TCP states break down with Scappy

tcp_syn_ack.py

from scapy.all import *
import time
import random

def sendit(src=None, dst=None, dport=None):

    """
    See the different status of a TCP connection
    Was initially written to identify a docker-proxy bug: https://github.com/moby/moby/issues/27539
    I suggest to keep a netstat running from the server side: $ watch -0.1 'netstat -laptn|grep 80'
    Note: linux will send an RST because the packet is not issued from the kernel. Disable it by executing:
    $ iptables -A OUTPUT -p tcp --tcp-flags RST RST -s <src_ip> -j DROP
    """
    sport = random.randint(1024,65535)
    ip=IP(src=src, dst=dst)
    SYN=TCP(sport=sport,dport=dport,flags='S',seq=1000)
    SYNACK=sr1(ip/SYN)
    print("SYN/ACK sent to {}:{} using {}:{}".format(ip.dst, dport, ip.src, sport))
    time.sleep(4)
    # SYN-RECV server-side
    
    ACK=TCP(sport=sport, dport=dport, flags='A', seq=SYNACK.ack + 1, ack=SYNACK.seq +1)
    send(ip/ACK)
    print('Sending ACK - Three-way handshake over')
    time.sleep(4)
    # ESTABLISHED
    
    FIN=TCP(sport=sport, dport=dport, flags="FA", seq=SYNACK.ack, ack=SYNACK.seq + 1)
    FINACK=sr1(ip/FIN)
    print("Sending FIN/ACK - End of tcp connection")
    time.sleep(4)
    # LAST-ACK
    
    LASTACK=TCP(sport=sport, dport=dport, flags="A", seq=FINACK.ack, ack=FINACK.seq + 1)
    print("Last ACK sent - The session {}:{} establised to {}:{} is now closed".format(ip.src, sport, dst, dport))
    time.sleep(4)
    send(ip/LASTACK)
    # connection closed

if __name__ == '__main__':
    sendit(src='172.31.29.249',dst='172.31.46.210',dport=80)