# Pseudo-header

The pseudo-header is appended to the layer 4 segment to compute the checksum . This is not sent with the layer 4 segment so has to be reproducible on the receiver’s side. The pseudo-header contains the source and destination IP address (found in the layer 3 header), the protocol used at layer 4 (which can be determined by the header fields) and the size of the layer 4 segment .

This breaks the independence of the layer 3 and layer 4 headers and means you can’t change the layer 3 header without updating the layer 4 header. This exists for historic reasons - when TCP and IP were one protocol .