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Request For Comments - RFC3993

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Network Working Group                                         R. Johnson
Request for Comments: 3993                                T. Palaniappan
Category: Standards Track                                       M. Stapp
                                                     Cisco Systems, Inc.
                                                              March 2005

                    Subscriber-ID Suboption for the
     Dynamic Host Configuration Protocol (DHCP) Relay Agent Option

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 (2005).


   This memo defines a new Subscriber-ID suboption for the Dynamic Host
   Configuration Protocol's (DHCP) relay agent information option.  The
   suboption allows a DHCP relay agent to associate a stable
   "Subscriber-ID" with DHCP client messages in a way that is
   independent of the client and of the underlying physical network

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Requirements Terminology . . . . . . . . . . . . . . . . . . .  2
   3.  The Subscriber-ID Suboption  . . . . . . . . . . . . . . . . .  2
       3.1.  Suboption Format . . . . . . . . . . . . . . . . . . . .  3
   4.  Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . .  3
   5.  DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . .  4
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  4
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  5
       9.1.  Normative References . . . . . . . . . . . . . . . . . .  5
       9.2.  Informative References . . . . . . . . . . . . . . . . .  5
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  6
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . .  7

Johnson, et al.             Standards Track                     [Page 1]

RFC 3993                Subscriber-ID Suboption               March 2005

1.  Introduction

   DHCP (RFC 2131 [2]) provides IP addresses and configuration
   information for IPv4 clients.  It includes a relay agent capability
   in which processes within the network infrastructure receive
   broadcast messages from clients and forward them to DHCP servers as
   unicast messages.  In network environments such as DOCSIS data-over-
   cable and xDSL, it has proven useful for the relay agent to add
   information to the DHCP message before forwarding it, by using the
   relay agent information option (RFC 3046 [3]).

   Servers that recognize the relay agent option echo it back in their
   replies, and some of the information that relays add may be used to
   help an edge device efficiently return replies to clients.  The
   information that relays supply can also be used in the server's
   decision making about the addresses and configuration parameters that
   the client should receive.

   In many service provider environments, it is desirable to associate
   some provider-specific information with clients' DHCP messages.  This
   is often done by using the relay agent information option.  RFC 3046
   defines Remote-ID and Circuit-ID suboptions that are used to carry
   such information.  The values of those suboptions, however, are
   usually based on a network resource such as an IP address of a
   network access device, an ATM Virtual Circuit identifier, or a DOCSIS
   cable-modem identifier.  As a result, the values carried in these
   suboptions are dependent on the physical network configuration.  If a
   client connects to the service provider network through different
   paths, different values are carried in network-dependent suboptions.

2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

3.  The Subscriber-ID Suboption

   In complex service provider environments, connecting a customer's
   DHCP configuration and administrative information is necessary.  The
   Subscriber-ID suboption carries a value that can be independent of
   the physical network configuration through which the subscriber is
   connected.  This value complements, and might well be used in
   addition to, the network-based relay agent option suboptions
   discussed in Section 2.  The "subscriber-id" assigned by the provider
   is intended to be stable as customers connect through different
   paths, and as network changes occur.

Johnson, et al.             Standards Track                     [Page 2]

RFC 3993                Subscriber-ID Suboption               March 2005

   The Subscriber-ID information allows the service provider to
   assign/activate subscriber-specific actions; e.g., assignment of host
   IP address and subnet mask, DNS configuration, or trigger accounting.
   This suboption is de-coupled from the access network's physical
   structure, so subscriber moves from one access-point to another, for
   example, would not require reconfiguration at the service provider's
   DHCP servers.

   The Subscriber-ID is an ASCII string; the encoding of the string is
   defined in Section 3.1.  The semantic contents of the Subscriber-ID
   string are, of course, provider-specific.  This specification does
   not establish any semantic requirements on the data in the string.

3.1.  Suboption Format

   This memo defines a new DHCP relay agent option suboption that
   carries a "Subscriber-ID" value.  The value is an ASCII string.  The
   suboption takes a form similar to that of many other relay
   information option suboptions:

       0     1     2     3     4     5
       |Code | Len | Subscriber-ID string ...

   The Code for the suboption is 6.

   The one-octet Len field is the length of the ID string, in octets.
   The minimum length of the ID string is 1 octet.

   The "Subscriber-ID" is an NVT ASCII [4] string.  The string MUST NOT
   be NULL terminated, as the length is specified in the "Len" field.

4.  Relay Agent Behavior

   DHCP relay agents MAY be configured to include a Subscriber-ID
   suboption if they include a relay agent information option in relayed
   DHCP messages.  The subscriber-id strings themselves are assigned and
   configured through mechanisms that are outside the scope of this

Johnson, et al.             Standards Track                     [Page 3]

RFC 3993                Subscriber-ID Suboption               March 2005

5.  DHCP Server Behavior

   This suboption provides additional information to the DHCP server.
   If it is configured to support this option, the DHCP server may use
   this information in addition to other relay agent option data and
   other options included in the DHCP client messages in order to assign
   an IP address and/or other configuration parameters to the client.
   There is no special additional processing for this suboption.

6.  Security Considerations

   Message authentication in DHCP for intradomain use where the out-of-
   band exchange of a shared secret is feasible is defined in RFC 3118
   [5].  Potential exposures to attacks are discussed in section 7 of
   the DHCP protocol specification in RFC 2131 [2].

   The DHCP relay agent option depends on a trusted relationship between
   the DHCP relay agent and the server, as described in section 5 of RFC
   3046.  Fraudulent relay agent option data could potentially lead to
   theft-of-service or exhaustion of limited resources (like IP
   addresses) by unauthorized clients.  A host that tampered with relay
   agent data associated with another host's DHCP messages could deny
   service to that host, or interfere with its operation by leading the
   DHCP server to assign it inappropriate configuration parameters.

   While the introduction of fraudulent relay agent options can be
   prevented by a perimeter defense that blocks these options unless the
   relay agent is trusted, a deeper defense using authentication for
   relay agent options via the Authentication Suboption [6] or IPSec [7]
   SHOULD be deployed as well.

   There are several data fields in a DHCP message conveying information
   that may identify an individual host on the network.  These include
   the chaddr, the client-id option, and the hostname and client-fqdn
   options.  Depending on the type of identifier selected, the
   Subscriber-ID suboption may also convey information that identifies a
   specific host or a specific user on the network.  In practice, this
   information isn't exposed outside the internal service-provider
   network, where DHCP messages are usually confined.  Administrators
   who configure data that's going to be used in DHCP Subscriber-ID
   suboptions should be careful to use identifiers that are appropriate
   for the types of networks they administer.  If DHCP messages travel
   outside the service-provider's own network, or if the suboption
   values may become visible to other users, that may raise privacy
   concerns for the access provider or service provider.

Johnson, et al.             Standards Track                     [Page 4]

RFC 3993                Subscriber-ID Suboption               March 2005

7.  IANA Considerations

   IANA has assigned a value of 6 from the DHCP Relay Agent Information
   Option [3] suboption codes for the Subscriber-ID Suboption described
   in this document.

8.  Acknowledgements

   This document is the result of work done within Cisco Systems.
   Thanks especially to Andy Sudduth for his review comments.

9.  References

9.1.  Normative References

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

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

   [3]  Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
        January 2001.

   [4]  Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD
        8, RFC 854, May 1983.

9.2.  Informative References

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

   [6]  Stapp, M., "The Authentication Suboption for the DHCP Relay
        Agent Option", Work in Progress.

   [7]  Droms, R., "Authentication of Relay Agent Options Using IPSec",
        Work in Progress.

Johnson, et al.             Standards Track                     [Page 5]

RFC 3993                Subscriber-ID Suboption               March 2005

Authors' Addresses

   Richard Johnson
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: 408.526.4000
   EMail: raj@cisco.com

   Theyn Palaniappan
   Cisco Systems, Inc.
   170 W. Tasman Dr.
   San Jose, CA  95134

   Phone: 408.526.4000
   EMail: athenmoz@cisco.com

   Mark Stapp
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: 978.936.0000
   EMail: mjs@cisco.com

Johnson, et al.             Standards Track                     [Page 6]

RFC 3993                Subscriber-ID Suboption               March 2005

Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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Johnson, et al.             Standards Track                     [Page 7]

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