wdiff rfc8168v5.original rfc8168.original

DHC Working Group T. Li
Internet-Draft C. Liu
Intended status: Standards Track Y. Cui
Expires: July 29, October 2, 2017 Tsinghua University
January 25,
March 31, 2017

DHCPv6 Prefix Length Hint Issues
draft-ietf-dhc-dhcpv6-prefix-length-hint-issue-05
draft-ietf-dhc-dhcpv6-prefix-length-hint-issue-06

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.

Status of This Memo

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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 . . . . . . . . . . . . 5 6
3.5. Receipt of Renew/Rebind Message . . . . . . . . . . . . . 6
3.6. General Recommendation . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Contributors List Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9

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 (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 which whose 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. 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 should be given more consideration.

[RFC3633] 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 it 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, 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 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 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 servers server usually has a record of the prefix it gave to the client
during previous interactions. The best way to assure a completely
new delegated prefix is to send a new IAID (Identity Association
IDentifier) in the IA_PD. IA_PD (Identity Association for Prefix
Delegation). However, this would require the client device to have
persistent storage, since rebooting the device would cause the client
to use the original IAID in the IA_PD.

Solution:

When the client prefers a prefix of specific length from the server,
the client MUST send a Solicit message using the same IAID in the
IAPD, 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 had gotten
before.

When the client wants the same prefix back from the server, it MUST
send a Solicit message using the same IAID in the IAPD, include the
previously delegated prefix value in the "IPv6 prefix" field of the
IAPREFIX 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.

When the client wants the same prefix back from the server, server and would
prefer to accept a prefix of specified length in case the requested
prefix is not available, the client MUST send a Solicit message using
the same IAID in the IAPD, include the previously delegated prefix in
one IAPREFIX option, and include the prefix-length hint in another
IAPREFIX option. There is no requirement to the order of the two
IAPREFIX options.

3.2. Receipt of Solicit message

Problem:

[RFC3633] allows a client to include a prefix-length hint in the
Solicit message, to signal its preference to the server. It is
unclear about how the prefix-length hint should be handled by the
server. 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. 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
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 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 length
matching the prefix-length hint value, then the server SHOULD provide
the prefix with the shortest whose length possible which 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
try to provide
check its prefix pool for a prefix with length matching the prefix-length value, or prefix-
length hint value. If the server does not have a prefix with length
matching the shortest prefix-length hint value, the server SHOULD provide the
prefix whose length possible which 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 [RFC3633].

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 where the client could not operate properly if it used a
prefix which 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 which is closest to
the prefix-length hint. The client SHOULD continue requesting for
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
[RFC7083].

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 for the desired IA_PD
prefix in subsequent DHCPv6 messages as specified in [RFC7550].

3.4. Creation of Renew/Rebind Message

Problem:

Servers might not be able to provide a prefix with length equal or
shorter than the prefix-length hint. If the client decided to use
the prefix provided by the server despite being longer than 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.

Solution:

During Renew/Rebind, if the client prefers a prefix length 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 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, 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, 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 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
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.

Solution:

Upon the receipt of Renew/Rebind, 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 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 prefix-length hint,
within the server's 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 lifetime of the original delegated prefix.

2. Extend 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 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
specific non-zero valid-lifetime, valid-lifetime depending on actual requirement, and
assign a new prefix of 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 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 requested length.

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 at least the hint length (or shorter) 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 introduces no new security considerations over those
already discussed in section 15 of RFC3633, as this document provides guidance on how the clients and servers
interact with regard to the DHCPv6 prefix-length hint mechanism introduced hint. Security
considerations in DHCP are described in Section 23 of [RFC3315].
Security considerations regarding DHCPv6 prefix delegation are
described in RFC3633. Section 15 of [RFC3633].

5. IANA Considerations

This document does not include an IANA request.

6. Contributors List Acknowledgements

Many thanks to Qi Sun, Bernie Volz, Ole Troan, Sunil Gandhewar,
Marcin Siodelski. Siodelski, Ted Lemon, Roni Even, Benoit Claise, Mirja
Kuehlewind, Kathleen Moriarty, Eric Rescorla, Alvaro Retana, Susan
Hares, Hilarie Orman for their review and comments.

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>.

[RFC3315] 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/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.

[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>.

Authors' Addresses

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

Yong Cui
Tsinghua University
Beijing 100084
P.R.China

Phone: +86-10-6260-3059
Email: yong@csnet1.cs.tsinghua.edu.cn