Traffic Shaping

Hello,

In the planning stages of implementing traffic shaping it occurred to me that Cable internet might throw a kink in things.  As I understand it, all rules/queues must be applied to a pipe.  That pipe must have a fixed bandwidth rate set.  The documentation makes it very clear that the total bandwidth of all pipes may not exceed available bandwidth.  However, Cable internet does not provide a fixed amount of bandwidth.  Sometimes you get more than your advertised rate, and sometimes you get less. 

To avoid problems it seems best to set the pipes up with a conservative amount of bandwidth.  But that means you lose out on any "extra" bandwidth your ISP might be providing at a given moment.  Even worse, if your available bandwidth drops below what you've set your pipe to, everything will grind to a halt.

It seems that if traffic shaping is to be implemented with cable, to avoid an occasional complete drop in service, the total pipe bandwidth must be set to the lowest speed your ISP will ever provide. 

I would love it if someone told me I was wrong, but with Cable it seems like this is a pretty serious drawback to using traffic shaping.   Any thoughts?

Measure average effective bandwidth that Your operator provides on longer time periods. Then use the collected average values.

I've similar problems with my ADSL line, which is fully rate adaptive, so it might change its speed while I'm online. Fortunately, my modem has a webgui where it shows at which speed it is syncronized. So I have a machine on my lan which runs 24/7 and checks these values every 5 minutes. If they change, it updates the traffic shaper values by accessing the m0n0wall webgui.
Before I found out my modem has a webgui, my idea was to have the computer on my LAN ping a server like google's every few seconds and take an average every few minutes. Then make a rule in the traffic shaper that pings at least from that machine have highest priority. Then some experimenting is neccessary to find out the average ping when the line is fully loaded, with pipes that do not overestimate your current bandwith. Then you just need a clever algorithm that decreases the bandwidth of the pipes if the ping goes to high, and slowly increases the bandwith as long as the ping is fine. You most probably want to focus on the upload here I guess, cause it is impossible to make any assumptions about whether the upstream or downstream is clogged if the delay increases. You would have to make speedtests all the time to find out, which would periodically block all the up- and download for other computers.
Fairly complicated, but to my knowledge the only way on a dynamic bw line.