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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 2012-11-05 (Latest revision 2012-04-28)
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)
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SAVI                                                               J. Bi
Internet-Draft                                                    G. Yao
Intended status: Standards Track                          Tsinghua Univ.
Expires: October 29, 2012                                     J. Halpern
                                                   E. Levy-Abegnoli, Ed.
                                                          April 27, 2012

           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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   This Internet-Draft will expire on October 29, 2012.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . . . 3
   3.  Problem Scope . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Recommendations for preventing collisions . . . . . . . . . . . 4
   5.  Handing binding collisions  . . . . . . . . . . . . . . . . . . 4
     5.1.  Same Address on Different Binding Anchors . . . . . . . . . 5
       5.1.1.  Basic preference  . . . . . . . . . . . . . . . . . . . 5
       5.1.2.  Overwritten preference  . . . . . . . . . . . . . . . . 5
       5.1.3.  Multiple SAVI Device Scenario . . . . . . . . . . . . . 6
     5.2.  Same Address on the Same Binding Anchor . . . . . . . . . . 6
   6.  Disscusion on Assumption Conflict . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . . . 7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1. Informative References  . . . . . . . . . . . . . . . . . . 8
     10.2. Normative References  . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   There are currently several documents [savi-fcfs], [savi-dhcp] and
   [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

   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.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [rfc2119].

3.  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 [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 [savi-fcfs] and [savi-framework].

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   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
   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.

4.  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

   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.

5.  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.

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   2.  The same address is bound on the same binding anchor by different
       SAVI solutions.

5.1.  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

5.1.1.  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 5.1.2) the SAVI device should choose the
   first-come entry, whether it was learnt from SLACC, SeND or DHCP.

5.1.2.  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.

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5.1.3.  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.

5.2.  Same Address on the Same Binding Anchor

   A binding may be set up on the same binding anchor by multiple
   solutions.  For example, if SAVI-FCFS and SAVI-DHCP are both enabled
   on one SAVI device, a DHCP address be bound by both SAVI instances.

   There is no conflict if the binding is valid in all the solutions.
   However, the binding lifetimes of different solutions can be
   different.  If one SAVI instance changes the state of a binding to
   invalid on lifetime expires, conflict will happen.

   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.  Disscusion on Assumption Conflict

   Different assumptions are made as the basis of solutions.  The
   assumptions of each solution specified which entity is the origin of
   the trust.  Indeed, the binding between address and binding anchor is
   actually the derivative of the assumptions based on the principles of
   binding set up.  The conflict in identifier field of address is
   specified in the above sections.  This section specifies the conflict
   in prefix field from different assumptions.

   SAVI FCFS and SAVI DHCP trust routers to get the legitimate prefixes
   for local link; however, only RADV validated by SEND is trusted by
   SAVI SEND.  In this solution, if any SAVI solution regards a prefix
   to be valid, the prefix is valid for the whole mechanism.

7.  Security Considerations

   As described in [savi-framework], this solution cannot strictly

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   prevent spoofing.  There are two scenarios in which spoofing can
   still happen:

   1.  The binding anchor is spoofable. if the binding anchor is
       spoofable, e.g., plain MAC address, an attacker can use forged
       binding anchor to send packet which will not be regarded as
       spoofing by SAVI device.  Indeed, using binding anchor that can
       be easily spoofed is dangerous.  An attacker can use the binding
       anchor of another host to perform a lot of DHCP procedures, and
       the SAVI device will refuse to set up new binding for the host
       whenever the binding number limitation has been reached.  Thus,
       it is RECOMMENDED to use strong enough binding anchor, e.g.,
       switch port, secure association in 802.11ae/af and 802.11i.

   2.  The binding anchor is shared by more than one host.  If the
       binding anchor is shared by more than one host, they can spoof
       the addresses of each other.  For example, a number of hosts can
       attach to the same switch port of a SAVI device through a hub.
       The SAVI device cannot distinguish packets from different hosts
       and thus the spoofing between them will not be detected.  This
       problem can be solved through not sharing binding anchor between

8.  IANA Considerations

   This memo asks the IANA for no new parameters.

   Note to RFC Editor: This section will have served its purpose if it
   correctly tells IANA that no new assignments or registries are
   required, or if those assignments or registries are created during
   the RFC publication process.  From the authors' perspective, it may
   therefore be removed upon publication as an RFC at the RFC Editor's

9.  Acknowledgment

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

   This document was generated using the xml2rfc tool.

10.  References

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10.1.  Informative References

   [rfc2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP 14, Match 1997.

10.2.  Normative References

   [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.

              Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-12 (work in progress),
              February 2012.

              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-14 (work
              in progress), February 2012.

              Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement Framework",
              draft-ietf-savi-framework-06 (work in progress),
              December 2011.

              Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
              Address Validation Implementation",
              draft-ietf-savi-send-06 (work in progress), October 2011.

Authors' Addresses

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


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


   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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