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RFC 6620 - FCFS SAVI: First-Come, First-Served Source Address Va


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Internet Engineering Task Force (IETF)                       E. Nordmark
Request for Comments: 6620                                 Cisco Systems
Category: Standards Track                                     M. Bagnulo
ISSN: 2070-1721                                                     UC3M
                                                        E. Levy-Abegnoli
                                                           Cisco Systems
                                                                May 2012

     FCFS SAVI: First-Come, First-Served Source Address Validation
            Improvement for Locally Assigned IPv6 Addresses

Abstract

   This memo describes First-Come, First-Served Source Address
   Validation Improvement (FCFS SAVI), a mechanism that provides source
   address validation for IPv6 networks using the FCFS principle.  The
   proposed mechanism is intended to complement ingress filtering
   techniques to help detect and prevent source address spoofing.

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 5741.

   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/rfc6620.

Copyright Notice

   Copyright (c) 2012 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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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................4
   2. Background to FCFS SAVI .........................................4
      2.1. Scope of FCFS SAVI .........................................4
      2.2. Constraints for FCFS SAVI Design ...........................5
      2.3. Address Ownership Proof ....................................5
      2.4. Binding Anchor Considerations ..............................6
      2.5. FCFS SAVI Protection Perimeter .............................6
      2.6. Special Cases .............................................10
   3. FCFS SAVI Specification ........................................11
      3.1. FCFS SAVI Data Structures .................................12
      3.2. FCFS SAVI Algorithm .......................................12
           3.2.1. Discovering On-Link Prefixes .......................12
           3.2.2. Processing of Transit Traffic ......................13
           3.2.3. Processing of Local Traffic ........................13
           3.2.4. FCFS SAVI Port Configuration Guidelines ............21
           3.2.5. VLAN Support .......................................22
      3.3. Default Protocol Values ...................................22
   4. Security Considerations ........................................22
      4.1. Denial-of-Service Attacks .................................22
      4.2. Residual Threats ..........................................23
      4.3. Privacy Considerations ....................................24
      4.4. Interaction with Secure Neighbor Discovery ................25
   5. Contributors ...................................................25
   6. Acknowledgments ................................................25
   7. References .....................................................26
      7.1. Normative References ......................................26
      7.2. Informative References ....................................26
   Appendix A.  Implications of Not Following the Recommended
                Behavior .............................................28
     A.1.  Implications of Not Generating DAD_NS Packets upon the
           Reception of Non-Compliant Data Packets ...................28
       A.1.1.  Lack of Binding State due to Packet Loss...............28
       A.1.2.  Lack of Binding State due to a Change in the
               Topology ..............................................31
       A.1.3.  Lack of Binding State due to State Loss ...............31
     A.2.  Implications of Not Discarding Non-Compliant Data
           Packets ...................................................35

1.  Introduction

   This memo describes FCFS SAVI, a mechanism that provides source
   address validation for IPv6 networks using the FCFS principle.  The
   proposed mechanism is intended to complement ingress filtering
   techniques to help detect and prevent source address spoofing.
   Section 2 gives the background and description of FCFS SAVI, and
   Section 3 specifies the FCFS SAVI protocol.

1.1.  Terminology

   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].

2.  Background to FCFS SAVI

2.1.  Scope of FCFS SAVI

   The application scenario for FCFS SAVI is limited to the local link.
   Hence, the goal of FCFS SAVI is to verify that the source address of
   the packets generated by the hosts attached to the local link have
   not been spoofed.

   In a link, hosts and routers are usually attached.  Hosts generate
   packets with their own address as the source address.  This is called
   "local traffic".  Routers send packets containing a source IP address
   other than their own, since they are forwarding packets generated by
   other hosts (usually located in a different link).  This is called
   "transit traffic".

   The applicability of FCFS SAVI is limited to the local traffic, i.e.,
   to verify if the traffic generated by the hosts attached to the local
   link contains a valid source address.  The verification of the source
   address of the transit traffic is out of the scope of FCFS SAVI.
   Other techniques, like ingress filtering [RFC2827], are recommended
   to validate transit traffic.  In that sense, FCFS SAVI complements
   ingress filtering, since it relies on ingress filtering to validate
   transit traffic, but it provides validation of local traffic, which
   is not provided by ingress filtering.  Hence, the security level is
   increased by using these two techniques.

   In addition, FCFS SAVI is designed to be used with locally assigned
   IPv6 addresses, in particular with IPv6 addresses configured through
   Stateless Address Autoconfiguration (SLAAC) [RFC4862].  Manually
   configured IPv6 addresses can be supported by FCFS SAVI, but manual
   configuration of the binding on the FCFS SAVI device provides higher
   security and seems compatible with manual address management.  FCFS

   SAVI can also be used with IPv6 addresses assigned via DHCPv6, since
   they ought to perform the Duplicate Address Detection (DAD)
   procedure, but there is a specific mechanism tailored for dealing
   with DHCP-assigned addresses defined in [SAVI-DHCP].  Additional
   considerations about how to use FCFS SAVI depending on the type of
   address management used and the nature of the addresses are discussed
   in the framework document [SAVI-FRAMEWORK].

2.2.  Constraints for FCFS SAVI Design

   FCFS SAVI is designed to be deployed in existing networks requiring a
   minimum set of changes.  For that reason, FCFS SAVI does not require
   any changes in the host whose source address is to be verified.  Any
   verification solely relies on the usage of already available
   protocols.  That is, FCFS SAVI does not define a new protocol, define
   any new message on existing protocols, or require that a host use an
   existent protocol message in a different way.  In other words, no
   host changes are required.

   FCFS SAVI validation is performed by the FCFS SAVI function.  The
   function can be placed in different types of devices, including a
   router or a Layer 2 (L2) bridge.  The basic idea is that the FCFS
   SAVI function is located in the points of the topology that can
   enforce the correct usage of the source address by dropping the non-
   compliant packets.

2.3.  Address Ownership Proof

   The main function performed by FCFS SAVI is to verify that the source
   address used in data packets actually belongs to the originator of
   the packet.  Since the FCFS SAVI scope is limited to the local link,
   the originator of the packet is attached to the local link.  In order
   to define a source address validation solution, we need to define the
   meaning of "address ownership", i.e., what it means that a given host
   owns a given address in the sense that the host is entitled to send
   packets with that source address.  With that definition, we can
   define how a device can confirm that the source address in a datagram
   is owned by the originator of the datagram.

   In FCFS SAVI, proof of address ownership is based on the First-Come,
   First-Served principle.  The first host that claims a given source
   address is the owner of the address until further notice.  Since no
   host changes are acceptable, we need to find the means to confirm
   address ownership without requiring a new protocol.  So, whenever a
   source address is used for the first time, a state is created in the
   device that is performing the FCFS SAVI function binding the source
   address to a binding anchor that consists of Layer 2 information that
   the FCFS SAVI box has available (e.g., the port in a switched LAN).

   Subsequent data packets containing that IP source address can be
   checked against the same binding anchor to confirm that the
   originator owns the source IP address.

   There are, however, additional considerations to be taken into
   account.  For instance, consider the case of a host that moves from
   one segment of a LAN to another segment of the same subnetwork and
   keeps the same IP address.  In this case, the host is still the owner
   of the IP address, but the associated binding anchor may have
   changed.  In order to cope with this case, the defined FCFS SAVI
   behavior implies verification of whether or not the host is still
   reachable using the previous binding anchor.  In order to do that,
   FCFS SAVI uses the Neighbor Discovery (ND) protocol.  If the host is
   no longer reachable at the previously recorded binding anchor, FCFS
   SAVI assumes that the new location is valid and creates a new binding
   using the new binding anchor.  In case the host is still reachable
   using the previously recorded binding anchor, the packets coming from
   the new binding anchor are dropped.

   Note that this only applies to local traffic.  Transit traffic
   generated by a router would be verified using alternative techniques,
   such as ingress filtering.  FCFS SAVI checks would not be fulfilled
   by the transit traffic, since the router is not the owner of the
   source address contained in the packets.

2.4.  Binding Anchor Considerations

   Any SAVI solution is not stronger than the binding anchor it uses.
   If the binding anchor is easily spoofable (e.g., a Media Access
   Control (MAC) address), then the resulting solution will be weak.
   The treatment of non-compliant packets needs to be tuned accordingly.
   In particular, if the binding anchor is easily spoofable and the FCFS
   SAVI device is configured to drop non-compliant packets, then the
   usage of FCFS SAVI may open a new vector of Denial-of-Service (DoS)
   attacks, based on spoofed binding anchors.  For that reason, in this
   specification, only switch ports MUST be used as binding anchors.
   Other forms of binding anchors are out of the scope of this
   specification, and proper analysis of the implications of using them,
   should be performed before their usage.

2.5.  FCFS SAVI Protection Perimeter

   FCFS SAVI provides perimetrical security.  FCFS SAVI devices form
   what can be called an FCFS SAVI protection perimeter, and they verify
   that any packet that crosses the perimeter is compliant (i.e., the
   source address is validated).  Once the packet is inside the
   perimeter, no further validations are performed on the packet.  This

   model has implications both on how FCFS SAVI devices are deployed in
   the topology and on the configuration of the FCFS SAVI boxes.

   The implication of this perimetrical security approach is that there
   is part of the topology that is inside the perimeter and part of the
   topology that is outside the perimeter.  So, while packets coming
   from interfaces connected to the external part of the topology need
   to be validated by the FCFS SAVI device, packets coming from
   interfaces connected to the internal part of the topology do not need
   to be validated.  This significantly reduces the processing
   requirements of the FCFS SAVI device.  It also implies that each FCFS
   SAVI device that is part of the perimeter must be able to verify the
   source addresses of the packets coming from the interfaces connected
   to the external part of the perimeter.  In order to do so, the FCFS
   SAVI device binds the source address to a binding anchor.

   One possible approach would be for every FCFS SAVI device to store
   binding information about every source address in the subnetwork.  In
   this case, every FCFS SAVI device would store a binding for each
   source address of the local link.  The problem with this approach is
   that it imposes a significant memory burden on the FCFS SAVI devices.
   In order to reduce the memory requirements imposed on each device,
   the FCFS SAVI solution described in this specification distributes
   the storage of FCFS SAVI binding information among the multiple FCFS
   SAVI devices of a subnetwork.  The FCFS SAVI binding state is
   distributed across the FCFS SAVI devices according to the following
   criterion: each FCFS SAVI device only stores binding information
   about the source addresses bound to anchors corresponding to the
   interfaces that connect to the part of the topology that is outside
   of the FCFS SAVI protection perimeter.  Since all the untrusted
   packet sources are by definition in the external part of the
   perimeter, packets generated by each of the untrusted sources will
   reach the perimeter through an interface of an FCFS SAVI device.  The
   binding information for that particular source address will be stored
   in the first FCFS SAVI device the packet reaches.

   The result is that the FCFS SAVI binding information will be
   distributed across multiple devices.  In order to provide proper
   source address validation, it is critical that the information
   distributed among the different FCFS SAVI devices be coherent.  In
   particular, it is important to avoid having the same source address
   bound to different binding anchors in different FCFS SAVI devices.
   Should that occur, then it would mean that two hosts are allowed to
   send packets with the same source address, which is what FCFS SAVI is
   trying to prevent.  In order to preserve the coherency of the FCFS
   SAVI bindings distributed among the FCFS SAVI devices within a realm,
   the Neighbor Discovery (ND) protocol [RFC4861] is used, in particular
   the Neighbor Solicitation (NS) and Neighbor Advertisement (NA)

   messages.  Following is a simplified example of how this might work.
   Before creating an FCFS SAVI binding in the local FCFS SAVI database,
   the FCFS SAVI device will send an NS message querying for the address
   involved.  Should any host reply to that message with an NA message,
   the FCFS SAVI device that sent the NS will infer that a binding for
   that address exists in another FCFS SAVI device and will not create a
   local binding for it.  If no NA message is received as a reply to the
   NS, then the local FCFS SAVI device will infer that no binding for
   that address exists in other FCFS SAVI device and will create the
   local FCFS SAVI binding for that address.

   To summarize, the proposed FCFS SAVI approach relies on the following
   design choices:

   o  An FCFS SAVI provides perimetrical security, so some interfaces of
      an FCFS SAVI device will connect to the internal (trusted) part of
      the topology, and other interfaces will connect to the external
      (untrusted) part of the topology.

   o  An FCFS SAVI device only verifies packets coming through an
      interface connected to the untrusted part of the topology.

   o  An FCFS SAVI device only stores binding information for the source
      addresses that are bound to binding anchors that correspond to
      interfaces that connect to the untrusted part of the topology.

   o  An FCFS SAVI uses NS and NA messages to preserve the coherency of
      the FCFS SAVI binding state distributed among the FCFS SAVI
      devices within a realm.

   So, in a link that is constituted of multiple L2 devices, some of
   which are FCFS SAVI capable and some of which are not, the FCFS-SAVI-
   capable devices MUST be deployed forming a connected perimeter (i.e.,
   no data packet can get inside the perimeter without passing through
   an FCFS SAVI device).  Packets that cross the perimeter will be
   validated while packets that do not cross the perimeter are not
   validated (hence, FCFS SAVI protection is not provided for these
   packets).  Consider the deployment of FCFS SAVI in the topology
   depicted in the following figure:

                                                +--------+
      +--+   +--+                          +--+ | +--+   |
      |H1|   |H2|                          |H3| | |R1|   |
      +--+   +--+                          +--+ | +--+   |
        |     |                              |  |  |     |
   +-------------SAVI-PROTECTION-PERIMETER------+  |     |
   |    |     |                              |     |     |
   |  +-1-----2-+                          +-1-----2-+   |
   |  |  SAVI1  |                          |  SAVI2  |   |
   |  +-3--4----+                          +--3------+   |
   |    |  |          +--------------+        |          |
   |    |  +----------|              |--------+          |
   |    |             |   SWITCH-A   |                   |
   |    |  +----------|              |--------+          |
   |    |  |          +--------------+        |          |
   |  +-1--2----+                          +--1------+   |
   |  |  SAVI3  |                          |  SAVI4  |   |
   |  +-3-----4-+                          +----4----+   |
   |    |     |                                 |        |
   |      +------SAVI-PROTECTION-PERIMETER---------------+
   |    | |   |                                 |
   |  +--+|  +--+                            +---------+
   |  |R2||  |H4|                            |SWITCH-B |
   |  +--+|  +--+                            +---------+
   +------+                                    |    |
                                             +--+  +--+
                                             |H5|  |H6|
                                             +--+  +--+

                    Figure 1: SAVI Protection Perimeter

   In Figure 1, the FCFS SAVI protection perimeter is provided by four
   FCFS SAVI devices, namely SAVI1, SAVI2, SAVI3, and SAVI4.  These
   devices verify the source address and filter packets accordingly.

   FCFS SAVI devices then have two types of ports: Trusted Ports and
   Validating Ports.

   o  Validating Ports (VPs) are those in which FCFS SAVI processing is
      performed.  When a packet is received through one of the
      Validating Ports, FCFS SAVI processing and filtering will be
      executed.

   o  Trusted Ports (TPs) are those in which FCFS SAVI processing is not
      performed.  So, packets received through Trusted Ports are not
      validated, and no FCFS SAVI processing is performed on them.

   Trusted Ports are used for connections with trusted infrastructure,
   including the communication between FCFS SAVI devices, the
   communication with routers, and the communication of other switches
   that, while not FCFS SAVI devices, only connect to trusted
   infrastructure (i.e., other FCFS SAVI devices, routers, or other
   trusted nodes).  So, in Figure 1, Port 3 of SAVI1 and Port 1 of SAVI3
   are trusted because they connect two FCFS SAVI devices.  Port 4 of
   SAVI1, Port 3 of SAVI2, Port 2 of SAVI3, and Port 1 of SAVI4 are
   trusted because they connect to SWITCH-A, to which only trusted nodes
   are connected.  In Figure 1, Port 2 of SAVI2 and Port 3 of SAVI3 are
   Trusted Ports because they connect to routers.

   Validating Ports are used for connection with non-trusted
   infrastructure.  In particular, hosts are normally connected to
   Validating Ports.  Non-SAVI switches that are outside of the FCFS
   SAVI protection perimeter also are connected through Validating
   Ports.  In particular, non-SAVI devices that connect directly to
   hosts or that have no SAVI-capable device between themselves and the
   hosts are connected through a Validating Port.  So, in Figure 1,
   Ports 1 and 2 of SAVI1, Port 1 of SAVI2, and Port 4 of SAVI 3 are
   Validating Ports because they connect to hosts.  Port 4 of SAVI4 is
   also a Validating Port because it is connected to SWITCH-B, which is
   a non-SAVI-capable switch that is connected to hosts H5 and H6.

2.6.  Special Cases

   Multi-subnet links: In some cases, a given subnet may have several
   prefixes.  This is directly supported by SAVI as any port can support
   multiple prefixes.  Forwarding of packets between different prefixes
   involving a router is even supported, as long as the router is
   connected to a Trusted Port, as recommended for all the routers.

   Multihomed hosts: A multihomed host is a host with multiple
   interfaces.  The interaction between SAVI and multihomed hosts is as
   follows.  If the different interfaces of the host are assigned
   different IP addresses and packets sent from each interface always
   carry the address assigned to that interface as the source address,
   then from the perspective of a SAVI device, this is equivalent to two
   hosts with a single interface, each with an IP address.  This is

   supported by SAVI without the need for additional considerations.  If
   the different interfaces share the same IP address or if the
   interfaces have different addresses but the host sends packets using
   the address of one of the interfaces through any of the interfaces,
   then SAVI does not directly support it.  It would require either
   connecting at least one interface of the multihomed host to a Trusted
   Port or manually configuring the SAVI bindings to allow binding the
   address of the multihomed host to multiple anchors simultaneously.

   Untrusted routers: One can envision scenarios where routers are
   dynamically attached to an FCFS SAVI network.  A typical example
   would be a mobile phone connecting to an FCFS SAVI switch where the
   mobile phone is acting as a router for other personal devices that
   are accessing the network through it.  In this case, the router does
   not seem to directly fall in the category of trusted infrastructure
   (if this was the case, it is likely that all devices would be
   trusted); hence, it cannot be connected to a Trusted Port and if it
   is connected to a Validating Port, the FCFS SAVI switch would discard
   all the packets containing an off-link source address coming from
   that device.  As a result, the default recommendation specified in
   this specification does not support such a scenario.

3.  FCFS SAVI Specification

3.1.  FCFS SAVI Data Structures

   The FCFS SAVI function relies on state information binding the source
   address used in data packets to the binding anchor that contained the
   first packet that used that source IP address.  Such information is
   stored in an FCFS SAVI database (DB).  The FCFS SAVI DB will contain
   a set of entries about the currently used IP source addresses.  Each
   entry will contain the following information:

   o  IP source address

   o  Binding anchor: port through which the packet was received

   o  Lifetime

   o  Status: either TENTATIVE, VALID, TESTING_VP, or TESTING_TP-LT

   o  Creation time: the value of the local clock when the entry was
      firstly created

   In addition, FCFS SAVI needs to know what prefixes are directly
   connected, so it maintains a data structure called the FCFS SAVI
   Prefix List, which contains:

   o  Prefix

   o  Interface where prefix is directly connected

3.2.  FCFS SAVI Algorithm

3.2.1.  Discovering On-Link Prefixes

   In order to distinguish local traffic from transit traffic, the FCFS
   SAVI device relies on the FCFS SAVI Prefix List, which contains the
   set of on-link IPv6 prefixes.  An FCFS SAVI device MUST support the
   following two methods for populating the Prefix List: manual
   configuration and Router Advertisement, as detailed next.

   Manual configuration: An FCFS SAVI device MUST support manual
   configuration of the on-link prefixes included in the Prefix List.
   For example, this can be used when there are no prefixes being
   advertised on the link.

   Router Advertisement: An FCFS SAVI device MUST support discovery of
   on-link prefixes through Router Advertisement messages in Trusted
   Ports.  For Trusted Ports, the FCFS SAVI device will learn the on-
   link prefixes following the procedure defined for a host to process
   the Prefix Information options described in Section 6.3.4 of
   [RFC4861] with the difference that the prefixes will be configured in
   the FCFS SAVI Prefix List rather than in the ND Prefix List.  In
   addition, when the FCFS SAVI device boots, it MUST send a Router
   Solicitation message as described in Section 6.3.7 of [RFC4861],
   using the unspecified source address.

3.2.2.  Processing of Transit Traffic

   The FCFS SAVI function is located in a forwarding device, such as a
   router or a Layer 2 switch.  The following processing is performed
   depending on the type of port through which the packet has been
   received:

   o  If the data packet is received through a Trusted Port, the data
      packet is forwarded, and no SAVI processing performed on the
      packet.

   o  If the data packet is received through a Validating Port, then the
      FCFS SAVI function checks whether the received data packet is
      local traffic or transit traffic.  It does so by verifying if the
      source address of the packet belongs to one of the directly
      connected prefixes available in the receiving interface.  It does
      so by searching the FCFS SAVI Prefix List.

      *  If the IP source address does not belong to one of the on-link
         prefixes of the receiving interface, the data packet is transit
         traffic, and the packet SHOULD be discarded.  (If for some
         reason, discarding the packets is not acceptable, logging or
         triggering of alarms MAY be used).  The FCFS SAVI function MAY
         send an ICMP Destination Unreachable Error back to the source
         address of the data packet, and ICMPv6, code 5 (Source address
         failed ingress/egress policy), should be used.

      *  If the source address of the packet does belong to one of the
         prefixes available in the receiving port, then the FCFS SAVI
         local traffic validation process is executed as described
         below.

      *  If the source address of the packet is an unspecified address,
         the packet is forwarded, and no SAVI processing is performed
         except for the case of the Neighbor Solicitation messages
         involved in the Duplicate Address Detection, which are treated
         as described in Section 3.2.3.

3.2.3.  Processing of Local Traffic

   We next describe how local traffic, including both control and data
   packets, is processed by the FCFS SAVI device using a state machine
   approach.

   The state machine described is for the binding of a given source IP
   address (called IPAddr) in a given FCFS SAVI device.  This means that
   all the packets described as inputs in the state machine above refer
   to that given IP address.  In the case of data packets, the source
   address of the packet is IPAddr.  In the case of the DAD_NS packets,
   the Target Address is IPAddr.  The key attribute is the IP address.
   The full state information is as follows:

   o  IP ADDRESS: IPAddr

   o  BINDING ANCHOR: P

   o  LIFETIME: LT

   The possible states are as follows:

   o  NO_BIND

   o  TENTATIVE

   o  VALID

   o  TESTING_TP-LT

   o  TESTING_VP

   We will use VP for Validating Port and TP for Trusted Port.

   After bootstrapping (when no binding exists), the state for all
   source IP addresses is NO-BIND, i.e., there is no binding for the IP
   address to any binding anchor.

   NO_BIND: The binding for a source IP address entry is in this state
   when it does not have any binding to an anchor.  All addresses are in
   this state by default after bootstrapping, unless bindings were
   created for them.

   TENTATIVE: The binding for a source address for which a data packet
   or an NS generated by the Duplicate Address Detection (DAD) procedure
   has been received is in this state during the waiting period during
   which the DAD procedure is being executed (either by the host itself
   or the FCFS SAVI device on its behalf).

   VALID: The binding for the source address is in this state after it
   has been verified.  It means that it is valid and usable for
   filtering traffic.

   TESTING_TP-LT: A binding for a source address enters this state due
   to one of two reasons:

   o  When a Duplicate Address Detection Neighbor Solicitation has been
      received through a Trusted Port.  This implies that a host is
      performing the DAD procedure for that source address in another
      switch.  This may be due to an attack or to the fact that the host
      may have moved.  The binding in this state is then being tested to
      determine which is the situation.

   o  The lifetime of the binding entry is about to expire.  This is due
      to the fact that no packets have been seen by the FCFS SAVI device
      for the LIFETIME period.  This may be due to the host simply being
      silent or because the host has left the location.  In order to
      determine which is the case, a test is performed to determine if
      the binding information should be discarded.

   TESTING_VP: A binding for a source address enters this state when a
   Duplicate Address Detection Neighbor Solicitation or a data packet
   has been received through a Validating Port other than the one
   address to which it is currently bound.  This implies that a host is
   performing the DAD procedure for that source address through a
   different port.  This may be due to an attack, the fact that the host

   may have moved, or just because another host tries to configure an
   address already used.  The binding in this state is then being tested
   to determine which is the situation.

   Next, we describe how the different inputs are processed depending on
   the state of the binding of the IP address (IPAddr).

   A simplified figure of the state machine is included in Figure 2
   below.

   NO_BIND

   o  Upon the reception through a Validating Port (VP) of a Neighbor
      Solicitation (NS) generated by the Duplicate Address Detection
      (DAD) procedure (hereafter named DAD_NS) containing Target Address
      IPAddr, the FCFS SAVI device MUST forward the NS, and T_WAIT
      milliseconds later, it MUST send a copy of the same message.
      These DAD_NS messages are not sent through any of the ports
      configured as Validating Ports.  The DAD_NS messages are sent
      through the Trusted Ports (but, of course, subject to usual switch
      behavior and possible Multicast Listener Discovery (MLD) snooping
      optimizations).  The state is moved to TENTATIVE.  The LIFETIME is
      set to TENT_LT (i.e., LT:=TENT_LT), the BINDING ANCHOR is set to
      VP (i.e., P:=VP), and the Creation time is set to the current
      value of the local clock.

   o  Upon the reception through a Validating Port (VP) of a DATA packet
      containing IPAddr as the source address, the SAVI device SHOULD
      execute the process of sending Neighbor Solicitation messages of
      the Duplicate Address Detection process as described in Section
      5.4.2 of [RFC4862] for the IPAddr using the following default
      parameters: DupAddrDetectTransmits set to 2 (i.e., 2 Neighbor
      Solicitation messages for that address will be sent by the SAVI
      device) and RetransTimer set to T_WAIT milliseconds (i.e., the
      time between two Neighbor Solicitation messages is T_WAIT
      milliseconds).  The implications of not following the recommended
      behavior are described in Appendix A.  The DAD_NS messages are not
      sent through any of the ports configured as Validating Ports.  The
      DAD_NSOL messages are sent through Trusted Ports (but, of course,
      subject to usual switch behavior and possible MLD snooping
      optimizations).  The SAVI device MAY discard the data packets
      while the DAD procedure is being executed, or it MAY store them
      until the binding is created.  In any case, it MUST NOT forward
      the data packets until the binding has been verified.  The state
      is moved to TENTATIVE.  The LIFETIME is set to TENT_LT (i.e., LT:
      =TENT_LT), the BINDING ANCHOR is set to VP (i.e., P:=VP), and the
      Creation time is set to the current value of the local clock.

   o  Data packets containing IPAddr as the source address received
      through Trusted Ports are processed and forwarded as usual (i.e.,
      no special SAVI processing).

   o  DAD_NS packets containing IPAddr as the Target Address that are
      received through a Trusted Port MUST NOT be forwarded through any
      of the Validating Ports, but they are sent through the Trusted
      Ports (but, of course, subject to usual switch behavior and
      possible MLD snooping optimizations).

   o  Neighbor Advertisement packets sent to all nodes as a reply to the
      DAD_NS (hereafter called DAD_NA) containing IPAddr as the Target
      Address coming through a Validating Port are discarded.

   o  Other signaling packets are processed and forwarded as usual
      (i.e., no SAVI processing).

   TENTATIVE

   o  If the LIFETIME times out, the state is moved to VALID.  The
      LIFETIME is set to DEFAULT_LT (i.e., LT:= DEFAULT_LT).  Stored
      data packets (if any) are forwarded.

   o  If a Neighbor Advertisement (NA) is received through a Trusted
      Port with the Target Address set to IPAddr, then the message is
      forwarded through port P, the state is set to NO_BIND, and the
      BINDING ANCHOR and the LIFETIME are cleared.  Data packets stored
      corresponding to this binding are discarded.

   o  If an NA is received through a Validating Port with the Target
      Address set to IPAddr, the NA packet is discarded

   o  If a data packet with source address IPAddr is received with
      binding anchor equal to P, then the packet is either stored or
      discarded.

   o  If a data packet with source address IPAddr is received through a
      Trusted Port, the data packet is forwarded.  The state is
      unchanged.

   o  If a data packet with source address IPAddr is received through a
      Validating Port other than P, the data packet is discarded.

   o  If a DAD_NS is received from a Trusted Port, with the Target
      Address set to IPAddr, then the message is forwarded to the
      Validating Port P, the state is set to NO_BIND, and the BINDING
      ANCHOR and LIFETIME are cleared.  Data packets stored
      corresponding to this binding are discarded.

   o  If a DAD_NS with the Target Address set to IPAddr is received from
      a Validating Port P' other than P, the message is forwarded to the
      Validating Port P and to the Trusted Ports, and the state remains
      in TENTATIVE; however, the BINDING ANCHOR is changed from P to P',
      and LIFETIME is set to TENT_LT.  Data packets stored corresponding
      to the binding with P are discarded.

   o  Other signaling packets are processed and forwarded as usual
      (i.e., no SAVI processing).

   VALID

   o  If a data packet containing IPAddr as the source address arrives
      from Validating Port P, then the LIFETIME is set to DEFAULT_LT and
      the packet is forwarded as usual.

   o  If a DAD_NS is received from a Trusted Port, then the DAD_NS
      message is forwarded to port P and is also forwarded to the
      Trusted Ports (but, of course, subject to usual switch behavior
      and possible MLD snooping optimizations).  The state is changed to
      TESTING_TP-LT.  The LIFETIME is set to TENT_LT.

   o  If a data packet containing source address IPAddr or a DAD_NA
      packet with the Target Address set to IPAddr is received through a
      Validating Port P' other than P, then the SAVI device will execute
      the process of sending DAD_NS messages as described in Section
      5.4.2 of [RFC4862] for the IPAddr using the following default
      parameters: DupAddrDetectTransmits set to 2 (i.e., two NS messages
      for that address will be sent by the SAVI device) and RetransTimer
      set to T_WAIT milliseconds (i.e., the time between two NS messages
      is T_WAIT milliseconds).  The DAD_NS message will be forwarded to
      the port P.  The state is moved to TESTING_VP.  The LIFETIME is
      set to TENT_LT.  The SAVI device MAY discard the data packet while
      the DAD procedure is being executed, or it MAY store them until
      the binding is created.  In any case, it MUST NOT forward the data
      packets until the binding has been verified.

   o  If a DAD_NS packet with the Target Address set to IPAddr is
      received through a Validating Port P' other than P, then the SAVI
      device will forward the DAD_NS packet, and T_WAIT milliseconds
      later, it will execute the process of sending DAD_NS messages as
      described in Section 5.4.2 of [RFC4862] for the IPAddr using the
      following default parameters: DupAddrDetectTransmits set to 1 and
      RetransTimer set to T_WAIT milliseconds.  The DAD_NS messages will
      be forwarded to the port P.  The state is moved to TESTING_VP.
      The LIFETIME is set to TENT_LT.  The SAVI device MAY discard the
      data packets while the DAD procedure is being executed, or it MAY

      store them until the binding is created.  In any case, it MUST NOT
      forward the data packets until the binding has been verified.

   o  If the LIFETIME expires, then the SAVI device will execute the
      process of sending DAD_NS messages as described in Section 5.4.2
      of [RFC4862] for the IPAddr using the following default
      parameters: DupAddrDetectTransmits set to 2 (i.e., two NS messages
      for that address will be sent by the SAVI device) and RetransTimer
      set to T_WAIT milliseconds (i.e., the time between two NS messages
      is T_WAIT milliseconds).  The DAD_NS messages will be forwarded to
      the port P.  The state is changed to TESTING_TP-LT, and the
      LIFETIME is set to TENT_LT.

   o  If a data packet containing IPAddr as a source address arrives
      from Trusted Port, the packet MAY be discarded.  The event MAY be
      logged.

   o  Other signaling packets are processed and forwarded as usual
      (i.e., no SAVI processing).  In particular, a DAD_NA coming from
      port P and containing IPAddr as the Target Address is forwarded as
      usual.

   TESTING_TP-LT

   o  If the LIFETIME expires, the BINDING ANCHOR is cleared, and the
      state is changed to NO_BIND.

   o  If an NA message containing the IPAddr as the Target Address is
      received through the Validating Port P as a reply to the DAD_NS
      message, then the NA is forwarded as usual, and the state is
      changed to VALID.  The LIFETIME is set to DEFAULT_LT

   o  If a data packet containing IPAddr as the source address is
      received through port P, then the packet is forwarded and the
      state is changed to VALID.  The LIFETIME is set to DEFAULT_LT.

   o  If a DAD_NS is received from a Trusted Port, the DAD_NS is
      forwarded as usual.

   o  If a DAD_NS is received from a Validating Port P' other than P,
      the DAD_NS is forwarded as usual, and the state is moved to
      TESTING_VP.

   o  If a data packet is received through a Validating Port P' that is
      other than port P, then the packet is discarded.

   o  If a data packet is received through a Trusted Port, then the
      packet MAY be discarded.  The event MAY be logged.

   TESTING_VP

   o  If the LIFETIME expires, the BINDING ANCHOR is modified from P to
      P', the LIFETIME is set to DEFAULT_LT, and the state is changed to
      VALID.  Stored data packet coming from P' are forwarded.

   o  If an NA message containing the IPAddr as the Target Address is
      received through the Validating Port P as a reply to the DAD_NS
      message, then the NA is forwarded as usual and the state is
      changed to VALID.  The LIFETIME is set to DEFAULT_LT.

   o  If a data packet containing IPAddr as the source address is
      received through port P, then the packet is forwarded.

   o  If a data packet containing IPAddr as the source address is
      received through a Validating Port P'' that is other than port P
      or P', then the packet is discarded.

   o  If a data packet containing IPAddr as the source address is
      received through a Trusted Port (i.e., other than port P), the
      state is moved to TESTING_TP-LT, and the packet MAY be discarded.

   o  If a DAD_NS is received through a Trusted Port, the packet is
      forwarded as usual, and the state is moved to TESTING_TP-LT.

   o  If a DAD_NS is received through Validating Port P'' other than P
      or P', the packet is forwarded as usual, and P'' is stored as the
      tentative port, i.e., P':=P''.  The state remains the same.

   +---------+  VP_NS, VP_DATA/2xNS                    +-----------+
   |         |---------------------------------------->|           |
   | NO_BIND |                                         | TENTATIVE |
   |         |<----------------------------------------|           |
   +---------+                    TP_NA, TP_NS/-       +-----------+
          ^                                                |
          |                                                | TimeOut
   Timeout|                                                |
          |                                                v
   +---------+  VP_NA/-                                +-----------+
   |         |---------------------------------------->|           |
   | TESTING |                                TP_NS/-  |           |
   |  TP-LT  |<----------------------------------------|   VALID   |
   |         |                           TimeOut/2xNS  |           |
   |         |<----------------------------------------|           |
   +---------+                                         +-----------+
     ^   |                                                ^    |
     |   |                                                |    |
     |   +---------------------      ---------------------+    |
     |       VP_NS/-          |     |  NP_NA, TimeOut/-        |
     |                        v     |                          |
     |                     +-----------+                       |
     |                     |           |                       |
     +---------------------|  TESTING  |<----------------------+
          VP_NS, VP_DATA/- |    VP     |  VP_DATA, VP_NS,
                           +-----------+  VP_NA/2xNS

                    Figure 2: Simplified State Machine

   MLD Considerations

   The FCFS SAVI device MUST join the solicited node multicast group for
   all the addresses with a state other than NO_BIND.  This is needed to
   make sure that the FCFS SAVI device will receive the DAD_NS for those
   addresses.  Please note that it may not be enough to rely on the host
   behind the Validating Port to do so, since the node may move, and
   after a while, the packets for that particular solicited node
   multicast group will no longer be forwarded to the FCFS SAVI device.
   Therefore, the FCFS SAVI device MUST join the solicited node
   multicast groups for all the addresses that are in a state other than
   NO_BIND.

3.2.4.  FCFS SAVI Port Configuration Guidelines

   The guidelines for port configuration in FCFS SAVI devices are as
   follows:

   o  The FCFS SAVI realm (i.e., the realm that is inside the FCFS SAVI
      protection perimeter) MUST be connected.  If this is not the case,
      legitimate transit traffic may be dropped.

   o  Ports that are connected to another FCFS SAVI device MUST be
      configured as Trusted Ports.  Not doing so will significantly
      increase the memory consumption in the FCFS SAVI devices and may
      result in legitimate transit traffic being dropped.

   o  Ports connected to hosts SHOULD be configured as Validating Ports.
      Not doing so will allow the host connected to that port to send
      packets with spoofed source addresses.  A valid exception is the
      case of a trusted host (e.g., a server) that could be connected to
      a Trusted Port, but untrusted hosts MUST be connected to
      Validating Ports.

   o  Ports connected to routers MUST be configured as Trusted Ports.
      Configuring them as Validating Ports should result in transit
      traffic being dropped.

   o  Ports connected to a chain of one or more legacy switches that
      have hosts connected SHOULD be configured as Validating Ports.
      Not doing so will allow the host connected to any of these
      switches to send packets with spoofed source addresses.  A valid
      exception is the case where the legacy switch only has trusted
      hosts attached, in which case it could be connected to a Trusted
      Port, but if there is at least one untrusted hosts connected to
      the legacy switch, then it MUST be connected to Validating Ports.

   o  Ports connected to a chain of one or more legacy switches that
      have other FCFS SAVI devices and/or routers connected but had no
      hosts attached to them MUST be configured as Trusted Ports.  Not
      doing so will at least significantly increase the memory
      consumption in the FCFS SAVI devices, increase the signaling
      traffic due to FCFS SAVI validation, and may result in legitimate
      transit traffic being dropped.

3.2.5.  VLAN Support

   If the FCFS SAVI device is a switch that supports customer VLANs
   [IEEE.802-1Q.2005], the FCFS SAVI implementation MUST behave as if
   there was one FCFS SAVI process per customer VLAN.  The FCFS SAVI
   process of each customer VLAN will store the binding information
   corresponding to the nodes attached to that particular customer VLAN.

3.3.  Default Protocol Values

   Following are the default values used in the FCFS SAVI specification.

   TENT_LT is 500 milliseconds

   DEFAULT_LT is 5 minutes

   T_WAIT is 250 milliseconds

   An implementation MAY allow these values to be modified, but tuning
   them precisely is considered out of the scope of this document.

4.  Security Considerations

4.1.  Denial-of-Service Attacks

   There are two types of Denial-of-Service (DoS) attacks [RFC4732] that
   can be envisaged in an FCFS SAVI environment.  On one hand, we can
   envision attacks against the FCFS SAVI device resources.  On the
   other hand, we can envision DoS attacks against the hosts connected
   to the network where FCFS SAVI is running.

   The attacks against the FCFS SAVI device basically consist of making
   the FCFS SAVI device consume its resources until it runs out of them.
   For instance, a possible attack would be to send packets with
   different source addresses, making the FCFS SAVI device create state
   for each of the addresses and waste memory.  At some point, the FCFS
   SAVI device runs out of memory and needs to decide how to react.  The
   result is that some form of garbage collection is needed to prune the
   entries.  When the FCFS SAVI device runs out of the memory allocated
   for the FCFS SAVI DB, it is RECOMMENDED that it create new entries by
   deleting the entries with a higher Creation time.  This implies that
   older entries are preserved and newer entries overwrite each other.
   In an attack scenario where the attacker sends a batch of data
   packets with different source addresses, each new source address is
   likely to rewrite another source address created by the attack
   itself.  It should be noted that entries are also garbage collected
   using the LIFETIME, which is updated using data packets.  The result
   is that in order for an attacker to actually fill the FCFS SAVI DB

   with false source addresses, it needs to continuously send data
   packets for all the different source addresses so that the entries
   grow old and compete with the legitimate entries.  The result is that
   the cost of the attack is highly increased for the attacker.

   In addition, it is RECOMMENDED that an FCFS SAVI device reserves a
   minimum amount of memory for each available port (in the case where
   the port is used as part of the L2 anchor).  The recommended minimum
   is the memory needed to store four bindings associated with the port.
   The motivation for this recommendation is as follows.  An attacker
   attached to a given port of an FCFS SAVI device may attempt to launch
   a DoS attack towards the FCFS SAVI device by creating many bindings
   for different addresses.  It can do so by sending DAD_NS for
   different addresses.  The result is that the attack will consume all
   the memory available in the FCFS SAVI device.  The above
   recommendation aims to reserve a minimum amount of memory per port,
   so that hosts located in different ports can make use of the reserved
   memory for their port even if a DoS attack is occurring in a
   different port.

   As the FCFS SAVI device may store data packets while the address is
   being verified, the memory for data packet storage may also be a
   target of DoS attacks.  The effects of such attacks may be limited to
   the lack of capacity to store new data packets.  The effect of such
   attacks will be that data packets will be dropped during the
   verification period.  An FCFS SAVI device MUST limit the amount of
   memory used to store data packets, allowing the other functions to
   have available memory even in the case of attacks such those
   described above.

   The FCFS SAVI device generates two DAD_NS packets upon the reception
   of a DAD_NS or a data packet.  As such, the FCFS SAVI device can be
   used as an amplifier by attackers.  In order to limit this type of
   attack, the FCFS SAVI device MUST perform rate limiting of the
   messages it generates.  Rate limiting is performed on a per-port
   basis, since having an attack on a given port should not prevent the
   FCFS SAVI device from functioning normally in the rest of the ports.

4.2.  Residual Threats

   FCFS SAVI performs its function by binding an IP source address to a
   binding anchor.  If the attacker manages to send packets using the
   binding anchor associated to a given IP address, FCFS SAVI validation
   will be successful, and the FCFS SAVI device will allow the packet
   through.  This can be achieved by spoofing the binding anchor or by
   sharing of the binding anchor between the legitimate owner of the
   address and the attacker.  An example of the latter is the case where
   the binding anchor is a port of a switched network and a legacy

   switch (i.e., not a SAVI-capable switch) is connected to that port.
   All the source addresses of the hosts connected to the legacy switch
   will share the same binding anchor (i.e., the switch port).  This
   means that hosts connected to the legacy switch can spoof each
   other's IP address and will not be detected by the FCFS SAVI device.
   This can be prevented by not sharing binding anchors among hosts.

   FCFS SAVI assumes that a host will be able to defend its address when
   the DAD procedure is executed for its addresses.  This is needed,
   among other things, to support mobility within a link (i.e., to allow
   a host to detach and reconnect to a different Layer 2 anchor of the
   same IP subnetwork without changing its IP address).  So, when a
   DAD_NS is issued for a given IP address for which a binding exists in
   an FCFS SAVI device, the FCFS SAVI device expects to see a DAD_NA
   coming from the binding anchor associated to that IP address in order
   to preserve the binding.  If the FCFS SAVI device does not see the
   DAD_NA, it may grant the binding to a different binding anchor.  This
   means that if an attacker manages to prevent a host from defending
   its source address, it will be able to destroy the existing binding
   and create a new one, with a different binding anchor.  An attacker
   may do so, for example, by intercepting the DAD_NA or launching a DoS
   attack to the host that will prevent it from issuing proper DAD
   replies.

   Even if routers are considered trusted, nothing can prevent a router
   from being compromised and sending traffic with spoofed IP source
   addresses.  Such traffic would be allowed with the present FCFS SAVI
   specification.  A way to mitigate this issue could be to specify a
   new port type (e.g., Router Port (RP)) that would act as Trusted Port
   for the transit traffic and as Validating Port for the local traffic.
   A detailed solution about this issue is outside the scope of this
   document.

4.3.  Privacy Considerations

   Personally identifying information MUST NOT be included in the FCFS
   SAVI DB with the MAC address as the canonical example, except when
   there is an attack attempt involved.  Moreover, compliant
   implementations MUST NOT log binding anchor information except where
   there is an identified reason why that information is likely to be
   involved in detection, prevention, or tracing of actual source
   address spoofing.  Information that is not logged MUST be deleted as
   soon as possible (i.e., as soon as the state for a given address is
   back to NO_BIND).  Information about the majority of hosts that never
   spoof SHOULD NOT be logged.

4.4.  Interaction with Secure Neighbor Discovery

   Even if the FCFS SAVI could get information from ND messages secured
   with Secure Neighbor Discovery (SEND) [RFC3971], in some case, the
   FCFS SAVI device must spoof DAD_NS messages but doesn't know the
   security credentials associated with the IPAddr (i.e., the private
   key used to sign the DAD_NS messages).  So, when SEND is deployed, it
   is recommended to use SEND SAVI [SAVI-SEND] rather than FCFS SAVI.

5.  Contributors

   Jun Bi
   CERNET
   Network Research Center, Tsinghua University
   Beijing 100084
   China
   EMail: junbi@cernet.edu.cn

   Guang Yao
   CERNET
   Network Research Center, Tsinghua University
   Beijing 100084
   China
   EMail: yaog@netarchlab.tsinghua.edu.cn

   Fred Baker
   Cisco Systems
   EMail: fred@cisco.com

   Alberto Garcia Martinez
   University Carlos III of Madrid
   EMail: alberto@it.uc3m.es

6.  Acknowledgments

   This document benefited from the input of the following individuals:
   Joel Halpern, Christian Vogt, Dong Zhang, Frank Xia, Jean-Michel
   Combes, Jari Arkko, Stephen Farrel, Dan Romascanu, Russ Housley, Pete
   Resnick, Ralph Droms, Wesley Eddy, Dave Harrington, and Lin Tao.

   Marcelo Bagnulo is partly funded by Trilogy, a research project
   supported by the European Commission under its Seventh Framework
   Program.

7.  References

7.1.  Normative References

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

   [RFC2827]   Ferguson, P. and D. Senie, "Network Ingress Filtering:
               Defeating Denial of Service Attacks which employ IP
               Source Address Spoofing", BCP 38, RFC 2827, May 2000.

   [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.

7.2.  Informative References

   [SAVI-FRAMEWORK]
               Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
               "Source Address Validation Improvement Framework", Work
               in Progress, December 2011.

   [SAVI-DHCP] Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
               DHCP", Work in Progress, February 2012.

   [SAVI-SEND] Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
               Address Validation Implementation", Work in Progress,
               March 2012.

   [RFC1958]   Carpenter, B., "Architectural Principles of the
               Internet", RFC 1958, June 1996.

   [RFC3971]   Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
               Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4732]   Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
               Service Considerations", RFC 4732, December 2006.

   [IEEE.802-1D.1998]
               Institute of Electrical and Electronics Engineers, "IEEE
               Standard for Local and Metropolitan Area Networks Media
               Access Control (MAC) Bridges", IEEE Standard 802.1D,
               1998.

   [IEEE.802-1D.2004]
               Institute of Electrical and Electronics Engineers, "IEEE
               Standard for Local and Metropolitan Area Networks Media
               Access Control (MAC) Bridges", IEEE Standard 802.1D,
               2004.

   [IEEE.802-1Q.2005]
               Institute of Electrical and Electronics Engineers, "IEEE
               Standard for Local and metropolitan area networks -
               Virtual Bridged Local Area Networks", IEEE Standard
               802.1Q, May 2005.

   [IEEE.802-1X.2004]
               Institute of Electrical and Electronics Engineers, "IEEE
               Standard for Local and metropolitan area networks - Port-
               Based Network Access Control", IEEE Standard 802.1X,
               2004.

Appendix A.  Implications of Not Following the Recommended Behavior

   This section qualifies some of the SHOULDs that are included in this
   specification by explaining the implications of not following the
   recommended behavior.  We start by describing the implication of not
   following the recommendation of generating DAD_NS upon the reception
   of a data packet for which there is no binding, and then we describe
   the implications of not discarding the non-compliant packets.

A.1.  Implications of Not Generating DAD_NS Packets upon the Reception
      of Non-Compliant Data Packets

   This specification recommends that SAVI implementations generate a
   DAD_NS message upon the reception of a data packet for which they
   have no binding.  In this section, we describe the implications of
   not doing so and simply discarding the data packet instead.

   The main argument against discarding the data packet is the overall
   robustness of the resulting network.  The main concern that has been
   stated is that a network running SAVI that discards data packets in
   this case may end up disconnecting legitimate users from the network,
   by filtering packets coming from them.  The net result would be a
   degraded robustness of the network as a whole, since legitimate users
   would perceive this as a network failure.  There are three different
   causes that resulted in the lack of state in the binding device for a
   legitimate address, namely, packet loss, state loss, and topology
   change.  We will next perform an analysis for each of them.

A.1.1.  Lack of Binding State due to Packet Loss

   The DAD procedure is inherently unreliable.  It consists of sending
   an NS packet, and if no NA packet is received back, success is
   assumed, and the host starts using the address.  In general, the lack
   of response is because no other host has that particular address
   configured in its interface, but it may also be the case that the NS
   packet or the NA packet has been lost.  From the perspective of the
   sending host, there is no difference, and the host assumes that it
   can use the address.  In other words, the default action is to allow
   the host to obtain network connectivity.

   It should be noted that the loss of a DAD packet has little impact on
   the network performance, since address collision is very rare, and
   the host assumes success in that case.  By designing a SAVI solution
   that would discard packets for which there is no binding, we are
   diametrically changing the default behavior in this respect, since
   the default would be that if the DAD packets are lost, then the node
   is disconnected from the network (as its packets are filtered).  What
   is worse, the node has little clue of what is going wrong, since it

   has successfully configured an address, but it has no connectivity.
   The net result is that the overall reliability of the network has
   significantly decreased as the loss of a single packet would imply
   that a host is disconnected from the network.

   The only mechanism that the DAD has to improve its reliability is
   sending multiple NSs.  However, [RFC4862] defines a default value of
   1 NS message for the DAD procedure, so requiring any higher value
   would imply manual configuration of all the hosts connected to the
   SAVI domain.

A.1.1.1.  Why Initial Packets May Be (Frequently) Lost

   The Case of LANs

   Devices connecting to a network may experience periods of packet loss
   after the link-layer becomes available for two reasons: Invalid
   Authentication state and incomplete topology assessment.  In both
   cases, physical-layer connection occurs initially and presents a
   medium where packets are transmissible, but frame forwarding is not
   available across the LAN.

   For the authentication system, devices on a controlled port are
   forced to complete 802.1X authentication, which may take multiple
   round trips and many milliseconds to complete (see
   [IEEE.802-1X.2004]).  In this time, initial DHCP, IPv6 Neighbor
   Discovery, Multicast Listener, or Duplicate Address Detection
   messages may be transmitted.  However, it has also been noted that
   some devices have the ability for the IP stack to not see the port as
   up until 802.1X has completed.  Hence, that issue needs investigation
   to determine how common it is now.

   Additionally, any system that requires user input at this stage can
   extend the authentication time and thus the outage.  This is
   problematic where hosts relying upon DHCP for address configuration
   time out.

   Upon completion of authentication, it is feasible to signal upper-
   layer protocols as to LAN forwarding availability.  This is not
   typical today, so it is necessary to assume that protocols are not
   aware of the preceding loss period.

   For environments that do not require authentication, addition of a
   new link can cause loops where LAN frames are forwarded continually.
   In order to prevent loops, all LANs today run a spanning tree
   protocol, which selectively disables redundant ports.  Devices that
   perform spanning tree calculations are either traditional Spanning
   Tree Protocol (STP) (see [IEEE.802-1D.1998]) or rapidly converging

   versions of the same (Rapid Spanning Tree Protocol (RSTP) / Multiple
   Spanning Tree Protocol (RSTP)) (see [IEEE.802-1D.2004] and
   [IEEE.802-1Q.2005]).

   Until a port is determined to be an edge port (RSTP/MSTP), the rapid
   protocol speaker has identified its position within the spanning tree
   (RSTP/MSTP) or completed a Listening phase (STP), its packets are
   discarded.

   For ports that are not connected to rapid protocol switches, it takes
   a minimum of three seconds to perform edge port determination (see
   [IEEE.802-1D.2004]).  Alternatively, completion of the Listening
   phase takes 15 seconds (see [IEEE.802-1D.1998]).  During this period,
   the link-layer appears available, but initial packet transmissions
   into and out of this port will fail.

   It is possible to pre-assess ports as edge ports using manual
   configuration of all the involved devices and thus make them
   immediately transmissible.  This is never default behavior though.

   The Case of Fixed Access Networks

   In fixed access networks such as DSL and cable, the end hosts are
   usually connected to the access network through a residential gateway
   (RG).  If the host interface is initialized prior to the RG getting
   authenticated and connected to the access network, the access network
   is not aware of the DAD packets that the host sent out.  As an
   example, in DSL networks, the Access Node (Digital Subscriber Link
   Access Multiplexer (DSLAM)) that needs to create and maintain binding
   state will never see the DAD message that is required to create such
   a state.

A.1.1.1.1.  Special Sub-Case:  SAVI Device Rate-Limiting Packets

   A particular sub-case is the one where the SAVI device itself "drops"
   ND packets.  In order to protect itself against DoS attacks and
   flash-crowds, the SAVI device will have to rate limit the processing
   of packets triggering the state-creation process (which requires
   processing from the SAVI device).  This implies that the SAVI device
   may not process all the ND packets if it is under heavy conditions.
   The result is that the SAVI device will fail to create a binding for
   a given DAD_NS packet, which implies that the data packets coming
   from the host that sent the DAD_NS packet will be filtered if this
   approach is adopted.  The problem is that the host will assume that
   the DAD procedure was successful and will not perform the DAD
   procedure again, which in turn will imply that the host will be
   disconnected from the network.  While it is true that the SAVI device
   will also have to rate limit the processing of the data packets, the

   host will keep on sending data packets, so it is possible to recover
   from the alternative approach where data packets trigger the binding-
   creation procedure.

A.1.2.  Lack of Binding State due to a Change in the Topology

   If SAVI is deployed in a switched Ethernet network, topology changes
   may result in a SAVI device receiving packets from a legitimate user
   for which the SAVI device does not have a binding.  Consider the
   following example:

          +------+             +--------+       +---------------+
          |SAVI I|-------------|SWITCH I|-------|rest of the net|
          +------+             +--------+       +---------------+
             |                    |
             |                 +--------+
             |                 | SAVI II|
             |                 +--------+
             |   +----------+     |
             +---|SWITCH II |-----+
                 +----------+
                             |
                          +-----+
                          | Host|
                          +-----+

                        Figure 3: Topology Example

   Suppose that after bootstrapping, all the elements are working
   properly and the spanning tree is rooted in the router and includes
   one branch that follows the path SWITCH I - SAVI I - SWITCH II, and
   another branch that follows SWITCH I-SAVI II.

   Suppose that the host boots at this moment and sends the DAD_NS.  The
   message is propagated through the spanning tree and is received by
   SAVI I but not by SAVI II.  SAVI I creates the binding.

   Suppose that SAVI I fails and the spanning tree reconverges to SWITCH
   I - SAVI II - SWITCH II.  Now, data packets coming from the host will
   be coursed through SAVI II, which does not have binding state and
   will drop the packets.

A.1.3.  Lack of Binding State due to State Loss

   The other reason a SAVI device may not have state for a legitimate
   address is simply because it lost it.  State can be lost due to a
   reboot of the SAVI device or other reasons such as memory corruption.
   So, the situation would be as follows.  The host performs the DAD

   procedure, and the SAVI device creates a binding for the host's
   address.  The host successfully communicates for a while.  The SAVI
   device reboots and loses the binding state.  The packets coming from
   the host are now discarded as there is no binding state for that
   address.  It should be noted that in this case, the host has been
   able to use the address successfully for a certain period of time.

   Architecturally, the degradation of the network robustness in this
   case can be easily explained by observing that this approach to SAVI
   implementation breaks the fate-sharing principle.  [RFC1958] reads:

      An end-to-end protocol design should not rely on the maintenance
      of state (i.e. information about the state of the end-to-end
      communication) inside the network.  Such state should be
      maintained only in the endpoints, in such a way that the state can
      only be destroyed when the endpoint itself breaks (known as fate-
      sharing).

   By binding the fate of the host's connectivity to the state in the
   SAVI device, we are breaking this principle, and the result is
   degraded network resilience.

   Moving on to more practical matters, we can dig deeper into the
   actual behavior by considering two scenarios, namely, the case where
   the host is directly connected to the SAVI device and the case where
   there is an intermediate device between the two.

A.1.3.1.  The Case of a Host Directly Connected to the SAVI Device

   The considered scenario is depicted in the following diagram:

         +------+             +-----------+       +---------------+
         | Host |-------------|SAVI device|-------|rest of the net|
         +------+             +-----------+       +---------------+

              Figure 4: Host Attached Directly to SAVI Device

   The key distinguishing element of this scenario is that the host is
   directly connected to the SAVI device.  As a result, if the SAVI
   device reboots, the host will see the carrier disappear and appear
   again.

   [RFC4862] requires that the DAD procedure is performed when the IP
   address is assigned to the interface (see [RFC4862], Section 5.4):

      Duplicate Address Detection:

      Duplicate Address Detection MUST be performed on all unicast
      addresses prior to assigning them to an interface, regardless of
      whether they are obtained through stateless autoconfiguration,
      DHCPv6, or manual configuration, with the following exceptions:
      ...

   However, it has been stated that some of the widely used OSs actually
   do perform DAD each time the link is up, but further data would be
   required for this to be taken for granted.  Assuming that behavior,
   this implies that if the loss of state in the SAVI device also
   results in the link to the host going down, then the host using the
   tested OSs would redo the DAD procedure allowing the recreation of
   the binding state in the SAVI device and preserving the connectivity
   of the host.  This would be the case if the SAVI device reboots.  It
   should be noted, however, that it is also possible that the binding
   state is lost because of an error in the SAVI process and that the
   SAVI link does not goes down.  In this case, the host would not redo
   the DAD procedure.  However, it has been pointed out that it would be
   possible to require the SAVI process to flap the links of the device
   it is running, in order to make sure that the link goes down each
   time the SAVI process restarts and to improve the chances the host
   will redo the DAD procedure when the SAVI process is rebooted.

A.1.3.2.  The Case of a Host Connected to the SAVI Device through One or
          More Legacy Devices

   The considered scenario is depicted in the following diagram:

     +------+    +-------------+     +-----------+    +---------------+
     | Host |----|Legacy device|-----|SAVI device|----|rest of the net|
     +------+    +-------------+     +-----------+    +---------------+

                Figure 5: Host Attached to a Legacy Device

   The key distinguishing element of this scenario is that the host is
   not directly connected to the SAVI device.  As a result, if the SAVI
   device reboots, the host will not see any changes.

   In this case, the host would get disconnected from the rest of the
   network since the SAVI device would filter all its packets once the
   state has gone.  As the node will not perform the DAD procedure
   again, it will remain disconnected until it reboots.

   As a final comment, it should be noted that it may not be obvious to
   the network admin which scenario its network is running.  Consider
   the case of a campus network where all the switches in the network
   are SAVI capable.  A small hub connected in the office would turn
   this into the scenario where the host is not directly connected to
   the SAVI device.  Moreover, consider the case of a host running
   multiple virtual machines connected through a virtual hub.  Depending
   on the implementation of such a virtual hub, this may turn a directly
   connected host scenario to the scenario where the multiple (virtual)
   hosts are connected through a legacy (virtual) hub.

A.1.3.2.1.  Enforcing Direct Connectivity between the SAVI Device and
            the Host

   It has been argued that enforcing direct connectivity between the
   SAVI device and the end host is actually a benefit.  There are
   several comments that can be made in this respect:

   o  First, it may well be the case in some scenarios that this is
      desirable, but it is certainly not the case in most scenarios.
      Because of that, the issue of enforcing direct connectivity must
      be treated as orthogonal to how data packets for which there is no
      binding are treated, since a general solution must support
      directly connected nodes and nodes connected through legacy
      switches.

   o  Second, as a matter of fact, the resulting behavior described
      above would not actually enforce direct connectivity between the
      end host and the SAVI device as it would work as long as the SAVI
      device does not reboot.  So, the argument being made is that this
      approach is not good enough to provide a robust network service,
      but it is not bad enough to enforce the direct connectivity of the
      host to the SAVI switch.

   o  Third, it should be noted that topology enforcement is not part of
      the SAVI problem space and that the SAVI problem by itself is
      complex enough without adding additional requirements.

A.2.  Implications of Not Discarding Non-Compliant Data Packets

   The FCFS SAVI mechanism is composed of two main functions, namely,
   the mechanisms for tracking compliant and non-compliant data packets
   and the actions to be performed upon the detection of a non-compliant
   packet.  Throughout this specification, we recommend discarding non-
   compliant data packets.  This is because forwarding non-compliant
   data packets is essentially allowing packets with spoofed source
   addresses to flow throughout the network.  However, there are
   alternative actions that can be taken with respect to these packets.

   For instance, it would be possible to forward the packets and trigger
   an alarm to network administrators to make them aware of the
   situation.  Similarly, it would be possible to log these events and
   allow the tracking down cases where packets with spoofed addresses
   were used for malicious purposes.  The reason a site deploying SAVI
   may not want to take milder actions like the ones mentioned above
   instead of discarding packets is because there may be cases where the
   non-compliant packets may be legitimate packets (for example, in the
   case that the SAVI device is malfunctioning and has failed to create
   the appropriate bindings upon the reception of a DAD packet).

Authors' Addresses

   Erik Nordmark
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035
   United States

   EMail: nordmark@acm.org

   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911
   Spain

   Phone: 34 91 6248814
   EMail: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es

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

   EMail: elevyabe@cisco.com

 

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