Date: Thu, 20 Dec 2001 11:18:50 +0100
From: Brian E Carpenter <[EMAIL PROTECTED]>
Message-ID: <[EMAIL PROTECTED]>
| The only way to restore the original
| value is if you carry it with the packet (nothing else is safe).
I'm not sure that is true. To take a (very stupid I admit) counter
example. A service provider might simply refuse to carry any traffic
with an odd flow id (they can throw away traffic for any reason they
like after all...). So, they know that the only packets that survive
entry filtering have an even flow id. Then they could use the lsb of
the flow id as some kind of signalling device inside the network.
Upon exit, the lsb of the flow id can be unilaterally cleared.
More likely, an ISP might arrange to use (internally) a certain flow ID
to get a specified service class. When a new flow for a particular
destination arrives, that requests that service class, they could send
notification to the destination of the original flow id, and the one
that will be used instead. Then for the rest of the flow, packets get
modified to use the internal flow id, with the exit router using the
pre-determined original flow ID to replace the one that was used internally.
This needs frills to be able to deal with multiple flows with the same
service class to the same destination but that's manageable (perhaps not
nice, but manageable).
So I'm not sure it is quite true to say that it would never make sense
for an ISP to do this (including putting the label back on exit) - but
as long as the label received is the same as the label transmitted,
there's no way to tell what is going on inside there, nor is there
any reason for anyone to care.
kre
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