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Network Working Group H. Schulzrinne Request for Comments: 3319 Columbia University Category: Standards Track B. Volz

                                                             July 2003
       Dynamic Host Configuration Protocol (DHCPv6) Options
           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 (2003).  All Rights Reserved.


 This document defines a Dynamic Host Configuration Protocol version 6
 (DHCPv6) option that contains a list of domain names or IPv6
 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

 This document uses the DHCP terminology defined in [1].
 A SIP server is 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 outbound SIP proxy
 server is running on.
 A SIP client is defined in RFC 3261 [2].  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 & Volz Standards Track [Page 1] RFC 3319 DHCPv6 Options for SIP Servers July 2003

 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119

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 two DHCPv6 options [1] that allow 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, such as
 when firewalls are present, or local dialing plans, local emergency
 and other services need to be provided, 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 DHCPv6 Option

 This document defines two DHCPv6 options that describe a local
 outbound SIP proxy: one carries a list of domain names (Section 3.1),
 the other a list of 128-bit (binary) IPv6 addresses (Section 3.2).
    Since DHCPv6 does not suffer from a shortage of option codes, we
    avoid the encoding byte found in the IPv4 DHCP option for SIP
    servers [6].  This makes the option shorter, easier to parse,
    simplifies appropriate word alignment for the numeric addresses
    and allows the client to request either numeric or domain name
    options using the "option request option".
 An implementation implementing this specification MUST support both

3.1 SIP Servers Domain Name List

 The option length is followed by a sequence of labels, encoded
 according to Section 3.1 of RFC 1035 [5], 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

Schulzrinne & Volz Standards Track [Page 2] RFC 3319 DHCPv6 Options for SIP Servers July 2003

    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
    RFC 1035 encoding was chosen to accommodate future
    internationalized domain name mechanisms.
 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.  Domain names MUST be listed in order of
    Use of multiple domain names is not meant to replace NAPTR or SRV
    records, but rather to allow a single DHCP server to indicate
    outbound proxy servers operated by multiple providers.
 The DHCPv6 option has the format shown in Fig. 1.
    option-code: OPTION_SIP_SERVER_D (21)
    option-length: Length of the 'SIP Server Domain Name List' field
    in octets; variable.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |      OPTION_SIP_SERVER_D      |         option-length         |
 |                 SIP Server Domain Name List                   |
 |                              ...                              |
        Figure 1: DHCPv6 option for SIP Server Domain Name List
    SIP Server Domain Name List: The domain names of the SIP outbound
    proxy servers for the client to use.  The domain names are encoded
    as specified in Section 8 ("Representation and use of domain
    names") of the DHCPv6 specification [1].

Schulzrinne & Volz Standards Track [Page 3] RFC 3319 DHCPv6 Options for SIP Servers July 2003

3.2 SIP Servers IPv6 Address List

 This option specifies a list of IPv6 addresses indicating SIP
 outbound proxy servers available to the client.  Servers MUST be
 listed in order of preference.
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |      OPTION_SIP_SERVER_A      |           option-len          |
 |                                                               |
 |                   SIP server (IP address)                     |
 |                                                               |
 |                                                               |
 |                                                               |
 |                   SIP server (IP address)                     |
 |                                                               |
 |                                                               |
 |                              ...                              |
    option-code: OPTION_SIP_SERVER_A (22)
    option-length: Length of the 'options' field in octets; must be a
    multiple of 16.
    SIP server: IPv6 address of a SIP server for the client to use.
                The servers are listed in the order of preference for
                use by the client.

4. Client Operation

 A client may request either or both of the SIP Servers Domain Name
 List and SIP Servers IPv6 Address List options in an Options Request
 Option (ORO) as described in [1],
 If a client receives both the SIP Servers Domain Name List and SIP
 Servers IPv6 Address List options, it SHOULD use the SIP Servers
 Domain Name List option.  Only if no server in the SIP Servers Domain
 Name List can be resolved or reached, the client MAY use the SIP
 Servers IPv6 Address List option.

Schulzrinne & Volz Standards Track [Page 4] RFC 3319 DHCPv6 Options for SIP Servers July 2003

5. Server Operation

 A server MAY send a client one or both of the SIP Servers Domain Name
 List and SIP Servers IPv6 Address List options.
 If a client requests both options and the server is configured for
 both, the server MAY send a client only one of these options and
 SHOULD send the SIP Servers Domain Name List.
 A server configured with the SIP Servers IPv6 Address List option
 MUST send a client the SIP Servers IPv6 Address List option if that
 client requested the SIP Servers IPv6 Address List option and not the
 SIP Servers Domain Name List option in an ORO (see [1]).
 The following table summarizes the server's response:
 Client sends in ORO            Domain Name List  IPv6 Address List
 Neither option                 SHOULD            MAY
 SIP Servers Domain Name List   SHOULD            MAY
 SIP Servers IPv6 Address List  MAY               MUST
 Both options                   SHOULD            MAY

6. Security Consideration

 The security considerations in RFC 3315 [1], RFC 3261 [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.

7. IANA Considerations

 The IANA has assigned a DHCPv6 option number of 21 for the "SIP
 Servers Domain Name List" and the DHCPv6 option number of 22 for the
 "SIP Servers IPv6 Address List" defined in this document.

8. Acknowledgements

 Erik Nordmark and Alex Zinin provided helpful comments.

Schulzrinne & Volz Standards Track [Page 5] RFC 3319 DHCPv6 Options for SIP Servers July 2003

9. Normative References

 [1] Droms, R., Editor, Bounds, J., Volz, B., Lemon, T., Perkins, C.
     and M. Carney, "Dynamic Host Configuration Protocol for IPv6
     (DHCPv6)", RFC 3315, July 2003.
 [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] Mockapetris, P., "Domain names - implementation and
     specification", STD 13, RFC 1035, November 1987.

10. Informative References

 [6] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-for-
     IPv4) Option for Session Initiation Protocol (SIP) Servers," RFC
     3361, August 2002.

11. Authors' Addresses

 Henning Schulzrinne
 Department of Computer Science
 Columbia University
 1214 Amsterdam Avenue, MC 0401
 New York, NY 10027
 Bernie Volz
 116 Hawkins Pond Road
 Center Harbor, NH  03226-3103

Schulzrinne & Volz Standards Track [Page 6] RFC 3319 DHCPv6 Options for SIP Servers July 2003

12. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  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 & Volz Standards Track [Page 7]

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