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SAVI                                                               J. Bi
Internet-Draft                                                     J. Wu
Intended status: Standards Track                                  G. Yao
Expires: September 8, 2014                                Tsinghua Univ.
                                                                F. Baker
                                                                   Cisco
                                                           March 7, 2014


                         SAVI Solution for DHCP
                        draft-ietf-savi-dhcp-19

Abstract

   This document specifies the procedure for creating a binding between
   a DHCPv4/DHCPv6 assigned IP address and a binding anchor on a SAVI
   (Source Address Validation Improvements) device.  The bindings set up
   by this procedure can be used to filter out packets with forged
   source IP address in DHCP scenario.  This mechanism is proposed as a
   complement to ingress filtering to provide finer-grained source IP
   address validation.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 8, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   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
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   This document may contain material from IETF Documents or IETF
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Deployment Scenario and Configuration  . . . . . . . . . . . .  8
     4.1.  Elements and Scenario  . . . . . . . . . . . . . . . . . .  9
     4.2.  Attribute  . . . . . . . . . . . . . . . . . . . . . . . . 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  . . . . . . . . . . . . . . . 12
       4.2.5.  Validating Attribute . . . . . . . . . . . . . . . . . 12
       4.2.6.  Table of Mutual Exclusions . . . . . . . . . . . . . . 12
     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 DHCP Server/Relay  . . . . . . . . 15
   5.  Binding State Table (BST)  . . . . . . . . . . . . . . . . . . 16
   6.  DHCP Snooping Process  . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.2.  Binding States Description . . . . . . . . . . . . . . . . 17
     6.3.  Events . . . . . . . . . . . . . . . . . . . . . . . . . . 18
       6.3.1.  Timer Expiration Event . . . . . . . . . . . . . . . . 18
       6.3.2.  Control Message Arriving Events  . . . . . . . . . . . 18
     6.4.  The State Machine of DHCP Snooping Process . . . . . . . . 19
       6.4.1.  From NO_BIND to Other States . . . . . . . . . . . . . 19
         6.4.1.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 19
         6.4.1.2.  Following Actions  . . . . . . . . . . . . . . . . 19
       6.4.2.  From INIT_BIND to Other States . . . . . . . . . . . . 20
         6.4.2.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 21
         6.4.2.2.  Following Actions  . . . . . . . . . . . . . . . . 21
       6.4.3.  From BOUND to Other States . . . . . . . . . . . . . . 23
         6.4.3.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 23
         6.4.3.2.  Following Actions  . . . . . . . . . . . . . . . . 23
     6.5.  Table of State Machine . . . . . . . . . . . . . . . . . . 24
   7.  Data Snooping Process  . . . . . . . . . . . . . . . . . . . . 25
     7.1.  Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 25
     7.2.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.3.  Additional Binding States Description  . . . . . . . . . . 27
     7.4.  Events . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     7.5.  State Machine of Binding Recovery Process  . . . . . . . . 28
       7.5.1.  From NO_BIND to Other States . . . . . . . . . . . . . 28
         7.5.1.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 28
         7.5.1.2.  Following Actions  . . . . . . . . . . . . . . . . 28
       7.5.2.  From DETECTION to Other States . . . . . . . . . . . . 29
         7.5.2.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 29
         7.5.2.2.  Following Actions  . . . . . . . . . . . . . . . . 29



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       7.5.3.  From RECOVERY to Other States  . . . . . . . . . . . . 30
         7.5.3.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 30
         7.5.3.2.  Following Actions  . . . . . . . . . . . . . . . . 30
       7.5.4.  After BOUND  . . . . . . . . . . . . . . . . . . . . . 31
         7.5.4.1.  Trigger Event  . . . . . . . . . . . . . . . . . . 31
         7.5.4.2.  Following Action . . . . . . . . . . . . . . . . . 31
     7.6.  Table of State Machine . . . . . . . . . . . . . . . . . . 31
   8.  Filtering Specification  . . . . . . . . . . . . . . . . . . . 32
     8.1.  Data Packet Filtering  . . . . . . . . . . . . . . . . . . 32
     8.2.  Control Packet Filtering . . . . . . . . . . . . . . . . . 33
   9.  State Restoration  . . . . . . . . . . . . . . . . . . . . . . 33
     9.1.  Attribute Configuration Restoration  . . . . . . . . . . . 34
     9.2.  Binding State Restoration  . . . . . . . . . . . . . . . . 34
   10. Constants  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   11. MLD Consideration  . . . . . . . . . . . . . . . . . . . . . . 35
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
     12.1. Security Problems about the Data Snooping Process  . . . . 35
     12.2. Issues about Leaving Clients . . . . . . . . . . . . . . . 35
     12.3. Duplicate Bindings of the Same Address . . . . . . . . . . 36
     12.4. Compatibility with DNA (Detecting Network Attachment)  . . 36
     12.5. Authentication in DHCPv6 Leasequery  . . . . . . . . . . . 37
     12.6. Binding Number Limitation  . . . . . . . . . . . . . . . . 38
     12.7. Residual Threats . . . . . . . . . . . . . . . . . . . . . 38
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   14. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 39
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     15.1. Informative References . . . . . . . . . . . . . . . . . . 39
     15.2. Normative References . . . . . . . . . . . . . . . . . . . 39
   Appendix A.  change log  . . . . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43





















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1.  Introduction

   This document describes a fine-grained source IP address validation
   mechanism.  This mechanism creates bindings between addresses
   assigned to network attachment points by DHCP and suitable binding
   anchors (refer to Section 3) of the attachments.  Then the bindings
   are used to identify and filter out packets originated from these
   attachments with forged source IP addresses.  In this way, this
   mechanism can prevent hosts from spoofing IP addresses assigned to
   the other attachment points.  Compared with [BCP38], which provides
   prefix granularity source IP address validity, this mechanism can
   benefit the network with finer-grained validity and traceability of
   source IP addresses.

   This mechanism primarily performs DHCP snooping to set up bindings
   between IP addresses assigned by DHCP and corresponding binding
   anchors.  This binding process is inspired by the work of [BA2007].
   Different from [BA2007], which designs specifications about DHCPv4,
   this mechanism covers the DHCPv6 snooping process, the data snooping
   process (refer to Section 7), as well as a number of other technical
   details.  Specially, the data snooping process is a data-triggered
   binding setup procedure designed to avoid permanent block of valid
   address in 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, because it has nothing to do with IP address allocation.  A
   client doing stateless DHCP acquires its IP address(es) using some
   other mechanism.  It is through that mechanism that SAVI must be
   accomplished.  Besides, this mechanism is primarily designed for pure
   DHCP scenarios in which only addresses assigned through DHCP are
   allowed.  However, it does not block any link-local address.  It is
   because link-local addresses are used by DHCPv6 clients before the
   clients are assigned a DHCPv6 address.  Considering that link-local
   addresses are generally self-generated, and the spoofing of link
   local address may disturb this mechanism, it is RECOMMENDED to enable
   a SAVI solution for link-local addresses, e.g., the SAVI-FCFS
   [savi-fcfs].


2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].





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3.  Terminology

   Binding anchor: A "binding anchor" is defined to be a link layer
   property of network attachment in [savi-framework].  A list of proper
   binding anchors can be found in Section 3.2 of [savi-framework].

   Attribute: A configurable property of each network attachment which
   indicates the actions to be performed on packets received from the
   network attachment.

   DHCP address: An IP address assigned to an interface via DHCP.

   SAVI-DHCP: The name of this SAVI function for DHCP address.

   SAVI device: A network device on which this SAVI function is enabled.

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

   DHCP Client-Server message: A message that is sent from a DHCP client
   to a DHCP server or DHCP servers.  Such a message is of 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]

   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 the Table 4 of [RFC2131]

   o   DHCPv4 Decline: DHCPDECLINE [RFC2131]

   o   DHCPv4 Release: DHCPRELEASE [RFC2131]

   o   DHCPv4 Inform: DHCPINFORM [RFC2131]

   o   DHCPv6 Request: REQUEST [RFC3315]




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   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]

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

   o   DHCPv4 ACK: DHCPACK [RFC2131]

   o   DHCPv4 NAK: DHCPNAK [RFC2131]

   o   DHCPv4 Offer: DHCPOFFER [RFC2131]

   o   DHCPv6 Reply: REPLY [RFC3315]

   o   DHCPv6 Advertise: ADVERTISE [RFC3315]

   o   DHCPv6 Reconfigure: RECONFIGURE [RFC3315]

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

   Binding entry: An 'permit' rule that defines a valid association
   between an IP address and a binding anchor.

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

   Binding entry limit: The maximum number of binding entries that may
   be associated with any one 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 should be an access device
   which is directly attached by hosts.  In such case, the hosts are
   direct attachments of the SAVI device.




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   Indirect attachment: A SAVI device can be an aggregration device
   which is connected with a number of access devices, which are
   attached by hosts.  In such case, the hosts are indirect attachments
   of the SAVI device.  Sometimes, it is expressed as "the hosts are
   indirectly attached to the SAVI device".

   Upstream link: Upstream links are links connected to non-SAVI devices
   from which the valid source address space of traffic contains the
   prefixes of other networks.  SAVI-DHCP will not set up bindings for
   addresses appearing on upstream links and will not check data traffic
   from upstream links.  The traffic from upstream links should be
   checked by a prefix granularity source address validation mechanism
   to avoid spoofing of local addresses from other networks.

   Upstream device: An upstream device is a non-SAVI device associated
   with an upstream link.  For example, the gateway router of the
   network.

   Downstream link: Downstream links are links connected to non-SAVI
   devices from which the valid source address space of traffic only
   contains the prefix(es) of the local network.  SAVI-DHCP may check
   traffic from downstream links.

   Downstream device: A downstream device is a non-SAVI device
   associated with an downstream link.  For example, an access switch in
   the network.

   (Note: To distinguish upstream/downstream links is essential for
   SAVI-DHCP.  Networks are not isolated and traffic from other networks
   may get into the network with SAVI-DHCP deployed.  It is unreasonable
   for SAVI-DHCP to set up bindings for addresses assigned in other
   networks.)

   CUT VERTEX: A cut vertex is 'any vertex whose removal increases the
   number of connected components'.  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]


4.  Deployment Scenario and Configuration







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4.1.  Elements and Scenario

   A list of essential elements in a SAVI-DHCP deployment scenario is
   given as follows:

   (1)  DHCP server

   (2)  DHCP client

   (3)  SAVI device

   And there may be following optional elements in a SAVI-DHCP
   deployment scenario:

   (1)  DHCP relay

   (2)  Non-SAVI device

   Figure 1 shows a deployment scenario that contains these elements.
   Note that a physical device can be multiple elements, e.g, a switch
   can be both a SAVI device and a DHCP relay.  In such cases, the links
   are logic links rather than physical links.



                  +--------+     +------------+
                  |DHCP    |-----|  Non-SAVI  |
                  |Server A|     |  Device 1  |
                  +--------+     +-----|------+
                 ......................|............................
                 .                     |  upstream link            .
                 . Protection      +---|------+                    .
                 . Perimeter       |  SAVI    |                    .
                 .                 |  Device C|                    .
                 .                 +---|------+                    .
                 .                     |                           .
                 . +----------+    +---|------+       +----------+ .
    downstream   . |  SAVI    |    |  Non SAVI|       |  SAVI    | .
    link    +----.-|  Device A|----|  Device 3|-------|  Device B| .
            |    . +----|--|--+    +----------+       +-|---|----+ .
            |    .      |  +----------+   ............  |   |      .
            |    '..............      |   .          .  |   |      .
            |           |      .      |   .    +--------+   |      .
       +----|-----+  +--|---+  . +----|-+ . +--|---+ .  +---|----+ .
       | Non-SAVI |  |Client|  . |DHCP  | . |Client| .  |DHCP    | .
       | Device 2 |  |A     |  . |Relay | . |B     | .  |Server B| .
       +----------+  +------+  . +------+ . +------+ .  +--------+ .
                               ............          ...............



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                       Figure 1: SAVI-DHCP Scenario

4.2.  Attribute

   As illustrated in Figure 1, an attachment to a SAVI device can be
   from either a DHCP client, or a DHCP relay/server, or a SAVI device,
   or a non-SAVI device.  Different actions are performed on traffic
   originated from different elements.  To distinguish different types
   of attachments, an attachment property named 'attribute' is
   configured on SAVI devices.  This section specifies the attributes
   used by SAVI-DHCP.

   Before configuration, an attachment is with no attribute.  An
   attachment MAY be configured to have one or more compatible
   attributes(refer to Section 4.2.6).  The attributes of each
   attachment MUST be configured before this SAVI-DHCP function is
   enabled on the attachment.  The procedure performed by SAVI devices
   on traffic from each attachment is determined by the attribute(s) set
   on the attachment.

   Particularly, if an attachment has no attribute, no actions will be
   performed by SAVI-DHCP on traffic from such attachments.  This
   prevents SAVI-DHCP from causing a break in the network when it is
   turned on without any binding anchors configured.  However, if a
   binding anchor has no attributes, this means that the SAVI-DHCP-Trust
   attribute is not present.  Because of this, DHCP server messages from
   that binding anchor will be discarded.  This prevents a host from
   connecting to an unconfigured binding anchor and acting as a DHCP
   server.  It is SUGGESTED to configure SAVI-DHCP-Trust on necessary
   binding anchors before turning on the SAVI-DHCP function.

   However, binding anchors associated with upstream links MAY have no
   attribute after configuration.  For example, in Figure 1, the
   attachment from the Non-SAVI Device 1 to the SAVI Device B should be
   configured with no attribute.  It means 1) SAVI devices will neither
   set up bindings for upstream hosts nor check traffic from upstream
   hosts; 2) SAVI devices will not snoop DHCP messages from upstream
   devices unless the DHCP-Trust attribute (refer to Section 4.2.2) is
   set on the corresponding attachment.  The reason that DHCP messages
   from upstream devices are not trusted is discussed in Section 4.3.3.

4.2.1.  Trust Attribute

   The "Trust Attribute" indicates the packets from the corresponding
   attachment are completely trustable.

   SAVI devices will not set up bindings for attachments with Trust
   attribute; DHCP messages and data packets from such attachments with



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   this attribute will not be checked.  If the DHCP Server-Client
   messages from attachments with this attribute can trigger the state
   transitions specified in Section 6 and Section 7, these messages will
   be handled by the corresponding processes in Section 6 and Section 7.

   This attribute is generally configured on the attachments from other
   SAVI devices.  For example, in Figure 1, the attachment from the SAVI
   Device A to the SAVI Device B and the attachment from the SAVI Device
   B to the SAVI Device A should be configured with this attribute.
   Besides, it can be configured on attachments from Non-SAVI devices
   only if the Non-SAVI devices will not introduce unchecked traffic
   from DHCP clients.  For example, the attachments from Non-SAVI device
   3 to SAVI device A, SAVI device B and SAVI device C can be configured
   with this attribute, only if Non-SAVI device 3 does not have
   attachment from DHCP clients.

4.2.2.  DHCP-Trust Attribute

   The "DHCP-Trust Attribute" indicates the DHCP Server-Client messages
   from the corresponding attachment is trustable.

   SAVI devices will forward DHCP Server-Client messages coming from the
   attachments with this attribute.  If the DHCP Server-Client messages
   can trigger the state transitions, they will be handled by the
   binding setup processes specified in Section 6 and Section 7.

   This attribute is generally used on the direct attachments from the
   trusted DHCP servers/relays.  In Figure 1, the attachment from the
   DHCP Relay to the SAVI Device B, and the attachment from the DHCP
   Server B to the SAVI Device B should be configured with this
   attribute.  It is NOT RECOMMENDED to configure this attribute on the
   indirect attachments from the non-neighboring DHCP servers/relays
   unless the attachments do not introduce bogus DHCP Server-Client
   messages.  For example, in Figure 1, the attachment from the Non-SAVI
   Device 1 to the SAVI Device C should not be configured with this
   attribute.  This issue is discussed in Section 4.3.3.

4.2.3.  DHCP-Snooping Attribute

   The "DHCP-Snooping Attribute" indicates bindings will be set up based
   on DHCP snooping.

   DHCP Client-Server messages from attachments with this attribute will
   trigger the setup of bindings.  SAVI devices will set up bindings on
   attachments with this attribute based on the DHCP snooping procedure
   described in Section 6.

   DHCP-Snooping attribute is configured on the attachments from DHCP



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   clients.  This attribute can be also used on the attachments from
   downstream Non-SAVI devices which are attached by DHCP clients.  In
   Figure 1, the attachment from the Client A to the SAVI Device A, the
   attachment from the Client B to the SAVI Device B, and the attachment
   from the Non-SAVI Device 2 to the SAVI Device A can be configured
   with this attribute.

4.2.4.  Data-Snooping Attribute

   The "Data-Snooping Attribute" indicates data packets from the
   corresponding attachment may trigger binding setup procedure.

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

   If DHCP-Snooping attribute is configured on an attachment, the
   bindings on this attachment are set up based on DHCP message
   snooping.  However, in some scenarios, a DHCP address may be used by
   a DHCP client without DHCP address assignment procedure performed on
   its current attachment.  For such attachments, 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.

4.2.5.  Validating Attribute

   The "Validating Attribute" indicates packets from the corresponding
   attachment will be checked based on binding entries on the
   attachment.

   Packets coming from attachments with this attribute will be checked
   based on binding entries on the attachment as specified in Section 8.

   Validating attribute is configured on the attachments from which the
   data packets should be checked.  For example, the DHCP clients.

4.2.6.  Table of Mutual Exclusions

   Different types of attributes may indicate mutually exclusive actions
   on packet.  Mutually exclusive attributes MUST NOT be set 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 on an attachment with Trust
   attribute.





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   +----------+----------+----------+----------+----------+----------+
   |          |          |          | 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 can form a perimeter separating untrusted and trusted
   areas, similarly to SAVI-FCFS (refer to Section 2.5 of [savi-fcfs]).
   Each SAVI device need only establish bindings for a client if it is
   connected to that client by a link that crosses the perimeter that
   encloses the SAVI device.

   The perimeter is primarily designed for scalability.  This has two
   implications.  First, SAVI devices only need to establish bindings
   for directly attached clients, or clients indirectly attached through
   non-SAVI device, rather than all the clients in the network.  Second,
   each SAVI device only need to check traffic from clients attached to
   it, without checking all the traffic passing by.

   Consider the example in Figure 1.  The protection perimeter is formed
   by SAVI Device A, B and C. In this case, SAVI device B doesn't create
   a binding for client A. SAVI device A doesn't create a binding for
   client B. But the SAVI device B is still protected from spoofing from
   client A and the SAVI device A is still protected from spoofing from
   client B.



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   There is three main differences between the SAVI-DHCP protection
   perimeter and SAVI-FCFS protection perimeter:

   (1)  SAVI-DHCP follows the state announced in DHCP messages, so there
        is no need to distribute state using Neighbor Solicitation/
        Neighbor Advertisement messages.

   (2)  The perimeter in SAVI-DHCP is not only a perimeter for data
        packets, but also a perimeter for DHCP messages.  The placement
        of DHCP Relay/Server, which is not involved in SAVI-FCFS , is
        related with the construction of the perimeter.  The requirement
        on the placement and configuration of DHCP Relay/Server are
        discussed in Section 4.3.3.

   (3)  The complexity caused by partial deployment, redundancy paths,
        dynamic Layer-2 routing is considered in SAVI-DHCP.  Thus,
        downstream/upstream links MUST be distinguished when configuring
        the perimeter to avoid estabilshing binding for addresses of
        other networks.  In SAVI-FCFS, it is underlyingly required all
        the links crossed the perimeter must be downstream links.
        However, it may not by the cases in real deployments.

4.3.2.  SAVI-DHCP Perimeter Configuration Guideline

   Through configuring attribute of each attachment properly, a
   perimeter separating untrusted area and trusted area can be formed:

   (1)  Configure Validating attribute on the direct attachments of all
        the DHCP clients.  Configure DHCP-Snooping attribute on these
        attachments.

   (2)  Configure Validating attribute on the indirect attachments of
        all the DHCP clients.  Configure DHCP-Snooping attribute on
        these attachments.

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

   (4)  If a Non-SAVI device, or a number of connected Non-SAVI devices,
        have only attachments from SAVI devices or upstream devices, set
        their attachments to SAVI devices with Trust attribute.

   (5)  Configure DHCP-Trust attribute on the direct attachments of
        trusted DHCP relays/servers.

   In this way, attachments with Validating attribute (and generally
   together with attachments of upstream devices) can form a perimeter
   separating DHCP clients and trusted devices.  Data packet check is



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   only performed on the perimeter.  The perimeter is also a perimeter
   for DHCP messages.  DHCP-Trust attribute is only configured on the
   inside links of the perimeter.  Only DHCP server-client messages
   originated in the perimeter is trusted.

4.3.3.  On the Placement of DHCP Server/Relay

   Based on the configuration guideline, it can be found that the SAVI
   devices only trust DHCP Server-Client messages originated inside the
   perimeter.  It means there MUST be at least one trusted DHCP relay/
   server in the perimeter.  DHCP server-client messages will be
   filtered on the perimeter (Note: server-relay messages will not be
   filtered).  In this way, DHCP server-client messages from bogus DHCP
   servers are filtered on the perimeter, and then the SAVI devices can
   be protected from fabricated DHCP messages.

   Such a requirement is due to the limitation of this binding based
   mechanism.  This document makes no assumption that the DHCP server-
   client messages arriving the perimeter from the outside can be
   trusted.  The binding anchor of a trusted remote DHCP server can be
   shared by a bogus DHCP server.  Thus, the SAVI device cannot
   distinguish bogus and valid DHCP messages only based on the
   associated binding anchor of DHCP messages in such case.

   Note that even if a DHCP server is valid, it may be not contained in
   the perimeter based on the guideline.  For example, in Figure 1, DHCP
   server A is valid, but it is attached to a Non-SAVI device.  The Non-
   SAVI device may be attached by attackers which generate fabricated
   DHCP messages.  This binding based mechanism may not have the ability
   to distinguish whether a message received from the attachment of the
   Non-SAVI device 1 is from DHCP server A or the attackers.  If the
   DHCP server A is contained in the perimeter, the Non-SAVI device 1
   will also be contained in the perimter.  However, the Non-SAVI device
   1 can introduce fabricated DHCP messages into the perimeter.  Thus,
   the DHCP server A cannot be contained in the perimeter.

   In this case, the SAVI devices can set up bindings for addresses
   assigned by DHCP server A through snooping the messages relayed by
   trusted relay in the network.  For example, the DHCP relay may relay
   messages between DHCP server A and the clients in the network, and
   the SAVI devices can snoop messages from the DHCP relay which is
   inside the perimeter.  The authentication mechanism (i.e., IPSec, as
   specified in section 21.1 of [RFC3315]) enforced between the DHCP
   relay and the DHCP server outside the perimeter can compensate this
   binding based mechanism.  It is SUGGESTED to configure IPSec between
   the DHCP relay and the DHCP server in such case.  If source address
   validation is enforced in the whole network, which makes the source
   IP address trustable, the DHCP relay and the DHCP server can simply



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   authenticate the messages from each other based on the source IP
   address without the requriement to deploy IPSec.


5.  Binding State Table (BST)

   Binding State Table is used to contain the bindings between the IP
   addresses assigned to the attachments and the corresponding binding
   anchors of the attachments.  Each entry of the table, i.e., binding
   entry, has 5 fields:

   o   Binding Anchor(Anchor): the binding anchor, i.e., a link-layer
       property of the attachment.

   o   IP Address(Address): the IP address assigned to the attachment by
       DHCP.

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

   o   Lifetime: the remaining seconds of the binding.  The Lifetime
       field counts down automatically.

   o   TID: the Transaction ID (TID) (refer to [RFC2131] [RFC3315]) of
       the corresponding DHCP transaction.  TID field is used to
       associate DHCP Server-Client messages with corresponding binding
       entries.

   IA does not present in the BST.  On the one hand, IA is not found to
   be necessary because the lease of each address in one IA is assigned
   respectively.  On the other hand, when the binding is set up based on
   data-snooping, IA cannot be recovered from the leasequery protocol.
   Besides, there is no IA for DHCPv4.

   An instance of this table is shown in Figure 3.



   +---------+----------+----------+-----------+-------+
   | Anchor  | Address  | State    | Lifetime  |TID    |
   +---------+----------+----------+-----------+-------+
   | A       | IP_1     | BOUND    |  65535    |TID_1  |
   +---------+----------+----------+-----------+-------+
   | A       | IP_2     | BOUND    |  10000    |TID_2  |
   +---------+----------+----------+-----------+-------+
   | B       | IP_3     |INIT_BIND |      1    |TID_3  |
   +---------+----------+----------+-----------+-------+




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                         Figure 3: Instance of BST


6.  DHCP Snooping Process

   This section specifies the process of setting up bindings based on
   DHCP snooping, named DHCP Snooping Process.  This process is
   illustrated making use of a state machine.

6.1.  Rationale

   The rationale of the DHCP Snooping Process is that if a DHCP client
   is legitimate to use a DHCP address, the DHCP address assignment
   procedure which assigns the IP address to the client must have been
   performed on the attachment of the client.  This basis stands 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
   disjoin the DHCP client and the remaining network containing the DHCP
   server(s)/relay(s).  For most of the layer-2 networks whose
   topologies are simple, it is possible to deploy this SAVI function at
   proper devices to meet this requirement.

   However, a deployment of this SAVI function may not meet the
   requirement.  For example, there are multiple paths from a DHCP
   client to the DHCP server and the SAVI device is only on one of them.
   Then the SAVI device may not be able to snoop the DHCP procedure.
   Host movement may also make this requirement can not be met.  For
   exmaple, when a DHCP client moves from one attachment to another
   attachment in the same network, it may not reinitialize its interface
   or send a Confirm message because of imcomplete 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

   Following binding states present in this process and the
   corresponding state machine:

   NO_BIND: The state before a binding has been set up.

   INIT_BIND: A potential binding has been set up.

   BOUND: The binding has been set up.





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6.3.  Events

   This section describes events in this process and the corresponding
   state machine.

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.

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

   EVE_DHCP_OPTION_RC: A DHCPv6 Solicitation message with 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_DCHP_LEASEQUERY: A successful DHCPv6 LEASEQUERY_REPLY (refer to
   section 4.3.3 of [RFC5007]) is received.

   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-Client messages and
       LEASEQUERY_REPLY should be from attachments with DHCP-Trust
       attribute; the DHCP Client-Server messages should be from
       attachments with DHCP-Snooping attribute.

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



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       which is nearest to the destination client.

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

   o   TID check: the DHCP Server-Client/Client-Server messages which
       may cause modification on existing binding entries must have
       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.

   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 12.6).

   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 transit and 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 the transits of each state and the
   corresponding actions.

6.4.1.  From NO_BIND to Other States

6.4.1.1.  Trigger Event

   EVE_DHCP_REQUEST, EVE_DHCP_OPTION_RC, EVE_DHCP_CONFIRM,
   EVE_DHCP_REBOOT.

6.4.1.2.  Following Actions

   If the triggering event is EVE_DHCP_REQUEST/EVE_DHCP_OPTION_RC/
   EVE_DHCP_REBOOT:

   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



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   Request or 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 4.



   +---------+-------+---------+-----------------------+-------+
   | Anchor  |Address| State   | Lifetime              |TID    |
   +---------+-------+---------+-----------------------+-------+
   | A       |       |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID   |
   +---------+-------+---------+-----------------------+-------+



       Figure 4: Binding entry in BST on Request/Rapid Commit/Reboot
                         triggered initialization

   If the triggering event is EVE_DHCP_CONFIRM:

   The SAVI device MUST forward the message.

   The SAVI device will generate corresponding entries in the BST for
   all the addresses in each the 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 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 5.



   +---------+--------+---------+-----------------------+-------+
   | Anchor  | Address| State   | Lifetime              |TID    |
   +---------+--------+---------+-----------------------+-------+
   | A       | Addr1  |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID   |
   +---------+--------+---------+-----------------------+-------+
   | A       | Addr2  |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID   |
   +---------+--------+---------+-----------------------+-------+


    Figure 5: Binding entry in BST on Confirm triggered initialization

6.4.2.  From INIT_BIND to Other States







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6.4.2.1.  Trigger Event

   EVE_DHCP_REPLY, EVE_ENTRY_EXPIRE.

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

6.4.2.2.  Following Actions

   If the trigger event is EVE_DHCP_REPLY:

   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 of 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 ACK message
   and MAX_DHCP_RESPONSE_TIME.  The State field is changed to BOUND.

   If the message is DHCPv6 Reply, there are following cases:

   1.  If the status code is not "Success", no modification on
   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 due to 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.

   2.1 If there are no IA options in the Reply message, the DHCP Reply
   message is in response to a Confirm message.  The state of the
   binding entries with matched TID is changed to BOUND.  Because
   [RFC3315] does not require lease time of addresses to be contained in
   the Reply message, the SAVI device MUST send a LEASEQUERY [RFC5007]
   message querying by IP address to All_DHCP_Servers multicast 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 MAX_LEASEQUERY_DELAY.  An example of the entries is
   illustrated in Figure 6.  The related security problem about DHCPv6
   LEASEQUERY is discussed in Section 12.5.



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   2.2 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 matched TID is set to the
   address.  The Lifetime field is set to the sum of the lease time in
   Reply message and MAX_DHCP_RESPONSE_TIME.  The State field is changed
   to BOUND.  If there are 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 7.  SAVI devices do not specially process IA options with
   NoAddrsAvail status, because there should be no address contained in
   such IA options.

   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    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr1    | BOUND | MAX_LEASEQUERY_DELAY   |TID    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr2    | BOUND | MAX_LEASEQUERY_DELAY   |TID    |
   +---------+----------+-------+------------------------+-------+




      Figure 6: From INIT_BIND to BOUND on DHCP Reply in response to
                                  Confirm



   +---------+----------+-------+------------------------+-------+
   | Anchor  | Address  | State | Lifetime               |TID    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr1    | BOUND |Lease time+             |TID    |
   |         |          |       |MAX_DHCP_RESPONSE_TIME  |       |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr2    | BOUND |Lease time+             |TID    |
   |         |          |       |MAX_DHCP_RESPONSE_TIME  |       |
   +---------+----------+-------+------------------------+-------+



      Figure 7: From INIT_BIND to BOUND on DHCP Reply in response to



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                                  Request

   If the trigger event is EVE_ENTRY_EXPIRE:

   The entry MUST be deleted from BST.

6.4.3.  From BOUND to Other States

6.4.3.1.  Trigger Event

   EVE_ENTRY_EXPIRE, EVE_DHCP_RELEASE, EVE_DHCP_DECLINE, EVE_DHCP_REPLY,
   EVE_DCHP_LEASEQUERY.

6.4.3.2.  Following Actions

   If the trigger event is EVE_ENTRY_EXPIRE:

   Remove the corresponding entry in BST.

   If the trigger event is EVE_DHCP_RELEASE/EVE_DHCP_DECLINE:

   The message MUST be forwarded.

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

   If the trigger event is EVE_DHCP_REPLY:

   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 just simply update the
   binding entry with matched TID, with the Lifetime field set to be the
   sum of the new lease time and MAX_DHCP_RESPONSE_TIME.

   If the message is DHCPv6 Reply, the SAVI device checks each IA
   Address option in each IA option.  If the valid lifetime of an IA
   address option is 0, the binding entry with matched TID and address
   is removed.  Or else, set the Lifetime field of the binding entry
   with matched TID and address to be the sum of the new valid lifetime
   and MAX_DHCP_RESPONSE_TIME.

   The SAVI device does not specially process IA options in Reply
   message with status NoBinding, because no address is contained in



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   such IA options and no actions will be performed.

   If the trigger event is EVE_DCHP_LEASEQUERY:

   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 IAADDR 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.

6.5.  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 lease query if no lease)
   INIT_BIND   Timeout          Remove entry                    NO_BIND
   BOUND       RLS/DCL          Remove entry                    NO_BIND
   BOUND       Timeout          Remove entry                    NO_BIND
   BOUND       RPL              Set new lifetime                  BOUND
   BOUND       LQR              Record lease time                 BOUND



                        Figure 8: Table of Transit

   RQ: EVE_DHCP_REQUEST

   CF: EVE_DHCP_CONFIRM

   RC: EVE_DHCP_OPTION_RC

   RE: EVE_DHCP_REBOOT

   RPL: EVE_DHCP_REPLY

   DCL: EVE_DHCP_DECLINE

   RLS: EVE_DHCP_RELEASE



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   LQR: EVE_DCHP_LEASEQUERY

   Timeout: EVE_ENTRY_EXPIRE





                             +-------------+
                             |             |
                   /---------|   NO_BIND   |<----------\
                   |  ------>|             |           |
                   |  |      +-------------+           |EVE_DHCP_RELEASE
 EVE_DHCP_REQUEST  |  |                                |EVE_DHCP_DECLINE
 EVE_DHCP_CONFIRM  |  |TIMEOUT                         |TIMEOUT
 EVE_DHCP_OPTION_RC|  |                                |
 EVE_DHCP_REBOOT   |  |                                |
                   |  |                                |
                   |  |                                |
                   v  |                                |
            +-------------+                        +------------+
            |             |      EVE_DHCP_REPLY    |            |
            |  INIT_BIND  ------------------------>|    BOUND   |<-\
            |             |                        |            |  |
            +-------------+                        +------------+  |
                                                          |        |
                                                          \--------/
                                              EVE_DHCP_REPLY
                                              EVE_DCHP_LEASEQUERY



                       Figure 9: 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 is legitimate to use a DHCP address, the
   corresponding DHCP address assignment procedure must have been
   finished on the attachment of the DHCP client.  This basis stands
   when the SAVI device is always on the path(s) from the DHCP client to
   the DHCP server(s)/relay(s).  However, there are two exceptions:






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   o   Multiple paths: there are more than one feasible layer-2 paths
       from the client to the DHCP server/relay, and the SAVI device is
       not on all of them.  The client may get the address through one
       of the paths not passing by the SAVI device, but packets from the
       client can travel through the other paths passing by the SAVI
       device.  Because the SAVI device does not snoop the DHCP
       assignment procedure, the DHCP snooping procedure will not set up
       the corresponding binding.

   o   Dynamic path: there is only one feasible layer-2 path from the
       client to the DHCP server/relay, but the path is dynamic due to
       topology change or layer-2 path change.  This situation also
       covers the local-link movement of clients without address
       confirm/re-configuration process.  In such cases, the DHCP
       snooping process will not set up the corresponding binding.

   Data Snooping Process is designed to avoid permanently blocking
   legitimate traffic in case of these two exceptions.  This process is
   performed on attachments with Data-Snooping attribute.  Data packets
   without matched binding entry may trigger this process to set up
   bindings.

   Snooping data traffic will introduce considerable burden on the
   processor and ASIC-to-Processor bandwidth of SAVI devices.
   Considering the overhead of this process, the implementation of this
   process is a conditional SHOULD.  This function SHOULD be implemented
   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 supposed to set up a binding whenever a data
   packet without matched binding entry is received.  Instead, unmatched
   data packets trigger this process with a probability and generally a
   number of unmatched packets will be discarded before the binding is
   set up.

7.2.  Rationale

   This process makes use of duplication detection 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 security issues about this process is discussed is Section 12.1.




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7.3.  Additional Binding States Description

   In addition to Section 6.2, new states used in this process are
   listed here:

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

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

7.4.  Events

   Additional events in this process are described here.  Also, if an
   event will trigger to set up a new binding entry, the binding entry
   limit on the binding anchor MUST NOT have not been reached.

   EVE_DATA_UNMATCH: A data packet without matched binding is received.

   EVE_DATA_CONFLICT: ARP Response/Neighbor Advertisement(NA) message
   against an address in DETECTION state is received.

   EVE_DATA_LEASEQUERY:

      IPv4: A DHCPLEASEACTIVE message with IP Address Lease Time option
      is received.

      IPv6: A successful LEASEQUERY-REPLY is 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
       Data-Snooping attribute; the DHCPLEASEACTIVE/LEASEQUERY_REPLY
       messages should be from attachments with DHCP-Snooping attribute.

   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 12.6).

   o   Address check: the address of the DHCP/ARP/NA messages should
       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.





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

7.5.  State Machine of Binding Recovery Process

   Through using additional states, the state machine of this process
   doesn't conflict the regular process described in Section 6.  Thus,
   it can be implemented separately without changing the state machine
   in Section 6.

7.5.1.  From NO_BIND to Other States

7.5.1.1.  Trigger Event

   EVE_DATA_UNMATCH.

7.5.1.2.  Following Actions

   Determine whether to process this event with a probability.  The
   probability can be configured or calculated based on the state of the
   SAVI device.  This probability should be low enough to mitigate the
   damage from DoS attack 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 be source address of the packet.  Set the State field to
   DETECTION.  Set the Lifetime of the created entry to 2*DAD_TIMEOUT.

   Check if the address has a local conflict (it violates an address
   being used by another node):

   (1)  IPv4 address: send an Address Resolution Protocol (ARP) Request
        [RFC826]or a ARP probe [RFC5227] on the address; if there is no
        response message after DAD_TIMEOUT, send another ARP Request or
        ARP probe;

   (2)  IPv6 address: perform Duplicate Address Detection (DAD)
        [RFC4862] on the address; if there is no response message after
        DAD_TIMEOUT, perform another DAD procedure.

   Because the delivery of detection message is unreliable, the
   detection message is of a certain possibility of not reaching the
   targeting node.  If the targeting node doesn't get the detection
   message, the address may be bound with a wrong binding anchor in the
   further stages.  This fault may introduce attack against this
   mechanism.  Thus, the detection is performed again if there is no
   response after the first detection.




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   The messages MUST NOT be sent to the attachment from which the
   triggering packet is received.

   The packet which triggers this event SHOULD be discarded.

   An example of the entry is illustrated in Figure 10.



   +---------+-------+---------+-----------------------+-------+
   | Anchor  |Address| State   | Lifetime              |TID    |
   +---------+-------+---------+-----------------------+-------+
   | A       | Addr1 |DETECTION|2*DAD_TIMEOUT          |       |
   +---------+-------+---------+-----------------------+-------+



     Figure 10: Binding entry in BST on data triggered initialization

7.5.2.  From DETECTION to Other States

7.5.2.1.  Trigger Event

   EVE_ENTRY_EXPIRE, EVE_DATA_CONFLICT.

7.5.2.2.  Following Actions

   If the trigger event is EVE_ENTRY_EXPIRE:

   (1)  IPv4 address: Send a DHCPLEASEQUERY [RFC4388] message querying
        by IP address to all DHCPv4 servers with IP Address Lease Time
        option (option 51).  The server addresses can be found through
        DHCPv4 Discovery or from configuration.  Change the state of the
        corresponding entry to RECOVERY.  Change the lifetime of the
        entry to be MAX_LEASEQUERY_DELAY.

   (2)  IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP
        address to All_DHCP_Relay_Agents_and_Servers multicast address
        or a configured server address.

   Change the state of the corresponding entry to RECOVERY.  Change the
   lifetime of the entry to be MAX_LEASEQUERY_DELAY.  The TID field is
   set to the TID used in the Leasequery message.

   An example of the entry is illustrated in Figure 11.






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   +---------+-------+---------+-----------------------+-------+
   | Anchor  |Address| State   | Lifetime              |TID    |
   +---------+-------+---------+-----------------------+-------+
   | A       | Addr1 |RECOVERY |MAX_LEASEQUERY_DELAY   |TID    |
   +---------+-------+---------+-----------------------+-------+



              Figure 11: Binding entry in BST on Lease Query

   If the trigger event is EVE_DATA_CONFLICT:

   Remove the entry.

7.5.3.  From RECOVERY to Other States

7.5.3.1.  Trigger Event

   EVE_ENTRY_EXPIRE, EVE_DATA_LEASEQUERY.

7.5.3.2.  Following Actions

   If the trigger event is EVE_DATA_LEASEQUERY:

   (1)  IPv4 address: Check if the 'chaddr' field (hardware address) of
        the DHCPLEASEACTIVE message matches the hardware address of the
        triggering message.  If the two addresses do not match, the
        following actions will not be performed.  Change the state of
        the corresponding binding to BOUND.  Set life time to the sum of
        the value encoded in IP Address Lease Time option of the
        DHCPLEASEACTIVE message and MAX_DHCP_RESPONSE_TIME.  Erase the
        TID field.

   (2)  IPv6 address: Change the state of the corresponding binding to
        BOUND.  Set the lifetime to the sum of the valid lifetime
        extracted from OPTION_CLIENT_DATA option in the LEASEQUERY-REPLY
        message and MAX_DHCP_RESPONSE_TIME.  Erase the TID field.

   If multiple addresses are specified in the LEASEQUERY-REPLY message,
   new entries MUST also be created correspondingly on the same binding
   anchor.

   If the trigger event is EVE_ENTRY_EXPIRE:

   Remove the entry.






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7.5.4.  After BOUND

   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.  Because TID is used to associate binding
   entries with messages from DHCP servers, it must be recovered; or
   else a number of state transits of this mechanism will be not
   executed normally.

7.5.4.1.  Trigger Event

   EVE_DHCP_RENEW/EVE_DHCP_REBIND.

7.5.4.2.  Following Action

   Set the TID field of the corresponding entry to the TID in the
   triggering message.

7.6.  Table of State Machine

   The main state transits are listed as follows.




 State       Event               Action                       Next State
 NO_BIND     EVE_DATA_UNMATCH    Duplication detection         DETECTION
 DETECTION   Timeout             Send Leasequery                RECOVERY
 DETECTION   EVE_DATA_CONFLICT   Remove entry                    NO_BIND
 RECOVERY    EVE_DATA_LEASEQUERY Set lease time                    BOUND
 RECOVERY    Timeout             Remove entry                    NO_BIND
 BOUND       RENEW/REBIND        Record TID                        BOUND



                        Figure 12: Table of Transit

   RENEW: EVE_DHCP_RENEW

   REBIND: EVE_DHCP_REBIND

   Timeout: EVE_ENTRY_EXPIRE









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                               +-------------+
                               |             |
                     /---------|   NO_BIND   |<----------\
                     |  ------>|             |           |
                     |  |      +-------------+           |
   EVE_DATA_UNMATCH  |  |EVE_DATA_CONFLICT               |
                     |  |                                |TIMEOUT
                     |  |                                |
                     |  |                                |
                     |  |                                |
                     |  |                                |
                     v  |                                |
              +-------------+        TIMEOUT         +------------+
              |             |                        |            |
              |  DETECTION  ------------------------>|  RECOVERY  |
              |             |                        |            |
              +-------------+                        +------------+
                                      EVE_DATA_LEASEQUERY|
                          /----------\                   |
            EVE_DHCP_RENEW|          |                   |
           EVE_DHCP_REBIND|    +-----v-------+           |
                          |    |             |           |
                          \----|   BOUND     |<----------/
                               |             |
                               +-------------+


                       Figure 13: Diagram of Transit


8.  Filtering Specification

   This section specifies how to use bindings to filter out spoofing
   packets.

   Filtering policies are different for data packet and control packet.
   DHCP and NDP (Neighbor Discovery Protocol) [RFC4861] messages that
   may cause state transit are classified into control packet.  Neighbor
   Advertisement (NA) and ARP Response are also included in control
   packet because the Target Address of NA and ARP Response should be
   checked to prevent spoofing.  All other packets are considered to be
   data packets.

8.1.  Data Packet Filtering

   Data packets from attachment with attribute Validating MUST be
   checked.




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   Packet whose source IP address is a link-local address SHOULD be
   forwarded.

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

   The SAVI device MAY record any violation.

8.2.  Control Packet Filtering

   For attachments with Validating attribute:

   Discard DHCPv4 Request message whose source IP address is neither all
   zeros nor a bound address in BST.

   Discard DHCPv6 Request message whose source IP address is neither a
   link-local address nor bound with the corresponding binding anchor in
   BST.

   Discard NDP messages whose source IP address is neither a link-local
   address nor bound with the corresponding binding anchor.  In
   addition, discard NA message whose target address is neither a link-
   local address nor bound with the corresponding binding anchor.

   Discard ARP messages whose protocol is IP and sender protocol address
   is neither all zeros address nor bound with the corresponding binding
   anchor.  In addition, discard ARP Reply messages whose target address
   is not bound with the corresponding binding anchor.

   For attachments with other attributes:

   Discard DHCP server/relay type message not from attachments with the
   DHCP-Trust attribute or Trust attribute.

   The SAVI device MAY record any violation.

   For attachments with no attribute:

   No action will be performed on traffic from such attachments.


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 BST.



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9.1.  Attribute Configuration Restoration

   The lost of attribute configuration will not break the network: no
   action will be performed on traffic from attachments with no
   attribute.  However, the lost 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 SAVI device, if the configuration of binding
   anchor attribute can be 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 discard
   legitimate traffic.  Purely using the Data Snooping Process to
   recover a large number of bindings is of heavy overhead and
   considerable delay.  Thus, to recover 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.

   Immediately after reboot, the SAVI device SHOULD restore binding
   states from the non-volatile storage.  The system time of save
   process MUST be stored.  After rebooting, the SAVI device MUST check
   whether each entry has been obsolete through comparing the saved
   lifetime and the difference between the current system time and saved
   system time.


10.  Constants

   MAX_DHCP_RESPONSE_TIME 120s

   DATA_SNOOPING_INTERVAL 60s and configurable

   MAX_LEASEQUERY_DELAY 10s

   OFFLINK_DELAY 30s

   DAD_TIMEOUT 0.5s







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11.  MLD Consideration

   To perform the duplicate detection in Data Snooping Process
   Section 7, the SAVI device MUST join the Solicited Node Multicast
   group of the source address of triggering IPv6 data packet whenever
   performing duplicate detection.


12.  Security Considerations

12.1.  Security Problems about the Data Snooping Process

   There are two security problems about 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 Services attack 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 have a minimum
        interval time DATA_SNOOPING_INTERVAL.  This constant SHOULD be
        configured prudently to avoid Denial of Service attacks.

   (2)  The Data Snooping Process may set up wrong bindings if the
        clients do not reply to the detection probes.  An attack will
        pass the duplicate detection if the client assigned the target
        address does not reply to the detection probes.  The DHCP
        Leasequery procedure performed by the SAVI device just tells
        whether the address is assigned in the network or not.  However,
        the SAVI device cannot determine whether the address is just
        assigned to the triggering attachment from the DHCP Leasequery
        Reply.

12.2.  Issues about Leaving Clients

   After a binding is set up, the corresponding client may leave its
   attachment point.  It may leave temporarily due to link flapping, or
   permanently due to it moves to a new attachment point or just leaves
   the network.  Considering the client may be back shortly, the binding
   should be kept, or else the legtimate traffic from the client will be
   blocked.  However, if the client leaves permanently, it may be
   insecure to keep the binding.  In case that the binding anchor is a
   property of the attachment point rather than the client, e.g., the
   switch port, an attacker which is attached to the attachment point of
   the leaving client can send spoofing packets with the addresses
   assigned to the client.  Even if the binding anchor is a property of
   the client, it is a waste of binding resource to keep bindings for



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   left clients.

   The following mechanism is designed to handle the leaving of client:

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

   (2)  If receiving DAD Neighbor Solicitation/Gratuitous ARP request
        targeting at the address during OFFLINK_DELAY, the entries MAY
        be removed.

   (3)  If the binding anchor turns on-link during OFFLINK_DELAY, turn
        off the timer.

   In this way, the bindings of a leaving client is kept for
   OFFLINK_DELAY.  In case of link flapping, the client will not be
   blocked.  If the client leaves permanently, the bindings will be
   removes after OFFLINK_DELAY.

12.3.  Duplicate Bindings of the Same Address

   The same address may be bound with multiple binding anchors, only if
   the binding setup processes are finished on each binding anchor
   successfully.  This mechanism is designed in consideration that a
   client may move on the local link, and a client may have multiple
   attachments to a SAVI device.

   There are two security issues about such a design:

   Firstly, due to allowing one address bound with multiple binding
   anchors, the traceability of address is weakened.  An address can be
   traced to multiple attachments.

   Secondly, in the local link movement scenario, the former binding may
   not be removed and it can be made use of by an attacker sharing the
   same binding anchor.  For example, when switch port is used as
   binding anchor and the port is shared by an attacker and a client
   with a hub, the attacker can make use of the address assigned to the
   client after the client leaves.

12.4.  Compatibility with DNA (Detecting Network Attachment)

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




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   Though DNA provides optimization for clients, there is not sufficient
   information for this mechanism to migrate the previous binding or
   establish a new binding.  If a binding is set up only through
   snooping the reachability test message, the binding can be invalid.
   For example, an attacker can perform reachability test with address
   bound to another client.  If binding is migrated to the attacker, the
   attacker can successful 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 re-configuration 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 Section 8.2.  If a client is re-
   attached 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 procedure at the same time, the binding will be
   recovered from the DHCP Snooping Process.  The DHCP Request messages
   will not be filtered out by this solution as they have link-local
   source addresses.  Before the DHCP procedure is completed, packets
   will be filtered out by the SAVI device.  In another word, if this
   SAVI function is enabled, Simple DNAv6 will not help reduce the
   handover latency.  If Data-Snooping attribute is configured on the
   new attachment of the client, the data triggered procedure may reduce
   the latency.

   In DNAv4 [RFC4436], the ARP probe will be discarded because unbound
   address is used as sender protocol address.  As a result, the client
   will regard the address under detection is 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 zero as required by [RFC2131].  Thus, if the address is
   still valid, the binding will be recovered from the DHCP Snooping
   Process.

12.5.  Authentication in DHCPv6 Leasequery

   As required in section 5 of RFC5007, DHCPv6 Leasequery 'Should' use
   IPsec-based authentication specified in the section 21.1 of RFC3315.
   However, with the deployment of this mechanism, there may be no need
   to enforce IPSec to perform DHCP Leasequery.

   Through containing the DHCP servers in the protection perimeter, the
   DHCP servers can be protected from spoofing based attacks.  Then
   through checking the source IP address of Leasequery messages, the



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   DHCP server can identify if the messages are from SAVI devices or
   not.  For the SAVI devices, because the perimeter filters out bogus
   DHCP messages, they can trust the DHCP Leasequery responses.  Thus,
   there is no need to enforce IPSec to validate the DHCP Leasequery
   messages in this mechanism.

12.6.  Binding Number Limitation

   A binding entry will cost a certain high-speed memory resource.  In
   general, a SAVI device can only afford a quite limited number of
   binding entries.  In order to prevent an attacker from overloading
   the resource of the SAVI device, binding entry limit is set on each
   attachment.  The binding entry limit is the upper bound of binding
   number for each attachment with Validating attribute.  No new binding
   should be set up after the limit has been reached.  Besides, 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.

12.7.  Residual Threats

   As described in [savi-framework], this solution cannot strictly
   prevent spoofing.  There are two scenarios in which spoofing can
   still happen:

   (1)  The binding anchor is spoofable.  If the binding anchor is
        spoofable, e.g., plain MAC address, an attacker can use forged
        binding anchor to send packet which will not be regarded as
        spoofing by SAVI device.  Indeed, using binding anchor that can
        be easily spoofed is more serious than allowing IP spoofing
        traffic.  For example, an attacker can use the binding anchor of
        another client to get a large number of addresses, and the SAVI
        device will refuse to set up new binding for the client whenever
        the binding number limitation has been reached.  Thus, it is
        RECOMMENDED to use strong enough binding anchor, e.g., switch
        port, secure association in 802.11ae/af and 802.11i.

   (2)  The binding anchor is shared by more than one clients.  If the
        binding anchor is shared by more than one clients, the clients
        can spoof the addresses of each other.  For example, if switch
        port is used as binding anchor a number of clients can attach to
        the same switch port of a SAVI device through a hub.  The SAVI
        device cannot distinguish packets from different clients and
        thus the spoofing between them will not be detected.  A number
        of the above security problems are caused by sharing binding
        anchor.  Besides, if binding anchor is shared, TID spoofing
        based attack is possible.  Thus, it is RECOMMENDED to use
        exclusive binding anchor.



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13.  IANA Considerations

   This memo asks the IANA for no new parameters.

   Note to RFC Editor: This section will have served its purpose if it
   correctly tells IANA that no new assignments or registries are
   required, or if those assignments or registries are created during
   the RFC publication process.  From the authors' perspective, it may
   therefore be removed upon publication as an RFC at the RFC Editor's
   discretion.


14.  Acknowledgment

   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, Ralph Droms and Alberto Garcia for
   careful review and valuation 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.

   This document was generated using the xml2rfc tool.


15.  References

15.1.  Informative References

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

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

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for  IPv6", RFC 3736, April 2004.

15.2.  Normative References

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.



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   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388, February 2006.

   [RFC4436]  Aboba, B., Carlson, J., and S. Cheshire, "Detecting
              Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, September 2007.

   [RFC5227]  Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              July 2008.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              November 2010.

   [RFC826]   Plummer, D., "Ethernet Address Resolution Protocol:  Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", RFC 826,
              November 1982.

   [savi-fcfs]
              Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS-
              SAVI: First-Come First-Serve Source-Address Validation for
              Locally Assigned Addresses", RFC 6620, May 2012.

   [savi-framework]
              Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement Framework",
              draft-ietf-savi-framework-06 (work in progress),
              December 2011.


Appendix A.  change log

   Main changes from 02 to 03:




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   (1)  Section 12, data trigger and counter trigger are combined to
        binding recovery process.  The expression "one of MUST" is
        changed to "conditional MUST.  Conditions related with the
        implementation are specified.  Related constants are changed in
        section 26."

   Main changes from 03 to 04:

   (1)  Section "Prefix configuration" is removed.

   (2)  Section "Supplemental binding process" is modified in
        requirement level.

   (3)  Sub-section 9.1 "Rationale" is added.

   (4)  Section "Filtering during Detection" is removed.

   (5)  Section "Handling layer 2 path change" is changed to
        "Consideration on Link layer routing complexity"

   (6)  Section "Background and related protocols" is removed.

   Main changes from 04 to 05:

   (1)  Trigger events are listed explicitly in section 8.

   (2)  Detection and Live states are deleted, together with
        corresponding sections.

   Main change from 05 to 06:

   (1)  Section 8.1: reference to section 20 is changed to section 15.

   Main changes from 06 to 07:

   (1)  So many changes in this modification.  We suggest to track
        http://www.ietf.org/mailarchive/web/savi/current/msg01543.ht ml.
        Changes are made according to the comments.

   Main changes from 07 to 08,09:

   (1)  The modifications are made according to the comments from Jean-
        Michel Combes.

   Main changes from 09 to 11:






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   (1)  DNA issues raised by Jari Arkko

   Main changes from 11 to 12:

   (1)  The modifications are made according to the comments from Eric,
        http://www.ietf.org/mail-archive/web/savi/current/msg01778.html.

   Main changes from 12 to 13:

   (1)  Main modifications are made based on comments from Elwyn Davies.
        http://www.ietf.org/mail-archive/web/gen-art/current/
        msg07297.html.

   (2)  Other modifications are made based on comments from Barry Leiba.

   Main changes from 13 to 14:

   (1)  A symbol error is corrected.

   Main changes from 14 to 15:

   (1)  In corresponding to "1.  Does section 8 describe the mechanism
        that a SAVI device must perform if it has been unable to snoop
        the DHCP traffic between a host and a DHCP server?  It appears
        that way in the document, but it would be good to explicitly
        state that early in the document when the discussion of
        topologies is being carried out.  This becomes important when
        arbitrary topologies do not provide a means for the SAVI device
        to eavesdrop on the DHCP traffic."  We specified in s7.1 p1 that
        arbitrary topologies may result in the regular process cannot
        set up correct bindings.  This is also specified in the
        beginning of s8.

   (2)  In corresponding to "2.  Section 12 refers to the "tentative
        address multicast group".  Do you really mean the Solicited Node
        Multicast address that is generated from the configured IPv6
        unicast address?"  Yes. We have changed s12 to "the SAVI device
        MUST join the Solicited Node Multicast group of the source
        address of triggering IPv6 data packet whenever performing
        duplicate detection."

   (3)  Other modifications are made according to the gen-art review.
        Refer to http://netarchlab.tsinghua.edu.cn/~yaog/review.txt.

   Main changes from 15 to 16:






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   (1)  Main modifications are made according to the second-round gen-
        art review.

   (2)  Improve the quality of writing.

   Main changes from 16 to 17:

   (1)  Main modifications are made according to the review from Ted
        Lemon.

   Main changes from 17 to 18: Main modifications are made according to
   the review from Ralph Droms and Ted Lemon.

   (1)  Add the definitions of upstream/downstream device/link.

   (2)  In Section 6.3.2, that message without triggering a valid event
        will be forwarded if it can pass Section 8.2 is clarified.

   (3)  All_DHCP_Servers is used instead of All_DHCP_Relays_and_Servers
        in Leasequery.  Besides, "A list of DHCP server addresses" is
        used instead of "a DHCP server address".

   Main changes from 18 to 19: Main modifications are made according to
   the second round review from Ted Lemon.

   (1)  Add the definitions of CUT VERTEX.

   (2)  Change "Besides, the movement of DHCP clients may make bindings
        are not set up on their new attachments." in section 6.2 to
        "Besides, when a DHCP client moves from one attachment to
        another attachment in the same network, it may not reinitialize
        its interface or send a Confirm message because of imcomplete
        protocol implementation."

   (3)  Modify the events and state transitions in section 6.

   (4)  Specify the DHCP relay/server placement and authentication
        problem in section Section 4.3.3.













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Authors' Addresses

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

   Email: junbi@tsinghua.edu.cn


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

   Email: jianping@cernet.edu.cn


   Guang Yao
   Tsinghua University
   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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