Use actual IP and TCP header lenght to determine cutoff point

[gsenkowski]

We noticed some inconsistencies in recorded packets and traced them back to the fixed lenghts for IP and TCP headers. To fix them we propose the following changes in recording.ts.

        // If IPv4
        if (ethType === 0x0800) {
            const ipHeaderStart = offset + 2;
            const ipVersionAndIHL = context.buffer[ipHeaderStart];
            const ipHeaderLength = (ipVersionAndIHL & 0x0F) * 4; // IHL field gives the length of the IP header

            // read protocol type (TCP/UDP/ICMP/etc.)
            const protocolType = context.buffer[ipHeaderStart + 9]; // Protocol field in IP header

            if (protocolType === 6) { // TCP
                const tcpHeaderStart = ipHeaderStart + ipHeaderLength;
                const tcpOffsetAndFlags = context.buffer[tcpHeaderStart + 12];
                const tcpHeaderLength = (tcpOffsetAndFlags >> 4) * 4; // Data offset in TCP header
                maxBytes = ipHeaderLength + tcpHeaderLength + 14; // Total length: IP header + TCP header + Ethernet header
            } else if (protocolType === 17) { // UDP
                maxBytes = ipHeaderLength + 8 + 14; // IP header + 8 bytes UDP header + Ethernet header
            } else if (protocolType === 1) { // ICMP
                maxBytes = 0;
            } else {
                maxBytes = 0;
            }
        } else if (ethTypeModified === 0x0800) {
            const ipHeaderStart = offset + 2 + 18; // What is 18?
            const ipVersionAndIHL = context.buffer[ipHeaderStart];
            const ipHeaderLength = (ipVersionAndIHL & 0x0F) * 4; // IHL field gives the length of the IP header

            // read protocol type (TCP/UDP/ICMP/etc.)
            const protocolType = context.buffer[ipHeaderStart + 9]; // Protocol field in IP header

            if (protocolType === 6) { // TCP
                const tcpHeaderStart = ipHeaderStart + ipHeaderLength;
                const tcpOffsetAndFlags = context.buffer[tcpHeaderStart + 12];
                const tcpHeaderLength = (tcpOffsetAndFlags >> 4) * 4; // Data offset in TCP header
                maxBytes = ipHeaderLength + tcpHeaderLength + 14 + 18; // Total length: IP header + TCP header + Ethernet header
            } else if (protocolType === 17) { // UDP
                maxBytes = ipHeaderLength + 8 + 14 + 18; // IP header + 8 bytes UDP header + Ethernet header
            } else if (protocolType === 1) { // ICMP
                maxBytes = 0;
            } else {
                maxBytes = 0;
            }
        }