





Internet Engineering Task Force (IETF)                       S. Krishnan
Request for Comments: 7559                                      Ericsson
Updates: 4861                                                  D. Anipko
Category: Standards Track                                   Unaffiliated
ISSN: 2070-1721                                                D. Thaler
                                                               Microsoft
                                                                May 2015


            Packet-Loss Resiliency for Router Solicitations

Abstract

   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.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7559.

















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Copyright Notice

   Copyright (c) 2015 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  Proposed Algorithm  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Stopping the Retransmissions  . . . . . . . . . . . . . .   3
   3.  Configuring the Use of Retransmissions  . . . . . . . . . . .   4
   4.  Known Limitations . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   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 (RS)
   messages, each separated by at least RTR_SOLICITATION_INTERVAL (4)
   seconds.  In certain scenarios, these Router Solicitations
   transmitted by the host might be lost.  For example, the host is
   connected to a bridged residential gateway over Ethernet or Wi-Fi.
   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 retransmits.

   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



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   MaxRtrAdvInterval seconds apart (maximum value 1800 seconds).  Thus,
   in the worst-case scenario a host would be without any connectivity
   for 30 minutes.  This delay may be unacceptable in some scenarios.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   RSs.

   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
   algorithm:

        IRT (Initial Retransmission Time):     4 seconds
        MRT (Maximum Retransmission Time):  3600 seconds
        MRC (Maximum Retransmission Count):    0
        MRD (Maximum Retransmission Duration): 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
   [RFC7083].  This is to ensure that the short-term behavior of the RSs
   is similar to what is experienced in current networks, and that
   longer-term persistent retransmission behavior trends towards being
   similar to that of DHCPv6 [RFC3315] [RFC7083].

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



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   in a default route is received).  If an RA is received 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 RSs as well.  For example,
   the mechanism specified in Section 8.3.4 of the Intra-Site Automatic
   Tunnel Addressing Protocol (ISATAP) [RFC5214] unicasts the RSs to
   specific routers.

3.  Configuring the Use of Retransmissions

   Implementations of this specification are encouraged to provide a
   configuration option to enable or disable potentially infinite RS
   retransmissions.  If a configuration option is provided, it MUST
   enable RS retransmissions by default.  Providing an option to enable/
   disable retransmissions on a per-interface basis allows network
   operators to configure RS behavior in the most applicable way for
   each connected link.

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.  Security Considerations

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











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6.  References

6.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [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,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC7083]  Droms, R., "Modification to Default Values of SOL_MAX_RT
              and INF_MAX_RT", RFC 7083, DOI 10.17487/RFC7083, November
              2013, <http://www.rfc-editor.org/info/rfc7083>.

6.2.  Informative References

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              DOI 10.17487/RFC5214, March 2008,
              <http://www.rfc-editor.org/info/rfc5214>.

Acknowledgements

   The authors would like to thank Steve Baillargeon, Erik Kline, Andrew
   Yourtchenko, Ole Troan, Erik Nordmark, Lorenzo Colitti, Thomas
   Narten, Ran Atkinson, Allison Mankin, Les Ginsberg, Brian Carpenter,
   Barry Leiba, Brian Haberman, Spencer Dawkins, Alia Atlas, Stephen
   Farrell, and Mehmet Ersue for their reviews and suggestions that made
   this document better.













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Authors' Addresses

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

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


   Dmitry Anipko
   Unaffiliated

   Phone: +1 425 442 6356
   EMail: dmitry.anipko@gmail.com


   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA
   United States

   EMail: dthaler@microsoft.com

























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