TCPDUMP and bad tcp CKSUM errors

Help! Over the past couple weeks ive been noticing declining conditions in the speed recieved over my cable modem. Before, I was getting around 4-500K/sec on average, now the most I seem to get is around 330K

I have tried communicating with the network tech at my isp, and he said that in order to further trace the problem, either he or I should attempt to run tcpdump to further ascertain more information about the problem. I have done so, but I have noticed that I seem to get this :

23:39:12.459888 192.168.1.100.49795 > 209.123.109.175.80: . [bad tcp cksum 5d78!] 1:1(0) ack 1 win 33304 <nop,nop,timestamp 3077701288 2904394004> (DF) (ttl 64, id 26354, len 52)
23:39:12.460010 192.168.1.100.49795 > 209.123.109.175.80: P [bad tcp cksum e605!] 1:500(499) ack 1 win 33304 <nop,nop,timestamp 3077701288 2904394004> (DF) (ttl 64, id 26355, len 551)
when downloading files, but not when uploading. I am not sure what to make of this, other than there is a problem.

So, is there anyone out there that can help please?

My Specs:
Powermac dual 1.25ghz
OS 10.2.6
1 gig ram

Further information:

Linksys router, 4 port, with a pc and 2 macs (laptop and desktop hooked up to it.)

Ive tried plugging the cable modem directly into the desktop in order to figure out if it is the router that is in some way causing the problem, but got the same thing. I’ve tried resetting the modem, renewing the DHCP lease, etc, but I am unable to figure out what the problem is.

I’d really appreciate some advice if possible

thanks in advance

p.s. the snipit above is just some random data transfer, and this problem seems to occur on all download transfers but not on ANY upload transfers.

any ideas?

Do uploads run at the 400-500K/sec speed while the downloads are at 330K/sec?

Does your ISP guarantee you the 400K+ speed? If not they may be throttling your bandwidth for their own purposes. Uploads are ‘cheap’ for the ISP as most traffic is down to you (you often see DSL sold with two numbers they define different speeds to upload and download).

Checksum errors (CKSUM) mean packets are getting corrupted in the transmission. As a result that packet needs to be resent. To some extent it is normal but if it gets to be too much your speeds may suffer (so essentially you still have all your bandwidth available but due to data needing to be resent your effective speed to get a complete file is reduced).

There could be many reasons for the corruption. The data lines could be dirty (and that includes from your computer to the router, router to the wall, the wiring in your building and the wiring to the ISP’s router). Your router could be bad. Your NIC could be flaking out. The ISP’s router and/or NICs might be flaking out. You might have a virus. You might have a flaky PC.

The list goes on…

The problem can be narrowed down but you will need the help of the ISP to track it. Your ISP may or may not want to be willing to go the extra mile for you but you may as well try. If you can get them on not providing your minimum contracted service you have a better shot. Unfortunately most home services leave a LOT of latitude to the ISP as to what minimum level of service they must provide. You can get guarantees for service (as businesses often do) but your prices jump drastically.

no. uploads are capped to 512kbit while downloads are uncapped. The bandwith im told is not throttled. By 400-500K i mean Kilobytes as opposed to bits. The corruption of packets theory seems feasable, but it seems to happen on EVERY packet.

my concern is not a minimum speed or maximum speed, its the change in transfer speeds from when I got the service to the way speeds are now. I think there is a problem with the routing or peering that the isp I have uses.

As far as a bad router, I think this is narrowed down by the fact that I tried connecting the cable modem directly to the pc, bypassing the router here. Same with the dirty lines theory, because im using coax as opposed to phone lines that DSL uses. It doesnt make sense to me that it would change over time, as I once got great speed from it, until very recently (2-3 weeks).

anyway, thanks for your help, i’ll keep bugging the people at the isp until i track the problem to its source. I dont intend to give up too easy, at least not yet =)

There is no way for me to know through this message board but a possibility is that your NIC (network interface card) is flaking out or the cable to the card is damaged.

Separate wires are used for receiving and transmitting information. Considering all seems to work well one way but not the other it would suggest the receiving wire in teh cable is damaged, the connectors at the interface of the NIC and the cable is damaged or the NIC itself is starting to flake out. Generally cables don’t ‘go bad’ unless someone steps on it or crimps it or something. Sometimes the cable gets yanked and damages the port on the NIC. NICs on the other hand do break from time to time. They rarely stop working altogether but they do start to flake out.

If you have a spare NIC lying about somewhere you might want to swap them and see if that helps (don’t forget appropriate drivers).

Understand that I am merely speculating here. Your problem may be something else entirely.

In theory, I should be able to plug the cable modem directly to the laptop to entertain the possiblility of a bad router or NIC card on the desktop. I’ll try that and see what happens. I’m pretty sure it isnt the NIC card, but who knows.

thanks for your advice