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Versions: (draft-troan-dhc-dhcpv6-stateful-issues) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7550

Network Working Group                                           O. Troan
Internet-Draft                                                   B. Volz
Updates: 3315,3633 (if approved)                     Cisco Systems, Inc.
Intended status: Standards Track                            May 13, 2013
Expires: November 14, 2013


              Issues with multiple stateful DHCPv6 options
              draft-ietf-dhc-dhcpv6-stateful-issues-04.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 [RFC6204] has shown multiple issues with the DHCPv6
   protocol in supporting multiple stateful options.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 14, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Handling of multiple IA options types . . . . . . . . . . . .   3
     4.1.  Advertisement message . . . . . . . . . . . . . . . . . .   4
     4.2.  Placement of Status codes . . . . . . . . . . . . . . . .   5
     4.3.  T1/T2 timers  . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Renew and Rebind messages . . . . . . . . . . . . . . . .   6
     4.5.  Confirm message . . . . . . . . . . . . . . . . . . . . .   7
     4.6.  Release messages  . . . . . . . . . . . . . . . . . . . .   8
     4.7.  Multiple provisioning domains . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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.





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   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
                               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
   were 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.

   If a client requests multiple IA option types, but the server is
   configured 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 and include the missing options on subsequent messages
   (Request, Renew, and Rebind) to the server.




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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.  The client SHOULD include the not offered IA option types
   in its Request.

   Replace Section 17.1.3 of [RFC3315]: (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:

   -  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.



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4.2.  Placement of Status codes

   In Reply messages IA specific status codes (i.e., NoAddrsAvail,
   NotOnlink, NoBinding, NoPrefixAvail) 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.

   To clarify further: when a client requests both IA_NA and IA_PD, and
   the server can offer IA_PD but not IA_NA, the server sends an
   Advertise response containing no IA_NA option, a status code option
   of NoAddrsAvail, and one or more IA_PD options containing IAPREFIX
   options.

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
   are ignored.




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4.4.  Renew and Rebind messages

   The Renew message, as described in [RFC3315], allows a client to only
   renew bindings assigned via a Request message.  The Rebind message,
   as described in [RFC3315] does not explicitly specify what a server
   should do when an IA option which contains no addresses is present.

   In a multiple IA option type model, the Renew does not support the
   ability for the client to renew one IA option type while requesting
   bindings for other IA option types that were not available when the
   client sent the Request.

   Proposed solution: The client should continue with the IA options
   received, while continuing to include the other IA options in
   subsequent messages to the server.  The client and server processing
   need to be modified.  Note that this change makes the server's IA
   processing of Renew and Rebind similar to the Request processing.

   Replace Section 18.1.3 of [RFC3315]:

   At time T1 for an IA, the client initiates a Renew/Reply message
   exchange to extend the lifetimes on any addresses in the IA.  The
   client includes an IA option with all addresses currently assigned
   to the IA in its Renew message.


   With:

   At time T1 for an IA, the client initiates a Renew/Reply message
   exchange to extend the lifetimes on any addresses in the IA.  The
   client includes an IA option with all addresses currently assigned
   to the IA in its Renew message. The client also includes an IA
   option for each binding it desires but has been unable to obtain.


   Replace Section 18.2.3 of [RFC3315]:

   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:








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   If the server cannot find a client entry for the IA the server
   creates the bindings for that client according to the server's
   policy and configuration information and records the IAs and
   other information requested by the client.


   Note that clients that communicate with servers that do not support
   this updated Renew processing will receive the NoBinding status for
   the IA which had no bindings.  The client MUST continue to process
   the other IAs in the Reply.  The client MAY attempt a Solicit/
   Advertise/Request/Reply sequence periodically to obtain bindings for
   these IAs.  However, it MUST limit the frequency at which is does
   this to no more often than the renewal frequency.

   Replace Section 18.1.4 of [RFC3315]:

   At time T2 for an IA (which will only be reached if the server to
   which the Renew message was sent at time T1 has not responded), the
   client initiates a Rebind/Reply message exchange with any available
   server.  The client includes an IA option with all addresses
   currently assigned to the IA in its Rebind message.


   With:

   At time T2 for an IA (which will only be reached if the server to
   which the Renew message was sent at time T1 has not responded), the
   client initiates a Rebind/Reply message exchange with any available
   server.  The client includes an IA option with all addresses
   currently assigned to the IA in its Rebind message.  The client
   also includes an IA option for each binding it desires but has been
   unable to obtain.


4.5.  Confirm message

   The Confirm message, as described in [RFC3315], is specific to
   address assignment.  It allows a server without a binding 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



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   to extend lease times or otherwise send back other configuration
   options.

   Proposed solution: Allow and specify the Confirm message for other IA
   option types.  The server performs the same test as for addresses on
   the delegated prefixes (see [RFC3315], section 18.2.2).

   Replace Section 12.1 of [RFC3633]:

      If such verification is needed the requesting router MUST initiate
      a Rebind/Reply message exchange as described in section 18.1.4,
      "Creation and Transmission of Rebind Messages" of RFC 3315, with
      the exception that the retransmission parameters should be set as
      for the Confirm message, described in section 18.1.2, "Creation
      and Transmission of Confirm Messages" of RFC 3315.  The requesting
      router includes any IA_PDs, along with prefixes associated with
      those IA_PDs in its Rebind message.

      ...

      The Confirm and Decline message types are not used with Prefix
      Delegation.


   With:

      If such verification is needed the requesting router MUST initiate
      a Confirm message exchange as described in section 18.1.2,
      "Creation and Transmission of Confirm Messages" of RFC 3315. The
      requesting router includes any IA_PDs, along with prefixes
      associated with those IA_PDs in its Confirm message.

      ...

      The Decline message type is not used with Prefix Delegation.


4.6.  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.









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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.  This was also assumed by [RFC3315] and [RFC3633].

   One could envision a network where the DHCP servers are in multiple
   provisioning domains, and it may be desirable 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.

7.  Acknowledgements

8.  References

8.1.  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.

8.2.  Informative References

   [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






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   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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