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IETF RFC 8168
Last modified on Thursday, June 1st, 2017
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Internet Engineering Task Force (IETF) T. Li
Request for Comments: 8168 C. Liu
Category: Standards Track Y. Cui
ISSN: 2070-1721 Tsinghua University
May 2017
DHCPv6 Prefix-Length Hint Issues
 Abstract
DHCPv6 Prefix Delegation allows a client to include a prefix-length
hint value in the IA_PD option to indicate a preference for the size
of the prefix to be delegated, but it is unclear about how the client
and server should act in different situations involving the prefix-
length hint. This document provides a summary of the existing
problems with the prefix-length hint and guidance on what the client
and server could do in different situations.
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 7841.
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/RFC 8168.
Copyright Notice
Copyright (c) 2017 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.
Li, et al. Standards Track PAGE 1 
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Description and Proposed Solutions . . . . . . . . . 3
3.1. Creation of Solicit Message . . . . . . . . . . . . . . . 3
3.2. Receipt of Solicit Message . . . . . . . . . . . . . . . 4
3.3. Receipt of Advertise Message . . . . . . . . . . . . . . 5
3.4. Creation of Renew/Rebind Message . . . . . . . . . . . . 6
3.5. Receipt of Renew/Rebind Message . . . . . . . . . . . . . 6
3.6. General Recommendation . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Normative References . . . . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DHCPv6 Prefix Delegation [RFC 3633] allows a client to include a
prefix-length hint value in the message sent to the server to
indicate a preference for the size of the prefix to be delegated. A
prefix-length hint is communicated by a client to the server by
including an IA_PD Prefix Option (IAPREFIX option), encapsulated in
an IA_PD option, with the "IPv6 prefix" field set to zero and the
"prefix-length" field set to a non-zero value. The servers are free
to ignore the prefix-length hint values depending on server policy.
However, some clients may not be able to function (or only in a
degraded state) when they're provided with a prefix whose length is
different from what they requested. For example, if the client is
asking for a /56 and the server returns a /64, the functionality of
the client might be limited because it might not be able to split the
prefix for all its interfaces. For other hints, such as requesting
for an explicit address, this might be less critical, as it just
helps a client that wishes to continue using what it used last time.
The prefix-length hint directly impacts the operational capability of
the client; thus, it should be given more consideration.
[RFC 3633] is unclear about how the client and server should act in
different situations involving the prefix-length hint. From the
client perspective, it should be able to use the prefix-length hint
to signal to the server its real-time need and should be able to
handle prefixes with lengths different from the prefix-length hint.
This document provides guidance on what a client should do in
different situations to help it operate properly. From the server
perspective, the server is free to ignore the prefix-length hints
depending on server policy; however, in cases where the server has a
Li, et al. Standards Track PAGE 2
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
policy for considering the hint, this document provides guidance on
how the prefix-length hint should be handled by the server in
different situations.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. Problem Description and Proposed Solutions
3.1. Creation of Solicit Message
Problem:
The Solicit message allows a client to ask servers for prefixes and
other configuration parameters. The client might want a different
prefix length due to configuration changes, or it might just want the
same prefix again after reboot. The client might also prefer a
prefix of a specific length in case the requested prefix is not
available. The server could decide whether to provide the client
with the preferred prefix depending on server policy, but the client
should be able to signal to the server its real-time need.
The server usually has a record of the prefix it gave to the client
during its most recent interaction. The best way to assure a
completely new delegated prefix is to send a new IAID (Identity
Association IDentifier) in the IA_PD (Identity Association for Prefix
Delegation). However, this would require the client device to have
persistent storage, because rebooting the device would cause the
client to use the original IAID in the IA_PD.
Solution:
When the client prefers a prefix of a specific length from the
server, the client MUST send a Solicit message using the same IAID in
the IA_PD, include the preferred prefix-length value in the "prefix-
length" field of the IAPREFIX option, and set the "IPv6 prefix" field
to zero. This is an indication to the server that the client prefers
a prefix of the specified length, regardless of what it received
before.
When the client wants the same prefix back from the server, it MUST
send a Solicit message using the same IAID in the IA_PD, include the
previously delegated prefix value in the "IPv6 prefix" field of the
Li, et al. Standards Track PAGE 3
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
IAPREFIX option, and include the length of the prefix in the "prefix-
length" field. This is an indication to the server that the client
wants the same prefix back.
When the client wants the same prefix back from the server and would
prefer to accept a prefix of a specified length in case the requested
prefix is not available, the client MUST send a Solicit message using
the same IAID in the IA_PD, include the previously delegated prefix
in one IAPREFIX option, and include the prefix-length hint in another
IAPREFIX option. There is no requirement regarding the order of the
two IAPREFIX options.
3.2. Receipt of Solicit Message
Problem:
[RFC 3633] allows a client to include a prefix-length hint in the
Solicit message to signal its preference to the server. How the
prefix-length hint should be handled by the server is unclear. The
client might want a different prefix length due to configuration
changes or it might just want the same prefix again after reboot.
The server should interpret these cases differently.
Many servers are configured to provide only prefixes of specific
lengths to the client, for example, if the client requested for a /54
but the server could only provide /30, /48, and /56. How should
these servers decide which prefix to give to the client based on the
prefix-length hint?
Solution:
Upon the receipt of Solicit message, if the client included only a
prefix-length hint in the message, the server SHOULD first check its
prefix pool for a prefix with a length matching the prefix-length
hint value, regardless of the prefix record from previous
interactions with the client. If the server does not have a prefix
with a length matching the prefix-length hint value, then the server
SHOULD provide the prefix whose length is shorter and closest to the
prefix-length hint value.
If the client included a specific prefix value in the Solicit
message, the server SHOULD check its prefix pool for a prefix
matching the requested prefix value. If the requested prefix is not
available in the server's prefix pool, and the client also included a
prefix-length hint in the same IA_PD option, then the server SHOULD
check its prefix pool for a prefix with a length matching the prefix-
length hint value. If the server does not have a prefix with a
length matching the prefix-length hint value, the server SHOULD
Li, et al. Standards Track PAGE 4
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
provide the prefix whose length is shorter and closest to the prefix-
length hint value.
If the server will not assign any prefixes to any IA_PDs in a
subsequent Request from the client, the server MUST send an Advertise
message to the client as described in Section 11.2 of [RFC 3633].
3.3. Receipt of Advertise Message
Problem:
The server might not be able to honor the prefix-length hint due to
server policy or lack of resources in its prefix pool. If the prefix
length provided by the server in the Advertise message is different
from what the client requested in the Solicit message, the question
would be whether the client should use the provided prefix length or
continue to ask for its preferred prefix length. There are certain
situations in which the client could not operate properly if it used
a prefix whose length is different from what it requested in the
prefix-length hint. However, if the client ignores the Advertise
messages and continues to solicit for the preferred prefix length,
the client might be stuck in the DHCP process. Another question is
whether the client should ignore other configuration parameters such
as available addresses.
Solution:
If the client could use the prefixes included in the Advertise
messages despite being different from the prefix-length hint, the
client SHOULD choose the shortest prefix length that is closest to
the prefix-length hint. The client SHOULD continue requesting the
preferred prefix in the subsequent DHCPv6 messages as defined in
Section 3.4 of this document.
If the client sent a Solicit with only IA_PDs and cannot use the
prefixes included in the Advertise messages, it MUST ignore the
Advertise messages and continue to send Solicit messages until it
gets the preferred prefix. To avoid traffic congestion, the client
MUST send Solicit messages at defined intervals, as specified in
[RFC 7083].
If the client also solicited for other stateful configuration options
such as IA_NAs and the client cannot use the prefixes included in the
Advertise messages, the client SHOULD accept the other stateful
configuration options and continue to request the desired IA_PD
prefix in subsequent DHCPv6 messages as specified in [RFC 7550].
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RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
3.4. Creation of Renew/Rebind Message
Problem:
Servers might not be able to provide a prefix with the length equal
to or shorter than the prefix-length hint. If the client decided to
use the prefix provided by the server despite it being longer than
the prefix-length hint but would still prefer the prefix-length hint
originally requested in the Solicit message, there should be some way
for the client to express this preference during Renew/Rebind. For
example, if the client requested for a /60 but got a /64, the client
should be able to signal to the server during Renew/Rebind that it
would still prefer a /60. This is to see whether the server has the
prefix preferred by the client available in its prefix pool during
Renew/Rebind. [RFC 3633] is not completely clear on whether the
client is allowed to include a prefix-length hint in the Renew/Rebind
message.
Solution:
During Renew/Rebind, if the client prefers a prefix length that is
different from the prefix it is currently using, then the client
SHOULD send the Renew/Rebind message with the same IA_PD, and include
two IAPREFIX options, one containing the currently delegated prefix
and the other containing the prefix-length hint. This is to extend
the lifetime of the prefix the client is currently using, get the
prefix the client prefers, and go through a graceful switch over.
If the server is unable to provide the client with the newly
requested prefix, but is able to extend lifetime of the old prefix,
the client SHOULD continue using the old prefix.
3.5. Receipt of Renew/Rebind Message
Problem:
The prefix preferred by the client might become available in the
server's prefix pool during Renew/Rebind, even though it was
unavailable during Solicit. This might be due to a server
configuration change or because some other client stopped using the
prefix.
The question is whether the server should remember the prefix-length
hint the client originally included in the Solicit message and check
it during Renew/Rebind to see if it has the prefix length the client
preferred. This would require the server to keep extra information
about the client. There is also the possibility that the client's
preference for the prefix length might have changed during this time
Li, et al. Standards Track PAGE 6
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
interval, so the prefix-length hint remembered by the server might
not be what the client prefers during Renew/Rebind.
Instead of having the server remember the prefix-length hint of the
client, another option is for the client to include the prefix-length
hint in the Renew/Rebind message. [RFC 3633] is unclear about what
the server should do if the client also included a prefix-length hint
value in the Renew/Rebind message and whether the server could
provide a different prefix to the client during Renew/Rebind.
Solution:
Upon the receipt of a Renew/Rebind message, if the client included in
the IA_PD both an IAPREFIX option with the delegated prefix value and
an IAPREFIX option with a prefix-length hint value, the server SHOULD
check whether it could extend the lifetime of the original delegated
prefix and whether it has any available prefix matching the prefix-
length hint (or determine the closest possible to the prefix-length
hint) within its limit.
If the server assigned the prefix included in IA_PD to the client,
the server SHOULD do one of the following, depending on its policy:
1. Extend the lifetime of the original delegated prefix.
2. Extend the lifetime of the original delegated prefix and assign a
new prefix of the requested length.
3. Mark the original delegated prefix as invalid by giving it 0
lifetimes, and assign a new prefix of the requested length. This
avoids the complexity of handling multiple delegated prefixes but
may break all the existing connections of the client.
4. Assign the original delegated prefix with 0 preferred-lifetime, a
specific non-zero valid-lifetime depending on actual requirement,
and assign a new prefix of the requested length. This allows the
client to finish up existing connections with the original prefix
and use the new prefix to establish new connections.
5. Do not include the original delegated prefix in the Reply message,
and assign a new prefix of the requested length. The original
prefix would be valid until its lifetime expires. This avoids
sudden renumbering on the client.
If the server does not know the client's bindings (e.g., a different
server receiving the message during Rebind), then the server SHOULD
ignore the original delegated prefix and try to assign a new prefix
of the requested length.
Li, et al. Standards Track PAGE 7
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
It's unnecessary for the server to remember the prefix-length hint
the client requested during Solicit. It is possible that the
client's preference for the prefix length might have changed during
this time interval, so the prefix-length hint in the Renew message is
reflecting what the client prefers at the time.
3.6. General Recommendation
The recommendation to address the issues discussed in this document
is for a client that wants (at least) to have a delegated prefix of a
specific prefix length to always include an IAPREFIX option with just
the prefix-length hint in addition to any IAPREFIX options it has
included for each IA_PD in any Solicit, Request, Renew, and Rebind
messages it sends. While a server is free to ignore the hint,
servers that do not choose to ignore the hint should attempt to
assign a prefix of the hint length (or assign the next closest length
that does not exceed the hint) if one is available. Whether a server
favors the hint or avoiding a renumbering event is a matter of server
policy.
4. Security Considerations
This document provides guidance on how the clients and servers
interact with regard to the DHCPv6 prefix-length hint. Security
considerations in DHCP are described in Section 23 of [RFC 3315].
Security considerations regarding DHCPv6 prefix delegation are
described in Section 15 of [RFC 3633].
5. IANA Considerations
This document does not require any IANA actions.
6. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC 3315, July
2003, <http://www.rfc-editor.org/info/RFC 3315>.
[RFC 3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC 3633, December 2003,
<http://www.rfc-editor.org/info/RFC 3633>.
Li, et al. Standards Track PAGE 8
RFC 8168 DHCPv6 Prefix-Length Hint Issues May 2017
[RFC 7083] Droms, R., "Modification to Default Values of SOL_MAX_RT
and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC 7083, November
2013, <http://www.rfc-editor.org/info/RFC 7083>.
[RFC 7550] Troan, O., Volz, B., and M. Siodelski, "Issues and
Recommendations with Multiple Stateful DHCPv6 Options",
RFC 7550, DOI 10.17487/RFC 7550, May 2015,
<http://www.rfc-editor.org/info/RFC 7550>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <http://www.rfc-editor.org/info/RFC 8174>.
Acknowledgements
Many thanks to Qi Sun, Bernie Volz, Ole Troan, Sunil Gandhewar,
Marcin Siodelski, Ted Lemon, Roni Even, Benoit Claise, Mirja
Kuehlewind, Kathleen Moriarty, Eric Rescorla, Alvaro Retana, Susan
Hares, and Hilarie Orman for their review and comments.
Authors' Addresses
Tianxiang Li
Tsinghua University
Beijing 100084
China
Phone: +86-18301185866
Email: peter416733@gmail.com
Cong Liu
Tsinghua University
Beijing 100084
China
Phone: +86-10-6278-5822
Email: gnocuil@gmail.com
Yong Cui
Tsinghua University
Beijing 100084
China
Phone: +86-10-6260-3059
Email: yong.cui.thu@gmail.com
Li, et al. Standards Track PAGE 9
RFC TOTAL SIZE: 21936 bytes
PUBLICATION DATE: Thursday, June 1st, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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