Internet Engineering Task Force                                   SIP WG
Internet Draft                                     G.Nair, H.Schulzrinne
draft-ietf-sip-dhcp-00.txt
draft-ietf-sip-dhcp-01.txt                           Columbia University
February 27,
April 6, 2000
Expires: August 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
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Abstract

   This document defines a DHCP options option that contain one or more 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 the a local oubound SIP servers. 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

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 application 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 application 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 signalling 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, in particular with firewalls, 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

   We identify two methods of notifying the

   The SIP client of the servers'
   location:

        1. obtains a DNS [5] SRV records: The host name can be resolved by the
             client, using DNS and the name string obtained via a DHCP option.  The SIP
   client first uses the SRV [6] resource records to resolve the host
   name. If this fails fails, the A resource records are tried.

        2.   List of IP addresses: Used in case there is no DNS server
             OR in the case that the client host is not DNS capable.

   Either or both of these methods MAY be used to notify the client of
   the servers' location through DHCP.

4 SIP server DHCP options

   We propose two new DHCP options. Our approach is defined in the
   following sections.

4.1 SIP server DNS name option

   This option specifies the DNS [5] string that is passed to the
   client. This string SHOULD be the domain name of the SIP server 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 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  |  ...
           +-----+-----+-----+-----+-----+-----+-----+--

   Clients SHOULD use this method to locate the SIP server.

   The reason
   to list for using the SRV string and use DNS to resolve obtain the IP address is that
   load sharing can be implemented more readily by an SRV-cognizant
   client.

4.2 SIP  The string sent by the DHCP server IP address option

   This option specifies SHOULD be the list domain name
   of IP addresses indicating the SIP servers
   available server. The client uses this string to construct the client. SRV
   query. The option SHOULD only be used if the client
   does not implement DNS (as in DHCP server MAY instead choose to send the case fully qualified
   domain name of some emebedded systems) OR
   if the DNS SIP server is not responding. We duplicate relevant parts of
   the SRV record [6] intended to be used in an A record
   query, but this option. Each item of is NOT RECOMMENDED.  The client however, MUST first
   treat the list consists of
   an ASCII string with 5 fields separated by a single space (ASCII
   0x20) as shown below:

               <IP Address> <prio> <wt> <port> <prot>

        IP address: This field contains the IP address of 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 dotted quad notation.

        prio: This field indicates constructing the priority of this target host. A
             client SRV query,
   it MUST attempt to contact NOT contain the target host with Service and Proto [6] fields. The client is
   aware of the
             lowest-numbered priority transport protocols that it can reach; target hosts with
             the same priority SHOULD be tried in a round-robin fashion
             starting with a randomly chosen address. The range of
             priorities is 0-65535.

        wt: This field support, therefore it is used by the load balancing mechanism.  When
             selecting a target host among those
   appropriate that have the same
             priority, Service and the chance of trying this one first SHOULD Proto fields be
             proportional to its weight. added by the
   client. The range of Service field in this number case is 1-
             65535. Domain administrators are urged to use Weight 0 when
             there isn't any load balancing to do, to make always _sip. The Proto
   fields may be _udp or _tcp depending on the option
             easier client's capabilities.
   The client adds the Service and Proto fields to read for humans (less noisy).

        port: This field indicates the port on this target SIP server.
             The range is 0-65535. This is often 5060 - as specified in string before
   making the IANA Assigned Numbers, but need not be.

        prot: This field indicates SRV query. If the protocol used by client's SRV query fails, the client
   MUST use the SIP server.
             This can be either `TCP' or `UDP'. The protocol name is
             case sensitive.

   The code for this option is TBD. The length of this option is
   specified in `Len'. The minimum length of this option is 17 octets.
   The maximum length of this option is 255 octets.

            Code  Len       string 1 string 2
          +-----+-----+-----+-----+-----+--   --+-----+-----+--
          | TBD |  n  | s1  | s2  | s3  |  ...  | s13 | s14 |  ...
          +-----+-----+-----+-----+-----+--   --+-----+-----+--

5 Multiple options

   More than one SIP server option MAY be originally returned by the DHCP server.
   More than one of any option types MAY be present. This permits the
   client to select server in an A
   record query (without adding the option that suits its capabilities (DNS-SRV,
   DNS-A, or no DNS capability).

6 Service and Proto fields).

5 Security Consideration

   There are no security considerations beyond those described in RFC
   2132.

7

6 Acknowledgements

   We would like to thank Robert Elz, Wenyu Jiang, Peter Koch, Erik
   Nordmark, Jonathan Rosenberg, Bernie Volz, Kundan Singh, Sven Ubik and Wenyu Jiang Bernie Volz
   for their contributions.

8

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

9

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 and Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
   the location of services (DNS SRV)," Request for Comments (Experimental)
   2052, (Proposed
   Standard) 2782, Internet Engineering Task Force, Oct. 1996. Feb. 2000.

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                           Table of Contents

   1          Terminology .........................................    1
   2          Introduction ........................................    2
   3          Overview ............................................    2
   4          SIP server DHCP options .............................    2
   4.1        SIP server DNS name option ..........................    3
   4.2        SIP server IP address option ........................    3
   5          Multiple options ....................................    4
   6          Security Consideration ..............................    4
   7          Acknowledgements ....................................    4
   8          Authors' Addresses ..................................    4
   9          Bibliography ........................................    5