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Versions: 01 02

Network Working Group                                           B. Liu
Internet Draft                                     Huawei Technologies
Intended status: Proposed Standard                           R. Bonica
Expires: March 21, 2014                               Juniper Networks
                                                    September 16, 2013



      DHCPv6/SLAAC Address Configuration Interaction Problem Statement
               draft-liu-bonica-dhcpv6-slaac-problem-02.txt


Status of this Memo

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   This Internet-Draft will expire on March 21, 2014.

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Abstract

   This document analyzes the host behavior of DHCPv6/SLAAC interaction
   issue. It reviews the standard definition of the host behaviors and



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   provides the test results of current mainstream implementations. Some
   potential operational gaps of the interaction are also described.

Table of Contents


   1. Introduction ................................................. 3
   2. Host Behavior of DHCPv6/SLAAC Interaction .................... 3
      2.1. Relevant RA Flags Defined in Standards .................. 4
         2.1.1. A (Autonomous) Flag ................................ 4
         2.1.2. M (Managed) Flag ................................... 4
         2.1.3. O (Otherconfig) Flag ............................... 4
      2.2. Behaviors of Current Implementations .................... 5
         2.2.1. A flag ................................... ......... 5
         2.2.2. M flag ..................................... ....... 5
         2.2.3. O flag ........................................ .... 6
   3. Possible Operational Gaps of DHCPv6/SLAAC Interaction ........ 6
      3.1. Renumbering ............................................. 6
      3.2. Cold Start Problems ..................................... 7
      3.3. Strong Management ....................................... 7
   4. Conclusions .................................................. 7
   5. Security Considerations ...................................... 7
   6. IANA Considerations .......................................... 7
   7. References ................................................... 8
      7.1. Normative References .................................... 8
      7.2. Informative References .................................. 8
   8. Acknowledgments .............................................. 8
   Appendix A. Test Details of Host Behaviors ..................... 10
      A.1 Host Initialing Behavior ................................ 10
      A.2 Host SLAAC/DHCPv6 Switching Behavior .................... 11
      A.3 Host Stateful/Stateless DHCPv6 Behavior ................. 12
   Authors' Addresses ............................................. 13
















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1. Introduction

   In IPv6, both of the DHCPv6 [RFC3315] and Neighbor Discovery [RFC4861]
   protocols can provide automatic IP address configuration for the
   hosts. They are known as stateful address auto-configuration and
   SLAAC (stateless address auto-configuration)[RFC4862], and are
   suitable for different scenarios respectively. Sometimes the two
   address configuration modes may be both available in one network.

   In ND protocol, there is a M (ManagedFlag) flag defined in RA message,
   indicating the hosts there is DHCPv6 service in the network if the
   flag is set. And there is an O "OtherConfigFlag", if set, indicating
   configure information other than addresses (e.g. DNS, Route .etc) is
   available through DHCPv6 configuration. Moreover, there's another A
   (Autonomous) flag defined in ND, which indicating the hosts to do
   SLAAC, may also influent the behavior of hosts.

   So with the A/M/O flags, the two separated address configuration
   modes are somehow correlated. But for some reason, the ND protocol
   didn't define the flags as prescriptive but only advisory. This
   ambiguous definition may vary the behavior of hosts when interpreting
   the flags. In section 2, we provided a brief test result to identify
   different host operating systems have taken different approaches.
   This would add additional complexity for both the hosts and the
   network management.

   This draft reviews the standard definition of the above mentioned
   flags, and provides a test result of several major desktop operating
   systems' behavior. And then identifies potential requirement/gaps of
   DHCPv6/SLAAC interaction.

2. Host Behavior of DHCPv6/SLAAC Interaction

   In this section, we analyzed A/M/O flags definition, and provide the
   test result of host behavior of interpreting these flags in
   mainstream operating systems implementations.

   Please note that, A flag has no direct relationship with DHCPv6, but
   it is somewhat correlated with M/O flags.








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2.1. Relevant RA Flags Defined in Standards

2.1.1. A (Autonomous) Flag

   In ND Prefix Information Option, the autonomous address-configuration
   flag (A flag).  When set indicates that this prefix can be used for
   stateless address configuration as specified in SLAAC.

   For the host behavior, there is an explicit rule in the SLAAC
   specification [RFC4862]: "If the Autonomous flag is not set, silently
   ignore the Prefix Information option."

   But when A flag is set, the SLAAC protocol didn't provide a
   prescriptive definition.

2.1.2. M (Managed) Flag

   In earlier SLAAC specification [RFC2462], the host behavior of
   interpreting M flag is as below:

   "On receipt of a valid Router Advertisement, a host copies the value
   of the advertisement's M bit into ManagedFlag. If the value of
   ManagedFlag changes from FALSE to TRUE, and the host is not already
   running the stateful address autoconfiguration protocol, the host
   should invoke the stateful address auto-configuration protocol,
   requesting both address information and other information. If the
   value of the ManagedFlag changes from TRUE to FALSE, the host should
   continue running the stateful address auto-configuration, i.e., the
   change in the value of the ManagedFlag has no effect.  If the value
   of the flag stays unchanged, no special action takes place. In
   particular, a host MUST NOT reinvoke stateful address configuration
   if it is already participating in the stateful protocol as a result
   of an earlier advertisement."

   But in the updated SLAAC specification [RFC4862], the relative
   description was removed, the reason was "considering the maturity of
   implementations and operational experiences. ManagedFlag and
   OtherConfigFlag were removed accordingly. (Note that this change does
   not mean the use of these flags is deprecated.)"

2.1.3. O (Otherconfig) Flag

   As mentioned above, the situation of O flag is similar with M. In
   earlier SLAAC [RFC2462], the host behavior is clear:

   "If the value of OtherConfigFlag changes from FALSE to TRUE, the host
   should invoke the stateful autoconfiguration protocol, requesting


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   information (excluding addresses if ManagedFlag is set to FALSE). If
   the value of the OtherConfigFlag changes from TRUE to FALSE, the host
   should continue running the stateful address autoconfiguration
   protocol, i.e., the change in the value of OtherConfigFlag has no
   effect. If the value of the flag stays unchanged, no special action
   takes place. In particular, a host MUST NOT reinvoke stateful
   configuration if it is already participating in the stateful protocol
   as a result of an earlier advertisement."

   And there's another description of the relationship of M and O flags
   in [RFC2462]:

   "In addition, when the value of the ManagedFlag is TRUE, the value of
   OtherConfigFlag is implicitely TRUE as well. It is not a valid
   configuration for a host to use stateful address autoconfiguration to
   request addresses only, without also accepting other configuration
   information."

2.2. Behaviors of Current Implementations

   We did tests of current 3 mainstream desktop operating systems on the
   behaviors; please refer to the appendix for details. This section
   illustrates the important results of the tests.

2.2.1. A flag

   A flag is a switch to control whether to do SLAAC, and it is
   independent with M/O flags, in another word, A is independent with
   DHCPv6.

   At the non-SLAAC-config state (either non-configured or DHCPv6-
   configured only), the 3 OSes acted the same with A flag, if A set,
   they all configured SLAAC, it is obvious and reasonable. But when
   SLAAC-configured, and A changed from 1 to 0, the behaviors varied,
   some deprecated SLAAC while some ignored the RA messages.

2.2.2. M flag

   M is a key flag to interact ND/DHCPv6, but the host behaviors on M
   flag were quite different.

   In our test, one OS treats the flag as instruction, it even released
   DHCPv6 session when M=0. But the other two just treat the flag as
   advisory, when SLAAC was done, it won't care about M=1, and M=0 won't
   cause operation for the already configured DHCPv6 addresses. Moreover,
   the two OSes even would not initiate DHCPv6 session until they



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   receives RA messages with M=1, this behavior has an implication that
   DHCPv6 somehow depends on ND.

   Please refer to [I-D.liu-6renum-dhcpv6-slaac-switching] for more
   details.

2.2.3. O flag

   In our tests, when M flag is set, the O flag is implicitly set as
   well; in another word, the hosts would not initial stateful DHCPv6
   and stateless DHCPv6 respectively. This is a reasonable behavior.

   But the O flag is not independent from A flag in some OSes. In our
   test, there are two OSes won't initiate stateless DHCPv6 when A flag
   is not set, that is to say, it is not applicable to have a "stateless
   DHCPv6 only" configuration state for some operating systems; it is
   also not applicable for these two OSes to switch between stateful
   DHCPv6 and stateless DHCPv6 (according to O flag changing from 0 to 1
   or verse vice).

3. Possible Operational Gaps of DHCPv6/SLAAC Interaction

   According to the abovementioned tests, there are possible operational
   issues as the following.

3.1. Renumbering

   During IPv6 renumbering, the SLAAC-configured hosts can reconfigure
   IP addresses by receiving ND Router Advertisement (RA) messages
   containing new prefix information. The DHCPv6-configured hosts can
   reconfigure addresses by initialing RENEW sessions when the current
   addresses' lease time is expired or receiving the reconfiguration
   messages initialed by the DHCPv6 servers.

   The above mechanisms have an implicit assumption that SLAAC-
   configured hosts will remain SLAAC while DHCPv6-managed hosts will
   remain DHCPv6-managed. But in some situations, SLAAC-configured hosts
   may need to switch to DHCPv6-managed, or verse vice. In [I-D.ietf-
   6renum-enterprise], it described several renumbering scenarios in
   enterprise network for this requirement; for example, the network may
   split, merge, relocate or reorganize. But due to current
   implementations, this requirement is not applicable and has been
   identified as a gap in [I-D.ietf-6renum-gap-analysis].






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3.2. Cold Start Problems

   If all nodes, or many nodes, restart at the same time after a power
   cut, the results might not consistent.

3.3. Strong Management

   Since the host behavior of address configuration is somehow un-
   controlled by the network side, it might cause gaps to the networks
   that need strong management (for example, the enterprise networks and
   the ISP CPE networks). Examples are:

   - the network wants the hosts to do DHCPv6-only configuration, it is
   not applicable for some operating systems due to current
   implementation unless manually configure the hosts to DHCPv6-only
   model
   - the hosts have been SLAAC-configured, then the network need the
   hosts to do DHCPv6 simultaneously (e.g. for multihoming)
   - the network wants the hosts to do statelssDHCPV6-only; for example,
   the hosts are configured with self-generated addresses (e.g. ULA),
   and they also need to contact the DHCPv6 server for info-
   configuration

4. Conclusions

   - The host behavior of SLAAC/DHCPv6 interaction is ambiguous in
   standard.

   - The implementations have been varied on this issue. In [RFC4862] it
   is said "Removed the text regarding the M and O flags, considering
   the maturity of implementations and operational experiences." The
   description seems not true anymore.

   - It is foreseeable that the un-uniformed host behavior can cause
   operational gaps, e.g. in renumbering and strong management.

5. Security Considerations

   No more security considerations than the Neighbor Discovery protocol
   [RFC4861].

6. IANA Considerations

   None.





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7. References

7.1. Normative References

   [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

   [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

   [RFC3315] R. Droms, 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.

   [RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
             Still Needs Work", RFC 5887, May 2010.

   [I-D.ietf-6renum-gap-analysis]
             Liu, B., and Jiang, S., "IPv6 Site Renumbering Gap
             Analysis", Working in Progress, March 2012

   [I-D.ietf-6renum-enterprise]
             Jiang, S., and B. Liu, "IPv6 Enterprise Network Renumbering
             Scenarios and Guidelines ", Working in Progress, March 2012.

8. Acknowledgments

   The test was done by our research partner BNRC-BUPT (Broad Network
   Research Centre in Beijing University of Posts and
   Telecommunications). Thanks for the hard efficient work of student
   Xudong Shi and the tutors Prof. Wendong Wang and Prof. Xiangyang Gong.

   Valuable comment was received from Brian E Carpenter to improve the
   draft.

   This document was prepared using 2-Word-v2.0.template.dot.




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Appendix A. Test Details of Host Behaviors

                                                /-----\
                 +---------+                  //       \\
                 |  DHCPv6 |                 |  Router   |
                 |  server |                  \\       //
                 +----+----+                    \--+--/
                      |                            |
                      |                            |
                      |                            |
                  ----+--+----------+----------+---+-----
                         |          |          |
                         |          |          |
                         |          |          |
                    +----+---+ +----+---+ +----+---+
                    |        | |        | |        |
                    |  Host1 | |  Host2 | |  Host3 |
                    +--------+ +--------+ +--------+

                         Figure 1 Test Environment


   The 5 elements were all created in Vmware in one computer, for ease
   of operation.

   - Router quagga 0.99-19 soft router installed on Ubuntu 11.04
     virtual host
   - DHCPv6 Server: dibbler-server installed on Ubuntu 11.04 virtual
     host
   - Host A Window 7  Virtual Host
   - Host B Ubuntu 12.10  Virtual Host
   - Host C Mac OS X v10.7 Virtual Host

A.1 Host Initialing Behavior

   Host from non-configured to configured, we tested different A/M/O
   combinations in each OS platform. The states are enumerated as the
   following, 3 operation systems respectively:

   o Window 7
     - A=0&M=O&O=0, non-config
     - A=1&M=0&O=0, SLAAC only
     - A=1&M=0&O=1, SLAAC + Stateless DHCPv6
     - A=1&M=1&O=0, SLAAC + DHCPv6


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     - A=1&M=1&O=1, SLAAC + DHCPv6
     - A=0&M=1&O=0, DHCPv6 only (A=0 or Non-PIO)
     - A=0&M=1&O=1, DHCPv6 only (A=0 or Non-PIO)
     - A=0&M=0&O=1, Stateless DHCPv6 only

   o Linux/MAC OS X
     - A=0&M=O&O=0, non-config
     - A=1&M=0&O=0, SLAAC only
     - A=1&M=0&O=1, SLAAC + Stateless DHCPv6
     - A=1&M=1&O=0, SLAAC + DHCPv6
     - A=1&M=1&O=1, SLAAC + DHCPv6
     - A=0&M=1&O=0, DHCPv6 only (A=0 or Non-PIO)
     - A=0&M=1&O=1, DHCPv6 only (A=0 or Non-PIO)
     - A=0&M=0&O=1, non-config

   As showed above, Linux and MAC OSX acted the same way, but differated
   from Windows 7. The only difference is when A=0&M=0&O=1, Windows 7
   did stateless DHCPv6 while Linux/MAC OSX did nothing.

   Result summary:

     - A is interpreted as prescript in each OS at the initial state
     - M is interpreted as prescript in each OS at the initial state
     - O is interpreted as prescript in Windows 7
     - A and M are independent in each OS at the initial state
     - A and O are not totally independent in Linux and Mac, A=1 is
       required for O=1 triggering DHCPv6 info-request
     - M and O are not totally independent in each OS. M=1 has the
       implication O=1

A.2 Host SLAAC/DHCPv6 Switching Behavior

   o SLAAC-only host receiving A=0&M=1
     - Window 7 would deprecate SLAAC and initiate DHCPv6
     - Linux/MAC would keep SLAAC and don't initiate DHCPv6 unless SLAAC
     is expired and no continuous RA

   o DHCPv6-only host receiving A=1&M=0
     - Window 7 would release DHCPv6 and do SLAAC
     - Linux/MAC would keep DHCPv6 and do SLAAC

   When the host has been configured, either by SLAAC or DHCPv6, the
   operating systems interpreting the M flag quite differently. Windows
   7 treats the flag as instruction, it even released DHCPv6 session
   when M=0. Linux and OS X were likely to treat the flag as advisory,



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   when SLAAC was done, it won't care about M=1, and M=0 won't cause
   operation for the already configured DHCPv6 addresses.

   Please refer to [I-D.liu-6renum-dhcpv6-slaac-switching] for more
   details.

A.3 Host Stateful/Stateless DHCPv6 Behavior

   o StatelessDHCPv6-configured host receiving M=1 (while keeping O=1)
     - Window 7 would initiate stateful DHCPv6, configuring address as
     well as re-configuring other information
     - Linux/MAC no action

   o StatefullDHCPv6-configured host receiving M=0 (while keeping O=1)
     - Window 7 would release all DHCPv6 configurations including
     address and other information, and initiate stateless DHCPv6
     - Linux/MAC no action






























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Authors' Addresses

   Bing Liu
   Q14-4-A Building
   Huawei Technologies Co., Ltd
   Zhong-Guan-Cun Environment Protection Park, No.156 Beiqing Rd.
   Hai-Dian District, Beijing
   P.R. China

   Email: leo.liubing@huawei.com


   Ron Bonica
   Juniper Networks
   Sterling, Virginia  20164
   USA

   Email: rbonica@juniper.net


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