[evanj]

Its actually very useful, although very weak. The link layer check, at least in the case of Ethernet, really only protects your data "on the wire". It doesn't protect it inside the switches. It turns out there are failure modes, such as the one that caused this issue, where the packets get corrupted inside the switch (probably due to bad RAM). Details about how this can happen: http://www.evanjones.ca/tcp-and-ethernet-checksums-fail.html

After this kernel bug was fixed, the TCP checksum did its job and discarded all the corrupt packets.