In persistent connections multiple HTTP requests and
responses are multiplex onto a single TCP
connection. Delimiters at the end of an HTTP request indicates the end of a
request and the content length allows the
receiver to identify how long a response
is. So, the server actually has to know the size of the transfer in advance.
Persistent connections can also be combined with
something call pipelining. In pipelining, a client
sends the next request as soon as it encounters a referenced object. So there's
as little as one round trip time for all referenced objects before they began
to be fetched. Persistent connections with pipelining
is the deafult behavior in, HTTP 1.1.
