Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-Hop Routers
draft-ietf-6man-grand-01

IPv6 Maintenance                                              J. Linkova
Internet-Draft                                                    Google
Updates: 4861 (if approved)                                July 25, 2020
Intended status: Standards Track
Expires: January 26, 2021

Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-
                              Hop Routers
                        draft-ietf-6man-grand-01

Abstract

   Neighbor Discovery (RFC4861) 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 RFC4861 to
   allow routers to proactively create a Neighbor Cache entry when a new
   IPv6 address is assigned to a node.  It also updates RFC4861 and
   recommends nodes to send unsolicited Neighbor Advertisements upon
   assigning a new IPv6 address.  The proposed change will minimize the
   delay and packet loss when a node initiate connections to off-link
   destination from a new IPv6 address.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 26, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Proposed Changes to Neighbor Discovery  . . . . . . . . . . .   4
     2.1.  Nodes Sending Gratuitous Neighbor Advertisements  . . . .   4
     2.2.  Routers Creating Cache Entries Upon Receiving Unsolicited
           Neighbor Advertisements . . . . . . . . . . . . . . . . .   5
   3.  Avoiding Disruption . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Neighbor Cache Entry Exists in Any State Other That
           INCOMPLETE  . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Neighbor Cache Entry is in INCOMPLETE state . . . . . . .   6
     3.3.  Neighbor Cache Entry Does Not Exist . . . . . . . . . . .   6
       3.3.1.  The Rightful Owner Is Not Sending Packets From The
               Address . . . . . . . . . . . . . . . . . . . . . . .   7
       3.3.2.  The Rightful Owner Has Started Sending Packets From
               The Address . . . . . . . . . . . . . . . . . . . . .   7
   4.  Modifications to RFC-Mandated Behavior  . . . . . . . . . . .   9
     4.1.  Modification to RFC4861 Neighbor Discovery for IP version
           6 (IPv6)  . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.1.1.  Modification to the section 7.2.5 . . . . . . . . . .   9
       4.1.2.  Modification to the section 7.2.6 . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

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

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   However when a host sends traffic to off-link destinations a
   different scenario is observed.  After 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 [RFC4861].  All subsequent packets arriving before
   the resolution process finishes are likely to be dropped.  It might
   cause user-visible packet loss and performance degradation.

   The detailed problem statement and the various solution approaches
   could be found in [I-D.ietf-v6ops-nd-cache-init].  This document
   summarizes the proposed neighbor discovery updates to address the
   issue.

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].

   SLAAC: IPv6 Stateless Address Autoconfiguration, [RFC4862].

   NS: Neighbor Solicitation, [RFC4861].

   NA: Neighbor Advertisement, [RFC4861].

   RS: Router Solicitation, [RFC4861].

   RA: Router Advertisement, [RFC4861].

   SLLA: Source link-layer Address, an option in the ND packets
   containing the link-layer address of the sender of the packet
   [RFC4861].

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   TLLA: Target link-layer Address, 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].

   Optimistic DAD: a modification of DAD, [RFC4429].

2.  Proposed Changes to Neighbor Discovery

   The following changes are proposed to minimize the delay in creating
   new entries in a router neighbor cache

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

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

   The following sections discuss these changes in more detail.

2.1.  Nodes Sending Gratuitous Neighbor Advertisements

   The section 7.2.6 of [RFC4861] discusses using unsolicited Neighbor
   Advertisement 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 gratuitous unsolicited Neighbor Advertisements upon assigning a
   new IPv6 address to its interface.

   To minimize the potential disruption in 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 Optimistic
   [RFC4429] state.

   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 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 MLD snooping [RFC4541] then those unsolicited NAs will be
   only sent to onlink routers instead of being flooded to all nodes.

   It should be noted that the proposed mechanism does not cause any
   significant increase in the multicast traffic.  The additional
   multicast unsolicited NA would proactively create a STALE cache entry
   on routers as discussed below.  When the router receives the return

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   traffic flows it does not need to send multicast NSes to the
   solicited node multicast address but would be sending unicast NSes
   instead.  Therefore total amount of multicast traffic should not
   increase.

2.2.  Routers Creating Cache Entries Upon Receiving Unsolicited Neighbor
      Advertisements

   The 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 onlink host-to-host communication but, as
   discussed in [I-D.ietf-v6ops-nd-cache-init] it does not really apply
   for the scenario when the host is announcing its address to routers.
   Therefore it would be beneficial to allow routers creating 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.
   The proposed changes do not modify routers behaviour specified in
   [RFC4861] for the scenario when the corresponding Neighbor Cache
   entry already exists.

3.  Avoiding Disruption

   If hosts following the recommendations in this document are using the
   DAD mechanism defined in [RFC4862], they would send unsolicited NA as
   soon as the address changes the state from tentative to preferred
   (after its uniqueness has been verified).  However hosts 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.  The 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
   traffic interruption of the rightful address owner in case of address
   conflict.  As hosts willing to speed up their network stack
   configuration are most likely to be affected by the problem outlined
   in this document 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 duplicated.

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   The following sections are discussing the address collision scenario
   when a host sends an unsolicited NA for an address in the Optimistic
   state, while another host has the same address assigned already.

3.1.  Neighbor Cache Entry Exists in Any State Other That INCOMPLETE

   If the router 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 the its Optimistic address the router cache entry
   would not be updated with the new Link-Layer address and no impact to
   the traffic for the rightful address owner is expected.

3.2.  Neighbor Cache Entry is in INCOMPLETE state

   Another corner case is the INCOMPLETE cache entry for the address.
   If the host sends an unsolicited NA from the Optimistic address it
   would update the entry with the host link-layer address and set the
   entry to the STALE state.  As the INCOMPLETE entry means that the
   router has started the ND process for the address and the multicast
   NS has been sent, the rightful owner is expected to reply with
   solicited NA with the Override flag set.  Upon receiving a solicited
   NA with the Override flag the cache entry will be updated with the
   TLLA supplied and (as the NA has the Solicited flag set), the entry
   state will be set to REACHABLE.  It would recover the cache entry and
   set the link-layer address to the one of the rightful owner.  The
   only potential impact would be for packets arriving to the router
   after the unsolicited NA from the host but before the rightful owner
   responded with the solicited NA.  Those packets would be sent to the
   host with the optimistic address instead of its rightful owner.
   However those packets would have been dropped anyway as until the
   solicited NA is received the router can not send the traffic.

3.3.  Neighbor Cache Entry Does Not Exist

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

   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.

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3.3.1.  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
   address.  Therefore it's safe to conclude that the proposed changes
   do not cause any disruption for the rightful owner.

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

   In this scenario the following events are happening:

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   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.

   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.

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4.  Modifications to RFC-Mandated Behavior

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

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

4.1.1.  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.

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

4.1.2.  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.

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   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.

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

5.  IANA Considerations

   This memo asks the IANA for no new parameters.

6.  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.

   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.

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   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].

7.  Acknowledgements

   Thanks to the following people (in alphabetical order) for their
   comments, review and feedback: Lorenzo Colitti, Fernando Gont, Tatuya
   Jinmei, Erik Kline, Warren Kumari, Erik Nordmark, Michael Richardson,
   Mark Smith, Dave Thaler, Pascal Thubert, Loganaden Velvindron, Eric
   Vyncke.

8.  References

8.1.  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>.

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   [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>.

8.2.  Informative References

   [I-D.ietf-v6ops-nd-cache-init]
              Linkova, J., "Neighbor Cache Entries on First-Hop Routers:
              Operational Considerations", draft-ietf-v6ops-nd-cache-
              init-03 (work in progress), July 2020.

   [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>.

Author's Address

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

   Email: furry@google.com

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