Client connect time is the delay from accept() system call till receiving a parse-able HTTP request.

It may seem that client connect time cannot depend on the result of the request (hit or miss) because the result is not known at that time. However, this is not the case: Connect time for hits is often longer than for misses.

Longer connect time for hits can be explained by the source of a [future] hit request. There are two sources for hits: clients and neighbors (siblings). There is only one major source for misses, that is clients, because, by ICP protocol, neighbors do not ask for an object if it will result in a miss! (Unless siblings are also parents, which is rare.) Note that clients are often closer to a cache server than neighbors. Thus, hit requests originated from neighbors may have longer connect time. Again, this does not happen to misses since they are not originated from neighbors.

This effect is especially noticeable on leaf proxies, but it can be visible on an intermediate proxy as well.

Proxy reply time is the time it takes to send a reply to a client after the document was retrieved from the cache or another server. Note that due to pipelining, a reply process may start prior to receiving the last byte from the server.

See also proxy reply time versus file size experiment.

To understand what affects proxy reply time it is important to distinguish several subclasses of replies. On this graph we isolate 200 hits from 304 hits. 200 hits require transmission of the entire document as in case of a miss. 304 hits require transmittion of minimal amount of information and are usually done in one system call.

Increase in proxy reply time may be affected by three factors:

• degradation of outbound connections (slows delivery of a large miss)
• proxy performance degradation (affects misses and 200 hits)
• degradation of inbound connections (affects all replies)

The behavior of the "304 hits" line is important. Neither outbound connections nor proxy performance can affect 304 replies. If reply time for 304 hits goes up, then inbound connections are congested.

Note that 200 hits and misses suffer from congestion more than 304 hits because of a larger size that may require several network I/Os. They may also depend on performance of the proxy, and misses may depend on outbound connections.

See also proxy reply time versus file size experiment.

Proxy reply time depends on the amount of information to be sent. This graph depicts the dependency based on 24 hour data.

Most documents served by proxies are smaller than 8 KB. Here we show reply time for small files. We plot 200 and 304 hits separately to compare reply time of a 200 hit with a miss in a fair setup.

Consider 1 KB files. If misses have smaller reply time then 200 hits, then destinations of hits are farther then destinations of misses. Note that the load on a destination machine should not affect reply time because the network connection is already established and we are transmitting very small files.

Slow hits are typical for leaf servers that cooperate with other proxies. This is consistent with slow hits during client connection phase.

You may want to click on a graph to see the details.