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Network Working Group                                     H. Schulzrinne
Request for Comments: 3361                           Columbia University
Category: Standards Track                                    August 2002

          Dynamic Host Configuration Protocol (DHCP-for-IPv4)
          Option for Session Initiation Protocol (SIP) Servers

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state

Copyright Notice



   This document defines a Dynamic Host Configuration Protocol
   (DHCP-for-IPv4) option that contains a list of domain names or IPv4
   addresses that can be mapped to one or more Session Initiation
   Protocol (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 SIP server.

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 3261 [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 3261.  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.

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   and "OPTIONAL" are to be interpreted as described in RFC 2119 [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.  A SIP system has a number of logical components:
   user agents, proxy servers, redirect servers and registrars.  User
   agents MAY contain SIP clients, proxy servers always do.

   This document specifies a DHCP option [1,5] that allows SIP clients
   to locate a local SIP server that is to be used for all outbound SIP
   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, for
   example, 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; manual configuration
   is an example of another.

3.  SIP Server DHCP Option

   The SIP server DHCP option carries either a 32-bit (binary) IPv4
   address or, preferably, a DNS (RFC 1035 [6]) fully-qualified domain
   name to be used by the SIP client to locate a SIP server.

   The option has two encodings, specified by the encoding byte ('enc')
   that follows the code byte.  If the encoding byte has the value 0, it
   is followed by a list of domain names, as described below (Section
   3.1).  If the encoding byte has the value 1, it is followed by one or
   more IPv4 addresses (Section 3.2).  All implementations MUST support
   both encodings.  The 'Len' field indicates the total number of octets
   in the option following the 'Len' field, including the encoding byte.

   A DHCP server MUST NOT mix the two encodings in the same DHCP
   message, even if it sends two different instances of the same option.
   Attempts to do so would result in incorrect client behavior as DHCP
   processing rules call for the concatenation of multiple instances of
   an option into a single option prior to processing the option [7].

   The code for this option is 120.

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3.1 Domain Name List

   If the 'enc' byte has a value of 0, the encoding byte is followed by
   a sequence of labels, encoded according to Section 3.1 of RFC 1035
   [6], quoted below:

         Domain names in messages are expressed in terms of a sequence
         of labels.  Each label is represented as a one octet length
         field followed by that number of octets.  Since every domain
         name ends with the null label of the root, a domain name is
         terminated by a length byte of zero.  The high order two bits
         of every length octet must be zero, and the remaining six bits
         of the length field limit the label to 63 octets or less.  To
         simplify implementations, the total length of a domain name
         255 octets or less.

   RFC 1035 encoding was chosen to accommodate future internationalized
   domain name mechanisms.

   The minimum length for this encoding is 3.

   The option MAY contain multiple domain names, but these SHOULD refer
   to different NAPTR records, rather than different A records.  The
   client MUST try the records in the order listed, applying the
   mechanism described in Section 4.1 of RFC 3263 [3] for each.  The
   client only resolves the subsequent domain names if attempts to
   contact the first one failed or yielded no common transport protocols
   between client and server or denote a domain administratively
   prohibited by client policy.

         Use of multiple domain names is not meant to replace NAPTR and
         SRV records, but rather to allow a single DHCP server to
         indicate outbound proxy servers operated by multiple providers.

   Clients MUST support compression according to the encoding in Section
   4.1.4 of "Domain Names - Implementation And Specification" [6].

         Since the domain names are supposed to be different domains,
         compression will likely have little effect, however.

   If the length of the domain list exceeds the maximum permissible
   within a single option (254 octets), then the domain list MUST be
   represented in the DHCP message as specified in [7].

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   The DHCP option for this encoding has the following format:

        Code  Len   enc   DNS name of SIP server
      | 120 |  n  |  0  |  s1 |  s2 |  s3 |  s4 | s5  |  ...

   As an example, consider the case where the server wants to offer two
   outbound proxy servers, "example.com" and "example.net".  These would
   be encoded as follows:

      |120|27 | 0 | 7 |'e'|'x'|'a'|'m'|'p'|'l'|'e'| 3 |'c'|'o'|'m'| 0 |
      |'e'|'x'|'a'|'m'|'p'|'l'|'e'| 3 |'n'|'e'|'t'| 0 | +---+---+---

3.2  IPv4 Address List

   If the 'enc' byte has a value of 1, the encoding byte is followed by
   a list of IPv4 addresses indicating SIP outbound proxy servers
   available to the client.  Servers MUST be listed in order of

   Its minimum length is 5, and the length MUST be a multiple of 4 plus
   one.  The DHCP option for this encoding has the following format:

       Code   Len   enc   Address 1               Address 2
      | 120 |  n  |  1  | a1  | a2  | a3  | a4  | a1  |  ...

4.  Security Considerations

   The security considerations in RFC 2131 [1], RFC 2543 [2] and RFC
   3263 [3] apply.  If an adversary manages to modify the response from
   a DHCP server or insert its own response, a SIP user agent could be
   led to contact a rogue SIP server, possibly one that then intercepts
   call requests or denies service.  A modified DHCP answer could also
   omit host names that translated to TLS-based SIP servers, thus
   facilitating intercept.

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5.  IANA Considerations

   IANA has assigned a DHCP option number of 120 for the "SIP Servers
   DHCP Option" defined in this document.

6.  Acknowledgements

   Ralph Droms, Robert Elz, Wenyu Jiang, Peter Koch, Gautam Nair, Thomas
   Narten, Erik Nordmark, Jonathan Rosenberg, Kundan Singh, Sven Ubik,
   Bernie Volz and Dean Willis provided useful feedback through the
   evolution of this document.

7.  Bibliography


       Session Initiation Protocol", RFC 3261, June 2002.

   [3] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol

       levels", BCP 14, RFC 2119, March 1997.

   [5] Alexander, S. and R. Droms, "DHCP options and BOOTP Vendor
       Extensions", RFC 2132, March 1997.

   [6] Mockapetris, P., "Domain names - implementation and
       specification", STD 13, RFC 1035, November 1987.

   [7] Lemon, T. and S. Cheshire, "Encoding Long DHCP Options", Work in

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8. Author's Address

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

   EMail:  schulzrinne@cs.columbia.edu

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9.  Full Copyright Statement


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