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

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IPv6 Maintenance                                              J. Linkova
Internet-Draft                                                    Google
Updates: 4861 (if approved)                            November 26, 2019
Intended status: Standards Track
Expires: May 29, 2020

Gratuitous Neighbor Discovery: Creating Neighbor Cache Entries on First-
                              Hop Routers
                      draft-linkova-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 host.  It also updates [RFC4862]
   and recommends hosts to send unsolicited Neighbor Advertisements upon
   assigning a new IPv6 address.  The proposed change will minimize the
   delay and packet loss when a host 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 May 29, 2020.

Copyright Notice

   Copyright (c) 2019 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
   carefully, as they describe your rights and restrictions with respect
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Proposed Changes to Neighbor Discovery  . . . . . . . . . . .   4
     2.1.  Hosts Sending Gratuitous Neighbor Advertisements  . . . .   4
     2.2.  Routers Creating Cache Entries Upon Receiving Unsolicited
           Neighbor Advertisements . . . . . . . . . . . . . . . . .   4
   3.  Avoiding Disruption . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Neighbor Cache Entry Exists in Any State Other That
           INCOMPLETE  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Neighbor Cache Entry Does Not Exist . . . . . . . . . . .   5
     3.3.  Neighbor Cache Entry is in INCOMPLETE state . . . . . . .   6
   4.  Modifications to RFC-Mandated Behavior  . . . . . . . . . . .   6
     4.1.  Modification to RFC4861 Neighbor Discovery for IP version
           6 (IPv6)  . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  Modification to the section 7.2.5 . . . . . . . . . .   6
       4.1.2.  Modification to the section 7.2.6 . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

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

   However when a host sends traffic to off-link destinations the
   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

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   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 [RFC4861] Section 7.2.2.  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 various solution approaches could
   be found in [I-D.ietf-v6ops-nd-cache-init].  This document summarized
   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

   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].

   LLA: Link-Layer Address.

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

   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].

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   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 host SHOULD send unsolicited NAs upon assigning a new IPv6
      address to its interface.

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

   The following sections discuss these changes in more detail.

2.1.  Hosts 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 host
   neighbors about the new network-layer address as well: the host can
   send gratuitous unsolicited Neighbor Advertisements upon assigning a
   new global IPv6 address to its interface.

   To minimize the potential disruption in case of duplicate addresses
   the host 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 SHOULD be sent to all-routers
   multicast address (ff02::2).  Limiting the recipients to routers only
   would help reduce the multicast noise level.

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

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   discussed in [I-D.ietf-v6ops-nd-cache-init] 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 suggests that routers SHOULD create a new Neighbor
   Cache entry when receive an unsolicited Neighbor Advertisement.

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 GUAs 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.

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 Does Not Exist

   If there is no entry then it would be created/updated with the
   supplied LLA and its state set to STALE.  In that case as soon as the
   entry is used for sending traffic to the host, the entry state will
   be changed to DELAY and the Neighbor Unreachability Detection would
   be started and the rightful owner LLA will be entered in the cache.
   So in the scenario when the rightful owner does not use the address
   for communication then it might be a short (a few seconds) period of

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   time when the data packets sent from the outside could reach the host
   with the optimistic address.  However it seems likely that hosts
   using Optimistic DAD would start sending/receiving traffic right
   away, so the first return packet would trigger the NUD process and
   rewrite the cache.

3.3.  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 LLA 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 would recover the cache entry and set the
   LLA 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.

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.

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   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:

   In such cases, a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT
   unsolicited Neighbor Advertisement messages to the all-nodes
   multicast address.  These advertisements MUST be separated by at
   least RetransTimer seconds.

   NEW TEXT:

   In such cases, a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT
   unsolicited Neighbor Advertisement messages to the all-nodes
   multicast address.  These advertisements MUST be separated by at
   least RetransTimer seconds.

   A host 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 host 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:

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

   o  if Optimistic DAD is not used: a GUA 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.

   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 it falls into 'Security through
   obscurity' category.  If peer-to-peer onlink communications are not
   desirable 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.

7.  Acknowledgements

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

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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>.

   [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-00 (work in progress), October 2019.

   [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>.

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Author's Address

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

   Email: furry@google.com

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