Dual Homing Experiment
Christian Huitema
Architect
Windows Networking & Communications
Microsoft Corporation
Simple dual homing problem statement
Simple bridged network
2 routers, 2 ISP
Internet
(IPv6)


ingress filtering
No ISP coordination
Example
ISP1

ISP2


R1
Several host
R2

Single link (bridge)
H
7/20/2002
H
2 DSL modems
DSL + cable
Cable + WiFi mesh
H

Simple = IPv6 basic
Advanced = multihoming aware
It must work!
Dual homing experiment
2
Define “it must work”
For the simple host


Can receive and send connection through
either ISP
If an ISP is down, continue with other
For the advanced host



7/20/2002
May choose the best connection based on
some heuristic
May use MIPv6 to redirect connections
With MIPv6, connection survives loss of
one ISP.
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First order issue: ingress filtering
Avoid the following failure scenario




Host choose source address A
Default route leads to router B
Router B forwards the packet to ISP B
Packet is dropped by ingress filtering.
Possible solutions

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7/20/2002
Exit router check source address, redirect packet
to “right” router in case of problem
Host knows which router advertises what prefix,
select default router that matches source address
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Second issue: avoid using dead router
Failure scenario



Router A is dead, or link to ISP A is dead
Host continue sending using source
address A
Router B cannot forward the packets
Possible solutions

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7/20/2002
Dead router stops advertising, or advertise
prefix as “deprecated”
Smart host detects that the router is dead,
switches to other address
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Third issue: maintain connection
Failure scenario



Host has connection with peer P using
address A
Router A or ISP A fails
Existing connections break
Solution

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7/20/2002
No solution for dumb host
Variation of MIPV6 for smart host
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Fourth issue: use the right exit/entrance
Failure mode



Peer picks address A rather than address B
Resulting traffic routed slow path
Performance looks terrible
Solution

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7/20/2002
Need information about which address will
get best performance
Need MIPV6 to move connection to “better
address”
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7
Further study
Large networks

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Case 1: multilink subnet, all exit routers on
same link.
Case 2: routed network, all exit routers on
same link.
Case 3: routed network, exit routers at
different location
PI addressing

7/20/2002
Use a “virtual IP” solution, implemented
with tunnels, as “a third connection”
Dual homing experiment
8