Internet DRAFT - draft-ietf-savi-fcfs
draft-ietf-savi-fcfs
Network Working Group E. Nordmark
Internet-Draft Cisco
Intended status: Standards Track M. Bagnulo
Expires: August 21, 2012 UC3M
E. Levy-Abegnoli
Cisco Systems
February 18, 2012
FCFS SAVI: First-Come First-Serve Source-Address Validation for Locally
Assigned IPv6 Addresses
draft-ietf-savi-fcfs-14
Abstract
This memo describes FCFS SAVI a mechanism to provide source address
validation for IPv6 networks using the First-Come First-Serve
principle. The proposed mechanism is intended to complement ingress
filtering techniques to help detect and prevent source address
spoofing.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Background to FCFS SAVI . . . . . . . . . . . . . . . . . . . 4
2.1. Scope of FCFS SAVI . . . . . . . . . . . . . . . . . . . . 4
2.2. Constraints for FCFS SAVI design . . . . . . . . . . . . . 5
2.3. Address ownership proof . . . . . . . . . . . . . . . . . 5
2.4. Binding Anchor considerations . . . . . . . . . . . . . . 6
2.5. FCFS SAVI protection perimeter . . . . . . . . . . . . . . 6
2.6. Special cases . . . . . . . . . . . . . . . . . . . . . . 10
3. FCFS SAVI specification . . . . . . . . . . . . . . . . . . . 11
3.1. FCFS SAVI Data structures . . . . . . . . . . . . . . . . 11
3.2. FCFS SAVI algorithm . . . . . . . . . . . . . . . . . . . 11
3.2.1. Discovering on-link prefixes . . . . . . . . . . . . . 11
3.2.2. Processing of transit traffic . . . . . . . . . . . . 12
3.2.3. Processing of local traffic. . . . . . . . . . . . . . 13
3.2.4. FCFS SAVI port configuration guidelines . . . . . . . 19
3.2.5. VLAN support . . . . . . . . . . . . . . . . . . . . . 20
3.3. Default Protocol Values . . . . . . . . . . . . . . . . . 20
4. Security Considerations . . . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . . 24
8.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Implications of not following the recommended
behaviour . . . . . . . . . . . . . . . . . . . . . . 25
A.1. Implications of not generating DAD-NS packets upon the
reception of non compliant data packets . . . . . . . . . 25
A.1.1. Lack of binding state due to packet loss . . . . . . . 25
A.1.2. Lack of binding state due to a change in the
topology . . . . . . . . . . . . . . . . . . . . . . . 28
A.1.3. Lack of binding state due to state loss . . . . . . . 29
A.2. Implications of not diacarding non compliant data
packets . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Introduction
This memo describes FCFS SAVI, a mechanism to provide source address
validation for IPv6 networks using the First-Come First-Serve
principle. The proposed mechanism is intended to complement ingress
filtering techniques to help detect and prevent source address
spoofing. Section 2 gives the background and description of FCFS
SAVI, and Section 3 specifies the FCFS SAVI protocol.
2. Background to FCFS SAVI
2.1. Scope of FCFS SAVI
The application scenario for FCFS SAVI is limited to the local link.
Hence, the goal of FCFS 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
generate packets with their own address as the source address. This
is called the local traffic. Routers send packets containing a
source IP address other than their own, since they are forwarding
packets generated by other hosts (usually located in a different
link). This is called the transit traffic.
The applicability of FCFS SAVI is limited to the local traffic i.e.
to verify if the traffic generated by the hosts attached to the local
link contains a valid source address. The verification of the source
address of the transit traffic is out of the scope of FCFS SAVI.
Other techniques, like ingress filtering [RFC2827], are recommended
to validate transit traffic. In that sense, FCFS SAVI complements
ingress filtering, since it relies on ingress filtering to validate
transit traffic but it provides validation of local traffic, which is
not provided by ingress filtering. Hence, the security level is
increased by using these two techniques.
In addition, FCFS SAVI is designed to be used with locally assigned
IPv6 addresses, in particular with IPv6 addresses configured through
Stateless Address Autoconfiguration (SLAAC) [RFC4862]. Manually
configured IPv6 addresses can be supported by FCFS SAVI, but manual
configuration of the binding on the FCFS SAVI device provides higher
security and seems compatible with manual address management. FCFS
SAVI can also be used with IPv6 addresses assigned via DHCPv6, since
they ought to perform the Duplicate Address Detection procedure, but
there is a specific mechanism tailored for dealing with DHCP assigned
addresses defined in [I-D.ietf-savi-dhcp]. Additional considerations
about how to use FCFS SAVI depending on the type of address
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management used and the nature of the addresses is discussed in the
framework document [I-D.ietf-savi-framework].
2.2. Constraints for FCFS SAVI design
FCFS SAVI is designed to be deployed in existing networks requiring a
minimum set of changes. For that reason, FCFS SAVI does not require
any changes in the hosts which source address is to be verified. Any
verification solely relies in the usage of already available
protocols. In other words, FCFS SAVI does not 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. In other
words, the requirement is no host changes.
FCFS SAVI validation is performed by the FCFS SAVI function. Such
function can be placed in different type of devices, including a
router or a layer-2 bridge. The basic idea is that the FCFS SAVI
function is located in the points of the topology that can enforce
the correct usage of source address by dropping the non-compliant
packets.
2.3. Address ownership proof
The main function performed by FCFS SAVI is to verify that the source
address used in data packets actually belongs to the originator of
the packet. Since FCFS SAVI scope is limited to the local link, the
originator of the packet is attached to the local link. In order to
define a source address validation solution, we need to define the
meaning of "address ownership"; i.e., what it means that a given host
owns a given address in the sense that the host is entitled to send
packets with that source address. With that definition, we can
define how a device can confirm that the source address in a datagram
is owned by the originator of the datagram.
In FCFS SAVI the address ownership proof is based in the First-Come
First- Serve principle. The first host that claims a given source
address is the owner of the address until further notice. Since no
host changes are acceptable, we need to find the means to confirm
address ownership without requiring a new protocol. So, whenever a
source address is used for the first time, a state is created in the
device that is performing the FCFS SAVI function binding the source
address to a binding anchor which consists on layer-2 information
that the FCFS SAVI box has available (e.g. the port in a switched
LAN). Subsequent data packets containing that IP source address can
be checked against the same binding anchor to confirm that the
originator owns the source IP address.
There are however additional considerations to be taken into account.
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For instance, consider the case of a host that moves from one segment
of a LAN to another segment of the same subnetwork and it keeps the
same IP address. In this case, the host is still the owner of the IP
address, but the associated binding anchor may have changed. In
order to cope with this case, the defined FCFS SAVI behaviour implies
the verification whether the host is still reachable using the
previous binding anchor. In order to do that FCFS SAVI uses the
Neighbour Discovery (ND) protocol. If the host is no longer
reachable at the previously recorded binding anchor, FCFS SAVI
assumes that the new location is valid and creates a new binding
using the new binding anchor. In case the host is still reachable
using the previously recorded binding anchor, the packets coming from
the new binding anchor are dropped.
Note that this only applies to local traffic. Transit traffic
generated by a router would be verified using alternative techniques,
such as ingress filtering. FCFS SAVI checks would not be fulfilled
by the transit traffic, since the router is not the owner of the
source address contained in the packets.
2.4. Binding Anchor considerations
Any SAVI solution is not stronger than the binding anchor it uses.
If the binding anchor is easily spoofable (e.g. a 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 FCFS SAVI device is configured to
drop non-compliant packets, then the usage of FCFS SAVI may open a
new vector of Denial of Service (DoS) attacks, based on spoofed
binding anchors. For that reason, in this specification only switch
ports MUST be used as binding anchors. Other forms of binding
anchors are out of the scope of this specification and proper
analysis of the implications of using them should be performed before
their usage.
2.5. FCFS SAVI protection perimeter
FCFS SAVI provides perimetrical security. FCFS SAVI devices form
what can be called a FCFS SAVI protection perimeter and they verify
that any packet that crosses the perimeter is compliant (i.e. the
source address is validated). Once the packet is inside the
perimeter, no further validations are performed to the packet. This
model has implications both on how FCFS SAVI devices are deployed in
the topology and on the configuration of the FCFS SAVI boxes.
The implication of this perimetrical security approach, is that there
is part of the topology that is inside the perimeter and part of the
topology that is outside the perimeter. So, while packets coming
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from interfaces connected to the external part of the topology need
to be validated by the FCFS SAVI device, packets coming from
interfaces connected to the internal part of the topology do not need
to be validated. This significantly reduces the processing
requirements of the FCFS SAVI device. It also implies that each FCFS
SAVI device that is part of the perimeter, must be able to verify the
source addresses of the packets coming from the interfaces connected
to the external part of the perimeter. In order to do so, the FCFS
SAVI device binds the source address to a binding anchor.
One possible approach would be for every FCFS SAVI device to store
binding information about every source addresses in the subnetwork.
In this case, every FCFS SAVI device would store a binding for each
source address of the local link. The problem with this approach is
that it imposes significant memory burden on the FCFS SAVI devices.
In order to reduce the memory requirements imposed to each device,
the FCFS SAVI solution described in this specification distributes
the storage of FCFS SAVI binding information among the multiple FCFS
SAVI devices of a subnetwork. The FCFS SAVI binding state is
distributed across the FCFS SAVI devices according to the following
criteria: each FCFS SAVI device only stores binding information about
the source addresses bound to anchors corresponding to the interfaces
that connect to the part of the topology that is outside of the FCFS
SAVI protection perimeter. Since all the untrusted packet sources
are by definition in the external part of the perimeter, packets
generated by each of the untrusted sources will reach the perimeter
through an interface of a FCFS SAVI device. The binding information
for that particular source address will be stored in this first FCFS
SAVI device the packet reaches to.
The result is that the FCFS SAVI binding information will be
distributed across multiple devices. In order to provide proper
source address validation, it is critical that the information
distributed among the different FCFS SAVI devices is coherent. In
particular, it is important to avoid that the same source address is
bound to different binding anchors in different FCFS SAVI devices.
Should that occur, then it would mean that two hosts are allowed to
send packets with the same source address, which is what FCFS SAVI is
trying to prevent. In order to preserve the coherency of the FCFS
SAVI bindings distributed among the FCFS SAVI devices within a realm,
the Neighbour Discovery (ND) protocol [RFC4861] is used, in
particular the Neighbour Solicitation (NS) and Neighbour
Advertisement (NA) messages. As a simplified example of how this
might work: before creating a FCFS SAVI binding in the local FCFS
SAVI database, the FCFS SAVI device will send a NS message querying
for the address involved. Should any host reply to that message with
a NA message, the FCFS SAVI device that sent the NS will infer that a
binding for that address exists in another FCFS SAVI device and will
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not create a local binding for it. If no NA message is received as a
reply to the NS, then the local FCFS SAVI device will infer that no
binding for that address exists in other FCFS SAVI device and will
create the local FCFS SAVI binding for that address.
So, summarizing, the proposed FCFS SAVI approach relies on the
following design choices:
o FCFS SAVI provides perimetrical security, so some interfaces of a
FCFS SAVI device will connect to the internal (trusted) part of
the topology and other interfaces will connect to the external
(untrusted) part of the topology.
o A FCFS SAVI device only verifies packets coming through an
interface connected to the untrusted part of the topology.
o A FCFS SAVI device only stores binding information for the source
addresses that are bound to binding anchors that correspond to
interfaces that connect to the untrusted part of the topology.
o FCFS SAVI uses the NS and NA messages to preserve the coherency of
the FCFS SAVI binding state distributed among the FCFS SAVI
devices within a realm.
So, in a link that is constituted of multiple L2 devices, some of
which are FCFS SAVI capable and some of which are not, the FCFS SAVI
capable devices MUST be deployed forming a connected perimeter (i.e.
that no data packet can get inside the perimeter without passing
through a FCFS SAVI device). Packets that cross the perimeter will
be validated while packets that do no cross the perimeter are not
validated (hence FCFS SAVI protection is not provided for these
packets). Consider the deployment of FCFS SAVI in the topology
depicted in the following picture:
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+--------+
+--+ +--+ +--+ | +--+ |
|H1| |H2| |H3| | |R1| |
+--+ +--+ +--+ | +--+ |
| | | | | |
+-------------SAVI-PROTECTION-PERIMETER------+ | |
| | | | | |
| +-1-----2-+ +-1-----2-+ |
| | SAVI1 | | SAVI2 | |
| +-3--4----+ +--3------+ |
| | | +--------------+ | |
| | +----------| |--------+ |
| | | SWITCH-A | |
| | +----------| |--------+ |
| | | +--------------+ | |
| +-1--2----+ +--1------+ |
| | SAVI3 | | SAVI4 | |
| +-3-----4-+ +----4----+ |
| | | | |
| +------SAVI-PROTECTION-PERIMETER---------------+
| | | | |
| +--+| +--+ +---------+
| |R2|| |H4| |SWICTH-B |
| +--+| +--+ +---------+
+------+ | |
+--+ +--+
|H5| |H6|
+--+ +--+
In the figure above, the FCFS SAVI protection perimeter is provided
by 4 FCFS SAVI devices, namely SAVI1, SAVI2, SAVI3 and SAVI4. These
devices verify the source address and filter packets accordingly.
FCFS SAVI devices then have two types of ports: trusted ports and
validating ports.
o Validating ports (VPs) are those in which FCFS SAVI processing is
performed. When a packet is received through one of the
validating ports, the FCFS SAVI processing and filtering will be
executed.
o Trusted ports (TPs) are those in which FCFS SAVI processing is not
performed. So, packets received through trusted ports are not
validated and no FCFS SAVI processing is performed in them.
Trusted ports are used for connections with the trusted
infrastructure, including the communication between FCFS SAVI
devices, the communication with routers and the communication of
other switches that while they are not FCFS SAVI devices, they only
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connect to trusted infrastructure (i.e. other FCFS SAVI devices,
routers or other trusted nodes). So, in the figure above, Port 3 of
SAVI1 and port 1 of SAVI3 are trusted because they connect two FCFS
SAVI devices. Port 4 of SAVI1, port 3 of SAVI2, port 2 of SAVI3 and
port 1 of SAVI4 are trusted because the connect to SWITCH-A to which
only trusted nodes are connected. In the figure above, port 2 of
SAVI2 and port 3 of SAVI3 are trusted ports because they connect to
routers.
Validating ports are used for connection with non-trusted
infrastructure. In particular, hosts are normally connected to
validating ports. Non-SAVI switches that are outside of the FCFS
SAVI protection perimeter also are connected through validating
ports. In particular, non-SAVI devices that connect directly to
hosts or that have no SAVI capable device between themselves and the
hosts are connected through a validating port. So, in the figure
above, ports 1 and 2 of SAVI1, port 1 of SAVI2, port 4 of SAVI 3 are
validating ports because they connect to hosts. Port 4 of SAVI4 is
also a validating port because it is connected to SWITCH-B which is a
non-SAVI capable switch which is connected to hosts H5 and H6.
2.6. Special cases
Multi-subnet links: In some cases, a given subnet may have several
prefixes. This is directly supported by SAVI as any port can support
multiple prefixes. Even the case where the forwarding of packets
between different prefixes involve a router is supported, as long as
the router is connected to a Trusted port, as recommended for all the
routers.
Multihomed hosts: A multihomed host is a host with multiple
interfaces. The interaction between 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 SAVI device perspective this is equivalent to two hosts with
a single interface each with an IP address each. This is supported
by SAVI without need for additional considerations. If the different
interfaces share the same IP address or if the interfaces have
different addresses but the host sends packets using the address of
one of the interfaces through any of the interfaces, then 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 SAVI bindings to allow binding the address of the
multihomed host to multiple anchors simultaneously.
Untrusted routers: One can envision scenarios where routers are
dynamically attached to a FCFS SAVI network. A typical example would
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be a mobile phone connecting to a FCFS SAVI switch where the mobile
phone is acting as a router for other personal devices that are
accessing the network through it. In this case, the router does not
seem to directly fall in the category of Trusted infrastructure (as
if this was the case, it is likely that all devices would be
trusted), hence it cannot be connected to a trusted port and if it is
connected to a Validating port, the FCFS SAVI switch would discard
all the packets containing an off link source address coming from
that device. As a result, the default recommendation specified in
this specification does not support such scenario.
3. FCFS 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 RFC 2119 [RFC2119].
3.1. FCFS SAVI Data structures
FCFS SAVI function relies on state information binding the source
address used in data packets to the binding anchor that contained the
first packet that used that source IP address. Such information is
stored in FCFS SAVI Data Base (DB). The FCFS SAVI DB will contain a
set of entries about the currently used IP source addresses. So each
entry will contain the following information:
o IP source address
o Binding anchor: port through which the packet was received
o Lifetime
o Status: either TENTATIVE, VALID, TESTING_VP or TESTING_TP-LT
o Creation time: the value of the local clock when the entry was
firstly created
In addition to this, FCFS SAVI needs to know what are the prefixes
that are directly connected, so it maintains a data structure called
the FCFS SAVI prefix list, which contains:
o Prefix
o Interface where prefix is directly connected
3.2. FCFS SAVI algorithm
3.2.1. Discovering on-link prefixes
In order to distinguish local traffic from transit traffic, the FCFS
SAVI device relies on the FCFS SAVI Prefix list, which contains the
set of on-link IPv6 prefixes. A FCFS SAVI device MUST support the
following two methods for populating the Prefix List: Manual
configuration and Router Advertisement, as detailed next.
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Manual configuration: A FCFS SAVI device MUST support manual
configuration of the on-link prefixes included in the Prefix List.
For example, this can be used when there are no prefixes being
advertised on the link.
Router Advertisement: A FCFS SAVI device MUST support discovery of
on-link prefixes through Router Advertisement messages in Trusted
Ports. For the Trusted Ports, the FCFS SAVI device will learn the
on-link prefixes following the procedure defined for a host to
process the Prefix Information options described in section 6.3.4 of
[RFC4861] with the difference that the prefixes will be configured in
the FCFS SAVI Prefix List rather than in the ND Prefix List. In
addition, when the FCFS SAVI device boots, it MUST send a Router
Solicitation message as described in section 6.3.7 of [RFC4861],
using the unspecified source address.
3.2.2. Processing of transit traffic
The FCFS SAVI function is located in a forwarding device, such as a
router or a layer-2 switch. The following processing is performed
depending on the type of port the packet has been received through:
o If the data packet is received through a Trusted port, the data
packet is forwarded and no SAVI processing performed to the
packet.
o If the data packet is received through a Validating port, then the
FCFS SAVI function checks whether the received data packet is
local traffic or transit traffic. It does so by verifying if the
source address of the packet belongs to one of the directly
connected prefixes available in the receiving interface. It does
so by searching the FCFS SAVI Prefix List.
* If the IP source address does not belong to one of the on-link
prefixes of the receiving interface, the data packet is transit
traffic and the packet SHOULD be discarded. (If for some
reason, discarding the packets is not acceptable, logging or
triggering of alarms MAY be used). The FCFS SAVI function MAY
send an ICMP Destination Unreachable Error back to the source
address of the data packet and ICMPv6, code 5 (Source address
failed ingress/egress policy), should be used.
* If the source address of the packet does belong to one of the
prefixes available in the receiving port, then the FCFS SAVI
local traffic validation process is executed as described
below.
* If the source address of the packet is the unspecified address,
the packet is forwarded and no SAVI processing is performed
except for the case of the Neighbor Solicitation messages
involved in the Duplicate Address Detection which are treated
as described in Section 3.2.3.
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3.2.3. Processing of local traffic.
We describe next how the local traffic, including both control and
data packets are processed by the FCFS SAVI device using a state
machine approach.
The state machine described is for the binding of a given source IP
address (called IPAddr) in a given FCFS SAVI device. So this means
that all the packets described as inputs in the state machine above
refer to that given IP address. In the case of data packets, the
source address of the packet is IPAddr. In the case of the DAD_NS
packets, the Target address is IPAddr. The key attribute is the IP
address. The full state information is:
o IP ADDRESS: IPAddr
o BINDING ANCHOR: P
o LIFETIME: LT
The possible states are:
o NO_BIND
o TENTATIVE
o VALID
o TESTING_TP-LT
o TESTING_VP
We will use VP for Validating Port and TP for Trusted Port.
After bootstrapping (when no binding exists), the state for all
source IP address is NO-BIND i.e. there is no binding for the IP
address to any binding anchor.
NO_BIND: The binding for a source IP address entry is in this state
when it does not have any binding to an anchor. All addresses are in
this state by default after bootstrapping, unless bindings were
created for it.
TENTATIVE: The binding for a source address for which a data packet
or a NS generated by the Duplicate Address Detection (DAD) procedure
has been received is in this state during the waiting period during
which the DAD procedure is being executed (either by the host itself
or the FCFS SAVI device on its behalf).
VALID: The binding for the source address is in this state after it
has been verified. It means that it is valid and usable for
filtering traffic.
TESTING_TP-LT: A binding for a source address enters in this state
due to one of two reasons:
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When a Duplicate Address Detection Neighbour Solicitation has been
received through a Trusted port. This implies that a host is
performing the DAD procedure for that source address in another
switch. This may be due to an attack or to the fact that the host
may have moved. The binding in this state is then being tested to
determine which is the situation.
The lifetime of the binding entry is about to expire. This is due
to the fact that no packets have been seen by the FCFS SAVI device
for the LIFETIME period. This may be due to the host simply being
silent or because the host has left the location. In order to
determine which is the case, a test is performed, in order to
determine if the binding information should be discarded.
TESTING_VP: A binding for a source address enters in this state when
a Duplicate Address Detection Neighbour Solicitation or a data packet
has been received through a Validating port other than the one
address is currently bound to. This implies that a host is
performing the DAD procedure for that source address through a
different port. This may due to an attack or to the fact that the
host may have moved or just because another host tries to configure
an address already used. The binding in this state is then being
tested to determine which is the situation.
We describe next how the different inputs are processed depending on
the state of the binding of the IP address (IPAddr).
A simplified figure of the state machine is included below.
NO_BIND
o Upon the reception through a Validating Port (VP) of a Neighbour
Solicitation (NS) generated by the Duplicate Address Detection
(DAD) procedure (hereafter named DAD_NS) containing Target Address
IPAddr, the FCFS SAVI device MUST forward the NS and T_WAIT
millisconds later it MUST send a copy of the same message. These
DAD_NS messages are not sent through any of the ports configured
as Validating Ports. The DAD_NS messages are sent through the
Trusted Ports (but of course subject to usual switch behavior and
possible Multicast Listener Discovery (MLD) snooping
optimizations). The state is moved to TENTATIVE. The LIFETIME is
set to TENT_LT (i.e. LT:=TENT_LT), the BINDING ANCHOR is set to
VP (i.e. P:=VP) and the Creation time is set to the current value
of the local clock.
o Upon the reception through a Validating Port (VP) of a DATA packet
containing IPAddr as the source address, the SAVI device SHOULD
execute the process of sending Neighbour Solicitation messages of
the Duplicate Address Detection process as described in section
5.4.2 of [RFC4862] for the IPAddr using the following default
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parameters: DupAddrDetectTransmits set to 2 (i.e. 2 Neighbour
Solicitation messages for that address will be sent by the SAVI
device) and RetransTimer set to T_WAIT Milliseconds (i.e. the time
between two Neighbour Solicitation messages is T_WAIT
Milliseconds). The implications of not following the recommended
behaviour are described in Appendix A. The DAD_NS messages are
not sent through any of the ports configured as Validating Ports.
The DAD_NSOL messages are sent through Trusted Ports (but of
course subject to usual switch behavior and possible MLD snooping
optimizations). The SAVI device MAY discard the data packet while
the DAD procedure is being executed or it MAY store them until the
binding is created. In any case, it MUST NOT forward the data
packets until the binding has been verified. The state is moved
to TENTATIVE. The LIFETIME is set to TENT_LT (i.e. LT:=TENT_LT),
the BINDING ANCHOR is set to VP (i.e. P:=VP) and the Creation
time is set to the current value of the local clock.
o Data packets containing IPAddr as the source address received
through Trusted ports are processed and forwarded as usual (i.e.
no special SAVI processing)
o DAD_NS packets containing IPAddr as the target address received
through a Trusted port MUST NOT forwarded through any of the
Validating ports but they are sent through the Trusted Ports (but
of course subject to usual switch behavior and possible MLD
snooping optimizations).
o Neighbor Advertisement packets sent to all nodes as a reply to the
DAD_NS (hereafter called DAD_NA) containing IPAddr as the target
address coming through a Validating port are discarded.
o Other signaling packets are processed and forwarded as usual (i.e.
no SAVI processing)
TENTATIVE
o If the LIFETIME times out, the state is moved to VALID. The
LIFETIME is set to DEFAULT_LT (i.e. LT:= DEFAULT_LT). Stored
data packets are forwarded (if any).
o If a Neighbour Advertisement (NA) is received through a Trusted
Port with Target Address set to IPAddr, then message is forwarded
through port P, the state is set to NO_BIND and the BINDING ANCHOR
and the LIFETIME are cleared. Data packets stored corresponding
to this binding are discarded.
o If a NA is received through a Validating Port with Target Address
set to IPAddr, the NA packet is discarded
o If a data packet with source address IPAddr is received with
binding anchor equal to P, then the packet is either stored or
discarded.
o If a data packet with source address IPAddr is received through a
Trusted port, the data packet is forwarded. The state is
unchanged .
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o If a data packet with source address IPAddr is received through a
Validating port other than P, the data packet is discarded.
o If a DAD_NS is received from a Trusted port, with target address
set to IPAddr, then the message is forwarded to the Validating
port P, the state is set to NO_BIND and the BINDING ANCHOR and
LIFETIME are cleared. Data packets stored corresponding to this
binding are discarded.
o If a DAD_NS with target address set to IPAddr is received from a
validating port P' other than P, the message is forwarded to the
Validating port P and to the Trusted ports, the state remains in
TENTATIVE, but the BINDING ANCHOR is changed from P to P' and
LIFETIME is set to TENT_LT. Data packets stored corresponding to
the binding with P are discarded.
o Other signaling packets are processed and forwarded as usual (i.e.
no SAVI processing)
VALID
o If a data packet containing IPAddr as a source address arrives
from Validating port P, then the LIFETIME is set to DEFAULT_LT and
the packet is forwarded as usual.
o If a DAD_NS is received from a Trusted port, then the DAD_NS
message is forwarded to port P and it is also forwarded to the
Trusted Ports (but of course subject to usual switch behavior and
possible MLD snooping optimizations). The state is changed to
TESTING_TP-LT. The LIFETIME is set to TENT_LT.
o If a data packet containing source address IPAddr or a DAD_NA
packet with target address set to IPAddr is received through a
Validating port P' other than P, then the SAVI device will execute
the process of sending DAD_NS messages as described in section
5.4.2 of [RFC4862] for the IPAddr using the following default
parameters: DupAddrDetectTransmits set to 2 (i.e. 2 NS messages
for that address will be sent by the SAVI device) and RetransTimer
set to T_WAIT Milliseconds (i.e. the time between two NS messages
is T_WAIT Milliseconds). The DAD_NS message will be forwarded to
the port P. The state is moved to TESTING_VP. The LIFETIME is set
to TENT_LT. The SAVI device MAY discard the data packet while the
DAD procedure is being executed or it MAY store them until the
binding is created. In any case, it MUST NOT forward the data
packets until the binding has been verified.
o If a DAD_NS packet with target address set to IPAddr is received
through a Validating port P' other than P, then the SAVI device
will forward the DAD_NS packet and T_WAIT Milliseconds later it
will execute the process of sending DAD_NS messages as described
in section 5.4.2 of [RFC4862] for the IPAddr using the following
default parameters: DupAddrDetectTransmits set to 1 and
RetransTimer set to T_WAIT Milliseconds. The DAD_NS messages will
be forwarded to the port P. The state is moved to TESTING_VP. The
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LIFETIME is set to TENT_LT. The SAVI device MAY discard the data
packet while the DAD procedure is being executed or it MAY store
them until the binding is created. In any case, it MUST NOT
forward the data packets until the binding has been verified.
o If the LIFETIME expires, then the SAVI device will execute the
process of sending DAD_NS messages as described in section 5.4.2
of [RFC4862] for the IPAddr using the following default
parameters: DupAddrDetectTransmits set to 2 (i.e. 2 NS messages
for that address will be sent by the SAVI device) and RetransTimer
set to T_WAIT Milliseconds (i.e. the time between two NS messages
is T_WAIT Milliseconds). The DAD_NS messages will be forwarded to
the port P. The state is changed to TESTING_TP-LT and the LIFETIME
is set to TENT_LT.
o If a data packet containing IPAddr as a source address arrives
from Trusted port, the packet MAY be discarded. The event MAY be
logged.
o Other signaling packets are processed and forwarded as usual (i.e.
no SAVI processing). In particular DAD_NA coming from port P and
containing IPAddr as the target address are forwarded as usual.
TESTING_TP-LT
o If the LIFETIME expires, the BINDING ANCHOR is cleared and the
state is changed to NO_BIND
o If a NA message containing the IPAddr as target address is
received through the Validating port P as a reply to the DAD_NS
message, then the NA is forwarded as usual and the state is
changed to VALID. The LIFETIME is set to DEFAULT_LT
o If a data packet containing IPAddr as the source address is
received through port P, then the packet is forwarded and the
state is changed to VALID. The LIFETIME is set to DEFAULT_LT.
o If a DAD_NS is received from a Trusted port, the DAD_NS is
forwarded as usual.
o If a DAD_NS is received from a Validating Port P' other than P,
the DAD_NS is forwarded as usual, the state is moved to
TESTING_VP.
o If a data packet is received through a Validating Port port P'
that is other than port P, then the packet is discarded.
o If a data packet is received through a Trusted Port port, then the
packet MAY be discarded. The event MAY be logged.
TESTING_VP
o If the LIFETIME expires, the BINDING ANCHOR is modified from P to
P', the LIFETIME is set to DEFAULT_LT and the state is changed to
VALID. Data packet stored coming from P' are forwarded.
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o If a NA message containing the IPAddr as target address is
received through the Validating port P as a reply to the DAD_NS
message, then the NA is forwarded as usual and the state is
changed to VALID. The LIFETIME is set to DEFAULT_LT
o If a data packet containing IPAddr as the source address is
received through port P, then the packet is forwarded.
o If a data packet containing IPAddr as the source address is
received through a Validating Port P'' that is other than port P
or P', then the packet is discarded.
o If a data packet containing IPAddr as the source address is
received through a Trusted Port port (i.e. other than port P), the
state is moved to TESTING_TP-LT, and the packet MAY is discarded.
o If a DAD_NS is received through Trusted Port, the packet is
forwarded as usual and the state is moved to TESTING_TP-LT.
o If a DAD_NS is received through Validating Port P'' other than P
or P', the packet is forwarded as usual and P'' is stored as the
tentative port i.e. P':=P''. The state remains the same.
Simplified state machine figure
+---------+ VP_NS, VP_DATA/2xNS +-----------+
| |---------------------------------------->| |
| NO_BIND | | TENTATIVE |
| |<----------------------------------------| |
+---------+ TP_NA, TP_NS/- +-----------+
^ |
| | TimeOut
Timeout| |
| v
+---------+ VP_NA/- +-----------+
| |---------------------------------------->| |
| TESTING | TP_NS/- | |
| TP-LT |<----------------------------------------| VALID |
| | TimeOut/2xNS | |
| |<----------------------------------------| |
+---------+ +-----------+
^ | ^ |
| | | |
| +--------------------- ---------------------+ |
| VP_NS/- | | NP_NA, TimeOut/- |
| v | |
| +-----------+ |
| | | |
+---------------------| TESTING |<----------------------+
VP_NS, VP_DATA/- | VP | VP_DATA, VP_NS,
+-----------+ VP_NA/2xNS
MLD considerations
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The FCFS 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 FCFS SAVI device will receive the DAD_NS for
those addresses. Please note that it may not be enough to rely 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 FCFS SAVI device.
So, the FCFS SAVI device MUST join the solicited node multicast
groups for all the addresses that are in a state other than NO_BIND
3.2.4. FCFS SAVI port configuration guidelines
The guidelines for port configuration in FCFS SAVI devices are:
o The FCFS SAVI realm (i.e. the realm that is inside the FCFS SAVI
protection perimeter) MUST be connected. If this is not the case,
legitimate transit traffic may be dropped.
o Ports that are connected to another FCFS SAVI device MUST be
configured as Trusted ports. Not doing so will significantly
increase the memory consumption in the FCFS SAVI devices and may
result in legitimate transit traffic being dropped.
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. A valid exception is the
case of a trusted host (e.g. a server) which could be connected to
a Trusted port, but untrusted hosts MUST be connected to
Validating ports.
o Ports connected to routers MUST be configured as Trusted ports.
Configuring them as Validating ports should result in transit
traffic being dropped.
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. A valid
exception is the case where the legacy switch only has trusted
hosts attached, in which case it could be connected to a Trusted
port, but if there is at least one untrusted hosts connected to
the legacy switch, then it MUST be connected to Validating ports.
o Ports connected to a chain of one or more legacy switches that
have other FCFS SAVI devices and/or routers connected but had no
hosts attached to them MUST be configured as Trusted ports. Not
doing so will at least significantly increase the memory
consumption in the FCFS SAVI devices and increase the signaling
traffic due to FCFS SAVI validation and may results in legitimate
transit traffic being dropped.
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3.2.5. VLAN support
In the case the FCFS SAVI device is a switch that supports customer
VLANs [IEEE.802-1Q.2005], the FCFS SAVI implementation MUST behave as
if there was one FCFS SAVI process per customer VLAN. The FCFS SAVI
process of each customer VLAN will store the binding information
corresponding the nodes attached to that particular customer VLAN.
3.3. Default Protocol Values
The following are the default values used in the FCFS SAVI
specification.
TENT_LT is 500 Milliseconds
DEFAULT_LT is 5 minutes
T_WAIT is 250 Milliseconds
An implementation MAY allow these values to be modified, but that
tuning them precisely is considered out of scope of this document.
4. Security Considerations
Denial of service attacks
There are two types of Denial of Service (DoS) attacks [RFC4732] that
can be envisaged in a FCFS SAVI environment. On one hand, we can
envision attacks against the FCFS SAVI device resources. On the
other hand, we can envision DoS attacks against the hosts connected
to the network where FCFS SAVI is running.
The attacks against the FCFS SAVI device basically consist on making
the FCFS SAVI device to 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 FCFS SAVI device to create
state for each of the addresses and waste memory. At some point the
FCFS SAVI device runs out of memory and it needs to decide how to
react in this situation. The result is that some form of garbage
collection is needed to prune the entries. It is RECOMMENDED that
when the FCFS SAVI device runs out of the memory allocated for the
FCFS SAVI DB, it creates new entries by deleting the entries which
Creation Time is higher. 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 address, each new source address is likely to
rewrite another source address created by the attack itself. It
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should be noted that entries are also garbage collected using the
LIFETIME, which is updated using data packets. The result is that in
order for an attacker to actually fill the FCFS SAVI DB with false
source addresses, it needs to continuously send data packets for all
the different source addresses, in order for the entries to grow old
and compete with the legitimate entries. The result is that the cost
of the attack for the attacker is highly increased.
In addition, it is also RECOMMENDED that a FCFS SAVI device reserves
a minimum amount of memory for each available port (in the case where
the port is used as part of the L2 anchor). The recommended minimum
is the memory needed to store 4 bindings associated to the port. The
motivation for this recommendation is as follows: an attacker
attached to a given port of a FCFS SAVI device may attempt to launch
a DoS attack towards the FCFS SAVI device by creating many bindings
for different addresses. It can do so, by sending DAD_NS for
different addresses. The result is that the attack will consume all
the memory available in the FCFS SAVI device. The above
recommendation aims to reserve a minimum amount of memory per port,
so that hosts located in different ports can make use of the reserved
memory for their port even if a DoS attack is occurring in a
different port.
As the FCFS SAVI device may store data packets while the address is
being verified, the memory for data packet storage may also be a
target of DoS attacks. The effects of such attacks may be limited to
the lack of capacity to store new data packets. The effect of such
attack will be then that data packets will be dropped during the
verification period. A FCFS SAVI device MUST limit the amount of
memory used to store data packets, allowing the other functions to
have available memory even in the case of attacks as the above
described.
The FCFS SAVI device generates 2 DAD_NS packets upon the reception of
a DAD_NS or a data packet. As such, the FCFS SAVI device can be used
as an amplifier by attackers. In order to limit this type of attack,
the FCFS SAVI device MUST performs rate limiting of the messages it
generates. The rate limiting is performed on a per port basis, since
having an attack on a given port should not prevent the FCFS SAVI
device to function normally in the rest of the ports.
Residual threats.
FCFS SAVI perform its function by binding an IP source address to a
binding anchor. If the attacker manages to send packets using the
binding anchor associated to a given IP address, FCFS SAVI validation
will be successful and the FCFS SAVI device will allow the packet
through. This can be achieved by spoofing the binding anchor or
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because the binding anchor is shared among the legitimate owner of
the address and the attacker. An example of the latter is the case
where the binding anchor is a port of a switched network and a legacy
switch (i.e. no SAVI capable switch) is connected to that port. All
the source addresses of the hosts connected to the legacy switch will
share the same binding anchor (i.e. the switch port). This means
that hosts connected to the legacy switch can spoof each other's IP
address and this will not be detected by the FCFS SAVI device. This
can be prevented by not sharing binding anchors among hosts.
FCFS SAVI assumes that a host will be able to defend its address when
the DAD procedure is executed for its addresses. 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). So, when a
DAD_NS is issued for a given IP address for which a binding exists in
a FCFS SAVI device, the FCFS SAVI device expects to see a DAD_NA
coming from the binding anchor associated to that IP address in order
to preserve the binding. If the FCFS SAVI device does not see the
DAD_NA, 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 intercepting the DAD_NA or launching a DoS
attack to the host that will prevent it to issue proper DAD replies.
Even if routers are considered as trusted, nothing can prevent that a
router could be compromised and send traffic with spoofed IP source
addresses. Such a traffic would be allowed with the present FCFS
SAVI specification. A way to mitigate this issue could be to specify
a new port type (e.g. Router Port, RP) that would act as Trusted
Port for the transit traffic and as Validating Port for the local
traffic. A detailed solution about this issue is outside the scope
of this document.
Privacy considerations
Personally identifying information MUST NOT be included in the FCFS
SAVI DB with the MAC address as the canonical example, except when
there is an attempt of attack involved. Moreover, compliant
implementation MUST NOT log binding anchor information except where
there is an identified reason why that information is likely to be
involved in detection, prevention or tracing of actual source address
spoofing. Information that is not logged MUST be deleted as soon as
possible (i.e. as soon as the the state for a given address is back
to NO_BIND). Information about the majority of hosts that never
spoof SHOULD NOT be logged.
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Interaction with Secure Neighbour Discovery
Even if FCFS SAVI could get information from ND messages secured with
SEND [RFC3971], in some case, the FCFS SAVI device must spoof DAD_NS
messages but doesn't know the security credentials associated with
the IPAddr (i.e. the private key used to sign the DAD_NS messages).
So, when SEND is deployed, it is recommended to use SEND SAVI
[I-D.ietf-savi-send] rather than FCFS SAVI."
5. IANA Considerations
This document has no actions for IANA.
6. Contributors
Jun Bi
CERNET
Network Research Center, Tsinghua University
Beijing 100084
China
Email: junbi@cernet.edu.cn
Guang Yao
CERNET
Network Research Center, Tsinghua University
Beijing 100084
China
Email: yaog@netarchlab.tsinghua.edu.cn
Fred Baker
Cisco Systems
Email: fred@cisco.com
Alberto Garcia Martinez
University Carlos III of Madrid
Email: alberto@it.uc3m.es
7. Acknowledgments
This draft benefited from the input from: Joel Halpern, Christian
Vogt, Dong Zhang, Frank Xia, Jean-Michel Combes, Jari Arkko, Stephen
Farrel, Dan Romascanu, Russ Housley, Pete Resnick, Ralph Droms,
Wesley Eddy, Dave Harrington and Lin Tao.
Marcelo Bagnulo is partly funded by Trilogy, a research project
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supported by the European Commission under its Seventh Framework
Program.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-06 (work in progress),
December 2011.
[I-D.ietf-savi-dhcp]
Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
DHCP", draft-ietf-savi-dhcp-12 (work in progress),
February 2012.
[I-D.ietf-savi-send]
Bagnulo, M. and A. Garcia-Martinez, "SEND-based Source-
Address Validation Implementation",
draft-ietf-savi-send-06 (work in progress), October 2011.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[IEEE.802-1Q.2005]
Institute of Electrical and Electronics Engineers, "IEEE
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Standard for Local and metropolitan area networks /
Virtual Bridged Local Area Networks", IEEE Standard
802.1Q, May 2005.
Appendix A. Implications of not following the recommended behaviour
This section qualifies some of the SHOULDs that are included in this
specification, by explaining the implications of not following the
recommended behaviour. We start by describing the implication of not
following the recommendation of generating DAD-NS upon the reception
of a data packet for which the there is no binding and then we
describe the implications of not discarding the non compliant
packets.
A.1. Implications of not generating DAD-NS packets upon the reception
of non compliant data packets
This specification recommends SAVI implementations to generate a
DAD_NS message upon the reception of a data packet for which they
have no binding for. In this section we describe the implications of
not doing so and simply discarding the data packet instead.
The main argument against discarding the data packet is the overall
robustness of the resulting network. The main concern that has been
stated is that a network running SAVI that discard data packets in
this case may end up disconnecting legitimate users from the network,
by filtering packets coming from them. The net result would a
degraded robustness of the network as a whole, since legitimate users
would perceive this as a network failure. There are three different
causes that resulted in the lack of state in the binding device for a
legitimate address, namely, packet loss, state loss and topology
change. We will next perform an analysis for each of them.
A.1.1. Lack of binding state due to packet loss
The DAD procedure is inherently unreliable. It consists on sending a
NS packet and if no NA packet is received back, success is assumed
and the host starts using the address. In general, the lack of
response is because no other host has that particular address
configured in their interface, but it may also be the case that the
NS packet or the NA packet has been lost. From the sending host
perspective there is no difference and the host assumes that it can
use the address. In other words, the default action is to allow the
host to obtain network connectivity.
It should be noted that the loss of a DAD packet has little impact on
the network performance, since address collision is very rare and the
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host assumes success in that case. By designing a SAVI solution that
would discard packets for which there is no binding, we are
diametrically changing the default behavior in this respect, since
the default would be that if the DAD packets are lost, then the node
is disconnected from the network (as its packets are filtered). What
is worse, the node has little clue of what is going wrong, since it
has successfully configured an address but it has no connectivity.
The net result is that the overall reliability of the network has
significantly decreased as the loss of a single packet would imply
that a host is disconnected from the network.
The only mechanism that the DAD has to improve its reliability is to
send multiple NS. However, current RFC4862 defines a default value
of 1 NS message for the DAD procedure, so requiring any higher value
would imply manual configuration of all the hosts connected to the
SAVI domain.
A.1.1.1. Why initial packets may be (frequently) lost
The case of LANs
Devices connecting to a network may experience periods of packet loss
after the link-layer becomes available for two reasons: Invalid
Authentication state and incomplete topology assessment. In both
cases, physical-layer connection occurs initially and presents a
medium where packets are transmissible, but frame forwarding is not
available across the LAN.
For the authentication system, devices on a controlled port are
forced to complete 802.1X authentication which may take multiple
round trips and many Milliseconds to complete (see IEEE 802.1X-2004).
In this time, initial DHCP, IPv6 Neighbour Discovery, Multicast
Listener or Duplicate Address Detection messages may be transmitted.
However, it has also been noted that some devices have the ability
for the IP stack to not see the port as up until 802.1x has
completed. Hence, that issue needs investigation to determine how
common it is now.
Additionally, any system which requires user input at this stage can
extend the authentication time, and thus the outage. This is
problematic where hosts relying upon DHCP for address configuration
time out.
Upon completion of authentication, it is feasible to signal upper
layer protocols as to LAN forwarding availability. This is not
typical today, so it is necessary to assume that protocols are not
aware of the preceding loss period.
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For environments which do not require authentication, addition of a
new link can cause loops where LAN frames are forwarded continually.
In order to prevent loops, all LANs today run a spanning-tree
protocol, which selectively disables redundant ports. Devices which
perform spanning-tree calculations are either traditional Spanning-
Tree Protocol (STP) (see IEEE802.1D-1998) or rapidly converging
versions of the same (RSTP/MSTP) (see IEEE 802.1D-2004 and IEEE
802.1Q-2005).
Until a port is determined to be an edge port (RSTP/MSTP), the rapid
protocol speaker has identified its position within the spanning-tree
(RSTP/MSTP) or completed a Listening phase (STP), its packets are
discarded.
For ports which are not connected to rapid protocol switches, it
takes a minimum three seconds to perform edge port determination (see
IEEE 802.1D-2004). Alternatively completion of Listening phase takes
15 seconds (see IEEE 802.1D-1998). During this period, the link-
layer appears available, but initial packet transmissions into and
out of this port will fail.
It is possible to pre-assess ports as edge ports using manual
configuration of all the involved devices and thus make them
immediately transmissible. This is never default behaviour though.
The case of fixed access networks
In fixed access networks such as DSL and Cable the end hosts are
usually connected to the access network through a residential gateway
(RG). If the host interface is initialized prior to the residential
gateway getting authenticated and connected to the access network,
the access network is not aware of the DAD packets that the host sent
out. As an example, in DSL networks the Access Node(DSLAM) that
needs to create and maintain binding state will never see the DAD
message that is required to create such state.
A.1.1.1.1. Special sub-case:SAVI device rate-limiting packets
A particular sub-case is the one where the SAVI device itself "drops"
ND packets. In order to protect itself against DoS attacks and
flash-crowds, the SAVI device will have to rate-limit the processing
of packets triggering the state creation process (which require
processing from the SAVI device). This implies that the SAVI device
may not process all the ND packets in case it is under heavy
conditions. The result is that the SAVI device will fail to create a
binding for a given DAD_NS packet, which implies that the data
packets coming from the host that sent the DAD_NS packet will be
filtered if this approach is adopted. The problem is that the host
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will assume that the DAD procedure was successful and will not
perform the DAD procedure again which in turn will imply that the
host will be disconnected from the network. While it is true that
the SAVI device will also have to rate limit the processing of the
data packets, the host will keep on sending data packets, so it is
possible to recover from the alternative approach where data packets
trigger the binding creation procedure.
A.1.2. Lack of binding state due to a change in the topology
In the case SAVI is being deployed in a switched Ethernet network,
topology changes may result in a SAVI device receiving packets from a
legitimate user for which the SAVI device does not have a binding
for. Consider the following example:
+------+ +--------+ +---------------+
|SAVI I|-------------|SWITCH I|-------|rest of the net|
+------+ +--------+ +---------------+
| |
| +--------+
| | SAVI II|
| +--------+
| +----------+ |
+---|SWITCH II |-----+
+----------+
|
+-----+
| Host|
+-----+
Suppose that after bootstrapping all the elements are working
properly and the spanning tree is rooted in the router and it
includes one branch that goes SWITCH I-SAVI I- SWITCH II and another
branch that goes SWITCH I-SAVI II.
Suppose that the Host boots at this moment and sends the DAD_NS. The
message is propagated through the spanning tree and it received by
SAVI I but not by SAVI II. SAVI I creates the binding.
Suppose that SAVI I fails and the spanning tree reconverges to SWITCH
I- SAVI II- SWITCH II. Now data packets coming from the Host will be
coursed through SAVI II which does not have binding state and will
drop the packets.
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A.1.3. Lack of binding state due to state loss
The other reason why a SAVI device may not have state for a
legitimate address is simply because it lost it. State can be lost
due to a reboot of the SAVI device or other reasons such as memory
corruption. So, the situation would be as follows: the host performs
the DAD procedure and the SAVI device creates a binding for the
host's address. The host successfully communicate for a while. The
SAVI device reboots and lost the binding state. The packets coming
from the host are now discarded as there is no binding state for that
address. It should be noted that in this case, the host has been
able to use the address successfully for a certain period of time.
Architecturally, the degradation of the network robustness in this
case can be easily explained by observing that this approach to SAVI
implementation breaks the fate-sharing principle. RFC 1958 reads:
An end-to-end protocol design should not rely on the maintenance
of state (i.e. information about the state of the end-to-end
communication) inside the network. Such state should be
maintained only in the endpoints, in such a way that the state can
only be destroyed when the endpoint itself breaks (known as fate-
sharing).
By binding the fate of the host's connectivity to the state in the
SAVI device, we are breaking this principle and the result is
degraded network resilience.
Moving on to more practical matters, we can dig deeper into the
actual behaviour by considering two scenarios, namely, the case where
the host is directly connected to the SAVI device and the case where
there is an intermediate device between the two.
A.1.3.1. The case of a host directly connected to the SAVI device
The considered scenario is depicted in the following picture:
+------+ +-----------+ +---------------+
| Host |-------------|SAVI device|-------|rest of the net|
+------+ +-----------+ +---------------+
The key distinguishing element of this scenario is that the host is
directly connected to the SAVI device. As a result, if the SAVI
device reboots, the host will see the carrier disappear and appear
again.
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RFC4862 requires that the DAD procedure is performed when the IP
address is assigned to the interface, quoting RFC4862 section 5.4.
Duplicate Address Detection:
Duplicate Address Detection MUST be performed on all unicast
addresses prior to assigning them to an interface, regardless of
whether they are obtained through stateless autoconfiguration,
DHCPv6, or manual configuration, with the following exceptions:...
However, it has been stated that some of the widely used OSes
actually do perform DAD each time the link is up, but further data
would be required to take this for granted. Assuming that behaviour,
that implies that if the lost of state in the SAVI device also
results in the link to the host going down, then the host using the
tested OSes would redo the DAD procedure allowing the recreation of
the binding state in the SAVI device and preserving the connectivity
of the host. This would be the case if the SAVI device reboots. It
should be noted though, that it is also possible that the binding
state is lost for whatever error in the SAVI process and that the
SAVI link does not goes down. In this case, the host would not redo
the DAD procedure. However, it has been pointed out that it would be
possible to require the SAVI process to flap the links of the device
it is running, in order to make sure that the links goes down each
time the SAVI process restarts and improving the chances the host
will redo the DAD procedure when the SAVI process is rebooted.
A.1.3.2. The case of a host connected to the SAVI device through one or
more legacy devices.
The considered scenario is depicted in the following picture:
+------+ +-------------+ +-----------+ +---------------+
| Host |----|Legacy device|-----|SAVI device|----|rest of the net|
+------+ +-------------+ +-----------+ +---------------+
The key distinguishing element of this scenario is that the host is
not directly connected to the SAVI device. As a result, if the SAVI
device reboots, the host will not see any changes.
In this case, the host would get get disconnected from the rest of
the network since the SAVI device would filter all its packets once
the state has gone. As the node will not perform the DAD procedure
again, it will remain disconnected until it reboots.
As a final comment, it should be noted that it may not be obvious to
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the network admin which scenario its network is running. Consider
the case of a campus network where all the switches in the network
are SAVI capable. A small hub connected in the office would turn
this into the scenario where the host is not directly connected to
the SAVI device. Moreover, consider the case of a host running
multiple virtual machines connected through a virtual hub, depending
on the implementation of such a virtual hub, may turn a directly
connected host scenario to the scenario where the multiple (virtual)
hosts are connected through a legacy (virtual) hub.
A.1.3.2.1. Enforcing direct connectivity between the SAVI device and
the host
It has been argued that enforcing the direct connectivity between the
SAVI device and the end host is actually a feature. There are
several comments that can be made in this respect:
First, it may well be the case in some scenarios this is
desirable, but it is certainly not the case in most scenarios.
Because of that, the issue of enforcing direct connectivity must
be treated as orthogonal to how data packets for which there is no
binding are treated, since a general solution must support
directly connected nodes and nodes connected through legacy
switches.
Second, as a matter of fact, the resulting behaviour described
above would not actually enforce direct connectivity between the
end host and the SAVI device as it would work as long as the SAVI
device would not reboot. So, the argument being made is that this
approach is not good enough to provide a robust network service,
but it is not bad enough to enforce the direct connectivity of
host to the SAVI switch.
Third, it should be noted that topology enforcement is not part of
the SAVI problem space and that the SAVI problem by itself is hard
enough to add additional requirements.
A.2. Implications of not diacarding non compliant data packets
The FCFS SAVI mechanism is composed of two main functions, namely,
the mechanisms for tracking compliant and non compliant data packets
and the actions to be performed upon the detection of a non compliant
packet. Throughout this specification, we recommend to discard non
compliant data packets. This is so because forwarding non compliant
data packets is essentially allowing packets with spoofed source
address to flow throughout the network. However, there are
alternative actions that can be taken with respect to this packets.
For instance, it would be possible to forward the packets and trigger
an alarm to the network administrator to make him aware of the
situation. Similarly, it would be possible to log these events and
allow to track down the cases where the packets with spoofed
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addresses were used for malicious purposes. The reason why a site
deploying SAVI may not want to take milder action like the ones
mentioned above instead of discarding packets is because there may be
cases where the non compliant packets may be legitimate packets (for
example in the case that the SAVI device is malfunctioning and it has
failed to create the appropriate bindings upon the reception of a DAD
packets).
Authors' Addresses
Erik Nordmark
Cisco
510 McCarthy Blvd.
Milpitas, California 95035
UNITED STATES
Email: nordmark@acm.org
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
Eric Levy-Abegnoli
Cisco Systems
Village d'Entreprises Green Side - 400, Avenue Roumanille
Biot-Sophia Antipolis - 06410
France
Email: elevyabe@cisco.com
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