Anycast 101

Fri Dec 17 15:59:52 UTC 2004

On Fri, 17 Dec 2004, Iljitsch van Beijnum wrote:

> As for TCP, it would be very useful if someone were to run the
> following experiment:
>                         +-------+
>                         |router2|
>                         +-------+
>                       /           \
> +------+   +-------+               +-------+   +------+
> |host a+---+router1|               |router4+---+host b|
> +------+   +-------+               +-------+   +------+
>                       \           /
>                         +-------+
>                         |router3|
>                         +-------+
>
> (Assuming the links to the hosts are (for instance) gigabit and the
> ones between the routers fast ethernet. If they're all the same speed
> you're only going to see out of order packets when the later packet is
> smaller than the earlier packet, which is inconsistent with a TCP
> session running at full blast.)

Now, let's say that your path through router 2 is several hundred, or
maybe a few thousand, miles longer than your path through router 3.  You
are, after all, arguing that the paths are different enough that the
packets are going to end up at different anycast hosts, which is
generally equivalent to going into another network via a different
exchange point.

Have you just come up with a way to overcome the speed of light, or are
you arguing that doing per packet load balancing over paths with
differences in latency of tens or hundreds of milliseconds wouldn't result
in out of order packets?

-Steve