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RFC 4039

Proposed STD
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Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)


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Network Working Group                                            S. Park
Request for Comments: 4039                                        P. Kim
Category: Standards Track                            SAMSUNG Electronics
                                                                 B. Volz
                                                           Cisco Systems
                                                              March 2005

                      Rapid Commit Option for the
         Dynamic Host Configuration Protocol version 4 (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 (2005).


   This document defines a new Dynamic Host Configuration Protocol
   version 4 (DHCPv4) option, modeled on the DHCPv6 Rapid Commit option,
   for obtaining IP address and configuration information using a
   2-message exchange rather than the usual 4-message exchange,
   expediting client configuration.

Table of Contents

   1. Introduction...................................................  2
   2. Requirements...................................................  2
   3. Client/Server Operations.......................................  2
      3.1.  Detailed Flow............................................  3
      3.2.  Administrative Considerations............................  6
   4. Rapid Commit Option Format.....................................  7
   5. IANA Considerations............................................  7
   6. Security Considerations........................................  7
   7. References.....................................................  7
      7.1.  Normative References.....................................  7
      7.2.  Informative References...................................  8
   8. Acknowledgements...............................................  8
   Authors' Addresses................................................  9
   Full Copyright Statement.......................................... 10

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1.  Introduction

   In some environments, such as those in which high mobility occurs and
   the network attachment point changes frequently, it is beneficial to
   rapidly configure clients.  And, in these environments it is possible
   to more quickly configure clients because the protections offered by
   the normal (and longer) 4-message exchange may not be needed.  The
   4-message exchange allows for redundancy (multiple DHCP servers)
   without wasting addresses, as addresses are only provisionally
   assigned to a client until the client chooses and requests one of the
   provisionally assigned addresses.  The 2-message exchange may
   therefore be used when only one server is present or when addresses
   are plentiful and having multiple servers commit addresses for a
   client is not a problem.

   This document defines a new Rapid Commit option for DHCPv4, modeled
   on the DHCPv6 Rapid Commit option [RFC3315], which can be used to
   initiate a 2-message exchange to expedite client configuration in
   some environments.  A client advertises its support of this option by
   sending it in DHCPDISCOVER messages.  A server then determines
   whether to allow the 2-message exchange based on its configuration
   information and can either handle the DHCPDISCOVER as defined in
   [RFC2131] or commit the client's configuration information and
   advance to sending a DHCPACK message with the Rapid Commit option as
   defined herein.

2.  Requirements

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

3.  Client/Server Operations

   If a client that supports the Rapid Commit option intends to use the
   rapid commit capability, it includes a Rapid Commit option in
   DHCPDISCOVER messages that it sends.  The client MUST NOT include it
   in any other messages.  A client and server only use this option when
   configured to do so.

   A client that sent a DHCPDISCOVER with Rapid Commit option processes
   responses as described in [RFC2131].  However, if the client receives
   a DHCPACK message with a Rapid Commit option, it SHOULD process the
   DHCPACK immediately (without waiting for additional DHCPOFFER or
   DHCPACK messages) and use the address and configuration information
   contained therein.

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   A server that supports the Rapid Commit option MAY respond to a
   DHCPDISCOVER message that included the Rapid Commit option with a
   DHCPACK that includes the Rapid Commit option and fully committed
   address and configuration information.  A server MUST NOT include the
   Rapid Commit option in any other messages.

   The Rapid Commit option MUST NOT appear in a Parameter Request List
   option [RFC2132].

   All other DHCP operations are as documented in [RFC2131].

3.1.  Detailed Flow

   The following is revised from Section 3.1 of [RFC2131], which
   includes handling of the Rapid Commit option.

      1. The client broadcasts a DHCPDISCOVER message on its local
         physical subnet.  If the client supports the Rapid Commit
         option and intends to use the rapid commit capability, it
         includes a Rapid Commit option in the DHCPDISCOVER message.
         The DHCPDISCOVER message MAY include options that suggest
         values for the network address and lease duration.  BOOTP relay
         agents may pass the message on to DHCP servers not on the same
         physical subnet.

      2. Each server may respond with either a DHCPOFFER message or a
         DHCPACK message with the Rapid Commit option (the latter only
         if the DHCPDISCOVER contained a Rapid Commit option and the
         server's configuration policies allow use of Rapid Commit).
         These would include an available network address in the
         'yiaddr' field (and other configuration parameters in DHCP
         options).  Servers sending a DHCPOFFER need not reserve the
         offered network address, although the protocol will work more
         efficiently if the server avoids allocating the offered network
         address to another client.  Servers sending the DHCPACK message
         commit the binding for the client to persistent storage before
         sending the DHCPACK.  The combination of 'client identifier' or
         'chaddr' and assigned network address constitute a unique
         identifier for the client's lease and are used by both the
         client and server to identify a lease referred to in any DHCP
         messages.  The server transmits the DHCPOFFER or DHCPACK
         message to the client, if necessary by using the BOOTP relay

         When allocating a new address, servers SHOULD check that the
         offered network address is not already in use; e.g., the server
         may probe the offered address with an ICMP Echo Request.

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         Servers SHOULD be implemented so that network administrators
         MAY choose to disable probes of newly allocated addresses.

         Figure 3 in [RFC2131] shows the flow for the normal 4-message
         exchange.  Figure 1 below shows the 2-message exchange.

                    Server          Client          Server
                (not selected)                    (selected)

                      v               v               v
                      |               |               |
                      |     Begins initialization     |
                      |               |               |
                      | _____________/|\____________  |
                      |/DHCPDISCOVER  | DHCPDISCOVER \|
                      | w/Rapid Commit| w/Rapid Commit|
                      |               |               |
                  Determines          |          Determines
                 configuration        |         configuration
                      |               |     Commits configuration
                      |       Collects replies        |
                      |\              |  ____________/|
                      | \________     | / DHCPACK     |
                      | DHCPOFFER\    |/w/Rapid Commit|
                      |  (discarded)  |               |
                      |    Initialization complete    |
                      |               |               |
                      .               .               .
                      .               .               .
                      |               |               |
                      |      Graceful shutdown        |
                      |               |               |
                      |               |\_____________ |
                      |               |  DHCPRELEASE \|
                      |               |               |
                      |               |        Discards lease
                      |               |               |
                      v               v               v

            Figure 1 Timeline diagram when Rapid Commit is used

      3. The client receives one or more DHCPOFFER or DHCPACK (if the
         Rapid Commit option is sent in DHCPDISCOVER) messages from one
         or more servers.  If a DHCPACK (with the Rapid Commit option)
         is received, the client may immediately advance to step 5 below
         if the offered configuration parameters are acceptable.  The
         client may choose to wait for multiple responses.  The client
         chooses one server from which to request or accept

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         configuration parameters, based on the configuration parameters
         offered in the DHCPOFFER and DHCPACK messages.  If the client
         chooses a DHCPACK, it advances to step 5.  Otherwise, the
         client broadcasts a DHCPREQUEST message that MUST include the
         'server identifier' option to indicate which server it has
         selected, the message MAY include other options specifying
         desired configuration values.  The 'requested IP address'
         option MUST be set to the value of 'yiaddr' in the DHCPOFFER
         message from the server.  This DHCPREQUEST message is broadcast
         and relayed through DHCP/BOOTP relay agents.  To help ensure
         that any BOOTP relay agents forward the DHCPREQUEST message to
         the same set of DHCP servers that received the original
         DHCPDISCOVER message, the DHCPREQUEST message MUST use the same
         value in the DHCP message header's 'secs' field and be sent to
         the same IP broadcast address as was the original DHCPDISCOVER
         message.  The client times out and retransmits the DHCPDISCOVER
         message if the client receives no DHCPOFFER messages.

      4. The servers receive the DHCPREQUEST broadcast from the client.
         Servers not selected by the DHCPREQUEST message use the message
         as notification that the client has declined those servers'
         offers.  The server selected in the DHCPREQUEST message commits
         the binding for the client to persistent storage and responds
         with a DHCPACK message containing the configuration parameters
         for the requesting client.  The combination of 'client
         identifier' or 'chaddr' and assigned network address constitute
         a unique identifier for the client's lease and are used by both
         the client and server to identify a lease referred to in any
         DHCP messages.  Any configuration parameters in the DHCPACK
         message SHOULD NOT conflict with those in the earlier DHCPOFFER
         message to which the client is responding.  The server SHOULD
         NOT check the offered network address at this point.  The
         'yiaddr' field in the DHCPACK messages is filled in with the
         selected network address.

         If the selected server is unable to satisfy the DHCPREQUEST
         message (e.g., the requested network address has been
         allocated), the server SHOULD respond with a DHCPNAK message.

         A server MAY choose to mark addresses offered to clients in
         DHCPOFFER messages as unavailable.  The server SHOULD mark an
         address offered to a client in a DHCPOFFER message as available
         if the server receives no DHCPREQUEST message from that client.

      5. The client receives the DHCPACK message with configuration
         parameters.  The client SHOULD perform a final check on the
         parameters (e.g., ARP for allocated network address), and it
         notes the duration of the lease specified in the DHCPACK

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         message.  At this point, the client is configured.  If the
         client detects that the address is already in use (e.g.,
         through the use of ARP), the client MUST send a DHCPDECLINE
         message to the server, and it restarts the configuration
         process.  The client SHOULD wait a minimum of ten seconds
         before restarting the configuration process to avoid excessive
         network traffic in case of looping.

         If the client receives a DHCPNAK message, the client restarts
         the configuration process.

         The client times out and retransmits the DHCPREQUEST message if
         the client receives neither a DHCPACK nor a DHCPNAK message.
         The client retransmits the DHCPREQUEST according to the
         retransmission algorithm in section 4.1 of [RFC2131].  The
         client should choose to retransmit the DHCPREQUEST enough times
         to give an adequate probability of contacting the server
         without causing the client (and the user of that client) to
         wait too long before giving up; e.g., a client retransmitting
         as described in section 4.1 of [RFC2131] might retransmit the
         DHCPREQUEST message four times, for a total delay of 60
         seconds, before restarting the initialization procedure.  If
         the client receives neither a DHCPACK nor a DHCPNAK message
         after employing the retransmission algorithm, the client
         reverts to INIT state and restarts the initialization process.
         The client SHOULD notify the user that the initialization
         process has failed and is restarting.

      6. The client may choose to relinquish its lease on a network
         address by sending a DHCPRELEASE message to the server.  The
         client identifies the lease to be released with its 'client
         identifier' or 'chaddr' and network address in the DHCPRELEASE
         message.  If the client used a 'client identifier' when it
         obtained the lease, it MUST use the same 'client identifier' in
         the DHCPRELEASE message.

3.2.  Administrative Considerations

   Network administrators MUST only enable the use of Rapid Commit on a
   DHCP server if one of the following conditions is met:

      1. The server is the only server for the subnet.

      2. When multiple servers are present, they may each commit a
         binding for all clients, and therefore each server must have
         sufficient addresses available.

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   A server MAY allow configuration for a different (likely shorter)
   initial lease time for addresses assigned when Rapid Commit is used
   to expedite reclaiming addresses not used by clients.

4.  Rapid Commit Option Format

   The Rapid Commit option is used to indicate the use of the two-
   message exchange for address assignment.  The code for the Rapid
   Commit option is 80.  The format of the option is:

           Code  Len
         |  80 |  0  |

   A client MUST include this option in a DHCPDISCOVER message if the
   client is prepared to perform the DHCPDISCOVER-DHCPACK message
   exchange described earlier.

   A server MUST include this option in a DHCPACK message sent in a
   response to a DHCPDISCOVER message when completing the DHCPDISCOVER-
   DHCPACK message exchange.

5.  IANA Considerations

   IANA has assigned value 80 for the Rapid Commit option code in
   accordance with [RFC2939].

6.  Security Considerations

   The concepts in this document do not significantly alter the security
   considerations for DHCP (see [RFC2131] and [RFC3118]).  However, use
   of this option could expedite denial of service attacks by allowing a
   mischievous client to consume all available addresses more rapidly or
   to do so without requiring two-way communication (as injecting
   DHCPDISCOVER messages with the Rapid Commit option is sufficient to
   cause a server to allocate an address).

7.  References

7.1.  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.

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7.2.  Informative References

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

   [RFC2939]  Droms, R., "Procedures and IANA Guidelines for Definition
              of New DHCP Options and Message Types", BCP 43, RFC 2939,
              September 2000.

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

   [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.

8.  Acknowledgements

   Special thanks to Ted Lemon and Andre Kostur for their many valuable
   comments.  Thanks to Ralph Droms for his review comments during WGLC.
   Thanks to Noh-Byung Park and Youngkeun Kim for their supports on this

   Particularly, the authors would like to acknowledge the
   implementation contributions by Minho Lee of SAMSUNG Electronics.

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Authors' Addresses

   Soohong Daniel Park
   Mobile Platform Laboratory
   SAMSUNG Electronics
   416, Maetan-3dong, Yeongtong-Gu
   Suwon, Korea

   Phone: +82-31-200-4508

   Pyungsoo Kim
   Mobile Platform Laboratory
   SAMSUNG Electronics
   416, Maetan-3dong, Yeongtong-Gu
   Suwon, Korea

   Phone: +82-31-200-4635

   Bernie Volz
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA 01719

   Phone:  +1-978-936-0382

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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
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