What is the purpose of content-length in HTTP?

Content-Length is a field in the HTTP header used to specify the length of the entity body in a request or response, measured in bytes. Its purpose is as follows:

1. Transmission Control: The content-length field in HTTP requests can inform the server of the length of the message body, allowing the server to read the body correctly. In HTTP responses, the content-length field can inform the client of the length of the message body, enabling the client to read the body correctly.
2. Save bandwidth: The content-length field can help the server and client determine the length of the entity body, thereby avoiding unnecessary data transmission. Servers can control the amount of data sent based on the content-length field, and clients can correctly receive data based on the content-length field to avoid receiving more data than expected.
3. Cache control: the content-length field can assist cache servers in properly storing and serving cached entity bodies. Cache servers can use the content-length field to check for any new entity bodies available and control the cache expiry based on the content length.

In summary, the purpose of the content-length field in HTTP is to specify the length of the entity body in a request or response, in order to achieve functions such as transmission control, bandwidth saving, and cache control.