Re: [LARTC] Shaping traffic on heavily oversubscribed links?

[Dimitris Kotsonis <jnk@comkey.spark.net.gr>]

Justin Schoeman wrote:

> Hi all,
>
> I am having some fun with traffic shaping, and have run into an 
> interesting situation.  Here is South Africa, most internet links are 
> heavily oversubscribed, which means that in most cases the local link 
> is _not_ the bottleneck, and shaping on the local link does not help 
> that much...
>

We have the same problem in DSL lines here in Greece.

I have found that while the average efective speed on such lines varies, 
tha average rate of packets is more or less constant. I have a theory 
for this. I believe that the routers that forward the traffic on 
congested lines - on ISPs and on the ATM circuits at the telcoms - don't 
take the extra time needed to calculate the size of the packets and 
distribute the traffic on a per packet basis. This leads to a 'fairness' 
among the end receivers based on packets/sec instead of  bandwidth.

To be more specific. In my ADSL line I usually achieve between 20-30 pps 
(measured with MRTG). With an average packet size of 1500 this is 20-45 
kbytes/sec. But packets sizes close to the MTU are found on single 
ftp/http connections and pretty much nowehere else. Packet sizes of 400 
to 500 are more realistic, especially when p2p programs are involved. 
20-30 packets give 8-10kbytes/sec. You can expect even less when using 
voip programs which utilize smaller packets.

If you find that single a FTP session tends to get more bandwidth thatn 
p2p programs or multiuser traffic then you have a simillar problem to 
our own. I would suggest that you setup MRTG to monitor packets to 
research further into this.



> Does anybody have some tips on shaping such links?  How can you get 
> interractive traffic if you don't know how much bandwidth to reserve 
> for it? How can you give fair access to a link if you don't know what 
> the link capacity is?
>

Well, I am working on one. Since I can't shape bandwidth because it 
flactuates erratically with time and usage I decided to shape packets. I 
have created a new queueing discipline based on TBF which uses packets 
instead of bytes for its tokens and I am allocating a constant 
packet/sec rate on each user of my ADSL line. A better solution would be 
to create an HTB alike packet-based qdisc for dynamic shaping.

If you find that you have the some problem as me and you want to 
experiment with a packet-based TBF qdisc I can send you a patch for 
linux-2.6.8 and iproute2 in this list.

I would like to here your thought on this anyway ...

Dimitris



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