Re: TCP Dup Ack Issues with Comcast vs.Cablevision

["William Howard" <whoward () spotonnetworks com>]

I didn't want to clutter up the e-mail with all of the different things
that were done to eliminate the wireless AP/wireless speed as the source
of the problem.  The AP is set to G only and we used a sniffer to
eliminate other sources of interference including 2.4 Ghz phones and the
like.  As mentioned below, we are connected at 54 Mbps with a "good"
signal.  The APs we tested were plugged directly into the circuit but it
didn't matter if they were plugged directly in or if they were through a
router.  It also didn't matter which manufacturer of the AP/Router was
used (Linksys - 3 types, Colubris/HP -2 Types, and Netgear -2 types) -
the symptoms were still the same.  

 

We did setup a "mini-Speedtest" server using the available software from
SpeedTest.net - we did not see the speed issues when going wireless or
wired to that server - regardless of what equipment was put in-between
the wireless client and the server.  This is one of the things we did as
we honed in on the fact that this issue was only present at Comcast
sites - sites with Cablevision or other providers do not exhibit this
issue.

 

Until the registry settings were changed - the speeds were consistently
6-9 Mbps.  After changing the settings - the speeds improved to 15-19 so
the wireless speed is not the overriding factor.  

 

What is concerning me is the quantity of TCP Dup Acks seen in the
traces.  We have since done another test at another person's home
Comcast circuit that is 30/5 and on this particular setup - the TCP Dup
Acks are not present and the speeds with/without the registry settings
are equivalent at the 16-18 Mbps range.  So it appears that certain
Comcast locations do not have this issue.  We have not tested at all of
our Comcast sites but where we are seeing this is in the Connecticut and
Massachusetts areas.

 

My primary concern is the volume of TCP Dup Acks and why we might be
seeing these given that they are present in both the wired and wireless
traces at the sites where the wireless speeds are slow (without the
registry changes).  Has anyone else seen this?

 

Our initial conversation with Comcast for one site in particular went
about as expected - basically they said if you are getting the speeds
wired - go pound sand....

 

I would have liked to believe this was strictly a wireless issue but the
facts/tests point toward Comcast in several regions.

 

William Howard

________________________________

From: wireshark-users-bounces () wireshark org
[mailto:wireshark-users-bounces () wireshark org] On Behalf Of Alan Emery
Sent: Friday, February 26, 2010 11:36 AM
To: Community support list for Wireshark
Subject: Re: [Wireshark-users] TCP Dup Ack Issues with Comcast
vs.Cablevision

 

This may be more related to a setting on the wireless access point. If
the wireless AP is not set to a fixed mode such as "g" or "n", and it
"hears" the presence of a "b" device associated or not, it will go into
a compatibility mode and effectively run at the lower "b" rate. In my
SOHO environment, I fix my wireless mode to "g" as that is compatible
with all the devices I want to have connected, but if a guest brings in
a "b" mode device, they cannot connect, but they don't downgrade the
throughput of my existing network. Your 6-9 Mbps throughput is more
typical of what I would expect in a "b" environment.

If you can isolate the wireless AP to a switch that will allow you to
connect a local target device, or to switch ports on the AP if
available, you can look at throughput just going through the AP to a
local target device to see if the issue lies with the AP or not.

Alan Emery

IBM Global Solution Center 
1177 S Beltline Road, Coppell, TX 75019 

 William Howard ---02/26/2010 09:32:29 AM---We have been investigating
what seems to be an obscure issue with regards to Comcast speeds wired v

 
From:

 
William Howard <wghoward () optonline net>


To:


wireshark-users () wireshark org


Date:


02/26/2010 09:32 AM


Subject:


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


Sent by:


wireshark-users-bounces () wireshark org

________________________________





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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