Internet Engineering Task Force (IETF)                             J. Bi
Request for Comments: 8074                           Tsinghua University
Category: Standards Track                                         G. Yao
ISSN: 2070-1721                                Tsinghua University/Baidu
                                                              J. Halpern
                                                                Ericsson
                                                   E. Levy-Abegnoli, Ed.
                                                                   Cisco
                                                           February 2017


              Source Address Validation Improvement (SAVI)
             for Mixed Address Assignment Methods Scenario

Abstract

   In networks that use multiple techniques for address assignment, the
   spoofing of addresses assigned by each technique can be prevented
   using the appropriate Source Address Validation Improvement (SAVI)
   methods.  This document reviews how multiple SAVI methods can coexist
   in a single SAVI device and how collisions are resolved when the same
   binding entry is discovered by two or more methods.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8074.

Copyright Notice

   Copyright (c) 2017 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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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Scope . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Recommendations for Assignment Separation . . . . . . . . . .   6
   6.  Resolving Binding Collisions  . . . . . . . . . . . . . . . .   6
     6.1.  Same Address on Different Binding Anchors . . . . . . . .   6
       6.1.1.  Basic Preference  . . . . . . . . . . . . . . . . . .   7
       6.1.2.  Exceptions  . . . . . . . . . . . . . . . . . . . . .   7
       6.1.3.  Multiple SAVI Device Scenario . . . . . . . . . . . .   8
     6.2.  Same Address on the Same Binding Anchor . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   There are currently several Source Address Validation Improvement
   (SAVI) documents ([RFC6620], [RFC7513], and [RFC7219]) that describe
   the different methods by which a switch can discover and record
   bindings between a node's IP 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
   technique used for their assignment: StateLess Address
   Autoconfiguration (SLAAC), the Dynamic Host Control Protocol (DHCP),
   and Secure Neighbor Discovery (SEND), respectively.  Each of these
   documents describes separately how one particular SAVI method deals
   with address collisions (same address but different binding anchor).

   While multiple IP assignment techniques can be used in the same layer
   2 domain, this means that a single SAVI device might have to deal
   with a combination or mix of SAVI methods.  The purpose of this
   document is to provide recommendations to avoid collisions and to
   review collision 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Problem Scope

   Three different IP address assignment techniques have been analyzed
   for SAVI:

   1.  StateLess Address Autoconfiguration (SLAAC) -- analyzed in FCFS
       SAVI (First-Come, First-Served) [RFC6620]

   2.  Dynamic Host Control Protocol address assignment (DHCP) --
       analyzed in SAVI-DHCP [RFC7513]

   3.  Secure Neighbor Discovery (SEND) address assignment -- analyzed
       in SEND SAVI [RFC7219]

   In addition, there is a fourth technique for managing (i.e.,
   creation, management, and deletion) a binding on the switch, referred
   to as "manual".  It is based on manual binding configuration.  How to
   manage manual bindings is determined by operators, so there is not a
   new SAVI method for manual addresses.

   All combinations of address assignment techniques can coexist within
   a layer 2 domain.  A SAVI device MUST implement the corresponding
   binding setup methods (referred to as "SAVI methods") for each such
   technique that is in use if it is to provide source address
   validation.

   SAVI methods are normally viewed as independent from each other, each
   one handling its own entries.  If multiple methods are used in the
   same device without coordination, each method will attempt to reject
   packets sourced with any addresses that method did not discover.  To
   prevent addresses discovered by one SAVI method from being filtered
   out by another method, the SAVI binding table SHOULD be shared by all
   the SAVI methods in use in the device.  This in turn could create
   some conflict when the same entry is discovered by two different
   methods.  The purpose of this document is twofold: to provide
   recommendations and methods to avoid conflicts and to resolve
   conflicts when they happen.  Collisions happening within a given
   method are outside the scope of this document.

4.  Architecture

   A SAVI device may implement and use multiple SAVI methods.  This
   mechanism, called "SAVI-MIX", is proposed as an arbiter of the
   binding generation algorithms from these multiple methods, generating
   the final binding entries as illustrated in Figure 1.  Once a SAVI
   method generates a candidate binding, it will request that SAVI-MIX
   set up a corresponding entry in the binding table.  Then, SAVI-MIX
   will check if there is any conflict in the binding table.  A new
   binding will be generated if there is no conflict.  If there is a
   conflict, SAVI-MIX will determine whether to replace the existing
   binding or reject the candidate binding based on the policies
   specified in Section 6.

   As a result of this, the packet filtering in the SAVI device will not
   be performed by each SAVI method separately.  Instead, the table
   resulting from applying SAVI-MIX will be used to perform filtering.
   Thus, the filtering is based on the combined results of the different
   SAVI mechanisms.  It is beyond the scope of this document to describe
   the details of the filtering mechanism and its use of the combined
   SAVI binding table.

   +--------------------------------------------------------+
   |                                                        |
   |                                        SAVI Device     |
   |                                                        |
   |                                                        |
   |     +------+    +------+    +------+                   |
   |     | SAVI |    | SAVI |    | SAVI |                   |
   |     |      |    |      |    |      |                   |
   |     | FCFS |    | DHCP |    | SEND |                   |
   |     +------+    +------+    +------+                   |
   |        |            |           |   Binding            |
   |        |            |           |   setup              |
   |        v            v           v   requests           |
   |     +------------------------------+                   |
   |     |                              |                   |
   |     |            SAVI-MIX          |                   |
   |     |                              |                   |
   |     +------------------------------+                   |
   |                     |                                  |
   |                     v               Final Binding      |
   |             +--------------+                           |
   |             |   Binding    |                           |
   |             |              |                           |
   |             |   Table      |                           |
   |             +--------------+                           |
   |                                                        |
   +--------------------------------------------------------+

                  Figure 1: SAVI-MIX Architecture

   Each entry in the binding table will contain the following fields:

   1.  IP source address

   2.  Binding anchor [RFC7039]

   3.  Lifetime

   4.  Creation time

   5.  Binding methods: the SAVI method used for this entry

5.  Recommendations for Assignment Separation

   If each address assignment technique uses a separate portion of the
   IP address space, collisions won't happen.  Using non-overlapping
   address space across address assignment techniques, and thus across
   SAVI methods, is therefore recommended.  To that end, one should:

   1.  DHCP and SLAAC: use a non-overlapping prefix for DHCP and SLAAC.
       Set the A bit in the Prefix Information option of the Router
       Advertisement for the SLAAC prefix, and set the M bit in the
       Router Advertisement for the DHCP prefix.  For detailed
       explanations of these bits, refer to [RFC4861] and [RFC4862].

   2.  SEND and non-SEND: avoid mixed environments (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 [RFC6085], in response to SEND/non-SEND Router
       Solicit.

6.  Resolving Binding Collisions

   In situations where collisions cannot be avoided by assignment
   separation, two cases should be considered:

   1.  The same address is bound on two different binding anchors by
       different SAVI methods.

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

6.1.  Same Address on Different Binding Anchors

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

6.1.1.  Basic Preference

   If there is any manually configured binding, the SAVI device SHOULD
   choose the manually configured binding anchor.

   For an address not covered by any manual bindings, the SAVI device
   must decide to which binding anchor the address should be bound
   (anchor-X or anchor-Y in this example).  Current standard documents
   of address assignment methods have implied the prioritization
   relationship based on order in time, i.e., First-Come, First-Served.

   o  SLAAC: Section 5.4.5 of [RFC4862]

   o  DHCPv4: Section 3.1, Point 5 of [RFC2131]

   o  DHCPv6: Section 18.1.8 of [RFC3315]

   o  SEND: Section 8 of [RFC3971]

   In the absence of any configuration or protocol hint (see
   Section 6.1.2), the SAVI device SHOULD choose the first-come binding
   anchor, whether it was learned from SLAAC, SEND, or DHCP.

6.1.2.  Exceptions

   There are two identified exceptions to the general prioritization
   model, one being Cryptographically Generated Addresses (CGA)
   [RFC3971] and the other controlled by the configuration of the
   switch.

6.1.2.1.  CGA Preference

   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 the "First-Come,
   First-Served" (FCFS) principle.

6.1.2.2.  Configuration Preference

   For configuration-driven exceptions, the SAVI device may allow the
   configuration of a triplet ("prefix", "anchor", "method") or
   ("address", "anchor", "method").  The "prefix" or "address"
   represents the address or address prefix to which this preference
   entry applies.  The "anchor" is the value of a known binding anchor
   that this device expects to see using this address or addresses from
   this prefix.  The "method" is the SAVI method that this device

   expects to use in validating address binding entries from the address
   or prefix.  At least one of "anchor" and "method" MUST be specified.
   Later, if a Duplicate Address Detection (DAD) message [RFC4861] is
   received with the following conditions verified:

   1.  The target in the DAD message does not exist in the binding
       table,

   2.  The target is within the configured "prefix" (or equal to
       "address"),

   3.  The anchor bound to the target is different from the configured
       anchor, when specified, and

   4.  The configured method, if any, is different from FCFS SAVI,

   then the switch SHOULD defend the address by responding to the DAD
   message, with a Neighbor Advertisement (NA) message, on behalf of the
   target node.  It SHOULD NOT install the entry into the binding table.
   The DAD message SHOULD be discarded and not forwarded.  Forwarding it
   may cause other SAVI devices to send additional defense NAs.  SEND
   nodes in the network MUST disable the option to ignore unsecured
   advertisements (see Section 8 of [RFC3971]).  If the option is
   enabled, the case is outside the scope of this document.  It is
   suggested to limit the rate of defense NAs to reduce security threats
   to the switch.  Otherwise, a malicious host could consume the
   resource of the switch heavily with flooding DAD messages.

   This will simply prevent the node from assigning the address and will
   de facto prioritize the configured anchor.  It is especially useful
   to protect well-known bindings (such as a static address of a server)
   against any other host, even when the server is down.  It is also a
   way to give priority to a binding learned from SAVI-DHCP over a
   binding for the same address, learned from FCFS SAVI.

6.1.3.  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 look up
   local bindings to identify a collision.  However, assuming DAD is
   performed throughout the security perimeter for all addresses
   regardless of the assignment method, then the DAD response will
   inform all SAVI devices about any collision.  In that case, "First-
   Come, First-Served" will apply the same way as in a single switch
   scenario.  If the admin configured a prefix (or a single static
   binding) on one of the switches to defend, the DAD response generated
   by this switch will also prevent the binding from being installed on

   other switches on the perimeter.  The SAVI-MIX preferences of all the
   SAVI devices in the same layer 2 domain should be consistent.
   Inconsistent configurations may cause network breaks.

6.2.  Same Address on the Same Binding Anchor

   A binding may be set up on the same binding anchor by multiple
   methods, typically FCFS SAVI and SAVI-DHCP.  If the binding lifetimes
   obtained from the two methods are different, priority should be given
   to 1) manual configuration, 2) SAVI-DHCP, 3) and FCFS SAVI as the
   least authoritative.  The binding will be removed when the
   prioritized lifetime expires, even if a less authoritative method had
   a longer lifetime.

7.  Security Considerations

   Combining SAVI methods (as in SAVI-MIX) does not improve or eliminate
   the security considerations associated with each individual SAVI
   method.  Therefore, security considerations for each enabled SAVI
   method should be addressed as described in that method's associated
   RFC.  Moreover, combining methods (as in SAVI-MIX) has two additional
   implications for security.  First, it may increase susceptibility to
   DoS attacks, because the SAVI binding setup rate will be the sum of
   the rates of all enabled SAVI methods.  Implementers must take these
   added resource requirements into account.  Second, because SAVI-MIX
   supports multiple binding mechanisms, it potentially reduces the
   security level to that of the weakest supported method, unless
   additional steps (e.g., requiring non-overlapping address spaces for
   different methods) are taken.

8.  Privacy Considerations

   When implementing multiple SAVI methods, privacy considerations of
   all methods apply cumulatively.

9.  IANA Considerations

   This document does not require any IANA registrations.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <http://www.rfc-editor.org/info/rfc3971>.

   [RFC6085]  Gundavelli, S., Townsley, M., Troan, O., and W. Dec,
              "Address Mapping of IPv6 Multicast Packets on Ethernet",
              RFC 6085, DOI 10.17487/RFC6085, January 2011,
              <http://www.rfc-editor.org/info/rfc6085>.

   [RFC6620]  Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
              SAVI: First-Come, First-Served Source Address Validation
              Improvement for Locally Assigned IPv6 Addresses",
              RFC 6620, DOI 10.17487/RFC6620, May 2012,
              <http://www.rfc-editor.org/info/rfc6620>.

   [RFC7219]  Bagnulo, M. and A. Garcia-Martinez, "SEcure Neighbor
              Discovery (SEND) Source Address Validation Improvement
              (SAVI)", RFC 7219, DOI 10.17487/RFC7219, May 2014,
              <http://www.rfc-editor.org/info/rfc7219>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <http://www.rfc-editor.org/info/rfc7513>.

10.2.  Informative References

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,
              <http://www.rfc-editor.org/info/rfc7039>.

Acknowledgments

   Thanks to Christian Vogt, Eric Nordmark, Marcelo Bagnulo Braun, David
   Lamparter, Scott G. Kelly, and Jari Arkko for their valuable
   contributions.

Authors' Addresses

   Jun Bi
   Tsinghua University
   Institute for Network Sciences and Cyberspace, Tsinghua University
   Beijing  100084
   China

   Email: junbi@tsinghua.edu.cn


   Guang Yao
   Tsinghua University/Baidu
   Baidu Science and Technology Park, Building 1
   Beijing  100193
   China

   Email: yaoguang.china@gmail.com


   Joel M. Halpern
   Ericsson

   Email: joel.halpern@ericsson.com


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

   Email: elevyabe@cisco.com