David Barrett wrote: > Got it. Excellent theory. > > I guess my question is: why bother with the whole neighbor-cache thing at > all? Why not stick with a classic IM architecture where clients just > maintain persistent TCP connections with the back end? I mean, there are IM > networks *far* bigger than Skype that deal with the same massive-restart > problems all the time.
It might be caused by non-technical reasons. Don't forget that Skype people are the Kazaa people. They might want to be better protected against legal attacks. The less you rely on a centralized service, the more difficult to shutdown your network by the court order. > So far as I can tell, the only thing unique about Skype versus a classic IM > network is Skype does a p2p relay service, but even that could be organized > in a centralized fashion far easier than with some p2p approach. > > It'd be interesting to do an analysis on uptime of AIM versus Skype -- > recognizing that AIM is 10x bigger. I don't use Skype enough to really have > an opinion on its uptime, but AIM seems pretty good: flaky on occasion, but > generally quite reliable. > > I'd love to see more rigorous numbers on that, especially given that Skype > has been widely celebrated for "better scalability and reliability through > the magic of p2p!" I wonder if it in fact achieves either of those. > > -david > >> -----Original Message----- >> From: [EMAIL PROTECTED] [mailto:p2p-hackers- >> [EMAIL PROTECTED] On Behalf Of Alexander Pevzner >> Sent: Tuesday, August 21, 2007 4:02 PM >> To: [email protected] >> Subject: Re: [p2p-hackers] what really happened to Skype? >> >> Hi, David, >> >> the Skype was not down completely all of this time. When it started, >> Skype more or less worked, but clients periodically lost connection to >> the server. Then situation began to become worse: the average period of >> disconnected state grown, the period of connected state shortened. In a >> worst state clients were able to connect for few seconds few times a >> day. The situation didn't recover one happy moment. Instead, it >> developed in a opposite direction: the clients were able to login more >> often and stay online for a longer time. >> >> I run 2 Skype clients on a different machines on a same time. There was >> absolutely no correlation between them when they were logged in or not. >> One of them was able to call Skype voice test, when other was unable to >> login. >> >> It brings me to conclusion that server was not down completely, but >> rejected (or was unable to complete in time) most of the client's >> requests performed during login process. >> >> Please note also 2 more facts: >> 1) This is not a first time, when millions of Windows users reboot >> their machines due to Windows upgrade. But this is a first time when it >> crashed a Skype network >> 2) The situation was fixed at the server side. Client upgrade was not >> required >> >> These observations together with Skype's claims that the problem is in >> the p2p algorithm leads me to the following hypothesis. Skype client, >> being online, tracks state of ~20 neighbors. There is a neighbor cache >> on a client, which only needs to be reloaded from the server when either >> client is new or nobody in a cache is online. Probably, this operation >> is not cheap for the server. When critical amount of clients becomes >> offline, too many orphaned clients try do update neighbor cache from a >> server, which leads to server overload, server becomes incapable to >> maintain already established connections, more clients leave the network >> and so on. >> >> I guess they did one of the following: >> 1) Optimized server's algorithms to fix the server-side bottleneck >> 2) Made a server-side limit of how many clients may connect per >> minute, to stabilize a network and reduce server load >> >> David Barrett wrote: >>> Reading the Skype blog post, it doesn't sound like the problem was due >> to >>> lack of central resources, but rather some catastrophic bug in the P2P >>> network itself -- like it was unable to reform the DHT (or whatever) in >> the >>> wake of a massive churn event. >>> >>> I mean, it shouldn't take *2 days* to log in 9 million users, and unless >>> this was coupled with servers actually suffering hardware malfunction >> (of >>> which there's no indication), I can't see any reason why it'd take that >> long >>> to simply deal with a big backlog of authentication requests. >>> >>> Does anyone know much about the Skype P2P/DHT/network algorithm, and can >>> they hypothesize what sort of event could cause it to take so long to >> get >>> back into operation? >>> >>> Also, was Skype 100% down for 2 days followed by it coming 100% back up >>> (indicative of a central server problem), or was it suffering from >> varying >>> levels of failure throughout that took a couple days to clear up >> (suggesting >>> P2P network problems)? >>> >>> -david >>> >>>> -----Original Message----- >>>> From: [EMAIL PROTECTED] [mailto:p2p-hackers- >>>> [EMAIL PROTECTED] On Behalf Of Alen Peacock >>>> Sent: Monday, August 20, 2007 1:39 PM >>>> To: theory and practice of decentralized computer networks >>>> Subject: Re: [p2p-hackers] what really happened to Skype? >>>> >>>> Absolutely true that Skype hasn't given us enough details to figure >>>> out exactly what happened or why, but that doesn't prevent the looser >>>> cannons among us from taking a shot: >>>> http://flud.org/blog/2007/08/20/p2ps-skype-induced-blackeye-or-why- >>>> diversity-is-good/ >>>> >>>> Alen >>>> >>>> >>>> On 8/20/07, zooko <[EMAIL PROTECTED]> wrote: >>>>> Folks: >>>>> >>>>> This is a fascinating case study, but we don't yet have enough >>>>> information to really learn from it! >>>>> >>>>> http://heartbeat.skype.com/2007/08/what_happened_on_august_16.html >>>> _______________________________________________ >>>> p2p-hackers mailing list >>>> [email protected] >>>> http://lists.zooko.com/mailman/listinfo/p2p-hackers >> _______________________________________________ >> p2p-hackers mailing list >> [email protected] >> http://lists.zooko.com/mailman/listinfo/p2p-hackers _______________________________________________ p2p-hackers mailing list [email protected] http://lists.zooko.com/mailman/listinfo/p2p-hackers
