I have evaluated separating TCP+UDP checksums from pseudo-header,
which gives some new applications:
1. ILNP - Identifier-Link Network Protocol.
2. IPsec-ESP-over-NAT-PT (one obstacle removed; not all)
But slows down other traffic by 0.4% (assuming 1000-byte packets) -
due to +4 bytes checksum at IP layer.
So separating checksums into TCP+IP layers has trade-offs, not a
simple improvement.
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-Alexey Eromenko "Technologov"