ext (arbitrary data of an application-defined format, up to 232 − 1 bytes)
timestamp (ext type = −1) (up to 64-bit seconds and 32-bit nanoseconds)
Comparison to other formats
MessagePack is more compact than JSON, but imposes limitations on array and integer sizes. On the other hand, it allows binary data and non-UTF-8 encoded strings. In JSON, map keys have to be strings, but in MessagePack there is no such limitation and any type can be a map key, including types like maps and arrays, and, like YAML, numbers.
Compared to BSON, MessagePack is more space-efficient. BSON is designed for fast in-memory manipulation, whereas MessagePack is designed for efficient transmission over the wire. For example, BSON requires null terminators at the end of all strings and inserts string indexes for list elements, while MessagePack doesn't. BSON represents both arrays and maps internally as documents, which are maps, where an array is a map with keys as decimal strings counting up from 0. MessagePack on the other hand represents both maps and arrays as arrays, where each map key-value pair is contiguous, making odd items keys and even items values.
The Protocol Buffers format provides a significantly more compact transmission format than MessagePack because it doesn't transmit field names. However, while JSON and MessagePack aim to serialize arbitrary data structures with type tags, Protocol Buffers requires a schema to define the data types. Protocol Buffers compiler creates boilerplate code in the target language to facilitate integration of serialization into the application code; MessagePack returns only a dynamically typed data structure and provides no automatic structure checks.
