ContentLength

ContentLength is a term most commonly associated with the HTTP protocol, where it designates the size of a message body expressed in octets (bytes). The value is conveyed using the Content‑Length header field, which appears in both request and response messages. When a client sends data to a server—such as in a POST request—the Content‑Length header informs the server how many bytes to read from the connection. Likewise, a server includes the header in responses to indicate the length of the payload, allowing the client to determine when the message has been fully received without relying on connection termination.

The header’s syntax follows the pattern “Content‑Length: <decimal‑number>”. The number must be a non‑negative integer; a zero value indicates an empty body. According to RFC 7230, the header is required for any message that contains a body unless another transfer coding (for example, chunked encoding) is used. If both Content‑Length and a transfer‑coding are present, the transfer‑coding takes precedence, and recipients must ignore the Content‑Length.

Improper or missing Content‑Length values can lead to errors such as premature termination of a request, lingering connections, or security vulnerabilities like request smuggling. To mitigate these issues, HTTP/1.1 implementations typically enforce strict validation, rejecting messages with contradictory or malformed length specifications.

Beyond HTTP, the concept of content length appears in other protocols (e.g., SMTP, SIP) where it similarly informs the receiver about the size of subsequent data. In all contexts, accurate reporting of content length is essential for reliable data transmission and resource allocation.