Issues with multiple stateful DHCPv6 options
draft-ietf-dhc-dhcpv6-stateful-issues-00

Network Working Group                                           O. Troan
Internet-Draft                                                   B. Volz
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: November 8, 2012                                    May 7, 2012

              Issues with multiple stateful DHCPv6 options
              draft-ietf-dhc-dhcpv6-stateful-issues-00.txt

Abstract

   Dynamic Host Configuration Protocol for IPv6 (DHCPv6) was not written
   with the expectation that additional stateful DHCPv6 options would be
   developed.  IPv6 Prefix Options for Dynamic Host Configuration
   Protocol (DHCP) version 6 shoe-horned the new options for Prefix
   Delegation into DHCPv6.  Implementation experience of the CPE model
   described in has shown multiple issues with the DHCPv6 protocol in
   supporting multiple stateful options.

Status of this Memo

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Handling of multiple IA options types . . . . . . . . . . . . . 4
     4.1.  Advertisement message . . . . . . . . . . . . . . . . . . . 4
     4.2.  Placement of Status codes . . . . . . . . . . . . . . . . . 5
     4.3.  T1/T2 timers  . . . . . . . . . . . . . . . . . . . . . . . 5
     4.4.  Confirm message . . . . . . . . . . . . . . . . . . . . . . 6
     4.5.  Release messages  . . . . . . . . . . . . . . . . . . . . . 6
     4.6.  Unanswered options  . . . . . . . . . . . . . . . . . . . . 6
     4.7.  Multiple provisioning domains . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   DHCPv6 [RFC3315] was not written with the expectation that additional
   stateful DHCPv6 options would be developed.  DHCPv6 Prefix Delegation
   [RFC3633] shoe-horned the new options for Prefix Delegation into
   DHCPv6.  Implementation experience of the CPE model described in
   [RFC6204] has shown multiple issues with the DHCPv6 protocol in
   supporting multiple stateful options.

   This document describes a number of problems encountered with
   multiple IA option types into DHCP and recommended changes to the
   DHCPv6 protocol specifications.

   The intention of this work is to modify the DHCP protocol
   specification to support multiple IA option types within a single
   DHCP session.  This problem can also be solved by implementing a
   separate DHCP session (separate client state machine) per IA option
   type.  This latter approach has a number of issues: additional DHCP
   protocol traffic, 'collisions' between stateless options also
   included with the IA options, divergence in that each IA option type
   specification specifies its 'own' version of the DHCP protocol.

   The changes described in this document will be incorporated in a new
   revision of the DHCPv6 protocol specification [RFC3315].

2.  Conventions

   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 RFC 2119 [RFC2119].

3.  Terminology

   Stateful options            Options that require dynamic binding
                               state per client on the server.

   Identity association (IA):  A collection of stateful options assigned
                               to a client.  Each IA has an associated
                               IAID.  A client may have more than one IA
                               assigned to it; for example, one for each
                               of its interfaces.  Each IA holds one
                               type of IA option; for example, an
                               identity association for temporary
                               addresses (IA_TA) holds temporary
                               addresses (see "identity association for
                               temporary addresses").  Throughout this

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                               document, "IA" is used to refer to an
                               identity association without identifying
                               the type of stateful option in the IA.

4.  Handling of multiple IA options types

   DHCPv6 was written with the assumption that the only stateful options
   where for assigning addresses.  DHCPv6 PD describes how to extend the
   DHCPv6 protocol to handle prefix delegation, but RFC3633 did not
   consider how DHCP address assignment and prefix delegation could co-
   exist.

4.1.  Advertisement message

   RFC3315 specifies that a client must ignore an Advertise message if a
   server will not assign any addresses to a client.  A client
   requesting both IA_NA and IA_PD, with a server that only offers one
   of them, is not supported in the current protocol specification.

   Proposed solution: a client should accept Advertise messages, even
   when not all IA option types are being offered.  A client should
   ignore an Advertise message when no bindings at all are being
   offered.

   Replace Section 17.1.3: (existing errata)

     The client MUST ignore any Advertise message that includes a Status
     Code option containing the value NoAddrsAvail, with the exception
     that the client MAY display the associated status message(s) to the
     user.

   With:

     The client MUST ignore any Advertise message that contains no
     bindings (if only IA_NA and/or IA_TA options were requested,
     this is a message that includes a Status Code option containing the
     value NoAddrsAvail), with the exception that the client MAY display
     the associated status message(s) to the user.

   And, replace:

      -  The client MAY choose a less-preferred server if that server
         has a better set of advertised parameters, such as the
         available addresses advertised in IAs.

   With:

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      -  The client MAY choose a less-preferred server if that server
         has a better set of advertised parameters, such as the
         available options advertised in IAs.

   It is important to note that the receipt of a Advertisement without
   any bindings does not imply that the client should restart the
   Solicit retransmissions timers.  Doing so would lead to a Solicit/
   Advertisement storm.

4.2.  Placement of Status codes

   In Reply messages IA specific status codes (NoAddrsAvail, NotOnlink,
   NoBinding) are encapsulated in the IA option.  In Advertisement
   messages the Status Code option with the NoAddrsAvail code is in the
   "global" scope.  That makes sense when the failure case is fatal.
   With the introduction of multiple IA option types, there might be a
   case where a server is not willing to offer addresses, but might be
   willing to offer other stateful option types.

   While a Status Code option is implicitly bound to a specific type of
   IA, e.g.  NoPrefixAvail is only applicable to IA_PD and NoAddrsAvail
   is only applicable to IA_NA/IA_TA, it may be problematic to make this
   assumption for all status codes.  Ideally the Status Code option
   should be encapsulated in the IA option for all DHCP messages.  This
   makes Advertisement messages equal to Reply messages.

   Proposed solution: No change.  For backwards compatibility, the
   NoAddrsAvail Status Code option when no addresses are available will
   be kept in the global scope for Advertise messages.  Other IA option
   types MUST encapsulate the Status Code option within the IA option.

4.3.  T1/T2 timers

   The T1 and T2 timers determine when the client will contact the
   server to extend lifetimes of information received in an IA.  How
   should a client handle the case where multiple IA options have
   different T1 and T2 timers?

   In a multiple IA option types model, the T1/T2 timers are protocol
   timers, that should be independent of the IA options themselves.  If
   we were to redo the DHCP protocol from scratch the T1/T2 timers
   should be carried in a separate DHCP option.

   Proposed solution: The server SHOULD set the T1/T2 timers in all IA
   options in Reply and Advertise messages to the same value.  To deal
   with the case where servers have not yet been updated to do that,
   clients MUST use the shortest (explicit or implicit) T1/T2 timer
   (larger than 0) in any IA options in the Reply.  Longer T1/T2 timers

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   are ignored.

4.4.  Confirm message

   The Confirm message, as described in [RFC3315], is specific to
   address assignment.  It lets a server without a binding to reply to
   the message, under the assumption that the server only needs
   knowledge about the prefix(es) on the link, to inform the client that
   the address is likely valid or not.  This message is sent when e.g.
   the client has moved and needs to validate its addresses.  Not all
   bindings can be validated by servers and the Confirm message provides
   for this by specifying that a server that is unable to determine the
   on-link status MUST NOT send a Reply.

   Note: Confirm has a specific meaning and does not overload Renew/
   Rebind.  It also is lower processing cost as the server does NOT need
   to extend lease times or otherwise send back other configuration
   options.

   Proposed solution: Allow and specify the Confirm message for other IA
   option types.  A server SHOULD respond to a Confirm message only if
   it has definitive knowledge, based on the network configuration and
   not the specific client's bindings, that the client is still on-link
   or not on-link.

4.5.  Release messages

   A client can release any individual lease at any time.  A client can
   get "back" a lease by using a Renew message.  It MAY do this at any
   time, though must avoid creating a Renew storm.  E.g. wait until T1.

4.6.  Unanswered options

   If a client requests multiple IA option types, but the server is
   willing to only offer a subset of them, the client could react in
   several ways.  Reset the state machine and continue to send Solicit
   messages, create separate DHCP sessions for each IA option type and
   continue to Solicit for the missing options, or it could continue
   with the single session, and include the missing options on
   subsequent messages to the server.

   Proposed solution: the client should keep a single session with the
   server.  The client should continue with the IA options received,
   while continuing to request the other IA options in subsequent
   messages to the server.  That means to continue to include the empty
   unanswered IAs in subsequent Renew and Rebind messages.

   For the IAs that the server will not offer a binding, it must reply

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   using the same behaviour as for a Request message.  That is not with
   the currently specified NoBinding status).  This behaviour will not
   require the server to remember the IAs that it is not willing to
   serve.  I.e. the change is to allow the client to include IAs in
   Renew/Rebind messages for which it has not received bindings (yet).

   A client can only use the Renew (or Rebind) to request new IA options
   if it already has one or more bindings.  A client MUST NOT use Renew
   (or Rebind) if it has no valid bindings it is renewing.

   Replace Section 18.2.3:

    If the server cannot find a client entry for the IA the server
    returns the IA containing no addresses with a Status Code option set
    to NoBinding in the Reply message.

   With:

     If the server cannot find a client entry for the IA but has one or
     more bindings for the client, the server SHOULD treat this like a
     Request message for the IA. If the server has no other bindings for
     the client, the server SHOULD return the IA containing no bindings
     with a Status Code option set to NoBinding in the Reply message.

4.7.  Multiple provisioning domains

   This document has assumed that all DHCP servers on a network are in a
   single provisioning domain and thus should be "equal" in the service
   that they offer.

   One could envision a network where the DHCP servers are in multiple
   provisioning domains, and it may be desireable to have the DHCP
   client obtain different IA types from different provisioning domains.
   How a client detects the multiple provisioning domains and how it
   would interact with the multiple servers in these different domains
   is outside the scope of this document and an area for future work.

5.  IANA Considerations

   This specification does not require any IANA actions.

6.  Security Considerations

   There are no new security considerations pertaining to this document.

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7.  Acknowledgements

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011.

Authors' Addresses

   Ole Troan
   Cisco Systems, Inc.
   Philip Pedersens vei 20
   N-1324 Lysaker,
   Norway

   Email: ot@cisco.com

   Bernie Volz
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA  01719,
   USA

   Email: volz@cisco.com

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