Connection HTTP header field plays a key role in defining how a client (or a proxy) and a server manage network connections. By providing instructions to either close the connection or keep it open for further requests, the
Connection header is integral to optimizing network resources and managing connection persistence.
Before diving into details, let’s understand the basic syntax of the
Connection HTTP header. In its simplest form, it is as follows:
<directive> can be either
close, each having a distinct impact on the lifecycle of the connection.
In HTTP/1.1, the default connection behavior is “persistent” or “keep-alive”, meaning the connection remains open for multiple requests/responses. It results in reduced latency for subsequent requests, improved performance, and optimal use of resources, as opening and closing TCP connections can be expensive.
The following example shows the use of the
Connection: keep-alive header:
GET /index.html HTTP/1.1 Host: www.example.com Connection: keep-alive
HTTP/1.1 200 OK Content-Type: text/html Connection: keep-alive Content-Length: 137
Though it’s not necessary to explicitly mention
keep-alive in HTTP/1.1 as it’s the default, this is helpful when communicating with older HTTP/1.0 servers that default to
Connection: close directive tells the server that the client (or a proxy) wishes to end the connection after the current request/response is complete. This behavior was default in HTTP/1.0 and is mostly useful for single, isolated requests where maintaining a connection isn’t beneficial.
GET /index.html HTTP/1.1 Host: www.example.com Connection: close
HTTP/1.1 200 OK Content-Type: text/html Connection: close Content-Length: 137
Connection header also has another key role: it can convey hop-by-hop headers, indicating that the mentioned headers should not be forwarded by proxies or other intermediaries, but are meant for the immediate connection.
For instance, consider the following request:
GET /index.html HTTP/1.1 Host: www.example.com Connection: keep-alive, Upgrade Upgrade: h2c
In the above example,
Upgrade is a hop-by-hop header. By including it in the
Connection header, we specify that it’s only relevant for the immediate, or “hop-by-hop,” connection.
Connection: keep-alive provides performance benefits, not all servers, clients, or proxies support persistent connections. Moreover, the HTTP/2 and HTTP/3 protocols handle connection management differently, making the
Connection header obsolete in these contexts. Always consider the entire communication path when determining whether to use
Connection HTTP header plays a key role in managing the lifecycle of network connections. With its two primary directives,
close, it allows optimal utilization of network resources. However, be mindful of the environment and protocol in use to ensure compatibility and expected behavior. The
Connection header’s secondary role in handling hop-by-hop headers provides granular control over the connection’s specifics, further expanding its functionality.