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This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

The REGISTER function is used in a SIP system primarily to associate a temporary contact address with an address-of-record. This contact is generally in the form of a URI, such as Contact: <sip:alice@pc33.atlanta.com> and is generally dynamic and associated with the IP address or hostname of the SIP UA. The problem is that network topology may be that there are one or more SIP proxies between the UA and the registrar, such that any message from the user's home network to the registered UA must traverse these proxies. The REGISTER method does not give us a mechanism to discover and record this sequence of proxies in the registrar for future use. This document defines an extension header, "Path" which provides such a mechanism.

Table of Contents

1.  Background
2.  Applicability Statement
3.  Path Header Definition and Syntax
4.  Usage of Path Header
4.1  Procedures at the UA
4.2  Procedures at Intermediate Proxies
4.3  Procedures at the Registrar
4.4  Procedures at the Home Proxy
4.5  Examples of Usage
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgements
§  Normative References
§  Non-Normative References
§  Author's Address
§  Full Copyright Statement

1. Background

3GPP established a requirement for discovering intermediate proxies during SIP registration and published this requirement in [3GPPReq]

Scenario:

UA----P1-----P2-----P3------REGISTRAR

UA wishes to register with REGISTRAR. However, due to network topology, UA must use P1 as an "outbound proxy", and all messages between UA1 and REGISTRAR must also traverse P1, P2, and P3 before reaching REGISTRAR. Likewise, all messages between REGISTRAR UA must also traverse P1, P2, and P3 before reaching UA.

UA has a standing relationship with REGISTRAR, which it considers its "Home"". How UA establishes this relationship is outside the scope of this document. UA discovers P1 as a result of a DHCP assignment or similar operation, also outside the scope of this document. REGISTRAR has a similar "default outbound proxy" relationship with P3.

Eventually, REGISTRAR or a service proxy closely related to it will receive a message for UA. It needs to know which proxies must be transited by that message in order to get back to UA. In some cases, this information may be deducible from SIP routing configuration tables or from DNS entries. In other cases, such as that raised by 3GPP, the information is not readily available outside of the SIP REGISTER transaction.

The proposed Path extension header allows accumulating and transmitting the list of proxies between UA and REGISTRAR. Intermediate nodes such as P1 may statefully retain Path information if needed by operational policy. This mechanism is in many ways similar to the operation of Record-Route in dialog-initiating messages.

2. Applicability Statement

The Path mechanism is applicable whenever there are intermediate proxies between a SIP UA and a SIP Registrar used by that UA where the following conditions are true:

1. One or more of the intermediate proxies MUST be visited by messages between REGISTRAR and UA.
2. The same set of intervening proxies MUST be visited by messages between a home service proxy and UA. That is, the proxy route between the UA and its home service proxy is identical to the proxy route between the UA and its registrar.
3. The network topology is such that the intermediate proxies which must be visited are NOT implied by SIP routing tables, DNS, or similar mechanisms.

3. Path Header Definition and Syntax

The Path header is a SIP extension header with syntax very similar to the Record-Route header. It is used in conjunction with SIP REGISTER messages and with 200 OK messages in response to REGISTER (a REGISTER response).

A Path header may be inserted into a REGISTER by any SIP node traversed by that message. Like the Route header, sequential Path headers are evaluated in the sequence in which they are present in the message, and Path header values may be combined into compound Path elements in a single Path header. The registrar reflects the accumulated Path back into the REGISTER response, and intermediate nodes propagate this back toward the originating UA. The originating UA is therefore informed of the inclusion of nodes on its registered Path, and MAY use that information in other capacities outside the scope of this document.

The primary difference between Path and Record-Route is that Path applies to REGISTER and 200 OK responses to REGISTER. Record-Route doesn't, and can't be defined in REGISTER for reasons of backward compatibility.

The syntax for Path can be given as:

Path = "Path" HCOLON path-value *( COMMA path-value )

path-value = name-addr *( SEMI rr-param )

The allowable usage of headers is described in Table 2 of [2]. The following additions to this table are needed for Path.

Addition of Path to SIP Table 2:

4. Usage of Path Header

4.1 Procedures at the UA

The UA executes its register operation as usual. The response may contain a Path header. The general operation of the UA is to ignore the Path header in the response. It MAY choose to display the contents of the Path header to the user or take other action outside the scope of this document.

It has been suggested that the UA MAY choose to store the contents of the Path header for future use as a preloaded Route for use when the UA wishes to send a SIP message that, for reasons of acquiring services in the home network, need to transit the service proxies in the home network. Such usage is explicitly outside the scope of this document.

4.2 Procedures at Intermediate Proxies

When a proxy processing a REGISTER request wishes to be on the path for future transmissions toward the UA originating that REGISTER request, the proxy inserts a URI for that proxy as the topmost element in the Path header (or inserts a new topmost Path header) before proxying that request. It is also possible for a proxy with specific knowledge of network topology to add a Path element referencing another node, thereby allowing construction of a Path which is discongruent with the route taken by the REGISTER request. Such a construction is implementation specific and outside the scope of this document.

4.3 Procedures at the Registrar

If a Path header exists in a successful REGISTER request, the registrar constructs an ordered list of route elements from the nodes listed in the Path elements, preserving the order as indicated in the Path header(s). The registrar then stores this route set in association with that contact and the addres-of-record indicated in the Register request (the "binding" as defined in RFC 2543[1]). The registrar copies the Path elements list as one or more Path headers into into the successful (200 OK) REGISTER response message.

4.4 Procedures at the Home Proxy

In the common SIP model, there is a home service proxy associated with the registrar for a user. Each incoming message targeted to the public address-of-record for the user is routed to this proxy, which consults the registrar's database in order to determine the contact to which the message should be retargeted. The home service proxy, in its basic mode of operation, rewrites the request-URI from the incoming message with the value of the registered contact and retransmits the message.

With the addition of Path, the home service proxy also copies the route set associated with the specific contact in the registrar database into the Route header(s) of the outgoing message as a preloaded route. This causes the outgoing message to transit the set of proxies that indicated that they were to be used in future messages to that contact by including themselves in the Path header on the REGISTER message.

4.5 Examples of Usage

As an example, we use the scenario from the Background section:

UA----P1-----P2----P3-----REGISTRAR

UA sends a REGISTER message to REGISTRAR. This message transits its default outbound proxy P1, an intermediate proxy P2, and the firewall proxy for the home domain, P3, before reaching REGISTRAR. Due to network topology and operational policy, P1 and and P3 need to be transited by messages from REGISTRAR or other nodes in the home network targeted to UA. P2 does not. P1 and P3 have been configured to include themselves in Path headers on REGISTER messages that they process.

Message sequence for REGISTER with Path:

5. Security Considerations

There are few security considerations for this draft beyond those in SIP [2]. From a security perspective, the Path extension and its usage are identical to the Record-Route header of basic SIP. Note that the transparency of the user expectations are preserved by returning the final Path to the originating UA -- that is, the UA is informed which additional proxies have been inserted into the path for the registration associated with that response.

6. IANA Considerations

This document defines the SIP extension header name "Path", which IANA will add to the registry of SIP header names defined in [2]

7. Acknowledgements

Min Huang and Stinson Mathai, who put together the original proposal in 3GPP for this mechanism, and worked out most of the 3GPP procedures in 24.229.

Keith Drage, Bill Marshall, and Miguel Angel Garcia-Martin who argued with everybody a lot about the idea as well as helped refine the requirements.

Juha Heinanen, who argued steadfastly against standardizing the function of discovering the home service proxy with this technique in this document.

Normative References

Non-Normative References

Author's Address

Full Copyright Statement

Copyright (C) The Internet Society (2002). All Rights Reserved.

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Acknowledgement

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