INTERNET DRAFT Weibin Zhao draft-zhao-slp-remote-da-discovery-00.txt Henning Schulzrinne July 13, 2001 Columbia University Expires: January 13, 2002 Chatschik Bisdikian William Jerome IBM Remote Directory Agent 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 presents a lightweight method for remote Directory Agent (DA) discovery in the Service Location Protocol (SLP), which enables User Agents (UAs) to discover services in remote domains. A UA first makes a DNS query for SRV records of SLP DA service to get a list of DAs in a remote domain, then the UA obtains DAAdverts of all remote DAs via a single "service:directory-agent:all" Service Request (SrvRqst) or via multiple "service:directory-agent" SrvRqsts. Thus, the UA can discover remote services by querying remote DAs via unicast in the same way as it does in the local domain. Zhao, et al. Expires: January 13, 2002 [Page 1] Internet Draft SLP Remote DA Discovery July 13, 2001 1. Introduction The Service Location Protocol (SLP [1]) provides a lightweight mechanism for service discovery within one administrative domain. A User Agent (UA) discovers a desired service by multicasting the Service Request (SrvRqst) to Service Agents (SAs) or unicasting the SrvRqst to a Directory Agent (DA). The local DA discovery is achieved via static configuration, DHCP [2], listening to DAAdvert multicast (passive DA discovery), or multicasting the "service:directory-agent" SrvRqst (active DA discovery). SLP is designed for local service discovery in an intranet - it does not aim for being used in the global Internet. However, beyond local domain, SLP can also be used for discovering services in a remote domain where the key issue is how to discover DAs in a remote domain. Currently, the mapping of a domain (such as example.com) to its SLP DAs (such as da1.example.com and da2.example.com) has not been standardized. In this document, we present a lightweight method for remote DA discovery in SLP, which enables UAs to discover services in remote domains. A UA first makes a DNS query [3] for SRV [4] records of SLP DA service to get a list of DAs in a remote domain, then the UA obtains DAAdverts of all remote DAs via a single "service:directory- agent:all" SrvRqst or via multiple "service:directory-agent" SrvRqsts. Thus, the UA can discover remote services by querying remote DAs via unicast in the same way as it does in the local domain. 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 [5]. 2. Remote DA Discovery via DNS SRV One way to enable remote users to discover DAs in a domain is to store DA URLs within the domain using DNS SRV records. The name of the DNS SRV record for SLP DA service has the following format: _slpda._. where is either "tcp" or "udp", and is a domain name (such as example.com). Note that "slpda" is the symbolic name for SLP DA service in Assigned Numbers [6], as required by [4]. For instance, if a UA wants to discover remote DAs at example.com, the UA makes standard DNS query [3] for SRV records of SLP DA service using the name: Zhao, et al. Expires: January 13, 2002 [Page 2] Internet Draft SLP Remote DA Discovery July 13, 2001 _slpda._tcp.example.com or _slpda._udp.example.com then the UA will receive a list of SRV records, which matches the query, from a DNS reply, such as: _slpda._tcp.example.com IN SRV 0 0 427 da1.example.com _slpda._tcp.example.com IN SRV 0 0 427 da2.example.com or _slpda._udp.example.com IN SRV 0 0 427 da1.example.com _slpda._udp.example.com IN SRV 0 0 427 da2.example.com 3. Retrieving DAAdverts of All DAs in a Remote Domain After a UA gets a list of DAs in a remote domain from DNS, the UA needs to obtain DAAdverts of all DAs in the remote domain to get their scope and attribute information. In SLPv2, the DAAdvert of a DA can be solicited by sending a "service:directory-agent" SrvRqst to the DA. Thus, to get all DAAdverts, the UA needs to unicast a "service:directory-agent" SrvRqst to each DA in the DA list obtained from DNS. This method is heavyweight as it needs to put all DA URLs within a domain into DNS to enable the DAs being discovered by remote users, and when DA URLs are changed, the corresponding DNS entries also need to be updated. A more lightweight and efficient way for a UA to obtain DAAdverts of all DAs in a remote domain is to use a special service type called "service:directory-agent:all" in the SrvRqst message. A "service:directory-agent:all" SrvRqst asks the receiving DA to return all matched DAAdverts the DA knows, not only the DA its own DAAdvert. To support the "service:directory-agent:all" SrvRqst, a DA needs to maintain a DAAdvert table for all DAs in its domain. As DAs multicast their DAAdverts periodically, a DA can easily get DAAdverts of other DAs in its domain. By using DAs that support the "service:directory- agent:all" SrvRqst, only a few such DAs (for reliability reason) need to be put into DNS. In summary, to get DAAdverts of all DAs in a remote domain, a UA SHOULD first unicast a "service:directory-agent:all" SrvRqst to any DA in the DA list obtained from DNS. If the UA receives an error from the DA indicating the "service:directory-agent:all" SrvRqst is not supported, then the UA has to use the "service:directory-agent" Zhao, et al. Expires: January 13, 2002 [Page 3] Internet Draft SLP Remote DA Discovery July 13, 2001 SrvRqst to get DAAdvert individually from each DA in the DA list obtained from DNS. 4. Example Consider the following example. Professor X is planning his first visit to University Y, and all services (such as video projectors and printers) at University Y are maintained by SLP DAs. If Professor X wants to learn the available services at University Y before leaving, he needs to first discover DAs at University Y, then query the DAs. The steps for Professor X to discover services at University Y (a remote domain) are as follows. First, he makes a DNS query for SRV records of SLP DA service at University Y, and gets a DA list. Second, he contacts one DA in the DA list obtained from DNS by issuing a "service:directory-agent:all" SrvRqst. If he succeeds, he will get DAAdverts of all DAs at University Y. Otherwise, he has to unicast a "service:directory-agent" SrvRqst individually to each DA in the DA list obtained from DNS. Third, he queries remote DAs at University Y via unicast to discover desired services. 5. Security Considerations To enable remote DA discovery and remote service discovery, local domain information is exposed to external users. Thus, security control is more important to protect valuable service information. Standard SLP authentication mechanism SHOULD be used. As this document describes a method for remote SLP DA discovery via the DNS SRV record, the security considerations for the DNS SRV record are inherited by this document. 6. References [1] E. Guttman, C. Perkins, J. Veizades and M. Day, "Service location protocol, version 2", RFC 2608, June 1999. [2] C. Perkins and E. Guttman, "DHCP options for service location protocol", RFC 2610, June, 1999. [3] P. Mockapetris, "Domain Names - Concepts and Facilities", STD 13, RFC 1034, November 1987. [4] A. Gulbrandsen, P. Vixie and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [5] S. Bradner, "Key words for use in RFCs to indicate requirement levels", BCP 14, RFC 2119, March 1997. Zhao, et al. Expires: January 13, 2002 [Page 4] Internet Draft SLP Remote DA Discovery July 13, 2001 [6] J. Reynolds and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. 7. 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 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 8. Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. 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