What is the Significance of Pseudo Header used in UDP/TCP

Question

Why is the Pseudo header prepended to the UDP datagram for the computation of the UDP checksum? What\'s the rational behind this?

Answer 1

When these protocols were being designed, a serious concern of theirs was a host receiving a packet thinking it was theirs when it was not. If a few bits were flipped in the IP header during transit and a packet changed course (but the IP checksum was still correct), the TCP/UDP stack of the redirected receiver can still know to reject the packet.

Though the pseudo-header broke the separation of layers idiom, it was deemed acceptable for the increased reliability.