Internet Engineering Task Force                                   SIP WG
Internet Draft                                             H.Schulzrinne
                                                     Columbia University
November 21, 2001
March 1, 2002
Expires: May August 2002


                     DHCPv4 Option for SIP Servers


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   all provisions of Section 10 of RFC2026.

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   This document defines a DHCP DHCP-for-IPv4 option that contains a single name list of
   domain names or IPv4 address addresses that can be mapped 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 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 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", "REQUIRED",
   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 draft 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 DHC message,
   even if it sends two different instances of the same option. Attempts
   to do so would result in incorrect client behavior as DHC 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 TBD.

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 (i.e., label octets and label
        length octets) is restricted to 255 octets or less.

        RFC 1035 encoding was chosen to accomodate 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 SRV NAPTR records, rather than different A records. Domain
   names SHOULD be listed in order of preference.

   A SIP The
   client obtains a domain name through the DHCP SIP server
   option, which MUST try the client then uses to locate records in the outbound proxy
   server by order listed, applying the
   mechanism described in Section 4.1 of RFC XXXX [3]. In summary, [3] for each. The
   client only resolves the subsequent domain name is used first in a DNS SRV lookup and, names if that fails
   because of a lack of matching DNS SRV records, attempts to
   contact the first one failed or yielded no common transport protocols
   between client and server or denote a domain name is
   used in an address record lookup. Normative details are contained in
   RFC XXXX [3]. 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

        An encoding

   Clients MUST support compression according to section the encoding in Section
   4.1.4 of "Domain Names - Implementation And Specification" [6] does not seem
        appropriate here, since [6].

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

   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 "Encoding Long DHCP
   Options". [7].

   The DHCP option for this encoding has the following format:

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

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

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

3.2 IPv4 Address List

   This option specifies

   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 SHOULD 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
           | TBD |  n  |  1  |  a1 |  a2 |  a3 |  a4 | a1  |  ...

4 Security Consideration

   There are no security

   The ecurity considerations beyond those described in RFC 2131 [1], RFC 2543 [2] and RFC XXXX [3].
   [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.

5 IANA Considerations

   IANA has assigned a DHCP option number of TBD 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 draft.

7 Authors' Addresses

   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 2131, Internet Engineering Task Force, Mar. 1997.

   [2] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 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, Mar. 2001.  Work in progress.

   [4] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

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

   [6] P. V. Mockapetris, "Domain names - implementation and
   specification," Request for Comments 1035, Internet Engineering Task
   Force, Nov. 1987.

   [7] T. Lemon and S. Cheshire, "Encoding long DHCP options," Internet
   Draft, Internet Engineering Task Force, Oct. 2001.  Work in progress.