IPTEL WG V. Gurbani
Internet-Draft Lucent Technologies, Inc./Bell
Expires: June 8, 2006 Laboratories
C. Jennings
Cisco Systems
Dec 5, 2005
Representing trunk groups in tel/sip Uniform Resource Identifiers (URIs)
draft-ietf-iptel-trunk-group-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a standardized mechanism to convey trunk
group-related information in sip and tel Uniform Resource Identifiers
(URIs). An extension to the "tel" URI is defined for this purpose.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem statement . . . . . . . . . . . . . . . . . . . . . 4
4. Requirements and rationale . . . . . . . . . . . . . . . . . 4
4.1 sip URI or tel URI? . . . . . . . . . . . . . . . . . . . 4
4.2 Trunk group namespace: global or local? . . . . . . . . . 5
4.3 Originating trunk group and terminating trunk group . . . 5
4.4 Intermediary processing of trunk groups . . . . . . . . . 6
5. Reference architecture . . . . . . . . . . . . . . . . . . . 7
6. Trunk group identifier: ABNF and examples . . . . . . . . . 8
7. Example call flows . . . . . . . . . . . . . . . . . . . . . 9
7.1 Basic Call Flow . . . . . . . . . . . . . . . . . . . . . 10
7.2 Inter-domain Call Flow . . . . . . . . . . . . . . . . . . 11
8. Security considerations . . . . . . . . . . . . . . . . . . 12
9. IANA considerations . . . . . . . . . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1 Normative References . . . . . . . . . . . . . . . . . . 14
11.2 Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. 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 RFC-2119 [1].
2. Definitions
Call routing in the Public Switched Telephone Network (PSTN) is
accomplished by routing calls over specific circuits (commonly
referred to as "trunks") between Time Division Multiplexed (TDM)
circuit switches. In large switches, a group of trunks that connects
to the same target switch or network is called a "trunk group."
Consequently, trunk groups have labels, which are used as the main
indication for the previous and next TDM switch participating in
routing the call.
Formally, we define a trunk and trunk group and related terminology
as follows (definition of "trunk" and "trunk group" is from [6]).
Trunk: In a network, a communication path connecting two
switching systems used in the establishment of an end-to-end
connection. In selected applications, it may have both its
terminations in the same switching system.
Trunk Group: A set of trunks, traffic engineered as a unit, for
the establishment of connections within or between switching
systems in which all of the paths are interchangeable.
Digital Signal 0 (DS0): Digital Signal X is a term for a series
of standard digital transmission rates based on DS0, a
transmission rate of 64kbps (the bandwidth normally used for one
telephone voice channel). The European E-carrier system of
transmission also operates using the DS series as a base multiple.
Since the introduction of ubiquitous digital trunking, which resulted
in the allocation of DS0s between end offices in minimum groups of 24
(in North America), it has become common to refer to bundles of DS0s
as a trunk. Strictly speaking, however, a trunk is a single DS0
between two PSTN end offices - however, for the purposes of this
document, the PSTN interface of a gateway acts effectively as an end
office (i.e. if the gateway interfaces with Signaling System 7 (SS7),
it has its own SS7 point code, and so on). A trunk group, then, is a
bundle of DS0s (that need not be numerically contiguous in an SS7
Trunk Circuit Identification Code numbering scheme) which are grouped
under a common administrative policy for routing.
A Session Initiation Protocol (SIP) [3] to PSTN gateway may have
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trunks that are connected to different carriers. It is entirely
reasonable for a SIP proxy to choose -- based on factors not
enumerated in this document -- which carrier a call is sent to when
it proxies a session setup request to the gateway. Since multiple
carriers can terminate a particular PSTN phone number, the phone
number itself is not sufficient enough to identify the carrier at the
gateway. An additional piece of information in the form of a trunk
group can be used to further pare down the choices at the gateway.
How the proxy picked a particular trunk group is outside the scope of
this document ([7] provides one such way); once the trunk group has
been decided upon, this document provides a standardized means to
represent it.
3. Problem statement
Currently, there isn't any standardized manner of transporting trunk-
groups between Internet signaling entities. This leads to ambiguity
on at least two fronts:
1. Positional ambiguity: A SIP proxy that wants to send a call to
an egress Voice over IP (VoIP) gateway may insert the trunk-group
as a parameter in the user portion of the Request-URI (R-URI), or
it may insert it as a parameter to the R-URI itself. This
ambiguity persists in the reverse direction as well, that is,
when an ingress VoIP gateway wants to send a incoming call
notification to its default outbound proxy.
2. Semantic ambiguity: The lack of any standardized grammar to
represent trunk groups leads to the unfortunate choice of ad hoc
names and values.
VoIP routing entities in the Internet, such as SIP proxies, may be
interested in using trunk-group information for normal operations.
To that extent, any standards-driven requirements will enable proxies
from one vendor to interoperate with gateways from yet another
vendor. Absent such guidelines, inter-operability will suffer as a
proxy vendor must conform to the expectations of a gateway as to
where it expects trunk-group information to be present (and vice
versa).
The aim of this specification is to outline how to structure and
represent the trunk group information as an extension to the tel URI
[4] in a standardized manner.
4. Requirements and rationale
4.1 sip URI or tel URI?
REQ 1: Trunk group information MUST be defined as an extension to
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the "tel" URI [4].
The trunk group information can be carried in either the "sip" URI or
the "tel" URI. Since trunks groups are intimately associated with
the PSTN, it seems reasonable to define them as extensions to the
"tel" URI (any SIP request that goes to a gateway could reasonably be
expected to have a tel URL, in whole or in part, in its R-URI
anyway). Furthermore, using the tel URL also allows this format to
be reused by non-SIP VoIP protocols (which could include anything
from MGCP or Megaco to H.323, if the proper information elements are
created).
Finally, once the trunk-group is defined for a "tel" URI, the
normative procedures of Section 19.1.6 of [3] can be used to derive
an equivalent "sip" URI from a "tel" URI, complete with the trunk
group information.
4.2 Trunk group namespace: global or local?
REQ 2: Inter-domain trunk group name collisions MUST be prevented.
Under normal operations, trunk groups are pertinent only within an
administrative domain (i.e. local scope). However, given that
inadvertent cross-domain trunk group name collisions may occur, it is
desirable to to prevent these. The judicious use of namespaces is a
solution to this problem. A trunk group in a URI MUST belong to a
namespace.
At first glance, it would appear that the use of the tel URI's
"phone-context" parameter provides a satisfactory means of
imposing a namespace on a trunk group. The "phone-context"
parameter identifies the scope of validity of a local telephone
number. And therein lies the problem. Semantically, a "phone-
context" tel URI parameter is applicable only to a local telephone
number and not a global one (i.e., one preceeded by a '+'). Trunk
groups, on the other hand, may appear in local or global telephone
numbers. Thus, what is needed is a new parameter with equivalent
functionality of the "phone-context" parameter of the tel URI, but
one that is equally applicable to local and global telephone
numbers.
4.3 Originating trunk group and terminating trunk group
REQ 3: Originating trunk group and destination trunk group SHOULD be
able to appear separately and concurrently in a SIP message.
SIP routing entities can make informed routing decisions based on
either the originating or the terminating trunk groups. Thus a
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requirement that both of these trunk groups need to be carried in SIP
requests.
Instead of having two parameters, one for the originating trunk group
and the other for a terminating trunk group, the placement of the
trunk group parameter in a SIP Contact header or the R-URI,
respectively, signifies the intent. A proxy (or any other
intermediary -- see next section) MAY insert a trunk group appears in
the R-URI; to the next downstream entity this signifies an egress (or
terminating) trunk group to be used.
Conversely, when a trunk group parameter appears in the Contact
header, this signifies the trunk group over which the call came over,
i.e., the ingress (or orgininating) trunk group.
4.4 Intermediary processing of trunk groups
REQ 4: SIP network intermediaries (proxy server and redirect
servers) should be able to add the destination trunk group attribute
to SIP sessions as a route is selected for a call.
If the trunk group parameter appears in a R-URI of a request, it
represents the destination trunk group.
This is consistent with using the R-URI as a routing element; SIP
routing entities may use the trunk group parameter in the R-URI to
make intelligent routing decisions. Furthermore, this also
satisfies REQ 4, since a SIP network intermediary can modify the
R-URI to include the trunk group information.
To the processing User Agent Server (UAS), a trunk group in the R-URI
implies that it should use the named trunk group for the outbound
call. If a UAS supports trunk groups but is not configured with the
particular trunk group identified in the R-URI, it SHOULD not use any
other trunk group other than the one requested.
A User Agent Client (UAC) that initiates a call and supports this
specification MAY include the trunk group in the Contact header. If
it does so, the trunk group in the Contact header represents the
originating trunk group. Subsequent requests destined to that UAC
MUST copy the trunk group from the Contact header into the R-URI.
Note that a Contact URI MUST be a sip or sips URI, thus, what appears
in the Contact header is a SIP translation of the tel URI, complete
with the trunk group information.
Arguably, the originating trunk group can be part of the From URI.
However, semantically, the URI in a From header is an abstract
identifier which represents the resource thus identified on a
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long-term basis. The presence of a trunk group, on the other
hand, signifies a binding that is valid for the duration of the
session only; a trunk group has no significance once the session
is over. Thus, the Contact URI is the best place to impart this
information since it has exactly those semantics.
5. Reference architecture
Consider Figure 1, which depicts a SIP proxy in a routing
relationship with three gateways in its domain, GW1, GW2, and GW3.
Among other sources of request arrival (not shown in Figure) at the
proxy is GW1. Gateways GW2 and GW3 are used as egress gateways from
the domain. GW2 has two trunk groups configured, TG2-1 and TG2-2.
GW3 has only one trunk group configured (TG3-1).
+-----+ TG2-1
| |--------> To
TG1-1 +-----+ +-------+ +---->| GW2 | TG2-2 PSTN
From ----->| | | SIP | | | |-------->
PSTN | GW1 |--->| Proxy |-----+ +-----+
----->| | +-------+ | +-----+ TG3-1
+-----+ | | |--------> To
+---->| GW3 | PSTN
| |-------->
+-----+
Figure 1: Reference architecture
GW1 in Figure 1 is always cognizant of any requests that arrive over
trunk group TG1-1. If it wishes to propagate the ingress trunk group
to the proxy, it MUST arrange for the trunk group to appear in the
Contact header of the SIP request destined to the proxy. The proxy
will, in turn, propagate the ingress trunk group in the Contact
header further downstream.
The proxy uses GW2 and GW3 as egress gateways to the PSTN. It is
assumed that the proxy has access to a routing table for GW2 and GW3
which includes the appropriate trunk groups to use when sending a
call to the PSTN (exactly how this table is constructed is out of
scope for this specification; [7] is one way to do so, a manually
created and maintained routing table is another). When the proxy
sends a request to either of the egress gateways, and the gateway
routing table is so configured that a trunk group is required by the
gateway, the proxy MUST arrange for the trunk group to appear in the
SIP R-URI of the request destined to that gateway.
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6. Trunk group identifier: ABNF and examples
The Augmented Backus Naur Form [2] syntax for a trunk group
identifier is given below and extends the "par" production rule of
the tel URI defined in [4]:
par = parameter / extension / isdn-subaddress / trunk-group /
trunk-context
trunk-group = ";tgrp=" trunk-group-label
trunk-context = ";trunk-context=" descriptor
trunk-group-label = *1( unreserved / escaped /
trunk-group-unreserved )
trunk-group-unreserved = "/" / "&" / "+" / "$"
trunk-group-unreserved is the intersection of param-unreserved
defined in [4] and user-unreserved defined in [3]. Since the
trunk group is an extension to the tel URI and will end up as the
user portion of a SIP URI, the ABNF for it has to ensure that it
can be adequately represented in both the constructs.
descriptor is defined in [4].
Trunk groups are identified by two parameters: "tgrp" and "trunk-
context"; both of these parameters MUST be present in a tel URI to
identify a trunk group. All implementations conforming to this
specification MUST generate both of these parameters when using trunk
groups. If an implementation receives a tel URI with only one of the
"tgrp" or "trunk-context" parameters, it MUST act as if there were
not any trunk group identifiers present at all in that URI. Whether
or not to further process such an URI is up to the discretion of the
implementation, however, if a decision is made to process it, the
implementation must act as if there were not any trunk group
identifiers present in the URI.
The "trunk-context" and "tgrp" parameter SHOULD use lower-case
characters as tel URIs may be used within contexts where comparisons
are case sensitive.
The "trunk-context" parameter imposes a namespace on the trunk group
by specifying a global number or any number of its leading digits
(e.g., +33), or a domain name. Syntactically, it is modeled after
the "phone-context" parameter of the tel URI [4], and semantically,
it serves an equivalent function except that unlike the "phone-
context" parameter, the "trunk-context" parameter can appear in
either a local or global tel URI.
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If the receiver of a SIP request is not the owner of the domain (or
global prefix) specified in the "trunk-context", it MUST treat the
trunk group as if it was not there.
For equivalency purposes, two URIs containing trunk group identifiers
are equivalent according to the base comparison rules of the URIs AND
the values of their "tgrp" and "trunk-context" parameters MUST match.
The base comparison rules of a tel URI are specified in Section 4 of
[4], and the base comparison rules of a sip URI are specified in
Section 19.1.4 of [3].
Note that the base comparison rules for a sip or sips URI that has
been derived from a tel URI mandate that the telephone-subscriber
parameters be ordered lexically by parameter name (see Section
19.1.6 of [3]). Thus, when comparing a sip URI that has been
derived from a tel URI, implementations MUST NOT assume that the
"tgrp" and "trunk-context" parameters will appear in a consecutive
manner because parameters defined by other tel URI extensions may
lexically fit between "tgrp" and "trunk-context".
Examples:
1. Trunk group in a local number, with a phone-context parameter
(mind the line breaks added for readibility):
tel:5551212;phone-context=+1-630;tgrp=TG-1;
trunk-context=example.com
Transforming this tel URI to a sip URI yields:
sip:5551212;phone-context=+1-630;tgrp=TG-1;
trunk-context=example.com@isp.example.net
2. Trunk group in a global number:
tel:+16305551212;tgrp=TG-1;trunk-context=example.com
Transforming this tel URI to a sip URI yields:
sip:+16305551212;tgrp=TG-1;trunk-context=example.com@
isp.example.net
3. Trunk group in a global number:
tel:+16305551212;tgrp=TG-1;trunk-context=+1-630
Transforming this tel URI to a sip URI yields:
sip:+16305551212;tgrp=TG-1;trunk-context=+1-630@isp.example.net
7. Example call flows
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7.1 Basic Call Flow
This example uses the reference architecture of Figure 1. Gateways
GW1, GW2, GW3 and the SIP proxy are assumed to be owned by a service
provider whose domain is example.com.
GW1 SIP Proxy GW2
From | | |
PSTN-->| | |
+---F1--------->| |
| +---F2----------->|
... ... ...
| | | Send to PSTN
| | | --> and receive Answer
| | | Complete Message
-----------------------------------------
Call in progress
-----------------------------------------
| | |
| |<-----------F3---+
+<--------------+ |
... ... ...
In the call flow below, certain headers and messages have been
omitted for brevity. In F1, GW1 receives a call setup request from
the PSTN over a certain trunk group. GW1 arranges for this trunk
group to appear in in the Contact header of the request destined to
the SIP proxy.
F1:
INVITE sip:+16305554554@example.com
...
Contact: <sip:4554;phone-context=example.com;tgrp=TG1-1;
trunk-context=example.com@gw1.example.com>
In F2, the SIP proxy translates the R-URI and adds a destination
trunk group to the R-URI. The request is then sent to GW2.
F2:
INVITE sip:+16305554554;tgrp=TG2-1;trunk-context=example.com@
gw2.example.com
...
Record-Route: <sip:proxy.example.com;lr>
Contact: <sip:4554;phone-context=example.com;tgrp=TG1-1;
trunk-context=example.com@gw1.example.com>
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Once the call is established, either end can tear the call down. For
illustrative purposes, F3 depicts GW2 tearing the call down. Note
that the Contact from F1, including the trunk group information, is
now the R-URI of the request. When GW1 gets this request, it can
associate the call with the appropriate trunk group to deallocate
resources.
F3:
BYE sip:4554;phone-context=example.com;tgrp=TG1-1;
trunk-context=example.com@gw1.example.com
Route: <sip:proxy.example.com;lr>
It is worth documenting the behavior when an incoming call from the
PSTN is received at a gateway without a calling party number.
Consider Figure 1; in it, GW1 gets a call request from the PSTN
without a calling party number. This is not an uncommon case, and
may happen for a variety of reasons, including privacy and
interworking between different signaling protocols before the request
reached GW1. Under normal circumstances (i.e., instances where the
calling party number is present in signaling), GW1 would derive a sip
URI to insert into the Contact header. This sip URI may contain, as
its user portion, the calling party number from from the incoming SS7
signaling information. The trunk group information then becomes part
of the user portion as discussed previously.
If a gateway receives an incoming call where the calling party number
is not available, it MUST create a sip URI that has, as its user
portion, a token that is generated locally and has local significance
to the gateway. Details of generating such a token are
implementation dependent; potential candidates include the gateway
line number or port number where the call was accepted. This sip URI
is subsequently inserted in the Contact header of the SIP request
going downstream from the gateway.
The tel scheme does not allow for an empty URI; thus the global-
number or local-number production rule of the tel URI [4] cannot
contain an empty string. Consequently, the behavior in the above
paragraph is mandated for cases where the incoming SS7 signaling
message does not contain a calling party number.
7.2 Inter-domain Call Flow
This example demonstrates the advantage of namespaces in trunk group.
In the example flow below, GW1 and SIP Proxy 1 belong to the
example.com domain and SIP Proxy 2 belongs to another domain,
example.net. A call arrives at GW1 (F1) and is routed to the
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example.net domain. In the call flow below, certain headers and
messages have been omitted for brevity.
example.com example.net
/-------------------------\ /-----------\
GW1 SIP Proxy 1 SIP Proxy 2
From | | |
PSTN-->| | |
+---F1--------->| |
| +---F2----------->|
... ... ...
F1:
INVITE sip:+16305554554@example.com
...
Contact: <sip:4554;phone-context=example.com;tgrp=TG1-1;
trunk-context=example.com@gw1.example.com>
In F2, the SIP proxy executes its routing logic and re-targets the
R-URI to refer to a resource in example.net domain.
F2:
INVITE sip:+16305554554@example.net
...
Record-Route: <sip:proxy.example.com;lr>
Contact: <sip:4554;phone-context=example.com;tgrp=TG1-1;
trunk-context=example.com@gw1.example.com>
Once the request is in the domain of example.net, the namespace
imposed by the "trunk-context" parameter in the Contact header
prevents collisions with similiar trunk groups maintained by the
example.net domain.
8. Security considerations
The extension defined in this document does not add any additional
security concerns beyond those already enumerated in [3]. The trunk
group information is carried in R-URIs and Contact headers; it is
simply a modifier of an address, and the trust imparted to that
address is not affected by such a modifier. The privacy information
revealed with trunk groups does not generally advertise much
information about a particular (human) user. It does, however,
convey two pieces of potentially private information which may be
considered sensitive by carriers. First, it may reveal how a carrier
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may be performing least-cost routing and peering; and secondly, it
does introduce an additional means for network topology and
information of a carrier. It is up to the discretionary judgment of
the carrier if it wants to include the trunk group information and
reveal potentially sensitive information on its network topology.
9. IANA considerations
The IANA registry for tel URIs is specified in [5]. Among other tel
URI parameters, the tel URI IANA registry specification is initially
seeded with the two trunk group parameters contained in this
specification. For the sake of completeness, the IANA template for
the two trunk group parameters is also specified below.
Parameter name: tgrp
Description: A trunk group on which an incoming call was received
at an ingress gateway or a trunk group on which an outgoing call
should be placed at an egress gateway.
This parameter is not mandatory.
Syntax restrictions: Details on the syntax are explained in RFC
AAAA. A phone-context parameter must occur in any tel URL that
contains a tgrp parameter.
Reference: RFC AAAA
[Note to RFC editor: Please replace AAAA with the RFC number
assigned to this document.]
Parameter name: trunk-context
Description: The context for a trunk group; imposes a namespace.
This parameter is not mandatory.
Syntax restrictions: Details on the syntax are explained in RFC
AAAA. A trunk-context parameter must occur in any tel URL that
contains a tgrp parameter.
Reference: RFC AAAA
[Note to RFC editor: Please replace AAAA with the RFC number
assigned to this document.]
10. Acknowledgments
The authors would like to acknowledge the efforts of the participants
of the SIPPING and IPTEL working group, especially Bryan Byerly, John
Hearty, Alan Johnston, Shan Lu, Rohan Mahy, Jon Peterson, Mike
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Pierce, Adam Roach, Brian Rosen, Jonathan Rosenberg, Dave Oran,
Takuya Sawada, Tom Taylor, and Al Varney.
Alex Mayrhofer brought up the issue of lexicographic ordering of tel
URI parameters when it is converted to a sip or sips URI.
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] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[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] Schulzrinne, H., "The tel URI for Telephone Calls", RFC 3966,
December 2004.
[5] Jennings, C. and V. Gurbani, "The Internet Assigned Number
Authority (IANA) tel Uniform Resource Identifier (URI)
Parameter Registry", draft-jennings-iptel-tel-reg-01.txt (work
in progress), December 2005.
11.2 Informative References
[6] "Bellcore Notes on the Network", Telcordia SR2275, Dec 1997,
<http://www.telcordia.com>.
[7] Bangalore, M., Kumar, R., Rosenberg, J., Salama, H., and D.
Shah, "A Telephony Gateway REgistration Protocol (TGREP)",
draft-ietf-iptel-tgrep-06.txt (work in progress), July 2005.
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Authors' Addresses
Vijay Gurbani
Lucent Technologies, Inc./Bell Laboratories
2000 Lucent Lane
Rm 6G-440
Naperville, IL 60566
USA
Phone: +1 630 224 0216
Email: vkg@lucent.com
Cullen Jennings
Cisco Systems
170 West Tasman Drive
Mailstop SJC-21/3
San Jose, CA 95134
USA
Phone: +1 408 421 9990
Email: fluffy@cisco.com
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Internet-Draft Trunk groups in tel/sip URIs Dec 2005
Intellectual Property Statement
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Gurbani & Jennings Expires June 8, 2006 [Page 16]
