INTERNET-DRAFT IPTEL WG
October 2002 Vijay K. Gurbani
Expires: April 2003 Lucent Technologies, Inc.
Cullen Jennings
Cisco Systems, Inc.
Jon Peterson
NeuStar
Document: draft-ietf-iptel-trunk-group-00.txt
Representing trunk groups in sip/tel Uniform Resource Identifiers (URIs)
STATUS OF THIS MEMO
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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.
1 Problem
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
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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: There isn't any standardized grammar to
represent trunk groups, leading to the 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 I-D is to outline how to structure and represent the
trunk group information in URIs. The next section contains
definitions for trunks, trunk groups and presents a reference
architecture to aid in the discussion that follows. Section 3
contains the various issues we have identified thus far and derives
requirements from these. It also provides recommendations on how to
meet the requirements. Section 4 provides the ABNF and examples for
a trunk group identifier. Section 5 has a call flow, and section 6
contains security considerations.
2 Introduction
2.1 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 [4].
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 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
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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 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.
2.2 Architecture
<Pull in architecture from Jon P.'s iptel-gwreg-arch I-D>
3 Issues
3.1 "sip" URI or "tel" URI?
REQ 1: Trunk group information MUST be carried in the "tel" URI [2].
The trunk group information can be carried in either the "sip" URI
[1] or the "tel" URI [2, 3]. 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 [1] can be used to derive
an equivalent "sip" URI from a "tel" URI, complete with the trunk-
group parameter.
3.2 Trunk-group namespace: global or local?
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.
REQ 2: To prevent inadverdent inter-domain trunk group naming
collisions, a name space MUST be defined which must be flexible
enough to both accomodate local naming conventions and provide global
naming semantics.
3.3 Originating trunk group and terminating trunk group
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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.
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.
If the trunk group parameter appears in a Contact header of a request
establishing a session (for the purpose of this I-D, that request is
an INVITE only), then it represents the trunk group that a UAC is
using for that dialog (originating trunk group). Subsequent requests
destined to that UA MUST copy the trunk group from the Contact header
into the R-URI.
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.
4 Trunk group identifier: ABNF and examples
The syntax for a trunk group identifier is as follows:
trunk-group = "tgrp" EQUAL trunk-group-token
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trunk-group-token = trunk-group-namespace "=" trunk-group-label
trunk-group-namespace = "local" / trunk-group-namespace-name
trunk-group-namespace-name = *1(unreserved / escaped /
trunk-group-unreserved )
trunk-group-label = *1( unreserved / escaped /
trunk-group-unreserved / "=" )
trunk-group-unreserved = "&" / "+" / "$" / "," / "?" / "/"
This I-D defines a "local" namespace for trunk group names having
local significance only (i.e. the name is valid for a particular
administrative domain). Organizations that need a global namespace
for their trunk groups MUST register a global namespace string with
IANA, thus guaranteeing uniqueness for the namespace.
Example: tel:+14085551212;tgrp=local=tg55/3
The example URI above extends the tel URI with a trunk group
identifier having local significance only. Transforming this "tel"
URI to a "sip" URI yields:
sip:+14085551212;tgrp=local=tg55/3@someprovider.il.us
5 Example call flows
The following call flow depicts a call request arriving at a SIP
proxy through a PSTN gateway on a certain trunk group. The gateway
treats the trunk group over which the call arrives as an originating
trunk group and stores this information in the Contact header (F1).
It then formats and sends an INVITE request to its controlling proxy
for further routing (F1). The proxy chooses an appropriate next hop
server (which may be yet another gateway) and modifies the R-URI,
adding a destination trunk group before sending it downstream (F2).
After the session has been established, the UA playing the part of
the UAS during session establishment sends a BYE request to teardown
the session (F3). Note that the R-URI of this request is the Contact
header from the INVITE request, complete with the trunk group
parameter.
Ingress Next downstream
PSTN Gateway Proxy SIP server
| | | |
| Call Request | | |
+------------->| F1 | |
| +--------------->| F2 |
| | +------------->|
... ... ... ...
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| | | F3 |
| | |<-------------+
| | | |
F1:
INVITE sip:+12025551212@someprovider.il.us SIP/2.0
...
Contact: <sip:gateway1.someprovider.il.us;tgrp=local=1001BSTAOMA01MN>
F2:
INVITE sip:+12025551212;tgrp=local=tg89@UA.someprovider.il.us SIP/2.0
...
Contact: <sip:gateway1.someprovider.il.us;tgrp=local=1001BSTAOMA01MN>
F3:
BYE sip:gateway1.someprovider.il.us;tgrp=local=1001BSTAOMA01MN SIP/2.0
...
6 Security considerations
The extension defined in this I-D does not add any additional
security concerns beyond the normal SIP one. The trunk group
information is carried in Request-URIs and Conatct headers; it is
simply a modifier of an address, and the trust imparted to that
address is not affected by such a modifier. It does, however,
introduce an additional means for network topology and information
about which trunks a domain uses to be propagated beyond that domain.
If this is a privacy concern, then the domain should take precautions
to hide that information before it leaves their trust boundary.
7 IANA considerations
<To fill in>
8 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, Tom Taylor, and Al Varney for insightful
discussions and comments.
AUTHORS' ADDRESSES
Vijay K. Gurbani Cullen Jennings
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Email: vkg@lucent.com Email: fluffy@cisco.com
Jon Peterson
jon.peterson@neustar.biz
Normative References:
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
Session Initiation Protocol", IETF RFC 3216, June 2002,
<http://www.ietf.org/rfc/rfc3261.txt>
[2] A. Vaha-Sipila, "URLs for Telephone Calls", IETF RFC 2806,
April 2000, <http://www.ietf.org/rfc/rfc2806.txt>
[3] H. Sculzrinne, A. Vaha-Sipila, "The tel URI for Telephone
Calls", IETF Internet-Draft, Expires December 2002, Work in
Progress, <http://www.ietf.org/internet-drafts/draft-antti-
rfc2806bis-05.txt>
Informative References
[4] Telcordia, "SR2275: Bellcore Notes on the Network", December
1997, <http://www.telcordia.com>.
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