SIMPLE Working Group B. Campbell Internet-Draft J. Rosenberg Expires:
December 29, 2003April 22, 2004 R. Sparks dynamicsoft P. Kyzivat Cisco Systems June 30,October 23, 2003 InstantThe Message Sessions in SIMPLE draft-ietf-simple-message-sessions-01Session Relay Protocol draft-ietf-simple-message-sessions-02 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. 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 Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 29, 2003.April 22, 2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document describes the Message Session Relay Protocol (MSRP), a mechanism for transmitting a series of Instant Messages within a session. MSRP sessions are managed using the SDPSession Description Protocol (SDP) offer/answer model carried by a signaling protocol such as SIP.the Session Initiation Protocol (SIP). MSRP supports end-to-end Instant Message Sessions, as well as sessions traversing one or two relays. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Motivation for Session-mode Messaging . . . . . . . . . . . 4 3. Scope of this Document . . . . . . . . . . . . . . . . . . . 5 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . 56 5. Architectural Considerations . . . . . . . . . . . . . . . . 87 5.1 Use of Relays . . . . . . . . . . . . . . . . . . . . . . . 8 5.2 Transferring Large Content . . . . . . . . . . . . . . . . . 98 5.3 Connection Sharing . . . . . . . . . . . . . . . . . . . . . 9 6. SDP Offer-Answer Exchanges for MSRP Sessions . . . . . . . . 10 6.1 Use of the SDP M-line . . . . . . . . . . . . . . . . . . . 10 6.2 The Direction Attribute . . . . . . . . . . . . . . . . . . 11 6.3 The Accept Types Attribute . . . . . . . . . . . . . . . . . 12 6.4 MIME Wrappers . . . . . . . . . . . . . . . . . . . . . . . 13 6.46.5 URL Negotiations . . . . . . . . . . . . . . . . . . . . . . 13 6.56.6 Example SDP Exchange . . . . . . . . . . . . . . . . . . . . 14 7. The Message Session Relay Protocol . . . . . . . . . . . . . 15 7.1 MSRP URLs . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1.1 MSRP URL Comparison . . . . . . . . . . . . . . . . . . . . 16 7.1.2 Resolving MSRP Host Device . . . . . . . . . . . . . . . . . 16 7.1.3 The msrps URL Scheme . . . . . . . . . . . . . . . . . . . . 17 7.2 MSRP messages . . . . . . . . . . . . . . . . . . . . . . . 17 7.3 MSRP Transactions . . . . . . . . . . . . . . . . . . . . . 1819 7.4 MSRP Sessions . . . . . . . . . . . . . . . . . . . . . . . 19 7.4.1 Initiating an MSRP session . . . . . . . . . . . . . . . . . 19 7.4.2 Handling VISIT requests . . . . . . . . . . . . . . . . . . 23 7.4.3 Sending Instant Messages on a Session . . . . . . . . . . . 23 7.4.4 Ending a Session . . . . . . . . . . . . . . . . . . . . . . 2425 7.4.5 Session Inactivity Timer . . . . . . . . . . . . . . . . . . 2426 7.4.6 Managing Session State and Connections . . . . . . . . . . . 2527 7.5 MSRP Relays . . . . . . . . . . . . . . . . . . . . . . . . 2627 7.5.1 Establishing Session State at a Relay . . . . . . . . . . . 2628 7.5.2 Removing Session State from a relay . . . . . . . . . . . . 2829 7.5.3 Sending IMs across an MSRP relay . . . . . . . . . . . . . . 2830 7.5.4 Relay Pairs . . . . . . . . . . . . . . . . . . . . . . . . 2830 7.5.5 Relay Shutdown . . . . . . . . . . . . . . . . . . . . . . . 31 7.6 Digest Authentication . . . . . . . . . . . . . . . . . . . 3031 7.6.1 The SHA1 Algorithm . . . . . . . . . . . . . . . . . . . . . 3133 7.7 Method Descriptions . . . . . . . . . . . . . . . . . . . . 3133 7.7.1 BIND . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3134 7.7.2 SEND . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234 7.7.3 VISIT . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234 7.8 Response Code Descriptions . . . . . . . . . . . . . . . . . 3234 7.8.1 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3235 7.8.2 400 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.3 401 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.4 403 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.5 415 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.6 426 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.7 481 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.8 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.8.9 506 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3335 7.9 Header Field Descriptions . . . . . . . . . . . . . . . . . 3336 7.9.1 TR-ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 3436 7.9.2 Exp . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3436 7.9.3 CAuth . . . . . . . . . . . . . . . . . . . . . . . . . . . 3436 7.9.4 SChal . . . . . . . . . . . . . . . . . . . . . . . . . . . 3537 7.9.5 Content-Type . . . . . . . . . . . . . . . . . . . . . . . . 3637 7.9.6 S-URL . . . . . . . . . . . . . . . . . . . . . . . . . . . 3637 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 3637 8.1 No Relay . . . . . . . . . . . . . . . . . . . . . . . . . . 3638 8.2 Single Relay . . . . . . . . . . . . . . . . . . . . . . . . 3940 8.3 Two Relays . . . . . . . . . . . . . . . . . . . . . . . . . 4243 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 46 9.1 MSRP Port . . . . . . . . . . . . . . . . . . . . . . . . . 46 9.2 MSRP URL Schemes . . . . . . . . . . . . . . . . . . . . . . 4647 9.2.1 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 4647 9.2.2 Character Encoding . . . . . . . . . . . . . . . . . . . . . 4647 9.2.3 Intended Usage . . . . . . . . . . . . . . . . . . . . . . . 47 9.2.4 Protocols . . . . . . . . . . . . . . . . . . . . . . . . . 47 9.2.5 Security Considerations . . . . . . . . . . . . . . . . . . 47 9.2.6 Relevant Publications . . . . . . . . . . . . . . . . . . . 47 9.3 SDP Parameters . . . . . . . . . . . . . . . . . . . . . . . 47 9.3.1 Direction . . . . . . . . . . . . . . . . . . . . . . . . . 47 9.3.2 Accept Types . . . . . . . . . . . . . . . . . . . . . . . . 48 9.3.3 Wrapped Types . . . . . . . . . . . . . . . . . . . . . . . 4748 10. Security Considerations . . . . . . . . . . . . . . . . . . 48 10.1 TLS and the MSRPS Scheme . . . . . . . . . . . . . . . . . . 48 10.2 Sensitivity of the Session URL . . . . . . . . . . . . . . . 49 10.3 End to End Protection of IMs . . . . . . . . . . . . . . . . 4950 10.4 CPIM compatibility . . . . . . . . . . . . . . . . . . . . . 50 10.5 PKI Considerations . . . . . . . . . . . . . . . . . . . . . 50 11. Changes from Previous Draft Versions . . . . . . . . . . . . 5051 11.1 draft-ietf-simple-message-sessions-01draft-ietf-simple-message-sessions-02 . . . . . . . . . . . 51 11.2 draft-ietf-simple-message-sessions-00draft-ietf-simple-message-sessions-01 . . . . . . . . . . . 51 11.3 draft-ietf-simple-message-sessions-00 . . . . . . . . . . . 52 11.4 draft-campbell-simple-im-sessions-01 . . . . . . . . . . . . 52 12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5253 Normative References . . . . . . . . . . . . . . . . . . . . 53 Informational References . . . . . . . . . . . . . . . . . . 5354 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 5455 Intellectual Property and Copyright Statements . . . . . . . 56 1. Introduction The MESSAGE  extension to SIP  allows SIP to be used to transmit instant messages. Instant messages sent using the MESSAGE method are normally independent of each other. This approach is often called page-mode messaging, since it follows a model similar to that used by many two-way pager devices. page-modePage-mode messaging makes sense for instant message exchanges where a small number of messages occur. Endpoints may treat page-mode messages as if they took place in an imaginative session, but there is no formal relationship between one message and another. There are also applications in which it is useful for instant messages to be formally associated togetherin some way.a session. For example, a user may wish to join a text conference, participate in the conference for some period of time, then leave the conference. This usage is analogous to regular media sessions that are typically initiated, managed, and terminated using SIP. We commonly refer to this model as session-mode messaging. One of the primary purposes of SIP and SDP (Section 6) is the management of media sessions. Session-mode messaging can be thought of as a media session like any other. This document describes the motivations for session-mode messaging, the Message Session Relay Protocol, and the use of the SDP offer/answer mechanism for managing MSRP session. 2. Motivation for Session-mode Messaging Message sessions offer several advantages over page-mode messages. For message exchanges that include more than a small number of message transactions, message sessions offer a way to remove messaging load from intervening SIP proxies. For example, a minimal session setup and tear-down requires one INVITE/ACK transaction, and one BYE transaction, for a total of 5 SIP messages. Normal SIP request routing allows for all but the initial INVITE transaction to bypass any intervening proxies that do not specifically request to be in the path for future requests. Session-mode messages never cross the SIP proxies themselves, unless proxies also act as message relays.themselves. Each page-mode message involves a complete SIP transaction, that is, a request and a response. Any page-mode message exchange that involves more than 2 MESSAGE requests will generate more SIP requests than a minimal session initiation sequence. Since MESSAGE is normally used outside of a SIP dialog, these requests will typically traverse the entire proxy network between the endpoints. Due to network congestion concerns, the MESSAGE method has significant limitations in message size, a prohibition against overlapping requests, etc. Much of this has been required because of perceived limitations in the congestion-avoidance features of SIP itself. Work is in progress to mitigate these concerns. However, session-mode messages are always sent over reliable, congestion-safe transports. Therefore, there are no restrictions on message sizes. There is no requirement to wait for acknowledgement,acknowledgement before sending another message, so that message transactions can be overlapped. Message sessions allow greater efficiency for secure message exchanges. The SIP MESSAGE request inherits the S/MIME features of SIP, allowing a message to be signed and/or encrypted. However, this approach requires public key operations for each message. With session-mode messaging, a session key can be established at the time of session initiation. This key can be used to protect each message that is part of the session. This requires only symmetric key operations for each subsequent IM, and no additional certificate exchanges are required after the initial exchange. The establishment of the session key can be done using standard techniques that apply to voice and video, in addition to instant messaging. Finally, SIP devices can treat message sessions like any other media sessions. Any SIP feature that can be applied to other sorts of media sessions can equally apply to message sessions. For example, conferencing , third party call control , call transfer , QoS integration , and privacy  can all be applied to message sessions. Messaging sessions can also reduce the overhead in each individual message. In page-mode, each message needs to include all of the SIP headers that are mandated by RFC 3261 . However, many of these headers are not needed once a context is established for exchanging messages. As a result, messaging session mechanisms can be designed with significantly less overhead. 3. Scope of this Document This document describes the use of MSRP between endpoints, or via one or two relays, where endpoints have advance knowledge of the relays. It does not provide a mechanism for endpoints to determine whether a relay is needed, or for endpoints to discover the presence of relays. This document describes the use of MSRP over TCP. MSRP may be used over other congestion-controlled protocols such as SCTP. However, the specific bindings for other such protocols are outside the scope of this document. 4. Protocol Overview The Message Session Relay Protocol (MSRP) provides a mechanism for transporting session-mode messages between endpoints. MSRP also contains primitives to allow the use of one or two relay devices. MSRP uses connection oriented, reliable network transport protocols only. It is intrinsically NAT and firewall friendly, as it allows participants to positively associate message sessions with specific connections, and does not depend upon connection source address, which may be obscured by NATs. MSRP uses the following primitives: SEND: Used to send message content from one endpoint to another. VISIT: Used by an endpoint to establish a session association to the opposite endpoint, or to a relay that was selected by the opposite endpoint. BIND: Used by an endpoint to establish a session at a relay, and allow the opposite endpoint to visit that relay. The simplest use case for MSRP is a session that goes directly between endpoints, with no intermediaries involved. Assume A is an endpoint that wishes to establish a message session, and B is the endpoint invited by A. A invites B to participate in a message session by sending a URL that represents the session. This URL is temporary, and must not duplicate the URL used for any other active sessions. B "visits" A by connecting to A and sending a VISIT request containing the URL that A provided. This associates the connection from B with the session. B then responds to the invitation, informing A that B has accepted the session. A and B may now exchange messages using SEND requests on the connection. When either party wishes to end the session, it informs the peer party with a SIP BYE request. A terminates the session by invalidating associated state, and dropping the connection. The end to end case looks something like the following. (Note that the example shows a logical flow only; syntax will come later in this document.) A->B (SDP): offer (msrp://A/123) B->A (MSRP): VISIT (msrp://A/123) A->B (MSRP): 200 OK B->A (SDP): answer(msrp://A/123) A->B (MSRP): SEND B->A (MSRP): 200 OK B->A (MSRP): SEND A->B (MSRP): 200 OK The session state has an associated inactivity timer. This timer is initialized when a successful VISIT request occurs, and is reset each time either endpoint sends a SEND request. If this timer expires without being reset, the hosting device invalidates the session state and terminates all associated connections. Endpoints that are otherwise idle may keep a session active by periodically sending SEND requests with no content. A slightly more complicated case involves a single relay, known about in advance by one of the parties. The endpoint that has the preexisting relationship with the relay uses the BIND method to establish session state in the relay. The relay returns a temporary URL, that identifies the session. For endpoints A and B, and relay R, the flow would look like the following: A->R: MSRP: BIND(msrp://r) R->A: MSRP: 200 OK (msrp://r/4uye) A->B (SDP): offer (msrp://r/4uye) B->R (MSRP): VISIT (msrp://r/4uye) R->B (MSRP): 200 OK B->A (SDP): answer(msrp://r/4uye) A->R (MSRP): SEND R->B (MSRP): SEND B->R (MSRP): 200 OK R->A (MSRP): 200 OK B->R (MSRP): SEND R->A (MSRP): SEND A->R (MSRP): 200 OK R->B (MSRP): 200 OK The BIND request contains an expiration time. If a successful VISIT request does not occur prior to the expiration, the relay will destroy the session. Additionally, when tearing down a session, the host endpoint invalidates the session state by issuing a BIND request with an expiration value of zero. 5. Architectural Considerations There are a number of considerations that, if handled in a reasonable fashion, will allow more effective use of the protocols described in this document. 5.1 Use of Relays The primary motivation for relay support in MSRP is to deal with situations where, due to issues of network topologies, neither endpoint is able to receive an inbound TCP connection from the other. For example, both endpoints may be behind separate firewalls that only allow outbound connections. Relays may also be needed for policy enforcement. For example, parts of the financial industry require the logging of all communication. However, the use of such relays has a significant impact on the scalability of MSRP. Each relay will require two TCP connections for each session in use, as well as memory for local session state storage. Most general purpose platforms on which one might implement MSRP relays will have relatively low limits on the number of simultaneous TCP connections they can handle. Therefore relays SHOULD NOT be used indescrimantly.indiscriminately. In the absence of strong reasons to use relays, MSRP endpoints SHOULD be configured to set up point-to-point sessions. MSRP supports the use of two relays, where each endpoint has a relay acting on its behalf. However, most of the network topology issues mentioned above can work with a single relay, if that relay is reachable by both endpoints. Dual relays are only needed for cases of very strict firewall policy, such as when only specific hosts are allowed to connect to the outside world; or situations requiring strict policy enforcement at both endpoint domains. If a given usage scenario can be solved with a single relay, then a second relay SHOULD NOT be used. In spite of these recommendations, relays serve a real purpose in that thethey increase the likelihood of two arbitrary endpoints being able to talk to one another. Therefore if a provider deploys MSRP endpoints in a network configuration that prevents them from receiving TCP connections from arbitrary peers, and does not wish to explicitly prevent MSRP communication with the outside world, then the provider SHOULD provide its endpoints with the use of an MSRP relay that is reachable from arbitrary peers. 5.2 Transferring Large Content MSRP endpoints may attempt to send very long messages onin a session. For example, most commercial instant messaging systems have a file transfer feature. Since MSRP does not impose message size limits, there is nothing to prevent endpoints from transferring files over it. An analysis of whether it makes sense to do this, rather than sending such content over FTP, HTTP, or some other such protocol, is beyond the scope of this document. However, implementers should be aware of the impact of sending very large messages over MSRP. The primary impact is, since MSRP is sent over TCP, is that any additional messages that the sender wishes to send will be blocked until the large transfer is complete. This includes responses to messages sent by the peer. Therefore, any SEND transactions initiated by the peer are likely to time out, even though they are received without problems. Further, there is no way to abort the sending of a very large message before it is complete. For the sake of efficiency, the framing mechanism in MSRP is very simple. There is no clean way to recover framing if the complete message is not sent. These issues can be mitigated greatly if the endpoint simply establishes a separate session for the transfer. This allows the transfer to be sent without interfering with any instant messages being sent on other sessions. Further, the endpoint can abort the transfer by simply tearing down the transfer session. Therefore, if a peer wishes to send very large content, it SHOULD establish a dedicated session for that purpose. 5.3 Connection Sharing The SIMPLE working group spent quiteOpen Issue: Do we need a bit of effort in themechanism to communicate the purpose of the session? It has been mentioned that the peer may not realize the purpose of the session, and start using it for normal messaging. Also, there has been discussion that we need a stronger mechanism to avoid transaction timeouts caused by long requests. 5.3 Connection Sharing The SIMPLE working group spent quite a bit of effort in the consideration of shared TCP connections. Connection sharing would offer value whenever a large number of message sessions cross the same two adjacent devices. This situation is likely to occur in the two relay model. It may also occur in the point-to-point model if the endpoints are multiuser devices, as is likely with web-hosted messaging services. Unfortunately, such connection sharing in TCP created significant problems. The biggest problem is it introduced a head-of-line blocking problem that spanned sessions. For example, if two different pairs of users had sessions that crossed the same shared connection, a large message sent on one session would block transfer of messages on the other session. The working group considered this an unacceptable property of shared connections. One possible solution was to put limits on message size, and possibly add mechanisms to allow breaking messages into many chunks. However, these solutions promised to add a great deal of complexity to the protocol, so the work group chose not to go that route. It may be possible to relax this requirement using other transport protocols, such as SCTP. The lack of connection sharing in this document should not be construed to prohibit shared connections on other such protocols. However, such specification is beyond the scope of this document. 6. SDP Offer-Answer Exchanges for MSRP Sessions MSRP sessions will typically be initiated using the Session Description Protocol (SDP)  offer-answer mechanism, carried in SIPthe Session Initiation Protocol (SIP)  or any other protocol supporting it. MSRP borrows the idea of the direction attributes from COMEDIA , but does not depend on that specification. 6.1 Use of the SDP M-line The SDP m-line"m"-line takes the following form: m=<media> <port> <protocol> <format list> For non-RTP media sessions, The media field specifies the top level MIME media type for the session. For MSRP sessions, the media field MUST have the value of "message". The port field is normally not used, and SHOULD be set to 9999. An exception is when the port field value is set to zero, according to normal SDP usage. The proto field MUST designate the message session mechanism and transport protocol, separated by a "/" character. For MSRP, left part of this value MUST be "msrp". For MSRP over TCP, the right part of this field MUST take the value "tcp". For MSRP over other transport protocols, the field value MUST be defined by the specification for that protocol binding. The format list MUST indicate the MIME content-types that the endpointlist is willingignored for MSRP. This is because MSRP formats are specified as MIME content types, which are not convenient to acceptencode in the payload of SEND requests. IfSDP format list syntax. Instead, the final entry inallowed formats are negotiated using "a"-line attributes. For MSRP sessions, the format list SHOULD contain a "*" character, and nothing else. The port field in the M-line is not normally used to determine the port to which to connect. Rather, the actual port is determined by the contents of the session URL (Section 7.1). However, a "*", this indicates thatport value of zero has the normal SDP meaning. The following example illustrates an m-line for a message session, where the endpoint is may bewilling to receive otheraccept root payloads of message/ cpim, plain text or HTML. The second two types could either be presented as well, butthe types listed explicitly are preferred.root body, or could be contained within message/cpim bodies. m=message 9999 msrp/tcp * 6.2 The format list inDirection Attribute Since MSRP uses connection oriented transport protocols, one goal of the SDP answer MUST be the same as, or a subset of,negotiation is to determine which participant initiates the list provided in the offer. A "*" in the format list indicates that the sender may attempt to send messages with other media types that have not been explicitly listed. If the receiver is able to process the media type, it does so. If not, it will respond with a 415. Note that all explicit entries in the format list should be considered preferred over any non-listed types. This feature is needed as, otherwise, the format list for IM devices may be prohibitively large. The m-line format list may include MIME wrapper types, that is, mime formats that contain other types internally. The types listed in the format field can be used both as the root payload, or may be contained in container types. (Note that the container type must also be listed in the format list.) A list of types that are only allowed when wrapped in containers can be communicated in the accept-wrapped-types (Section 6.3) attribute. The port field in the M-line is not normally used to determine the port to which to connect. Rather, the actual port is determined by the contents of the session URL (Section 7.1). However, a port value of zero has the normal SDP meaning. The following example illustrates an m-line for a message session, where the endpoint is willing to accept root payloads of message/ cpim, plain text or HTML. The second two types could either be presented as the root body, or could be contained within message/cpim bodies. m=message 9999 msrp/tcp message/cpim text/plain text/html 6.2 The Direction Attribute Since MSRP uses connection oriented transport protocols, one goal of the SDP negotiation is to determine which participant initiates the transport connection. The direction attribute advertises whethertransport connection. The direction attribute advertises whether the offerer or answerer wishes to initiate the connection, wishes the peer endpoint to initiate the connection, or doesn't care. The endpoint that accepts the connection, or has a relay accept the connection on its behalf, is said to "host" the session, and is known as the hosting endpoint. The endpoint that initiates the connection is said to "visit" the session, and is known as the visiting endpoint. The direction attribute is included in an SDP a-line, with a value taking the following syntax: direction = direction-label ":" role direction-label = "direction" role = active / passive / both active = "active" passive = "passive" [sp timeout]both = "both" [sp timeout] timeout = 1*DIGIT ; timeout value in seconds The values for the role field are as follows: passivepassive: The endpoint wishes to host the session activeactive: The endpoint wishes the peer to host the session. bothboth: The endpoint is willing to act as either host or visitor. If "both" is selected, it may contain an optional timeout value. This timeout specifies how much time the answerer should wait before giving up on a connection and attempting to take over as host device. If the timeout value is not specified, it defaults to 30 seconds. The SDP offer for an MSRP session MUST contain a direction attribute, which MAY take any of the defined values. If the offerer is capable of hosting the session, or can arrange for a relay to host the session on its behalf, then it SHOULD select "both". The endpoint SHOULD NOT select "active" unless it cannot host the session under any circumstances. The endpoint SHOULD NOT select "passive" unless it has no option but to host the session. The SDP answer also MUST contain a direction attribute, but its value choices are limited based on the value in the offer. If the offer contained "active", then the answerer MUST either select "passive" or reject the offer. Likewise, if the offer contained "passive", then the answerer MUST select"active"select "active" or reject the offer. If the offer contained "both", the answerer SHOULD select "active", but MAY select "passive" if it is unable to reach the host device, or if local policy requires it to act as host. 6.3host device, or if local policy requires it to act as host. 6.3 The Accept Types Attribute MSRP can carry any MIME encoded payload. Endpoints specify MIME content types that they are willing to receive in the accept types "a"-line attribute. This attribute has the following syntax: accept-types = accept-types-label ":" format-list accept-types-label = "accept-types" format-list = format-entry *( SP format-entry) format-entry = (type "/" subtype) / ("*") type = token subtype = token SDP offers for MSRP sessions MUST include an accept-types attribute. SDP answers MUST also include the attribute, which MUST contain either the same list as in the offer or a subset of that list. A "*" entry in the accept-types attribute indicates that the sender may attempt to send messages with media types that have not been explicitly listed. If the receiver is able to process the media type, it does so. If not, it will respond with a 415. Note that all explicit entries SHOULD be considered preferred over any non-listed types. This feature is needed as, otherwise, the list of formats for rich IM devices may be prohibitively large. The accept-types attribute may include container types, that is, mime formats that contain other types internally. If compound types are used, the types listed in the accept-types attribute may be used both as the root payload, or may be wrapped in a listed container type. (Note that the container type MUST also be listed in the accept-types attribute.) 6.4 MIME Wrappers The MIME content-types in the M-line format listaccept-types attribute will often include compoundcontainer types; that is, types that contain other types. For example, "message/cpim" or "multipart/mixed." Occasionally andan endpoint will need to specify a MIME body type that can only be used if wrapped inside a listed container type. Endpoints MAY specify MIME types that are only allowed to be wrapped inside compound types using the "accept-wrapped-types" attribute in an SDP a-line. This attribute has the following syntax: accept-wrapped-types = wrapped-types-label ":" format-list wrapped-types-label = "accept-wrapped-types" The format-list element has the identical syntax as defined for the format list in the m-line.accept-types attribute. The semantics for this attribute are identical to those of the m-line format list,accept-types attribute, with the exception that the specified types may only be used when wrapped inside containers. The containerOnly types wouldlisted in accept-types may be specified onused as the m-line normally."root" type for the entire body. Since any type listed on the m-linein accept-types may be used both as a root body, orand wrapped in other bodies, format entries from the m-line SHOULD NOT be repeated in this attribute. This approach does not allow for specifying distinct lists of acceptable wrapped types for different types of containers. If an endpoint understands a MIME type in the context of one wrapper, it is assumed to understand it in the context of any other acceptable wrappers, subject to any constraints defined by the wrapper types themselves. The approach of specifying types that are only allowed inside of containers separately from the primary payload types allows an endpoint to force the use of certain wrappers. For example, a CPIM gateway device may require all messages to be wrapped inside message/cpim bodies, but may allow several content types inside the wrapper. If the gateway were to specify the wrapped types in the m-line format list,accept-types attribute, its peer could choose to use those types without the wrapper. 6.46.5 URL Negotiations An MSRP session is identified by an MSRP URL, which is determined by the hosting endpoint, and negotiated in the SDP exchange. Any SDP offer or answer that creates a possibility that the sender will host the session, that is, it contains a direction value of "passive" or "both", MUST contain an MSRP URL in a session attribute. This attribute has the following syntax: a=session:<MSRP_URL> where <MSRP_URL> is an MSRP or MSRPS URL as defined in Section 7.1. The visitor will use the session URL established by the host both to resolve the host address and port, and to identify the session when connecting. For MSRP sessions, the address field in the C-line is not relevant, and MUST be ignored. The port field in the M-line MUST be ignored if non-zero. Zero values have the normal meetingmeaning for SDP. The following example shows an SDP offer with a session URL of "msrp://example.com:7394/2s93i" c=IN IP4 useless.host.name m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction:both a=session:msrp://example.com:7394/2s93i The session URL MUST be a temporary URL assigned just for this particular session. It MUST NOT duplicate any URL in use for any other session hosted by the endpoint or relay. Further, since the peer endpoint will use the session URL to identify itself when connecting, it SHOULD be hard to guess, and protected from eavesdroppers. This will be discussed in more detail in Section 10. 6.56.6 Example SDP Exchange Endpoint A wishes to invite Endpoint B to a MSRP session. A offers the following session description containing the following lines: c=IN IP4 alice.example.com m=message 9999 msrp/tcp * a=accept-types: message/cpim text/plain text/html a=direction:both a=session:msrp://alice.example.com:7394/2s93i9 Endpoint B chooses to participate in the role of visitor, opens a TCP connection to alice.example.com:7394, and successfully performs a VISIT transaction passing the URL of msrp://alice.example.com:7394/ 2s93i9;.2s93i9. B indicates that it has accomplished this by answering with: c=IN IP4 dontlookhere m=message 9999 msrp/tcp message/cpim* a=accept-types:message/cpim text/plain a=direction:active A may now send IMs to B by executing SEND transactions on the same connection on which B sent the VISIT request. 7. The Message Session Relay Protocol The Message Session Relay Protocol (MSRP) is a text based, message oriented protocol for the transfer of instant messages in the context of a session. MSRP uses the UTF8 character set. MSRP messages MUST be sent over a reliable, congestion-controlled, connection-oriented transport protocols, such asprotocol. This document specifies the use of MSRP over TCP. Other documents may specify bindings for other such protocols. 7.1 MSRP URLs MSRP sessions are identified by MSRP URLs. An MSRP URL follows a subset of the URL syntax in Appendix A of RFC2396 , with a scheme of "msrp": msrp_url = "msrp" ":" "//" [userinfo] hostport ["/' resource] resource = 1*unreserved The constructions for "userinfo", "hostport", and "unreserved" are detailed in RFC2396 . An MSRP URL server part identifies the hosting device of an MSRP session. If the server part contains a numeric IP address, it MUST also contain a port. The resource part identifies a particular session at that host device. The absence of the resource part indicates a reference to an MSRP host device, but does not specifically refer to a particular session resource. MSRP has an IANA registered recommended port defined in Section 9.1. However, thisThis value should notSHOULD NOT be considered a default, as the URL process described herein will always explicitly resolve a port number. However, the URLs SHOULD be configured so that the recommended port is used whenever appropriate. This makes life easier for network administrators who need to manage firewall policy for MSRP. The server part will typically not contain a userinfo component, but MAY do so to indicate a user account for which the session is valid. Note that this is not the same thing as identifying the session itself. If a userinfo component exists, MUST be constructed only from "unreserved" characters, to avoid a need for escape processing. Escaping MUST NOT be used in an MSRP URL. Furthermore, a userinfo part MUST NOT contain password information. The following is an example of a typical MSRP URL: msrp://host.example.com:8493/asfd34 7.1.1 MSRP URL Comparison MSRP URL comparisons MUST be performed according to the following rules: 1. The host part is compared as case insensitive. 2. If the port exists explicitly in either URL, then it must match exactly. An URL with an explicit port is never equivalent to another with no port specified. 3. The resource part is compared as case insensitive. A URL without a resource part is never equivalent to one that includes a resource part. 4. Userinfo parts are not considered for URL comparison. Path normalization is not relevant for MSRP URLs. Escape normalization is not required, since the relevant parts are limited to unreserved characters. 7.1.2 Resolving MSRP Host Device An MSRP host device is identified by the server part of an MSRP URL. If the server part contains a numeric IP address and port, they MUST be used as listed..listed. If the server part contains a host name and a port, the connecting device MUST determine a host address by doing an A or AAAA DNS query, and use the port as listed. If the server part contains a host name but no port, the connecting device MUST perform the following steps: 1. Construct an SRV  query string by prefixing the host name with the service field "_msrp" and the protocol field ("_tcp" for TCP). For example, "_msrp._tcp.host.example.com". 2. Perform a DNS SRV query using this query string. 3. Select a resulting record according to the rules in RFC2782 . Determine the port from the chosen record. 4. If necessary, determine a host device address by performing an A or AAAA query on the host name field in the selected SRV result record. If multiple A or AAAA records are returned, the first entry SHOULD be chosen for the initial connection attempt. This allows any ordering created in the DNS to be preserved. 5. If the connection attempt fails, the device SHOULD attempt to connect to the addresses returned in any additional A or AAAA records, in the order the records were presented. If all of these fail, the device SHOULD attempt to use any additional SRV records that may have been returned, following the normal rules for SRV record selection. Note that in most cases, the transport protocol will be determined separately from the resolution process. For example, if the MSRP URL was communicated in an SDP offer or answer, the SDP M-line will contain the transport protocol. When an MSRP URL is communicated outside of SDP, the protocol SHOULD also be communicated. For example, a client may be configured to use a particular relay that is referenced with an MSRP URL. The client MUST also be told what protocol to use. If a device needs to resolve an MSRP URL and does not know the protocol, it SHOULD assume TCP. 7.1.3 The msrps URL Scheme The "msrps" URL Scheme indicates that each hop MUST be secured with TLS. Otherwise, it is used identically as an MSRP URL, except that a MSRPS URL MUST NOT be considered equivalent to an MSRP URL. The MSRPS scheme is further discussed in Section 10. 7.2 MSRP messages MSRP messages are either requests or responses. Requests and responses are distinguished from one another by the first line. The first line of a Request takes the form of the request-start entry below. Likewise, the first line of a response takes the form of response-start. The syntax for an MSRP message is as follows: msrp-message = request-start/response-start *(header CRLF) [CRLF body] request-start = "MSRP" SP length SP Method CRLF response-start=response-start = "MSRP" SP length SP Status-Code SP Reason CRLF length = 1*DIGIT ; the length of the message, ; exclusive of the start line. Method = SEND / BIND / VISIT / other-method other-method = token header = Client-Authenticate / Server-Challenge / Transaction-ID / Session-URL/ Content-Type / Expires Status-Code = 200 ;Success / 400 ;Bad Request / 401 ;Authentication Required / 403 ;Forbidden403 ;Forbidden / 415 ;Unsupported Content Type / 426 ;Upgrade Required / 481 ;No session / 500 ;Cannot Deliver / 506 ;duplicate session Reason = token ; Human readable text describing status Client-Authenticate = "CAuth" ":" credentials Server-Challenge = "SChal" ":" challenge Transaction-ID = "Tr-ID" ":" token Content-Type = "Content-Type" ":" quoted-string Session-URL = "S-URL" ":" msrp_url Expires = "Exp"":" delta-seconds delta-seconds = 1*DIGIT ; Integer number of seconds challenge = digest-scheme SP digest-challenge *("," digest-challenge) digest-scheme = "Digest" digest-challenge = nonce / algorithm / auth-param nonce = "nonce" "=" nonce-value nonce-value = quoted-string algorithm = "algorithm" "=" ( "SHA1" / token ) credentials = "Digest" digest-response *("," digest-response) digest-response = username / nonce / response / algorithm / auth-param username = "username" "=" username-value username-value = quoted-string response = "response" "=" request-digest request-digest = <"> 40LHEX <"> LHEX = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" / 415 ;Unsupported Content Type"a" / 426 ;Upgrade Required"b" / 481 ;No session"c" / 500 ;Cannot Deliver"d" / 506 ;duplicate session Reason = token ; Human readable text describing status Client-Authenticate = "CAuth" credentials Server-Challenge = "SChal" ":" challenge Transaction-ID = "Tr-ID" ":" token Content-Type = "Content-Type" ":" quoted-string Session-URL = "S-URL" ":" msrp_url Expires = "Exp"":" delta-seconds delta-seconds= 1*DIGIT ; Integer number of seconds"e" / "f" All requests and responses MUST contain at least a TR-ID header field. Messages MAY contain other fields, depending on the method or response code. 7.3 MSRP Transactions An MSRP transaction consists of exactly one request and one response. A response matches a transaction if it share the same TR-ID value, and arrives on the same connection on which the transaction was sent. BIND is always hop by hop. VISIT transactions are usually hop-by-hop, but may be relayed in situations where the visiting endpoint uses a relay. However, SEND transactions are end-to-end, meaning that under normal circumstances the response is sent by the peer endpoint, even if there are intervening relays. Endpoints MUST select TR-ID header field values in requests so that they are not repeated by the same endpoint in scope of the given session. TR-ID values SHOULD be globally unique. The TR-ID space of each endpoint is independent of that of its peer. Endpoints MUST NOT infer any semantics from the TR-ID header field beyond what is stated above. In particular, TR-ID values are not required to follow any sequence. MSRP Transactions complete when a response is received, or after a timeout interval expires with no response. Endpoints MUST treat such timeouts in exactly the same way they would treat a 500 response. The size of thetimeout interval isSHOULD be 30 seconds, but other values may be established as a matter of local policy, with a default of 30 seconds after a request has been completely sent.policy. 7.4 MSRP Sessions AN MSRP session is a context in which a series of instant messages are exchanged, using SEND requests. A session has two endpoints (a host and a visitor) and may have one or two relays. A session is identified by an MSRP URL. 7.4.1 Initiating an MSRP session When an endpoint wishes to engage a peer endpoint in a message session, it invites the peer to communicate using an SDP offer, carried over SIP or some other protocol supporting the SDP offer/ answer model. For the purpose of this document, we will refer to the endpoint choosing to initiate communication as the offerer, and the peer being invited as the answerer. The offerer SHOULD volunteer to act as the hosting endpoint if allowed by policy and network topology. An endpoint is said to host a session if one of two conditions are true. The host either directly listens for a connection from the peer endpoint, and maintains session state itself, or it uses a BIND request to initialize session state at a relay that will listen for a connection from the peer. The peer that is not the host is designated as the visitor. The offerer MAY request the answerer to act as host if it is prevented from accepting connections by network topology or policy, and is not able to bind to a relay to act on its behalf. If the offerer wishes to host the session directly, that is without using a relay, it MUST perform the following steps: 1. Construct a session MSRP URL . This URL MUST be resolvable to the offerer. The URL SHOULD be temporary, SHOULD be hard to guess, and MUST not duplicate the URL of any other session currently hosted by the offerer. 2. Listen for a connection from the peer. 3. Construct an SDP offer as described in Section 6, including the list of allowed IM payload formats in the format list.accept-types attribute. The offerer maps the session URL to the session attribute, as described in Section 22.214.171.124. 4. Insert a direction attribute. This value SHOULD be "both", indicating that the offerer will allow the answerer to override the offerer's decision to host. If "both" is selected, the offerer SHOULD leave the timeout at the default value (by leaving out the value entirely."entirely. However, the offerer MAY select a different timeout if circumstances warrant it. The direction value MAY be "passive" if the offerer is prevented from allowing the answerer override this choice. 5. Send the SDP offer using the normal processing for the signaling protocol. If the offerer chooses to force the answerer to host the session, it MUST perform the following steps instead: 1. Construct an SDP offer as described above, but with no session attribute. 2. Insert a direction attribute with a value of "active". 3. Send the offer using normal processing for the signaling protocol. When the answerer receives the SDP offer and chooses to participate in the session, it must choose whether ofto act as the host or the visitor. A direction attribute value of "both" in the offer indicates that the offerer wishesprefers to host, but will allow the answerer to host, in whichhost. In this case the answerer SHOULD act as the visitor, but MAY choose to host. A value of "passive" means the offerer insists upon hosting, in which case the answerer MUST act as visitor or decline the offer. If the answerer chooses to participate as a visitor, it MUST perform the following steps: 1. Determine the host address and port from the session URL, following the procedures in section Section 7.1 2. Connect to the host address and port, using the transport protocol from the M-line. 3. Construct a VISIT request, which MUST contain the following information: 1. An S-URL header field containing the session URL. 2. A TR-ID header field containing a unique transaction ID. 3. A size field containing size of the message subsequent to the start-line. 4. Send the request and wait for a response 5. If the transaction succeeds, send a SDP answer via the signaling protocol, according to the following rules: 1. The C-line is copied unmodified from the offer. 2. The M-Line contains a dummy port value, the protocol field from the original offer, and a format list describingthe accept-types attribute contains the SEND payload media types that the answerer is willing to accept. The format listaccept-types attribute in the answer MUST be either the same as the format list inthat of the offer, or it MUST be a subset. 3. A direction attribute containing the value "active". 6. If the transaction fails, the answerer MAY choose to act as host, if allowed by the direction attribute of the answer. If the answerer is unable or unwilling to host, then it should return an error response as appropriate for the signaling protocol. Some TCP connection failure conditions may ordinarily take some time to notice. For example, if the offerer is unable to open a TCP connection to the host device, this connection attempt may take a fairly large number of seconds to timeout. This length of time will not be acceptable for many call flow scenarios. Therefore, the devices SHOULD limit the time they wait for the TCP connection to a shorter timeout value, which will default to 30 seconds. However, the offerer MAY supply a different time in the timeout parameter of the "both" direction value. If the offerer supplies a value, the answerer SHOULD use that value for the TCP connection timeout, interpreted as an integer number of seconds. If the answerer chooses to host the session, it MUST perform the following steps: 1. Construct a new session URL . This MUST be a MSRP or MSRPS URL, MUST resolve to the answerer, and MUST not be the same as the session URL in the offer. The URL SHOULD be temporary, SHOULD be hard to guess, and MUST not duplicate URLs currently identifying any active sessions hosted by the answerer. 2. Listen for a connection from the peer. 3. Construct an SDP answer as described in Section 6, mapping the new session URL to the session attribute, and inserting a direction attribute with the value of "passive". 4. Send the SDP offer using the normal processing for the signaling protocol. When the offerer receives the SDP answer, it must determine who will continue to host the session. If the answer contained a direction attribute value of "active", the offerer MUST continue as host. If the offer contained "active" or "both" and the answer contains "passive", then the offerer MUST allow the answerer to host the session. If the offerer chooses not to continue as host, it MUST perform the following steps: 1. Release resources it acquired in expectation of hosting the session, if any. 2. Determine the host address and port from the session URL of the answer, following the procedures in section Section 7.1 3. Connect to the host address and port, using the transport protocol from the M-line. 4. Construct a VISIT request, which MUST contain the following information: 1. A S-URL header field containing the session URL. 2. A TR-ID header field containing a unique transaction ID. 3. A size field containing size of the message subsequent to the start-line. 5. Send the request and wait for a response 6. If the transaction succeeds, set the actual expiration time to the value in the Exp header field in the response, and acknowledge the answer via the signaling protocol. If either the connection attempt or the VISIT transaction fail, acknowledge the answer, then initiate the tear-down of the session using the signaling protocol. 7.4.2 Handling VISIT requests An MSRP endpoint that is hosting a session will receive a VISIT request from the visiting endpoint. When an endpoint receives a VISIT request, it MUST perform the following procedures: 1. Check if state exists for a session with a URL that matches the S-URL of the VISIT request. If so, and if no visitor connection has been associated with the session, then return a 200 response, and save state designating the connection on which the request was received as the visitor leg of the session. 2. If the session exists, and the visitor connection has already been established, return a 506 response and do not change session state in any way. 3. If no matching session exists, return a 481 request, and do not change session state in any way. 7.4.3 Sending Instant Messages on a Session Once a MSRP session has been established, either endpoint may send instant messages to its peer using the SEND method. When an endpoint wishes to do so, it MUST construct a SEND request according to the following process: 1. Insert the message payload in the body, and the media type in the Content-Type header field. The media type MUST match one of the types in the format list negotiated in the SDP exchange. If a "*" iswas present in the format list,accept-types attribute, then the media type SHOULD match one of the explicitly listed entries, but MAY be any other arbitrary value. 2. Set the TR-ID header field to a unique value. 3. Send the request on the connection associated with the session. 4. If a 2xx response code is received, the transaction was successful. 5. If a 5xx response code is received, the transaction failed, but other transactions may possibly be successful if retried.still succeed in the future. The endpoint MAY retryattempt to send the request asmessage content again in a new transaction,request, that is, with a new TR-ID value. If the endpoint receives 5xx responses more than some threshold number of times in a row, it SHOULD assume the session has failed, and initiate tear-down via the signaling protocol. The threshold value is a matter of local policy. 6. If a 415 response is received, this indicates the recipient is unable or unwillingrecipient is unable or unwilling to process the media type. The sender SHOULD NOT attempt to send that particular media type again in the context of this session. 7. If any other response code is received, the endpoint SHOULD assume the session has failed, and initiate tear-down. Normally transaction timeouts are treated the same as transactions that receive 5xx responses But, unlike transactions that fail explicitly, requests that have been timed out may in fact have been delivered to the peer endpoint, and even presented to the user. Attempting to resend such messages may result in the peer user seeing duplicate messages. Therefore a client implementation should take such action carefully, and should clearly indicate the situation to processthe media type. The sender SHOULD NOT attemptuser. Open Issue: Do we need to send that particular media type again in the context of this session. 7.create a duplicate suppression mechanism? If any other response code is received,retries were sent with with the endpoint SHOULD assumeTR-ID, then the session has failed, and initiate tear-down.recipient could recognize a duplicate message if it occurs in the same session. When an endpoint receives a SEND request, it MUST perform the following steps. 1. Determine that it understands the media type in the body, if any exists. 2. If it does, return a 200 response and render the message to the user. The method of rendering is a matter of local policy. 3. If it does not understand the media type, return a 415 response. 7.4.4 Ending a Session When either endpoint in an MSRP session wishes to end the session, it first signals its intent using the normal processing for the signaling protocol. For example, in SIP, it would send a BYE request to the peer. After agreeing to end the session, the host endpoint MUST release any resources acquired as part of the session. The process for this differs depending on whether the session is hosted directly by the host, or anby a relay. The host MUST destroy local state for the session. This involves completely removing the state entry for this session and invalidating session URL. If the host is using an MSRP relay, it MUST send a BIND containing an expires value of zero. This request MUST be sent on the host connection established by the original BIND request. This BIND request MUST include the session URL in the S-URL header field. Since these host actions completely destroy the session state at the hosting device, the visitor is not required to take further action beyond cleaning up any local state. If for some reasontake further action beyond cleaning up any local state. If for some reason the host fails to destroy session state, the state will be invalidated anyway when the inactivity timer expires. When an endpoint chooses to close a session, it may have SEND transactions outstanding. For example, it may have send SEND requests to which it has not yet received a response, or it may have received SEND requests that to which it has not responded. Once an endpoint has decided to close the connection, it SHOULD wait for such outstanding transactions to complete. It SHOULD NOT generate any new SEND transactions, and it MAY choose not to respond to any new SEND requests that are received after it decides to close the session. It SHOULD not respond to any new messages that arrive after it signals its intent to close the session. When an endpoint is signaled of its peer's intent to close a session, it SHOULD NOT initiate any more SEND requests. It SHOULD wait for any outstanding transactions that it initiated to complete, and it SHOULD attempt respond to any open SEND transactions received prior to being signaled. It is not possible to completely eliminate the host fails to destroychance of a session state,terminating with incomplete SEND transactions. When this occurs, an endpoint SHOULD clearly inform the state will be invalidated anyway whenuser that the inactivity timer expires.messages mat not have been delivered. 7.4.5 Session Inactivity Timer State associated with MSRP sessions, either at the host endpoint, or a hosting or visiting relay, is soft-state; that is, it expires over time if no message activity occurs. Each such device maintains a pair of inactivity timer, each with an initial value of 1 minute.12 minutes. One of these timers is assigned for each endpoint. All devices use the same, predetermined timer expiration value. While there might be some utility in negotiating this timer on a per device basis, such negotiation would add a great deal of complexity to MSRP. The choice of 12 minutes is somewhat arbitrary, but is intended to balance the bandwidth overhead against how quickly a relay can shed stale sessions. Since host endpoints will normally explicitly destroy sessions, stale sessions should only occur under failure conditions. Open Issue: In the 2 relay use case, the visitor does not explicitly remove state from the visiting relay. Rather, the visiting relay must infer that a session has been removed when the host device closes the connection, or when the inactivity timer expires. When a hosting device or visiting relay returns a successful response to a VISIT request, it MUST initialize both timers. The device MUST reset a timer anytime the associated endpoint sends a SEND request. If either timer expires without being reset, the device MUST invalidate the session, using normal procedures depending on the device's role in the session. Each endpoint MUST keep a similar timer, which it initializes when the session is created from its perspective. For the host endpoint, this is when it receives a successful response to a BIND request. For a visiting endpoint, this is when it sees a successful response to a VISIT request. Each endpoint resets its timer whenever it sends a SEND request. If an endpoint inactivity timer approaches expiration, and the endpoint wishes to continue participating in the session, it MUST send a SEND request. This request MAY be sent without a body if there is no user data to send. Endpoints MUST select the timer value so that there is sufficient time for the SEND request to traverse to the opposite endpoint. If the endpoint waits to the last moment, there is a danger that it will not be received by all relevant devices in time to prevent session destruction. Open Issue: There has been list discussion suggesting we should have a separate KEEPALIVE method for this purpose, rather than using SEND requests. 7.4.6 Managing Session State and Connections A MSRP session is represented by state at the host device. As mention previously, session state is identified by an MSRP URL. An active session also has two associated network connections. The connection between the hosting device and the host endpoint is known as the host connection. The connection with the visiting endpoint is the visiting connection. Note that when the session state is hosted directly by an endpoint, the host connection may not involve a physical network connection; rather it is a logical connection the device maintains with itself. When session state is destroyed for any reason, the hosting device SHOULD drop the connection(s). If a connection fails for any reason, the session hosting device MUST invalidate the session state. This is true regardless of whether the dropped connection is the host or visiting connection. Once a connection is dropped, the associated session state MUST NOT be reused. If the endpoints wish to continue to communicate after a connection failure, they must initiate a new session. An endpoint detecting a connection failure SHOULD attempt to tear down the session using the rules of the signaling protocol. It would be nice to allow sessions to be recovered after a connection failure, perhaps by allowing the opposite endpoint to reconnect, and send a new VISIT or BIND request. However, this approach creates a race condition between the time that the hosting device notices the failed connection, and the time that the endpoint tries to recover the session. If the endpoint attempts to reconnect prior to the hosting device noticing the failure, the hosting device will interpret the recovery attempt as a conflict. The only way around this would be to force the hosting device to do a liveness check on the original connection, which would create a lot of complexity and overhead that do not seem to be worth the trouble. 7.5 MSRP Relays MSRP supports the use of message relays. This specification describes the use of one or two relays. While more than two relays are not forbidden by MSRP, a solution for an arbitary number of relays is beyond the scope of this document. 7.5.1 Establishing Session State at a Relay An endpoint that wishes to host a MSRP session MAY do so by initiating session state at a MSRP relay, rather than hosting directly. An endpoint may wish to do this because network topology or local policy prevents a peer from connecting directly to the endpoint. The use of a relay should not be the default case, that is, a hosting endpoint that is not prevented from doing so by topology or policy SHOULD host the session directly. In order to use a relay, an MSRP endpoint MUST have knowledge of that relay's existence and location..location. We previously mentioned how an endpoint wishing to host a MSRP session constructs the session URL. When using a relay, the endpoint delegates that responsibility to the relay. To establish session state at a relay, the endpoint MUST perform the following steps: 1. Open a network connection to the relay at the relays address and port. Normally, this information will be resolved from an MSRP URL representing the relay, although the relay MAY be configured with an explicit address and port, rather than a URL. 2. Construct a BIND request with a S-URL that refers to the relay. 3. Set the ExpireExp header field to a desired value. 4. Send the BIND request on the connection. 5. Respond to any authentication request from the relay. 6. If the response has a 2xx status code, use the URL in the S-URL header field as the session URL. The endpoint uses this URL in exactly the same manner as it had constructed it itself. Additionally, accept the expires value in the response as pre-visit expiration time. A MSRP relay listens for connections at all times. When it receives a BIND request, it SHOULD authenticate the request, either using digest-authentication, TLS authentication, or some other authentication mechanism. If authentication succeeds, the relay performs the following steps: 1. Verify the client is authorized to BIND to this relay. If not, return a 403 response and make no state change. 2. If the client is authorized, construct a session MSRP URL. The URL MUST resolve to the relay. It SHOULD be temporary, and hard to guess. It MUST not duplicate any URL used in any active sessions hosted by the relay. If the relay wishes the visiting endpoint to connect over a pointport other than the MSRP relay well-know port, it MUST explicitly add the port number to visitor URL. 3. Establish the pre-visit expiration time for the session according to sectionSection 7.4.5. 4. Create state for the session. The relay MUST associate the connection on which the BIND request arrived as the host connection for the session. 5. Return a 200 response, with the session URL in the S-URL header field, and the pre-visit session expiration time in the Exp header field. When an MSRP relay receives a VISIT request, it MUST perform the following steps: 1. Check the S-URL header field value to see it matches the URL for an existing session state entry. 2. If not, return a 481 response and make no state changes 3. If it matches, but another connection has already been associated with the session URL, return a 506 response and make no state changes. If the session has been previously associated with this connection, treat the request as a refresh. 4. If it matches, and no visiting connection has been previously associated with the session, then the VISIT succeeds. The relay assigns the connection on which it received the VISIT request as the visiting connection for the session, and returns a 200 response. 7.5.2 Removing Session State from a relay An MSRP relay SHOULD remove state for a session when any of the following conditions occur: o The session inactivity timer expires. o The pre-visit timer expires before a VISIT request has occurred. o The host sends a BIND refresh request matching with an expiration value of zero. o Either the host or visitor network connection fails for any reason. 7.5.3 Sending IMs across an MSRP relay Once a session is established at a relay, the host and visitor may exchange IMs by sending SEND requests. Under normal circumstances, the relay does not respond to SEND requests in any way. Rather, the relay MUST forward the request to the peer connection unchanged. Likewise, if the relay receives a response it MUST forward the request unchanged on the peer connection. If a SEND request arrives on a connection that is not associated with a session, the relay MUST return a 481 response. 7.5.4 Relay Pairs In rare circumstances, two relays may be required in a session. For example, two endpoints may exist in separate administrative domains, where each domain's policy insist that all sessions must cross that domain's relay. A relay operating on behalf of the visiting endpoint is known as a visiting relay. An MSRP relay MAY be capable of acting as a visiting relay. This document does not describe a mechanism for an endpoint to discover that it needs to use a visiting relay. We assume that an endpoint is globally configured to use or not use such a relay, and does not make this decision on a session-by-session basis. This, of course, does not preclude using some other mechanism to make such a decision. In a two relay scenario, the visitor connects to a relay operating on its behalf, rather than connecting directly to the hosting device. The visitor sends a VISIT request as it would if it had connected directly to the hosting device. The visiting relay then connects to the hosting device and performs a VISIT request on behalf of the visitor. When a relay that is capable of acting as a visiting relay receives a VISIT request, it MUST check to see if the S-URL of the request matches a domain that the relay hosts. If the URL matches, then the visitor is not requesting the relay act as a visiting relay, and it SHOULD operate normally. If the URL does not match, then the relay SHOULD perform the following steps: 1. The relay SHOULD authenticate the VISIT request, using digest authentication or some other mechanism. 2. Determine that the visiting endpoint is authorized to use this device as a visiting relay. If not, return a 403 response and drop the connection. 3. Attempt to open a connection to the hosting device, determining the address and port from the S-URL exactly as if it were a visiting endpoint connecting directly. If this connection is successful, continue with the remaining steps. Otherwise, return a 500 response. 4. Create local state to associate the connection to the host device with the connection to the visiting device. 5. Relay the VISIT request unchanged to the hosting device. 6. Relay the response to the VISIT request unchanged to the visiting endpoint. 7. Relay all subsequent requests arriving on one of the associated connections to the peer connection. If either associated connection fails for any reason, the visiting relay MUST invalidate the session state, and MUST dropMUST drop the peer connection. 7.5.5 Relay Shutdown Relay administrators will occasionally need to take MSRP relays out of service. A relay implementation SHOULD allow a graceful shutdown that minimizes the occurrence of "lost", or timed out, messages. When a relay effects a graceful shutdown, it SHOULD refuse all new connection attempts, and refuse all MSRP requests, returning 481 responses. In order to allow any open transactions a high chance of completion, the relay SHOULD wait at least one transaction timeout period (normally 30 seconds) between the time it starts refusing requests and the peer connection.time it closes existing connections and shuts down. Open Issue: We have discussed that an endpoint implementation may attempt to establish a new session (perhaps using a different relay) with its peer. Do we wish to specify anything at all about such behavior? 7.6 Digest Authentication MSRP relays may use the digest authentication scheme to authenticate users. MSRP digest authentication is a simplified version of HTTP digest authentication , but this specification does not normatively depend on that document. MSRP digest authentication does not support the concept of a protection domain, nor does it support integrity protection. Since a user of a relay is expected to have credentials for that particular relay, it does not support the realm concept. Finally, since digest authentication is only expected for the initial BIND or VISIT request, MSRP does not support HTTP digest optimizations such as MD5-sess and preemptive credential loading by the client. Typically, a hosting user that uses a relay will have a preexisting relationship with that relay. This relationship SHOULD include authentication credentials. An MSRP relay SHOULD authenticate initial BIND requests. It is less likely that the visiting user will have an account at the hosting relay, so in most cases the authentication of VISIT requests is not useful. However a relay MAY authenticate initial VISIT requests. A visiting relay SHOULD authenticate initial VISIT requests, as it is much more likely to share credentials with the visiting user. There has been some discussion that a hosting relay SHOULD also authenticate VISIT requests. However, it will be common for visiting users to have no preexisting relationship with the host relay. Using authentication here would require the host endpoint to send temporary credentials in the SDP exchange, perhaps as part of the session URL. However, these temporary credentials would necessarily be transferred via the same channels as the session URL itself. If the credentials are sufficiently protected in transfer, then so is the session URL. Further, since the session URL is intended for a one time use, and is expected to be hard to guess, that URL itself should be sufficient for this purpose. Any situation where this is not adequate can be covered by the use of the MSRPS scheme. MSRP relays MUST NOT request authentication for any method other than BIND and VISIT. If a relay wishes to authenticate a request using digest authentication, it MAY challenge the request by responding with a 401 response, which MUST include a SChal header field. If an endpoint wishes to respond to a digest authentication challenge received in a 401 response, it MAY do so by sending a new VISIT or BIND request, identical to the previous request, but with a CAuth header field containing the response to the challenge. 7.6.1 The SHA1 Algorithm The only digest authentication algorithm defined in this specification is SHA1.  Other algorithms can be added as extensions. SHA1 is the default algorithm if no algorithm directive is present in the challenge. All MSRP devices MUST support SHA1. Open Issue: Do we need to specify how to offer more than one algorithm in a challenge? Do we need multiple algorithms possible for a particular challenge, or should we follow the HTTP digest approach of multiple challenges. It has been suggested that SHA1 MUST always be offered, to ensure that the client and server will have at least one common algorithm. The SHA1 digest is defined as follows: Let KD(secret, data) denote the string obtained by performing the digest algorithm to the data "data" with the secret "secret". Let H(data) denote the string obtained by performing the checksum algorithm on the data "data". For the "SHA1" algorithm, H(data) = SHA1(data), and KD(secret,data) = H(concat(secret, ":", data) TheSection 7.2 describes the syntax for the request-digest value in a CAuth header field takesas 40 digits in lower case hexadecimal notation. The actual structure of the following format.field is defined as follows. Note that unq(quoted-string) denotes the value of the string with the quotes removed. request-digest = <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) > <"> A1 = unq(username-value) ":" shared-secret ; "unq" denotes removal of quotes A2 = concat(Method,TR-ID,S-URI) When the relay receives a CAuth header, it SHOULD check its validity by looking up the shared secret, or H(A1), performing the same digest operation as performed by the client, and comparing the results to the request-digest value. 7.7 Method Descriptions This section summarizes the purpose of each MSRP method. All MSRP messages MUST contain the TR-ID header fields. All messages MUST contain a length field in the start line that indicates the overall length of the request, including any body, but not including the start line itself. Additional requirements exist depending on the individual method. Except where otherwise noted, all requests are hop by hop. 7.7.1 BIND The BIND method is used by a host endpoint to establish or refresh session state at a hosting relay. BIND requests SHOULD be authenticated. BIND requests MUST contain the S-URL and Exp header fields and MAY contain the CAuth header fields. A successful response to a BIND request MUST contain the S-URL and Exp header fields. 7.7.2 SEND The SEND method is used by both the host and visitor endpoints to send instant messages to its peer endpoint. SEND requests SHOULD contain a MIME body part. The body MUST be of a media type included in the format list negotiated in the SDP exchange. If a body is present, the request MUST contain a Content-Type header field identifying the media type of the body. Unlike other methods, SEND requests are end to end in nature. This means the request is consumed only by the opposite endpoint. Under normal conditions, any intervening relays merely forward the request on towards the peer endpoint. 7.7.3 VISIT The visiting endpoint uses the VISIT method to associate a network connection with the session state at the hosting device, which could be either the host endpoint or a relay operating on behalf of the host endpoint. The request MUST contain a S-URL header matching the session URL. There is normally no authentication operation for the VISIT request. This is because the session URL acts as a shared secret between host and the visitor. This puts certain requirements on the handling of the session URLs that are discussed in Section 10. However, if a visiting relay is used, it SHOULD authenticate VISIT requests. 7.8 Response Code Descriptions This section summarizes the various response codes. Except where noted, all responses MUST contain a TR-ID header field matching the TR-ID header field of the associated request. Responses are never consumed by relays. 7.8.1 200 The 200 response code indicates a successful transaction. 7.8.2 400 A 400 response indicates a request was unintelligible. 7.8.3 401 A 401 response indicates authentication is required. 401 responses MUST NOT be used in response to any method other than BIND and VISIT. A 401 response MUST contain a SChal header field. 7.8.4 403 A 403 response indicates the user is not authorized to perform the action. 7.8.5 415 A 415 response indicates the SEND request contained a MIME content-type that is not understood by the receiver. 7.8.6 426 A 426 response indicates that the request is only allowed over TLS protected connections. Open Issue: Do we need to make 426 extensible to support other types of protection?7.8.7 481 A 481 response indicates that no session exists for the connection. 7.8.8 500 A 500 response indicates that a relay was unable to deliver a SENDrequest to the target. 7.8.9 506 A 506 response indicates that a VISIT request occurred in which the S-URL indicates a session that is already associated with another connection. A 506 response MUST NOT be returned in response to any method other than VISIT. 7.9 Header Field Descriptions This section summarizes the various header fields. MSRP header fields are single valued; that is, they MUST NOT occur more than once in a particular request or response. 7.9.1 TR-ID The TR-ID header field contains a transaction identifier used to map a response to the corresponding request. A TR-ID value MUST be unique among all values used by a given endpoint inside a given session. MSRP elements MUST NOT assume any additional semantics for TR-ID. 7.9.2 Exp The Exp header field specifies when the state associated with a BIND request will expire, if no successful VISIT request has been received..received. The value is specified as an integer number of seconds from the time the request is received. BIND requests MUST contain this header field. Furthermore, successful responses to BIND requests MUST also contain the Exp header. The maximum value for the Exp header field is (2**32)-1 seconds. Exp has no meaning if it occurs in MSRP messages other than BIND requests, and responses to those requests. MSRP compliant devices SHOULD NOT use Exp in other requests or responses, unless that usage is defined in an extension to this specification. 7.9.3 CAuth The CAuth header field is used by a host endpoint to respond to offer digest authentication credentials to a relay, usually in response to a digest authentication challenge. CAuth SHOULD NOT be present in a request of any method other than BIND and VISIT. The CAuth credentials adhere to the following syntax: credentials = "Digest" digest-response digest-response = 1#( username | nonce | response | [ algorithm ] | [auth-param] ) username = "username" "=" username-value username-value = quoted-string response = "response" "=" request-digest request-digest = <"> 32LHEX <"> LHEX = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" | "b" | "c" | "d" | "e" | "f"used by a host endpoint to offer digest authentication credentials to a relay, in response to a digest authentication challenge. CAuth SHOULD NOT be present in a request of any method other than BIND and VISIT. The syntax of the CAuth credentials is described in Section 7.2 The meaning of each value is as follows: username: The user's account name. nonce: The nonce value copied from the challenge. response: A 32 hex digit string that proves user knowledge of the shared secret. algorithm: The algorithm value copied from the challenge. auth-param: Additional parameters for the sake of extensibility. 7.9.4 SChal The SChal header field is used by a relay to carry the challenge in a digest authentication attempt. Exactly one SChal header field MUST exist in a 401 response. The SChal header MUST NOT be used in any message except for a 401 response. The syntax for the SChal header field value is made up of achallenge according to the following syntax: challenge= digest-scheme SP digest-challenge digest-scheme = "Digest" digest-challenge = 1#( nonce | [ algorithm ] | [auth-param] ) nonce = "nonce" "=" nonce-value nonce-value = quoted-string algorithm = "algorithm" "=" ( "SHA1" | token )is described in Section 7.2 The meaning of each value is as follows: digest scheme: A token to identify the particular authentication scheme. ForSince MSRP only supports digest, thethis value MUST be "Digest." This token is present for the sake of extensibility.set to "Digest" nonce: A server-specified string, which the relay SHOULD uniquely generate each time it sends a 401 response. This string SHOULD take the form of base64 or hexadecimal data, to avoid the presence of a double-quote character, which is not allowed. algorithm: A token indicating the algorithms to be used to generate the digest and checksum. This directive exists for the sake of extensibility; the only value defined by this document is "SHA1". Absence of this directive indicates a value of "SHA1". 7.9.5 Content-Type The Content-Type header field is used to indicate the MIME media type of the body. Content-Type MUST be present if a body is present. Open Issue: We may need to clean up our MIME usage. This includes better defining the Content-Type usage possibly moving content-type into the body, indicating MIME version, etc. 7.9.6 S-URL The S-URL header field is used to identify the session. The S-URI header field MUST be present in a BIND request, a successful response to a BIND request, or a VISIT request. 8. Examples This section shows some example message flows for various common scenarios. The examples assume SIP is used to transport the SDP exchange. Details of the SIP messages and SIP proxy infrastructure are omitted for the sake of brevity. In the examples, assume the offerer is sip:firstname.lastname@example.org and the answerer is sip:email@example.com. In any given MSRP message, an "xx" in the length field indicates the actual length of the rest of the message. 8.1 No Relay In this scenario, the session goes directly between endpoints with no MSRP relays involved. Alice Bob | | | | |(1) (SIP) INVITE | |----------------------->| |(2) (MSRP) VISIT | |<-----------------------| |(3) (MSRP) 200 OK | |----------------------->| |(4) (SIP) 200 OK | |<-----------------------| |(5) (SIP) ACK | |----------------------->| |(6) (MSRP) SEND | |----------------------->| |(7) (MSRP) 200 OK | |<-----------------------| |(8) (MSRP) SEND | |<-----------------------| |(9) (MSRP) 200 OK | |----------------------->| |(10) (SIP) BYE | |----------------------->| |(11) (SIP) 200 OK | |<-----------------------| | | | | 1. Alice constructs a session URL of msrp://alicepc.atlanta.com/ iau39msrp:// alicepc.atlanta.com:7777/iau39 and listens for a connection on TCP port 7777. Alice->Bob (SIP): INVITE sip:firstname.lastname@example.org c=IN IP4 fillername m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction:both a=session:msrp://alicepc.atlanta.com/iau39:7777a=session:msrp://alicepc.atlanta.com:7777/iau39 2. Bob opens a TCP connection to alicepc.atlanta.com:7777: Bob->Alice (MSRP): MSRP xx VISIT S-URL:msrp://alicepc.atlanta.com/iau39:7777S-URL:msrp://alicepc.atlanta.com:7777/iau39 Tr-ID: sie09s 3. Alice->Bob (MSRP): MSRP xx 200 OK Tr-ID: sie09s Exp:300 4. Bob->Alice (SIP): 200 OK c=IN IP4 ignorefield m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction:active 5. Alice->Bob (SIP): ACK 6. Alice->Bob (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 7. Bob->Alice (MSRP): MSRP xx 200 OK TR-ID: 123 8. Bob->Alice (MSRP): MSRP xx SEND TR-ID: 456 Content-Type: "text/plain" Hi, Alice! I'm Bob! 9. Alice->Bob (MSRP): MSRP xx 200 OK TR-ID: 456 10. Alice->Bob (SIP): BYE Alice invalidates session and drops connection. 11. Bob invalidates local state for the session. Bob->Alice (SIP): 200 OK 8.2 Single Relay This scenario introduces an MSRP relay at relay.atlanta.com. Alice Relay Bob | | | | | | |(1) (MSRP) BIND | | |----------------------->| | |(2) (MSRP) 200 OK | | |<-----------------------| | |(3) (SIP) INVITE | | |------------------------------------------------>| | |(4) (MSRP) VISIT | | |<-----------------------| | |(5) (MSRP) 200 OK | | |----------------------->| |(6) (SIP) 200 OK | | |<------------------------------------------------| |(7) (SIP) ACK | | |------------------------------------------------>| |(8) (MSRP) SEND | | |----------------------->| | | |(9) (MSRP) SEND | | |----------------------->| | |(10) (MSRP) 200 OK | | |<-----------------------| |(11) (MSRP) 200 OK | | |<-----------------------| | | |(12) (MSRP) SEND | | |<-----------------------| |(13) (MSRP) SEND | | |<-----------------------| | |(14) (MSRP) 200 OK | | |----------------------->| | | |(15) (MSRP) 200 OK | | |----------------------->| |(16) (SIP) BYE | | |------------------------------------------------>| |(17) (MSRP) BIND | | |----------------------->| | |(18) (MSRP) 200 OK | | |<-----------------------| | |(19) (SIP) 200 OK | | |<------------------------------------------------| | | | | | | 1. Alice->Relay (MSRP): Alice opens a connection to the relay, and sends the following: MSRP xx BIND S-URL:msrp://relay.atlanta.com TR-ID: 321 Exp:600 2. Relay->Alice (MSRP): MSRP xx 200 OK TR-ID: 321 S-URL: msrp://relay.atlanta.com:7777/iau39 Exp:300 3. Alice->Bob (SIP): INVITE sip:email@example.com c=IN IP4 dummyvalue m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction:passive a=session:msrp://relay.atlanta.com:7777/iau39 4. Bob->Alice: Open connection to relay.atlanta.com:7777. Bob->Relay (MSRP): MSRP xx VISIT S-URL:msrp://relay.atlanta.com:7777/iau39 TR-ID: msrp:sie09ssie09s 5. Relay->Bob (MSRP): MSRP xx 200 OK TR-ID: sie09s Exp:300 6. Bob->Alice (SIP): 200 OK c=IN IP4 nobodybutuschickens m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction:active 7. Alice->Bob (SIP): ACK 8. Alice->Relay (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 9. Relay->Bob (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 10. Bob->Relay (MSRP): MSRP xx 200 OK TR-ID: 123 11. Relay->Alice (MSRP): MSRP xx 200 OK TR-ID: 123 12. Bob->Relay (MSRP): MSRP xx SEND TR-ID: 456 Content-Type:"text/plain" Hi, Alice! I'm Bob! 13. Relay->Alice (MSRP): MSRP xx SEND TR-ID: 456 Content-Type: "text/plain" Hi, Alice! I'm Bob! 14. Alice->relay (MSRP): MSRP xx 200 OK TR-ID: 456 15. Relay->Bob (MSRP): MSRP xx 200 OK TR-ID: 456 16. Alice->Bob (SIP): BYE 17. Alice->Relay (MSRP): MSRP xx BIND S-URL: msrp://relay.atlanta.com:7777/iau39 TR-ID: 42 Exp:0 18. Relay->Alice (MSRP): Relay invalidates session state. MSRP xx 200 OK TR-ID: 42 Exp:0 19. Bob invalidates local state for the session. Bob->Alice (SIP): 200 OK 8.3 Two Relays In this scenario, both Alice and Bob are each required by local policy to route all sessions through a different local relay. Alice AtlantaRelay BiloxiRelay Bob | | | | | | | | |(1) (MSRP) BIND | | |------------->| | | |(2) (MSRP) 200 OK | | |<-------------| | | |(3) (SIP) INVITE | | |------------------------------------------->| | | |(4) (MSRP) VISIT | | |<-------------| | |(5) (MSRP) VISIT | | |<-------------| | | |(6) (MSRP) 200 OK | | |------------->| | | | |(7) (MSRP) 200 OK | | |------------->| |(8) (SIP) 200 OK | | |<-------------------------------------------| |(9) (SIP) ACK | | | |------------------------------------------->| |(10) (MSRP) SEND | | |------------->| | | | |(11) (MSRP) SEND | | |------------->| | | | |(12) (MSRP) SEND | | |------------->| | | |(13) (MSRP) 200 OK | | |<-------------| | |(14) (MSRP) 200 OK | | |<-------------| | |(15) (MSRP) SEND | | |<-------------| | | |(16) (SIP) BYE| | | |------------------------------------------->| |(17) (MSRP) BIND | | |------------->| | | |(18) (MSRP) 200 OK | | |<-------------| | | |(19) (SIP) 200 OK | | |<-------------------------------------------| | | | | | | | | 1. Alice->AtlantaRelay (MSRP): Alice opens a connection to her relay, and sends the following: MSRP xx BIND S-URL: msrp://relay.atlanta.com TR-ID: 321 Exp:600 2. AtlantaRelay->Alice (MSRP): MSRP xx 200 OK TR-ID: 321 S-URL: msrp://relay.atlanta.com:7777/iau39 Exp:600 3. Alice->Bob (SIP): INVITE sip:firstname.lastname@example.org c=IN IP4 blahblahblah m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=session:msrp://relay.atlanta.com:7777/iau39 a=direction:passive 4. Bob determines that, due to local policy, he must connect through his own relay. Bob->BiloxiRelay (MSRP): Bob opens a connection to his relay, and sends the following: MSRP xx VISIT S-URL: msrp://relay.atlanta.com:7777/iau39 TR-ID: 934 5. BiloxiRelay->AtlantaRelay (MSRP): BiloxiRelay resolves the URL, opens a connection to relay.atlanta.com:7777, and sends the following: MSRP xx VISIT S-URL: msrp://relay.atlanta.com:7777/iau39 TR-ID: 934 6. AtlantaRelay->BiloxiRelay(MSRP): MSRP xx 200 OK TR-ID: 934 7. BiloxiRelay->Bob(MSRP): MSRP xx 200 OK TR-ID: 934 8. Bob->Alice (SIP): 200 OK c=IN IP4 stuff m=message 9999 msrp/tcp text/plain* a=accept-types:text/plain a=direction: active 9. Alice->Bob (SIP): ACK 10. Alice->AtlantaRelay (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 11. AtlantaRelay ->BiloxiRelay (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 12. BiloxiRelay->Bob (MSRP): MSRP xx SEND TR-ID: 123 Content-Type: "text/plain" Hi, I'm Alice! 13. Bob->BiloxiRelay (MSRP): MSRP xx 200 OK TR-ID: 123 14. BiloxiRelay->AtlantaRelay (MSRP): MSRP xx 200 OK TR-ID: 123 15. AtlantaRelay->Alice (MSRP): MSRP xx 200 OK TR-ID: 123 16. Alice->Bob (SIP): BYE 17. Alice->AtlantaRelay (MSRP): MSRP xx BIND S-URL: msrp://relay.atlanta.com:7777/iau39 TR-ID: 42 Exp:0 18. Relay->Alice (MSRP): Relay invalidates session state. MSRP xx 200 OK TR-ID: 42 Exp:0 19. Bob->Alice (SIP): 200 OK 9. IANA Considerations 9.1 MSRP Port MSRP uses TCP port XYX, to be determined by IANA after this document is approved for publication. Usage of this value is described in Section 7.1 9.2 MSRP URL Schemes This document defines the URL schemes of "msrp" and "msrps". 9.2.1 Syntax See Section 7.1. 9.2.2 Character Encoding See Section 7.1. 9.2.3 Intended Usage See Section 7.1. 9.2.4 Protocols The Message Session Relay Protocol (MSRP). 9.2.5 Security Considerations See Section 10. 9.2.6 Relevant Publications RFCXXXX [Note to RFC Editor: Please replace RFCXXXX in the above paragraph with the actual number assigned to this document. 9.3 SDP Parameters This document registers the following SDP parameters in the sdp-parameters registry: 9.3.1 Direction Attribute-name: direction Long-form Attribute Name Direction Type: Media level Subject to Charset Attribute No Purpose and Appropriate Values See Section 6.2. 9.3.2 Accept Types Attribute-name: accept-types Long-form Attribute Name Acceptable MIME Types Type: Media level Subject to Charset Attribute No Purpose and Appropriate Values See Section 6.3. 9.3.3 Wrapped Types Attribute-name: accept-wrapped-types Long-form Attribute Name Acceptable MIME Types Inside Wrappers Type: Media level Subject to Charset Attribute No Purpose and Appropriate Values See Section 126.96.36.199. 10. Security Considerations There are a number of security considerations for MSRP, some of which are mentioned elsewhere in this document. This section discusses those further, and introduces some new ones. Open Issue: There have been suggestions that we need more here covering the multiple authentication possibilities, MITM attack possibility on digest if not over TLS, and possible bid-down attacks on the digest algorithm selection. 10.1 TLS and the MSRPS Scheme All MSRP devices must support TLS, with at least the TLS_RSA_WITH_AES_128_CBC_SHA  cipher suite. Other cipher suites MAY be supported. MSRP does not define a separate TCP port for TLS connections. This means that all MSRP server devices, that is, all devices that listen for TCP connections, MUST be prepared to handle both TLS and plain text connections on the same port. When a device accepts a TCP connection, it MUST watch for the TLS handshake messages to determine if a particular connection uses TLS. If the first data received is not part of a start TLS handshakerequest, the device ceases to watch for the TLS handshake and begins normal MSRP processing.until it reads the entire message. Once the message has been completely received, the device resumes watching for the start TLS message. An MSRP device MAY refuse to accept a given request over a non-TLS connection by returning a 426 response, after which it MUST either immediately close the connection, or begin watching for a TLS handshake request as it would if it had just acceptednon-TLS connection by returning a connection.426 response. MSRP devices acting in the role of TCP client MAY perform a TLS handshake either immediately upon connection, or immediately after receiving a 426 response. Theyat any time, as long as the request occurs between MSRP messages. The endpoint MUST NOT attempt to upgrade tosend a start TLS at any other time. Allowing clientsrequest in the middle of an MSRP message. The working group considered only requiring the endpoint to upgradewatch for a TLS handshake at any time would requirethe server device to check every single requestbeginning of the session. However, the endpoint should be able to determine if ita new message is a start TLS request or an MSRP request or a TLS handshake request. The specified approachby only requires this check onreading ahead three bytes. Therefore, the initial data received on a connection, or on data received immediately afterworking group chose to allow a 426 response. In both cases, the receiver will havesession to peek ahead in the received data streamswitch to look for the TLS, then read again from the start once the presence or absence ofTLS has been determined.in mid-stream, as long as the switch occurs between MRSP messages. The MSRPS URI scheme indicates that all hops in an MSRP session MUST be protected with TLS. Ensuring this implies some additional rules. A relay MUST return an MSRPS URL to a BIND request if the request arrived over TLS and included a MSRPS URI in the S-URI header field. The relay MAY return an MSRPS URI to any BIND request that arrives over TLS, but MUST NOT return an MSRP URI to a BIND request that does not arrive over TLS. If a relay receives a BIND request with an MSRPS S-URI, over a non-TLS connection, it MUST reject the request with a 426 response. A relay may insist on always using MSRPS by returning a 426 to any bind received over an unprotected connection, and always returning MSRPS URLs to BIND requests over protected connections. A VISIT request for an MSRPS URL MUST be sent over a TLS protected connection. If a visiting relay receives a VISIT request for an MSRPS URL over an unprotected connection, it MUST reject the request with a 426 response. 10.2 Sensitivity of the Session URL The URL of a MSRP session is used by the visiting endpoint to identify itself to the hosting device, regardless of whether the session is directly hosted by the host endpoint, or is hosted by a relay. If an attacker were able to acquire the session URL, either by guessing it or by eavesdropping, there is a window of opportunity in which the attacker could hijack the session by sending a VISIT request to the host device before the true visiting endpoint. Because of this sensitivity, the session URL SHOULD be constructed in a way to make it difficult to guess, and should be sufficiently random so that it is unlikely to be reused. All mechanisms used to transport the session URL to the visitor and back to the host SHOULD be protected from eavesdroppers and man-in-the-middle attacks. Therefore an MSRP device MUST support the use of TLS for at least the VISIT request, which by extension indicates the endpoint MUST support the use of TLS for all MSRP messages. Further, MSRP connections SHOULD actually be protected with TLS. Further, an MSRP endpoint MUST be capable of using the security features of the signaling protocol in order to protect the SDP exchange and SHOULD actually use them on all such exchanges. End-to-end protection schemes SHOULD be preferred over hop-by-hop schemes for protection of the SDP exchange. 10.3 End to End Protection of IMs Instant messages can contain very sensitive information. As a result, as specified in RFC 2779 , instant messaging protocols need to provide for encryption, integrity and authentication of instant messages. Therefore MSRP endpoints MUST support the end-to-end encryption and integrity of bodies sent via SEND requests using Secure MIME (S/MIME) . Note that while each protected body could use separate keying material, this is inefficient in that it requires an independent public key operation for each message. Endpoints wishing to invoke end-to-end protection of message sessions SHOULD exchange symmetric keys in SDP k-lines, and use secret key encryption on for each MSRP message. When symmetric keys are present in the SDP, the offer-answer exchange MUST be protected from eavesdropping and tampering using the appropriate facilities of the signaling protocol. For example, if the signaling protocol is SIP, the SDP exchange MUST be protected using S/MIME. 10.4 CPIM compatibility MSRP sessions may be gatewayed to other CPIM compatible protocols. If this occurs, the gateway MUST maintain session state, and MUST translate between the MSRP session semantics and CPIM semantics that do not include a concept of sessions. Furthermore, when one endpoint of the session is a CPIM gateway, instant messages SHOULD be wrapped in "message/cpim"  bodies. Such a gateway MUST include "message/cpim" as the first entry in its SDP format list.accept-types attribute. MSRP endpoints sending instant messages to a peer that has included 'message/cpim" as the first entry in the format listaccept-types attribute SHOULD encapsulate all instant message bodies in "message/cpim""message/ cpim" wrappers. All MSRP endpoints MUST support the message/cpim type, and SHOULD support the S/MIME features of that format. 10.5 PKI Considerations Several aspects of MSRP will benefit from being used in the context of a public key infrastructure. For example, the MSRPS scheme allows, and even encourages, TLS connections between endpoint devices. And while MSRP allows for a symmetric session key to protect all messages in a session, it is most likely that session key itself would be exchanged in a signaling protocol such as SIP. Since that key is extremely sensitive, its exchange would also need to be protected. In SIP, the preferred mechanism for this would be S/MIME, which would also benefit from a PKI. However, all of these features may be used without PKI. Each endpoint could instead use self signed certificates. This will, of course, be less convenient than with a PKI, in that there would be no certificate authority to act as a trusted introducer. Peers would be required to exchange certificates prior to securely communicating. Since, at least for the immediate future, any given MSRP implementation is likely to communicate with at least some peers that do not have a PKI available, MSRP implementations SHOULD support the use of self-signed certificates, and SHOULD support the ability to configure lists of trusted certificates. 11. Changes from Previous Draft Versions This section to be deleted prior to publication as an RFC 11.1 draft-ietf-simple-message-sessions-02 Moved all content type negotiation from the "m"-line format list into "a"-line attributes. Added the accept-types attribute. This is due to the fact that the sdp format-list syntax is not conducive to encoding MIME content types values. Added "other-method" construction to the message syntax to allow for extensible methods. Consolidated all syntax definitions into the same section. Cleaned up ABNF for digest challenge and response syntax. Changed the session inactivity timeout to 12 minutes. Required support for the SHA1 alogorithm. Required support for the message/cpim format. Fixed lots of editorial issues. Documented a number of open issues from recent list discussions. 11.2 draft-ietf-simple-message-sessions-01 Abstract rewritten. Added architectural considerations section. The m-line format list now only describes the root body part for a request. Contained body part types may be described in the "accept-wrapped-types" a-line attribute. Added a standard dummy value for the m-line port field. Clarified that a zero in this field has normal SDP meaning. Clarified that an endpoint is globally configured as to whether or not to use a relay. There is no relay discovery mechanism intrinsic to MSRP. Changed digest algorithm to SHA1. Added TR-ID and S-URI to the hash for digest authentication. CMS usage replaced with S/MIME. TLS and MSRPS usage clarified. Session state timeout is now based on SEND activity, rather than BIND and VISIT refreshes. Default port added. Added sequence diagrams to the example message flows. Added discussion of self-signed certificates in the security considerations section. 11.211.3 draft-ietf-simple-message-sessions-00 Name changed to reflect status as a work group item. This version no longer supports the use of multiple sessions across a single TCP session. This has several related changes: There is now a single session URI, rather than a separate one for each endpoint. The session URI is not required to be in requests other than BIND and VISIT, as the session can be determined based on the connection on which it arrives. BIND and VISIT now create soft state, eliminating the need for the RELEASE and LEAVE methods. The MSRP URL format was changed to better reflect generic URL standards. URL comparison and resolution rules were added. SRV usage added. Determination of host and visitor roles now uses a direction attribute much like the one used in COMEDIA. Format list negotiation expanded to allow a "prefer these formats but try anything" semantic Clarified handling of direction notification failures. Clarified signaling associated with session failure due to dropped connections. Clarified security related motivations for MSRP. Removed MIKEY dependency for session key exchange. Simple usage of k-lines in SDP, where the SDP exchange is protected end-to-end seems sufficient. 11.311.4 draft-campbell-simple-im-sessions-01 Version 01 is a significant re-write. References to COMEDIA were removed, as it was determined that COMEDIA would not allow connections to be used bidirectionallybidirectional in the presence of NATs. Significantly more discussion of a concrete mechanism has been added to make up for no longer using COMEDIA. Additionally, this draft and draft-campbell-cpimmsg-sessions (which would have also changed drastically) have now been combined into this single draft. 12. Contributors The following people contributed substantially to this ongoing effort: Rohan Mahy Allison Mankin Jon Peterson Brian Rosen Dean Willis Adam Roach Cullen Jennings Aki Niemi Hisham Khartabil Pekka Pessi Chris Boulton Normative References  Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.  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.  Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging / Presence Protocol Requirements", RFC 2779, February 2000.  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URL): Generic Syntax", RFC 2396, August 1998.  Atkins, D. and G. Klyne, "Common Presence and Instant Messaging Message Format", draft-ietf-impp-cpim-msgfmt-08 (work in progress), January 2003.  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.  Ramsdell, B., "S/MIME Version 3 Message Specification", RFC 2633, June 1999.  Chown, P., ""Advanced Encryption Standard (AES) Ciphersuites for Transport Layer Security (TLS)", RFC 3268, June 2002.  Eastlake, 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. Informational References  Campbell, B. and J. Rosenberg, "Session Initiation Protocol Extension for Instant Messaging", RFC 3428, September 2002.  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996.  Mahy, R., Campbell, B., Sparks, R., Rosenberg, J., Petrie, D. and A. Johnston, "A Multi-party Application Framework for SIP", draft-ietf-sipping-cc-framework-02 (work in progress), May 2003.  Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo, "Best Current Practices for Third Party Call Control in the Session Initiation Protocol", draft-ietf-sipping-3pcc-03draft-ietf-sipping-3pcc-04 (work in progress), MarchJune 2003.  Sparks, R. and A. Johnston, "Session Initiation Protocol Call Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in progress), February 2003.  Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002.  Peterson, J., "A Privacy Mechanism for the Session Initiation Protocol (SIP)", RFC 3323 , November 2002.  Peterson, J., "A Common Profile for Instant Messaging (CPIM)", draft-ietf-impp-im-03draft-ietf-impp-im-04 (work in progress), MayAugust 2003.  Yon, D., "Connection-Oriented Media Transport in SDP", draft-ietf-mmusic-sdp-comedia-05 (work in progress), March 2003.  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. Authors' Addresses Ben Campbell dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 EMail: email@example.com Jonathan Rosenberg dynamicsoft 600 Lanidex Plaza Parsippany, NJ 07054 EMail: firstname.lastname@example.org Robert Sparks dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 EMail: email@example.com Paul Kyzivat Cisco Systems Mail Stop LWL3/12/2 900 Chelmsford St. Lowell, MA 01851 EMail: firstname.lastname@example.org Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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