Network Working Group M. Garcia-Martin

Network Working Group M. Garcia-Martin
Request for Comments: 3455 Ericsson
Category: Informational E. Henrikson
Lucent
D. Mills
Vodafone
January 2003

Private Header (P-Header) Extensions to the Session Initiation
Protocol (SIP) for the 3rd-Generation Partnership Project (3GPP)

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

This document describes a set of private Session Initiation Protocol
(SIP) headers (P-headers) used by the 3rd-Generation Partnership
Project (3GPP), along with their applicability, which is limited to
particular environments. The P-headers are for a variety of purposes
within the networks that the partners use, including charging and
information about the networks a call traverses.

Table of Contents

1. Overall Applicability . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. SIP Private Headers . . . . . . . . . . . . . . . . . . . . . 3
4.1 The P-Associated-URI header. . . . . . . . . . . . . . . . 3
4.1.1 Applicability statement for the
P-Associated-URI header. . . . . . . . . . . . . . . 4
4.1.2 Usage of the P-Associated-URI header . . . . . . . . 4
4.2 The P-Called-Party-ID header . . . . . . . . . . . . . . . 6
4.2.1 Applicability statement for the
P-Called-Party-ID header. . . . . . . . . . . . . . . 9
4.2.2 Usage of the P-Called-Party-ID header. . . . . . . . 10
4.3 The P-Visited-Network-ID header. . . . . . . . . . . . . . 11
4.3.1 Applicability statement for the
P-Visited-Network-ID header. . . . . . . . . . . . . 11

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4.3.2 Usage of the P-Visited-Network-ID header . . . . . . 12
4.4 The P-Access-Network-Info header . . . . . . . . . . . . . 15
4.4.1 Applicability Statement for the
P-Access-Network-Info header . . . . . . . . . . . . 16
4.4.2 Usage of the P-Access-Network-Info header . . . . . 17
4.5 The P-Charging-Function-Addresses header . . . . . . . . . 18
4.5.1 Applicability Statement for the
P-Charging-Function-Addresses header . . . . . . . . 18
4.5.2 Usage of the P-Charging-Function-Addresses
headerd. . . . . . . . . . . . . . . . . . . . . . . 19
4.6 The P-Charging-Vector header . . . . . . . . . . . . . . . 21
4.6.1 Applicability Statement for the
P-Charging-Vector header . . . . . . . . . . . . . . 22
4.6.2 Usage of the P-Charging-Vector header . . . . . . . 23
5. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1 P-Associated-URI header syntax . . . . . . . . . . . . . . 25
5.2 P-Called-Party-ID header syntax. . . . . . . . . . . . . . 25
5.3 P-Visited-Network-ID header syntax . . . . . . . . . . . . 25
5.4 P-Access-Network-Info header syntax. . . . . . . . . . . . 25
5.5 P-Charging-Function-Addresses header syntax. . . . . . . . 26
5.6 P-Charging-Vector header syntax. . . . . . . . . . . . . . 26
5.7 Table of new headers . . . . . . . . . . . . . . . . . . . 27
6. Security Considerations . . . . . . . . . . . . . . . . . . . 28
6.1 P-Associated-URI . . . . . . . . . . . . . . . . . . . . . 28
6.2 P-Called-Party-ID. . . . . . . . . . . . . . . . . . . . . 28
6.3 P-Visited-Network-ID . . . . . . . . . . . . . . . . . . . 28
6.4 P-Access-Network-Info. . . . . . . . . . . . . . . . . . . 29
6.5 P-Charging-Function-Addresses. . . . . . . . . . . . . . . 30
6.6 P-Charging-Vector. . . . . . . . . . . . . . . . . . . . . 30
7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 30
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
9. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . 32
10. Normative References . . . . . . . . . . . . . . . . . . . . 32
11. Informative References . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 34

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1. Overall Applicability

The SIP extensions specified in this document make certain
assumptions regarding network topology, linkage between SIP and lower
layers, and the availability of transitive trust. These assumptions
are generally NOT APPLICABLE in the Internet as a whole. The
mechanisms specified here were designed to satisfy the requirements
specified in the 3GPP Release 5 requirements on SIP [4] for which
either no general-purpose solution was planned, where insufficient
operational experience was available to understand if a general
solution is needed, or where a more general solution is not yet
mature. For more details about the assumptions made about these
extensions, consult the Applicability subsection for each extension.

2. Conventions

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 [2].

3. Overview

The Third Generation Partnership Project (3GPP) has selected SIP as
the protocol used to establish and tear down multimedia sessions in
the context of its IP Multimedia Subsystem (IMS). (For more
information on the IMS, a detailed description can be found in 3GPP
TS 23.228 [14] and 3GPP TS 24.229 [15]). 3GPP notified the IETF SIP
and SIPPING working groups that existing SIP documents provided
almost all the functionality needed to satisfy the requirements of
the IMS, but that they required some additional functionality in
order to use SIP for this purpose. These requirements [4] are
documented in an Internet Draft which was submitted to the SIPPING
Working Group. Some of these requirements are satisfied by chartered
extensions, while other requirements were applicable to SIP, but not
sufficiently general for the SIP Working Group to adopt. This
document describes private extensions to address those requirements.
Each extension, or set of related extensions is described in its own
section below.

4. SIP Private Headers

4.1 The P-Associated-URI header

This extension allows a registrar to return a set of associated URIs
for a registered address-of-record. We define the P-Associated-URI
header field, used in the 200 OK response to a REGISTER request. The
P-Associated-URI header field transports the set of Associated URIs
to the registered address-of-record.

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An associated URI is a URI that the service provider has allocated to
a user for his own usage. A registrar contains information that
allows an address-of-record URI to be associated with zero or more
URIs. Usually, all these URIs (the address-of-record URI and the
associated URIs) are allocated for the usage of a particular user.
This extension to SIP allows the UAC to know, upon a successful
authenticated registration, which other URIs, if any, the service
provider has associated to an address-of-record URI.

Note that, generally speaking, the registrar does not register the
associated URIs on behalf of the user. Only the address-of-record
which is present in the To header field of the REGISTER is registered
and bound to the contact address. The only information conveyed is
that the registrar is aware of other URIs to be used by the same
user.

It may be possible, however, that an application server (or even the
registrar itself) registers any of the associated URIs on behalf of
the user by means of a third party registration. However, this third
party registration is out of the scope of this document. A UAC MUST
NOT assume that the associated URIs are registered.

If a UAC wants to check whether any of the associated URIs is
registered, it can do so by mechanisms specified outside this
document, e.g., the UA may send a REGISTER request with the To header
field value set to any of the associated URIs and without a Contact
header. The 200 OK response will include a Contact header with the
list of registered contact addresses. If the associated URI is not
registered, the UA MAY register it prior to its utilization.

4.1.1 Applicability statement for the P-Associated-URI header

The P-Associated-URI header is applicable in SIP networks where the
SIP provider is allocating the set of identities that a user can
claim (in headers like the From field) in requests that the UA
generates. It furthermore assumes that the provider knows the entire
set of identities that a user can legitimately claim, and that the
user is willing to restrict its claimed identities to that set. This
is in contrast to normal SIP usage, where the From field is
explicitly an end-user specified field.

4.1.2 Usage of the P-Associated-URI header

The registrar inserts the P-Associated-URI header field into the 200
OK response to a REGISTER request. The header field value is
populated with a list containing zero or more URIs that are
associated to the address-of-record.

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If the registrar supports the P-Associated-URI header extension, then
the registrar MUST always insert the P-Associated-URI header field in
all the 200 OK responses to a REGISTER request, regardless of whether
the REGISTER was an initial registration, re-registration, or
de-registration and regardless of whether there are zero or more
associated URIs.

4.1.2.1 Procedures at the UA

A UAC may receive a P-Associated-URI header field in the 200 OK
response for a REGISTER. The presence of the header field in the 200
OK response for a REGISTER request implies that the extension is
supported at the registrar.

The header value contains a list of zero or more associated URIs to
the address-of-record URI. The UAC MAY use any of the associated
URIs to populate the From header value, or any other SIP header value
that provides information of the identity of the calling party, in a
subsequent request.

The UAC MAY check whether the associated URI is registered or not.
This check can be done, e.g., by populating the To header value in a
REGISTER sent to the registrar and without a Contact header. The 200
OK response will include a Contact header with the list of registered
contact addresses. As described in SIP [1], the 200 OK response may
contain a Contact header field with zero or more values (zero meaning
the address-of-record is not registered).

4.1.2.2 Procedures at the registrar

A registrar that receives and authorizes a REGISTER request, may
associate zero or more URIs with the address-of-record.

A registrar that supports this specification MUST include a
P-Associated-URI header field in the 200 OK response to a REGISTER
request. The header MUST be populated with a comma-separated list of
SIP or SIPS URIs which are associated to the address-of-record under
registration.

In case the address-of-record under registration does not have any
other SIP or SIPS URIs associated, the registrar MUST include an
empty P-Associated-URI header value.

4.1.2.3 Procedures at the proxy

This memo does not define any procedure at the proxy.

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4.2 The P-Called-Party-ID header

A proxy server inserts a P-Called-Party-ID header, typically in an
INVITE request, en-route to its destination. The header is populated
with the Request-URI received by the proxy in the request. The UAS
identifies which address-of-record, out of several registered
address-of-records, the invitation was sent to (for example, the user
may be simultaneously using a personal and a business SIP URIs to
receive invitation to sessions). The UAS may use the information to
render different distinctive audiovisual alerting tones, depending on
the URI used to receive the invitation to the session.

Users in the 3GPP IP Multimedia Subsystem (IMS) may get one or
several SIP URIs (address-of-record) to identify the user. For
instance, a user may get a business SIP URI and a personal one. As
an example of utilization, the user may make available the business
SIP URI to co-workers and may make available the personal SIP URI to
members of the family.

At a certain point in time, both the business SIP URI and the
personal SIP URI are registered in the SIP registrar, so both URIs
can receive invitations to new sessions. When the user receives an
invitation to join a session, he/she should be aware of which of the
several registered SIP URIs this session was sent to.

This requirement is stated in the 3GPP Release 5 requirements on SIP
[4].

The problem arises during the terminating side of a session
establishment, when the SIP proxy that is serving a UA gets an
INVITE, and the SIP server retargets the SIP URI which is present in
the Request-URI field, and replaces it by the SIP URI published by
the user in the Contact header field of the REGISTER request at
registration time. When the UAS receives the SIP INVITE, it cannot
determine which address-of-record the request was sent to.

One can argue that the To header field conveys the semantics of the
called user, and therefore, this extension to SIP is not needed.
Although the To header field in SIP may convey the called party ID in
most situations, there are two particular cases when the above
assumption is not correct:

1. The session has been forwarded, redirected, etc., by previous SIP
proxies, before arriving to the proxy which is serving the called
user.

2. The UAC builds an INVITE request and the To header field is not
the same as the Request-URI.

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The problem of using the To header field is that this field is
populated by the UAC and not modified by proxies in the path. If the
UAC, for any reason, did not populate the To header field with the
address-of-record of the destination user, then the destination user
is not able to distinguish which address-of-record the session was
destined.

Another possible solution to the problem is built upon the
differentiation of the Contact header value between different
address-of-record at registration time. The UA can differentiate
each address-of-record it registers by assigning a different Contact
header value. For instance, when the UA registers the address-of-
record sip:id1, the Contact header value can be sip:id1@ua; the
registration of sip:id2 can be bound to the Contact value sip:id2@ua.

The solution described above assumes that the UA explicitly registers
each of its address-of-record URIs, and therefore, it has full
control over the contact address values assigned to each
registration. However, in the case the UA does not have full control
of its registered address-of-record, because of, e.g., a third party
registration, the solution does not work. This may be the case of
the 3GPP registration, where the UA may have previously indicated the
network, by means outside of SIP, that some other address-of-record
URIs may be automatically registered when the UA registers a
particular address-of-record. The requirement is covered in the 3GPP
Release 5 requirements on SIP [4].

In the next paragraphs we show an example of the problem, in the case
there has been some sort of call forwarding in the session, so that
the UAC is not aware of the intended destination URI in the current
INVITE.

We assume that a User Agent (UA) is registering to his proxy (P1).

Scenario UA --- P1

F1 Register UA -> P1
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:[email protected];tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>

The user also registers his personal URI to his/her registrar.

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F2 Register UA -> P1
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashdt8
To: sip:[email protected]
From: sip:[email protected];tag=346249
Call-ID: 2Q3817637684230998sdasdh10
CSeq: 1827 REGISTER
Contact: <sip:[email protected]>

Later, the proxy/registrar (P1) receives an INVITE from another proxy
(P2) destined to the user's business SIP address-of-record. We
assume that this SIP INVITE has undergone some sort of forwarding in
the past, and as such, the To header field is not populated with the
SIP URI of the user. In this case we assume that the session was
initially addressed to sip:[email protected]. The SIP
server at othernetwork.com has forwarded this session to
sip:[email protected]

Scenario UA --- P1 --- P2

F3 Invite P2 -> P1
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
To: sip:[email protected]
From: sip:[email protected];tag=938s0
Call-ID: 843817637684230998sdasdh09
CSeq: 101 INVITE

The proxy P1 retargets the user and replaces the Request-URI with the
SIP URI published during registration time in the Contact header
value.

F4 Invite P1 -> UA
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.10:5060;branch=z9hG4bKg48sh128
Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
To: sip:[email protected]
From: sip:[email protected];tag=938s0
Call-ID: 843817637684230998sdasdh09
CSeq: 101 INVITE

When the UAS receives the INVITE, it cannot determine whether it got
the session invitation due to his registration of the business or the
personal address-of-record. Neither the UAS nor proxies or
application servers can provide this user a service based on the
destination address-of-record of the session.

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We solve this problem by allowing the proxy that is responsible for
the home domain (as defined in SIP) of the user to insert a
P-Called-Party-ID header that identifies the address-of-record to
which this session is destined.

If this SIP extension is used, the proxy serving the called user will
get the message flow F5, it will populate the P-Called-Party-ID
header in message flow F6 with the contents of the Request-URI in F4.
This is show in flows F5 and F6 below:

F5 Invite P2 -> P1
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
To: sip:[email protected]
From: sip:[email protected];tag=938s0
Call-ID: 843817637684230998sdasdh09
CSeq: 101 INVITE

F6 Invite P1 -> UA
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.10:5060;branch=z9hG4bKg48sh128
Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
To: sip:[email protected]
From: sip:[email protected];tag=938s0
Call-ID: 843817637684230998sdasdh09
P-Called-Party-ID: sip:[email protected]
CSeq: 101 INVITE

When the UA receives the INVITE request F6 it can determine the
intended address-of-record of the session, and apply whatever service
is needed for that address-of-record.

4.2.1 Applicability statement for the P-Called-Party-ID header

The P-Called-Party-ID is applicable when the UAS needs to be aware of
the intended address-of-record that was present in the Request-URI of
the request, before the proxy retargets to the contact address. The
UAS may be interested in applying different audiovisual alerting
effects or other filtering services, depending on the intended
destination of the request. It is specially valuable when the UAS
has registered several address-of-record URIs to his registrar, and
therefore, the UAS is not aware of the address-of-record that was
present in the INVITE request when it hit his proxy/registrar, unless
this extension is used.

Requirements for a more general solution are proposed in [12], but
have not been adopted by SIP, nor a solution has been developed.

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4.2.2 Usage of the P-Called-Party-ID header

The P-Called-Party-ID header field provides proxies and the UAS with
the address-of-record that was present in the Request-URI of the
request, before a proxy retargets the request. This information is
intended to be used by subsequent proxies in the path or by the UAS.

Typically, a SIP proxy inserts the P-Called-Party-ID header prior to
retargetting the Request-URI in the SIP request. The header value is
populated with the contents of Request-URI, prior to replacing it
with the Contact address.

4.2.2.1 Procedures at the UA

A UAC MUST NOT insert a P-Called-Party-ID header field in any SIP
request or response.

A UAS may receive a SIP request that contains a P-Called-Party-ID
header field. The header will be populated with the address-of-
record received by the proxy in the Request-URI of the request, prior
to its forwarding to the UAS.

The UAS may use the value in the P-Called-Party-ID header field to
provide services based on the called party URI, such as, e.g.,
filtering of calls depending on the date and time, distinctive
presentation services, distinctive alerting tones, etc.

4.2.2.2 Procedures at the proxy

A proxy that has access to the Contact information of the user, MAY
insert a P-Called-Party-ID header field in any of the requests
indicated in the Table 1 (Section 5.7). The proxy MUST populate the
header value with the contents of the Request-URI present in the SIP
request that the proxy received.

It is necessary that the proxy which inserts the P-Called-Party-ID
header has information about the user, in order to prevent a wrong
delivery of the called party ID. This information may have been
learned through a registration process, for instance.

A proxy or application server that receives a request containing a
P-Called-Party-ID header may use the contents of the header to
provide a service to the user based on the URI of that header value.

A SIP proxy MUST NOT insert a P-Called-Party-ID header in REGISTER
requests.

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4.3 The P-Visited-Network-ID header

3GPP networks are composed of a collection of so called home
networks, visited networks and subscribers. A particular home
network may have roaming agreements with one or more visited
networks. This has the effect that when a mobile terminal is
roaming, it can use resources provided by the visited network in a
transparent fashion.

One of the conditions for a home network to accept the registration
of a UA roaming to a particular visited network, is the existence of
a roaming agreement between the home and the visited network. There
is a need to indicate to the home network which one is the visited
network that is providing services to the roaming UA.

3GPP user agents always register to the home network. The REGISTER
request is proxied by one or more proxies located in the visited
network towards the home network. For the sake of a simple approach,
it seems sensible that the visited network includes an identification
that is known at the home network. This identification should be
globally unique, and takes the form of a quoted text string or a
token. The home network may use this identification to verify the
existence of a roaming agreement with the visited network, and to
authorize the registration through that visited network.

4.3.1 Applicability statement for the P-Visited-Network-ID header

The P-Visited-Network-ID is applicable whenever the following
circumstances are met:

1. There is transitive trust in intermediate proxies between the UA
and the home network proxy via established relationships between
the home network and the visited network, and generally supported
by the use of standard security mechanisms, e.g., IPsec, AKA, or
TLS.

2. An endpoint is using resources provided by one or more visited
networks (a network to which the user does not have a direct
business relationship).

3. A proxy that is located in one of the visited networks wants to be
identified at the user's home network.

4. There is no requirement that every visited network needs to be
identified at the home network. Those networks that want to be
identified make use of the extension defined in this document.
Those networks that do not want to be identified do nothing.

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5. A commonly pre-agreed text string or token identifies the visited
network at the home network.

6. The UAC sends a REGISTER or dialog-initiating request (e.g.,
INVITE) or a standalone request outside a dialog (e.g., OPTIONS)
to a proxy in a visited network.

7. The request traverses, en route to its destination, a first proxy
located in the visited network, and a second proxy located in the
home network or its destination is the registrar in the home
network.

8. The registrar or home proxy verifies and authorizes the usage of
resources (e.g., proxies) in the visited network.

4.3.2 Usage of the P-Visited-Network-ID header

The P-Visited-Network-ID header field is used to convey to the
registrar or home proxy in the home network the identifier of a
visited network. The identifier is a text string or token that is
known by both the registrar or the home proxy at the home network and
the proxies in the visited network.

Typically, the home network authorizes the UA to roam to a particular
visited network. This action requires an existing roaming agreement
between the home and the visited network.

While it is possible for a home network to identify one or more
visited networks by inspecting the domain name in the Via header
fields, this approach has a heavy dependency on DNS. It is an option
for a proxy to populate the via header with an IP address, for
example, and in the absence of a reverse DNS entry, the IP address
will not convey the desired information.

Any SIP proxy that receives any of the requests indicated in Table 1
(Section 5.7) MAY insert a P-Visited-Network-ID header when it
forwards the request. In case a REGISTER or other request is
traversing different administrative domains (e.g., different visited
networks), a SIP proxy MAY insert a new P-Visited-Network-ID header
if the request does not contain a P-Visited-Network-ID header with
the same network identifier as its own network identifier (e.g., if
the request has traversed other different administrative domains).

Note also that, there is not requirement for the header value to be
readable in the proxies. Therefore, a first proxy may insert an
encrypted header that only the registrar can decrypt. If the request
traverses a second proxy located in the same administrative domain as
the first proxy, the second proxy may not be able to read the

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contents of the P-Visited-Network-ID header. In this situation, the
second proxy will consider that its visited network identifier is not
already present in the value of the header, and therefore, it will
insert a new P-Visited-Network-ID header value (hopefully with the
same identifier that the first proxy inserted, although perhaps, not
encrypted). When the request arrives at the registrar or proxy in
the home network, it will notice that the header value is repeated
(both the first and the second proxy inserted it). The decrypted
values should be the same, because both proxies where part of the
same administrative domain. While this situation is not desirable,
it does not create any harm at the registrar or proxy in the home
network.

The P-Visited-Network-ID is normally used at registration. However,
this extension does not preclude other usages. For instance, a proxy

located in a visited network that does not maintain registration
state may insert a P-Visited-Network-ID header into any standalone
request outside a dialog or a request that creates a dialog. At the
time of writing this document, the only requests that create dialogs
are INVITE [1], SUBSCRIBE [6] and REFER [11].

In order to avoid conflicts with identifiers, especially when the
number of roaming agreements between networks increase, care must be
taken when selecting the value of the P-Visited-Network-ID. The
identifier should be a globally unique to avoid duplications.
Although there are many mechanism to create globally unique
identifiers across networks, one of such as mechanisms is already in
operation, and that is DNS. The P-Visited-Network-ID does not have
any connection to DNS, but the values in the header can be chosen
from the own DNS entry representing the domain name of the network.
This guarantees the uniqueness of the value.

4.3.2.1 Procedures at the UA

User agent clients SHOULD NOT insert a P-Visited-Network-ID header in
any SIP message.

4.3.2.2 Procedures at the registrar and proxy

A SIP proxy which is located in a visited network MAY insert a
P-Visited-Network-ID header field in any of the requests indicated in
the Table 1 (Section 5.7). The header MUST be populated with the
contents of a text string or a token that identifies the
administrative domain of the network where the proxy is operating at
the user's home network.

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A SIP proxy or registrar which is located in the home network may use
the contents of the P-Visited-Network-ID as an identifier of one or
more visited networks that the request traversed. The proxy or
registrar in the home network may take local policy driven actions
based on the existence or not of a roaming agreement between the home
and the visited networks. This means, for instance, authorize the
actions of the request based on the contents of the
P-Visited-Network-ID header.

A SIP proxy which is located in the home network MUST delete this
header when forwarding the message outside the home network
administrative domain, in order to retain the user's privacy.

A SIP proxy which is located in the home network SHOULD delete this
header when the home proxy has used the contents of the header or the
request is routed based on the called party, even when the request is
not forwarded outside the home network administrative domain.

4.3.2.3 Examples of Usage

We present example in the context of the scenario presented in the
following network diagram:

Scenario UA --- P1 --- P2 --- REGISTRAR

This example shows the message sequence for an REGISTER transaction
originating from UA1 eventually arriving at REGISTRAR. P1 is an
outbound proxy for UA1. In this case P1 also inserts the
P-Visited-Network-ID header. P1 then routes the REGISTER request to
the Registrar via P2.

Message sequence for REGISTER using P-Visited-Network-ID header:

F1 Register UA -> P1
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:[email protected];tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>

In flow F2, proxy P2 adds its own identifier to the
P-Visited-Network-ID header.

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F2 Register P1 -> P2
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP p1.visited.net;branch=z9hG4bK203igld
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashdt8
To: sip:[email protected]
From: sip:[email protected];tag=346249
Call-ID: 2Q3817637684230998sdasdh10
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
P-Visited-Network-ID: "Visited network number 1"

Finally, in flow F3, proxy P2 decides to insert his own identifier,
derived from its own domain name.

F3 Register P2 -> REGISTRAR
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/UDP p2.other.net;branch=z9hG4bK2bndnvk
Via: SIP/2.0/UDP p1.visited.net;branch=z9hG4bK203igld
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashdt8
To: sip:[email protected]
From: sip:[email protected];tag=346249
Call-ID: 2Q3817637684230998sdasdh10
CSeq: 1826 REGISTER
Contact: <sip:[email protected]>
P-Visited-Network-ID: other.net, "Visited network number 1"

4.4 The P-Access-Network-Info header

This section describes the P-Access-Network-Info header. This header
is useful in SIP-based networks that also provide layer 2/layer 3
connectivity through different access technologies. SIP User Agents
may use this header to relay information about the access technology
to proxies that are providing services. The serving proxy may then
use this information to optimize services for the UA. For example, a
3GPP UA may use this header to pass information about the access
network such as radio access technology and radio cell identity to
its home service provider.

For the purpose of this extension, we define an access network as the
network providing the layer 2/layer 3 IP connectivity which in turn
provides a user with access to the SIP capabilities and services
provided.

In some cases, the SIP server that provides the user with services
may wish to know information about the type of access network that
the UA is currently using. Some services are more suitable or less

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suitable depending on the access type, and some services are of more
value to subscribers if the access network details are known by the
SIP proxy which provides the user with services.

In other cases, the SIP server that provides the user with services
may simply wish to know crude location information in order to
provide certain services to the user. For example, many of the
location based services available in wireless networks today require
the home network to know the identity of the cell the user is being
served by.

Some regulatory requirements exist mandating that for cellular radio
systems, the identity of the cell where an emergency call is
established is made available to the emergency authorities.

The SIP server that provides services to the user may desire
knowledge about the access network. This is achieved by defining a
new private SIP extension header, P-Access-Network-Info. This header
carries information relating to the access network between the UAC
and its serving proxy in the home network.

4.4.1 Applicability Statement for the P-Access-Network-Info header

This mechanism is appropriate in environments where SIP services are
dependent on SIP elements knowing details about the IP and lower
layer technologies used by a UA to connect to the SIP network.
Specifically, the extension requires that the UA know the access
technology it is using, and that a proxy desires such information to
provide services. Generally, SIP is built on the "Everything over IP
and IP over everything" principle, where the access technology is not
relevant for the operation of SIP. Since SIP systems generally
should not care or even know about the access technology, this SIP
extension is not for general SIP usage.

The information revealed in the P-Access-Network-Info header is
potentially very sensitive. Proper protection of this information
depends on the existence of specific business and security
relationships amongst the proxies that will see SIP messages
containing this header. It also depends on explicit knowledge of the
UA of the existence of those relationships. Therefore, this
mechanism is only suitable in environments where the appropriate
relationships are in place, and the UA has explicit knowledge that
they exist.

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4.4.2 Usage of the P-Access-Network-Info header

When a UA generates a SIP request or response which it knows is going
to be securely sent to its SIP proxy that is providing services, the
UA inserts a P-Access-Network-Info header into the SIP message. This
header contains information on the access network that the UA is
using to get IP connectivity. The header is typically ignored by
intermediate proxies between the UA and the SIP proxy that is
providing services. The proxy providing services can inspect the
header and make use of the information contained there to provide
appropriate services, depending on the value of the header. Before
proxying the request onwards, this proxy strips the header from the
message.

4.4.2.1 UA behavior

A UA that supports this extension and is willing to disclose the
related parameters MAY insert the P-Access-Network-Info header in any
SIP request or response.

The UA inserting this information MUST trust the proxy that is
providing services to protect its privacy by deleting the header
before forwarding the message outside of the proxy's domain. This
proxy is typically located in the home network.

In order to do the deletion of the header, there must also be a
transitive trust in intermediate proxies between the UA and the proxy
that provides the services. This trust is established by business
agreements between the home network and the access network, and
generally supported by the use of standard security mechanisms, e.g.,
IPsec, AKA, and TLS.

4.4.2.2 Proxy behavior

A proxy MUST NOT insert or modify the value of the
P-Access-Network-Info header.

A proxy which is providing services to the UA, may act upon any
information present in the P-Access-Network-Info header value, if is
present, to provide a different service depending on the network or
the location through which the UA is accessing the server. For
example, for cellular radio access networks the SIP proxy located in
the home network may use the cell ID to provide basic localized
services.

A proxy that provides services to the user, the proxy typically
located in the home network, and therefore trusted, MUST delete the
header when the SIP signaling is forwarded to a SIP server located in

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a non-trusted administrative network domain. The SIP server
providing services to the UA uses the access network information and
is of no interest to other proxies located in different
administrative domains.

4.5 The P-Charging-Function-Addresses header

3GPP has defined a distributed architecture that results in multiple
network entities becoming involved in providing access and services.
There is a need to inform each SIP proxy involved in a transaction
about the common charging functional entities to receive the
generated charging records or charging events.

The solution provided by 3GPP is to define two types of charging
functional entities: Charging Collection Function (CCF) and Event
Charging Function (ECF). CCF is used for off-line charging (e.g.,
for postpaid account charging). ECF is used for on-line charging
(e.g., for pre-paid account charging). There may be more than a
single instance of CCF and ECF in a network, in order to provide
redundancy in the network. In case there are more than a single
instance of either the CCF or the ECF addresses, implementations
SHOULD attempt sending the charging data to the ECF or CCF address,
starting with the first address of the sequence (if any) in the
P-Charging-Function-Addresses header. The CCF and ECF addresses may
be passed during the establishment of a dialog or in a standalone
transaction. More detailed information about charging can be found
in 3GPP TS 32.200 [16] and 3GPP TS 32.225 [17].

We define the SIP private header P-Charging-Function-Addresses. A
proxy MAY include this header, if not already present, in either the
initial request or response for a dialog, or in the request and
response of a standalone transaction outside a dialog. Only one
instance of the header MUST be present in a particular request or
response.

The mechanisms by which a SIP proxy collects the values to populate
the P-Charging-Function-Addresses header values are outside the scope
of this document. However, as an example, a SIP proxy may have
preconfigured these addresses, or may obtain them from a subscriber
database.

4.5.1 Applicability Statement for the P-Charging-Function-Addresses
header

The P-Charging-Function-Addresses header is applicable within a
single private administrative domain where coordination of charging
is required, for example, according to the architecture specified in
3GPP TS 32.200 [16].

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The P-Charging-Function-Addresses header is not included in a SIP
message sent outside of the own administrative domain. The header is
not applicable if the administrative domain does not provide a
charging function.

The P-Charging-Function-Addresses header is applicable whenever the
following circumstances are met:

1. A UA sends a REGISTER or dialog-initiating request (e.g., INVITE)
or a standalone transaction request outside a dialog to a proxy
located in the administrative domain of a private network.

2. A registrar, proxy or UA that is located in the administrative
domain of the private network wants to generate charging records.

3. A registrar, proxy or UA that is located in the private network
has access to the addresses of the charging function entities for
that network.

4. There are other proxies located in the same administrative domain
of the private network, that are generated charging records or
charging events. The proxies want to send, by means outside SIP,
the charging information to the same charging collecting entities
than the first proxy.

4.5.2 Usage of the P-Charging-Function-Addresses header

A SIP proxy that receives a SIP request may insert a
P-Charging-Function-Addresses header prior to forwarding the request,
if the header was not already present in the SIP request. The header
value contains one or more parameters that contain the hostnames or
IP addresses of the nodes that are willing to receive charging
information.

A SIP proxy that receives a SIP request that includes a
P-Charging-Function-Addresses may use the hostnames or IP addresses
included in the value, as the destination of charging information or
charging events. The means to send those charging information or
events are outside the scope of this document, and usually, do not
use SIP for that purpose.

4.5.2.1 Procedures at the UA

This document does not specify any procedure at the UA, with regard
to the P-Charging-Function-Addresses header. UAs need not understand
this header.

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However, it might be possible that a UA is located within the
administrative domain of a private network (e.g., a PSTN gateway, or
conference mixer), and it may have access to the addresses of the
charging entities. In this cases, a UA MAY insert the
P-Charging-Function-Addresses header in a SIP request or response
when the next hop for the message is a proxy located in the same
administrative domain.

4.5.2.2 Procedures at the Proxy

A SIP proxy that supports this extension and receives a request or
response without the P-Charging-Function-Addresses MAY insert a
P-Charging-Function-Addresses header prior to forwarding the message.
The header is populated with a list of the addresses of one or more
charging entities where the proxy should send charging related
information.

If a proxy that supports this extension receives a request or
response with the P-Charging-Function-Addresses, it may retrieve the
information from the header value to use with application specific
logic, i.e., charging. If the next hop for the message is within the
administrative domain of the proxy, then the proxy SHOULD include the
P-Charging-Function-Addresses header in the outbound message.
However, if the next hop for the message is outside the
administrative domain of the proxy, then the proxy MUST remove the
P-Charging-Function-Addresses header.

4.5.2.3 Examples of Usage

We present example in the context of the scenario presented in the
following network diagram:

Scenario UA1 --- P1 --- P2 --- UA2

In the scenario we assume that P1 and P2 belong to the same
administrative domain.

The example below shows the message sequence for an INVITE
transaction originating from UA1 eventually arriving at UA2. P1 is
an outbound proxy for UA1. In this case P1 also inserts charging
information. P1 then routes the call via P2 to UA2.

Message sequence for INVITE using P-Charging-Function-Addresses:

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F1 Invite UA1 -> P1
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:[email protected];tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 18 INVITE
Contact: sip:[email protected]

F2 Invite P1 -> P2
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP p1.home1.net:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:ua1home1.net;tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 18 INVITE
Contact: sip:[email protected]
P-Charging-Function-Addresses: ccf=192.1.1.1; ccf=192.1.1.2;
ecf=192.1.1.3; ecf=192.1.1.4

Now both P1 and P2 are aware of the IP addresses of the entities that
collect charging record or charging events. Both proxies can send
the charging information to the same entities.

4.6 The P-Charging-Vector header

3GPP has defined a distributed architecture that results in multiple
network entities becoming involved in providing access and services.
Operators need the ability and flexibility to charge for the access
and services as they see fit. This requires coordination among the
network entities (e.g., SIP proxies), which includes correlating
charging records generated from different entities that are related
to the same session.

The correlation information includes, but it is not limited to, a
globally unique charging identifier that makes easy the billing
effort.

A charging vector is defined as a collection of charging information.
The charging vector may be filled in during the establishment of a
dialog or standalone transaction outside a dialog. The information
inside the charging vector may be filled in by multiple network
entities (including SIP proxies) and retrieved by multiple network
entities. There are three types of correlation information to be
transferred: the IMS Charging Identity (ICID) value, the address of
the SIP proxy that creates the ICID value, and the Inter Operator
Identifiers (IOI).

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ICID is a charging value that identifies a dialog or a transaction
outside a dialog. It is used to correlate charging records. ICID
MUST be a globally unique value. One way to achieve globally
uniqueness is to generate the ICID using two components: a locally
unique value and the host name or IP address of the SIP proxy that
generated the locally unique value.

The IOI identifies both the originating and terminating networks
involved in a SIP dialog or transaction outside a dialog. There may
an IOI generated from each side of the dialog to identify the network
associated with each side.

There is also expected to be access network charging information,
which consists of network specific identifiers for the access level
(e.g., UMTS radio access network or IEEE 802.11b). The details of
the information for each type of network are not described in this
memo.

We define the SIP private header P-Charging-Vector. A proxy MAY
include this header, if not already present, in either the initial
request or response for a dialog, or in the request and response of a
standalone transaction outside a dialog. Only one instance of the
header MUST be present in a particular request or response.

The mechanisms by which a SIP proxy collects the values to populate
in the P-Charging-Vector are outside the scope of this document.

4.6.1 Applicability Statement for the P-Charging-Vector header

The P-Charging-Vector header is applicable within a single private
administrative domain or between different administrative domains
where there is a trust relationship between the domains.

The P-Charging-Vector header is not included in a SIP message sent to
another network if there is no trust relationship. The header is not
applicable if the administrative domain manages charging in a way
that does not require correlation of records from multiple network
entities (e.g., SIP proxies).

The P-Charging-Vector header is applicable whenever the following
circumstances are met:

1. A UA sends a REGISTER or dialog-initiating request (e.g., INVITE)
or a standalone transaction request outside a dialog to a proxy
located in the administrative domain of a private network.

2. A registrar, proxy or UA that is located in the administrative
domain of the private network wants to generate charging records.

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3. A proxy or UA that is located in the administrative domain of the
private network has access to the charging correlation information
for that network.

4. Optionally, a registrar, proxy or UA that is part of a second
administrative domain in another private network, whose SIP
request and responses are traversed through, en-route to the first
private network, wants to generate charging records and correlate
those records with those of the first private network. This
assumes that there is a trust relationship between both private
networks.

4.6.2 Usage of the P-Charging-Vector header

The P-Charging-Vector header is used to convey charging related
information, such as the globally unique IMS charging identifier
(ICID) value.

Typically, a SIP proxy that receives a SIP request that does not
contain a P-Charging-Vector header may insert it, with those
parameters that are available at the SIP proxy.

A SIP proxy that receives a SIP request that contains a
P-Charging-Vector header may use the values, such as the globally
unique ICID, to produce charging records.

4.6.2.1 Procedures at the UA

This document does not specify any procedure at the UA, with regard
to the P-Charging-Vector header. UAs need not understand this
header.

4.6.2.2 Procedures at the Proxy

A SIP proxy that supports this extension and receives a request or
response without the P-Charging-Vector header MAY insert a
P-Charging-Vector header prior to forwarding the message. The header
is populated with one ore more parameters, as described in the
syntax, including but not limited to, a globally unique charging
identifier.

If a proxy that supports this extension receives a request or
response with the P-Charging-Vector header, it may retrieve the
information from the header value to use with application specific
logic, i.e., charging. If the next hop for the message is within the
trusted domain, then the proxy SHOULD include the P-Charging-Vector

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header in the outbound message. If the next hop for the message is
outside the trusted domain, then the proxy MAY remove the
P-Charging-Function-Addresses header.

Per local application specific logic, the proxy MAY modify the
contents of the P-Charging-Vector header prior to sending the
message.

4.6.2.3 Examples of Usage

We present example in the context of the scenario presented in the
following network diagram:

Scenario UA1 --- P1 --- P2 --- UA2

This example shows the message sequence for an INVITE transaction
originating from UA1 eventually arriving at UA2. P1 is an outbound
proxy for UA1. In this case P1 also inserts charging information.
P1 then routes the call via P2 to UA2.

Message sequence for INVITE using P-Charging-Vector:

F1 Invite UA1 -> P1
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:[email protected];tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 18 INVITE
Contact: sip:[email protected]

F2 Invite P1 -> P2
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP P1.home1.net:5060;branch=z9hG4bK34ghi7a
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: sip:[email protected]
From: sip:[email protected];tag=456248
Call-ID: 843817637684230998sdasdh09
CSeq: 18 INVITE
Contact: sip:[email protected]
P-Charging-Vector: icid-value=1234bc9876e;
icid-generated-at=192.0.6.8;
orig-ioi=home1.net

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5. Formal Syntax

All of the mechanisms specified in this document are described in
both prose and an augmented Backus-Naur Form (BNF) defined in RFC
2234 [3]. Further, several BNF definitions are inherited from SIP
and are not repeated here. Implementors need to be familiar with the
notation and contents of SIP [1] and RFC 2234 [3] to understand this
document.

5.1 P-Associated-URI header syntax

The syntax of the P-Associated-URI header is described as follows:

P-Associated-URI = "P-Associated-URI" HCOLON
(p-aso-uri-spec)
*(COMMA p-aso-uri-spec)
p-aso-uri-spec = name-addr *(SEMI ai-param)
ai-param = generic-param

5.2 P-Called-Party-ID header syntax

The syntax of the P-Called-Party-ID header is described as follows:

P-Called-Party-ID = "P-Called-Party-ID" HCOLON
called-pty-id-spec
called-pty-id-spec = name-addr *(SEMI cpid-param)
cpid-param = generic-param

5.3 P-Visited-Network-ID header syntax

The syntax of the P-Visited-Network-ID header is described as
follows:

P-Visited-Network-ID = "P-Visited-Network-ID" HCOLON
vnetwork-spec
*(COMMA vnetwork-spec)
vnetwork-spec = (token / quoted-string)
*(SEMI vnetwork-param)
vnetwork-param = generic-param

5.4 P-Access-Network-Info header syntax

The syntax of the P-Access-Network-Info header is described as
follows:

P-Access-Network-Info = "P-Access-Network-Info" HCOLON
access-net-spec
access-net-spec = access-type *(SEMI access-info)

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access-type = "IEEE-802.11a" / "IEEE-802.11b" /
"3GPP-GERAN" / "3GPP-UTRAN-FDD" /
"3GPP-UTRAN-TDD" /
"3GPP-CDMA2000" / token
access-info = cgi-3gpp / utran-cell-id-3gpp /
extension-access-info
extension-access-info = gen-value
cgi-3gpp = "cgi-3gpp" EQUAL
(token / quoted-string)
utran-cell-id-3gpp = "utran-cell-id-3gpp" EQUAL
(token / quoted-string)

The access-info may contain additional information relating to the
access network. The values for "cgi-3gpp" and "utran-cell-id-3gpp"
are defined in 3GPP TS 24.229 [15].

5.5 P-Charging-Function-Addresses header syntax

The syntax for the P-Charging-Function-Addresses header is described
as follows:

P-Charging-Addr = "P-Charging-Function-Addresses" HCOLON
charge-addr-params
*(SEMI charge-addr-params)
charge-addr-params = ccf / ecf / generic-param
ccf = "ccf" EQUAL gen-value
ecf = "ecf" EQUAL gen-value

5.6 P-Charging-Vector header syntax

The syntax for the P-Charging-Vector header is described as
follows:

P-Charging-Vector = "P-Charging-Vector" HCOLON icid-value
*(SEMI charge-params)
charge-params = icid-gen-addr / orig-ioi /
term-ioi / generic-param
icid-value = "icid-value" EQUAL gen-value
icid-gen-addr = "icid-generated-at" EQUAL host
orig-ioi = "orig-ioi" EQUAL gen-value
term-ioi = "term-ioi" EQUAL gen-value

The P-Charging-Vector contains icid-value mandatory parameter. The
icid-value represents the IMS charging ID, and contains an identifier
used for correlating charging records and events. The first proxy
that receives the request generates this value.

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The icid-gen-addr parameter contains the host name or IP address of
the proxy that generated the icid-value.

The orig-ioi and term-ioi parameters represent, respectively, the
originating and terminating interoperator identifiers. They are used
to correlate charging records between different operators. The
originating ioi represents the network responsible for the charging
records in the originating part of the session or standalone request.
Similarly, the terminating ioi represents the network responsible for
the charging records in the terminating part of the session or
standalone request.

5.7 Table of new headers

Table 1 extends the headers defined in this document to Table 2 in
SIP [1], section 7.1 of the SIP-specific event notification [6],
tables 1 and 2 in the SIP INFO method [8], tables 1 and 2 in
Reliability of provisional responses in SIP [7], tables 1 and 2 in
the SIP UPDATE method [9], tables 1 and 2 in the SIP extension for
Instant Messaging [10], and table 1 in the SIP REFER method [11]:

Header field where proxy ACK BYE CAN INV OPT REG
___________________________________________________________
P-Associated-URI 2xx - - - - - o
P-Called-Party-ID R amr - - - o o -
P-Visited-Network-ID R ad - - - o o o
P-Access-Network-Info dr - o - o o o
P-Charging-Vector admr - o - o o o
P-Charging-Function- adr - o - o o o
Addresses

Header field SUB NOT PRA INF UPD MSG REF
___________________________________________________________
P-Associated-URI - - - - - - -
P-Called-Party-ID o - - - - o o
P-Visited-Network-ID o - - - - o o
P-Access-Network-Info o o o o o o o
P-Charging-Vector o o o o o o o
P-Charging-Function- o o o o o o o
Addresses

Table 1: Header field support

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6. Security Considerations

6.1 P-Associated-URI

The information returned in the P-Associated-URI header is not viewed
as particularly sensitive. Rather, it is simply informational in
nature, providing openness to the UAC with regard to the automatic
association performed by the registrar. If end-to-end protection is
not used at the SIP layer, it is possible for proxies between the
registrar and the UA to modify the contents of the header value.
This attack, while potentially annoying, should not have significant
impacts.

The lack of encryption, either end-to-end or hop-by-hop, may lead to
leak some privacy regarding the list of authorized identities. For
instance, a user who registers an address-of-record of
sip:[email protected] may get another SIP URI associated as
sip:[email protected] returned in the P-Associated-URI header
value. An eavesdropper could collect this information. If the user
does not want to disclose the associated URIs, the eavesdropper could
have gain access to private URIs. Therefore it is RECOMMENDED that
this extension is used in a secured environment, where encryption of
SIP messages is provided either end-to-end or hop-by-hop.

6.2 P-Called-Party-ID

Due to the nature of the P-Called-Party-ID header, this header does
not introduce any significant security concern. It is possible for
an attacker to modify the contents of the header. However, this
modification will not cause any harm to the session establishment.

An eavesdropper may collect the list of identities a user is
registered. This may have privacy implications. To mitigate this
problem, this extension SHOULD only be used in a secured environment,
where encryption of SIP messages is provided either end-to-end or
hop-by-hop.

6.3 P-Visited-Network-ID

The P-Visited-Network-ID header assumes that there is trust
relationship between a home network and one or more transited visited
networks. It is possible for other proxies between the proxy in the
visited network that inserts the header, and the registrar or the
home proxy, to modify the value of P-Visited-Network-ID header.
Therefore intermediaries participating in this mechanism MUST apply a
hop-by-hop integrity protection mechanism such us IPsec or other
available mechanisms in order to prevent such attacks.

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6.4 P-Access-Network-Info

A Trust Domain is formally defined in the Short term requirements for
Network Asserted Identity [13] document. For the purpose of this
document, we refer to the 3GPP trust domain as the collection of SIP
proxies and application servers that are operated by a 3GPP network
operator and are compliant with the requirements expressed in 3GPP TS
24.229 [15].

This extension assumes that the access network is trusted by the UA
(because the UA's home network has a trust relationship with the
access network), as described earlier in this document.

This extension assumes that the information added to the header by
the UAC should be sent only to trusted entities and should not be
used outside of the trusted administrative network domain.

The SIP proxy that provides services to the user, utilizes the
information contained in this header to provide additional services
and UAs are expected to provide correct information. However, there
are no security problems resulting from a UA inserting incorrect
information. Networks providing services based on the information
carried in the P-Access-Network-Info header will therefore need to
trust the UA sending the information. A rogue UA sending false
access network information will do no more harm than to restrict the
user from using certain services.

The mechanism provided in this document is designed primarily for
private systems like 3GPP. Most security requirements are met by way
of private standardized solutions.

For instance, 3GPP will use the P-Access-Network-Info header to carry
relatively sensitive information like the cell ID. Therefore the
information MUST NOT be sent outside of the 3GPP domain.

The UA is aware - if it is a 3GPP UA - that it is operating within a
trusted domain.

The 3GPP UA is aware of whether or not a secure association to the
home network domain for transporting SIP signaling, is currently
available, and as such the sensitive information carried in the
P-Access-Network-Info header SHOULD NOT be sent in any initial
unauthenticated and unprotected requests (e.g., REGISTER).

Any UA that is using this extension and is not part of a private
trusted domain should not consider the mechanism as secure and as
such SHOULD NOT send sensitive information in the
P-Access-Network-Info header.

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Any proxy that is operating in a private trust domain where the
P-Access-Network-Info header is supported is required to delete the
header, if it is present, from any message prior to forwarding it
outside of the trusted domain.

Therefore, a network that requires its UA to send information in the
P-Access-Network-Info header must ensure that either that information
is not of a sensitive nature or that the information is not sent
outside of the trust domain.

A proxy receiving a message containing the P-Access-Network-Info
header from a non-trusted entity is not able to guarantee the
validity of the contents.

6.5 P-Charging-Function-Addresses

It is expected as normal behavior that proxies within a closed
network will modify the values of the P-Charging-Function-Addresses
and insert it into a SIP request or response. However, these proxies
that share this information MUST have a trust relationship.

If an untrusted entity were inserted between trusted entities, it
could potentially substitute a different charging function address.
Therefore, an integrity protection mechanism such as IPsec or other
available mechanisms MUST be applied in order to prevent such
attacks. Since each trusted proxy may need to view or modify the
values in the P-Charging-Function-Addresses header, the protection
should be applied on a hop-by-hop basis.

6.6 P-Charging-Vector

It is expected as normal behavior that proxies within a closed
network will modify the values of the P-Charging-Vector and insert it
into a SIP request or response. However, these proxies that share
this information MUST have a trust relationship.

If an untrusted entity were inserted between trusted entities, it
could potentially interfere with the charging correlation mechanism.
Therefore, an integrity protection mechanism such as IPsec or other
available mechanisms MUST be applied in order to prevent such
attacks. Since each trusted proxy may need to view or modify the
values in the P-Charging-Vector header, the protection should be
applied on a hop-by-hop basis.

7. IANA Considerations

This document defines several private SIP extension header fields
(beginning with the prefix "P-" ).

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These extension headers have been included in the registry of SIP
header fields defined in SIP [1]. Expert review as required for this
process was provided by the SIP Working Group.

The following extensions are registered as private extension header
fields:

RFC Number: RFC3455
Header Field Name: P-Associated-URI
Compact Form: none

RFC Number: RFC3455
Header Field Name: P-Called-Party-ID
Compact Form: none

RFC Number: RFC3455
Header Field Name: P-Visited-Network-ID
Compact Form: none

RFC Number: RFC3455
Header Field Name: P-Access-Network-Info
Compact Form: none

RFC Number: RFC3455
Header Field Name: P-Charging-Function-Addresses
Compact Form: none

RFC Number: RFC3455
Header Field Name: P-Charging-Vector
Compact Form: none

8. Contributors

The extensions described in this document were originally specified
in several documents. Miguel Garcia-Martin authored the
P-Associated-URI, P-Called-Party-ID, and P-Visited-Network-ID
headers. Duncan Mills authored the P-Access-Network-Info header.
Eric Henrikson authored the P-Charging-Function-Addresses and
P-Charging-Vector headers. Rohan Mahy assisted in the incorporation
of these extensions into a single document.

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9. Acknowledgments

The authors would like to thank Andrew Allen, Gabor Bajko, Gonzalo
Camarillo, Keith Drage, Georg Mayer, Dean Willis, Rohan Mahy,
Jonathan Rosenberg, Ya-Ching Tan and the 3GPP CN1 WG members for
their comments on this document.

10. 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., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

11. Informative References

[4] Garcia-Martin, M., "3rd-Generation Partnership Project (3GPP)
Release 5 requirements on the Session Initiation Protocol
(SIP)", Work in Progress.

[5] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002.

[6] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.

[7] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in Session Initiation Protocol (SIP)", RFC 3262, June
2002.

[8] Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

[9] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.

[10] Campbell, B., Editor, Rosenberg, J., Schulzrinne, H., Huitema,
C. and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.

[11] Sparks, R., "The SIP Refer Method", Work in Progress.

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[12] Barnes, M., "SIP Generic Request History Capability
Requirements", Work in Progress.

[13] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, November 2002.

[14] 3GPP, "TS 23.228: IP Multimedia Subsystem (IMS); Stage 2
(Release 5)", 3GPP 23.228, September 2002, <ftp://ftp.3gpp.org/
Specs/archive/23_series/23.228/>.

[15] 3GPP, "TS 24.229: IP Multimedia Call Control Protocol based on
SIP and SDP; Stage 3 (Release 5)", 3GPP 24.229, September 2002,
<ftp://ftp.3gpp.org/Specs/archive/24_series/24.229/>.

[16] 3GPP, "TS 32.200: Telecommunication Management; Charging
management; Charging principles (Release 5)", 3GPP 32.200, June
2002, <ftp://ftp.3gpp.org/Specs/archive/32_series/32.200/>.

[17] 3GPP, "TS 32.225: Telecommunication Management; Charging
management; Charging Data Description for IP Multimedia
Subsystem (Release 5)", 3GPP 32.225, September 2002, <ftp://
ftp.3gpp.org/Specs/archive/32_series/32.225/>.

Authors' Addresses

Miguel A. Garcia-Martin
Ericsson
Hirsalantie 11
Jorvas FIN-02420
Finland
EMail: [email protected]

Eric Henrikson
Lucent
11601 Willows Rd, Suite 100
Redmond, WA 98052
USA
EMail: [email protected]

Duncan Mills
Vodafone
The Courtyard, 2-4 London Road
Newbury, Berkshire RG14 1JX
UK
EMail: [email protected]

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Full Copyright Statement

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