talking between python tcp server and a c++ client

Question

I am having an issue trying to communicate between a python TCP server and a c++ TCP client. After the first call, which works fine, the subsequent calls cause issues.

As far as WinSock is concerned, the send() function worked properly, it returns the proper length and WSAGetLastError() does not return anything of significance.

However, when watching the packets using wireshark, i notice that the first call sends two packets, a PSH,ACK with all of the data in it, and an ACK right after, but the subsequent calls, which don't work, only send the PSH,ACK packet, and not a subsequent ACK packet

the receiving computers wireshark corroborates this, and the python server does nothing, it doesnt have any data coming out of the socket, and i cannot debug deeper, since socket is a native class

when i run a c++ client and a c++ server (a hacked replica of what the python one would do), the client faithfully sends both the PSH,ACk and ACK packets the whole time, even after the first call.

Is the winsock send function supposed to always send a PSH,ACK and an ACK? If so, why would it do so when connected to my C++ server and not the python server? Has anyone had any issues similar to this?

Answer 1

What size of packets do you send?

If they are small - may be Nagle's Algorith & Delayed ACK Algorithm is your headache? From what you described think Delayed ACK is involved...

Answer 2

client sends a PSH,ACK and then the server sends a PSH,ACK and a FIN,PSH,ACK

There is a FIN, so could it be that the Python version of your server is closing the connection immediately after the initial read?

If you are not explicitly closing the server's socket, it's probable that the server's remote socket variable is going out of scope, thus closing it (and that this bug is not present in your C++ version)?

Assuming that this is the case, I can cause a very similar TCP sequence with this code for the server:

# server.py
import socket
from time import sleep

def f(s):
        r,a = s.accept()
        print r.recv(100)

s = socket.socket()
s.bind(('localhost',1234))
s.listen(1)

f(s)
# wait around a bit for the client to send it's second packet
sleep(10)

and this for the client:

# client.py
import socket
from time import sleep

s = socket.socket()
s.connect(('localhost',1234))

s.send('hello 1')
# wait around for a while so that the socket in server.py goes out of scope
sleep(5)
s.send('hello 2')

Start your packet sniffer, then run server.py and then, client.py. Here is the outout of tcpdump -A -i lo, which matches your observations:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 96 bytes
12:42:37.683710 IP localhost:33491 > localhost.1234: S 1129726741:1129726741(0) win 32792 <mss 16396,sackOK,timestamp 640881101 0,nop,wscale 7>
E..<R.@[email protected]|....@....
&3..........
12:42:37.684049 IP localhost.1234 > localhost:33491: S 1128039653:1128039653(0) ack 1129726742 win 32768 <mss 16396,sackOK,timestamp 640881101 640881101,nop,wscale 7>
E..<..@.@.<.............C<..CVC.....Ia....@....
&3..&3......
12:42:37.684087 IP localhost:33491 > localhost.1234: . ack 1 win 257 <nop,nop,timestamp 640881102 640881101>
E..4R.@[email protected]<......1......
&3..&3..
12:42:37.684220 IP localhost:33491 > localhost.1234: P 1:8(7) ack 1 win 257 <nop,nop,timestamp 640881102 640881101>
E..;R.@[email protected]<......./.....
&3..&3..hello 1
12:42:37.684271 IP localhost.1234 > localhost:33491: . ack 8 win 256 <nop,nop,timestamp 640881102 640881102>
E..4.(@[email protected]<..CVC.....1}.....
&3..&3..
12:42:37.684755 IP localhost.1234 > localhost:33491: F 1:1(0) ack 8 win 256 <nop,nop,timestamp 640881103 640881102>
E..4.)@[email protected]<..CVC.....1{.....
&3..&3..
12:42:37.685639 IP localhost:33491 > localhost.1234: . ack 2 win 257 <nop,nop,timestamp 640881104 640881103>
E..4R.@[email protected]<......1x.....
&3..&3..
12:42:42.683367 IP localhost:33491 > localhost.1234: P 8:15(7) ack 2 win 257 <nop,nop,timestamp 640886103 640881103>
E..;R.@[email protected]<......./.....
&3%W&3..hello 2
12:42:42.683401 IP localhost.1234 > localhost:33491: R 1128039655:1128039655(0) win 0
E..(..@.@.<.............C<......P...b...

9 packets captured
27 packets received by filter
0 packets dropped by kernel