SAVI J. Bi
Internet-Draft J. Wu
Intended status: Standards Track G. Yao
Expires: September 12, 2014 Tsinghua Univ.
F. Baker
Cisco
March 11, 2014
SAVI Solution for DHCP
draft-ietf-savi-dhcp-20
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
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This Internet-Draft will expire on September 12, 2014.
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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 . . . . . . . . . . . . . . . . . . 8
4.2. Attribute . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2.1. Trust Attribute . . . . . . . . . . . . . . . . . . . 11
4.2.2. DHCP-Trust Attribute . . . . . . . . . . . . . . . . . 11
4.2.3. DHCP-Snooping Attribute . . . . . . . . . . . . . . . 12
4.2.4. Data-Snooping Attribute . . . . . . . . . . . . . . . 12
4.2.5. Validating Attribute . . . . . . . . . . . . . . . . . 13
4.2.6. Table of Mutual Exclusions . . . . . . . . . . . . . . 13
4.3. Perimeter . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. SAVI-DHCP Perimeter Overview . . . . . . . . . . . . . 14
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 . . . . . . . . . . . . . . . . 18
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 . . . . . . . . 20
6.4.1. From NO_BIND to Other States . . . . . . . . . . . . . 20
6.4.1.1. Trigger Events . . . . . . . . . . . . . . . . . . 20
6.4.1.2. Following Actions . . . . . . . . . . . . . . . . 20
6.4.2. From INIT_BIND to Other States . . . . . . . . . . . . 21
6.4.2.1. Trigger Events . . . . . . . . . . . . . . . . . . 21
6.4.2.2. Following Actions . . . . . . . . . . . . . . . . 22
6.4.3. From BOUND to Other States . . . . . . . . . . . . . . 24
6.4.3.1. Trigger Events . . . . . . . . . . . . . . . . . . 24
6.4.3.2. Following Actions . . . . . . . . . . . . . . . . 24
6.5. Table of State Machine . . . . . . . . . . . . . . . . . . 25
7. Data Snooping Process . . . . . . . . . . . . . . . . . . . . 26
7.1. Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 27
7.3. Additional Binding States Description . . . . . . . . . . 28
7.4. Events . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5. State Machine of Binding Recovery Process . . . . . . . . 29
7.5.1. From NO_BIND to Other States . . . . . . . . . . . . . 29
7.5.1.1. Trigger Event . . . . . . . . . . . . . . . . . . 29
7.5.1.2. Following Actions . . . . . . . . . . . . . . . . 29
7.5.2. From DETECTION to Other States . . . . . . . . . . . . 30
7.5.2.1. Trigger Event . . . . . . . . . . . . . . . . . . 30
7.5.2.2. Following Actions . . . . . . . . . . . . . . . . 30
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7.5.3. From RECOVERY to Other States . . . . . . . . . . . . 31
7.5.3.1. Trigger Event . . . . . . . . . . . . . . . . . . 31
7.5.3.2. Following Actions . . . . . . . . . . . . . . . . 31
7.5.4. After BOUND . . . . . . . . . . . . . . . . . . . . . 32
7.5.4.1. Trigger Event . . . . . . . . . . . . . . . . . . 33
7.5.4.2. Following Action . . . . . . . . . . . . . . . . . 33
7.6. Table of State Machine . . . . . . . . . . . . . . . . . . 33
8. Filtering Specification . . . . . . . . . . . . . . . . . . . 34
8.1. Data Packet Filtering . . . . . . . . . . . . . . . . . . 34
8.2. Control Packet Filtering . . . . . . . . . . . . . . . . . 35
9. State Restoration . . . . . . . . . . . . . . . . . . . . . . 36
9.1. Attribute Configuration Restoration . . . . . . . . . . . 36
9.2. Binding State Restoration . . . . . . . . . . . . . . . . 36
10. Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 36
11. Security Considerations . . . . . . . . . . . . . . . . . . . 37
11.1. Security Problems about the Data Snooping Process . . . . 37
11.2. Issues about Leaving Clients . . . . . . . . . . . . . . . 37
11.3. Duplicate Bindings to the Same Address . . . . . . . . . . 38
11.4. Compatibility with DNA (Detecting Network Attachment) . . 38
11.5. Authentication in DHCPv6 Leasequery . . . . . . . . . . . 39
11.6. Binding Number Limitation . . . . . . . . . . . . . . . . 40
11.7. Residual Threats . . . . . . . . . . . . . . . . . . . . . 40
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
13. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 41
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
14.1. Informative References . . . . . . . . . . . . . . . . . . 41
14.2. Normative References . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
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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
procedure which snoops the header of data packet to set up bindings.
It is 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 the client uses that
SAVI must be accomplished. For example, for hosts using Stateless
Auto-configuration address, SAVI-FCFS [savi-fcfs] should be enabled.
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 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.
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.
Downstream device: A downstream device is a non-SAVI device
associated with an downstream link. For example, an access switch in
the network.
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]
Detection message: a DAD or ARP message intended to detect a
duplicate address by the Data Snooping Process.
4. Deployment Scenario and Configuration
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
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(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| .
+----------+ +------+ . +------+ . +------+ . +--------+ .
............ ...............
Figure 1: SAVI-DHCP Scenario
Note: To distinguish upstream/downstream links is essential for SAVI-
DHCP.
Networks are not isolated and traffic from other networks, i.e.,
transit traffic specified in RFC6620, may get into the network with
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SAVI-DHCP deployed through the upstream links. Since SAVI solutions
are limited to check traffic generated from local link, SAVI-DHCP is
not to set up bindings for addresses assigned in other networks.
Thus, 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. How to generate and deploy such
a mechanism is out of the scope of this document.
However, traffic from downstream links are generated from local
network. For example, a hub, which is attached by some DHCP clients,
is on the downstream link of a SAVI device. The traffic from
downstream links should be checked by SAVI-DHCP if possible.
However, because DHCP clients on the downstream links are indirectly
attached, a number of security problems Section 11.7 can be
introduced.
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, data traffic from
such attachments will not be checked by SAVI-DHCP and will be
forwarded directly. 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-client 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.
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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 drop DHCP server-client 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
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 any
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indirect attachment point of the non-neighboring DHCP servers and
relays, unless all the elements that can be reached through that
attachment point can be trusted, i.e., bogus DHCP Server-Client
messages will not be generated by these elements. 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
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.
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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.
+----------+----------+----------+----------+----------+----------+
| | | | 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
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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.
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) Downstream/upstream links MUST be distinguished when configuring
the perimeter to avoid estabilshing binding for addresses of
other networks.
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:
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(1) Configure Validating and DHCP-Snooping attribute on the direct
attachments of all the DHCP clients.
(2) Configure Validating and DHCP-Snooping attribute on the indirect
attachments of all the DHCP clients(i.e., DHCP clients on the
downstream links).
(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.
(6) Optional: configure filters on the upstream links to filter out
spoofing of local addresses from other networks.
In this way, the points of attachments with Validating attribute (and
generally together with attachments of upstream devices) on SAVI
devices can form a perimeter separating DHCP clients and trusted
devices. Data packet check is 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 the trusted DHCP relays/servers must be placed
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.
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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
authenticate the messages from each other based on the source IP
address without the requriement to deploy IPSec.
Another considration on the placement is that if the DHCP server/
relay is not inside the perimeter, the SAVI devices may not be able
to set up bindings correctly, because the SAVI devices may not be on
the path between the clients and the server/relay, or the DHCP
messages are encapsulated (e.g., Relay-reply and Relay-forward).
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.
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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 |
+---------+----------+----------+-----------+-------+
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
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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 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.
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.
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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.
Note: the events listed here do not cover all the DHCP messages in
section 3. The messages which do not really determine address usage
(DHCPv4 Discover, DHCPv4 Inform, DHCPv6 Solicit without Rapid Commit,
DHCPv6 Information-Request, DHCPv4 Offer, DHCPv6 Advertise, DHCPv6
Reconfigure), and which are not neccessary to snoop (DHCPv4 NAK,
refer to section 6.4.2.1), are not included.
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
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
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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 11.6).
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 Events
Trigger events: 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
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.
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+---------+-------+---------+-----------------------+-------+
| 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
6.4.2.1. Trigger Events
Trigger events: 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.
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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 2*MAX_LEASEQUERY_DELAY. If there is no response message after
MAX_LEASEQUERY_DELAY, send the LEASEQUERY message again. An example
of the entries is illustrated in Figure 6. The related security
problem about DHCPv6 LEASEQUERY is discussed in Section 11.5.
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
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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 | 2*MAX_LEASEQUERY_DELAY |TID |
+---------+----------+-------+------------------------+-------+
| A | Addr2 | BOUND | 2*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
Request
If the trigger event is EVE_ENTRY_EXPIRE:
The entry MUST be deleted from BST.
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6.4.3. From BOUND to Other States
6.4.3.1. Trigger Events
Trigger events: 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
such IA options and no actions will be performed.
If the trigger event is EVE_DCHP_LEASEQUERY:
The message MUST be forwarded.
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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
LQR: EVE_DCHP_LEASEQUERY
Timeout: EVE_ENTRY_EXPIRE
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+-------------+
| |
/---------| 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's use of a DHCP address is legitimate, the
corresponding DHCP address assignment procedure must have been
finished on the attachment of the DHCP client. This is the case
stands when the SAVI device is persistently on the path(s) from the
DHCP client to the DHCP server(s)/relay(s). However, there are two
case when this does not work:
o Multiple paths: there is more than one feasible layer-2 paths
from the client to the DHCP server/relay, and the SAVI device is
not on everyone of them. The client may get its address through
one of the paths not passing by the SAVI device, but packets from
the client can travel through paths that pass through the SAVI
device. Because the SAVI device could not snoop the DHCP packet
exchange procedure, the DHCP snooping procedure cannot set up the
corresponding binding.
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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 (for example, some link turns broken due to
failure or as planned) or layer-2 path change. This situation
also covers the local-link movement of clients without address
confirm/re-configuration process. For example, a host changes
its attached switch port in a very short time. In such cases,
the DHCP snooping process will not set up the corresponding
binding.
Data Snooping Process prevents permanently blocking legitimate
traffic in case of these two exceptions. This process is performed
on attachments with the Data-Snooping attribute. Data packets
without matching binding entry may trigger this process to set up
bindings.
Snooping data traffic introduces considerable burden on the processor
and ASIC-to-Processor bandwidth of SAVI devices. Because of the
overhead of this process, the implementation of this process is a
conditional SHOULD. This function SHOULD be enabled unless the
implementation is known to be used in the scenarios without the above
exceptions. For example, if the implementation is to be used in
networks with tree topology and without host local-link movement,
there is no need to implement this process in such scenarios.
This process is not intended to set up a binding whenever a data
packet without matched binding entry is received. Instead, unmatched
data packets trigger this process probabilistically and generally a
number of unmatched packets will be discarded before the binding is
set up.
To perform this process, the SAVI device MUST join the Solicited Node
Multicast group of the source address of triggering IPv6 data packet
whenever performing duplicate detection.
7.2. Rationale
This process makes use of DAD/ARP and DHCP Leasequery to set up
bindings. If an address is not used by another client in the
network, and the address has been assigned in the network, the
address can be bound with the binding anchor of the attachment from
which the unmatched packet is received.
The security issues about this process is discussed is Section 11.1.
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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 the creation of a new binding entry, the binding
entry limit on the binding anchor MUST NOT be exceeded.
EVE_DATA_UNMATCH: A data packet without matched binding is received.
EVE_DATA_CONFLICT: ARP Reply/Neighbor Advertisement(NA) message
against an address in DETECTION state is received from a host other
than the one for which the entry was added.
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 11.6).
o Address check: For EVE_DATA_LEASEQUERY, the source address of the
DHCP Leasequery messages must pass the check specified in
Section 8.2. For EVE_DATA_CONFLICT, the source address and
target address of the ARP or NA messages must pass the check
specified in Section 8.2.
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o Interval check: the interval between two successive
EVE_DATA_UNMATCH events triggered by an attachment MUST be no
smaller than DATA_SNOOPING_INTERVAL.
o TID check: the DHCPLEASEACTIVE/LEASEQUERY-REPLY messages must
have 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
Trigger event: EVE_DATA_UNMATCH.
7.5.1.2. Following Actions
Make a probabilistic determination whether to act on this event. 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 may fail to reach the targeting node. If there is
a node that has the IP address seen in the Data Snooping Process, it
may not get the detection messages. This failure mode enables an
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attack against the Data Snooping Process. Thus, the detection is
performed again if there is no response after the first detection.
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
Trigger events: 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 each DHCPv4 server with IP Address Lease Time
option (option 51). A list of authorized DHCP servers are kept
by the SAVI device. The list should be pre-configured or
discovered by sending DHCPv4 Discover messages and parsing the
replied DHCPv4 Offer messages. Change the state of the
corresponding entry to RECOVERY. Change the lifetime of the
entry to be 2*MAX_LEASEQUERY_DELAY. The TID field is set to the
TID used in the DHCPLEASEQUERY message. If there is no response
message after MAX_LEASEQUERY_DELAY, send a DHCPLEASEQUERY to
each DHCPv4 server again.
(2) IPv6 address: Send a LEASEQUERY [RFC5007] message querying by IP
address to All_DHCP_Relay_Agents_and_Servers multicast address
or a list of pre-configured DHCPv6 server addresses. Change the
state of the corresponding entry to RECOVERY. Change the
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lifetime of the entry to be 2*MAX_LEASEQUERY_DELAY. The TID
field is set to the TID used in the LEASEQUERY message. If
there is no response message after MAX_LEASEQUERY_DELAY, send
the LEASEQUERY message again.
An example of the entry is illustrated in Figure 11.
+---------+-------+---------+-----------------------+-------+
| Anchor |Address| State | Lifetime |TID |
+---------+-------+---------+-----------------------+-------+
| A | Addr1 |RECOVERY |2*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
Trigger events: EVE_ENTRY_EXPIRE, EVE_DATA_LEASEQUERY.
7.5.3.2. Following Actions
If the trigger event is EVE_DATA_LEASEQUERY:
IPv4 address:
(1) Send an ARP Request with the Target Protocol Address set to the
IP address in the corresponding entry. The ARP Request is only
sent to the attachment which triggers the binding. If there is
no response after DAD_TIMEOUT, send another ARP Request. If
there is still no response, the following actions will not be
performed. If there is only one identical response, get the
sender hardware address. Check if the 'chaddr' field (hardware
address) of the DHCPLEASEACTIVE message matches the sender
hardware address. If the two addresses do not match, the
following actions will not be performed. If there is more than
one response, if any of the sender hardware addresses matches
the 'chaddr' field (hardware address) of the DHCPLEASEACTIVE
message, the following actions are to be performed.
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(2) 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.
IPv6 address:
(1) Send a Neighbor Solicitation message with the target address set
to the IP address in the corresponding entry. The Neighbor
Solicitation is only sent to the attachment which triggers the
binding. If there is no response after DAD_TIMEOUT, send
another Neighbor Solicitation. If there is still no response,
the following actions will not be performed. If there is only
one identical response, get the source hardware address from the
response. Check if the 'chaddr' field (hardware address) of the
LEASEQUERY-REPLY message matches the source hardware address.
If the two addresses do not match, the following actions will
not be performed. If there is more than one response, if any of
the source hardware addresses matches the 'chaddr' field
(hardware address) of the LEASEQUERY-REPLY message, the
following actions are to be performed.
(2) 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.
(3) After the above checks, if multiple addresses are specified in
the LEASEQUERY-REPLY message and there are no corresponding
binding entries, new entries MUST also be created
correspondingly on the same binding anchor.
If responses are received from multiple DHCP servers, the conflict
resolution mechanisms specified in section 6.8 of [RFC4388] and
section 4.3.4 of [RFC5007] will be used to determine which message
should be used.
If the trigger event is EVE_ENTRY_EXPIRE:
Remove the entry.
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
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executed normally.
7.5.4.1. Trigger Event
Trigger events: 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 or NO_BIND
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
LQ_DELAY: MAX_LEASEQUERY_DELAY
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+-------------+
| |
/---------| NO_BIND |<--------\
| ------>| | | TIMEOUT
| | +-------------+ |(2nd LQ_DELAY)
EVE_DATA_UNMATCH | | |
| | |
| | |
1st DAD_TIMEOUT | | | 1st LQ_DELAY
/------\ | | | /---------\
| | | | EVE_DATA_CONFLICT | | |
| v v | | v |
| +-------------+ TIMEOUT +------------+ |
| | | (2nd DAD_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 packets and control
packets. DHCP and NDP (Neighbor Discovery Protocol) [RFC4861]
messages that may cause state transit are classified as control
packet. Neighbor Advertisement (NA) and ARP Reply are also included
in control packet because the Target Address of NA and ARP Reply
should be checked to prevent spoofing. All other packets are
classified as data packets.
8.1. Data Packet Filtering
Data packets from attachments with the Validating attribute MUST be
checked.
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Packet whose source IP address is a link-local address will not be
checked. Note: as explained in Section 1, a SAVI solution for link-
local addresses, e.g., the SAVI-FCFS [savi-fcfs], can be enabled to
check packets with link-local source address.
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 Section 7 if Data-Snooping attribute is set on the
attachment.
Data packets from attachments with no attribute will forwarded
without checking.
The SAVI device MAY record any violation.
8.2. Control Packet Filtering
For attachments with the Validating attribute:
Discard DHCPv4 Client-Server message messages whose source IP address
is neither all zeros nor bound with the corresponding binding anchor
in the BST.
Discard DHCPv6 Client-Server message messages whose source IP address
is neither a link-local address nor bound with the corresponding
binding anchor in the BST.
Discard NDP messages whose source IP address is neither a link-local
address nor bound with the corresponding binding anchor. Especially,
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. Especially, discard ARP Reply messages whose target protocol
address is not bound with the corresponding binding anchor.
For attachments with other attributes:
Discard DHCP Server-Client message not from attachments with the
DHCP-Trust attribute or Trust attribute.
For attachments with no attribute:
Discard DHCP Server-Client message from such attachments.
The SAVI device MAY record any violation.
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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.
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. The
time when each binding entry is established is also saved.
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 by comparing the saved lifetime
and the difference between the current time and time when the binding
entry is established.
10. Constants
MAX_DHCP_RESPONSE_TIME 120s
DATA_SNOOPING_INTERVAL 60s and configurable
MAX_LEASEQUERY_DELAY 10s
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OFFLINK_DELAY 30s
DAD_TIMEOUT 0.5s
11. Security Considerations
11.1. 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.
11.2. 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 by moving to a new attachment point or leaving the
network. Since the client may return shortly, the binding should be
kept, or legtimate traffic from the client will be blocked. However,
if the client leaves permanently, it may be insecure to keep the
binding. If 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 resources to keep bindings for departed clients.
The following mechanism is designed to handle the leaving of client:
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(1) Whenever a client with the Validating attribute leaves, a timer
of duration OFFLINK_DELAY is set on the corresponding binding
entries.
(2) If a DAD Neighbor Solicitation/Gratuitous ARP request is
received that targets the address during OFFLINK_DELAY, the
entry MAY be removed.
(3) If the client returns on-link during OFFLINK_DELAY, cancel the
timer.
In this way, the bindings of a departing client are kept for
OFFLINK_DELAY. In case of link flapping, the client will not be
blocked. If the client leaves permanently, the bindings will be
removed after OFFLINK_DELAY.
11.3. Duplicate Bindings to the Same Address
The same address may be bound to multiple binding anchors only if the
binding setup processes successfully complete for each binding
anchor. This mechanism is designed to address the case where a
client moves on the local link, and the case where a client has
multiple attachments to a SAVI device.
There are two security issues with such a design:
First, by allowing one address to be bound to multiple binding
anchors, the traceability of the address is weakened. An address can
be traced to multiple attachments.
Second, in the local link movement scenario, the former binding may
not be removed and it can be used by an attacker sharing the same
binding anchor. For example, when a 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.
11.4. 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 by sending unicast Neighbor
Solicitation/Router Solicitation/ARP Request message to determine
whether a previously configured address is still valid.
Although DNA provides optimization for clients, there is insufficient
information for this mechanism to migrate the previous binding or
establish a new binding. If a binding is set up only by snooping the
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reachability test message, the binding may be invalid. For example,
an attacker can perform reachability test with an address bound to
another client. If binding is migrated to the attacker, the attacker
can successfully obtain the binding from the victim. Because this
mechanism wouldn't set up a binding based on snooping the DNA
procedure, it cannot achieve perfect compatibility with DNA.
However, it only means the 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 exchange at the same time, the binding will be recovered
from the DHCP Snooping Process. The DHCP Request messages will not
be filtered out in this case because they have link-local source
addresses. Before the DHCP procedure is completed, packets will be
filtered out by the SAVI device. In other words, if this SAVI
function is enabled, Simple DNAv6 will not help reduce the handover
latency. If Data-Snooping attribute is configured on the new
attachment of the client, the data triggered procedure may reduce
latency.
In DNAv4 [RFC4436], the ARP probe will be discarded because an
unbound address is used as the sender protocol address. As a result,
the client will regard the address under detection 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.
11.5. 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.
By containing the DHCP servers in the protection perimeter, the DHCP
servers can be protected from spoofing based attacks. Then by
checking the source IP address of Leasequery messages, the 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
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is no need to enforce IPSec to validate the DHCP Leasequery messages
in this mechanism.
11.6. Binding Number Limitation
A binding entry will consume a certain high-speed memory resources.
In general, a SAVI device can afford only a quite limited number of
binding entries. In order to prevent an attacker from overloading
the resource of the SAVI device, a binding entry limit is set on each
attachment. The binding entry limit is the maximum number of
bindings supported on each attachment with Validating attribute. No
new binding should be set up after the limit has been reached. If a
DHCP Reply assigns more addresses than the remaining binding entry
quota of each client, the message will be discarded and no binding
will be set up.
11.7. 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 each other addresses. For example, if a 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
anchors. If binding anchor is shared, TID spoofing based attack
is possible. Thus, it is RECOMMENDED to use exclusive binding
anchor.
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12. 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.
13. 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.
14. References
14.1. 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.
14.2. 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.
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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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