Content-Length

The anticipated size of the request or response body, as a decimal number of octets, sent to the recipient.

Fastly reads this header from requests and responses and writes it into requests and responses. It is defined by an external standard.

When a message does not have a Transfer-Encoding header field, a Content-Length header field can provide the anticipated size, as a decimal number of octets, for a potential payload body. For messages that do include a payload body, the Content-Length field-value provides the framing information necessary for determining where the body (and message) ends. For messages that do not include a payload body, the Content-Length indicates the size of the selected representation (Section 3 of RFC7231).