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

Copyright Notice

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


 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.

Schulzrinne Standards Track [Page 1] RFC 3361 DHCPv4 Option for SIP Servers August 2002

 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 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.

Schulzrinne Standards Track [Page 2] RFC 3361 DHCPv4 Option for SIP Servers August 2002

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].

Schulzrinne Standards Track [Page 3] RFC 3361 DHCPv4 Option for SIP Servers August 2002

 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, "" and "".  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
 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.

Schulzrinne Standards Track [Page 4] RFC 3361 DHCPv4 Option for SIP Servers August 2002

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
 [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

Schulzrinne Standards Track [Page 5] RFC 3361 DHCPv4 Option for SIP Servers August 2002

8. Author's Address

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

Schulzrinne Standards Track [Page 6] RFC 3361 DHCPv4 Option for SIP Servers August 2002

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
 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


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

Schulzrinne Standards Track [Page 7]

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