HTTP
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, ...
persistent connection, also called HTTP keep-alive, or HTTP connection reuse, is the idea of using a single
TCP connection to send and receive multiple
HTTP requests
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, w ...
/responses, as opposed to opening a new connection for every single request/response pair. The newer
HTTP/2
HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working ...
protocol uses the same idea and takes it further to allow multiple concurrent requests/responses to be multiplexed over a single connection.
Operation
HTTP 1.0
Under HTTP 1.0, connections should always be closed by the server after sending the response.
Since late 1996, developers of popular products (browsers, web servers, etc.) using HTTP/1.0, started to add an unofficial extension (to the protocol) named "keep-alive" in order to allow the reuse of a connection for multiple requests/responses.
If the client supports keep-alive, it adds an additional header to the request:
Connection: keep-alive
When the server receives this request and generates a response, if it supports keep-alive then it also adds the same above header to the response. Following this, the connection is not dropped, but is instead kept open. When the client sends another request, it uses the same connection.
This will continue until either the client or the server decides that the conversation is over and in this case they omit the
"Connection:"
header from the last message sent or, better, they add the keyword "close" to it:
Connection: close
After that the connection is closed following specified rules.
Since 1997, the various versions of HTTP/1.1 specifications acknowledged the usage of this unofficial extension and included a few caveats regarding the interoperability between HTTP/1.0 (keep-alive) and HTTP/1.1 clients / servers.
HTTP 1.1
In HTTP 1.1, all connections are considered persistent unless declared otherwise.
[Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing, Persistence](_blank)
/ref> The HTTP persistent connections do not use separate keepalive messages, they just allow multiple requests to use a single connection. However, the default connection timeout of Apache httpd 1.3 and 2.0 is as little as 15 seconds and just 5 seconds for Apache httpd 2.2 and above. The advantage of a short timeout is the ability to deliver multiple components of a web page quickly while not consuming resources to run multiple server processes or threads for too long.
Keepalive with
chunked transfer encoding
Chunked transfer encoding is a streaming data transfer mechanism available in Hypertext Transfer Protocol (HTTP) version 1.1, defined in RFC 9112 §7.1. In chunked transfer encoding, the data stream is divided into a series of non-overlapping "c ...
Keepalive makes it difficult for the client to determine where one response ends and the next response begins, particularly during pipelined HTTP operation. This is a serious problem when Content-Length
cannot be used due to streaming. To solve this problem, HTTP 1.1 introduced a chunked transfer coding
Chunked transfer encoding is a streaming data transfer mechanism available in Hypertext Transfer Protocol (HTTP) version 1.1, defined in RFC 9112 §7.1. In chunked transfer encoding, the data stream is divided into a series of non-overlapping "c ...
that defines a last-chunk
bit. The last-chunk
bit is set at the end of each response so that the client knows where the next response begins.
Advantages
* Reduced latency in subsequent requests (no handshaking
A handshake is a globally widespread, brief greeting or parting tradition in which two people grasp one of each other's like hands, in most cases accompanied by a brief up-and-down movement of the grasped hands. Customs surrounding handshakes a ...
and no slow start).
* Reduced CPU usage and round-trips because of fewer new connections and TLS handshakes.
* Enables HTTP pipelining
HTTP pipelining is a feature of HTTP/1.1 which allows multiple HTTP requests to be sent over a single TCP connection without waiting for the corresponding responses. HTTP/1.1 requires servers to respond to pipelined requests correctly, with non-p ...
of requests and responses.
* Reduced network congestion
Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...
(fewer TCP connection
TCP may refer to:
Science and technology
* Transformer coupled plasma
* Tool Center Point, see Robot end effector
Computing
* Transmission Control Protocol, a fundamental Internet standard
* Telephony control protocol, a Bluetooth communication s ...
s).
* Errors can be reported without the penalty of closing the TCP connection.
According t
RFC 7230, section 6.4
"a client ought to limit the number of simultaneous open connections that it maintains to a given server". The previous version of the HTTP/1.1 specificatio
stated specific maximum values
but in the words of RFC 7230 "this was found to be impractical for many applications... instead... be conservative when opening multiple connections". These guidelines are intended to improve HTTP response times and avoid congestion. If HTTP pipelining is correctly implemented, there is no performance benefit to be gained from additional connections, while additional connections may cause issues with congestion.
Disadvantages
If the client does not close the connection when all of the data it needs has been received, the resources needed to keep the connection open on the server will be unavailable for other clients. How much this affects the server's availability and how long the resources are unavailable depend on the server's architecture and configuration.
Also a race condition
A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of t ...
can occur where the client sends a request to the server at the same time that the server closes the TCP connection. A server should send a 408 Request Timeout status code to the client immediately before closing the connection. When a client receives the 408 status code, after having sent the request, it may open a new connection to the server and re-send the request. Not all clients will re-send the request, and many that do will only do so if the request has an idempotent HTTP method.
Use in web browsers
All modern web browsers including Google Chrome
Google Chrome is a cross-platform web browser developed by Google. It was first released in 2008 for Microsoft Windows, built with free software components from Apple WebKit and Mozilla Firefox. Versions were later released for Linux, macOS ...
, Firefox
Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current and ...
, Internet Explorer
Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical user interface, graphical web browsers developed by Microsoft which was used in the Microsoft Wind ...
(since 4.01), Opera
Opera is a form of theatre in which music is a fundamental component and dramatic roles are taken by singers. Such a "work" (the literal translation of the Italian word "opera") is typically a collaboration between a composer and a librett ...
(since 4.0) and Safari
A safari (; ) is an overland journey to observe wild animals, especially in eastern or southern Africa. The so-called "Big Five" game animals of Africa – lion, leopard, rhinoceros, elephant, and Cape buffalo – particularly form an importa ...
use persistent connections.
By default, Internet Explorer
Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical user interface, graphical web browsers developed by Microsoft which was used in the Microsoft Wind ...
versions 6 and 7 use two persistent connections while version 8 uses six. Persistent connections time out after 60 seconds of inactivity which is changeable via the Windows Registry.
In Firefox
Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements current and ...
, the number of simultaneous connections can be customized (per-server, per-proxy, total). Persistent connections time out after 115 seconds (1.92 minutes) of inactivity which is changeable via the configuration.
See also
* HTTP pipelining
HTTP pipelining is a feature of HTTP/1.1 which allows multiple HTTP requests to be sent over a single TCP connection without waiting for the corresponding responses. HTTP/1.1 requires servers to respond to pipelined requests correctly, with non-p ...
, whereby multiple requests can be sent without waiting for a response
* HTTP/2
HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working ...
, which allows out-of-order pipelining of requests and responses, and also predictive pushing of content before it has been requested
References
External links
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing, Connection Management, Persistence
{{DEFAULTSORT:Http Persistent Connection
Hypertext Transfer Protocol