SIP -- Session Initiation Protocol D. Willis
Working Group dynamicsoft Inc.
Internet-Draft B. Hoeneisen
Expires: October 28, 2002 Nokia
April 29, 2002
SIP Extension for Registering Non-Adjacent Contacts
draft-willis-sip-path-04
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The REGISTER function is used in a SIP system primarily to associate
a temporary contact address with an address-of-record. This contact
is generally in the form of a URI, such as Contact:
<sip:alice@pc33.atlanta.com> and is generally dynamic and associated
with the IP address or hostname of the SIP UA. The problem is that
network topology may be that there are one or more SIP proxies
between the UA and the registrar, such that any message from the
user's home network to the registered UA must traverse these proxies.
The REGISTER method does not give us a mechanism to discover and
record this sequence of proxies in the registrar for future use.
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This document defines an extension header, "Path" which provides such
a mechanism.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. Path Header Definition and Syntax . . . . . . . . . . . . . . 4
4. Usage of Path Header . . . . . . . . . . . . . . . . . . . . . 5
4.1 Procedures at the UA . . . . . . . . . . . . . . . . . . . . . 5
4.2 Procedures at Intermediate Proxies . . . . . . . . . . . . . . 5
4.3 Procedures at the Registrar . . . . . . . . . . . . . . . . . 5
4.4 Procedures at the Home Proxy . . . . . . . . . . . . . . . . . 6
4.5 Examples of Usage . . . . . . . . . . . . . . . . . . . . . . 6
4.6 Example of Mechanism in REGISTER Transaction . . . . . . . . . 7
4.7 Example of Mechanism in INVITE Transaction . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1 Considerations in REGISTER Request Processing . . . . . . . . 12
5.2 Considerations in REGISTER Response Processing . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . . 13
Non-Normative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Background
3GPP established a requirement for discovering intermediate proxies
during SIP registration and published this requirement in draft-
garcia-sipping-3gpp-reqs [5].
Scenario:
UA----P1-----P2-----P3------REGISTRAR
UA wishes to register with REGISTRAR. However, due to network
topology, UA must use P1 as an "outbound proxy", and all messages
between UA1 and REGISTRAR must also traverse P1, P2, and P3 before
reaching REGISTRAR. Likewise, all messages between REGISTRAR and UA
must also traverse P1, P2, and P3 before reaching UA.
UA has a standing relationship with REGISTRAR, which it considers its
"Home". How UA establishes this relationship is outside the scope of
this document. UA discovers P1 as a result of a DHCP assignment or
similar operation, also outside the scope of this document.
REGISTRAR has a similar "default outbound proxy" relationship with
P3.
Eventually, REGISTRAR or a service proxy closely related to it will
receive a message for UA. It needs to know which proxies must be
transited by that message in order to get back to UA. In some cases,
this information may be deducible from SIP routing configuration
tables or from DNS entries. In other cases, such as that raised by
3GPP, the information is not readily available outside of the SIP
REGISTER transaction.
The proposed Path extension header allows accumulating and
transmitting the list of proxies between UA and REGISTRAR.
Intermediate nodes such as P1 may statefully retain Path information
if needed by operational policy. This mechanism is in many ways
similar to the operation of Record-Route in dialog-initiating
messages.
2. Applicability Statement
The Path mechanism is applicable whenever there are intermediate
proxies between a SIP UA and a SIP Registrar used by that UA where
the following conditions are true:
1. One or more of the intermediate proxies MUST be visited by
messages between REGISTRAR and UA.
2. The same set of intervening proxies MUST be visited by messages
between a home service proxy and UA. That is, the proxy route
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between the UA and its home service proxy is identical to the
proxy route between the UA and its registrar.
3. The network topology is such that the intermediate proxies which
must be visited are NOT implied by SIP routing tables, DNS, or
similar mechanisms.
3. Path Header Definition and Syntax
The Path header is a SIP extension header with syntax very similar to
the Record-Route header. It is used in conjunction with SIP REGISTER
messages and with 200 OK messages in response to REGISTER (a REGISTER
response).
A Path header may be inserted into a REGISTER by any SIP node
traversed by that message. Like the Route header, sequential Path
headers are evaluated in the sequence in which they are present in
the message, and Path header values may be combined into compound
Path elements in a single Path header. The registrar reflects the
accumulated Path back into the REGISTER response, and intermediate
nodes propagate this back toward the originating UA. The originating
UA is therefore informed of the inclusion of nodes on its registered
Path, and MAY use that information in other capacities outside the
scope of this document.
The primary difference between Path and Record-Route is that Path
applies to REGISTER and 200 OK responses to REGISTER. Record-Route
doesn't, and can't be defined in REGISTER for reasons of backward
compatibility.
The syntax for Path can be given as:
Path = "Path" HCOLON path-value *( COMMA path-value )
path-value = name-addr *( SEMI rr-param )
The allowable usage of headers is described in Tables 2 and 3 of
SIPbis [2]. The following additions to this table are needed for
Path.
Addition of Path to SIP Table 3:
Header field where proxy ACK BYE CAN INV OPT REG PRA
_______________________________________________________________
Path R ar - - - - - - -
Path 2xx - - - - - - o -
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4. Usage of Path Header
4.1 Procedures at the UA
The UA executes its register operation as usual. The response may
contain a Path header. The general operation of the UA is to ignore
the Path header in the response. It MAY choose to display the
contents of the Path header to the user or take other action outside
the scope of this document. The Path information in the REGISTER
response lets the UA know what intermediate proxies were added during
registration. Examination of this information may be important from
a security perspective.
It has been suggested that the UA MAY choose to store the contents of
the Path header for future use as a preloaded Route for use when the
UA wishes to send a SIP message that, for reasons of acquiring
services in the home network, need to transit the service proxies in
the home network. Such usage is explicitly outside the scope of this
document.
4.2 Procedures at Intermediate Proxies
When a proxy processing a REGISTER request wishes to be on the path
for future transmissions toward the UA originating that REGISTER
request, the proxy inserts a URI for that proxy as the topmost
element in the Path header (or inserts a new topmost Path header)
before proxying that request. It is also possible for a proxy with
specific knowledge of network topology to add a Path element
referencing another node, thereby allowing construction of a Path
which is discongruent with the route taken by the REGISTER request.
Such a construction is implementation specific and outside the scope
of this document.
Proxies processing a REGISTER response MUST NOT alter the value of
any Path headers that may be present in the response.
4.3 Procedures at the Registrar
If a Path header exists in a successful REGISTER request, the
registrar constructs an ordered list of route elements from the nodes
listed in the Path elements, preserving the order as indicated in the
Path header(s). The registrar then stores this route set in
association with that contact and the addres-of-record indicated in
the Register request (the "binding" as defined in RFC 2543 [1]). The
registrar copies the Path elements list as one or more Path headers
into into the successful (200 OK) REGISTER response message.
Note that the inserted Path elements conform to the syntax of a Route
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element as defined in [2]. As suggested therein, such route elements
MUST include the loose-routing indicator parameter ";lr" for full
compliance with [2]
4.4 Procedures at the Home Proxy
In the common SIP model, there is a home service proxy associated
with the registrar for a user. Each incoming message targeted to the
public address-of-record for the user is routed to this proxy, which
consults the registrar's database in order to determine the contact
to which the message should be retargeted. The home service proxy,
in its basic mode of operation, rewrites the request-URI from the
incoming message with the value of the registered contact and
retransmits the message.
With the addition of Path, the home service proxy also copies the
route set associated with the specific contact in the registrar
database into the Route header(s) of the outgoing message as a
preloaded route. This causes the outgoing message to transit the set
of proxies that indicated that they were to be used in future
messages to that contact by including themselves in the Path header
on the REGISTER message.
Loose routes may interact with routing policy in interesting ways.
The specifics of how the stored 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 stored route set is appended to any locally
configured route needed to egress the service cluster. Systems
designers must match the Path recording policy of their nodes with
the routing poilicy in order to get a workable system.
4.5 Examples of Usage
As an example, we use the scenario from the Background section:
UA1----P1-----P2----P3-----REGISTRAR
Note that the names used, such as "UA1", are symbols for "real" host
names or IP addresses. The substitution provides a shorter and
hopefully more readable presentation. The node marked REGISTRAR is
also a proxy and serves as a home service proxy.
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4.6 Example of Mechanism in REGISTER Transaction
In this example, UA1 sends a REGISTER message to REGISTRAR. This
message transits its default outbound proxy P1, an intermediate proxy
P2, and the firewall proxy for the home domain, P3, before reaching
REGISTRAR. Due to network topology and operational policy, P1 and
and P3 need to be transited by messages from REGISTRAR or other nodes
in the home network targeted to UA1. P2 does not. P1 and P3 have
been configured to include themselves in Path headers on REGISTER
messages that they process. UA1 has a current IP address of
"192.0.2.4".
Message sequence for REGISTER with Path:
F1 Register UA1 -> P1
REGISTER sip:REGISTRAR SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
. . .
F2 Register P1 -> P2
REGISTER sip:REGISTRAR 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@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTAR <sip:UA1@REGISTAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P1;lr>
. . .
Note: P1 has added itself to the Path.
F3 Register P2 -> P3
REGISTER sip:REGISTRAR 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
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To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P1;lr>
. . .
Note: P2 did NOT add itself to the Path.
F4 Register P3 -> REGISTRAR
REGISTER sip:REGISTRAR SIP/2.0
Via: SIP/2.0/UDP P3:5060;branch=p3wer654363
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@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P3;lr>
Path: <sip:P1;lr>
. . .
Note: P3 added itself to the Path.
F5 REGISTRAR executes Register
REGISTRAR Stores:
For UA1@REGISTRAR
Contact = <sip:UA1@192.0.2.4>
Stored Route Set = <sip:P3;lr>,<sip:P1;lr>
F6 Register Response REGISTRAR -> P3
SIP/2.0 200 OK
Via: SIP/2.0/UDP P3:5060;branch=p3wer654363
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@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
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Path: <sip:P3;lr>,<sip:P1;lr>
. . .
Note: The Path header in the response is identical to the one
received in the REGISTER request.
F7 Register Response P3 -> P2
SIP/2.0 200 OK
Via: SIP/2.0/UDP 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@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P3;lr>,<sip:P1;lr>
. . .
F8 Register Response P2 -> P1
SIP/2.0 200 OK
Via: SIP/2.0/UDP P1:5060;branch=34ghi7ab04
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P3;lr>,<sip:P1;lr>
. . .
F9 Register Response P1 -> UA1
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA1@REGISTRAR <sip:UA1@REGISTRAR>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:UA1@192.0.2.4>
Path: <sip:P3;lr>,<sip:P1;lr>
. . .
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4.7 Example of Mechanism in INVITE Transaction
This example shows the message sequence for an INVITE transaction
originating from UA2 eventually arriving at UA1. REGISTRAR inserts a
preloaded Route toward UA1 and retargets the request by replacing the
request URI with the registered Contact. It then sends the
retargetted INVITE along the Path towards UA1. Note that this
example introduces foreign user agent UA2 (address "71.91.180.10")
and foreign domain FOREIGNDOMAIN
Message sequence for INVITE using Path:
F1 Invite UA2 -> REGISTRAR
INVITE UA1@REGISTRAR SIP/2.0
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=e2i95c5st3R
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA2@FOREIGNDOMAIN <sip:UA2@FOREIGNDOMAIN>;tag=224497
Call-ID: 48273181116@71.91.180.10
CSeq: 29 INVITE
Contact: <sip:UA2@71.91.180.10>
. . .
F2 REGISTRAR processing
REGISTRAR looks up name "UA1@REGISTRAR" and returns:
- Contact = <sip:UA1@192.0.2.4>
- Route Set = <sip:P3;lr>,<sip:P1;lr>
Note: The Contact replaces the request-URI. The Route Set is pushed
onto the Route stack (preloaded Route) of the outgoing INVITE
request. The topmost Route is used for making the routing decision.
F3 Invite REGISTRAR -> P3
INVITE UA1@192.0.2.4 SIP/2.0
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=lj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=e2i95c5st3R
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA2@FOREIGNDOMAIN <sip:UA2@FOREIGNDOMAIN>;tag=224497
Call-ID: 48273181116@71.91.180.10
CSeq: 29 INVITE
Contact: <sip:UA2@71.91.180.10>
Route: <sip:P3;lr>,<sip:P1;lr>
. . .
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Note: In this example REGISTRAR does not want to stay on the
Route and therefore does not insert a Record-Route.
F4 Invite P3 -> P1
INVITE UA1@192.0.2.4 SIP/2.0
Via: SIP/2.0/UDP P3:5060;branch=jasg7li7nc9e
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=lj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=e2i95c5st3R
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA2@FOREIGNDOMAIN <sip:UA2@FOREIGNDOMAIN>;tag=224497
Call-ID: 48273181116@71.91.180.10
CSeq: 29 INVITE
Contact: <sip:UA2@71.91.180.10>
Record-Route: <sip:P3;lr>
Route: <sip:P1;lr>
. . .
Note: P3 has added a Record-Route entry, indicating that it wants
to be traversed by future messages in this dialog.
F5 Invite P1 -> UA1
INVITE UA1@192.0.2.4 SIP/2.0
Via: SIP/2.0/UDP P1:5060;branch=k5l1833o43p
Via: SIP/2.0/UDP P3:5060;branch=jasg7li7nc9e
Via: SIP/2.0/UDP 143.70.6.83:5060;branch=lj25C107a7b176
Via: SIP/2.0/UDP 71.91.180.10:5060;branch=e2i95c5st3R
To: UA1@REGISTRAR <sip:UA1@REGISTRAR>
From: UA2@FOREIGNDOMAIN <sip:UA2@FOREIGNDOMAIN>;tag=224497
Call-ID: 48273181116@71.91.180.10
CSeq: 29 INVITE
Contact: <sip:UA2@71.91.180.10>
Record-Route: <sip:P1;lr>
Record-Route: <sip:P3;lr>
. . .
Note: P1 has added a Record-Route entry, indicating that it wants
to be traversed by future messages in this dialog.
5. Security Considerations
There are few security considerations for this draft beyond those in
SIPbis [2]. From a security perspective, the Path extension and its
usage are identical to the Record-Route header of basic SIP. Note
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that the transparency of the user expectations are preserved by
returning the final Path to the originating UA -- that is, the UA is
informed which additional proxies have been inserted into the path
for the registration associated with that response.
The PATH header accumulates information in a hop-by-hop manner during
REGISTER processing. The return information is essentially end-to-
end, that is it is not altered by intermediate proxies. This leads
to two slightly different security approaches.
5.1 Considerations in REGISTER Request Processing
Information accumulated in REGISTER processsing causes additional
proxies to be included in future messages between the registrar's
location and that of the UA. An attack that allowed an intruding
proxy to add itself to this chain would allow the attacker to
intercept future calls directed toward the UA.
An attacker could conceivably alter the Path either by altering data
"on the wire" or by other manipulations (such as impersonation) that
would cause it to be included in the SIP routing chain (a "node
insertion" attack). Altering data "on the wire" is addressed
adequately by the use of transport-layer integrity protection
mechanisms such as TLS or IPSEC. Proxy insertion can be addressed by
mutual authentication at the proxy layer, which can also be provided
by TLS or IPSEC. The "sips:" URI class defined in [2] provides a
mechanism by which a UA may request that intermediate proxies provide
integrity protection and mutual authentication.
Systems using the Path mechanism SHOULD use appropriate mechanisms
(TLS, IPSEC, etc.) to provide message integrity and mutual
authentication. UAs SHOULD use "sips:" to request transitive
protection.
5.2 Considerations in REGISTER Response Processing
The data returned to the UA by the Path header in the response to the
REGISTER request is there to provide openness to the UA. The
registrar is telling the UA "These are the intermediate proxies that
will be included on future requests to you processed through me". By
inspection of this header, the UA may be able to detect node
insertion attacks that involve inserting an proxy into the SIP
routing chain. Of course, the attacker may try to alter returned
Path headers, perhaps deleting self-references.
As specified, there is no requirement for proxies between the UA and
the registrar to modify the Path header in the REGISTER response.
Consequently, we may use an end-to-end protection technique. The S/
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MIME technique defined in [2] provides an effective mechanism. Using
this technique, the registrar makes a copy of the complete response,
signs it, and attaches it as a body to the response. The UA may then
verify this response, assuring an unmodifed Path header is received.
In addtion to the hop-by-hop integrity protection and mutual
authentication measures suggested for REGISTER request processing in
the preceding section, systems using Path headers SHOULD implement
end-to-end protection using S/MIME. More specifically, registrars
returning a Path header SHOULD attach a signed S/MIME of the the
response, and UAs receiving a REGISTER response containing a Path
header SHOULD validate the message using the S/MIME technique.
Furthermore, UAs receiving a Path header in a REGISTER response
SHOULD render it to the user, or (where feasible) check it
programmatically.
6. IANA Considerations
This document defines the SIP extension header name "Path", which
IANA will add to the registry of SIP header names defined in SIPbis
[2]
7. Acknowledgements
Min Huang and Stinson Mathai, who put together the original proposal
in 3GPP for this mechanism, and worked out most of the 3GPP
procedures in 24.229.
Keith Drage, Bill Marshall, and Miguel Angel Garcia-Martin who argued
with everybody a lot about the idea as well as helped refine the
requirements.
Juha Heinanen, who argued steadfastly against standardizing the
function of discovering the home service proxy with this technique in
this document.
Normative References
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[2] Rosenberg, J., "SIP: Session Initiation Protocol", draft-ietf-
sip-rfc2543bis-09 (work in progress), March 2002.
[3] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[4] Postel, J. and J. Reynolds, "Instructions to RFC Authors", RFC
Willis & Hoeneisen Expires October 28, 2002 [Page 13]
Internet-Draft Path Extension Header for SIP April 2002
2223, October 1997.
Non-Normative References
[5] Garcia-Martin, MA., "3GPP Requirements On SIP", draft-garcia-
sipping-3gpp-reqs-03 (work in progress), March 2002.
[6] Mankin, A., "SIP Change Process", draft-tsvarea-sipchange-01
(work in progress), March 2002.
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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Internet-Draft Path Extension Header for SIP April 2002
Full Copyright Statement
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