SIP Working Group W. Marshall Internet Draft K. Ramakrishnan Document: <draft-dcsgroup-sip-state-01.txt> AT&T Category: Informational E. Miller G. Russell CableLabs B. Beser M. Mannette K. Steinbrenner 3Com D. Oran F. Andreasen Cisco J. Pickens Com21 P. Lalwaney J. Fellows Motorola D. Evans Secure Cable Solutions K. Kelly NetSpeak March, 2000 SIP Extensions for supporting Distributed Call State Status of this Memo This document is an Internet-Draft and is NOT offered in accordance with Section 10 of RFC2026[1], and the author does not provide the IETF with any rights other than to publish as an Internet-Draft. 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 DCS Group Internet Draft - Expiration 09/30/00 1 SIP Extensions for Distributed Call State March 2000 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. The distribution of this memo is unlimited. It is filed as <draft- dcsgroup-sip-state-01.txt>, and expires September 30, 2000. Please send comments to the authors. 1. Abstract This document describes extensions to the Session Initiation Protocol (RFC2543) for supporting telephony services using the Distributed Call Signaling architecture described in [2]. This document discusses the State header that is used for supporting a call signaling architecture where call state is distributed to the clients during call setup and is stored there for the duration of the call while the proxy server remains stateless. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [3]. 3. Introduction The Distributed call signaling (DCS) architecture provides signaling support for creating a session using a signaling scheme so that call state is distributed to the clients and network resources reserved prior to alerting the called party. The SIP proxy server in the DCS architecture is referred to as a DCS-Proxy. The SIP user agent is referred to as a client or endpoint. From a call signaling perspective, the DCS Proxies are involved in setting up a call. During a successful call setup, call state and the associated billing information is encrypted and signed by the proxies and sent to the clients using the proposed "State" header. This is sent in the initial INVITE to the "called" client and in the first 1xx (except 100) or a 200 response to the "calling" client/caller. The DCS-Proxy in effect, transfers call state to the clients and other network entities during the call-setup phase and then remains stateless for the duration of the call. The state may be encrypted and also contain an integrity check value. If the client wishes to change call characteristics that affect bearer path and/or require billing changes, it passes the saved proxy encrypted and signed state information in a SIP INVITE request to its proxy server, which may verify integrity of the state and decrypt it. DCS Group Internet Draft - Expiration 09/30/00 2 SIP Extensions for Distributed Call State March 2000 In the DCS architecture, there are three kinds of state associated with a call - transaction state, connection state and call state. The DCS goal with managing state is to store state about the call at places where it is needed. Transaction state refers to billing and accounting information about a call and this is stored in record keeping servers. Connection state refers to the state associated with the media path. This includes the characteristics of the flow, admission control and policing parameters and is stored in devices in the network/media path where admission control and policing decisions are made. The Call state refers to endpoint identification, caller and callee preferences that affect active call characteristics, and network and transactions state hooks or identifiers in the active call that can be used by the proxy to modify the characteristics of the call. This state information is encrypted and signed by the proxy and distributed to the endpoints during call setup. The proxy associated with the endpoint can therefore remain stateless during the call. The proxy also distributes transaction state to the record keeping servers and connection state to network entities during call setup. The call state is returned to the proxy when the characteristics of the active call need to be changed. An example of this is a mid-call codec change that requires re-authorization from the network and a possible change to the connection and transaction states. The focus of this draft is on issues related to the call state that is transferred between the proxy and the endpoint and between proxies during the signaling phase of call setup. There are three candidate cases that DCS has identified where the proxy needs to be involved in the call after the initial call setup: 1. Mid-call changes that need proxy assistance, e.g. changes to higher-bandwidth codecs, or call transfers 2. Placing a new call to a previous destination, e.g. call- return or call-trace. 3. Identifying a party to be added to a call, using the Also: header. All the above require the client to send to its proxy the encrypted Remote-Party-ID header [5] (if privacy is requested for the call) so that the proxy can identify the other endpoint that is affected in the requested call change. In addition, the information in the State header (especially connection state and billing/transaction info) may be needed for case 1 and 3. Call return and call trace as currently defined in the PSTN are examples of services that are "new calls" that require remote party identification. The state information distribution described above between the proxy and the endpoint can be generalized to a network of proxies in the signaling path. The state information would benefit from the stackable properties of via's and record-routes in SIP. Proxies could encrypt information pertaining to the call in the state header. This information can be retrieved from the opaque state DCS Group Internet Draft - Expiration 09/30/00 3 SIP Extensions for Distributed Call State March 2000 information passed back to the proxy when the characteristics of a particular call need to be changed. This requires that there be a field in the state header that identifies the proxy that inserted the state information. Once the client has cached state information, the rules for determining when and how the state information is returned to the proxy are discussed in the section below. 4. SIP State Header Extension In this section, we propose a new SIP header called "State" to carry information about an active call. 4.1 State Header Syntax The State extension conveys state information usually between a proxy and a client and may be used between proxies. The state information will typically be encrypted and protected by an integrity check value, e.g. a digital signature or seal. This state information allows the proxy to reliably and securely store state information in the client that may be needed for subsequent feature invocation, allowing the proxy to remain stateless during the call. The following syntax includes the generalization where signaling proxies can transfer relevant state information related to the call being setup in the state header which can later be retrieved when a change in call characteristics is initiated by an endpoint. The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC-2234 [4]. State = "State" ":" 1#(host ";" state-token *(";" state-token)) state-token = token ["=" (*token | quoted-string)] The host field identifies the proxy that inserted the state information. State headers may be nested. In that case, each proxy in the signaling path that wishes to transfer state information takes the state header it received in the incoming signaling message (previous host; token form), adds its state information, optionally encrypts and signs the combined information, and adds its hostname. State-tokens may be encrypted and signed, and the last token in a sequence is likely to be an integrity check over the previous. It is an encoding of an encrypted structure containing multiple pieces of information needed by the proxy to perform various mid-call features. The encrypted structure is returned from the client to the Proxy for call services that affect the current call. The State header information is not used to identify the other endpoint in the call. When an endpoint invokes services that require the proxy to DCS Group Internet Draft - Expiration 09/30/00 4 SIP Extensions for Distributed Call State March 2000 determine the other endpoint in the call, the Remote-Party-ID header [5] is also included in the message. The following example illustrates the use of the distribution of state during call setup. Endpoint-o and Endpoint-t refer to the originating and terminating UAC/UAS for the call. P1 is the proxy associated with Endpoint-o and P2 is the proxy associated with Endpoint-t. eP1{*} refers to the state token encrypted by P1. Endpoint-o -> P1 -> P2 -> Endpoint-t Endpoint-o->P1: invite, no state header P1->P2: invite, State:P1;state=eP1{"cached translation of Endpoint-t number"} P2->Endpoint-t: invite, State:P2;state=eP2{"hunt group ID, billing ID,P1;state=eP1{"cached translation of Endpoint-t number"}"} Endpoint-t saves the above state header information received from its proxy P2 for the duration of the call. Endpoint-t->P2: response, State:P2; state=eP2{"hunt group ID, billing ID,P1;state=eP1{"cached translation of Endpoint-t number"}"} P2->P1: response, State:P2;state=eP2{"hunt group ID, billing ID"},P1;state=eP1{"cached translation of Endpoint-t number"} P1->Endpoint-o: response, state:P1;state=eP1{"billing ID, cached translation of Endpoint-t number, P2;state=eP2{"hunt group ID, billing ID"}"} Endpoint-o saves the above state info received from P1 for the duration of the call. When call begins, state at Endpoint-o is: State:P1;state=eP1{"billing ID, cached translation of Endpoint- t number", P2;state=eP2{"hunt group ID, billing ID"}"} State at endpoint-t is: State:P2;state=eP2{"hunt group ID, billing ID,P1;state=eP1{" cached translation of Endpoint-t number"}"} 4.2 Rules at Endpoints for determining inclusion of State Headers The rules at the endpoint for returning state information to its proxy are listed below: DCS Group Internet Draft - Expiration 09/30/00 5 SIP Extensions for Distributed Call State March 2000 1. Every received State header is saved for the duration of the call along with From, To, and Call-ID. 2. On a subsequent invite, a State header is included if the From, To (including ones with From/To reversed), Call-ID match those associated with the saved State, and Request-URI matches the hostname of the saved State header. If Route header is present, also include State headers that have hostname matching a component (in case state was not "nested") 3. A State header is included in every response where a match on the From, To (including ones with From/To reversed), Call-ID is found, and the topmost Via header matching the hostname of the State header. 4. If the request includes a Also header that includes a Call-ID header and includes a Replaces header, append all State headers to the Also: that (1) match the Call-ID, (2) either the From or To value matches the Replaces header, and (3) the hostname in the State header appears either in the request-uri or in a route header. 5. When call ends, all saved State headers MAY be deleted. 4.3 Proxy rules for State header insertion and processing The rules at the proxy for processing State headers received from the clients (endpoints) or other proxies are listed below: 1. After extracting relevant State information, a proxy MAY discard the State headers with its hostname. 2. A State header addressed to another proxy MUST be passed on. 3. Any proxy MAY take the set of all State headers and concatenate them into a single State header; any proxy MAY take this concatenated State header and combine it into a single State header with local state (nesting). This action is REQUIRED for any proxy that hides Via headers. 4. Any proxy can generate multiple State headers. 4.4 State Header and HTTP Cookie/Pcookie Comparison The State header field discussed in this section should not be confused with HTTP1.1 Cookies as described in [6]. The intended use of the two is very different. HTTP uses the Cookie for "state" management, or as a handle to pass session context change from server to client where the server is the other endpoint of the session. On the otherhand, the State header is sent by the SIP proxy to the client so that call state can be securely stored at the DCS Group Internet Draft - Expiration 09/30/00 6 SIP Extensions for Distributed Call State March 2000 endpoint making the associated proxies "stateless" during the call. The state header can be considered to be a handle to request session change by the endpoint from its proxy. In addition, there are no attribute value pairs associated with the state header as in the Cookie that clients make use of. 5. Security Considerations The clients/endpoints are untrusted entities in the DCS architecture. DCS-Proxies are responsible for verifying the contents and consistency of the State header discussed in this document. 6. References 1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2. DCS Group, "Architectural Considerations for Providing Carrier Class Telephony Services Utilizing SIP-based Distributed Call Control Mechanisms", draft-dcsgroup-sip-arch-01.txt, March 2000. 3. Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 4. Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997 5. "SIP Extensions for Caller Identity, Privacy and Operator Services", Internet Draft: <draft-dcsgroup-sip-privacy-01.txt>, March 2000. 6. Kristol, D. and Montulli, L., "HTTP State Management Mechanism", RFC 2109, February 1997. See current working draft <draft-ietf- http-state-man-mec-12.txt> modified by the same authors based on field implementation feedback. 7. Acknowledgments The Distributed Call Signaling work in the PacketCable project is the work of a large number of people, representing many different companies. The authors would like to recognize and thank the following for their assistance: John Wheeler, Motorola; David Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jon Fellows, Jay Strater, Jeff Ollis, Clive Holborow, Motorola; Doug Newlin, Guido Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; DCS Group Internet Draft - Expiration 09/30/00 7 SIP Extensions for Distributed Call State March 2000 Farzi Khazai, Nortel; John Chapman, Bill Guckel, Michael Ramalho, Cisco; Chuck Kalmanek, Doug Nortz, John Lawser, James Cheng, Tung- Hai Hsiao, Partho Mishra, AT&T; Telcordia Technologies; and Lucent Cable Communications. 8. Author's Addresses Bill Marshall AT&T Florham Park, NJ 07932 Email: wtm@research.att.com K. K. Ramakrishnan AT&T Florham Park, NJ 07932 Email: kkrama@research.att.com Ed Miller CableLabs Louisville, CO 80027 Email: E.Miller@Cablelabs.com Glenn Russell CableLabs Louisville, CO 80027 Email: G.Russell@Cablelabs.com Burcak Beser 3Com Rolling Meadows, IL 60008 Email: Burcak_Beser@3com.com Mike Mannette 3Com Rolling Meadows, IL 60008 Email: Michael_Mannette@3com.com Kurt Steinbrenner 3Com Rolling Meadows, IL 60008 Email: Kurt_Steinbrenner@3com.com Dave Oran Cisco Acton, MA 01720 Email: oran@cisco.com Flemming Andreasen Cisco Edison, NJ Email: fandreas@cisco.com DCS Group Internet Draft - Expiration 09/30/00 8 SIP Extensions for Distributed Call State March 2000 John Pickens Com21 San Jose, CA Email: jpickens@com21.com Poornima Lalwaney Motorola San Diego, CA 92121 Email: plalwaney@gi.com Jon Fellows Motorola San Diego, CA 92121 Email: jfellows@gi.com Doc Evans Secure Cable Solutions Westminster, CO 30120 Email: drevans@securecable.com Keith Kelly NetSpeak Boca Raton, FL 33587 Email: keith@netspeak.com DCS Group Internet Draft - Expiration 09/30/00 9 SIP Extensions for Distributed Call State March 2000 Full Copyright Statement "Copyright (C) The Internet Society (date). 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