SAVI Working Group M. Bagnulo
Internet-Draft A. Garcia-Martinez
Intended status: Standards Track UC3M
Expires: October 28, 2013 April 26, 2013
SEND-based Source-Address Validation Implementation
draft-ietf-savi-send-10
Abstract
This memo describes SEND SAVI, a mechanism to provide source address
validation using the SEND protocol. The proposed mechanism is
intended to complement ingress filtering techniques to provide a
finer granularity on the control of the source addresses used.
Status of this Memo
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This Internet-Draft will expire on October 28, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background to SEND SAVI . . . . . . . . . . . . . . . . . . . 4
2.1. Address Validation Scope . . . . . . . . . . . . . . . . . 4
2.2. Binding Creation for SEND SAVI . . . . . . . . . . . . . . 4
2.3. SEND SAVI Protection Perimeter . . . . . . . . . . . . . . 7
2.4. Special cases . . . . . . . . . . . . . . . . . . . . . . 8
3. SEND SAVI Specification . . . . . . . . . . . . . . . . . . . 9
3.1. SEND SAVI Data Structures . . . . . . . . . . . . . . . . 9
3.2. SEND SAVI Device Configuration . . . . . . . . . . . . . . 10
3.3. Traffic Processing . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Transit Traffic Processing . . . . . . . . . . . . . . 11
3.3.2. Local Traffic Processing . . . . . . . . . . . . . . . 11
3.4. SEND SAVI Port Configuration Guidelines . . . . . . . . . 24
3.5. VLAN Support . . . . . . . . . . . . . . . . . . . . . . . 25
3.6. Protocol Constants . . . . . . . . . . . . . . . . . . . . 25
4. Protocol Walkthrough . . . . . . . . . . . . . . . . . . . . . 26
4.1. Change of the attachment point of a host . . . . . . . . . 26
4.1.1. Moving to a port of the same switch . . . . . . . . . 26
4.1.2. Moving to a port of a different switch . . . . . . . . 27
4.2. Attack of a malicious host . . . . . . . . . . . . . . . . 28
4.2.1. M attaches to the same switch as the victim's
switch . . . . . . . . . . . . . . . . . . . . . . . . 28
4.2.2. M attaches to a different switch to the victim's
switch . . . . . . . . . . . . . . . . . . . . . . . . 29
5. Security Considerations . . . . . . . . . . . . . . . . . . . 30
5.1. Protection Against Replay Attacks . . . . . . . . . . . . 30
5.2. Protection Against Denial of Service Attacks . . . . . . . 31
5.3. Residual threats . . . . . . . . . . . . . . . . . . . . . 32
5.4. Privacy considerations . . . . . . . . . . . . . . . . . . 33
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.1. Normative References . . . . . . . . . . . . . . . . . . . 33
8.2. Informative References . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
This memo describes SEND SAVI (SEcure Neighbor Discovery Source
Address Validation Implementation), a mechanism to provide source
address validation for IPv6 networks using the SEND protocol
[RFC3971]. The proposed mechanism is intended to complement ingress
filtering techniques to provide a finer granularity on the control of
the source addresses used.
SEND SAVI uses the DAD_NSOL (Duplicate Address Detection Neighbor
SOLicitation) and the DAD_NADV (DAD Neighbor ADVertisement) messages
defined in [RFC4862], and the NUD_NSOL (Neighbor Unreachability
Detection Neigbor SOLicitation) and NUD_NADV (NUD Neighbor
ADVertisement) messages defined in [RFC4861] to validate the address
ownership claim of a node. In addition, SEND SAVI uses RADV (Router
ADVertisement) messages defined in [RFC4861] to identify routers, and
therefore restrict the nodes which can generate packets containing
off-link IPv6 source addresses. Using the information contained in
these messages, host and router IPv6 addresses are associated to
switch ports, so that data packets will be validated by checking for
consistency in this binding, as described in
[I-D.ietf-savi-framework].
Scalability of a distributed SAVI system comprised of multiple SEND
SAVI devices is preserved by means of a deployment scenario in which
SEND SAVI devices form a "protection perimeter". In this deployment
scenario, validation is only performed when the packet ingress to the
protection perimeter.
The SEND SAVI specification, as defined in this document, is limited
to links and prefixes in which every IPv6 host and every IPv6 router
uses the SEND protocol [RFC3971] to protect the exchange of Neighbor
Discovery information.
SEND SAVI is designed to be deployed in SEND networks with a minimum
set of changes. In particular, SEND SAVI does not require any
changes in the nodes whose source address is to be verified. This is
because verification solely relies in the usage of already available
protocols. Therefore, SEND SAVI does neither define a new protocol,
nor define any new message on existing protocols, nor require that a
host or router uses an existing protocol message in a different way.
An overview of the general framework about Source Address Validation
Implementation is presented in [I-D.ietf-savi-framework].
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2. Background to SEND SAVI
2.1. Address Validation Scope
The application scenario of SEND SAVI is limited to the local link.
This means that the goal of SEND SAVI is to verify that the source
addresses of the packets generated by the nodes attached to the local
link have not been spoofed, and that only legitimate routers generate
packets with off-link IPv6 source addresses.
In a link there usually are hosts and routers attached. Hosts
generate packets with their own addresses as the source address.
This is called local traffic. Routers may send packets containing a
source address other than their own, since they can forward packets
generated by other hosts (usually located in a different link). This
is the so-called transit traffic.
SEND SAVI allows the validation of the source address of the local
traffic, i.e., it allows to verify that the source addresses of the
packets generated by the nodes attached to the local link have not
been spoofed. In addition, since SEND does provide the means to
verify that a node claiming to act as a router is indeed authorized
to do so, SEND SAVI also provides means to prevent hosts from
generating packets with source addresses derived from off-link
prefixes. However, SEND SAVI does not provide the means to verify if
a given router is actually authorized to forward packets containing a
particular off-link source address. Other techniques, like ingress
filtering [RFC2827], are recommended to validate transit traffic.
2.2. Binding Creation for SEND SAVI
Filtering is performed according to bindings between a layer-2 anchor
(the binding anchor) and an IPv6 address. These bindings should
allow legitimate nodes to use the bounded IPv6 address as source
address, and prevent illegitimate nodes to do so.
Any SAVI solution is not stronger than the binding anchor it uses.
If the binding anchor is easily spoofable (e.g., a Media Access
Control (MAC) address), then the resulting solution will be weak.
The treatment of non-compliant packets needs to be tuned accordingly.
In particular, if the binding anchor is easily spoofable and the SEND
SAVI device is configured to drop non-compliant packets, then the
usage of FCFS SAVI may open a new vector of Denial-of-Service (DoS)
attacks, based on spoofed binding anchors. For that reason, in this
specification, only switch ports MUST be used as binding anchors.
Other forms of binding anchors are out of the scope of this
specification, and proper analysis of the implications of using them,
should be performed before their usage.
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SEND [RFC3971] provides tools to assure that a ND (Neighbor
Discovery) message containing a CGA (Cryptographically Generated
Addresses) option and signed by a RSA option has been generated by
the legitimate owner of the CGA IPv6 address. It also provides tools
to verify that a Router Advertisement (RADV) message signed by a RSA
option with a key bounded to a CGA [RFC3972] or a certificate, has
been generated by a legitimate router.
SEND SAVI uses SEND validated messages to create bindings between the
CGA and the port of the SEND SAVI device from which it is reasonable
to receive packets with the CGA as source addresses. The events that
trigger the binding creation process in a SEND SAVI device are:
o The reception of a DAD_NSOL message, indicating the attempt of a
node to configure an address. This may occur when a node
configures an address for the first time or after being idle for
some time, or when the node has changed the physical attachment
point to the layer-2 infrastructure.
o The reception of any other packet (including data packets) with a
source address for which no binding exists. This would occur if
DAD_NSOL messages were lost, a node has changed the physical
attachment point to the layer-2 infrastructure without issuing a
DAD_NSOL message, a SAVI device loses a binding (for example, due
to a restart), or the link topology changed.
When the binding creation process is triggered, the SEND SAVI device
has to assure that the node for which the binding is to be created is
the legitimate owner of the address. For the case in which the
binding creation process initiated by a DAD_NSOL exchange, the SEND
SAVI device waits for the reception of a validated DAD_NADV message
indicating that other node had configured the address before, or
validated DAD_NSOL messages arriving from other locations indicating
that another node is trying to configure the same address at the same
time. For the case in which other packets than a DAD_NSOL initiate
the creation of the binding, the SEND SAVI device explicitly requires
the node sending those packets to prove address ownership by issuing
a secured NUD_NSOL which has to be answered by a secured NUD_NADV by
the probed node.
Bindings are refreshed periodically by means of secured NUD_NSOL
message issued by the SEND SAVI device, which had to be answered by a
valid NUD_NADV message by the node for which the binding exists.
Validated RADV messages are used to associate router authorization to
existing bindings (i.e., to an IPv6 address which is also associated
to a port). Packets with off-link source addresses are only
forwarded if they are received from a port associated to the IPv6
address of a router.
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SEND SAVI needs to be protected against replay attacks, i.e., attacks
in which a secured SEND message is replayed by another node. As
discussed before, the SEND SAVI specification uses SEND messages to
create a binding between the address contained in the message (that
must be signed by a node possessing the private key associated to the
address) and the port through which the message is received. If an
attacker manages to obtain such a message from another node, for
example because the message was sent to the all-nodes multicast
address or because the attacker has subscribed to the Solicited Node
multicast address associated to a remote node, it could replay it
preserving the original signature. This may create an illegitimate
binding in the SEND SAVI device, or could be used to abort address
configuration at other node. While SEND provides some means to limit
the impact of the replay of ND messages, the emphasis for SEND anti-
replay protection is to limit to a short period of time the validity
of the ND information transmitted in the message, for example, the
relationship between an IPv6 address and a layer-2 address. Note
that the period must be long enough to assure that the information
sent by the legitimate sender is considered valid despite the
possible differences in clock synchronization between sender and
receiver(s). For example, with the values recommended by [RFC3971]
for TIMESTAMP_FUZZ and TIMESTAMP_DRIFT, a node receiving a DAD_NSOL
message would not discard replays of this message being received
within a period of approximately 2 seconds (more precisely, 2/0.99
seconds). The underlying assumption for SEND security is that even
if the message is replayed by another node during this period of
time, the information disseminated by ND is still the same. However,
allowing a node to replay a SEND message do have impact to SEND SAVI
operation, regardless the time elapsed since it was generated, since
it can create a new binding in a SEND SAVI device for the port to
which an illegitimate node attaches. As can be concluded, the
protection provided by SEND is not enough in all cases for SEND SAVI.
SEND SAVI is designed to increase the protection against the replay
attacks compared to SEND. First, each node is required to connect to
the SEND SAVI topology through a different port to prevent
eavesdropping before entering to the SAVI protection perimeter.
Then, SEND SAVI bindings are updated only according to messages whose
dissemination can be restricted in the SEND SAVI topology without
interfering with normal SEND operation. The messages used by SEND
SAVI to create bindings are DAD_NSOL messages, for which SEND SAVI
limits its propagation to the ports through which a previous binding
for the same IPv6 address existed (see Section 3.3.2), and NUD_NADV
messages in response to a secured NUD_NSOL sent by the SEND SAVI
device only through the tested port. Finally, SEND SAVI filtering
rules prevent nodes from replaying messages generated by the SEND
SAVI devices themselves. Section 5.1 discusses in more detail the
protection provided by SEND SAVI against replay attacks.
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2.3. SEND SAVI Protection Perimeter
In order to reduce computing and state requirements in SEND SAVI
devices, SEND SAVI devices can be deployed to form a "protection
perimeter" [I-D.ietf-savi-framework]. With this deployment strategy,
source address validation is performed only when packets enter in the
protected realm defined through the protection perimeter. The
perimeter is defined by appropriate configuration of the roles of
each port, which can be 'Validating' or 'Trusted':
o Validating ports (VPs) are those in which SEND SAVI filtering and
binding creation is performed.
o Trusted ports (TPs) are ports in which limited processing is
performed. Only SEND messages related with certificates, prefix
information and DAD operation are processed, in order to update
the state of the SEND SAVI device or the state related with any of
the Validating ports of the switch.
The following figure shows a typical topology involving trusted and
untrusted infrastructure.
+--+ +--+ +--+ +--+
|H1| |H2| |H3| |R1|
+--+ +--+ +--+ +--+
| | | |
+----------SEND SAVI PROTECTION PERIMETER-------------+
| | | | | |
| +-1-----2-+ +-1-----2-+ |
| | SEND- | | SEND- | |
| | SAVI1 | | SAVI2 | |
| +-3--4----+ +--3--4---+ |
| | | +--------------+ | | |
| | +----------| |--------+ | |
| | | SWITCH-A | | |
| | +----------| | | |
| | | +--------------+ | |
| +-1--2----+ +-----1---+ |
| | SEND- | | SEND- | |
| | SAVI3 | | SAVI4 | |
| +-3-----4-+ +----4----+ |
| | | | |
+------------SEND SAVI PROTECTION PERIMETER-----------+
| | |
+--+ +--+ +--+
|R2| |H4| |H5|
+--+ +--+ +--+
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Trusted ports are used for connections with trusted infrastructure,
such as other SEND SAVI devices. Port 3 of SEND-SAVI1 and port 1 of
SEND-SAVI3, and port 4 of SEND-SAVI2 and port 1 of SEND-SAVI4 are
trusted because they connect two SAVI devices. Port 4 of SEND-SAVI1,
port 3 of SEND-SAVI2 and port 2 of SEND-SAVI3 are trusted because
they connect to SWITCH-A to which only trusted nodes are connected.
Validating ports are used for connection with non-trusted
infrastructure. Therefore, hosts are normally connected to
Validating ports. Routers are also recommended to be connected to
Validating ports, although they could also be attached to Trusted
ports. For a more detailed discussion on this, see Section 3.4. So,
in the figure above, ports 1 and 2 of SEND-SAVI1, port 1 of SEND-
SAVI2, port 4 of SEND-SAVI3 are Validating ports because they connect
to hosts. Port 2 of SEND-SAVI2 and port 3 of SEND-SAVI3 are
Validating ports because they connect to routers. Port 4 of SEND-
SAVI4 is also a Validating port because it is connected to host H5.
2.4. Special cases
Multi-subnet links: In some cases, a given subnet may have several
prefixes. This is supported by SEND SAVI as any port can support
multiple prefixes.
Multihomed hosts: A multihomed host is a host with multiple
interfaces. The interaction between SEND SAVI and multihomed hosts
is as follows. If the different interfaces of the host are assigned
different IP addresses and packets sent from each interface always
carry the address assigned to that interface as source address, then
from the perspective of a SEND SAVI device, this is equivalent to two
hosts with a single interface, each with an IP address. This is
supported by SAVI without need for additional considerations. If the
different interfaces share the same IP address or if the interfaces
have different addresses but the host sends packets using the address
of one of the interfaces through any of the interfaces, then SEND
SAVI does not directly support it. It would require either
connecting at least one interface of the multihomed host to a Trusted
port, or manually configure the SEND SAVI bindings to allow binding
the address of the multihomed host to multiple anchors
simultaneously.
Untrusted routers: One can envision scenarios where routers are
dynamically attached to a SEND SAVI network. A typical example would
be a mobile phone connecting to a SEND SAVI switch where the mobile
phone is acting as a router for other personal devices that are
accessing the network through it. In this case, the router does not
seem to directly fall in the category of Trusted infrastructure (as
if this was the case, it is likely that all devices would be
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trusted), hence it cannot be connected to a trusted port and if it is
connected to a Validating port, the SEND SAVI switch would discard
all the packets containing an off-link source address coming from
that device. As a result, the default recommendation specified in
this specification does not support such a scenario.
3. SEND SAVI Specification
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3.1. SEND SAVI Data Structures
The following three data structures are defined for SEND SAVI
operations:
SEND SAVI Data Base. The SEND SAVI function relies on state
information binding the source IPv6 address used in data packets to
the port through which the legitimate node connects. Such
information is stored in the SEND SAVI Data Base. The SEND SAVI Data
Base is populated with the contents of validated SEND messages. Each
entry contains the following information:
o IPv6 source address
o Binding anchor: port through which the packet was received
o Lifetime
o Status: TENTATIVE_DAD, TENTATIVE_NUD, VALID, TESTING_VP,
TESTING_VP'
o Alternative binding anchor: port from which a DAD_NSOL message or
any data packet has been received while a different port was
stored in the binding anchor for the address.
o Creation time: the value of the local clock when the entry was
firstly created
SEND SAVI Prefix list. SEND SAVI devices need to know which are the
link prefixes in order to identify local and off-link traffic. A
SEND SAVI device MUST support discovering this information from the
Prefix Information option [RFC4861] with the L set bit set of
validated RADV messages, either coming from Validating or Trusted
ports, as described in Section 3.3.2. The list of prefixes MAY also
be configured manually. This information is not specific to a given
port. The SEND SAVI Prefix list contains one entry per prefix in
use, as follows:
o Prefix: prefix included in a Prefix Information option
o Prefix lifetime: time in seconds that the prefix is valid.
Initially set to the Valid Lifetime value of the Prefix
Information option of a valid RADV message, or set to a value of
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all one bits (0xffffffff), which represents infinity, if
configured manually.
When the SEND SAVI device boots, it MUST send a Router Solicitation
(RSOL) message, which does not need to be secured if the unspecified
address is used (see [RFC3971], sections 5.1.1 and 5.2.1). The SAVI
device SHOULD issue a RSOL message in case the prefix entry is about
to expire.
SEND SAVI Router list. SEND SAVI keeps a table with one entry for
each authorized router in use connected to a Validating port of the
SAVI device. A SEND SAVI device MUST support discovering this
information from a validated RADV message received from a Validating
port, addressed to the all-nodes multicast address or to the IPv6
address of the SEND SAVI device. Alternatively, the list of routers
MAY be configured manually. The information stored in the table is
the following:
o IPv6 address of the Router. There MUST be an entry in the SEND
SAVI Data Base for the same IPv6 address. If the corresponding
entry in the SEND SAVI Data Base expires, the entry in this table
MUST be removed.
o Router lifetime: Lifetime associated with the default router in
units of seconds. Initially set to the Router Lifetime of a valid
RADV message. If the router lifetime expires, the entry in this
table is removed.
3.2. SEND SAVI Device Configuration
In order to perform SEND SAVI operation, some basic parameters of the
SEND SAVI device have to be configured. Since a SEND SAVI device
operates as a SEND node to generate NUD_NSOL, RSOL or Certification
Path Solicitation (CPS) messages,
o the SEND SAVI device MUST be configured with a valid CGA address.
This CGA address SHOULD be a link-local address, to recover from
the following situation: the DAD_NSOL message used by a router
when it configures its link-local address has not been received,
so a binding has not been created for the router address. If the
port to which the router connects is a Validating port, the SEND
SAVI device cannot accept any packet, so no RADV issued by the
router will be accepted. Then, the SEND SAVI device may not
receive prefix configuration to configure any other address than a
link-local. However, if the SEND SAVI device configures a link-
local CGA, it can issue a NUD_NSOL to the router, and create the
binding according to the process described in Section 3.3.2.
When the SEND SAVI device configures this address, it MUST behave
as regular SEND node, i.e., using secured NSOL messages to perform
DAD, etc., in addition to fulfill the requirements stated for
regular IPv6 nodes [RFC6434].
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o the SEND SAVI device MUST be configured with at least one trust
anchor to validate the Certification Paths that is used to
validate router information.
o the SEND SAVI device MAY be configured with Certification Paths.
The alternative is obtaining them by means of issuing
Certification Path Solicitation messages, as detailed in the SEND
specification [RFC3971].
In addition, the port role for each port of the SEND SAVI device
SHOULD be configured. The guidelines for this configuration are
specified in Section 3.4. Unconfigured ports MUST be labeled as
Validating ports; in this case performance may be degraded, as
discussed in [I-D.ietf-savi-framework].
3.3. Traffic Processing
In this section we describe how packets are processed by a SEND SAVI
device. Behavior varies depending on if the packet belongs to local
or transit traffic. This is determined by checking if the prefix of
the source address is included in the SEND SAVI prefix list or the
unspecified address (local traffic), or not included in the SEND SAVI
prefix list (transit traffic).
3.3.1. Transit Traffic Processing
Transit traffic processing occurs as follows:
o If the transit traffic packet is received through a Trusted port,
the data packet is forwarded and no SAVI processing performed.
o If the transit traffic packet is received through a Validating
port, the packet is only forwarded if the port through which the
packet has been received is associated to the port of an IPv6
address for which an entry in the Router list exists. If transit
traffic is received from a Validating port which is not associated
to an entry in the SEND SAVI Router list, the SEND SAVI device
SHOULD discard the packets, and MAY send a RSOL message to the
all-routers multicast address to the port through which the packet
was received.
3.3.2. Local Traffic Processing
If the verification of the source address of a packet shows that it
belongs to local traffic, this packet is processed using the state
machine described in this section. SEND SAVI is designed to perform
source address validation for both hosts and routers, so in the
following description we refer to nodes.
For the rest of the section, the following assumptions hold:
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o When it is stated that a secured NUD_NSOL message is issued by a
SEND SAVI device through a port P, this means the following: the
SEND SAVI device generates a NUD_NSOL message according to the
Neighbor Unreachability Detection procedure described in
[RFC4861], addressed to the IPv6 target address, which is the
source address of the packet triggering the procedure. This
message is secured by SEND as defined in [RFC3971]. The source
address used for issuing the NUD_NSOL message is the source
address of the SEND SAVI device. The message is sent only through
port P.
o When it is stated that a validated NUD_NADV message is received by
a SEND SAVI device, this means that: a SEND secured NUD_NADV
message has been received by the same port P through which the
corresponding NUD_NSOL message was issued, and the NUD_NADV
message has been validated according to [RFC3971] to prove
ownership for the IPv6 address under consideration and to prove
that it is a response for the previous NUD_NSOL message issued by
the SEND SAVI device (containing the same nonce value as the
NUD_NSOL message to which it answers).
We use VP to refer to a Validating port, and TP to refer to a Trusted
port.
The state machine is defined for a binding of a given source IPv6
address in a given SEND SAVI device. In the transitions considered,
packets described as inputs refer to the IPaddr IPv6 address
associated to the state machine.
The possible states for a given IPaddr are: NO_BIND, TENTATIVE_DAD,
TENTATIVE_NUD, VALID, TESTING_VP and TESTING_VP'. The NO_BIND state
represents that no binding exists for IPaddr; this is the state for
all addresses unless a binding is explicitly created.
The states can be classified into 'forwarding' states, i.e., states
in which packets received from the port associated to the IPv6
address are forwarded, and 'non-forwarding' states, i.e., states in
which packets different to the ones used for signaling are not
forwarded. VALID, TENTATIVE_DAD, TESTING_VP and TESTING_VP' are
forwarding states, and NO_BIND and TENTATIVE_NUD are non-forwarding
states.
The SEND SAVI device MUST join the Solicited Node Multicast group for
all the addresses whose state is other than NO_BIND. This is needed
to make sure that the SEND SAVI device receives DAD_NSOL messages
issued for those addresses. Note that it may not be enough to relay
on the Multicast Listener Discovery (MLD) messages being sent by the
node attached to a Validating port for which a binding for the
corresponding address exist, since the node may move and packets sent
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to that particular Solicited Node Multicast group may no longer be
forwarded to the SEND SAVI device.
SEND SAVI devices MUST support the processing of validated
Certification Path Advertisement (CPA) messages, sent in reply to CPS
messages, to acquire certificates used to validate ND messages. In
order to process a CPA message received from a Validating port, an
entry for the source address of the message MUST exist in the SEND
SAVI Data Base. CPA messages received from Trusted ports are always
checked and processed.
SEND SAVI devices MUST use validated RADV messages to update the SEND
SAVI Prefix list and the SEND SAVI Router list. SEND SAVI devices
MAY only consider for updating these structures RADV messages
addressed to either its own IPv6 address or to the all-nodes
multicast address. Validated RADV messages received from Trusted
ports MUST be used to update the SEND SAVI Prefix and Router lists in
the SEND SAVI device. RADV messages received from Validating ports
are only processed for updating the SEND SAVI Router and Prefix lists
if a binding for the source IPv6 address of the RADV message is in a
forwarding state.
In order to determine which traffic is on-link and off-link, the SEND
SAVI device MUST support discovery of this information from the
Prefix Information option with the L set bit set of validated RADV
messages. In this case, at least one router MUST be configured to
advertise RADV messages containing a Prefix Information option with
the prefixes that the untrusted nodes can use as source addresses,
and the bit L set. An alternative to this is to configure manually
the SEND SAVI prefix list.
The state machine defined for SEND SAVI operation adheres to the
following design guidelines:
o The only events which trigger state changes from forwarding to
non-forwarding states and vice versa are the reception of
DAD_NSOL, DAD_NADV and NUD_NADV, or the expiration of a timer.
The other possible input to consider is 'any other packet', which
could generate changes to states belonging to the same forwarding
or non-forwarding class as the original state. In other words,
when 'any other packet' is received, the state cannot move from
being forwarding to non-forwarding and vice versa. A special case
of 'any other packet' is when validated RADV are received, which
can result in the update of the SEND SAVI Prefix or Router lists.
The reduced set of messages being able to trigger a change
simplifies the processing at SEND SAVI devices.
o DAD_NADV and NUD_NADV are only processed when they are a response
to a DAD_NSOL or a NUD_NSOL message.
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o ND messages are only used by SEND SAVI devices if they are valid.
If any of the ND messages used by SEND SAVI is not valid, it is
discarded. SEND SAVI devices SHOULD assume that such messages
received from Trusted ports have been validated by other SEND SAVI
devices, so they SHOULD NOT attempt to validate them in order to
reduce processing load at the SEND SAVI device.
o The only messages the SEND SAVI device is required to generate
specifically per each source IP address are MLD and NUD_NSOL
messages. This also keeps the state machine simple.
o Well-behaved nodes are expected to initiate communication by
sending secured DAD_NSOL messages. The SEND SAVI state machine is
tailored to efficiently process these events. The reception of
other packet types without receiving previously validated DAD_NSOL
messages is assumed to be consequence of bad-behaving nodes or
infrequent events (such as packet loss, a change in the topology
connecting the switches, etc.) While a binding will ultimately be
created for nodes affected by such events, simplicity of the state
machine is prioritized over any possible optimization for these
cases.
o If a node has an address configured, and it can prove that it owns
this address, the binding is preserved regardless of any
indication that a binding for the same source address could be
configured in other SEND SAVI device. Bindings for the same
source address in two or more SEND SAVI devices may occur due to
several reasons, for example when a host moves (the two bindings
exist just for a short period of time), or when many nodes
generate the same address and the DAD procedure has failed. In
these infrequent cases, SEND SAVI preserves connectivity for the
resulting bindings.
We next describe how different inputs are processed depending on the
state of the binding of the IP address 'IPaddr'. Note that every ND
message is assumed to be validated according to SEND specification.
To facilitate the reader understanding the most relevant transitions
of the SEND SAVI state machine, a simplified version, which does not
contain every possible transition, is depicted in the next figure:
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+-------------+
| |
| TESTING_VP' |
| |
+-------------+
Timeout/VP=VP' | ^
| |
VP_NUD_NADV/- | | VP'_DAD_NSOL/
| | VP_NUD_NSOL
| |
v |
VP_DAD_NSOL/- +--------+
+------------- | |
| | VALID |< -------------------+
| +-------- >| | |
| | +--------+ |
| | ^ | |
| | VP_NUD_ | | Timeout, |
| | NADV/- | | TP_DAD_NSOL/VP_NUD_NSOL|
| | | v |
| | +------------+ |
| | | | |
| | | TESTING_VP | |
| | | | |
| | +------------+ |
| | | |
| | | Timeout/- |
| | VP*, | |
| | Timeout/- | VP_NUD_NADV/- |
v | | |
+---------------+ | +---------------+
| | | | |
| TENTATIVE_DAD | | | TENTATIVE_NUD |
| | | | |
+---------------+ | +---------------+
^ | | | ^
| | | Timeout/- | |
| | TP_DAD_NSOL, | | |
| | TP_DAD_NADV/- | | |
| | v | |
| | +---------+ | |
| +--------- >| |< -----+ |
| | NO_BIND | |
+--------------| |-----------------+
VP_DAD_NSOL/- +---------+ VP*/VP_NUD_NSOL
Simplified SEND SAVI state machine
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Each state transition is characterized by any of the events which may
trigger the change and the message(s) generated as a result of this
change. The meaning of some terms are referred next:
o VP_DAD_NSOL as a triggering event means that a validated DAD_NSOL
message has been received from the current BINDING_ANCHOR port VP.
o VP* means any packet (data packet) received from the current
BINDING_ANCHOR port VP.
o TP_DAD_NSOL as a triggering event means that a DAD_NSOL message
was received from a Trusted Port.
o - means that no message is sent. VP=VP' means that the
BINDING_ANCHOR is set to VP'.
The notation
Timeout, TP_DAD_NSOL/VP_NUD_NSOL
means that the transition is triggered by either a timeout expiration
or the reception of a DAD_NSOL message from a Trusted Port, and in
addition to the transition, a NUD_NSOL message is sent through port
VP.
For the rest of the description, it is assumed the following:
o When a validated message is required (i.e., a 'validated
DAD_NSOL'), messages are check for validity in the considered
switch according to [RFC3971], and messages not fulfilling these
conditions are discarded.
o When any SEND message is received from a validated port, the SEND
SAVI SHOULD assume that the message has been validated by the SEND
SAVI device through which the message accessed to the SEND SAVI
protection perimeter (unless the SEND SAVI perimeter has been
breached), or the device generating it is trusted. In this case,
the SAVI device does not perform any further validation.
Performing validation for SEND messages received through a Trusted
port may affect performance negatively.
o When a RADV message is received through a Validating port, and the
SEND SAVI device is in a forwarding state (VALID, TENTATIVE_DAD,
TESTING_VP and TESTING_VP') for the source address of the RADV
message, the message is forwarded to the appropriate Trusted
ports. In addition, either an entry for this IPv6 source address
in the SEND SAVI Router List is created, or the LIFETIME of an
existing entry is updated with the information received in this
message. The SEND SAVI Prefix list MUST also be updated according
to the content of the RADV message. The SEND SAVI device MAY not
process (although it MUST forward them) RADV messages addressed to
destinations other than the all-nodes multicast address or to the
IPv6 address of the SEND SAVI device.
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NO_BIND
When the node is in this state, there are no unresolved NUD_NSOL
messages generated by SEND SAVI or DAD_NSOL propagated to any
Validating port, so the only relevant inputs are DAD_NSOL messages
coming either from a Validating port (VP) or Trusted port (TP), or
any packet other than DAD_NSOL coming from VP or TP. There are no
timers configured for this state.
Messages received from a validating port
o If a validated DAD_NSOL message is received from a Validating port
VP, the SEND SAVI device forwards this message to all appropriate
Trusted ports (the subset of Trusted ports which belong to the
forwarding layer-2 topology, with the restrictions imposed by the
MLD snooping mechanism, if applied). DAD_NSOL messages are not
sent through any of the ports configured as Validating Ports. The
SEND SAVI device sets the LIFETIME to TENT_LT, stores all the
information required for future validation of the corresponding
DAD_NADV message (such as the nonce of the message), creates a new
entry in the SEND SAVI Data Base for IPaddr, sets BINDING_ANCHOR
to VP, and changes the state to TENTATIVE_DAD. Creation time is
set to the current value of the local clock.
Note that in this case it is not possible to check address
ownership by sending a NUD_NSOL because while the node is waiting
for a possible DAD_NADV its address is in tentative state and the
node cannot respond to NSOL messages [RFC4862].
o If any packet other than a DAD_NSOL is received through a
Validating port VP, the SEND SAVI device issues a secured NUD_NSOL
through port VP. The SEND SAVI device sets the LIFETIME to
TENT_LT. The SEND SAVI device creates a new entry in the SEND
SAVI Data Base for IPaddr, sets BINDING_ANCHOR to VP, and the
state is changed to TENTATIVE_NUD. Creation time is set to the
current value of the local clock. The SAVI device MAY discard the
packet while the NUD procedure is being executed, or MAY store it
in order to send it if the next transitions are (strictly)
TENTATIVE_NUD and then VALID.
Messages received from a trusted port
o If a DAD_NSOL message containing IPaddr as the target address is
received through a Trusted port, it MUST NOT be forwarded through
any of the Validating ports but it is sent through the proper
Trusted ports. The state is not changed.
o Any packet other than a DAD_NSOL received from a Trusted port is
forwarded to its destination. This packet is assumed to come from
a SEND SAVI device that has securely validated the binding
according to SEND SAVI rules (unless the SEND SAVI perimeter has
been breached). The state is not changed.
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TENTATIVE_DAD
To arrive to this state, the SEND SAVI device has received a
validated DAD_NSOL coming from the BINDING_ANCHOR port and it has
forwarded it to the appropriate TPs. The relevant events occurring
in this state are: the reception of a DAD_NADV message from a TP, a
DAD_NSOL message from the BINDING_ANCHOR port, other Validating port
or TP, a data packet from the BINDING_ANCHOR port, and the expiration
of the LIFETIME timer initiated when the DAD_NSOL was received at
port the BINDING_ANCHOR port.
Messages received from a trusted port
o The reception of a valid DAD_NADV message from a Trusted port
indicates that the binding cannot be configured for the
BINDING_ANCHOR port. The state is changed to NO_BIND, and the
LIFETIME cleared.
o The reception of a valid DAD_NSOL from a Trusted port indicates
that a node connected to another SEND SAVI device may be trying to
configure the same address at the same time. The DAD_NSOL message
is forwarded to the BINDING_ANCHOR port, so that the node at this
port will not configure the address, as stated in [RFC4862]. The
DAD_NSOL message is also forwarded to all appropriate Trusted
ports. Then, the LIFETIME is cleared, and the state is changed to
NO_BIND.
o Any packet other than a validated DAD_NSOL or DAD_NADV received
from a Trusted port is forwarded to its destination. This packet
is assumed to come from a SEND SAVI device that has securely
validated the binding according to SEND SAVI rules (unless the
SEND SAVI perimeter has been breached). The state is not changed.
Messages received from a validating port different from the
BINDING_ANCHOR
o A validated DAD_NSOL is received from a Validating port VP'
different the BINDING_ANCHOR port. The reception of a valid
DAD_NSOL from port VP' indicates that a node connected to VP' may
be trying to configure the same address at the same time. The
DAD_NSOL message is forwarded to the BINDING_ANCHOR port, so that
the node at this port will not configure the address, as stated in
[RFC4862]. The DAD_NSOL message is also forwarded to all
appropriate Trusted ports. Then, the BINDING_ANCHOR is set to VP'
(through which the DAD_NSOL message was received), the LIFETIME is
set to TENT_LT, and the state remains in TENTATIVE_DAD.
o Any other packet than a validated DAD_NSOL is received from a
Validating port VP' different from the BINDING_ANCHOR port is
discarded. The state is not changed.
Messages received from the BINDING_ANCHOR port
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o If a validated DAD_NSOL is received from the BINDING_ANCHOR port,
the LIFETIME is set to TENT_LT, and the state remains in
TENTATIVE_DAD.
o If any packet other than a DAD_NSOL is received from the
BINDING_ANCHOR port, it is assumed that the node has configured
its address, although it has done it in less time than expected by
the SEND SAVI device (less than TENT_LT). Since the node proved
address ownership by means of the validated DAD_NSOL message, the
LIFETIME is set to DEFAULT_LT, and the state is changed to VALID.
LIFETIME expires
o If LIFETIME expires, it is assumed that no other node has
configured this address. Therefore, the Validating port VP
(currently stored in the BINDING_ANCHOR) could be bound to this
IPv6 address. The LIFETIME is set to DEFAULT_LT, and the state is
changed to VALID.
VALID
To arrive to this state, successful validation of address ownership
has been completed and a binding for IPaddr has been created.
Relevant transitions for this state are triggered by the reception of
DAD_NSOL from the BINDING_ANCHOR port, other Validating port or a TP,
and any packet other than DAD_NSOL from other validating port than
the BINDING_ANCHOR or a TP. The expiration of LIFETIME is also
relevant to trigger a check for address ownership for the node at the
BINDING_ANCHOR port.
Messages received from the BINDING_ANCHOR port
o If a validated DAD_NSOL with IPaddr as source address is received
through the BINDING_ANCHOR port, it is forwarded to the
appropriate Trusted ports. The LIFETIME is set to TENT_LT and the
state is changed to TENTATIVE_DAD.
o Any packet other than a DAD_NSOL containing IPaddr as a source
address arriving from the BINDING_ANCHOR port is forwarded
appropriately. The state is not changed.
Messages received from a trusted port
o If a DAD_NSOL with IPaddr as source address is received through a
Trusted port, the message is forwarded to VP. The LIFETIME is set
to TENT_LT, a secured NUD_NSOL message is sent to IPaddr through
VP and the state is changed to TESTING_VP.
o If any packet other than a DAD_NSOL with IPaddr as source address
is received through a Trusted port, the packet is forwarded to VP
and to other appropriate Trusted ports. A secured NUD_NSOL is
sent to the BINDING_ANCHOR port, the LIFETIME is set to TENT_LT,
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and the state is changed to TESTING_VP.
Messages received from a validating port different from the
BINDING_ANCHOR
o If a validated DAD_NSOL packet with IPaddr as source address is
received through a Validating Port VP' (VP' different from the
current BINDING ANCHOR), the message is forwarded to the
BINDING_ANCHOR port. In addition, a secured NUD_NSOL is sent to
BINDING_ANCHOR port, the ALTERNATIVE BINDING ANCHOR is set to port
VP' (for future use if the node at VP' is finally selected), the
LIFETIME is set to TENT_LT, and the state is changed to
TESTING_VP'.
o If any packet other than a DAD_NSOL with IPaddr as source address
is received from a Validating port VP', different from the current
BINDING_ANCHOR for this binding, VP, the packet is discarded. The
SEND SAVI device MAY issue a secured NUD_NSOL through the
BINDING_ANCHOR port, store VP' in the ALTERNATIVE BINDING ANCHOR
for possible future use, set the LIFETIME to TENT_LT, and change
the state to TESTING_VP'. An alternative to this behavior is that
the SEND SAVI device MAY not do anything (in this case, the state
would eventually change after a maximum DEFAULT_LT time, if the
node at VP does not respond to a NUD_NSOL at TESTING_VP, the state
is moved to NO_BIND). Then a packet arriving from VP' would
trigger a process that may end up with binding for the node
connecting to VP'.
LIFETIME expires
o If LIFETIME expires, a secured NUD_NSOL message is sent through
the BINDING_ANCHOR port to IPaddr, the LIFETIME is set to TENT_LT,
and the state is changed to TESTING_VP. In the TESTING_VP state
packets are still being forwarded until the timer expires without
receiving a NUD_NADV.
TESTING_VP
When the SEND SAVI device enters in the TESTING_VP state, the current
Validating port is under check through a secured NUD_NSOL message
generated by the SEND SAVI device. While testing, packets from the
current Validating port are forwarded. Packets coming from Trusted
ports are also forwarded. The relevant events for this state are the
reception of a NUD_NADV message from VP, the reception of a DAD_NSOL
message from VP, VP' or TP, the reception of any packet other than
the previous cases from VP, VP' or TP, and the expiration of the
timer associated to the reception of NUD_NADV.
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Messages received from the BINDING_ANCHOR port
o If a validated NUD_NADV is received from VP, the LIFETIME is
changed to DEFAULT_LT, and the state is changed to VALID. The
message is not forwarded to any other port.
o If a validated DAD_NSOL message is received from VP, it is
forwarded to the appropriate Trusted ports, the LIFETIME is set to
DEFAULT_LT, and the state is changed to TENTATIVE_DAD.
o Any packet other than DAD_NSOL or NUD_NADV containing IPaddr as a
source address arriving from the BINDING_ANCHOR port is forwarded.
Neither the LIFETIME nor the state are changed.
Messages received from a trusted port
o If a DAD_NSOL packet is received from a Trusted port, the message
is forwarded to VP and the appropriate Trusted ports. Neither the
LIFETIME nor the state are changed. The node at the
BINDING_ANCHOR port is under check: if it still is at this port,
it should answer with a NUD_NADV, and also with a DAD_NADV. If it
is not there, neither the NUD_NADV nor the DAD_NADV will be
received, the timer will expire and the local state will move to
NO_BIND.
o If a packet other than a DAD_NSOL arrives from a Trusted port, the
packet is forwarded. Neither the LIFETIME nor the state are
changed.
Messages received from a validating port different from the
BINDING_ANCHOR
o If a valid DAD_NSOL is received from a Validating port VP' other
than the current BINDING_ANCHOR port, the message is forwarded to
the BINDING_ANCHOR port and to the appropriate Trusted ports. In
addition, a secured NUD_NSOL is sent to the BINDING_ANCHOR port,
the ALTERNATIVE BINDING ANCHOR is set to VP' (for future use if
the node at VP' is finally selected), the LIFETIME is set to
TENT_LT, and the state is changed to TESTING_VP'.
o Any other packet received from a Validating port VP' other than
the BINDING_ANCHOR port is discarded. This may occur because the
node has moved but have not issued a DAD_NSOL or the DAD_NSOL
message has been lost. The state will eventually move to NO_BIND,
and then the packets sent from VP' will trigger the creation of
the binding for VP'.
LIFETIME expires
o If the LIFETIME expires, the LIFETIME is cleared and the state is
changed to NO_BIND.
TESTING_VP'
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To arrive to this state an indication that a node at VP' different
from the BINDING_ANCHOR port wants to send data with IPaddr as source
address occurred while a binding existed for VP. The port VP' which
triggered the change of the state to TESTING_VP' was stored at the
ALTERNATIVE_BINDING_ANCHOR, so that it can be retrieved if the node
at VP' is determined as the legitimate owner of IPaddr. The SEND
SAVI device has issued a NUD_NSOL to IPaddr through the
BINDING_ANCHOR port. The relevant events that may occur in this case
are the reception of a NUD_NADV from port VP (the BINDING_ANCHOR
port), the reception of DAD_NSOL from VP, VP', TP and VP" (VP"
different from VP and VP'), the reception of any other packet from
VP, VP', TP or VP", and the expiration of the timer.
Messages received from the BINDING_ANCHOR port
o A validated NUD_NADV is received from the BINDING_ANCHOR port.
The reception of a valid NUD_NADV indicates that the node at VP is
defending its address. The BINDING_ANCHOR in use is kept, the
LIFETIME is set to DEFAULT_LT, and the state is changed to VALID.
o If a valid DAD_NSOL is received from the BINDING_ANCHOR port, it
is forwarded to VP' (the port stored in the
ALTERNATIVE_BINDING_ANCHOR). The BINDING_ANCHOR in use is kept,
the LIFETIME is set to TENT_LT and the state is changed to
TENTATIVE_DAD. When the DAD_NSOL message is received by the node
at VP', this node is expected to unconfigure its address.
o If a valid RADV is received from the BINDING_ANCHOR port, the
message is forwarded appropriately. Either an entry for this IPv6
source address in the SEND SAVI Router List is created, or the
lifetime of an existing entry is updated with the information
received in this message. The SEND SAVI Prefix list MUST also be
updated according to the content of the RADV message. The SEND
SAVI device MAY ignore and discard RADV messages addressed to
destinations other than the all-nodes multicast address or to the
IPv6 address of the SEND SAVI device. The state remains in
TESTING_VP' and the LIFETIME is left unchanged. Note that if the
timeout expires later, while still in the TESTING_VP' state, the
entry of the SEND SAVI Router List will also be removed.
o Any packet other than a validated DAD_NSOL, a validated NUD_NADV
or a validated RADV coming from the BINDING_ANCHOR port, is
forwarded, and the state is not changed.
Messages received from the ALTERNATIVE_BINDING_ANCHOR validating port
o If a valid DAD_NSOL is received from the port stored in the
ALTERNATIVE_BINDING_ANCHOR, it is forwarded to the BINDING_ANCHOR
port. The BINDING_ANCHOR and the ALTERNATIVE BINDING ANCHOR are
kept, the LIFETIME is set to DEFAULT_LT, and the state is not
changed.
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o Any packet other than a validated DAD_NSOL coming from the
ALTERNATIVE_BINDING_ANCHOR port is discarded, and the state is not
changed.
Messages received from a validating port different from the
BINDING_ANCHOR and the ALTERNATIVE_BINDING_ANCHOR ports
o If a validated DAD_NSOL is received from port VP", different from
BINDING_ANCHOR and the ALTERNATIVE_BINDING_ANCHOR ports, it is
forwarded to the BINDING_ANCHOR and the ALTERNATIVE_BINDING_ANCHOR
ports. The node at ALTERNATIVE BINDING ANCHOR port is expected to
unconfigure its address if the message triggering the transition
to this state was a DAD_NSOL message received from the
ALTERNATIVE_BINDING_ANCHOR port (and not any other packet). The
state remains in TESTING_VP' although VP" is stored in the
ALTERNATIVE_BINDING_ANCHOR for future use if the node at VP" is
finally selected. The LIFETIME is not changed.
o Any packet other than a validated DAD_NSOL received from port VP"
is discarded and does not affect to the state.
Messages received from a trusted port
o If a DAD_NSOL is received from a Trusted port, the message is
forwarded to the BINDING_ANCHOR and the ALTERNATIVE_BINDING_ANCHOR
ports and other appropriate Trusted ports. The LIFETIME is left
unchanged and the state is changed to TESTING_VP. The node at the
ALTERNATIVE_BINDING_ANCHOR port is expected to unconfigure its
address if the packet triggering the transition to this state was
a DAD_NSOL message received from the ALTERNATIVE_BINDING_ANCHOR
port.
o Any packet other than a DAD_NSOL coming from a Trusted port is
forwarded appropriately, but the state is not changed.
LIFETIME expires
o If LIFETIME expires, it is assumed that the node for which the
binding existed is no longer connected through the BINDING_ANCHOR
port. Therefore, the BINDING_ANCHOR is set to the
ALTERNATIVE_BINDING_ANCHOR port value. The LIFETIME is set to
DEFAULT_LT and the state is changed to VALID.
TENTATIVE_NUD
To arrive to this state, a data packet has been received through the
BINDING_ANCHOR port without any existing binding in the SEND SAVI
device. The SEND SAVI device has sent a NUD_NSOL message to the
BINDING_ANCHOR port. The relevant events for this case are the
reception of a NUD_NADV from port the BINDING_ANCHOR port; the
reception of DAD_NSOL from the BINDING_ANCHOR port, other VP
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different from the BINDING_ANCHOR port, or a TP; and the reception of
any packet other than DAD_NSOL and NUD_NADV from the BINDING_ANCHOR
port, and other than DAD_NSOL for other VP different from the
BINDING_ANCHOR port, or TP. In addition, the LIFETIME may expire.
Messages received from the BINDING_ANCHOR port
o If a validated NUD_NADV message is received through the
BINDING_ANCHOR port, the LIFETIME is set to TENT_LT, and the state
is changed to VALID. The message is not forwarded to any port.
o If a validated DAD_NSOL message is received through the
BINDING_ANCHOR port, it is forwarded to the appropriate Trusted
ports, the LIFETIME is set to TENT_LT and the state is changed to
TENTATIVE_DAD.
o Any packet other than NUD_NADV or DAD_NSOL received through the
BINDING_ANCHOR port is discarded.
Messages received from a validating port different from the
BINDING_ANCHOR
o If a validated DAD_NSOL message is received through port VP'
different from the BINDING_ANCHOR port, it is forwarded to the
appropriate Trusted ports, the LIFETIME is set to TENT_LT, the
BINDING_ANCHOR is set to VP', and the state is changed to
TENTATIVE_DAD.
o Any packet other than validated DAD_NSOL received through port VP'
MUST NOT be forwarded unless the next state for the binding is
VALID. The packets received MAY be discarded or MAY be stored for
being sent if the state changes later to VALID. The state is left
unchanged.
Messages received from a trusted port
o If a DAD_NSOL message is received through a Trusted port, it is
forwarded to the BINDING_ANCHOR port, and the state is left
unchanged.
o Any other packet received from a Trusted port is forwarded
appropriately. This packet may come from a SEND SAVI device that
has securely validated the attachment of the node to its
Validating port according to SEND SAVI rules. The state is left
unchanged.
LIFETIME expires
o If LIFETIME expires, the LIFETIME is cleared and the state is
changed to NO_BIND.
3.4. SEND SAVI Port Configuration Guidelines
The detailed guidelines for port configuration in SEND SAVI devices
are:
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o Ports that are connected to another SEND SAVI devices SHOULD be
configured as Trusted ports. Not doing so will increase
significantly the CPU time, memory consumption and signaling
traffic due to SEND SAVI validation, in both the SEND SAVI devices
and the node whose address is being validated.
o Ports connected to hosts SHOULD be configured as Validating ports.
Not doing so will allow the host connected to that port to send
packets with spoofed source address.
o No more than one host SHOULD be connected to each port.
Connecting more than one host to a port | will allow hosts to
generate packets with the same source address as the other hosts
connected to the same port, and will allow performing replaying
attacks as described in Section 5.1.
o Ports connected to routers SHOULD be configured as Validating
ports. However, the SEND SAVI specification also allows the
routers to be connected to Trusted ports, as they are assumed to
be part of the trusted infrastructure. When connected through a
Trusted port, a router can generate traffic with any source
address, even those belonging to the link, while when connected
through a Validating port it can only send traffic using off-link
source addresses, or its own source addresses. When routers are
connected to Validating ports, authorization for the routing
function is bound to the binding anchor of the router itself,
instead of being bound to a port configured in a switch, so in
this case changing the port through which a router attaches to the
SAVI protection perimeter does not require SEND SAVI-specific
configuration.
o Ports connected to a chain of one or more legacy switches that
have other SEND SAVI devices but had no routers or hosts attached
to them SHOULD be configured as Trusted ports. Not doing so will
significantly increase the memory consumption in the SEND SAVI
devices and increase the signaling traffic due to SEND SAVI
validation.
3.5. VLAN Support
In the case the SEND SAVI device is a switch that supports customer
VLANs [IEEE.802-1Q.2005], the SEND SAVI implementation MUST behave as
if there was one SEND SAVI process per customer VLAN. The SEND SAVI
process of each customer VLAN will store the binding information
corresponding the nodes attached to that particular customer VLAN.
3.6. Protocol Constants
TENT_LT is 500 milliseconds.
DEFAULT_LT is 5 minutes.
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4. Protocol Walkthrough
In this section we include two cases which illustrate the behavior of
SEND SAVI, the change of the attachment port of a host, and the
attack of a malicious host. We use the topology depicted in the
following figure.
+-1-----2-+ +-1-----2-+
| | | |
| SAVI1 | | SAVI2 |
| | | |
+-3-----4-+ +-3-----4-+
| |
-------------------
4.1. Change of the attachment point of a host
There are two cases, depending on if the switch to which H moves is
the same switch or a different one.
4.1.1. Moving to a port of the same switch
Host H is connected to port 1 of SAVI1 and moves to port 2 of the
same switch. Before moving, the SEND SAVI state associated to IPH,
the IP address of H is
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
In the general case, H issues a DAD_NSOL message for IPH when it is
connected to a different port. When SAVI1 receives this message, it
validates it and changes its state to
SAVI1=TESTING_VP', BINDING_ANCHOR=1, ALTERNATIVE_BINDING_ANCHOR=2,
TIMER=TENT_LT / SAVI2=NO_BIND
The DAD_NSOL message is propagated to port 1, because it is the
current BINDING_ANCHOR, and the trusted port 3; but it is not
propagated to Validating port 4. SAVI1 configures a timer for
TENT_LT seconds. In addition, SAVI1 generates a NUD_NSOL and sends
it through port 1. When SAVI2 receives this message through it
trusted port, it discards it and remains in the NO_BIND state.
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SAVI1 waits for a NUD_NADV message being received from port 1. Since
there is no node attached to 1, there is no response for neither of
these messages. When TENT_LT expires at SAVI1, the state changes to
SAVI1=VALID, BINDING_ANCHOR=2 / SAVI2=NO_BIND
If the node moving does not issue a DAD_NSOL when it attaches to port
2, then SAVI1 will receive a data packet through this port. The data
packet is discarded, SAVI1 issues a secured NUD_NSOL through port 1,
and changes the state to TESTING_VP'.
SAVI1=TESTING_VP', BINDING_ANCHOR=1, ALTERNATIVE_BINDING_ANCHOR=2
TIMER=TENT_LT / SAVI2=NO_BIND
SAVI1 waits for a NUD_NADV message being received from port 1. Since
there is no node attached to 1, there is no response for neither of
these messages. When TENT_LT expires at SAVI1, the state changes to
SAVI1=VALID, BINDING_ANCHOR=2 / SAVI2=NO_BIND
An alternative behavior allowed by the specification for the case in
which the host does not issue a DAD_NSOL is that SAVI1 does nothing.
In this case, after some time (bounded by DEFAULT_LT), the switch
will change the state for IPH to TESTING_VP, check if H is still at
port 1 (which is not), and move the state to NO_BIND. Then, a packet
arriving from port 2 would trigger a process that finishes with a
VALID stated with BINDING_ANCHOR=2.
4.1.2. Moving to a port of a different switch
Host H, connected to port 1 of SAVI1, moves to port 4 of SAVI2.
Before moving, the SEND SAVI state associated to IPH, the IP address
of H is
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
If H issues a DAD_NSOL message for IPH when it connects to port 4 of
SAVI2, the state is changed to
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=TENTATIVE_DAD,
BINDING_ANCHOR=4, TIMER=TENT_LT
The DAD_NSOL message is propagated only through the trusted port of
SAVI2. Then, SAVI1 changes its state as follows:
SAVI1=TESTING_VP, BINDING_ANCHOR=1, TIMER=TENT_LT /
SAVI2=TENTATIVE_DAD, BINDING_ANCHOR=4, TIMER=TENT_LT
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SAVI1 propagates the DAD_NSOL message to port 1. Since he only node
which can answer with a secured DAD_NUD has moved, the timer at SAVI2
expires, and SAVI2 changes its state to VALID:
SAVI1=TESTING_VP, BINDING_ANCHOR=1, TIMER=TENT_LT / SAVI2=VALID,
BINDING_ANCHOR=4
Just a very short time after, the timer at SAVI1 expires, and the
state changes to NO_BIND.
SAVI1=NO_BIND / SAVI2=VALID, BINDING_ANCHOR=4
If host H does not send a DAD_NSOL when it moves to SAVI2, but a data
packet, SAVI2 changes its state to TENTATIVE_NUD.
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=TENTATIVE_NUD,
BINDING_ANCHOR=4, TIMER=TENT_LT
SAVI2 issues a secured NUD_NSOL through port 4. H is assumed to have
the address configured (otherwise it should not have generated a data
packet), so it can respond with a NUD_NADV. When SAVI1 receives the
NUD_NADV and validates it, the state is changed to VALID
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=VALID, BINDING_ANCHOR=4
After some time (bounded by DEFAULT_LT), the state in SAVI1 will
expire, and SAVI1 will perform a check for host H.
SAVI1=TESTING_VP, BINDING_ANCHOR=1, TIMER=TENT_LT / SAVI2=VALID,
BINDING_ANCHOR=4
SAVI1 issues a NUD_NSOL through port 1 for IPH. No response is
received in this case, so SAVI1 changes its state to NO_BIND
SAVI1=NO_BIND / SAVI2=VALID, BINDING_ANCHOR=4
4.2. Attack of a malicious host
Host H is attached to the SEND SAVI infrastructure through port 1 of
SAVII1. We consider that host M starts sending data packets using
IPH (the IP address of H) as source address, without issuing a
DAD_NSOL (a similar analysis can be done for this case).
4.2.1. M attaches to the same switch as the victim's switch
The initial state before the attack of M is:
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
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M attaches to port 2 of SAVI1, and starts sending data packets. When
SAVI1 receives the data packet, the packet is discarded. SEND SAVI
may issue a secured NUD_NSOL through port 1, and changes the state to
SAVI1=TESTING_VP', BINDING_ANCHOR=1, ALTERNATIVE_BINDING_ANCHOR=2,
TIMER=TENT_LT / SAVI2=NO_BIND
Host H is still attached to port 1, so it receives the NUD_NSOL and
responds with a secured NUD_NADV. SAVI1 receives this message,
validates it and changes its state again to
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
To prevent the drain of CPU resources in SAVI1, the processing of
further packets received from port 2 may be rate-limited, as
discussed in Section 5.2.
An alternative to the previous behavior is that SAVI1 does nothing
when node M starts sending packets from port 2. In this case, when
the timer to renew the state triggers (this time is bounded by
DEFAULT_LT), SAVI1 moves the state to TESTING_VP, sends a NUD_NSOL
through port 1, host H responds, and the state remains in VALID for
BINDING_ANCHOR=1. In this way, communication of host H is also
defended.
4.2.2. M attaches to a different switch to the victim's switch
The initial state before the attack of M is:
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
M attaches to port 2 of SAVI2, and starts sending data packets. When
SAVI2 receives the data packet, it changes the state to
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=TENTATIVE_DAD,
BINDING_ANCHOR=2, TIMER=TENT_LT
SAVI2 issues a secured NUD_NSOL through port 2. Since M does not own
the IPH CGA, it cannot respond to the message. When the timer
expires, the state is moved back to:
SAVI1=VALID, BINDING_ANCHOR=1 / SAVI2=NO_BIND
To prevent the drain of CPU resources in SAVI2, the processing of
further packets received from port 2 may be rate-limited, as
discussed in Section 5.2.
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5. Security Considerations
SEND SAVI is defined to operate only with validated SEND messages.
The interaction in a mixed scenario comprising SEND and non-SEND
devices should be addressed in other document. However, nodes MUST
NOT assume that all SEND messages received from a SEND SAVI device
are validated, since these devices only validate the messages
strictly required for SEND SAVI operation. Among the number of
messages which are not validated, we can name NUD_NSOL messages
generated by other nodes and its corresponding NUD_NADV responses, or
RSOL messages.
SEND SAVI improves protection compared to conventional SAVI, as a
result of the increased ability of SEND nodes to prove address
ownership.
A critical security consideration regarding to SEND SAVI deals with
the need of proper configuration of the roles of the ports in a SEND
SAVI deployment scenario. Regarding to security, the main
requirement is that ports defining the protected perimeter SHOULD be
configured as Validating ports. Not doing so will allow an attacker
sending packets using any source address, regardless of the bindings
established in other SEND SAVI devices.
5.1. Protection Against Replay Attacks
One possible concern about SEND SAVI is its behavior when an attacker
tries to forge the identity of a legitimate node by replaying SEND
messages used by the SEND SAVI specification. An attacker could
replay any of these messages to interfere with SEND SAVI operation.
For example, it could replay a DAD_NSOL message to abort the
configuration of an address for a legitimate node and to gain the
right to use the address for DEFAULT_LT seconds.
There are two different cases to analyse when considering SEND SAVI
reply attacks:
o When the SEND message replayed is used to create or update binding
information for SEND SAVI, since the port through which this
message is received is key to SEND SAVI operation. SEND SAVI
creates and maintains bindings as a result of the reception of
DAD_NSOL messages and of the exchange of NUD_NSOL/NUD_NADV
messages.
o When the SEND message replayed does not result in the update of
binding information for SEND SAVI, and thus it is not related to
the specific port through which it was received. Such situations
are the reception of CPA messages containing certificates, and the
processing of an RADV message coming from a Trusted port, which
can be used in SEND SAVI to populate the SEND SAVI Prefix list.
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In this two cases, the security risks are equivalent to those of
SEND operation, i.e., we can consider that the information will
not be changed by its legitimate sender for the time during which
the SEND specification allows replaying (which depends on the
values of TIMESTAMP_FUZZ and TIMESTAMP_DRIFT, [RFC3971]).
A special case is the processing of a RADV message coming from a
Validating port. Although part of the information obtained (the
router condition of the node connecting to the port) is
(indirectly) associated to the binding, the replay of this RADV
message does not provide an advantage to an attacker. This is so
because SEND SAVI requires a binding to exist (between the IPv6
address and the port of the SEND SAVI device) prior to consider
the RADV message, so protecting the creation of the binding also
protects the ability of an attacker to become a router.
For replay of messages belonging to the second case, i.e., messages
which does not result in changes in the SEND SAVI binding
information, the security provided by SEND is sufficient. For the
replay of messages belonging to the first case, DAD_NSOL and
NUD_NSOL/NUD_NADV messages, protection results from the behavior of
SEND SAVI, specified in Section 3.3.2, which restricts the ports to
which the messages involved in SEND SAVI binding updates are
disseminated. SEND SAVI devices only forward these messages to ports
for which a binding to the address being tested by the DAD_NSOL
message existed. Therefore, it is not enough for an attacker to
subscribe to a Solicited Node address to receive DAD_NSOL messages
sent to that address, but the attacker needs to generate a valid
DAD_NSOL message associated to the address for which the binding is
being tested, which is deemed unfeasible [RFC3971].
5.2. Protection Against Denial of Service Attacks
The attacks against the SEND SAVI device basically consist of making
the SEND SAVI device consume its resources until it runs out of them.
For instance, a possible attack would be to send packets with
different source addresses, making the SEND SAVI device create state
for each of the addresses and waste memory. At some point, the SEND
SAVI device runs out of memory and needs to decide how to react. The
result is that some form of garbage collection is needed to prune the
entries. When the SEND SAVI device runs out of the memory allocated
for the SEND SAVI Data Base, it is RECOMMENDED that it create new
entries by deleting the entries with a higher Creation time. This
implies that older entries are preserved and newer entries overwrite
each other. In an attack scenario where the attacker sends a batch
of data packets with different source addresses, each new source
address is likely to rewrite another source address created by the
attack itself. It should be noted that entries are also garbage
collected using the DEFAULT_LT, which is updated by NUD_NSOL/NUD_NADV
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exchange. The result is that in order for an attacker to actually
fill the FCFS SAVI Data Base with false source addresses, it needs to
continuously answer to NUD_NSOL for all the different source
addresses so that the entries grow old and compete with the
legitimate entries. The result is that the cost of the attack is
highly increased for the attacker.
In addition, it is also RECOMMENDED that a SEND SAVI device reserves
a minimum amount of memory for each available port (in the case where
the port is used as part of the L2 anchor). The recommended minimum
is the memory needed to store 4 bindings associated to the port. The
motivation for this recommendation is as follows. An attacker
attached to a given port of a SEND SAVI device may attempt to launch
a DoS attack towards the SEND SAVI device by creating many bindings
for different addresses. It can do so, by sending DAD_NSOL for
different addresses. The result is that the attack will consume all
the memory available in the SEND SAVI device. The above
recommendation aims to reserve a minimum amount of memory per port,
so that nodes located in different ports can make use of the reserved
memory for their port even if a DoS attack is occurring in a
different port.
As the SEND SAVI device may store data packets while the address is
being verified, the memory for data packet storage may also be a
target of DoS attacks. The effects of such attacks may be limited to
the lack of capacity to store new data packets. The effect of such
attack will be then that data packets will be dropped during the
verification period. A SEND SAVI device MUST limit the amount of
memory used to store data packets, allowing the other functions to
have available memory even in the case of an attacks such those
described above.
It is worth to note that the potential of Denial of Service attacks
against the SEND SAVI network is increased due to the use of costly
cryptographic operations in order to validate the address of the
nodes. An attacker could generate packets using new source addresses
in order to make the closest SEND SAVI device spend CPU time to
validate DAD_NSOL messages or to generate a secure NUD_NSOL. This
attack can be used to drain CPU resources of SEND SAVI devices with a
very low cost for the attacker. In order to solve this problem,
rate-limiting the processing of packets which may trigger SEND SAVI
events SHOULD be enforced in a per-port basis.
5.3. Residual threats
SEND SAVI assumes that a host will be able to defend its address when
the DAD procedure is executed for its addresses, and that it will
answer to a NUD_NSOL with a NUD_NADV when required. This is needed,
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among other things, to support mobility within a link (i.e., to allow
a host to detach and reconnect to a different Layer_2 anchor of the
same IP subnetwork, without changing its IP address). If the SEND
SAVI device does not see the DAD_NADV or the NUD_NADV, it may grant
the binding to a different binding anchor. This means that if an
attacker manages to prevent a host from defending its source address,
it will be able to destroy the existing binding and create a new one,
with a different binding anchor. An attacker may do so for example
by launching a DoS attack to the host that will prevent it to issue
proper replies.
5.4. Privacy considerations
A SEND SAVI MUST NOT log binding anchor information except where
there is an identified reason why that information is likely to be
involved in detection, prevention or tracing of actual source address
spoofing. Information that is not logged MUST be deleted as soon as
possible (i.e., as soon as the state for a given address is back to
NO_BIND). Information about the majority of hosts that never spoof
SHOULD NOT be logged.
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgments
Thanks to Jean-Michel Combes and Ana Kukec for their review and
comments on this document. The text has also benefited from feedback
provided by Tony Cheneau and Greg Daley.
Marcelo Bagnulo is partly funded by Trilogy, a research project
supported by the European Commission under its Seventh Framework
Program.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
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[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[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.
8.2. Informative References
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[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-06 (work in progress),
January 2012.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
[IEEE.802-1Q.2005]
Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and metropolitan area networks /
Virtual Bridged Local Area Networks", IEEE Standard
802.1Q, May 2005.
Authors' Addresses
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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Alberto Garcia-Martinez
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6248782
Email: alberto@it.uc3m.es
URI: http://www.it.uc3m.es
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