Modification to Default Values of SOL_MAX_RT and INF_MAX_RT
Increases DHCPv6 retransmission timeouts and lets servers override them
Droms, R., "Modification to Default Values of SOL_MAX_RT and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC7083, November 2013, <https://www.rfc-editor.org/info/rfc7083>.
@misc{rfc7083,
series = {Request for Comments},
number = 7083,
howpublished = {RFC 7083},
publisher = {RFC Editor},
doi = {10.17487/RFC7083},
url = {https://www.rfc-editor.org/info/rfc7083},
author = {R. Droms},
title = {{Modification to Default Values of SOL_MAX_RT and INF_MAX_RT}},
pagetotal = 7,
year = 2013,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Droms
Request for Comments: 7083 Cisco Systems
Updates: 3315 November 2013
Category: Standards Track
ISSN: 2070-1721
Modification to Default Values of SOL_MAX_RT and INF_MAX_RT
Abstract
This document updates RFC 3315 by redefining the default values for
SOL_MAX_RT and INF_MAX_RT and defining options through which a DHCPv6
server can override the client's default value for SOL_MAX_RT and
INF_MAX_RT with new values.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7083.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Updates to SOL_MAX_RT and INF_MAX_RT in RFC 3315 . . . . . . 2
4. SOL_MAX_RT option . . . . . . . . . . . . . . . . . . . . . . 3
5. INF_MAX_RT Option . . . . . . . . . . . . . . . . . . . . . . 4
6. Updates for SOL_MAX_RT and INF_MAX_RT Options to RFC 3315 . . 4
7. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . . . 5
8. DHCPv6 Server Behavior . . . . . . . . . . . . . . . . . . . 6
9. DHCPv6 Relay Agent Behavior . . . . . . . . . . . . . . . . . 6
10. Security Considerations . . . . . . . . . . . . . . . . . . . 6
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
13.1. Normative References . . . . . . . . . . . . . . . . . . 7
13.2. Informative References . . . . . . . . . . . . . . . . . 7
1. Introduction
Section 5.5 of the DHCPv6 specification [RFC3315] defines the default
values of SOL_MAX_RT and INF_MAX_RT to be 120 seconds. In some
circumstances, these defaults will lead to an unacceptably high
volume of aggregated traffic at a DHCPv6 server.
The change to SOL_MAX_RT is in response to DHCPv6 message rates
observed on a DHCPv6 server in a deployment in which many DHCPv6
clients are sending Solicit messages, but the DHCPv6 server has been
configured not to respond to those Solicit messages. While no
explicit observations of traffic due to INF_MAX_RT have been
conducted, this document updates INF_MAX_RT for consistency with
SOL_MAX_RT.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Updates to SOL_MAX_RT and INF_MAX_RT in RFC 3315
This document changes Section 5.5 of RFC 3315 as follows:
OLD:
SOL_MAX_RT 120 secs Max Solicit timeout value
NEW:
SOL_MAX_RT 3600 secs Max Solicit timeout value
OLD:
INF_MAX_RT 120 secs Max Information-request timeout value
NEW:
INF_MAX_RT 3600 secs Max Information-request timeout value
With this change, a DHCPv6 client that does not receive a
satisfactory response will send Solicit or Information-request
messages with the same initial frequency and exponential backoff as
specified in Sections 17.1.2 and 18.1.5 of RFC 3315. However, the
long-term behavior of these DHCPv6 clients will be to send a Solicit
or Information-request message every 3600 seconds rather than every
120 seconds, significantly reducing the aggregated traffic at the
DHCPv6 server.
4. SOL_MAX_RT option
A DHCPv6 server sends the SOL_MAX_RT option to a client to override
the default value of SOL_MAX_RT. The value of SOL_MAX_RT in the
option replaces the default value defined in Section 3. One use for
the SOL_MAX_RT option is to set a longer value for SOL_MAX_RT, which
reduces the Solicit traffic from a client that has not received a
response to its Solicit messages.
The format of the SOL_MAX_RT option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SOL_MAX_RT value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SOL_MAX_RT (82)
option-len 4
SOL_MAX_RT value Overriding value for SOL_MAX_RT
in seconds; MUST be in range:
60 <= "value" <= 86400 (1 day)
Figure 1: SOL_MAX_RT Option Format
5. INF_MAX_RT Option
A DHCPv6 server sends the INF_MAX_RT option to a client to override
the default value of INF_MAX_RT. The value of INF_MAX_RT in the
option replaces the default value defined in Section 3. One use for
the INF_MAX_RT option is to set a longer value for INF_MAX_RT, which
reduces the Information-request traffic from a client that has not
received a response to its Information-request messages.
The format of the INF_MAX_RT option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| INF_MAX_RT value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_INF_MAX_RT (83)
option-len 4
INF_MAX_RT value Overriding value for INF_MAX_RT
in seconds; MUST be in range:
60 <= "value" <= 86400 (1 day)
Figure 2: INF_MAX_RT Option Format
6. Updates for SOL_MAX_RT and INF_MAX_RT Options to RFC 3315
Update to RFC 3315, Section 17.1.3:
OLD:
The client MUST ignore any Advertise message that includes a Status
Code option containing the value NoAddrsAvail, with the exception
that the client MAY display the associated status message to the
user.
NEW:
The client MUST ignore any Advertise message that includes a Status
Code option containing the value NoAddrsAvail, with the exception
that the client MUST process an included SOL_MAX_RT option, MUST
process an included INF_MAX_RT option, and MAY display the
associated status message to the user.
Update to RFC 3315, Section 17.2.2:
OLD:
If the server will not assign any addresses to any IAs in a
subsequent Request from the client, the server MUST send an
Advertise message to the client that includes only a Status Code
option with code NoAddrsAvail and a status message for the user, a
Server Identifier option with the server's DUID, and a Client
Identifier option with the client's DUID.
NEW:
If the server will not assign any addresses to any IAs in a
subsequent Request from the client, the server MUST send an Advertise
message to the client that includes only a Status Code option with
code NoAddrsAvail and a status message for the user, a Server
Identifier option with the server's DUID, a Client Identifier option
with the client's DUID, and (optionally) SOL_MAX_RT and/or INF_MAX_RT
options.
Update to RFC 3315, Section 14 (Add text, clarifying client behavior
while waiting for a response from a server):
NEW:
A client is not expected to listen for a response during the entire
period between transmission of Solicit or Information-request
messages.
7. DHCPv6 Client Behavior
A DHCPv6 client MUST include the SOL_MAX_RT option code in any Option
Request option [RFC3315] it sends as required by RFC 3315.
A DHCPv6 client MUST include the INF_MAX_RT option code in any Option
Request option it sends as required by RFC 3315.
A DHCPv6 client MUST silently ignore any SOL_MAX_RT or INF_MAX_RT
option values that are less than 60 or more than 86400.
If a DHCPv6 client receives a message containing a SOL_MAX_RT option
that has a valid value for SOL_MAX_RT, the client MUST set its
internal SOL_MAX_RT parameter to the value contained in the
SOL_MAX_RT option. This value of SOL_MAX_RT is then used by the
retransmission mechanism defined in Sections 14 and 17.1.2 of RFC
3315.
If a DHCPv6 client receives a message containing an INF_MAX_RT option
that has a valid value for INF_MAX_RT, the client MUST set its
internal INF_MAX_RT parameter to the value contained in the
INF_MAX_RT option. This value of INF_MAX_RT is then used by the
retransmission mechanism defined in Sections 14 and 18.1.5 of RFC
3315.
Updated SOL_MAX_RT and INF_MAX_RT values apply only to the network
interface on which the client received SOL_MAX_RT and/or INF_MAX_RT
options.
8. DHCPv6 Server Behavior
Sections 17.2.2 and 18.2 of RFC 3315 govern server operation in
regard to option assignment. As a convenience to the reader, we
mention here that the server will send options SOL_MAX_RT and
INF_MAX_RT only if configured with specific values for them, and the
client requested those options using the Option Request option.
The DHCPv6 server MAY include the SOL_MAX_RT option in any response
it sends to a client that has included the SOL_MAX_RT option code in
an Option Request option. The SOL_MAX_RT option is sent in the main
body of the message to client, not as an encapsulated option in,
e.g., an IA_NA, IA_TA [RFC3315], or IA_PD [RFC3633] option.
The DHCPv6 server MAY include the INF_MAX_RT option in any response
it sends to a client that has included the INF_MAX_RT option code in
an Option Request option. The INF_MAX_RT option is sent in the main
body of the message to client, not as an encapsulated option in,
e.g., an IA_NA, IA_TA, or IA_PD option.
9. DHCPv6 Relay Agent Behavior
There are no additional requirements for relays.
10. Security Considerations
This document introduces one security consideration beyond those
described in RFC 3315. A malicious DHCPv6 server might cause a
client to set its SOL_MAX_RT and INF_MAX_RT parameters to an
unreasonably high value with the SOL_MAX_RT and INF_MAX_RT options,
which may cause an undue delay in a client completing its DHCPv6
protocol transaction in the case no other valid response is received.
Assuming the client also receives a response from a valid DHCPv6
server, large values for SOL_MAX_RT and INF_MAX_RT will not have any
effect.
11. Acknowledgments
Tomek Mrugalski edited the text for compliance with "Guidelines for
Creating New DHCPv6 Options" [DHC-OPTION] and added important details
to the Security Considerations section.
12. IANA Considerations
IANA has assigned one option code each for OPTION_SOL_MAX_RT (82) and
OPTION_INF_MAX_RT (83) from the "DHCP Option Codes" table of the
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" registry.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
13.2. Informative References
[DHC-OPTION]
Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
Work in Progress, September 2013.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
Author's Address
Ralph Droms
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Phone: +1 978 936 1674
EMail: rdroms@cisco.com
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On this page
§1The Problem with Default Timeouts
DHCPv6, defined in RFC 3315, allows IPv6 hosts to request network configuration from servers. When a client sends a Solicit message looking for a DHCPv6 server or an Information-request message requesting configuration parameters, the client must retransmit if it does not receive a response within a certain interval.
RFC 3315 originally set the maximum retransmission interval (SOL_MAX_RT and INF_MAX_RT) to 120 seconds. In practice, deployments experienced problems: when a DHCPv6 server is configured not to respond to certain clients or when the server is unreachable, clients retransmit every 120 seconds indefinitely. In a large deployment with many unresponsive clients, this creates an unacceptably high volume of aggregated traffic at the server.
When a computer can't reach a server, it asks again every 2 minutes. If there are lots of computers, the server gets asked thousands of times a minute, which is too much.
§2New Default Timeout Values
RFC 7083 updates RFC 3315 by changing the default maximum retransmission intervals for both SOL_MAX_RT and INF_MAX_RT from 120 seconds to 3600 seconds (one hour). With this change, DHCPv6 clients still use exponential backoff for initial retransmissions, as defined in RFC 3315, but the long-term steady-state behavior changes dramatically: instead of retransmitting every 2 minutes, clients now retransmit every hour.
This significantly reduces the aggregated traffic sent to DHCPv6 servers, especially in deployments where many clients are unable to receive server responses. The exponential backoff still causes rapid retransmission for the first few attempts, providing responsive behavior when servers are briefly unavailable.
Instead of asking again every 2 minutes forever, computers now ask every hour. They still ask more often at first, but after they give up, they wait much longer.
§3SOL_MAX_RT and INF_MAX_RT Options
RFC 7083 defines two new DHCPv6 options that allow a server to override the client's default timeout values on a per-client basis. The SOL_MAX_RT option (option code 82) carries a 32-bit unsigned integer specifying the maximum timeout for Solicit messages in seconds. The INF_MAX_RT option (option code 83) specifies the maximum timeout for Information-request messages.
Both options have the same format: a 16-bit option code and 16-bit length (4 bytes), followed by a 32-bit timeout value. The timeout value MUST be between 60 seconds and 86400 seconds (one day). A DHCPv6 server MAY include these options in any response to a client that requested them via the Option Request option.
The server can now send the client a number that says how long to wait before asking again. It can set different timeouts for different types of requests, and the timeouts can be anywhere from 1 minute to 1 day.
§4Client and Server Behavior
A DHCPv6 client MUST include the SOL_MAX_RT and INF_MAX_RT option codes in its Option Request option when communicating with a server, indicating that it understands these options. When a client receives a message containing a valid SOL_MAX_RT or INF_MAX_RT option, it MUST set its internal parameter to the value in the option. Clients MUST silently ignore any option values less than 60 or greater than 86400 seconds.
A DHCPv6 server MAY send these options in responses, but only if configured with specific values and only to clients that requested the options. The options are sent in the main body of the message, not nested inside other options. Importantly, the updated RFC 3315 behavior specifies that a client MUST process these options even when the server responds with a NoAddrsAvail status code, allowing servers to manage retransmission behavior even when they cannot provide addresses.
Clients ask if the server understands these options. If the server sends back a timeout, the client uses it. The client ignores unreasonable values (too short or too long), and the server only sends these options if it's configured to do so.
§5Deployment Considerations
The change in default values from 120 to 3600 seconds applies automatically to any DHCPv6 client updated to conform to RFC 7083. This reduces traffic immediately for clients that do not receive server responses. Servers that wish to use non-default values can deploy these options to actively manage client retransmission behavior, providing fine-tuned control.
The updated RFC 3315 specifies that a client is not expected to listen for a response during the entire period between transmission of Solicit or Information-request messages, clarifying that clients may enter sleep or low-power states between retransmissions. This addition acknowledges that the longer timeout intervals are intended for scenarios where continuous listening is not required.
When you update computers with the new version, they automatically stop asking as often. Servers can fine-tune this by telling clients to use different timeouts, and computers don't have to stay awake waiting for answers.