[GUFSC] BBC News - Skype crash: Software bug and server overloads blamed

[Altamir Dias]

Skype crash: Software bug and server overloads blamed

Server overloads and a bug in Skype for Windows caused the two-day 
outage for the net phone firm.

Details of what caused the service to be unusable for millions of users 
prior to Christmas have been posted on the firm's blog.

The two events combined to create a cascade of problems that managed to 
knock out much of the network underpinning the phone service.

Skype is assessing how its network is built to stop the problem recurring.
Traffic cascade

Writing on the Skype blog, Lars Rabbe, chief information officer at the 
company, said the problems started on 22 December, when some of its 
servers that handle instant messaging started getting overloaded.

This meant that the responses they sent to Windows machines running 
Skype were slightly delayed. Unfortunately, a bug in one version of 
Skype for Windows meant this delay caused the program to crash.

About 50% of all Skype users ran the buggy 5.0.0.152 version of the 
software, said Mr Rabbe.

This caused problems for Skype because of the way the network supporting 
it is organised. Some of the data travelling round Skype's network are 
passed through all those machines logged on to the service.
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Those participating machines act as what Skype calls "supernodes" and 
carry out some of the administrative tasks of the global network and 
help to ensure calls get through.

With a huge number of these machines offline because of the crash, the 
rest of the network quickly became overloaded.

Mr Rabbe wrote that the disappearance of the supernodes meant the 
remaining ones were swamped by traffic.

"The initial crashes happened just before our usual daily peak-hour and 
very shortly after the initial crash," wrote Mr Rabbe, "which resulted 
in traffic to the supernodes that was about 100 times what would 
normally be expected at that time of day."

Traffic levels were so high that they blew through the safe operating 
specifications supernodes usually use. As a result, more supernodes shut 
down.
Compensation offer

The "confluence of events", said Mr Rabbe, led to Skype being offline 
for about 24 hours as engineers put in place hundreds of dedicated 
supernodes and gradually brought the service back to life.

To ensure the outage does not happen again, Mr Rabbe said Skype would 
look at its update policy, to see if it should automatically move users 
to newer versions of its software.

A version of Skype for Windows that is free of the bug already exists, 
but is not automatically given to users.

It said it would also look at its network to improve capacity and get on 
with an investment programme that would boost this resilience.

Mr Rabbe apologised again on behalf of the company and added: "We know 
that we fell short in both fulfilling your expectations and 
communicating with you during this incident."

Skype has offered compensation to customers in the form of vouchers for 
pre-pay users and a free week of service for subscribers.


http://www.bbc.co.uk/news/technology-12092795
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      _/_/_/_/ _/_/_/_/_/ _/_/_/_/_/   Prof. Altamir Dias, Dr. Eng.
     _/       _/  _/  _/ _/            Departamento de Eng. Mecânica
    _/_/_/   _/  _/  _/ _/ UFSC        Universidade Federal de SC
   _/       _/  _/  _/ _/              88.040-900 - Florianópolis-SC
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                                       Phone: 55-48-3721-9264 - Ramal 210
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http://www.emc.ufsc.br/professores/altamir/