[e2e] Skype and congestion collapse.

[James Kempf]

Jon,

Maybe for the backbone, but what about the access networks? Capacity there
is usually a lot less.

See RFC 3714 for some musings Sally Floyd and I went through on this topic.

            jak

----- Original Message ----- 
From: "Jon Crowcroft" <Jon.Crowcroft at cl.cam.ac.uk>
To: "Emmanuel Papirakis" <papiraki at gmail.com>
Cc: <Jon.Crowcroft at cl.cam.ac.uk>; <end2end-interest at postel.org>
Sent: Friday, March 04, 2005 6:30 AM
Subject: Re: [e2e] Skype and congestion collapse.


> well so a back of the envelope calculation on skype would tell you that
> even if everyone ran it, 24*7,
> there must be enough capacity in the world for it -
>
> 1/ the PSTN has virtually zero call blocking probability in most europe
> and north america-  so the underlying network has enough for 64kbps, and
> skype is using 6 times less than that
>
> 2/ the internet is no longer an overlay on the core PSTN -= the core
> internet has its own capacity - in the UK, just one ISP for example, the
> UK academic net, has a 10Gbps backbone - there are several that big in
> the UK and many european countries are similar
>
> 10Gbps is enough for 10M skype calls - thats 1/5 of the UK population
> talking simulataneously assuming there is NO locality of calls...
> if there's any locality at all (and there usually is, even ifd just
> becuase of timezone differences (ok i know there's not much timezone in
> the uK - ok, language differences, work, lunch breaks etc) then its no
> problem...
>
> it would not be hard to upgrade the natl net to 40Gbps -
>
> in th next few years it ought to be easy to go another order of
> magnitude without layering any more core fiber (just more lambdas)
>
> if there is a bandwidth crunch, imho, the solution is to do
probe/measurement
> based end user call admission control. There is not a lot of room for
> congestion control _below_ 10kbps
>
> oh, skype isnt truly p2p afaik, it has servers- they will get
> congested LONG before the net does. If they don't, it will be because
> they have to raise revenue to scale them up - to do that, they will have
> to charge people - if they generate more than a modest amount of traffic
> they will have to co-lo in POPs which means they'll have to rent rack
> space and other facilities - this will quicky mean they rate limit
> themselves at the aggregate level
>
> why use skype anyhow when you can use ichat, marratech and other fine
> tools that allow video, shared whiteboards and have really really nice
> interfaces?
>
> oh, ok, i admit it - skype is pretty cool:)
>
>
> (I am tempted to say that the internet is in fact a shared illusion
> caused by a deficit of information, and that in reality, all those
> pixels lighting up on the screen in front of You<---->Right Now,
> are just fragements of a dream.)
>
> (p.s.  I am even more tempted after recent food scares in the UK to say:
>
> - Additive Increase: just say no.
> - Multiplicative Decrease: its too too devisive.
> - On or off, that should be good enough for the law of large numbers...
> just given a big enough dice.
>
> this is the voice of the mysterons... ... ...
>
> In missive <11ad0fa8050304053342514f51 at mail.gmail.com>, Emmanuel Papirakis
typed:
>
>  >>Hello,
>  >>
>  >>a VoIP application called Skype is gaining more and more popularity. I
>  >>did a basic capture using Ethereal. It seems that it continuously
>  >>sends data at a rate of 10 KB/sec.
>  >>
>  >>Obviously, it does not use a sliding window mechanism nor does it
>  >>consider the rate at  which the receiver is able to receive data.
>  >>Furthermore, it does not attempt to detect periods of silence and
>  >>during those, it continues to send data at its 10KB/sec constant bit
>  >>rate.
>  >>
>  >>A colleague of mine is very enthousiastic about Skype as it saves him
>  >>a lot of money on  his long distance bills. According to his vision of
>  >>Skype, one day, everybody is going to use Skype....
>  >>
>  >>This scares me. Intuitively, this looks like the perfect recipe for a
>  >>congestion collapse. But, he argues that this could not be the case as
>  >>there are miles and miles of unused copper and fiber opticts out there
>  >>and that more bandwidth is widely available across the world...
>  >>
>  >>IMHO, I think that applications like Skype should be responsible for
>  >>managing the congestion they could potentially cause. This brings me
>  >>to my question. If more and more applications start to behave like
>  >>Skype and selfishly worry more about their business model than about
>  >>the health of the global Internet, is there still a possibility of a
>  >>congestion collapse today ? Or, are those worries well behind because
>  >>the problem can be compensated by introducing more bandwidth into the
>  >>network ?
>  >>
>  >>Thank you
>  >>
>  >>Emmanuel
>  >>
>  >>-- 
>  >>UNIX IS very user friendly. It is just selective about its friends...
>
>  cheers
>
>    jon
>
>