Skype bottleneck using ZyXEL firewall

All of our PC’s in the house are using skype and that has apparently caused some bottleneck issues on the network. I noted an extreme slow starup time when launching the first browser window. It was just like I would have to wait for some kind of timeout in the network, before I had any success at all.

By some kind of luck, I found an advice on the network that I should increase the IP NAT Session table in the firewall. And that fixes the problem everytime:

Telnet <your internal gateway-address>
Type in your password
Select 24 for system maintenance in the menu
Select 8 for command interpreter mode
enter “IP NAT SESSSION 1024

that takes care of the issue.

Make this change permanent
This fix will not work forever – after a while everything seems to halt again and repeating the steps above will sort the problem again. But it may be done in a permanent way by storing the new table size as a configuration setting.

In command interpreter mode, type:
VIEW AUTOEXEC.NET

Here is how my output is listed:

fw10> sys view autoexec.net

ip nat loopback on
bridge mode 1
sys errctl 0
sys trcl level 5
ip nat session 100
sys trcl type 1180
sys trcp cr 96 128
sys trcl sw off
ip tcp mss 1400
ip tcp limit 2
ip tcp irtt 65000
ip tcp window 16
ip tcp ceiling 6000
ip rip activate
ip rip merge on
ip icmp disc enif0 off
ppp ipcp com off
sys wd sw on
sys wd cnt 600
sys mbuf debug off
fw10>

Now, to change all that, the command apparently is sys edit autoexec.net. The thing is, if you are not already familiar with the sys edit command, you will probably need a few hints. Here is what I have done:

1. type sys edit autoexec.net
2. Then continue to type n until seeing the line ip nat session 100
3. Type r and hit return, follwed by the new text ip nat session 1024
4. Enter by hitting return and then save this session by typing x
5. Confirm everything by typing sys view autoexec.net

This worked for me.