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SaltyRTC Chunking

This specification describes the binary data chunking algorithm used by the SaltyRTC WebRTC task. It allows the user to split up large binary messages into multiple chunks of a certain size.

This specification has been created originally to work around the current size limitation of WebRTC data channel messages, but it can also be used in other, generic contexts.


The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.


This specification defines the following two modes:

  • Reliable/Ordered: Intended for reliable and ordered transmission of chunks. The application that is reassembling chunks into a message MUST ensure that chunks of a message are not reordered. Furthermore, chunks of different messages SHALL NOT be interleaved.
  • Unreliable/Unordered: Intended for transmission of chunks where chunks MAY be lost or reordered. Additionally, an implementation MAY optionally be able to handle duplicated chunks.

Chunk Size

The term chunk size is referring to the resulting size from the concatenation of chunk header and chunk data in bytes.

Chunk Header

When converting data to chunks, a header MUST be prepended to each chunk of the following byte size:

  • 1 byte short header for reliable/ordered mode, and
  • 9 byte long header for unreliable/unordered mode.

Both header variants are encoded in binary using network-oriented format (most significant byte first, also known as big-endian).

Short Header

The short header contains a single byte called the options bit field:

Options bit field

The options bit field field is used to encode additional information about a chunk.

MSB           LSB
|R R R R R M M E|

- R: Reserved, MUST be 0.

- M: Mode with the following values:

     11 - reliable/ordered
     10 - reserved
     01 - reserved
     00 - unreliable/unordered

- E: End-of-message, this MUST be set to 1 if this is the
     last chunk of the message. Otherwise, it MUST be set
     to 0.

Long Header

The long header contains a total of 9 bytes and is structured in the following way:


- O: Options bit field (1 byte)
- I: Message id (4 bytes)
- S: Serial number (4 bytes)

Options bit field

Identical to the options bit field of the Short Header format.

Message id

The message id SHALL be any 32 bit unsigned integer. It is RECOMMENDED to start with 0 and to increment the counter for each message. The message id MAY wrap back to 0 on integer overflow.

Serial number

The serial number SHALL be a 32 bit unsigned integer. It MUST start with 0 and MUST be incremented by 1 for every chunk.

Chunk data

The chunk data MUST be appended to the chunk header. It MUST contain at least 1 byte of data.

Every chunk MUST contain up to chunk size - header size bytes of data unless it is the last chunk of a message in which case it MAY contain less than chunk size - header size bytes of data.

The chunk data MUST be chunked in a non-overlapping, sequential way.


Reliable/Ordered Mode

When chunking the byte sequence 12345678 (where each digit is an 8 bit unsigned integer) with a chunk size of 6, the data is being chunked into the following two chunks:

- First chunk:  `0b00000110 || 0x0102030405`
- Second chunk: `0b00000111 || 0x060708`

Unreliable/Unordered Mode

When chunking the byte sequence 12345678 (where each digit is an 8 bit unsigned integer) with a chunk size of 12 and the message id 42, the data is being chunked into the following three chunks:

- First chunk:  `0b00000000 || 0x0000002a || 0x00000000 || 0x010203`
- Second chunk: `0b00000000 || 0x0000002a || 0x00000001 || 0x040506`
- Third chunk:  `0b00000001 || 0x0000002a || 0x00000002 || 0x0708`


In unordered/unreliable mode, implementations MUST support unchunking of chunks that arrive in arbitrary order. This is usually done by keeping track of messages and the corresponding chunks. In order to prevent memory leaks when chunks are lost in transmission, implementations SHOULD provide a way to clean up incomplete messages.

Implementations of the unreliable/unordered mode MAY optionally be able to handle duplicated chunks.



Version 1.1

Important: Backwards compatibility to 1.0 can only be ensured by using the unreliable/unordered mode.

  • Add reliable/ordered mode with 1 byte header


Version 1.0

  • Define unreliable/unordered mode with 9 byte header