Over buffered networks can lead to high latencies

[Tharic-Nar]

Ordinarily, I'd write this up as news, but it be a tad over my head, still interesting none the less.

http://arstechnica.com/tech-policy/...ferbloat-and-the-network-buffer-arms-race.ars

In a nutshell, the large buffers on home routers and switches can cause high latencies for other traffic when they become full. Normally you'd think that large buffers are a good thing, smooths out traffic, etc, but when the buffer can hold up to 2 or 3 seconds (or even 10) of traffic during a download, and you want to browse the net... then you have to wait for the buffer to clear before your web packet can get through...

Now, this is normally when Quality of Service (QoS) is brought up, and it indeed does mitigate the problem. However, it wouldn't really be needed if the buffers were smaller (not too small mind), since the packet would be dropped and resent instead of held up for several seconds in a buffer. Problems with TCP transfer protocols with Flow rate and such.

I guess there is more to the humble balancing act of the home router than meets the eye. Rather strange to think about this in a different way... a slow connection to a website with a download running in the background may not be the result of bandwidth saturation, but of a full buffer... who'd of thought... I guess that explains why long distance downloads can result in the same delays, even though you are not maxing out your connection.

 

[OriginalJoeCool]

I've long been having an issue with my router (Linksys WRT54G). Particularly when I'm running uTorrent, but also when I'm just browsing, my connection becomes extremely slow,drops off. It seems almost random. However, I have noticed that my buffer is consistently nearly full (96% currently - 5460 kB / 5680 kB ). Could this be the problem?

 

[Tharic-Nar]

I believe I had the ADSL version of that router and also suffered problems with any kind of torrent. It would lockup or become unresponsive to login attempts... port forwarding changes would result in it rebooting itself, and so forth. So yeah, i'm inclined to believe the router is at fault rather than the buffer, i don't think it can handle multiple simultaneous connections very easily. A different firmware may be able to help, but I no longer use that router (the power adapter decided to melt and the router was never the same after). Been using a WAG354G V.2 for about 4 or 5 years now, handles torrents fine, but it is ADSL rather than DSL or Cable.

 

[OriginalJoeCool]

I have DD-wrt firmware on the router now. I might switch to tomato eventually.

 

[Glider]

We had to produce new hardware cards at work for this, for a big customer. They have very old DSLAMs, which have buffers of 1-2 packets. Our boxes, with huge buffers, showed a "burst" behaviour (if a buffer in a QoS class is emptied, all packets are dumped at max speed), causing them to flood the DSLAMS, leading to stability issues.

This issue exists also on the access side (your home router ) So QoS is not the solution to this, because the buffers are emptied in sequence, at full rate, causing the burst behavior.

 

[Psi*]

I wonder if this can cause HD time outs when transferring very large files between computers in a LAN across a router.

 

[Tharic-Nar]

Is there any way to fix this behaviour on the home side? My only guess would be a different firmware with a choke on the buffer... apart from that, just a new router.

So does QoS only help when the buffer is full - as in, completely useless under these circumstances? If that's the case, what is the use of QoS? Is it largely a wireless function and has no real bearing on internet traffic? Or is it useful for small - streamed packets, rather than larger data packets? This issue has me kind of confused... well, not kind of, lol, I am confused.