Network Working Group                                        Jianping Wu
Internet-Draft                                                    Jun Bi
Intended status:  Informational                                   CERNET
Expires:  March 26,  April 28, 2011                                 Marcelo Bagnulo
                                                              Fred Baker
                                                     Christian Vogt, Ed.
                                                      September 22,
                                                        October 25, 2010

            Source Address Validation Improvement Protocol Framework


   The Source Address Validation Improvement protocol method was developed to
   complement ingress filtering with finer-grained, standardized IP
   source address validation.  This document describes and motivates the
   design of the SAVI protocol. method.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Protocol  Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Deployment Options  . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Scalability Optimizations . . . . . . . . . . . . . . . . . . . 6
   5.  Reliability Optimizations . . . . . . . . . . . . . . . . . . . 8
   6.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . . . 9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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.
   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], 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 be helpful to enable IP-source-
   address-based authentication, authorization, and host localization,
   as well as to efficiently 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 protocol 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 protocol method was designed to be modular and
   extensible.  This document describes and motivates the design of the
   SAVI protocol. method.

2.  Protocol  Model

   To enable network operators to deploy fine-grained IP source address
   validation without a dependency on supportive functionality on hosts,
   the SAVI protocol method was designed to be purely network-based.  A SAVI
   instance is located on the path of hosts' packets, enforcing the
   hosts' use of legitimate IP source addresses 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 a SAVI protocol instance to be located anywhere on the link
   to which the hosts attach, hence enabling different locations for a protocol
   SAVI instance.  One way to locate a SAVI protocol 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 protocol 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 protocol instance is located to the hosts, the more
   effective the SAVI protocol method is.  This is because each of the three
   steps of the SAVI protocol 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 protocol instance, and hence cannot be
      monitored, increases with the distance between the SAVI
      protocol 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 protocol instance, and hence do not
      undergo IP source address validation, increases with the distance
      between the protocol SAVI instance and the host.

   The preferred location of SAVI protocol instances is therefore close to hosts,
   such as in switches that directly attach to the hosts whose IP source
   addresses are being validated.

3.  Deployment Options

   The model of the SAVI protocol, 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 protocol method in networks of
   various kinds, the SAVI protocol 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 protocol method accommodates this.

3.1.  IP Address Assignment Methods

   Since the SAVI protocol method traces IP address assignment packets, it
   necessarily needs to incorporate logic that is specific to particular
   IP address assignment methods.  However, developing SAVI protocol 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 protocol method hence comes in multiple variants:
   for links with Stateless Address Autoconfiguration, for links with
   DHCP, for links with Secure Neighbor Discovery, and for links that
   use any combination of IP address assignment methods.

   The reason to develop SAVI protocol 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 protocol, method, separate IP address assignment
   methods into different IP address prefixes.  The SAVI protocol method for such
   links can be simpler than the SAVI protocol method for links with multiple IP
   address assignment methods per IP address prefix.

3.2.  Binding Anchors

   The SAVI protocol method supports a range of binding anchors:

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

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

   To reduce memory requirements for SAVI protocol instances that are located on
   a switch, the SAVI protocol method enables the suppression of binding
   replication on links with multiple protocol SAVI instances.  This requires
   manual disabling of IP source address validation on switch ports that
   connect to other switches running a SAVI protocol instance.  Each SAVI protocol
   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 protocol instance.  On ports towards other switches running a
   SAVI protocol instance, IP source addresses are not validated.  The switches
   running SAVI protocol instances thus form a "protection perimeter".  The IP
   source addresses in packets passing the protection perimeter are
   validated by the ingress SAVI protocol instance, but no further validation
   takes place as long as the packets remain within, or leave the
   protection perimeter.

                       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 protocol instance.  The protection perimeter created by
   the four SAVI protocol 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 protocol instances because every binding is kept
   only once, namely, by the SAVI protocol 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 protocol instances would then be higher:  Since IP
   source address validation would be activated on the two ports
   connecting to the legacy switch, the SAVI
   protocol 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
   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 protocol instance, are lost before
      reaching the SAVI protocol instance.

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

   o  The link topology changes, resulting in hosts to communicate
      through SAVI protocol 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 protocol 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 protocol 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 protocol instance supports:  The SAVI protocol method variant for Stateless
   Address Autoconfiguration and for Secure Neighbor Discovery verifies
   an IP address through the Duplicate Address Detection procedure.  The
   SAVI protocol 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 protocol
   instance creates a binding for the IP address.  Otherwise, no binding
   is created, and packets sent from the IP address continue to be

6.  Acknowledgment

   The author 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.

7.  References

   [BA2007]  Fred,  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.

Authors' Addresses

   Jianping Wu
   Computer Science, Tsinghua University
   Beijing  100084


   Jun Bi
   Network Research Center, Tsinghua University
   Beijing  100084


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


   Fred Baker
   Cisco Systems
   Santa Barbara, CA  93117
   United States


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