


Network Working Group                                        Jianping Wu
Internet-Draft                                                    Jun Bi
Intended status:  Informational                           Tsinghua Univ.
Expires:  July 24, 2012                                  Marcelo Bagnulo
                                                                    UC3M
                                                              Fred Baker
                                                                   Cisco
                                                     Christian Vogt, Ed.
                                                                Ericsson
                                                       December 27, 2011


            Source Address Validation Improvement Framework
                      draft-ietf-savi-framework-06

Abstract

   Source Address Validation Improvement methods were developed to
   prevent nodes attached to the same IP link from spoofing each other's
   IP addresses, so as to complement ingress filtering with finer-
   grained, standardized IP source address validation.  This document is
   a framework document, which describes and motivates the design of the
   SAVI methods.  Particular SAVI methods are described in other
   documents.

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
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   This Internet-Draft will expire on April 24, 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



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Model  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Deployment Options . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Scalability Optimizations  . . . . . . . . . . . . . . . . . .  8
   5.  Reliability Optimizations  . . . . . . . . . . . . . . . . . . 10
   6.  Scenario with Multiple Assignment Methods  . . . . . . . . . . 10
   7.  Prefix Configuration . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     11.1.  Informative References  . . . . . . . . . . . . . . . . . 13
     11.2.  Normative References  . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14



































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1.  Introduction

   Since IP source addresses are used by hosts and network entities to
   determine the origin of a packet and as a destination for return
   data, spoofing of IP source addresses can enable impersonation,
   concealment, and malicious traffic redirection.  Unfortunately, the
   Internet architecture does not prevent IP source address spoofing
   [draft-ietf-savi-threat-scope].  Since the IP source address of a
   packet generally takes no role in forwarding the packet, it can be
   selected arbitrarily by the sending host without jeopardizing packet
   delivery.  Extra methods are necessary for IP source address
   validation, to augment packet forwarding with an explicit check of
   whether a given packet's IP source address is legitimate.

   IP source address validation can happen at different granularity:
   Ingress filtering [BCP38] [BCP84], a widely deployed standard for IP
   source address validation, functions at the coarse granularity of
   networks.  It verifies that the prefix of an IP source address routes
   to the network from which the packet was received.  An advantage of
   ingress filtering is simplicity:  the decision of whether to accept
   or to reject an IP source address can be made solely based on the
   information available from routing protocols.  However, the
   simplicity comes at the cost of not being able to validate IP source
   addresses at a finer granularity, due to the aggregated nature of the
   information available from routing protocols.  Finer-grained IP
   source address validation would ensure that source address
   information is accurate, reduce the ability to launch denial-of-
   service attacks, and help with localizing hosts and identify
   misbehaving hosts.  Partial solutions [BA2007] exist for finer-
   grained IP source address validation, but are proprietary and hence
   often unsuitable for corporate procurement.

   The Source Address Validation Improvement method was developed to
   complement ingress filtering with standardized IP source address
   validation at the maximally fine granularity of individual IP
   addresses:  It prevents hosts attached to the same link from spoofing
   each other's IP addresses.  To facilitate deployment in networks of
   various kinds, the SAVI method was designed to be modular and
   extensible.  This document describes and motivates the design of the
   SAVI method.


2.  Model

   To enable network operators to deploy fine-grained IP source address
   validation without a dependency on supportive functionality on hosts,
   the SAVI method was designed to be purely network-based.  A SAVI
   instance enforces the hosts' use of legitimate IP source addresses



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   according to the following three-step model:

   1.  Identify which IP source addresses are legitimate for a host,
       based on monitoring packets exchanged by the host.

   2.  Bind a legitimate IP address to a link layer property of the
       host's network attachment.  This property, called a "binding
       anchor", must be verifiable in every packet that the host sends,
       and harder to spoof than the host's IP source address itself.

   3.  Enforce that the IP source addresses in packets match the binding
       anchors to which they were bound.

   This model allows SAVI functionality (a SAVI instance) to be located
   anywhere on the link to which the hosts attach, hence enabling
   different locations for a SAVI instance.  One way to locate a SAVI
   instance is in the hosts' default router.  IP source addresses are
   then validated in packets traversing the default router, yet the IP
   source addresses in packets exchanged locally on the link may bypass
   validation.  Another way to locate a SAVI instance is in a switch
   between the hosts and their default router.  Thus, packets may
   undergo IP source address validation even if exchanged locally on the
   link.

   The closer a SAVI instance is located to the hosts, the more
   effective the SAVI method is.  This is because each of the three
   steps of the SAVI model can best be accomplished in a position close
   to the host:

   o  Identifying a host's legitimate IP source addresses is most
      efficient close to the host, because the likelihood that the
      host's packets bypass a SAVI instance, and hence cannot be
      monitored, increases with the topological distance between the
      SAVI instance and the host.

   o  Selecting a binding anchor for a host's IP source address is
      easiest close to the host, because many link layer properties are
      unique for a given host only on a link segment directly attaching
      to the host.

   o  Enforcing a host's use of a legitimate IP source address is most
      reliable when pursued close to the host, because the likelihood
      that the host's packets bypass a SAVI instance, and hence do not
      undergo IP source address validation, increases with the
      topological distance between the SAVI instance and the host.

   The preferred location of SAVI instances is therefore close to hosts,
   such as in switches that directly attach to the hosts whose IP source



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   addresses are being validated.

   Nevertheless, it is useful for SAVI mechanisms to be able to handle
   situations where hosts are not directly attached to the SAVI-capable
   device.  For instance, deployments with both SAVI-capable and legacy
   switches could be supported.  In general, a SAVI solution needs to
   specify how it deals with a number of deployment scenarios and
   exceptional situations, including interaction with legacy devices,
   hosts moving between wireless attachment points, network partitions,
   and so on.

   Besides, in the case of legacy switches, the security level is lower,
   as there is no full protection for the hosts connected to the legacy
   server.


3.  Deployment Options

   The model of the SAVI method, as explained in Section 2, is
   deployment-specific in two ways:

   o  The identification of legitimate IP source addresses is dependent
      on the IP address assignment method in use on a link, since it is
      through assignment that a host becomes the legitimate user of an
      IP source address.

   o  Binding anchors are dependent on the technology used to build the
      link on which they are used, as binding anchors are link layer
      properties of a host's network attachment.

   To facilitate the deployment of the SAVI method in networks of
   various kinds, the SAVI method is designed to support different IP
   address assignment methods, and to function with different binding
   anchors.  Naturally, both the IP address assignment methods in use on
   a link and the available binding anchors have an impact on the
   functioning and the strength of IP source address validation.  The
   following two sub-sections explain this impact, and describe how the
   SAVI method accommodates this.

3.1.  IP Address Assignment Methods

   Since the SAVI method traces IP address assignment packets, it
   necessarily needs to incorporate logic that is specific to particular
   IP address assignment methods.  However, developing SAVI method
   variants for each IP address assignment method is alone not
   sufficient, since multiple IP address assignment methods may co-exist
   on a given link.  The SAVI method hence comes in multiple variants,
   e.g. for links with DHCP [rfc2131] [rfc3315], Stateless Address



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   Autoconfiguration [rfc4862] with or without Secure Neighbor Discovery
   [rfc3971], IKEv2 [rfc5996] [rfc5739] [rfc5026] and combinations
   thereof.

   The reason to develop SAVI method variants for each single IP address
   configuration method, in addition to the variant that handles all IP
   address assignment methods, is to minimize the complexity of the
   common case:  many link deployments today either are constrained to a
   single IP address assignment methods or, equivalently from the
   perspective of the SAVI method, separate IP address assignment
   methods into different IP address prefixes.  The SAVI method for such
   links can be simpler than the SAVI method for links with multiple IP
   address assignment methods per IP address prefix.

3.2.  Binding Anchors

   The SAVI method supports a range of binding anchors:

   o  The IEEE extended unique identifier, EUI-48 or EUI-64, of a host's
      interface.

   o  The port on an Ethernet switch to which a host attaches.

   o  The security association between a host and the base station on
      wireless links.

   o  The combination of a host interface's link-layer address and a
      customer relationship in cable modem networks.

   o  An ATM virtual channel, a PPP session identifier, or an L2TP
      session identifier in a DSL network.

   o  A tunnel that connects to a single host, such as an IP-in-IP
      tunnel, a GRE tunnel, or an MPLS label-switched path.

   The various binding anchors differ significantly in the security they
   provide.  IEEE extended unique identifiers, for example, fail to
   render a secure binding anchor because they can be spoofed with
   little effort.  And switch ports alone may be insufficient because
   they may connect to more than a single host, such as in the case of
   concatenated switches.

   Given this diversity in the security provided, one could define a set
   of possible binding anchors, and leave it up to the administrator to
   choose one or more of them.  Such a selection of binding anchors
   would, of course, have to be accompanied by an explanation of the
   pros and cons of the different binding anchors.  In addition, SAVI
   devices may have a default binding anchor depending on the lower



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   layers.  Such a default could be to use switch ports when available,
   and MAC addresses otherwise.  Or to use MAC addresses, and switch
   ports in addition if available.


4.  Scalability Optimizations

   The preference to locate a SAVI instance close to hosts implies that
   multiple SAVI instances must be able to co-exist in order to support
   large links.  Although the model of the SAVI method is independent of
   the number of SAVI instances per link, co-existence of multiple SAVI
   instances without further measures can lead to higher-than-necessary
   memory requirements:  since a SAVI instance creates bindings for the
   IP source addresses of all hosts on a link, bindings are replicated
   if multiple SAVI instances co-exist on the link.  High memory
   requirements, in turn, increase the cost of a SAVI instance.  This is
   problematic in particular for SAVI instances that are located on a
   switch, since it may significantly increase the cost of such a
   switch.

   To reduce memory requirements for SAVI instances that are located on
   a switch, the SAVI method enables the suppression of binding
   replication on links with multiple SAVI instances.  This requires
   manual disabling of IP source address validation on switch ports that
   connect to other switches running a SAVI instance.  Each SAVI
   instance is then responsible for validating IP source addresses only
   on those ports to which hosts attach either directly, or via switches
   without a SAVI instance.  On ports towards other switches running a
   SAVI instance, IP source addresses are not validated.  The switches
   running SAVI instances thus form a "protection perimeter".  The IP
   source addresses in packets passing the protection perimeter are
   validated by the ingress SAVI instance, but no further validation
   takes place as long as the packets remain within, or leave the
   protection perimeter.

















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                                                 ..............
                       protection perimeter -->  : +--------+ :
          +---+  +---+                           : |  SAVI  | :
          | A |  | B |  <-- hosts                : | switch | :
          +---+  +---+                           : +--------+ :
         ...|......|.............................:        |   :
         : +--------+          +--------+          +--------+ :
         : |  SAVI  |----------| legacy |          |  SAVI  | :
         : | switch |          | switch |----------| switch | :
         : +--------+          +--------+          +--------+ :
         :   |        ...............................|........:
         : +--------+ :                            +--------+
         : |  SAVI  | :                            | legacy |
         : | switch | :                            | switch |
         : +--------+ :                            +--------+
         :............:                             |      |
                                                  +---+  +---+
                                       hosts -->  | C |  | D |
                                                  +---+  +---+


                  Figure 1: Protection perimeter concept

   Figure 1 illustrates the concept of the protection perimeter.  The
   figure shows a link with six switches, of which four, denoted "SAVI
   switch", run a SAVI instance.  The protection perimeter created by
   the four SAVI instances is shown as a dotted line in the figure.  IP
   source address validation is enabled on all switch ports on the
   protection perimeter, and it is disabled on all other switch ports.
   Four hosts, denoted A through D in the figure, attach to the
   protection perimeter.

   In the example of figure Figure 1, the protection perimeter
   encompasses one of the legacy switches, located in the middle of the
   depicted link topology.  This enables a single, unpartitioned
   protection perimeter.  A single protection perimeter minimizes memory
   requirements for the SAVI instances because every binding is kept
   only once, namely, by the SAVI instance that attaches to the host
   being validated.  Excluding the legacy switch from the protection
   perimeter would result in two smaller protection perimeters to the
   left and to the right of the depicted link topology.  The memory
   requirements for the SAVI instances would then be higher:  since IP
   source address validation would be activated on the two ports
   connecting to the legacy switch, the SAVI instances adjacent to the
   legacy switch would replicate all bindings from the respectively
   other protection perimeter.  The reason why it is possible to include
   the legacy switch in the protection perimeter is because the depicted
   link topology guarantees that packets cannot enter the protection



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   perimeter via this legacy switch.  Without this guarantee, the legacy
   switch would have to be excluded from the protection perimeter in
   order to ensure that packets entering the protection perimeter
   undergo IP source address validation.


5.  Reliability Optimizations

   The explicit storage of legitimate IP addresses in the form of
   bindings implies that failure to create a binding, or the premature
   removal of bindings, can lead to loss of legitimate packets.  There
   are three situations in which this can happen:

   o  Legitimate IP address configuration packets, which should trigger
      the creation of a binding in a SAVI instance, are lost before
      reaching the SAVI instance.

   o  A SAVI instance loses a binding, for example, due to a restart.

   o  The link topology changes, resulting in hosts to communicate
      through SAVI instances that do not have a binding for those hosts'
      IP addresses.

   To limit the disruption that missing bindings for legitimate IP
   addresses can have, the SAVI method includes a mechanism for reactive
   binding creation based on regular packets.  This mechanism
   supplements the proactive binding creation based on IP address
   configuration packets.  Reactive binding creation occurs when a SAVI
   instances recognizes excessive drops of regular packets originating
   from the same IP address.  The SAVI instance then verifies whether
   said IP address is unique on the link.  How the verification is
   carried out depends on the IP address configuration method that the
   SAVI instance supports:  the SAVI method variant for Stateless
   Address Autoconfiguration and for Secure Neighbor Discovery verifies
   an IP address through the Duplicate Address Detection procedure.  The
   SAVI method variant for DHCP verifies an IP address through a DHCP
   Lease Query message exchange with the DHCP server.  If verification
   indicates that the IP address is unique on the link, the SAVI
   instance creates a binding for the IP address.  Otherwise, no binding
   is created, and packets sent from the IP address continue to be
   dropped.  These reliability issues should be addressed in all the
   SAVI protocols describing particular SAVI methods.


6.  Scenario with Multiple Assignment Methods

   While multiple assignment methods can be used on the same link, the
   SAVI device may have to deal with a mix of binding discovery methods.



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   If the address prefix used for each assignment method is different,
   mix scenario can handle the same as scenario with only one assignment
   method.  If different address assignment methods are used to assign
   addresses from the same prefix, additional considerations are needed
   because one binding mechanism may create a binding violating an
   existing binding from another binding mechanism, e.g., binding from
   SAVI-FCFS [savi-fcfs] may violate binding from SAVI-DHCP [savi-dhcp].
   Thus, the collision between different SAVI mechanisms in mix scenario
   must be handled in case more than one address assignment method is
   used to assign addresses from the same prefix.

   Prioritization relationship between different address assignment
   methods is used as the basis to solve possible collisions.  Current
   standard documents of address assignment methods have implied the
   prioritization relationship in general cases.  However, considering
   in some scenarios, default prioritization level may not be quite
   suitable.  Configurable prioritization level should be supported in a
   document of SAVI solution for the mix scenario.


7.  Prefix Configuration

   Before setting up a host-level granularity binding, it is important
   to configure correct prefixes on the SAVI device.  This document
   suggests set 3 prefix configuration mechanisms at SAVI device:

   o  Manual Prefix Configuration:  The allowed prefix scope of IPv4
      Addresses, IPv6 static addresses, IPv6 addresses assigned by
      SLAAC, and IPv6 addresses assigned by DHCPv6 can be set manually
      at SAVI device.  FE80::/64 is always a feasible prefix in IPv6.

   o  Prefix Configuration by RA Snooping:  The allowed prefix scope of
      IPv6 static addresses and IPv6 addresses assigned by Stateless
      Address Autoconfiguration (SLAAC) can be set at SAVI device
      through snooping RA message at SAVI device.

   o  Prefix Configuration by DHCP Prefix Delegation (DHCP-PD) Snooping:
      The allowed prefix scope of IPv6 static addresses, IPv6 addresses
      assigned by SLAAC, and IPv6 addresses assigned by DHCPv6 can be
      set through snooping DHCP-PD message at SAVI device.

   If some of the prefix scopes is set to have no prefix, it implies
   corresponding address assignment method is not allowed in the
   network.

   There is no need to explicitly present these prefix scopes, but these
   restrictions should be used as premier check in binding set up.




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   When SAVI is partially deployed, binding may fail as RA messages and
   DHCP-PD can be spoofed.  So it is recommended that Manual Prefix
   Configuration is used unless SAVI gets fully deployed.


8.  Acknowledgment

   The authors would like to thank the SAVI working group for a thorough
   technical discussion on the design and the framework of the SAVI
   method, as captured in this document, in particular Erik Nordmark,
   Guang Yao, Eric Levy-Abegnoli, and Alberto Garcia.  Thanks also to
   Torben Melsen for reviewing this document.

   This document was generated using the xml2rfc tool.


9.  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
   discretion.


10.  Security Considerations

   This document only discusses the possible methods to mitigate the
   usage of forged IP addresses.  Some such methods may rely on
   cryptographic methods, but not all do.  As a result, it is generally
   not possible to prove address ownership in any strong sense.  If
   binding anchor is not exclusive for each IP address, or is without
   strong security, addresses can still be forged.  Besides, the binding
   may not accord with the address management requirement, which can be
   more specified for each client.  However, given no new protocol is
   introduced, the improvements are still acceptable.  If there is
   requirement the usage of IP address must be of strong security, the
   only way is using cryptographic based authentication.


11.  References







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11.1.  Informative References

   [BA2007]   Baker, F., "Cisco IP Version 4 Source Guard", IETF
              Internet draft (work in progress), November 2007.

   [BCP38]    Paul, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", RFC 2827, BCP 38, May 2000.

   [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", RFC 3704, BCP 84, March 2004.

11.2.  Normative References

   [draft-ietf-savi-threat-scope]
              McPherson, D., Baker, F., and J. Halpern, "SAVI Threat
              Scope", draft-ietf-savi-threat-scope-04 (work in
              progress), March 2011.

   [rfc2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [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.

   [rfc4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [rfc5026]  Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6
              Bootstrapping in Split Scenario", RFC 5026, October 2007.

   [rfc5739]  Eronen, P., Laganier, J., and C. Madson, "IPv6
              Configuration in Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 5739, February 2010.

   [rfc5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [savi-dhcp]
              Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
              DHCP", draft-ietf-savi-dhcp-07 (work in progress),
              November 2010.




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   [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.


Authors' Addresses

   Jianping Wu
   Tsinghua University
   Computer Science, Tsinghua University
   Beijing  100084
   China

   Email:  jianping@cernet.edu.cn


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

   Email:  junbi@tsinghua.edu.cn


   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Avenida de la Universidad 30
   Leganes, Madrid  28911
   Spain

   Email:  marcelo@it.uc3m.es


   Fred Baker
   Cisco Systems
   Santa Barbara, CA  93117
   United States

   Email:  fred@cisco.com









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   Christian Vogt (editor)
   Ericsson
   200 Holger Way
   San Jose, CA  95134
   United States

   Email:  christian.vogt@ericsson.com












































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