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Title says it all. For a bit more info though:

Basically, I have Time Warner cable internet. My speeds maintain a stable 2Mbit/s upload and 20Mbit/s download with average ping times around 30ms.

This crazy thing happens though when I upload anything. I went to upload a 200M file to my server today through sftp and my internet completely choked up. I speed tested it during this upload and my ping time was around 800ms, download speeds of 0.2Mbit/s and Upload speeds of 0.3Mbit/s. Note, I wasn't downloading anything during this time either. It is just straight upload.

What is it that causes this phenomenon? My router is OpenBSD. Is there anything I could set up to fix this problem(by queues or some such), or is this a problem with cable internet?

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Latency is the problem. You'll need to speak to your ISP. – user3463 Sep 10 '12 at 3:10
@RandolphWest but latency is only a problem when uploading? – Earlz Sep 10 '12 at 3:18
Yep, which is why you have to speak to your ISP. I'm currently facing a similar problem with my connection. – user3463 Sep 10 '12 at 3:26
You may need to implement QoS somehow on your router, but how you do this with OpenBSD, I don't know. Basically, it sounds like your upload is hogging almost all the upstream bandwidth. Downloads and other activity still require some of that upstream bandwidth to work responsively. – jjlin Sep 10 '12 at 5:03
+1 for QoS. Also you should try with other protocol like plain FTP or HTTP. Maybe your router is prioritizing the sftp traffic. – Rufo El Magufo Sep 10 '12 at 22:25
up vote 3 down vote accepted

I run OpenBSD on my egress device at home and have had this addressed for quite a while. I recommend you read about this well-known phenomenon at for more information on how to address this using pf (the author is the original developer of pf).

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This looks very promising! I'll try this tomorrow – Earlz Sep 14 '12 at 5:51
I put that bit of queuing on to my OpenBSD router and now it works like a champ! Also I don't have such sporadic ping times in general now which helps with online games. Do you know if something like this should also be done with UDP or if it'd hurt? – Earlz Sep 17 '12 at 14:57
Glad to hear that did the trick for you. As for UDP traffic, you’ll want to prioritize any important traffic. For example, I prioritize my VoIP, which is UDP, over my TCK ACK traffic—because a clear phone call is more important than perfectly fast downloads! – Bink Sep 18 '12 at 16:33

You ran a speed test whilst uploading. The upload then showed 0.3 mbit/s. I assime your file upload speed to the server was somewhere near (2 mbit/s - 0.3 mbit/s), right? As jjlin saus: downloading also needs uploading. Limit the upload speed to your server (sftp -l) for example to 1.5 mbit/s and then see if your downloads run smoothly again when you are uploading.

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Another thing to keep in mind is that service providers use a number of techniques to keep residential users from operating commercial servers on their connections. One technique is to monitor the aggregate upload megabytes over a few minutes, and, if it's high, start to "choke" the connection.

So a medium-sized upload may run fairly fast, but a larger one will slow down and also slow down the download rate with it.

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