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The Dynamic Host Configuration Protocol Version 4 (DHCPv4) Relay Agent Flags Suboption
RFC 5010

Network Working Group                                         K. Kinnear
Request for Comments: 5010                                   M. Normoyle
Category: Standards Track                                       M. Stapp
                                                     Cisco Systems, Inc.
                                                          September 2007

      The Dynamic Host Configuration Protocol Version 4 (DHCPv4)
                      Relay Agent Flags Suboption

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.

Abstract

   This memo defines a new suboption of the Dynamic Host Configuration
   Protocol (DHCP) relay agent information option that allows the DHCP
   relay to specify flags for the forwarded packet.  One flag is defined
   to indicate whether the DHCP relay received the packet via a unicast
   or broadcast packet.  This information may be used by the DHCP server
   to better serve clients based on whether their request was originally
   broadcast or unicast.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Requirements Terminology  . . . . . . . . . . . . . . . . . . . 2
   3.  The Flags Suboption . . . . . . . . . . . . . . . . . . . . . . 3
   4.  DHCP 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

Kinnear, et al.             Standards Track                     [Page 1]
RFC 5010              Relay Agent Flags Suboption         September 2007

1.  Introduction

   Any time a client's DHCP packet is broadcast, a local DHCP relay will
   process its request and forward it on to the DHCP server.  When the
   DHCP relay performs this function, it can be configured to use the
   DHCP relay agent information option to forward additional information
   to the DHCP server, which the server may then use to alter its
   processing algorithms.  Once the lease has been granted, however,
   future DHCP DHCPREQUEST/RENEWAL messages are unicast directly to the
   DHCP Server [RFC2131] [RFC2132] [RFC3046].

   In general, DHCP servers may also make subtle (and sometimes not so
   subtle) changes in their processing algorithms depending on whether
   or not the DHCP server received the message as a unicast packet from
   the DHCP client directly, a broadcast packet from the DHCP client on
   a locally connected network, or a unicast packet from a DHCP Relay
   Agent, which has forwarded on a packet broadcast from a DHCP client
   connected to a network local to the DHCP Relay Agent.

   In some situations, DHCP Clients may unicast their DHCPREQUEST/RENEW
   packets to the DHCP Relay Agent, which will forward the packet on to
   the DHCP server.  In these cases, the DHCP server cannot tell whether
   the packet was broadcast or unicast by the DHCP client, and so it may
   be unable to process the DHCP client packets in the manner that it
   would if it knew whether the original DHCP packet was broadcast or
   unicast.  For example, a server might be willing to NAK a client in
   the REBINDING state based on a determination that the client's
   address does not match its location in the network, but might not be
   willing to do so if the client is in the RENEWING state.

   The purpose of the suboption described in this document is to allow
   the DHCP relay to specify flags for the forwarded packet.  These
   flags can be used to describe DHCP client attributes that are useful
   to the DHCP server, but can only be detected by the local DHCP relay.
   The DHCP server can use the information provided by the DHCP relay to
   improve its processing algorithms.

   One flag is defined to indicate whether the DHCP relay received the
   packet via a unicast or broadcast packet.  This allows the DHCP
   server to know if a packet forwarded on by a DHCP Relay Agent was
   broadcast or unicast to the DHCP Relay Agent.

2.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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