2. 7/11/2003 Simple Multi-Homing Experiment 2
Simple dual homing problem statement
Simple bridged network
2 routers, 2 ISP
ingress filtering
No ISP coordination
Example
T1 + DSL back-up
2 DSL modems
DSL + cable
Cable + WiFi mesh
Several hosts
Simple = IPv6 basic
Advanced = multi-
homing aware
It must work!
Single link (bridge)
H H
H
R1 R2
ISP1 ISP2
Internet
(IPv6)
3. 7/11/2003 Simple Multi-Homing Experiment 3
IPv4 equivalent: back-up
Use for Back-Up
Switch R1 on if R2 is
down
May incur small
delay
In general, loose TCP
connections
Typically combined
with NAT & DHCP
Private addresses, no
renumbering
Single link (bridge)
H H
H
R1 R2
ISP1 ISP2
Internet
(IPv4)
4. 7/11/2003 Simple Multi-Homing Experiment 4
Broad Lines of the Solution
No coordination between ISP
Use of Provider Addresses
Each ISP allocates a prefix
or each ISP allocates an IPv4 address, and the
routers use 6to4
Multi-Addressing
Each router advertises a prefix
Hosts configure addresses with each prefix
Five issues need resolution
5. 7/11/2003 Simple Multi-Homing Experiment 5
Multi-addressing issues
Ingress filtering
host pick address from ISP1, send through R2?
Dead default exit router, or dead ISP
host keeps sending packets through a black hole?
Inbound connection through wrong ISP
Peers send packet to the black-holes address?
Maintaining TCP connections
Keep TCP going if the Router or the ISP fails?
Use the right exit/entrance
Maybe some amount of load balancing
6. 7/11/2003 Simple Multi-Homing Experiment 6
Ideas, Ingress Filtering
Choice by host
Host treats multiple “auto-config” prefixes
as “sub-interfaces”, associates individual
IPv6 address and default router.
Easy to implement in “new hosts”.
Redirect at routers
No need for tunnels in single link network.
Guarantees that “old hosts” keep working.
There may be other solutions
New services, ISP involvement, etc.
7. 7/11/2003 Simple Multi-Homing Experiment 7
Ideas, dead exit router or dead ISP
If the router notices the problem
Advertises prefix as “deprecated”, or stop
advertising
Will not be used for new connections
Will work for old and new hosts.
If the problem is not really detected
New host tries multiple source addresses
when establishing a new connection
Host may keep track of the quality of each
router connection
8. 7/11/2003 Simple Multi-Homing Experiment 8
Ideas, Peer using dead address
If the problem is detected
Update the name server?
If the problem is not detected
DNS advertises multiple addresses
Peer tries several addresses
Issue: TCP timers?
9. 7/11/2003 Simple Multi-Homing Experiment 9
Ideas, Maintaining TCP Connections
No good solution for old hosts
But there is no solution in a similar IPv4 set-up
either
Many applications will automatically reconnect
New hosts may use MIPv6
See “Application of the MIPv6 protocol to the
multi-homing problem”
draft-bagnulo-multi6-mnm-00
SCTP may also be used
See “multi-homing issues in SCTP”
draft-coene-sctp-multihome-04.txt
10. 7/11/2003 Simple Multi-Homing Experiment 10
Idea, Selecting the right exit/entrance
Right entrance: DNS tricks
In asymmetric scenarios (back-up), only
publish the “best address” in the DNS
In symmetric scenarios, publish both
Right exit: Routing tricks
Provide information in router
announcement, as in “Default Router
Preferences, More-Specific Routes, and
Load Sharing “
draft-ietf-ipv6-router-selection-02.txt
11. 7/11/2003 Simple Multi-Homing Experiment 11
Summary
It looks good on paper
All issues have plausible solutions
No change required to IPv6 standards
No need to rewrite the IPv6 address at site exit
We would benefit from “mobile IPv6” and “router
selection” work
But we would like an actual deployment…
In theory, there is no difference between theory
and practice, but in practice there is!
12. 7/11/2003 Simple Multi-Homing Experiment 12
Range of solutions
Small sites
Do as we just explained
Medium sites
Ask the ISP to cooperate, allow both
source addresses in ingress filtering
E.g. add a local route to the other prefix
Very large sites
Treat as ISP, get their own prefix & AS#
