[e2e] Skype and congestion collapse.

[Jon Crowcroft]

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