Link Selection sub-option for the Relay Agent Information Option for DHCPv4
RFC 3527

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Network Working Group                                         K. Kinnear
Request for Comments: 3527                                      M. Stapp
Category: Standards Track                                     R. Johnson
                                                           J. Kumarasamy
                                                           Cisco Systems
                                                              April 2003

                       Link Selection sub-option
           for the Relay Agent Information Option for DHCPv4

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.

Abstract

   This document describes the link selection sub-option of the relay-
   agent-information option for the Dynamic Host Configuration Protocol
   (DHCPv4).  The giaddr specifies an IP address which determines both a
   subnet, and thereby a link on which a Dynamic Host Configuration
   Protocol (DHCP) client resides as well as an IP address that can be
   used to communicate with the relay agent.  The subnet-selection
   option allows the functions of the giaddr to be split so that when
   one entity is performing as a DHCP proxy, it can specify the
   subnet/link from which to allocate an IP address, which is different
   from the IP address with which it desires to communicate with the
   DHCP server.  Analogous situations exist where the relay agent needs
   to specify the subnet/link on which a DHCP client resides, which is
   different from an IP address that can be used to communicate with the
   relay agent.

1.  Introduction

   In RFC 2131, the giaddr specifies an IP address which determines a
   subnet (and from there a link) on which a DHCP client resides as well
   as an IP address which can be used to communicate with the relay
   agent.  The subnet-selection option [RFC 3011] allows these functions
   of the giaddr to be split, so that when one entity is performing as a

Kinnear, et al.             Standards Track                     [Page 1]
RFC 3527               Link Selection sub-option              April 2003

   DHCP proxy, it can specify the subnet/link from which to allocate an
   IP address that is different from the IP address with which it
   desires to communicate with the DHCP server.

   Analogous situations exist where the relay agent needs to specify the
   subnet/link on which a DHCP client resides, which is different from
   an IP address that can be used to communicate with the relay agent.
   Consider the following architecture:

      +--------+         +---------------+
      |  DHCP  |     IP x|               |IP y
      | Server |-.......-|  Relay Agent  |----+------------+
      +--------+         |               |    |            |
                         +---------------+    |         +------+
                                              |         |Modem |
                                              |         +------+
                                              |          |    |
                                           +-----+  +-----+ +-----+
                                           |Host1|  |Host2| |Host3|
                                           +-----+  +-----+ +-----+

   In the usual approach, the relay agent would put IP address Y into
   the giaddr of any packets that it forwarded to the DHCP server.
   However, if for any reason, IP address Y is not accessible from the
   DHCP server, this approach will fail.  There are several reasons why
   IP y might be inaccessible from the DHCP server:

      o  There might be some firewall capability in the network element
         in which the relay agent resides that does not allow the DHCP
         server to access the relay agent via IP y.

      o  There might not be an IP y.  An example would be the case where
         there was only one host and this was a point to point link.

   In any of these or other cases, the relay agent needs to be able to
   communicate to the DHCP server the subnet/link from which to allocate
   an IP address.  The IP address, which will communicate to the DHCP
   server the subnet/link information, cannot be used as a way to
   communicate between the DHCP server and the relay agent.

   Since the relay agent can modify the client's DHCP DHCPREQUEST in
   only two ways, the giaddr and the relay-agent-info option, there is a
   need to extend the relay-agent-info option with a new sub-option, the
   link-selection sub-option, to allow separation of the specification
   of the subnet/link from the IP address to use when communicating with
   the relay agent.