DHCPv6 Prefix-Length Hint Issues
RFC 8168

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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/rfc8168.

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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 [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 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.

   [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 should be able to
   handle prefixes with lengths different from the prefix-length hint.