Look up how HTTP/1.1 address this problem and summarize what you found. Answer:

When a HTTP server sends the contents of a document to a client in a response body, it uses the Content-Length header line to specify the byte length of the body. For static documents, the byte length is provided by the file system. But for dynamically generated web pages, such as those generated by a CGI script, the length must be determined at runtime, on the fly.

According to section 4.4 of RFC206:
In addition to the Content-Length specification required by HTTP/1.0, two alternatives are allowed:
? "chunked" transfer coding, which allows the length to be defined by "chunked" transfer coding, as follows:
“The chunked encoding modifies the body of a message in order to
transfer it as a series of chunks, each with its own size indicator,
followed by an optional footer containing entity-header fields. This
allows dynamically-produced content to be transferred along with the
information necessary for the recipient to verify that it has
received the full message.”
? If the message uses the media type "multipart/byteranges", which is self-delimiting, then that defines the length.