V6OPS                                                             B. Liu
Internet-Draft                                                  S. Jiang
Intended status: Informational                       Huawei Technologies
Expires: April 17, 30, 2015                                        R. Bonica
                                                        Juniper Networks
                                                                 X. Gong
                                                                 W. Wang
                                                         BUPT University
                                                        October 14, 27, 2014

    DHCPv6/SLAAC Address Configuration Interaction Problem Statement
                draft-ietf-v6ops-dhcpv6-slaac-problem-02
                draft-ietf-v6ops-dhcpv6-slaac-problem-03

Abstract

   The IPv6 Neighbor Discovery (ND) Protocol includes an ICMPv6 Router
   Advertisement (RA) message.  The RA message contains three flags,
   indicating which autoconfiguration mechanisms are available to on-
   link hosts.  These are the M, O and A flags.  The M, O M and A O flags are
   advisory, not prescriptive.

   This document describes divergent host behaviors observed in popular
   operating systems.  It also describes operational problems that
   divergent behaviors cause.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The M, O and A Flags  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  M (Managed) Flag  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  O (Otherconfig) Flag  . . . . . . . . . . . . . . . . . .   4
     2.3.  A (Autonomous) Flag . . . . . . . . . . . . . . . . . . .   4
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Divergent Host Behaviors  . . . . . . . . . . . . . . . .   4
     3.2.  Operational Problems  . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Inappropriate Sources . . . . . . . . . . . . . . . .   5
       3.2.2.  Renumbering . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Test Results . . . . . . . . . . . . . . . . . . . .   7
     A.1.  Test Environment  . . . . . . . . . . . . . . . . . . . .   7
     A.2.  Host Behavior in the Initial State  . . . . . . . . . . .   8
     A.3.  Host Behavior in State Transitions  . . . . . . . . . . .   9  10
   Appendix B.  Analysis of the Ambiguities  . . . . . . . . . . . .  10  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11  12

1.  Introduction

   IPv6 [RFC2460] hosts invoke Neighbor Discovery (ND) [RFC4861]
   procedures in order to discover which autoconfiguration mechanisms
   are available to them.  The following is a list of autoconfiguration
   mechanisms:

   o  DHCPv6 [RFC3315]

   o  Stateless Address Autoconfiguration (SLAAC) [RFC4862]

   ND specifies an ICMPv6 [RFC4443] Router Advertisement (RA) message.
   Routers periodically broadcast the RA message to all on-link nodes.

   They also unicast RA messages in response to solicitations.  The RA
   message contains:

   o  an M (Managed) flag

   o  an O (OtherConfig) flag

   o  zero or more Prefix Information (PI) Options

   The M flag indicates that addresses are available from DHCPv6.  The O
   flag indicates that other configuration information (e.g., DNS-
   related information) is available from DHCPv6.  The PI Option
   includes a prefix, an A (Autonomous) flag and other fields.  The A
   flag indicates that the prefix can be used for SLAAC.  The M, O M and A O
   flags are advisory, not prescriptive. prescriptive (although A flag is also
   advisory in definition in standard, it is quite prescriptive in
   implementations).  For example, the M flag indicates that addresses
   are available from DHCPv6.  It does not indicate that hosts are
   required to acquire addresses from DHCPv6.  Similar statements can be
   made about the O and A flags. flag.

   In most cases, the M, O and A flags elicit identical behaviors from
   most popular operating systems.  However, in several cases, the M, O
   and A flags elicit divergent behaviors.  For example, when a router
   changes the settings of the M, O, and A flag from one RA message to
   the next, it is likely to elicit one behavior from hosts running one
   operating system and another behavior from hosts running a different
   operating system.

   This document describes divergent host behaviors observed in popular
   operating systems.  It also describes operational problems that
   divergent behaviors cause.

2.  The M, O and A Flags

   This section briefly reviews how the M, O and A flags are defined in
   [RFC4861].

2.1.  M (Managed) Flag

   The M flag indicates that addresses are available from IPv6. DHCPv6.  If
   the M flag is set, the O flag is redundant and can be ignored because
   DHCPv6 will return all available configuration information.

   M and A flag semantics are independent of one another.  The M flag
   indicates that addresses are available from DHCPv6, regardless of the
   A flag setting.  The following setting settings are all allowed:

   o  M=0 A=0

   o  M=0 A=1

   o  M=1 A=0

   o  M=1 A=1

2.2.  O (Otherconfig) Flag

   The O flag indicates that other configuration information (e.g., DNS-
   related information) is available from IPv6. DHCPv6.  If the M flag is set,
   the O flag is redundant and can be ignored because DHCPv6 will return
   all available configuration information.

   O and A flag semantics are independent of one another.  The O flag
   indicates that other configuration is available from DHCPv6,
   regardless of the A flag setting.  The following setting settings are all
   allowed:

   o  O=0 A=0

   o  O=0 A=1

   o  O=1 A=0

   o  O=1 A=1

2.3.  A (Autonomous) Flag

   The A flag indicates that the prefix that is also carried by the PI
   option can be used for SLAAC.  A flag semantics are independent of M
   and O flag semantics.  The A flag indicates that the prefix can be
   used by SLAAC, regardless of the M and O flag settings.

3.  Problem Statement

3.1.  Divergent Host Behaviors

   The authors tested several popular operating systems in order to
   determine what behaviors the M, O and A flag elicit.  In some cases,
   the M, A and O flags elicit identical behaviors from most popular
   operating systems.  However, in several cases, the M, O and A flags
   elicit divergent behaviors.  The table below characterizes those
   cases:

   Host State         Input  Behavior

   Host has not       No RA  Some popular operating systems acquire
   acquired any              addresses from DHCPv6.  Others do not.
   addresses

   Host has not       RA     Some popular operating systems acquire
   acquired any       with   other information from DHCPv6, regardless
   addresses          M=0,   of the A flag setting. Others do so, but
                      O=1    only if A=1

   Host has acquired  RA     Some operating systems release DHCPv6
   addresses from     with M addresses immediately. Some release DHCPv6
   DHCPv6 only (M =   =0     addresses when they expire.
   1)

   Host has acquired  RA     Some operating systems acquire DHCPv6
   addresses from     with M addresses immediately.  Others do so only
   SLAAC only (A=1)   = 1    if their SLAAC addresses expire and cannot
                             be refreshed.

3.2.  Operational Problems

   This section describes operational issues caused by the divergent
   behaviors, described above.

3.2.1.  Inappropriate Sources

   Some operating systems base their decision to acquire configuration
   information upon inappropriate sources.  For example, some operating
   systems acquire other configuration information if M = 0, O = 1, and
   A = 1, but not if M = 0, 0 = 1 and A = 0.  In other words, on some
   operating systems, it is impossible to acquire other information from
   DHCPv6 unless addresses are acquired from either DHCPv6 or SLAAC.

3.2.2.  Renumbering

   According to [RFC6879] a renumbering exercise can include the
   following steps:

   o  Causing hosts that have acquired addresses from one
      autoconfiguration mechanism to release those addresses and acquire
      new addresses from another autoconfiguration mechanism

   o  Causing hosts that have acquired addresses from one
      autoconfiguration mechanism to release those addresses and acquire
      new addresses from the same autoconfiguration mechanism

   o  Causing hosts that have acquired addresses from one
      autoconfiguration mechanism to retain those addresses and acquire
      new addresses from another autoconfiguration mechanism

   Ideally, these steps could be initiated by broadcasting RA message
   onto the subnetwork that is being renumbered.  Sadly, this is not
   possible, because the RA message may elicit a different behavior from
   each host.  According to Section 3.1, renumbering operations would
   have the following limitations:

   o  During a flash switch from DHCPv6 to SLAAC,  When M flag is turned off, some operating systems release DHCPv6
      acquired addresses immediately, while other will retain then until
      they expire.  Therefore, results are
      unpredictable.  This means a flash switch from DHCPv6 to SLAAC would
      happen on some hosts.  Normally, the "make-before-break" approach
      proposed in [RFC4192] is considered better than flash renumbering.

   o  On some operating systems, if a host has acquired addresses from
      SLAAC, it is impossible to acquire additional addresses from
      DHCPv6.  This may be required as part of a renumbering operation.

4.  Security Considerations

   As this memo does not introduce any new protocols or procedures, it
   does not introduce any new security vulnerabilities.

5.  IANA Considerations

   This draft does not request any IANA action.

6.  Acknowledgements

   The authors wish to acknowledge BNRC-BUPT (Broad Network Research
   Centre in Beijing University of Posts and Telecommunications) for
   their testing efforts.  Special thanks to Xudong Shi, Longyun Yuan
   and Xiaojian Xue for their extraordinary effort.

   The authors also wish to acknowledge Brian E Carpenter, Ran Atkinson,
   Mikael Abrahamsson, Tatuya Jinmei, Mark Andrews and Mark Smith for
   their helpful comments.

7.  References

7.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

7.2.  Informative References

   [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.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005.

   [RFC6879]  Jiang, S., Liu, B., and B. Carpenter, "IPv6 Enterprise
              Network Renumbering Scenarios, Considerations, and
              Methods", RFC 6879, February 2013.

Appendix A.  Test Results

A.1.  Test Environment
                                                /-----\
                 +---------+                  //       \\
                 |  DHCPv6 |                 |  Router   |
                 |  server |                  \\       //
                 +----+----+                    \--+--/
                      |                            |
                      |                            |
                      |                            |
                  ----+--+----------+----------+---+-----
                         |          |          |
                         |          |          |
                         |          |          |
                    +----+---+ +----+---+ +----+---+
                    |        | |        | |        |
                    | Host 1 | | Host 2 | | Host 3 |
                    +--------+ +--------+ +--------+

                        Figure 1: Test Environment

   The test environment depicted Figure 1 in was replicated on a single
   server using VMware.  For simplicity of operation, only one host was
   run at a time.  Network elements were as follows:

   o  Router: Quagga 0.99-19 soft router installed on Ubuntu 11.04
      virtual host

   o  DHCPv6 Server: Dibbler-server installed on Ubuntu 11.04 virtual
      host

   o  Host 1: Window 7 / Window 8.1 Virtual Host

   o  Host 2: Ubuntu 14.04 (Linux Kernel 3.12.0) Virtual Host

   o  Host 3: Mac OS X v10.9 Virtual Host

   o  Host 4: IOS 8.0 (model: Apple iPhone 5S, connected via wifi)

A.2.  Host Behavior in the Initial State

   The bullet list below describes host behavior in the initial state,
   when the host has not yet acquired any autoconfiguration information.
   Each bullet item represents an input and the behavior elicited by
   that input.

   o  A=0, M=0, O=0

      *  Windows 8.1 acquired addresses and other information from
         DHCPv6.

      *  All other hosts acquired no configuration information.

   o  A=0, M=0, O=1

      *  Windows 8.1 acquired addresses and other information from
         DHCPv6.

      *  Windows 7, OSX 10.9 and IOS 8.0 acquired other information from
         DHCPv6.

      *  Ubuntu 14.04 acquired no configuration information.

   o  A=0, M=1, O=0

      *  All hosts acquired addresses and other information from DHCPv6.

   o  A=0, M=1, O=1

      *  All hosts acquired addresses and other information from DHCPv6.

   o  A=1, M=0, O=0

      *  Windows 8.1 acquired addresses from SLAAC and DHCPv6.  It also
         acquired non-address information from DHCPv6.

      *  All the other host acquired addresses from SLAAC

   o  A=1, M=0, O=1

      *  Windows 8.1 acquired addresses from SLAAC and DHCPv6.  It also
         acquired other information from DHCPv6.

      *  All the other hosts acquired addresses from SLAAC and other
         information from DHCPv6.

   o  A=1, M=1, O=0

      *  All hosts acquired addresses from SLAAC and DHCPv6.  They also
         acquired other information from DHCPv6.

   o  A=1, M=1, O=1

      *  All hosts acquired addresses from SLAAC and DHCPv6.  They also
         acquired other information from DHCPv6.

   As showed above, four inputs result in divergent behaviors.

A.3.  Host Behavior in State Transitions

   The bullet list below describes behavior elicited during state
   transitions.  The value x can represents both 0 and 1.

   o  Old state (M = x, O = x, A = 1) , New state (M = x, O = x, A = 0)
      (This means a SLAAC-configured host, which is regardless of DHCPv6
      configured or not, reveiving receiving A transitiong transitioning from 1 to 0. )

      *  All the hosts retain SLAAC addresses until they expire

   o  Old state (M = 0, O = x, A = 1), New state (M = 1, O = x, A = 1)
      (This means a SLAAC-only host receiving M transisioning transitioning from 0 to
      1.)

      *  Windows 7 acquires addresses from DHCPv6, immediately.

      *  Ubuntu 14.04/OSX 10.9/IOS 8.0 acquires addresses from DHCPv6
         only if the SLAAC addresses are allowed to expire

      *  Windows 8.1 was not tested because it always acquire addresses
         from DHCPv6 regardless of the M flag setting.

   o  Old state (M = 1, O = x, A = x), New state (M = 0, O = x, A = x)
      (This means a DHCPv6-configured host receiving M transitioning
      from 1 to 0.)

      *  Windows 7 immediately released the DHCPv6 address

      *  Windows 8.1/Ubuntu 14.04/OSX 10.9/IOS 8.0 keep the DHCPv6
         addresses until they expire

   o  Old state (M = 1, O = x, A = 0), New state (M = 1, O = x, A = 1)
      (This means a DHCPv6-only host receiving A transisioning transitioning from 0 to
      1.)

      *  All host acquire addresses from SLAAC

   o  Old state (M = 0, O = 1, A = x), New state (M = 1, O = 1, A = x)
      (This means a Stateless DHCPv6-configured host [RFC3736], which is
      regardless of SLAAC configured or not, receiving M transisioning transitioning
      from 0 to 1 with keeping O=1 )

      *  Windows 7 acquires addresses and refreshes other information
         from DHCPv6

      *  Ubuntu 14.04/OSX 10.9/IOS 8.0 does nothing
      *  Windows 8.1 was not tested because it always acquire addresses
         from DHCPv6 regardless of the M flag setting.

   o  Old state (M = 1, O = 1, A = x), New state (M = 0, O = 1, A = x)
      (This means a Stateful DHCPv6-configured host, which is regardless
      of SLAAC configured or not, receiving M transisioning transitioning from 0 to 1
      with keeping O=1 )

      *  Windows 7 released all DHCPv6 addresses and refreshes all
         DHCPv6 other information.

      *  Windows 8.1/Ubuntu 14.04/OSX 10.9/IOS 8.0 does nothing

Appendix B.  Analysis of the Ambiguities

   Following is a comprehensive analysis of the ambiguities as defined
   in the standards.  In theory, all the ambiguities might cause
   divergent host behavior.  Some of the divergence has been identified
   by the tests while some haven't.  It is worth to document all the
   ambiguities.

   1.  Dependency between DHCPv6 and RA

      In standards, behavior of DHCPv6 and Neighbor Discovery protocols
      is specified respectively.  But it is not clear that whether there
      should be any dependency between them.  More specifically, is RA
      (with M=1) required to trigger DHCPv6?  If there are no RAs at
      all, should hosts initiate DHCPv6 by themselves?

   2.  Behaviors of Flag Transition

      When flags are in transition, e.g. the host is already SLAAC-
      configured, then M flag changes from FALSE to TRUE, it is not
      clear whether the host should start DHCPv6 or not; or vise versa,
      the host is already both SLAAC/DHCPv6 configured, then M flag
      change from TRUE to FALSE, it is also not clear whether the host
      should turn DHCPv6 off or not.

   3.  Distinction between "Address Configuring Method" and "Address
   Lifetime"

      When one address configuration method is off, that is, the A flag
      or M flag changes from TRUE to FALSE, it is not clear whether the
      host should immediately release the corresponding address(es) or
      just retain it(them) until expired.

   4.  Dependencies between the flags
      The semantics of the flags seems not totally independent, but the
      standards didn't clearly clarify it.  For example, when both M and
      O flags are TRUE, it is not clear whether the host should initiate
      one stateful DHCPv6 session to get both address and info-
      configuration or initiate two independent sessions of which one is
      dedicated for address provisioning and the other is for
      information provision.  When A and M flags are FALSE and O flag is
      TRUE, it is not clear whether the host should initiate a stand-
      alone stateless DHCPv6 session.

Authors' Addresses

   Bing Liu
   Huawei Technologies
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

   Email: leo.liubing@huawei.com

   Sheng Jiang
   Huawei Technologies
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

   Email: jiangsheng@huawei.com

   Ron Bonica
   Juniper Networks
   Sterling, Virginia
   20164
   USA

   Email: rbonica@juniper.net

   Xiangyang Gong
   BUPT University
   No.3 Teaching Building
   Beijing University of Posts and Telecommunications (BUPT)
   No.10 Xi-Tu-Cheng Rd.
   Hai-Dian District, Beijing
   P.R. China

   Email: xygong@bupt.edu.cn
   Wendong Wang
   BUPT University
   No.3 Teaching Building
   Beijing University of Posts and Telecommunications (BUPT)
   No.10 Xi-Tu-Cheng Rd.
   Hai-Dian District, Beijing
   P.R. China

   Email: wdwang@bupt.edu.cn