![]() ![]() & (previous_packets.saddr =iph->daddr) // Same for IP dest addreses If ((previous_packets.daddr = iph->saddr) // Swapped IP source addresses Find packets going to the reverse direction Now Check previous communication to check for retransmission At this point the packets are almost identical & (tcph->syn=1 || tcph->fin=1 ||segmentlength>0)) // Check if SYN or FIN are & (previous_tcp.th_flags = tcph->th_flags) // Same flags & (previous_tcp.th_win = tcph->th_win) // Same window & (previous_tcp.th_ack=tcph->th_ack) // Same acknowledge number & (previous_tcp.th_seq = tcph->th_seq) // Same sequence number & (previous_tcp.dest = tcph->dest) // Same destination port If((previous_tcp.source = tcph->source) // Same source port & (temphdrlen = iphdrlen)) // Same header length & (previous_packets.protocol = iph->protocol) //Same protocol & (previous_packets.daddr = iph->daddr) // Same destination Ip address ![]() * First check if a same TCP packet has been received */įor(int i=0 isaddr) // Same source IP address Int header_size = sizeof(struct ethhdr) + iphdrlen + tcph->doff*4 Struct tcphdr *tcph=(struct tcphdr*)(Buffer Struct iphdr *iph = (struct iphdr *)(Buffer + sizeof(struct ethhdr)) Void find_retransmissions(const u_char * Buffer, int Size) Struct iphdr *iph = (struct iphdr*)(buffer +sizeof(struct ethhdr)) ![]() Struct ethhdr *eth = (struct ethhdr *)buffer Pcap_loop(handle, -1, process_packet, NULL) Handle = pcap_open_offline("smallFlows.pcap", errbuff) Void find_retransmissions(const u_char *, int ) Void process_packet(u_char *,const struct pcap_pkthdr *, const u_char *) The function find_retransmissions is where the packet is analyzed. This is an MRE that reads a pcap file and analyzes the TCP packets sent over IPv4. What I actually want to achieve is, to do on a basic level, what the filter does in wireshark. After searching extensively the web, I've concluded that in order to so, I need to track the traffic behaviour and this means also analyzing previously received packets. I want to be able to deduce whether a TCP packet I parsed is a TCP retransmission or not. I can parse the packets one by one and analyze them up to a point. If the recipient should empty its receive buffers at all (in other words, the application makes even a partial pickup), it will announce the new “space available” with a TCP Window Update.I've written a simple source file that can read pcap files using the libpcap library in C. Also, it might be that the application does not pick up the packets in a timely fashion from the TCP buffer. Or it could be that there is an error in the TCP receiver. ![]() It could be that the machine is running too many processes at that moment, and its processor is maxed. This means that the machine is not able to receive further information at the moment, and the TCP transmission should be halted until it can process the information that is pending in its buffer. TCP Zero Window is when the Window size in a machine remains at zero for a specified amount of time. If you want to filter on TCP duplicates use this Wireshark filter: These are called fast retransmissions.Ĭonnections with more latency between the client and server will typically have more duplicate acknowledgment packets when a segment is lost. In most cases, once the sender receives three duplicate acknowledgments, it will immediately retransmit the missing packet instead of waiting for a timer to expire. They are a common symptom of packet loss. Typically, duplicate acknowledgments mean that one or more packets have been lost in the stream and the connection is attempting to recover. Most packet analyzers will indicate a duplicate acknowledgment condition when two ACK packets are detected with the same ACK numbers. If you want to filter on TCP transmissions use this Wireshark filter: Above you can see that after more than 1s a frame get’s sent again. ![]()
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