Network Working Group M. Bagnulo
Internet-Draft A. Garcia-Martinez
Intended status: Standards Track UC3M
Expires: April 28, 2011 October 25, 2010
SEND-based Source-Address Validation Implementation
draft-ietf-savi-send-04
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
higher granularity on the control of the source addresses used.
Status of this Memo
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This Internet-Draft will expire on April 28, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Non-normative Background to SEND SAVI . . . . . . . . . . . . 3
2.1. Address Validation Scope . . . . . . . . . . . . . . . . . 3
2.2. SEND SAVI Enforcement Perimeter . . . . . . . . . . . . . 4
2.3. Binding Creation for SEND SAVI . . . . . . . . . . . . . . 5
3. Perimeter Configuration Guidelines for SEND SAVI . . . . . . . 6
4. SEND SAVI Specification . . . . . . . . . . . . . . . . . . . 9
4.1. SEND SAVI Data Structures . . . . . . . . . . . . . . . . 9
4.2. SEND SAVI Device Configuration . . . . . . . . . . . . . . 11
4.3. SEND SAVI Algorithm . . . . . . . . . . . . . . . . . . . 11
4.3.1. Traffic Processing . . . . . . . . . . . . . . . . . . 11
4.4. VLAN Support . . . . . . . . . . . . . . . . . . . . . . . 22
4.5. Protocol Constants . . . . . . . . . . . . . . . . . . . . 22
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
7. Normative References . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
This memo describes SEND SAVI (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 higher granularity on the control of the source addresses
used.
SEND SAVI uses DAD_NSOL (Duplicate Address Detection Neighbor
Solicitation), DAD_NADV (DAD Neighbor Advertisement), NUD_NSOL
(Neighbor Unreachability Detection NSOL) and NUD_NADV (NUD Neighbor
Advertisement) messages to validate the address ownership claim of a
node. Ports and other layer-2 binding anchors can be associated to
the IPv6 address of the neighbor, so that source address validation
could be performed.
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" and 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 in which every IPv6 host and every IPv6 router uses the SEND
protocol [RFC3971] to protect the exchange of Neighbor Discovery
information. However, SEND SAVI is designed to be deployed in
existing SEND networks requiring a minimum set of changes. In
particular, SEND SAVI does not require any changes in the hosts whose
source address is to be verified. Any verification must solely rely
in the usage of already available protocols. This means that SEND
SAVI does neither define a new protocol, nor define any new message
on existing protocols, nor require that a host uses an existent
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].
2. Non-normative 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
address of the packets generated by the hosts attached to the local
link have not been spoofed.
In a link there usually are hosts and routers attached. Hosts
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generate packets with their own addresses as the source address.
This is the so-called local traffic, while routers send packets
containing a source address other than their own, since they are
forwarding 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 address of the
packets generated by the hosts attached to the local link has 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 act as one, SEND SAVI also provides the means to verify that
packets containing off-link prefixes in the source address are
forwarded by authorized routers. However, SEND SAVI does not provide
the means to verify if a given router is actually authorized to
forward packets containing a specific off-link source address. Other
techniques, like ingress filtering [RFC2827], are recommended to
validate transit traffic. Hence, the security level is increased by
the use of both SAVI and ingress filtering.
2.2. SEND SAVI Enforcement Perimeter
SAVI devices prevent address spoofing by verifying that a layer-2
anchor is associated to the IPv6 address used as source address for
the packets being exchanged. The layer-2 anchor, which must be
difficult to spoof can be the port of the layer-2 switch through
which a packet containing a given IPv6 address is received, or a
layer-2 address. In this document we assume that MAC-specific
mechanisms to secure data packets, such as IEEE 802.1AE, are not
generally available, so SEND SAVI is defined to operate with ports as
the only available layer-2 anchor.
In order to reduce computing and state requirements in SEND SAVI
devices, SEND SAVI is designed according to the perimetrical
protection deployment model presented in the SAVI framework document
[I-D.ietf-savi-framework]. In this model, source address validation
is performed only when packets enter in a protected realm defined
through the protection perimeter. This perimeter must be deployed in
such a way that packets for which validation must be performed can
only enter in the protected realm through a port belonging to the
border performing the validation. The perimeter is defined by
appropriate configuration of the roles of each port, which can be
Validating ports and Trusted ports. Validating ports are the ports
forming the protection perimeter, so they are the ports in which
validation for incoming packets is performed. Trusted ports (TPs)
are those in which SEND SAVI filtering is not performed.
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2.3. Binding Creation for SEND SAVI
Filtering is performed according to the bindings existing between a
layer-2 anchor and an IPv6 address. These bindings should allow
legitimate nodes to use the binding IPv6 address as source address,
and prevent illegitimate nodes to do so. When a protection perimeter
is defined, the binding must be created for a port of the border to
which a legitimate node is attached to, and must not be created in
other case.
SEND provides tools to assure that a ND message containing a CGA
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 RADV message signed by a RSA option with a key bounded
to a CGA or a certificate has been generated by a legitimate router.
SEND SAVI benefits from SEND ability to prove address ownership and
router authorization to create SAVI bindings. SEND SAVI assumes that
a successfully validated SEND message ingressing to the protection
perimeter from a validating port guarantees that the host
legitimatelly issuing the message is connected to that port. In this
case, a binding for the host to this layer-2 port is created.
The events that trigger the binding creation process in a Validating
port of a SEND SAVI device are:
o The reception from a Validating port of a DAD_NSOL message,
indicating the attempt of a node to configure an address. This
may occur when a node configure an address after being idle for
sometime, or because the node has changed the physical attachment
point to the layer-2 infrastructure.
o The reception from a Validating port of any other packet
(including data packets) with a source address for which no
binding exists. This would occur if a DAD_NSOL message was lost
before arriving to the Validating port, or if a node has changed
the physical attachment point to the layer-2 infrastructure
without issuing a DAD_NSOL message.
When the binding creation process is triggered, the SEND SAVI device
has to assure that the host for which the binding is to be created is
the legitimate owner of the address. For a binding creation process
initiated by a DAD_NSOL exchange, the messages to consider for
address ownership validation are other DAD_NSOL messages arriving
from other locations or a DAD_NADV message indicating that other host
has configured the address before. For other packets initiating the
creation of the binding, the SEND SAVI device asks the host to prove
address ownership by issuing a NUD_NSOL which has to be answered by a
NUD_NADV by the probed node. Note that it is not required to ask
other SEND SAVI devices, as it is done in the non-SEND FCFS
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specification [I-D.ietf-savi-fcfs], since in this case a SEND host
can prove authoritatively the ownership of its address.
Bindings are refreshed periodically by means of a NUD_NSOL message
issued by the SEND SAVI device through the bounded port which has to
be answered by a valid NUD_NADV message by the node for which the
binding exist.
SEND SAVI could be sensible to replay attacks, i.e. situations in
which a secured SEND message is replayed by a non-legitimate node.
For example, a node could immediatelly re-inject a valid SEND message
being received from other node, to force the creation of a binding
for which it is not authorized. SEND provides some means to prevent
the replaying of ND messages, in particular, the use of nonces to
validate advertisements that were previously solicited, and the use
of timestamps to validate solicitation messages and unsolicited
advertisements. However, the emphasis for SEND anti-replay
protection is to assure that confidence in some information (for
example, the relationship between an IPv6 address and a layer-2
address) is not hold for more time than reasonable, while in SEND
SAVI truthful information (in SEND sense, like the relationship of an
IPv6 address and a layer-2 address) can be used to create a SAVI
binding in a time span shorter than the time reasonable to consider
the information aged. As a consequence, SEND SAVI relies only in
messages with a low chance of being replayed from different ports to
the legitimate one and still being considered valid by SEND. The
messages being used by SEND SAVI to create bindings are:
o Unsolicited DAD_NSOL messages. According to the SEND SAVI
specification Section 4.3.1.1 These messages can only be forwarded
to ports through which a previous binding for the same IPv6
address existed.
o NUD_NADV messages in response to a NUD_NSOL sent by the SEND SAVI
device, both exchanged through the same Validating port. In this
case, anti-replay protection is assured by nonce exchange. This
message exchange is also used to refresh the binding.
Any validated RADV can be used to determine that a node for which a
binding exists in a Validating port is a router, since the
topological part of the binding has been assured before. In
addition, the acquisition of prefix information, required to
determine local and transit traffic, is not tied to topological
considerations too, so for this case regular SEND validating rules
are applied.
3. Perimeter Configuration Guidelines for SEND SAVI
As it has been discussed before, the perimeter is defined by
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appropriate port configuration. Ports in SEND SAVI devices may
assume two roles according to its behavior when filtering and
validating SEND messages: Validating ports and Trusted ports.
o Validating ports (VPs) are those in which SEND SAVI filtering and
binding creation is performed.
o Trusted ports (TPs) are those in which neither SEND SAVI filtering
nor binding creation are performed. So, packets received through
Trusted ports are not filtered by SEND SAVI. The only SEND
messages received through a Trusted port which are processed are
those related with certificates, prefix information and Neighbor
Advertisements for Duplicate Address Detection (DAD_NADV).
The following figure shows a typical topology involving trusted and
untrusted infrastructure.
+--------+
+--+ +--+ +--+ | +--+ |
|H1| |H2| |H3| | |R1| |
+--+ +--+ +--+ | +--+ |
| | | | | |
+----------SEND SAVI-ENFORCEMENT-PERIMETER---+ |
| | | | | |
| +-1-----2-+ +-1-----2-+ |
| | SEND- | | SEND- | |
| | SAVI1 | | SAVI2 | |
| +-3--4----+ +--3------+ |
| | | +--------------+ | |
| | +----------| |--------+ |
| | | SWITCH-A | |
| | +----------| |--------+ |
| | | +--------------+ | |
| +-1--2----+ +--1------+ |
| | SEND- | | SEND- | |
| | SAVI3 | | SAVI4 | |
| +-3-----4-+ +----4----+ |
| | | | |
| +----SEND SAVI-ENFORCEMENT-PERIMETER----------+
| | | | |
| +--+ | +--+ +---------+
| |R2| | |H4| |SWITCH-B |
| +--+ | +--+ +---------+
| | | |
+-------+ +--+ +--+
|H5| |H6|
+--+ +--+
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Trusted ports are used for connections with trusted infrastructure,
including the communication between SEND SAVI devices, the
communication with other switches which are not SEND SAVI devices,
routers or other trusted nodes.
Port 3 of SEND-SAVI1 and port 1 of SEND-SAVI3 are trusted because the
connect two SAVI devices. Port 4 of SEND-SAVI1, port 3 of SEND-
SAVI2, port 2 of SEND-SAVI3 and port 1 of SEND-SAVI4 are trusted
because they connect to SWITCH-A to which only trusted nodes are
connected. Port 2 of SEND-SAVI2 and port 3 of SEND-SAVI3 are trusted
ports, because they connect to routers.
Validating ports are used for connection with non-trusted
infrastructure. Therefore, hosts are normally connected to
validating ports. Non-SEND SAVI switches that are outside of the
SAVI enforcement perimeter also are connected through validating
port. In particular, non-SEND SAVI devices which connect directly to
hosts or which have no SEND SAVI capable device between themselves
and the hosts are connected through a validating port. 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 4 of SEND-SAVI4 is also a validating port because it is
connected to SWITCH-B which is a non- SEND SAVI capable switch which
is connected to hosts H5 and H6.
SEND SAVI requires all devices performing SAVI function to implement
SEND SAVI (for example, coexistence with non-SEND aware FCFS SAVI
[I-D.ietf-savi-fcfs] switches is not allowed).
The detailed guidelines for port configuration in SEND SAVI devices
are:
o Ports that are connected to another SEND SAVI device SHOULD be
configured as Trusted ports. Not doing so will at least 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 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, authorization for the routing function is
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bound to the router itself, instead of being bound to a port
configured in a switch.
o Ports connected to a chain of one or more legacy switches that
have hosts connected SHOULD be configured as Validating ports.
Not doing so will allow the host connected to any of these
switches to send packets with spoofed source address.
o Ports connected to a chain of one or more legacy switches that
have other SEND SAVI devices and/or routers connected but had no
hosts attached to them SHOULD be configured as Trusted ports. Not
doing so will at least significantly increase the memory
consumption in the SEND SAVI devices and increase the signaling
traffic due to SEND SAVI validation.
o Ports connected to a chain of one or more legacy switches that
have a mix of SEND SAVI devices and/or routers with 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.
4. SEND SAVI Specification
4.1. SEND SAVI Data Structures
The following data structures are defined for SEND SAVI operation:
SEND SAVI Port list. This structure defines an entry per port in the
SAVI device. Each entry indicates the role configured for the port
(Trusted port or Validating port). In addition, only for Validating
ports, the entry indicates the presence or absence of a router
connected through the port has been stated by successful validation
of a RADV message received from this port. This data structure is
used to determine the filtering behavior for each port when local-
link and off-link traffic is received. If the port is a Trusted
Port, both local-link and off-link traffic coming from the port is
accepted. If the port is a Validating port but not a Routing port,
then only local-link traffic coming from the port for which a binding
exists is accepted. If the port is a Validating port and a Routing
port, then off-link traffic coming from the port is accepted, but
only local-link traffic coming from the port for which a binding
exists is accepted.
Each entry of the SEND SAVI Port list contains the following
information:
o Layer-2 Validating port
o Configured port role (either Trusted port or Validating port).
The default configuration is Validating port.
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o Router port bit. This value is only meaningful for ports with a
configured port role set to Validating port. It indicates whether
a RADV message received from the port has been successfully
validated, indicating that a router is connected to the port. For
this bit to be set, an entry containing this layer-2 port MUST
exist in the SEND SAVI Address table for an IP address of an entry
in the SEND SAVI Router table.
SEND SAVI Address list. The SEND SAVI function relies on state
information binding the source IPv6 address used in data packets to
the port through which the legitimate host connects. Such
information is stored in SEND SAVI Address table. The SEND SAVI
Address table contains one entry for each of the IPv6 source
addresses in use on a Validating port of the SEND SAVI device. The
SEND SAVI Address list is populated with the contents of successfully
validated SEND messages. Each entry contains the following
information:
o IP source address
o Layer-2 Validating port to which the host is connected.
o Lifetime
o Status: TENTATIVE_DAD, TENTATIVE_NUD, VALID, TESTING_VP,
TESTING_VP'.
SEND SAVI Prefix list. In addition to this, a SEND SAVI device needs
to know which are the link prefixes in order to identify local and
off-link traffic. This information is obtained from validated RADV
messages. This information is not specific to a given port. Note
that the information in this table is equivalent to the Prefix List
conceptual data structure defined in [RFC4861]. The SEND SAVI Prefix
list contains one entry per prefix in use, as follows:
o Prefix
o Lifetime
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. In particular, it contains the address for which a
successfully validated RADV has been received. The information in
this table is used to populate the SEND SAVI port table when at least
one router has been validated in a layer-2 Validating port (the
layer-2 port can be obtained by looking-up for the IPv6 address of
the router in the SEND SAVI Address list). It can also be used to
issue a RSOL in case the entry is about to expire, in order to ensure
that the node is still performing as a router. Note that the
information in this table is equivalent to the Default Router List
conceptual data structure defined in [RFC4861]. The information
stored in the table is the following:
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o Router IPv6 address
o Lifetime
4.2. SEND SAVI Device Configuration
In order to perform SEND SAVI operation, some basic parameters of a
SEND SAVI device have to be configured.
A SEND SAVI device operates as a full-fledged SEND node in some
cases: it may generate NUD_NSOL, RSOL or CPS messages. Therefore, a
SEND SAVI device
o MUST be configured with a valid CGA address. Note that when the
SEND SAVI device configures this address, it must follow the same
rules as regular SEND hosts (such as using secured NSOL messages
to perform DAD, etc.)
o MUST be configured with at least one Trust anchor to validate the
Certification Paths that authorizes route operation.
o MUST be configured with Certification Paths, either manually or 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 Port list
SHOULD be configured. Otherwise, every port would be labeled as
Validating port, and performance may be degraded, as discussed in
[I-D.ietf-savi-framework].
4.3. SEND SAVI Algorithm
4.3.1. Traffic Processing
In this section we describe how packets are processed.
First, the source address of packet is analysed to determine if it is
local or transit traffic, by checking if the prefix of the source
address is included in the SEND SAVI Prefix List (local traffic) or
not included (transit traffic). A special case of local traffic is
the traffic destined to the SEND SAVI device itself, either
specifically, or through a multicast address to which the SEND SAVI
device is registered (such as the all-nodes address, ff02::1).
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 appears with the
Routing bit set in the SEND SAVI Port list, indicating that a
router has been validated through SEND procedures at this port.
If transit traffic is received from a Validating port, and the
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port does not appear with the Routing bit set in the SEND SAVI
Port list, the SAVI SEND device SHOULD send a RSOL message through
the considered port.
Processing of traffic addressed to the SEND SAVI device itself occurs
as follows:
o Packets received from Trusted ports are not filtered. In
particular, if a successfully validated CPA message is received
through a Trusted port, the certificate information is accepted by
the SEND SAVI device. If a successfully validated RADV message is
received through a Trusted port, the SEND SAVI Prefix list in the
SEND SAVI device is updated accordingly.
o NUD_NADV messages corresponding to SEND SAVI operation are
processed according to the specification of Section 4.3.1.1.
o Packets received from Validating ports are only processed by the
SEND SAVI device if a binding exists for the source IPv6 address
of the packet, and the state for the binding is VALID or TESTING
(see next section). In particular, If a successfully validated
RADV message is received through a Trusted port, the SEND SAVI
Prefix list in the SEND SAVI device is updated accordingly. The
Router bit of the SEND SAVI Port list is set only if the
destination address of the RADV message is not a multicast
address. If a SEND SAVI device receives a RADV sent to a
multicast address, it SHOULD issue a RSOL message to the port
through which this message has been received.
We next consider how local traffic is processed.
4.3.1.1. Processing of Local Traffic
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.
For the rest of the section, the following assumptions hold:
o When it is stated that a secured NUD_NSOL message is issued by a
SEND SAVI device through a given port, this means the following:
the SEND SAVI device performs a Neighbor Unreachability Detection
procedure as described in [RFC4861] with SEND secured messages as
defined in [RFC3971] addressed to the IPv6 target address (source
address of the packet triggering the procedure). The source
address used for issuing the NUD_NSOL is the source address of the
SEND SAVI device.
o When it is stated that a validated NUD_NADV message is received by
a SEND SAVI device through a port P, this means that: a SEND
secured NUD_NADV message has been received by the same port
through which the corresponding NUD_NSOL message was issued, and
the NUD_NADV message has been validated according to [RFC3971] to
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prove ownership for the IPv6 address under consideration, and
being 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 for Trusted port.
The state machine is defined for a binding of a given source IP
address in a given SAVI device. In the transitions considered,
packets described as inputs refer to the IPaddr IPv6 address
associated to the state machine.
The possible states are
o NO_BIND. This state represents that no binding exists for the
address. This is the state for all addresses unless a binding is
explicitly created.
o TENTATIVE_DAD. This state is reached when the SEND SAVI device
has received a validated DAD_NSOL message. The SEND SAVI device
waits for a possible DAD_NADV. Packets with the source address of
the binding are not forwarded.
o TENTATIVE_NUD. A packet different from a valid DAD_NSOL message
has been received from port VP and the SEND SAVI device has sent a
NUD_NSOL message to the port. Packets with the source address of
the binding are not forwarded.
o VALID. The binding for the source address has been verified.
Packets with the source address of the binding are forwarded.
o TESTING_VP. The lifetime of the binding has expired so SEND SAVI
device has sent a NUD_NSOL message to the port, or a DAD_NSOL
coming from other SEND SAVI device has been received. The SEND
SAVI device waits for a validated NADV. Packets with the source
address of the binding are allowed to be forwarded.
o TESTING_VP'. A validated DAD_NSOL message has been received from
a Validating port of the SEND SAVI device. The device waits for a
DAD_NADV coming from port VP, or changes the binding to port VP'
if no response is received after TENT_LT milliseconds. Packets
coming from port VP with the source address of the binding are
allowed to be forwarded.
The states can be classified into forwarding states, i.e. states in
which packets coming for the port associated to the IPv6 address
different to the ones used for signalling are forwarded (VALID,
TESTING_VP and TESTING_VP'), and non-forwarding states, i.e. states
in which packets coming from the port associated to the IPv6 address
different to the ones used for signalling are not forwarded (NO_BIND,
TENTATIVE_DAD and TENTATIVE_NUD).
The state machine defined for SEND SAVI operation adheres to the
following design guidelines:
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o The only events which triggers 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.
Besides, DAD_NADV and NUD_NADV are only processed when expected as
a response to a DAD_NSOL or a NUD_NSOL message. The other
possible input to consider is 'any other packet', which could
generate changes to states belonging to the same class as the
original state (i.e. when 'any other packet' is received, the
state cannot move from being forwarding to non-forwarding and vice
versa). Note that non-validated SEND messages always belong to
the 'any other packet' cathegory. The reduced set of messages
being able to trigger a change simplifies the processing at SEND
SAVI devices. It is also convenient for defining a comprehensive
model regarding to anti-replay protection.
o The SEND SAVI device is only required to generate NUD_NSOL
messages for SEND SAVI operation. This also simplifies the state
machine.
o Well-behaved hosts are expected to initiate communication by
sending secured DAD_NSOL messages. The SEND SAVI state machine is
designed to process these events in an optimal way. The reception
of other packet types without receiving previously validated
DAD_NSOL messages is assumed to be the result of either bad-
behaving hosts or the lost of packets. While these events may
occur and a binding will ultimately be created for such hosts, the
case in which data packets are received without receiving
previously a DAD_NSOL message is not always optimized, for the
sake of simplicity of the state machine. It is also worth to note
that a validated DAD_NSOL provides a reliable hint about the
address ownership of a host attached to a given port, while this
is not the case for data packets, for example.
o If a host has an address configured, and it can prove the
ownership of this address, the state 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), if accidentally two hosts
generate the same address and the DAD procedure has failed. In
these unfrequent cases, connectivity is honored over security.
The SEND SAVI device must join the Solicited Node Multicast group for
all the addresses which state is other than NO_BIND. This is needed
to make sure that the SEND SAVI device will receive the DAD_NSOL for
those addresses. Please note that it may not be enough to relay on
the host behind the Validating port doing so, since the node may move
and after a while, the packets for that particular solicited node
multicast group will no longer be forwarded to the SEND SAVI device.
So, the SAVI device SHOULD join the solicited node multicast groups
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for all the addresses that are in a state other than NO_BIND.
We next describe how different inputs are processed depending on the
state of the binding of the IP address 'IPaddr'. A Waiting_lifetime
timer is associated to each binding.
A simplified version is depicted in the next figure:
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+-------------+
| |
| TESTING_VP' |
| |
+-------------+
| ^
Timeout / VP=VP' | |
VP_NUD_NADV | | VP'_DAD_NSOL/
| | NUD_NSOL
| |
v |
VP_DAD_NSOL +--------+
+------------- | |
| | VALID |< -------------------+
| +-------- >| | |
| | +--------+ |
| | ^ | |
| | VP_NUD_ | | Timeout, |
| | NADV/- | | TP_DAD_NSOL/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
NO_BIND
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Relevant inputs for this state: When the node is in this state, there
are no unresolved DAD_NSOL or NUD_NSOL messages (generated by SEND
SAVI), so the only relevant inputs are DAD_NSOL messages coming
either from VP or TP, or any packet other than DAD_NSOL coming from
VP or TP. There are no timers too.
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, and with the restrictions imposed by
the MLD snooping mechanism, if applied). The DAD_NSOL messages
are not sent through any of the ports configured as Validating
Ports. The SEND SAVI device sets the Waiting_timer to TENT_LT,
stores all the information required for future validation of the
corresponding DAD_NADV message (such as the nonce of the message)
and changes the state to TENTATIVE_DAD. Note that in this case it
is not possible to check address ownership by sending a NUD_NSOL
because while the host is waiting for a possible DAD_NADV its
address is in tentative state and it 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 Waiting_timer to
TENT_LT. The state is changed to TENTATIVE_NUD.
o Validated DAD_NSOL message containing IPAddr as the target address
received through a Trusted port are NOT forwarded through any of
the Validating ports but they are sent through the proper Trusted
Ports (as defined by the switch behavior that will depend on
whether it performs MLD snooping or not). The SEND SAVI device
MAY assume that any DAD_NSOL message received from a Trusted port
has been successfully validated by other SEND SAVI device, so that
no additional validation is required. 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 attachment of
the host to its Validating port according to SEND SAVI rules
(unless the SEND SAVI perimeter has been breached). The state is
not changed.
TENTATIVE_DAD
To arrive to this state, the SEND SAVI device has received a
validated DAD_NSOL coming from port VP and forwarded to TP. The
possible events occuring in this state are: the reception of a
DAD_NADV message from a TP (the corresponding DAD_NSOL was forwarded
to this port), a DAD_NSOL message from VP, other validating port VP'
or TP, and the expiration of the timer initiated when the DAD_NSOL
was received at port VP.
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o If a validated DAD_NADV is received from a Trusted port, the
binding cannot be configured for port VP. The state is changed to
NO_BIND, and the Waiting_timer cleared.
o If a validated DAD_NSOL is received from a Trusted port, a host
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 port VP, so that the host at port VP will not
configure the address, as stated in [RFC4862]. The DAD_NSOL
message is also forwarded to all appropriate Trusted ports. Then,
the Waiting_timer 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 attachment of the host to its Validating port
according to SEND SAVI rules (unless the SEND SAVI perimeter has
been breached). The state is not changed.
o If a validated DAD_NSOL is received from a Validating port VP'
different to VP, a host connected to VP' may be trying to
configure the same address at the same time. The DAD_NSOL message
is forwarded to port VP, so that the host at port VP will not
configure the address, as stated in [RFC4862]. The DAD_NSOL
message is also forwarded to all appropriate Trusted ports. Then,
the Waiting_timer is set to TENT_LT, and the state remains in
TENTATIVE_DAD, although in this case with VP=VP'.
o Any other packet than a validated DAD_NSOL is received from a
Validating port VP' different from VP is discarded. The state is
not changed.
o If a validated DAD_NSOL is received from port VP, the
Waiting_timer is set to TENT_LT, and the state remains in
TENTATIVE_DAD.
o If any packet other than a validated DAD_NSOL is received from VP,
it is assumed that the host 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 host proved address ownership by
means of the validated DAD_NSOL message, the binding is created.
The Waiting_timer is set to DEFAULT_LT, and the state is changed
to VALID.
o If Waiting_timer expires, it is assumed that no other host has
configured this address. Therefore, the Validating port VP could
be bound to this IPv6 address. The Waiting_timer 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. Relevant transitions for this state are
triggered by the reception of DAD_NSOL from ports VP, VP' or TP, and
any packet other than DAD_NSOL from VP' or TP. The expiration of
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Waiting_timer is also relevant to check again for address ownership.
o If a validated DAD_NSOL with IPaddr as source address is received
through Validating port VP, this message is forwarded to the
appropriate trusted ports. The Waiting_timer is set to TENT_LT
and the state is changed to TENTATIVE_DAD.
o Any packet other than a validated DAD_NSOL containing IPaddr as a
source address arriving from Validating port VP is forwarded
appropriately. The state is not changed. Note that in the SEND
SAVI case Timeout_valid for the entry MUST NOT be set to
DEFAULT_LT (as occurs for FCFS SAVI), since regular sending of
packets does not provide the required security, which is achieved
by performing secured NUD periodically with the sending host.
o If a validated DAD_NSOL with IPaddr as source address is received
through a Trusted port, the message is forwarded to VP. The
Waiting_timer 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 VP, the Waiting_timer is set to TENT_LT, and the state is
changed to TESTING_VP.
o If a DAD_NSOL packet with IPaddr as source address is received
through a Validating Port VP' (VP' different from the current
Validating port for this binding), the message is forwarded to VP.
In addition, a secured NUD_NSOL is sent to VP, the Waiting_timer
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
Validating port for this binding, VP, the packet is discarded.
The SEND SAVI device MAY issue a secured NUD_NSOL through port VP,
set the Waiting_timer 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, and a packet arrives from VP'.
o If Waiting_timer expires, a secured NUD_NSOL message is sent
through port VP to the IPv6 address, the Waiting_timer 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
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ports are also forwarded. The relevant events for this state are the
reception of a secured NUD_NADV message from VP, the reception of a
secured 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 waiting for NUD_NADV.
o If a validated NUD_NADV message is received from VP, the message
is discarded, the Waiting_timer is changed to DEFAULT_LT, and the
state is changed to VALID.
o If a validated DAD_NSOL message is received from VP, the message
is forwarded to the appropriate Trusted ports, the Waiting_timer
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 Validating port VP is forwarded.
Neither the Waiting_timer nor the state are changed.
o If a DAD_NSOL message is received from a Trusted port, the message
is forwarded to VP and the appropriate Trusted ports. Neither the
Waiting_timer nor the state are changed. The host at VP port is
under check: if it still is at port VP, 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, the local state will move to NO_BIND, and the state at the
remote node will change to VALID.
o If a packet other than a DAD_NSOL arrives from a Trusted port, the
packet is forwarded. Neither the Waiting_timer nor the state are
changed.
o If a DAD_NSOL is received from a validating port VP', the message
is forwarded to VP and the appropriate Trusted ports. In
addition, a secured NUD_NSOL is sent to VP, the Waiting_timer is
set to TENT_LT, and the state is changed to TESTING_VP'.
o Any other packet received from a validating port VP' is discarded.
This may occur because the host 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'.
o If the Waiting_timer expires, the Waiting_timer is cleared and the
state is changed to NO_BIND.
TESTING_VP'
To arrive to this state an indication that a host at VP' wants to
send data with IPaddr as source address while a binding existed for
VP. The SEND SAVI device has issued a NUD_NSOL to the host through
port VP. The possible events that may occur in this case are the
reception of a NUD_NADV from port VP, 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.
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o If a validated NUD_NADV is received from port VP, then the host at
VP is defending its address. VP is kept as the Validating port,
the Waiting_timer is set to DEFAULT_LT, and the state is changed
to VALID.
o If a validated DAD_NSOL is received from port VP, the message is
forwarded to VP'. The Waiting_timer is set to TENT_LT and the
state is changed to TENTATIVE_DAD. If the host at VP is
reconfiguring its address; when forwarding the DAD_NSOL message,
the node at VP' is expected to unconfigure its address.
o Any packet other than a validated DAD_NSOL or a validated NUD_NADV
coming from port VP is forwarded, and the state is not changed.
o If a validated DAD_NSOL is received from port VP', the message is
forwarded to VP. The Waiting_timer is set to DEFAULT_LT, and the
state is not changed.
o Any packet other than a validated DAD_NSOL coming from port VP is
discarded, and the state is not changed.
o If a validated DAD_NSOL is received from port VP", different from
VP and VP', the message is forwarded to VP and VP'. VP' is
expected to unconfigure its address if it was a VP'_DAD_NSOL
message (and not any other packet) the message triggering the
transition to this state. The state remains in TESTING_VP'
although with VP'=VP". The Waiting_timer 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.
o If a validated DAD_NSOL is received from a Trusted port, the
message is forwarded to ports VP, VP' and other appropriate
Trusted ports. The Waiting_timer is left unchanged and the state
is changed to TESTING_VP. VP' is expected to unconfigure its
address if it was a VP'_DAD_NSOL message (and not any other
packet) the message triggering the transition to this state.
o Any packet other than a validated DAD_NSOL coming from a Trusted
port is forwarded appropriately, but the state is not changed.
o If Waiting_timer expires, it is assumed that the host for which
the binding existed is no longer connected through port VP.
Therefore, the Validating port VP' could be bound to this IPv6
address. The Waiting_timer 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 port
VP without any existing binding in the SEND SAVI device. The SEND
SAVI device has sent a NUD_NSOL message to VP. The relevant events
for this case are the reception of a NUD_NADV from port VP, the
reception of DAD_NSOL from port VP, VP' or TP, and the reception of
any packet other than DAD_NSOL and NUD_NADV for port VP, and
different from DAD_NSOL for VP' or TP. In addition, the
Waiting_timer may expire.
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o If a validated NUD_NADV message is received through port VP, the
message is discarded, the Waiting_timer is set to TENT_LT, and the
state is changed to VALID.
o If a validated DAD_NSOL message is received through port VP, the
message is forwarded to the appropriate Trusted ports, the
Waiting_timer 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 port
VP is discarded.
o If a validated DAD_NSOL message is received through port VP'
different from port VP, the message is forwarded to the
appropriate Trusted ports, the Waiting_timer is set to TENT_LT
with VP=VP', and the state is changed to TENTATIVE_DAD.
o Any packets other than DAD_NSOL received through port VP' are
discarded, and the state is left unchanged.
o If a validated DAD_NSOL message is received through a Trusted
port, the message is forwarded to port VP, and the state is left
unchanged.
o Any other packet received from a Trusted port are forwarded
appropriately. These packets may come from a SEND SAVI device
that has securely validated the attachment of the host to its
Validating port according to SEND SAVI rules. The state is left
unchanged.
o If Waiting_timer expires, the Waiting_timer is cleared and the
state is changed to NO_BIND.
4.4. VLAN Support
In the case the SAVI device is a switch that supports VLANs, the SAVI
implementation will behave as if there was one SAVI process per VLAN.
The SAVI process of each VLAN will store the binding information
corresponding the nodes attached to that particular VLAN.
4.5. Protocol Constants
TENT_LT is 500 milliseconds.
DEFAULT_LT is 5 minutes.
5. Security Considerations
It should be noted that any SAVI solution is as strong as the lower
layer anchor that it uses. In particular, if the lower layer anchor
is forgeable, then the resulting SAVI solution will be weak. For
example, if the lower layer anchor is a MAC address that can be
easily spoofed, then the resulting SAVI will not be stronger than
that. On the other hand, if we use switch ports as lower layer
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anchors (and there is only one host connected to each port) it is
likely that the resulting SAVI solution will be considerably more
secure.
SEND SAVI improves protection compared to conventional SAVI, as a
result of the increased ability of SEND hosts 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. Not doing so will generate security
breaches through which an attacker could send packets using any
source address, regardless of the bindings established in other SEND
SAVI devices. However, SEND SAVI is designed to allow even in this
case communication for legitimate users. The worst case for the
misconfiguration of the perimeter is then that two hosts may use the
same source IPv6 address. The reasons for having a misconfigured
perimeter, apart from initial misconfiguration, are the dynamic
operations performed by layer-2 routing mechanisms, for example, as a
result of a failure in a link or switching device. To prevent the
security risks associated, in the case of changes in the topology of
the SEND SAVI devices, all ports of a SEND SAVI device MAY be changed
automatically to Validating. Note that neither connectivity nor the
protection offered are compromised by operating in a mode in which
all ports of the SEND SAVI devices operate in Validating mode (only
performance is affected by this setting).
SEND SAVI does not protect against spoofers being attached to the
same port as a legitimate host. For this reason it is RECOMMENDED
that only one host attaches at the same time to a given port.
One possible concern about SEND SAVI is its behavior when an attacker
tries to forge the identity of a legitimate host by replaying
messages. Note that information that can be valid for SEND a short
period after being generated (the binding between an IPv6 address and
a layer-2 MAC address) is not valid for SEND SAVI if it arrives from
an non-legitimate port. We now perform a security analysis of such a
replay attack for SEND SAVI. On one hand, there is some information
for which the security risks are equivalent to those of SEND
operation, which are situations in which the information received is
not tied to port-related state in SEND SAVI operation. Such
situations are the reception of CPA messages containing certificates,
or the processing of an unsolicited RADV message, which can be used
in SEND SAVI to associate the router condition to the IPv6 address of
an existing binding in the SEND SAVI Port list. On the other hand,
all the messages which can be create a SEND SAVI binding may be
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sensible for the replaying of valid SEND messages. 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 To prevent DAD_NSOL replay attacks, DAD_NSOL messages are not
forwarded to ports through which an existing binding existed.
Therefore, to capture a message that could be used to launch a
replay attack, an attacker must be located either in the port
through which the legitimate host is (in which case the attack is
useless), or in a port in which a legitimate host was before and
for which a binding still exists. For this latter case, an
attacker can prevent the configuration of binding for a legitimate
host in other port (which could have moved from the initial
location), and the binding would be available for the attacker for
DEFAULT_LT ms. The attacker can do this either in the port for
which a binding existed, or in other port to which it is connected
or to which it can convey this information for a third node to
perform this attack. This risk is inherent to allowing layer-2
host mobility in an scenario in which many hosts can attach to the
same port (either at the same time or in instants very close one
to the other). Another consideration is that this situation
reflect the fact that it is impossible to determine the legitimacy
of a node with a more secure NUD_NSOL/NUD_NADV exchange when the
nodes claim to be configuring the address.
o When a NUD_NSOL/NUD_NADV exchange is used to create or maintain a
state, the messages are only forwarded to the port in which the
host claiming to be legitimate is located. In this case, an
attacker must be connected to the same port of the legitimate host
to be able to capture a message which could be replayed. The
replay of NUD_NSOL is useless, since it is not used to trigger the
creation of a binding. The replay of a NUD_NADV message through
the same port is useless, since SEND SAVI does not protect against
spoofers attached to the same port. The replay of a NUD_NADV
message through a different port does result neither in the
creation of a binding in other SEND SAVI device, nor in the
binding created in the SEND SAVI device originating the NUD_NSOL
message, since this SEND SAVI device only considers NUD_NADV
message received from the same port through which the NUD_NSOL
message was sent.
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
hosts. 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 generate a NUD_NSOL and create a state
in which a NUD_NADV is waited for. 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, a rate-limiting mechanism
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SHOULD be enforced in a per-port basis.
6. Acknowledgments
Thanks to Ana Kukec for her review and comments on this document.
The text has also benefited from feedback provided by Tony Cheneau.
Marcelo Bagnulo is partly funded by Trilogy, a research project
supported by the European Commission under its Seventh Framework
Program.
Alberto Garcia-Martinez is partly funded by T2C2, a Spanish R&D
project.
7. Normative 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.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[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.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Protocol
Framework", draft-ietf-savi-framework-00 (work in
progress), September 2010.
[I-D.ietf-savi-fcfs]
Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS-
SAVI: First-Come First-Serve Source-Address Validation for
Locally Assigned Addresses", draft-ietf-savi-fcfs-05 (work
in progress), October 2010.
Bagnulo & Garcia-Martinez Expires April 28, 2011 [Page 25]
Internet-Draft SEND SAVI October 2010
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
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
Bagnulo & Garcia-Martinez Expires April 28, 2011 [Page 26]
