[e2e] Question about propagation and queuing delays

Marc Herbert marc.herbert at free.fr
Tue Aug 23 11:49:09 PDT 2005

On Tue, 23 Aug 2005, Detlef Bosau wrote:

> When I read what you say, I would like to invite you into my ISP´s
> support newsgroup, I think much of the readers can learn a lot from you!
> 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. 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!
> What is even more disastrous: In fact, in DSL TCP appears to be based
> upon AAL5/UBR. Unspecified bitrate. Hence, all congestion control is
> done at the TCP endpoints. I´m totally with you that this requires well
> behaved participants in a network. IIRC, LANE works with ABR and that
> will alleviate the problem.

FYI the second biggest ISP in France (about 1.2M subscribers) gives
its subscribers a write access to this "fastpath" interleave level,
through a simple web interface.


Of course you also have access to error stats on the DSL line.

All gamers know about and love this feature. It helps them gain about
30ms, a huge benefit for "real-time" games. And they don't care much
about the rest. Other subscribers don't care and use the default,
conservative setting. So everyone is happy with this well-designed
feature... I guess that if this feature was "flooding the network with
malicious frames" or something, the ISP would obviously not have
offered it.

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