HTTP Chunked Transfer Encoding can be used when the HTTP body length is not know ahead of the transmission. The data to transmit will be split in chunks. Each chunk is preceded by the hexadecimal code of the chunk's size. The last chunk is empty to inform the receiver that the data has been fully transferred. Chunked Transfer Encoding can be used by HTTP client and server, although many servers don't support it in reception yet.
µC/HTTP-server can support the Chunked Transfer Encoding in transmission only.