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CDN node traffic distribution

    Question

  • We just pushed some of our resources to CDN a couple of days ago to improve performance for our Asian customers and we are monitoring the CDN traffic. I created 2 affinity groups for East Asia and anywhere Asia and the storage account itself was created with 'Anywhere in US' region option.  I am using webpagetest.org to do a simple simulation of http request location detection. Most of the simulated requests from Asian countries seem to be getting the content from US CDN nodes. I tried Paris and Moscow requests and they seem to be served up from London node. So the question is, why aren't they served up from Asian nodes or Moscow node etc? I understand that the CDN may not be served up always from the nearest edge server, but for almost 2 days, none of the Asian requests are served up from Asian nodes? If I keep monitoring would I see the nodes changing? I am trying to explain to our business users that if a request is from Hong Kong, then content may not be served up from Hong Kong but the CDN is indeed working as expected.

    Wednesday, February 02, 2011 11:01 PM

Answers

  • Sorry this has taken so long to get a response, Jason left us.

    You are correct.  Using tools such as Keynote or Gomez is a much better (although not perfect) way to test network locality.  What they have going for them is a wide distribution of testing agents, each with their own DNS configuration, giving a much clearer view of how traffic is actually served from the CDN.

    More questions?  Direct to stcarter [at] microsoft {dot} com.

    -Steve

    Tuesday, February 21, 2012 11:40 AM

All replies

  • I created 2 affinity groups for East Asia and anywhere Asia and the storage account itself was created with 'Anywhere in US' region option. 

    I don't think affinity groups have any bearing on the situation. Affinity groups are used to associate an Azure Service and related storage. Are you sure the CDN is configured properly with appropriate TTL, etc?

    Thursday, February 03, 2011 2:08 AM
    Answerer
  • Could you explain how TTL would affect what node CDN selects? Regarding CDN being configured properly, as you know, there isn't much configuration for CDN. I have a CDN with custom domain mapped and the storage account itself was created with 'Anywhere in US' as the region as I mentioned.  For the same content, the Paris and Moscow requests picked the London node (not US node). Why would Asian requests pick US node always?

    Thursday, February 03, 2011 6:46 PM
  • Hi,

    I'm with the Azure CDN team and can help explain this a bit more in depth.

    Neil, you are correct that affinity groups have no bearing on CDN delivery from the edge. The choice of Storage location can have a minor impact on first-load cache performance but I don't have hard and fast data on that difference today.

    TTL has no bearing on edge node selection. They're critical settings in themselves but are entirely unrelated.

    So, to answer the origin question for Valli -- it's probably working as expected; don't confuse physical locality with logical/network locality. The *client ISP* selects the "nearest" node in terms of BGP routing and peering -- and different ISPs within the same geography can behave very differently depending on their routing and peering.

    It's likely that the ISP that you're testing from is primarily peered with the Azure CDN network in the US. This is a totally standard behavior in CDNs and is by design. As you point out, we can (and do) shift traffic between nodes, but this is more or less an exception. Traffic will be routed to the preferred logical location, not the closest physical location. If you were to test on different ISPs with different peering/routing, you would see different behaviors.

    Happy to discuss further as needed. Direct mail me at jason (dot) sherron [at] microsoft {dot} com

    Jason

    • Proposed as answer by JasonSherron Wednesday, August 03, 2011 10:55 PM
    Monday, March 28, 2011 12:09 AM
  • Jason,

    Thanks for taking time to reply to my question. If I understand correclty, what you mean is that the "webpagetest.org" site that I am using to test, is the client ISP that is primarily peered with US and that's why I always see US nodes for Asia CDN?. Does that mean if I use a different test tool, I might get a different result?  Are there any recommendations on these tools?

    Thanks for sharing your email. I would definitely like to understand more and will email you. 

    Friday, April 01, 2011 11:01 PM
  • Sorry this has taken so long to get a response, Jason left us.

    You are correct.  Using tools such as Keynote or Gomez is a much better (although not perfect) way to test network locality.  What they have going for them is a wide distribution of testing agents, each with their own DNS configuration, giving a much clearer view of how traffic is actually served from the CDN.

    More questions?  Direct to stcarter [at] microsoft {dot} com.

    -Steve

    Tuesday, February 21, 2012 11:40 AM