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SAVI for Mixed Address Assignment Methods Scenario

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8074.
Expired & archived
Authors Jun Bi , Guang Yao , Joel M. Halpern , Eric Levy-Abegnoli
Last updated 2012-04-28 (Latest revision 2011-10-26)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Jean-Michel Combes
IESG IESG state Became RFC 8074 (Proposed Standard)
Consensus boilerplate Unknown
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Responsible AD (None)
Send notices to (None)
Network Working Group                                              J. Bi
Internet-Draft                                                    CERNET
Intended status: Standards Track                                  G. Yao
Expires: April 28, 2012                              Tsinghua University
                                                              J. Halpern
                                                  Newbridge Networks Inc
                                                   E. Levy-Abegnoli, Ed.
                                                           Cisco Systems
                                                        October 26, 2011

           SAVI for Mixed Address Assignment Methods Scenario


   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
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   Drafts is at

   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 April 28, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Recommendations for preventing collisions . . . . . . . . . . . 4
   4.  Handing binding collisions  . . . . . . . . . . . . . . . . . . 4
     4.1.  Same Address on Different Binding Anchors . . . . . . . . . 4
       4.1.1.  Basic preference  . . . . . . . . . . . . . . . . . . . 5
       4.1.2.  Overwritten preference  . . . . . . . . . . . . . . . . 5
       4.1.3.  Multiple SAVI Device Scenario . . . . . . . . . . . . . 5
     4.2.  Same Address on the Same Binding Anchor . . . . . . . . . . 6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Appendix A.  Contributors and Acknowledgments . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   There are currently several documents [I-D.ietf-savi-fcfs],
   [I-D.ietf-savi-dhcp] and [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.  Each of these
   documents specifies how to learn on-link addresses, based on the
   method used for their assignment, respectively: StateLess
   Autoconfiguration (SLAAC), Dynamic Host Control Protocol (DHCP) and
   Secure Neighbor Discovery (SeND).  Each of these documents describes
   separately how one particular discovery method deals with address
   collisions (same address, different anchor).

   While multiple assignment methods can be used in the same layer2
   domain, a SAVI device 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.  Problem Scope

   There are three address assignment methods identified and reviewed in
   one of the SAVI document:
   1.  StateLess Address AutoConfiguration (SLAAC) - reviewed in
   2.  Dynamic Host Control Protocol address assignment (DHCP) -
       reviewed in [I-D.ietf-savi-dhcp]
   3.  Secure Neighbor Discovery (SeND) address assignment, reviewed in

   Each address assignment method corresponds to a binding discovery
   method: SAVI-FCFS, SAVI-DHCP and SAVI-SeND.  In addition, there is a
   fourth method for installing a bindings on the switch, referred to as
   "manual".  It is based on manual (address or prefix) binding
   configuration and is reviewed in [I-D.ietf-savi-fcfs] and

   All combinations of address assignment methods can coexist within a
   layer2 domain.  A SAVI device will have to implement the
   corresponding SAVI discovery methods (referred to as a "SAVI
   solution") to enable Source Address Validation.  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.

   SAVI solutions are independent from each other, each one handling its
   own entries.  In the absence of reconciliation, each solution will

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   reject packets sourced with an address it did not 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 purpose of this document is
   of two folds: provide recommendations and method to avoid conflicts,
   and resolve conflicts if and when they happen.  Collisions happening
   within a given solution are outside the scope of this document.

3.  Recommendations for preventing collisions

   If each solution has a dedicated address space, collisions won't
   happen.  Using non overlapping address space across SAVI solutions is
   therefore recommended.  To that end, one should:

   1.  DHCP/SLAAC: use non-overlapping prefix for 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.  For detail explanations on these bits, refer to
       [RFC4861] [RFC4862].
   2.  SeND/non-SeND: avoid mixed environment (where SeND and non-SeND
       nodes are deployed) or separate the prefixes announced to SeND
       and non-SenD nodes.  One way to separate the prefixes is to have
       the router()s announcing different (non-overlapping) prefixes to
       SeND and to non-SeND nodes, using unicast Router Advertisements,
       in response to SeND/non-SeND Router Solicit.

4.  Handing binding collisions

   In situations 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.

4.1.  Same Address on Different Binding Anchors

   This would typically occur in case assignment address spaces could
   not be separated.  For instance,overl an address is assigned by SLAAC
   on node X, installed in the binding table using SAVI-FCFS, anchored
   to "anchor-X".  Later, the same address is assigned by DHCP to node
   Y, as a potential candidate in the same binding table, anchored to

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4.1.1.  Basic preference

   The SAVI device must decide whom the address should be bound with
   (anchor-X or anchor-Y in this example).  Current standard documents
   of address assignment methods have implied the prioritization
   relationship (first-come).  In the absence of any configuration or
   protocol hint (see Section 4.1.2) the SAVI device should choose the
   first-come entry, whether it was learnt from SLACC, SeND or DHCP.

4.1.2.  Overwritten preference

   There are two identified exceptions to the general prioritization
   model, one of them being CGA addresses, another one controlled by the
   configuration of the switch:

   1.  When CGA addresses are used, and a collision is detected,
       preference should be given to the anchor that carries the CGA
       credentials once they are verified, in particular the CGA
       parameters and the RSA options.  Note that if an attacker was
       trying to replay CGA credentials, he would then compete on the
       base of fcfs (first-come, first-serve).
   2.  The SAVI device should allow the configuration of a triplet
       ("prefix", "anchor", "method") or ("address", "anchor",
       "method").  Later, if a DAD message is received for a target
       within "prefix" (or equal "address") bound to "anchor1"
       (different from "anchor"), or via a discovery method different
       from "method", the switch should defend the address by responding
       to the DAD message.  It should not at this point install the
       entry into the binding table.  It will simply prevent the node to
       assign the address, and will de-facto prioritize the configured
       anchor or configured assignment method for that address.  This is
       especially useful to protect well known bindings such as a static
       address of a server over anybody, even when the server is down.
       It is also a way to give priority to a binding learnt from SAVI-
       DHCP over a binding for the same address, learnt from SAVI-FCFS.

4.1.3.  Multiple SAVI Device Scenario

   A single SAVI device doesn't have the information of all bound
   addresses on the perimeter.  Therefore it is not enough to lookup
   local bindings to identify a collision.  However, assuming DAD is
   performed throughout the security perimeter for all addresses
   regardless of the assignment method, then DAD response will inform
   all SAVI devices about any collision.  In that case, FCFS will apply
   the same way as in a single switch scenario.  If the admin configured
   on one the switches a prefix (or a single static binding) to defend,
   the DAD response generated by this switch will also prevent the
   binding to be installed on other switches of the perimeter.

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4.2.  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.

5.  References

5.1.  Normative References

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

5.2.  Informative References

              Wu, J., Yao, G., Bi, J., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-10 (work in progress),
              July 2011.

              Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come First-Serve Source-Address Validation for
              Locally Assigned IPv6 Addresses", draft-ietf-savi-fcfs-09
              (work in progress), April 2011.

              Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
              "Source Address Validation Improvement Framework",
              draft-ietf-savi-framework-05 (work in progress),
              July 2011.

              Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
              Address Validation Implementation",

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              draft-ietf-savi-send-06 (work in progress), October 2011.

   [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
   Network Research Center, Tsinghua University
   Beijing 100084


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


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   Joel M. Halpern
   Newbridge Networks Inc


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


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