Sequence number comparisons need to be done "modulo"

[ambrop72]

Sequence numbers must not be compared for inequality "naively", since they are designed to wrap around. For example there is an issue here when checking if an incoming segment has a valid sequence number: https://github.com/saminiir/level-ip/blob/c1950ea0e0f9feceb5602432f1751b8ce71c4952/src/tcp_input.c#L106

Consider the case tcb->rcv_nxt=4294967290, tcb->rcv_wnd=20, th->seq=5. This is an acceptable segment (it is 11 more than tcb->rcv_nxt, so it is within the receive window), but your code will decide it is not acceptable.

One way to fix this specific part of code is like this:

uint32_t seq_relative_to_rcv_nxt = (uint32_t)(th->seq - tcb->rcv_nxt);
if (seq_relative_to_rcv_nxt > tcb->rcv_wnd) {
    // invalid segment
}

Note that the conversion of the result of the subtraction to uint32_t is imperative to ensure modulo reduction.