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 This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 draft-iptel-trunk-group-00.txt [Page 1] Representing trunk groups in sip/tel URIs October 2002 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 draft-iptel-trunk-group-00.txt [Page 2] Representing trunk groups in sip/tel URIs October 2002 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 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 draft-iptel-trunk-group-00.txt [Page 3] Representing trunk groups in sip/tel URIs October 2002 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 draft-iptel-trunk-group-00.txt [Page 4] Representing trunk groups in sip/tel URIs October 2002 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 | | | +------------->| ... ... ... ... draft-iptel-trunk-group-00.txt [Page 5] Representing trunk groups in sip/tel URIs October 2002 | | | F3 | | | |<-------------+ | | | | F1: INVITE sip:+12025551212@someprovider.il.us SIP/2.0 ... Contact: F2: INVITE sip:+12025551212;tgrp=local=tg89@UA.someprovider.il.us SIP/2.0 ... Contact: 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 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 draft-iptel-trunk-group-00.txt [Page 6] Representing trunk groups in sip/tel URIs October 2002 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, [2] A. Vaha-Sipila, "URLs for Telephone Calls", IETF RFC 2806, April 2000, [3] H. Sculzrinne, A. Vaha-Sipila, "The tel URI for Telephone Calls", IETF Internet-Draft, Expires December 2002, Work in Progress, Informative References [4] Telcordia, "SR2275: Bellcore Notes on the Network", December 1997, . FULL COPYRIGHT STATEMENT "Copyright (C) The Internet Society (date). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING draft-iptel-trunk-group-00.txt [Page 7] Representing trunk groups in sip/tel URIs October 2002 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. draft-iptel-trunk-group-00.txt [Page 8]