





Internet Engineering Task Force (IETF)                             J. Bi
Request for Comments: 7513                                         J. Wu
Category: Standards Track                                         G. Yao
ISSN: 2070-1721                                           Tsinghua Univ.
                                                                F. Baker
                                                                   Cisco
                                                                May 2015


     Source Address Validation Improvement (SAVI) Solution for DHCP

Abstract

   This document specifies the procedure for creating a binding between
   a DHCPv4/DHCPv6-assigned IP address and a binding anchor on a Source
   Address Validation Improvement (SAVI) device.  The bindings set up by
   this procedure are used to filter packets with forged source IP
   addresses.  This mechanism complements BCP 38 (RFC 2827) ingress
   filtering, providing finer-grained source IP address validation.

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/rfc7513.

Copyright Notice

   Copyright (c) 2015 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Deployment Scenario and Configuration . . . . . . . . . . . .   8
     4.1.  Elements and Scenario . . . . . . . . . . . . . . . . . .   8
     4.2.  SAVI Binding Type Attributes  . . . . . . . . . . . . . .  10
       4.2.1.  Trust Attribute . . . . . . . . . . . . . . . . . . .  10
       4.2.2.  DHCP-Trust Attribute  . . . . . . . . . . . . . . . .  11
       4.2.3.  DHCP-Snooping Attribute . . . . . . . . . . . . . . .  11
       4.2.4.  Data-Snooping Attribute . . . . . . . . . . . . . . .  11
       4.2.5.  Validating Attribute  . . . . . . . . . . . . . . . .  12
       4.2.6.  Table of Mutual Exclusions  . . . . . . . . . . . . .  13
     4.3.  Perimeter . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.3.1.  SAVI-DHCP Perimeter Overview  . . . . . . . . . . . .  13
       4.3.2.  SAVI-DHCP Perimeter Configuration Guideline . . . . .  14
       4.3.3.  On the Placement of the DHCP Server and Relay . . . .  15
       4.3.4.  An Alternative Deployment . . . . . . . . . . . . . .  15
       4.3.5.  Considerations regarding Binding Anchors  . . . . . .  16
     4.4.  Other Device Configuration  . . . . . . . . . . . . . . .  17
   5.  Binding State Table (BST) . . . . . . . . . . . . . . . . . .  17
   6.  DHCP Snooping Process . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Binding States Description  . . . . . . . . . . . . . . .  19
     6.3.  Events  . . . . . . . . . . . . . . . . . . . . . . . . .  19
       6.3.1.  Timer Expiration Event  . . . . . . . . . . . . . . .  19
       6.3.2.  Control Message Arriving Events . . . . . . . . . . .  19
     6.4.  The State Machine of DHCP Snooping Process  . . . . . . .  21
       6.4.1.  Initial State: NO_BIND  . . . . . . . . . . . . . . .  21
       6.4.2.  Initial State: INIT_BIND  . . . . . . . . . . . . . .  24
       6.4.3.  Initial State: BOUND  . . . . . . . . . . . . . . . .  27
       6.4.4.  Table of State Machine  . . . . . . . . . . . . . . .  30
   7.  Data Snooping Process . . . . . . . . . . . . . . . . . . . .  31
     7.1.  Scenario  . . . . . . . . . . . . . . . . . . . . . . . .  31
     7.2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .  32
     7.3.  Additional Binding States Description . . . . . . . . . .  33
     7.4.  Events  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     7.5.  Message Sender Functions  . . . . . . . . . . . . . . . .  35
       7.5.1.  Duplicate Detection Message Sender  . . . . . . . . .  35
       7.5.2.  Leasequery Message Sender . . . . . . . . . . . . . .  36
       7.5.3.  Address Verification Message Sender . . . . . . . . .  36
     7.6.  Initial State: NO_BIND  . . . . . . . . . . . . . . . . .  37
       7.6.1.  Event: EVE_DATA_UNMATCH: A data packet without a
               matched binding is received . . . . . . . . . . . . .  37
       7.6.2.  Events Not Observed in NO_BIND for Data Snooping  . .  38





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     7.7.  Initial State: DETECTION  . . . . . . . . . . . . . . . .  39
       7.7.1.  Event: EVE_ENTRY_EXPIRE . . . . . . . . . . . . . . .  39
       7.7.2.  Event: EVE_DATA_CONFLICT: ARP Reply / NA Message
               Received from Unexpected System . . . . . . . . . . .  39
       7.7.3.  Events Not Observed in DETECTION  . . . . . . . . . .  39
     7.8.  Initial State: RECOVERY . . . . . . . . . . . . . . . . .  40
       7.8.1.  Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE
               or successful LEASEQUERY-REPLY is received  . . . . .  40
       7.8.2.  Event: EVE_ENTRY_EXPIRE . . . . . . . . . . . . . . .  41
       7.8.3.  Events Not Observed in RECOVERY . . . . . . . . . . .  41
     7.9.  Initial State: VERIFY . . . . . . . . . . . . . . . . . .  41
       7.9.1.  Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE
               or successful LEASEQUERY-REPLY is received  . . . . .  41
       7.9.2.  Event: EVE_DATA_VERIFY: A valid ARP Reply or NA is
               received from the device attached via the binding
               anchor  . . . . . . . . . . . . . . . . . . . . . . .  42
       7.9.3.  Event: EVE_ENTRY_EXPIRE . . . . . . . . . . . . . . .  42
       7.9.4.  Event: EVE_DATA_EXPIRE  . . . . . . . . . . . . . . .  43
       7.9.5.  Events Not Observed in VERIFY . . . . . . . . . . . .  43
     7.10. Initial State: BOUND  . . . . . . . . . . . . . . . . . .  43
     7.11. Table of State Machine  . . . . . . . . . . . . . . . . .  44
   8.  Filtering Specification . . . . . . . . . . . . . . . . . . .  45
     8.1.  Data Packet Filtering . . . . . . . . . . . . . . . . . .  46
     8.2.  Control Packet Filtering  . . . . . . . . . . . . . . . .  46
   9.  State Restoration . . . . . . . . . . . . . . . . . . . . . .  47
     9.1.  Attribute Configuration Restoration . . . . . . . . . . .  47
     9.2.  Binding State Restoration . . . . . . . . . . . . . . . .  47
   10. Constants . . . . . . . . . . . . . . . . . . . . . . . . . .  48
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  48
     11.1.  Security Problems with the Data Snooping Process . . . .  48
     11.2.  Securing Leasequery Operations . . . . . . . . . . . . .  49
     11.3.  Client Departure Issues  . . . . . . . . . . . . . . . .  49
     11.4.  Compatibility with Detecting Network Attachment (DNA)  .  50
     11.5.  Binding Number Limitation  . . . . . . . . . . . . . . .  51
     11.6.  Privacy Considerations . . . . . . . . . . . . . . . . .  51
     11.7.  Fragmented DHCP Messages . . . . . . . . . . . . . . . .  51
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     12.2.  Informative References . . . . . . . . . . . . . . . . .  53
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54










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1.  Introduction

   This document describes a fine-grained source address validation
   mechanism for IPv4 and IPv6 packets.  This mechanism creates bindings
   between IP addresses assigned to network interfaces by DHCP and
   suitable binding anchors (Section 4.3.5).  As discussed in Section 3
   and [RFC7039], a "binding anchor" is an attribute that is immutable
   or difficult to change that may be used to identify the system an IP
   address has been assigned to; common examples include a Media Access
   Control (MAC) address found on an Ethernet switch port or Wi-Fi
   security association.  The bindings are used to identify and filter
   packets originated by these interfaces using forged source IP
   addresses.  In this way, this mechanism can prevent hosts from using
   IP addresses assigned to any other attachment point in or not
   associated with the network.  This behavior is referred to as
   "spoofing" and is key to amplification attacks, in which a set of
   systems send messages to another set of systems claiming to be from a
   third set of systems, and sending the replies to systems that don't
   expect them.  Whereas BCP 38 [RFC2827] protects a network from a
   neighboring network by providing prefix granularity source IP address
   validity, this mechanism protects a network, including a Local Area
   Network, from itself by providing address granularity source IP
   validity when DHCP/DHCPv6 is used to assign IPv4/IPv6 addresses.
   Both provide a certain level of traceability, in that packet drops
   indicate the presence of a system that is producing packets with
   spoofed IP addresses.

   SAVI-DHCP snoops DHCP address assignments to set up bindings between
   IP addresses assigned by DHCP and corresponding binding anchors.  It
   includes the DHCPv4 and DHCPv6 Snooping Process (Section 6) and the
   Data Snooping Process (Section 7), as well as a number of other
   technical details.  The Data Snooping Process is a data-triggered
   procedure that snoops the IP header of data packets to set up
   bindings.  It is designed to avoid a permanent blockage of valid
   addresses in the case that DHCP snooping is insufficient to set up
   all the valid bindings.

   This mechanism is designed for the stateful DHCP scenario [RFC2131]
   [RFC3315].  Stateless DHCP [RFC3736] is out of scope for this
   document, as it has nothing to do with IP address allocation.  An
   alternative SAVI method would have be used in those cases.  For hosts
   using Stateless Address Autoconfiguration (SLAAC) to allocate
   addresses, First-Come, First-Served Source Address Validation
   Improvement (FCFS SAVI) [RFC6620] should be enabled.  SAVI-DHCP is
   primarily designed for pure DHCP scenarios in which only addresses
   assigned through DHCP are allowed.  However, it does not block link-





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   local addresses, as they are not assigned using DHCP.  It is
   RECOMMENDED that the administration deploy a SAVI solution for link-
   local addresses, e.g., FCFS SAVI [RFC6620].

   This mechanism works for networks that use DHCPv4 only, DHCPv6 only,
   or both DHCPv4 and DHCPv6.  However, the DHCP address assignment
   mechanism in IPv4/IPv6 transition scenarios, e.g., [RFC7341], are
   beyond the scope of this document.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Terminology

   Binding anchor: A "binding anchor" is defined to be a physical and/or
   link-layer property of an attached device, as in [RFC7039].  A list
   of sample binding anchors can be found in Section 3.2 of that
   document.  To the degree possible, a binding anchor associates an IP
   address with something unspoofable that identifies a single-client
   system or one of its interfaces.  See Section 4.3.5 for more detail.

   Attribute: A configurable property of each binding anchor (port, MAC
   address, or other information) that indicates the actions to be
   performed on packets received from the attached network device.

   DHCP address: An IP address assigned via DHCP.

   SAVI-DHCP: The name of this SAVI function for DHCP-assigned
   addresses.

   SAVI device: A network device on which SAVI-DHCP is enabled.

   Non-SAVI device: A network device on which SAVI-DHCP is not enabled.

   DHCP Client-to-Server message: A message that is sent from a DHCP
   client to a DHCP server or DHCP servers and is one of the following
   types:

   o  DHCPv4 Discover: DHCPDISCOVER [RFC2131].

   o  DHCPv4 Request: DHCPREQUEST generated during SELECTING state
      [RFC2131].

   o  DHCPv4 Renew: DHCPREQUEST generated during RENEWING state
      [RFC2131].



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   o  DHCPv4 Rebind: DHCPREQUEST generated during REBINDING state
      [RFC2131].

   o  DHCPv4 Reboot: DHCPREQUEST generated during INIT-REBOOT state
      [RFC2131].

   o  Note: DHCPv4 Request/Renew/Rebind/Reboot messages can be
      identified based on Table 4 of [RFC2131].

   o  DHCPv4 Decline: DHCPDECLINE [RFC2131].

   o  DHCPv4 Release: DHCPRELEASE [RFC2131].

   o  DHCPv4 Inform: DHCPINFORM [RFC2131].

   o  DHCPv4 DHCPLEASEQUERY: A message sent to inquire about the lease
      that might exist for an IPv4 address [RFC4388].

   o  DHCPv6 Request: REQUEST [RFC3315].

   o  DHCPv6 Solicit: SOLICIT [RFC3315].

   o  DHCPv6 Confirm: CONFIRM [RFC3315].

   o  DHCPv6 Decline: DECLINE [RFC3315].

   o  DHCPv6 Release: RELEASE [RFC3315].

   o  DHCPv6 Rebind: REBIND [RFC3315].

   o  DHCPv6 Renew: RENEW [RFC3315].

   o  DHCPv6 Information-Request: INFORMATION-REQUEST [RFC3315].

   o  DHCPv6 LEASEQUERY: A message sent to inquire about the lease that
      might exist for an IPv6 address [RFC5007].

   DHCP Server-to-Client message: A message that is sent from a DHCP
   server to a DHCP client and is one of the following types:

   o  DHCPv4 ACK: DHCPACK [RFC2131].

   o  DHCPv4 NAK: DHCPNAK [RFC2131].

   o  DHCPv4 Offer: DHCPOFFER [RFC2131].

   o  DHCPv4 DHCPLEASEACTIVE: A response to a DHCPLEASEQUERY request
      containing lease information [RFC4388].



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   o  DHCPv4 DHCPLEASEUNKNOWN: A response to a DHCPLEASEQUERY request
      indicating that the server does not manage the address [RFC4388].

   o  DHCPv4 DHCPLEASEUNASSIGNED: A response to a DHCPLEASEQUERY request
      indicating that the server manages the address and there is no
      current lease [RFC4388].

   o  DHCPv6 Reply: REPLY [RFC3315].

   o  DHCPv6 Advertise: ADVERTISE [RFC3315].

   o  DHCPv6 Reconfigure: RECONFIGURE [RFC3315].

   o  DHCPv6 LEASEQUERY-REPLY: A response to a LEASEQUERY request
      [RFC5007].

   Lease time: The lease time in IPv4 [RFC2131] or the valid lifetime in
   IPv6 [RFC3315].

   Binding entry: A rule that associates an IP address with a binding
   anchor.

   Binding State Table (BST): The data structure that contains the
   binding entries.

   Binding entry limit: The maximum number of binding entries that may
   be associated with a binding anchor.  Limiting the number of binding
   entries per binding anchor prevents a malicious or malfunctioning
   node from overloading the binding table on a SAVI device.

   Direct attachment: Ideally, a SAVI device is an access device that
   hosts are attached to directly.  In such a case, the hosts are direct
   attachments (i.e., they attach directly) to the SAVI device.

   Indirect attachment: A SAVI device MAY be an aggregation device that
   other access devices are attached to and that hosts in turn attach
   to.  In such a case, the hosts are indirect attachments (i.e., they
   attach indirectly) to the SAVI device.

   Unprotected link: Unprotected links are links that connect to hosts
   or networks of hosts that receive their DHCP traffic by another path
   and are therefore outside the SAVI perimeter.

   Unprotected device: An unprotected device is a device associated with
   an unprotected link.  One example might be the gateway router of a
   network.





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   Protected link: If DHCP messages for a given attached device always
   use a given link, the link is considered to be "protected" by the
   SAVI device and is therefore within the SAVI perimeter.

   Protected device: A protected device is a device associated with a
   protected link.  One example might be a desktop switch in the
   network, or a host.

   Cut vertex: A cut vertex is any vertex whose removal increases the
   number of connected components in a (network) graph.  This is a
   concept in graph theory.  This term is used in Section 6.1 to
   accurately specify the required deployment location of SAVI devices
   when they only perform the DHCP Snooping Process.

   Identity Association (IA): "A collection of addresses assigned to a
   client" [RFC3315].

   Detection message: A Neighbor Solicitation or ARP message intended by
   the Data Snooping Process to detect a duplicate address.

   DHCP_DEFAULT_LEASE: Default lifetime for a DHCPv6 address when the
   binding is triggered by a DHCPv6 Confirm message but a DHCPv6
   Leasequery exchange [RFC5007] cannot be performed by the SAVI device
   to fetch the lease.

4.  Deployment Scenario and Configuration

4.1.  Elements and Scenario

   The essential elements in a SAVI-DHCP deployment scenario include at
   least one DHCP server (which may or may not be assigned an address
   using DHCP and therefore may or may not be protected), zero or more
   protected DHCP clients, and one or more SAVI devices.  It may also
   include DHCP relays, when the DHCP server is not co-located with a
   set of clients, and zero or more protected non-SAVI devices.  Outside
   the perimeter, via unprotected links, there may be many unprotected
   devices.














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                                 +-------------+
                                 | Unprotected |
                                 |   Device    |
                                 +------+------+
                                        |
                   +--------+     +-----+------+    +----------+
                   |DHCP    +-----+  Non-SAVI  +----+Bogus DHCP|
                   |Server A|     |  Device 1  |    |Server    |
                   +--------+     +-----+------+    +----------+
                                        |trusted, unprotected link
       . . . . . . . . . . . . . . . . .|. . . . . . . . . . . . . .
      .                                 |                           .
      .             Protection      +---+------+ trusted link       .
      .             Perimeter       | SAVI     +--------------+     .
      .                             | Device C |              |     .
      .                             +---+------+              |     .
      .                                 |                     |     .
      .  untrusted, +----------+    +---+------+       +------+---+ .
      .  protected  | SAVI     |    | Non-SAVI |       | SAVI     | .
      .  link+------+ Device A +----+ Device 3 +-------+ Device B | .
      .      |      +----+--+--+    +----------+       +-+---+----+ .
      .      |           |  +----------+    . . . .  .   |   |      .
      .      |       . . . . . .       |   .          .  |   |      .
      .      |      .    |      .      |   .    +--------+   |      .
      . +----+-----+. +--+---+  . +----+-+ . +--+---+ .  +---+----+ .
      . | Non-SAVI |. |Client|  . |DHCP  | . |Client| .  |DHCP    | .
      . | Device 2 |. |A     |  . |Relay | . |B     | .  |Server B| .
      . +----------+. +------+  . +------+ . +------+ .  +--------+ .
       . . . . . . .             . . . . .             . . . . . . .

                       Figure 1: SAVI-DHCP Scenario

   Figure 1 shows a deployment scenario that contains these elements.
   Note that a physical device can instantiate multiple elements, e.g.,
   a switch can be both a SAVI device and a DHCP relay, or in a cloud-
   computing environment, a physical host may contain a virtual switch
   plus some number of virtual hosts.  In such cases, the links are
   logical links rather than physical links.

   Networks are not usually isolated.  As a result, traffic from other
   networks, including transit traffic as specified in [RFC6620] (e.g.,
   traffic from another SAVI switch or a router) may enter a SAVI-DHCP
   network through the unprotected links.  Since SAVI solutions are
   limited to validating traffic generated from a local link, SAVI-DHCP
   does not set up bindings for addresses assigned in other networks and
   cannot validate them.  Traffic from unprotected links should be
   checked by an unprotected device or mechanisms described in




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   [RFC2827].  The generation and deployment of such a mechanism is
   beyond the scope of this document.

   Traffic from protected links is, however, locally generated and
   should have its source addresses validated by SAVI-DHCP if possible.
   In the event that there is an intervening protected non-SAVI device
   between the host and the SAVI device, however, use of the physical
   attachment point alone as a binding anchor is insufficiently secure,
   as several devices on a port or other point of attachment can spoof
   each other.  Hence, additional information such as a MAC address
   SHOULD be used to disambiguate them.

4.2.  SAVI Binding Type Attributes

   As illustrated in Figure 1, a system attached to a SAVI device can be
   a DHCP client, a DHCP relay/server, a SAVI device, or a non-SAVI
   device.  Different actions are performed on traffic originated from
   different elements.  To distinguish among their requirements, several
   properties are associated with their point of attachment on the SAVI
   device.

   When a binding association is uninstantiated, e.g., when no host is
   attached to the SAVI device using a given port or other binding
   anchor, the binding port attributes take default values unless
   overridden by configuration.  By default, a SAVI switch does not
   filter DHCP messages, nor does it attempt to validate source
   addresses, which is to say that the binding attributes are ignored
   until SAVI-DHCP is itself enabled.  This is because a SAVI switch
   that depends on DHCP cannot tell, a priori, which ports have valid
   DHCP servers attached, or which have routers or other equipment that
   would validly appear to use an arbitrary set of source addresses.
   When SAVI has been enabled, the attributes take effect.

4.2.1.  Trust Attribute

   The "Trust Attribute" is a Boolean value.  If TRUE, it indicates that
   the packets from the corresponding attached device need not have
   their source addresses validated.  Examples of a trusted attachment
   would be a port to another SAVI device, or to an IP router, as shown
   in Figure 1.  In both cases, traffic using many source IP addresses
   will be seen.  By default, the Trust attribute is FALSE, indicating
   that any device found on that port will seek an address using DHCP
   and be limited to using such addresses.

   SAVI devices will not set up bindings for points of attachment with
   the Trust attribute set TRUE; no packets, including DHCP messages,
   from devices with this attribute on their attachments will be
   validated.  However, DHCP Server-to-Client messages will be snooped



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   on attachment points with the Trust attribute set TRUE in the same
   way as if they had the DHCP-Trust attribute set (see Section 4.2.2).

4.2.2.  DHCP-Trust Attribute

   The "DHCP-Trust Attribute" is similarly a Boolean attribute.  It
   indicates whether the attached device is permitted to initiate DHCP
   Server-to-Client messages.  In Figure 1, the points of attachment of
   the DHCP server and the DHCP relay would have this attribute set
   TRUE, and attachment points that have Trust set TRUE are implicitly
   treated as if DHCP-Trust is TRUE.

   If the DHCP-Trust attribute is TRUE, SAVI devices will forward DHCP
   Server-to-Client messages from the points of attachment with this
   attribute.  If the DHCP Server-to-Client messages can trigger the
   state transitions, the binding setup processes specified in Sections
   6 and 7 will handle them.  By default, the DHCP-Trust attribute is
   FALSE, indicating that the attached system is not a DHCP server.

   A DHCPv6 implementor can refer to [DHCPv6-SHIELD] for more details.

4.2.3.  DHCP-Snooping Attribute

   The "DHCP-Snooping Attribute" is similarly a Boolean attribute.  It
   indicates whether bindings will be set up based on DHCP snooping.

   If this attribute is TRUE, DHCP Client-to-Server messages to points
   of attachment with this attribute will trigger creation of bindings
   based on the DHCP Snooping Process described in Section 6.  If it is
   FALSE, either the Trust attribute must be TRUE (so that bindings
   become irrelevant) or another SAVI mechanism such as FCFS SAVI must
   be used on the point of attachment.

   The DHCP-Snooping attribute is configured on the DHCP client's point
   of attachment.  This attribute can be also used on the attachments to
   protected non-SAVI devices that are used by DHCP clients.  In
   Figure 1, the attachment from Client A to SAVI Device A, the
   attachment from Client B to SAVI Device B, and the attachment from
   Non-SAVI Device 2 to SAVI Device A can be configured with this
   attribute.

4.2.4.  Data-Snooping Attribute

   The "Data-Snooping Attribute" is a Boolean attribute.  It indicates
   whether data packets from the corresponding point of attachment may
   trigger the binding setup procedure.





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   Data packets from points of attachment with this attribute may
   trigger the setup of bindings.  SAVI devices will set up bindings on
   points of attachment with this attribute based on the data-triggered
   process described in Section 7.

   If the DHCP-Snooping attribute is configured on a point of
   attachment, the bindings on this attachment are set up based on DHCP
   message snooping.  However, in some scenarios, a DHCP client may use
   a DHCP address without the DHCP address assignment procedure being
   performed on its current attachment.  For such attached devices, the
   Data Snooping Process, which is described in Section 7, is necessary.
   This attribute is configured on such attachments.  The usage of this
   attribute is further discussed in Section 7.

   Since some networks require DHCP deployment and others avoid it,
   there is no obvious universal default value for the Data-Snooping
   attribute.  Hence, the Data-Snooping attribute should default to
   FALSE, and a mechanism should be implemented to conveniently set it
   to TRUE on all points of attachment for which the Trust attribute is
   FALSE.

4.2.5.  Validating Attribute

   The "Validating Attribute" is a Boolean attribute.  It indicates
   whether packets from the corresponding attachment will have their IP
   source addresses validated based on binding entries on the
   attachment.

   If it is TRUE, packets coming from attachments with this attribute
   will be validated based on binding entries on the attachment as
   specified in Section 8.  If it is FALSE, they will not.  Since the
   binding table is used in common with other SAVI algorithms, it merely
   signifies whether the check will be done, not whether it will be done
   for SAVI-DHCP originated bindings.

   This attribute is by default the inverse of the Trust attribute;
   source addresses on untrusted links are validated by default.  It MAY
   be set FALSE by the administration.

   The expected use case is when SAVI is used to monitor but not block
   forged transmissions.  The network manager, in that case, may set the
   DHCP-Snooping and/or Data-Snooping attribute TRUE but the Validating
   attribute FALSE.








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4.2.6.  Table of Mutual Exclusions

   Different types of attributes may indicate mutually exclusive actions
   on a packet.  Mutually exclusive attributes MUST NOT be set TRUE on
   the same attachment.  The compatibility of different attributes is
   listed in Figure 2.  Note that although Trust and DHCP-Trust are
   compatible, there is no need to configure DHCP-Trust to TRUE on an
   attachment with Trust attribute TRUE.

    +----------+----------+----------+----------+----------+----------+
    |          |          |          | DHCP-    | Data-    |          |
    |          |  Trust   |DHCP-Trust| Snooping | Snooping |Validating|
    +----------+----------+----------+----------+----------+----------+
    |          |          |          | mutually | mutually | mutually |
    |  Trust   |    -     |compatible| exclusive| exclusive| exclusive|
    +----------+----------+----------+----------+----------+----------+
    |          |          |          |          |          |          |
    |DHCP-Trust|compatible|    -     |compatible|compatible|compatible|
    +----------+----------+----------+----------+----------+----------+
    |DHCP-     |mutually  |          |          |          |          |
    |Snooping  |exclusive |compatible|     -    |compatible|compatible|
    +----------+----------+----------+----------+----------+----------+
    |Data-     |mutually  |          |          |          |          |
    |Snooping  |exclusive |compatible|compatible|    -     |compatible|
    +----------+----------+----------+----------+----------+----------+
    |          |mutually  |          |          |          |          |
    |Validating|exclusive |compatible|compatible|compatible|    -     |
    +----------+----------+----------+----------+----------+----------+

                   Figure 2: Table of Mutual Exclusions

4.3.  Perimeter

4.3.1.  SAVI-DHCP Perimeter Overview

   SAVI devices form a perimeter separating trusted and untrusted
   regions of a network, as FCFS SAVI does (Section 2.5 of [RFC6620]).
   The perimeter is primarily designed for scalability.  It has two
   implications.

   o  SAVI devices only need to establish bindings for directly attached
      clients, or clients indirectly attached through a non-SAVI
      protected device, rather than all of the clients in the network.

   o  Each SAVI device only needs to validate the source addresses in
      traffic from clients attached to it, without checking all the
      traffic passing by.




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   Consider the example in Figure 1.  The protection perimeter is formed
   by SAVI Devices A, B, and C.  In this case, SAVI Device B does not
   create a binding for Client A.  However, because SAVI Device A
   filters spoofed traffic from Client A, SAVI Device B can avoid
   receiving spoofed traffic from Client A.

   The perimeter in SAVI-DHCP is not only a perimeter for data packets
   but also a perimeter for DHCP messages.  DHCP server response
   messages incoming across the perimeter will be dropped (Section 8).
   The placement of the DHCP relay and DHCP server, which are not
   involved in [RFC6620], is related to the construction of the
   perimeter.  The requirement on the placement and configuration of the
   DHCP relay and DHCP server is discussed in Section 4.3.3.

4.3.2.  SAVI-DHCP Perimeter Configuration Guideline

   A perimeter separating trusted and untrusted regions of the network
   is formed as follows:

   (1)  Configure the Validating and DHCP-Snooping attributes TRUE on
        the direct attachments of all DHCP clients.

   (2)  Configure the Validating and DHCP-Snooping attributes TRUE on
        the indirect attachments of all DHCP clients (i.e., DHCP clients
        on protected links).

   (3)  Configure the Trust attribute TRUE on the attachments to other
        SAVI devices.

   (4)  If a non-SAVI device, or a number of connected non-SAVI devices,
        are attached only to SAVI devices, set the Trust attribute TRUE
        on their attachments.

   (5)  Configure the DHCP-Trust attribute TRUE on the direct
        attachments to trusted DHCP relays and servers.

   In this way, the points of attachments with the Validating attribute
   TRUE (and generally together with attachments of unprotected devices)
   on SAVI devices can form a perimeter separating DHCP clients and
   trusted devices.  Data packet checks are only performed on the
   perimeter.  The perimeter is also a perimeter for DHCP messages.  The
   DHCP-Trust attribute is only TRUE on links inside the perimeter.
   Only DHCP Server-to-Client messages originated within the perimeter
   are trusted.







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4.3.3.  On the Placement of the DHCP Server and Relay

   As a result of the configuration guidelines, SAVI devices only trust
   DHCP Server-to-Client messages originated inside the perimeter.
   Thus, the trusted DHCP relays and DHCP servers must be placed within
   the perimeter.  DHCP Server-to-Client messages will be filtered on
   the perimeter.  Server-to-Relay messages will not be filtered, as
   they are within the perimeter.  In this way, DHCP Server-to-Client
   messages from bogus DHCP servers are filtered on the perimeter,
   having entered through untrusted points of attachment.  The SAVI
   devices are protected from forged DHCP messages.

   DHCP Server-to-Client messages arriving at the perimeter from outside
   the perimeter are not trusted.  There is no distinction between a
   DHCP server owned and operated by the correct administration but
   outside the SAVI perimeter and a bogus DHCP server.  For example, in
   Figure 1, DHCP Server A is valid, but it is attached to Non-SAVI
   Device 1.  A bogus DHCP server is also attached to Non-SAVI Device 1.
   While one could imagine a scenario in which the valid one had a
   statistically configured port number and MAC address, and therefore a
   binding, by default SAVI-DHCP cannot distinguish whether a message
   received from the port of Non-SAVI Device 1 is from DHCP Server A or
   the bogus DHCP server.  If DHCP Server A is contained in the
   perimeter, Non-SAVI Device 1 will also be contained in the perimeter.
   Thus, DHCP Server A cannot be contained within the perimeter apart
   from manual configuration of the binding anchor.

   Another consideration on the placement is that if the DHCP server/
   relay is not inside the perimeter, the SAVI devices may not be able
   to set up bindings correctly because the SAVI devices may not be on
   the path between the clients and the server/relay, or the DHCP
   messages are encapsulated (e.g., Relay-reply and Relay-forward).

4.3.4.  An Alternative Deployment

   In common deployment practice, the traffic from the unprotected
   network is treated as trustworthy, which is to say that it is not
   filtered.  In such a case, the Trust attribute can be set TRUE on the
   unprotected link.  If non-SAVI devices, or a number of connected non-
   SAVI devices, are only attached to SAVI devices and unprotected
   devices, their attachment to SAVI devices can have the Trust
   attribute set TRUE.  Then an unclosed perimeter will be formed, as
   illustrated in Figure 3.








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           |             .             .           Protection |
           |             |             |           Perimeter  |
           |             |             |                      |
           | Unprotected |             | Unprotected          |
           | Link        |             | Link                 |
           |             |             |                      |
           |             |             |                      |
           |        +----+---+    +----+---+    +--------+    |
           |        |SAVI    +----+Non-SAVI+----+SAVI    |    |
           |        |Device  |    |Device  |    |Device  |    |
           |        +----+---+    +--------+    +----+---+    |
           |             |                           |        |
           \_____________+___________________________+________/
                         |                           |
                         |                           |
                    +--------+                  +--------+
                    |DHCP    |                  |DHCP    |
                    |Client  |                  |Client  |
                    +--------+                  +--------+

               Figure 3: Alternative Perimeter Configuration

4.3.5.  Considerations regarding Binding Anchors

   The strength of this binding-based mechanism depends on the strength
   of the binding anchor.  The sample binding anchors in [RFC7039] have
   the property in which they associate an IP address with a direct
   physical or secure virtual interface such as a switch port, a
   subscriber association, or a security association.  In addition,
   especially in the case where a protected non-SAVI device such as a
   desktop switch or a hub is between the client and SAVI devices, they
   MAY be extended to also include a MAC address or other link-layer
   attribute.  In short, a binding anchor is intended to associate an IP
   address with something unspoofable that identifies a single-client
   system or one of its interfaces; this may be a physical or virtual
   interface or that plus disambiguating link-layer information.

   If the binding anchor is spoofable, such as a plain MAC address, or
   non-exclusive, such as a switch port extended using a non-SAVI
   device, an attacker can use a forged binding anchor to evade
   validation.  Indeed, using a binding anchor that can be easily
   spoofed can lead to worse outcomes than allowing spoofed IP traffic.
   Thus, a SAVI device MUST use a non-spoofable and exclusive binding
   anchor.







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4.4.  Other Device Configuration

   In addition to a possible binding anchor configuration specified in
   Section 4.2, an implementation has the following configuration
   requirements:

   (1)  Address configuration.  For DHCPv4: the SAVI device MUST have an
        IPv4 address.  For DHCPv6: the client of a SAVI device MUST have
        a link-local address; when the DHCPv6 server is not on the same
        link as the SAVI device, the SAVI device MUST also have an IPv6
        address of at least the same scope as the DHCPv6 Server.

   (2)  DHCP server address configuration: a SAVI device MUST store the
        list of the DHCP server addresses that it could contact during a
        leasequery process.

   (3)  A SAVI device may also require security parameters, such as
        preconfigured keys to establish a secure connection for the
        leasequery process [RFC4388] [RFC5007] connection.

5.  Binding State Table (BST)

   The Binding State Table, which may be implemented centrally in the
   switch or distributed among its ports, is used to contain the
   bindings between the IP addresses assigned to the attachments and the
   corresponding binding anchors of the attachments.  Note that in this
   description, there is a binding entry for each IPv4 or IPv6 address
   associated with each binding anchor, and there may be several of each
   such address, especially if the port is extended using a protected
   non-SAVI device.  Each binding entry has six fields:

   o  Binding Anchor (listed as "Anchor" in subsequent figures): the
      binding anchor, i.e., one or more physical and/or link-layer
      properties of the attachment.

   o  IP Address (listed as "Address" in subsequent figures): the IPv4
      or IPv6 address assigned to the attachment by DHCP.

   o  State: the state of the binding.  Possible values of this field
      are listed in Sections 6.2 and 7.3.

   o  Lifetime: the remaining seconds of the binding.  Internally, this
      MAY be stored as the timestamp value at which the lifetime
      expires.

   o  Transaction ID (TID): the Transaction ID [RFC2131] [RFC3315] of
      the corresponding DHCP transaction.  The TID field is used to




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      associate DHCP Server-to-Client messages with corresponding
      binding entries.

   o  Timeouts: the number of timeouts that expired in the current state
      (only used in the Data Snooping Process; see Section 7).

   The IA is not present in the BST for three reasons:

   o  The lease of each address in one IA is assigned separately.

   o  When the binding is set up based on data snooping, the IA cannot
      be recovered from the leasequery protocol.

   o  DHCPv4 does not define an IA.

   An example of such a table is shown in Figure 4.

    +---------+----------+-----------+-----------+--------+----------+
    | Anchor  | Address  | State     | Lifetime  | TID    | Timeouts |
    +---------+----------+-----------+-----------+--------+----------+
    | Port_1  | IP_1     | BOUND     |  65535    | TID_1  |     0    |
    +---------+----------+-----------+-----------+--------+----------+
    | Port_1  | IP_2     | BOUND     |  10000    | TID_2  |     0    |
    +---------+----------+-----------+-----------+--------+----------+
    | Port_2  | IP_3     | INIT_BIND |      1    | TID_3  |     0    |
    +---------+----------+-----------+-----------+--------+----------+

                   Figure 4: Example Binding State Table

6.  DHCP Snooping Process

   This section specifies the process of setting up bindings based on
   DHCP snooping.  This process is illustrated using a state machine.

6.1.  Rationale

   The rationale of the DHCP Snooping Process is that if a DHCP client
   is legitimately using a DHCP-assigned address, the DHCP address
   assignment procedure that assigns the IP address to the client must
   have been performed via the client's point of attachment.  This
   assumption works when the SAVI device is always on the path(s) from
   the DHCP client to the DHCP server(s)/relay(s).  Without considering
   the movement of DHCP clients, the SAVI device should be the cut
   vertex whose removal will separate the DHCP client and the remaining
   network containing the DHCP server(s)/relay(s).  For most of the
   networks whose topologies are simple, it is possible to deploy this
   SAVI function at proper devices to meet this requirement.




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   However, if there are multiple paths from a DHCP client to the DHCP
   server and the SAVI device is only on one of them, there is an
   obvious failure case: the SAVI device may not be able to snoop the
   DHCP procedure.  Host movement may also make this requirement
   difficult to meet.  For example, when a DHCP client moves from one
   attachment to another attachment in the same network, it may fail to
   reinitialize its interface or send a Confirm message because of
   incomplete protocol implementation.  Thus, there can be scenarios in
   which only performing this DHCP Snooping Process is insufficient to
   set up bindings for all the valid DHCP addresses.  These exceptions
   and the solutions are discussed in Section 7.

6.2.  Binding States Description

   The following binding states are present in this process and the
   corresponding state machine:

   NO_BIND: No binding has been set up.

   INIT_BIND: A potential binding has been set up.

   BOUND: The binding has been set up.

6.3.  Events

   This section describes events in this process and the corresponding
   state machine transitions.  The DHCP message categories (e.g., DHCPv4
   Discover) defined in Section 3 are used extensively in the
   definitions of events and elsewhere in the state machine definition.
   If an event will trigger the creation of a new binding entry, the
   binding entry limit on the binding anchor MUST NOT be exceeded.

6.3.1.  Timer Expiration Event

   EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires.

6.3.2.  Control Message Arriving Events

   EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
   received.

   EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received.

   EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received.

   EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
   received.




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   EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is received.

   EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with the Rapid
   Commit option is received.

   EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received.

   EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
   received.

   EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
   received.

   EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY-REPLY (refer to
   Section 4.3.3 of [RFC5007]) is received.

   Note: the events listed here do not cover all the DHCP messages in
   Section 3.  The messages that do not really determine address usage
   (DHCPv4 Discover, DHCPv4 Inform, DHCPv6 Solicit without Rapid Commit,
   DHCPv6 Information-Request, DHCPv4 Offer, DHCPv6 Advertise, and
   DHCPv6 Reconfigure) and that are not necessary to snoop (DHCPv4
   Negative Acknowledgment (NAK); refer to Section 6.4.2.3) are not
   included.  Note also that DHCPv4 DHCPLEASEQUERY is not used in the
   DHCP Snooping Process to avoid confusion with Section 7.  Also, since
   the LEASEQUERY should have been originated by the SAVI device itself,
   the destination check should verify that the message is directed to
   this SAVI device, and it should not be forwarded once it has been
   processed here.

   Moreover, only if a DHCP message can pass the following checks, the
   corresponding event is regarded as a valid event:

   o  Attribute check: the DHCP Server-to-Client messages and
      LEASEQUERY-REPLY should be from attachments with the DHCP-Trust
      attribute; the DHCP Client-to-Server messages should be from
      attachments with the DHCP-Snooping attribute.

   o  Destination check: the DHCP Server-to-Client messages should be
      destined to attachments with the DHCP-Snooping attribute.  This
      check is performed to ensure the binding is set up on the SAVI
      device that is nearest to the destination client.

   o  Binding anchor check: the DHCP Client-to-Server messages that may
      trigger modification or removal of an existing binding entry must
      have a matching binding anchor with the corresponding entry.






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   o  TID check: the DHCP Server-to-Client/Client-to-Server messages
      that may cause modification of existing binding entries must have
      a matched TID with the corresponding entry.  Note that this check
      is not performed on LEASEQUERY and LEASEQUERY-REPLY messages as
      they are exchanged between the SAVI devices and the DHCP servers.
      Besides, this check is not performed on DHCP Renew/Rebind
      messages.

   o  Binding limitation check: the DHCP messages must not cause new
      binding setup on an attachment whose binding entry limitation has
      been reached (refer to Section 11.5).

   o  Address check: the source address of the DHCP messages should pass
      the check specified in Section 8.2.

   On receiving a DHCP message without triggering a valid event, the
   state will not change, and the actions will not be performed.  Note
   that if a message does not trigger a valid event but it can pass the
   checks in Section 8.2, it MUST be forwarded.

6.4.  The State Machine of DHCP Snooping Process

   This section specifies state transitions and their corresponding
   actions.

6.4.1.  Initial State: NO_BIND

6.4.1.1.  Event: EVE_DHCP_REQUEST - A DHCPv4 Request or a DHCPv6 Request
          message is received

   The SAVI device MUST forward the message.

   The SAVI device will generate an entry in the BST.  The Binding
   Anchor field is set to the binding anchor of the attachment from
   which the message is received.  The State field is set to INIT_BIND.
   The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME.  The TID
   field is set to the TID of the message.  If the message is DHCPv4
   Request, the Address field can be set to the address to request,
   i.e., the 'requested IP address'.  An example of the entry is
   illustrated in Figure 5.











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   +--------+-------+---------+-----------------------+-----+----------+
   | Anchor |Address| State   | Lifetime              | TID | Timeouts |
   +--------+-------+---------+-----------------------+-----+----------+
   | Port_1 |       |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |     0    |
   +--------+-------+---------+-----------------------+-----+----------+

       Figure 5: Binding Entry in BST on Initialization Triggered by
                   Request/Rapid Commit/Reboot Messages

   Resulting state: INIT_BIND - A potential binding has been set up.

6.4.1.2.  Event: EVE_DHCP_REBOOT - A DHCPv4 Reboot message is received

   The SAVI device MUST forward the message.

   The SAVI device will generate an entry in the BST.  The Binding
   Anchor field is set to the binding anchor of the attachment from
   which the message is received.  The State field is set to INIT_BIND.
   The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME.  The TID
   field is set to the TID of the message.  If the message is DHCPv4
   Reboot, the Address field can be set to the address to request, i.e.,
   the 'requested IP address'.  An example of the entry is illustrated
   in Figure 5.

   Resulting state: INIT_BIND - A potential binding has been set up.

6.4.1.3.  Event: EVE_DHCP_SOLICIT_RC - A DHCPv6 Solicitation message
          with the Rapid Commit option is received

   The SAVI device MUST forward the message.

   The SAVI device will generate an entry in the BST.  The Binding
   Anchor field is set to the binding anchor of the attachment from
   which the message is received.  The State field is set to INIT_BIND.
   The Lifetime field is set to be MAX_DHCP_RESPONSE_TIME.  The TID
   field is set to the TID of the message.  An example of the entry is
   illustrated in Figure 5.

   Resulting state: INIT_BIND - A potential binding has been set up.

6.4.1.4.  Event: EVE_DHCP_CONFIRM - A DHCPv6 Confirm message is received

   The SAVI device MUST forward the message.

   The SAVI device will generate corresponding entries in the BST for
   each address in each Identity Association (IA) option of the Confirm
   message.  The Binding Anchor field is set to the binding anchor of
   the attachment from which the message is received.  The State field



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   is set to INIT_BIND.  The Lifetime field is set to be
   MAX_DHCP_RESPONSE_TIME.  The TID field is set to the TID of the
   message.  The Address field is set to the address(es) to confirm.  An
   example of the entries is illustrated in Figure 6.

   +--------+-------+---------+-----------------------+-----+----------+
   | Anchor |Address| State   | Lifetime              | TID | Timeouts |
   +--------+-------+---------+-----------------------+-----+----------+
   | Port_1 | Addr1 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |    0     |
   +--------+-------+---------+-----------------------+-----+----------+
   | Port_1 | Addr2 |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID |    0     |
   +--------+-------+---------+-----------------------+-----+----------+

    Figure 6: Binding Entry in BST on Confirm-Triggered Initialization

   Resulting state: INIT_BIND - A potential binding has been set up.

6.4.1.5.  Events That Cannot Happen in the NO_BIND State

   o  EVE_ENTRY_EXPIRE: The lifetime of a binding entry expires

   o  EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
      received

   o  EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is
      received

   o  EVE_DHCP_REPLY: A DHCPv4 ACK or a DHCPv6 Reply message is received

   o  EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
      received

   o  EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
      received

   o  EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY-REPLY is
      received

   These cannot happen because they are each something that happens
   AFTER a binding has been created.











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6.4.2.  Initial State: INIT_BIND

6.4.2.1.  Event: EVE_DHCP_REPLY - A DHCPv4 ACK or a DHCPv6 Reply message
          is received

   The message MUST be forwarded to the corresponding client.

   If the message is DHCPv4 ACK, the Address field of the corresponding
   entry (i.e., the binding entry whose TID is the same as the message)
   is set to the address in the message (i.e., 'yiaddr' in DHCPv4 ACK).
   The Lifetime field is set to the sum of the lease time in the ACK
   message and MAX_DHCP_RESPONSE_TIME.  The State field is changed to
   BOUND.

   If the message is DHCPv6 Reply, note the following cases:

   1.  If the status code is not "Success", no modification of
       corresponding entries will be made.  Corresponding entries will
       expire automatically if no "Success" Reply is received during the
       lifetime.  The entries are not removed immediately because the
       client may be able to use the addresses whenever a "Success"
       Reply is received ("If the client receives any Reply messages
       that do not indicate a NotOnLink status, the client can use the
       addresses in the IA and ignore any messages that indicate a
       NotOnLink status" [RFC3315]).

   2.  If the status code is "Success", the SAVI device checks the IA
       options in the Reply message.

       A.  If there are IA options in the Reply message, the SAVI device
           checks each IA option.  When the first assigned address is
           found, the Address field of the binding entry with a matched
           TID is set to the address.  The Lifetime field is set to the
           sum of the lease time in the Reply message and
           MAX_DHCP_RESPONSE_TIME.  The State field is changed to BOUND.
           If there is more than one address assigned in the message,
           new binding entries are set up for the remaining address
           assigned in the IA options.  An example of the entries is
           illustrated in Figure 8.  SAVI devices do not specially
           process IA options with a NoAddrsAvail status because there
           should be no address contained in such IA options.

       B.  Otherwise, the DHCP Reply message is in response to a Confirm
           message.  The state of the binding entries with a matched TID
           is changed to BOUND.  Because [RFC3315] does not require the
           lease time of addresses to be contained in the Reply message,
           the SAVI device SHOULD send a LEASEQUERY [RFC5007] message
           querying by IP address to the All_DHCP_Servers multicast



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           address [RFC3315] or a list of configured DHCP server
           addresses.  The LEASEQUERY message is generated for each IP
           address if multiple addresses are confirmed.  The lifetime of
           corresponding entries is set to 2*MAX_LEASEQUERY_DELAY.  If
           there is no response message after MAX_LEASEQUERY_DELAY, send
           the LEASEQUERY message again.  An example of the entries is
           illustrated in Figure 7.  If the SAVI device does not send
           the LEASEQUERY message, a preconfigured lifetime
           DHCP_DEFAULT_LEASE MUST be set on the corresponding entry.
           (Note: it is RECOMMENDED to use T1 configured on DHCP servers
           as the DHCP_DEFAULT_LEASE.)

   Note: the SAVI devices do not check if the assigned addresses are
   duplicated because in SAVI-DHCP scenarios, the DHCP servers are the
   only source of valid addresses.  However, the DHCP servers should be
   configured to make sure no duplicated addresses are assigned.

   +--------+-------+-------+------------------------+-----+----------+
   | Anchor |Address| State | Lifetime               | TID | Timeouts |
   +--------+-------+-------+------------------------+-----+----------+
   | Port_1 | Addr1 | BOUND | 2*MAX_LEASEQUERY_DELAY | TID |    0     |
   +--------+-------+-------+------------------------+-----+----------+
   | Port_1 | Addr2 | BOUND | 2*MAX_LEASEQUERY_DELAY | TID |    0     |
   +--------+-------+-------+------------------------+-----+----------+

      Figure 7: From INIT_BIND to BOUND on DHCP Reply in Response to
                                  Confirm

   Transition
   +--------+-------+-------+------------------------+-----+----------+
   | Anchor |Address| State | Lifetime               | TID | Timeouts |
   +--------+-------+-------+------------------------+-----+----------+
   | Port_1 | Addr1 | BOUND |Lease time+             | TID |    0     |
   |        |       |       |MAX_DHCP_RESPONSE_TIME  |     |          |
   +--------+-------+-------+------------------------+-----+----------+
   | Port_1 | Addr2 | BOUND |Lease time+             | TID |    0     |
   |        |       |       |MAX_DHCP_RESPONSE_TIME  |     |          |
   +--------+-------+-------+------------------------+-----+----------+

      Figure 8: From INIT_BIND to BOUND on DHCP Reply in Response to
                                  Request

   Resulting state: BOUND - The binding has been set up.








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6.4.2.2.  Event: EVE_ENTRY_EXPIRE - The lifetime of a binding entry
          expires

   The entry MUST be deleted from the BST.

   Resulting state: An entry that has been deleted from the BST may be
   considered to be in the "NO_BIND" state - No binding has been set up.

6.4.2.3.  Events That Are Ignored in INIT_BIND

   If no DHCP Server-to-Client messages that assign addresses or confirm
   addresses are received, corresponding entries will expire
   automatically.  Thus, other DHCP Server-to-Client messages (e.g.,
   DHCPv4 NAK) are not specially processed.

   As a result, the following events, should they occur, are ignored
   until either a DHCPv4 ACK or a DHCPv6 Reply message is received or
   the lifetime of the binding entry expires.

   o  EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
      received

   o  EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received

   o  EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received

   o  EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
      received

   o  EVE_DHCP_RENEW: A DHCPv4 Renew or a DHCPv6 Renew message is
      received

   o  EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with the Rapid
      Commit option is received

   o  EVE_DHCP_DECLINE: A DHCPv4 Decline or a DHCPv6 Decline message is
      received

   o  EVE_DHCP_RELEASE: A DHCPv4 Release or a DHCPv6 Release message is
      received

   o  EVE_DHCP_LEASEQUERY: A successful DHCPv6 LEASEQUERY-REPLY is
      received

   In each case, the message MUST be forwarded.

   Resulting state: INIT_BIND - A potential binding has been set up.




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6.4.3.  Initial State: BOUND

6.4.3.1.  Event: EVE_ENTRY_EXPIRE - The lifetime of a binding entry
          expires

   The entry MUST be deleted from the BST.

   Resulting state: An entry that has been deleted from the BST may be
   considered to be in the "NO_BIND" state - No binding has been set up.

6.4.3.2.  Event: EVE_DHCP_DECLINE - A DHCPv4 Decline or a DHCPv6 Decline
          message is received

   The message MUST be forwarded.

   First, the SAVI device gets all the addresses ("Requested IP address"
   in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, and addresses in all
   the IA options of DHCPv6 Decline/Release) to decline/release in the
   message.  Then, the corresponding entries MUST be removed.

   Resulting state in each relevant BST entry: An entry that has been
   deleted from the BST may be considered to be in the "NO_BIND" state -
   No binding has been set up.

6.4.3.3.  Event: EVE_DHCP_RELEASE - A DHCPv4 Release or a DHCPv6 Release
          message is received

   The message MUST be forwarded.

   First, the SAVI device gets all the addresses ("Requested IP address"
   in DHCPv4 Decline, "ciaddr" in DHCPv4 Release, and addresses in all
   the IA options of DHCPv6 Decline/Release) to decline/release in the
   message.  Then, the corresponding entries MUST be removed.

   Resulting state in each relevant BST entry: An entry that has been
   deleted from the BST may be considered to be in the "NO_BIND" state -
   No binding has been set up.

6.4.3.4.  Event: EVE_DHCP_REBIND - A DHCPv4 Rebind or a DHCPv6 Rebind
          message is received

   The message MUST be forwarded.

   In such a case, a new TID will be used by the client.  The TID field
   of the corresponding entries MUST be set to the new TID.  Note that
   the TID check will not be performed on such messages.

   Resulting state: BOUND: The binding has been set up.



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6.4.3.5.  Event: EVE_DHCP_RENEW - A DHCPv4 Renew or a DHCPv6 Renew
          message is received

   The message MUST be forwarded.

   In such a case, a new TID will be used by the client.  The TID field
   of the corresponding entries MUST be set to the new TID.  Note that
   the TID check will not be performed on such messages.

   Resulting state: BOUND: The binding has been set up.

6.4.3.6.  Event: EVE_DHCP_REPLY - A DHCPv4 ACK or a DHCPv6 Reply message
          is received

   The message MUST be forwarded.

   The DHCP Reply messages received in current states should be in
   response to DHCP Renew/Rebind.

   If the message is DHCPv4 ACK, the SAVI device updates the binding
   entry with a matched TID, with the Lifetime field set to be the sum
   of the new lease time and MAX_DHCP_RESPONSE_TIME, leaving the entry
   in the BOUND state.

   If the message is DHCPv6 Reply, the SAVI device checks each IA
   Address option in each IA option.  For each:

   1.  If the IA entry in the REPLY message has the status "NoBinding",
       there is no address in the option, and no operation on an address
       is performed.

   2.  If the valid lifetime of an IA Address option is 0, the binding
       entry with a matched TID and address is removed, leaving it
       effectively in the NO_BIND state.

   3.  Otherwise, set the Lifetime field of the binding entry with the
       matched TID and address to be the sum of the new valid lifetime
       and MAX_DHCP_RESPONSE_TIME, leaving the entry in the BOUND state.

   Resulting state: NO_BIND or BOUND, as specified.











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6.4.3.7.  Event: EVE_DHCP_LEASEQUERY - A successful DHCPv6
          LEASEQUERY_REPLY is received

   The message MUST be forwarded.

   The message should be in response to the LEASEQUERY message sent in
   Section 6.4.2.  The related binding entry can be determined based on
   the address in the IA Address option in the LEASEQUERY-REPLY message.
   The Lifetime field of the corresponding binding entry is set to the
   sum of the lease time in the LEASEQUERY-REPLY message and
   MAX_DHCP_RESPONSE_TIME.

   Resulting state: BOUND: The binding has been set up.

6.4.3.8.  Events Not Processed in the State BOUND

   The following events are ignored if received while the indicated
   entry is in the BOUND state.  Any required action will be the result
   of the next message in the client/server exchange.

   o  EVE_DHCP_REQUEST: A DHCPv4 Request or a DHCPv6 Request message is
      received

   o  EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received

   o  EVE_DHCP_REBOOT: A DHCPv4 Reboot message is received

   o  EVE_DHCP_SOLICIT_RC: A DHCPv6 Solicitation message with the Rapid
      Commit option is received






















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6.4.4.  Table of State Machine

   The main state transits are listed as follows.  Note that not all the
   details are specified in the table and the diagram.

   State       Event             Action                       Next State
   ---------------------------------------------------------------------
   NO_BIND     RQ/RC/CF/RE       Generate entry                INIT_BIND

   INIT_BIND   RPL               Record lease time                 BOUND
                                 (send leasequery if no lease)

   INIT_BIND   EVE_ENTRY_EXPIRE  Remove entry                    NO_BIND

   BOUND       RLS/DCL           Remove entry                    NO_BIND

   BOUND       EVE_ENTRY_EXPIRE  Remove entry                    NO_BIND

   BOUND       RPL               Set new lifetime                  BOUND

   BOUND       LQR               Record lease time                 BOUND

                     Figure 9: State Transition Table

   RQ:  EVE_DHCP_REQUEST
   RC:  EVE_DHCP_SOLICIT_RC
   CF:  EVE_DHCP_CONFIRM
   RE:  EVE_DHCP_REBOOT
   RPL: EVE_DHCP_REPLY
   RLS: EVE_DHCP_RELEASE
   DCL: EVE_DHCP_DECLINE
   LQR: EVE_DHCP_LEASEQUERY



















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                               +-------------+
                               |             |
                      /--------+   NO_BIND   |<--------\
                      |  ----->|             |         |
                      |  |     +-------------+         |EVE_DHCP_RELEASE
   EVE_DHCP_REQUEST   |  |                             |EVE_DHCP_DECLINE
   EVE_DHCP_CONFIRM   |  |EVE_ENTRY_EXPIRE             |EVE_ENTRY_EXPIRE
   EVE_DHCP_SOLICIT_RC|  |                             |
   EVE_DHCP_REBOOT    |  |                             |
                      |  |                             |
                      |  |                             |
                      v  |                             |
              +-------------+                      +------------+
              |             |    EVE_DHCP_REPLY   |            |
              |  INIT_BIND  --------------------->|    BOUND   |<-\
              |             |                     |            |  |
              +-------------+                     +------------+  |
                                                         |        |
                                                         \--------/
                                               EVE_DHCP_REPLY
                                               EVE_DHCP_LEASEQUERY

                       Figure 10: Diagram of Transit

7.  Data Snooping Process

7.1.  Scenario

   The rationale of the DHCP Snooping Process specified in Section 6 is
   that if a DHCP client's use of a DHCP address is legitimate, the
   corresponding DHCP address assignment procedure must have been
   finished during the attachment of the DHCP client.  This is the case
   when the SAVI device is continuously on the path(s) from the DHCP
   client to the DHCP server(s)/relay(s).  However, there are two cases
   in which this does not work:

   o  Multiple paths: there is more than one feasible link-layer path
      from the client to the DHCP server/relay, and the SAVI device is
      not on every one of them.  The client may get its address through
      one of the paths that does not pass through the SAVI device, but
      packets from the client can travel on paths that pass through the
      SAVI device, such as when the path through the link-layer network
      changes.  Because the SAVI device could not snoop the DHCP packet
      exchange procedure, the DHCP Snooping Process cannot set up the
      corresponding binding.






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   o  Dynamic path: there is only one feasible link-layer path from the
      client to the DHCP server/relay, but the path is dynamic due to
      topology change (for example, some link becomes broken due to
      failure or some planned change) or link-layer path change.  This
      situation also covers the local-link movement of clients without
      the address confirm/reconfiguration process.  For example, a host
      changes its attached switch port in a very short time.  In such
      cases, the DHCP Snooping Process will not set up the corresponding
      binding.

   The Data Snooping Process can avoid the permanent blocking of
   legitimate traffic in case one of these two exceptions occurs.  This
   process is performed on attachments with the Data-Snooping attribute.
   Data packets without a matching binding entry may trigger this
   process to set up bindings.

   Snooping data traffic introduces a considerable burden on the
   processor and ASIC-to-Processor bandwidth of SAVI devices.  Because
   of the overhead of this process, the implementation of this process
   is OPTIONAL.  This function SHOULD be enabled unless the
   implementation is known to be used in the scenarios without the above
   exceptions.  For example, if the implementation is to be used in
   networks with tree topology and without host local-link movement,
   there is no need to implement this process in such scenarios.

   This process is not intended to set up a binding whenever a data
   packet without a matched binding entry is received.  Instead,
   unmatched data packets trigger this process probabilistically, and
   generally a number of unmatched packets will be discarded before the
   binding is set up.  The parameter(s) of this probabilistic process
   SHOULD be configurable, defaulting to a situation where data snooping
   is disabled.

7.2.  Rationale

   This process makes use of NS/ARP and DHCP Leasequery to set up
   bindings.  If an address is not used by another client in the
   network, and the address has been assigned in the network, the
   address can be bound with the binding anchor of the attachment from
   which the unmatched packet is received.

   The Data Snooping Process provides an alternative path for binding
   entries to reach the BOUND state in the exceptional cases explained
   in Section 7.1 when there are no DHCP messages that can be snooped by
   the SAVI device.






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   In some of the exceptional cases (especially the dynamic topology
   case), by the time the binding has reached the BOUND state, the DHCP
   messages may be passing through the SAVI device.  In this case, the
   events driven by DHCP messages that are expected in the BOUND state
   in the DHCP Snooping Process may occur, and the binding can be
   handled by the DHCP Snooping Process state machine.

   In any event, the lease expiry timeout event will occur even if no
   others do.  This will cause the binding to be deleted and the state
   to logically return to NO_BIND state.  Either the DHCP or the Data
   Snooping Process will be reinvoked if the lease is still in place.
   If DHCP messages are still not passing through the SAVI device, there
   will be a brief disconnection during which data packets passing
   through the SAVI device will be dropped.  The probabilistic
   initiation of the Data Snooping Process can then take over again and
   return the binding state to BOUND in due course.

   The security issues concerning this process are discussed in
   Section 11.1.

7.3.  Additional Binding States Description

   In addition to NO_BIND and BOUND from Section 6.2, three new states
   used in this process are listed here.  The INIT_BIND state is not
   used, as it is entered by observing a DHCP message.

   DETECTION: The address in the entry is undergoing local duplication
   detection.

   RECOVERY: The SAVI device is querying the assignment and lease time
   of the address in the entry through DHCP Leasequery.

   VERIFY: The SAVI device is verifying that the device connected to the
   attachment point has a hardware address that matches the one returned
   in the DHCP Leasequery.

   Because the mechanisms used for the operations carried out while the
   binding is in these three states operate over unreliable protocols,
   each operation is carried out twice with a timeout that is triggered
   if no response is received.

7.4.  Events

   To handle the Data Snooping Process, six extra events, described
   here, are needed in addition to those used by the DHCP Snooping
   Process (see Section 6.3).  If an event will trigger the creation of
   a new binding entry, the binding entry limit on the binding anchor
   MUST NOT be exceeded.



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   EVE_DATA_UNMATCH: A data packet without a matched binding is
   received.

   EVE_DATA_CONFLICT: An ARP Reply / Neighbor Advertisement (NA) message
   against an address in the DETECTION state is received from a host
   other than the one for which the entry was added (i.e., a host
   attached at a point other than the one on which the triggering data
   packet was received).

   EVE_DATA_LEASEQUERY:

   o  IPv4: A DHCPLEASEACTIVE message with the IP Address Lease Time
      option is received.  Note that the DHCPLEASEUNKNOWN and
      DHCPLEASEUNASSIGNED replies are ignored.

   o  IPv6: A successful LEASEQUERY-REPLY is received.

   EVE_DATA_VERIFY: An ARP Reply / NA message has been received in the
   VERIFY state from the device connected to the attachment point on
   which the data packet was received.

   The triggering packet should pass the following checks to trigger a
   valid event:

   o  Attribute check: the data packet should be from attachments with
      the Data-Snooping attribute; the DHCPLEASEACTIVE/LEASEQUERY-REPLY
      messages should be from attachments with the DHCP-Snooping
      attribute.

   o  Binding limitation check: the data messages must not cause new
      binding setup on an attachment whose binding entry limitation has
      been reached (refer to Section 11.5).

   o  Address check: For EVE_DATA_LEASEQUERY, the source address of the
      DHCPLEASEQUERY messages must pass the check specified in
      Section 8.2.  For EVE_DATA_CONFLICT and EVE_DATA_VERIFY, the
      source address and target address of the ARP or NA messages must
      pass the check specified in Section 8.2.

   o  Interval check: the interval between two successive
      EVE_DATA_UNMATCH events triggered by an attachment MUST be no
      smaller than DATA_SNOOPING_INTERVAL.

   o  TID check: the DHCPLEASEACTIVE/LEASEQUERY-REPLY messages must have
      a matched TID with the corresponding entry.

   o  Prefix check: the source address of the data packet should be of a
      valid local prefix, as specified in Section 7 of [RFC7039].



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   EVE_DATA_EXPIRE: A timer expires indicating that a response to a
   hardware address verification message sent in the VERIFY state has
   not been received within the specified DETECTION_TIMEOUT period.

   EVE_ENTRY_EXPIRE: A timer expires after the Lifetime indicated in the
   relevant BST entry has elapsed.  This is identical to the usage in
   the DHCP Snooping Process.

7.5.  Message Sender Functions

   The Data Snooping Process involves sending three different messages
   to other network devices.  Each message may be sent up to two times
   since they are sent over unreliable transports and are sent in
   different states.  The functions defined in this section specify the
   messages to be sent in the three cases.  In each case, the message to
   be sent depends on whether the triggering data packet is an IPv4 or
   an IPv6 packet.

7.5.1.  Duplicate Detection Message Sender

   Send a message to check if the source address in the data packet that
   triggered the Data Snooping Process has a local conflict (that is, it
   uses an address that is being used by another node):

   IPv4 address:  Broadcast an Address Resolution Protocol (ARP) Request
         [RFC826] or an ARP Probe [RFC5227] for the address to the local
         network.  An ARP Response will be expected from the device on
         the attachment point on which the triggering data packet was
         received.  An ARP Reply received on any other port indicates a
         duplicate address.

   IPv6 address:  Send a Duplicate Address Detection (DAD) message
         (Neighbor Solicitation message) to the solicited-node multicast
         address [RFC4861] targeting the address.  Ideally, only the
         host on that point of attachment responds with a Neighbor
         Advertisement.  A Neighbor Advertisement received on any other
         port indicates a duplicate address.

   As both the ARP and DAD processes are unreliable (the packet either
   to or from the other system may be lost in transit; see [RFC6620]),
   if there is no response after the DETECTION_TIMEOUT, an
   EVE_ENTRY_EXPIRE is generated.









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7.5.2.  Leasequery Message Sender

   Send a DHCPLEASEQUERY message to the DHCP server(s) to determine if
   it has given out a lease for the source address in the triggering
   data packet.  A list of authorized DHCP servers is kept by the SAVI
   device.  The list should be either preconfigured with the IPv4 and/or
   IPv6 addresses or dynamically discovered: For networks using IPv4,
   this can be done by sending DHCPv4 Discover messages and parsing the
   returned DHCPv4 Offer messages; for networks using IPv6, discovery
   can be done by sending DHCPv6 SOLICIT messages and parsing the
   returned ADVERTISE messages.  The same TID should be used for all
   LEASEQUERY messages sent in response to a triggering data message on
   an attachment point.  The TID is generated if the TID field in the
   BST entry is empty and recorded in the TID field of the BST entry
   when the first message is sent.  Subsequent messages use the TID from
   the BST entry.

   (1)  IPv4 address: Send a DHCPLEASEQUERY [RFC4388] message querying
        by IP address to each DHCPv4 server in the list of authorized
        servers with an IP Address Lease Time option (option 51).  If
        the server has a valid lease for the address, the requested
        information will be returned in a DHCPLEASEACTIVE message.

   (2)  IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP
        address to each DHCPv6 server in the list of authorized servers
        using the server address as the link-address in the LEASEQUERY
        message.  If the server has a valid lease for the address, the
        requested information will be returned in a LEASEQUERY-REPLY
        message marked as successful (i.e., without an
        OPTION_STATUS_CODE in the reply).  The IA Address option(s)
        returned contains any IPv6 addresses bound to the same link
        together with the lease validity time.

   As DHCP Leasequeries are an unreliable process (the packet either to
   or from the server may be lost in transit), if there is no response
   after the MAX_LEASEQUERY_DELAY, an EVE_DATA_EXPIRE is generated.
   Note that multiple response messages may be received if the list of
   authorized servers contains more than one address of the appropriate
   type and, in the case of DHCPv6, the responses may contain additional
   addresses for which leases have been allocated.

7.5.3.  Address Verification Message Sender

   Send a message to verify that the link-layer address in the attached
   device that sent the triggering data packet matches the link-layer
   address contained in the leasequery response:





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   IPv4 address:  Send an ARP Request with the Target Protocol Address
         set to the IP address in the BST entry.  The ARP Request is
         only sent to the attachment that triggered the binding.  If the
         attached device has the IP address bound to the interface
         attached to the SAVI device, an ARP Reply should be received
         containing the hardware address of the interface on the
         attached device that can be compared with the leasequery value.

   IPv6 address:  Send a Neighbor Solicitation (NS) message with the
         target address set to the IP address in the BST entry.  The NS
         is only sent to the attachment that triggered the binding.  If
         the attached device has the IP address bound to the interface
         attached to the SAVI device, an NA should be received
         indicating that the attached device has the IP address
         configured on the interface.

   As both the ARP and NS/NA processes are unreliable (the packet either
   to or from the other system may be lost in transit; see [RFC6620]),
   if there is no response after the DETECTION_TIMEOUT, an
   EVE_DATA_EXPIRE is generated.

7.6.  Initial State: NO_BIND

7.6.1.  Event: EVE_DATA_UNMATCH: A data packet without a matched binding
        is received

   Make a probabilistic determination as to whether to act on this
   event.  The probability may be configured or calculated based on the
   state of the SAVI device.  This probability should be low enough to
   mitigate the damage from DoS attacks against this process.

   Create a new entry in the BST.  Set the Binding Anchor field to the
   corresponding binding anchor of the attachment.  Set the Address
   field to the source address of the packet.

   Address conflicts MUST be detected and prevented.

   If local address detection is performed:
         Set the State field to DETECTION.  Set the Lifetime of the
         created entry to DETECTION_TIMEOUT.  Set the Timeouts field to
         0.  Start the detection of any local address conflicts by
         sending a Duplicate Address Detection Message (Section 7.5.1).
         Transition to DETECTION state.








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   If local address detection is not performed:
         Set the State field to RECOVERY.  Set the Lifetime of the
         created entry to LEASEQUERY_DELAY.  Set the Timeouts field to
         0.  Start the recovery of any DHCP lease associated with the
         source IP address by sending one or more LEASEQUERY messages
         (Section 7.5.2).  Transition to RECOVERY state.

   The packet that triggers this event SHOULD be discarded.

   An example of the BST entry during duplicate address detection is
   illustrated in Figure 11.

   +--------+-------+---------+-----------------------+-----+----------+
   | Anchor |Address|  State  | Lifetime              | TID | Timeouts |
   +--------+-------+---------+-----------------------+-----+----------+
   | Port_1 | Addr1 |DETECTION| DETECTION_TIMEOUT     |     |    0     |
   +--------+-------+---------+-----------------------+-----+----------+

     Figure 11: Binding Entry in BST on Data-Triggered Initialization

   Resulting state: DETECTION - The address in the entry is undergoing
   local duplication detection - or RECOVERY - The DHCP lease(s)
   associated with the address is being queried.

7.6.2.  Events Not Observed in NO_BIND for Data Snooping

   EVE_DATA_CONFLICT: An ARP Reply / NA message is received from an
   unexpected system.

   EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or LEASEQUERY-REPLY is
   received.

   EVE_DATA_VERIFY: A valid ARP Reply or NA message is received from the
   attached device.

   All EVE_DHCP_* events defined in Section 6.3.2 are treated as
   described in the DHCP Snooping Process (Section 6.4.1) and may result
   in that process being triggered.

   EVE_ENTRY_EXPIRE: Expiration of the DECTECTION_TIMEOUT

   EVE_DATA_EXPIRE: Expiration of the DECTECTION_TIMEOUT









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7.7.  Initial State: DETECTION

7.7.1.  Event: EVE_ENTRY_EXPIRE

   When this event occurs, no address conflict has been detected during
   the previous DETECTION_TIMEOUT period.

   If the Timeouts field in the BST entry is 0:
         Set the Lifetime of the BST entry to DETECTION_TIMEOUT.  Set
         the Timeouts field to 1.  Restart the detection of any local
         address conflicts by sending a second Duplicate Address
         Detection Message (Section 7.5.1).  Remain in DETECTION state.

   If the Timeouts field in the BST entry is 1:

         Assume that there is no local address conflict.  Set the State
         field to RECOVERY.  Set the Lifetime of the BST entry to
         LEASEQUERY_DELAY.  Set the Timeouts field to 0.  Start the
         recovery of any DHCP lease associated with the source IP
         address by sending one or more LEASEQUERY messages
         (Section 7.5.2).  Transition to RECOVERY state.

   An example of the entry is illustrated in Figure 12.

   +--------+-------+----------+----------------------+-----+----------+
   | Anchor |Address|  State   | Lifetime             | TID | Timeouts |
   +--------+-------+----------+----------------------+-----+----------+
   | Port_1 | Addr1 | RECOVERY | MAX_LEASEQUERY_DELAY | TID |    0     |
   +--------+-------+----------+----------------------+-----+----------+

               Figure 12: Binding Entry in BST on Leasequery

   Resulting state: DETECTION - If a second local conflict period is
   required - or RECOVERY - The SAVI device is querying the assignment
   and lease time of the address in the entry through DHCP Leasequery.

7.7.2.  Event: EVE_DATA_CONFLICT: ARP Reply / NA Message Received from
        Unexpected System

   Remove the entry.

   Resulting state: NO_BIND - No binding has been set up.

7.7.3.  Events Not Observed in DETECTION

   EVE_DATA_UNMATCH: A data packet without a matched binding is received

   All EVE_DHCP_* events defined in Section 6.3.2



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   EVE_DHCP_REBIND: A DHCPv4 Rebind or a DHCPv6 Rebind message is
   received

7.8.  Initial State: RECOVERY

7.8.1.  Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or
        successful LEASEQUERY-REPLY is received

   Set the State in the BST entry to VERIFY.  Depending on the type of
   triggering source IP address, process the received DHCP Leasequery
   response:

   IPv4 address:  Update the Lifetime field in the BST entry to the sum
         of the value encoded in the IP Address Lease Time option of the
         DHCPLEASEACTIVE message and MAX_DHCP_RESPONSE_TIME.  Record the
         value of the "chaddr" field (hardware address) in the message
         for checking against the hardware address received during
         verification in the next state.  Set the Timeouts field to 0.
         Start the verification process by sending an Address
         Verification Message (see Section 7.5.3).  Transition to VERIFY
         state.  Start an additional verification timer with a duration
         of DETECTION_TIMEOUT.  When this expires, an EVE_DATA_EXPIRE
         event will be generated.

   IPv6 address:  Update the Lifetime field in the BST entry to the sum
         of the valid lifetime extracted from the OPTION_CLIENT_DATA
         option in the LEASEQUERY-REPLY message and
         MAX_DHCP_RESPONSE_TIME.  Set the Timeouts field to 0.  Start
         the verification process by sending an Address Verification
         Message (see Section 7.5.3).  Transition to VERIFY state.
         Start an additional verification timer with a duration of
         DETECTION_TIMEOUT.  When this expires, an EVE_DATA_EXPIRE event
         will be generated.

         If multiple addresses are received in the LEASEQUERY-REPLY, new
         BST entries MUST be created for the additional addresses using
         the same binding anchor.  The entries are created with state
         set to VERIFY and the other fields set as described in this
         section for the triggering source IP address.  Also, start the
         verification process and start verification timers for each
         additional address.

   Resulting state: VERIFY - Awaiting verification or otherwise of the
   association of the IP address with the connected interface.







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7.8.2.  Event: EVE_ENTRY_EXPIRE

   Depending on the value of the Timeouts field in the BST entry, either
   send repeat LEASEQUERY messages or discard the binding:

   If the Timeouts field in the BST entry is 0:
         No responses to the LEASEQUERY message(s) sent have been
         received during the first LEASEQUERY_DELAY period.  Set the
         Lifetime of the BST entry to LEASEQUERY_DELAY.  Set the
         Timeouts field to 1.  Restart the recovery of any DHCP lease
         associated with the source IP address by sending one or more
         LEASEQUERY messages (Section 7.5.2).  Remain in RECOVERY state.

   If the Timeouts field in the BST entry is 1:
         No responses to the LEASEQUERY messages sent during two
         LEASEQUERY_DELAY periods were received.  Assume that no leases
         exist and hence that the source IP address is bogus.  Delete
         the BST entry.  Transition to NO_BIND state.

   Resulting state: RECOVERY - If repeat leasequeries are sent - or
   NO_BIND - If no successful responses to LEASEQUERY messages have been
   received.

7.8.3.  Events Not Observed in RECOVERY

   EVE_DATA_UNMATCH: A data packet without a matched binding is received

   EVE_DATA_CONFLICT: An ARP Reply / NA message is received from an
   unexpected system

   EVE_DATA_VERIFY: A valid ARP Reply or NA message is received from the
   attached device

   All EVE_DHCP_* events defined in Section 6.3.2

   EVE_DATA_EXPIRE: Expiration of the DECTECTION_TIMEOUT

7.9.  Initial State: VERIFY

7.9.1.  Event: EVE_DATA_LEASEQUERY: A valid DHCPLEASEACTIVE or
        successful LEASEQUERY-REPLY is received

   If LEASEQUERY messages were sent to more than one DHCP server during
   RECOVERY state, additional successful leasequery responses may be
   received relating to the source IP address.  The conflict resolution
   mechanisms specified in Section 6.8 of [RFC4388] and Section 4.3.4 of
   [RFC5007] can be used to determine the message from which values are
   used to update the BST Lifetime entry and the hardware address



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   obtained from DHCP, as described in Section 7.8.1.  In the case of
   DHCPv6 queries, the LEASEQUERY-REPLY may contain additional addresses
   as described in Section 7.8.1.  If so, additional BST entries MUST be
   created or ones previously created updated as described in that
   section.

   Resulting state: VERIFY (no change).

7.9.2.  Event: EVE_DATA_VERIFY: A valid ARP Reply or NA is received from
        the device attached via the binding anchor

   Depending on the type of triggering source IP address, this event may
   indicate that the device attached via the binding anchor in the BST
   entry is configured by DHCP using the IP address:

   IPv4 address:  Check that the value of the sender hardware address in
         the ARP Reply matches the saved "chaddr" field (hardware
         address) from the previously received DHCPLEASEACTIVE message.
         If not, ignore this event; a subsequent retry may provide
         verification.  If the hardware addresses match, the binding
         entry has been verified.

   IPv6 address:  Simple receipt of a valid NA from the triggering
         source IP address at the binding anchor port provides
         verification for the binding entry.

   If the binding entry has been verified, set the state in the BST
   entry to BOUND.  Clear the TID field.  Cancel the verification timer.

   Resulting state: VERIFY (no change) - If the IPv4 DHCPLEASEQUERY
   "chaddr" address does not match the ARP Reply hardware address.
   Otherwise, the resulting state is BOUND.

7.9.3.  Event: EVE_ENTRY_EXPIRE

   The DHCP lease lifetime has expired before the entry could be
   verified.  Remove the entry.  Transition to NO_BIND state.

   Resulting state: NO_BIND - No binding has been set up.












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7.9.4.  Event: EVE_DATA_EXPIRE

   Depending on the value of the Timeouts field in the BST entry, either
   send a repeat validation message or discard the binding:

   If the Timeouts field in the BST entry is 0:
         No response to the verification message sent has been received
         during the first DETECTION_TIMEOUT period.  Set the Timeouts
         field to 1.  Restart the verification process by sending an
         Address Verification Message (see Section 7.5.3).  Start a
         verification timer with a duration of DETECTION_TIMEOUT.  When
         this expires, an EVE_DATA_EXPIRE event will be generated.
         Remain in VERIFY state.

   If the Timeouts field in the BST entry is 1:
         No responses to the verification messages sent during two
         DETECTION_TIMEOUT periods were received.  Assume that the
         configuration of the triggering source IP address cannot be
         verified and hence that the source IP address is bogus.  Delete
         the BST entry.  Transition to NO_BIND state.

   Resulting state: VERIFY - Additional verification message sent - or
   NO_BIND - No binding has been set up.

7.9.5.  Events Not Observed in VERIFY

   EVE_DATA_UNMATCH: A data packet without a matched binding is received

   EVE_DATA_CONFLICT: An ARP Reply / NA message is received from an
   unexpected system

   All EVE_DHCP_* events defined in Section 6.3.2

7.10.  Initial State: BOUND

   Upon entry to the BOUND state, control of the system continues as if
   a DHCP message assigning the address has been observed, as in
   Section 6.4.3.  The BST entry has been restored.

   Note that the TID field contains no value after the binding state
   changes to BOUND.  The TID field is recovered from snooping DHCP
   Renew/Rebind messages if these are observed as described in the DHCP
   Snooping Process.  Because TID is used to associate binding entries
   with messages from DHCP servers, it must be recovered or else a
   number of state transitions of this mechanism will not be executed
   normally.





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7.11.  Table of State Machine

   The main state transitions are listed as follows.

   State      Event               Action                      Next State
   ---------------------------------------------------------------------
   NO_BIND    EVE_DATA_UNMATCH    Start duplicate detect       DETECTION

   DETECTION  EVE_ENTRY_EXPIRE 1  Repeat duplicate detect      DETECTION

   DETECTION  EVE_ENTRY_EXPIRE 2  Start leasequery              RECOVERY

   DETECTION  EVE_DATA_CONFLICT   Remove entry                   NO_BIND

   RECOVERY   EVE_ENTRY_EXPIRE 1  Repeat leasequery             RECOVERY

   RECOVERY   EVE_ENTRY_EXPIRE 2  No lease found; remove entry   NO_BIND

   RECOVERY   EVE_DATA_LEASEQUERY Set lease time; start verify    VERIFY

   VERIFY     EVE_ENTRY_EXPIRE    Lease expiry; remove entry     NO_BIND

   VERIFY     EVE_DATA_LEASEQUERY Resolve lease conflict(s)       VERIFY

   VERIFY     EVE_DATA_VERIFY     Finish validation     BOUND or NO_BIND

   VERIFY     EVE_DATA_EXPIRE 1   Repeat verify                   VERIFY

   VERIFY     EVE_DATA_EXPIRE 2   Verify failed; remove entry    NO_BIND

   BOUND      EVE_ENTRY_EXPIRE    Lease expiry; remove entry     NO_BIND

   BOUND      RENEW/REBIND        Record TID                       BOUND

                     Figure 13: State Transition Table
















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                               +-------------+         EVE_ENTRY_EXPIRE
                     /---------+             |<------------------------\
                     |         |   NO_BIND   |         EVE_DATA_EXPIRE |
    EVE_DATA_UNMATCH |  /----->|             |<----\   (2nd VRF_DELAY) |
                     |  |      +-------------+     |                   |
                     |  |         EVE_ENTRY_EXPIRE |                   |
                     |  |           (2nd LQ_DELAY) |                   |
   EVE_ENTRY_EXPIRE  |  |                          |  EVE_ENTRY_EXPIRE |
   (1st DAD_DELAY)   |  |                          |   (1st LQ_DELAY)  |
         /------\    |  |                          |        /--------\ |
         |      |    |  | EVE_DATA_CONFLICT        \---\    |        | |
         |      v    v  |                              |    v        | |
         |    +-------------+ EVE_ENTRY_EXPIRE       +------------+  | |
         |    |             | (2nd DAD_DELAY)        |            |  | |
         \----+  DETECTION  ------------------------>|  RECOVERY  +--/ |
              |             |                        |            |    |
              +-------------+   (To NO_BIND)         +------------+    |
                                ^                               |      |
                                |           EVE_DATA_LEASEQUERY |      |
                  /----------\  |                               |      |
                  |          |  | EVE_ENTRY_EXPIRE              |      |
    EVE_DHCP_RENEW|          v  |                               v      |
   EVE_DHCP_REBIND|    +-------------+                +-------------+  |
                  |    |             |                |             +--/
                  \----+   BOUND     |<---------------+   VERIFY    |
                       |             | EVE_DATA_VERIFY|             |<-\
                       +-------------+                +-------------+  |
                                                            |          |
                                                            \----------/
                                                     EVE_DATA_LEASEQUERY
                                                         EVE_DATA_EXPIRE
                                                         (1st VRF_DELAY)

                       Figure 14: Diagram of Transit

   LQ_DELAY:  MAX_LEASEQUERY_DELAY
   VRF_DELAY: DETECTION_TIMEOUT

8.  Filtering Specification

   This section specifies how to use bindings to filter out packets with
   spoofed source addresses.

   Filtering policies are different for data packets and control
   packets.  DHCP, ARP, and Neighbor Discovery Protocol (NDP) [RFC4861]
   messages are classified as control packets.  All other packets are
   classified as data packets.




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8.1.  Data Packet Filtering

   Data packets from attachments with the Validating attribute TRUE MUST
   have their source addresses validated.  There is one exception to
   this rule.

   A packet whose source IP address is a link-local address cannot be
   checked against DHCP assignments, as it is not assigned using DHCP.
   Note: as explained in Section 1, a SAVI solution for link-local
   addresses, e.g., FCFS SAVI [RFC6620], can be enabled to check packets
   with a link-local source address.

   If the source IP address of a packet is not a link-local address, but
   there is not a matching entry in the BST with BOUND state, this
   packet MUST be discarded.  However, the packet may trigger the Data
   Snooping Process (Section 7) if the Data-Snooping attribute is set on
   the attachment.

   Data packets from an attachment with the Validating attribute set
   FALSE will be forwarded without having their source addresses
   validated.

   The SAVI device MAY log packets that fail source address validation.

8.2.  Control Packet Filtering

   For attachments with the Validating attribute:

   DHCPv4 Client-to-Server messages in which the source IP address is
   neither all zeros nor bound with the corresponding binding anchor in
   the BST MUST be discarded.

   DHCPv6 Client-to-Server messages in which the source IP address is
   neither a link-local address nor bound with the corresponding binding
   anchor in the BST MUST be discarded.

   NDP messages in which the source IP address is neither a link-local
   address nor bound with the corresponding binding anchor MUST be
   discarded.

   NA messages in which the target address is neither a link-local
   address nor bound with the corresponding binding anchor MUST be
   discarded.

   ARP messages in which the protocol is IP and the sender protocol
   address is neither all zeros nor bound with the corresponding binding
   anchor MUST be discarded.




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   ARP Reply messages in which the target protocol address is not bound
   with the corresponding binding anchor MUST be discarded.

   For attachments with other attributes:

   DHCP Server-to-Client messages not from attachments with the DHCP-
   Trust attribute or Trust attribute MUST be discarded.

   For attachments with no attribute:

   DHCP Server-to-Client messages from such attachments MUST be
   discarded.

   The SAVI device MAY record any messages that are discarded.

9.  State Restoration

   If a SAVI device reboots, the information kept in volatile memory
   will be lost.  This section specifies the restoration of attribute
   configuration and the BST.

9.1.  Attribute Configuration Restoration

   The loss of attribute configuration will not break the network: no
   action will be performed on traffic from attachments with no
   attribute.  However, the loss of attribute configuration makes this
   SAVI function unable to work.

   To avoid the loss of binding anchor attribute configuration, the
   configuration MUST be able to be stored in non-volatile storage.
   After the reboot of the SAVI device, if the configuration of binding
   anchor attributes is found in non-volatile storage, the configuration
   MUST be used.

9.2.  Binding State Restoration

   The loss of binding state will cause the SAVI devices to discard
   legitimate traffic.  Simply using the Data Snooping Process to
   recover a large number of bindings is a heavy overhead and may cause
   considerable delay.  Thus, recovering bindings from non-volatile
   storage, as specified below, is RECOMMENDED.

   Binding entries MAY be saved into non-volatile storage whenever a new
   binding entry changes to BOUND state.  If a binding with BOUND state
   is removed, the saved entry MUST be removed correspondingly.  The
   time when each binding entry is established is also saved.





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   If the BST is stored in non-volatile storage, the SAVI device SHOULD
   restore binding state from the non-volatile storage immediately after
   reboot.  Using the time when each binding entry was saved, the SAVI
   device should check whether the entry has become obsolete by
   comparing the saved lifetime and the difference between the current
   time and time when the binding entry was established.  Obsolete
   entries that would have expired before the reboot MUST be removed.

10.  Constants

   The following constants are recommended for use in this context:

   o  MAX_DHCP_RESPONSE_TIME (120s): Maximum Solicit timeout value
      (SOL_MAX_RT from [RFC3315])

   o  MAX_LEASEQUERY_DELAY (10s): Maximum LEASEQUERY timeout value
      (LQ_MAX_RT from [RFC5007])

   o  DETECTION_TIMEOUT (0.5s): Maximum duration of a hardware address
      verification step in the VERIFY state (TENT_LT from [RFC6620])

   o  DATA_SNOOPING_INTERVAL: Minimum interval between two successive
      EVE_DATA_UNMATCH events triggered by an attachment.
      Recommended interval: 60s and configurable

   o  OFFLINK_DELAY: Period after a client is last detected before the
      binding anchor is being removed.  Recommended delay: 30s

11.  Security Considerations

11.1.  Security Problems with the Data Snooping Process

   There are two security problems with the Data Snooping Process
   (Section 7):

   (1)  The Data Snooping Process is costly, but an attacker can trigger
        it simply through sending a number of data packets.  To avoid
        Denial-of-Service attacks against the SAVI device itself, the
        Data Snooping Process MUST be rate limited.  A constant
        DATA_SNOOPING_INTERVAL is used to control the frequency.  Two
        Data Snooping Processes on one attachment MUST be separated by a
        minimum interval time of DATA_SNOOPING_INTERVAL.  If this value
        is changed, the value needs to be large enough to minimize DoS
        attacks.

   (2)  The Data Snooping Process may set up incorrect bindings if the
        clients do not reply to the detection probes (Section 7.6.1).
        An attack will pass the duplicate detection if the client



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        assigned the target address does not reply to the detection
        probes.  The DHCP Leasequery procedure performed by the SAVI
        device just tells whether or not the address is assigned in the
        network.  However, the SAVI device cannot determine whether the
        address is just assigned to the triggering attachment from the
        DHCPLEASEQUERY Reply.

11.2.  Securing Leasequery Operations

   In [RFC4388] and [RFC5007], the specific case of DHCP Leasequeries
   originated by "access concentrators" is addressed extensively.  SAVI
   devices are very similar to access concentrators in that they snoop
   on DHCP traffic and seek to validate source addresses based on the
   results.  Accordingly, the recommendations for securing leasequery
   operations for access concentrators in Section 7 of [RFC4388] and
   Section 5 of [RFC5007] MUST be followed when leasequeries are made
   from SAVI devices.  [RFC5007] RECOMMENDS that communications between
   the querier and the DHCP server are protected with IPsec.  It is
   pointed out that there are relatively few devices involved in a given
   administrative domain (SAVI devices, DHCP relays, and DHCP servers)
   so that manual configuration of keying material would not be overly
   burdensome.

11.3.  Client Departure Issues

   After a binding is set up, the corresponding client may leave its
   attachment point.  It may depart temporarily due to signal fade or
   permanently by moving to a new attachment point or leaving the
   network.  In the signal fade case, since the client may return
   shortly, the binding should be kept momentarily, lest legitimate
   traffic from the client be blocked.  However, if the client leaves
   permanently, keeping the binding can be a security issue.  If the
   binding anchor is a property of the attachment point rather than the
   client, e.g., the switch port but not incorporating the MAC address,
   an attacker using the same binding anchor can send packets using IP
   addresses assigned to the client.  Even if the binding anchor is a
   property of the client, retaining binding state for a departed client
   for a long time is a waste of resources.

   Whenever a direct client departs from the network, a link-down event
   associated with the binding anchor will be triggered.  SAVI-DHCP
   monitors such events and performs the following mechanism.

   (1)  Whenever a client with the Validating attribute leaves, a timer
        of duration OFFLINK_DELAY is set on the corresponding binding
        entries.





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   (2)  If a DAD Neighbor Solicitation / Gratuitous ARP request is
        received that targets the address during OFFLINK_DELAY, the
        entry MAY be removed.

   (3)  If the client returns on-link during OFFLINK_DELAY, cancel the
        timer.

   In this way, the bindings of a departing client are kept for
   OFFLINK_DELAY.  In cases of link flapping, the client will not be
   blocked.  If the client leaves permanently, the bindings will be
   removed after OFFLINK_DELAY.

   SAVI-DHCP does not handle the departure of indirect clients because
   it will not be notified of such events.  Switches supporting indirect
   attachment (e.g., through a separate non-SAVI switch) SHOULD use
   information specific to the client such as its MAC address as part of
   the binding anchor.

11.4.  Compatibility with Detecting Network Attachment (DNA)

   DNA [RFC4436] [RFC6059] is designed to decrease the handover latency
   after reattachment to the same network.  DNA mainly relies on
   performing a reachability test by sending unicast Neighbor
   Solicitation / Router Solicitation / ARP Request messages to
   determine whether a previously configured address is still valid.

   Although DNA provides optimization for clients, there is insufficient
   information for this mechanism to migrate the previous binding or
   establish a new binding.  If a binding is set up only by snooping the
   reachability test message, the binding may be invalid.  For example,
   an attacker can perform the reachability test with an address bound
   to another client.  If a binding is migrated to the attacker, the
   attacker can successfully obtain the binding from the victim.
   Because this mechanism wouldn't set up a binding based on snooping
   the DNA procedure, it cannot achieve perfect compatibility with DNA.
   However, it only means the reconfiguration of the interface is slowed
   but not prevented.  Details are discussed as follows.

   In Simple DNAv6 [RFC6059], the probe is sent with the source address
   set to a link-local address, and such messages will not be discarded
   by the policy specified in Section 8.2.  If a client is reattached to
   a previous network, the detection will be completed, and the address
   will be regarded as valid by the client.  However, the candidate
   address is not contained in the probe.  Thus, the binding cannot be
   recovered through snooping the probe.  As the client will perform
   DHCP exchange at the same time, the binding will be recovered from
   the DHCP Snooping Process.  The DHCP Request messages will not be
   filtered out in this case because they have link-local source



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   addresses.  Before the DHCP procedure is completed, packets will be
   filtered out by the SAVI device.  In other words, if this SAVI
   function is enabled, Simple DNAv6 will not help reduce the handover
   latency.  If the Data-Snooping attribute is configured on the new
   attachment of the client, the data-triggered procedure may reduce
   latency.

   In DNAv4 [RFC4436], the ARP Probe will be discarded because an
   unbound address is used as the sender protocol address.  As a result,
   the client will regard the address under detection as valid.
   However, the data traffic will be filtered.  The DHCP Request message
   sent by the client will not be discarded because the source IP
   address field should be all zeros as required by [RFC2131].  Thus, if
   the address is still valid, the binding will be recovered from the
   DHCP Snooping Process.

11.5.  Binding Number Limitation

   A binding entry will consume certain high-speed memory resources.  In
   general, a SAVI device can afford only a quite limited number of
   binding entries.  In order to prevent an attacker from overloading
   the resources of the SAVI device, a binding entry limit is set on
   each attachment.  The binding entry limit is the maximum number of
   bindings supported on each attachment with the Validating attribute.
   No new binding should be set up after the limit has been reached.  If
   a DHCP Reply assigns more addresses than the remaining binding entry
   quota of each client, the message will be discarded and no binding
   will be set up.

11.6.  Privacy Considerations

   A SAVI device MUST delete binding anchor information as soon as
   possible (i.e., as soon as the state for a given address is back to
   NO_BIND), except where there is an identified reason why that
   information is likely to be involved in the detection, prevention, or
   tracing of actual source-address spoofing.  Information about hosts
   that never spoof (probably the majority of hosts) SHOULD NOT be
   logged.

11.7.  Fragmented DHCP Messages

   This specification does not preclude reassembly of fragmented DHCP
   messages, but it also does not require it.  If DHCP fragmentation
   proves to be an issue, the issue will need to be specified and
   addressed.  (This topic is beyond the scope of this document.)






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12.  References

12.1.  Normative References

   [RFC826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <http://www.rfc-editor.org/info/rfc826>.

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

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

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

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388,
              DOI 10.17487/RFC4388, February 2006,
              <http://www.rfc-editor.org/info/rfc4388>.

   [RFC4436]  Aboba, B., Carlson, J., and S. Cheshire, "Detecting
              Network Attachment in IPv4 (DNAv4)", RFC 4436,
              DOI 10.17487/RFC4436, March 2006,
              <http://www.rfc-editor.org/info/rfc4436>.

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

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, DOI 10.17487/RFC5007,
              September 2007, <http://www.rfc-editor.org/info/rfc5007>.

   [RFC5227]  Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              DOI 10.17487/RFC5227, July 2008,
              <http://www.rfc-editor.org/info/rfc5227>.





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   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,
              <http://www.rfc-editor.org/info/rfc6059>.

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

12.2.  Informative References

   [DHCPv6-SHIELD]
              Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
              Protecting Against Rogue DHCPv6 Servers", Work in
              Progress, draft-ietf-opsec-dhcpv6-shield-07, May 2015.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, DOI 10.17487/RFC3736,
              April 2004, <http://www.rfc-editor.org/info/rfc3736>.

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

   [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
              RFC 7341, DOI 10.17487/RFC7341, August 2014,
              <http://www.rfc-editor.org/info/rfc7341>.















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Acknowledgments

   Special thanks to Jean-Michel Combes, Christian Vogt, Joel M.
   Halpern, Eric Levy-Abegnoli, Marcelo Bagnulo Braun, Jari Arkko, Elwyn
   Davies, Barry Leiba, Ted Lemon, Leaf Yeh, Ralph Droms, and Alberto
   Garcia for careful review and evaluation comments on the mechanism
   and text.

   Thanks to Mark Williams, Erik Nordmark, Mikael Abrahamsson, David
   Harrington, Pekka Savola, Xing Li, Lixia Zhang, Bingyang Liu, Duanqi
   Zhou, Robert Raszuk, Greg Daley, John Kaippallimalil, and Tao Lin for
   their valuable contributions.

Authors' Addresses

   Jun Bi
   Network Research Center, Tsinghua University
   Beijing  100084
   China

   EMail: junbi@tsinghua.edu.cn


   Jianping Wu
   Dept. of Computer Science, Tsinghua University
   Beijing  100084
   China

   EMail: jianping@cernet.edu.cn


   Guang Yao
   Network Research Center, Tsinghua University
   Beijing  100084
   China

   EMail: yaoguang@cernet.edu.cn


   Fred Baker
   Cisco Systems
   Santa Barbara, CA  93117
   United States

   EMail: fred@cisco.com






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