DHC Working Group Y. Cui
Internet-Draft T. Li
Intended status: Standards Track C. Liu
Expires: April 13, 2016 Tsinghua University
October 11, 2015
DHCPv6 Prefix Length Hint Issues
draft-cui-dhc-dhcpv6-prefix-length-hint-issue-01
Abstract
DHCPv6 Prefix Delegation [RFC3633] 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 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Description . . . . . . . . . . . . . . . . . . . . . 3
3.1. Creation of Solicit Message by the Client . . . . . . . . 3
3.2. Receipt of Solicit message by the Server . . . . . . . . 3
3.3. Receipt of Advertise Message by the Client . . . . . . . 4
3.4. Creation of Renew/Rebind Message by the Client . . . . . 4
3.5. Receipt of Renew/Rebind Message by the Server . . . . . . 4
4. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Creation of Solicit Message by the Client . . . . . . . . 5
4.2. Receipt of Solicit message by the Server . . . . . . . . 5
4.3. Receipt of Advertise Message by the Client . . . . . . . 6
4.4. Creation of Renew/Rebind Message by the Client . . . . . 6
4.5. Receipt of Renew/Rebind Message by the Server . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Contributors List . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
DHCPv6 Prefix Delegation [RFC3633] 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, 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
hint values depending on server policy. This would not cause
problems for some hint values such as T1 and T2 lifetimes, but it
would be an issue for the prefix-length hint. Some clients can't
function normally when they're provided with a prefix which length is
different from what they requested. E.g. 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. The clients usually have higher preference on
the prefix-length hint than the other option hints, and it should be
given more consideration.
The current specification is unclear about how the client and server
should act in different situations involving the prefix-length hint.
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From the client perspective, it should be able to use the prefix-
length hint to signal to the server its real time need and it should
be able to handle the prefixes which lengths are different from the
prefix-length hint. This document provides guidance on what a client
should do in different situations, to prevent it from failing. From
the server perspective, the server is free to ignore the prefix-
length hints depending on server policy, but in cases where the
server has a 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Problem Description
3.1. Creation of Solicit Message by the Client
The Solicit message allows a client to ask servers for addresses and
configuration parameters. When the client's configuration changes,
it might require a prefix length different from what it had
previously gotten. The server usually has a record of the prefix it
delegated to the client during previous interactions. How should the
client avoid getting the same prefix back from the server?
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 that it wants a different prefix. The
best way to assure a completely new delegated prefix is to send a new
IAID in the IA_PD. However, this would require the client device to
have persistant storage, since rebooting the device would cause the
client to use the original IAID in the IA_PD.
3.2. Receipt of Solicit message by the Server
[RFC3633] allows a client to include a prefix-length hint in the
Solicit message, to signal its preference to the server. However, it
is unclear about how this prefix-length hint should be handled by the
server. Some servers will keep a record about prefixes it gave to
the client during previous interactions, and give the client the same
prefix. When the client includes a prefix-length hint in the Solicit
message, the server has to decide whether to honor the newly
requested prefix-length hint or give the client the recorded prefix.
The client might want a different prefix length due to configuration
changes or it might just want the same prefix again after reboot.
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The server should interpret these cases differently.
Many servers are configured to provide only prefixes of specific
lengths to the client. E.g. If the client requested for a /54, and
the server could only provide /30,/48, and /56. How should these
servers decide which prefix to give to the client based on the
client's prefix-length hint?
3.3. Receipt of Advertise Message by the Client
The server might not be able to honor the prefix-length hint due to
server policy. 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 where the client would
fail if it used a prefix which 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.
3.4. Creation of Renew/Rebind Message by the Client
Servers might not be able to provide a prefix matching the prefix-
length hint requested by the client. If the client decided to use
the prefix provided by the server which doesn't match the prefix-
length hint, but would still prefer the prefix-length hint it
originally requested in the Solicit message, there should be some way
for the client to express this preference during Renew/Rebind. E.g.
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.[RFC3633] is not completely clear on whether the client is
allowed to include a prefix-length hint in the Renew/Rebind message.
3.5. Receipt of Renew/Rebind Message by the Server
The prefix preferred by the client might become available in the
server's prefix pool during Renew/Rebind, but was unavailable during
Solicit. This might be due to 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
during Renew/Rebind 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
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preference for the prefix length might have changed during this time
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. The current specification is
unclear about what the server should do if the client also included
in the Renew/Rebind message a prefix-length hint value, and whether
the server could provide a different prefix to the client during
Renew/Rebind.
4. Proposed Solution
4.1. Creation of Solicit Message by the Client
When the client prefers a prefix of specific length from the server,
the client should send a Solicit message including the preferred
prefix-length value in the "prefix-length" field of the IA_PD Prefix
option, and set the "IPv6 prefix" field to zero. This is an
indiction to the server that the client prefers a prefix of specific
length, regardless of what it had gotten before.
When the client wants the same prefix back from the server, it should
include the prefix value in the "IPv6 prefix" field of the IA_PD
Prefix option, and 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.
4.2. Receipt of Solicit message by the Server
Upon the receipt of Solicit message, if the client included a prefix-
length hint in the message, the server should try to honor the
prefix-length hint within bounds of what the server is configured to
return, regardless of the prefix record from previous interactions
with the client. The server should regard the prefix-length hint in
the Solicit message as the prefix length most preferred by the client
at the time.
Many servers are configured to provide prefixes of specific lengths
to the client. In this situation, the server should provide the
shortest prefix length possible which is closest to the prefix-length
hint. E.g. If the server could only provide prefixes with lengths
/30,/48, and /56, and the client is requesting for a /50 in the
prefix-length hint, then the server should provide the /48 to the
client.
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4.3. Receipt of Advertise Message by the Client
If none of the prefixes provided by the server in the Advertise
messages match the prefix-length hint the client included in the
Solicit message, the client could choose to either accept or ignore
the prefixes provided by the servers depending on functional need.
If the client could use the prefixes provided by the servers despite
being different from the prefix-length hint, the client should choose
a prefix length closest to the prefix-length hint.
There are certain situations where the client will fail if it used a
prefix which length does not meet its requirement. If the client
cannot use the prefixes provided by the servers, it should ignore the
Advertise messages and continue to send Solicit messages until it
gets the preferred prefix. To avoid traffic congestion, the client
should send Solicit messages at defined intervals, as specified in
[RFC7083]. To prevent the client from not functioning, the client
should not ignore other configuration parameters provided by the
server such as available IA_NA addresses.
4.4. Creation of Renew/Rebind Message by the Client
During the Renew process, if the client prefers a prefix length
different from the prefix it is currently using, then the client
should send the Renew message with the same IA_PD, and include two
IA_PD Prefix options, one containing the currently delegated prefix
and the other containing the prefix-length hint. This is to extend
lifetime of the prefix the client is currently using and also 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, the client should continue using the prefix it
currently has.
4.5. Receipt of Renew/Rebind Message by the Server
Upon the receipt of Renew message, if the client included in the
IA_PD both the delegated prefix value and a prefix-length hint value,
the server should check to see whether it could extend the lifetime
of the original delegated prefix and whether it has any available
prefix matching the prefix-length hint, or as close a possible to the
requested length, within the server's limit.
The server could do one of the following depending on server policy:
1. Renew just the original delegated prefix.
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2. Renew 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 asssign a new prefix of 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
short non-zero valid-lifetime, and asssign a new prefix of requested
length. This is to provide the original delegated prefix with a
short lifetime so the client can go through a graceful switch over.
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.
5. Security Considerations
TBD.
6. IANA Considerations
This document does not include an IANA request.
7. Contributors List
Many thanks to Qi Sun, Bernie Volz, Ole Troan, Sunil Gandhewar.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<http://www.rfc-editor.org/info/rfc3633>.
[RFC7083] Droms, R., "Modification to Default Values of SOL_MAX_RT
and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC7083, November
2013, <http://www.rfc-editor.org/info/rfc7083>.
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Authors' Addresses
Yong Cui
Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6260-3059
Email: yong@csnet1.cs.tsinghua.edu.cn
Tianxiang Li
Tsinghua University
Beijing 100084
P.R.China
Phone: +86-18301185866
Email: peter416733@gmail.com
Cong Liu
Tsinghua University
Beijing 100084
P.R.China
Phone: +86-10-6278-5822
Email: gnocuil@gmail.com
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