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Versions: 00 01                                                         
Network Working Group                                     A. Yourtchenko
Internet-Draft                                                     cisco
Intended status: Informational                          October 27, 2014
Expires: April 30, 2015


     A survey of issues related to IPv6 Duplicate Address Detection
                  draft-yourtchenko-6man-dad-issues-00

Abstract

   This document enumerates the practical issues observed with respect
   to Duplicate Address Detection.

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
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   This Internet-Draft will expire on April 30, 2015.

Copyright Notice

   Copyright (c) 2014 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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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Duplicate L2 address detection  . . . . . . . . . . . . . . .   2
   3.  Interaction with delay in forwarding on the link  . . . . . .   3
   4.  Behavior on links with unreliable multicast . . . . . . . . .   3
   5.  Interaction with looped interfaces  . . . . . . . . . . . . .   3
   6.  Delays before an address can be used  . . . . . . . . . . . .   3
   7.  Partition-join tolerance  . . . . . . . . . . . . . . . . . .   4
   8.  Behavior on collision . . . . . . . . . . . . . . . . . . . .   4
   9.  Energy efficiency . . . . . . . . . . . . . . . . . . . . . .   4
   10. Wake-up and L2 events . . . . . . . . . . . . . . . . . . . .   4
   11. Usage of DAD to create state  . . . . . . . . . . . . . . . .   5
   12. Support of multi-link subnets . . . . . . . . . . . . . . . .   5
   13. Anycast Addresses and Duplicate Address Detection . . . . . .   5
   14. Implementations doing DAD once per IID  . . . . . . . . . . .   5
   15. Backwards compatibility and presence of the DAD proxies . . .   6
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   18. Security Considerations . . . . . . . . . . . . . . . . . . .   6
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     19.1.  Informative References . . . . . . . . . . . . . . . . .   6
     19.2.  Normative References . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   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 [RFC2119].

   Duplicate Address Detection is a procedure in IPv6 performed before
   any address can be assigned to the interface.  On one hand, it is
   mandatory for all addresses.  On the other hand, it is a "best
   effort" activity.  These somewhat counter-intuitive properties result
   in some issues that arise related to DAD.  They are listed below.

2.  Duplicate L2 address detection

   DAD does not detect duplicate L2 addresses in all cases.  Depending
   on the medium, it may be impossible to detect a duplicate L2 address
   - e.g. if this address itself is used as a determinant in order to
   establish the L2 connection.








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3.  Interaction with delay in forwarding on the link

   The DAD makes an assumption that if a link layer is up, the traffic
   can be immediately forwarded, which is frequently not the case in
   modern networks.  Two prominent cases include the switches running
   Spanning Tree Protocol (STP), and bridging modems.

   When a port on an STP-enabled switch comes up, it goes through three
   phases of Listening then Learning then Forwarding.  The default is to
   keep it for 15 seconds in Listening and 15 seconds in Learning
   states.  During this time no user traffic is forwarded by the switch
   from and to this port.  Therefore, if a DAD process happens during
   this period it is guaranteed to not detect any duplicates.  This
   results in DAD being ineffective for link-local and otherwise pre
   configured addresses.

   Similarly, a DSL or cable modem whose line status is invisible to IP
   stack either within the modem or to a host connected on the Ethernet
   side, also renders the DAD ineffective - the delay before the
   connectivity is established can be much longer than any DAD wait.

4.  Behavior on links with unreliable multicast

   DAD requires two multicast messages to pass through - the NS and NA.
   Thus it shows a noticeable failure rate on links that do not pass
   multicast reliably (e.g. the 802.11a/b/g/n series of technologies).
   Author's ad-hoc experimentation at IETF90 revealed the success rate
   of detecting the duplicate address being about 4 in 5.  This may
   violate the assumptions that other protocols make.

5.  Interaction with looped interfaces

   [RFC4862] explicitly defines that the case of a physically looped
   back interface is not a failure: "If the solicitation is from the
   node itself (because the node loops back multicast packets), the
   solicitation does not indicate the presence of a duplicate address."

   However, the practical experiences show that the measures described
   in [RFC4862] are either incomplete or incorrectly implemented: a
   loopback on the interface causes DAD failure.

   [I-D.ietf-6man-enhanced-dad] discusses the solution to this issue.

6.  Delays before an address can be used

   Section "5.4.  Duplicate Address Detection" of [RFC4862] specifies
   that until the DAD procedure completes, the address remains in
   Tentative state.  In this state, any traffic to this address other



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   than that related to DAD-related is dropped.  This introduces delay
   between the interface getting connected to the network and an address
   on this interface becoming usable.  For fast-moving nodes it may be a
   problem.

   [RFC4429] introduces "Optimistic DAD" process, which addresses this.

7.  Partition-join tolerance

   [RFC4862] explicitly mentions this problem: "Note that the method for
   detecting duplicates is not completely reliable, and it is possible
   that duplicate addresses will still exist (e.g., if the link was
   partitioned while Duplicate Address Detection was performed)."

   In contrast, IPv4 stacks typically implement the Address Conflict
   Detection (ACD) from [RFC5227].  This disparity results in a less
   robust operation of IPv6 compared to IPv4 and is undesirable.

8.  Behavior on collision

   [RFC4862] in its section "5.4.5.  When Duplicate Address Detection
   Fails" is much more prescriptive than [RFC2462] that it superceeds.
   However, it has been observed that some implementations may simply
   reset the network interface and attempt the DAD process again.  This
   behavior, while being more resilient in case the DAD failure is
   happening erroneously, is different from what is recommended in the
   standard.

9.  Energy efficiency

   If a node wants to "defend" its address using DAD, it has to be awake
   and listening on the solicited node multicast address in order to
   receive the DAD NS.  In the low-power environments this may
   significantly impact the battery life of the devices.

10.  Wake-up and L2 events

   In mobile environments, node may roam in different parts of the
   network and also take "naps".  The specification in [RFC4862] does
   not explicitly discuss this scenario, so there is a room for
   ambiguity in implementation.  This may either result in less robust
   DAD coverage (if the node does not perform the DAD again when an L2
   event happens), or an excessive amount of multicast packets (when a
   node performs the dad every time L2 event happens and there is a lot
   of them moving within a segment).






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11.  Usage of DAD to create state

   [RFC4862] in section "5.4.  Duplicate Address Detection" states that
   DAD must be performed on all addresses.  Given the potentially
   decentralized nature of address assignment in IPv6, this property is
   being used to prebuild the state in the network about the host's
   addresses - e.g. for "First Come First Served" security as described
   in section "3.2.3.  Processing of Local Traffic" of [RFC6620].

   If the delivery of the DAD_NS packets is unreliable or there are
   nodes on the segment which use the Optimistic DAD mechanism, state
   created purely on DAD_NS packets might be also unreliable.  The
   specific case of [RFC6620] solves the issue by triggering the
   recreation of state based on data packets as well, however it might
   not be possible in some scenarios.

12.  Support of multi-link subnets

   DAD doesn't support multi-link subnets: a multicast DAD_NS sent on
   one link will not be seen on the other.

   [RFC6275] specifically provides one way to construct a multi-link
   subnet (consisting of a broadcast link and a collection of point to
   point tunnels).  It explicitly defines the procedures for making DAD
   work in that topology.

   [RFC4903] discusses the issues related to multi-link subnets - and
   given the multi-link subnets might be created in many ways, it might
   be prudent to keep enhancements to DAD whose sole purpose is related
   to multi-link subnets, to be out of scope.

13.  Anycast Addresses and Duplicate Address Detection

   Section 5.4 "Duplicate Address Detection" of [RFC4862] specifies that
   Duplicate Address Detection MUST NOT be performed on anycast
   addresses.  This, stems from the fact that the anycast addresses are
   syntactically indistinguishable from unicast addresses.  One can
   argue that this allows for misconfiguration if an address deemed to
   be anycast already exist on the network.

14.  Implementations doing DAD once per IID

   Section 5.4 of [RFC4862] mentions the implementations performing a
   single DAD per interface identifier, and discourages that
   "optimization".  As the practice is emerging in the industry is to
   move away from the fixed interface identifiers anywhere, the
   necessity to perform a DAD on a per-address basis might be useful to
   elevate to a requirement status.



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15.  Backwards compatibility and presence of the DAD proxies

   While not being an issue as such, this is a reminder that the
   operation of DAD has to remain backwards compatible, both to remain
   cooperative with the existing hosts, and the potentially present DAD
   proxies as described in [RFC6957].

16.  Acknowledgements

   Thanks to Ole Troan for creating and curating the original list.
   Thanks a lot to Suresh Krishnan and Erik Nordmark for the reviews and
   useful suggestions.

17.  IANA Considerations

   None.

18.  Security Considerations

   There are no additional security considerations as this document only
   outlines the issues observed with the current Duplicate Address
   Detection protocol.

19.  References

19.1.  Informative References

   [I-D.ietf-6man-enhanced-dad]
              Asati, R., Singh, H., Beebee, W., Pignataro, C., Dart, E.,
              and W. George, "Enhanced Duplicate Address Detection",
              draft-ietf-6man-enhanced-dad-07 (work in progress),
              October 2014.

19.2.  Normative References

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

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

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, April 2006.

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





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   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903, June
              2007.

   [RFC5227]  Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              July 2008.

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses", RFC
              6620, May 2012.

   [RFC6957]  Costa, F., Combes, J-M., Pougnard, X., and H. Li,
              "Duplicate Address Detection Proxy", RFC 6957, June 2013.

Author's Address

   Andrew Yourtchenko
   cisco
   6b de Kleetlaan
   Diegem  1831
   Belgium

   Email: ayourtch@cisco.com

























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