Sometimes, to simulate one corner case, i.e., the TCP layer splits one application record into multiple packets, I need to use libpcap to tweak pcap files. E.g., for following diagram:
I split frame 8 into 3 parts, one will be appended to frame 7, and the remaining 2 parts will be 2 separated packets.
Generally speaking, only length field in IP header and Sequence number field in TCP header need to be modified. The source code and pcap file can be referenced here.