DHCPv4 Rapid Commit: Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)
A DHCPv4 option that lets a client get its address in two messages instead of four.
Park, S., Kim, P., and B. Volz, "Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)", RFC 4039, DOI 10.17487/RFC4039, March 2005, <https://www.rfc-editor.org/info/rfc4039>.
@misc{rfc4039,
series = {Request for Comments},
number = 4039,
howpublished = {RFC 4039},
publisher = {RFC Editor},
doi = {10.17487/RFC4039},
url = {https://www.rfc-editor.org/info/rfc4039},
author = {S. Park and P. Kim and B. Volz},
title = {{Rapid Commit Option for the Dynamic Host Configuration Protocol version 4 (DHCPv4)}},
pagetotal = 10,
year = 2005,
month = mar,
} RFC text
Contents
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
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.
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.
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
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
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.
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.
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.
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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AI ELI5
On this page
§1Why Two Messages Can Be Enough
In environments with high mobility, where the network attachment point changes frequently, it helps to configure clients quickly. RFC 4039 observes that in such places the protections of the normal 4-message exchange may not be needed.
The 4-message exchange lets several DHCP servers cooperate without wasting addresses, because addresses are only provisionally assigned until the client chooses one and requests it. A 2-message exchange may therefore be used when only one server is present, or when addresses are plentiful and having several servers commit addresses for one client is not a problem.
The new Rapid Commit option is modeled on the DHCPv6 Rapid Commit option from RFC 3315. If the server does not agree to use it, it simply handles the DHCPDISCOVER as defined in RFC 2131.
Usually you ask for an address, get offers, pick one and then get a final yes. If you are the only one asking and there are plenty of addresses, you can skip the middle steps.
§3How Client and Server Use the Option
A client that wants rapid commit includes a Rapid Commit option in the DHCPDISCOVER messages it sends. It MUST NOT include the option in any other message. Both client and server only use the option when configured to do so.
The client still processes responses as in RFC 2131. But if it receives a DHCPACK with a Rapid Commit option, it SHOULD process that DHCPACK immediately, without waiting for further DHCPOFFER or DHCPACK messages, and use the address and configuration in it.
A server that supports the option MAY answer such a DHCPDISCOVER with a DHCPACK that includes the Rapid Commit option and fully committed address and configuration information. A server MUST NOT include the option in any other message. The option MUST NOT appear in a Parameter Request List option (RFC 2132).
The server decides. It may ignore the option and send an ordinary DHCPOFFER, and the client then continues with the usual four-message exchange.
The client says 'I would take an address right away, if you are willing'. A willing server hands one over and writes it down. An unwilling server just makes an offer the usual way.
§3.1The Revised Step-by-Step Flow
The RFC rewrites the six-step flow of Section 3.1 of RFC 2131 to include rapid commit. The client broadcasts a DHCPDISCOVER, with the Rapid Commit option if it wants it. Each server answers with either a DHCPOFFER or, only if the DISCOVER carried the option and local policy allows it, a DHCPACK with the Rapid Commit option.
A server sending the DHCPACK commits the binding to persistent storage before sending it. When allocating a new address, servers SHOULD check that it is not already in use, for example by probing it with an ICMP Echo Request, and administrators MAY disable these probes.
If the client gets a DHCPACK with Rapid Commit and the parameters are acceptable, it may go straight to the final step. Otherwise it picks a server and broadcasts a DHCPREQUEST as before. In the final step the client SHOULD perform a last check such as ARP. If the address is already in use, it MUST send a DHCPDECLINE and restart configuration, and it SHOULD wait at least ten seconds before restarting. A DHCPNAK also makes the client restart.
The client may later give up its lease with a DHCPRELEASE, using the same 'client identifier' it used to obtain the lease.
The client shouts for an address. If a server answers 'yours is 10, written down', the client does a quick check that nobody else is using it and starts using it. If the check fails, it says no thanks and tries again after a short wait.
- DHCPDISCOVER w/ Rapid CommitBroadcast to begin initialization.
- DHCPACK w/ Rapid CommitThe server commits the configuration first.
- DHCPRELEASELater, on graceful shutdown.
§3.2Administrative Considerations
Because a Rapid Commit server commits a binding for every client that asks, administrators MUST only enable the option on a server when one of two conditions holds. Either the server is the only server for the subnet, or, when several servers are present, each may commit a binding for all clients and therefore each must have enough addresses available.
A server MAY also allow a different, likely shorter, initial lease time for addresses assigned using Rapid Commit. This helps to reclaim addresses that clients never actually use.
Handing out an address to everyone who asks only works if you have enough to go round. A shorter first loan lets the server get unused ones back sooner.
§4The Rapid Commit Option Format
The option is very small. Its code is 80 and its length is 0, so it has no data and its presence is the signal.
A client MUST include it in a DHCPDISCOVER if it is prepared to perform the DHCPDISCOVER-DHCPACK exchange. A server MUST include it in the DHCPACK it sends in response to such a DHCPDISCOVER when completing that exchange. IANA has assigned value 80 in accordance with RFC 2939.
It is a tiny flag with a number on it and nothing inside. Seeing the flag is enough to know what is wanted.
§6Security Considerations
The RFC says the concepts do not significantly alter the security considerations of DHCP (see RFC 2131 and RFC 3118). There is one exception: the option could help denial of service attacks.
A mischievous client could use up all available addresses more rapidly. It could also do so without needing two-way communication, because injecting DHCPDISCOVER messages carrying the Rapid Commit option is enough to make a server allocate an address.
If a server gives away an address to anyone who asks, a troublemaker can ask over and over and use them all up faster.