IPTEL WG V. Gurbani
Internet-Draft Lucent Technologies
Expires: July 8, 2004 C. Jennings
Cisco Systems
J. Peterson
NeuStar
January 8, 2004
Representing trunk groups in tel/sip URIs
draft-ietf-iptel-trunk-group-01
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a standardized mechanism to convey trunk
group- related information in SIP and TEL URIs. An extension to the
"tel" URI is defined for this purpose.
This work is being discussed on the iptel@ietf.org mailing list.
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem statement . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . 5
5. Reference architecture . . . . . . . . . . . . . . . . . . . 6
6. Trunk group identifier: ABNF and examples . . . . . . . . . 7
7. Example call flows . . . . . . . . . . . . . . . . . . . . . 8
7.1 Trunk group in a Contact header . . . . . . . . . . . . . . 8
7.2 Trunk group in the R-URI . . . . . . . . . . . . . . . . . . 9
8. Security considerations . . . . . . . . . . . . . . . . . . 9
9. IANA considerations . . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 10
11. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1 Changes from trunk-group-00 . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . 12
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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. 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 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. Absence 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 Internet draft is to outline how to structure and
represent the trunk group information as an extension to the tel URI
[5] in a standardized manner.
3. Definitions
Before we take the discussion of trunks any further, we must define
both a trunk and a trunk group and explain the difference between the
two. The following definitions are taken 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
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systems in which all of the paths are interchangeable except where
subgrouped.
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 Public Switched Telephone Network (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 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 (TCIC)
numbering scheme) which are grouped under a common administrative
policy for routing.
A SIP-PSTN gateway may have 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 (reference [7]
provides one such way); once the trunk group has been decided upon,
this document provides a standardized means to represent it.
4. Requirements and rationale
4.1 sip URI or tel URI?
REQ 1: Trunk group information MUST be carried in the "tel" URI [5].
The trunk group information can be carried in either the "sip" URI
[3] or the "tel" URI [4,5]. Since trunks groups are intimately
associated with the PSTN (Public Switched Telephone Network), 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-U anyway).
Furthermore, using the tel URL also allows this format to be re-used
by non-SIP VoIP protocols (which could include anything from MGCP or
Megaco to H.323, if the proper IEs are created).
Finally, once the trunk-group is defined for a "tel" URI, the
normative procedures of Section 19.1.6 in [3] can be used to derive
an equivalent "sip" URI from a "tel" URI, complete with the trunk
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group information.
4.2 Trunk group namespace: global or local?
REQ 2: To prevent inadvertent inter-domain trunk group naming
collisions, a name space MUST be defined which must be flexible
enough to both accommodate local naming conventions and provide
global naming semantics.
Under normal operations, trunk groups have meaning only within an
administrative domain (i.e. local scope). However, to prevent
inadvertent cross-domain trunk group collisions (which, given
Murphy's law, will happen), a global scope appears to be useful. The
"phone-context" parameter of the tel URI is used to impose a
namespace by specifying a domain where the trunk groups are
understood.
The use of the "phone-context" parameter in conjunction with this
draft is mandatory; implementations choosing to include trunk groups
in SIP signaling MUST be capable of parsing and generating the
"phone-context" parameter of the tel URI. If a receiver of a SIP
request is not the owner of the domain specified in the
"phone-context", it MUST treat the trunk group as if it was not
there.
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
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.
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.
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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 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 UAC that initiates a call and supports this draft 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 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
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.
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+-----+ TG2-1
| |--------> To
TG1-1 +-----+ +-------+ +---->| GW2 | TG2-2 PSTN
From ----->| | | SIP | | | |-------->
PSTN | GW1 |--->| Proxy |-----+ +-----+
----->| | +-------+ | +-----+ TG3-1
+-----+ | | |--------> To
+---->| GW3 | PSTN
| |-------->
+-----+
Figure 1: Reference architecture
On requests arriving to the proxy from GW1, the proxy will have
access to the trunk group TG1-1 if the request arrived on that
particular trunk. If the receiving gateway (GW1, in this case) wants
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 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 draft; [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.
6. Trunk group identifier: ABNF and examples
The ABNF syntax [2] for a trunk group identifier is given below and
extends the "par" production rule of the tel URI defined in [5]:
par = parameter / extension / isdn-subaddress / trunk-group
trunk-group = ";tgrp=" trunk-group-label
trunk-group-label = *1( unreserved / escaped /
trunk-group-unreserved )
trunk-group-unreserved = "/" / "&" / "+" / "$"
trunk-group-unreserved is the intersection of param-unreserved
defined in [5] and user-unreserved defined in [3]. Since the
trunk group is an extension to the tel URI and will end up as the
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user portion of a SIP URI, the ABNF for it has to ensure that it
can be adequately representated in both the constructs.
Examples:
tel:+15555551212;tgrp=tg55;phone-context=telco.example.com
tel:0216;tgrp=TG-1;phone-context=+1-555-555
The example URIs above extends the tel URI with a trunk group
identifier. Transforming these "tel" URI to "sip" URIs yields,
respectively:
sip:+15555551212;tgrp=tg55;phone-context=telco.example.com@isp.example.net
sip:0216;tgrp=TG-1;phone-context=+1-555-555@isp.example.net
7. Example call flows
7.1 Trunk group in a Contact header
This call flow depicts a gateway accepting a call from the PSTN and
conversing with its SIP proxy in order to further route the call
(F1). At some later time after the call has been established, the
gateway receives a BYE (F2) to tear down the call (note the R-URI in
the BYE).
PSTN Ingress Proxy
Gateway
| | |
|Call Request | |
+------------->| |
| +---F1----->|
... ... ...
| |
| |<----F2---
F1:
INVITE sip:40216@isp.example.net SIP/2.0
...
Contact: <sip:40216;tgrp=tg55;phone-context=isp.example.net@igwy.isp.example.net>
...
F2:
BYE sip:40216;tgrp=tg55;phone-context=isp.example.net@igwy.isp.example.net
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...
7.2 Trunk group in the R-URI
This call flow depicts a proxy sending a request to a gateway with a
particular trunk group (F1). The gateway sends the request to the
PSTN; when the callee picks up the phone, a 200 OK is generated
towards the UAC (F2). Note the Contact of the 200 OK; it contains
the trunk group information. Subsequent requests from the UAC will
contain this URI as the R-URI.
UAC Proxy Egress
Gateway
| | |
|Call Request | |
+------------->| |
| +---F1------>|
| | |---> Interface with PSTN and
| | | received Answer Complete Message (ACM)
| +<-------F2--+
| | |
... ... ...
F1:
INVITE sip:+15555551212;tgrp=TG2-1;phone-context=isp.example.net@egwy.isp.example.net SIP/2.0
...
F2:
SIP/2.0 200 OK
...
Contact: <sip:1212;tgrp=TG2-1;phone-context=isp.example.net@egwy.isp.example.net>
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 Request-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 reveal much information
about a particular (human) user. It does, however, reveal two pieces
of potentially private information which may be considered sensitive
by carriers. First, it may reveal how a carrier may be performing
least-cost routing and peering; and secondly, it does introduce an
additional means for network topology and information of a carrier.
If revealing this information is considered a privacy concern by a
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carrier, it should take precautions to hide it.
9. IANA considerations
Section 9 of [5] creates a registry for tel URI parameters. This
document updates the registry with the following entry:
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.]
10. Acknowledgments
The authors would like to acknowledge the efforts of all the
participants in the SIPPING and IPTEL working group. Special thanks
to John Hearty, Alan Johnston, Rohan Mahy, Mike Pierce, Adam Roach,
Jonathan Rosenberg, Bryan Byerly, Dave Oran, Tom Taylor, and Al
Varney for insightful discussions and comments.
11. Changes
11.1 Changes from trunk-group-00
1. Changed tgrp=local syntax.
2. Introduction of "phone-context" parameter.
3. Redid the Examples section and added an architecture diagram.
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] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April
2000.
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[5] Schulzrinne, H. and A. Vaha-Sipila, "The tel URI for Telephone
Calls", draft-ietf-iptel-rfc2806bis-02 (work in progress), July
2003.
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-02.txt (work in progress), December 2003.
Authors' Addresses
Vijay Gurbani
Lucent Technologies
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
Jon Peterson
NeuStar
1800 Sutter St.
Suite 570
Concord, CA 94520
USA
Phone: +1 925 363 8720
EMail: jon.peterson@neustar.biz
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