[e2e] Question about propagation and queuing delays

grenville armitage garmitage at swin.edu.au
Tue Aug 23 11:27:55 PDT 2005

Detlef Bosau wrote:
> Just to give one example from there: We recently had a discussion about 
> "Fastpath". In DSL lines, you need error recovery on the last mile. Now, 
>  to save overhead you do codespreading/interleaing. Some "well informed 
> guys" want the ISP to turn interleaving off in order to spare some "ping 
> time". First of all, it´s simply ridiculous, theat individual customers 
> without any technical knowledge will prescribe the provider the 
> appropriate line coding for one individual wire pair.

I'm curious if you have any stats on the typical error rates that will
be experienced by the people who switch from Interleave to Fastpath mode
on their DSL links. It is certainly true that e.g. gamers find Interleave
mode to be a pain (at least 20ms additional latency), with good reason.
Stats on how much packet loss the gamer will experience (in order to gain
the latency improvement of Fastpath) would be interesting to know. If the
error rates are low, then in fact it seems like an entirely reasonable
thing for a customer to desire Fastpath.

> Second: Not only 
> these customers may be affected by increasing error rates: These guys 
> flood large portions of the network with defictive frames, more 
> precisely with defective ATM cells with corrupted payload, which is 
> eventually being detected at the customers AAL 5 peer. (At least AFAIK.)
> This is thoughtless waste of bandwidth, but it is nearly impossible to 
> convince those guys that this is malicious in quite a number of cases!

I'm also curious about this "large portions of the network" which is carrying
useless ATM AAL5 cells. If the bit error rate is so bad that a noticable fraction
of ATM cells are useless then gamers going to have a bad packet loss rate and
fairly quickly go back to interleave mode (despite the higher latency). Yet if
the gamers are happy with the typical loss rate using Fastpath, then there's
probably not that many wasted/useless ATM cells floating around.

(Naturally, if the actual AAL_PDU loss rate starts to become more than an
integer of a % the gamer's use of TCP for p2p, web surfing and email becomes
problematic. But it is hard to argue this by hand-waving - we need stats on
likely bit error rates a typical DSL customers islikely to see using Fastpath.)


More information about the end2end-interest mailing list