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Wireshark-users: [Wireshark-users] TCP Dup Ack Issues with Comcast vs. Cablevision

From: William Howard <wghoward@xxxxxxxxxxxxx>
Date: Thu, 25 Feb 2010 21:31:46 -0500

We have been investigating what seems to be an obscure issue with regards to Comcast speeds wired vs. wireless "G" speeds on a 30/5 circuit. 

Here are the symptoms:

Wired (directly to modem): Speeds are what one would expect - 25-30 Mbps down and 4-5 Mbps up.

Wireless: Speeds are in the 6-9 Mbps.  We have tried a variety of consumer and higher end APs/Wireless routers.  All with the same basic results - the speeds are significantly slower.
  • The wireless NIC was connected with a "good" signal at 54 Mbps.
  • I verified that wireless interference was not an issue.
  • I tried several different laptops to make sure that the particular wireless NIC was an issue.
  • The AP/Router were the only items on the circuit.  Time of day did not matter as I tried going back and forth between wired and wireless - both produced consistent speeds each time.
What we did discover is that when testing the same equipment on a cablevision/optimum online 30/5 circuit, the problems virtually disappear.  Wired speeds are equivalent to Comcast but wireless speeds were in the 15-19 Mbps range. 

In order to dig deeper, I captured wireshark traces for both wired/wireless on Comcast and Optimum Online circuits.  The biggest difference I could find is that on the Comcast circuit both wired and wireless, there were many: TCP Dup ACK packets (see below for an example)
TCP    [TCP Dup ACK 17802#55] http > apc-3052 [ACK] Seq=8154484 Ack=307815 Win=206848 Len=0 SLE=370595 SRE=447975 SLE=331175 SRE=335555
I have seen the "tcp optimizers" and they have produced good results and have improved the Comcast speeds to 12-16 Mbps but it seems very odd that only Comcast seems to suffer from packets arriving out of order (or whatever is causing this) but Cablevision does not.  I don't like the idea of having to change a client device when it seems like this problem lies within the Comcast network.

Has anyone seen this before?  Is there a solution without changing the client laptop?  We would like to have a solution that is hardware based (router or firmware) rather than telling users they must all make registry changes which makes us nervous (liability) and end-users irritated that "it works on other networks without a problem"

Any insight on this would be greatly appreciated.

Will Howard