--- 1/draft-ietf-mmusic-sdp-comedia-05.txt 2006-02-05 00:28:22.000000000 +0100 +++ 2/draft-ietf-mmusic-sdp-comedia-06.txt 2006-02-05 00:28:22.000000000 +0100 @@ -1,679 +1,547 @@ -INTERNET-DRAFT D. Yon -Document: draft-ietf-mmusic-sdp-comedia-05.txt Dialout.Net -Expires September 2003 March 2003 +MMUSIC Working Group D. Yon +Internet-Draft Dialout.Net, Inc +Expires: November 12, 2004 G. Camarillo + Ericsson + May 14, 2004 - Connection-Oriented Media Transport in SDP - + Connection-Oriented Media Transport in the Session Description + Protocol (SDP) + draft-ietf-mmusic-sdp-comedia-06.txt Status of this Memo - This document is an Internet-Draft and is in full conformance with - all provisions of Section 10 of RFC2026. + By submitting this Internet-Draft, I certify that any applicable + 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. Internet-Drafts are working documents of the Internet Engineering - Task Force (IETF), its areas, and its working groups. Note that - other groups may also distribute working documents as Internet- - Drafts. + Task Force (IETF), its areas, and its working groups. Note that other + groups may also distribute working documents as Internet-Drafts. - Internet-Drafts are draft documents valid for a maximum of six - months and may be updated, replaced, or obsoleted by other documents - at any time. It is inappropriate to use Internet-Drafts as reference + Internet-Drafts are draft documents valid for a maximum of six months + and may be updated, replaced, or obsoleted by other documents at any + time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - The list of current Internet-Drafts can be accessed at: - http://www.ietf.org/ietf/1id-abstracts.txt + The list of current Internet-Drafts can be accessed at http:// + www.ietf.org/ietf/1id-abstracts.txt. - The list of Internet-Draft Shadow Directories can be accessed at: + The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - Copyright (C) The Internet Society (2002). All Rights Reserved. + This Internet-Draft will expire on November 12, 2004. + +Copyright Notice + + Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes how to express media transport over connection-oriented protocols using the Session Description Protocol - (SDP). It defines two new protocol identifiers: TCP and TLS. It - also defines the syntax and semantics for an SDP "direction" - attribute that describes the connection setup procedure. + (SDP). It defines two new protocol identifiers: TCP and TCP/TLS. It + also defines the SDP setup attribute, which describes the connection + 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 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 - multimedia session with sufficient information to discover and - participate in that session. Session descriptions may be sent using - arbitrary existing application protocols for transport (e.g., SAP, - SIP, RTSP, email, HTTP, etc.). + multimedia session with sufficient information to participate in that + session. Session descriptions may be sent using arbitrary existing + application protocols for transport (e.g., SAP [9], SIP [10], RTSP + [7], email, HTTP [8], etc.). - [SDP] describes two protocol identifiers: RTP/AVP and UDP, both of - which are unreliable, connectionless protocols, an appropriate - choice for multimedia streams. There are, however, applications for - which the connection-oriented transports such as TCP are more - appropriate, but [SDP] provides no way to describe a session that - uses protocols other than RTP or UDP. + SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of + which represent unreliable connectionless protocols. While these + transports are appropriate choices for multimedia streams, there are + applications for which connection-oriented transports such as TCP are + more appropriate. We define two new protocol identifiers: TCP and + TCP/TLS. Both represent connection-oriented reliable transports. Connection-oriented protocols introduce a new factor when describing - 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 - connection setup procedure. This memo defines two new protocol - identifiers, TCP and TLS, along with the syntax and semantics of the - a=direction and a=reconnect attributes. + a session: how should end points perform the connection setup + procedure. We define two new attributes to describe connection setup: + setup and reconnect. -2 Terminology +2. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", - "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", - and "OPTIONAL" are to be interpreted as described in RFC 2119 [7] - and indicate requirement levels for compliant implementations. - -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: - - The 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: + "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT + RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as + described in BCP 14, RFC 2119 [2] and indicate requirement levels for + compliant implementations. - c=IN IP4 10.1.1.1 - m=image 9 TCP t38 - a=direction:active IN IP4 +3. Protocol Identifiers -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 - 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 - the m= line of the other endpoint. + media-field = "m=" media space port ["/" integer] + space proto 1*(space fmt) CRLF -Yon INTERNET-DRAFT - Expires September 2003 3 - 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. + We define two new values for the proto field: TCP and TCP/TLS. - Endpoints may choose to specify direction:both for one or more of - the following reasons: +3.1 TCP - 1) The endpoint has no preference as to whether it accepts or - initiates the connection, and therefore is offering the remote - endpoint a choice of connection setup procedures. + The TCP protocol identifier is similar to the UDP protocol identifier + in that it only describes the transport protocol, and not the + 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 - the media, but it is not known whether firewall issues will - 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. + Media lines with the TCP protocol identifier are carried using TCP + [1]. - If one endpoint specifies either direction:active or - 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. +3.2 TCP/TLS - If both endpoints specify direction:both then each endpoint MUST - initiate a connection to the port number specified on the m= line of - the opposite endpoint. There is one exception to this requirement: - 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. + The TCP/TLS protocol identifier specifies that the session will use + the Transport Layer Security (TLS) protocol [3] on top on a TCP [1] + connection. - If only one connection succeeds, then that connection will be used - to carry the media. Once it has transmitted data on this - connection, the initiating endpoint MUST NOT perform another - connection attempt to the accepting endpoint. This allows the - accepting endpoint to release or recycle the listening port for - another session once it has received data from the initiating - endpoint. + An m= line that contain the TCP/TLS protocol identifier MUST further + qualify the protocol using a fmt identifier. - 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, - it MUST use that connection exclusively for transmission. + setup-attr = "a=setup:" role + role = "active" / "passive" / "actpass" - c) Once an endpoint has transmitted AND received data, if one of the - connections is determined to be idle, the endpoint SHOULD close - the idle connection. + Active: The endpoint will initiate an outgoing connection. + Passive: The endpoint will accept an incoming 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 - two connections will be created when only one is needed. While - rules in the previous section accommodate the closing of an idle - connection, they do not prevent a race condition where the endpoints - simultaneously start sending data on opposite connections thereby - causing two connections to be used where one would have sufficed. - While it is not possible to entirely eliminate this race condition, - 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: + The offer/answer model, defined in RFC 3264 [5], provides endpoints + with a means to obtain shared view of a session. Some session + parameters are negotiated (e.g., codecs to use), while others are + simply communicated from one endpoint to the other (e.g., IP + addresses). The value of the setup attribute falls into the first + category. That is, both endpoints negotiate its value using the + offer/answer model. - a) There comes a point during the exchange of SDP where one endpoint - is prepared to send the final message that will complete the - negotiation and allow the session to begin. For the purposes of - this discussion, the endpoint that will send this final message - will be called the Initiator, and the endpoint that will receive - this message will be called the Acceptor. + The negotiation of the value of the setup attribute takes places as + follows. The offerer states which role or roles is willing to perform + and the answerer, taking the offerer's willingness into + consideration, chooses which roles both endpoints will actually + perform during connection establishment. The following are the values + that the setup attribute can take in an offer/answer exchange: - b) The Initiator, upon receiving sufficient information to initiate a - connection, MUST attempt to connect to the Acceptor as soon as - possible. + Offer Answer + _______________ + active passive + passive active + actpass active / passive / actpass - c) In order to lower the likelihood that the Acceptor will also - attempt to initiate a connection, the Initiator SHOULD incorporate - a short delay between initiating the connection and sending the - final SDP to the Acceptor. + The value active indicates that the endpoint SHOULD 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 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 - unacceptable session setup delay should the connection to the - Acceptor not succeed. + The value passive indicates that the endpoint SHOULD be ready to + accept a connection on the port number specified in the m= line. -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 - the intent is for media to flow in both directions, both endpoints - must specify SDP that describes where to deliver the media and what - media type(s) to use. For example, if only Endpoint A presents SDP - then media can only flow towards Endpoint A, as Endpoint B has not - specified where and how to send media to it. + Endpoints typically use the actpass value for the following reasons: + 1. The offerer has no preference as to whether it accepts or + initiates the connection and, so, is letting the answerer choose. + 2. The endpoints intend to use a single connection to transport the + media, but it is not known whether NAT (Network Address + 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, - 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. +4.2 Multiple-Connection Avoidance when Using Actpass - It is, however, perfectly acceptable for an endpoint to transmit - data on the same connection it is using to receive data, so long as + When an offer/answer exchange results in actpass, each endpoint + 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 - the other endpoint has advertised its willingness to accept data. - Likewise, it is perfectly acceptable for an endpoint to receive data - on the same connection it is using to transmit data to the - corresponding remote endpoint. In other words, for a bi-directional - application-level session, a connection may be used to send data in - both directions (contingent to rules outlined in Section 2.3) as - long as one side of the connection is attached to either of the - advertised SDP transport addresses. + First of all, if the endpoints follow the recommendation of setting + the port number in their m= line to the source port number which they + will use to establish the connection towards the remote endpoint, the + transport layer should take care of simultaneous opens (at least if + TCP is the transport protocol). If, for some reason, any of the + endpoints does not follow this recommendation, both endpoints should + follow the rules below. -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 - media. This indicates that the endpoint would like to treat this - media as a type of connection-oriented media. (The endpoint may do - this to facilitate NAT traversal for example.) Note that for - 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. + In case two connections are established, if an endpoint receives data + (i.e., media) over one of the connections before having sent any data + on any of the connections, the endpoint SHOULD terminate the + connection that has not carried any data. - Endpoints that specify direction:passive MUST NOT send any media, - any packets whatsoever (including control packets such as RTCP), - from their passive ports until they receive a packet on these ports - and record the source address and port of the sender. The passive - endpoint then assumes that the first packet received corresponds to - its active peer. From this point onward, passive endpoints MUST - 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 port on which they expect to receive it (typically the RTP port - number plus 1). + When two connections are established and both endpoints start sending + data before receiving anything from the other endpoint, it may happen + that each of the endpoints choose a different connection to send + data. If the answerer receives data over a connection after having + sent data on the other connection, it SHOULD continue sending data on + the other connection until an application-layer data boundary. At + that point, the answerer SHOULD terminate this connection and start + using the connection on which the offerer was sending data. - Endpoints that specify direction:active MUST be prepared to receive - 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 - 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 - 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. + Note that different applications may define application-layer + boundaries in different ways. A typical suitable point for the + answerer to change connections is the end of an application-layer + message and the beginning of the next one. -5 Reconnect Attribute +5. The Reconnect Attribute - 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 preceding description of the setup attribute has been in the + context of using SDP to initiate a session. Still, SDP may be exchanged between endpoints at various stages of a session to accomplish tasks such as terminating a session, redirecting media to - a new endpoint, renegotiating the media parameters for a session, - etc. After the initial session has been established, it may be - ambiguous as to whether subsequent SDP exchange represents a - confirmation that the endpoint is to continue using the current - media connection unchanged, or is a request to make a new media - -Yon INTERNET-DRAFT - Expires September 2003 6 - connection. The reconnect attribute is used to disambiguate these - two scenarios, and the syntax is as follows: - - a=reconnect - - SDP containing a=reconnect signals that the specified session does - NOT refer to an existing connection between the two endpoints. If - the endpoints agree to continue the session, the endpoints MUST - close the existing connection for the currently negotiated session, - and MUST create a new connection according to the a=direction - attribute in the SDP. If an endpoint receives SDP that contains - a=reconnect, the endpoint's response MUST also contain a=reconnect. - Endpoints MUST NOT include a=reconnect in SDP that negotiates the - start of a session. - - See section 6, "Connection and Listener Lifetime Considerations" for - more information on scenarios that are relevant to the a=reconnect - attribute. - -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 + a new endpoint, or renegotiating the media parameters for a session. + After the initial session has been established, it may be ambiguous + as to whether subsequent SDP exchange represents a confirmation that + the endpoint is to continue using the current media connection + unchanged, or is a request to make a new media connection. The + reconnect attribute, which is charset-independent and can be a + session-level or a media-level attribute, is used to disambiguate + these two scenarios. The following is the ABNF of the reconnect + attribute: - b) The endpoint has received the expected number of incoming - connections on that address and port, or + reconnect-attr = "a=reconnect" - c) Subsequent exchanges have superceded the SDP that originally - advertised the availability of the address and port. + On reception of an m= line with a reconnect attribute, the endpoints + 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 - on a specific address and port, it SHOULD terminate the listener. - The endpoint is then free to re-use the address and port for - subsequent session advertisements. + Either the offerer or the answerer can include a reconnect attribute + in an m= line. In any event, if the offer contained this attribute, + the answer MUST contain it as well. -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 so, even if it does not intend to immediately begin sending media to the remote endpoint. This allows media to flow from the remote - endpoint. - - An endpoint MUST NOT close the connection until the session has - expired, been explicitly terminated, or the media stream is - redirected to a different address or port. + endpoint. An endpoint SHOULD NOT close the connection until the + session has 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 - MAY attempt to re-establish the connection. The decision to do so - is application and/or context dependant. If the endpoint opts to - 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. + MAY attempt to re-establish the connection. The decision to do so is + application and context dependant. -6.3 Session Renegotiation and Connection Lifetime +6.1 Session Renegotiation There are scenarios where SDP is sent by an endpoint in order to renegotiate an existing session. These include muting/unmuting a session, renegotiating the attributes of the media used by the session, or extending the length of a session about to expire. Connection-oriented media introduces some ambiguities into session renegotiation as to when the direction attribute must be obeyed and when it is ignored. The scenario of extending the duration of an existing session is a good example: in order to extend an existing session, endpoints will typically resend the original SDP with updated time information. In connectionless media the result is no change to the existing media streams. The problem with connection oriented media is that the - original SDP will contain a direction attribute which can be - construed as a request to create a new connection, as opposed to a - request to maintain steady state. To avoid this ambiguity, the - 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. + original SDP will contain a setup attribute which can be considered + as a request to create a new connection, as opposed to a request to + maintain steady state. The following rule help avoid this ambiguity: - This disambiguates most session renegotiation scenarios, with the - exception of muting. Muting a media stream is accomplished by - sending the original session SDP but with an "a=inactive" or - "a=sendonly/recvonly" attribute. This is still valid for connection - oriented media, with the additional caveat that the endpoints MUST - NOT close the connection described by that SDP. + If the transport section (the c= and m= lines) of an SDP + description describes an existing connection between two endpoints + and the m= line does not contain a reconnect attribute, the + endpoints SHOULD use that connection to carry the media described + in the remainder of the message. The endpoints SHOULD NOT attempt + 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 - usage of the direction attribute combined with TCP-based media - descriptions. For the purpose of brevity, the main portion of the - session description is omitted in the examples and is assumed to be - the following: + What follows are a number of examples that show the most common usage + of the setup attribute combined with TCP-based media descriptions. + For the purpose of brevity, the main portion of the session + description is omitted in the examples and is assumed to be the + following: -Yon INTERNET-DRAFT - Expires September 2003 8 v=0 o=me 2890844526 2890842807 IN IP4 10.1.1.2 s=Call me using TCP 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: - c=IN IP4 10.1.1.2 + c=IN IP4 192.0.2.2 m=image 54111 TCP t38 - a=direction:passive + a=setup:passive - An endpoint at 10.1.1.1 receiving this description responds with the - following: + An answerer at 192.0.2.1 receiving this offer responds with the + following answer: - c=IN IP4 10.1.1.1 + c=IN IP4 192.0.2.1 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 - 54111 at 10.1.1.2. + The endpoint at 192.0.2.1 then initiates the TCP connection to port + 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 - TCP connection fails and the endpoints wish to reestablish the - connection for the session. The endpoint at 10.1.1.2 signals this - intent with the following SDP: + Continuing the preceding example, consider the scenario where the TCP + connection fails and the endpoints wish to reestablish the connection + for the session. The endpoint at 192.0.2.2 signals this intent with + the following SDP: - c=IN IP4 10.1.1.2 + c=IN IP4 192.0.2.2 m=image 54111 TCP t38 - a=direction:passive + a=setup:passive 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 transport, rather than continuing with the original connection. - Because the endpoint at 10.1.1.1 may not yet be aware that the TCP - connection has failed, this eliminates any ambiguity. If 10.1.1.1 + Because the endpoint at 192.0.2.1 may not yet be aware that the TCP + connection has failed, this eliminates any ambiguity. If 192.0.2.1 agrees to continue the session using a new connection, it responds with: - c=IN IP4 10.1.1.1 + c=IN IP4 192.0.2.1 m=image 9 TCP t38 - a=direction:active IN IP4 + a=setup:active IN IP4 a=reconnect -7.3 Example: agnostic both - - 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 +7.3 Actpass - An endpoint at 10.1.1.2 uses the same description as the previous - example: + An offerer at 192.0.2.2 signals its availability for a T.38 fax + session at TCP port 54111. Additionally, this offerer is also willing + to set up the media stream by initiating the TCP connection: - c=IN IP4 10.1.1.2 + c=IN IP4 192.0.2.2 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 following description: + The endpoint at 192.0.2.1 responds with the following description: - c=IN IP4 10.1.1.1 + c=IN IP4 192.0.2.1 m=image 54321 TCP t38 - a=direction:both - - 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. + a=setup:actpass - 3) In order to minimize the chance of a duplicate connection, the - endpoint at 10.1.1.1 pauses for a short time to allow the - endpoint at 10.1.1.2 to receive the TCP connection initiation. + This will cause the offerer (at 192.0.2.2) to initiate a connection + to port 54321 at 192.0.2.1 and the answerer (at 192.0.2.1) to + 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 - response as listed above. +8. Security Considerations - The pause in #3 gives the first TCP connection attempt a chance to - succeed, since withholding the SDP response deprives the endpoint at - 10.1.1.2 of the information it needs to attempt its own TCP - connection. + See RFC 2327 [4] for security and other considerations specific to + the Session Description Protocol in general. -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 - public address. + This document touches upon NAT traversal. Implementers should be + 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 - m=audio 9 RTP/AVP 0 - a=direction:active + When an endpoint receives a session description with a private IP + address that belongs to a different address space, in most of the + 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 - receive RTP and RTCP packets from its active peer. +9. IANA Considerations - c=IN IP4 1.2.3.4 - m=audio 18240 RTP/AVP 0 - a=direction:passive + This document defines two session and media level SDP attributes: + setup and reconnect. Their formats are defined in Section 4 and + 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 - 1.2.3.4 port 18240. A NAT translates the source address to 5.6.7.8 - port 1542. The passive endpoint receives this RTP packet and stores - 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 - destination address back to 10.1.1.2 port 9012 and delivers it to - the active endpoint. + This document defines two proto values: TCP and TCP/TLS. Their + formats are defined in Section 3.1 and Section 3.2 respectively. + These two proto values should be registered by the IANA on http:// + www.iana.org/assignments/sdp-parameters under "proto". - 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 - 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. +10. Acknowledgements -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 - Session Description Protocol in general. +11. References -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 - attributes described in this document should be registered with - IANA, as should the "TCP" and "TLS" protocol identifiers. + [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement + Levels", BCP 14, RFC 2119, March 1997. -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, - Anders Kristensen, Jeorg Ott, Paul Kyzivat, and Robert Fairlie- - Cuninghame for their valuable insights and contributions. + [4] Handley, M. and V. Jacobson, "SDP: Session Description + Protocol", RFC 2327, April 1998. -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 - 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. +11.2 Informational References - 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 - Specifications: ABNF," RFC 2234, November 1997 +Authors' Addresses - [SDP] M. Handley, V. Jacobson, "SDP: Session Description - Protocol," RFC 2327, April 1998 + David Yon + Dialout.Net, Inc + One Indian Head Plaza + Nashua, NH 03060 + USA - [T38] International Telecommunication Union, "Procedures for - Real-Time Group 3 Facsimile Communications over IP - Networks," Recommendation T.38, June 1998 + EMail: yon@dialout.net - [TLS] T. Dierks, C. Allen, "The TLS Protocol," RFC 2246, - January 1999 + Gonzalo Camarillo + Ericsson + Hirsalantie 11 + Jorvas 02420 + Finland - [UTF-8] F. Yergeau, "UTF-8, a transformation format of Unicode - and ISO 10646," RFC 2044, October 1996 + EMail: Gonzalo.Camarillo@ericsson.com -Author's Address +Intellectual Property Statement - David Yon - Dialout.Net, Inc. - One Indian Head Plaza - Nashua, NH 03060 + The IETF takes no position regarding the validity or scope of any + Intellectual Property Rights or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; nor does it represent that it has + made any independent effort to identify any such rights. 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