Smaller than a /24 for BGP?

William Herrin bill at
Sat Jan 28 15:05:39 UTC 2023

On Fri, Jan 27, 2023 at 9:49 PM Masataka Ohta
<mohta at> wrote:
> That multihomed sites are relying on the entire Internet
> for computation of the best ways to reach them is not
> healthy way of multihoming.

This was studied in the IRTF RRG about a decade ago. There aren't any
other workable ways of multihoming compatible with the TCP protocol,
not even in theory. Every other mechanism imagined failed some basic
system constraint, usually the requirement that packets have
administrative permission to cross an intermediate network. So,
another way of multihoming critically depends on replacing the layer-4
protocols with something that doesn't intermingle the IP address with
the connection identifier.

For clarity: TCP's connection identifier consists of the source and
destination IP addresses plus the source and destination ports. Those
four elements, unique when combined, identify exactly one ongoing TCP
connection. Because of this, the connection must fail if the source or
destination IP addresses are no longer available to the source or
destination hosts. From this fact, we get the requirement that the
entire Internet learn when a particular IP address has changed its
position within the network.

Bill Herrin

For hire.

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