Partial checksums:
...
>I'm not convinced this is a real problem. Is there some network
>where this actually happens to a substantial degree where one wouldn't be
>better off just using FEC?
There are a number of issues here:
(1) Are bit errors sufficiently frequent in the current Internet
to make partial checksums worth the trouble? If so, it could be
because the app can make use of the errored data, or because the
app would prefer to know that this was corruption and not congestion,
and therefore avoid a congestion control response.
[Aside from Sally: This is a case where our underlying models
affect our design. If our conceptual model is one of a wireless
Internet with bit errors, then partial checksums seem more attractive.
If our model is one of a wireless Internet with lots of FEC and
such, in part to accommodate the needs of TCP, and therefore no bit
errors, the partial checksums seem less pressing.]
This is again the "chicken & egg" that was discussed wrt UDP-Lite. I
believe that bit errors are extremely rare in *today's* wireless Internet.
This is true at least for the cellular world (2.5G & 3G) where link layer
error detection (checksumming) filters out broken IP-Packets/L2-Frames no
matter how much FEC/L2-ARQ was applied before.