6man WG                                                      E. Nordmark
Internet-Draft                                           Arista Networks
Updates: 4861 (if approved)                               A. Yourtchenko
Intended status: Standards Track                                   Cisco
Expires: April 30, 2015                                      S. Krishnan
                                                        October 27, 2014

         IPv6 Neighbor Discovery Optional Unicast RS/RA Refresh


   IPv6 Neighbor Discovery relies on periodic multicast Router
   Advertisement messages to update timer values and to distribute new
   information (such as new prefixes) to hosts.  On some links the use
   of periodic multicast messages to all host becomes expensive, and in
   some cases it results in hosts waking up frequently.  Many
   implementations of RFC 4861 also use multicast for solicited Router
   Advertisement messages, even though that behavior is optional.

   This specification provides an optional mechanism for hosts and
   routers where instead of periodic multicast Router Advertisements the
   hosts are instructed (by the routers) to use unicast Router
   Solicitations to request refreshed Router Advertisements.  This
   mechanism is enabled by configuring the router to include a new
   option in the Router Advertisement in order to allow the network
   administrator to choose host behavior based on whether periodic
   multicast are more efficient on their link or not.  The routers can
   also tell whether the hosts are capable of the new behavior through a
   new flag in the Router Solicitations.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on April 30, 2015.

Copyright Notice

   Copyright (c) 2014 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
   (http://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
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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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Goals and Requirements . . . . . . . . . . . . . . . . . . . .  5
   3.  Definition Of Terms  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  5
   5.  New Neighbor Discovery Flags and Options . . . . . . . . . . .  6
     5.1.  Introducing a Router Solicitation Flag . . . . . . . . . .  6
     5.2.  Refresh Time option  . . . . . . . . . . . . . . . . . . .  6
   6.  Conceptual Data Structures . . . . . . . . . . . . . . . . . .  7
   7.  Host Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  7
     7.1.  Sleep and Wakeup . . . . . . . . . . . . . . . . . . . . .  8
     7.2.  Movement . . . . . . . . . . . . . . . . . . . . . . . . .  8
   8.  Router Behavior  . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Router and/or Interface Initialization . . . . . . . . . .  9
     8.2.  Periodic Multicast RA for unmodified hosts . . . . . . . .  9
     8.3.  Unsolicited RAs to share new information . . . . . . . . .  9
   9.  Router Advertisement Consistency . . . . . . . . . . . . . . . 10
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   13. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     14.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   IPv6 Neighbor Discovery [RFC4861] was defined at a time when local
   area networks had different properties than today.  A common link was
   the yellow-coax shared wire Ethernet, where a link-layer multicast
   and unicast worked the same - send the packet on the wire and the
   interested receivers will pick it up.  Thus the network cost
   (ignoring any processing cost on the receivers that might not
   completely filter out Ethernet multicast addresses that they did not
   want) and the reliability of sending a link-layer unicast and
   multicast was the same.  Furthermore, the hosts at the time was
   always on and connected.  Powering on and off the workstation/PC
   hosts at the time was slow and disruptive process.

   Under the above assumptions it was quite efficient to maintain the
   shared state of the link such as the prefixes and their lifetimes
   using periodic multicast Router Advertisement messages.  It was also
   efficient to use multicast Neighbor Solicitations for address
   resolution as a slight improvement over the broadcast use in ARP.
   And finally, checking for a potential duplicate IPv6 address using
   multicast was efficient and natural.

   There are still links, such a satellite links, where periodic
   multicast advertisements is the most efficient and reliable approach
   to keep the hosts up to date.  However other links have different
   performance and reliability for multicast than for unicast (see for
   instance [I-D.vyncke-6man-mcast-not-efficient] which discusses WiFi
   links).  Cellular networks which employ paging and support sleeping
   hosts have different issues (see e.g.,
   [I-D.garneij-6man-nd-m2m-issues] that would benefit from having the
   hosts wake up and request information from the routers instead of the
   routers periodically multicasting the information.

   Since different links types and deployments have different needs,
   this specification provides mechanism by which the routers can
   determine whether all the hosts support the RS refresh, and the hosts
   only employ the RS refresh when instructed by the routers using an
   option in the Router Advertisement.

   The operator retains the option to use unsolicited multicast Router
   Advertisement to announce new or removed information.  That can be
   useful for uncommon cases while allowing using a higher refresh time
   for normal network operations.

   The specification does not assume that all hosts on the link
   implement the new capability.  As soon as there are router(s) on a
   link which supports these optimizations, then the updated hosts on
   the link can sleep better, while co-existing on the same link with

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   unmodified hosts.

2.  Goals and Requirements

   The key goal is to allow the operator to choose whether unicast RS
   refresh is more efficient than periodic multicast RAs, while
   preserving the timely and scalable reconfiguration capabilities that
   a periodic RA model provides.

   In addition, an operator might want to be notified whether the link
   includes hosts that do not support the new mechanism.  Potential
   router implementations can react dynamically to that information, or
   can log events to system management when hosts appear which do not
   implement this new capability.

   The assumption is that host which implement this specification also
   implement [I-D.ietf-6man-resilient-rs] as that ensures resiliency to
   packet loss at host initialization.

3.  Definition Of Terms

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

4.  Protocol Overview

   The hosts include a new flag in the Router Solicitation message,
   which allows the routers to report to system management whether there
   are hosts that do not support the RS refresh on the link.

   If the network administrator has configured the routers to send the
   new Refresh Timer option, then the option will be included in all the
   Router Advertisements.  This option includes the time interval when
   the hosts should unicast Router Solicitations.

   The host maintains the value of the Refresh Timer option (RTO) by
   recording it in the default router list.  A value of zero can be used
   to indicate that a router did not include a Refresh Timer option.

   The host calculates a timeout after it has received a RTO - either
   per router or per link.  If it is maintained per link then the host
   SHOULD use the minimum Refresh Timer it has received from the routers
   on the link.  The timeout is a random value uniformly distributed
   between 0.5 and 1.5 times the Refresh Timer value (in order to avoid

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   synchronization of the timers across hosts.  [TBD: Add SYNC reference
   from RFC 4861.]  When this timer fires the host sends one unicast
   Router Solicitation to the router (if maintained per router) or to
   all the routers in the default router list for link (if maintained
   per link.)

5.  New Neighbor Discovery Flags and Options

   This specification introduces a option used in the RAs which both
   indicates that the router can handle RS refresh using unicast RA, and
   a flag for the RS that indicates to the router that the host will do
   RS refresh if the router so wishes.

5.1.  Introducing a Router Solicitation Flag

   A node which implements this specification sets the R flag in all the
   Router Solicitation messages it sends.  That allows the router to
   determine whether there are legacy hosts on the link.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |     Code      |          Checksum             |
   |R|                          Reserved                           |

   New fields:

   R-flag:        When set indicates that the sending node is capable of
                  doing unicast RS refresh.

   Reserved:      Field is reduced from 32 bits to 31 bits.  It MUST be
                  initialized to zero by the sender and MUST be ignored
                  by the receiver.

5.2.  Refresh Time option

   A router which implements this specification can be configured to
   operate without periodic multicast Router Advertisements.  When the
   operator configures this mode of operation, then the router MUST
   include this new option in the RA.

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |   Length=1    |          Refresh Time         |
   |                            Reserved                           |


   Type:          TBD ND option code value (IANA)

   Length:        8-bit unsigned integer.  The length of the option
                  (including the type and length fields) in units of 8
                  bytes.  The value 0 is invalid.  Value is 1 for this

   Refresh Time:  16-bit unsigned integer.  Units is seconds.  The all-
                  ones value (65535) means infinite.

   Reserved:      32 bits.  This field is unused.  It MUST be
                  initialized to zero by the sender and MUST be ignored
                  by the receiver.

6.  Conceptual Data Structures

   In addition to the Conceptual Data structures in [RFC4861] a host
   records the received RTO value in the default router list.  It also
   maintains a timeout - either per link or per default router.

7.  Host Behavior

   See Protocol Overview section above.

   A host implementing this specification SHOULD also implement
   [I-D.ietf-6man-resilient-rs].  That ensures that a router that has
   been configured to not send periodic RA messages will always receive
   an RS from the host as the host initializes.

   If there is no RTO in the received Router Advertisements, then the
   host behavior does not change.  However, if RTOs start appearing in
   RAs after the initial RAs, the host SHOULD start performing RS
   refresh.  As the last router that included RTO options time out from
   the default router list, the host SHOULD stop sending RS refresh

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   The host MUST join the all-nodes multicast address as in [RFC4861]
   since the routers MAY send multicast RAs for important changes.

   Some links might have routers with different configuration where some
   router includes RTO in the RA and others do not.  Hosts MAY make the
   simplifying assumption that if any router on the link includes RTO
   then the host can use RS refresh to all the routers in the default
   router list.  Also, the routers might advertise different refresh
   time, and hosts MAY use the minimum of the time received from any
   router that remains in the default router list.  Note that section
   Section 9 says that routers SHOULD report such inconsistencies to
   system management.

7.1.  Sleep and Wakeup

   The protocol allows the sleepy nodes to complete its sleep schedule
   without waking up due to multicast Router Advertisement messages and
   the host is not required to wake up solely for the purposes of
   performing RS refresh.  This assumes that sleepy nodes perform a RS
   refresh when they wake up.  If hosts do wake up due to multicast RAs,
   then the host only needs to perform a refresh on wakeup if the
   Refresh timeout has expired while the host was sleeping.

7.2.  Movement

   When a host wakes up it can combine movement detecting (DNA), NUD,
   and refreshing its prefixes etc by sending a unicast RS to each of
   its existing default router(s).  If it receives unicast RA from a
   router, then it can mark the router as REACHABLE.

   Note that DNA [RFC6059] specifies using NS messages since many IPv6
   routers delay (and multicast) solicited RAs and DNA wants to avoid
   that delay.  Routers which implement this specification SHOULD
   unicast solicited RAs, hence if a router included the RTO then the
   host can use RS for DNA.  For non-RTO routers the host MAY choose to
   use NS for DNA as in [RFC6059].

8.  Router Behavior

   See Protocol Overview section.

   A router implementing this specification (and including RTO in the
   RAs) SHOULD also respond to unicast RS messages (that do not have an
   unspecified source address) with unicast RAs.  If a RS message has an
   unspecified source address then the host MAY respond with a RA
   unicast at layer 2 (sent to the link-layer source address of the RS),
   or it MAY follow the rate-limited multicast RA procedure in

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   The RECOMMENDED default configuration for routers is to have RTO

8.1.  Router and/or Interface Initialization

   This specification does not change the initialization procedure.
   Thus a router multicasts some initial Router Advertisements
   (MAX_INITIAL_RTR_ADVERTISEMENTS) at system startup or interface
   initialization as specified in [RFC4861] and its updates.

8.2.  Periodic Multicast RA for unmodified hosts

   By default a router MUST send periodic multicast RAs as specified in
   [RFC4861].  A router can be configured to omit those, which can be
   used in particular deployments.  If they are omitted, then there MUST
   be a mechanism to prevent or detect the existence of unmodified hosts
   on the link.  That be be performed at deployment time (e.g., only
   hosts which are known to support RTO are configured with the layer 2
   security keys), or the routers detect any RSs which do not include
   the R-flag and report this to system management, or dynamically
   enable periodic multicast RAs when observing at least one RS without
   the R-flag.

   Note that such dynamic detection is not bullet proof.  If a host does
   not implement RS refresh nor implements resilient RS
   [I-D.ietf-6man-resilient-rs], then the host might receive a multicast
   RA (from router initialization or the periodic multicast RAs) without
   the router ever receiving a RS from the host.  Such a host would
   function as long as the routers are sending periodic multicast RAs.

8.3.  Unsolicited RAs to share new information

   When a router has new information to share (new prefixes, prefixes
   that should be immediately deprecated, etc) it MAY multicast up to
   MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements.

   On links where multicast is expensive the router MAY instead unicast
   up to MAX_INITIAL_RTR_ADVERTISEMENTS number of Router Advertisements
   to the hosts in its neighbor cache.

   .  Note that such new information is not likely to reach sleeping
   hosts until those hosts refresh by sending a RS.

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9.  Router Advertisement Consistency

   The routers follows section 6.2.7 in [RFC4861] by receiving RAs from
   other routers on the link.  In addition to the checks in that
   section, the routers SHOULD verify that the RTO have the same Refresh
   Time, and report to system management if they differ.  While the host
   will pick the lowest time and operate correctly, it is not useful to
   use different Refresh Times for different routers.

10.  Security Considerations

   These optimizations are not known to introduce any new threats
   against Neighbor Discovery beyond what is already documented for IPv6

   Section 11.2 of [RFC4861] applies to this document as well.

   The mechanisms in this document work with SeND [RFC3971].

11.  IANA Considerations

   A new flag (R-flag) in the Router Solicitation message has been
   introduced by carving out a bit from the Reserved field.  There is
   currently no IANA registry for RS flags.  Perhaps one should be

   This document needs a new Neighbor Discovery option type for the RTO.

12.  Acknowledgements

   The original idea came up in a discussion with Suresh Krishnan.
   Comments from Erik Kline and Samita Chakrabarti have helped improve
   the document.

   This document has been discussed in the efficient-nd design team.

13.  Open Issues

      Should we make the Refresh Time 32 bits instead of 16? 16 bits
      implies maximum of 18 hours and in some deployments a refresh time
      measured in days might be desirable.

      Should we update the DNA procedures [RFC6059]?  We can use a
      unicast RS with this approach since that will result in an

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      immediate unicast RA which would include any updated prefixes.
      Note that a RS can not have an unspecified source and a SLLAO,
      hence some care would be needed in the interaction with DAD.

      Would it be worth-while to try to remove unchanged information
      from the refreshed RAs?  If so it could be done by including some
      epoch number in the RS and RA, and if the RS contains the current
      epoch then the RA would not need to include any options except the
      epoch number indicating that none of the options are the same as

14.  References

14.1.  Normative References

              Krishnan, S., Anipko, D., and D. Thaler, "Packet loss
              resiliency for Router Solicitations",
              draft-ietf-6man-resilient-rs-04 (work in progress),
              October 2014.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

14.2.  Informative References

              Garneij, F., Chakrabarti, S., and S. Krishnan, "Impact of
              IPv6 Neighbor Discovery on Cellular M2M Networks",
              draft-garneij-6man-nd-m2m-issues-00 (work in progress),
              July 2014.

              Vyncke, E., Thubert, P., Levy-Abegnoli, E., and A.
              Yourtchenko, "Why Network-Layer Multicast is Not Always
              Efficient At Datalink Layer",
              draft-vyncke-6man-mcast-not-efficient-01 (work in
              progress), February 2014.

   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor

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              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

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

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              November 2010.

Authors' Addresses

   Erik Nordmark
   Arista Networks
   Santa Clara, CA

   Email: nordmark@acm.org

   Andrew Yourtchenko
   7a de Kleetlaan
   Diegem, 1831

   Phone: +32 2 704 5494
   Email: ayourtch@cisco.com

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Phone: +1 514 345 7900 x42871
   Email: suresh.krishnan@ericsson.com

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