[kwlug-disc] [OT] QOS router

Jeff Welling jeff.w.bulk at gmail.com
Mon Nov 1 11:19:56 EDT 2010


On 2010-11-01, at 9:39 AM, John Van Ostrand wrote:

> ----- Original Message -----
> As I understand it (but have not practised it) QoS is an Ethernet thing. So you would use QoS on your LAN switch to prioritise traffic. QoS on switches is generally unneeded in small offices. On your router you need to traffic shape. I suppose a router could recognize QoS in an Ethernet packet and prioritize the packet going out.

Right.  
QoS is a way of classifying packets, commonly done by looking at the DiffServ/TOS field in IP packets and grouping them together into different classes of service.  Packets are then pulled from the highest priority queue and are sent out over the line until that queue is empty, at which point it moves on to the next highest priority queue and repeats until the outbound speed limit is met or it runs out of packets to send.

> 
> The problem is that you can't prioritize the incoming packets, at least not in a useful way.
> 
> So if the remote end of your calls says that you sound fine, your QoS and traffic shaping are probably working correctly or not needed at all.
> 
> If you find that the remote end breaks up, then you may have incoming traffic shaping problems.
> 
> Only your ISP can be effective at shaping incoming traffic. I only know of one that provides automatic VoIP traffic shaping, that's Unlimitel. I'm sure there are others.

While you likely won't see many benefits from policing (shaping on ingress is called policing), what you can do is apply traffic shaping to egress traffic, that is, apply traffic shaping to the traffic as it exits the router en route to your LAN.  

The main difference between policing and shaping is that shaping has a queue, policing does not.  Ergo, policing simply drops packets, while shaping enqueues them for later transmission (until your buffer is full anyway).  You can see now why it's a better idea to shape on egress than it is to police on ingress.

So you _can_ implement shaping/policing on incoming traffic, but it's inadvisable if alternatives are available, like having your ISP do it for you.  I'm not aware of any ISPs that are willing to do that for you though.  
Also, it probably goes without saying, but don't expect your ISP to honour the DiffServ (QoS) field on your packets unless they've explicitly told you that they do.

I just think its slightly disingenuous to say that shaping incoming queues can only be done effectively by your ISP.  I think it _can_ be done effectively if its done on the egress queue to your LAN, and you choose a limit just below your maximum allowed download speed.  
Of course, any traffic generated from your gateway itself (such as doing 'apt-get update') will throw this all out of whack because this additional traffic is not throttled along with the rest, so in that situation, yes only your ISP can do it effectively. 

I have briefly tried implementing shaping on my gateway on both the egress to my modem and the egress to my LAN before, though to be honest I saw no noteworthy changes.  Perhaps I was doing it wrong, or wasn't testing it the way that I should have.

Just my 0.02$ :)
Jeff Welling


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