Variable-length integer encoding

[treiher]

Context and Problem Statement

In some protocols, the length field is encoded using a variable-length encoding scheme. In MQTT, such an encoding is used for the Remaining Length field. There are multiple variants of variable-length encoding of integers.

Examples

Variable-Length Quantity (VLQ)

VLQ is used by:

• MIDI
• ASN.1 BER (e.g., SNMP) / DER (e.g., X.509)
  • Identifier octets
  • Length octets

Little Endian Base 128 (LEB128)

LEB128 is used by:

• WebAssembly
• DWARF

MQTT

Variable Byte Integer (equivalent to ULEB128)

QUIC

Variable-Length Integer Encoding

Bluetooth L2CAP

Protocol/Service Multiplexer - PSM (2 octets (minimum)) The PSM field is at least two octets in length. All PSM values shall have the least significant bit of the most significant octet equal to 0 and the least significant bit of all other octets equal to 1. Note: This means that all PSMs are odd numbers and that the end of a PSM can be easily found by searching for the first even octet.

(Bluetooth Core Specification v5.2 - Vol 3, Part A, 4.2)

[treiher]

It seems not to be possible to specify MQTT with ASN.1 in combination with ACN. ACN only supports the following encodings for integers: pos-int, twos-complement, BCD, ASCII.