Chunked transfer encoding

Chunked transfer encoding (CTE) is a mechanism in which the encoder sends data to the player in a series of chunks. The player doesn’t have to wait until the complete segment is available. CTE is available in HTTP 1.1.

In CTE, each chunk is preceded by its size in bytes. The transmission ends when a zero-length chunk is received. The chunked keyword in the Transfer-Encoding header is used to indicate chunked transfer.

The Content-Length header is omitted in CTE and at the beginning of each chunk, you should add the current chunk length in hexadecimal format, followed by \r\n and the chunk, followed by another \r\n. The terminating chunk is a regular chunk with zero length. It is followed by the trailer, which consists of (may be empty) sequence of entity header fields.

The following example shows data encoded using CTE:

HTTP/1.1 200 OK 
Content-Type: text/plain
Transfer-Encoding: chunked
5\r\n
Media\r\n
8\r\n
Services\r\n
4\r\n
Live\r\n
0\r\n
\r\n

Decoded data

Media Services Live

A standard CMAF media segment is encoded with a single moof and mdat atom, as shown in the following figure. The mdat holds a single IDR (instantaneous decoder refresh) frame, which is required to begin every segment. This figure shows chunk encoding of a CMAF segment:

To achieve ULL, the encoder needs to provide the following:

  • HTTP 1.1 chunked transfer encoding is required in the header for each POST.
  • Your encoder should be compliant with RFC 2616.
  • You cannot define the content length in the header.
  • Chunks should be uploaded immediately once it is available, and not throttled at the beginning of the upload.
  • There should be a ‘moof’ box for every chunk.
  • To enable better monitoring, the encoder can optionally insert AKAM-ENC-CONTENT-CAP-TIME in the HTTP header, which is passed to the ingest server and included in Akamai-Path-Timestamp. The ingest server does not add the encoder timestamp, if the encoder does not set AKAM-ENC-CONTENT-CAP-TIME.

Recommended options for the encoder:

  • Allow control of the number of chunks per segment. This gives added flexibility to the end user.
  • Provide a toggle switch to let the user enable CTE. This clarifies whether your post is chunked transfer or not.