draft-ietf-mmusic-sdp-comedia-02.txt   draft-ietf-mmusic-sdp-comedia-03.txt 
INTERNET-DRAFT D. Yon INTERNET-DRAFT D. Yon
Document: draft-ietf-mmusic-sdp-comedia-02.txt Dialout.Net Document: draft-ietf-mmusic-sdp-comedia-03.txt Dialout.Net
Expires October 2002 April 2002 Expires December 2002 June 2002
Connection-Oriented Media Transport in SDP Connection-Oriented Media Transport in SDP
<draft-ietf-mmusic-sdp-comedia-02.txt> <draft-ietf-mmusic-sdp-comedia-03.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 51 skipping to change at line 51
1 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., arbitrary existing application protocols for transport (e.g., SAP,
SAP, SIP, RTSP, email, HTTP, etc.). SIP, RTSP, email, HTTP, etc.).
[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. This memo defines two new protocol connection setup procedure. This memo defines two new protocol
identifiers, TCP and TLS, along with the syntax and semantics of the identifiers, TCP and TLS, along with the syntax and semantics of the
a=direction attribute. a=direction and a=reconnect attributes.
Terminology 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", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [7] and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
and indicate requirement levels for compliant implementations. and indicate requirement levels for compliant implementations.
2 Protocol Identifiers 2 Protocol Identifiers
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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).
2.2 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.
3 Direction Attribute 3 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:
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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.
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 SHOULD be specified
direction:active or direction:both. Within the <source-address>, with direction:active or direction:both, and MUST NOT be specified
the source port number is RECOMMENDED but may be omitted. with direction:passive. Within the <source-address>, the source
port number is RECOMMENDED but may be omitted.
3.1 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.
3.2 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
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the a=direction line. The following SDP fragment shows an example the a=direction line. The following SDP fragment shows an example
of direction:active: of direction:active:
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
3.3 Semantics of direction:both 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
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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. There is one exception to this requirement: the opposite endpoint. There is one exception to this requirement:
if an endpoint receives the incoming connection from the opposite if an endpoint receives the incoming connection from the opposite
endpoint prior to initiating its own outbound connection, then that endpoint prior to initiating its own outbound connection, then that
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endpoint MAY use that connection rather than attempt to make an endpoint MAY use that connection rather than attempt to make an
outbound connection to the opposite endpoint. outbound 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, the following rules SHALL apply: If both connections succeed, the following rules SHALL apply:
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3.4 Optimizing direction:both 3.4 Optimizing direction:both
As discussed in the previous section, there is the possibility that As discussed in the previous section, there is the possibility that
two connections will be created when only one is needed. While two connections will be created when only one is needed. While
rules in the previous section accommodate the closing of an idle rules in the previous section accommodate the closing of an idle
connection, they do not prevent a race condition where the endpoints connection, they do not prevent a race condition where the endpoints
simultaneously start sending data on opposite connections thereby simultaneously start sending data on opposite connections thereby
causing two connections to be used where one would have sufficed. causing two connections to be used where one would have sufficed.
While it is not possible to entirely eliminate this race condition, While it is not possible to entirely eliminate this race condition,
it is in the endpoints interest to minimize its occurrence. it is in the endpoints' interest to minimize its occurrence.
Therefore, when a session is negotiated through interactive exchange Therefore, when a session is negotiated through interactive exchange
of SDP between endpoints (as in the case of SIP) AND the result of of SDP between endpoints (as in the case of SIP) AND the result of
the negotiation is that each endpoint specifies direction:both, it the negotiation is that each endpoint specifies direction:both, it
is RECOMMENDED that the endpoints use the following guidelines: is RECOMMENDED that the endpoints use the following guidelines:
a) There comes a point during the exchange of SDP where one endpoint a) There comes a point during the exchange of SDP where one endpoint
is prepared to send the final message that will complete the is prepared to send the final message that will complete the
negotiation and allow the session to begin. For the purposes of negotiation and allow the session to begin. For the purposes of
this discussion, the endpoint that will send this final message this discussion, the endpoint that will send this final message
will be called the Initiator, and the endpoint that will receive will be called the Initiator, and the endpoint that will receive
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b) The Initiator, upon receiving sufficient information to initiate a b) The Initiator, upon receiving sufficient information to initiate a
connection, MUST attempt to connect to the Acceptor as soon as connection, MUST attempt to connect to the Acceptor as soon as
possible. possible.
c) In order to lower the likelihood that the Acceptor will also c) In order to lower the likelihood that the Acceptor will also
attempt to initiate a connection, the Initiator SHOULD incorporate attempt to initiate a connection, the Initiator SHOULD incorporate
a short delay between initiating the connection and sending the a short delay between initiating the connection and sending the
final SDP to the Acceptor. final SDP to the Acceptor.
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d) The delay time chosen by the Initiator MUST NOT introduce an d) The delay time chosen by the Initiator MUST NOT introduce an
unacceptable session setup delay should the connection to the unacceptable session setup delay should the connection to the
Acceptor not succeed. Acceptor not succeed.
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3.5 Bidirectional versus Unidirectional Media 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,
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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.
3.6 Treating UDP and RTP/AVP like Connection Oriented Media 3.6 Treating UDP and RTP/AVP like Connection Oriented Media
Endpoints MAY specify a direction attribute for UDP or RTP/AVP Endpoints MAY specify a direction attribute for UDP or RTP/AVP
media. This indicates that the endpoint would like to treat this media. This indicates that the endpoint would like to treat this
media as a type of connection oriented media. (The endpoint may do media as a type of connection-oriented media. (The endpoint may do
this to facilitate NAT traversal for example.) Note that for this to facilitate NAT traversal for example.) Note that for
backwards compatibility, an endpoint which can specify backwards compatibility, an endpoint which can specify
direction:active MUST include valid addresses and ports in the SDP direction:active MUST include valid addresses and ports in the SDP
as always. If the peer's SDP does not include a direction as always. If the peer's SDP does not include a direction
attribute, it knows that the peer does not support connection- attribute, it knows that the peer does not support connection-
oriented media, and media exchange will proceed normally, as if oriented media, and media exchange will proceed normally, as if
connection-oriented media were not offered. connection-oriented media were not offered.
Endpoints that specify direction:passive MUST NOT send any media, Endpoints that specify direction:passive MUST NOT send any media,
any packets whatsoever (including control packets such as RTCP), any packets whatsoever (including control packets such as RTCP),
from their passive ports until they receive a packet on these ports from their passive ports until they receive a packet on these ports
and record the source address and port of the sender. The passive and record the source address and port of the sender. The passive
endpoint then assumes that the first packet received corresponds to endpoint then assumes that the first packet received corresponds to
its active peer. From this point onward, passive endpoints MUST its active peer. From this point onward, passive endpoints MUST
send UDP or RTP media from the same port as the port indicated in 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 m= line. Passive endpoints MUST send RTCP media (if any) from
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the port on which they expect to receive it (typically the RTP port the port on which they expect to receive it (typically the RTP port
number plus 1). number plus 1).
Endpoints that specify direction:active MUST be prepared to receive Endpoints that specify direction:active MUST be prepared to receive
on the ports from which they send. Once they learn the IP address 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 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 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 these ports. This is so the peer can learn their IP address and
port, in order to send media back without additional delay. port, in order to send media back without additional delay.
Effectively, the exchange of the first media packet completes a bi- Effectively, the exchange of the first media packet completes a bi-
directional handshake between the active and passive peer. directional handshake between the active and passive peer.
4 Source-Address Considerations 4 Reconnect Attribute
In the cases where the endpoint is initiating the connection, it is The preceding description of the a=direction attribute has been in
RECOMMENDED that a source address be specified on the a=direction the context of using SDP to initiate a session. However, SDP may be
line by that endpoint. It is also RECOMMENDED that the source port exchanged between endpoints at various stages of a session to
be included in the source address. In most environments, the source accomplish tasks such as terminating a session, redirecting media to
port number can be determined by binding the socket before a new endpoint, renegotiating the media parameters for a session,
initiating the connect, as shown in the sample C code below: etc. After the initial session has been established, it may be
ambiguous as to whether subsequent SDP exchange represents a
confirmation that the endpoint is to continue using the current
media connection unchanged, or is a request to make a new media
connection. The reconnect attribute is used to disambiguate these
two scenarios, and the syntax is as follows:
a=reconnect
SDP containing a=reconnect signals that the specified session does
NOT refer to an existing connection between the two endpoints. If
the endpoints agree to continue the session, the endpoints MUST
close the existing connection for the currently negotiated session,
and MUST create a new connection according to the a=direction
attribute in the SDP. If an endpoint receives SDP that contains
a=reconnect, the endpoint's response MUST also contain a=reconnect.
Endpoints MUST NOT include a=reconnect in SDP that negotiates the
start of a session.
See section 6, "Connection and Listener Lifetime Considerations" for
more information on scenarios that are relevant to the a=reconnect
attribute.
5 Source-Address Considerations
In the cases where the endpoint is initiating the connection, the
endpoint SHOULD specify a source address on the a=direction line.
In addition, the endpoint SHOULD include the source port in the
source address. In most environments, the source port number can be
determined by binding the socket before 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;
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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);
// Bind the socket to any IP address and port // Bind the socket to any IP address and port
bzero((char *)&cli_sin,sizeof(cli_sin)); bzero((char *)&cli_sin,sizeof(cli_sin));
cli_sin.sin_family = AF_INET; cli_sin.sin_family = AF_INET;
cli_sin.sin_addr.s_addr = htonl(INADDR_ANY); cli_sin.sin_addr.s_addr = htonl(INADDR_ANY);
cli_sin.sin_port = 0; cli_sin.sin_port = 0;
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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 twofold. First, it aids Application-Level Proxies
(ALP) by explicitly announcing the source of the (ALP) by explicitly announcing the source of the
outbound connection. This allows, for example, a outbound connection. This allows, for example, a
dynamic firewall pinhole to be created that will allow dynamic firewall pinhole to be created that will allow
the connection to pass. Or as another example, an ALP the connection to pass. Or as another example, an ALP
integrated with a Network Address Translation (NAT) integrated with a Network Address Translation (NAT)
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gateway could create a dynamic address/port binding gateway could create a dynamic address/port binding
and rewrite the SDP accordingly. and rewrite the SDP accordingly.
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 NAT can render the source incoming connections, as NAT can render the source
address unreliable. In addition if the originating address unreliable. In addition if the originating
endpoint omits the source port, the source address can endpoint omits the source port, the source address can
be ambiguous if multiple, logical endpoints share the be ambiguous if multiple, logical endpoints share the
same network address. Therefore it is NOT RECOMMENDED same network address. Therefore it is NOT RECOMMENDED
that the source address be used for this purpose that the source address be used for this purpose
unless the SDP occurs in the context of a controlled unless the SDP occurs in the context of a controlled
network topology that guarantees that the source network topology that guarantees that the source
address is both correct (i.e., no NAT, or a NAT with address is both correct (i.e., no NAT, or a NAT with
an Application-Level Proxy that rewrites the SDP) and an Application-Level Proxy that rewrites the SDP) and
unambiguous (i.e., the source port is specified). unambiguous (i.e., the source port is specified).
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5.1 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.
5 Connection and Listener Lifetime Considerations 6 Connection and Listener Lifetime Considerations
5.1 Listener Lifetime 6.1 Listener Lifetime
An endpoint that has specified direction:both or direction:passive An endpoint that has specified direction:both or direction:passive
MUST be ready to accept a connection on the appropriate address and MUST be ready to accept a connection on the appropriate address and
port during the time slot(s) advertised for that session. The port during the time slot(s) advertised for that session. The
endpoint MUST keep the address and port available for incoming endpoint MUST keep the address and port available for incoming
connections until either: connections until either:
a) The time window for the session has expired, or a) The time window for the session has expired, or
b) The endpoint has received the expected number of incoming b) The endpoint has received the expected number of incoming
connections on that address and port, or connections on that address and port, or
c) Subsequent exchanges have superceded the SDP that originally c) Subsequent exchanges have superceded the SDP that originally
advertised the availability of the address and port. advertised the availability of the address and port.
Once the endpoint has determined that a listener is no longer needed Once the endpoint has determined that a listener is no longer needed
on a specific address and port, it SHOULD terminate the listener. on a specific address and port, it SHOULD terminate the listener.
The endpoint is then free to re-use the address and port for The endpoint is then free to re-use the address and port for
subsequent session advertisements. subsequent session advertisements.
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5.2 Connection Lifetime
An endpoint that intends to initiate the connection MUST initiate An endpoint that intends to initiate the connection MUST initiate
the connection immediately after it has sufficient information to do the connection immediately after it has sufficient information to do
so, even if it does not intend to immediately begin sending media to so, even if it does not intend to immediately begin sending media to
the remote endpoint. This allows media to flow from the remote the remote endpoint. This allows media to flow from the remote
endpoint. endpoint.
An endpoint MUST NOT close the connection until the session has An endpoint MUST NOT close the connection until the session has
expired, been explicitly terminated, or the media stream is expired, been explicitly terminated, or the media stream is
redirected to a different address or port. redirected to a different address or port.
If the endpoint determines that the connection has been closed, it If the endpoint determines that the connection has been closed, it
MAY attempt to re-establish the connection. The decision to do so MAY attempt to re-establish the connection. The decision to do so
is application and/or context dependant. If the endpoint opts to is application and/or context dependant. If the endpoint opts to
re-establish the connection, it MUST NOT assume that the original re-establish the connection, it MUST NOT assume that the original
address and port advertised by the remote endpoint is still valid. address and port advertised by the remote endpoint is still valid.
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Instead, the endpoint MUST renegotiate the session parameters by Instead, the endpoint MUST renegotiate the session parameters by
exchanging new SDP. exchanging new SDP.
5.3 Session Renegotiation and Connection Lifetime 6.3 Session Renegotiation and Connection Lifetime
There are scenarios where SDP is sent by an endpoint in order to There are scenarios where SDP is sent by an endpoint in order to
renegotiate an existing session. These include muting/unmuting a renegotiate an existing session. These include muting/unmuting a
session, renegotiating the attributes of the media used by the session, renegotiating the attributes of the media used by the
session, or extending the length of a session about to expire. session, or extending the length of a session about to expire.
Connection-oriented media introduces some ambiguities into session Connection-oriented media introduces some ambiguities into session
renegotiation as to when the direction attribute must be obeyed and renegotiation as to when the direction attribute must be obeyed and
when it is ignored. when it is ignored.
The scenario of extending the duration of an existing session is a The scenario of extending the duration of an existing session is a
good example: in order to extend an existing session, endpoints will good example: in order to extend an existing session, endpoints will
typically resend the original SDP with updated time information. In typically resend the original SDP with updated time information. In
connectionless media the result is no change to the existing media connectionless media the result is no change to the existing media
streams. The problem with connection oriented media is that the streams. The problem with connection oriented media is that the
original SDP will contain a direction attribute which can be original SDP will contain a direction attribute which can be
construed as a request to create a new connection, as opposed to a construed as a request to create a new connection, as opposed to a
request to maintain steady state. To avoid this ambiguity, the request to maintain steady state. To avoid this ambiguity, the
following rule SHALL apply to subsequent exchanges of SDP: following rule SHALL apply to subsequent exchanges of SDP:
If the transport section combined with the direction If the transport section (the c= and m= lines)
attribute of an SDP message describes an existing combined with the direction attribute of an SDP
connection between two endpoints, then the endpoints message describes an existing connection between two
MUST use that connection to carry the media described endpoints, AND the SDP does not contain a=reconnect,
in the remainder of the message. The endpoints MUST then the endpoints MUST use that connection to carry
NOT attempt to set up a new connection, regardless of the media described in the remainder of the message.
what is specified in the direction attribute. 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 This disambiguates most session renegotiation scenarios, with the
exception of muting. Muting a media stream is accomplished by exception of muting. Muting a media stream is accomplished by
sending the original session SDP but with the port number set to sending the original session SDP but with an "a=inactive" or
zero. This is still valid for connection oriented media, with the "a=sendonly/recvonly" attribute. This is still valid for connection
oriented media, with the additional caveat that the endpoints MUST
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additional caveat that the endpoints MUST NOT close the connection
described by that SDP.
6 Examples 7 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
s=Call me using TCP s=Call me using TCP
t=3034423619 3042462419 t=3034423619 3042462419
6.1 Example: simple passive/active Yon INTERNET-DRAFT - Expires December 2002 10
7.1 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
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:
skipping to change at line 528 skipping to change at line 562
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=direction line, the endpoint By adding the source address to the a=direction line, the endpoint
at 10.1.1.1 must now use a source port of 1892 when initiating the at 10.1.1.1 must now use a source port of 1892 when initiating the
TCP connection to port 54111 at 10.1.1.2. TCP connection to port 54111 at 10.1.1.2.
6.2 Example: agnostic both 7.2 Example: simple passive/active with reconnect
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 10.1.1.2 signals this
intent with the following SDP:
c=IN IP4 10.1.1.2
m=image 54111 TCP t38
a=direction:passive
a=reconnect
The a=reconnect attribute informs the endpoint at 10.1.1.1 that this
SDP represents the intent to establish a new connection for media
transport, rather than continuing with the original connection.
Because the endpoint at 10.1.1.1 may not yet be aware that the TCP
connection has failed, this eliminates any ambiguity. If 10.1.1.1
agrees to continue the session using a new connection, it responds
with:
c=IN IP4 10.1.1.1
m=image 9 TCP t38
a=direction:active IN IP4 10.1.1.1 1893
a=reconnect
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Note that the source port is different in this message, since the OS
will have likely chosen a new ephemeral port number for the new
connection.
7.3 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.
6.3 Example: redundant both 7.4 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:
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 3.3.
6.4 Example: "Bidirectional" RTP and RTCP Yon INTERNET-DRAFT - Expires December 2002 12
In order to minimize the chance that two connections are created,
the endpoint at 10.1.1.1 may opt to use the recommendation in
section 3.4, which would result in events occurring in the following
sequence:
1) The endpoint at 10.1.1.2 sends SDP as listed above. The
endpoint MUST enable a listener on port 54111 at this time,
but is not able to initiate a TCP connection due to the fact
that it does not have sufficient information from the endpoint
at 10.1.1.1.
2) The endpoint at 10.1.1.1, upon receiving the SDP, immediately
initiates a TCP connection to 10.1.1.2:54111.
3) In order to minimize the chance of a duplicate connection, the
endpoint at 10.1.1.1 pauses for a short time to allow the
endpoint at 10.1.1.2 to receive the TCP connection initiation.
4) After the short pause, the endpoint at 10.1.1.1 sends the SDP
response as listed above.
The pause in #3 gives the first TCP connection attempt a chance to
succeed, since withholding the SDP response deprives the endpoint at
10.1.1.2 of the information it needs to attempt its own TCP
connection.
7.5 Example: "Bidirectional" RTP and RTCP
An endpoint at 10.1.1.2 is behind a NAT and does not know its own An endpoint at 10.1.1.2 is behind a NAT and does not know its own
public address. public address.
c=IN IP4 10.1.1.2 c=IN IP4 10.1.1.2
m=audio 9 RTP/AVP 0 m=audio 9 RTP/AVP 0
a=direction:active a=direction:active
A peer with a public IP address responds as follows and waits to A peer with a public IP address responds as follows and waits to
receive RTP and RTCP packets from its active peer. receive RTP and RTCP packets from its active peer.
Yon INTERNET-DRAFT - Expires October 2002 11
c=IN IP4 1.2.3.4 c=IN IP4 1.2.3.4
m=audio 18240 RTP/AVP 0 m=audio 18240 RTP/AVP 0
a=direction:passive a=direction:passive
The endpoint at 10.1.1.2 immediately sends RTP from port 9012 to 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 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 port 1542. The passive endpoint receives this RTP packet and stores
this source address. When the passive endpoint wants to send RTP 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 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 destination address back to 10.1.1.2 port 9012 and delivers it to
the active endpoint. the active endpoint.
Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port 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 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 passive endpoint receives the RTCP packet and stores the source
Yon INTERNET-DRAFT - Expires December 2002 13
address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which 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 is translated back to 10.1.1.2:9013 and delivered to the active
endpoint. endpoint.
7 Security Considerations 8 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. Session Description Protocol in general.
A possible security concern arises if a firewall were to monitor and 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. act on the source address as described in the note in Section 4.
Firewall implementers must take care to ensure that the SDP came Firewall implementers must take care to ensure that the SDP came
from a trusted source before deciding whether to change the network from a trusted source before deciding whether to change the network
traffic restrictions currently imposed by the firewall. traffic restrictions currently imposed by the firewall.
8 IANA Considerations 9 IANA Considerations
As recommended by [SDP] Appendix B, the direction attribute As recommended by [SDP] Appendix B, the direction and reconnect
described in this document should be registered with IANA, as should attributes described in this document should be registered with
the "TCP" and "TLS" protocol identifiers. IANA, as should the "TCP" and "TLS" protocol identifiers.
Acknowledgements Acknowledgements
The author would like to thank Jonathan Rosenberg, Rohan Mahy, The author would like to thank Jonathan Rosenberg, Rohan Mahy,
Anders Kristensen, Paul Kyzivat, and Robert Fairlie-Cuninghame for Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie-
their valuable insights and contributions. Cuninghame for 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.
skipping to change at line 657 skipping to change at line 748
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" | "passive" 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
reconnect-attribute = "reconnect"
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
Protocol," RFC 2327, April 1998 Protocol," RFC 2327, April 1998
[T38] International Telecommunication Union, "Procedures for [T38] International Telecommunication Union, "Procedures for
Real-Time Group 3 Facsimile Communications over IP Real-Time Group 3 Facsimile Communications over IP
skipping to change at line 689 skipping to change at line 782
One Indian Head Plaza One Indian Head Plaza
Nashua, NH 03060 Nashua, NH 03060
Phone: (603) 324-4100 Phone: (603) 324-4100
EMail: yon@dialout.net EMail: yon@dialout.net
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
Yon INTERNET-DRAFT - Expires October 2002 13 Yon INTERNET-DRAFT - Expires December 2002 15
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
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developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
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The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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