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Chapter 7
Hierarchical Caching
parent_name:port_number; parent_name:port_number;
6. Click the Make These Changes button.
ICP cache hierarchies
The Internet Cache Protocol (ICP) is a protocol used by proxy caches to exchange information about their
content. ICP query messages ask other caches if they are storing a particular URL. ICP response messages
reply with a hit or miss answer.
A cache exchanges ICP messages only with specific ICP peers, which are neighboring caches that can receive
ICP messages. An ICP peer can be a sibling cache, which is at the same level in the hierarchy, or a parent
cache, which is one level up in the hierarchy.
If Traffic Server has ICP caching enabled, it sends out ICP queries to its sibling caches in the event of a cache
miss on an HTTP request. If there are no hits on siblings, Traffic Server sends ICP queries to ICP parents. If
there are no hits on ICP parents, Traffic Server forwards the request to its HTTP parents. If there are no HTTP
parent caches established, Traffic Server forwards the request to a selected ICP parent cache (which resolves
the request by communicating with the origin server).
Configuring Traffic Server to use an ICP cache hierarchy
When you configure a Traffic Server node to be part of an ICP cache hierarchy, you must:
•
Enable ICP caching and set options to:
o
Determine if the Traffic Server can receive ICP messages only or both send and receive ICP messages
o
Determine if Traffic Server can send messages directly to each ICP peer or send a single message on a
specified multicast channel
o
Specify the port used for ICP messages
o
Set the ICP query timeout
•
Identify the ICP peers with which Traffic Server can communicate
You can set ICP options and identify ICP peers by using Traffic Manager or by editing a configuration file
manually. Both procedures are provided below.
NOTE
When you use Traffic Manager to identify parent caches for parent failover, Traffic
Server sends requests to the parents in the order that they appear in the Parent
Cache field. For example, when the first parent cache listed in the Parent Cache
field is not available, Traffic Server sends requests to the next parent cache in the
list. If all parent caches are unavailable, requests are sent directly to the origin server.
You can change the order in which parent caches are queried by setting up a parent
proxy rule in the
parent.config
file using the
round_robin
action. Refer to
parent.config‚ on page 179
.
NOTE
If Traffic Server receives a hit message from an ICP peer, Traffic Server sends the
HTTP request to that peer. However, it may be a cache miss, because the original HTTP
request contains header information that is not communicated by the ICP query. For
example, the hit might not be the requested alternate. If an ICP hit turns out to be a miss,
Traffic Server forwards the request to either its HTTP parent caches or to the origin
server.
