Hosts a and b are communicating over a tcp connection, and host b has already received from a all bytes up through byte 126. suppose host a then sends two segments back-to-back to host b, with 800 and 400 bytes of data in the first and second segments, respectively. in the first segment, the sequence number is 127 (for simplicity, we assume the sequence number starts from 0, although it typically starts with a random number in practice), the source port number is 50002, and the destination port number is 80. host b sends an ack to a whenever it receives a segment from a.