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Versions: 00 01 02                                                      
DNA Working Group                                           S. Narayanan
Internet-Draft                                                 Panasonic
Expires: April 26, 2006                                         G. Daley
                                                  Monash University CTIE
                                                            N. Montavont
                                                             LSIIT - ULP
                                                            Oct 23, 2005


   Detecting Network Attachment in IPv6 - Best Current Practices for
                                Routers
                     draft-ietf-dna-routers-01.txt

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   This Internet-Draft will expire on April 26, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Hosts experiencing rapid link-layer changes may require to do
   configuration change detection procedures more frequently than
   traditional fixed hosts.  This document describes practices available
   to network deployers in order to support such hosts in Detecting



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   Network Attachment in IPv6 networks.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Terms and Abbreviations  . . . . . . . . . . . . . . . . .  3
     1.2   Relevant Host Issues . . . . . . . . . . . . . . . . . . .  4
     1.3   Relevant Router Issues . . . . . . . . . . . . . . . . . .  4
     1.4   Summary  . . . . . . . . . . . . . . . . . . . . . . . . .  5

   2.  Configuration Practices for DNAv6 Routers  . . . . . . . . . .  5
     2.1   Multicast and Unicast RA Responses . . . . . . . . . . . .  5
       2.1.1   Recommendations  . . . . . . . . . . . . . . . . . . .  7
     2.2   Router Advertisement Parameters  . . . . . . . . . . . . .  7
       2.2.1   Recommendations  . . . . . . . . . . . . . . . . . . .  8
     2.3   Router Advertisement Options . . . . . . . . . . . . . . .  8
       2.3.1   Recommendations  . . . . . . . . . . . . . . . . . . .  9
     2.4   Triggered Router Advertisements  . . . . . . . . . . . . .  9
     2.5   Split Advertisements . . . . . . . . . . . . . . . . . . .  9
     2.6   Router Configurations  . . . . . . . . . . . . . . . . . . 10

   3.  Topological Practices for DNAv6 Networks . . . . . . . . . . . 10
     3.1   Link Extent and Composition  . . . . . . . . . . . . . . . 10
     3.2   Multiple Router Links  . . . . . . . . . . . . . . . . . . 10
     3.3   Point-to-point Links . . . . . . . . . . . . . . . . . . . 11

   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     4.1   Providing Router Authorization . . . . . . . . . . . . . . 12

   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1   Normative References . . . . . . . . . . . . . . . . . . . 12
     5.2   Informative References . . . . . . . . . . . . . . . . . . 13

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14

   A.  Summary of Recommendations . . . . . . . . . . . . . . . . . . 14

   B.  Router Advertisement Rates . . . . . . . . . . . . . . . . . . 16

       Intellectual Property and Copyright Statements . . . . . . . . 19











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1.  Introduction

   Hosts on the Internet may be connected by various media.  It has
   become common that hosts have access through wireless media and are
   mobile.  The frequency of configuration change for wireless and
   nomadic devices are elevated, due to the vagaries of wireless
   propagation or the motion of the hosts themselves.

   Such hosts need to determine if they have moved to a new IPv6 link
   rapidly, in order that configuration procedures may be run and
   application packet delivery services restored.  Detecting Network
   Attachment (DNA) is a strategy to assist such configuration changes
   by rapidly determining whether they are required.

   Several network-side factors may impact on the effectiveness and
   speed of DNA procedures.  This document provides guidelines embodying
   the best current practice for network deployers wishing to support
   detection of network attachment by IPv6 hosts.

   It should be noted that many already deployed routers will not
   support these recommendations, and that hosts SHOULD NOT rely on
   their being in place, unless they have particular reason to do so.

1.1  Terms and Abbreviations

   Access network: A network where hosts are present.  Especially, a
      network used for the support of visiting wireless hosts.

   Link: A link is the range across which communications can pass
      without being forwarded through a router [1].

   Link-Change: Link-Change occurs when a host moves from a point-of-
      attachment on a link, to another point-of-attachment where it is
      unable to reach devices belonging to the previous link, without
      being forwarded through a router.

   Point-of-Attachment: A link-layer base-station, VLAN or port through
      which a device attempts to reach the network.  Changes to a
      host's point-of-attachment may cause link-change.

   Reachability Detection: Determination that a device (such as a
      router) is currently reachable, over both a wireless medium, and
      any attached fixed network.  This is typically achieved using
      Neighbor Unreachability Detection procedure [1].







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   Wireless Medium: A physical layer which incorporates free space
      electromagnetic or optical propagation.  Such media are
      susceptible to mobility and interference effects, potentially
      resulting in high packet loss probabilities.


1.2  Relevant Host Issues

   Hosts attempting to discover link change are likely to send Router
   Solicitations (RSs) in order to identify the routers and prefixes
   available on a link.  Additionally, they may wish to send Neighbour
   Solicitations (NSs) to known routers for reachability detection
   purposes.

   The following is a list of critical issues for hosts undertaking link
   change detection in IPv6:

      Hosts require Router Advertisements (RAs) rapidly in order to
      minimize reconfiguration latencies in the case of link change or
      link failure.

      Host wishes to identify if their current prefix is still valid on
      a link before the prefix expires.  Existing IPv6 Neighbour
      Discovery procedures make this difficult.  If the host can
      determine that the target router is still reachable through a
      NS/NA exchange, it does not mean that the prefix is still valid.
      Conversely, if host sends an RS, the RA received in response may
      not contain the prefix of interest for the hosts.

      Hosts wish to detect if a particular router is reachable in order
      to use it for routing.

      Hosts may require some assurance that a device is actually
      present, and is authorized to act as a router.

   Consideration for these issues underlie the practices recommended in
   this document.

1.3  Relevant Router Issues

      The IPv6 Neighbour Discovery RFC [1] provides mechanisms where
      hosts can send Router Solicitations and receive Router
      Advertisements, from each of the routers on a link.

      Responses may either be unicast or multicast, but in all cases, a
      random delay of between 0 and 500 milliseconds is required before
      responses are sent.  This is to prevent multiple routers
      responding at the same time, and also may mitigate the effects of



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      simultaneous solicitations.  This results in a basic time delay
      incurred by hosts receiving response RAs, which cannot be avoided
      within current standards [1].

      As described in Section 2.1, additional delays may occur if
      multicast responses are required.

      Routers should also be careful not to increase the network
      overhead by frequently transmitting router advertisements (see
      Section 2.4).

      Routers should include all advertised prefixes within the same RA
      and indicate whether a previous advertised prefix is not valid
      anymore.  Multiple prefixes advertised in different RAs by a
      single router may lead to host configurations errors.


1.4  Summary

   The practices embodied in this document are considered to provide
   minimal support for hosts wishing to detect network attachment.
   Current work within the DNA working group aims to provide
   substantially improved performance for link change detection.

   Existing limitations in base protocols such as IPv6 Neighbour
   Discovery preclude support of real-time applications in some
   environments.  Future deployers and implementers are encouraged to
   consider the protocols under development at this time in order to
   provide a generic service to support hosts detecting change.

2.  Configuration Practices for DNAv6 Routers

   Routers which are being deployed to support hosts' change detection
   procedures should attempt to use appropriate configurations, which
   limit advertisement latency, and provide appropriate service
   considering the constraints of the deployed access network
   technology.

   This section describes several configuration parameters which may
   exist on IPv6 routers, and how their tuning may affect DNA hosts.

2.1  Multicast and Unicast RA Responses

   While IPv6 Neighbour Discovery assumes that responses to
   solicitations will be sent multicast, the specification allows any
   router to respond to RS message with a unicast RA if it desires [1].

   The advantage in responding with an unicast RA message is to allow



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   the IP host to conclusively infer bi-directional reachability from
   the RS-RA exchange.  Neighbour Discovery does not provide any
   mechanism to match multicast RA responses with their solicitation,
   and therefore it is not possible for the hosts to find out whether at
   least one of its RS messages was received and processed by the
   router.  Since unicast RAs are only sent in response to solicitation,
   a host can infer that at least one of its Router Solicitations
   reached the router.

   The dis-advantage in sending unicast RA is that the router will not
   be able to aggregate its response for multiple RS messages from
   multiple hosts received during the waiting period before RA
   transmission.

   For multicast Router Advertisements, a minimum separating delay
   exists so that these RAs may not be scheduled close to each other.
   When a host solicits and attempts to schedule a multicast RA within
   MIN_DELAY_BETWEEN_RAS (or MinDelayBetweenRAs from Mobile IPv6 [3]) of
   the previous multicast Router Advertisement, the scheduling of a
   response will be deferred until the minimum separation expires.

   This separation delay does not affect unicast Router Advertisement
   responses.  Routers MAY choose to respond to RS messages with a
   unicast RA response to avoid the delay introduced by the
   MIN_DELAY_BETWEEN_RAS restriction [1].

   Where many unicast responses are scheduled awaiting transmission,
   Routers MAY consider aggregating them into a single multicast
   response if a multicast advertisement may be sent before the
   advertisements' scheduled transmission time.

   It is noted that computational requirements for SEND may preclude
   this subsequent aggregation in some environments.

   Where multiple unicast transmissions for the same destination await
   transmission, routers MAY remove all transmissions after the first
   without ill-effect, if a multicast RA is scheduled for the next
   possible response time.

   In some cases it is not possible to provide unicast responses, since
   solicitations may be sent with an unspecified address, or
   solicitations do not provide enough link-layer addressing information
   to send an unicast response without neighbour discovery exchange.  In
   these cases, a router may need to send multicast responses, even if
   the expected delay is greater.






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2.1.1  Recommendations

      Routers SHOULD respond to a RS message with unicast RS message.

      Routers SHOULD aggregate RA messages into a multi-cast RA message
      if more than 3 unicast RA messages are queued for transmission.

      Where multiple unicast transmissions for the same destination
      await transmission, routers MAY remove all transmissions after the
      first without ill-effect.


2.2  Router Advertisement Parameters

   Where hosts have changeable connectivity, there may be a number of
   prefixes or routers stored in the host's memory, which are no longer
   directly reachable.  This additional storage may make movement
   detection slower where hosts rapidly pass through networks, or pass
   through networks which have very long advertised timeouts.

   Routers SHOULD be configured to advertise non-default Valid and
   Preferred lifetimes in order to provide DNA hosts with link-specific
   address lifetime information.

   Administrators are advised to set the advertised Preferred and Valid
   timers of prefixes to the maximum duration for which any host may be
   required to continue functioning without receiving a particular
   advertised prefix.

   Where hosts with ongoing transactions, or well known services (such
   as DNS) are present on a network, this duration SHOULD be greater
   than the maximum expected outage time (for example 9 hours for 0.999
   uptime, with a single outage/year).

   Upon links where fixed hosts are unlikely to be present,
   administrators SHOULD reduce the Router Lifetime, and Prefix Valid
   and Preferred Lifetimes on routers used to support DNA.

   One potential configuration heuristic would be to configure lifetimes
   to be a low number (for example: 15) of times the MaxRtrAdvInterval,
   or greater than the lower quartile cell residence time of hosts on
   the network (if known).  This allows reuse of configuration in the
   case where hosts are moving back and forth rapidly between links, but
   allows rapid timeouts of old configurations.

   The Router Lifetime MUST NOT be advertised as less than the
   MaxRtrAdvInterval unless the router is not to be used as a default
   [1].



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   Routers MUST NOT be configured with Valid or Preferred lifetime
   values lower than the MaxRtrAdvInterval.  These minima ensure that
   lifetimes do not expire in between periodic Router Advertisements.

2.2.1  Recommendations

      Routers SHOULD be configured to advertise non-default Valid and
      Preferred lifetimes in order to provide DNA hosts with link-
      specific address lifetime information.

      Upon links where fixed hosts are unlikely to be present,
      administrators SHOULD reduce the Router Lifetime, and Prefix Valid
      and Preferred Lifetimes on routers used to support DNA.

      The Router Lifetime MUST NOT be advertised as less than the
      MaxRtrAdvInterval unless the router is not to be used as a default
      [1].

      Routers MUST NOT be configured with Valid or Preferred lifetime
      values lower than the MaxRtrAdvInterval.


2.3  Router Advertisement Options

   When receiving a Router Advertisement from a particular router for
   the first time, a host needs to determine if the information
   contained in the RA indicates link change or that the transmitting
   router is part of the same link as another router it has already
   seen.  It is not possible to do this unless global prefix information
   is included in the advertisement.

   Routers SHOULD include at least one global Prefix Information Option
   in every Router Advertisement.

   Mobile IPv6 introduced a new option for Router Advertisements, which
   indicates the current MaxRtrAdvInterval of router [3].  Reception of
   this option allows hosts to estimate whether they have missed Router
   Advertisements, and allows them to check reachability or discover new
   routers.

   Routers SHOULD include Advertisement Interval options in Router
   Advertisements.

   Mobile IPv6 adds the Router Address 'R' Flag to Prefix Information
   options [3].  This flag, when set indicates that the router's entire
   global address is configured and sent in the prefix information
   option.  Bits beyond those specified in the prefix length field
   identify the router's Interface Identifier [6].



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   Hosts which are detecting network attachment can use a global router
   address to uniquely identify the router and link, rather than using
   link-local source addresses, which may be present on multiple links.

   Routers SHOULD advertise at least one global address consistently in
   a Prefix Information Option, by setting the Router Address 'R' Flag.

2.3.1  Recommendations

      Routers SHOULD include at least one global Prefix Information
      Option in every Router Advertisement.

      Routers SHOULD include Advertisement Interval options in Router
      Advertisements.

      Routers SHOULD advertise at least one global address consistently
      in a Prefix Information Option, by setting the Router Address 'R'
      Flag.


2.4  Triggered Router Advertisements

   There are proposals for IPv6 Router Advertisements to be sent to
   hosts based on network side link-layer information.

   Where these mechanisms exist they can provide Router Advertisements
   in the quickest possible time without need for Router Solicitation.
   These systems rely upon link-layer facilities are not available in
   all environments.  Therefore, interested readers are referred to the
   individual methods' documentation [15].

2.5  Split Advertisements

   A router may choose to split the options in the RA and send multiple
   RAs to reduce bandwidth overhead or to reduce the size of the RA to
   below the link MTU (section 6.2.3 of [1]).

   If such a choice is made, average multicast RA time discussed in
   Appendix B increases for each subset of the prefixes included in the
   split RA messages.

   Routers SHOULD consistently include one prefix in both sets of its RA
   messages.  This provide the host with a unique identifier based on
   the combination of link-local address and the constant prefix, to
   identify the router every time a RA message is received.






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2.6  Router Configurations

   Each router can have its own configuration with respect to sending
   RAs, and the treatment of router and neighbour solicitations.
   Different timers and constants might be used by different routers,
   such as the delay between Router Advertisements or delay before
   replying to a multicast RS.  If a host is changing its IPv6 link, a
   newly seen router on that link may have a different configuration and
   may introduce more delay than the previous default router of the
   host.

   While transitions between links under different administrative
   control are considered to be common, it is  RECOMMENDED that network
   deployers adopt uniform configuration practices across routers on
   different links within the same logical domain, in order to provide
   consistent performance.

3.  Topological Practices for DNAv6 Networks

   IPv6 does not prefer one particular network topology over another and
   allows multiple routers and subnet prefixes to exist on one link.
   Different deployments of network elements and their configuration may
   impact on link change detection though.  Effects and recommended
   practices for dealing with different network topologies are presented
   below.

3.1  Link Extent and Composition

   Most of today's access networks deploy link-layer bridging
   technologies in order to extend their logical range beyond a single
   Medium Access Control domain.

   Consequently, while many routers will come with traditional wired or
   optic-fibre interfaces, packets travelling within the same link may
   have been bridged across from a wired segment to a wireless segment.

   In many of cases, the router will not have accurate information about
   the transmission rates or media of particular segments on the link.
   Routers with interfaces whose technology is bridgeable SHOULD NOT
   assume that all segments and devices on the link have the same
   bandwidth available.

3.2  Multiple Router Links

   IPv6 Neighbour Discovery allows multiple routers to be advertising on
   the same link [1].  These routers are not required to advertise the
   same prefixes as each other.  This section provides some guidelines
   for deploying multiple routers on the same link.



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   While many routers may exist on a link, it is preferable to limit the
   number of advertising routers.  There SHOULD NOT be more than three
   (3) routers advertising on a link.  This will provide robustness in
   the case of RA packet loss, but provides a bound for bandwidth
   consumption.

   Multiple routers responding to Router Solicitation will reduce the
   mean delay for solicitation, at the cost of additional traffic.  For
   unicast responses, the delays may be halved for three responding
   routers.

         +-----------------------+---------+----------+----------+
         |Num advertising routers|      1  |       2  |       3  |
         +-----------------------+---------+----------+----------+
         |Expected Unicast Delay |  0.250s |   0.167s |   0.125s |
         +-----------------------+---------+----------+----------+

   If using advertising intervals lower than those specified in IPv6
   Neighbour Discovery, only one router MAY advertise at the elevated
   rate.  Other routers beyond the first SHOULD NOT have
   MinDelayBetweenRAs, MinRtrAdvInterval or MaxRtrAdvInterval less than
   the minima specified in IPv6 Neighbour Discovery [1][3].

   Where it is possible, routers SHOULD include at least one common
   prefix in all of their Router Advertisement messages.  This allows
   hosts to immediately see that both routers are on the same link.

3.3  Point-to-point Links

   IPv6 Router Discovery mandates the delay of RA responses by stating
   (in section 6.2.6 of [1]):

       "In all cases, Router Advertisements sent in response to a
        Router Solicitation MUST be delayed by a random time
        between 0 and MAX_RA_DELAY_TIME seconds."

   Cases where the router is on a point-to-point link, this restriction
   is too stringent as the router in question will be the only router on
   the link.  Routers on such point-to-point links MAY avoid the delay
   by not waiting for the prescribed random time before responding for
   the Router Solicitation message [8] [14].

4.  Security Considerations

   When operating a network in support of hosts performing link change
   detection, both the operational security of the hosts and network
   infrastructure are important.  DNA procedures rely upon rapid
   delivery of information to hosts using IPv6 Neighbour Discovery.



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   Neighbour Discovery as a critical service in IPv6 networks is subject
   to various attacks as described in [7].

   The following sections describe issues and practices to provide
   additional functional security for both operators and hosts.

4.1  Providing Router Authorization

   In DNA, some hosts will begin configuration procedures based on a
   single message transmitted by a router.  As such the ability of
   routing infrastructure to prove its authenticity and authorization is
   important to support correct operation of hosts.  Authentication and
   authorization mechanisms exist which allow hosts to check security of
   routers when they receive Router Advertisements indicating link
   change.

   Today these mechanisms require additional message exchanges and
   public key operations to check the authorization chain back to a
   trusted root.  Considering the computational cost for verifying
   certificates, it will useful for administrators to attempt to
   minimize the length of these authorization chains.

   Where a Router Advertisement is sent by a router, it SHOULD contain
   sufficient information to prove that the router is on the same link
   as previously seen advertisers, or is indeed the same router.  This
   may prevent expensive checks by hosts which will not need to
   immediately test the authenticity of the router through signature
   verification, or additional transmissions.

   As described in section Section 3.2,  advertising common prefixes
   achieves this goal.

   Hosts which wish to have secured exchanges with neighbours and on-
   link routers may use Secured Neighbour Discovery (SEND) [4].  SEND
   provides authenticity as well as response matching, using nonces
   copied from solicitations into advertisements.

   Routers supporting DNA SHOULD provide secured router discovery
   services using SEND [4].

5.  References

5.1  Normative References

   [1]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement



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        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.

   [4]  Arkko, J., Kempf, J., Sommerfeld, B., Zill, B., and P. Nikander,
        "SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-06
        (work in progress), July 2004.

5.2  Informative References

   [5]   Thomson, S. and T. Narten, "IPv6 Stateless Address
         Autoconfiguration", RFC 2462, December 1998.

   [6]   Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
         Addressing Architecture", RFC 3513, April 2003.

   [7]   Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
         Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

   [8]   Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
         December 1998.

   [9]   O'Hara, B. and G. Ennis, "Wireless LAN Medium Access Control
         (MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE
         Std 802.11, 1999.

   [10]  Yamamoto, S., "Detecting Network Attachment Terminology",
         draft-yamamoto-dna-term-00 (work in progress), February 2004.

   [11]  Manner, J. and M. Kojo, "Mobility Related Terminology",
         draft-ietf-seamoby-mobility-terminology-06 (work in progress),
         February 2004.

   [12]  Moore, N., "Optimistic Duplicate Address Detection for IPv6",
         draft-ietf-ipv6-optimistic-dad-02 (work in progress),
         September 2004.

   [13]  Christensen, M., Kimball, K., and F. Solensky, "Considerations
         for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-12
         (work in progress), February 2005.

   [14]  "3GPP TS 29.061 V5.5.0 (2003-03)  Interworking between the
         Public Land Mobile Network (PLMN) supporting packet based
         services and Packet Data Networks (PDN) (Release 5)",
         TS 29.061, March 2003.

   [15]  Choi, J., "Fast Router Discovery with RA Caching",



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         draft-jinchoi-dna-frd-00 (work in progress), July 2004.


Authors' Addresses

   Sathya Narayanan
   Panasonic Digital Networking Lab
   Two Research Way, 3rd Floor
   Princeton, NJ  08536
   USA

   Phone: 609 734 7599
   Email: sathya@Research.Panasonic.COM
   URI:


   Greg Daley
   Centre for Telecommunications and Information Engineering
   Department of Electrical adn Computer Systems Engineering
   Monash University
   Clayton, Victoria  3800
   Australia

   Phone: +61 3 9905 4655
   Email: greg.daley@eng.monash.edu.au


   Nicolas Montavont
   LSIIT - Univerity Louis Pasteur
   Pole API, bureau C444
   Boulevard Sebastien Brant
   Illkirch  67400
   FRANCE

   Phone: (33) 3 90 24 45 87
   Email: montavont@dpt-info.u-strasbg.fr
   URI:   http://www-r2.u-strasbg.fr/~montavont/

Appendix A.  Summary of Recommendations

   It should be noted that many already deployed routers will not
   support these recommendations, and that hosts SHOULD NOT rely on
   their being in place, unless they have particular reason to do so.

   Where many unicast responses are scheduled awaiting transmission,
   Routers MAY consider aggregating them into a single multicast
   response if a multicast advertisement may be sent before the
   advertisements' scheduled transmission time.



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   Where multiple unicast transmissions for the same destination await
   transmission, routers MAY remove all transmissions after the first
   without ill-effect, if a multicast RA is scheduled for the next
   possible response time.

   Routers MAY choose to respond to RS messages with a unicast RA
   response to avoid the delay introduced by the MIN_DELAY_BETWEEN_RAS
   restriction [1].

   Routers SHOULD be configured to advertise non-default Valid and
   Preferred lifetimes in order to provide DNA hosts with link-specific
   address lifetime information.

   Where hosts with ongoing transactions, or well known services are
   present on a network, this duration SHOULD be greater than the
   maximum expected outage time.

   Upon links where fixed hosts are unlikely to be present,
   administrators SHOULD reduce the Router Lifetime, and Prefix Valid
   and Preferred Lifetimes on routers used to support DNA.

   The Router Lifetime MUST NOT be advertised as less than the
   MaxRtrAdvInterval unless the router is not to be used as a default
   [1].

   Routers MUST NOT be configured with Valid or Preferred lifetime
   values lower than the MaxRtrAdvInterval.

   Routers SHOULD include at least one global Prefix Information Option
   in every Router Advertisement.

   Routers SHOULD include Advertisement Interval options in Router
   Advertisements.

   Routers SHOULD advertise at least one global address consistently in
   a Prefix Information Option, by setting the Router Address 'R' Flag.

   A router MAY choose to split the options in the RA and send multiple
   RAs to reduce bandwidth overhead or to reduce the size of the RA to
   below the link MTU (see section 6.2.3 of [1]).

   While transitions between links under different administrative
   control are considered to be common, it is  RECOMMENDED that network
   deployers adopt uniform configuration practices across routers on
   different links within the same logical domain, in order to provide
   consistent performance.

   Routers with interfaces whose technology is bridgeable SHOULD NOT



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   assume that all segments and devices on the link have the same
   bandwidth available.

   There SHOULD NOT be more than three (3) routers advertising on a
   link.

   If using advertising intervals lower than those specified in IPv6
   Neighbour Discovery, only one router MAY advertise at the elevated
   rate.  Other routers beyond the first SHOULD NOT have
   MinDelayBetweenRAs, MinRtrAdvInterval or MaxRtrAdvInterval less than
   the minima specified in IPv6 Neighbour Discovery [1][3].

   Where it is possible, routers SHOULD include at least one common
   prefix in all of their Router Advertisement messages.

   Routers on point-to-point links MAY avoid delay by not waiting for
   the prescribed random time before responding for the Router
   Solicitation message [8] [14].

   Considering the computational cost for verifying certificates,
   administrators SHOULD attempt to minimize the length of authorization
   chains.

   Where a Router Advertisement is sent by a router, it SHOULD contain
   sufficient information to prove that the router is on the same link
   as previously seen advertisers, or is indeed the same router.

   Routers supporting DNA SHOULD provide secured router discovery
   services using SEND [4].

   On access networks supporting Detecting Network Attachment,
   administrators SHOULD configure routers to advertise at the shortest
   safe intervals.

Appendix B.  Router Advertisement Rates

   Unsolicited Router Advertisements are scheduled to be transmitted at
   a time between MinRtrAdvInterval and MaxRtrAdvInterval after the last
   multicast Router Advertisement.  These parameters may be configured
   in the way which best suits the network.  The table below summarizes
   the parameters as described by IPv6 Neighbour Discovery [1].










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         +-----------------------+----+----+----+-----+----+-----+
         |   Timer               |Maximum  |Default   |Minimum   |
         +-----------------------+----+----+----+-----+----+-----+
         | MaxRtrAdvInterval     |  1800   |   600    |    4     |
         +-----------------------+----+----+----+-----+----+-----+
         | MinRtrAdvInterval     |   594   |   198    |    3     |
         +-----------------------+----+----+----+-----+----+-----+
         |Avg. Multicast RA time |  1197   |   399    |   3.5    |
         +-----------------------+----+----+----+-----+----+-----+

   The load on the network, and the timeliness of any received
   information updates are therefore influenced by the router's
   advertisement parameters.

   On access networks supporting Detecting Network Attachment,
   administrators SHOULD configure routers to advertise at the shortest
   safe intervals.  Determination of the shortest safe intervals depends
   on topology, and the composition of the link, as described in
   Section 3.1.

   Mobile IPv6 attempts to address the delays associated with hosts'
   movement and change detection by reducing the minimum settings for
   MinRtrAdvInterval to 30ms and MaxRtrAdvInterval to 70ms.  Not all
   IPv6 routers support these configuration values today.  Where hosts
   have no reactive way of detecting change, and do not solicit for
   Router Advertisements, these intervals may allow change detection
   sufficiently fast to support real-time applications.

   The effect of these timers are summarized in the table below.

         +-----------------------+----+----+----+-----+----+-----+
         |   Timer               |Maximum  |Default   |Minimum   |
         +-----------------------+----+----+----+-----+----+-----+
         | MaxRtrAdvInterval     |  1800   |   600    |  0.07    |
         +-----------------------+----+----+----+-----+----+-----+
         | MinRtrAdvInterval     |   594   |   198    |  0.03    |
         +-----------------------+----+----+----+-----+----+-----+
         |Avg. Multicast RA time |  1197   |   399    |  0.05    |
         +-----------------------+----+----+----+-----+----+-----+

   Where Mobile IPv6 is supported, the minimum values change, but the
   default timers are unmodified.  If administrators wish to take
   advantage of shorter intervals between unsolicited RAs, explicit
   configuration is required.  This is because the elevated rate of
   multicast RA transmission can have detrimental effects on some
   constrained links [3].

   The minimum average for un-solicited Router Advertisements would be



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   20 messages per second.  Assuming the minimum packet size for an RA
   with one prefix as 88 bytes, the bandwidth used will be 14 kbps.
   With SEND Options, and (somewhat weak) 1024-bit RSA keys, a single RA
   could be around 432 octets.  This would consume approximately 69 kbps
   without considering link-layer overheads [4].

   As described in Section 2.1, parameters may be chosen to optimize
   solicited behaviour in a way which limits the mean bandwidth overhead
   for unsolicited RAs.

   A good example would be setting a MinRtrAdvInterval (along with
   MinDelayBetweenRAs) as 0.5 s, and the MaxRtrAdvInterval to 4s.  This
   makes the mean delay before receiving an unsolicited RA 2.25 seconds,
   and limits the bandwidth utilization for unsolicited RAs (using the
   SEND example above) to 1.5 kbps, and the maximum multicast solicited
   rate to 6.9 kbps (one multicast RA each 0.5s).



































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