draft-ietf-mmusic-sdp-comedia-00.txt		draft-ietf-mmusic-sdp-comedia-01.txt
	INTERNET-DRAFT D. Yon		INTERNET-DRAFT D. Yon
	Document: draft-ietf-mmusic-sdp-comedia-00.txt Dialout.Net		Document: draft-ietf-mmusic-sdp-comedia-01.txt Dialout.Net
	Expires August 2001 February 2001		Expires April 2002 October 2001
	Connection-Oriented Media Transport in SDP		Connection-Oriented Media Transport in SDP
	<draft-ietf-mmusic-sdp-comedia-00.txt>		<draft-ietf-mmusic-sdp-comedia-01.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 36		skipping to change at line 36
	The list of Internet-Draft Shadow Directories can be accessed at:		The list of Internet-Draft Shadow Directories can be accessed at:
	http://www.ietf.org/shadow.html.		http://www.ietf.org/shadow.html.
	Copyright (C) The Internet Society (2001). All Rights Reserved.		Copyright (C) The Internet Society (2001). 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 three new protocol identifiers: TCP, TLS and		(SDP). It defines two new protocol identifiers: TCP and TLS. It
	SCTP. It also defines the syntax and semantics for an SDP		also defines the syntax and semantics for an SDP "direction"
	"direction" attribute that describes the connection setup procedure.		attribute that describes the connection setup procedure.
	Yon 1		Yon 1
	Introduction		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		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 or SCTP is 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.
	1 Protocol Identifiers		1 Protocol Identifiers
	1.1 TCP		1.1 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 SCTP		1.2 TLS
	The SCTP protocol identifier, like TCP above, only describes the
	transport protocol without any connotation as to the upper-layer
	protocol. An m= line that specifies SCTP indicates that media will
	be transports using the SCTP protocol [SCTP], with an upper-layer
	protocol specified by the fmt identifier.
	1.3 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.
	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.
	Yon INTERNET-DRAFT û Expires August 2001 2		Yon INTERNET-DRAFT û Expires January 2002 2
	2 Direction Attribute		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-port>		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.
	The <source-port> is an optional value that may only be specified in		reuse: The endpoint will use the connection that has already
	the context of direction:active or direction:both.		been established with the opposite endpoint.
			The <source-address> is a sequence of values that describe the
			address and port number from where the connection will originate,
			and consists of the following values:
			nettype addrtype unicast-address [port]
			The <source-address> is an optional value that may be specified with
			direction:active, direction:both, or direction:reuse. Within the
			<source-address>, the source port number is RECOMMENDED but may be
			omitted.
	2.1 Semantics of direction:passive		2.1 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.		connection from the other endpoint. The endpoint MUST NOT specify a
			<source-address> after direction:passive.
	2.2 Semantics of direction:active		2.2 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 is		endpoint. The port number on its own m= line is irrelevant, and the
	to be ignored by the other endpoint. Nevertheless, since the m=		opposite endpoint MUST NOT attempt to initiate a connection to the
	line must contain a valid port number, the endpoint specifying		port number specified there. Nevertheless, since the m= line must
	direction:active should specify a port number of 9 (the discard		contain a valid port number, the endpoint specifying
	port) on its m= line. The endpoint must not specify a port number		direction:active SHOULD specify a port number of 9 (the discard
			port) on its m= line. The endpoint MUST NOT specify a port number
	of zero, as that carries other semantics in [SDP].		of zero, as that carries other semantics in [SDP].
	The endpoint may optionally specify the port number from which it		Yon INTERNET-DRAFT û Expires January 2002 3
	will initiate the connection in the <source-port> position on the a=		The endpoint SHOULD specify the address and port number from which
	line.		it will initiate the connection in the <source-address> position on
			the a= line.
	2.3 Semantics of direction:both		2.3 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. As with direction:active, the		the m= line of the other endpoint.
	endpoint may optionally specify the port number from which it will
	initiate the connection in the <source-port> position on the a=		As with direction:active, the endpoint SHOULD specify the address
	line.		and port number from which it will initiate the connection in the
			<source-address> position on the a= line.
	Yon INTERNET-DRAFT û Expires August 2001 3
	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.
	skipping to change at line 174		skipping to change at line 187
	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		3) The endpoints intend to use two connections to transport the
	media, and one must be initiated by the remote endpoint and		media, and one must be initiated by the remote endpoint and
	the other must be initiated by the local endpoint.		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 should 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 should 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 only one connection succeeds, then that		the opposite endpoint. If a single connection is needed (case #1 or
	connection will be used to carry the media. If both connections		#2 above), there is one exception to this requirement: if an
	succeed but only one was needed (case #2 above), the following rules		endpoint receives the incoming connection from the opposite endpoint
	shall apply:		prior to initiating its own outbound connection, then that endpoint
			MAY use that connection rather than attempt to make an outbound
			connection to the opposite endpoint.
			Yon INTERNET-DRAFT û Expires January 2002 4
			If only one connection succeeds, then that connection will be used
			to carry the media. Once it has transmitted data on this
			connection, the initiating endpoint MUST NOT perform another
			connection attempt to the accepting endpoint. This allows the
			accepting endpoint to release or recycle the listening port for
			another session once it has received data from the initiating
			endpoint.
			If both connections succeed but only one was needed (case #2 above),
			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, it MUST use that connection exclusively for		connections, 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 connections is determined to be idle, the endpoint MAY		the connections is determined to be idle, the endpoint MAY
	close the idle connection.		close the idle connection.
	3 Source-Port Considerations		2.4 Semantics of direction:reuse
	In the cases where the endpoint is initiating the connection, a		By specifying direction:reuse, the endpoint indicates that it is
	source port number may optionally be specified on the a= line by		changing the parameter(s) of an existing session on a previously
	that endpoint. In most environments, the source port number can be		established connection with the opposite endpoint. Therefore no new
			connections are to be created. This is intended for cases where
			media types are added, removed, or changed during a session. For
			example, an endpoint adding a video stream to an existing audio
			session may elect to multiplex the new stream over the same
			connection that is currently transporting the audio stream.
	Yon INTERNET-DRAFT û Expires August 2001 4		2.5 Bidirectional versus Unidirectional Media
	determined by binding the socket before initiating the connect, as
	shown in the sample C code below:		In traditional SDP transport types the flow is unidirectional. If
			the intent is for media to flow in both directions, both endpoints
			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
			then media can only flow towards Endpoint A, as Endpoint B has not
			specified where and how to send media to it.
			Because most connection-oriented media is inherently bi-directional,
			endpoints may encounter a situation where only one side presented
			SDP yet there is now a network path that can carry media in either
			direction. In keeping with traditional SDP semantics, an endpoint
			MUST NOT send data to the other endpoint unless it has specified SDP
			information describing the type of media it can accept.
			It is, however, perfectly acceptable for an endpoint to transmit
			data on the same connection it is using to receive data, so long as
			the other endpoint has advertised its willingness to accept data.
			Likewise, it is perfectly acceptable for an endpoint to receive data
			Yon INTERNET-DRAFT û Expires January 2002 5
			on the same connection it is using to transmit data to the
			corresponding remote endpoint. In other words, for a bi-directional
			application-level session, a connection may be used to send data in
			both directions (contingent to rules outlined in Section 2.3) as
			long as one side of the connection is attached to either of the
			advertised SDP transport addresses.
			3 Source-Address Considerations
			In the cases where the endpoint is initiating the connection, it is
			RECOMMENDED that a source address be specified on the a= line by
			that endpoint. It is also RECOMMENDED that the source port be
			included 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;
	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
	skipping to change at line 232		skipping to change at line 298
	bind(s_id,(SOCKADDR *)&cli_sin,sizeof(cli_sin));		bind(s_id,(SOCKADDR *)&cli_sin,sizeof(cli_sin));
	// Find the port number that was bound		// Find the port number that was bound
	namelen = sizeof(cli_sin);		namelen = sizeof(cli_sin);
	getsockname(s_id,(SOCKADDR *)&cli_sin,&namelen);		getsockname(s_id,(SOCKADDR *)&cli_sin,&namelen);
	// Print the port number		// Print the port number
	printf("Source Port = %d\n",ntohs(cli_sin.sin_port));		printf("Source Port = %d\n",ntohs(cli_sin.sin_port));
	}		}
	The motivation for specifying the source port is to allow topologies		If the source address is omitted, the receiver of the SDP packet
	where one or more endpoints use a single, fixed TCP port for		MUST NOT make any assumptions in regards to the address or port from
	incoming connections. Non-RTP protocols transported over TCP		where the connection will originate. In particular, the receiver
	commonly use this technique. By specifying the source port, an		MUST NOT assume that the address information listed on the c= line
	endpoint avoids a potential ambiguity when more than one session is		has any implication as to where the media connection originates.
	set up between two endpoints.
	For example, consider two endpoints with IP addresses of 10.1.1.1		NOTE:
	and 10.1.1.2. The endpoint at 10.1.1.1 signals the availability of		The motivation for specifying the source address is
	a session on TCP port 2393 (passive). Before the endpoint at		twofold. First, it aids Application-Level Proxies by
	10.1.1.2 has a chance to initiate the connection, events transpire		explicitly announcing the source of the outbound
	that cause the endpoint at 10.1.1.1 to signal the availability of a		connection. This allows, for example, a dynamic
	separate session that is also found at TCP port 2393 (passive).		firewall pinhole to be created that will allow the
	Shortly thereafter, both entities at 10.1.1.2 initiate connections		connection to pass.
	to 10.1.1.1 on port 2393.
	The problem is this: how does the endpoint at 10.1.1.1 differentiate		Yon INTERNET-DRAFT û Expires January 2002 6
	the two connections? To which entity at 10.1.1.2 does each		Second, it allows the passive endpoint to correlate
	connection correspond? By specifying the source port prior to		the incoming connection with the session being
	connecting, the entities at 10.1.1.2 can avoid this ambiguity,		negotiated. Note that great care must be taken when
	because now the endpoint at 10.1.1.1 can simply inspect the port		using the source address as a means to identify
	number from which the connection originated to determine which		incoming connections, as Network Address Translation
	entity has initiated the connection.		(NAT) can render the source address unreliable. In
			addition if the originating endpoint omits the source
			port, the source address can be ambiguous if multiple,
			logical endpoints share the same network address.
			Therefore it is NOT RECOMMENDED that the source
			address be used for this purpose unless the SDP occurs
			in the context of a controlled network topology that
			guarantees that the source address is both correct
			(i.e., no NAT, or a NAT with an Application-Level
			Proxy that rewrites the SDP) and unambiguous (i.e.,
			the source port is specified).
	Caution must be exercised when designing systems that rely on this		3.1 Source Address Timing Considerations
	feature, as not all environments are able to determine the source
	port prior to initiating the connection.
	Yon INTERNET-DRAFT û Expires August 2001 5		When used in conjunction with a session signaling protocol such as
			SIP, there may be cases where an endpoint initiates a connection
			prior to the opposite endpoint receiving the SDP that describe the
			source address of the initiating endpoint. Therefore, an endpoint
			that has advertised an address and port number with direction:both
			or direction:passive MUST be ready to accept a connection on that
			address and port immediately. If the accepting endpoint requires
			the source address to identify the initiating endpoint, it MUST keep
			the connection active and allow sufficient time for the source
			address to arrive before discarding the connection.
	4 Examples		4 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 <me@ietf.org>		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=0 0
	4.1 Example: simple passive/active		4.1 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 10.1.1.2/127		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:passive		a=direction:passive
			Yon INTERNET-DRAFT û Expires January 2002 7
	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 10.1.1.1/127		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 port. The endpoint at 10.1.1.1 could have optionally committed		any address or port. The endpoint at 10.1.1.1 could have optionally
	to a source port with a simple modification:		committed to a source address with a simple modification:
	c=IN IP4 10.1.1.1/127		c=IN IP4 10.1.1.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active 1892		a=direction:active IN IP4 10.1.1.1 1892
	By adding the "1892" to the a= line, the endpoint at 10.1.1.1 must		By adding the source address to the a= line, the endpoint at
	now use a source port of 1892 when initiating the TCP connection to		10.1.1.1 must now use a source port of 1892 when initiating the TCP
	port 54111 at 10.1.1.2.		connection to port 54111 at 10.1.1.2.
	4.2 Example: agnostic both		4.2 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 10.1.1.2/127		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:both		a=direction:both
	The endpoint at 10.1.1.1 has three choices:		The endpoint at 10.1.1.1 has three choices:
	Yon INTERNET-DRAFT û Expires August 2001 6
	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 10.1.1.1/127		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		4.3 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:
	c=IN IP4 10.1.1.2/127		Yon INTERNET-DRAFT û Expires January 2002 8
			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 10.1.1.1/127		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		5 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. There are no new security
	considerations introduced by these protocol identifiers and		considerations introduced by these protocol identifiers and
	attributes.		attributes.
	6 IANA Considerations		6 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, TLS, and SCTP protocol identifiers.		the "TCP" and "TLS" protocol identifiers.
	Acknowledgements		Acknowledgements
	Yon INTERNET-DRAFT û Expires August 2001 7
	The author would like to thank Jonathan Rosenberg, Anders		The author would like to thank Jonathan Rosenberg, Anders
	Kristensen, and Robert Fairlie-Cuninghame for their valuable		Kristensen, Paul Kyzivat, and Robert Fairlie-Cuninghame for their
	insights.		valuable insights.
	Yon INTERNET-DRAFT û Expires August 2001 8		Yon INTERNET-DRAFT û Expires January 2002 9
	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 port		qualified-direction = direction-ident | direction-ident source
	direction-ident = "both" | "active"		direction-ident = "both" | "active" | "reuse"
			source = nettype addrtype unicast-address |
			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
	[SCTP] Stewart et al, "Stream Control Transmission Protocol,"
	RFC 2960, October 2000
	[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
	Networks," Recommendation T.38, June 1998		Networks," Recommendation T.38, June 1998
	[TLS] T. Dierks, C. Allen, "The TLS Protocol," RFC 2246,		[TLS] T. Dierks, C. Allen, "The TLS Protocol," RFC 2246,
	January 1999		January 1999
	[UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode		[UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode
	and ISO 10646," RFC 2044, October 1996		and ISO 10646," RFC 2044, October 1996
	AuthorÆs Address		AuthorÆs Address
	David Yon		David Yon
	Dialout.Net, Inc.		Dialout.Net, Inc.
	402 Amherst St		One Indian Head Plaza
	Nashua, NH 03063		Nashua, NH 03060
	Phone: (603) 577-8708		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 û Expires August 2001 9		Yon INTERNET-DRAFT û Expires January 2002 10
	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
	the copyright notice or references to the Internet Society or other		the copyright notice or references to the Internet Society or other
	Internet organizations, except as needed for the purpose of		Internet organizations, except as needed for the purpose of
	developing Internet standards in which case the procedures for		developing Internet standards in which case the procedures for
	skipping to change at line 455		skipping to change at line 537
	The limited permissions granted above are perpetual and will not be		The limited permissions granted above are perpetual and will not be
	revoked by the Internet Society or its successors or assigns.		revoked by the Internet Society or its successors or assigns.
	This document and the information contained herein is provided on an		This document and the information contained herein is provided on an
	"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING		"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
	TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING		TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
	BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION		BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
	HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF		HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
	MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."		MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
	Yon INTERNET-DRAFT û Expires August 2001 10		Yon INTERNET-DRAFT û Expires January 2002 11
End of changes.
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