[ietf-nntp] Decreasing low water mark

[Clive D.W. Feather]

In 6.1.1.2 we write:

    Except when the group is empty and all three numbers are zero, whenever
    a subsequent GROUP command for the same newsgroup is issued, either by
    the same client or a different client, the reported low water mark in
    the response MUST be no less than that in any previous response for
    that newsgroup sent to any client.

In a cluster system where each server does its own expiry, it is possible
that one server may expire earlier than another. Thus we see something
like:

    [Connection to cluster]
    [S] 200 Welcome to server 1, last expiry 03:00
    [C] GROUP misc.test
    [S] 211 10 1011 1020 misc.test
    [C] QUIT
    [S] 205
    [Connection ends]

    [Connection to cluster]
    [S] 200 Welcome to server 2, last expiry 01:00
    [C] GROUP misc.test
    [S] 211 12 1000 1020 misc.test
    [C] QUIT
    [S] 205
    [Connection ends]

which breaks the requirement.

Russ's suggestion was to change the MUST to a SHOULD. I feel that we need
to say something about the times this is broken, but I can't offhand think
what.

-- 
Clive D.W. Feather  | Work:  <clive at demon.net>   | Tel:    +44 20 8495 6138
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