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Network Working Group                                     H. SchulzrinneRequest for Comments: 3361                           Columbia UniversityCategory: Standards Track                                    August 2002Dynamic Host Configuration Protocol (DHCP-for-IPv4)Option for Session Initiation Protocol (SIP) ServersStatus 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 inRFC 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 inRFC 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",   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",   and "OPTIONAL" are to be interpreted as described inRFC 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 (Section3.1).  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 20023.1 Domain Name List   If the 'enc' byte has a value of 0, the encoding byte is followed by   a sequence of labels, encoded according toSection 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 inSection 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 inSection4.1.4 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, "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 inRFC 2131 [1],RFC 2543 [2] andRFC3263 [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 20025.  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.Schulzrinne                 Standards Track                     [Page 5]

RFC 3361             DHCPv4 Option for SIP Servers           August 20028. Author's Address   Henning Schulzrinne   Dept. of Computer Science   Columbia University   1214 Amsterdam Avenue, MC 0401   New York, NY 10027   USA   EMail:  schulzrinne@cs.columbia.eduSchulzrinne                 Standards Track                     [Page 6]

RFC 3361             DHCPv4 Option for SIP Servers           August 20029.  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.Schulzrinne                 Standards Track                     [Page 7]