Identify ACK packets from wget in Mininet

In mininet, I was able to make a TCP connection between a server h1 and a host h4 in my custom POX controller c0 as the following:

h1.cmd('python -m SimpleHTTPServer 80 &')
thread.start_new_thread(tcp_thread, h4)

def tcp_thread(src):
    for i in range(10):
        src.cmd('wget -O - 10.0.0.1')
        time.sleep(5)

h4 requests HTTP page from the server and keeps acknowledging the server normally based on TCP standard. Here, I want to be able to force h4 to send ACK packets using another path between them. I don't have a problem with forwarding or making paths but I do have an issue with how to capture or extract Ack packets before h4 sending them so I can forward them as I want.

Thank you.

Solution

If you have a PacketIn function you can add some lines of code to catch ACK flag

packet = event.parsed
tcp_found = packet.find('tcp')
if tcp_found:
  if tcp_found.ACK:
    print "ACK found"
    # do your things

The ACK flag is a tcp attribute as stated in the POX wiki

TCP (tcp) Attributes: ...... FIN (bool) - True when FIN flag set ACK (bool) - True when ACK flag set ......

To get those attributes we assign the tcp part of the packet to a variable using the find method, then we access the attributes using a dot (.) ex.

variable.attribute
tcp_found.ACK