268 PART II Core Protocols 1 byte 1 byte 1 byte 1 byte Source IPv4 Address Destination IPv4 Address All 0 byte Protocol (517) UDP Length FIGURE 10.3 The UDP IPv4 pseudo-header. These fields are used for checksum computation and include fields in the IP header. At the receiver, the value of the Checksum is copied and the field again set to zero. The checksum is again computed on the pseudo-header and compared to the received value. If they match, the datagram is processed by the receiving application indicated by the destination port number. If they do not match, the datagram is silently discarded (i.e., no error message is sent to the source).