INTERNET DRAFT Weibin Zhao draft-zhao-slp-remote-da-discovery-01.txt Henning Schulzrinne January 22, 2002 Columbia University Expires: July 22, 2002 Chatschik Bisdikian William Jerome IBM Remote Discovery in SLP 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. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract This document describes remote discovery in the Service Location Protocol (SLP) via DNS SRV. It defines the name of DNS SRV RR for the SLP service, and gives the steps for remote discovery in SLP. To map the SLP service, a medium SLP domain (SLPD) can list its Directory Agents (DAs) as DNS SRV RRs directly. But a small or large SLPD needs to deploy Gateway DAs (GDAs), and list these GDAs as DNS SRV RRs. Zhao, et al. Expires: July 22, 2002 [Page 1] Internet Draft SLP Remote Discovery January 22, 2002 1. Introduction The Service Location Protocol (SLP [1]) provides a lightweight mechanism for service discovery within one administrative domain. SLP is not designed for global service discovery as it uses multicast and flat scope names. However, beyond the local domain, SLP can be used for service discovery in specific remote domains. There are three configurations for an SLP domain (SLPD): small SLPD without Directory Agents (DAs), medium SLPD with all DAs in the same scope, and large SLPD with DAs in different scopes. The key issue for remote discovery in SLP is to enable a User Agent (UA) to learn about remote DAs (in a medium/large SLPD) or remote SAs (in a small SLPD) without relying on multicast. DNS SRV [2] is a mechanism to map a specific service in a specific domain to a list of server names. All servers in the list should be equivalent except for their priorities and weights for selection purpose. To map the SLP service, a medium SLPD can list its DAs as DNS SRV RRs directly. But a small or large SLPD cannot list its SAs or DAs as DNS SRV RRs because these SAs or DAs are not equivalent. Instead, a small or large SLPD needs to deploy Gateway DAs (GDAs), and list these GDAs as DNS SRV RRs. In this document, we describe how to deploy GDAs for small and large SLPDs, define the name of DNS SRV RR for the SLP service, and give the steps for remote discovery in SLP. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted according to RFC 2119 [3]. 2. Gateway DA A GDA is a special DA. It MUST have an empty scope (""), have the "slpd-gateway" attribute, and be configured to serve in either a small SLPD or a large SLPD. In a small SLPD, a GDA listens for SAAdvert multicast, and records all SAAdverts in its SLPD. In a large SLPD, a GDA listens for DAAdvert multicast, and records all DAAdverts in its SLPD. A GDA does not accept any SrvReg or SrvDeReg from SAs as it has an empty scope. Like regular DAs, a GDA answers a "service:directory-agent" SrvRqst multicast with its own DAAdvert. But when a GDA receives a "service:directory-agent" or "service:service-agent" SrvRqst via TCP from a UA, it replies with a list of all DAAdverts or SAAdverts in its SLPD accordingly. Zhao, et al. Expires: July 22, 2002 [Page 2] Internet Draft SLP Remote Discovery January 22, 2002 3. DNS SRV for SLP service The name of DNS SRV RR for the SLP service has the following format: _slp._tcp. where is a domain name (such as example.com). Note that "slp" is the symbolic name for the SLP service in Assigned Numbers [4], as required by RFC 2782 [2]. For instance, if a UA makes a standard DNS query [5] for SRV RRs of the SLP service using the name: _slp._tcp.example.com then the UA will receive a list of SRV RRs (which matches the query) from a DNS reply, such as _slp._tcp.example.com IN SRV 0 0 427 gda1.example.com _slp._tcp.example.com IN SRV 0 0 427 gda2.example.com 4. Steps for Remote Discovery in SLP Assume that all services in domain D are maintained by SLP, and D uses DNS SRV to map its SLP service. The steps for a remote client C to discover services in D are as follows. (1) C makes a DNS query for SRV RRs of the SLP service in D, and gets a DA list (regular DAs for a medium SLPD, GDAs otherwise) from a DNS reply. (2) C selects one DA (say Z) from the DA list based on some metrics described in RFC 2782 [2] or randomly, then makes a TCP connection to Z. (3) C issues a "service:directory-agent" SrvRqst via TCP to Z, and gets a list of DAAdverts. (4) There are three cases for this DAAdvert list: o If it has one DAAdvert of a regular DA, then D is a medium SLPD. o If it only has DAAdverts of GDAs, then D is a small SLPD. o If it has DAAdverts of both GDAs and regular DAs, then D is a large SLPD. Zhao, et al. Expires: July 22, 2002 [Page 3] Internet Draft SLP Remote Discovery January 22, 2002 (5) If D is a small SLPD, C issues a "service:service-agent" SrvRqst via TCP to Z, and gets a list of SAAdverts. (6) After finding out regular DAs or SAs in D, C can query these remote DAs or SAs via unicast to discover desired services. 5. Security Considerations To enable remote discovery in SLP, local domain information is exposed to external users. Thus, access control is important to protect valuable service information. Standard SLP authentication mechanism SHOULD be used. As this document describes a method for remote discovery in SLP via DNS SRV, the security considerations for DNS SRV are inherited by this document. 6. Acknowledgements Erik Guttman and Kevin Arnold provided valuable comments for this document. 7. References [1] E. Guttman, C. Perkins, J. Veizades and M. Day, "Service location protocol, version 2", RFC 2608, June 1999. [2] A. Gulbrandsen, P. Vixie and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [3] S. Bradner, "Key words for use in RFCs to indicate requirement levels", BCP 14, RFC 2119, March 1997. [4] J. Reynolds and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. [5] P. Mockapetris, "Domain Names - Concepts and Facilities", STD 13, RFC 1034, November 1987. 8. Authors' Addresses Weibin Zhao Henning Schulzrinne Department of Computer Science Columbia University 1214 Amsterdam Avenue, MC 0401 New York, NY 10027-7003 Email: {zwb,hgs}@cs.columbia.edu Zhao, et al. Expires: July 22, 2002 [Page 4] Internet Draft SLP Remote Discovery January 22, 2002 Chatschik Bisdikian William F. Jerome IBM T. J. Watson Research Center P.O.Box 218 Yorktown Heights, NY 10598-0218 Email: {bisdik,wfj}@us.ibm.com 9. Full Copyright Statement Copyright (C) The Internet Society (2002). 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 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 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. Zhao, et al. Expires: July 22, 2002 [Page 5]