Week 2, Checksum, CRC and MAC

Although Checksums, CRC and MAC can’t guarantee many types of errors, but remember that “guarantee” is a strong statement, while in reality they still have great chance of detecting errors. Moreover, in practice you tend to have multiple layers of error detection, e.g.

End-to-end check是必须的，也就是每一层之间需要进行，而不能说Eternet layer check后IP就可以略过了，因为Eternet card might introduce an error after check. So IP has to do its own end-to-end check at the network layer.

MAC, message authentication code, usually used in TLS (transport layer security).它不保证error detection, 但保证安全。坏人改了message中的bit后，很难猜出MAC是什么，即c是什么。 c = MAC(M, s) M=message, s=security code。