CDN Debugging

20 May 2013 12:28

Working with a CDN can be a whole world of pain sometimes, especially when you’re trying to figure out what the heck is going on.

Firstly, CDN’s quite often will have a pair of POP’s. Someone like LLNW will return you this:

$ host media.limelight.com
media.limelight.com is an alias for llnw.vo.llnwd.net.
llnw.vo.llnwd.net has address 87.248.213.101
llnw.vo.llnwd.net has address 178.79.207.22
llnw.vo.llnwd.net has IPv6 address 2a02:3d0:600:15:230:48ff:feda:d0c0
llnw.vo.llnwd.net has IPv6 address 2a02:3d0:600:212:225:90ff:fe4a:ef16

Each POP could return you different content. Whilst not recommended you could target a POP directly like this:

curl -H 'Host: media.limelight.com' -H 'X-LDebug:2' \
  -I http://87.248.213.101/designimages/limelight-content-delivery.jpg

LimeLight Networks offer some debugging via a header which is shown in the command above, “X-LDebug:2”. If you make a call with it in you’ll see some extra X-Cache information:

$ curl -H 'Host: media.limelight.com' -H 'X-LDebug:2' \
  -I http://87.248.213.101/designimages/limelight-content-delivery.jpg
HTTP/1.1 200 OK
Server: Apache
Cache-Control: max-age=3600
X-Server-Name: la-c1-r3-u30-b1
Content-Type: image/jpeg
Via: 1.1 la-c1-r2-u20-b2:3128 (squid)
X-Cache: MISS from cds1020.iad.llnw.net
X-Cache: MISS from cds822.lga.llnw.net
Age: 308
Date: Mon, 20 May 2013 11:15:05 GMT
Last-Modified: Thu, 08 Mar 2012 22:18:12 GMT
Expires: Mon, 20 May 2013 12:09:57 GMT
X-Cache: HIT from cds163.lon.llnw.net
Content-Length: 5486
X-Cache: MISS from cds761.lon.llnw.net
Connection: keep-alive

Akamai are a much larger network and as such can be a bit more complex to find POPs, but, they also offer a lot more in terms of debugging. Most of this is lifted from Mesmor.

Akamai provide an extension for both Internet Explore and Firefox which can be found in the control panel. You can also quickly see the information from an Akamai asset using curl is:

curl -I -H "Pragma: akamai-x-cache-on, akamai-x-cache-remote-on, 
akamai-x-check-cacheable, akamai-x-get-cache-key, akamai-x-get-extracted-values, 
akamai-x-get-nonces, akamai-x-get-ssl-client-session-id, akamai-x-get-true-cache-key, 
akamai-x-serial-no" http://www.website.com/style.css

Some explanation of the results:

X-Cache : Cache State

X-Cache TCP_MEM_HIT from a118-214-191-15 (AkamaiGHost/6.7.0.2-9183648)

a118-214-191-15      : Akamai IP Identifier (The Akamai 
                       server that serves this request)
TCP_HIT              : The object was fresh in cache and 
                       object from disk cache.
TCP_MISS             : The object was not in cache, server 
                       fetched object from origin.
TCP_REFRESH_HIT      : The object was stale in cache and we 
                       successfully refreshed with the origin 
                       on an If-modified-Since request.
TCP_REFRESH_MISS     : Object was stale in cache and refresh 
                       obtained a new object from origin 
                       in response to our IF-Modified-Since request.
TCP_REFRESH_FAIL_HIT : Object was stale in cache and we 
                       failed on refresh (couldn't reach origin) 
                       so we served the stale object.
TCP_IMS_HIT          : IF-Modified-Since request from client 
                       and object was fresh in cache and served.
TCP_NEGATIVE_HIT     : Object previously returned a "not found"
                       (or any other negatively cacheable response)
                       and that cached response was a hit for this
                       new request.
TCP_MEM_HIT          : Object was on disk and in the memory 
                       cache. Server served it without hitting 
                       the disk.
TCP_DENIED           : Denied access to the client for whatever reason.
TCP_COOKIE_DENY      : Denied access on cookie authentication 
                       (if centralized or decentralized authorization 
                       feature is being used in config).

X-Cache-Key : Internal Cache Key

X-Cache-Key /L/1745/15811/3h/www.website.com/common/css/style.css
15811           : Akamai CP Code
3h              : Cache TTL
www.website.com : Origin Server Name

X-Check-Cacheable : Cacheability

This determines if the request object is cache-able as set in your Akamai configuration.

You can read more about CDN testing pre-live and also accessing some other providers headers.

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