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Typical Scenario of IPv6 Multihoming
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THE PROBLEM
• Full Routing Table Too Large– because of random IPv4 address allocation
• to be solved with IPv6
– because of Multihoming
• # of TLAs should be controlled– below what?– below 1,000 seems to be easy– below 100 is not difficult, hopefully
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TLI
SLI
Subscribers
Typical Scenario of IPv6 Multihoming
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TLI
SLI
Subscribers 3 8 2 4
3 3 2 2
1 1 1 1
5
2
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Number of Prefixes with E2E Multihoming
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Multihoming
• Typical IPv4 multihoming– Advertise an address range through multiple
(not necessarily 2) routes– Explosion of
• # of routing table entries
• # of ASes
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MultihomedSite
H
ISP A ISP B
Rest of the Internet
Singly HomedSite
7Multihoming by (Intelligent) Routing
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１３１．１１３．０．０／１６
MultihomedSite
H
ISP A ISP B
Rest of the Internet
Singly HomedSite
8Multihoming by (Intelligent) Routing
１３１．１１２．０．０／１６１３１．１１２．０．０／１６
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MultihomedSite
H
ISP A ISP B
Rest of the Internet
Singly HomedSite
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End to EndMultihoming (1)
• A host has multiple addresses
• Application or transport tries all the destination addresses– Each address range can be aggregated
• No routing table entry explosion
• No AS number explosion
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End to EndMultihoming (2)
– When to Try Alternative Addresses?• Application/Transport dependent
– Controlled by the intelligence of end systems
• TCP will have a default timeout period
– Which Address Should be Tried Next?• Routing table can give hints
– Lack of a routing table entry means lack of reachability to a host
» Existence of an aggregated routing table entry does not mean reachability to the host
– Metric information in routing table can help too
11End to End Multihoming
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１３３．０．０．０／８ １３１．０．０．０／８
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１３３．１１２．３２．１３２，１３１．１１２．３２．１３２
MultihomedSite
H
ISP A ISP B
Rest of the Internet
Singly HomedSite
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Because It IS End to End
• No change to router functionality
• MUST change API on hosts– Or the hosts are singly homed
• Wrong to assume– Intelligent routers help dumb nodes– A host can and is recommended to have a
default free global (but now small) routing table– A real dumb host is dumb and singly homed
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Do Intelligent End SystemsRequire Standard IGP?
• No.
• Standard protocol to distribute (but not compute) routing table to hosts is required
• RIPv2 (with metric >15) seems to be good enough. Or RA?
• BGP routers generate metric from policy based preference
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Source Address Selection?
• Wrong topic for multihoming
• Both sources select destination addresses– At the destination, reverse & forward DNS
lookup of source address gives all the address of the source
• The destination selects an appropriate destination address for reply
• No source address selection by source meaningful
– Or, protocols may be modified to carry them
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TCP API/Protocol Changes
• Source have no reachability information of SYNACK reply addresses at the destination– Destination should select the address
• Multiple PCB entries for a connection• How to give the multiple addresses of the
source to the destination?– DNS?– Let SYN carry all the addresses?
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DNS Changes
• DNS can not give addresses for DNS reply• Clients should choose source address
reachable from the name server– First, choose randomly– Should (re)try with other source addresses– A lot of delay
• Or, change protocol?– Query carries all the addresses of the source
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End to End Multihomingand DNS/SMTP
• DNS and SMTP servers already deploy E2E multihoming– NS/MX servers may have multiple A records– If a server has multiple addresses
• All the addresses are tried
• It is of course as the most important, required-to-be-rubust applications of the Internet
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8+8
• DNS reverse lookup by lower 8 bytes only
• Hosts are identified by lower 8 bytes (IID)– A compact DNS name carried by all the packets
of all the protocols– Makes modification to application/transport
protocols for E2E multihoming easier
• Not Mike O’dell’s GSE one (violate E2E)
• Teraoka san will present his version (8+5)
