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Internet Engineering Task Force (IETF)                       S. KrishnanRequest for Comments: 7559                                      EricssonUpdates:4861                                                  D. AnipkoCategory: Standards Track                                   UnaffiliatedISSN: 2070-1721                                                D. Thaler                                                               Microsoft                                                                May 2015Packet-Loss Resiliency for Router SolicitationsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7559.Krishnan, et al.             Standards Track                    [Page 1]

RFC 7559               Resilient RS Retransmission              May 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Conventions Used in This Document . . . . . . . . . . . .32.  Proposed Algorithm  . . . . . . . . . . . . . . . . . . . . .32.1.  Stopping the Retransmissions  . . . . . . . . . . . . . .33.  Configuring the Use of Retransmissions  . . . . . . . . . . .44.  Known Limitations . . . . . . . . . . . . . . . . . . . . . .45.  Security Considerations . . . . . . . . . . . . . . . . . . .46.  References  . . . . . . . . . . . . . . . . . . . . . . . . .56.1.  Normative References  . . . . . . . . . . . . . . . . . .56.2.  Informative References  . . . . . . . . . . . . . . . . .5   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .5   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  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 inSection 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 mostKrishnan, et al.             Standards Track                    [Page 2]

RFC 7559               Resilient RS Retransmission              May 2015   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 inSection 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 resultsKrishnan, et al.             Standards Track                    [Page 3]

RFC 7559               Resilient RS Retransmission              May 2015   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 inSection 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].Krishnan, et al.             Standards Track                    [Page 4]

RFC 7559               Resilient RS Retransmission              May 20156.  References6.1.  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.Krishnan, et al.             Standards Track                    [Page 5]

RFC 7559               Resilient RS Retransmission              May 2015Authors' 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.comKrishnan, et al.             Standards Track                    [Page 6]