DHCPv6 Prefix Length Hint Issues
draft-ietf-dhc-dhcpv6-prefix-length-hint-issue-03

quot;
   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 specified length in case the requested
   prefix is not avaiable, 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.

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 length possible which is 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 a prefix matching the prefix-length value, or the
   prefix with the shortest length possible which is closest to the
   prefix-length hint value.

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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 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 Solicted for 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

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   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.

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   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 non-zero valid-lifetime, 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 it's 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.

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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
   prefix-length hint mechanism introduced in RFC3633.

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>.

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   [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

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