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wccp bypass packets

I read an interesting paragraph in the article about how to configure wccp (http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp/configuration/12-4t/iap-12-4t-book/iap-wccp.html).

 

WCCPv2 Web Cache Packet Return

If a content engine is unable to provide a requested object it has cached due to error or overload, the content engine will return the request to the router for onward transmission to the originally specified destination server. WCCPv2 provides a check on packets that determines which requests have been returned from the content engine unserviced. Using this information, the router can then forward the request to the originally targeted server (rather than attempting to resend the request to the content engine cluster). This process provides error handling transparency to clients.

Typical reasons why a content engine would reject packets and initiate the packet return feature include the following

Instances when the content engine is overloaded and has no room to service the packets

Instances when the content engine is filtering for certain conditions that make caching packets counterproductive (for example, when IP authentication has been turned on)

 

And I can't find any more details about how this stuff works. How it's determined that request is unserviced? What flags and where in the packet does this indicate? Does this mean that wccp router caches such unserviced requests and all further requests will go directly to target server or it happens on per-packet basis?

 

Thanks in advance!

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