Network Working Group|
Request for Comments: 4039
Category: Standards Track
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 © 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.
3. Client/Server Operations
3.1. Detailed Flow
3.2. Administrative Considerations
4. Rapid Commit Option Format
5. IANA Considerations
6. Security Considerations
7.1. Normative References
7.2. Informative References
Full Copyright Statement
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.
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].
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.
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].
The following is revised from Section 3.1 of [RFC2131], which includes handling of the Rapid Commit option.
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.
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
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.
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.
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.
Network administrators MUST only enable the use of Rapid Commit on a DHCP server if one of the following conditions is met:
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.
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:
+-----+-----+ | 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.
IANA has assigned value 80 for the Rapid Commit option code in accordance with [RFC2939].
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).
[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.
[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.
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.
Soohong Daniel Park
Mobile Platform Laboratory
416, Maetan-3dong, Yeongtong-Gu
Mobile Platform Laboratory
416, Maetan-3dong, Yeongtong-Gu
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
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