dhc Working Group                                               R. Droms
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                       September 8,                        November 3, 2008
Expires: March 12, May 7, 2009

               Container Option for Server Configuration

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   In some DHCP service deployments, it is desirable for a DHCP server
   in one administrative domain to pass configuration options to a DHCP
   server in a different administrative domain.  This DHCP option
   carries a set of DHCP options that can be used by another DHCP

1.  Introduction

   In some DHCP service deployments, it is desirable to pass
   configuration options from a DHCP server in one administrative domain
   to another DHCP server in a different administrative domain.  In one
   example of such a deployment, an IPTV service provider (SP) may need
   to provide certain SP domain-specific information to IPTV device(s)
   located in the consumer domain.  This information is sent from the
   IPTV SP DHCP server to the consumer DHCP server located in the
   Residential Gateway (RG), which can then be passed along to IPTV
   device(s) in the subscriber network.

   Existing RGs may pass some configuration information received by the
   RG DHCP client to the RG server for configuration of devices attached
   to the consumer network.  There are several motivations for this

   o  The devices attached to the consumer network may require different
      configuration information than the DHCP options provided to the RG

   o  Existing RG DHCP clients are typically not be coded to process new
      DHCP options and, therefore, will be unable to pass those new
      options to the RG DHCP server

   o  Existing RG DHCP clients are typically coded to pass only a fixed
      list of DHCP options to the RG DHCP server and, therefore, will be
      unable to pass newly defined options to the RG DHCP server.

   The DHCP Container option defined in this document provides a
   mechanism through which the RG DHCP client can pass DHCP options to
   the RG DHCP server without explicit knowledge of the semantics of
   those options.  With this option, the SP DHCP server can pass both
   current and future DHCP options to the RG DHCP server.

   The DHCP Container option does not carry IP addresses, IPv6 prefixes
   or other information about leases.  It carries other configuration

2.  Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be
   interpreted as described in RFC2119 [RFC2119].

   The following terms and acronyms are used in this document:

   DHCPv4              "Dynamic Host Configuration Protocol" [RFC2131]

   DHCPv6              "Dynamic Host Configuration Protocol for IPv6"
   DHCP                DHCPv4 and/or DHCPv6

   RG                  "home gateway"; the device through which the
                       consumer network connects to the broadband WAN;
                       typically a layer 3 forwarding device

   RG DHCP client      (or "RG client") the DHCP client in the RG

   RG DHCP server      (or "RG server") the DHCP server in the RG

   SP DHCP server      (or "SP server") the DHCP server managed by the
                       service provider (SP)

   This document uses other terminology for DHCPv4 and DHCPv6 as defined
   RFCs RFC 2131 and 2132, RFC 3315, respectively.

3.  Problem statement and requirements for RG DHCP server configuration

   The following diagram shows the components in a network deployment
   using the DHCP Container option:

   Client STB/CPE -+  +---------+           +------+
                   |  |   RG    |           |  SP  |
   Client STB/CPE -+  |   Client+--- ... ---+ DHCP |
                   +--+Server   |           |server|
   Client STB/CPE -+  +---------+           +------+

   In this diagram, the RG client engages in DHCP message exchanges with
   the SP server to obtain its IP address and other configuration

   The problem under consideration in this document is to transmit
   configuration information from the SP DHCP server to devices attached
   to the consumer network.  The problem solution has the following

   o  The SP server MUST be able to transmit different configuration
      information to the consumer devices than the DHCP options provided
      to the RG

   o  The SP server MUST be able to control which DHCP options are
      transmitted to the consumer device
   o  There MUST be a way for the SP server to pass DHCP options to be
      defined in the future to consumer devices

4.  Design alternatives

   The following three designs meet the solution requirements:

   o  SP server passes container option to RG client, which forwards
      contents to RG server; this alternative is the preferred solution

   o  RG server does direct DHCP info request to SP server; this
      alternative is not preferred:

      *  requires that the RG server include a DHCP client

      *  requires that the SP server be able to differentiate between RG
         client and server requests

      *  does not scale well, as it at least doubles the load on the SP

   o  RG server passes device requests to SP DHCP server; this
      alternative is not preferred:

      *  requires that the RG also function as a DHCP relay

      *  requires that the RG relay function be configured with the IP
         addresses of the SP DHCP server(s)

      *  requires that the RG relay function differentiate between DHCP
         messages that are processed by the RG server and DHCP messages
         that are processes by the SP server

   A variant on the preferred design would allow the inclusion of
   multiple sets of DHCP options intended for different classes of
   devices in the consumer network; e.g., the design would allow for one
   set of options for video set-top boxes and a second set of options
   for VoIP MTAs.  The variant would require the specification of rules
   to be provided by the SP server through which the RG server would
   differentiate its clients and send the appropriate set of options to
   each device.  At present, there is no requirement for differential
   configuration of consumer devices and this alternative is not defined
   in this document.

5.  Semantics and syntax of the Container option

   Along with configuration information intended for the RG, the SP
   server can include the DHCP Container option.  When the RG client
   receives the DHCP Container option, it passes the contents of the
   option to the RG server.  The means through which the information is
   passed between the RG client and the RG server is out of the scope of
   this document and left unspecified.

   The DHCP options in this container are carried in DHCP message format
   (option-code/length/value).  In this format, the contained options
   can be passed through a DHCP client to a co-located DHCP server
   without specific knowledge on the part of the client or the server of
   the semantics of the options.

5.1.  DHCPv4 Container option

   The DHCPv4 Container option has the following format:

        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
       |     Code      |      len      |   DHCP Options for RG server  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
       .                                                               .
       .                                                               .
       .                                                               .

   Code                OPTION_CONTAINER_V4 (TBD)

   len                 Length of options for RG server, in octets

5.2.  DHCPv6 Container option

   The DHCPv6 Container option has the following format:

        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_CONTAINER_V6      |        option-len             |
       |                  DHCP Options for RG server                   |
       .                                                               .
       .                                                               .
       .                                                               .
   option-code         OPTION_CONTAINER_V6 (TBD).

   option-len          Length of options for RG server, in octets

5.3.  SP server behavior

   The SP server MAY include the Container option in any DHCP message
   sent to an RG client.

   The policy through which the SP server is instructed to include a
   Container option for an RG client, and the policy determining the
   contents of the Container object are out of scope of this document
   and left unspecified.

5.4.  RG client behavior

   The RG client MUST pass the contents of the received Container option
   to the RG server without alteration.  The details of the
   implementation through which the RG client parses the content of the
   Container option and passes the options to the RG server are out of
   scope for this document and left unspecified.

5.5.  RG server behavior

   The RG server MUST discard any options related to IP address
   assignment, IPv6 prefix delegation or operation of the DHCP protocol
   itself.  Appendices TBD give a list of DHCPv4 and DHCPv6 options that
   the RG server MUST discard.

   The Container option provides a mechanism through which the SP might
   be able to unilaterally control the configuration settings passed
   from a CPE DHCP server to a CPE device.  This configuration channel
   must be handled with some care if the subscriber is to retain desired
   control over the CPE configurations.  The following behaviors limit
   the degree to which the SP con control CPE configuration:

   o  The RG server MAY discard any undesired options, as determined by
      policy in the RG.

   o  The RG server MUST return to any DHCP client only those options
      requested by the DHCP client in a Parameter Request List option
      (DHCPv4 option code 55) or an Option Request option (DHCPv6 option
      code 6).

6.  Security Considerations

   A rogue server can use this option to pass invalid information to the
   RG client, which would then be passed to the Client STB/CPEs.  This
   invalid information could be used to mount a denial of service attack
   or a man-in-the-middle attack against some applications.

   Authentication of DHCP messages(RFC 3118 [RFC3118] for DHCPv4 or
   section 20 of RFC 3315 [RFC3315]) can be used to ensure that the
   contents of this option are not altered in transit between the DHCP
   server and client.

7.  IANA Considerations

   When this document is published, IANA is asked to assign an option
   tag from the "BOOTP Vendor Extensions and DHCP Options" registry for

   When this document is published, IANA is asked to assign an option
   code from the "DHCPv6 Option Codes" registry for OPTION_CONTAINER_V6.

8.  Change Log

   If this document is accepted for publication as an RFC, this change
   log is to be removed before publication.

   o  Corrected a cut-and-paste error in section "DHCPv6 Container
      option": The Time Protocol Servers option -> The DHCPv4 Container

   o  Added text to section "RG Server Behavior" to address policy
      management concerns

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

Author's Address

   Ralph Droms
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA  01719

   Phone: +1 978.936.1674
   Email: rdroms@cisco.com

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