MUST is an aspiration

[Mark Smith <[e-mail]>]

We're seeing drafts that keep saying the inserted EH MUST be removed at
network egress, and therefore then use that to justify saying EH insertion
is fine.

I'd like to meet the people who have developed protocol implementations
that are and always will be bug free, never partially fail, and are always
100% configured correctly.

If they don't exist (they don't), then such a MUST in an RFC is an
aspiration. It's a theoretical goal, one that in practice has been proven
not to be 100% true in similar scenarios where things "MUST" not leak  (RFC
1918 leaks, route leaks, DNS PTR lookup for private addresses etc.)

Identifying packets with inserted EHs to then remove the EH is more
complicated that DA matching, which would be used with EH addition through
encapsulation. More complexity results in more likelihood of bugs and more
failure modes.

MUST is aspirational for DA matching (due to bugs, misconfiguration etc.),
it is even more so for complex EH insertion/subtraction operations.

If an additional IPv6 packet header causes too much overhead when used to
add local network information to an existing packet transiting a domain,
then IPv6 is the wrong tool for the job.

IPv6 packet headers are large because IPv6 addresses are large, and they're
large because they need to uniquely identify all devices attached to the
global IPv6 Internet.

For additional supplemental information limited to the local network, the
devices that only need to receive it is limited to those within the local
network that is adding the supplemental information. The only addressing
requirement here is to be able to uniquely identify the devices within the
network that will be adding and processing the local network's supplemental
information.

A much smaller encapsulation, to carry the local network supplimentary
information,  with much smaller addresses can be used, such as MPLS or IPv4.

IPv6 would be fine to use without modification if it didn't cause
unacceptable problems. Apparently does, motivating EH insertion to reduce
overhead. It's not the right tool for the job if its architecture has to
compromised to suit.