The message-body (if any) of an HTTP message is used to carry the entity-body associated with the request or response. The message-body differs from the entity-body only when a transfer coding has been applied, as indicated by the Transfer-Encoding header field (section 14.40).
message-body = entity-body | <entity-body encoded as per Transfer-Encoding>
Transfer-Encoding MUST be used to indicate any transfer codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding is a property of the message, not of the entity, and thus can be added or removed by any application along the request/response chain.
The rules for when a message-body is allowed in a message differ for requests and responses.
The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers. A message-body MAY be included in a request only when the request method (section 5.1.1) allows an entity-body.
For response messages, whether or not a message-body is included with a message is dependent on both the request method and the response status code (section 6.1.1). All responses to the HEAD request method MUST NOT include a message-body, even though the presence of entity- header fields might lead one to believe they do. All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a message-body. All other responses do include a message-body, although it may be of zero length.