TCP Dup Ack Issues with Comcast vs. Cablevision

[William Howard <wghoward () optonline net>]

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

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