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SAVI                                              J. Bi, J. Wu, G. Yao
Internet Draft                                          Tsinghua Univ.
Intended status: Standard Tracks                             F. Baker
Expires: August 2012                                     Cisco Systems
                                                      February 8, 2012


                          SAVI Solution for DHCP
                        draft-ietf-savi-dhcp-12.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79. This document may not be modified,
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   This Internet-Draft will expire on August 3, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Abstract

   This document specifies the procedure for creating bindings between a
   DHCPv4 [RFC2131]/DHCPv6 [RFC3315] assigned source IP address and a
   binding anchor [I-D.ietf-savi-framework] on SAVI (Source Address
   Validation Improvements) device. The bindings can be used to filter
   packets generated on the local link with forged source IP address.

Table of Contents


   Copyright Notice ............................................... 1
   Abstract ....................................................... 2
   1. Introduction ................................................ 3
   2. Conventions used in this document............................ 4
   3. Terminology ................................................. 4
   4. SAVI-DHCP Scenario .......................................... 4
   5. Data Structures ............................................. 5
      5.1. Control Plane Data Structure: Binding State Table (BST). 5
      5.2. Data Plane Data Structure: Filtering Table (FT)......... 6
      5.3. Mapping Table from Link Layer Address to Binding Anchor. 6
   6. Binding Anchor Attributes.................................... 7
      6.1. No Attribute ........................................... 7
      6.2. SAVI-Validation Attribute............................... 7
      6.3. SAVI-DHCP-Trust Attribute............................... 7
      6.4. SAVI-SAVI Attribute..................................... 8
      6.5. SAVI-BindRecovery Attribute............................. 8
   7. Binding Set Up .............................................. 8
      7.1. Rationale .............................................. 9
      7.2. Binding States Description.............................. 9
      7.3. Events ................................................. 9
         7.3.1. Timer Expiration Event............................. 9
         7.3.2. Control Message Arriving Events ................... 9
      7.4. Process of DHCP Packet Snooping ....................... 10
         7.4.1. From NO_BIND to Other States ..................... 10
            7.4.1.1. Trigger Event................................ 10
            7.4.1.2. Following Actions............................ 10
         7.4.2. From INIT_BIND to Other States ................... 12
            7.4.2.1. Trigger Event................................ 12
            7.4.2.2. Following Actions............................ 12
         7.4.3. From BOUND to Other States ....................... 13
            7.4.3.1. Trigger Event................................ 13
            7.4.3.2. Following Actions............................ 13
      7.5. State Machine of DHCP Snooping ........................ 14


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   8. Supplemental Binding Process................................ 15
      8.1. Binding Recovery Process............................... 15
   9. Filtering Specification..................................... 16
      9.1. Data Packet Filtering.................................. 17
      9.2. Control Packet Filtering............................... 17
   10. State Restoration ......................................... 18
   11. Handle Binding Anchor Off-link Event ...................... 18
   12. Constants ................................................. 18
   13. Security Considerations.................................... 19
      13.1. Security Problem about Binding Triggered by
      EVE_DHCP_REPLY_NULL ........................................ 19
      13.2. Binding Number Limitation............................. 20
      13.3. Risk from Link Layer Routing Dynamic ................. 20
      13.4. Duplicate Bindings of Same Address ................... 20
      13.5. Security Problems about Binding Recovery Process...... 21
      13.6. Compatibility with DNA (Detecting Network Attachment). 21
      13.7. Residual Threats...................................... 22
   14. IANA Considerations........................................ 22
   15. References ................................................ 23
      15.1. Normative References.................................. 23
      15.2. Informative References................................ 24
   16. Acknowledgments ........................................... 24
   17. Change Log ................................................ 25

1. Introduction

   This document describes the procedure for creating binding between
   DHCP address and binding anchor on SAVI device [I-D.ietf-savi-
   framework]. The removal and restoration of the bindings are also
   specified in this document.

   Bindings can be used to filter or identify packets with forged source
   IP address. Section 9 suggests usage of these bindings for common
   practice.

   The mechanism specified in this document is designed to provide a
   binding anchor granularity validation, as a supplement to BCP38
   [BCP38]. This mechanism is deployed on the access device (including
   access switch, wireless access point/controller, etc), and performs
   mainly DHCP snooping to set up bindings between IP addresses assigned
   by DHCP and corresponding binding anchors. The binding process is
   inspired by the work of IP Source Guard [IP Source Guard].

   This solution is designed for stateful DHCP scenario [RFC2131],
   [RFC3315]. In stateless DHCP scenarios [RFC3736], a node must have
   obtained its IPv6 addresses through some other mechanisms and so the
   address of the client SHOULD be bound based on other SAVI solutions


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   This solution is primarily designed for a pure DHCP scenario in which
   only DHCP address is legitimate global address. How to use this
   mechanism in multiple address assignments scenario is discussed in
   [I-D.ietf-savi-mix].

2. Conventions used in this document

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

3. Terminology

   Lease time:   Lease time in IPv4 [RFC2131] and valid lifetime in
   IPv6 [RFC3315]

4. SAVI-DHCP Scenario

   Figure 1 shows the main elements in a DHCP network. At least one DHCP
   server must be deployed in the network, and DHCP relay may be used to
   relay message between client and server. Multiple SAVI devices and
   non-SAVI devices can co-exist on link. A SAVI device can be connected
   to DHCP client, DHCP relay (even DHCP server), SAVI device and non-
   SAVI device.

   Other address assignment mechanisms may be also used in such a
   network. However, this solution is primarily designed for a pure DHCP
   scenario, in which only DHCP servers can assign valid global address.

   Note that in IPv6 environment, every interface has a link-local
   address, which is not assigned by DHCP. This solution will not
   validate link-local address. It is SUGGESTED to enable a SAVI
   solution for link-local addresses, e.g. [I-D.ietf-savi-fcfs].














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                             /------------\
              +--------+     |            |
              | DHCP   |-----|   Router A |
              | Server |     |            |
              +--------+     \------------/
             ......................|..........
             .                     |         .
             . Protection          |         .
             . Perimeter           |         .
             .                     |         .
             .                     |         .
             .                     |         .
             . +----------+    +----'-----+  .   +----------+
             . |  SAVI    |    |  SAVI    |  .   |  Non SAVI|
             . |  Device A|----|  Device B|------|  Device  |
             . +/---.-----+    +-.------.-+  .   +-----.----+
             .................................         |
                    |            |      |              |
                    |            |      |              |
               +----/-----+ +----\-+   +\-----+     +--\---+
               | Router B | |DHCP  |   |Client|     |Client|
               |          | |Relay |   | A    |     | B    |
               +----------+ +------+   +------+     +------+
                          Figure 1 DHCP Scenario



5. Data Structures

   This section describes the data structures used in this mechanism.

   Two main data structures are used to record bindings and their states
   respectively. There is redundancy between the two structures, for the
   consideration of separation of data plane and control plane.

   Besides the two main data structures, a mapping table from link layer
   address to binding anchor may be required, as described in section
   7.4.1.

  5.1.   Control Plane Data Structure: Binding State Table (BST)

   This table contains the state of binding between source address and
   binding anchor. Entries are keyed on the binding anchor and source IP
   address. Each entry has a lifetime field recording the remaining
   lifetime of the entry, a state field recording the state of the
   binding and a TID field recording transaction ID of DHCP message. The


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   lifetime field is used to help remove expired bindings. The state
   field is used to identify state. The TID field is used to keep the
   Transaction ID (TID) [RFC2131][RFC3315] in DHCP request. The TID
   field can be cleared after the state is changed to BOUND.

           +---------+----------+----------+-----------+-------+
           | 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  |
           +---------+----------+----------+-----------+-------+
                         Figure 2 Instance of BST


  5.2.   Data Plane Data Structure: Filtering Table (FT)

   This table contains the bindings between binding anchor and address,
   keyed on binding anchor and address. This table doesn't contain any
   state of the binding. This table is only used to filter packets. An
   Access Control List can be regarded as a practical instance of this
   table. This table SHOULD be updated by other SAVI mechanisms [I-
   D.ietf-savi-mix] when DHCP SAVI is deployed in IPv6 environment, as
   explained in section 4.

                          +---------+----------+
                          | Anchor  |Address   |
                          +---------+----------+
                          |A        |IP_1      |
                          +---------+----------+
                          |A        |IP_2      |
                          +---------+----------+
                          Figure 3 Instance of FT

  5.3.  Mapping Table from Link Layer Address to Binding Anchor

   As described in section 7.4.1, whenever binding anchor must be
   recovered from DCHP Reply, such a mapping table is required. This
   table maps link layer address to binding anchor, so as that the SAVI
   device can determine on which binding anchor to set up a binding only
   based on a DHCP Reply message.

   Such a table can already exist on SAVI device. For example, if the
   binding anchor is switch port, the mapping table from MAC address to
   switch port is required on switch for switching frames. We don't


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   require SAVI device to set up a different mapping table from the
   existing one. Instead, SAVI device MUST only use the existing one. If
   there is not such a mapping table yet, SAVI device MUST NOT set up
   binding based on EVE_DHCP_REPLY_NULL (cf. 7.4.1).

   The set up and update of this table is out of the scope of this
   document.

6. Binding Anchor Attributes

   This section specifies the binding anchor attributes used in this
   mechanism.

   Attribute of each binding anchor is configurable. By default, binding
   anchor has no attribute. A binding anchor MAY be configured to have
   one or more compatible attributes.

  6.1. No Attribute

   Before configuration, by default, a binding anchor has no attribute.
   To filter bogus DHCP server message by default, server type DHCP
   message from binding anchor with no attribute MUST be dropped.
   However, to avoid discarding legitimate traffic, other packets SHOULD
   NOT be dropped before any binding is setup on such binding anchor.
   Generally, each binding anchor is configured to have one or more
   attributes after configuration. However, a binding anchor MAY always
   have no attribute if it is connected to another link. For example, in
   Figure 1, on the binding anchor of SAVI Device A connected to Router
   B, it is unreasonable either to set up bindings for host behind
   Router B, or filter out traffic from Router B, or allow bogus DHCP
   message from Router B. Thus, no attribute should be configured on
   this binding anchor.

  6.2. SAVI-Validation Attribute

   SAVI-Validation attribute is used on binding anchor on which the
   source address of data packet and DHCP message is to be validated.
   The filtering process on binding anchor with such attribute is
   described in section 9. In Figure 1, the binding anchor between SAVI
   Device B and Client A, and the binding anchor between SAVI Device B
   and Non SAVI Device should be configured to have this attribute.

  6.3. SAVI-DHCP-Trust Attribute

   SAVI-DHCP-Trust Attribute is used on binding anchor on the path to a
   trustable DHCP server/relay. DHCP server/relay message coming from
   binding anchor with this attribute will be forwarded. In Figure 1,


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   the binding anchor between SAVI Device B and DHCP Relay, and the
   binding anchor between SAVI Device B and Router A, should be
   configured to have this attribute.

  6.4. SAVI-SAVI Attribute

   This attribute is used on binding anchor from which the data traffic
   is not to be checked. Binding will not be set up on binding anchor
   with this attribute. All data packets will be allowed directly. In
   Figure 1, the binding anchor between SAVI Device A and SAVI Device B
   should be configured with this attribute.

   Through configuring this attribute on binding anchor that joins two
   or more SAVI devices, SAVI-Validation and SAVI-SAVI attributes
   implement the security perimeter concept in [I-D.ietf-savi-
   framework]. Since no binding entry is needed on such binding anchor,
   the resource requirement can be reduced greatly.

   Though there is no factual difference in packet process between a
   binding anchor with no attribute and a binding anchor only with SAVI-
   SAVI attribute, their connotations are different. SAVI-SAVI attribute
   is configured on binding anchor between SAVI devices on the same link
   inside the protection perimeter. But only when a binding anchor is on
   the protection perimeter and connected to another link, it can have
   no attribute after configuration.

   This attribute is mutually exclusive with SAVI-Validation.

  6.5. SAVI-BindRecovery Attribute

   This attribute is used on binding anchor that requires data-triggered
   binding recovery described in section 8.1. It can be configured on
   any binding anchor with SAVI-Validation attribute, especially, the
   binding anchor not directly attached by client. In Figure 1, it is
   suggested to configure this attribute on binding anchor between SAVI
   Device B and Non SAVI Device.

   This attribute is mutually exclusive with SAVI-SAVI.

7. Binding Set Up

   This section specifies the procedure of setting up bindings based on
   DHCP message snooping. The binding procedure specified here is
   exclusively designed for binding anchor with SAVI-Validation
   attribute.




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  7.1. Rationale

   The rationale of this mechanism is that if a node attached to a
   binding anchor is legitimate to use a DHCP address, the DHCP
   procedure which assigns the address to the node must have been
   performed on the same binding anchor. This basis stands when the link
   layer routing is stable. However, layer-2 mobility and unstable link
   layer routing may result in that data packet is received from a
   different binding anchor. Infrequent link layer path change can be
   handled (but not perfectly) by the mechanism described in section 8.
   Section 13.3 discusses the situation that link layer routing is
   naturedly unstable. A solution for this issue is outside the scope of
   this document.

  7.2. Binding States Description

   This section describes the binding states of this mechanism.

   NO_BIND       The state before a binding has been set up.

   INIT_BIND       A DHCP request (or a DHCPv6 Confirm, or a DHCPv6
   Solicitation with Rapid Commit option) has been received from host,
   and it may trigger a new binding.

   BOUND         The address is authorized to the client.

  7.3. Events

7.3.1. Timer Expiration Event

   EVE_ENTRY_EXPIRE: The lifetime of an entry expires

7.3.2. Control Message Arriving Events

   Only if a control message can pass the check in section 9.2, the
   corresponding event is a valid event.

   EVE_DHCP_REQUEST: A DHCP Request message is received from a binding
   anchor with SAVI-Validation attribute, and the binding entry limit
   (cf. section 13.2) on the binding anchor has not been reached.

   EVE_DHCP_CONFIRM: A DHCPv6 Confirm message is received from a binding
   anchor with SAVI-Validation attribute, and the binding entry limit on
   the binding anchor has not been reached.

   EVE_DHCP_OPTION_RC: A DHCPv6 Solicitation message with Rapid Commit
   option is received from a binding anchor with SAVI-Validation


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   attribute, and the binding entry limit on the binding anchor has not
   been reached.

   EVE_DHCP_REPLY: A DHCPv4 Acknowledgement or DHCPv6 Reply message is
   received from a binding anchor with SAVI-DHCP-Trust attribute, and
   the message should be forwarded to a binding anchor with SAVI-
   Validation attribute.

   EVE_DHCP_REPLY_NULL: A DHCPv4 Acknowledgement or DHCPv6 Reply message
   is received from a binding anchor with SAVI-DHCP-Trust attribute, and
   there is no entry in state INIT_BIND contains the same TID as the
   message.

   EVE_DHCP_DECLINE: A DHCP Decline message is received from a binding
   anchor with SAVI-Validation attribute.

   EVE_DHCP_RELEASE: A DHCP Release message is received from a binding
   anchor with SAVI-Validation attribute.

   EVE_LEASEQUERY_REPLY: A successful DHCP LEASEQUERY_REPLY is received
   from a binding anchor with SAVI-DHCP-Trust attribute.

  7.4. Process of DHCP Packet Snooping

7.4.1. From NO_BIND to Other States

7.4.1.1. Trigger Event

   EVE_DHCP_REQUEST, EVE_DHCP_CONFIRM, EVE_DHCP_OPTION_RC,
   EVE_DHCP_REPLY_NULL.

7.4.1.2. Following Actions

   If the triggering event is EVE_DHCP_REQUEST/EVE_DHCP_OPTION_RC:

               The SAVI device MUST forward the message.

     The SAVI device MUST generate an entry for the binding anchor in
     the Binding State Table (BST) and set the state field to INIT_BIND.
     The lifetime of this entry MUST set to be MAX_DHCP_RESPONSE_TIME.
     The TID field of the request packet MUST be recorded in the entry.








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   +---------+-------+---------+-----------------------+-------+
   | Anchor  |Address| State   | Lifetime              |TID    |
   +---------+-------+---------+-----------------------+-------+
   | A       |       |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID   |
   +---------+-------+---------+-----------------------+-------+
      Figure 4 Binding entry in BST on client triggered initialization

     The TID is stored because it will be used in a future step (cf.
     section 7.4.2.2) to correctly associate the assigned address to the
     binding anchor.

   If the triggering event is EVE_DHCP_CONFIRM:

     Besides forwarding the message and generating corresponding entry,
     the address to be confirmed MUST be recorded in the entry. Because
     no lease time will be contained in the REPLY from DHCP server, the
     SAVI device MUST lookup in BST to determine whether there is an
     entry with the same address. If there is such an entry, it can be a
     local movement and the lifetime can be recovered from the entry. If
     there is not, the SAVI device MUST send a LEASEQUERY [RFC5007]
     message querying by IP address to All_DHCP_Relay_Agents_and_Servers
     multicast address [RFC3315] or a configured server address.

   +---------+--------+---------+-----------------------+-------+
   | Anchor  | Address| State   | Lifetime              |TID    |
   +---------+--------+---------+-----------------------+-------+
   | A       | Addr   |INIT_BIND|MAX_DHCP_RESPONSE_TIME | TID   |
   +---------+--------+---------+-----------------------+-------+
     Figure 5 Binding entry in BST on Confirm triggered initialization

   If the triggering event is EVE_DHCP_REPLY_NULL:

     If the binding anchor is not link layer address and there is not a
     mapping table from link layer address to binding anchor, the
     message SHOULD be discarded.

     Else:

     The SAVI device MUST deliver the message to the destination.

     The SAVI device MUST generate as many new entries in BST and FT as
     the number of IADDR found in the message. If the binding anchor
     type inside the BST is not link layer address, the binding anchor
     for an entry is recovered from the mapping table from link layer
     address to binding anchor (cf. section 5.3) based on the
     destination link layer address inside the DHCP message.



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     The states of the corresponding entries are set to be BOUND. The
     lifetime of the entries MUST be set to be the lease time.

     The binding entry limit can be exceeded when setting up bindings
     for all addresses in a REPLY message. If there is enough binding
     entry resources, corresponding new entries MUST be generated even
     the binding number limit is exceeded. In case that there is not
     enough resource left, as many as possible entries SHOULD be set up.

     If the binding anchor is a switch port, there can be vulnerability
     in this process which is discussed in section 13.1. Similar problem
     can happen with other binding anchors.

   +---------+----------+-------+------------------------+-------+
   | Anchor  | Address  | State | Lifetime               |TID    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr1    | BOUND |   Lease time 1         |TID    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr2    | BOUND |   Lease time 2         |TID    |
   +---------+----------+-------+------------------------+-------+
      Figure 6 Binding entry in BST on Reply triggered initialization

                          +---------+----------+
                          | Anchor  |Address   |
                          +---------+----------+
                          |A        |Addr1     |
                          +---------+----------+
                          |A        |Addr2     |
                          +---------+----------+

       Figure 7 Binding entry in FT on Reply triggered initialization

7.4.2. From INIT_BIND to Other States

7.4.2.1. Trigger Event

   EVE_DHCP_REPLY, EVE_ENTRY_EXPIRE, EVE_LEASEQUERY_REPLY.

7.4.2.2. Following Actions

   If the trigger event is EVE_DHCP_REPLY:

     The SAVI device MUST deliver the message to the destination.

      If the Address field is null, the lease time in Reply message MUST
      be recorded in the entry with matched TID. The state of the entry
      is changed to be BOUND. If more than one IADDR is found in the


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      message, if there is enough binding entry resource, corresponding
      new entries MUST be generated even the binding number limit is
      exceeded. In case that there is not enough resource left, as many
      as possible entries SHOULD be set up.

      If the Address field is not null, the Reply is in response to a
      Confirm message. If the Reply message is of Status Code Success,
      and there is an entry with the same address, set the Lifetime to
      be the remaining Lifetime of the existing entry, and remove the
      existing entry. If there is no entry with the same address, set
      the Lifetime of corresponding entry to MAX_LEASEQUERY_DELAY. The
      state of the entry is changed to be BOUND.

   +---------+----------+-------+------------------------+-------+
   | Anchor  | Address  | State | Lifetime               |TID    |
   +---------+----------+-------+------------------------+-------+
   | A       | Addr     | BOUND |  Lease time            |TID    |
   +---------+----------+-------+------------------------+-------+
                     Figure 8 From INIT_BIND to BOUND

      A corresponding entry MUST also be generated in FT.

   If the trigger event is EVE_ENTRY_EXPIRE:

      The entry MUST be deleted from BST.

   If the trigger event is EVE_LEASEQUERY_REPLY:

     The Lifetime field of entry with corresponding IP address MUST be
   set to the lease time in the LEASEQUERY_REPLY. The state of the entry
   is changed to BOUND.

7.4.3. From BOUND to Other States

7.4.3.1. Trigger Event

   EVE_ENTRY_EXPIRE, EVE_DHCP_RELEASE, EVE_DHCP_DECLINE,
   EVE_DHCP_REPLY_RENEW.

7.4.3.2. Following Actions

   If the trigger event is EVE_ENTRY_EXPIRE:

     Remove the corresponding entry in BST and FT.

   If the trigger event is EVE_DHCP_RELEASE or EVE_DHCP_DECLINE:



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     Remove the corresponding entry in BST and FT. The Release or
     Decline message MUST be forwarded.

   If the trigger event is EVE_DHCP_REPLY_RENEW:

     Set the lifetime of the address to be the new lease time.

  7.5. State Machine of DHCP Snooping

   The main state transits are listed as follows.

   State        Event            Action                       Next State

   NO_BIND       REQ/RC    Generate entry                      INIT_BIND

   NO_BIND       CFM       Generate entry and send Leasequery  INIT_BIND

   *NO_BIND      RPL_NULL  Generate entry with lease               BOUND

   INIT_BIND     RPL       Record lease time/set LQ_DLY            BOUND

   INIT_BIND     Timeout   Remove entry                          NO_BIND

   INIT_BIND     LQR       Record lease time                       BOUND

   BOUND         RLS/DCL   Remove entry                          NO_BIND

   BOUND         Timeout   Remove entry                          NO_BIND

   BOUND         RNW       Set new lifetime                        BOUND

   *: optional but NOT RECOMMENDED.

   REQ: EVE_DHCP_REQUEST

   CFM: EVE_DHCP_CONFIRM

   RC:  EVE_DHCP_OPTION_RC

   RPL: EVE_DHCP REPLY

   RPL_NULL: EVE_DHCP REPLY_NULL

   DCL: DHCP DECLINE

   RLS: DHCP RELEASE



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   RNW: EVE_DHCP_RPL_RENEW

   LQR: EVE_LEASEQUERY_REPLY

   Timeout: EVE_ENTRY_EXPIRE

  LQ_DLY: MAX_LEASEQUERY_DELAY

8. Supplemental Binding Process

   Supplemental binding process is designed to cover scenarios where a
   packet is sent by node but no previous DHCP exchanges have occurred
   to update correctly the SAVI device's BST. A typical situation is
   when the link topology changes after the binding has been set up, and
   then the node will send packet through a different port than the
   bound port. Another scenario is that a node moves on the local link
   without re-configuration process.

   Supplemental binding process is designed to avoid blocking
   permanently legitimate traffic. It is not supposed to set up a
   binding whenever a data packet with unbound source address is
   received. Generally, longer time and more packets are needed to
   trigger supplemental binding processes, as explained in the following
   section.

   Binding Recovery Process is a conditional SHOULD. This function
   SHOULD be implemented if the vendor has such ability, unless the
   implementation is known to be directly attached to host. If an
   implementation is directly attached to host, change in link topology
   will not affect the bindings, and host will always start re-
   configuration process after interface is re-connected. Thus, there is
   no need to use additional process to recovery bindings. If the
   mechanism is not implemented and managed hosts are not directly
   attached, permanent legitimate traffic blocking can happen until the
   node is reconfigured.

  8.1. Binding Recovery Process

   If a binding anchor is set to have SAVI-BindRecovery attribute,
   packet without matched binding can trigger the SAVI device to check
   if the source address can be used by corresponding node:

   1. Check if the address has a local conflict through:

       IPv4 address: performing Address Resolution Protocol (ARP)
       [RFC826] or Address Conflict Detection [RFC5227] twice on the
       address;


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       IPv6 address: performing Duplicate Address Detection (DAD)
       [RFC4862] twice on the address.

      If duplicate detection fails, the packet MUST be discarded.
      Otherwise, go to the next step.

   2.

   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. If no DHCPLEASEACTIVE message with IP Address Lease
     Time option is received, discard the packet; otherwise generate a
     new binding entry for the address, with the life time set to the
     value encoded in IP Address Lease Time option of the
     DHCPLEASEACTIVE message.

   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. If no successful LEASEQUERY-REPLY is received,
     discard the packet; otherwise generate a new binding entry for the
     address, with the lifetime set to the valid lifetime extracted from
     OPTION_CLIENT_DATA option in the LEASEQUERY-REPLY message. The SAVI
     device MAY repeat this process if a LEASEQUERY-REPLY with
     OPTION_CLIENT_LINK is received, in order to set up binding entries
     for all the address of the client.

   In case that SAVI device is pure layer-2 device without IP address,
   it is impossible to perform DHCP LEASEQUERY. Besides, this process
   may fail if any DHCP server doesn't support DHCP LEASEQUERY.

   The security issues about this process is discussed is section 13.5.

9. Filtering Specification

   This section specifies how to use bindings to filter 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



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   checked to prevent spoofing. All other packets are considered to be
   data packets.

  9.1. Data Packet Filtering

   Data packets with a binding anchor which has attribute SAVI-
   Validation MUST be checked.

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

   If the source IP address of a packet is not a link-local address, but
   the address is not bound with the corresponding binding anchor, this
   packet MUST be discarded.

   The SAVI device SHOULD record any violation.

  9.2. Control Packet Filtering

   For binding anchors with SAVI-Validation attribute:

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

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

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

     Discard ARP messages whose protocol is IP and sender protocol
     address is neither link-local address nor bound with the
     corresponding binding anchor. In addition, discard ARP Reply
     messages whose target address is neither link-local address nor
     bound with the corresponding binding anchor.

   For other binding anchors:

     Discard DHCP Reply/ACK messages not from binding anchor with the
     SAVI-DHCP-Trust attribute or SAVI-SAVI attribute.

   The SAVI device SHOULD record any violation of the previous rules.





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10. State Restoration

   If a SAVI device reboots accidentally or designedly, the states kept
   in volatile memory will get lost. This may cause legitimate traffic
   from hosts indirectly attached to the SAVI device to be blocked until
   a binding recovery. Purely using the Binding Recovery Process is
   expansive and results delay to recover a large number of bindings.
   Thus, recovery from non-volatile storage, as specified below, is
   recommended.

   If this function is supported by hardware, binding entries MUST be
   saved into non-volatile storage whenever a new binding entry changes
   to BOUND state or a binding with BOUND state is removed.

   Immediately after reboot, the SAVI device MUST 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 current system time and saved system time.

11. Handle Binding Anchor Off-link Event

   Port DOWN event MUST be handled if switch port is used as binding
   anchor. In more general case, if a binding anchor turns off-link,
   this event MUST be handled.

   Whenever a binding anchor with attribute SAVI-Validation turns down,
   set a timer with OFFLINK_DELAY. Until the timer becomes zero, the
   bindings with the binding anchor SHOULD be kept. As an exception, to
   handle node movement, if receiving DAD Neighbor
   Solicitation/Gratuitous ARP request targeting at the address during
   OFFLINK_DELAY, the entry MAY be removed.

   If the binding anchor turns on-link during OFFLINK_DELAY, turn off
   the timer and keep corresponding bindings.

12. Constants

   MAX_DHCP_RESPONSE_TIME     120s

   BIND_RECOVERY_INTERVAL     60s and configurable

   MAX_LEASEQUERY_DELAY       10s

   OFFLINK_DELAY              30s




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13. Security Considerations

  13.1. Security Problem about Binding Triggered by
     EVE_DHCP_REPLY_NULL

    When the binding anchor is switch port, binding based on
    EVE_DHCP_REPLY_NULL can result in security threats. The assigned
    address could be bound to a wrong switch port if an attack can
    poison the table mapping a link layer address to switch port (cf.
    section 5.3).

    For example, host A requests address from port 1. When SAVI switch
    receives a DHCP REPLY with assigned address IP_A and destination
    link layer address MAC_A, it will check its MAC/port table to find
    the right binding port. But MAC/port table might be polluted by an
    attacker host B attached to port 2. Then SAVI switch will find the
    MAC_A is at port 2 from the polluted MAC/port table and it will
    result in a wrong binding which binds IP_A and port 2.

    If one of the following conditions is satisfied, the security can be
    ensured.

   1. DHCP Option 82 is used to keep binding anchor in DHCP Request and
      Reply.

                   DHCP Option 82 can be used to keep the circuit information of the
       client and returned by the DHCP server. Thus the binding anchor
       can be determined from the circuit information in the Option. It
       can be used whenever an implementation doesn't want to create an
       entry on the DHCP Request message.

   2. Unspoofable MAC is used as binding anchor (802.11i, 802.1ae/af).

   3. The mapping table from MAC to binding anchor is secure.

       If the binding anchor is a link layer address, or there are
       mechanisms preventing the corruption of the table mapping the
       link layer to a switch port, mapping link layer address to
       binding anchor may be considered as secure.

   It is NOT RECOMMENDED to initialize a binding based on DHCP Reply,
   unless a mechanism protecting the mapping table from corruption is
   also implemented.

   Similar problem may happen with binding anchors not based on link
   layer addresses.



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  13.2. Binding Number Limitation

   It is suggested to configure some mechanism in order to prevent a
   single node from exhausting the binding table entries on the SAVI
   device. Either of the following mechanism is sufficient to prevent
   such attack.

   1. Set the upper bound of binding number for each binding anchor with
      SAVI-Validation.

   2. Reserve a number of binding entries for each binding anchor with
      SAVI-Validation attribute and all binding anchors share a pool of
      the other binding entries.

   3. Limit DHCP Request rate per binding anchor.

  13.3. Risk from Link Layer Routing Dynamic

   An implicit assumption of this solution is that data packet must
   arrive at the same binding anchor with the binding anchor that the
   control packets have arrived at. If this assumption is not valid,
   this control packet based solution will fail or at least discard
   legitimate packet. Unfortunately, the link layer routing between host
   and SAVI device can be inconsistent from time to time. Time
   consistency of link layer routing is not assured by link layer
   routing protocol. For example, TRILL, a recent link layer routing
   protocol, is flexible and multiple link layer paths are allowed.

   To make the basic assumption stand, the best way is enforcing that
   there should be only one topology path from downstream host to the
   SAVI device. For example, SAVI device is directly attached by hosts.

   If the assumption doesn't stand, a better solution is requiring
   inter-operation between SAVI protocol and the link layer routing
   protocol to make SAVI protocol sensitive to the link layer routing
   change. This solution is above the scope of this document.

  13.4. Duplicate Bindings of Same Address

   The same address may be bound with multiple binding anchors, only if
   the binding processes are finished on each binding anchor
   successfully respectively. This mechanism is designed in
   consideration that a node may move on the local ink, and a node may
   have multiple binding anchors. However, the traceability of address
   is reduced.




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   Note that the local link movement scenario is not handled perfectly.
   The former binding may not be removed, unless the node is directly
   attached to SAVI device. The nodes sharing the same former binding
   anchor of the moving node have the ability to use its address.

  13.5. Security Problems about Binding Recovery Process

   The Binding Recovery Process (cf. section 8.1) MUST be rate limited
   to avoid Denial of Services attack against the SAVI device itself. A
   constant BIND_RECOVERY_INTERVAL is used to control the frequency. Two
   data-triggered recovery processes on one binding anchor MUST have a
   minimum interval time BIND_RECOVERY_INTERVAL. This constant SHOULD be
   configured prudently to avoid Denial of Service attacks.

   This process is not strict secure. The node with SAVI-BindRecovery
   binding anchor has the ability to use the address of an inactive
   node, which doesn't reply to the detection probes.

  13.6. 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. Though DNA
   provides an optimization for host, it doesn't provide sufficient
   information for this mechanism to migrate or establish a 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 host. 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 interface is slowed but not prevented. Details
   are discussed as follows.

   In Simple DNAv6 [RFC6059], the probe is sent with source address set
   to link-local address, and such messages will not be filtered by the
   policy specified in section 9.2. If an interface is re-attached to a
   previous network, the detection will be complete and the address will
   be regarded as valid by host. The candidate address is not contained
   in the probe. Thus, the binding cannot be recovered through snooping
   the probe. The binding can only be recovered from the DHCP snooping
   procedure. The DHCP REQUEST messages wouldn't be filtered by this
   solution as their source address is link-local address. Before the
   DHCP procedure is completed, packets will be filtered by SAVI device.


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   In another word, in SAVI scenarios, Simple DNAv6 will not help reduce
   the handover latency. If SAVI-BindRecovery attribute is configured on
   the new binding anchor, data triggered procedure may reduce the
   latency.

   In DNAv4 [RFC4436], the ARP probe will be filtered because unbound
   address is used as sender protocol address. As a result, the
   detection will not complete and false negative is caused. The DHCP
   REQUEST message sent by the node will not be filtered, 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 procedure.

  13.7. Residual Threats

   As described in [I-D.ietf-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 dangerous.
   An attacker can use the binding anchor of another host to perform a
   lot of DHCP procedures, and the SAVI device will refuse to set up new
   binding for the host 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 host

   If the binding anchor is shared by more than one host, they can spoof
   the addresses of each other. For example, a number of hosts can
   attach to the same switch port of a SAVI device through a hub. The
   SAVI device cannot distinguish packets from different hosts and thus
   the spoofing between them will not be detected. This problem can be
   solved through not sharing binding anchor between hosts.

14. IANA Considerations

   This document has no IANA actions.

   [RFC Editor: please remove this section prior to publication.]



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15. References

  15.1. Normative References

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

   [I-D.ietf-savi-framework]

             Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
             "Source Address Validation Improvement Framework", draft-
             ietf-savi-framework-04 (work in progress), March 2011.

   [I-D.ietf-savi-fcfs]

             Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS-
             SAVI: First-Come First-Serve Source-Address Validation for
             Locally Assigned IPv6 Addresses", draft-ietf-savi-fcfs-09
             (work in progress), April 2011.

   [I-D.ietf-savi-mix]

             Jun Bi, Guang Yao, J. Halpern and E. Levy-Abegnoli, "SAVI
             for Mixed Address Assignment Methods Scenario", draft-ietf-
             savi-mix-00 (work in progress), March 2011.

   [RFC826]  Plummer, D.C., "Ethernet Address Resolution Protocol:  Or
             converting network protocol addresses to 48.bit Ethernet
             address for transmission on Ethernet hardware", STD 37, RFC
             826, November 1982.

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

   [RFC3315] R. Droms, Ed. "Dynamic Host Configuration Protocol for IPv6
             (DHCPv6)", RFC3315, July 2003.

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

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

   [RFC4426] Aboba, B., Carlson, J., and S. Cheshire, "Detecting Network
             Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.


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

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

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

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

  15.2. Informative References

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

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

   [IP Source Guard]

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

16. Acknowledgments

Special thanks to Jean-Michel Combes, Christian Vogt, Joel M. Halpern,
Eric Levy-Abegnoli and Alberto Garcia for careful review and valuation
comments on the state machine and text.
Thanks to Marcelo Bagnulo Braun, Mark Williams, Erik Nordmark, Mikael
Abrahamsson, Jari Arkko, David Harrington, Pekka Savola, Xing Li, Lixia
Zhang, Robert Raszuk, Greg Daley, John Kaippallimalil and Tao Lin for
their valuable contributions.












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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
      Computer Science, Tsinghua University
      Beijing 100084
      China
      Email: yaog@netarchlab.tsinghua.edu.cn

    Fred Baker
       Cisco Systems
       Santa Barbara, CA  93117
       United States
       Email:  fred@cisco.com


17. Change Log

   From 02 to 03:

         -  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."





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   Main changes from 03 to 04:

         -  Section "Prefix configuration" is removed.

         -  Section "Supplemental binding process" is modified in
            requirement level.

         -  Sub-section 9.1 "Rationale" is added.

         -  Section "Filtering during Detection" is removed.

         -  Section "Handling layer 2 path change" is changed to
            "Consideration on Link layer routing complexity"

         -  Section "Background and related protocols" is removed.

   Main changes from 04 to 05:

         -  Trigger events are listed explicitly in section 8.

         -  Detection and Live states are deleted, together with
            corresponding sections.

   Main change from 05 to 06:

         -  Section 8.1: reference to section 20 is changed to section
            15.

   Main changes from 06 to 07:

         -  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:

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

   Main changes from 09 to 11:

         -  DNA issues







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