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
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

  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
            address.

       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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  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 [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
        (i.e., label octets and label length octets) is restricted to
        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 |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     +---+---+---+---+---+---+---+---+---+---+---+---+---+ | 7
     |'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
  preference.

  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

  [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
      1997.

  [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
      Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
      Session Initiation Protocol", RFC 3261, June 2002.

  [3] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
      (SIP): Locating SIP Servers", RFC 3263, June 2002.

  [4] Bradner, S., "Key words for use in RFCs to indicate requirement
      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
      Progress.
















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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
  USA

  EMail:  [email protected]









































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9.  Full Copyright Statement

  Copyright (C) The Internet Society (2002).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  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.

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.



















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