Network Working Group                                     A. Yourtchenko
Internet-Draft                                                     cisco
Intended status: Informational                               E. Nordmark
Expires: September 1, 2015                               Arista Networks
                                                       February 28, 2015

     A survey of issues related to IPv6 Duplicate Address Detection


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

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Robustness: Interaction with delay in forwarding . . . . .  3
     2.2.  Robustness: Behavior on links with unreliable multicast  .  4
     2.3.  Robustness: Partition-join tolerance . . . . . . . . . . .  4
     2.4.  Robustness: Behavior on collision  . . . . . . . . . . . .  4
     2.5.  Energy Efficiency  . . . . . . . . . . . . . . . . . . . .  5
     2.6.  Wake-up and L2 events  . . . . . . . . . . . . . . . . . .  5
   3.  Solved Issues  . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Interaction with looped interfaces . . . . . . . . . . . .  5
     3.2.  Delays before an address can be used . . . . . . . . . . .  6
   4.  Observations . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Duplicate L2 address detection . . . . . . . . . . . . . .  6
     4.2.  Usage of DAD to create state . . . . . . . . . . . . . . .  6
     4.3.  No support of multi-link subnets . . . . . . . . . . . . .  7
     4.4.  Anycast Addresses and Duplicate Address Detection  . . . .  7
     4.5.  Implementations doing DAD once per IID . . . . . . . . . .  7
     4.6.  Backwards compatibility and presence of the DAD proxies  .  8
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Duplicate Address Detection (DAD) is a procedure in IPv6 performed on
   an address before it can be assigned to an interface [RFC2462].  By
   default it consists of sending a single multicast Neighbor
   Soliciation message and waiting for a response for one second.  If no
   response is received, the address is declared to not be a duplicate.
   Once the address has been tested once, there is no further attempts
   to check for duplicates (unless the interface is re-initialized).

   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.  The issues have been grouped to facilitate discussing

2.  Open Issues

   Whether it is due to the assumptions made in 1995, or changes in how
   networks are built or deployed, there are many reasons why DAD would
   fail to detect a duplicate even when one exists.  From a historical
   perspective it is important to keep in mind that when DAD was
   designed we had two forms of IPv6 addresses; those derived from
   EUI-64 and statically assigned.  Since the IETF has developed
   additional methods for address assignment like DHCPv6 and addresses
   that improve privacy by reducing linkability.

2.1.  Robustness: Interaction with delay in forwarding

   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.

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   Similarly, a modem-like device 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.

   Some of the link types, notably cable modems, have link-specific
   standards to address this issue by requiring a new DAD each time the
   RF-side interface bounces, as well as bouncing the LAN interface
   triggered by the bounce of the RF interface.

   Note that [I-D.ietf-6man-resilient-rs] makes the router solicitation
   resilent to the above cases, but there is no counterpart to make DAD

2.2.  Robustness: 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.
   See [I-D.vyncke-6man-mcast-not-efficient] for more information.

   The author's ad-hoc experimentation at IETF90 revealed the success
   rate of detecting the duplicate address on the IETF WiFi network
   being about 4 in 5.  This may violate the assumptions that other
   protocols make.

2.3.  Robustness: 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.

   Note that solutions along the lines of ACD, while improving
   robustness, might result in more resource usage in on the links and
   nodes by multicasting more ND packets.

2.4.  Robustness: 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

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   happening erroneously, is different from what is recommended in the

   TBD: Do the other RFCs for address allocation require some retry

2.5.  Energy Efficiency

   The use of multicast messages for DAD results in some inefficiencies
   for both the network, in particular when multicast uses more layer 2
   resources than unicast, and also has efficiency implications for
   hosts.  Potential techniques for making DAD reliably detect and
   recover from duplicates might result in reduced efficiency.  The
   impact for WiFi is shown in

   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.

2.6.  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, nor does DNA [RFC6059], 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).

   Thus this item could be categorized as being either in the robustness
   or efficiency group of items.

3.  Solved Issues

   Some issues have been or are in the process of being solved.

3.1.  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

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   in [RFC4862] are either incomplete or incorrectly implemented: a
   loopback on the interface causes DAD failure.

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

3.2.  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
   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

   [RFC4429] introduces "Optimistic DAD" process, which addresses this.
   That document has some notes about potentially causing TCP RST when
   there is a duplicate, which can reset an existing TCP connection for
   the existing user of the IPv6 address.  That has some overall impact
   on the robustness of the network and implicitly assumes that all
   application protocols will always retry in order to handle such an

4.  Observations

   Some issues we can't do much about in that they are more observations
   of what can be done.

4.1.  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.

4.2.  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

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   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.

4.3.  No 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.

   One may also argue that since [RFC4861] defers the clarifications on
   IPv6 operation on NBMA networks to [RFC2491], it is unreasonable to
   expect [RFC4862] describe the operation of DAD on NBMA type links,
   and it is up to a link-specific document to describe such operation.
   (An example is cable industry, where the cable standards define it).

   However, it is then unclear where to address the frequently used
   scenario of WiFi with blocked direct communication between the
   stations - whether it is supposed to be an IEEE document or IETF
   document ?  And is there enough fundamental differences between the
   different NBMA models to warrant the link-specific approaches to DAD

4.4.  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.

4.5.  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 anyhow, the necessity
   to perform a DAD on a per-address basis might be useful to elevate to

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   a requirement status.

4.6.  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].

   There are also various forms of sleep proxies [ECMA-393]
   [] which perform
   handoffs of Neighbor Discovery protocol processing that need to be

5.  Acknowledgements

   Thanks to Ole Troan for creating and curating the original list.
   Thanks a lot to Lorenzo Colitti, Suresh Krishnan, Hemant Singh,
   Hesham Soliman, Eric Vyncke, and James Woodyatt for the reviews and
   useful suggestions.

6.  IANA Considerations


7.  Security Considerations

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

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.

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

   [RFC2491]  Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
              over Non-Broadcast Multiple Access (NBMA) networks",
              RFC 2491, January 1999.

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

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

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

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              November 2010.

   [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.

8.2.  Informative References

              Desmouceaux, Y., "Power consumption due to IPv6 multicast
              on WiFi devices",
              draft-desmouceaux-ipv6-mcast-wifi-power-usage-01 (work in
              progress), August 2014.

              Asati, R., Singh, H., Beebee, W., Pignataro, C., Dart, E.,
              and W. George, "Enhanced Duplicate Address Detection",
              draft-ietf-6man-enhanced-dad-13 (work in progress),
              February 2015.

              Krishnan, S., Anipko, D., and D. Thaler, "Packet loss
              resiliency for Router Solicitations",

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              draft-ietf-6man-resilient-rs-04 (work in progress),
              October 2014.

              Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
              Yourtchenko, "Why Network-Layer Multicast is Not Always
              Efficient At Datalink Layer",
              draft-vyncke-6man-mcast-not-efficient-01 (work in
              progress), February 2014.

Authors' Addresses

   Andrew Yourtchenko
   6b de Kleetlaan
   Diegem  1831


   Erik Nordmark
   Arista Networks
   Santa Clara, CA


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