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Experiential Learning of Networking Technologies

Non-persistent and Persistent Connections


The default approach in both HTTP/0.9 and HTTP/1.0 protocols is to establish a separate TCP connection for each URL access. Such connections are called non-persistent. Although non-persistent connections are simple, they have three main disadvantages:

i. A new TCP connection must be established for each URL access. This results in an additional round trip delay for setting up a TCP connection.
ii. For each new TCP connection, the web server must allocate resources (buffers, TCP state variables, etc.) which results in significant overhead for the web server.
iii. Most web page today consists of multiple objects, so the web server needs to concurrently serve several TCP connections for a single client web browser.

In HTTP/1.1, the default2 is to use persistent connections. These overcome the disadvantages listed above because a single TCP connection is reused to serve multiple HTTP requests (concurrently, if needed). This change has led to such a significant performance improvement that it has been back ported to HTTP/1.0. The HTTP protocol defines two headers (Connection and Keep-Alive) that identify whether a web request is using persistent or non-persistent connections.

The header line Connection: Close means that the TCP connection should be closed immediately after the web request is served, implying non-persistence. In contrast, the header line Connection: Keep-Alive means that the underlying TCP connection should remain open for serving web requests in the future, implying persistence. To establish a persistent connection, both the client and the web server should support such connections. If either one does not support persistence, then the connection is non-persistent.

A persistent TCP connection should not remain open forever. For instance, when the client navigates to a different web site, keeping the underlying TCP connection open will lead to unnecessary resource consumption on the original web server, causing its performance to degrade. Further, the web server will quickly reach the limit on the maximum number of open TCP connections it can support, after which it will no longer be able to cater to new requests. Thus, there is a need to define a timeout after which a persistent TCP connection needs to be closed. Further, it is prudent to impose a limit on the total number of web requests that can be served on a given persistent TCP connection. To understand why, suppose a web server has a bug that leads to resource inconsistency (e.g., a memory leak) while serving a web request. The greater the number of web requests served on a TCP connection, the greater the resource leak. Hence, permitting several web requests on one TCP connection could exhaust web server resources. The header Keep-Alive has two parameters (timeout and max) to handle these two concerns.

For example, the header line Keep-Alive: timeout=100, max=250 states that the underlying TCP connection can be kept idle for up to 100 seconds. If no new HTTP requests are received for 100 seconds after serving the last request, the underlying TCP connection will be closed. Further, the parameter max=250 states that after the first request, a maximum of 250 additional HTTP requests can be served on the underlying TCP connection. If another (251st) request is sent, the web server sends the HTTP header Connection: Close to indicate that the TCP connection is now closed. To service the next (252nd) request from the same web client, a new TCP connection must be established before web content can be exchanged. Further details about persistent and non-persistent connections can be found in the textbook by Kurose and Ross [6].

Although RFC 2616 [3] suggests that only two concurrent persistent connections should be established between a client and a web server, modern browsers allow a greater number of concurrent connections. The default values for Firefox and Google Chrome browsers is 6, whereas IE10 sets the default value at 8 and Edge (Windows 10) sets the default number of concurrent connections to a website even higher. Among these browsers, only Firefox allows users to easily change the number of concurrent persistent connections. Enter about:config in the URL bar(this triggers a warning), find the field max-persistent-connections-per-server, and double-click on it to change the value as shown in Figure 2.









Figure 2: Configuring the maximum number of concurrent persistent connections per server

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