Question

I always knew web browsers could do parallel downloads. But then the other day I heard about pipelining. I thought pipelining was just another name for parallel downloads, but then found out even firefox has pipelining disabled by default. What is the difference between these things and how do work together?

Answer 1

I think this MDC article explains HTTP pipelining pretty darn well.

What is HTTP pipelining?

Normally, HTTP requests are issued sequentially, with the next request being issued only after the response to the current request has been completely received. Depending on network latencies and bandwidth limitations, this can result in a significant delay before the next request is seen by the server.

HTTP/1.1 allows multiple HTTP requests to be written out to a socket together without waiting for the corresponding responses. The requestor then waits for the responses to arrive in the order in which they were requested. The act of pipelining the requests can result in a dramatic improvement in page loading times, especially over high latency connections.

Pipelining can also dramatically reduce the number of TCP/IP packets. With a typical MSS (maximum segment size) in the range of 536 to 1460 bytes, it is possible to pack several HTTP requests into one TCP/IP packet. Reducing the number of packets required to load a page benefits the internet as a whole, as fewer packets naturally reduces the burden on IP routers and networks.

HTTP/1.1 conforming servers are required to support pipelining. This does not mean that servers are required to pipeline responses, but that they are required to not fail if a client chooses to pipeline requests. This obviously has the potential to introduce a new category of evangelism bugs, since no other popular web browsers implement pipelining.

I recommend reading the whole article since there’s more than what I copied into my answer.