Network Working Group D. Willis

Network Working Group D. Willis
Request for Comments: 3327 dynamicsoft Inc.
Category: Standards Track B. Hoeneisen
Switch
December 2002

Session Initiation Protocol (SIP) Extension Header Field
for Registering Non-Adjacent Contacts

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

The REGISTER function is used in a Session Initiation Protocol (SIP)
system primarily to associate a temporary contact address with an
address-of-record. This contact is generally in the form of a
Uniform Resource Identifier (URI), such as Contact:
<sip:[email protected]> and is generally dynamic and associated
with the IP address or hostname of the SIP User Agent (UA). The
problem is that network topology may have one or more SIP proxies
between the UA and the registrar, such that any request traveling
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 field, "Path"
which provides such a mechanism.

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Table of Contents

1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability Statement . . . . . . . . . . . . . . . . . . 3
4. Path Header Field Definition and Syntax . . . . . . . . . . 3
5. Usage of Path Header Field . . . . . . . . . . . . . . . . . 5
5.1 Procedures at the UA . . . . . . . . . . . . . . . . . . . . 5
5.2 Procedures at Intermediate Proxies . . . . . . . . . . . . . 5
5.3 Procedures at the Registrar . . . . . . . . . . . . . . . . 6
5.4 Procedures at the Home Proxy . . . . . . . . . . . . . . . . 6
5.5 Examples of Usage . . . . . . . . . . . . . . . . . . . . . 7
5.5.1 Example of Mechanism in REGISTER Transaction . . . . . . . . 7
5.5.2 Example of Mechanism in INVITE Transaction . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . 13
6.1 Considerations in REGISTER Request Processing . . . . . . . 13
6.2 Considerations in REGISTER Response Processing . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . 16
Non-Normative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
Full Copyright Statement . . . . . . . . . . . . . . . . . . 17

1. Background

3GPP established a requirement for discovering intermediate proxies
during SIP registration and published this requirement in [5].

Scenario:

UA1----P1-----P2-----P3------REGISTRAR

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

UA1 has a standing relationship with REGISTRAR. How UA1 establishes
this relationship is outside the scope of this document. UA1
discovers P1 as a result of configuration, DHCP assignment or other
similar operation, also outside the scope of this document.
REGISTRAR has a similar "default outbound proxy" relationship with
P3.

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Eventually, REGISTRAR or a "home proxy" (a proxy serving as the
terminal point for routing an address-of-record) closely related to
it will receive a request destined for UA1. It needs to know which
proxies must be transited by that request in order to get back to
UA1. 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 Path extension header field allows accumulating and transmitting
the list of proxies between UA1 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 requests. The routing
established by the Path header field mechanism applies only to
requests transiting or originating in the home domain.

2. Terminology

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 BCP 14, RFC 2119 [3].

3. 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 are visited by
registration requests from the UA to the Registrar.
2. The same intermediate proxies or a set of proxies known to the
intermediate proxies must be traversed, in reverse order, by
requests sent through a home proxy to the UA. In the simplest
form, the route between the home proxy and the UA is the exact
inverse of the route between the UA and the route between the UA
and the 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.

4. Path Header Field Definition and Syntax

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

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A Path header field MAY be inserted into a REGISTER by any SIP node
traversed by that request. Like the Route header field, sequential
Path header fields are evaluated in the sequence in which they are
present in the request, and Path header fields MAY be combined into
compound Path header in a single Path header field. 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 difference between Path and Record-Route is that Path applies to
REGISTER and 200 class responses to REGISTER. Record-Route doesn't,
and can't be defined in REGISTER for reasons of backward
compatibility. Furthermore, the vector established by Record-Route
applies only to requests within the dialog that established that
Record-Route, whereas the vector established by Path applies to
future dialogs.

The syntax for Path is defined as follows:

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

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

Note that the Path header field values conform to the syntax of a
Route element as defined in [1]. As suggested therein, such values
MUST include the loose-routing indicator parameter ";lr" for full
compliance with [1].

The allowable usage of header fields is described in Tables 2 and 3
of SIP [1]. The following additions to this table are needed for
Path.

Support for the Path header field MAY be indicated by a UA by
including the option-tag "path" in a Supported header field.

Addition of Path to SIP Table 3:

Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________
Path R ar - - - - - o
Path 2xx - - - - - - o

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5. Usage of Path Header Field

5.1 Procedures at the UA

The UA executes its register operation as usual. The response MAY
contain a Path header field. The general operation of the UA is to
ignore the Path header field in the response. It MAY choose to
display the contents of the Path header field to the user or take
other action outside the scope of this document. The Path
information in the REGISTER response lets the UA know what
intermediate proxies were added during registration. Examination of
this information may be important from a security perspective, as
such inspection might allow the UA to detect intermediate proxies
that have inappropriately added themselves.

The UA SHOULD include the option tag "path" as a header field value
in all Supported header fields, and SHOULD include a Supported header
field in all requests.

The UA MAY include a Path header field in a request. This is not
broadly applicable and caution must be taken to insure proper
function, as the Path header field inserted by the UA may have
additional Path header field values appended by intermediate proxies.
Such proxies are not aware that the Path header field value was
inserted by a UA, and will treat it as if it had been inserted by a
previously traversed proxy, which could result in unexpected routing
behavior wherein the UA is asked to act as a proxy.

5.2 Procedures at Intermediate Proxies

When a proxy processing a REGISTER request wishes to be on the path
for future requests toward the UA originating that REGISTER request,
the proxy inserts a URI for that proxy as the topmost value in the
Path header field (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 header field value
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.

Intermediate proxies SHOULD NOT add a Path header field to a request
unless the UA has indicated support for this extension with a
Supported header field value. If the UA has indicated support and
the proxy requires the registrar to support the Path extension, then
the proxy SHOULD insert a Requires header field value for this
extension. If the UA has not indicated support for the extension and
the proxy requires support for it in the registrar, the proxy SHOULD

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reject the request with a 421 response indicating a requirement for
the extension.

Proxies processing a REGISTER response SHOULD NOT alter any Path
header field values that may be present in the response. The
registrar MAY protect the Path header field in the response by
including it in a protected S/MIME body, and alterations of the Path
by an intermediate proxy can therefore be detected by the UA as man-
in-the-middle attacks. Proxies SHOULD only consider altering the
value of a Path header field in the REGISTER response if they have
the credentials to correctly alter the S/MIME body to account for the
change.

5.3 Procedures at the Registrar

If a Path header field exists in a successful REGISTER request, the
registrar constructs an ordered list of route elements (a path
vector) from the nodes listed in the Path header field values,
preserving the order as indicated in the Path header field values.
The registrar then stores this path vector in association with that
contact and the address-of-record indicated in the REGISTER request
(the "binding" as defined in [1]). The registrar copies the Path
header field values into a Path header field in the successful (200
class) REGISTER response. In the event that the home proxy and
registrar are not co-located, the registrar MAY apply a locally-
determined transformation to the stored path vector.

If a registrar receives a REGISTER request containing a Path header
field and there is no indication of support for the extension in the
UA (via a Supported header field), the registrar must rely on local
policy in determining how to treat this request. The recommended
policy is for the registrar to reject the request with a 420 "Bad
Extension" response indicating the Path extension. This approach
allows the UA to detect that an intermediate proxy has
inappropriately added a Path header field. However, the Path
mechanism should technically work in the absence of UA support (at
some compromise to security), so some registrars MAY choose to
support the extension in the absence of a Supported header field
value in the request.

5.4 Procedures at the Home Proxy

In the common SIP model, there is a home proxy associated with the
registrar for a user. Each incoming request 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 request should be retargeted. The home proxy, in its
basic mode of operation, rewrites the request-URI from the incoming

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request with the value of the registered contact and retransmits the
request.

With the addition of Path, the home proxy also copies the stored path
vector associated with the specific contact in the registrar database
into the Route header field of the outgoing request as a preloaded
route. This causes the outgoing request to transit the proxies that
were included in the Path header field of the REGISTER request.

In normal processing, the home proxy is the "terminal point" for the
user's address-of-record (AOR). Consequentially, the Route header
field on the incoming request will have been exhausted in reaching
the home proxy. If it isn't, then things get interesting. In the
most common case, the home proxy generates the outgoing Route header
field by inserting the stored path vector ahead of the Route header
field values contained in the incoming request. This procedure may be
altered by a local policy at the home proxy.

Loose routes may interact with routing policy in interesting ways.
The specifics of how the stored path vector integrates with any
locally required default route and local policy are implementation
dependent. For example, some devices will use locally-configured
explicit loose routing to reach a next-hop proxy, and others will use
a default outbound-proxy routing rule. However, for the result to
function, the combination must provide valid routing in the local
environment. In general, the stored path vector is appended to any
locally configured route needed to egress the service cluster. The
service proxy (or registrar, as noted earlier) MAY also transform the
stored path vector as needed to provide correct functionality.
Systems designers must match the Path recording policy of their nodes
with the routing policy in order to get a workable system.

5.5 Examples of Usage

Note that some header fields (e.g. Content-Length) and session
descriptions are omitted to provide a shorter and hopefully more
readable presentation. The node marked REGISTRAR is a registrar and a
proxy and serves as a home proxy. Thus, in the DNS the domain
EXAMPLEHOME.COM points to the same host as REGISTRAR.EXAMPLEHOME.COM.

5.5.1 Example of Mechanism in REGISTER Transaction

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

UA1----P1-----P2----P3-----REGISTRAR

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In this example, UA1 sends a REGISTER request to REGISTRAR. This
request 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 requests from REGISTRAR or other nodes
in the home network targeted to UA1. P2 does not. P1 and P3 have
been configured to include themselves in Path header fields on
REGISTER requests that they process. UA1 has a current IP address of
"192.0.2.4".

Message sequence for REGISTER with Path:

F1 Register UA1 -> P1

REGISTER sip:REGISTRAR.EXAMPLEHOME.COM SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
. . .

F2 Register P1 -> P2

REGISTER sip:REGISTRAR.EXAMPLEHOME.COM SIP/2.0
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: P1 has added itself to the Path.

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F3 Register P2 -> P3

REGISTER sip:REGISTRAR.EXAMPLEHOME.COM SIP/2.0
Via: SIP/2.0/UDP 178.73.76.230:5060;branch=z9hG4bKiokioukju908
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: P2 did NOT add itself to the Path.

F4 Register P3 -> REGISTRAR

REGISTER sip:REGISTRAR.EXAMPLEHOME.COM SIP/2.0
Via: SIP/2.0/UDP 19.31.97.3:5060;branch=z9hG4bKp3wer654363
Via: SIP/2.0/UDP 178.73.76.230:5060;branch=z9hG4bKiokioukju908
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: P3 added itself to the Path.

F5 REGISTRAR executes Register

REGISTRAR Stores:
For [email protected]
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>

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F6 Register Response REGISTRAR -> P3

SIP/2.0 200 OK
Via: SIP/2.0/UDP 19.31.97.3:5060;branch=z9hG4bKp3wer654363
Via: SIP/2.0/UDP 178.73.76.230:5060;branch=z9hG4bKiokioukju908
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>;tag=251077
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: The Path header field in the response is identical to the
one received in the REGISTER request.

F7 Register Response P3 -> P2

SIP/2.0 200 OK
Via: SIP/2.0/UDP 178.73.76.230:5060;branch=z9hG4bKiokioukju908
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>;tag=251077
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

F8 Register Response P2 -> P1

SIP/2.0 200 OK
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>;tag=251077
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

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F9 Register Response P1 -> UA1

SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1 <sip:[email protected]>;tag=251077
From: UA1 <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
Supported: path
Path: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

5.5.2 Example of Mechanism in INVITE Transaction

This example shows the message sequence for an INVITE transaction
originating from UA2 eventually arriving at UA1. REGISTRAR inserts a
preloaded Route toward UA1 and retargets the request by replacing the
request URI with the registered Contact. It then sends the
retargeted INVITE along the Path towards UA1. Note that this example
introduces foreign user agent UA2 (address "71.91.180.10") and
foreign domain FOREIGN.ELSEWHERE.ORG. We have extended the diagram
from the previous example by adding UA2, and by showing P2 out-of-
line indicating that it did not include itself in the path during
registration.

Scenario

UA1----P1---------P3-----REGISTRAR
| |
P2 |
|
UA2--------------------------

Message sequence for INVITE using Path:

F1 Invite UA2 -> REGISTRAR

INVITE [email protected] SIP/2.0
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=z9hG4bKe2i95c5st3R
To: UA1 <sip:[email protected]>
From: UA2 <sip:[email protected]>;tag=224497
Call-ID: [email protected]
CSeq: 29 INVITE
Contact: <sip:[email protected]>
. . .

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F2 REGISTRAR processing

REGISTRAR looks up name "[email protected]" and returns:
- Contact = <sip:[email protected]>
- Path vector = <sip:P3.EXAMPLEHOME.COM;lr>,
<sip:P1.EXAMPLEVISITED.COM;lr>

Note: The Contact replaces the request-URI. The path vector is
pushed onto the Route stack (preloaded Route) of the outgoing
INVITE request. The topmost Route is used for making the
routing decision (in conjunction with local policy).

F3 Invite REGISTRAR -> P3

INVITE [email protected] SIP/2.0
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=z9hG4bKlj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=z9hG4bKe2i95c5st3R
To: UA1 <sip:[email protected]>
From: UA2 <sip:[email protected]>;tag=224497
Call-ID: [email protected]
CSeq: 29 INVITE
Contact: <sip:[email protected]>
Route: <sip:P3.EXAMPLEHOME.COM;lr>,<sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: In this example REGISTRAR does not want to stay on the
Route and therefore does not insert a Record-Route.

F4 Invite P3 -> P1

INVITE [email protected] SIP/2.0
Via: SIP/2.0/UDP 19.31.97.3:5060;branch=z9hG4bKjasg7li7nc9e
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=z9hG4bKlj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=z9hG4bKe2i95c5st3R
To: UA1 <sip:[email protected]>
From: UA2 <sip:[email protected]>;tag=224497
Call-ID: [email protected]
CSeq: 29 INVITE
Contact: <sip:[email protected]>
Record-Route: <sip:P3.EXAMPLEHOME.COM;lr>
Route: <sip:P1.EXAMPLEVISITED.COM;lr>
. . .

Note: P3 has added a Record-Route entry, indicating that it wants
to be traversed by future messages in this dialog.

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F5 Invite P1 -> UA1

INVITE [email protected] SIP/2.0
Via: SIP/2.0/UDP 112.68.155.4:5060;branch=z9hG4bKk5l1833o43p
Via: SIP/2.0/UDP 19.31.97.3:5060;branch=z9hG4bKjasg7li7nc9e
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=z9hG4bKlj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=z9hG4bKe2i95c5st3R
To: UA1 <sip:[email protected]>
From: UA2 <sip:[email protected]>;tag=224497
Call-ID: [email protected]
CSeq: 29 INVITE
Contact: <sip:[email protected]>
Record-Route: <sip:P1.EXAMPLEVISITED.COM;lr>
Record-Route: <sip:P3.EXAMPLEHOME.COM;lr>
. . .

Note: P1 has added a Record-Route entry, indicating that it wants
to be traversed by future messages in this dialog.

6. Security Considerations

There are few security considerations for this document beyond those
in SIP [1]. From a security perspective, the Path extension and its
usage are identical to the Record-Route header field 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.

The Path header field accumulates information in a hop-by-hop manner
during REGISTER processing. The return information is essentially
end-to-end, that is, it is not altered by intermediate proxies. This
leads to two slightly different security approaches.

6.1 Considerations in REGISTER Request Processing

Information accumulated in REGISTER processing causes additional
proxies to be included in future requests between the registrar's
location and the UA. An attack that allowed an intruding proxy to
add itself to this chain would allow the attacker to intercept future
calls intended for the UA.

An attacker could conceivably alter the Path either by altering data
"on the wire" or by other manipulations (such as impersonation) that
would cause it to be included in the SIP routing chain (a "node
insertion" attack). Altering data "on the wire" may be addressed
adequately by the use of transport-layer integrity protection
mechanisms such as TLS or IPSEC. Proxy insertion can be addressed by

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mutual authentication at the proxy layer, which can also be provided
by TLS or IPSEC. The "sips:" URI class defined in [1] provides a
mechanism by which a UA may request that intermediate proxies provide
integrity protection and mutual authentication.

Systems using the Path mechanism SHOULD use appropriate mechanisms
(TLS, IPSEC, etc.) to provide message integrity and mutual
authentication. UAs SHOULD use "sips:" to request transitive
protection.

The registering UA SHOULD use S/MIME mechanisms to provide a
protected copy of the original request to the registrar. In this
case, the UA SHOULD include a Supported header field with a value
indicating support for the Path extension in the protected copy.
Registrars receiving such as request MUST honor the Path extension
only if support is indicated in the protected header field. Further,
they SHOULD compare the unprotected Supported header field with the
protected Supported header field and take appropriate action in the
event that an intermediate has altered the message to indicate
support for Path when it was not indicated by the requesting UA.

6.2 Considerations in REGISTER Response Processing

The data returned to the UA by the Path header field in the response
to the REGISTER request is there to provide openness to the UA. The
registrar is telling the UA, "These are the intermediate proxies that
will be included on future requests to you processed through me". By
inspection of this header field, the UA may be able to detect node
insertion attacks that involve inserting a proxy into the SIP routing
chain. S/MIME techniques may be used to prevent alteration of this
header field by intermediate proxies during response processing.

As specified, there is no requirement for arbitrary proxies between
the UA and the registrar to modify the Path header field in the
REGISTER response. Consequently, we may use an end-to-end protection
technique. The S/MIME technique defined in [1] provides an effective
mechanism. Using this technique, the registrar makes a copy of the
complete response, signs it, and attaches it as a body to the
response. The UA may then verify this response, assuring an
unmodified Path header field is received.

In addition to the hop-by-hop integrity protection and mutual
authentication measures suggested for REGISTER request processing in
the preceding section, systems using Path header fields SHOULD
implement end-to-end protection using S/MIME. More specifically,
registrars returning a Path header field SHOULD attach a signed
S/MIME of the response, and UAs receiving a REGISTER response
containing a Path header field SHOULD validate the message using the

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S/MIME technique. Furthermore, UAs receiving a Path header field in
a REGISTER response SHOULD render it to the user, or (where feasible)
check it programmatically.

7. IANA Considerations

This document defines the SIP extension header field "Path", which
the IANA has added to the registry of SIP header fields defined in
SIP [1].

This document also defines the SIP option tag "path" which IANA has
added to the registry of SIP option tags defined in SIP [1].

The following is the registration for the Path header field:

RFC Number: RFC3327

Header Field Name: Path

Compact Form: none

The following is the registration for the path option tag:

RFC Number: RFC3327

Option Tag: path

8. 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 proxy with this technique in this
document.

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RFC 3327 Path Extension Header Field for SIP December 2002

Normative References

[1] 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.

[2] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.

[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[4] Postel, J. and J. Reynolds, "Instructions to RFC Authors", RFC
2223, October 1997.

Non-Normative References

[5] Garcia-Martin, MA., "3GPP Requirements On SIP", Work in Progress.

[6] Mankin, A., "SIP Change Process", Work in Progress.

Authors' Addresses

Dean Willis
dynamicsoft Inc.
5100 Tennyson Parkway
Suite 1200
Plano, TX 75028
US

Phone: +1 972 473 5455
EMail: [email protected]
URI: http://www.dynamicsoft.com/

Bernie Hoeneisen
Switch
Limmatquai 138
CH-8001 Zuerich
Switzerland

Phone: +41 1 268 1515
EMail: [email protected], [email protected]
URI: http://www.switch.ch/

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Full Copyright Statement

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Acknowledgement

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