6man Working Group                                           S. Krishnan
Internet-Draft                                                  Ericsson
Updates: RFC4861 (if approved)                                 D. Anipko
Intended status: Standards Track                               D. Thaler
Expires: April 24, October 23, 2014                                      Microsoft
                                                          April 21, 2013 2014

            Packet loss resiliency for Router Solicitations


   When an interface on a host is initialized, the host transmits Router
   Solicitations in order to minimize the amount of time it needs to
   wait until the next unsolicited multicast Router Advertisement is
   received.  In certain scenarios, these router solicitations
   transmitted by the host might be lost.  This document specifies a
   mechanism for hosts to cope with the loss of the initial Router
   Solicitations.  Furthermore, on some links, unsolicited multicast
   Router Advertisements are never sent and the mechanism in this
   document is intended to work even in such scenarios.

Status of this Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Conventions used in this document . . . . . . . . . . . . . 3
   2.  Proposed algorithm  . . . . . . . . . . . . . . . . . . . . . . 4
     2.1.  Stopping the retransmissions  . . . . . . . . . . . . . . . 4
   3.  Configuring the use of retransmissions  . . . . . . . . . . . . 5
   4.  Known Limitations . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

1.  Introduction

   As specified in [RFC4861], when an interface on a host is
   initialized, in order to obtain Router Advertisements quickly, a host
   transmits up to MAX_RTR_SOLICITATIONS (3) Router Solicitation
   messages, each separated by at least RTR_SOLICITATION_INTERVAL (4)
   seconds.  In certain scenarios, these router solicitations
   transmitted by the host might be lost.

   The generic scenario is that the interface on the host comes up
   before it gets access to a router.  Examples include:

   a.  The host is connected to a bridged residential gateway over
       Ethernet or WiFi.  LAN connectivity is achieved at interface
       initialization, but the upstream WAN connectivity is not active
       yet.  In this case, the host just gives up after the initial RS
   b.  Access networks/links that turn off periodic RAs and only send
       RAs in response to RSs.  In this case, if the link between the
       Access Point (AP) and the host comes up before the link between
       the AP and the Controller/Router, the host will never be able to
       connect.  This technique of turning off periodic RAs is commonly
       used in several wireless LAN and datacenter networks to reduce
       the amount of multicast traffic.
   c.  Links that are not multicast capable.  In this case, sending an
       RA can only be triggered by an RS (as is the case, for instance,
       on ISATAP [RFC5214] links).

   Once the initial RSs are lost, the host gives up and assumes that
   there are no routers on the link as specified in Section 6.3.7 of
   [RFC4861].  The host will not have any form of Internet connectivity
   until the next unsolicited multicast Router Advertisement is
   received.  These Router Advertisements are transmitted at most
   MaxRtrAdvInterval seconds apart (maximum value 1800 seconds).  Thus
   in the worst case scenario a host would be without any connectivity
   for 30 minutes.  In general, the delay may be unacceptable in some

1.1.  Conventions used in this document

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

2.  Proposed algorithm

   To achieve resiliency to packet loss, the host needs to continue
   retransmitting the Router Solicitations until it receives a Router
   Advertisement, or until it is willing to accept that no router
   exists.  If the host continues retransmitting the RSs at
   RTR_SOLICITATION_INTERVAL second intervals, it may cause excessive
   network traffic if a large number of such hosts exists.  To achieve
   resiliency while keeping the aggregate network traffic low, the host
   can use some form of exponential backoff algorithm to retransmit the

   Hosts complying to this specification MUST use the exponential
   backoff algorithm for retransmits that is described in Section 14 of
   [RFC3315] in order to continuously retransmit the Router
   Solicitations until a Router Advertisement is received.  The hosts
   SHOULD use the following variables as input to the retransmission

     IRT  4 seconds
     MRT  3600 seconds
     MRC  0
     MRD  0

   The initial value IRT was chosen to be in line with the current
   retransmission interval (RTR_SOLICITATION_INTERVAL) that is specified
   by [RFC4861] and the maximum retransmission time MRT was chosen to be
   in line with the new value of SOL_MAX_RT as specified by [SOLMAXRT].
   This is to ensure that the short term behavior of the RSs is similar
   to what is experienced in current networks, and longer term
   persistent retransmission behavior trends towards being similar to
   that of DHCPv6 [RFC3315] [SOLMAXRT].

2.1.  Stopping the retransmissions

   On multicast-capable links, the hosts following this specification
   SHOULD stop retransmitting the RSs when Router Discovery is
   successful (i.e. an RA with a non-zero Router Lifetime that results
   in a default route is received).  Hosts MAY continue retransmitting
   the RSs even after router discovery is successful.  If the host
   continues to retransmit RSs, it is RECOMMENDED that such
   retransmissions be rate-limited to one every MRT.  If an RA is
   recieved from a router and it does not result in a default route
   (i.e.  Router Lifetime is zero) the host MUST continue retransmitting
   the RSs.

   On non-multicast links, the hosts following this specification MUST
   continue retransmitting the RSs even after an RA that results in a
   default route is received.  This is required because, in such links,
   sending an RA can only be triggered by an RS.  Please note that such
   links have special mechanisms for sending RSes as well. e.g.  The
   mechanism specified in Section 8.3.4. of ISATAP [RFC5214] unicasts
   the RSes to specific routers.

3.  Configuring the use of retransmissions

   Implementations of this specification MAY provide a configuration
   option to enable or disable the use of such potentially infinite
   retransmissions.  If the implementation provides such a configuration
   option, it MUST be able to enable/disable retransmissions on a per-
   interface basis.

4.  Known Limitations

   When an IPv6-capable host attaches to a network that does not have
   IPv6 enabled, it transmits 3 (MAX_RTR_SOLICITATIONS) Router
   Solicitations as specified in [RFC4861].  If it receives no Router
   Advertisements, it assumes that there are no routers present on the
   link and it ceases to send further RSs.  With the mechanism specified
   in this document, the host will continue to retransmit RSs
   indefinitely at the rate of approximately 1 RS per hour.  It is
   unclear how to differentiate between such a network with no IPv6
   routers and a link where an IPv6 router is temporarily unreachable
   but could become reachable in the future.

5.  IANA Considerations

   This document does not require any IANA actions.

6.  Security Considerations

   This document does not present any additional security issues beyond
   those discussed in [RFC4861].

7.  Acknowledgements

   The author authors would like to thank Steve Baillargeon, Erik Kline, Andrew
   Yourtchenko, and Ole Troan Troan, Erik Nordmark, Lorenzo Colitti, Thomas Narten
   and Ran Atkinson for their reviews and suggestions that made this
   document better.

8.  References

8.1.  Normative References

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

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

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

              Droms, R., "Modification to Default Value of SOL_MAX_RT",
              draft-droms-dhc-dhcpv6-solmaxrt-update-02 (work in
              progress), January 2012.

8.2.  Informative References

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

Authors' Addresses

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

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

   Dmitry Anipko
   One Microsoft Way
   Redmond, WA

   Phone: +1 425 703 7070
   Email: danipko@microsoft.com
   Dave Thaler
   One Microsoft Way
   Redmond, WA

   Email: dthaler@microsoft.com