Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop Routers
RFC 9131

Document Type RFC - Proposed Standard (October 2021; No errata)
Updates RFC 4861
Author Jen Linkova 
Last updated 2021-10-01
Replaces draft-linkova-6man-grand, draft-ietf-v6ops-nd-cache-init
Stream Internet Engineering Task Force (IETF)
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Internet Engineering Task Force (IETF)                        J. Linkova
Request for Comments: 9131                                        Google
Updates: 4861                                               October 2021
Category: Standards Track                                               
ISSN: 2070-1721

   Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on
                           First-Hop Routers

Abstract

   Neighbor Discovery (RFC 4861) is used by IPv6 nodes to determine the
   link-layer addresses of neighboring nodes as well as to discover and
   maintain reachability information.  This document updates RFC 4861 to
   allow routers to proactively create a Neighbor Cache entry when a new
   IPv6 address is assigned to a node.  It also updates RFC 4861 and
   recommends that nodes send unsolicited Neighbor Advertisements upon
   assigning a new IPv6 address.  These changes will minimize the delay
   and packet loss when a node initiates connections to an off-link
   destination from a new IPv6 address.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9131.

Copyright Notice

   Copyright (c) 2021 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Terminology
   2.  Problem Statement
   3.  Solution Requirements
   4.  Changes to Neighbor Discovery
     4.1.  Nodes Sending Gratuitous Neighbor Advertisements
     4.2.  Routers Creating Cache Entries upon Receiving Unsolicited
           Neighbor Advertisements
   5.  Avoiding Disruption
     5.1.  Neighbor Cache Entry Exists in Any State Other Than
           INCOMPLETE
     5.2.  Neighbor Cache Entry Is in INCOMPLETE State
     5.3.  Neighbor Cache Entry Does Not Exist
       5.3.1.  The Rightful Owner Is Not Sending Packets from the
               Address
       5.3.2.  The Rightful Owner Has Started Sending Packets from the
               Address
   6.  Modifications to RFC-Mandated Behavior
     6.1.  Modification to RFC 4861 (Neighbor Discovery for IP version
           6 (IPv6))
       6.1.1.  Modification to Section 7.2.5 of RFC 4861
       6.1.2.  Modification to Section 7.2.6 of RFC 4861
   7.  Solution Limitations
   8.  Solutions Considered but Discarded
     8.1.  Do Nothing
     8.2.  Change to the Registration-Based Neighbor Discovery
     8.3.  Host Sending NS to the Router Address from Its GUA
     8.4.  Host Sending Router Solicitation from Its GUA
     8.5.  Routers Populating Their Caches by Gleaning from Neighbor
           Discovery Packets
     8.6.  Initiating Host-to-Router Communication
     8.7.  Making the Probing Logic on Hosts More Robust
     8.8.  Increasing the Buffer Size on Routers
     8.9.  Transit Data Plane Traffic from a New Address to Trigger
           Address Resolution
   9.  IANA Considerations
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   The Neighbor Discovery state machine defined in [RFC4861] assumes
   that communications between IPv6 nodes are, in most cases,
   bidirectional and if a node A is trying to communicate to its
   neighbor, node B, the return traffic flows could be expected.  So,
   when node A starts the address resolution process, the target node B
   would also create an entry containing A's IPv6 and link-layer
   addresses in its Neighbor Cache.  That entry will be used for sending
   the return traffic to A.

   In particular, Section 7.2.5 of [RFC4861] states:

   |  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.

   While this approach is perfectly suitable for host-to-host on-link
   communications, it does not work so well when a host sends traffic to
   off-link destinations.  After joining the network and receiving a
   Router Advertisement, the host populates its Neighbor Cache with the
   default router IPv6 and link-layer addresses and is able to send
   traffic to off-link destinations.  At the same time, the router does
   not have any cache entries for the host global addresses yet and only
   starts address resolution upon receiving the first packet of the
   return traffic flow.  While waiting for the resolution to complete,
   routers only keep a very small number of packets in the queue, as
   recommended in Section 7.2.2 of [RFC4861].  Any additional packets
   arriving before the resolution process finishes are likely to result
   in dropped packets.  It can cause packet loss and performance
   degradation that can be visible to users.

   This document updates the Neighbor Discovery protocol [RFC4861] to
   avoid packet loss in the scenario described above.  Section 4
   discusses the changes and analyzes the potential impact, while
   normative changes to [RFC4861] are specified in Section 6.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   Node:  A device that implements IP [RFC4861].

   Host:  Any node that is not a router [RFC4861].

   ND:  Neighbor Discovery [RFC4861].

   NC:  Neighbor Cache [RFC4861].  The Neighbor Cache entry can be in
      one of five states, as described in Section 7.3.2 of [RFC4861]:
      INCOMPLETE, REACHABLE, STALE, DELAY, or PROBE.

   SLAAC:  IPv6 Stateless Address Autoconfiguration [RFC4862].

   NS:  Neighbor Solicitation [RFC4861].

   NA:  Neighbor Advertisement [RFC4861].

   RS:  Router Solicitation [RFC4861].

   RA:  Router Advertisement [RFC4861].

   SLLAO:  Source Link-Layer Address Option.  An option in the ND
      packets containing the link-layer address of the sender of the
      packet [RFC4861].

   TLLAO:  Target Link-Layer Address Option.  An option in the ND
      packets containing the link-layer address of the target [RFC4861].

   GUA:  Global Unicast Address [RFC4291].

   DAD:  Duplicate Address Detection [RFC4862].

   Preferred Address:  An address assigned to an interface whose
      uniqueness has been verified using DAD and whose use by upper-
      layer protocols is unrestricted [RFC4862].  Preferred addresses
      may be used as the source address of packets sent from the
      interface.

   Optimistic DAD:  A modification of DAD [RFC4429].

2.  Problem Statement

   The most typical scenario when the problem described in this document
   may arise is a host joining the network, forming a new address, and
   using that address for accessing the Internet:

   1.  A host joins the network and receives a Router Advertisement (RA)
       packet from the first-hop router (either a periodic unsolicited
       RA or a response to a Router Solicitation sent by the host).  The
       RA contains information the host needs to perform SLAAC and to
       configure its network stack.  The RA is sent from the router's
       link-local address to a link-local destination address and may
       contain the link-layer address of the router.  As a result, the
       host can populate its Neighbor Cache with the router's link-local
       and link-layer addresses.

   2.  The host starts opening connections to off-link destinations.  A
       very common use case is a mobile device sending probes to detect
       Internet connectivity and/or the presence of a captive portal on
       the network.  To speed up that process, many implementations use
       Optimistic DAD, which allows them to send probes before the DAD
       process is completed.  At that moment, the device's Neighbor
       Cache contains all information required to send those probes
       (such as the default router link-local and link-layer addresses).
       The router's Neighbor Cache, however, might contain an entry for
       the device's link-local address (if the device has been
       performing address resolution for the router's link-local
       address), but there are no entries for any of the device's global
       addresses.

   3.  Return traffic is received by the first-hop router.  As the
       router does not have any cache entry for the host's global
       address yet, the router starts the Neighbor Discovery process by
       creating an INCOMPLETE cache entry and then sending a Neighbor
       Solicitation to the solicited-node multicast address
       (Section 7.3.2 of [RFC4861]).  As per Section 7.2.2 of [RFC4861],
       routers MUST buffer at least one data packet and MAY buffer more,
       while resolving the packet destination address.  However, most
       router implementations limit the buffer size to a few packets
       only, and some implementations are known to buffer just one
       packet.  So, any subsequent packets arriving before the address
       resolution process is completed cause packet loss by replacing
       older packets in the buffer.

   4.  If the host sends multiple probes in parallel, in the worst case,
       it would consider all but one of them failed.  That leads to
       user-visible delay in connecting to the network, especially if
       the host implements some form of backoff mechanism and does not
       retransmit the probes as soon as possible.

   This scenario illustrates the problem occurring when the device
   connects to the network for the first time or after an inactivity
   period long enough for the device's address to be removed from the
   router's Neighbor Cache.  However, the same sequence of events
   happens when the host starts using a new global address previously
   unseen by the router, such as a new privacy address [RFC8981] or if
   the router's Neighbor Cache has been flushed.

   While in dual-stack networks this problem might be hidden by Happy
   Eyeballs [RFC8305], it manifests quite clearly in IPv6-only
   environments, especially wireless environments, leading to poor user
   experience and contributing to a negative perception of IPv6-only
   solutions as unstable and non-deployable.

3.  Solution Requirements

   It would be highly desirable to improve the Neighbor Discovery
   mechanics so routers have a usable cache entry for a host address by
   the time the router receives the first packet for that address.  In
   particular:

   *  If the router does not have a Neighbor Cache entry for the
      address, a STALE entry needs to be created proactively, prior to
      arrival of the first packet intended for that address.

   *  The solution needs to work for Optimistic Addresses as well.
      Devices implementing Optimistic DAD usually attempt to minimize
      the delay in connecting to the network and therefore are more
      likely to be affected by the problem described in this document.

   *  In the case of duplicate addresses present in the network, the
      solution should not override the existing entry.

   *  In topologies with multiple first-hop routers, the cache needs to
      be updated on all of them, as traffic might be asymmetric:
      outgoing flows leaving the network via one router while the return
      traffic enters the segment via another one.

   In addition, the solution must not exacerbate issues described in
   [RFC6583] and needs to be compatible with the recommendations
   provided in [RFC6583].

4.  Changes to Neighbor Discovery

   The following changes are required to minimize the delay in creating
   new entries in a router's Neighbor Cache:

   *  A node sends unsolicited NAs upon assigning a new IPv6 address to
      its interface.

   *  A router creates a new cache entry upon receiving an unsolicited
      NA from a host.

   The following sections discuss these changes in more detail.
   Normative changes are specified in Section 6.

4.1.  Nodes Sending Gratuitous Neighbor Advertisements

   Section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor
   Advertisements to inform node neighbors of the new link-layer address
   quickly.  The same mechanism could be used to notify the node
   neighbors about the new network-layer address as well: the node can
   send unsolicited Neighbor Advertisements upon assigning a new IPv6
   address to its interface.

   To minimize potential disruption in the case of duplicate addresses,
   the node should not set the Override flag for a preferred address and
   must not set the Override flag if the address is in the Optimistic
   state [RFC4429].

   As the main purpose of sending unsolicited NAs upon configuring a new
   address is to proactively create a Neighbor Cache entry on the first-
   hop routers, the gratuitous NAs are sent to the all-routers multicast
   address (ff02::2).  Limiting the recipients to routers only would
   help reduce the multicast noise level.  If the link-layer devices are
   performing Multicast Listener Discovery (MLD) snooping [RFC4541],
   then those unsolicited NAs will only be sent to routers on the given
   network segment/link, instead of being flooded to all nodes.

   It should be noted that the mechanism discussed here does not cause
   any significant increase in multicast traffic.  The additional
   multicast unsolicited NAs would proactively create a STALE cache
   entry on the router, as discussed below.  When the router receives
   the return traffic flows, it does not need to send multicast NSes to
   the solicited-node multicast address but would send unicast NSes
   instead.  Therefore, this procedure would only produce an increase in
   the overall amount of multicast traffic if no return traffic arrives
   for the address that sent the unsolicited NA or if the router does
   not create a STALE entry upon receiving such an NA.  The increase
   would be negligible, as that additional traffic is a few orders of
   magnitude less than the usual level of Neighbor Discovery multicast
   traffic.

4.2.  Routers Creating Cache Entries upon Receiving Unsolicited Neighbor
      Advertisements

   Section 7.2.5 of [RFC4861] states:

   |  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.

   The reasoning behind dropping unsolicited Neighbor Advertisements
   ("the recipient has apparently not initiated any communication with
   the target") is valid for on-link host-to-host communication but, as
   discussed in Section 1, it does not really apply to the scenario when
   the host is announcing its address to routers.  Therefore, it would
   be beneficial to allow routers to create new entries upon receiving
   an unsolicited Neighbor Advertisement.

   This document updates [RFC4861] so that routers create a new Neighbor
   Cache entry upon receiving an unsolicited Neighbor Advertisement for
   an address that does not already have a Neighbor Cache entry.  These
   changes do not modify the router behavior specified in [RFC4861] for
   the scenario when the corresponding Neighbor Cache entry already
   exists.

   The next section analyzes various scenarios of duplicate addresses
   and discusses the potential impact of creating a STALE entry for a
   duplicate IPv6 address.

5.  Avoiding Disruption

   If nodes following the recommendations in this document are using the
   DAD mechanism defined in [RFC4862], they would send unsolicited NAs
   as soon as the address changes state from tentative to preferred
   (after its uniqueness has been verified).  However, nodes willing to
   minimize network stack configuration delays might be using Optimistic
   Addresses, which means there is a possibility of the address not
   being unique on the link.  Section 2.2 of [RFC4429] discusses
   measures to ensure that ND packets from the Optimistic Address do not
   override any existing Neighbor Cache entries, as it would cause
   interruption of the rightful address owner's traffic in the case of
   an address conflict.  Nodes that are willing to speed up their
   network stack configuration are most likely to be affected by the
   problem outlined in this document; therefore, it seems reasonable for
   such hosts to advertise their Optimistic Addresses by sending
   unsolicited NAs.  The main question to consider is the potential risk
   of overriding the cache entry for the rightful address owner if the
   Optimistic Address happens to be a duplicate.

   The following sections discuss the address collision scenario when a
   node sends an unsolicited NA for an address in the Optimistic state,
   while another node (the rightful owner) already has the same address
   assigned.  This document uses the term "the rightful owner", as the
   same terminology is used in [RFC4429].  The analysis assumes that the
   host performs DAD, as Section 5.4 of [RFC4862] requires that DAD MUST
   be performed on all unicast addresses prior to assigning them to an
   interface.

5.1.  Neighbor Cache Entry Exists in Any State Other Than INCOMPLETE

   If the router's Neighbor Cache entry for the target address already
   exists in any state other than INCOMPLETE, then as per Section 7.2.5
   of [RFC4861], an unsolicited NA with the Override flag cleared would
   change the entry state from REACHABLE to STALE but would not update
   the entry in any other way.  Therefore, even if the host sends an
   unsolicited NA from its Optimistic Address, the router's cache entry
   would not be updated with the new link-layer address, and no impact
   on the traffic for the rightful address owner is expected.

   The return traffic intended for the host with the Optimistic Address
   would be sent to the rightful owner.  However, this is unavoidable
   with or without the unsolicited NA mechanism.

5.2.  Neighbor Cache Entry Is in INCOMPLETE State

   Another corner case is the INCOMPLETE cache entry for the address.

   1.  The router receives a packet for the rightful owner of the
       address.

   2.  The router starts the address resolution process by creating an
       INCOMPLETE entry and sends the multicast NS.

   3.  More packets arrive at the router for the address in question.

   4.  The host configures an Optimistic Address and sends an
       unsolicited NA.

   5.  The router creates a STALE entry and sends the buffered packet(s)
       to the host (while at least some of those packets are actually
       intended for the rightful owner).

   6.  As the STALE entry was used to send packets, the router changes
       the entry state to DELAY and waits up to DELAY_FIRST_PROBE_TIME
       (5 seconds) [RFC4861] before sending a unicast NS.

   7.  The rightful owner responds to the multicast NS sent at Step 2
       with a solicited NA with the Override flag set.

   8.  The router updates the entry with the TLLAO supplied (the
       rightful owner's link-layer address) and sets the entry state to
       REACHABLE (as the NA has the Solicited flag set).

   As a result, some packets (packets in the buffer at Step 6 and all
   packets arriving between Step 6 and Step 8) are delivered to the host
   with the Optimistic Address, while some of them, if not all, are
   intended for the rightful owner.  Without the unsolicited NA, one or
   more packets that are in the buffer at Step 8 (usually just one
   packet, but some routers may buffer a few) would have been delivered
   to the rightful owner and the rest of the packets would have been
   dropped.  However, the probability of such a scenario is rather low,
   as it would require the following things to happen almost
   simultaneously (within tens of milliseconds in most cases):

   *  One host starts using a new IPv6 address and sending traffic
      without sending an unsolicited NA first.

   *  Another host configures the same IPv6 address in Optimistic mode
      before the router completes the address resolution process for the
      rightful owner.

   It should be noted that in this scenario the rightful owner does not
   send any unsolicited NAs before sending packets.  If the rightful
   owner implements the functionality described in this document and
   sends unsolicited NAs upon configuring its address, then the router
   creates a STALE entry for the address, causing all packets to be
   delivered to the rightful owner (see Section 5.1).  The rightful
   owner would experience no disruption but might receive some packets
   intended for the host with an Optimistic Address.

   This section focuses on the scenario when the solicited NA from the
   rightful owner arrives after the unsolicited one sent from the
   Optimistic Address (Step 7 and Step 4, respectively).  If the
   solicited NA arrives first, it changes the NC entry state from
   INCOMPLETE to REACHABLE.  As discussed in Section 5.1, there will be
   no disruption for the rightful owner if the router already has a
   REACHABLE entry for the address when an unsolicited NA is received.

5.3.  Neighbor Cache Entry Does Not Exist

   There are two distinct scenarios that can lead to the situation when
   the router does not have an NC entry for the IPv6 address:

   1.  The rightful owner of the address has not been using it for off-
       link communication recently or has never used it at all.

   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 duplicate address.
       As the NC entry is STALE, it sends traffic using that entry,
       changes it to DELAY, and waits up to DELAY_FIRST_PROBE_TIME
       seconds [RFC4861].

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

   7.  As the host has already detected the address conflict, it does
       not respond to the unicast NSes.  (It is unlikely that the host
       has not completed the DAD process at this stage, as
       DELAY_FIRST_PROBE_TIME (5 seconds) is much higher than the DAD
       duration (DupAddrDetectTransmits*RetransTimer*1000 +
       MAX_RTR_SOLICITATION_DELAY seconds) (Section 5.4 of [RFC4862]).)
       The default value for the DAD process would be 1*1*1000 + 1 = 2
       seconds [RFC4861].  If the host has completed DAD but did not
       detect the address conflict, then there are two hosts with the
       same address in the preferred state and disruption is inevitable
       anyway.

   8.  As the router receives no response for the unicast NSes, it
       deletes the NC entry.

   9.  If return packets for communication initiated at Step 2 are still
       arriving, the router buffers a small number of those packets and
       starts the address resolution process again by sending a
       multicast NS to the solicited-node multicast address.  The
       rightful owner responds, and the router's NC entry is updated
       with the rightful owner's link-local address.  The buffered
       packet or packets are sent to that address.  Any packets still
       arriving after the address resolution process has completed are
       sent to the rightful address owner as well.

   The rightful owner is not experiencing any disruption, as it does not
   send any traffic.  It would only start receiving packets intended for
   another host after Step 8 is completed and only if return packets for
   the communication initiated at Step 2 are still arriving.

   However, the same behavior would be observed if the changes specified
   in this document are not implemented.  If the host starts sending
   packets from its Optimistic Address but then detects that the address
   is a duplicate, the first return packet would trigger the address
   resolution process and would be buffered until the resolution is
   completed.  The buffered packet(s) and any packets still arriving
   after the address is resolved would be forwarded to the rightful
   owner of the address.  So, the rightful owner might still receive one
   or more packets from the flows intended for another host.  Therefore,
   it's safe to conclude that the changes specified in this document do
   not introduce any disruption for the rightful owner of the duplicated
   address.

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 for the IPv6 address in
        question.  Some flows are intended for the rightful owner of the
        duplicate address, while some are for the new host.  As the NC
        entry is STALE, it sends traffic using that entry, changes it to
        DELAY, and waits up to DELAY_FIRST_PROBE_TIME seconds [RFC4861].

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

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

   9.   As the router receives no response for the unicast NSes, it
        deletes the NC entry.

   10.  The next packet recreates the entry and triggers the resolution
        process.  The router buffers the packet and sends a multicast NS
        to the solicited-node multicast address.  The rightful owner
        responds, and the router's NC entry is updated with the rightful
        owner's link-local address.

   As a result, the traffic for the address of the rightful owner would
   be sent to the host with the duplicate 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 = 7000 milliseconds, or 7 seconds.

   However, it should be noted that the probability of such a scenario
   is rather low.  Similar to the scenario discussed in Section 5.2, it
   would require the following things to happen almost simultaneously
   (within tens of milliseconds in most cases):

   *  One host starts using a new IPv6 address and sending traffic
      without sending an unsolicited NA first.

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

   As discussed in Section 5.2, the disruption for the rightful owner
   can easily be prevented if that node implements the mechanism
   described in this document.  Sending unsolicited NAs before
   initiating off-link communication would create a STALE entry in the
   router's NC and prevent any traffic to that address from being sent
   to the host with the Optimistic Address (see Section 5.1).

6.  Modifications to RFC-Mandated Behavior

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

6.1.  Modification to RFC 4861 (Neighbor Discovery for IP version 6
      (IPv6))

6.1.1.  Modification to Section 7.2.5 of RFC 4861

   This document makes the following changes to Section 7.2.5 of
   [RFC4861]:

   The text in RFC 4861 is as follows:

   |  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.

   This document updates the text as follows:

   |  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's reachability state MUST be
   |     set to STALE.  If the received Neighbor Advertisement does not
   |     contain the Target Link-Layer Address Option, the advertisement
   |     SHOULD be silently discarded.

6.1.2.  Modification to Section 7.2.6 of RFC 4861

   This document makes the following changes to Section 7.2.6 of
   [RFC4861]:

   The text in RFC 4861 is as follows:

   |  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.

   This document updates the text as follows:

   |  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:
   |  If Optimistic DAD [RFC4429] is used:  A new Optimistic Address is
   |     assigned to the node interface.
   |  
   |  If Optimistic DAD is not used:  An address changes the state from
   |     tentative to preferred.

7.  Solution Limitations

   The solution described in this document provides some improvement for
   a node configuring a new IPv6 address and starting to send traffic
   from it.  However, that approach does not completely eliminate the
   scenario when a router receives some transit traffic for an address
   without the corresponding Neighbor Cache entry.  For example:

   *  If the host starts using an already-configured IPv6 address after
      a long period of inactivity, the router might not have the NC
      entry for that address anymore, as old/expired entries are
      deleted.

   *  Clearing the router's Neighbor Cache would trigger packet loss for
      all actively used addresses removed from the cache.

8.  Solutions Considered but Discarded

   There are other possible approaches to address the problem.  For
   example:

   *  Just do nothing.

   *  Migrate from the "reactive" Neighbor Discovery [RFC4861] to the
      registration-based mechanisms [RFC8505].

   *  Create new entries in the router's Neighbor Cache by gleaning from
      Neighbor Discovery DAD messages.

   *  Initiate bidirectional communication from the host to the router
      using the host GUA.

   *  Make the probing logic on hosts more robust.

   *  Increase the buffer size on routers.

   *  Transit data plane traffic from an unknown address (an address
      without the corresponding Neighbor Cache entry) to trigger 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.

8.1.  Do Nothing

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

   *  Unhappy users.

   *  Many support tickets.

   *  More resistance to deploying IPv6 and IPv6-only networks.

8.2.  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 IPv6 over Low-Power Wireless Personal
   Area Networks (6LoWPANs) to the rest of the IPv6 deployments.  This
   option requires some investigation and discussion.  However,
   significant changes to the existing IPv6 implementations would be
   needed, so an unclear adoption timeline makes this approach less
   preferable than the approach specified in this document.

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

   The host could force the creation of a STALE entry for its GUA in the
   router's Neighbor Cache by sending the following Neighbor
   Solicitation message:

   *  The NS source address is the host GUA.

   *  The destination address is the default router IPv6 address.

   *  The Source Link-Layer Address Option contains the host link-layer
      address.

   *  The target address is the host's default router address (the
      default router address the host received in the RA).

   The main disadvantages of this approach are as follows:

   *  It would not work for Optimistic Addresses, as Section 2.2 of
      [RFC4429] explicitly prohibits sending Neighbor Solicitations from
      an Optimistic Address.

   *  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.

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

8.4.  Host Sending Router Solicitation from Its GUA

   The host could send a Router Solicitation message to the 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
   Section 6.2.6 of [RFC4861].  However, this approach cannot be used if
   the GUA is in the 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 cause
   disruption for the rightful owner of the address in the case of a
   collision.  So, for the Optimistic Addresses, the host can send an RS
   without a SLLAO included.  In that case, the router may respond with
   either a multicast or unicast RA (only the latter would create a
   cache entry).

   This approach has the following drawbacks:

   *  If the address is in the Optimistic state, the RS cannot contain a
      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.

   *  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.

   *  Some wireless devices are known to intercept ND packets and
      perform various nonobvious forms of ND proxy actions.  In some
      cases, the RS might not even reach the routers.

8.5.  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 process for that entry.

   The same solution was proposed in [ND-ADDR-RES].  Some routing
   vendors already support such optimization.  However, this approach
   has a number of drawbacks and therefore should not be used as the
   only solution:

   *  Routers need to receive all multicast Neighbor Discovery packets;
      this might negatively impact a router's CPU.

   *  If the router starts the address resolution process as soon as it
      receives the DAD Neighbor Solicitation, the host might still be
      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 Section 5.4.3 of [RFC4862].
      As a result, the router might not be able to complete the address
      resolution process before the return traffic arrives.

8.6.  Initiating Host-to-Router 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 the 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:

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

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

   *  Unless the data packet is sent to the 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.

8.7.  Making the Probing Logic on Hosts More Robust

   Theoretically, the probing logic on hosts might be modified to better
   deal with initial packet loss.  For example, only one probe can be
   sent, or probe retransmit intervals can be reduced.  However, this
   approach has a number of drawbacks:

   *  It would require updating all possible applications that perform
      probing, while the solution described in this document is
      implemented at the operating-system level.

   *  Some implementations need to send multiple probes.  Examples
      include but are not limited to:

      -  Sending AAAA and A record DNS probes in parallel.

      -  Detecting captive portals, which often requires sending
         multiple packets.

   *  While it would increase the probability that the probing will
      complete successfully, there are multiple cases when packet loss
      would still occur:

      -  The probe response consists of multiple packets, so all but the
         first one are dropped.

      -  There are multiple applications on the same host sending
         traffic, and return packets arrive simultaneously.

      -  There are multiple first-hop routers in the network.  The first
         probe packet creates the NC entry on one of them.  The
         subsequent return traffic flows might cross other routers and
         still experience the issue.

   *  Reducing the probe retransmit interval unnecessarily increases
      network utilization and might cause network congestion.

8.8.  Increasing the Buffer Size on Routers

   Increasing the buffer size and buffering more packets would
   exacerbate issues described in [RFC6583] and make the router more
   vulnerable to ND-based denial-of-service attacks.

8.9.  Transit Data Plane Traffic from a New Address to Trigger Address
      Resolution

   When a router receives a transit packet sourced by an on-link
   neighbor node, it might check for the presence of a Neighbor Cache
   entry for the packet source address and, if the entry does not exist,
   start the 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 off-link traffic.  However,
   this approach has a number of limitations.  In particular:

   *  If traffic flows are asymmetrical, the return traffic might not
      transit the same router as the original traffic that triggered the
      address resolution process.  So, the Neighbor Cache entry is
      created on the "wrong" router, not the one that actually needs the
      Neighbor Cache entry for the host address.

   *  The functionality needs to be limited to explicitly configured
      networks/interfaces, as the router needs to distinguish between
      on-link addresses (addresses for which the router needs to have
      Neighbor Cache entries) and the rest of the address space.  The
      proactive address resolution process must only be triggered by
      packets from the prefixes known to be on-link.  Otherwise, traffic
      from spoofed source addresses or any transit traffic could lead to
      Neighbor Cache exhaustion.

   *  Implementing such functionality is much more complicated than all
      other solutions, as it would involve complex interactions between
      the data plane and the control plane.

9.  IANA Considerations

   This document has no IANA actions.

10.  Security Considerations

   One of the potential attack vectors to consider is cache spoofing,
   where 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 Neighbor Cache for a given IPv6
   address already exists.  Therefore, there are no new vectors for an
   attacker to override an existing cache entry.

   Section 5 describes some corner cases when a host with a duplicate
   Optimistic Address might get some packets intended for the rightful
   owner of the address.  However, such scenarios do not introduce any
   new attack vectors: even without the changes discussed in this
   document, an attacker can easily override the router's Neighbor Cache
   and redirect the traffic by sending NAs with the Solicited flag set.
   As discussed in Section 5.3.2, the worst-case scenario might cause a
   disruption for up to 7 seconds.  Because this scenario is highly
   unlikely, this risk of disruption is considered acceptable.  More
   importantly, for all cases described in Section 5, the rightful owner
   can prevent disruption caused by an accidental address duplication
   just by implementing the mechanism described in this document.  If
   the rightful owner sends unsolicited NAs before using the address,
   the STALE entry would be created on the router's NC, and any
   subsequent unsolicited NAs sent from the host with an Optimistic
   Address would not override the NC entry.

   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 the mechanism specified in this
   document does not increase the risk of such an attack: the attacker
   could do it, for example, by sending an NS or RS packet with a SLLAO
   included.  All recommendations from [RFC6583] still apply.

   Announcing a new address to the 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 if MLD snooping is not enabled.
   Network administrators can also mitigate this issue by enabling MLD
   snooping on the link-layer devices to prevent IPv6 link-local
   multicast packets from being flooded to all on-link nodes.  If peer-
   to-peer on-link communications are not desirable for a given network
   segment, they should be prevented by proper Layer 2 security
   mechanisms.  Therefore, the risk of allowing hosts to send
   unsolicited Neighbor Advertisements to the all-routers multicast
   address is low.

   It should be noted that the mechanism discussed in this document
   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 Neighbor
   Cache exhaustion DoS attacks as described in Section 4.3.2 of
   [RFC3756] and in [RFC6583].

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>.

   [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>.

   [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>.

11.2.  Informative References

   [ND-ADDR-RES]
              Chen, I. and J. Halpern, "Triggering ND Address Resolution
              on Receiving DAD-NS", Work in Progress, Internet-Draft,
              draft-halpern-6man-nd-pre-resolve-addr-00, 10 January
              2014, <https://datatracker.ietf.org/doc/html/draft-
              halpern-6man-nd-pre-resolve-addr-00>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   comments, review, and feedback: Mikael Abrahamsson, Stewart Bryant,
   Lorenzo Colitti, Roman Danyliw, Owen DeLong, Martin Duke, Igor
   Gashinsky, Carles Gomez, Fernando Gont, Tatuya Jinmei, Benjamin
   Kaduk, Scott Kelly, Erik Kline, Warren Kumari, Barry Leiba, Jordi
   Palet Martinez, Erik Nordmark, Michael Richardson, Dan Romascanu,
   Zaheduzzaman Sarker, Michael Scharf, John Scudder, Mark Smith, Dave
   Thaler, Pascal Thubert, Loganaden Velvindron, and Éric Vyncke.

Author's Address

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont NSW 2009
   Australia

   Email: furry@google.com