draft-ietf-mmusic-sdp-comedia-03.txt		draft-ietf-mmusic-sdp-comedia-04.txt
	INTERNET-DRAFT D. Yon		INTERNET-DRAFT D. Yon
	Document: draft-ietf-mmusic-sdp-comedia-03.txt Dialout.Net		Document: draft-ietf-mmusic-sdp-comedia-04.txt Dialout.Net
	Expires December 2002 June 2002		Expires January 2003 July 2002
	Connection-Oriented Media Transport in SDP		Connection-Oriented Media Transport in SDP
	<draft-ietf-mmusic-sdp-comedia-03.txt>		<draft-ietf-mmusic-sdp-comedia-04.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 68		skipping to change at line 68
	appropriate, but [SDP] provides no way to describe a session that		appropriate, but [SDP] provides no way to describe a session that
	uses protocols other than RTP or UDP.		uses protocols other than RTP or UDP.
	Connection-oriented protocols introduce a new factor when describing		Connection-oriented protocols introduce a new factor when describing
	a session: not only must it be possible to express that a protocol		a session: not only must it be possible to express that a protocol
	will be based on this protocol, but it must also describe the		will be based on this protocol, but it must also describe the
	connection setup procedure. This memo defines two new protocol		connection setup procedure. This memo defines two new protocol
	identifiers, TCP and TLS, along with the syntax and semantics of the		identifiers, TCP and TLS, along with the syntax and semantics of the
	a=direction and a=reconnect attributes.		a=direction and a=reconnect attributes.
	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", "MAY",
	and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]		and "OPTIONAL" are to be interpreted as described in RFC 2119 [7]
	and indicate requirement levels for compliant implementations.		and indicate requirement levels for compliant implementations.
	2 Protocol Identifiers		3 Protocol Identifiers
	The m= line in [SDP] is where an endpoint specifies the protocol		The m= line in [SDP] is where an endpoint specifies the protocol
	used for the media in the session. See the "Media Announcements"		used for the media in the session. See the "Media Announcements"
	section of [SDP] for a discussion on protocol identifiers.		section of [SDP] for a discussion on protocol identifiers.
	2.1 TCP		3.1 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).
	2.2 TLS		3.2 TLS
	The TLS protocol identifier specifies that the session will use the		The TLS protocol identifier specifies that the session will use the
	Transport Layer Security protocol [TLS] with an implied transport		Transport Layer Security protocol [TLS] with an implied transport
	protocol of TCP. To describe a media session that uses TLS over		protocol of TCP. To describe a media session that uses TLS over
	TCP, the protocol identifier "TLS" must be specified in the m= line.		TCP, the protocol identifier "TLS" must be specified in the m= line.
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	An m= line that specifies TLS MUST further qualify the protocol		An m= line that specifies TLS MUST further qualify the protocol
	using a fmt identifier.		using a fmt identifier.
	3 Direction Attribute		4 Direction Attribute
	An important attribute of connection-oriented protocols is the setup		An important attribute of connection-oriented protocols is the setup
	procedure. One endpoint needs to initiate the connection and the		procedure. One endpoint needs to initiate the connection and the
	other endpoint needs to accept the connection. The direction		other endpoint needs to accept the connection. The direction
	attribute is used to describe these roles, and the syntax is as		attribute is used to describe these roles, and the syntax is as
	follows:		follows:
	a=direction:<role> [<source-address>]		a=direction:<role> [<source-address>]
	The <role> is one of the following:		The <role> is one of the following:
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	address and port number from where the connection will originate,		address and port number from where the connection will originate,
	and consists of the following values:		and consists of the following values:
	nettype addrtype unicast-address [port]		nettype addrtype unicast-address [port]
	The <source-address> is an optional value that SHOULD be specified		The <source-address> is an optional value that SHOULD be specified
	with direction:active or direction:both, and MUST NOT be specified		with direction:active or direction:both, and MUST NOT be specified
	with direction:passive. Within the <source-address>, the source		with direction:passive. Within the <source-address>, the source
	port number is RECOMMENDED but may be omitted.		port number is RECOMMENDED but may be omitted.
	3.1 Semantics of direction:passive		4.1 Semantics of direction:passive
	By specifying direction:passive, the endpoint indicates that the		By specifying direction:passive, the endpoint indicates that the
	port number specified in the m= line is available to accept a		port number specified in the m= line is available to accept a
	connection from the other endpoint. The endpoint MUST NOT specify a		connection from the other endpoint. The endpoint MUST NOT specify a
	<source-address> after direction:passive.		<source-address> after direction:passive.
	3.2 Semantics of direction:active		4.2 Semantics of direction:active
	By specifying direction:active, the endpoint indicates that it will		By specifying direction:active, the endpoint indicates that it will
	initiate a connection to the port number on the m= line of the other		initiate a connection to the port number on the m= line of the other
	endpoint. The port number on its own m= line is irrelevant, and the		endpoint. The port number on its own m= line is irrelevant, and the
	opposite endpoint MUST NOT attempt to initiate a connection to the		opposite endpoint MUST NOT attempt to initiate a connection to the
	port number specified there. Nevertheless, since the m= line must		port number specified there. Nevertheless, since the m= line must
	contain a valid port number, the endpoint specifying		contain a valid port number, the endpoint specifying
	direction:active SHOULD specify a port number of 9 (the discard		direction:active SHOULD specify a port number of 9 (the discard
	port) on its m= line. The endpoint MUST NOT specify a port number		port) on its m= line. The endpoint MUST NOT specify a port number
	of zero, as that carries other semantics in [SDP].		of zero, as that carries other semantics in [SDP].
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	The endpoint SHOULD specify the address and port number from which		The endpoint SHOULD specify the address and port number from which
	it will initiate the connection in the <source-address> position on		it will initiate the connection in the <source-address> position on
	the a=direction line. The following SDP fragment shows an example		the a=direction line. The following SDP fragment shows an example
	of direction:active:		of direction:active:
	c=IN IP4 10.1.1.1		c=IN IP4 10.1.1.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active IN IP4 10.1.1.1 1892		a=direction:active IN IP4 10.1.1.1 1892
	3.3 Semantics of direction:both		4.3 Semantics of direction:both
	By specifying direction:both, the endpoint indicates that it will		By specifying direction:both, the endpoint indicates that it will
	both accept a TCP connection on the port number of its own m= line,		both accept a TCP connection on the port number of its own m= line,
	and that it will also initiate a connection to the port number on		and that it will also initiate a connection to the port number on
	the m= line of the other endpoint.		the m= line of the other endpoint.
	As with direction:active, the endpoint SHOULD specify the address		As with direction:active, the endpoint SHOULD specify the address
	and port number from which it will initiate the connection in the		and port number from which it will initiate the connection in the
	<source-address> position on the a=direction line.		<source-address> position on the a=direction line.
	skipping to change at line 203		skipping to change at line 203
	direction:passive. Conversely, specifying direction:both when the		direction:passive. Conversely, specifying direction:both when the
	other endpoint specifies direction:passive SHALL cause both		other endpoint specifies direction:passive SHALL cause both
	endpoints to behave as if the former had specified direction:active.		endpoints to behave as if the former had specified direction:active.
	If both endpoints specify direction:both then each endpoint MUST		If both endpoints specify direction:both then each endpoint MUST
	initiate a connection to the port number specified on the m= line of		initiate a connection to the port number specified on the m= line of
	the opposite endpoint. There is one exception to this requirement:		the opposite endpoint. There is one exception to this requirement:
	if an endpoint receives the incoming connection from the opposite		if an endpoint receives the incoming connection from the opposite
	endpoint prior to initiating its own outbound connection, then that		endpoint prior to initiating its own outbound connection, then that
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	endpoint MAY use that connection rather than attempt to make an		endpoint MAY use that connection rather than attempt to make an
	outbound connection to the opposite endpoint.		outbound connection to the opposite endpoint.
	If only one connection succeeds, then that connection will be used		If only one connection succeeds, then that connection will be used
	to carry the media. Once it has transmitted data on this		to carry the media. Once it has transmitted data on this
	connection, the initiating endpoint MUST NOT perform another		connection, the initiating endpoint MUST NOT perform another
	connection attempt to the accepting endpoint. This allows the		connection attempt to the accepting endpoint. This allows the
	accepting endpoint to release or recycle the listening port for		accepting endpoint to release or recycle the listening port for
	another session once it has received data from the initiating		another session once it has received data from the initiating
	endpoint.		endpoint.
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	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 connections,		b) Once an endpoint has transmitted data to one of the connections,
	it MUST use that connection exclusively for transmission.		it MUST use that connection exclusively for transmission.
	c) Once an endpoint has transmitted AND received data, if one of the		c) Once an endpoint has transmitted AND received data, if one of the
	connections is determined to be idle, the endpoint SHOULD close		connections is determined to be idle, the endpoint SHOULD close
	the idle connection.		the idle connection.
	3.4 Optimizing direction:both		4.4 Optimizing direction:both
	As discussed in the previous section, there is the possibility that		As discussed in the previous section, there is the possibility that
	two connections will be created when only one is needed. While		two connections will be created when only one is needed. While
	rules in the previous section accommodate the closing of an idle		rules in the previous section accommodate the closing of an idle
	connection, they do not prevent a race condition where the endpoints		connection, they do not prevent a race condition where the endpoints
	simultaneously start sending data on opposite connections thereby		simultaneously start sending data on opposite connections thereby
	causing two connections to be used where one would have sufficed.		causing two connections to be used where one would have sufficed.
	While it is not possible to entirely eliminate this race condition,		While it is not possible to entirely eliminate this race condition,
	it is in the endpoints' interest to minimize its occurrence.		it is in the endpoints' interest to minimize its occurrence.
	Therefore, when a session is negotiated through interactive exchange		Therefore, when a session is negotiated through interactive exchange
	skipping to change at line 257		skipping to change at line 257
	b) The Initiator, upon receiving sufficient information to initiate a		b) The Initiator, upon receiving sufficient information to initiate a
	connection, MUST attempt to connect to the Acceptor as soon as		connection, MUST attempt to connect to the Acceptor as soon as
	possible.		possible.
	c) In order to lower the likelihood that the Acceptor will also		c) In order to lower the likelihood that the Acceptor will also
	attempt to initiate a connection, the Initiator SHOULD incorporate		attempt to initiate a connection, the Initiator SHOULD incorporate
	a short delay between initiating the connection and sending the		a short delay between initiating the connection and sending the
	final SDP to the Acceptor.		final SDP to the Acceptor.
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	d) The delay time chosen by the Initiator MUST NOT introduce an		d) The delay time chosen by the Initiator MUST NOT introduce an
	unacceptable session setup delay should the connection to the		unacceptable session setup delay should the connection to the
	Acceptor not succeed.		Acceptor not succeed.
	3.5 Bidirectional versus Unidirectional Media		4.5 Bidirectional versus Unidirectional Media
	In traditional SDP transport types the flow is unidirectional. If		In traditional SDP transport types the flow is unidirectional. If
	the intent is for media to flow in both directions, both endpoints		the intent is for media to flow in both directions, both endpoints
	must specify SDP that describes where to deliver the media and what		must specify SDP that describes where to deliver the media and what
	media type(s) to use. For example, if only Endpoint A presents SDP		media type(s) to use. For example, if only Endpoint A presents SDP
	then media can only flow towards Endpoint A, as Endpoint B has not		then media can only flow towards Endpoint A, as Endpoint B has not
	specified where and how to send media to it.		specified where and how to send media to it.
	Because most connection-oriented media is inherently bi-directional,		Because most connection-oriented media is inherently bi-directional,
	endpoints may encounter a situation where only one side presented		endpoints may encounter a situation where only one side presented
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	data on the same connection it is using to receive data, so long as		data on the same connection it is using to receive data, so long as
	the other endpoint has advertised its willingness to accept data.		the other endpoint has advertised its willingness to accept data.
	Likewise, it is perfectly acceptable for an endpoint to receive data		Likewise, it is perfectly acceptable for an endpoint to receive data
	on the same connection it is using to transmit data to the		on the same connection it is using to transmit data to the
	corresponding remote endpoint. In other words, for a bi-directional		corresponding remote endpoint. In other words, for a bi-directional
	application-level session, a connection may be used to send data in		application-level session, a connection may be used to send data in
	both directions (contingent to rules outlined in Section 2.3) as		both directions (contingent to rules outlined in Section 2.3) as
	long as one side of the connection is attached to either of the		long as one side of the connection is attached to either of the
	advertised SDP transport addresses.		advertised SDP transport addresses.
	3.6 Treating UDP and RTP/AVP like Connection Oriented Media		4.6 Treating UDP and RTP/AVP like Connection Oriented Media
	Endpoints MAY specify a direction attribute for UDP or RTP/AVP		Endpoints MAY specify a direction attribute for UDP or RTP/AVP
	media. This indicates that the endpoint would like to treat this		media. This indicates that the endpoint would like to treat this
	media as a type of connection-oriented media. (The endpoint may do		media as a type of connection-oriented media. (The endpoint may do
	this to facilitate NAT traversal for example.) Note that for		this to facilitate NAT traversal for example.) Note that for
	backwards compatibility, an endpoint which can specify		backwards compatibility, an endpoint which can specify
	direction:active MUST include valid addresses and ports in the SDP		direction:active MUST include valid addresses and ports in the SDP
	as always. If the peer's SDP does not include a direction		as always. If the peer's SDP does not include a direction
	attribute, it knows that the peer does not support connection-		attribute, it knows that the peer does not support connection-
	oriented media, and media exchange will proceed normally, as if		oriented media, and media exchange will proceed normally, as if
	skipping to change at line 311		skipping to change at line 311
	Endpoints that specify direction:passive MUST NOT send any media,		Endpoints that specify direction:passive MUST NOT send any media,
	any packets whatsoever (including control packets such as RTCP),		any packets whatsoever (including control packets such as RTCP),
	from their passive ports until they receive a packet on these ports		from their passive ports until they receive a packet on these ports
	and record the source address and port of the sender. The passive		and record the source address and port of the sender. The passive
	endpoint then assumes that the first packet received corresponds to		endpoint then assumes that the first packet received corresponds to
	its active peer. From this point onward, passive endpoints MUST		its active peer. From this point onward, passive endpoints MUST
	send UDP or RTP media from the same port as the port indicated in		send UDP or RTP media from the same port as the port indicated in
	the m= line. Passive endpoints MUST send RTCP media (if any) from		the m= line. Passive endpoints MUST send RTCP media (if any) from
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	the port on which they expect to receive it (typically the RTP port		the port on which they expect to receive it (typically the RTP port
	number plus 1).		number plus 1).
	Endpoints that specify direction:active MUST be prepared to receive		Endpoints that specify direction:active MUST be prepared to receive
	on the ports from which they send. Once they learn the IP address		on the ports from which they send. Once they learn the IP address
	and port of their peer from the peer's SDP, they SHOULD immediately		and port of their peer from the peer's SDP, they SHOULD immediately
	send some kind of media (even if just comfort noise) to each of		send some kind of media (even if just comfort noise) to each of
	these ports. This is so the peer can learn their IP address and		these ports. This is so the peer can learn their IP address and
	port, in order to send media back without additional delay.		port, in order to send media back without additional delay.
	Effectively, the exchange of the first media packet completes a bi-		Effectively, the exchange of the first media packet completes a bi-
	directional handshake between the active and passive peer.		directional handshake between the active and passive peer.
	4 Reconnect Attribute		5 Reconnect Attribute
	The preceding description of the a=direction attribute has been in		The preceding description of the a=direction attribute has been in
	the context of using SDP to initiate a session. However, SDP may be		the context of using SDP to initiate a session. However, 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, renegotiating the media parameters for a session,
	etc. After the initial session has been established, it may be		etc. After the initial session has been established, it may be
	ambiguous as to whether subsequent SDP exchange represents a		ambiguous as to whether subsequent SDP exchange represents a
	confirmation that the endpoint is to continue using the current		confirmation that the endpoint is to continue using the current
	media connection unchanged, or is a request to make a new media		media connection unchanged, or is a request to make a new media
	skipping to change at line 354		skipping to change at line 354
	and MUST create a new connection according to the a=direction		and MUST create a new connection according to the a=direction
	attribute in the SDP. If an endpoint receives SDP that contains		attribute in the SDP. If an endpoint receives SDP that contains
	a=reconnect, the endpoint's response MUST also contain a=reconnect.		a=reconnect, the endpoint's response MUST also contain a=reconnect.
	Endpoints MUST NOT include a=reconnect in SDP that negotiates the		Endpoints MUST NOT include a=reconnect in SDP that negotiates the
	start of a session.		start of a session.
	See section 6, "Connection and Listener Lifetime Considerations" for		See section 6, "Connection and Listener Lifetime Considerations" for
	more information on scenarios that are relevant to the a=reconnect		more information on scenarios that are relevant to the a=reconnect
	attribute.		attribute.
	5 Source-Address Considerations		6 Source-Address Considerations
	In the cases where the endpoint is initiating the connection, the		In the cases where the endpoint is initiating the connection, the
	endpoint SHOULD specify a source address on the a=direction line.		endpoint SHOULD specify a source address on the a=direction line.
	In addition, the endpoint SHOULD include the source port in the		In addition, the endpoint SHOULD include the source port in the
	source address. In most environments, the source port number can be		source address. In most environments, the source port number can be
	determined by binding the socket before initiating the connect, as		determined by binding the socket before initiating the connect, as
	shown in the sample C code below:		shown in the sample C code below:
	{		{
	SOCKET s_id		SOCKET s_id
	SOCKADDR_IN cli_sin;		SOCKADDR_IN cli_sin;
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	int namelen;		int namelen;
	// Create the socket		// Create the socket
	s_id = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);		s_id = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
	// Bind the socket to any IP address and port		// Bind the socket to any IP address and port
	bzero((char *)&cli_sin,sizeof(cli_sin));		bzero((char *)&cli_sin,sizeof(cli_sin));
	cli_sin.sin_family = AF_INET;		cli_sin.sin_family = AF_INET;
	cli_sin.sin_addr.s_addr = htonl(INADDR_ANY);		cli_sin.sin_addr.s_addr = htonl(INADDR_ANY);
	cli_sin.sin_port = 0;		cli_sin.sin_port = 0;
	skipping to change at line 421		skipping to change at line 421
	endpoint omits the source port, the source address can		endpoint omits the source port, the source address can
	be ambiguous if multiple, logical endpoints share the		be ambiguous if multiple, logical endpoints share the
	same network address. Therefore it is NOT RECOMMENDED		same network address. Therefore it is NOT RECOMMENDED
	that the source address be used for this purpose		that the source address be used for this purpose
	unless the SDP occurs in the context of a controlled		unless the SDP occurs in the context of a controlled
	network topology that guarantees that the source		network topology that guarantees that the source
	address is both correct (i.e., no NAT, or a NAT with		address is both correct (i.e., no NAT, or a NAT with
	an Application-Level Proxy that rewrites the SDP) and		an Application-Level Proxy that rewrites the SDP) and
	unambiguous (i.e., the source port is specified).		unambiguous (i.e., the source port is specified).
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	5.1 Source Address Timing Considerations		6.1 Source Address Timing Considerations
	When used in conjunction with a session signaling protocol such as		When used in conjunction with a session signaling protocol such as
	SIP, there may be cases where an endpoint initiates a connection		SIP, there may be cases where an endpoint initiates a connection
	prior to the opposite endpoint receiving the SDP that describe the		prior to the opposite endpoint receiving the SDP that describe the
	source address of the initiating endpoint. Therefore, an endpoint		source address of the initiating endpoint. Therefore, an endpoint
	that has advertised an address and port number with direction:both		that has advertised an address and port number with direction:both
	or direction:passive MUST be ready to accept a connection on that		or direction:passive MUST be ready to accept a connection on that
	address and port immediately. If the accepting endpoint requires		address and port immediately. If the accepting endpoint requires
	the source address to identify the initiating endpoint, it MUST keep		the source address to identify the initiating endpoint, it MUST keep
	the connection active and allow sufficient time for the source		the connection active and allow sufficient time for the source
	address to arrive before discarding the connection.		address to arrive before discarding the connection.
	6 Connection and Listener Lifetime Considerations		7 Connection and Listener Lifetime Considerations
	6.1 Listener Lifetime		7.1 Listener Lifetime
	An endpoint that has specified direction:both or direction:passive		An endpoint that has specified direction:both or direction:passive
	MUST be ready to accept a connection on the appropriate address and		MUST be ready to accept a connection on the appropriate address and
	port during the time slot(s) advertised for that session. The		port during the time slot(s) advertised for that session. The
	endpoint MUST keep the address and port available for incoming		endpoint MUST keep the address and port available for incoming
	connections until either:		connections until either:
	a) The time window for the session has expired, or		a) The time window for the session has expired, or
	b) The endpoint has received the expected number of incoming		b) The endpoint has received the expected number of incoming
	connections on that address and port, or		connections on that address and port, or
	c) Subsequent exchanges have superceded the SDP that originally		c) Subsequent exchanges have superceded the SDP that originally
	advertised the availability of the address and port.		advertised the availability of the address and port.
	Once the endpoint has determined that a listener is no longer needed		Once the endpoint has determined that a listener is no longer needed
	on a specific address and port, it SHOULD terminate the listener.		on a specific address and port, it SHOULD terminate the listener.
	The endpoint is then free to re-use the address and port for		The endpoint is then free to re-use the address and port for
	subsequent session advertisements.		subsequent session advertisements.
	6.2 Connection Lifetime		7.2 Connection Lifetime
	An endpoint that intends to initiate the connection MUST initiate		An endpoint that intends to initiate the connection MUST initiate
	the connection immediately after it has sufficient information to do		the connection immediately after it has sufficient information to do
	so, even if it does not intend to immediately begin sending media to		so, even if it does not intend to immediately begin sending media to
	the remote endpoint. This allows media to flow from the remote		the remote endpoint. This allows media to flow from the remote
	endpoint.		endpoint.
	An endpoint MUST NOT close the connection until the session has		An endpoint MUST NOT close the connection until the session has
	expired, been explicitly terminated, or the media stream is		expired, been explicitly terminated, or the media stream is
	redirected to a different address or port.		redirected to a different address or port.
	If the endpoint determines that the connection has been closed, it		If the endpoint determines that the connection has been closed, it
	MAY attempt to re-establish the connection. The decision to do so		MAY attempt to re-establish the connection. The decision to do so
	is application and/or context dependant. If the endpoint opts to		is application and/or context dependant. If the endpoint opts to
	re-establish the connection, it MUST NOT assume that the original		re-establish the connection, it MUST NOT assume that the original
	address and port advertised by the remote endpoint is still valid.		address and port advertised by the remote endpoint is still valid.
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	Instead, the endpoint MUST renegotiate the session parameters by		Instead, the endpoint MUST renegotiate the session parameters by
	exchanging new SDP.		exchanging new SDP.
	6.3 Session Renegotiation and Connection Lifetime		7.3 Session Renegotiation and Connection Lifetime
	There are scenarios where SDP is sent by an endpoint in order to		There are scenarios where SDP is sent by an endpoint in order to
	renegotiate an existing session. These include muting/unmuting a		renegotiate an existing session. These include muting/unmuting a
	session, renegotiating the attributes of the media used by the		session, renegotiating the attributes of the media used by the
	session, or extending the length of a session about to expire.		session, or extending the length of a session about to expire.
	Connection-oriented media introduces some ambiguities into session		Connection-oriented media introduces some ambiguities into session
	renegotiation as to when the direction attribute must be obeyed and		renegotiation as to when the direction attribute must be obeyed and
	when it is ignored.		when it is ignored.
	The scenario of extending the duration of an existing session is a		The scenario of extending the duration of an existing session is a
	skipping to change at line 518		skipping to change at line 518
	connection, regardless of what is specified in the		connection, regardless of what is specified in the
	direction attribute.		direction attribute.
	This disambiguates most session renegotiation scenarios, with the		This disambiguates most session renegotiation scenarios, with the
	exception of muting. Muting a media stream is accomplished by		exception of muting. Muting a media stream is accomplished by
	sending the original session SDP but with an "a=inactive" or		sending the original session SDP but with an "a=inactive" or
	"a=sendonly/recvonly" attribute. This is still valid for connection		"a=sendonly/recvonly" attribute. This is still valid for connection
	oriented media, with the additional caveat that the endpoints MUST		oriented media, with the additional caveat that the endpoints MUST
	NOT close the connection described by that SDP.		NOT close the connection described by that SDP.
	7 Examples		8 Examples
	What follows are a number of examples that show the most common		What follows are a number of examples that show the most common
	usage of the direction attribute combined with TCP-based media		usage of the direction attribute combined with TCP-based media
	descriptions. For the purpose of brevity, the main portion of the		descriptions. For the purpose of brevity, the main portion of the
	session description is omitted in the examples and is assumed to be		session description is omitted in the examples and is assumed to be
	the following:		the following:
	v=0		v=0
	o=me 2890844526 2890842807 IN IP4 10.1.1.2		o=me 2890844526 2890842807 IN IP4 10.1.1.2
	s=Call me using TCP		s=Call me using TCP
	t=3034423619 3042462419		t=3034423619 3042462419
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	7.1 Example: simple passive/active		8.1 Example: simple passive/active
	An endpoint at 10.1.1.2 signals the availability of a T.38 fax		An endpoint at 10.1.1.2 signals the availability of a T.38 fax
	session at port 54111:		session at port 54111:
	c=IN IP4 10.1.1.2		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:passive		a=direction:passive
	An endpoint at 10.1.1.1 receiving this description responds with the		An endpoint at 10.1.1.1 receiving this description responds with the
	following:		following:
	skipping to change at line 562		skipping to change at line 562
	committed to a source address with a simple modification:		committed to a source address with a simple modification:
	c=IN IP4 10.1.1.1		c=IN IP4 10.1.1.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active IN IP4 10.1.1.1 1892		a=direction:active IN IP4 10.1.1.1 1892
	By adding the source address to the a=direction line, the endpoint		By adding the source address to the a=direction line, the endpoint
	at 10.1.1.1 must now use a source port of 1892 when initiating the		at 10.1.1.1 must now use a source port of 1892 when initiating the
	TCP connection to port 54111 at 10.1.1.2.		TCP connection to port 54111 at 10.1.1.2.
	7.2 Example: simple passive/active with reconnect		8.2 Example: simple passive/active with reconnect
	Continuing the preceding example, consider the scenario where the		Continuing the preceding example, consider the scenario where the
	TCP connection fails and the endpoints wish to reestablish the		TCP connection fails and the endpoints wish to reestablish the
	connection for the session. The endpoint at 10.1.1.2 signals this		connection for the session. The endpoint at 10.1.1.2 signals this
	intent with the following SDP:		intent with the following SDP:
	c=IN IP4 10.1.1.2		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:passive		a=direction:passive
	a=reconnect		a=reconnect
	skipping to change at line 587		skipping to change at line 587
	Because the endpoint at 10.1.1.1 may not yet be aware that the TCP		Because the endpoint at 10.1.1.1 may not yet be aware that the TCP
	connection has failed, this eliminates any ambiguity. If 10.1.1.1		connection has failed, this eliminates any ambiguity. If 10.1.1.1
	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 10.1.1.1
	m=image 9 TCP t38		m=image 9 TCP t38
	a=direction:active IN IP4 10.1.1.1 1893		a=direction:active IN IP4 10.1.1.1 1893
	a=reconnect		a=reconnect
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	Note that the source port is different in this message, since the OS		Note that the source port is different in this message, since the OS
	will have likely chosen a new ephemeral port number for the new		will have likely chosen a new ephemeral port number for the new
	connection.		connection.
	7.3 Example: agnostic both		8.3 Example: agnostic both
	An endpoint at 10.1.1.2 signals the availability of a T.38 fax		An endpoint at 10.1.1.2 signals the availability of a T.38 fax
	session at TCP port 54111, but is also willing to set up the media		session at TCP port 54111, but is also willing to set up the media
	stream by initiating the TCP connection:		stream by initiating the TCP connection:
	c=IN IP4 10.1.1.2		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:both		a=direction:both
	The endpoint at 10.1.1.1 has three choices:		The endpoint at 10.1.1.1 has three choices:
	skipping to change at line 621		skipping to change at line 621
	c=IN IP4 10.1.1.1		c=IN IP4 10.1.1.1
	m=image 54321 TCP t38		m=image 54321 TCP t38
	a=direction:passive		a=direction:passive
	In this case the endpoint at 10.1.1.2 must initiate a		In this case the endpoint at 10.1.1.2 must initiate a
	connection to port 54321 at 10.1.1.1.		connection to port 54321 at 10.1.1.1.
	3) It can respond with a description that specifies		3) It can respond with a description that specifies
	direction:both, which is covered in the next example.		direction:both, which is covered in the next example.
	7.4 Example: redundant both		8.4 Example: redundant both
	An endpoint at 10.1.1.2 uses the same description as the previous		An endpoint at 10.1.1.2 uses the same description as the previous
	example:		example:
	c=IN IP4 10.1.1.2		c=IN IP4 10.1.1.2
	m=image 54111 TCP t38		m=image 54111 TCP t38
	a=direction:both		a=direction:both
	Unlike the previous example, the endpoint at 10.1.1.1 responds with		Unlike the previous example, the endpoint at 10.1.1.1 responds with
	the following description:		the following description:
	skipping to change at line 643		skipping to change at line 643
	c=IN IP4 10.1.1.1		c=IN IP4 10.1.1.1
	m=image 54321 TCP t38		m=image 54321 TCP t38
	a=direction:both		a=direction:both
	This will cause the endpoint at 10.1.1.2 to initiate a connection to		This will cause the endpoint at 10.1.1.2 to initiate a connection to
	port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a		port 54321 at 10.1.1.1, and the endpoint at 10.1.1.1 to initiate a
	connection to port 54111 at 10.1.1.2. Whichever TCP connection		connection to port 54111 at 10.1.1.2. Whichever TCP connection
	succeeds will be used. If both succeed, one of the connections may		succeeds will be used. If both succeed, one of the connections may
	be closed as an optimization, using the rules in section 3.3.		be closed as an optimization, using the rules in section 3.3.
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	In order to minimize the chance that two connections are created,		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		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		section 3.4, which would result in events occurring in the following
	sequence:		sequence:
	1) The endpoint at 10.1.1.2 sends SDP as listed above. The		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,		endpoint MUST enable a listener on port 54111 at this time,
	but is not able to initiate a TCP connection due to the fact		but is not able to initiate a TCP connection due to the fact
	that it does not have sufficient information from the endpoint		that it does not have sufficient information from the endpoint
	at 10.1.1.1.		at 10.1.1.1.
	skipping to change at line 670		skipping to change at line 670
	endpoint at 10.1.1.2 to receive the TCP connection initiation.		endpoint at 10.1.1.2 to receive the TCP connection initiation.
	4) After the short pause, the endpoint at 10.1.1.1 sends the SDP		4) After the short pause, the endpoint at 10.1.1.1 sends the SDP
	response as listed above.		response as listed above.
	The pause in #3 gives the first TCP connection attempt a chance to		The pause in #3 gives the first TCP connection attempt a chance to
	succeed, since withholding the SDP response deprives the endpoint at		succeed, since withholding the SDP response deprives the endpoint at
	10.1.1.2 of the information it needs to attempt its own TCP		10.1.1.2 of the information it needs to attempt its own TCP
	connection.		connection.
	7.5 Example: "Bidirectional" RTP and RTCP		8.5 Example: "Bidirectional" RTP and RTCP
	An endpoint at 10.1.1.2 is behind a NAT and does not know its own		An endpoint at 10.1.1.2 is behind a NAT and does not know its own
	public address.		public address.
	c=IN IP4 10.1.1.2		c=IN IP4 10.1.1.2
	m=audio 9 RTP/AVP 0		m=audio 9 RTP/AVP 0
	a=direction:active		a=direction:active
	A peer with a public IP address responds as follows and waits to		A peer with a public IP address responds as follows and waits to
	receive RTP and RTCP packets from its active peer.		receive RTP and RTCP packets from its active peer.
	skipping to change at line 698		skipping to change at line 698
	port 1542. The passive endpoint receives this RTP packet and stores		port 1542. The passive endpoint receives this RTP packet and stores
	this source address. When the passive endpoint wants to send RTP		this source address. When the passive endpoint wants to send RTP
	media it sends it back to 5.6.7.8 port 1542. The NAT translates this		media it sends it back to 5.6.7.8 port 1542. The NAT translates this
	destination address back to 10.1.1.2 port 9012 and delivers it to		destination address back to 10.1.1.2 port 9012 and delivers it to
	the active endpoint.		the active endpoint.
	Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port		Likewise the endpoint at 10.1.1.2 immediately sends RTCP from port
	9013 to 1.2.3.4:18241. The NAT translates this to 5.6.7.8:1984. The		9013 to 1.2.3.4:18241. The NAT translates this to 5.6.7.8:1984. The
	passive endpoint receives the RTCP packet and stores the source		passive endpoint receives the RTCP packet and stores the source
	Yon INTERNET-DRAFT - Expires December 2002 13		Yon INTERNET-DRAFT - Expires January 2003 13
	address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which		address. The passive endpoint sends its RTCP to 5.6.7.8:1984 which
	is translated back to 10.1.1.2:9013 and delivered to the active		is translated back to 10.1.1.2:9013 and delivered to the active
	endpoint.		endpoint.
	8 Security Considerations		9 Security Considerations
	See [SDP] for security and other considerations specific to the		See [SDP] for security and other considerations specific to the
	Session Description Protocol in general.		Session Description Protocol in general.
	A possible security concern arises if a firewall were to monitor and		A possible security concern arises if a firewall were to monitor and
	act on the source address as described in the note in Section 4.		act on the source address as described in the note in Section 4.
	Firewall implementers must take care to ensure that the SDP came		Firewall implementers must take care to ensure that the SDP came
	from a trusted source before deciding whether to change the network		from a trusted source before deciding whether to change the network
	traffic restrictions currently imposed by the firewall.		traffic restrictions currently imposed by the firewall.
	9 IANA Considerations		10 IANA Considerations
	As recommended by [SDP] Appendix B, the direction and reconnect		As recommended by [SDP] Appendix B, the direction and reconnect
	attributes described in this document should be registered with		attributes described in this document should be registered with
	IANA, as should the "TCP" and "TLS" protocol identifiers.		IANA, as should the "TCP" and "TLS" protocol identifiers.
	Acknowledgements		Acknowledgements
	The author would like to thank Jonathan Rosenberg, Rohan Mahy,		The author would like to thank Jonathan Rosenberg, Rohan Mahy,
	Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie-		Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie-
	Cuninghame for their valuable insights and contributions.		Cuninghame for their valuable insights and contributions.
	Yon INTERNET-DRAFT - Expires December 2002 14		Yon INTERNET-DRAFT - Expires January 2003 14
	Appendix A: Direction Attribute Syntax		Appendix A: Direction Attribute Syntax
	This appendix provides an Augmented BNF [ABNF] grammar for		This appendix provides an Augmented BNF [ABNF] grammar for
	expressing the direction attribute for connection setup. It is		expressing the direction attribute for connection setup. It is
	intended as an extension to the grammar for the Session Description		intended as an extension to the grammar for the Session Description
	Protocol, as defined in [SDP]. Specifically, it describes the		Protocol, as defined in [SDP]. Specifically, it describes the
	syntax for the new "connection-setup" attribute field, which MAY be		syntax for the new "connection-setup" attribute field, which MAY be
	either a session-level or media-level attribute.		either a session-level or media-level attribute.
	connection-setup = "direction" ":" direction-spec		connection-setup = "direction" ":" direction-spec
	direction-spec = "passive" | qualified-direction		direction-spec = "passive" / qualified-direction
	qualified-direction = direction-ident | direction-ident source		qualified-direction = direction-ident / direction-ident source
	direction-ident = "both" | "active" | "passive"		direction-ident = "both" / "active" / "passive"
	source = nettype addrtype unicast-address |		source = nettype addrtype unicast-address /
	nettype addrtype unicast-address port		nettype addrtype unicast-address port
	reconnect-attribute = "reconnect"		reconnect-attribute = "reconnect"
	References		References
	[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax		[ABNF] D. Crocker, P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF," RFC 2234, November 1997		Specifications: ABNF," RFC 2234, November 1997
	[SDP] M. Handley, V. Jacobson, "SDP: Session Description		[SDP] M. Handley, V. Jacobson, "SDP: Session Description
	skipping to change at line 768		skipping to change at line 768
	[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		Author's Address
	David Yon		David Yon
	Dialout.Net, Inc.		Dialout.Net, Inc.
	One Indian Head Plaza		One Indian Head Plaza
	Nashua, NH 03060		Nashua, NH 03060
	Phone: (603) 324-4100		Phone: (603) 324-4100
	EMail: yon@dialout.net		EMail: yon@dialout.net
	Full Copyright Statement		Full Copyright Statement
	Copyright (C) The Internet Society (2001). All Rights Reserved.		Copyright (C) The Internet Society (2001). All Rights Reserved.
	Yon INTERNET-DRAFT - Expires December 2002 15		Yon INTERNET-DRAFT - Expires January 2003 15
	This document and translations of it may be copied and furnished to		This document and translations of it may be copied and furnished to
	others, and derivative works that comment on or otherwise explain it		others, and derivative works that comment on or otherwise explain it
	or assist in its implementation may be prepared, copied, published		or assist in its implementation may be prepared, copied, published
	and distributed, in whole or in part, without restriction of any		and distributed, in whole or in part, without restriction of any
	kind, provided that the above copyright notice and this paragraph		kind, provided that the above copyright notice and this paragraph
	are included on all such copies and derivative works. However, this		are included on all such copies and derivative works. However, this
	document itself may not be modified in any way, such as by removing		document itself may not be modified in any way, such as by removing
	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 807		skipping to change at line 807
	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 - Expires December 2002 16		Yon INTERNET-DRAFT - Expires January 2003 16
End of changes.
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