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Network Working Group                                              J. Bi
Internet-Draft                                                    CERNET
Intended status: Standards Track                                  G. Yao
Expires: May 20, 2011                                Tsinghua University
                                                              J. Halpern
                                                  Newbridge Networks Inc
                                                        E. Levy-Abegnoli
                                                           Cisco Systems
                                                       November 16, 2010


           SAVI for Mixed Address Assignment Methods Scenario
                       <draft-bi-savi-mix-03.txt>

Abstract

   This document reviews how multiple address discovery methods can
   coexist in a single savi device and collisions are resolved when the
   same binding entry is discovered by two or more methods.

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 May 20, 2011.

Copyright Notice

   Copyright (c) 2010 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
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   (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



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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Mixed Address Assignment Methods Scenario . . . . . . . . . . . 3
   4.  Basic Structure . . . . . . . . . . . . . . . . . . . . . . . . 3
   5.  Problem Scope, Statement and Solution . . . . . . . . . . . . . 4
     5.1.  Problem Scope . . . . . . . . . . . . . . . . . . . . . . . 4
     5.2.  Recommendations for preventing collisions . . . . . . . . . 4
     5.3.  Binding on the Same Address . . . . . . . . . . . . . . . . 4
       5.3.1.  Same Address on Different Binding Anchors . . . . . . . 5
         5.3.1.1.  Basic preference  . . . . . . . . . . . . . . . . . 5
         5.3.1.2.  Issues in Multiple SAVI Device Scenario . . . . . . 6
         5.3.1.3.  Conflict Announcement . . . . . . . . . . . . . . . 7
       5.3.2.  Same Address on the Same Binding Anchor . . . . . . . . 8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Contributors and Acknowledgments . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9


























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1.  Introduction

   There are currently several documents [I-D.ietf-savi-fcfs],
   [I-D.ietf-savi-dhcp], [I-D.ietf-savi-send] that describe the
   different methods by which a switch can discover and record bindings
   between a node's layer3 address and a binding anchor and use that
   binding to perform Source Address Validation.

   The method used by nodes to assign the address drove the break down
   into these multiple documents, whether StateLess Autoconfiguration
   (SLACC), Dynamic Host Control Protocol (DHCP), Secure Neighbor
   Discovery (SeND) or manual.  Each of these documents describes
   separately how one particular discovery method deals with address
   collisions.

   While multiple assignment methods can be used in the same layer2
   domain, a savi-switch might have to deal with a mix of binding
   discovery methods.  The purpose of this document is to provide
   recommendations to avoid collisions and to review collisions handling
   when two or more such methods come up with competing bindings.


2.  Terminology


3.  Mixed Address Assignment Methods Scenario

   Currently, there are four SAVI solutions which cover different types
   of address assignment methods:
   1.  SAVI-FCFS: SLAAC
   2.  SAVI-DHCP: stateful DHCP, static DHCP
   3.  SAVI-SeND: CGA with certificate, CGA without certificate
   4.  Manually configuration: static address manually configured by
       administrator on SAVI device.

   Any combination of address assignment methods can be potentially
   found within a layer2 domain, and a savi device will have to
   implement the corresponding savi discovery methods (savi solutions)
   to prevent packets from valid sources to be filtered out.  If more
   than one SAVI solution is enabled on a SAVI device, the method is
   referred to as "mix address assignment method" in this document.


4.  Basic Structure

   Different savi solutions are independent from each other, each one
   handling its own entries.  In the absence of a reconciliation, each
   solution will reject packets sourced with an address it did not



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   discovered.  To prevent addresses discovered by one solution to be
   filtered out by another, the binding table should be shared by all
   the solutions.  However this could create some conflict when the same
   entry is discovered by two different methods: the main purpose of
   this document is to resolve such conflicts if and when they happen.


5.  Problem Scope, Statement and Solution

5.1.  Problem Scope

   This document reviews the case of collisions between different SAVI
   solutions.  Collision happening within a given solution is not in the
   scope of this document.

5.2.  Recommendations for preventing collisions

   If each solution has a dedicated address space, collisions won't
   happen.  Thus, it is recommended to avoid overlap in the address
   space across SAVI solutions enabled on any particular savi switch.
   More specifically:

   1.  DHCP/Static: exclude the static address from the DHCP pool.
   2.  DHCP/SLAAC: separate the prefix scope of DHCP and SLAAC.  Set the
       A bit in Prefix information option of Router Advertisement for
       SLAAC prefix.  And set the M bit in Router Advertisement for DHCP
       prefix.  [RFC4861] [RFC4862].
   3.  SLAAC/Static: separate the prefix scope of SLAAC and Static.  It
       may be impossible in practice.  SAVI device can perform DAD proxy
       for static address to hold the address from SLAAC node.
   4.  SEND/non-SEND: In an environment where SeND is deployed, the only
       way to avoid collisions in the SAVI devices is to have SeND-only
       nodes.  In a mixed environment, two nodes, SeND and non-SeND,
       could configure the same address and the SAVI-device will have to
       deal with a collision.

5.3.  Binding on the Same Address

   In situation where collisions could not be avoided, two cases should
   be considered:
   1.  The same address is bound on two different binding anchors by
       different SAVI solutions.
   2.  The same address is bound on the same binding anchor by different
       SAVI solutions.







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5.3.1.  Same Address on Different Binding Anchors

5.3.1.1.  Basic preference

   Within the SAVI perimeter, one address bound to a binding anchor by
   one SAVI solution could also be bound by another SAVI solution to a
   different binding anchor.  For example an address could be initially
   bound to a binding anchor by SAVI-FCFS solution.  If another host is
   assigned the same address from DHCP and the DAD procedure is not
   performed, the same address will also be bound to the new binding
   anchor.  Both bindings are legitimate in the corresponding solution.

   Though it is possible that the hosts and network can still work in
   such scenario, the uniqueness of address is not assured.  The SAVI
   device must decide whom the address should be bound with.  A binding
   preference level based solution is proposed here.

   To determine a proper preference level, following evidences are used:
   1.  "Duplicate Address Detection MUST be performed on all unicast
       addresses prior to assigning them to an interface, regardless of
       whether they are obtained through stateless autoconfiguration,
       DHCPv6, or manual configuration,..."  [RFC4862]
   2.  "A tentative address that is determined to be a duplicate as
       described above MUST NOT be assigned to an interface,..."
       [RFC4862]
   3.  "The client SHOULD perform duplicate address detection on each of
       the addresses in any IAs it receives in the Reply message before
       using that address for traffic."  [RFC3315]
   4.  "A SEND node that uses the CGA authorization method to protect
       Neighbor Solicitations SHOULD perform Duplicate Address Detection
       as follows.  If Duplicate Address Detection indicates that the
       tentative address is already in use, the node generates a new
       tentative CGA.  If after three consecutive attempts no non-unique
       address is generated, it logs a system error and gives up
       attempting to generate an address for that interface.

       When performing Duplicate Address Detection for the first
       tentative address, the node accepts both secured and unsecured
       Neighbor Advertisements and Solicitations received in response to
       the Neighbor Solicitations.  When performing Duplicate Address
       Detection for the second or third tentative address, it ignores
       unsecured Neighbor Advertisements and Solicitations."  [RFC3971]
   5.  "The node MAY have a configuration option whereby it ignores
       unsecured advertisements, even when performing Duplicate Address
       Detection for the first tentative address.  This configuration
       option SHOULD be disabled by default.  This is a recovery
       mechanism for cases in which attacks against the first address
       become common."  [RFC3971]



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   From the above materials, FCFS is found to be a universal principle
   with only one exception: SEND node may use a duplicate address if the
   DAD NA is only from non-SEND node.  And Duplicate Address Detection
   is enforced to detect the uniqueness of address (though in [RFC3315],
   "SHOULD" is used but not "MUST").  The static address is not covered
   in any document, as we believe the "manual configuration" in
   [RFC4862] means address configured on host by user, but not static
   address must be protected for servers and special purpose.

   The following preference level can be inferred from listed materials
   and above analysis:
   1.  SLAAC, DHCP and manually configured address by user have the same
       priority.
   2.  SEND can have higher priority because it may configure an address
       bound by non-SEND node.
   3.  Static address should have the highest priority to ensure
       administrator having the right to manage the usage of address.

   Combined solution preference with binding sequence, there will be 16
   scenarios (Denote solutions by FCFS, DHCP, SEND, and Admin
   correspondingly):

   Existing        Candidate               PREFERENCE
   FCFS            FCFS                    In the scope of SAVI-SLAAC
   FCFS            DHCP                    FCFS
   FCFS            SEND                    SEND
   FCFS            Admin                   Admin
   DHCP            FCFS                    DHCP
   DHCP            DHCP                    In the scope of SAVI-DHCP
   DHCP            SEND                    SEND
   DHCP            Admin                   Admin
   SEND            FCFS                    SEND
   SEND            DHCP                    SEND
   SEND            SEND                    In the scope of SAVI-SEND
   SEND            Admin                   Admin
   Admin           FCFS                    Admin
   Admin           DHCP                    Admin
   Admin           SEND                    Admin
   Admin           Admin                   Candidate binding

5.3.1.2.  Issues in Multiple SAVI Device Scenario

   A single SAVI device doesn't have the information of all bound
   addresses on the perimeter.  Therefore a collision may not be
   explicit based only on local bindings.  To make the perimeter-scope
   collision explicit to each SAVI device requires:





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   1.  A SAVI device must have the ability to know whether a local
       binding setup request violate a binding on other SAVI devices or
       not.
   2.  A SAVI device must have the ability to know whether a local
       binding should be removed because the address is bound on another
       SAVI device by solution with higher priority.

   The first requirement is relatively easy to meet, as DAD must have
   been performed on address bound by SAVI-SLAAC and SAVI-SEND, and
   there is no need to check if a static address violates an existing
   binding.  However DAD is not required by SAVI-DHCP, and static
   addresses must be prevented from being grabbed by other solutions.
   Thus, following mechanisms MUST be enforced:

   1.  SAVI device MUST perform DAD procedure on DHCP address or track
       if DAD performed by DHCP client itself is successful before
       binding a DHCP address.  Only if the DAD succeeds, the DHCP
       address can be bound.
   2.  SAVI device MUST perform DAD proxy for static address.  Or all
       the other SAVI devices MUST be configured to deny static address
       bound on other SAVI devices, in condition that SAVI-SEND is
       enabled and it may bind a static address.
   3.  The second requirement is relatively hard to satisfy.  Whenever
       SAVI-SEND decides to bind an address even it is used by a non-
       SEND node, and a bound address is bound manually to another
       binding anchor, the SAVI device with the existing binding must
       get noticed and delete the binding.  Following mechanisms MUST be
       enforced:
       1.  If the SAVI-SEND solution decides to bind an address despite
           that the binding collides with an existing FCFS/DHCP address,
           a SEND NA MUST be sent by the SAVI device.
       2.  If a SAVI device receives a SEND NA targeting at a local
           bound address by FCFS and DHCP, it MUST remove the binding,
           and announce the conflict to the host with the binding.
       3.  If a static address bound manually collides with any exiting
           binding, the existing binding MUST be removed manually by
           administrator, and the conflict MUST be announced to the host
           with existing binding.

5.3.1.3.  Conflict Announcement

   If a host is prohibited from using a bound address, the violation
   MUST be announced to it, through delivering one (or more) Neighbor
   Advertisement message to the host.







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5.3.2.  Same Address on the Same Binding Anchor

   A binding may be set up on the same binding anchor by multiple
   solutions.  Generally, the binding lifetimes of different solutions
   are different.  Potentially, if one solution requires to remove the
   binding, the node using the address may be taken the use right.

   For example, a node performs DAD procedure after being assigned an
   address from DHCP, then the address will also be bound by SAVI-FCFS.
   If the SAVI-FCFS lifetime is shorter than DHCP lifetime, when the
   SAVI-FCFS lifetime expires, it will request to remove the binding.
   If the binding is removed, the node will not be able to use the
   address even the DHCP lease time doesn't expire.

   The solution proposed is to keep a binding as long as possible.  A
   binding is kept until it has been required to be removed by all the
   solutions that ever set up it.


6.  References

6.1.  Normative References

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

6.2.  Informative References

   [I-D.ietf-savi-dhcp]
              Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-06 (work in progress),
              September 2010.

   [I-D.ietf-savi-fcfs]
              Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS-
              SAVI: First-Come First-Serve Source-Address Validation for
              Locally Assigned Addresses", draft-ietf-savi-fcfs-05 (work
              in progress), October 2010.

   [I-D.ietf-savi-framework]
              Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
              "Source Address Validation Improvement Framework",
              draft-ietf-savi-framework-01 (work in progress),
              October 2010.

   [I-D.ietf-savi-send]
              Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
              Address Validation Implementation",



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              draft-ietf-savi-send-04 (work in progress), October 2010.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [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.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [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.


Appendix A.  Contributors and Acknowledgments

   Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun and
   Jari Arkko for their valuable contributions.


Authors' Addresses

   Jun Bi
   CERNET
   Network Research Center, Tsinghua University
   Beijing 100084
   China

   Email: junbi@cernet.edu.cn


   Guang Yao
   Tsinghua University
   Network Research Center, Tsinghua University
   Beijing 100084
   China

   Email: yaog@netarchlab.tsinghua.edu.cn




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   Joel M. Halpern
   Newbridge Networks Inc

   Email: jmh@joelhalpern.com


   Eric Levy-Abegnoli
   Cisco Systems
   Village d'Entreprises Green Side - 400, Avenue Roumanille
   Biot-Sophia Antipolis - 06410
   France

   Email: elevyabe@cisco.com






































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