Internet Engineering Task Force                                   SIP WG
Internet Draft                                     G.Nair, H.Schulzrinne
draft-ietf-sip-dhcp-02.txt                           Columbia University
April 6, 2000
January 15, 2001
Expires: October 2000 June 2001

                      DHCP Option for SIP Servers


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This document defines a DHCP option that contains a pointers to one
   or more SIP outbound proxy servers. This is one of the many methods
   that a SIP client can use to obtain the addresses of such a local oubound SIP

                              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

1 Terminology

        DHCP client: A DHCP [1] client is an Internet host that uses
             DHCP to obtain configuration parameters such as a network

        DHCP server: A DHCP server is an Internet host that returns
             configuration parameters to DHCP clients.

        SIP server: As defined in RFC 2543 [2]. This server MUST be an
             outbound proxy server, as defined in [3]. 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. The client can be a user
             agent client or the client portion of a proxy server. 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", "MUSTNOT", "REQUIRED",
   "OPTIONAL" are to be interpreted as described in RFC 2119 [3]. [4].

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 a number of logical components:
   agents agents, proxy servers, redirect servers and servers.  The user agent is the SIP end system that acts
   on behalf of someone who wants to participate in a registrars. User
   agents MAY contain SIP call. clients, proxy servers always do.

   This draft specifies a DHCP option [1,4] [1,5] that allows SIP user agents
   (clients) clients to
   locate a local SIP server that is to be used for all outbound SIP requests.
   requests, a so-called outbound proxy server. (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. server; manual configuration is an example of another.

3 Overview

   The SIP client obtains a DNS [5] [6] string via a DHCP option.  The SIP
   client first uses  This
   string is then used by the SRV [6] resource records mechanism described in [3] to resolve locate the host
   name. If this fails,
   outbound proxy server. In summary, the A resource records domain name encoded in the
   string is used first in a DNS SRV lookup and, if that fails because
   of a lack of matching DNS SRV records, in an address record lookup.
   Normative details are tried. contained in [3].

4 SIP server DHCP options

   This option specifies the DNS [5] [6] string that is passed to the
   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 server
   (rather than a textual representation of the same string in an A record query. network address). 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. For example, a value may be

             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

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. provided useful

7 Authors' Addresses

   Gautam Nair
   Dept. of Computer Science
   Columbia University 1214 Amsterdam Avenue, MC 0401
   New York, NY 10027
   electronic mail:

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University 1214 Amsterdam Avenue, MC 0401
   New York, NY 10027
   electronic mail:

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] H. Schulzrinne and J. Rosenberg, "SIP: session initiation
   protocol -- locating SIP servers," Internet Draft, Internet
   Engineering Task Force, Jan. 2001.  Work in progress.

   [4] 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.


   [5] S. Alexander and R. Droms, "DHCP options and BOOTP vendor
   extensions," Request for Comments (Draft Standard) 2132, Internet Engineering Task
   Force, Mar. 1997.


   [6] 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.

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