SIPPING Working Group J. Hautakorpi
Internet-Draft G. Camarillo
Expires: July 5, 2007 Ericsson
Jan 2007
The Session Initiation Protocol (SIP) P-Refused-URI-List Private-Header
(P-Header)
draft-hautakorpi-sipping-uri-list-handling-refused-01.txt
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Abstract
This document specifies the Session Initiation Protocol (SIP)
P-Refused-URI-List Private-Header (P-Header). This P-Header is used
in the Open Mobile Alliance's (OMA) Push to talk over Cellular (PoC)
system. It enables URI-list servers to refuse the handling of an
incoming URI-list, which has an embedded URI-list inside it. This
P-Header also makes it possible for the URI-list server to inform the
client about the embedded URI-list that caused the rejection and the
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individual URIs that form such a URI-list.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6
5. Syntax of P-Refused-URI-List Header Field . . . . . . . . . . 6
6. Response Generation . . . . . . . . . . . . . . . . . . . . . 6
7. Message Sequence Example . . . . . . . . . . . . . . . . . . . 7
8. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
The Open Mobile Alliance (OMA) has specified the Push to talk over
Cellular (PoC) service, which uses the Session Initiation Protocol
(SIP) [3] and Uniform Resource Identifier (URI)-list services [5]
(more information about OMA PoC can be found at [8]).
OMA PoC needs a mechanism for servers to refuse the handling of
incoming URI-lists when these have embedded URI-lists inside them.
Such a mechanism is intended to be used to establish a particular
type of PoC session called ad-hoc PoC group.
The remainder of this document is organized as follows. Section 3
describes the scenario where the mechanism will be used. Section 4
provides an overview of the mechanism, which includes a new P-Header
called P-Refused-URI-List. Section 5 defines the syntax of this new
P-Header. Section 6 specifies how to use the P-Header. Section 7
provides a usage example. Section 8 describes the applicability of
the P-Header. Security considerations are discussed in Section 9 and
finally the IANA consideration are stated in Section 10.
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 RFC 2119 [1].
3. Usage Scenario
An ad-hoc PoC group is a type of multi-party PoC session. The
originator or a particular ad-hoc PoC group chooses in an ad-hoc
manner (e.g., selecting them from an address book) the set of
participants in the ad-hoc PoC group. In order to establish the ad-
hoc PoC group, the originator sends an INVITE request with a URI
list, which contains the URIs of those participants.
The PoC network, following the procedures defined in [6], receives
such an INVITE request and generates an individual INVITE request
towards each of the URIs in the URI list.
In previous versions of the OMA PoC service, the originator of an ad-
hoc PoC group was only allowed to populate the initial URI list with
URIs identifying individual users. Later versions of the service
allow the originator to also include entries that do not represent
individual users, but also represent URI lists themselves. That is,
the initial URI list would carry entries that are URI lists
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themselves.
The expected service behavior in such a case is that all the members
of the embedded URI lists are also invited to join the ad-hoc PoC
group. Figure 1 illustrates this point. The originator places the
URI list friends@example.org in inside its initial URI list. The PoC
network resolves friends@example.org into its members, which are
bob@example.org and carol@example.net, and sends INVITE requests to
all the recipients.
2. INVITE
+------------------>
| alice@example.com
|
|
+-------------+
| |
1. INVITE | | 3. INVITE
------------------>| PoC Network |---------------->
alice@example.com | | bob@example.org
friends@example.org | |
+-------------+
|
|
|
| 4. INVITE
+------------------>
carol@example.net
Figure 1: PoC Expected Behavior
The PoC network in Figure 1 consists of PoC servers, which are SIP
entities that can behave as proxies or B2BUAs (Back-to-back User
Agents). There are two types of PoC servers: controlling and
participating.
In an ad-hoc PoC group, there is always exactly one controlling PoC
server. However, there can be any number of participating PoC
servers. The controlling PoC server of an ad-hoc PoC group is the
only PoC server that can resolve an incoming URI list and send
INVITEs to its members. Every participant in an ad-hoc PoC group has
an associated participating PoC server.
Figure 2 shows how the PoC network behaves in the scenario shown in
Figure 1. A PoC user agent sends an INVITE request (1) with URI list
in its body to its PoC server. The PoC server receives the INVITE
request becomes the controlling PoC server for the ad-hoc PoC group
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and sends an INVITE request to each of the URIs in the URI list.
+-------------+
2. INVITE | Particip. |
+------------------>| PoC server |->
| alice@example.com | example.com |
| +-------------+
|
+-------------+ 3. INVITE +-------------+
| |-------------------->| |
1. INVITE | Controlling | friends@example.org | Particip. |
---------------->| PoC server | | PoC server |->
alice@example.com | | 4. 403 Forbidden | example.org |
friends@example.org| |<--------------------| |
+-------------+ bob@example.org +-------------+
| | carol@example.net ^
| | |
| | 5. INVITE |
| +--------------------------------+
| bob@example.org
|
| +------------+
| 6. INVITE | Particip. |
+------------------>| PoC server |->
carol@example.net | example.net|
+------------+
Figure 2: PoC Network Behavior
The first URI, alice@example.com, identifies a single user. However,
the second URI in the URI list, friends@example.org, identifies a URI
list. In PoC terminology, the URI identifies a pre-arranged PoC
group. The PoC server at example.org cannot send INVITE requests to
the member of friends@example.org because it is not the controlling
PoC server for the ad-hoc PoC group. Instead, it informs the
controlling PoC server that friends@example.org is a list whose
members are bob@example.org and carol@example.net. On receiving this
information, the controlling PoC server generates INVITE requests
towards the members of the list.
At present, there is no mechanism for a participating PoC server to
inform a controlling PoC server about the fact that a URI identifies
a list and about the members of that list. This document defines
such a mechanism: the P-Refused-URI-List P-Header.
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4. Overview of Operation
When a URI-list server receives an INVITE request with a URI list,
some of which entries are URI lists themselves that the server cannot
handle, it returns a 403 (Forbidden) response with a P-Refused-URI-
List P-Header, as shown in Figure 3. The P-Refused-URI P-Header
contains the members of the URI list or lists that caused the
rejection of the request. This way, the client can send requests
directly to those member URIs.
+---------+ INVITE request +----------+
| |------------------------------>| |
| | [URI-list in a URI-list] | URI-list |
| Client | | server |
| | 403 Forbidden | |
| |<------------------------------| |
| | [Content of refused URI-list] | |
+---------+ +----------+
Figure 3: Operational Overview
5. Syntax of P-Refused-URI-List Header Field
The following is the augmented Backus-Naur Form (BNF) [4] syntax of
the P-Refused-URI-List P-Header:
P-Refused-URI-List = "P-Refused-URI-List" HCOLON
uri-list-entry
*(COMMA uri-list-entry)
uri-list-entry = ( name-addr / addr-spec )
*( SEMI refused-param )
refused-param = members-param / generic-param
members-param = "members" EQUAL
LDQUOT *( qdtext / quoted-pair ) RDQUOT
The members P-Header parameter MUST contain a cid-url, which is
defined in RFC 2392 [2].
The HCOLON, SEMI, EQUAL, LDQUOT, RDQUOT, and generic-param are
defined in [3].
6. Response Generation
A 403 (Forbidden) response can contain more than one P-Refused-URI-
List entries. The P-Refused-URI-List header field MUST NOT be used
with any other response. The P-Refused-URI-List P-Header contains
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one or more URIs, which were present in the URI-list in the incoming
request and could not be handled by the server. Additionally, the
P-Refused-URI-List can optionally carry the members of the URI lists
identified by those URIs.
The 403 (Forbidden) response MAY contain body parts which contain
URI-lists. Those body parts can be referenced by the P-Refused-URI-
List entries through their Content-IDs [2]. If there is a Content-ID
defined in the P-Refused-URI-List, one of the body parts MUST have an
equivalent Content-ID. The format of a URI-list is service specific.
This kind of message structure enables clients to determine which URI
relates to which URI-list, if the URI-list server is willing to
disclose that information. Furthermore, the information enclosed in
the URI lists enable clients to take further actions to remedy the
rejection situation (e.g., send individual requests to the members of
the URI list).
7. Message Sequence Example
In the following message sequence example, a controlling PoC server
sends an INVITE request to a participating PoC server. The
participating PoC server rejects the request with a 403 (Forbidden)
response. The 403 response has a P-Refused-URI-List P-Header that
carries the members of the rejected URI-lists in the body of the
message.
Controlling PoC server Participating PoC server
example.com example.net
| |
| INVITE |
|-------------------------------->|
| |
| 403 Forbidden |
|<--------------------------------|
| |
Figure 4: Message Sequence Example
The INVITE request shown in Figure 4 is the following (Via header
fields are not shown for simplicity):
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INVITE sip:poc-service@example.net SIP/2.0
Max-Forwards: 70
From: PoC service <sip:poc-service@example.com>;tag=4fxaed73sl
To: PoC service <sip:poc-service@example.net>
Call-ID: 7xTn9vxNit65XU7p4@example.com
CSeq: 1 INVITE
Contact: <sip:poc-service@poc-as.example.com>
Require: recipient-list-invite
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 538
--boundary1
Content-Type: application/sdp
(SDP not shown)
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="sip:bob@example.net"/>
<entry uri="sip:friends-list@example.net"/>
<entry uri="sip:colleagues-list@example.net"/>
</list>
</resource-lists>
--boundary1--
The URIs sip:friends-list@example.net and
sip:colleagues-list@example.net in the example above are actually
references to URI lists (i.e., pre-arranged PoC groups). In the
following response, the URI lists are in the XML resource list format
[7].
The content of the 403 (Forbidden) response in Figure 4 is the
following (Via header fields are not shown for simplicity):
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SIP/2.0 403 Forbidden
From: PoC service <sip:poc-service@example.com>;tag=4fxaed73sl
To: PoC service <sip:poc-service@example.com>;tag=814254
Call-ID: 7xTn9vxNit65XU7p4@example.com
CSeq: 1 INVITE
P-Refused-URI-List: sip:friends-list@example.net;
members=<cid:an3bt8jf03@example.net>
P-Refused-URI-List: sip:colleagues-list@example.net;
members=<cid:bn35n8jf04@example.net>
Conten-Type: multipart/mixed;boundary="boundary1"
Content-Length: 745
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: uri-list
Content-ID: <an3bt8jf03@example.net>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="sip:bill@example3.com"/>
<entry uri="sip:randy@example2.net"/>
<entry uri="sip:eddy@example4.com"/>
</list>
</resource-lists>
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: uri-list
Content-ID: <bn35n8jf04@example.net>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
<list>
<entry uri="sip:joe@example3.org"/>
<entry uri="sip:carol@example5.net"/>
</list>
</resource-lists>
--boundary1--
From the 403 (Forbidden) response above controlling PoC server can
determine the members of sip:friend-list@example.net and
sip:colleagues-list@example.net from the message body. Furthermore,
the controlling PoC server can deduce that the participating PoC
server has not sent any outgoing requests, per regular URI-list
server procedures.
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8. Applicability
The P-Refused-URI-list header field is intended to be used in OMA PoC
networks. This header field is used between PoC servers and carries
information about those URI-lists that were rejected by the server
receiving the request.
The OMA PoC services is designed so that, in a given session, only
one server can resolve incoming URI lists and send INVITEs to its
members. This restriction is not present on services developed to be
used on the public Internet. Therefore, the P-Refused-URI-List
P-Header does not seem to have general applicability outside the OMA
PoC service.
Additionally, the use of the P-Refused-URI-List P-Header requires
special trust relationships between servers that do not typically
exist on the public Internet.
It is important to note that the P-Refused-URI-List is optional and
does not change the basic behavior of a SIP URI-list service. The
P-Refused-URI-List only provides clients with additional information
about the refusal of the request.
9. Security Considerations
It is assumed that the network elements handling the P-Refused-URI-
List P-Header are trusted. Also, attackers are not supposed to have
access to the protocol messages between those elements. This is
because the P-Refused-URI-List is intended to be used in the OMA PoC
environment, which is implemented in the operators' core network.
Traffic protection between network elements is achieved by using IP
Security (IPsec) and sometimes by physically protecting the network.
However, implementors and administrators should be aware of two
special security consideration related to the use of P-Refused-URI-
List:
Eavesdropping: 403 (Forbidden) responses may contain information
about the members of a given URI list. Eavesdroppers can acquire
this information if the 403 (Forbidden) responses are not
encrypted. Therefore it is RECOMMENDED that either hop-by-hop or
end-to-end encryption is used.
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Disclosing information: A rogue entity may be able to acquire
information about the members of a given URI list if the URI-list
server sends information about those URI-lists also to
unauthorized users. Therefore, it is RECOMMENDED that URI-list
server discloses the content of URI-list only to authorized
clients.
10. IANA Considerations
The IANA is requested to make two additions to the Session Initiation
Protocol (SIP) Parameters registry. The first addition is to add the
following header field to the already existing Header Fields sub-
registry
Header Name compact Reference
----------------- ------- ---------
P-Refused-URI-List [RFCxxxx]
The second addition is to add the following header field parameter to
the already existing Header Field Parameters and Parameter Values
sub-registry.
Predefined
Header Field Parameter Name Values Reference
---------------------------- --------------- --------- ---------
P-Refused-URI-List members No [RFCxxxx]
Note for the RFC Editor: The two occurrences of 'RFCxxxx' in the
above should be a reference to the coming RFC number of this draft.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[3] 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.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
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[5] Camarillo, G. and A. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) Uniform
Resource Identifier (URI)-List Services",
draft-ietf-sipping-uri-services-05 (work in progress),
January 2006.
[6] Camarillo, G. and A. Johnston, "Conference Establishment Using
Request-Contained Lists in the Session Initiation Protocol
(SIP)", draft-ietf-sip-uri-list-conferencing-00 (work in
progress), September 2006.
11.2. Informative References
[7] Rosenberg, J., "Extensible Markup Language (XML) Formats for
Representing Resource Lists",
draft-ietf-simple-xcap-list-usage-05 (work in progress),
February 2005.
[8] "OMA PoC System Description: Draft Version 2.0", April 2006.
Authors' Addresses
Jani Hautakorpi
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Jani.Hautakorpi@ericsson.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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