

Understanding Today's Skype Outage: Explaining Supernodes - danyork
http://www.disruptivetelephony.com/2010/12/understanding-todays-skype-outage-explaining-supernodes.html

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drats
My theory: Tons of supernodes (static ip with no NAT, broadband, constantly
on) are in the 20s-30s age group and are on holiday. Plus office and
university machines that are normally left on have been turned off. The people
I know in universities across Europe normally have skype on at work
constantly, or at least during the day, and most have static ips on their uni
networks so I imagine they get selected as supernodes quite often. I know many
uni network admins have complained about skype using bandwidth. All those
people just went on holiday over the last week or so along with all the people
in uni dormitories. I'd expect that would have lost Skype thousands upon
thousands of supernodes. Add to that an increase in calls due to the approach
of Christmas; calls that are personal and use video and audio rather than just
audio (which is more common in business usage patterns during the rest of the
year). Holiday gamers turning off skype to stop lag would factor as well
(doesn't matter if it's true or not, gamers blame everything but themselves
during losses and would no doubt turn off skype). So a 20% or more drop in
supernodes, 20% or more increase in calls and bandwidth, plus holiday
torrents/youtube/hulu on the remaining nodes slowing them down and you have
the perfect situation for this.

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rix0r
That could very well be the case, but if that were really the cause you'd
expect to see this pattern every year. However, it apparently didn't happen
for at least two years in a row[1]. Also, I would have expected Skype
engineers to have wised up to the situation by now if it were really a
seasonal thing.

[1] Since the last outage was 3 years ago. I can't find a source for that
right now -- would be interesting to see if the outage was also around
Christmas time.

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danyork
No, the outage in 2007 was in August, and had to do with churn of the
supernodes related to updates. I wrote about it here:

[http://www.disruptivetelephony.com/2007/08/skype-offers-
fu.h...](http://www.disruptivetelephony.com/2007/08/skype-offers-fu.html)

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itsnotvalid
That is basically why it's free to use Skype. It's free as many of their
infrastructure is provided by users.

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arn
That Skype uses supernodes is interesting, but makes some sense since it was
developed by the same people who developed Kazaa (a P2P filesharing service),
which as I recall has a similar architecture.

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nedrichards
So much so that they actually licenced the Joltid technology that they spun
out of Kazaa. There was a lawsuit:

<http://blogs.skype.com/en/2009/11/joltid_settlement.html>

There were in fact several lawsuits related to licencing this stuff.

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mseebach
I was wondering if Skype couldn't benefit from a product that is essentially a
limited, local supernode.

In my job we try to use Skype video-calls, but the quality is very flaky,
which I think is because we rely on someone else relaying us (since both ends
are NAT'ed and likely also firewalled). Individually port-forwarding for
everybody is unfeasible, but running a single service on a server, and telling
everyone to use that as their super-node/proxy would be fine.

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mustpax
OT, is there an equivalent semi-technical analysis of the Tumblr outage? I've
been searching through their status blog and all I can find is “we
accidentally brought the db cluster down.” Does anyone know how this happened?

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iwwr
In other words, supernodes are nodes connected behind a routable IP with a
minimum of bandwidth. They are used to provide NAT traversal for unroutable
clients.

I can imagine ipv6 would greatly benefit Skype.

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ZoFreX
It's not just to solve the NAT problem. There are no servers in the
traditional sense, it's a "cloud" of peer-to-peer clients (that are also,
strictly speaking, servers), and you need some way of "bootstrapping" yourself
into that cloud when you sign in. Supernodes fix this problem easily, but I
have to wonder why it was a relatively short list hardcoded into the client...
it wouldn't be hard to make it a bit more adaptable.

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bdonlan
Perhaps the problem is that it was, in fact, adaptable, meaning that if all
the supernodes your client knows about go down you're no longer able to
bootstrap.

In fact, I found I was able to reconnect faster after the outage by deleting
the shared.xml file that stores known supernodes - presumably connecting back
to Skype-operated bootstrap servers.

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grammaton
Is it just me, or does the Skype setup sound an awful lot like Gnutella and
it's Ultrapeers back in the day?

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eelco
It not just you. Skype was founded by the people that also founded KaZaA and
they (obviously) took the technology/knowledge they already had, modified it
and applied to a different purpose.

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webuiarchitect
Today's? Like it used work perfectly otherwise? Huh!

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syaz1
Awesome dumbed-down explanation and visualization!

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danyork
Thank you!

