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Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop Routers
draft-ietf-6man-grand-02

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9131.
Author Jen Linkova
Last updated 2020-09-13
Replaces draft-linkova-6man-grand
RFC stream Internet Engineering Task Force (IETF)
Formats
Reviews
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Bob Hinden
Shepherd write-up Show Last changed 2020-08-12
IESG IESG state Became RFC 9131 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Erik Kline
Send notices to Bob Hinden <bob.hinden@gmail.com>
draft-ietf-6man-grand-02
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   1.  The rightful owner of the address has not been using it for
       communication.

   2.  The rightful owner just started sending packets from that address
       but the router has not received any return traffic yet.

   The impact on the rightful owner's traffic flows would be different
   in those cases.

5.3.1.  The Rightful Owner Is Not Sending Packets From The Address

   In this scenario the following events are expected to happen:

   1.  The host configures the address and sets its state to Optimistic.

   2.  The host sends an unsolicited NA with the Override flag set to
       zero and starts sending traffic from the Optimistic address.

   3.  The router creates a STALE entry for the address and the host
       link-layer address.

   4.  The host starts DAD and detects the address duplication.

   5.  The router receives the return traffic for the duplicated
       address.  As the NC entry is STALE it sends traffic using that
       entry, changes it to DELAY and wait up to DELAY_FIRST_PROBE_TIME
       ([RFC4861]) seconds.

   6.  The router changes the NC entry state to PROBE and sends up to
       MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by
       RetransTimer milliseconds ([RFC4861]) to the host link-layer
       address.

   7.  As the host has detected the address conflict already it does not
       respond to the unicast NSes.

   8.  The router sends a multicast NS to the solicited node multicast
       address, the rightful owner responds and the router NC entry is
       updated with the rightful owner link-local address.

   The rightful owner is not experiencing any disruption as it does not
   send/receive any traffic.  If after step 7 the router keeps receiving
   any return traffic for communication initiated at step 2, those
   packets would be forwarded to the rightful owner.  However the same
   behaviour would be observed if changes proposed in this document are
   implemented: if the host starts sending packets from its Optimistic
   address but then changed the address state to Duplicated, almost all
   return traffic would be forwarded to the rightful owner of the said

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   address.  Therefore it's safe to conclude that the proposed changes
   do not cause any disruption for the rightful owner.

5.3.2.  The Rightful Owner Has Started Sending Packets From The Address

   In this scenario the following events are happening:

   1.  The rightful owner starts sending traffic from the address (e.g.
       the address has just been configured or has not been recently
       used).

   2.  The host configures the address and sets its state to Optimistic.

   3.  The host sends an unsolicited NA with the Override flag set to
       zero and starts sending traffic from the Optimistic address.

   4.  The router creates a STALE entry for the address and the host
       link-layer address.

   5.  The host starts DAD and detects the address duplication.

   6.  The router receives the return traffic flows for both the
       rightful owner of the duplicated address and the new host.  As
       the NC entry is STALE it sends traffic using that entry, changes
       it to DELAY and wait up to DELAY_FIRST_PROBE_TIME ([RFC4861])
       seconds.

   7.  The router changes the NC entry state to PROBE and sends up to
       MAX_UNICAST_SOLICIT ([RFC4861]) unicast NSes separated by
       RetransTimer milliseconds ([RFC4861]) to the host link-layer
       address.

   8.  As the host has detected the address conflict already it does not
       respond to the unicast NSes.

   9.  The router sends a multicast NS to the solicited node multicast
       address, the rightful owner responds and the router NC entry is
       updated with the rightful owner link-local address.

   As a result the traffic for the address rightful owner would be sent
   to the host with the duplicated address instead.  The duration of the
   disruption can be estimated as DELAY_FIRST_PROBE_TIME*1000 +
   (MAX_UNICAST_SOLICIT - 1)*RetransTimer milliseconds.  As per the
   constants defined in Section 10 of [RFC4861] this interval is equal
   to 5*1000 + (3 - 1)*1000 = 7000ms or 7 seconds.

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   However it should be noted that the probability of such scenario is
   rather low as it would require the following things to happen almost
   simultaneously (within tens of milliseconds):

   o  One host starts using a new IPv6 address and sending traffic.

   o  Another host configures the same IPv6 address in Optimistic mode
      before the router receives the return traffic for the first host.

6.  Modifications to RFC-Mandated Behavior

   All normative text in this memo is contained in this section.

6.1.  Modification to RFC4861 Neighbor Discovery for IP version 6 (IPv6)

6.1.1.  Modification to the section 7.2.5

   This document proposes the following changes to the section 7.2.5 of
   [RFC4861]:

   ------------------------------------------------------------------

   OLD TEXT:

   When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists, the advertisement SHOULD be silently discarded.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the
   target.

   NEW TEXT:

   When a valid Neighbor Advertisement is received (either solicited or
   unsolicited), the Neighbor Cache is searched for the target's entry.
   If no entry exists, hosts SHOULD silently discard the advertisement.
   There is no need to create an entry if none exists, since the
   recipient has apparently not initiated any communication with the
   target.  Routers SHOULD create a new entry for the target address
   with the link-layer address set to the Target link-layer address
   option (if supplied).  The entry its reachability state MUST also be
   set to STALE.  If the received Neighbor Advertisement does not
   contain the Target link-layer address option the advertisement SHOULD
   be silently discarded.

   ------------------------------------------------------------------

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6.1.2.  Modification to the section 7.2.6

   This document proposes the following changes to the section 7.2.6 of
   [RFC4861]:

   OLD TEXT:

   Also, a node belonging to an anycast address MAY multicast
   unsolicited Neighbor Advertisements for the anycast address when the
   node's link-layer address changes.

   NEW TEXT:

   Also, a node belonging to an anycast address MAY multicast
   unsolicited Neighbor Advertisements for the anycast address when the
   node's link-layer address changes.

   A node may also wish to notify its first-hop routers when it
   configures a new global IPv6 address so the routers can proactively
   populate their neighbor caches with the corresponding entries.  In
   such cases a node SHOULD send up to MAX_NEIGHBOR_ADVERTISEMENT
   Neighbor Advertisement messages.  If the address is preferred then
   the Override flag SHOULD NOT be set.  If the address is in the
   Optimistic state then the Override flag MUST NOT be set.  The
   destination address SHOULD be set to the all-routers multicast
   address.  These advertisements MUST be separated by at least
   RetransTimer seconds.  The first advertisement SHOULD be sent as soon
   as one of the following events happens:

   o  if Optimistic DAD [RFC4429] is used: a new Optimistic address is
      assigned to the node interface.

   o  if Optimistic DAD is not used: an address changes the state from
      tentative to preferred.

   ------------------------------------------------------------------

7.  Solutions Considered but Discarded

   There are other possible approaches to address the problem, for
   example:

   o  Just do nothing.

   o  Migrating from the "reactive" Neighbor Discovery ([RFC4861]) to
      the registration-based mechanisms ([RFC8505]).

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   o  Creating new entries in routers Neighbor Cache by gleaning from
      Neighbor Discovery DAD messages.

   o  Initiates bidirectional communication from the host to the router
      using the host GUA.

   o  Making the probing logic on hosts more robust.

   o  Increasing the buffer size on routers.

   o  Transit dataplane traffic from an unknown address (an address w/o
      the corresponding neighbor cache entry) triggers an address
      resolution process on the router.

   It should be noted that some of those options are already implemented
   by some vendors.  The following sections discuss those approaches and
   the reasons they were discarded.

7.1.  Do Nothing

   One of the possible approaches might be to declare that everything is
   working as intended and let the upper-layer protocols to deal with
   packet loss.  The obvious drawbacks include:

   o  Unhappy users.

   o  Many support tickets.

   o  More resistance to deploy IPv6 and IPv6-Only networks.

7.2.  Change to the Registration-Based Neighbor Discovery

   The most radical approach would be to move away from the reactive ND
   as defined in [RFC4861] and expand the registration-based ND
   ([RFC6775], [RFC8505]) used in Low-Power Wireless Personal Area
   Networks (6LoWPANs) to the rest of IPv6 deployments.  This option
   requires some investigation and discussions and seems to be excessive
   for the problem described in this document.

7.3.  Host Sending NS to the Router Address from Its GUA

   The host could force creating a STALE entry for its GUA in the router
   ND cache by sending the following Neighbor Solicitation message:

   o  The NS source address is the host GUA.

   o  The destination address is the default router IPv6 address.

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   o  The Source Link-Layer Address option contains the host link-layer
      address.

   o  The target address is the host default router address (the default
      router address the host received in the RA).

   The main disadvantages of this approach are:

   o  Would not work for Optimistic addresses as section 2.2 of
      [RFC4429] explicitly prohibits sending Neighbor Solicitations from
      an Optimistic Address.

   o  If first-hop redundancy is deployed in the network, the NS would
      reach the active router only, so all backup routers (or all active
      routers except one) would not get their neighbor cache updated.

   o  Some wireless devices are known to alter ND packets and perform
      various non-obvious forms of ND proxy actions.  In some cases,
      unsolicited NAs might not even reach the routers.

7.4.  Host Sending Router Solicitation from its GUA

   The host could send a router solicitation message to 'all routers'
   multicast address, using its GUA as a source.  If the host link-layer
   address is included in the Source Link-Layer Address option, the
   router would create a STALE entry for the host GUA as per the section
   6.2.6 of [RFC4861].  However, this approach can not be used if the
   GUA is in optimistic state: section 2.2 of [RFC4429] explicitly
   prohibits using an Optimistic Address as the source address of a
   Router Solicitation with a SLLAO as it might disrupt the rightful
   owner of the address in the case of a collision.  So for the
   optimistic addresses the host can send an RS without SLLAO included.
   In that case the router may respond with either a multicast or a
   unicast RA (only the latter would create a cache entry).

   This approach has the following drawbacks:

   o  If the address is in the Optimistic state the RS can not contain
      SLLAO.  As a result the router would only create a cache entry if
      solicited RAs are sent as unicast.  Routers sending solicited RAs
      as multicast would not create a new cache entry as they do not
      need to send a unicast packet back to the host.

   o  There might be a random delay between receiving an RS and sending
      a unicast RA back (and creating a cache entry) which might
      undermine the idea of creating the cache entry proactively.

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   o  Some wireless devices are known to intercept ND packets and
      perform various non-obvious forms of ND proxy actions.  In some
      cases the RS might not even reach the routers.

7.5.  Routers Populating Their Caches by Gleaning From Neighbor
      Discovery Packets

   Routers may be able to learn about new addresses by gleaning from the
   DAD Neighbor Solicitation messages.  The router could listen to all
   solicited node multicast address groups and upon receiving a Neighbor
   Solicitation from the unspecified address search its Neighbor Cache
   for the solicitation's Target Address.  If no entry exists, the
   router may create an entry, set its reachability state to
   'INCOMPLETE' and start the address resolution for that entry.

   The same solution was proposed in
   [I-D.halpern-6man-nd-pre-resolve-addr].  Some routing vendors support
   such optimization already.  However, this approach has a number of
   drawbacks and therefore should not be used as the only solution:

   o  Routers need to receive all multicast Neighbor Discovery packets
      which might negatively impact the routers CPU.

   o  If the router starts the address resolution as soon as it receives
      the DAD Neighbor Solicitation the host might be still performing
      DAD and the target address might be tentative.  In that case, the
      host SHOULD silently ignore the received Neighbor Solicitation
      from the router as per the Section 5.4.3 of [RFC4862].  As a
      result the router might not be able to complete the address
      resolution before the return traffic arrives.

7.6.  Initiating Hosts-to-Routers Communication

   The host may force the router to start address resolution by sending
   a data packet such as ping or traceroute to its default router link-
   local address, using the GUA as a source address.  As the RTT to the
   default router is lower than RTT to any off-link destinations it's
   quite likely that the router would start the neighbor discovery
   process for the host GUA before the first packet of the returning
   traffic arrives.

   This approach has the following drawbacks:

   o  Data packets to the router link-local address could be blocked by
      security policy or control plane protection mechanism.

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   o  It introduces an additional overhead for routers control plane (in
      addition to processing ND packets, the data packet needs to be
      processed as well).

   o  Unless the data packet is sent to 'all routers' ff02::2 multicast
      address, if the network provides a first-hop redundancy then only
      the active router would create a new cache entry.

7.7.  Transit Dataplane Traffic From a New Address Triggering Address
      Resolution

   When a router receives a transit packet, it might check the presence
   of the neighbor cache entry for the packet source address and if the
   entry does not, exist start address resolution process.  This
   approach does ensure that a Neighbor Cache entry is proactively
   created every time a new, previously unseen GUA is used for sending
   offlink traffic.  However this approach has a number of limitations,
   in particular:

   o  If traffic flows are asymmetrical the return traffic might not
      transit the same router as the original traffic which triggered
      the address resolution.  So the neighbor cache entry is created on
      the "wrong" router, not the one which actually needs the neighbor
      cache entry for the host address.

   o  The functionality needs to be limited to explicitly configured
      networks/interfaces, as the router needs to distinguish between
      onlink addresses (ones the router needs to have Neighbor Cache
      entries for) and the rest of the address space.

   o  Implementing such functionality is much more complicated than all
      other solutions as it would involve complex data-control planes
      interaction.

8.  IANA Considerations

   This memo asks the IANA for no new parameters.

9.  Security Considerations

   One of the potential attack vectors to consider is a cache spoofing
   when the attacker might try to install a cache entry for the victim's
   IPv6 address and the attacker's Link-Layer address.  However it
   should be noted that this document does not propose any changes for
   the scenario when the ND cache for the given IPv6 address already
   exists.  Therefore it is not possible for the attacker to override
   any existing cache entry.

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   A malicious host could attempt to exhaust the neighbor cache on the
   router by creating a large number of STALE entries.  However this
   attack vector is not new and this document does not increase the risk
   of such an attack: the attacker could do it, for example, by sending
   a NS or RS packet with SLLAO included.  All recommendations from
   [RFC6583] still apply.

   Announcing a new address to all-routers multicast address may inform
   an on-link attacker about IPv6 addresses assigned to the host.
   However hiding information about the specific IPv6 address should not
   be considered a security measure as such information is usually
   disclosed via DAD to all nodes anyway.  Network administrators can
   also mitigate this issue by enabling MLD snooping on the link-layer
   devices to prevent IPv6 link-local multicast packets being flooded to
   all onlink nodes.  If peer-to-peer onlink communications are not
   desirable for the given network segment they should be prevented by
   proper layer2 security mechanisms.  Therefore the risk of allowing
   hosts to send unsolicited Neighbor Advertisements to all-routers
   multicast address is low.

   It should be noted that the proposed mechanism allows hosts to
   proactively inform their routers about global IPv6 addresses existing
   on-link.  Routers could use that information to distinguish between
   used and unused addresses to mitigate ND cache exhaustion DoS attacks
   described in Section 4.3.2 [RFC3756] and [RFC6583].

10.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   comments, review and feedback: Mikael Abrahamsson, Stewart Bryant,
   Lorenzo Colitti, Owen DeLong, Igor Gashinsky, Fernando Gont, Tatuya
   Jinmei, Erik Kline, Warren Kumari, Barry Leiba, Jordi Palet Martinez,
   Erik Nordmark, Michael Richardson, Mark Smith, Dave Thaler, Pascal
   Thubert, Loganaden Velvindron, Eric Vyncke.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

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   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,
              <https://www.rfc-editor.org/info/rfc6775>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

11.2.  Informative References

   [I-D.halpern-6man-nd-pre-resolve-addr]
              Chen, I. and J. Halpern, "Triggering ND Address Resolution
              on Receiving DAD-NS", draft-halpern-6man-nd-pre-resolve-
              addr-00 (work in progress), January 2014.

   [RFC3756]  Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
              Neighbor Discovery (ND) Trust Models and Threats",
              RFC 3756, DOI 10.17487/RFC3756, May 2004,
              <https://www.rfc-editor.org/info/rfc3756>.

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   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <https://www.rfc-editor.org/info/rfc4541>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <https://www.rfc-editor.org/info/rfc4941>.

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583,
              DOI 10.17487/RFC6583, March 2012,
              <https://www.rfc-editor.org/info/rfc6583>.

Author's Address

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont, NSW  2009
   AU

   Email: furry@google.com

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