Subject: RE: (ngtrans) shipworm, 6to4 and anycast [...]

From: "Christian Huitema" <[email-masked]>
Date: Thu, 31 Jan 2002 16:43:40 -0800

A point that has not been made yet in the discussion is the trade-off
between use of anycast addresses and load of the teredo servers. Coarse
studies of the NAT market show that:

* about 45% of the NAT on the market are "cone" NAT, i.e. establish a
port mapping that is independent of the peer address and port, and allow
packet coming from any source to use that mapping,

* about 50% are "restricted cone" or "port restricted cone" NAT, i.e.
establish a port mapping that is independent of the peer address and
port, but limit use of that mapping to peers to whom a packet has
already been sent,

* about 5% are "symmetric" NAT, for which the mapping is a function of
both the internal and external addresses and ports.

The bubble mechanism allows direct communication between the 95% of
Teredo clients that are not behind a symmetric NAT, i.e. in about 90% of
the direct exchanges. The actual reduction of traffic on the servers is
a function of how much traffic is between Teredo clients, versus how
much is between them and regular IPv6 clients. 

The relation with anycast is the following. If we use option 1, i.e. a
single anycast address, then we can effectively direct-route 90% of the
client-client traffic. If we split the clients along multiple clouds, as
in option 2 or 3, we reduce the scope in which anycast addresses are
used, but we also reduce the fraction of the traffic that can follow a
direct route, since the bubbles will only be sent to the members of "the
same cloud".

-- Christian Huitema