Network Working Group E. Nordmark
Internet-Draft Sun
Intended status: Standards Track M. Bagnulo
Expires: January 13, 2011 UC3M
E. Levy-Abegnoli
Cisco Systems
July 12, 2010
FCFS-SAVI: First-Come First-Serve Source-Address Validation for Locally
Assigned Addresses
draft-ietf-savi-fcfs-04
Abstract
This memo describes FCFS SAVI a mechanism to provide source address
validation for IPv6 networks using the First-Come First-Serve
approach. The proposed mechanism is intended to complement ingress
filtering techniques to provide a higher granularity on the control
of the source addresses used.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 13, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Design considerations . . . . . . . . . . . . . . . . . . . . 4
2.1. Scope of FCFS SAVI . . . . . . . . . . . . . . . . . . . . 4
2.2. Constraints for FCFS SAVI . . . . . . . . . . . . . . . . 5
2.3. Address ownership proof . . . . . . . . . . . . . . . . . 5
2.4. Layer-2 Anchor considerations . . . . . . . . . . . . . . 6
2.5. Special cases . . . . . . . . . . . . . . . . . . . . . . 6
3. SAVI topology and port configuration . . . . . . . . . . . . . 6
3.1. SAVI enforcement perimeter . . . . . . . . . . . . . . . . 7
3.2. SAVI port configuration guidelines . . . . . . . . . . . . 10
3.3. VLAN support . . . . . . . . . . . . . . . . . . . . . . . 11
4. FCFS SAVI specification . . . . . . . . . . . . . . . . . . . 11
4.1. FCFS SAVI Data structures . . . . . . . . . . . . . . . . 11
4.2. FCFS SAVI algorithm . . . . . . . . . . . . . . . . . . . 11
4.2.1. Discovering on-link prefixes . . . . . . . . . . . . . 11
4.2.2. Processing of transit traffic . . . . . . . . . . . . 12
4.2.3. Processing of local traffic. . . . . . . . . . . . . . 12
4.3. Protocol Constants . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Implications of not following the reccomended
behaviour . . . . . . . . . . . . . . . . . . . . . . 21
A.1. Lack of binding state due to packet loss . . . . . . . . . 21
A.1.1. Why initial packets may be (frequently) lost . . . . . 22
A.2. Lack of binding state due to a change in the topology . . 24
A.3. Lack of binding state due to state loss . . . . . . . . . 24
A.3.1. The case of a host directly connected to the SAVI
device . . . . . . . . . . . . . . . . . . . . . . . . 25
A.3.2. The case of a host connected to the SAVI device
through one or more legacy devices. . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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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
approach. The proposed mechanism is intended to complement ingress
filtering techniques to provide a higher granularity on the control
of the source addresses used.
2. Design considerations
2.1. Scope of FCFS SAVI
The application scenario for FCFS SAVI is limited to the local-link.
This means that the goal of FCFS SAVI is verify that the source
address of the packets generated by the hosts attached to the local
link have not been spoofed.
In any link there usually are hosts and routers attached. Hosts
generate packets with their own address as the source address. This
is the so-called local traffic. While routers send packets
containing a source address other than their own, since they are
forwarding packets generated by other hosts (usually located in a
different link). This what the so-called 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 is 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
addresses, in particular with address configured through stateless
address autoconfiguration [RFC4862]. Manually configured addresses
can be supported by FCFS SAVI, but manual configuration of the
binding on the SAVI device provides higher security and seems
compatible with manual address management. Additional considerations
about how to use FCFS SAVI depending on the type of address
management used and the nature of the addresses is discussed in the
framework document (add reference when available).
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2.2. Constraints for FCFS SAVI
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 must solely rely in the usage of already available
protocols. This means that FCFS SAVI cannot 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 any source address validation solution, we need to define some
address ownership proof concept i.e. what it means to be able to
proof that a given host owns a given address in the sense that the
host is entitled to send packet with that source address.
Since no host changes are acceptable, we need to find the means to
proof address ownership without requiring a new protocol. In FCFS
SAVI the address ownership proof is based in the First-Come first
Serve approach. This means that the first host that claims a given
source address is the owner of the address until further notice.
More precisely, 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 the layer-2 information that
the FCFS SAVI box has available (e.g. the port in a switched LAN).
Following data packets containing that IP source address must use the
same layer-2 information in order to be compliant.
There are however additional considerations to be taken into account.
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 layer-2 information has changed. In
order to cope with this case, the defined FCFS SAVI behaviour implies
the verification whether the host is still reachable using the
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previous layer-2 information. In order to do that FCFS SAVI uses the
Neighbour Discovery (ND) protocol. If the host is no longer
reachable at the previously recorded layer-2 information, FCFS SAVI
assumes that the new location is valid and creates a new binding
using the new layer-2 information. In case the host is still
reachable using the previously recorded information, the packets
coming from the new layer-2 information 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. 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. Layer-2 Anchor considerations
Any SAVI solution is not stronger than the Layer-2 anchor it uses.
If the Layer-2 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 Layer-2
anchor is easily spoofable and the SAVI device is configured to drop
no compliant packets, then the usage of SAVI may open a new vector of
Denial of Service attacks, based on spoofed Layer-2 anchors. For
that reason, in this document, we assume that the Layer-2 anchors
used by the SAVI solution are not easily spoofable (e.g. ports of a
switched network) and that the SAVI device MAY be configured to drop
non- compliant packets. If the SAVI solution proposed in this
document is to be used with weaker Layer-2 anchors (such as MAC
addresses), logging is RECOMMENDED instead of dropping non-compliant
packets. For the rest of the document, we will assume that the
Layer-2 anchors are ports of a switched network.
2.5. Special cases
The following special cases that need to be considered
o Hosts with multiple physical interfaces connected to the same
link.
o Anycast i.e. multiple hosts using the same source address to send
packets.
o Proxy ND i.e. host sending packets on behalf of other, in a
layer-3 transparent manner.
o Optimistic Duplicate Address Detection (DAD) [RFC4429].
3. SAVI topology and port configuration
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3.1. SAVI enforcement perimeter
SAVI provides perimetrical security. This means that the SAVI
devices form what can be called a SAVI enforcement perimeter and they
verify that any packet that crosses the perimeter is compliant (i.e.
the source address related information is validated). Once the
packet is inside the perimeter, no further validations are performed
to the packet. This model has implications both on how SAVI devices
are deployed in the topology and on the configuration of the 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. This means that while
packets coming from interfaces connected to the external part of the
topology need to be validated by the SAVI device, packets coming from
interfaces connected to the the internal part of the topology do not
need to be validated. This significantly reduces the processing
requirements of the SAVI device. It also implies that each 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 SAVI
device binds the source address to a layer-2 anchor.
One possible approach would be for every SAVI device to store binding
information about every source addresses in the subnetwork. This
means that every SAVI device would store binding for each source
address to the local layer-2 anchor through packets with that source
address can be received through. The problem with this approach is
that it imposes significant memory burden on the SAVI devices. In
order to reduce the memory requirements imposed to each device, the
SAVI solution described in this specification distributes the storage
of SAVI binding information among the multiple SAVI devices of a
subnetwork. The SAVI binding state is distributed across the SAVI
devices according to the following criteria: each SAVI device will
store binding information about the source addresses bound to layer-2
anchors corresponding to the interfaces that connect to the part of
the topology that is outside of the SAVI enforcement perimeter.
Since all the untrusted packet sources are by definition in the
external part of the perimeter, this means that the packets generated
by each of the untrusted sources will reach the perimeter through one
interface of a SAVI device. The binding information for that
particular source address will be stored in this first SAVI device
the packet reaches to.
This means the 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
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different SAVI devices is coherent. In particular, it is important
to avoid that the same source address is bound to different layer-2
anchors in different 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 we are trying to prevent. In order to
preserve the coherency of the SAVI bindings distributed among the
SAVI devices within a realm, the Neighbour Discovery (ND) protocol is
used, in particular the Neighbour Solicitation (NSOL) and Neighbour
Advertisement (NADV) messages. Before creating a SAVI binding in the
local SAVI database, the SAVI device will send a NSOL message
querying for the address involved. Should any host reply to that
message with a NADV message, the SAVI device that sent the NADV will
infer that a binding for that address exists in another SAVI device
and will not create a local binding for it. If no NADV message is
received as a reply to the NSOL, then the local SAVI device will
infer that no binding for that address exists in other SAVI device
and will create the local SAVI binding for that address. (NOTE that
the description included here is overly simplified to illustrate the
mechanism used to preserve the coherency of the binding databases of
the different SAVI devices. The actual SAVI mechanism as described
below varies in the fact that in some cases it is the SAVI device
that generates the NSOL while in other cases it simply forwards the
NSOL generated by the end host, and that the SAVI device will send
multiple copies of the NSOL in order to improve the reliability of
the message exchange).
So, summarizing, the proposed SAVI approach relies on the following
design choices:
o SAVI provides perimetrical security, so some interfaces of a 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 SAVI device only verifies packets coming though one interface
connected to the untrusted part of the topology.
o A SAVI device only stores binding information for the source
addresses that are bound to layer-2 anchors that correspond to
interfaces that connect to the untrusted part of the topology.
o SAVI uses the NSOL and NADV messages to preserve the coherency of
the SAVI binding state distributed among the SAVI devices within a
realm.
So, in a link that is constituted of multiple L2 devices, some of
which are SAVI capable and some of which are not, the SAVI capable
devices SHOULD be deployed forming a connected perimeter (i.e. that
no data packet can get inside the perimeter without passing through a
SAVI device). Packets that cross the perimeter will be validated
while packets that do no cross the perimeter are not validated (hence
SAVI protection is not provided for these packets). Consider the
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deployment of SAVI in the topology depicted in the following picture:
+--+ +--+ +--+ +--+
|H1| |H2| |H3| |R1|
+--+ +--+ +--+ +--+
| | | |
+-------------SAVI-ENFORCEMENT-PERIMETER--------------+
| | | | | |
| +-1-----2-+ +-1-----2-+ |
| | SAVI1 | | SAVI2 | |
| +-3--4----+ +--3------+ |
| | | +--------------+ | |
| | +----------| |--------+ |
| | | SWITCH-A | |
| | +----------| |--------+ |
| | | +--------------+ | |
| +-1--2----+ +--1------+ |
| | SAVI3 | | SAVI4 | |
| +-3---4---+ +----4----+ |
| | | | |
+-------------SAVI-ENFORCEMENT-PERIMETER--------------+
| | |
+--+ +--+ +---------+
|R2| |H4| |SWICTH-B |
+--+ +--+ +---------+
| |
+--+ +--+
|H5| |H6|
+--+ +--+
In the figure above, the SAVI enforcement perimeter is provided by 4
SAVI devices, namely SAVI1, SAVI2, SAVI3 and SAVI4. These devices
verify information related to the source address both in data and in
ND packets and filter packets accordingly.
SAVI devices then have two types of ports: trusted ports and
validating ports.
o Validating ports (VPs) are those in which SAVI processing is
performed. This means that when a packet is received through one
of the validating ports, the SAVI processing and filtering will be
executed.
o Trusted ports (TPs) are those in which SAVI processing is not
performed. So, packets received through trusted ports are not
validated and no SAVI processing is performed in them.
Trusted ports are used for connections with trusted infrastructure,
including the communication between SAVI devices, the communication
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with routers and the communication of other switches that while they
are not SAVI devices, they only connect to trusted infrastructure
(i.e. other SAVI devices, routers or other trusted nodes). So, in
the figure above, Port 3 of SAVI1 and port 1 of SAVI3 are trusted
because the connect two 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 SAVI
enforcement perimeter also are connected through validating port. 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.
3.2. SAVI port configuration guidelines
The guidelines for port configuration in SAVI devices are:
o Ports that are connected to another SAVI device SHOULD be
configured as Trusted ports. Not doing so will at least
significantly increase the memory consumption in the SAVI devices.
o Ports connected to hosts SHOULD be configured as Validating ports.
Not doing so will allow the host connected to that port to send
packets with spoofed source address.
o Ports connected to routers SHOULD be configured as Trusted ports.
Configuring them as Validating ports would increase the signaling
due to SAVI. The implication is that a router can generate
traffic with any source address as they are assumed to be part of
the trusted infrastructure.
o Ports connected to a chain of one or more legacy switches that
have hosts connected SHOULD be configured as Validating ports.
Not doing so will allow the host connected to any of these
switches to send packets with spoofed source address.
o Ports connected to a chain of one or more legacy switches that
have other SAVI devices and/or routers connected but had no hosts
attached to them SHOULD be configured as Trusted ports. Not doing
so will at least significantly increase the memory consumption in
the SAVI devices and increase the signaling traffic due to SAVI
validation.
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o Ports connected to a chain of one or more legacy switches that
have a mix of SAVI devices and/or routers with hosts, SHOULD be
configured as Validating ports. Not doing so will allow the host
connected to that port to send packets with spoofed source
address. Nevertheless, this topology will result in increased
SAVI signaling and memory consumption compared to a topology where
SAVI-hosts communications and inter SAVI communications are kept
through different legacy switches.
3.3. VLAN support
In the case the SAVI device is a switch that supports VLANs, the SAVI
implementation will behave as if there was one SAVI process per VLAN.
The SAVI process of each VLAN will store the binding information
corresponding the the nodes attached to that particular VLAN.
4. FCFS SAVI specification
4.1. FCFS SAVI Data structures
FCFS SAVI function relies on state information binding the source
address used in data packets to the layer-2 information 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 Layer-2 information, such as Layer-2 address, port through which
the packet was received, etc
o Lifetime
o Status:either tentative or valid
o Creation time: the value of the local clock when the entry was
firstly created
In addition to this, FCFS SAVI need to know what are the prefixes
that are directly connected, so it maintains a data structure called
the the FCFS SAVI prefix list, which contains:
o Prefix
o Interface where prefix is directly connected
4.2. FCFS SAVI algorithm
4.2.1. Discovering on-link prefixes
In order to distinguish local traffic form transit traffic, the SAVI
device relies on the FCFS SAVI Prefix list, which contains the set of
on-link prefixes. A SAVI device MUST support the following two
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methods for populating the Prefix List: Manual configuration and
Router Advertisement, as detailed next.
Manual configuration: A SAVI device MUST support manual configuration
of the on-link prefixes included in the Prefix List.
Router Advertisement: A SAVI device MUST support discovery of on-link
prefixes through Router Advertisement messages. The 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 instead than in the ND Prefix List. In
addition, when the SAVI device boots, it MUST send a Router
Solicitation message as described in section 6.3.7 of [RFC4861],
using the unspecified source address.
4.2.2. Processing of transit traffic
The FCFS SAVI function is located in a forwarding device, such as a
router or a layer-2 bridge. 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
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 local
prefixes of the receiving interface, this means that the data
packet is transit traffic and the packet SHOULD be discarded.
The FCFS SAVI function MAY send an ICMP Destination Unreachable
Error back to the source address of the data packet. (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 the receiving port, then the SAVI
local traffic validation processes is executed as described
below.
4.2.3. Processing of local traffic.
We describe next how the local traffic, including both control and
data packets are processed by the SAVI device using a state machine
approach.
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The state machine described is for the binding of a given source IP
address in a given SAVI device. So this means that all the packets
described as inputs in the state machine above refer to that given IP
address. The key attribute is the IP address. The full state
information is:
o IP ADDRESS: IPAddr
o LAYER_2 ANCHOR: P
o LIFETIME: LT
The possible states are:
o NO_BIND
o TENTATIVE
o VALID
o TESTING_TP
o TESTING_VP
o TESTING_LIFETIME
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 Layer-2 anchor.
NO_BIND: The binding for a source IP address entry is in this state
when it does not have any binding to a Layer 2 anchor. All addresses
are in this state by default after bootstrapping, unless bindings
were created for it.
TENTATIVE: The binding for a source address is in this state during
the waiting period during which the DAD procedure is being executed
(either directly by the host itself or by the SAVI device on its
behalf).
VALID: The binding for the source address has been verified, it is
valid and usable for filtering traffic.
TESTING_TP: A binding for a source address enters in this state 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 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.
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
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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.
TESTING_LIFETIME: A binding for a source address is in this state
cause the lifetime of the entry is about to expire. This is due to
the fact that no packets has been seen by the 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.
We describe next how the different inputs are processed depending on
the state of the binding of the IP address.
A simplified figure of the state machine can be found at
http://www.it.uc3m.es/~marcelo/SAVI_state_machine.pdf
NO_BIND
o Upon the reception through a Validating Port (VP) of a Neighbour
Solicitation (NSOL) generated by the Duplicate Address Detection
(DAD) procedure (hereafter named DAD_NSOL) containing Target
Address IPAddr, the SAVI device MUST 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 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 500 milliseconds (i.e. the time between two
Neighbour Solicitation messages is 500 millisecs and also the wait
time for replies in the form of Neighbour Advertisement for the
queried address). This is equivalent to sending the received
DAD_NSOL message twice. The DAD_NSOL messages are not sent
through any of the ports configured as Validating Ports. The NSOL
messages are sent through the proper Trusted Ports (as defined by
the switch behaviour that will depend on whether it performs MLD
snooping or not).
* EDITOR NOTE: We need to rate limit the emission of NSOL of the
SAVI device as a whole.
* EDITOR NOTE 2: should we send the NSOL through the port the
packet was received through?. --JMC-- IMHO, if the SAVI device
has lost all the bindings (e.g. due to a crash) and, in fact,
had already before a binding for this IPAddr/Port, if the
action is triggered by a data packet, it should send the
DAD_NSOL through this port to regenerate the binding. --JMC--
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The state is moved to TENTATIVE. The LIFETIME is set to TENT_LT
(i.e. LT==TENT_LT) and the LAYER_2 ANCHOR is set to VP (i.e.
P==VP)
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
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 500 milliseconds (i.e. the time
between two Neighbour Solicitation messages is 500 millisecs and
also the wait time for replies in the form of Neighbour
Advertisement for the queried address). The implications of not
following the reccomended behaviour are described in Appendix A
The DAD_NSOL messages are not sent through any of the ports
configured as Validating Ports. The NSOL messages are sent
through the proper Trusted Ports (as defined by the switch
behaviour that will depend on whether it performs MLD snooping or
not). The SAVI device MAY discard the data packet while the DAD
procedure is being executed.
* EDITOR NOTE: We need to rate limit the emission of NSOL of the
SAVI device as a whole.
* EDITOR NOTE 2: should we send the NSOL through the port the
packet was received through?. --JMC-- IMHO, if the SAVI device
has lost all the bindings (e.g. due to a crash) and, in fact,
had already before a binding for this IPAddr/Port, if the
action is triggered by a data packet, it should send the
DAD_NSOL through this port to regenerate the binding. --JMC--
The state is moved to TENTATIVE. The LIFETIME is set to TENT_LT
(i.e. LT==TENT_LT) and the LAYER_2 ANCHOR is set to VP (i.e.
P==VP)
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_NSOL packets containing IPAddr as the target address received
through a Trusted port are NOT forwarded through any of the
Validating ports but they are sent through the proper Trusted
Ports (as defined by the switch behaviour that will depend on
whether it performs MLD snooping or not)
o Neighbor Advertisement packets sent to all nodes as a reply to the
DAD_NSOL (hereafter called DAD_NADV) 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
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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 (NADV) is received through a Trusted
Port with Target Address set to IPAddr, then state is set to
NO_BIND and the LAYER_2 ANCHOR and the LIFETIME are cleared. Data
packets stored corresponding to this binding are discarded.
o If a NADV is received through a Validating Port with Target
Address set to IPAddr, the NADV packet is discarded
o If a data packet with source address IPAddr is received with
Layer_2 anchor equal to P, then the packet is either stored or
discarded.
* EDITOR NOTE: we need to define a maximum storage space for the
data packets. Having a single default value may be hard since
it will heavily depend on the capability of the device. Maybe
it would be enough that the device has a maximum and that the
value can be configured?
o If a data packet with source address IPAddr is received through a
Trusted port, the data packet is forwarded. The state is
unchanged. ( waiting for the NADV?)
o If a data packet with source address IPAddr is received through a
Validating port other than P, the data packet is discarded.
o Other signaling packets are processed and forwarded as usual (i.e.
no SAVI processing)
* EDITOR NOTE: this may need more thought
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.
* EDITOR NOTE: Is this feasible? i.e. to reset a timer each time
a data packet arrives? We could just have a long lifetime and
actively check for the host when the lifetime is about to
expire.
o If a DAD_NSOL is received from a Trusted port, then the DAD_NSOL
message is forwarded to port P and it is also forwarded to the
proper Trusted Ports (as defined by the switch behaviour that will
depend on whether it performs MLD snooping or not). The state is
changed to TESTING_TP. The LIFETIME is set to TENT_LT.
o If a data packet containing source address IPAddr or a DAD_NSOL
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_NSOL 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 NSOL messages
for that address will be sent by the SAVI device) and RetransTimer
set to 500 milliseconds (i.e. the time between two NSOL messages
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is 500 millisecs and also the wait time for replies in the form of
Neighbour Advertisement for the queried address). The DAD_NSOL
message will be forwarded to the port P.
* EDITOR NOTE: should we also forward it though the TP?
Theoretically, there shouldn't be another binding in any other
SAVI device, so there should not be a need for this.
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.
o If the LIFETIME expires, then the SAVI device will execute the
process of sending DAD_NSOL 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 NSOL messages
for that address will be sent by the SAVI device) and RetransTimer
set to 500 milliseconds (i.e. the time between two NSOL messages
is 500 millisecs and also the wait time for replies in the form of
Neighbour Advertisement for the queried address). The DAD_NSOL
messages will be forwarded to the port P. The state is changed to
TESTING_LIFETIME 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 is discarded.
* EDITOR NOTE: receiving such a packet means that another SAVI
device has created a binding for this address, or that the
perimeter has been breached. This should be logged?
o Other signaling packets are processed and forwarded as usual (i.e.
no SAVI processing). In particular DAD_NADV containing IPAddr as
the target address are forwarded as usual.
TESTING_TP
o If the LIFETIME expires, the LAYER_2 ANCHOR is cleared and the
state is changed to NO_BIND
o If a NADV message containing the IPAddr as target address is
received through the Validating port P as a reply to the DAD_NSOL
message, then the NADV 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.
* EDITOR NOTE: should we move back to VALID? --JMC-- As the host
has been able to perform DAD before (i.e. as result, creation
of a Binding in the SAVI device), the same device should be
able to reply to the DAD_NSOL with a DAD_NADV: so, unless there
is a specific case I missed, I would suggest to keep the STATE
to TESTING_TP --JMC--
o If a data packet is received through a port that is other than
port P, then the packet is discarded.
TESTING_VP
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o If the LIFETIME expires, the LAYER_2 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.
o If a NADV message containing the IPAddr as target address is
received through the Validating port P as a reply to the DAD_NSOL
message, then the NADV 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.
* EDITOR NOTE: should we move back to VALID? --JMC-- see similar
comment above --JMC--
o If a data packet is received through a port that is other than
port P, then the packet is discarded.
TESTING_LIFETIME
o If the LIFETIME expires, the LAYER_2 ANCHOR is cleared and the
state is changed to NO_BIND
o If a NADV message containing the IPAddr as target address is
received through the Validating port P as a reply to the DAD_NSOL
message, then the NADV 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.
Rate limiting of messages: TBD
MLD considerations
The 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 SAVI device will receive the DAD_NSOL for those
addresses. Please note that it may not be enough to relay on the
host behind the Validating port doing so, since the node may move and
after a while, the packets for that particular solicited node
multicast group will no longer be forwarded to the SAVI device. So,
the SAVI device SHOULD join the solicited node multicast groups for
all the addresses that are in a state other than NO_BIND
4.3. Protocol Constants
TENT_LT
DEFAULT_LT
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5. Security Considerations
First of all, it should be noted that any SAVI solution will be as
strong as the lower layer anchor that it uses. In particular, if the
lower layer anchor is forgeable, then the resulting SAVI solution
will be weak. For example, if the lower layer anchor is a MAC
address that can be easily spoofed, then the resulting SAVI will not
be stronger than that. On the other hand, if we use switch ports as
lower layer anchors (and there is only one host connected to each
port) it is likely that the resulting SAVI solution will be
considerably more secure.
Denial of service attacks
There are two types of DoS attacks that can be envisaged in a SAVI
environment. On one hand, we can envision attacks against the SAVI
device resources. On the other hand, we can envision DoS attacks
against the hosts connected to the network where SAVI is running.
The attacks against the SAVI device basically consist on making the
SAVI device to consume its resource until it runs out of them. For
instance, a possible attack would be to send packets with different
source addresses, making the SAVI device to create state for each of
the addresses and waste memory. At some point the 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 SAVI device runs out of
the memory allocated for the 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 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 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.
The other type of attack is when an attacker manages to create state
in the SAVI device that will result in blocking the data packets sent
by the legitimate owner of the address. In IPv6 these attacks are
not an issue thanks to the 2^64 addresses available in each link.
Compare with Threat analysis and identify residual threats: TBD
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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 and Lin Tao.
Marcelo Bagnulo is partly funded by Trilogy, a research project
supported by the European Commission under its Seventh Framework
Program.
8. References
8.1. Normative References
[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
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Address Autoconfiguration", RFC 4862, September 2007.
8.2. Informative References
[RFC4429] Moore, N., "Optimistic Duplicate Address Detection (DAD)
for IPv6", RFC 4429, April 2006.
Appendix A. Implications of not following the reccomended behaviour
This specification recommends SAVI implementations to generate a
DAD_NSOL 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 w 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. Lack of binding state due to packet loss
The DAD procedure is inherently unreliable. It consists on sending a
NSOL packet and if no NADV 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
NSOL packet or the NADV 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 coalition is very rare and the
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
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significantly decreased as the lost 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 NSOL. However, current RFC4862 defines a default value
of 1 NSOL 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. 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 packeted are transmissable, 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.
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).
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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). This means that 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 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. 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 NSOL packet, which implies that the data
packets coming from the host that sent the DAD NSOL packet will be
filtered if this approach is adopted. The problem is that the host
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.
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A.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 NSOL.
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.
A.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
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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.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.
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
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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.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
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.
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A.3.2.1. Enforcing direct connectivty between the SAVI device and the
host
Some people have 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 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.
Authors' Addresses
Erik Nordmark
Sun Microsystems, Inc.
17 Network Circle
Menlo Park, CA 94025
USA
Phone: +1 650 786 2921
Email: Erik.Nordmark@Sun.COM
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: 34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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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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