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8. generate, create denso qr bar code none in .net projects PLANET The directi QR Code for .NET ve prefer_direct modifies Squid"s behavior only for cacheable requests. If we want to route all requests through a firewall proxy, we should use never_direct instead.

. nonhierarch ical_direct Non-hierarchical requests are the requests that are either identified by the hierarchy_ stoplist directive or can"t be cached by Squid. Such requests should not be sent to cache peers because they will not result in cache hits. Therefore it"s a good idea to forward these requests directly to remote servers.

We achieve this behavior by setting the value of the directive nonhierarchical_direct to on. If we set this directive"s value to off, these requests will not be sent to remote servers directly. Please note that although HTTPS requests are not cacheable, nonhierarchical_direct must be set to off for HTTPS requests to be relayed through a firewall parent proxy.

. It"s not re visual .net QR Code 2d barcode commended to set the value of nonhierarchical_ direct to off. If we want to direct all requests via a firewall proxy, we should use the never_direct directive instead.

. Have a go hero proxy server behind a firewall Configure y Quick Response Code for .NET our proxy server so that it forwards all the requests to a parent proxy server and never contacts the remote servers directly..

Peer communication protocols We have lea .net framework qr codes rned about configuring Squid to be a part of a cache hierarchy. When many proxy servers are a part of cache hierarchy, they need to communicate to share information about the objects present in their cache so that neighbors can utilize these cached objects as hits.

For communication among peers, Squid implements ICP, HTCP, and Cache Digest protocols. Later on, we"ll have a brief look at ICP, HTCP, and Cache Digest protocols..

Internet Cache Protocol ICP or Inte qrcode for .NET rnet Cache Protocol is a simple web-caching protocol used to query proxy servers (cache peers) about the existence of a particular object in their cache. Depending on the replies received from the neighbors, Squid will decide the forwarding path for the particular request.

. [ 215 ]. Building a Hierarchy of Squid Caches As we saw i n the peer selection algorithms, ICP is also used to calculate the round trip time and also for detecting dead peers in a hierarchy. The round trip time calculation is an important measure as it can help Squid in dynamically rerouting the traffic to a less congested network route. Although ICP is a simple protocol and it"s very easy to configure proxy servers to communicate with each other using it, ICP also suffers from a lot of problems.

The first one is latency. Squid doesn"t know whether an object is present in a peer cache or not. It has to query all the peer caches for each object, which in some cases (if the number of peers is large), will introduce a significant delay as it will take time to query all of them.

So, if we have a large number of peers, there will be a lot of ICP packets floating around on the network which may end up causing congestion. To get around the congestion issue here, we can use the multicast ICP protocol. Other flaws in the ICP protocol are false hits, security, and so on.

For more details on the ICP protocol, please visit

txt. Another interesting read about the application of the ICP protocol is at http://tools.ietf.

org/html/rfc2187. The HTCP protocol is recommended over the ICP protocol to avoid problems like latency, false hits, and so on..

Cache digests Squid keeps a list of all the cached objects in the main memory in the form of a hash, so that it can quickly guess whether a URI will result in a hit or miss without actually searching for the files on disk. Cache digest is a summary of these cached objects into a compact bitmap using the Bloom Filter data structure (for more information on the Bloom Filter, please visit http://en.wikipedia.

org/wiki/Bloom_filter). The value of the bit determines whether a particular object is present in the cache or not. If the bit is on or set to 1, the object is present in the cache, otherwise it"s not in the cache.

This summary is available to other peers via a special URL over the HTTP protocol. When a cache digest is retrieved by peers they can determine, by checking the digest, whether a particular URI is present in the cache or not. So, cache digests significantly reduce the number of packets flowing on the network which are just for querying the other peers, but the total data transfer amount increases as the cache digests are fetched by all the peers periodically.

However, this helps in significantly decreasing the delay introduced by ICP queries. With the cache digest protocol, the problem of false hits get worse as the digest grows older. The digest is rebuilt only periodically (hourly by default).

This also introduces the problems of false misses. The false misses are for web objects which were cached after the cache digest was built..

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