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Versions: 00 01

SAVI                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                            June 4, 2012
Expires: December 6, 2012

                Throttling RAs on constrained interfaces


   In a large switched topology with either many routers or routers that
   transmit a high rate of multicast advertisements per router, as
   suggested to support mobile nodes, the cost of distributing the many
   resulting multicast messages to certain classes of devices might be
   prohibitive.  This is the case of a device that runs on batteries, or
   a device that is reachable over a wireless interface.  For this
   device, it can be beneficial to filter a certain amount of multicast
   messages such as the Router Advertisement while preserving the
   functionality expected in the IPv6 Neighbor Discovery Protocol.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

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."

   This Internet-Draft will expire on December 6, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

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   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
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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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem statement  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Routers behavior . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Wireless Mobility domain . . . . . . . . . . . . . . . . .  5
   4.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Throttling scope and period  . . . . . . . . . . . . . . .  7
     4.2.  Pending Hosts List . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Advertising Routers List . . . . . . . . . . . . . . . . .  8
     4.4.  RA with an Advertisement Interval Option . . . . . . . . .  8
     4.5.  Final RA . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.6.  Throttling Policy  . . . . . . . . . . . . . . . . . . . . 10
   5.  Manageability  . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   The protection of the network is not necessarily a security function
   such as the defense against a specific attack.  It might also have to
   do with other activities that include the control of multicast
   storms, as provided to some extent by Multicast Listener Discovery
   (MLD) Snooping [RFC4541], or of the overuse of the network that might
   degrade the service for all users as provided for instance by Call
   Admission Control [RFC5865].

   In particular, the wireless edge of a large Layer 2 topology will
   require some special protections to conserve the limited bandwidth
   that is available over the radio medium, such protections certainly
   involving the reduction of multicast operations.

   MLD Snooping helps control the impact of multicast messages but:

      It does not apply to the all-nodes link-scoped multicast address
      (FF02::1) as defined in the IP Version 6 Addressing Architecture

      MLD snooping is generally not implemented for link scope multicast
      messages anyway.

      If MLD snooping runs in instance as opposed to the Access Point
      (AP) and if there is at least one listener associated to the AP
      then the AP will still get the multicast and transmit it to all
      the devices that are associated to the AP.

      MLD snooping has the granularity of a group as opposed to a
      binding table that has the granularity of a target - a host.

   This document focusses on the protection against an excessive
   bandwidth consumption by multicast IPv6 Neighbor Discovery (ND)
   [RFC4861] Protocol (NDP) Router Advertisement (RA) messages over the
   wireless edge of a switched network.

   RA messages are link-scoped messages that provide the recipient node
   with link information such as availability and characteristics of
   routers that are present on the link and a list of prefixes that are
   usable for IPv6 NDP Stateless Address Autoconfiguration (SLAAC)

   RA messages coming from different routers may carry different
   information, including information about the router itself, but also
   different prefix lists and other information such as the period of
   transmission [RFC6275].

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   In a number of cases, the fact that a node misses an RA does not
   impact the node operation in a notable fashion, either because the
   information is fully redundant with information that the node already
   has (e.g. multiple RAs of a same content from a same router in rapid
   sequence) or because the information is not critical to the node
   (e.g. yet another router that is not preferable as default gateway).
   In other cases, the loss of an RA is eventually recovered, but the
   node will not operate optimally in the meantime and such a loss
   should be avoided.

   This document studies situations when an Ethernet Switch, an IEEE
   802.11 Access Point, or a Control And Provisioning of Wireless Access
   Points (CAPWAP) Access Controller (AC) [RFC5415] can, with no notable
   effect, make the decision not to copy an RA message onto a port or a
   set of ports, typically one or more ports that connect to an IEEE
   802.11 wireless domain, the consequences of doing so and the eventual
   recovery.  In the remainder of this document, the term throttling
   will refer to the decision not to copy a message over such ports, and
   the layer 2 device in charge of making that decision will simply be
   referred to as switch, whether it is a classical Ethernet switch or
   any one of the sorts of devices listed above.

2.  Terminology

   The draft uses the following terminology:

   Switch:  A layer 2 device that distributes packets over one or more
      ports.  This broad definition includes but is not limited to
      Ethernet and IEEE802.3 switches, CAPWAP Access Controllers and
      IEEE 802.11 Access Points.

   Throttling:  The decision not to copy a given multicast message onto
      a given port or a set of ports after the determination that the RA
      would be redundant for most hosts across the port(s).  A multicast
      packet that is throttled over a given port might still be copied
      for unicast delivery to selected hosts on a that port if it is
      determined that they will benefit from receiving the RA.  The
      packet might still be passed on to other ports such as trunk
      ports, for further switching along a VLAN for instance.

   Throttled port:  A port on the switch where throttling is active.
      The port might be an access port that is directly connected to a
      host, but it might also be a multipoint port, for instance if it
      is connected to another switch such as an Access Point.

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3.  Problem statement

3.1.  Routers behavior

   Assuming that the solicitor's source address is not the unspecified
   address, a router may choose to respond to an ND Router solicitation
   (RS) with a unicast RA message directly to the soliciting host's
   address.  But it is common that the router aggregates multiple
   requests and sends a single multicast response to the all-nodes
   group.  This RA is received by all the nodes on link, though a host
   that did not issue an RS is probably not very interested in receiving
   the solicited RA response message.  Yet, in a wireless environment, a
   host will usually issue an RS each time it reassociates, which can be
   quite often if the host is as mobile as a smartphone.

   A traditional (wireline) router will typically not rate limit its RA
   emissions based on consistent RA messages received from other
   routers, though such a behavior is required in the Routing Protocol
   for Low Power and Lossy Networks (RPL) [I-D.ietf-roll-rpl] that is
   specifically designed for constrained environments such as wireless
   mesh networks.  As a result, the number of RAs on a switched topology
   increase roughly linearly with the number of deployed routers.

   As it goes, the whole RA operation denotes an implicit expectation
   that the cost for a multicast operation is not substantially
   different from that of a unicast transmission and that the cost is
   roughly similar on all segments of the link.  Sadly, this is not true
   in the case of a composite network with a switched Ethernet backbone
   and an IEEE 802.11 Extended Service Set (ESS) wireless edge.  The
   situation is even worse if the edge is a mesh network, e.g. an IEEE
   802.11S or a Low Power Lossy Network as described in
   [I-D.phinney-roll-rpl-industrial-applicability] and
   [I-D.thubert-lowpan-backbone-router].  In any case, a rate of RAs
   that might appear acceptable on the backbone can rapidly become
   excessive on the wireless edge.

3.2.  Wireless Mobility domain

   A number of (layer 3) Network-Based Localized Mobility Management
   (NetLMM) techniques have been deployed that enable IP mobility
   transparently to the host, that is without requiring the active
   participation of the host in any mobility-related signaling.  This is
   achieved by hiding its mobility to the host and more specifically by
   presenting the host with a consistent link appearance as it roams at
   layer 3, in particular through tailored RA messages.  An example of
   such NetLMM solutions would be the adaption of Proxy Mobile IPv6
   (PMIPv6) [RFC5213] within a proprietary mobility framework.

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   As a result, within a same radio environment (say an IEEE 802.11
   Service Set Identification or SSID), some of the associated nodes may
   be local nodes and some other nodes may be roaming devices that are
   virtually part of some other link or VLAN in a remote location.  If
   all the associated nodes received a local RA announcing an local IPv6
   prefix, roaming devices would detect their movement, form new
   addresses and defeat the mobility functionality to the point that the
   entire mobility domain would appear as one flat single IPv6 link.

   To avoid that problem, a dedicated RA is unicast to each of the
   associated devices as opposed to sent once as a layer 2 broadcast to
   all devices in a single shot.  A very common method consists in
   rewriting the link layer multicast address in a frame that carries
   the layer 3 multicast message onto a layer 2 unicast address.  This
   isolates which L3 multicast packet gets to which host, and more
   importantly which multicast packet will not get to a given host for
   whih it is not destined.

   When a multicast packet is converted into multiple unicast frames by
   a siwtch such as an Access Point or an Access Controller, a single
   packet that is sent to the all-nodes group can consume a large amount
   of bandwidth that is roughly a factor of the number of associated
   devices, and disrupt sensitive applications such as voice over IEEE
   802.11.  The NDP assumption that a multicast does not cost more than
   a unicast is severely broken.  It results that the ND Protocol is not
   really suited for the wireless medium, and that some tailoring is
   required in instance to reduce the impact of the multicast messages,
   in particular RA messages.

4.  Operation

   The NDP messages Router Advertisements are scoped to a link.  They
   are sent on a given IPv6 link (e.g. a Virtual Local Area Network) and
   should be delivered only to IPv6 nodes that reside on that link.  An
   RA message can be transmitted over the medium either as unicast
   response or as a multicast message that is sent to the link-scoped
   all-nodes multicast address (FF02::1) as defined in [RFC4291], which
   all IPv6 nodes on the link listen to.

   If a Router Advertisement is sent to a unicast destination address,
   instance MUST forward the packet to the destination device.  But as
   opposed to other ND Protocol operations such as the Duplicate Address
   Detection (DAD) that occurs only when a node obtains or forms a new
   address, multicast RAs are sent periodically and might be quite
   frequent for the duration of the network activity.  In that case,
   instance MAY drop the multicast RA if it is redundant.  The question
   becomes to determine whether a multicast RA is redundant.

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   A switch might connect ports of different natures.  Some ports may
   need throttling of the RA messages, and some node.  It is expected
   that some mechanism is in place to determine which ports require
   throttling, for instance a configured policy or an automatic

4.1.  Throttling scope and period

   The scope of a throttling activity is a link (a VLAN).  Within that
   scope, some ports on instance are determined to be throttled, while
   others are not.  A throttling period is associated to that scope.  A
   policy dictates how many and under which conditions multicast RAs are
   throttled.  The policy is based on counters that count RAs per router
   and counters that aggregate the numbers to the throttling scope (the
   VLAN).  The counters are reset at each throttling period.

   NDP does not mandate that routers on a link expose the same prefixes.
   It is possible that a router advertises a prefix that none of the
   other routers does for instance.  Or a router might advertise a
   better preference for a given destination [RFC4291].  It is this
   important that the throttling mechanism does not starve any given
   router. instance SHOULD attempt to distribute fairly the amount of
   RAs per source router, and to serve at least once any given router on
   the link (VLAN) within a given period of time.

4.2.  Pending Hosts List

   A multicast RA that is the response to an RS is probably redundant
   for all nodes that did not solicit the RA in the first place.  But it
   is certainly useful to nodes that issued an RS over a throttled port
   since the last multicast RA happened. instance needs to keep track of
   all those hosts as discovered through their RS messages, in an
   abstract list referred to as the Pending Hosts List (PHL).  There is
   one PHL per link (that is, typically, per VLAN).

   A host is anchored to a port on instance, a link layer address, and
   eventually one or more VLAN identifier(s) depending on the
   deployment.  An IPv6 Link Local Address [RFC4291] might be available
   to qualify the host.  A PHL entry SHOULD contain all the anchor
   parameter and MAY indicate additional information such as the host
   Link Local Address.

   instance SHOULD add a host to the PHL when it receives an RS from
   that host over a throttled port, and upon a layer 2 trigger that
   indicates that the port has flapped, typically an association or a
   reassociation event in an Access Point or an Accesss Controller.
   instance MAY remove a host from the PHL when a RA is forwarded to the
   host, either as a unicast, a multicast, or a unicast copy of a

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   throttled multicast, and SHOULD remove the entry after a number or
   RAs are forwarded, depending on the policy that applies to the host.

4.3.  Advertising Routers List

   The primary cause of RA redundancies is a router that sends multiple
   identical RAs in a short sequence, for instance as stimulated by
   hosts joining the link. instance identifies such redundancies by
   keeping track of all the routers as discovered through RA messages,
   and eventually of the content of those RA messages, in an abstract
   list referred to as the Advertising Routers List (ARL).  There is one
   ARL per link (VLAN).

   A router is anchored to a port on instance, a link layer address, and
   eventually one or more VLAN identifier(s) depending on the
   deployment.  The router Link Local Address is found as the source
   address of the RA.  A ARL entry SHOULD contain all the anchor
   parameters, the router Link Local Address, and a number of counters
   that indicate the router activity over the last period and MAY
   contain additional information from the RA such as a prefix list or
   the router preferences [RFC4191].  The entry MUST also contain
   counters that are necessary for the throttling operation, typically
   the number of multicast RAs that where copied and the number that
   were throttled during the current throttling period.

   instance SHOULD add a router to the ARL when it receives an RA from
   that router on any port that belong to that link (VLAN). instance
   SHOULD remove the router from the ARL when the throttle period
   elapses. instance MAY maintain a list of routers that were part of
   the ARL for the previous period in an alternate list to keep
   additional history and improve runtime performances.

4.4.  RA with an Advertisement Interval Option

   The Mobility Support in IPv6 (MIPv6) [RFC4191] section 7.3 introduces
   the Advertisement Interval Option (AIO), used in RA messages to
   advertise the interval at which the advertising router sends
   unsolicited multicast Router Advertisements.  When this option is
   present, a switch SHOULD NOT interfere with a routers attempt to live
   up to its claim that at least one RA message will be posted every
   advertisement interval.

   There is more than one way for instance to comply with this
   requirement, as controlled by a policy that applies to the throttling

      instance MAY never copy RAs from a given router that carry the AIO
      over throttled ports.

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      Or instance MAY copy all RAs from a given router that carry the
      AIO over throttled ports.

      Alternatively, instance MAY monitor the timing of RA emissions
      from a given router and refrain from throttling at least one RA
      per advertisement interval from that router.  It might then happen
      that the router arms its timer on a message that instance
      throttles later.  In that case, the next RA that is not throttled
      can be separated by substantially more time than one advertisement
      interval though less than 2 intervals.  This should not impact the
      MIPv6 operation that does not take action until no RA is received
      within two and a half advertisement intervals.

   It can be noted that the advertisement interval that is used to
   support mobility can be very short and load the radio medium quite
   dramatically, depending on the available bandwidth on that medium.
   The policy in place SHOULD probably make it so that RAs with too
   short intervals are not copied on throttled ports unless no other
   option is available.  If mobile devices are expected on the wireless
   link, then it might be preferred to block all routers advertising AIO
   but one or two that would preferably use an acceptable interval.

4.5.  Final RA

   Section 6.2.5 of the Neighbor Discovery specification [RFC4861]
   describe the router operation when it ceases to advertise on a given
   interface.  In particular, the router needs to transmit one or more
   final (multicast) RA messages on the interface with a Router Lifetime
   field of zero.

   This information is critical to any host that utilizes the router
   either as default gateway or more preferred gateway for a given
   destination prefix since filtering out a final RA might leave such
   host without connectivity till the host discovers that the router is
   gone.  A switch SHOULD NOT take actions that would prevent such a
   host from receiving at least one final RA that indicates that a given
   router ceases to be a available as a IPv6 gateway on the link (VLAN)
   where throttling applies.

   There is more than one way for instance to comply with that
   requirement, as controlled by a policy that applies to the throttling

      instance MAY never throttle an RA with a Router Lifetime field set
      to zero.

      Alternatively, instance MAY throttle further multicast final RAs
      arrive immediately after a first final RA from a same router.

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   It can be noted that the advertisement interval that is used to
   support mobility can be very short and load the system quite
   dramatically.  The policy in place should probably make it so that
   RAs with short intervals are not copied on throttled ports unless no
   other option is available.

4.6.  Throttling Policy

   An implementation SHOULD allow to configure a policy whereby the RA
   throttling operation is based on the history of received RAs during
   the current throttling period.

   Suggested policy parameters per link (VLAN) include:

   throttle-period:  This is the duration of the throttling period.  A
      suggestion is to keep this value under the highest
      MaxRtrAdvInterval used in the network.  MaxRtrAdvInterval is
      defined in [RFC4861] with a default of 600 seconds.  The policy
      that provides that parameter MAY apply to the link (VLAN) or

   max-through:  This is a maximum number of RAs that may pass before
      for all routers during a throttling period. rAdvInterval is
      defined in [RFC4861] with a default of 600 seconds.  The policy
      that provides that parameter MAY apply to the link (VLAN) or
      instance.  A suggested default is 1.

   at-least:  This is the minimum guaranteed number of RAs that pass
      before the first RA is throttled for a given router.  The policy
      that provides that parameter MAY apply to an individual router, a
      port, the link (VLAN) or instance.  A suggested default is 1.
      This parameter takes precedence over the max-through parameter
      that is defined at the link (VLAN) level so as not to starve any

   at-most:  This is a maximum number of RAs that may pass before for a
      given router during a throttling period.  The policy that provides
      that parameter MAY apply to the router, the port, the link (VLAN)
      or instance.  A suggested default is 1.

   interval-option:  This parameter indicates the behaviour upon RAs
      with the IAO as discussed in Section 4.4.  The policy that
      provides that parameter MAY apply to the router, the port, the
      link (VLAN) or instance.  A suggested default is never to copy RAs
      with IAO on a throttled port.

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5.  Manageability

   An implementation SHOULD allow the administrator to define one or
   more throttling policies and to apply them on the relevant targets
   (routers, ports, links and switch).  The implementation should count
   the number of RAs that passed and RAs that are throttled per target.

6.  IANA Considerations

   This specification does not require IANA action.

7.  Security Considerations

   This specification is not found to introduce new security threat.

8.  Acknowledgements

   The author wishes to thank Eric Levy-Abegnoli for his kind mentorship
   all along this project.

9.  References

9.1.  Normative References

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

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

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

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

9.2.  Informative References

              Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P.,
              Levis, P., Struik, R., Kelsey, R., Clausen, T., and T.
              Winter, "RPL: IPv6 Routing Protocol for Low power and
              Lossy Networks", draft-ietf-roll-rpl-19 (work in

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              progress), March 2011.

              Phinney, T., Thubert, P., and R. Assimiti, "RPL
              applicability in industrial networks",
              draft-phinney-roll-rpl-industrial-applicability-00 (work
              in progress), October 2011.

              Thubert, P., "LoWPAN Backbone Router",
              draft-thubert-lowpan-backbone-router-00 (work in
              progress), November 2007.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, November 2005.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, May 2006.

   [RFC5213]  Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
              and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.

   [RFC5415]  Calhoun, P., Montemurro, M., and D. Stanley, "Control And
              Provisioning of Wireless Access Points (CAPWAP) Protocol
              Specification", RFC 5415, March 2009.

   [RFC5865]  Baker, F., Polk, J., and M. Dolly, "A Differentiated
              Services Code Point (DSCP) for Capacity-Admitted Traffic",
              RFC 5865, May 2010.

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

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Author's Address

   Pascal Thubert (editor)
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

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