Network Working Group                                           R. Asati
Internet-Draft                                                  H. Singh
Updates: 4862, 4861, 3971 (if approved)                        W. Beebee
Intended status: Standards Track                            C. Pignataro
Expires: April 26, May 14, 2015                                Cisco Systems, Inc.
                                                                 E. Dart
                                   Lawrence Berkeley National Laboratory
                                                               W. George
                                                       Time Warner Cable
                                                        October 23,
                                                       November 10, 2014

                  Enhanced Duplicate Address Detection
                    draft-ietf-6man-enhanced-dad-07
                    draft-ietf-6man-enhanced-dad-08

Abstract

   IPv6 Loopback Suppression and Duplicate Address Detection (DAD) are
   discussed in Appendix A of [RFC4862].  That specification mentions a
   hardware-assisted mechanism to detect looped back DAD messages.  If
   hardware cannot suppress looped back DAD messages, a software
   solution is required.  Several service provider communities have
   expressed a need for automated detection of looped backed Neighbor
   Discovery (ND) messages used by DAD.  This document includes
   mitigation techniques and outlines the Enhanced DAD algorithm to
   automate the detection of looped back IPv6 ND messages used by DAD.
   For network loopback tests, the Enhanced DAD algorithm allows IPv6 to
   self-heal after a loopback is placed and removed.  Further, for
   certain access networks the document automates resolving a specific
   duplicate address conflict.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 26, May 14, 2015.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Two Deployment Problems . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  Operational Mitigation Options  . . . . . . . . . . . . . . .   5
     3.1.  Disable DAD on an Interface . . . . . . . . . . . . . . .   5
     3.2.  Dynamic Disable/Enable of DAD Using Layer-2 Protocol  . .   5
     3.3.  Operational Considerations  . . . . . . . . . . . . . . .   6
   4.  The Enhanced DAD Algorithm  . . . . . . . . . . . . . . . . .   6
     4.1.  General Rules . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Processing Rules for Senders  . . . . . . . . . . . . . .   7
     4.3.
     4.2.  Processing Rules for Receivers  . . . . . . . . . . . . .   7
     4.4.
     4.3.  Impact on SEND  . . . . . . . . . . . . . . . . . . . . .   8
     4.5.
     4.4.  Changes to RFC 4862 . . . . . . . . . . . . . . . . . . .   8
     4.6.
     4.5.  Changes to RFC 4861 . . . . . . . . . . . . . . . . . . .   9
     4.7.   8
     4.6.  Changes to RFC 3971 . . . . . . . . . . . . . . . . . . .   9
   5.  Actions to Perform on Detecting a Genuine Duplicate . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   IPv6 Loopback Suppression and Duplicate Address Detection (DAD) are
   discussed in Appendix A of [RFC4862].  That specification mentions a
   hardware-assisted mechanism to detect looped back DAD messages.  If
   hardware cannot suppress looped back DAD messages, a software
   solution is required.  One specific DAD message is the Neighbor
   Solicitation (NS), specified in [RFC4861].  The NS is issued by the
   network interface of an IPv6 node for DAD.  Another message involved
   in DAD is the Neighbor Advertisement (NA).  The Enhanced DAD
   algorithm specified in this document focuses on detecting an NS
   looped back to the transmitting interface during the DAD operation.
   Detecting a looped back NA does not solve the looped back DAD
   problem.  Detection of any other looped back ND messages during the
   DAD operation is outside the scope of this document.  This document
   also includes a section on Mitigation that discusses means already
   available to mitigate the DAD loopback problem.

1.1.  Two Deployment Problems

   In each problem articulated below, the IPv6 link-local address DAD
   operation fails due to a looped back DAD probe.  However, the looped
   back DAD probe exists for any IPv6 address type including global
   addresses.

   Recently, service providers have reported a problem with DAD that is
   caused by looped back NS messages.  The following is a description of
   the circumstances under which the problem arises.  Loopback testing
   for troubleshooting purposes is underway on a circuit connected to an
   interface on a router.  The interface on the router is enabled for
   IPv6.  The interface issues a NS for the IPv6 link-local address DAD.
   The NS is reflected back to the router interface due to the loopback
   condition of the circuit, and the router interface enters a DAD-
   failed state.  After the circuit troubleshooting has concluded and
   the loopback condition is removed, IPv4 will return to operation
   without further manual intervention.  However, IPv6 will remain in
   DAD-failed state until manual intervention on the router restores
   IPv6 to operation.

   There are other conditions which will also trigger similar problems
   with DAD Loopback.  While the following example is not a common
   configuration, it has occurred in a large service provider network.
   It is necessary to address it in the proposed solution because the
   trigger scenario has the potential to cause significant IPv6 service
   outages when it does occur.  Two  In this scenario, two broadband modems
   in the same home are served by the same service provider and both
   modems are served by one access concentrator and one layer-3
   interface on the access concentrator.  The two modems have the
   Ethernet ports of each modem connected to a network hub.  The access
   concentrator serving the modems is the first-hop IPv6 router for the
   modems.  The network interface of the access concentrator serving the
   two broadband modems is enabled for IPv6 and the interface issues a
   NS(DAD) message for the IPv6 link-local address.  The NS message
   reaches one modem first and this modem sends the message to the
   network hub which sends the message to the second modem which
   forwards the message back to the access concentrator.  The looped
   back NS message causes the network interface on the access
   concentrator to be in a DAD-failed state.  Such a network interface
   typically serves up to a hundred thousand broadband modems causing
   all the modems (and hosts behind the modems) to fail to get IPv6
   online on the access network.  Additionally, it may be tedious for
   the access concentrator to find out which of the hundred thousand or
   more homes looped back the DAD message.  Clearly there is a need for
   automated detection of looped back NS messages during DAD operations
   by a node.

2.  Terminology

   o  DAD-failed state - Duplication Address Detection failure as
      specified in [RFC4862].  Note even Optimistic DAD as specified in
      [RFC4429] can fail due to a looped back DAD probe.  This document
      covers looped back detection for Optimistic DAD as well.

   o  Looped back message - also referred to as a reflected message.
      The message sent by the sender is received by the sender due to
      the network or an Upper Layer Protocol on the sender looping the
      message back.

   o  Loopback - A function in which the router's layer-3 interface (or
      the circuit to which the router's interface is connected) is
      looped back or connected to itself.  Loopback causes packets sent
      by the interface to be received by the interface and results in
      interface unavailability for regular data traffic forwarding.  See
      more details in section 9.1 of [RFC1583].  The Loopback function
      is commonly used in an interface context to gain information on
      the quality of the interface, by employing mechanisms such as
      ICMPv6 pings and bit-error tests.  In a circuit context, this
      function is used in wide area environments including optical Dense
      Wave Division Multiplexing (DWDM) and SONET/SDH for fault
      isolation (e.g. by placing a loopback at different geographic
      locations along the path of a wide area circuit to help locate a
      circuit fault).  The Loopback function may be employed locally or
      remotely.

   o  NS(DAD) - shorthand notation to denote an Neighbor Solicitation
      (NS) (as specified in [RFC4861]) with unspecified IPv6 source-
      address issued during DAD.

2.1.  Requirements Language

   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].

3.  Operational Mitigation Options

   Two mitigation options are described below.  The mechanisms do not
   require any change to existing implementations.

3.1.  Disable DAD on an Interface

   One can disable DAD on an interface so that there is no NS(DAD)
   issued to be looped back.  DAD is disabled by setting the interface's
   DupAddrDetectTransmits variable to zero.  While this mitigation may
   be the simplest, the mitigation has three drawbacks.

   This mitigation would likely require careful analysis of
   configuration on such point-to-point interfaces, a one-time manual
   configuration on each of such interfaces, and more importantly,
   genuine duplicates in the link will not be detected.

   A Service Provider router, such as an access concentrator, or network
   core router, SHOULD support this mitigation strategy.

3.2.  Dynamic Disable/Enable of DAD Using Layer-2 Protocol

   One or more layer-2 protocols MAY include provisions to detect the
   existence of a loopback on an interface circuit, usually by comparing
   protocol data sent and received.  For example, PPP the Point-to-Point
   Protocol (PPP) uses a magic number (section 6.4 of [RFC1661]) to
   detect a loopback on an interface.

   When a layer-2 protocol detects that a loopback is present on an
   interface circuit, the device MUST temporarily disable DAD on the
   interface.  When the protocol detects that a loopback is no longer
   present (or the interface state has changed), the device MUST
   (re-)enable DAD on that interface.

   This solution requires no protocol changes.  This solution SHOULD be
   enabled by default, and MUST be a configurable option if the layer-2
   technology provides means for detecting loopback messages on an
   interface circuit.

   This mitigation has several benefits.  They are

   1.  It leverages layer-2 protocol's built-in loopback detection
       capability, if available.

   2.  It scales better since it relies on an event-driven model which
       requires no additional state or timer.  This may be a significant
       scaling consideration on devices with hundreds or thousands of
       interfaces that may be in loopback for long periods of time (such
       as while awaiting turn-up or during long-duration intrusive bit
       error rate tests).

3.3.  Operational Considerations

   The mitigation options discussed in the document do not require the
   devices on both ends of the circuit to support the mitigation
   functionality simultaneously, and do not propose any capability
   negotiation.  The mitigation options discussed in this document are
   effective for unidirectional circuit or interface loopback (i.e. the
   the loopback is placed in one direction on the circuit, rendering the
   other direction non-operational).

   The mitigation options may not be effective for the bidirectional
   loopback (i.e. the loopback is placed in both directions of the
   circuit interface, so as to identify the faulty segment) if only one
   device followed a mitigation option specified in this document, since
   the other device would follow current behavior and disable IPv6 on
   that interface due to DAD until manual intervention restores it.

   This is nothing different from what happens today (without the
   solutions proposed by this document) in case of unidirectional
   loopback.  Hence, it is expected that an operator would resort to
   manual intervention for the devices not compliant with this document,
   as usual.

4.  The Enhanced DAD Algorithm

   The Enhanced DAD algorithm covers detection of a looped back NS(DAD)
   message.  The document proposes use of the Nonce Option specified in
   the SEND document of [RFC3971].  The nonce is a random number as
   specified in [RFC3971].  If SEND is enabled on the router and the
   router also supports the Enhanced DAD algorithm (specified in this
   document), there is integration with the Enhanced DAD algorithm and
   SEND.  (See more details in the Impact on SEND section section 4.4.) 4.3.)  Since a
   nonce is used only once, The NS(DAD) for each IPv6 address of an
   interface uses a different nonce.  Additional details of the
   algorithm are included in section 4.2.

   Six bytes of random nonce is sufficiently large to minimize
   collisions.  However, if there is a collision because two nodes that
   are using
   the same Target Address in their NS(DAD) and generated the same
   random nonce, then the algorithm will incorrectly detect a looped
   back NS(DAD) when a genuine address collision has occurred.  Since
   each looped back NS(DAD) event is logged to system management, the
   administrator of the network will have access to the information
   necessary to intervene manually.  Also, because the nodes will have
   detected what appear to be looped back NS(DAD) messages, they will
   continue to probe, and it is unlikely that they will choose the same
   nonce the second time (assuming quality random number generators).

   The algorithm is capable of detecting any ND solicitation (NS and
   Router Solicitation) or advertisement (NA and Router Advertisement)
   that is looped back.  However, saving for a sender to store a nonce and
   nonce related data state for all ND messages has impact on memory requirements of the node and
   also adds
   causes more complexity for the algorithm state to a substantially larger number of ND
   messages. sender node.  Therefore, this document
   does not recommend using the algorithm outside of the DAD operation
   by an interface on a node.

4.1.  General Rules

   If an IPv6 node implements the Enhanced DAD algorithm, the node MUST
   implement detection of looped back NS(DAD) messages during DAD for an
   interface address.

4.2.  Processing Rules for Senders

   If a node has been configured to use the Enhanced DAD algorithm, when
   sending an NS(DAD) for a tentative or optimistic interface address
   the sender MUST generate a random nonce associated with the interface
   address, MUST save store the nonce, nonce internally, and MUST include the nonce
   in the Nonce Option included in the NS(DAD).  If the interface does
   not receive any DAD failure indications within RetransTimer
   milliseconds (see [RFC4861]) after having sent DupAddrDetectTransmits
   Neighbor Solicitations, the interface moves the Target Address to the
   assigned state.

   If any probe is looped back within RetransTimer milliseconds after
   having sent DupAddrDetectTransmits NS(DAD) messages, the interface
   continues with another MAX_MULTICAST_SOLICIT number of NS(DAD)
   messages transmitted RetransTimer millseconds milliseconds apart.  Section 2 of
   [RFC3971] defines a single-use nonce, so each Enhanced DAD probe uses
   a different nonce.  If no probe is looped back within RetransTimer
   milliseconds after MAX_MULTICAST_SOLICIT NS(DAD) messages are sent,
   the probing stops.  The probing MAY be stopped via manual
   intervention.  When probing is stopped, the interface moves the
   Target Address to the assigned state.

4.3.

4.2.  Processing Rules for Receivers

   If the node has been configured to use the Enhanced DAD algorithm and
   an interface on the node receives any NS(DAD) message where the
   Target Address matches the interface address (in tentative or
   optimistic state), the receiver compares the nonce included in the
   message, with any saved stored nonce on the receiving interface.  If a
   match is found, the node SHOULD log a system management message,
   SHOULD update any statistics counter, and MUST drop the received
   message.  If the received NS(DAD) message includes a nonce and no
   match is found with any saved stored nonce, the node SHOULD log a system
   management message for a DAD-failed state, and SHOULD update any
   statistics counter.  If the interface does not receive any DAD failure
   indications within RetransTimer milliseconds after having sent
   DupAddrDetectTransmits Neighbor Solicitations, the interface moves
   the Target Address to the assigned state.

4.4.

4.3.  Impact on SEND

   The SEND document uses the Nonce Option in the context of matching an
   NA with an NS.  However, no text in SEND has an explicit mention of
   detecting looped back ND messages.  As this document updates
   [RFC4862], SEND should be updated to integrate with the Enhanced DAD
   algorithm.  A minor update to SEND would be to explicitly mention
   that the nonce in SEND is also used by SEND to detect looped back
   NS(DAD) messages during DAD operations by the node.  In a mixed SEND
   environment with SEND and unsecured nodes, the lengths of the nonce
   used by SEND and unsecured nodes MUST be identical.

4.5.

4.4.  Changes to RFC 4862

   The following text is added to the end of the Introduction section of
   [RFC4862].

   A network interface of an IPv6 node SHOULD should implement the Enhanced DAD
   algorithm.  For example, if the interface on an IPv6 node is
   connected to a circuit that supports loopback testing, then the node
   SHOULD
   should implement the Enhanced DAD algorithm that allows the IPv6
   interface to self-heal after loopback testing is ended on the
   circuit.  Another example is when the IPv6 interface resides on an
   access concentrator running DAD Proxy.  The interface supports up to
   a hundred thousand IPv6 clients (broadband modems) connected to the
   interface.  If the interface performs DAD for its IPv6 link-local
   address and the DAD probe is reflected back to the interface, the
   interface is stuck in DAD-failed state and IPv6 services to the
   hundred thousand clients is denied.  Disabling DAD for such an IPv6
   interface on an access concentrator is less desirable than
   implementing the algorithm because the network also needs to detect
   genuine duplicates in the interface downstream network.  The Enhanced
   DAD algorithm also facilitates detecting a genuine duplicate for the
   interface on the access concentrator.  (See the Actions to Perform on
   Detecting a Genuine Duplicate section of the Enhanced DAD document.)

   The following text is added to the end of Appendix A of [RFC4862].

   The Enhanced DAD algorithm from draft-ietf-6man-enhanced-dad is
   designed to detect looped back DAD probes.  A network interface of an
   IPv6 node SHOULD implement the Enhanced DAD algorithm.

4.6.

4.5.  Changes to RFC 4861

   The following text is appended to the RetransTimer variable
   description in section 6.3.2 of [RFC4861]:

   The RetransTimer may be overridden by a link-specific document if a
   node supports the Enhanced DAD algorithm.

   The following text is appended to the Source Address definition in
   section 4.3 of [RFC4861]:

   If a node has been configured to use the Enhanced DAD algorithm, an
   NS with an unspecified source address adds the Nonce option to the
   message and implements the state machine of the Enhanced DAD
   algorithm.

4.7.

4.6.  Changes to RFC 3971

   The following text is changed in section 5.3.2 of [RFC3971]:

   The purpose of the Nonce option is to make sure that an advertisement
   is a fresh response to a solicitation sent earlier by the node.

   The new text is included below:

   The purpose of the Nonce option is to make sure that an advertisement
   is a fresh response to a solicitation sent earlier by the node.  The
   nonce is also used to detect looped back NS messages when the network
   interface performs DAD [RFC4862].  Detecting looped back DAD messages
   is covered by the Enhanced DAD algorithm as specified in draft-ietf-
   6man-enhanced-dad.  In a mixed SEND environment with SEND and
   unsecured nodes, the lengths of the nonce used by SEND and unsecured
   nodes MUST be identical.

5.  Actions to Perform on Detecting a Genuine Duplicate

   As described in the paragraphs above, the nonce can also serve to
   detect genuine duplicates even when the network has potential for
   looping back ND messages.  When a genuine duplicate is detected, the
   node follows the manual intervention specified in section 5.4.5 of
   [RFC4862].  However, in certain networks such as an access network,
   if the genuine duplicate matches the tentative or optimistic IPv6
   address of a network interface of the access concentrator, automated
   actions are recommended.

   One example of a type of access network is a cable broadband deployment
   where the access concentrator is the first-hop IPv6 router to several
   hundred thousand broadband modems.  The router also supports proxying
   of DAD messages.  The network interface on the access concentrator
   initiates DAD for an IPv6 address and detects a genuine duplicate due
   to receiving an NS(DAD) or an NA message.  On detecting such a
   duplicate, the access concentrator logs a system management message,
   drops the received ND message, and blocks the modem on whose layer-2
   service identifier the NS(DAD) or NA message was received on.

   The network described above follows a trust model where a trusted
   router serves un-trusted IPv6 host nodes.  Operators of such networks
   have a desire to take automated action if a network interface of the
   trusted router has a tentative or optimistic address duplicated by a
   host.  Any other network that follows the same trust model MAY use
   the automated actions proposed in this section.

6.  Security Considerations

   This document does not improve nor reduce the security posture of
   [RFC4862].  The nonce can be exploited by a rogue deliberately
   changing the nonce to fail the looped back detection specified by the
   Enhanced DAD algorithm.  SEND is recommended to circumvent this
   exploit.  Additionally, the nonce does not protect against the DoS
   caused by a rogue node replying by a fake NA to all DAD probes.  SEND
   is recommended to circumvent this exploit also.  Disabling DAD has an
   obvious security issue before a remote node on the link can issue
   reflected NS(DAD) messages.  Again, SEND is recommended for this
   exploit.  Source Address Validation Improvement (SAVI) [RFC6620] also
   protects against various attacks by on-link rogues.

7.  IANA Considerations

   None.

8.  Acknowledgements

   Thanks (in alphabetical order by first name) to Bernie Volz, Dmitry
   Anipko, Eric Levy-Abegnoli, Eric Vyncke, Erik Nordmark, Fred Templin,
   Jouni Korhonen, Michael Sinatra, Ole Troan, Pascal Thubert, Ray
   Hunter, Suresh Krishnan, and Tassos Chatzithomaoglou for their
   guidance and review of the document.  Thanks to Thomas Narten for
   encouraging this work.  Thanks to Steinar Haug and Scott Beuker for
   describing the use cases.

9.  Normative References

   [RFC1583]  Moy, J., "OSPF Version 2", RFC 1583, March 1994.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

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

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [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.

   [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.

Authors' Addresses

   Rajiv Asati
   Cisco Systems, Inc.
   7025 Kit Creek road
   Research Triangle Park, NC  27709-4987
   USA

   Email: rajiva@cisco.com
   URI:   http://www.cisco.com/

   Hemant Singh
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   Phone: +1 978 936 1622
   Email: shemant@cisco.com
   URI:   http://www.cisco.com/
   Wes Beebee
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   Phone: +1 978 936 2030
   Email: wbeebee@cisco.com
   URI:   http://www.cisco.com/

   Carlos Pignataro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709
   USA

   Email: cpignata@cisco.com
   URI:   http://www.cisco.com/

   Eli Dart
   Lawrence Berkeley National Laboratory
   1 Cyclotron Road, Berkeley, CA 94720
   USA

   Email: dart@es.net
   URI:   http://www.es.net/

   Wes George
   Time Warner Cable
   13820 Sunrise Valley Drive
   Herndon, VA  20171
   USA

   Email: wesley.george@twcable.com