Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)
RFC 4039

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).
Abstract
 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
Park, et al. Standards Track [Page 1]
RFC 4039 Rapid Commit Option for DHCPv4 March 2005
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
 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 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.
Park, et al. Standards Track [Page 2]
RFC 4039 Rapid Commit Option for DHCPv4 March 2005
 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
 agent.
 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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RFC 4039 Rapid Commit Option for DHCPv4 March 2005
 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
Park, et al. Standards Track [Page 4]
RFC 4039 Rapid Commit Option for DHCPv4 March 2005
 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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RFC 4039 Rapid Commit Option for DHCPv4 March 2005
 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.
Park, et al. Standards Track [Page 6]
RFC 4039 Rapid Commit Option for DHCPv4 March 2005
 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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RFC 4039 Rapid Commit Option for DHCPv4 March 2005
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
 work.
 Particularly, the authors would like to acknowledge the
 implementation contributions by Minho Lee of SAMSUNG Electronics.
Park, et al. Standards Track [Page 8]
RFC 4039 Rapid Commit Option for DHCPv4 March 2005
Authors' Addresses
 Soohong Daniel Park
 Mobile Platform Laboratory
 SAMSUNG Electronics
 416, Maetan-3dong, Yeongtong-Gu
 Suwon, Korea
 Phone: +82-31-200-4508
 EMail: soohong.park@samsung.com
 Pyungsoo Kim
 Mobile Platform Laboratory
 SAMSUNG Electronics
 416, Maetan-3dong, Yeongtong-Gu
 Suwon, Korea
 Phone: +82-31-200-4635
 EMail: kimps@samsung.com
 Bernie Volz
 Cisco Systems, Inc.
 1414 Massachusetts Ave.
 Boxborough, MA 01719
 USA
 Phone: +1-978-936-0382
 EMail: volz@cisco.com
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RFC 4039 Rapid Commit Option for DHCPv4 March 2005
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Park, et al. Standards Track [Page 10]