Persistent Connections

A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior of any HTTP connection. That is, unless otherwise indicated, the client should assume that the server will maintain a persistent connection, even after error responses from the server. Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling takes place using the Connection¹ header field. Once a close has been signaled, the client must not send any more requests on that connection. Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers and causing congestion on the Internet. The use of inline images and other associated data often require a client to make multiple requests of the same server in a short amount of time. Persistent HTTP connections have a number of advantages:

• By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways, tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
• HTTP requests and responses can be pipelined in a connection. Pipelining allows a client to make multiple requests without waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
• Network congestion is reduced by reducing the number of packets caused by TCP opens (three-way handshakes' cost), and by allowing TCP sufficient time to determine the congestion state of the network.
• Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.
• HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using future versions of HTTP might optimistically try a new feature, but if communicating an older server, retry with old semantics after an error is reported.