HTTP: The Definitive Guide

Parallel Connections

As we mentioned previously, a browser could naively process each embedded object serially by completely requesting the original HTML page, then the first embedded object, then the second embedded object, etc. But this is too slow!

HTTP allows clients to open multiple connections and perform multiple HTTP transactions in parallel, as sketched in Figure 4-11. In this example, four embedded images are loaded in parallel, with each transaction getting its own TCP connection.^[12]

Figure 4-11. Each component of a page involves a separate HTTP transaction

Parallel Connections May Make Pages Load Faster

Composite pages consisting of embedded objects may load faster if they take advantage of the dead time and bandwidth limits of a single connection. The delays can be overlapped, and if a single connection does not saturate the client’s Internet bandwidth, the unused bandwidth can be allocated to loading additional objects.

Figure 4-12 shows a timeline for parallel connections, which is significantly faster than Figure 4-10. The enclosing HTML page is loaded first, and then the remaining three transactions are processed concurrently, each with their own connection.^[13] Because the images are loaded in parallel, the connection delays are overlapped.

Figure 4-12. Four transactions ...