Internet Draft Internet Engineering Task Force SIP WG Internet Draft G.Nair, H.Schulzrinne draft-ietf-sip-dhcp-01.txt Columbia University April 6, 2000 Expires: October 2000 DHCP Option for SIP Servers 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 defines a DHCP option that contains a pointers to one or more SIP servers. This is one of the many methods that a SIP client can use to obtain the addresses of a local oubound SIP server. Table of Contents 1 Terminology ......................................... 2 2 Introduction ........................................ 2 3 Overview ............................................ 2 4 SIP server DHCP options ............................. 2 5 Security Consideration .............................. 3 6 Acknowledgements .................................... 3 7 Authors' Addresses .................................. 4 8 Bibliography ........................................ 4 G.Nair, H.Schulzrinne [Page 1] Internet Draft April 6, 2000 1 Terminology DHCP client: A DHCP [1] client is an Internet host that uses DHCP to obtain configuration parameters such as a network address. DHCP server: A DHCP server is an Internet host that returns configuration parameters to DHCP clients. SIP server: As defined in RFC 2543 [2]. In the context of this document, a SIP server refers to the host the SIP server is running on. SIP client: As defined in RFC 2543. In the context of this document, a SIP client refers to the host the SIP client is running on. In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [3]. 2 Introduction The Session Initiation Protocol (SIP) [2] is an application-layer control protocol that can establish, modify and terminate multimedia sessions or calls. In particular, it is used for signaling of Internet telephony calls. A SIP system has two components: user agents and servers. The user agent is the SIP end system that acts on behalf of someone who wants to participate in a SIP call. This draft specifies a DHCP option [1,4] that allows SIP user agents (clients) to locate a local SIP server that is to be used for all outbound SIP requests. (SIP clients MAY contact the address identified in the SIP URL directly, without involving a local SIP server. However in some circumstances, when firewalls are present, SIP clients need to use a local server for outbound requests.) This is one of many possible solutions for locating the outbound SIP server. 3 Overview The SIP client obtains a DNS [5] string via a DHCP option. The SIP client first uses the SRV [6] resource records to resolve the host name. If this fails, the A resource records are tried. 4 SIP server DHCP options This option specifies the DNS [5] string that is passed to the G.Nair, H.Schulzrinne [Page 2] Internet Draft April 6, 2000 client. This string SHOULD be the domain name of the SIP server. The client SHOULD first use this string to send an SRV query to the DNS server. If the client is not SRV-cognizant or the SRV query fails, the client sends the same string in an A record query. The code for this option is TBD. The length of the DNS name string is specified in `Len'. The maximum length of this string is 255 octets and minimum length is 1 octet. Code Len DNS name of SIP server +-----+-----+-----+-----+-----+-----+-----+-- | TBD | n | s1 | s2 | s3 | s4 | s5 | ... +-----+-----+-----+-----+-----+-----+-----+-- The reason for using the SRV string to obtain the IP address is that load sharing can be implemented more readily by an SRV-cognizant client. The string sent by the DHCP server SHOULD be the domain name of the SIP server. The client uses this string to construct the SRV query. The DHCP server MAY instead choose to send the fully qualified domain name of the SIP server intended to be used in an A record query, but this is NOT RECOMMENDED. The client however, MUST first treat the string as a domain name and use it to construct the SRV query. SIP clients usually support either UDP or TCP, but SIP servers usually support both UDP and TCP. Thus, if the string sent by the DHCP server is intended for use in constructing the SRV query, it MUST NOT contain the Service and Proto [6] fields. The client is aware of the transport protocols that it can support, therefore it is appropriate that the Service and the Proto fields be added by the client. The Service field in this case is always _sip. The Proto fields may be _udp or _tcp depending on the client's capabilities. The client adds the Service and Proto fields to the string before making the SRV query. If the client's SRV query fails, the client MUST use the string originally returned by the DHCP server in an A record query (without adding the Service and Proto fields). 5 Security Consideration There are no security considerations beyond those described in RFC 2132. 6 Acknowledgements We would like to thank Robert Elz, Wenyu Jiang, Peter Koch, Erik Nordmark, Jonathan Rosenberg, Kundan Singh, Sven Ubik and Bernie Volz for their contributions. G.Nair, H.Schulzrinne [Page 3] Internet Draft April 6, 2000 7 Authors' Addresses Gautam Nair Dept. of Computer Science Columbia University 1214 Amsterdam Avenue, MC 0401 New York, NY 10027 USA electronic mail: gnair@cs.columbia.edu Henning Schulzrinne Dept. of Computer Science Columbia University 1214 Amsterdam Avenue, MC 0401 New York, NY 10027 USA electronic mail: schulzrinne@cs.columbia.edu 8 Bibliography [1] R. Droms, "Dynamic host configuration protocol," Request for Comments (Draft Standard) 2131, Internet Engineering Task Force, Mar. 1997. [2] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP: session initiation protocol," Request for Comments (Proposed Standard) 2543, Internet Engineering Task Force, Mar. 1999. [3] S. Bradner, "Key words for use in RFCs to indicate requirement levels," Request for Comments (Best Current Practice) 2119, Internet Engineering Task Force, Mar. 1997. [4] S. Alexander and R. Droms, "DHCP options and BOOTP vendor extensions," Request for Comments (Draft Standard) 2132, Internet Engineering Task Force, Mar. 1997. [5] P. V. Mockapetris, "Domain names - implementation and specification," Request for Comments (Standard) 1035, Internet Engineering Task Force, Nov. 1987. [6] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)," Request for Comments (Proposed Standard) 2782, Internet Engineering Task Force, Feb. 2000. Full Copyright Statement Copyright (c) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to G.Nair, H.Schulzrinne [Page 4] Internet Draft April 6, 2000 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. G.Nair, H.Schulzrinne [Page 5]