DHC Working Group Y. Cui
Internet-Draft T. Li
Intended status: Informational C. Liu
Expires: May 16, 2016 Tsinghua University
November 13, 2015
DHCPv6 Prefix Length Hint Issues
draft-cui-dhc-dhcpv6-prefix-length-hint-issue-02
Abstract
DHCPv6 Prefix Delegation [RFC3633] allows a requesting router 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 requesting router and delegating router 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 requesting router and delegating router
could do in different situations.
Status of This Memo
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This Internet-Draft will expire on May 16, 2016.
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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
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Contributors List . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
DHCPv6 Prefix Delegation [RFC3633] allows a requesting router to
include a prefix-length hint value in the message sent to the
delegating router, to indicate a preference for the size of the
prefix to be delegated. A prefix-length hint is communicated by a
requesting router to the delegating router 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 delegating routers 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 requesting routers can't
function normally when they're provided with a prefix which length is
different from what they requested. E.g. if the requesting router is
asking for a /56 and the delegating router returns a /64, the
functionality of the requesting router might be limited because it
might not be able to split the prefix for all its interfaces. The
requesting routers 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 requesting router
and delegating router should act in different situations involving
the prefix-length hint. From the requesting router perspective, it
should be able to use the prefix-length hint to signal to the
delegating router its real time need and it should be able to handle
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the prefixes which lengths are different from the prefix-length hint.
This document provides guidance on what a requesting router should do
in different situations, to prevent it from failing. From the
delegating router perspective, the delegating router is free to
ignore the prefix-length hints depending on server policy, but in
cases where the delegating router has a policy for considering the
hint, this document provides guidance on how the prefix-length hint
should be handled by the delegating router 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 and Proposed Solutions
3.1. Creation of Solicit Message
Problem:
The Solicit message allows a requesting router to ask delegating
routers for prefixes and other configuration parameters. When the
requesting router's configuration changes, it might require a prefix
length different from what it had previously gotten. The delegating
router usually has a record of the prefix it delegated to the
requesting router during previous interactions. How should the
requesting router avoid getting the same prefix back from the
delegating router?
The delegating router could decide whether to provide the requesting
router with the preferred prefix depending on server policy, but the
requesting router should be able to signal to the delegating router
whether it wants a different prefix or the same 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 requesting router device
to have persistant storage, since rebooting the device would cause
the requesting router to use the original IAID in the IA_PD.
Solution:
When the requesting router prefers a prefix of specific length from
the delegating router, the requesting router 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 delegating router that
the requesting router prefers a prefix of specific length, regardless
of what it had gotten before.
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When the requesting router wants the same prefix back from the
delegating router, 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
delegating router that the requesting router wants the same prefix
back.
3.2. Receipt of Solicit message
Problem:
[RFC3633] allows a requesting router to include a prefix-length hint
in the Solicit message, to signal its preference to the delegating
router. However, it is unclear about how this prefix-length hint
should be handled by the delegating router. Some delegating routers
will keep a record about prefixes it gave to the requesting router
during previous interactions, and give the requesting router the same
prefix. When the requesting router includes a prefix-length hint in
the Solicit message, the delegating router has to decide whether to
honor the newly requested prefix-length hint or give the requesting
router the recorded prefix. The requesting router might want a
different prefix length due to configuration changes or it might just
want the same prefix again after reboot. The delegating router
should interpret these cases differently.
Many delegating routers are configured to provide only prefixes of
specific lengths to the requesting router. E.g. If the requesting
router requested for a /54, and the delegating router could only
provide /30,/48, and /56. How should these delegating routers decide
which prefix to give to the requesting router based on the requesting
router's prefix-length hint?
Solution:
Upon the receipt of Solicit message, if the requesting router
included only a prefix-length hint in the message, the delegating
router should try to honor the prefix-length hint within bounds of
what the delegating router is configured to return, regardless of the
prefix record from previous interactions with the requesting router.
The delegating router should regard the prefix-length hint in the
Solicit message as the prefix length most preferred by the requesting
router at the time.
If the requesting router included a specific prefix value and the
corresponding prefix-length value in the Solicit message, the
delegating router should first try to provide the requested prefix to
the requesting router. If the requested prefix is not available in
the delegating router's prefix pool, then the delegating router
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should try to provide a prefix matching the prefix-length value.
The delegating router might not have prefixes exactly matching the
prefix-length hint. In this situation, the delegating router should
provide the shortest prefix length possible which is closest to the
prefix-length hint. E.g. If the delegating router could only
provide prefixes pf lengths /30,/48, and /56, and the requesting
router is requesting for a /50 in the prefix-length hint, then the
delegating router should provide the /48 to the requesting router.
3.3. Receipt of Advertise Message
Problem:
The delegating router might not be able to honor the prefix-length
hint due to server policy. If the prefix length provided by the
delegating router in the Advertise message is different from what the
requesting router requested in the Solicit message, the question
would be whether the requesting router should use the provided prefix
length or continue to ask for its preferred prefix length. There are
certain situations where the requesting router would fail if it used
a prefix which length is different from what it requested in the
prefix-length hint. However, if the requesting router ignores the
Advertise messages, and continues to solicit for the preferred prefix
length, the requesting router might be stuck in the DHCP process.
Solution:
If none of the prefixes provided by the delegating router in the
Advertise messages match the prefix-length hint the requesting router
included in the Solicit message, the requesting router could choose
to either accept or ignore the prefixes provided by the delegating
routers depending on functional need.
If the requesting router could use the prefixes provided by the
delegating routers despite being different from the prefix-length
hint, the requesting router should choose the shortest prefix length
which is closest to the prefix-length hint.
There are certain situations where the requesting router will fail if
it used a prefix which length does not meet its requirement. If the
requesting router cannot use the prefixes provided by the delegating
routers, it should ignore the Advertise messages and continue to send
Solicit messages until it gets the preferred prefix. To avoid
traffic congestion, the requesting router should send Solicit
messages at defined intervals, as specified in [RFC7083]. If the
requesting router also Solicited for IA_NAs, the requesting router
should accept the IA_NA addresses and continue to request for the
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desired IA_PD prefix in subsequent DHCPv6 messages as specified in
[RFC7550].
3.4. Creation of Renew/Rebind Message
Problem:
delegating routers might not be able to provide a prefix matching the
prefix-length hint requested by the requesting router. If the
requesting router decided to use the prefix provided by the
delegating router 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 requesting
router to express this preference during Renew/Rebind. E.g. If the
requesting router requested for a /60 but got a /64, the requesting
router should be able to signal to the delegating router during
Renew/Rebind that it would still prefer a /60. This is to see
whether the delegating router has the prefix preferred by the
requesting router available in its prefix pool during Renew/
Rebind.[RFC3633] is not completely clear on whether the requesting
router is allowed to include a prefix-length hint in the Renew/Rebind
message.
Solution:
During the Renew process, if the requesting router prefers a prefix
length different from the prefix it is currently using, then the
requesting router 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 requesting router is
currently using and also get the prefix the requesting router
prefers, and go through a graceful switch over.
If the delegating router is unable to provide the requesting router
with the newly requested prefix, the requesting router should
continue using the prefix it currently has.
3.5. Receipt of Renew/Rebind Message
Problem:
The prefix preferred by the requesting router might become available
in the delegating router's prefix pool during Renew/Rebind, but was
unavailable during Solicit. This might be due to delegating router
configuration change or because some other requesting router stopped
using the prefix.
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The question is whether the delegating router should remember the
prefix-length hint the requesting router originally included in the
Solicit message and check during Renew/Rebind see if it has the
prefix length the requesting router preferred. This would require
the delegating router to keep extra information about the requesting
router. There is also the possibility that the requesting router's
preference for the prefix length might have changed during this time
interval, so the prefix-length hint remembered by the delegating
router might not be what the requesting router prefers during Renew/
Rebind.
Instead of having the delegating router remember the prefix-length
hint of the requesting router, another option is for the requesting
router to include the prefix-length hint in the Renew/Rebind message.
The current specification is unclear about what the delegating router
should do if the requesting router also included in the Renew/Rebind
message a prefix-length hint value, and whether the delegating router
could provide a different prefix to the requesting router during
Renew/Rebind.
Solution:
Upon the receipt of Renew message, if the requesting router included
in the IA_PD both the delegated prefix value and a prefix-length hint
value, the delegating router 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 delegating router's
limit.
The delegating router could do one of the following depending on
server policy:
1. Renew just the original delegated prefix.
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 requesting router.
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 allows the requesting router to finish up existing
connections with the original prefix, and use the new prefix to
establish new connections.
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5. Do not include the original delegated prefix in the Reply
message, and assign a new prefix of requested length. The original
prefix would be valid until it's lifetime expires. This avoids
sudden renumbering on the requesting router.
It's unnecessary for the delegating router to remember the prefix-
length hint the requesting router requested during Solicit. It is
possible that the requesting router'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 requesting
router prefers at the time.
4. Security Considerations
TBD.
5. IANA Considerations
This document does not include an IANA request.
6. Contributors List
Many thanks to Qi Sun, Bernie Volz, Ole Troan, Sunil Gandhewar,
Marcin Siodelski.
7. 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>.
[RFC7550] Troan, O., Volz, B., and M. Siodelski, "Issues and
Recommendations with Multiple Stateful DHCPv6 Options",
RFC 7550, DOI 10.17487/RFC7550, May 2015,
<http://www.rfc-editor.org/info/rfc7550>.
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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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