I'm having trouble finding a definite specification of this in the standard. I have an HTTP client that's not including a 'Content-Length: 0' header when doing a PUT request where I don't specify a body, and a server that gets confused by such requests, and I'm wondering which program I should be blaming.
The content length field is required as per the following section in the HTTP/1.1 standard http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.13
What is being PUT (in the verb sense) onto the server if there's no content? The spec refers to the content as "the enclosed entity", but a request with no content would have no enclosed entity, and therefore nothing to put on the server.
Unless, of course, you wanted to PUT nothing onto the server, in which case you'd probably want a DELETE instead.
HTTP requests have a body if they have a Content-Length or Transfer-Encoding header (RFC 2616 4.3). If the request has neither, it has no body, and your server should treat it as such.
That said it is unusual for a PUT request to have no body, and so if I were designing a client that really wanted to send an empty body, I'd pass Content-Length: 0. Indeed, depending on one's reading of the POST and PUT method definitions (RFC 2616 9.5, 9.6) one might argue that the body is implied to be required - but a reasonable way to handle no body would be to assume a zero-length body.