draft-ietf-mmusic-sdp-comedia-08.txt		draft-ietf-mmusic-sdp-comedia-09.txt
	MMUSIC Working Group D. Yon		MMUSIC Working Group D. Yon
	Internet-Draft Dialout.Net, Inc		Internet-Draft Tactical Software, LLC
	Expires: January 14, 2005 G. Camarillo		Expires: March 30, 2005 G. Camarillo
	Ericsson		Ericsson
	July 16, 2004		September 29, 2004
	Connection-Oriented Media Transport in the Session Description		Connection-Oriented Media Transport in the Session Description
	Protocol (SDP)		Protocol (SDP)
	draft-ietf-mmusic-sdp-comedia-08.txt		draft-ietf-mmusic-sdp-comedia-09.txt
	Status of this Memo		Status of this Memo
	By submitting this Internet-Draft, I certify that any applicable		This document is an Internet-Draft and is subject to all provisions
	patent or other IPR claims of which I am aware have been disclosed,		of section 3 of RFC 3667. By submitting this Internet-Draft, each
	and any of which I become aware will be disclosed, in accordance with		author represents that any applicable patent or other IPR claims of
			which he or she is aware have been or will be disclosed, and any of
			which he or she become aware will be disclosed, in accordance with
	RFC 3668.		RFC 3668.
	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 other		Task Force (IETF), its areas, and its working groups. Note that
	groups may also distribute working documents as Internet-Drafts.		other groups may also distribute working documents as
			Internet-Drafts.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		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 http://		The list of current Internet-Drafts can be accessed at
	www.ietf.org/ietf/1id-abstracts.txt.		http://www.ietf.org/ietf/1id-abstracts.txt.
	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.
	This Internet-Draft will expire on January 14, 2005.		This Internet-Draft will expire on March 30, 2005.
	Copyright Notice		Copyright Notice
	Copyright (C) The Internet Society (2004). All Rights Reserved.		Copyright (C) The Internet Society (2004).
	Abstract		Abstract
	This document describes how to express media transport over		This document describes how to express media transport over
	connection-oriented protocols using the Session Description Protocol		connection-oriented protocols using the Session Description Protocol
	(SDP). It defines the SDP TCP protocol identifier, the SDP setup		(SDP). It defines the SDP TCP protocol identifier, the SDP setup
	attribute, which describes the connection setup procedure, and the		attribute, which describes the connection setup procedure, and the
	SDP connid attribute, which provides a connection identifier.		SDP connection attribute, which handles connection reestablishment.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3		3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3
	4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4		4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
	4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4		4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4
	5. The Connid Attribute . . . . . . . . . . . . . . . . . . . . . 5		5. The Connection Attribute . . . . . . . . . . . . . . . . . . . 5
	5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6		5.1 Offerer Behaviour . . . . . . . . . . . . . . . . . . . . 6
	5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7		5.2 Answerer Behaviour . . . . . . . . . . . . . . . . . . . . 7
	6. Connection Management . . . . . . . . . . . . . . . . . . . . 7		6. Connection Management . . . . . . . . . . . . . . . . . . . . 7
			6.1 Connection Establishment . . . . . . . . . . . . . . . . . 7
			6.2 Connection Reestablishment . . . . . . . . . . . . . . . . 8
			6.3 Connection Termination . . . . . . . . . . . . . . . . . . 8
	7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8		7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
	7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8		7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
	7.2 Passive/Active with Connection Reestablishment . . . . . . 9		7.2 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9
	7.3 Actpass/Passive . . . . . . . . . . . . . . . . . . . . . 9		7.3 Existing Connection Reuse . . . . . . . . . . . . . . . . 9
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 10		7.4 Existing Connection Refusal . . . . . . . . . . . . . . . 10
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10		8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
	10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10		9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10		10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
	11.1 Normative References . . . . . . . . . . . . . . . . . . . . 10		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
	11.2 Informative References . . . . . . . . . . . . . . . . . . . 11		11.1 Normative References . . . . . . . . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11		11.2 Informative References . . . . . . . . . . . . . . . . . . . 12
	Intellectual Property and Copyright Statements . . . . . . . . 13		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
			Intellectual Property and Copyright Statements . . . . . . . . 14
	1. Introduction		1. Introduction
	The Session Description Protocol [3] provides a general-purpose		The Session Description Protocol [4] provides a general-purpose
	format for describing multimedia sessions in announcements or		format for describing multimedia sessions in announcements or
	invitations. SDP uses an entirely textual data format (the US-ASCII		invitations. SDP uses an entirely textual data format (the US-ASCII
	subset of UTF-8 [10]) to maximize portability among transports. SDP		subset of UTF-8 [11]) to maximize portability among transports. SDP
	does not define a protocol, but only the syntax to describe a		does not define a protocol, but only the syntax to describe a
	multimedia session with sufficient information to participate in that		multimedia session with sufficient information to participate in that
	session. Session descriptions may be sent using arbitrary existing		session. Session descriptions may be sent using arbitrary existing
	application protocols for transport (e.g., SAP [8], SIP [9], RTSP		application protocols for transport (e.g., SAP [9], SIP [10], RTSP
	[5], email, HTTP [7], etc.).		[6], email, HTTP [8], etc.).
	SDP [3] defines two protocol identifiers: RTP/AVP and UDP, both of		SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of
	which represent unreliable connectionless protocols. While these		which represent unreliable connectionless protocols. While these
	transports are appropriate choices for multimedia streams, there are		transports are appropriate choices for multimedia streams, there are
	applications for which connection-oriented transports, such as TCP,		applications for which connection-oriented transports, such as TCP,
	are more appropriate. This document defines a new protocol		are more appropriate. This document defines a new protocol
	identifier, TCP, to describe TCP connetions in SDP.		identifier, TCP, to describe TCP connetions in SDP.
	Connection-oriented protocols introduce two new factor when		Connection-oriented protocols introduce two new factor when
	describing a session: how and when should end points perform the		describing a session: how and when should end points perform the
	connection setup procedure. This document defines two new attributes		connection setup procedure. This document defines two new attributes
	to describe connection setups: setup and connid.		to describe connection setups: setup and connection.
	2. Terminology		2. Terminology
	In this document, the key words "MUST", "MUST NOT", "REQUIRED",		In this document, the key words "MUST", "MUST NOT", "REQUIRED",
	"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT		"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
	RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as		RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
	described in BCP 14, RFC 2119 [2] and indicate requirement levels for		described in BCP 14, RFC 2119 [3] and indicate requirement levels for
	compliant implementations.		compliant implementations.
	3. Protocol Identifier		3. Protocol Identifier
	The following is the ABNF for an m= line, as specified by RFC 2327		The following is the ABNF for an m= line, as specified by RFC 2327
	[3].		[4].
	media-field = "m=" media space port ["/" integer]		media-field = "m=" media space port ["/" integer]
	space proto 1*(space fmt) CRLF		space proto 1*(space fmt) CRLF
	This document defines a new value for the proto field: TCP.		This document defines a new value for the proto field: TCP.
	The TCP protocol identifier is similar to the UDP protocol identifier		The TCP protocol identifier is similar to the UDP protocol identifier
	in that it only describes the transport protocol, and not the		in that it only describes the transport protocol, and not the
	upper-layer protocol. An m= line that specifies "TCP" MUST further		upper-layer protocol. An m= line that specifies "TCP" MUST further
	qualify the application-layer protocol using an fmt identifier. Media		qualify the application-layer protocol using an fmt identifier.
	lines with the TCP protocol identifier are carried using TCP [1].		Media described using an m= lines containing the TCP protocol
			identifier are carried using TCP [1].
	It is RECOMMENDED that documents defining new SDP protocol		It is RECOMMENDED that documents defining new SDP protocol
	identifiers that involve extra protocol layers between TCP and the		identifiers that involve extra protocol layers between TCP and the
	media itself (e.g., TLS [6] over TCP) start with the string "TCP/"		media itself (e.g., TLS [7] over TCP) start with the string "TCP/"
	(e.g., TCP/TLS).		(e.g., TCP/TLS).
	The following sections define the setup and the connid attributes.		The following sections define the setup and the connection
	While both attributes are applicable to m= lines that use the TCP		attributes. While both attributes are applicable to m= lines that
	protocol identifier, they are not limited to them. These attributes		use the TCP protocol identifier, they are not limited to them. These
	MAY be used in any m= line which uses a connection-oriented transport		attributes MAY be used in conjunction with any m= line which uses a
	protocol, even if the protocol identifier of the m= line is not TCP.		connection- oriented transport protocol, even if the protocol
			identifier of the m= line is not TCP.
	4. Setup Attribute		4. Setup Attribute
	The setup attribute indicates which of the end points should initiate		The setup attribute indicates which of the end points should initiate
	the connection establishment (e.g., send the initial TCP SYN). The		the connection establishment (e.g., send the initial TCP SYN). The
	setup attribute is charset-independent and can be a session-level or		setup attribute is charset-independent and can be a session-level or
	a media-level attribute. The following is the ABNF of the setup		a media-level attribute. The following is the ABNF of the setup
	attribute:		attribute:
	setup-attr = "a=setup:" role		setup-attr = "a=setup:" role
	skipping to change at page 4, line 40		skipping to change at page 4, line 42
	Passive: The endpoint will accept an incoming connection.		Passive: The endpoint will accept an incoming connection.
	ActPass: The endpoint is willing to accept an incoming connection		ActPass: The endpoint is willing to accept an incoming connection
	or to initiate an outgoing connection.		or to initiate an outgoing connection.
	Holdconn: The endpoint does not want the connection to be		Holdconn: The endpoint does not want the connection to be
	established for the time being.		established for the time being.
	4.1 The Setup Attribute in the Offer/answer Model		4.1 The Setup Attribute in the Offer/answer Model
	The offer/answer model, defined in RFC 3264 [4], provides endpoints		The offer/answer model, defined in RFC 3264 [5], provides endpoints
	with a means to obtain shared view of a session. Some session		with a means to obtain shared view of a session. Some session
	parameters are negotiated (e.g., codecs to use), while others are		parameters are negotiated (e.g., codecs to use), while others are
	simply communicated from one endpoint to the other (e.g., IP		simply communicated from one endpoint to the other (e.g., IP
	addresses). The value of the setup attribute falls into the first		addresses). The value of the setup attribute falls into the first
	category. That is, both endpoints negotiate its value using the		category. That is, both endpoints negotiate its value using the
	offer/answer model.		offer/answer model.
	The negotiation of the value of the setup attribute takes places as		The negotiation of the value of the setup attribute takes places as
	follows. The offerer states which role or roles it is willing to		follows. The offerer states which role or roles it is willing to
	perform and the answerer, taking the offerer's willingness into		perform and the answerer, taking the offerer's willingness into
	consideration, chooses which roles both endpoints will actually		consideration, chooses which roles both endpoints will actually
	perform during connection establishment. The following are the values		perform during connection establishment. The following are the
	that the setup attribute can take in an offer/answer exchange:		values that the setup attribute can take in an offer/answer exchange:
	Offer Answer		Offer Answer
	________________		________________
	active passive / holdconn		active passive / holdconn
	passive active / holdconn		passive active / holdconn
	actpass active / passive / holdconn		actpass active / passive / holdconn
	holdconn holdconn		holdconn holdconn
	The active endpoint SHOULD initiate a connection to the port number		The active endpoint SHOULD initiate a connection to the port number
	on the m= line of the other endpoint. The port number on its own m=		on the m= line of the other endpoint. The port number on its own m=
	skipping to change at page 5, line 40		skipping to change at page 5, line 43
	offer. That is, the offerer has no preference as to whether it		offer. That is, the offerer has no preference as to whether it
	accepts or initiates the connection and, so, is letting the answerer		accepts or initiates the connection and, so, is letting the answerer
	choose.		choose.
	A value of holdconn indicates that the connection should not be		A value of holdconn indicates that the connection should not be
	established for the time being.		established for the time being.
	The default value of the setup attribute in an offer/answer exchange		The default value of the setup attribute in an offer/answer exchange
	is active in the offer and passive in the answer.		is active in the offer and passive in the answer.
	5. The Connid Attribute		5. The Connection Attribute
	The preceding description of the setup attribute has been in the		The preceding description of the setup attribute has been in the
	context of using SDP to initiate a session. Still, SDP may be		context of using SDP to initiate a session. Still, SDP may be
	exchanged between endpoints at various stages of a session to		exchanged between endpoints at various stages of a session to
	accomplish tasks such as terminating a session, redirecting media to		accomplish tasks such as terminating a session, redirecting media to
	a new endpoint, or renegotiating the media parameters for a session.		a new endpoint, or renegotiating the media parameters for a session.
	After the initial session has been established, it may be ambiguous		After the initial session has been established, it may be ambiguous
	as to whether subsequent SDP exchange represents a confirmation that		as to whether subsequent SDP exchange represents a confirmation that
	the endpoint is to continue using the current media connection		the endpoint is to continue using the current media connection
	unchanged, or is a request to make a new media connection. The		unchanged, or is a request to make a new media connection. The
	media-level connid attribute, which is charset-independent, is used		media-level connection attribute, which is charset-independent, is
	to disambiguate these two scenarios. The following is the ABNF of the		used to disambiguate these two scenarios. The following is the ABNF
	connid attribute:		of the connection attribute:
	connid = "a=connid:" connection-identifier
	connection-identifier = token
	The connid attribute provides an identifier for the transport-layer		connection-attr = "a=connection:" conn-value
	connection used by the m= line. Connid values are meaningful in the		conn-value = "new" / "existing"
	context of a particular m= line. So, different m= lines in the same
	session description MAY have the same connid value.
	5.1 Offerer Behaviour		5.1 Offerer Behaviour
	Offerers and answerers use the connid attribute to decide whether a		Offerers and answerers use the connection attribute to decide whether
	new transport connection needs to be established or, on the other		a new transport connection needs to be established or, on the other
	hand, the existing transport connection should still be used.		hand, the existing transport connection should still be used. The
			connection value resulting from an offer/answer exchange is the
			connection value in the answer. If the connection value in the
			answer is "new", the end-points SHOULD establish a new connection.
			If the connection value in the answer is "existing", the end-points
			SHOULD continue using the exiting connection.
	When an offerer generates an m= line which uses a connection-oriented		When an offerer generates an m= line which uses a connection-oriented
	transport, it SHOULD provide such an m= line with a connection		transport, it SHOULD provide a connection attribute for the m= line
	identifier using a connid attribute, unless the application using the		unless the application using the m= line has other means to deal with
	m= line has other means to deal with connection reestablishment. The		connection reestablishment. The connection attribute in an initial
	connid attribute in an initial offer (i.e., no transport connection		offer (i.e., no transport connection has been established yet) takes
	has been established yet) can take any value. This value identifies		the value of "new".
	the initial connection that the endpoints will attempt to establish.
	After the initial offer/answer exchange, any of the endpoints can		After the initial offer/answer exchange, any of the endpoints can
	generate a new offer to change some characteristics of the session		generate a new offer to change some characteristics of the session
	(e.g., the direction attribute). If such an offerer wants to continue		(e.g., the direction attribute). If such an offerer wants to
	using the previously-established transport-layer connection for the		continue using the previously-established transport-layer connection
	m= line, the offerer MUST use the same connid value for the m= line.		for the m= line, the offerer MUST use use a connection value of
	If, on the other hand, the offerer wants to establish a new		"existing" for the m= line. If, on the other hand, the offerer wants
	transport-layer connection for the m= line, it MUST use a new connid		to establish a new transport-layer connection for the m= line, it
	value. This new connid value MUST be different from the current		MUST use a connection value of "new".
	connid value in use and SHOULD be different than any connid value
	used previously in the same m= line.
	The connid value in an offer is only compared with the connid
	value currently in use. So, having a connid value different than
	the one in use is enough to trigger the establishment of a new
	connection. Still, it is recommended to use a value different than
	all the previous ones used in the m= line to make debugging
	easier.
	Note that, according to the rules in this section, an offer that		Note that, according to the rules in this section, an offer that
	changes the transport address (IP address plus port number) of an		changes the transport address (IP address or port number) of an m=
	m= line will have a new connid value for this m= line.		line will have a connection value of "new".
	5.2 Answerer Behaviour		The default value of the connection attribute in an offer/answer
			exchange is "new".
	The connid value for an m= line is negotiated using the offer/answer		5.2 Answerer Behaviour
	model. The resulting connid value after an offer/answer exchange is
	the connid value in the answer.
	For an m= line, if the offer contains a new connid value (i.e.,		The connection value for an m= line is negotiated using the offer/
	different from the one in use) the answerer MUST use this value in		answer model. The resulting connection value after an offer/answer
	the answer. If the offer contains the connid value in use and the		exchange is the connection value in the answer. If the connection
	answerer wishes to continue using the existing transport-layer		value in the offer is "new", the answerer MUST also use a value of
	connection, the answerer MUST use this connid value in the answer. If		"new" in the answer. If the connection value in the offer is
	the offer contains the connid value in use but the answerer wishes to		"existing", the answerer uses a value of "existing" in the answer if
	establish a new transport-layer connection, the answerer MUST use a		it wishes to continue using the existing connection and a value of
	new connid value in the answer.		"new" if it wants a new connection to be established.
	If the connid value for an m= line resulting from an offer/answer		In some scenarios where third party call control [12] is used, an
	exchange is different than the connid in use so far, the endpoints		endpoint may receive an initial offer with a connection value of
	SHOULD establish a new transport-layer connection as indicated by the		"existing". Following the previous rules, such an answerer would
	setup attribute. If a previous connection is still up, the endpoint		use a connection value of "new" in the answer.
	responsible for establishing the new connection performing the active
	role SHOULD close it as soon as the offer/answer exchange is
	completed. It is up to the application to ensure proper data
	synchornization between the two connections.
	If the connid value for an m= line resulting from an offer/answer		If the connection value for an m= line resulting from an offer/answer
	exchange is the same as the connid in use so far, the endpoints		exchange is "new", the endpoints SHOULD establish a new
	SHOULD continue using the existing connection.		transport-layer connection as indicated by the setup attribute. If a
			previous connection is still up, the endpoints SHOULD close it as
			soon as the offer/answer exchange is completed. It is up to the
			application to ensure proper data synchornization between the two
			connections.
	In the past, it was proposed to use the presence of a media-level		If the connection value for an m= line resulting from an offer/answer
	SDP attribute as a flag to indicate that a new connection needed		exchange is "existing", the endpoints SHOULD continue using the
	to be established. It was decided not to follow the flag approach		existing connection.
	because an offerer whose intent was to signal "no changes" in a
	session would need to issue a different offer than the previous
	one (i.e., it would need to remove the flag from the m= line). By
	using the connid attribute instead, an offerer signals "no
	changes" in a session by issuing an identical offer to the one in
	use.
	6. Connection Management		6. Connection Management
			This section addresses connection establishment, connection
			reestablishment, and connection termination.
			6.1 Connection Establishment
	An endpoint that according to an offer/answer exchange is supposed to		An endpoint that according to an offer/answer exchange is supposed to
	initiate a new connection SHOULD initiate it as soon as the offer/		initiate a new connection SHOULD initiate it as soon as it is able
	answer exchange is completed, even if the endpoint does not intend to		to, even if the endpoint does not intend to immediately begin sending
	immediately begin sending media to the remote endpoint. This allows		media to the remote endpoint. This allows media to flow from the
	media to flow from the remote endpoint if needed.		remote endpoint if needed.
	Typically, endpoints do not close the connection until the session		Note that some endpoints need to wait for some event to happen
	has expired, been explicitly terminated, or a new connid value has		before being able to establish the connection. For example, a
	been provided for the m= line. Additionaly, specific applications can		wireless terminal may need to set up a radio bearer before being
	describe further scenarios where an end-point may close a given		able to initiate a connection.
	connection. In case the session is explicitly terminated by one of
	the endpoints (e.g., the endpoint sends a SIP [9] BYE), the end point		6.2 Connection Reestablishment
	terminating the session is responsible for closing the
	transport-connection.
	If an endpoint determines that the transport-connection for an m=		If an endpoint determines that the transport-connection for an m=
	line has been closed and it should be reestablished, it SHOULD		line has been closed and it should be reestablished, it SHOULD
	perform a new offer/answer exchange using a new connid value for this		perform a new offer/answer exchange using a connection value of "new"
	m= line.		for this m= line.
	Note that the SDP direction attribute (e.g., a=sendonly) deals		Note that the SDP direction attribute (e.g., a=sendonly) deals
	with the media sent over the transport-connection, but has no		with the media sent over the transport-connection, but has no
	impact on the transport-connection itself.		impact on the transport-connection itself.
			6.3 Connection Termination
			Typically, endpoints do not close the connection until the session
			has expired, been explicitly terminated, or a new connection value
			has been provided for the m= line. Additionaly, specific
			applications can describe further scenarios where an end-point may
			close a given connection. As soon as an end-point notices that it
			needs to terminate a connection, it SHOULD do so.
			While in TCP both end-points need to close a connection, other
			connection-oriented transport protocols may not have the concept of
			half-close connections. In this case, a connection would be
			terminated as soon as one of the end-points closed it, making it
			unnecessary for the other end-point to perform any further action to
			terminate the connection.
			In any case, individual applications may provide further
			considerations on how to achieve a graceful connection termination.
			For example, a file application using TCP receiving a FIN from the
			remote endpoint may need to finish the ongoing transmission of a file
			before sending its own FIN.
	7. Examples		7. Examples
	The following examples show the most common usage of the setup		The following examples show the most common usage of the setup
	attribute combined with TCP-based media descriptions. For the purpose		attribute combined with TCP-based media descriptions. For the
	of brevity, the main portion of the session description is omitted in		purpose of brevity, the main portion of the session description is
	the examples, which only show m= lines and their attributes		omitted in the examples, which only show m= lines and their
	(including c= lines).		attributes (including c= lines).
	7.1 Passive/Active		7.1 Passive/Active
	An offerer at 192.0.2.2 signals its availability for a T.38 fax		An offerer at 192.0.2.2 signals its availability for a T.38 fax
	session at port 54111:		session at port 54111:
	m=image 54111 TCP t38		m=image 54111 TCP t38
	c=IN IP4 192.0.2.2		c=IN IP4 192.0.2.2
	a=setup:passive		a=setup:passive
	a=connid:1		a=connection:new
	An answerer at 192.0.2.1 receiving this offer responds with the		An answerer at 192.0.2.1 receiving this offer responds with the
	following answer:		following answer:
	c=IN IP4 192.0.2.1
	m=image 9 TCP t38		m=image 9 TCP t38
			c=IN IP4 192.0.2.1
	a=setup:active		a=setup:active
	a=connid:1		a=connection:new
	The endpoint at 192.0.2.1 then initiates the TCP connection to port		The endpoint at 192.0.2.1 then initiates the TCP connection to port
	54111 at 192.0.2.2.		54111 at 192.0.2.2.
	7.2 Passive/Active with Connection Reestablishment		7.2 Actpass/Passive
	Continuing the preceding example, consider the scenario where the TCP
	connection fails and the endpoints wish to reestablish the connection
	for the session. The endpoint at 192.0.2.2 signals this intent with
	the following SDP:
	m=image 54111 TCP t38
	c=IN IP4 192.0.2.2
	a=setup:passive
	a=connid:2
	The new connid value informs the endpoint at 192.0.2.1 that this SDP
	represents the intent to establish a new connection for media
	transport, rather than continuing with the original connection. If
	192.0.2.1 agrees to continue the session using a new connection, it
	responds with:
	m=image 9 TCP t38
	c=IN IP4 192.0.2.1
	a=setup:active
	a=connid:2
	7.3 Actpass/Passive
	In another example, an offerer at 192.0.2.2 signals its availability		In another example, an offerer at 192.0.2.2 signals its availability
	for a T.38 fax session at TCP port 54111. Additionally, this offerer		for a T.38 fax session at TCP port 54111. Additionally, this offerer
	is also willing to set up the media stream by initiating the TCP		is also willing to set up the media stream by initiating the TCP
	connection:		connection:
	m=image 54111 TCP t38		m=image 54111 TCP t38
	c=IN IP4 192.0.2.2		c=IN IP4 192.0.2.2
	a=setup:actpass		a=setup:actpass
	a=connid:3		a=connection:new
	The endpoint at 192.0.2.1 responds with the following description:		The endpoint at 192.0.2.1 responds with the following description:
	m=image 54321 TCP t38		m=image 54321 TCP t38
	c=IN IP4 192.0.2.1		c=IN IP4 192.0.2.1
	a=setup:passive		a=setup:passive
	a=connid:3		a=connection:new
	This will cause the offerer (at 192.0.2.2) to initiate a connection		This will cause the offerer (at 192.0.2.2) to initiate a connection
	to port 54321 at 192.0.2.1.		to port 54321 at 192.0.2.1.
			7.3 Existing Connection Reuse
			Subsequent to the exchange in Section 7.2, another offer/answer
			exchange is initiated in the opposite direction. The endpoint at
			192.0.2.1 wishes to continue using the existing connection:
			m=image 54321 TCP t38
			c=IN IP4 192.0.2.1
			a=setup:passive
			a=connection:existing
			The endpoint at 192.0.2.2 also wishes to use the existing connection
			and responds with the following description:
			m=image 9 TCP t38
			c=IN IP4 192.0.2.2
			a=setup:active
			a=connection:existing
			The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.
			Note that the endpoint at 192.0.2.2 uses setup:active in response
			to the offer of setup:passive, and uses port 9 because it is
			active.
			7.4 Existing Connection Refusal
			Subsequent to the exchange in Section 7.3, another offer/answer
			exchange is initiated by the endpoint at 192.0.2.2, again wishing to
			reuse the existing connection:
			m=image 54111 TCP t38
			c=IN IP4 192.0.2.2
			a=setup:actpass
			a=connection:existing
			However, this time the answerer is unaware of the old connection and
			so wishes to establish a new one. (This could be the result of a
			transfer via 3pcc.) It is unable to act in the passive mode so
			responds as active:
			m=image 9 TCP t38
			c=IN IP4 192.0.2.3
			a=setup:active
			a=connection:new
			The endpoint at 192.0.2.3 then initiates the TCP connection to port
			54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old
			connection.
			Note that the endpoint at 192.0.2.2, while specifying connection:
			existing has reverted to setup:actpass and its real port number,
			rather than repeating setup:active and port 9 from the previous
			cycle. Had it not done this, this negotiation would have failed.
	8. Security Considerations		8. Security Considerations
	See RFC 2327 [3] for security and other considerations specific to		See RFC 2327 [4] for security and other considerations specific to
	the Session Description Protocol in general.		the Session Description Protocol in general.
	An attacker may attempt to modify the values of the connid and setup		An attacker may attempt to modify the values of the connection and
	attributes to have endpoints reestablish connections unnecesaryly or		setup attributes to have endpoints reestablish connections
	to keep them from establishing a connection. So, it is STRONGLY		unnecesaryly or to keep them from establishing a connection. So, it
	RECOMMENDED that integrity protection be applied to the SDP session		is STRONGLY RECOMMENDED that integrity protection be applied to the
	descriptions. For session descriptions carried in SIP [9], S/MIME is		SDP session descriptions. For session descriptions carried in SIP
	the natural choice to provide such end-to-end integrity protection,		[10], S/MIME is the natural choice to provide such end-to-end
	as described in RFC 3261 [9]. Other applications MAY use a different		integrity protection, as described in RFC 3261 [10]. Other
	form of integrity protection.		applications MAY use a different form of integrity protection.
	9. IANA Considerations		9. IANA Considerations
	This document defines two session and media level SDP attributes:		This document defines two session and media level SDP attributes:
	setup and connid. Their formats are defined in Section 4 and Section		setup and connection. Their formats are defined in Section 4 and
	5 respectively. These two attributes should be registered by the IANA		Section 5 respectively. These two attributes should be registered by
	on		the IANA on
	http://www.iana.org/assignments/sdp-parameters		http://www.iana.org/assignments/sdp-parameters
	under "att-field (both session and media level)".		under "att-field (both session and media level)".
	This document defines a proto values: TCP. Its format is defined in		This document defines a proto value: TCP. Its format is defined in
	Section 3. This proto value should be registered by the IANA on		Section 3. This proto value should be registered by the IANA on
	http://www.iana.org/assignments/sdp-parameters		http://www.iana.org/assignments/sdp-parameters
	under "proto".		under "proto".
			Specifications defining new proto values, like this one, must define
			the rules by which their media format (fmt) namespace is managed.
			For the TCP protocol, new formats SHOULD have an associated MIME
			registration. Use of an existing MIME subtype for the format is
			encouraged. If no MIME subtype exists, it is RECOMMENDED that a
			suitable one is registered through the IETF process [2] by production
			of, or reference to, a standards-track RFC that defines the transport
			protocol for the format.
	10. Acknowledgements		10. Acknowledgements
	Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul		Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul
	Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer		Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer
	Holmberg provided valuable insights and contributions.		Holmberg provided valuable insights and contributions.
	11. References		11. References
	11.1 Normative References		11.1 Normative References
	[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,		[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
	September 1981.		September 1981.
	[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement		[2] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet
			Mail Extensions (MIME) Part Four: Registration Procedures", BCP
			13, RFC 2048, November 1996.
			[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
	Levels", BCP 14, RFC 2119, March 1997.		Levels", BCP 14, RFC 2119, March 1997.
	[3] Handley, M. and V. Jacobson, "SDP: Session Description		[4] Handley, M. and V. Jacobson, "SDP: Session Description
	Protocol", RFC 2327, April 1998.		Protocol", RFC 2327, April 1998.
	[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with		[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
	Session Description Protocol (SDP)", RFC 3264, June 2002.		Session Description Protocol (SDP)", RFC 3264, June 2002.
	11.2 Informative References		11.2 Informative References
	[5] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming		[6] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
	Protocol (RTSP)", RFC 2326, April 1998.		Protocol (RTSP)", RFC 2326, April 1998.
	[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC		[7] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
	2246, January 1999.		2246, January 1999.
	[7] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,		[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
	Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --		Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
	HTTP/1.1", RFC 2616, June 1999.		HTTP/1.1", RFC 2616, June 1999.
	[8] Handley, M., Perkins, C. and E. Whelan, "Session Announcement		[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
	Protocol", RFC 2974, October 2000.		Protocol", RFC 2974, October 2000.
	[9] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,		[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
	Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:		Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
	Session Initiation Protocol", RFC 3261, June 2002.		Session Initiation Protocol", RFC 3261, June 2002.
	[10] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD		[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
	63, RFC 3629, November 2003.		63, RFC 3629, November 2003.
			[12] Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
			"Best Current Practices for Third Party Call Control (3pcc) in
			the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
			2004.
	Authors' Addresses		Authors' Addresses
	David Yon		David Yon
	Dialout.Net, Inc		Tactical Software, LLC
	One Indian Head Plaza		670 N Commercial St
	Nashua, NH 03060		Manchester, NH 03101
	USA		USA
	EMail: yon@dialout.net		EMail: yon-comedia@rfdsoftware.com
	Gonzalo Camarillo		Gonzalo Camarillo
	Ericsson		Ericsson
	Hirsalantie 11		Hirsalantie 11
	Jorvas 02420		Jorvas 02420
	Finland		Finland
	EMail: Gonzalo.Camarillo@ericsson.com		EMail: Gonzalo.Camarillo@ericsson.com
	Intellectual Property Statement		Intellectual Property Statement
	The IETF takes no position regarding the validity or scope of any		The IETF takes no position regarding the validity or scope of any
	Intellectual Property Rights or other rights that might be claimed to		Intellectual Property Rights or other rights that might be claimed to
	pertain to the implementation or use of the technology described in		pertain to the implementation or use of the technology described in
	this document or the extent to which any license under such rights		this document or the extent to which any license under such rights
	might or might not be available; nor does it represent that it has		might or might not be available; nor does it represent that it has
	made any independent effort to identify any such rights. Information		made any independent effort to identify any such rights. Information
	on the IETF's procedures with respect to rights in IETF Documents can		on the procedures with respect to rights in RFC documents can be
	be found in BCP 78 and BCP 79.		found in BCP 78 and BCP 79.
	Copies of IPR disclosures made to the IETF Secretariat and any		Copies of IPR disclosures made to the IETF Secretariat and any
	assurances of licenses to be made available, or the result of an		assurances of licenses to be made available, or the result of an
	attempt made to obtain a general license or permission for the use of		attempt made to obtain a general license or permission for the use of
	such proprietary rights by implementers or users of this		such proprietary rights by implementers or users of this
	specification can be obtained from the IETF on-line IPR repository at		specification can be obtained from the IETF on-line IPR repository at
	http://www.ietf.org/ipr.		http://www.ietf.org/ipr.
	The IETF invites any interested party to bring to its attention any		The IETF invites any interested party to bring to its attention any
	copyrights, patents or patent applications, or other proprietary		copyrights, patents or patent applications, or other proprietary
End of changes.
This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/