, the receiver of the SDP packet
MUST NOT make any assumptions in regards to the address or port from MUST NOT make any assumptions in regards to the address or port from
where the connection will originate. In particular, the receiver where the connection will originate. In particular, the receiver
MUST NOT assume that the address information listed on the c= line MUST NOT assume that the address information listed on the c= line
has any implication as to where the media connection originates. has any implication as to where the media connection originates.
NOTE: NOTE:
The motivation for specifying the source address is The motivation for specifying the source address is
twofold. First, it aids Application-Level Proxies by twofold. First, it aids Application-Level Proxies
explicitly announcing the source of the outbound (ALP) by explicitly announcing the source of the
connection. This allows, for example, a dynamic outbound connection. This allows, for example, a
firewall pinhole to be created that will allow the dynamic firewall pinhole to be created that will allow
connection to pass. the connection to pass. Or as another example, an ALP
integrated with a Network Address Translation (NAT)
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gateway could create a dynamic address/port binding
and rewrite the SDP accordingly.
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Second, it allows the passive endpoint to correlate Second, it allows the passive endpoint to correlate
the incoming connection with the session being the incoming connection with the session being
negotiated. Note that great care must be taken when negotiated. Note that great care must be taken when
using the source address as a means to identify using the source address as a means to identify
incoming connections, as Network Address Translation incoming connections, as NAT can render the source
(NAT) can render the source address unreliable. In address unreliable. In addition if the originating
addition if the originating endpoint omits the source endpoint omits the source port, the source address can
port, the source address can be ambiguous if multiple, be ambiguous if multiple, logical endpoints share the
logical endpoints share the same network address. same network address. Therefore it is NOT RECOMMENDED
Therefore it is NOT RECOMMENDED that the source that the source address be used for this purpose
address be used for this purpose unless the SDP occurs unless the SDP occurs in the context of a controlled
in the context of a controlled network topology that network topology that guarantees that the source
guarantees that the source address is both correct address is both correct (i.e., no NAT, or a NAT with
(i.e., no NAT, or a NAT with an Application-Level an Application-Level Proxy that rewrites the SDP) and
Proxy that rewrites the SDP) and unambiguous (i.e., unambiguous (i.e., the source port is specified).
the source port is specified).
3.1 Source Address Timing Considerations 4.1 Source Address Timing Considerations
When used in conjunction with a session signaling protocol such as When used in conjunction with a session signaling protocol such as
SIP, there may be cases where an endpoint initiates a connection SIP, there may be cases where an endpoint initiates a connection
prior to the opposite endpoint receiving the SDP that describe the prior to the opposite endpoint receiving the SDP that describe the
source address of the initiating endpoint. Therefore, an endpoint source address of the initiating endpoint. Therefore, an endpoint
that has advertised an address and port number with direction:both that has advertised an address and port number with direction:both
or direction:passive MUST be ready to accept a connection on that or direction:passive MUST be ready to accept a connection on that
address and port immediately. If the accepting endpoint requires address and port immediately. If the accepting endpoint requires
the source address to identify the initiating endpoint, it MUST keep the source address to identify the initiating endpoint, it MUST keep
the connection active and allow sufficient time for the source the connection active and allow sufficient time for the source
address to arrive before discarding the connection. address to arrive before discarding the connection.
4 Examples 5 Connection and Listener Lifetime Considerations
5.1 Listener Lifetime
An endpoint that has specified direction:both or direction:passive
MUST be ready to accept a connection on the appropriate address and
port during the time slot(s) advertised for that session. The
endpoint MUST keep the address and port available for incoming
connections until either:
a) The time window for the session has expired, or
b) The endpoint has received the expected number of incoming
connections on that address and port, or
c) Subsequent exchanges have superceded the SDP that originally
advertised the availability of the address and port.
Once the endpoint has determined that a listener is no longer needed
on a specific address and port, it SHOULD terminate the listener.
The endpoint is then free to re-use the address and port for
subsequent session advertisements.
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5.2 Connection Lifetime
An endpoint that intends to initiate the connection MUST initiate
the connection immediately after it has sufficient information to do
so, even if it does not intend to immediately begin sending media to
the remote endpoint. This allows media to flow from the remote
endpoint.
An endpoint MUST NOT close the connection until the session has
expired, been explicitly terminated, or the media stream is
redirected to a different address or port.
If the endpoint determines that the connection has been closed, it
MAY attempt to re-establish the connection. The decision to do so
is application and/or context dependant. If the endpoint opts to
re-establish the connection, it MUST NOT assume that the original
address and port advertised by the remote endpoint is still valid.
Instead, the endpoint MUST renegotiate the session parameters by
exchanging new SDP.
5.3 Session Renegotiation and Connection Lifetime
There are scenarios where SDP is sent by an endpoint in order to
renegotiate an existing session. These include muting/unmuting a
session, renegotiating the attributes of the media used by the
session, or extending the length of a session about to expire.
Connection-oriented media introduces some ambiguities into session
renegotiation as to when the direction attribute must be obeyed and
when it is ignored.
The scenario of extending the duration of an existing session is a
good example: in order to extend an existing session, endpoints will
typically resend the original SDP with updated time information. In
connectionless media the result is no change to the existing media
streams. The problem with connection oriented media is that the
original SDP will contain a direction attribute which can be
construed as a request to create a new connection, as opposed to a
request to maintain steady state. To avoid this ambiguity, the
following rule SHALL apply to subsequent exchanges of SDP:
If the transport section combined with the direction
attribute of an SDP message describes an existing
connection between two endpoints, then the endpoints
MUST use that connection to carry the media described
in the remainder of the message. The endpoints MUST
NOT attempt to set up a new connection, regardless of
what is specified in the direction attribute.
This disambiguates most session renegotiation scenarios, with the
exception of muting. Muting a media stream is accomplished by
sending the original session SDP but with the port number set to
zero. This is still valid for connection oriented media, with the
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additional caveat that the endpoints MUST NOT close the connection
described by that SDP.
6 Examples
What follows are a number of examples that show the most common What follows are a number of examples that show the most common
usage of the direction attribute combined with TCP-based media usage of the direction attribute combined with TCP-based media
descriptions. For the purpose of brevity, the main portion of the descriptions. For the purpose of brevity, the main portion of the
session description is omitted in the examples and is assumed to be session description is omitted in the examples and is assumed to be
the following: the following:
v=0 v=0
o=me 2890844526 2890842807 IN IP4 10.1.1.2 o=me 2890844526 2890842807 IN IP4 10.1.1.2
e=Me <me@ietf.org>
s=Call me using TCP s=Call me using TCP
t=0 0 t=3034423619 3042462419
4.1 Example: simple passive/active 6.1 Example: simple passive/active
An endpoint at 10.1.1.2 signals the availability of a T.38 fax An endpoint at 10.1.1.2 signals the availability of a T.38 fax
session at port 54111: session at port 54111:
c=IN IP4 10.1.1.2 c=IN IP4 10.1.1.2
m=image 54111 TCP t38 m=image 54111 TCP t38
a=direction:passive a=direction:passive
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An endpoint at 10.1.1.1 receiving this description responds with the An endpoint at 10.1.1.1 receiving this description responds with the
following: following:
c=IN IP4 10.1.1.1 c=IN IP4 10.1.1.1
m=image 9 TCP t38 m=image 9 TCP t38
a=direction:active a=direction:active
The endpoint at 10.1.1.1 then initiates the TCP connection to port The endpoint at 10.1.1.1 then initiates the TCP connection to port
54111 at 10.1.1.2. Note that the TCP connection may originate from 54111 at 10.1.1.2. Note that the TCP connection may originate from
any address or port. The endpoint at 10.1.1.1 could have optionally any address or port. The endpoint at 10.1.1.1 could have optionally
committed to a source address with a simple modification: committed to a source address with a simple modification:
c=IN IP4 10.1.1.1 c=IN IP4 10.1.1.1
m=image 9 TCP t38 m=image 9 TCP t38
a=direction:active IN IP4 10.1.1.1 1892 a=direction:active IN IP4 10.1.1.1 1892
By adding the source address to the a= line, the endpoint at By adding the source address to the a=direction line, the endpoint
10.1.1.1 must now use a source port of 1892 when initiating the TCP at 10.1.1.1 must now use a source port of 1892 when initiating the
connection to port 54111 at 10.1.1.2. TCP connection to port 54111 at 10.1.1.2.
4.2 Example: agnostic both 6.2 Example: agnostic both
An endpoint at 10.1.1.2 signals the availability of a T.38 fax An endpoint at 10.1.1.2 signals the availability of a T.38 fax
session at TCP port 54111, but is also willing to set up the media session at TCP port 54111, but is also willing to set up the media
stream by initiating the TCP connection: stream by initiating the TCP connection:
c=IN IP4 10.1.1.2 c=IN IP4 10.1.1.2
m=image 54111 TCP t38 m=image 54111 TCP t38
a=direction:both a=direction:both
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The endpoint at 10.1.1.1 has three choices: The endpoint at 10.1.1.1 has three choices:
1) It can respond with either of the two direction:active 1) It can respond with either of the two direction:active
descriptions listed in the previous example. In this case the descriptions listed in the previous example. In this case the
endpoint at 10.1.1.1 must initiate a connection to port 54111 endpoint at 10.1.1.1 must initiate a connection to port 54111
at 10.1.1.2. at 10.1.1.2.
2) It can respond with a description similar to the following: 2) It can respond with a description similar to the following:
c=IN IP4 10.1.1.1 c=IN IP4 10.1.1.1
m=image 54321 TCP t38 m=image 54321 TCP t38
a=direction:passive a=direction:passive
In this case the endpoint at 10.1.1.2 must initiate a In this case the endpoint at 10.1.1.2 must initiate a
connection to port 54321 at 10.1.1.1. connection to port 54321 at 10.1.1.1.
3) It can respond with a description that specifies 3) It can respond with a description that specifies
direction:both, which is covered in the next example. direction:both, which is covered in the next example.
4.3 Example: redundant both 6.3 Example: redundant both
An endpoint at 10.1.1.2 uses the same description as the previous An endpoint at 10.1.1.2 uses the same description as the previous
example: example:
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c=IN IP4 10.1.1.2 c=IN IP4 10.1.1.2
m=image 54111 TCP t38 m=image 54111 TCP t38
a=direction:both a=direction:both
Unlike the previous example, the endpoint at 10.1.1.1 responds with Unlike the previous example, the endpoint at 10.1.1.1 responds with
the following description: the following description:
c=IN IP4 10.1.1.1 c=IN IP4 10.1.1.1
m=image 54321 TCP t38 m=image 54321 TCP t38
a=direction:both a=direction:both
This will cause the endpoint at 10.1.1.2 to initiate a connection to This will cause the endpoint at 10.1.1.2 to initiate a connection to
port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a
connection to port 54111 at 10.1.1.2. Whichever TCP connection connection to port 54111 at 10.1.1.2. Whichever TCP connection
succeeds will be used. If both succeed, one of the connections may succeeds will be used. If both succeed, one of the connections may
be closed as an optimization, using the rules in section 2.3. be closed as an optimization, using the rules in section 2.3.
5 Security Considerations 6.4 Example: "Bidirectional" RTP and RTCP
An endpoint at 10.1.1.2 is behind a NAT and does not know its own
public address.
c=IN IP4 10.1.1.2
m=audio 9 RTP/AVP 0
a=direction:active
A peer with a public IP address responds as follows and waits to
receive RTP and RTCP packets from its active peer.
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c=IN IP4 1.2.3.4
m=audio 18240 RTP/AVP 0
a=direction:passive
The endpoint at 10.1.1.2 immediately sends RTP from port 9012 to
1.2.3.4 port 18240. A NAT translates the source address to 5.6.7.8
port 1542. The passive endpoint receives this RTP packet and stores
this source address. When the passive endpoint wants to send RTP
media it sends it back to 5.6.7.8 port 1542. The NAT translates this
destination address back to 10.1.1.2 port 9012 and delivers it to
the active endpoint.
Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port
9013 to 1.2.3.4:18241. The NAT translates this to 5.6.7.8:1984. The
passive endpoint receives the RTCP packet and stores the source
address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which
is translated back to 10.1.1.2:9013 and delivered to the active
endpoint.
7 Security Considerations
See [SDP] for security and other considerations specific to the See [SDP] for security and other considerations specific to the
Session Description Protocol in general. There are no new security Session Description Protocol in general.
considerations introduced by these protocol identifiers and
attributes.
6 IANA Considerations A possible security concern arises if a firewall were to monitor and
act on the source address as described in the note in Section 4.
Firewall implementers must take care to ensure that the SDP came
from a trusted source before deciding whether to change the network
traffic restrictions currently imposed by the firewall.
8 IANA Considerations
As recommended by [SDP] Appendix B, the direction attribute As recommended by [SDP] Appendix B, the direction attribute
described in this document should be registered with IANA, as should described in this document should be registered with IANA, as should
the "TCP" and "TLS" protocol identifiers. the "TCP" and "TLS" protocol identifiers.
Acknowledgements Acknowledgements
The author would like to thank Jonathan Rosenberg, Anders The author would like to thank Jonathan Rosenberg, Rohan Mahy,
Kristensen, Paul Kyzivat, and Robert Fairlie-Cuninghame for their Anders Kristensen, Paul Kyzivat, and Robert Fairlie-Cuninghame for
valuable insights. their valuable insights and contributions.
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Appendix A: Direction Attribute Syntax Appendix A: Direction Attribute Syntax
This appendix provides an Augmented BNF [ABNF] grammar for This appendix provides an Augmented BNF [ABNF] grammar for
expressing the direction attribute for connection setup. It is expressing the direction attribute for connection setup. It is
intended as an extension to the grammar for the Session Description intended as an extension to the grammar for the Session Description
Protocol, as defined in [SDP]. Specifically, it describes the Protocol, as defined in [SDP]. Specifically, it describes the
syntax for the new "connection-setup" attribute field, which MAY be syntax for the new "connection-setup" attribute field, which MAY be
either a session-level or media-level attribute. either a session-level or media-level attribute.
connection-setup = "direction" ":" direction-spec connection-setup = "direction" ":" direction-spec
direction-spec = "passive" | qualified-direction direction-spec = "passive" | qualified-direction
qualified-direction = direction-ident | direction-ident source qualified-direction = direction-ident | direction-ident source
direction-ident = "both" | "active" | "reuse" direction-ident = "both" | "active" | "passive"
source = nettype addrtype unicast-address | source = nettype addrtype unicast-address |
nettype addrtype unicast-address port nettype addrtype unicast-address port
References References
[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax [ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications: ABNF," RFC 2234, November 1997 Specifications: ABNF," RFC 2234, November 1997
[SDP] M. Handley, V. Jacobson, "SDP: Session Description [SDP] M. Handley, V. Jacobson, "SDP: Session Description
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