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rims

join:2000-10-22
Phoenix, AZ
reply to Umm4

Re: I think Qwest is blocking YOUTUBE

Interesting post. YouTube wants all its content servers hit equally, so its DNS server rotates which IP address is handed out on each request. Even if Qwest had a cached IP address, it would still reach a server with the content I wanted.

But, you say, the content at each of these servers is also changed frequently, so my video is on serverA in the morning and serverB in the afternoon. And Qwest is ignoring the TTL of the you tube DNS entry, so it sends out the IP address of serverA in the afternoon.

Not knowing much about DNS, 1) why would YouTube regularly move content off one machine and onto another, and 2) what is the advantage to Qwest to ignore TTL entries?


Umm4

join:2009-10-03
Bellevue, WA

said by rims:

Not knowing much about DNS, 1) why would YouTube regularly move content off one machine and onto another, and 2) what is the advantage to Qwest to ignore TTL entries?
1) As was mentioned earlier, it is the cache servers that hang the most. By definition, these servers would not hold videos for a long time. Last night I did a test and it was v6.lscache5.youtube.com that was hanging, and the opendns servers returned a different IP for that URI than the Qwest servers did.

2) The "advantage" would be that Qwests DNS servers would not have to do as many upstream queries. Not having to send and wait for an off network update every time someone clicks on a youtube video would increase the number of customers that each DNS server was able to accommodate. That said, I think they could probably get away with fixing it without too much of a performance problem, if that is indeed what the problem is, as most resources only have a low TTL if they actually need one. It could very well just be a mistake. One line in a configuration file that only affects 1% of all of your DNS responses is very easy to overlook.

That said, I can not be certain that is what the problem is, but it is very reminiscent of it, as when I query a hanging cache server with Qwest's DNS I do get a response, just not the correct one. If it was a matter of Qwest DNS not being able to talk to Google's it would most likely hang at the query stage also, instead of quickly returning an incorrect result. As for it being a problem with Qwest giving out the wrong TTL, well that would affect all ISP's equally, and as we have seen, that is not happening.

FYI, While I may be misdiagnosing the problem, I am actaully 7+ years CompTIA Net+ certified, and run our network at work, so I am not completely speaking out of my you know what. Although I have certainly never worked on an a huge network like Qwest's, I have put some thought into this, and it it is the only answer than makes sense to me.

Chris