One document matched: draft-dcsgroup-sip-state-01.txt
Differences from draft-dcsgroup-sip-state-00.txt
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
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DCS Group Internet Draft - Expiration 09/30/00 1
SIP Extensions for Distributed Call State March 2000
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The distribution of this memo is unlimited. It is filed as <draft-
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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.
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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
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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
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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:
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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
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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;
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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
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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
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Expiration Date: This memo is filed as <draft-dcsgroup-sip-state-
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DCS Group Internet Draft - Expiration 09/30/00 10
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