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

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Dimitris Kotsonis wrote:
> 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.

It is normal for an FTP download to take over from p2ps the latter are 
likely to be higher latency, so TCP will let a lower latency FTP grab 
more bandwidth.

Try shaping with HTB and sfq - It should help.

> 
> 
> 
>> 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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