draft-ietf-mmusic-sdp-comedia-05.txt		draft-ietf-mmusic-sdp-comedia-06.txt
	INTERNET-DRAFT D. Yon		MMUSIC Working Group D. Yon
	Document: draft-ietf-mmusic-sdp-comedia-05.txt Dialout.Net		Internet-Draft Dialout.Net, Inc
	Expires September 2003 March 2003		Expires: November 12, 2004 G. Camarillo
			Ericsson
			May 14, 2004
	Connection-Oriented Media Transport in SDP		Connection-Oriented Media Transport in the Session Description
	<draft-ietf-mmusic-sdp-comedia-05.txt>		Protocol (SDP)
			draft-ietf-mmusic-sdp-comedia-06.txt
	Status of this Memo		Status of this Memo
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	all provisions of Section 10 of RFC2026.		patent or other IPR claims of which I am aware have been disclosed,
			and any of which I become aware will be disclosed, in accordance with
			RFC 3668.
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	Copyright (C) The Internet Society (2002). All Rights Reserved.		This Internet-Draft will expire on November 12, 2004.
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			Copyright (C) The Internet Society (2004). All Rights Reserved.
	Abstract		Abstract
	This document describes how to express media transport over		This document describes how to express media transport over
	connection-oriented protocols using the Session Description Protocol		connection-oriented protocols using the Session Description Protocol
	(SDP). It defines two new protocol identifiers: TCP and TLS. It		(SDP). It defines two new protocol identifiers: TCP and TCP/TLS. It
	also defines the syntax and semantics for an SDP "direction"		also defines the SDP setup attribute, which describes the connection
	attribute that describes the connection setup procedure.		setup procedure, and the SDP reconnect attribute.
	Yon 1		Table of Contents
	1 Introduction		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
			2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
			3. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 3
			3.1 TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
			3.2 TCP/TLS . . . . . . . . . . . . . . . . . . . . . . . . . 4
			4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
			4.1 The Setup Attribute in the Offer/answer Model . . . . . . 4
			4.2 Multiple-Connection Avoidance when Using Actpass . . . . . 5
			5. The Reconnect Attribute . . . . . . . . . . . . . . . . . . . 6
			6. Connection Lifetime . . . . . . . . . . . . . . . . . . . . . 7
			6.1 Session Renegotiation . . . . . . . . . . . . . . . . . . 7
			7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
			7.1 Passive/Active . . . . . . . . . . . . . . . . . . . . . . 8
			7.2 Passive/Active with Reconnect . . . . . . . . . . . . . . 9
			7.3 Actpass . . . . . . . . . . . . . . . . . . . . . . . . . 9
			8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
			9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
			10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
			11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
			11.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
			11.2 Informational References . . . . . . . . . . . . . . . . . . 11
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
			Intellectual Property and Copyright Statements . . . . . . . . 13
	The Session Description Protocol [SDP] provides a general-purpose		1. Introduction
			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]) to maximize portability among transports. SDP		subset of UTF-8 [6]) 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 participate in that
	participate in that session. Session descriptions may be sent using		session. Session descriptions may be sent using arbitrary existing
	arbitrary existing application protocols for transport (e.g., SAP,		application protocols for transport (e.g., SAP [9], SIP [10], RTSP
	SIP, RTSP, email, HTTP, etc.).		[7], email, HTTP [8], etc.).
	[SDP] describes two protocol identifiers: RTP/AVP and UDP, both of		SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of
	which are unreliable, connectionless protocols, an appropriate		which represent unreliable connectionless protocols. While these
	choice for multimedia streams. There are, however, applications for		transports are appropriate choices for multimedia streams, there are
	which the connection-oriented transports such as TCP are more		applications for which connection-oriented transports such as TCP are
	appropriate, but [SDP] provides no way to describe a session that		more appropriate. We define two new protocol identifiers: TCP and
	uses protocols other than RTP or UDP.		TCP/TLS. Both represent connection-oriented reliable transports.
	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: how should end points perform the connection setup
	will be based on this protocol, but it must also describe the		procedure. We define two new attributes to describe connection setup:
	connection setup procedure. This memo defines two new protocol		setup and reconnect.
	identifiers, TCP and TLS, along with the syntax and semantics of the
	a=direction and a=reconnect attributes.
	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", "MAY",		"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
	and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]		RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
	and indicate requirement levels for compliant implementations.		described in BCP 14, RFC 2119 [2] and indicate requirement levels for
			compliant implementations.
	3 Protocol Identifiers
	The m= line in [SDP] is where an endpoint specifies the protocol
	used for the media in the session. See the "Media Announcements"
	section of [SDP] for a discussion on protocol identifiers.
	3.1 TCP
	The TCP protocol identifier is similar to the UDP protocol
	identifier in that it only describes the transport protocol without
	any connotation as to the upper-layer protocol. An m= line that
	specifies "TCP" MUST further qualify the protocol using a fmt
	identifier (see [SDP] Appendix B).
	3.2 TLS
	The TLS protocol identifier specifies that the session will use the
	Transport Layer Security protocol [TLS] with an implied transport
	protocol of TCP. To describe a media session that uses TLS over
	TCP, the protocol identifier "TLS" must be specified in the m= line.
	Yon INTERNET-DRAFT - Expires September 2003 2
	An m= line that specifies TLS MUST further qualify the protocol
	using a fmt identifier.
	4 Direction Attribute
	An important attribute of connection-oriented protocols is the setup
	procedure. One endpoint needs to initiate the connection and the
	other endpoint needs to accept the connection. The direction
	attribute is used to describe these roles, and the syntax is as
	follows:
	a=direction:<role>
	The <role> is one of the following:
	passive: The endpoint will accept an incoming connection.
	active: The endpoint will initiate an outgoing connection.
	both: The endpoint will both accept an incoming connection
	and will initiate an outgoing connection.
	4.1 Semantics of direction:passive
	By specifying direction:passive, the endpoint indicates that the
	port number specified in the m= line is available to accept a
	connection from the other endpoint.
	4.2 Semantics of direction:active
	By specifying direction:active, the endpoint indicates that it will
	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
	opposite endpoint MUST NOT attempt to initiate a connection to the
	port number specified there. Nevertheless, since the m= line must
	contain a valid port number, the endpoint specifying
	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]. The following
	SDP fragment shows an example of direction:active:
	c=IN IP4 10.1.1.1		3. Protocol Identifiers
	m=image 9 TCP t38
	a=direction:active IN IP4
	4.3 Semantics of direction:both		The following is the ABNF for an m= line, as specified by RFC 2327
			[4].
	By specifying direction:both, the endpoint indicates that it will		media-field = "m=" media space port ["/" integer]
	both accept a TCP connection on the port number of its own m= line,		space proto 1*(space fmt) CRLF
	and that it will also initiate a connection to the port number on
	the m= line of the other endpoint.
	Yon INTERNET-DRAFT - Expires September 2003 3		We define two new values for the proto field: TCP and TCP/TLS.
	Since this attribute describes behavior that is similar to
	connectionless media descriptions in [SDP], it is the default value
	for the direction attribute and is therefore optional.
	Endpoints may choose to specify direction:both for one or more of		3.1 TCP
	the following reasons:
	1) The endpoint has no preference as to whether it accepts or		The TCP protocol identifier is similar to the UDP protocol identifier
	initiates the connection, and therefore is offering the remote		in that it only describes the transport protocol, and not the
	endpoint a choice of connection setup procedures.		upper-layer protocol. An m= line that specifies "TCP" MUST further
			qualify the application-layer protocol using an fmt identifier.
	2) The endpoints intend to use a single connection to transport		Media lines with the TCP protocol identifier are carried using TCP
	the media, but it is not known whether firewall issues will		[1].
	prevent either endpoint from initiating or accepting the
	connection. Therefore both endpoints will attempt to initiate
	a connection in hopes that at least one will succeed.
	If one endpoint specifies either direction:active or		3.2 TCP/TLS
	direction:passive and the other specifies direction:both, both
	endpoints MUST behave as if the latter had specified the inverse
	direction of the former. For example, specifying direction:both
	when the other endpoint specifies direction:active SHALL cause both
	endpoints to behave as if the former had specified
	direction:passive. Conversely, specifying direction:both when the
	other endpoint specifies direction:passive SHALL cause both
	endpoints to behave as if the former had specified direction:active.
	If both endpoints specify direction:both then each endpoint MUST		The TCP/TLS protocol identifier specifies that the session will use
	initiate a connection to the port number specified on the m= line of		the Transport Layer Security (TLS) protocol [3] on top on a TCP [1]
	the opposite endpoint. There is one exception to this requirement:		connection.
	if an endpoint receives the incoming connection from the opposite
	endpoint 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.
	If only one connection succeeds, then that connection will be used		An m= line that contain the TCP/TLS protocol identifier MUST further
	to carry the media. Once it has transmitted data on this		qualify the protocol using a fmt identifier.
	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, the following rules SHALL apply:		4. Setup Attribute
	a) Each endpoint MUST accept data from either connection.		The setup attribute indicates which of the end points should initiate
			the connection establishment (e.g., send the initial TCP SYN). The
			setup attribute is charset-independent and can be a session-level or
			a media-level attribute. The following is the ABNF of the setup
			attribute:
	b) Once an endpoint has transmitted data to one of the connections,		setup-attr = "a=setup:" role
	it MUST use that connection exclusively for transmission.		role = "active" / "passive" / "actpass"
	c) Once an endpoint has transmitted AND received data, if one of the		Active: The endpoint will initiate an outgoing connection.
	connections is determined to be idle, the endpoint SHOULD close		Passive: The endpoint will accept an incoming connection.
	the idle connection.		ActPass: The endpoint will both accept an incoming connection and
			will initiate an outgoing connection.
	Yon INTERNET-DRAFT - Expires September 2003 4		The default value of the setup attribute is actpass. That is, an m=
			line without an associated setup line is considered to be actpass.
	4.4 Optimizing direction:both		4.1 The Setup Attribute in the Offer/answer Model
	As discussed in the previous section, there is the possibility that		The offer/answer model, defined in RFC 3264 [5], provides endpoints
	two connections will be created when only one is needed. While		with a means to obtain shared view of a session. Some session
	rules in the previous section accommodate the closing of an idle		parameters are negotiated (e.g., codecs to use), while others are
	connection, they do not prevent a race condition where the endpoints		simply communicated from one endpoint to the other (e.g., IP
	simultaneously start sending data on opposite connections thereby		addresses). The value of the setup attribute falls into the first
	causing two connections to be used where one would have sufficed.		category. That is, both endpoints negotiate its value using the
	While it is not possible to entirely eliminate this race condition,		offer/answer model.
	it is in the endpoints' interest to minimize its occurrence.
	Therefore, when a session is negotiated through interactive exchange
	of SDP between endpoints (as in the case of SIP) AND the result of
	the negotiation is that each endpoint specifies direction:both, it
	is RECOMMENDED that the endpoints use the following guidelines:
	a) There comes a point during the exchange of SDP where one endpoint		The negotiation of the value of the setup attribute takes places as
	is prepared to send the final message that will complete the		follows. The offerer states which role or roles is willing to perform
	negotiation and allow the session to begin. For the purposes of		and the answerer, taking the offerer's willingness into
	this discussion, the endpoint that will send this final message		consideration, chooses which roles both endpoints will actually
	will be called the Initiator, and the endpoint that will receive		perform during connection establishment. The following are the values
	this message will be called the Acceptor.		that the setup attribute can take in an offer/answer exchange:
	b) The Initiator, upon receiving sufficient information to initiate a		Offer Answer
	connection, MUST attempt to connect to the Acceptor as soon as		_______________
	possible.		active passive
			passive active
			actpass active / passive / actpass
	c) In order to lower the likelihood that the Acceptor will also		The value active indicates that the endpoint SHOULD initiate a
	attempt to initiate a connection, the Initiator SHOULD incorporate		connection to the port number on the m= line of the other endpoint.
	a short delay between initiating the connection and sending the		The port number on its own m= line is irrelevant, and the opposite
	final SDP to the Acceptor.		endpoint MUST NOT attempt to initiate a connection to the port number
			specified there. Nevertheless, since the m= line must contain a valid
			port number, the endpoint specifying using the value 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.
	d) The delay time chosen by the Initiator MUST NOT introduce an		The value passive indicates that the endpoint SHOULD be ready to
	unacceptable session setup delay should the connection to the		accept a connection on the port number specified in the m= line.
	Acceptor not succeed.
	4.5 Bidirectional versus Unidirectional Media		The value actpass indicates that the endpoint SHOULD initiate a
			connection to the port number on the m= line of the other endpoint
			and that the endpoint SHOULD be ready to accept a connection on the
			port number specified in the m= line. It is RECOMMENDED that, if
			possible, endpoints set the port number on their m= line to the
			source port number which they will use to establish the connection
			towards the remote endpoint. This way, the transport-layer protocol
			(e.g., TCP) can take care of simultaneous opens.
	In traditional SDP transport types the flow is unidirectional. If		Endpoints typically use the actpass value for the following reasons:
	the intent is for media to flow in both directions, both endpoints		1. The offerer has no preference as to whether it accepts or
	must specify SDP that describes where to deliver the media and what		initiates the connection and, so, is letting the answerer choose.
	media type(s) to use. For example, if only Endpoint A presents SDP		2. The endpoints intend to use a single connection to transport the
	then media can only flow towards Endpoint A, as Endpoint B has not		media, but it is not known whether NAT (Network Address
	specified where and how to send media to it.		Translator) issues will prevent either endpoint from initiating
			or accepting the connection. So, both endpoints will attempt to
			initiate a connection hoping that at least one will succeed.
	Because most connection-oriented media is inherently bi-directional,		4.2 Multiple-Connection Avoidance when Using Actpass
	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		When an offer/answer exchange results in actpass, each endpoint
	data on the same connection it is using to receive data, so long as		attempts to establish a transport connection towards the other
			endpoint. If only one of the connections succeeds, this connection is
			used to transfer media. Nevertheless, if both connections succeed,
			one of them needs to be terminated so that both endpoints exchange
			data over a single connection. In this section, we provide rules to
			choose which of the two connections should be terminated (or not even
			initiated).
	Yon INTERNET-DRAFT - Expires September 2003 5		First of all, if the endpoints follow the recommendation of setting
	the other endpoint has advertised its willingness to accept data.		the port number in their m= line to the source port number which they
	Likewise, it is perfectly acceptable for an endpoint to receive data		will use to establish the connection towards the remote endpoint, the
	on the same connection it is using to transmit data to the		transport layer should take care of simultaneous opens (at least if
	corresponding remote endpoint. In other words, for a bi-directional		TCP is the transport protocol). If, for some reason, any of the
	application-level session, a connection may be used to send data in		endpoints does not follow this recommendation, both endpoints should
	both directions (contingent to rules outlined in Section 2.3) as		follow the rules below.
	long as one side of the connection is attached to either of the
	advertised SDP transport addresses.
	4.6 Treating UDP and RTP/AVP like Connection Oriented Media		If an endpoint is notified about a connection establishment attempt
			from the other endpoint before performing its own connection attempt,
			it SHOULD behave as a passive endpoint and SHOULD NOT attempt to
			establish any other connection.
	Endpoints MAY specify a direction attribute for UDP or RTP/AVP		In case two connections are established, if an endpoint receives data
	media. This indicates that the endpoint would like to treat this		(i.e., media) over one of the connections before having sent any data
	media as a type of connection-oriented media. (The endpoint may do		on any of the connections, the endpoint SHOULD terminate the
	this to facilitate NAT traversal for example.) Note that for		connection that has not carried any data.
	backwards compatibility, an endpoint which can specify
	direction:active MUST include valid addresses and ports in the SDP
	as always. If the peer's SDP does not include a direction
	attribute, it knows that the peer does not support connection-
	oriented media, and media exchange will proceed normally, as if
	connection-oriented media were not offered.
	Endpoints that specify direction:passive MUST NOT send any media,		When two connections are established and both endpoints start sending
	any packets whatsoever (including control packets such as RTCP),		data before receiving anything from the other endpoint, it may happen
	from their passive ports until they receive a packet on these ports		that each of the endpoints choose a different connection to send
	and record the source address and port of the sender. The passive		data. If the answerer receives data over a connection after having
	endpoint then assumes that the first packet received corresponds to		sent data on the other connection, it SHOULD continue sending data on
	its active peer. From this point onward, passive endpoints MUST		the other connection until an application-layer data boundary. At
	send UDP or RTP media from the same port as the port indicated in		that point, the answerer SHOULD terminate this connection and start
	the m= line. Passive endpoints MUST send RTCP media (if any) from		using the connection on which the offerer was sending data.
	the port on which they expect to receive it (typically the RTP port
	number plus 1).
	Endpoints that specify direction:active MUST be prepared to receive		Note that different applications may define application-layer
	on the ports from which they send. Once they learn the IP address		boundaries in different ways. A typical suitable point for the
	and port of their peer from the peer's SDP, they SHOULD immediately		answerer to change connections is the end of an application-layer
	send some kind of media (even if just comfort noise) to each of		message and the beginning of the next one.
	these ports. This is so the peer can learn their IP address and
	port, in order to send media back without additional delay.
	Effectively, the exchange of the first media packet completes a bi-
	directional handshake between the active and passive peer.
	5 Reconnect Attribute		5. The Reconnect Attribute
	The preceding description of the a=direction attribute has been in		The preceding description of the setup attribute has been in the
	the context of using SDP to initiate a session. However, 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, renegotiating the media parameters for a session,		a new endpoint, or renegotiating the media parameters for a session.
	etc. After the initial session has been established, it may be		After the initial session has been established, it may be ambiguous
	ambiguous as to whether subsequent SDP exchange represents a		as to whether subsequent SDP exchange represents a confirmation that
	confirmation that the endpoint is to continue using the current		the endpoint is to continue using the current media connection
	media connection unchanged, or is a request to make a new media		unchanged, or is a request to make a new media connection. The
			reconnect attribute, which is charset-independent and can be a
	Yon INTERNET-DRAFT - Expires September 2003 6		session-level or a media-level attribute, is used to disambiguate
	connection. The reconnect attribute is used to disambiguate these		these two scenarios. The following is the ABNF of the reconnect
	two scenarios, and the syntax is as follows:		attribute:
	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.
	6 Connection and Listener Lifetime Considerations
	6.1 Listener Lifetime
	An endpoint that has specified direction:both or direction:passive
	MUST be ready to accept a connection on the appropriate address and
	port during the time slot(s) advertised for that session. The
	endpoint MUST keep the address and port available for incoming
	connections until either:
	a) The time window for the session has expired, or
	b) The endpoint has received the expected number of incoming		reconnect-attr = "a=reconnect"
	connections on that address and port, or
	c) Subsequent exchanges have superceded the SDP that originally		On reception of an m= line with a reconnect attribute, the endpoints
	advertised the availability of the address and port.		SHOULD close the existing connection, in case it was still up, and
			SHOULD establish a new connection according to the setup attribute in
			the m= line.
	Once the endpoint has determined that a listener is no longer needed		Either the offerer or the answerer can include a reconnect attribute
	on a specific address and port, it SHOULD terminate the listener.		in an m= line. In any event, if the offer contained this attribute,
	The endpoint is then free to re-use the address and port for		the answer MUST contain it as well.
	subsequent session advertisements.
	6.2 Connection Lifetime		6. Connection Lifetime
	An endpoint that intends to initiate the connection MUST initiate		An endpoint that intends to initiate the connection SHOULD 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 SHOULD NOT close the connection until the
			session has expired, been explicitly terminated, or the media stream
	An endpoint MUST NOT close the connection until the session has		is redirected to a different address or port.
	expired, been explicitly terminated, or the media stream is
	redirected to a different address or port.
	Yon INTERNET-DRAFT - Expires September 2003 7
	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
	is application and/or context dependant. If the endpoint opts to		application and context dependant.
	re-establish the connection, it MUST NOT assume that the original
	address and port advertised by the remote endpoint is still valid.
	Instead, the endpoint MUST renegotiate the session parameters by
	exchanging new SDP.
	6.3 Session Renegotiation and Connection Lifetime		6.1 Session Renegotiation
	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 setup attribute which can be considered
	construed as a request to create a new connection, as opposed to a		as a request to create a new connection, as opposed to a request to
	request to maintain steady state. To avoid this ambiguity, the		maintain steady state. The following rule help avoid this ambiguity:
	following rule SHALL apply to subsequent exchanges of SDP:
	If the transport section (the c= and m= lines)
	combined with the direction attribute of an SDP
	message describes an existing connection between two
	endpoints, AND the SDP does not contain a=reconnect,
	then the endpoints MUST use that connection to carry
	the media described in the remainder of the message.
	The endpoints MUST NOT attempt to set up a new
	connection, regardless of what is specified in the
	direction attribute.
	This disambiguates most session renegotiation scenarios, with the		If the transport section (the c= and m= lines) of an SDP
	exception of muting. Muting a media stream is accomplished by		description describes an existing connection between two endpoints
	sending the original session SDP but with an "a=inactive" or		and the m= line does not contain a reconnect attribute, the
	"a=sendonly/recvonly" attribute. This is still valid for connection		endpoints SHOULD use that connection to carry the media described
	oriented media, with the additional caveat that the endpoints MUST		in the remainder of the message. The endpoints SHOULD NOT attempt
	NOT close the connection described by that SDP.		to set up a new connection, regardless of what is specified in the
			setup attribute.
			Note that if the port number in the m= line changes, there is no
			need to use the reconnect attribute because the new port will
			trigger the establishment of a new connection anyway.
	7 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
	usage of the direction attribute combined with TCP-based media		of the setup attribute combined with TCP-based media descriptions.
	descriptions. For the purpose of brevity, the main portion of the		For the purpose of brevity, the main portion of the session
	session description is omitted in the examples and is assumed to be		description is omitted in the examples and is assumed to be the
	the following:		following:
	Yon INTERNET-DRAFT - Expires September 2003 8
	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
	7.1 Example: simple passive/active		7.1 Passive/Active
	An endpoint at 10.1.1.2 signals the availability of a T.38 fax		An offerer at 192.0.2.2 signals its availability for a T.38 fax
	session at port 54111:		session at port 54111:
	c=IN IP4 10.1.1.2		c=IN IP4 192.0.2.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:passive		a=setup:passive
	An endpoint at 10.1.1.1 receiving this description responds with the		An answerer at 192.0.2.1 receiving this offer responds with the
	following:		following answer:
	c=IN IP4 10.1.1.1		c=IN IP4 192.0.2.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active		a=setup:active
	The endpoint at 10.1.1.1 then initiates the TCP connection to port		The endpoint at 192.0.2.1 then initiates the TCP connection to port
	54111 at 10.1.1.2.		54111 at 192.0.2.2.
	7.2 Example: simple passive/active with reconnect		7.2 Passive/Active with Reconnect
	Continuing the preceding example, consider the scenario where the		Continuing the preceding example, consider the scenario where the TCP
	TCP connection fails and the endpoints wish to reestablish the		connection fails and the endpoints wish to reestablish the connection
	connection for the session. The endpoint at 10.1.1.2 signals this		for the session. The endpoint at 192.0.2.2 signals this intent with
	intent with the following SDP:		the following SDP:
	c=IN IP4 10.1.1.2		c=IN IP4 192.0.2.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:passive		a=setup:passive
	a=reconnect		a=reconnect
	The a=reconnect attribute informs the endpoint at 10.1.1.1 that this		The reconnect attribute informs the endpoint at 192.0.2.1 that this
	SDP represents the intent to establish a new connection for media		SDP represents the intent to establish a new connection for media
	transport, rather than continuing with the original connection.		transport, rather than continuing with the original connection.
	Because the endpoint at 10.1.1.1 may not yet be aware that the TCP		Because the endpoint at 192.0.2.1 may not yet be aware that the TCP
	connection has failed, this eliminates any ambiguity. If 10.1.1.1		connection has failed, this eliminates any ambiguity. If 192.0.2.1
	agrees to continue the session using a new connection, it responds		agrees to continue the session using a new connection, it responds
	with:		with:
	c=IN IP4 10.1.1.1		c=IN IP4 192.0.2.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active IN IP4		a=setup:active IN IP4
	a=reconnect		a=reconnect
	7.3 Example: agnostic both		7.3 Actpass
	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
	stream by initiating the TCP connection:
	Yon INTERNET-DRAFT - Expires September 2003 9
	c=IN IP4 10.1.1.2
	m=image 54111 TCP t38
	a=direction:both
	The endpoint at 10.1.1.1 has three choices:
	1) It can respond with either of the two direction:active
	descriptions listed in the previous example. In this case the
	endpoint at 10.1.1.1 must initiate a connection to port 54111
	at 10.1.1.2.
	2) It can respond with a description similar to the following:
	c=IN IP4 10.1.1.1
	m=image 54321 TCP t38
	a=direction:passive
	In this case the endpoint at 10.1.1.2 must initiate a
	connection to port 54321 at 10.1.1.1.
	3) It can respond with a description that specifies
	direction:both, which is covered in the next example.
	7.4 Example: redundant both
	An endpoint at 10.1.1.2 uses the same description as the previous		An offerer at 192.0.2.2 signals its availability for a T.38 fax
	example:		session at TCP port 54111. Additionally, this offerer is also willing
			to set up the media stream by initiating the TCP connection:
	c=IN IP4 10.1.1.2		c=IN IP4 192.0.2.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:both		a=setup:actpass
	Unlike the previous example, the endpoint at 10.1.1.1 responds with		The endpoint at 192.0.2.1 responds with the following description:
	the following description:
	c=IN IP4 10.1.1.1		c=IN IP4 192.0.2.1
	m=image 54321 TCP t38		m=image 54321 TCP t38
	a=direction:both		a=setup:actpass
	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
	connection to port 54111 at 10.1.1.2. Whichever TCP connection
	succeeds will be used. If both succeed, one of the connections may
	be closed as an optimization, using the rules in section 3.3.
	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
	Yon INTERNET-DRAFT - Expires September 2003 10
	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		This will cause the offerer (at 192.0.2.2) to initiate a connection
	endpoint at 10.1.1.1 pauses for a short time to allow the		to port 54321 at 192.0.2.1 and the answerer (at 192.0.2.1) to
	endpoint at 10.1.1.2 to receive the TCP connection initiation.		initiate a connection to port 54111 at 192.0.2.2. Ideally, the
			offerer would use 192.0.2.2:5411 as the source of its connection
			attempt and the answerer would use 192.0.2.1:54321 as its.
	4) After the short pause, the endpoint at 10.1.1.1 sends the SDP		8. Security Considerations
	response as listed above.
	The pause in #3 gives the first TCP connection attempt a chance to		See RFC 2327 [4] for security and other considerations specific to
	succeed, since withholding the SDP response deprives the endpoint at		the Session Description Protocol in general.
	10.1.1.2 of the information it needs to attempt its own TCP
	connection.
	7.5 Example: "Bidirectional" RTP and RTCP		An attacker may attempt to substitute TCP/TLS with only TCP in a
			session description. So, it is STRONGLY RECOMMENDED that integrity
			protection be applied to the SDP session descriptions. For session
			descriptions carried in SIP [10], S/MIME is the natural choice to
			provide such end-to-end integrity protection, as described in RFC
			3261 [10]. Other applications MAY use a different form of integrity
			protection.
	An endpoint at 10.1.1.2 is behind a NAT and does not know its own		This document touches upon NAT traversal. Implementers should be
	public address.		aware of some issues that relate to the use of private IP addresses
			within the offer/answer model (i.e., they are not specific to this
			document).
	c=IN IP4 10.1.1.2		When an endpoint receives a session description with a private IP
	m=audio 9 RTP/AVP 0		address that belongs to a different address space, in most of the
	a=direction:active		cases, the endpoint will not be able to reach such an address.
			Nevertheless, if this particular address also exists in the
			endpoint's address space, the endpoint may end up reaching a
			different peer than the one that generated the session description.
			It is RECOMMENDED that endpoints authenticate their peer somehow
			(e.g., using TLS [3]) or that they encrypt their media.
	A peer with a public IP address responds as follows and waits to		9. IANA Considerations
	receive RTP and RTCP packets from its active peer.
	c=IN IP4 1.2.3.4		This document defines two session and media level SDP attributes:
	m=audio 18240 RTP/AVP 0		setup and reconnect. Their formats are defined in Section 4 and
	a=direction:passive		Section 5 respectively. These two attributes should be registered by
			the IANA on http://www.iana.org/assignments/sdp-parameters under
			"att-field (both session and media level)".
	The endpoint at 10.1.1.2 immediately sends RTP from port 9012 to		This document defines two proto values: TCP and TCP/TLS. Their
	1.2.3.4 port 18240. A NAT translates the source address to 5.6.7.8		formats are defined in Section 3.1 and Section 3.2 respectively.
	port 1542. The passive endpoint receives this RTP packet and stores		These two proto values should be registered by the IANA on http://
	this source address. When the passive endpoint wants to send RTP		www.iana.org/assignments/sdp-parameters under "proto".
	media it sends it back to 5.6.7.8 port 1542. The NAT translates this
	destination address back to 10.1.1.2 port 9012 and delivers it to
	the active endpoint.
	Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port		10. Acknowledgements
	9013 to 1.2.3.4:18241. The NAT translates this to 5.6.7.8:1984. The
	passive endpoint receives the RTCP packet and stores the source
	address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which
	is translated back to 10.1.1.2:9013 and delivered to the active
	endpoint.
	8 Security Considerations		The authors would like to thank Jonathan Rosenberg, Rohan Mahy,
			Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert
			Fairlie-Cuninghame, and Colin Perkins for their valuable insights and
			contributions.
	See [SDP] for security and other considerations specific to the		11. References
	Session Description Protocol in general.
	Yon INTERNET-DRAFT - Expires September 2003 11		11.1 Normative References
	9 IANA Considerations		[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
			September 1981.
	As recommended by [SDP] Appendix B, the direction and reconnect		[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
	attributes described in this document should be registered with		Levels", BCP 14, RFC 2119, March 1997.
	IANA, as should the "TCP" and "TLS" protocol identifiers.
	Acknowledgements		[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
			2246, January 1999.
	The author would like to thank Jonathan Rosenberg, Rohan Mahy,		[4] Handley, M. and V. Jacobson, "SDP: Session Description
	Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie-		Protocol", RFC 2327, April 1998.
	Cuninghame for their valuable insights and contributions.
	Yon INTERNET-DRAFT - Expires September 2003 12		[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
			Session Description Protocol (SDP)", RFC 3264, June 2002.
	Appendix A: Direction Attribute Syntax		[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
			63, RFC 3629, November 2003.
	This appendix provides an Augmented BNF [ABNF] grammar for		11.2 Informational References
	expressing the direction attribute for connection setup. It is
	intended as an extension to the grammar for the Session Description
	Protocol, as defined in [SDP]. Specifically, it describes the
	syntax for the new "connection-setup" attribute field, which MAY be
	either a session-level or media-level attribute.
	connection-setup = "direction" ":" direction-spec		[7] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
			Protocol (RTSP)", RFC 2326, April 1998.
	direction-spec = "both" / "active" / "passive"		[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
			Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
			HTTP/1.1", RFC 2616, June 1999.
	reconnect-attribute = "reconnect"		[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
			Protocol", RFC 2974, October 2000.
	References		[10] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
			Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
			Session Initiation Protocol", RFC 3261, June 2002.
	[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax		Authors' Addresses
	Specifications: ABNF," RFC 2234, November 1997
	[SDP] M. Handley, V. Jacobson, "SDP: Session Description		David Yon
	Protocol," RFC 2327, April 1998		Dialout.Net, Inc
			One Indian Head Plaza
			Nashua, NH 03060
			USA
	[T38] International Telecommunication Union, "Procedures for		EMail: yon@dialout.net
	Real-Time Group 3 Facsimile Communications over IP
	Networks," Recommendation T.38, June 1998
	[TLS] T. Dierks, C. Allen, "The TLS Protocol," RFC 2246,		Gonzalo Camarillo
	January 1999		Ericsson
			Hirsalantie 11
			Jorvas 02420
			Finland
	[UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode		EMail: Gonzalo.Camarillo@ericsson.com
	and ISO 10646," RFC 2044, October 1996
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