SIP -- Session Initiation Protocol D. Willis
Working Group dynamicsoft Inc.
Internet-Draft B. Hoeneisen
Expires: November 3, 2002 Nokia
May 5, 2002
SIP Extension Header for Service Route Discovery in Private Networks
draft-willis-sip-scvrtdisco-03
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document proposes a private SIP extension header used in
conjunction with responses to REGISTER messages to provide a
mechanism by which a registrar may inform a registering UA of a
service route that the UA may use to request outbound services from
the registrar's domain.
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Discussion of Mechanism . . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . 5
4. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1 Procedures at the UA . . . . . . . . . . . . . . . . . . . . 6
5.2 Procedures at the Proxy . . . . . . . . . . . . . . . . . . 6
5.3 Procedures at the Registrar . . . . . . . . . . . . . . . . 7
5.4 Examples of Usage . . . . . . . . . . . . . . . . . . . . . 7
5.4.1 Example of Mechanism in REGISTER Transaction . . . . . . . . 8
5.4.2 Example of Mechanism in INVITE Transaction . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . 13
Non-Normative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . 15
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1. Background
3GPP established a requirement for discovering home proxies during
SIP registration and published this requirement in draft-garcia-
sipping-3gpp-reqs [6]. Unlike many other network environments, the
3GPP network dynamically assigns a home service proxy to each
address-of-record. This assignment may occur in conjunction with a
REGISTER operation, or out-of-band as needed to support call services
when the address-of-record has no registrations. This home service
proxy may provide both inbound (UA terminated) and outbound (UA
originated) services.
For inbound (UA terminated) session cases, the home proxy network
routes messages having a request-URI targeting the address-of-record
associated with the UA to the assigned home service proxy by using
some sort of look-up-mechanism outside the scope of this document.
Outbound (UA originated) session cases raise another issue.
Specifically, "How does the UA know which service proxy to use and
how to get there?"
Several mechanisms have been proposed in list discussions, including:
1. Configuration data in the UA. This raises questions of UA
configuration management and updating, especially if proxy
assignment is very dynamic, such as in load-balancing scenarios.
2. Use of some other protocol, such as HTTP, to get configuration
data from a configuration server in the home network. While
functional, this solution requires additional protocol engines,
firewall complexity, operations overhead, and a significant
additional "over the air" traffic.
3. Use of lookup tables in the home network, as is done for inbound
messages. This has a relatively high overhead in terms of
database operations.
4. Returning a 302 response indicating the service proxy as a new
contact, causing the upstream node processing the 302 (ostensibly
the UA) to retransmit the message toward the service proxy.
While this shares the database operation of the previous
alternative, it does explicitly allow for caching the 302
response thereby potentially reducing the frequency and number of
database operations.
5. Performing an operation equivalent to record-routing in a
REGISTER transaction between the UA and the associated registrar,
then storing that route in the UA and reusing it as a service
route on future messages originating from the UA. While
efficient, this constrains the service route for proxy operations
to be congruent with the route taken by the REGISTER message.
6. Returning service route information as the value of a header in
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the REGISTER response. While similar to the previous
alternative, this approach grants the ability for the registrar
to selectively apply knowledge about the topology of the home
network in constructing the service route.
This document discusses this final alternative: using a header in the
REGISTER response to indicate a service route that the UA may wish to
use if requesting services from the proxy network associated with the
registrar generating the response.
Scenario
UA1----P1-----| |--R-------|
| | |
P2---| DBMS
| | |
UA2-----------| |--HSP-----|
In this scenario, we have a "home network" containing routing proxy
P2, registrar R, home service proxy HSP, and database DBMS used by
both R and HSP. P2 represents the "edge" of the home network from a
SIP perspective, and might be called an "edge proxy". UA1 is an
external UA behind proxy P1. UA1 discovers P1 via DHCP. UA2 is
another UA on the Internet, and does not use a default outbound
proxy. We do not show DNS elements in this diagram, but will assume
their reasonable availability in the discussion. The mission is for
UA1 to discover HSP so that outbound messages from UA1 may be routed
(at the discretion of UA1) through HSP, thereby receiving outbound
services from HSP.
2. Discussion of Mechanism
The proposed mechanism uses a private header "P-Service-Route" in the
REGISTER response to indicate a service route that the UA may wish to
use if requesting services from the proxy network associated with the
registrar generating the response. The routing established by the P-
Service-Route mechanism applies only to to requests originating in
the user agent.
Simply put, the registrar generates a service route for the
registering UA and returns it in the response to each successful
REGISTER request. This service route has the form of a Route header
that the registering UA may use to send messages through the service
proxy selected by the registrar. The UA would use this route by
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inserting it as a preloaded Route header in messages originated by
the UA intended for routing through the service proxy.
The mechanism by which the registrar constructs the header value is
specific to the local implementation and outside the scope of this
document.
3. Applicability Statement
The P-Service-Route mechanism is applicable when:
1. The UA registers with a REGISTRAR in a given domain.
2. The domain dynamically assigns a service proxy for the UA.
3. The registrar(s) in the domain has/have sufficient knowledge of
the network topology, policy, and situation such that a
reasonable service route can be constructed.
4. Other mechanisms for proposing a service route to the UA are not
available or are inappropriate for use within the administrative
domain.
4. Syntax
The syntax for the P-Service-Route header is:
P-Service-Route = "P-Service-Route" HCOLON 1#( p-sr-value)
p-sr-value = name-addr *( SEMI rr-param )
rr-param = generic-param
The allowable usage of headers is described in Tables 2 and 3 of
SIPbis [1]. The following additions to this table are needed for P-
Service-Route.
Addition of P-Service-Route to SIP Table 3:
Header field where proxy ACK BYE CAN INV OPT REG PRA
_______________________________________________________________
P-Service-Route 2xx ar - - - - - o -
5. Usage
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5.1 Procedures at the UA
The UA performs a register as usual. The register response may
contain a P-Service-Route header. If so, the UA MAY store the value
of the P-Service-Route header in an association with the address-of-
record for which the REGISTER message had registered a contact. If
the UA supports multiple address of records, it may be able to store
multiple service routes, one per address-of-record. If the UA
refreshes the registration, the stored value of the P-Service-Route
is updated according to the P-Service-Route header of the latest 200
OK response. If there is no P-Service-Route header in the response,
the UA clears any service route for that registrar previously stored
by the UA.
Loose routes may interact with routing policy in interesting ways.
The specifics of how the service route set 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 service route set is appended to any
locally configured route needed to egress the access proxy chain.
Systems designers must match the service routing policy of their
nodes with the basic SIP routing poilicy in order to get a workable
system.
Note: A Fetching Bindings operation, i.e. no Contact header field is
present in the REGISTER request, does not affect any stored value
of P-Service-Route.
The UA MAY choose to exercise a service route for future messages
associated with a given address-of-record for which a service route
is known. If so, it appends the given service route to any locally
required Route headers, and uses the result as a preloaded Route
header in outgoing messages. The UA MUST preserve the order, in case
there is more than one P-Service-Route header or element.
5.2 Procedures at the Proxy
The P-Service-Route header is generally treated like any other
unknown header by intermediate proxies. They simply forward it on
towards the destination.
There is a question of whether proxies processing a REGISTER response
may add themselves to the route set in the P-Service-Route header.
While this would enable dynamic construction of service routes, it
has two significant problems. The first is one of transparency, as
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seen by the registrar: Intermediate proxies could add themselves
without the knowledge or consent of the registrar. The second
problem is interaction with end-to-end security. If the registrar
uses S/MIME techniques to protect the REGISTER response, such
additions would be visible to the UA as "man in the middle"
alterations in the response. Consequently, intermediate proxies
SHOULD NOT alter the value of P-Service-Route in REGISTER responses,
and if they do, acceptance of the alteration by the UA MUST NOT be
required.
5.3 Procedures at the Registrar
When a registrar receives a successful REGISTER message, it MAY
choose to return one or more P-Service-Route header(s) in the 200 OK
response. The determinations of whether to include these header(s)
into the 200 OK response and what value(s) to insert are a matter of
local policy and outside the scope of this document.
Having inserted a P-Service-Route header, the registrar returns the
200 OK response to the UA in accordance with standard procedures.
Certain network topologies MAY require a specific proxy (e.g.
firewall proxy) to be traversed before the home service proxy. Thus,
a registrar with specific knowledge of the network topology MAY
return more than one P-Service-Route header or element in the 200 OK
response; the order is specified as top-down, meaning the topmost P-
Service-Route entry will be visited first. Such constructions are
implementation specific and outside the scope of this document.
In general, the P-Service-Route header contains references to
elements strictly within the administrative domain of the registrar
and home service proxy. For example, consider a case where a user
leaves the "home" network and roams into a "visited" network. The
registar cannot be assumed to have knowledge of the topology of the
visited network, so the P-Service-Route it returns contains elements
only within the home network.
Note that the inserted P-Service-Route element(s) MUST conform to the
syntax of a Route element as defined in [1]. As suggested therein,
such route elements MUST include the loose-routing indicator
parameter ";lr" for full compliance with [1]
5.4 Examples of Usage
We present an example in the context of the scenario presented in the
Background section earlier in this document. The network diagram is
replicated below:
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Scenario
UA1----P1-----| |--R-------|
| | |
P2---| DBMS
| | |
UA2-----------| |--HSP-----|
5.4.1 Example of Mechanism in REGISTER Transaction
This example shows the message sequence for user agent UA1
registering to HOMEDOMAIN using registrar R. R returns a P-Service-
Route indicating that UA1 may use home service proxy HSP to receive
outbound services from HOMEDOMAIN.
Please note that the name UA1, HOMEDOMAIN, etc. are placeholders for
approprate user and host names or addresses.
Message sequence for REGISTER returning P-Service-Route:
F1 Register UA1 -> P1
REGISTER sip:HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
. . .
F2 Register P1 -> P2
REGISTER sip:HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@REGISTAR <sip:UA1@REGISTAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
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. . .
F3 Register P2 -> R
REGISTER sip:HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP P2:5060;branch=iokioukju908
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
. . .
F4 R executes Register
R Stores:
For <sip:UA1@HOMEDOMAIN>
Contact = <sip:UA1@192.0.2.4>
F5 R calculates Service Route
In this example, R is statically configured to reference HSP as a
service route, so P-Service-Route = <sip:HSP;lr>
F6 Register Response r -> P2
SIP/2.0 200 OK
Via: SIP/2.0/UDP P2:5060;branch=iokioukju908
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
P-Service-Route: <sip:HSP;lr>
. . .
F7 Register Response P2 -> P1
SIP/2.0 200 OK
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Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
P-Service-Route: <sip:HSP;lr>
. . .
F8 Register Response P1 -> UA1
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
P-Service-Route: <sip:HSP;lr>
. . .
F9 UA1 stores service route for HOMEDOMAIN
5.4.2 Example of Mechanism in INVITE Transaction
This example shows the message sequence for an INVITE transaction
originating from UA1 eventually arriving at UA2 using outbound
services from HOMEDOMAIN, where UA1 has previously registered with
HOMEDOMAIN and been informed of a service route through HSP. The
service being provided by HOMEDOMAIN is a "logging" service, which
provides a record of the call for UA1's use (perhaps the user of UA1
is an attorney who bills for calls to customers).
Message sequence for INVITE using P-Service-Route:
F1 INVITE UA1 -> P1
INVITE sip:UA2@HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOMEDOMAIN>
From: Lawyer <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
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CSeq: 18 INVITE
Contact: <sip:UA1@192.0.2.4>
Route: <sip:HSP;lr>
. . .
Note: P1 is selected using the "outbound proxy" rule in UA1.
F2 INVITE P1 -> P2
INVITE sip:UA2@HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOMEDOMAIN>
From: Lawyer <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 18 INVITE
Contact: <sip:UA1@192.0.2.4>
Record-Route: <sip:P1;lr>
Route: <sip:HSP;lr>
. . .
Note: P2 is selected using a DNS lookup on the domain of HSP.
P1 has added itself to the Record Route.
F3 INVITE P2 -> HSP
INVITE sip:UA2@HOMEDOMAIN SIP/2.0
Via: SIP/2.0/UDP P2:5060;branch=iokioukju908
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOMEDMAIN>
From: Lawyer <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 18 INVITE
Contact: <sip:UA1@192.0.2.4>
Record-Route: <sip:P2;lr>
Record-Route: <sip:P1;lr>
Route: <sip:HSP;lr>
. . .
Note: HSP is selected using a DNS lookup for HSP within HOMEDOMAIN.
P2 has addded itself to the Record Route.
F4 HSP executes service
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HSP identifies the service to be executed from UA1's stored
profile. The specifics of this are outside the scope of this
document. HSP writes a record to "Lawyer"s log book, then looks up
name "sip:UA2@HOMEDOMAIN" and discovers that the current contact for
UA2 is address 18.19.20.21. This will be the request-URI of the
next-hop INVITE
F5 INVITE HSP->P2
INVITE sip:UA2@18.19.20.21
Via: SIP/2.0/USP HSP:5060;branch=HSP10120323
Via: SIP/2.0/UDP P2:5060;branch=iokioukju908
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOMEDOMAIN>
From: UA1@HOMEDOMAIN <sip:UA1@HOMEDOMAIN>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 18 INVITE
Contact: <sip:UA1@192.0.2.4>
Record-Route: <sip:HSP;lr>
Record-Route: <sip:P2;lr>
Record-Route: <sip:P1;lr>
. . .
Note: P2 selected by outbound proxy rule on HSP.
INVITE propagates toward UA2 as usual.
6. Security Considerations
It is possible for proxies between the UA and the registrar during
the REGISTER transaction to modify the value of P-Service-Route
returned by the registrar, or to insert a P-Service-Route even when
one was not returned by the registrar. It is also possible for
proxies on the INVITE path to execute many different attacks. It is
therefore desirable to apply transitive mutual authentication using
sips: or other available mechanisms in order to prevent such attacks.
The "sips:" URI as defined in [1] defines a mechanism by which a UA
may request transport-level message integrity and mutual
authentication. Since there is no requirement for proxies to modify
message, S/MIME signed bodies may be used to provide end-to-end
protection for the returned value.
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Systems using P-Service-Route SHOULD provide hop-by-hop message
integrity and mutual authentication. UAs SHOULD request this support
by using a "sips:" URI. Registrars returning a P-Service-Route
SHOULD provide end-to-end protection on the return using S/MIME. UAs
receiving P-Service-Route SHOULD authenticate attached S/MIME bodies.
7. IANA Considerations
This document defines the SIP extension header "P-Service-Route"
which should be included in the registry of SIP headers defined in
SIP bis [1]. As required by the SIP change process draft-tsvarea-
sipchange [7] the SIP extension header name "Service-Route" should
also be registered in association with this extension. However,
"Service-Route" MUST not be used until documented by a standards-
track RFC. Expert review as required for this process is to be
provided by the SIP Working Group.
Normative References
[1] Rosenberg, J., "SIP: Session Initiation Protocol", draft-ietf-
sip-rfc2543bis-09 (work in progress), March 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.
[5] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
Non-Normative References
[6] Garcia-Martin, MA., "3GPP Requirements On SIP", draft-garcia-
sipping-3gpp-reqs-03 (work in progress), March 2002.
[7] Mankin, A., "SIP Change Process", draft-tsvarea-sipchange-01
(work in progress), March 2002.
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Authors' Addresses
Dean Willis
dynamicsoft Inc.
5100 Tennyson Parkway
Suite 1200
Plano, TX 75028
US
Phone: +1 972 473 5455
EMail: dwillis@dynamicsoft.com
URI: http://www.dynamicsoft.com/
Bernie Hoeneisen
Nokia
Helsinki, Hiomo 3/6
P.O. Box 312
00045 NOKIA Group
Finland
Phone: +358-40-821 9 831
EMail: bernhard.honeisen@nokia.com, b.hoeneisen@ieee.org
URI: http://www.nokia.com/
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