Internet DRAFT - draft-ietf-savi-send
draft-ietf-savi-send
SAVI Working Group M. Bagnulo
Internet-Draft A. Garcia-Martinez
Intended status: Standards Track UC3M
Expires: July 24, 2014 January 20, 2014
SEND-based Source-Address Validation Improvement
draft-ietf-savi-send-11
Abstract
This memo specifies SEND SAVI, a mechanism to provide source address
validation using the SEND protocol. The proposed mechanism
complements ingress filtering techniques to provide a finer
granularity on the control of IPv6 source addresses.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 24, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . 10
3.1. SEND SAVI Data Structures . . . . . . . . . . . . . . . . 10
3.2. SEND SAVI Device Configuration . . . . . . . . . . . . . . 11
3.3. Traffic Processing . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Transit Traffic Processing . . . . . . . . . . . . . . 12
3.3.2. Local Traffic Processing . . . . . . . . . . . . . . . 12
3.4. SEND SAVI Port Configuration Guidelines . . . . . . . . . 25
3.5. VLAN Support . . . . . . . . . . . . . . . . . . . . . . . 26
3.6. Protocol Constants . . . . . . . . . . . . . . . . . . . . 26
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 . . . . . . . . 28
4.2. Attack of a malicious host . . . . . . . . . . . . . . . . 29
4.2.1. M attaches to the same switch as the victim's
switch . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2.2. M attaches to a different switch to the victim's
switch . . . . . . . . . . . . . . . . . . . . . . . . 30
5. Security Considerations . . . . . . . . . . . . . . . . . . . 30
5.1. Protection Against Replay Attacks . . . . . . . . . . . . 31
5.2. Protection Against Denial of Service Attacks . . . . . . . 32
5.3. Considerations on the deployment model for trust
anchors . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.4. Residual threats . . . . . . . . . . . . . . . . . . . . . 34
5.5. Privacy considerations . . . . . . . . . . . . . . . . . . 34
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Normative References . . . . . . . . . . . . . . . . . . . 35
8.2. Informative References . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
This memo specifies SEND SAVI (SEcure Neighbor Discovery Source
Address Validation Improvement), a mechanism to provide source
address validation for IPv6 networks using the SEND protocol
[RFC3971]. The proposed mechanism complements 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. Using the information contained in these
messages, host IPv6 addresses are associated to switch ports, so that
data packets will be validated by checking for consistency in this
binding, as described in [RFC7039]. In addition, SEND SAVI prevents
hosts from generating packets containing off-link IPv6 source
addresses.
Scalability of a distributed SAVI system comprising 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, the distributed SAVI system only validates the packets when
they ingress to the protection perimeter, not in every SEND SAVI
device traversed.
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. If the SEND protocol is not used, we can
deploy other SAVI solutions relying on monitoring different address
configuration mechanisms to prove address ownership. For example,
FCFS (First-Come, First-Served) SAVI [RFC6620] can be used by nodes
locally configuring IPv6 addresses by means of the Stateless Address
Autoconfiguration mechanism [RFC4862].
SEND SAVI is designed to be deployed in SEND networks with as few
changes to the deployed implementations as possible. 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
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Improvement is presented in [RFC7039].
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. 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
SEND SAVI devices filter packets 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 SEND SAVI may open a new vector of Denial-of-Service (DoS)
attacks, based on spoofed binding anchors. For that reason,
implementations of this specification use switch ports as their
binding anchors. Other forms of binding anchors are out of the scope
of this specification, and proper analysis of the implications of
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using them, should be performed before their usage.
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.
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 may 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 with a secured NUD_NADV
by the probed node.
SEND SAVI devices issue secured NUD_NSOL messages periodically in
order to refresh bindings, which had to be answered with a valid
NUD_NADV message by the node for which the binding exists.
SEND SAVI devices only forward packets with off-link source addresses
if they are received from a port manually configured to connect to a
router.
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
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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
the node 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 increases 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" [RFC7039]. With this deployment strategy, SEND SAVI
devices perform source address validation 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 ports 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 routers and other SEND SAVI devices. Port 2 of SEND-SAVI2
and port 3 of SEND-SAVI3 are Validating ports because they connect to
routers. 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. Finally, 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 connect normally to Validating
ports. 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 host H5.
For a more detailed discussion on this, see Section 3.4.
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. SEND SAVI
supports this without 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.
Virtual switches: A hypervisor or a host Operating System may perform
bridging functions between virtual hosts running on the same machine.
The hypervisor or host OS may in turn connect to a SEND SAVI system.
This scenario is depicted in the next figure, with two virtual
machines VM1 and VM2 connected through a virtual switch VS1 to SEND
SAVI device SEND-SAVI1. The attachment points of VS1 to VM1 and VM2
are configured as Validating.
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Host1
+----------------+
| +---+ +---+ |
| |VM1| |VM2| |
| +---+ +---+ |
| | | |
| +-1-----2--+ |
| | VS1 | |
| +--3-------+ |
| | |
+----|-----------+
|
|
+--1-----2--+
| SEND- |
| SAVI1 |
+--3---4----+
| |
In order to provide proper security against replay attacks, it is
recommended to perform SEND SAVI filtering as close to untrusted
hosts as possible (see Section 3.4 and Section 5.1). In this
scenario, this objective can be achieved by enabling SEND SAVI
validation in VS1. Ideally, VS1 could be integrated into the SEND
SAVI protection perimeter, if the hypervisor or host OS at Host1 can
be trusted (even though VM1 and VM2 could not be trusted). To do so,
both the attachment to SEND-SAVI1 at VS1, and port 1 at SEND-SAVI1,
are configured as Trusted.
If the administrator of the network does not trust on VS1, port 1 of
SEND-SAVI1 is configured as Validating, so that every address being
used at Host1 is validated at SEND-SAVI1 by SEND SAVI. The
attachment point to the physical network at VS1 should be configured
as Trusted, if the host administrator knows that it is connected to a
SEND SAVI device; in this case, VS1 relies on the infrastructure
comprised by the physical SEND SAVI devices, but not the other way.
Packets egressing from VM1 are validated twice, first at VS1, and
then at SEND-SAVI1; packets going in the reverse direction (from an
external host to VM1) are validated once, when they first reach a
SEND SAVI device. If the administrator of VS1 does not trust on the
physical switch to which it attaches, it can configure the attachment
to SEND-SAVI1 as Validating. In the figure above, this means that a
packet going from another host to VM1 would be validated twice, once
when entering the SEND SAVI perimeter formed by the physical devices,
and the other when entering at VS1.
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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. Regarding to the validation of the
source address performed in a SEND SAVI device, such an untrusted
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 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. Although the SEND SAVI device to
which this router attaches could be configured to permit the transit
of packets with source addresses belonging to the set of prefixes
reachable through the untrusted router, such a mechanism is out of
the scope of this document. As a result, the default mechanism
described in this specification cannot be applied in 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
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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 RADV
messages coming from 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
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.
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.
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].
o the SEND SAVI device MAY be configured with at least one trust
anchor, if it is configured to validate RADV messages (see
Section 3.3.2). In this case 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 SEND specification
[RFC3971].
In addition, the port role for each port of the SEND SAVI device MUST
be configured. The guidelines for this configuration are specified
in Section 3.4.
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
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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 SEND SAVI device receives a transit traffic packet through
a Trusted port, it forwards it without any SAVI processing.
o If the SEND SAVI device receives a transit traffic packet through
a Validating port, it discards the packet.
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.
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 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,
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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
to that particular Solicited Node Multicast group may no longer be
forwarded to the SEND SAVI device.
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 RADV messages.
In this case, at least one router SHOULD 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, or restrict to the use of link-local
addresses.
SEND SAVI devices MUST discard RADV messages received from Validating
ports. RADV messages are only accepted and processed when received
through Trusted ports.
SEND SAVI devices SHOULD NOT validate RADV messages to update the
SEND SAVI Prefix list and forward them to other nodes. These
messages can only be received from Trusted ports, and we assume that
routers are trusted. Validating RADV messages would be required in
any SEND SAVI device the node is traversing. Besides, hosts will
validate this message before using the information it contains.
In case SEND SAVI devices are configured to validate RADV message,
SEND SAVI devices SHOULD support the processing of validated
Certification Path Advertisement (CPA) messages, sent in reply to CPS
messages, to acquire certificates used to validate router messages,
or alternatively SHOULD be configured with a certification path.
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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. 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.
o SEND SAVI devices MUST only use ND messages received through
Validating ports if they are valid, otherwise they discard them.
SEND SAVI devices SHOULD assume that such messages received from
Trusted ports have been validated by other SEND SAVI devices, or
come from a trusted device such a router, 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.
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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, we assume 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.
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
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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.
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
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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
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.
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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, the SEND SAVI device has successfully
validated address ownership, and has created a binding for IPaddr.
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,
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
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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.
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.
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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'
To arrive to this state, the SEND SAVI device has received 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"
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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 Any packet other than a validated DAD_NSOL, or a validated
NUD_NADV 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.
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
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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
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.
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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:
o Ports connected to another SEND SAVI devices MUST be configured as
Trusted ports. Not doing so will prevent off-link traffic from
being forwarded, along with the following effects for on-link
traffic: 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 MUST be configured as Trusted ports.
Not doing so results in SEND SAVI devices discarding off-link
traffic. Note that this means that, since routers are connected
through Trusted ports, they can generate traffic with any source
address, even those belonging to the link.
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
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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 specification 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.
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.
+---+
| H |
+---+
|
|
+-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 whether the host H moves to a
different port on the same switch, or to a different switch.
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,
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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.
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.
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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
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.
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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
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.
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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.
5. Security Considerations
SEND SAVI operates only with validated SEND messages to create
bindings. Note that IPv6 packets generated by non-SEND nodes will be
discarded by the first SEND SAVI device receiving it. Therefore,
attackers cannot obtain any benefit by not using SEND. In order to
perform address validation in a mixed scenario comprising SEND and
non-SEND devices, a different solution is required, which should be
addressed in other document.
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 by SEND SAVI, 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
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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.
We can analyse two different cases when considering SEND SAVI replay
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.
In these 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]).
For replay of messages belonging to the second case, i.e., messages
which do 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].
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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
exchange. The result is that in order for an attacker to actually
fill the SEND 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 REQUIRED minimum is
the memory needed to store four bindings associated to the port,
although it SHOULD be raised if the ratio between the maximum number
of bindings allowed in the device and the number of ports is high.
The motivation for setting a minimum number of bindings per port 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.
The SEND SAVI device may store data packets while the address is
being verified, for example, when a DAD_NSOL was lost before arriving
to the SEND SAVI device to which the host attaches, and the host
sends data packets, these data packets may be stored until the SEND
SAVI device verifies the binding by means of a NUD packet exchange.
In this case, the memory for data packet storage may also be a target
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of DoS attacks. A SEND SAVI device MUST limit the amount of memory
used to store data packets, allowing the other functions (such as
being able to store new bindings) to have available memory even in
the case of an attack 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 trigger SEND SAVI
events SHOULD be enforced in a per-port basis.
5.3. Considerations on the deployment model for trust anchors
The SEND specification [RFC3971] proposes two deployment models for
trust anchors, either a centralized model relaying on a globally
rooted public key infrastructure, or a more local, decentralized
deployment model, in which end hosts are configured with a collection
of public keys which are trusted only on a domain.
The appeal of a centralized model is the possibility for hosts to use
SEND to validate routers as they move through links belonging to
different organizations without additional configuration. However,
without any further protection, it also enables routers authorized
with a certificate path rooted on a global trust anchor to appear as
legitimate routers in a link in which they were not intended to act
as such. This threat already existed for SEND deployments, for which
links configured to accept centralized trust anchors may send
outgoing traffic and use prefix information from alien routers. In a
SEND SAVI deployment, such routers may be able to deliver off-link
traffic to any node of the link.
In order to cope with this threat, SEND SAVI specifies that nodes are
only allowed to behave as routers if they connect through Trusted
ports. In particular, RADV messages and traffic with off-link source
addresses are discarded when received through Validating ports, which
are the ports intended for non-trusted infrastructure, as moving
nodes. The protection provided by filtering RADV messages prevents
SEND nodes from identifying alien routers as legitimate routers, even
though the trust anchor of these routers is valid.
Besides, it is worth to say that SEND SAVI supports a decentralized
deployment model.
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5.4. 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,
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.
5.5. Privacy considerations
A SEND SAVI device MUST delete binding anchor information as soon as
possible (i.e., as soon as the state for a given address is back to
NO_BIND), 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 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, Ana Kukec, Ted Lemon, Adrian Farrel,
Barry Leiba, Brian Haberman, Vicent Roca and Benoit Claise 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
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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.
[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.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
[RFC6620] Nordmark, E., Bagnulo, M., and E. Levy-Abegnoli, "FCFS
SAVI: First-Come, First-Served Source Address Validation
Improvement for Locally Assigned IPv6 Addresses",
RFC 6620, May 2012.
[RFC7039] Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement (SAVI) Framework",
RFC 7039, October 2013.
[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.
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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
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