The spec for the friendly Stream Multiplexer (that works in 3 languages!)
| Lifecycle Stage | Maturity | Status | Latest Revision |
|---|---|---|---|
| 3A | Recommendation | Active | r0, 2018-10-10 |
Authors: @daviddias, @Stebalien, @tomaka
Interest Group: @yusefnapora, @richardschneider, @jacobheun
See the lifecycle document for context about the maturity level and spec status.
Mplex is a Stream Multiplexer protocol used by js-ipfs and go-ipfs in their implementations. The origins of this protocol are based in multiplex, the JavaScript-only Stream Multiplexer. After many battle field tests, we felt the need to improve and fix some of its bugs and mechanics, resulting on this new version used by libp2p.
This document will attempt to define a specification for the wire protocol and algorithm used in both implementations.
Mplex is a very simple protocol that does not provide many features offered by other stream multiplexers. Notably, mplex does not provide backpressure at the protocol level.
Implementations in:
Every communication in mplex consists of a header, and a length prefixed data segment.
The header is an unsigned base128 varint. The lower three bits are the message flags, and the rest of the bits (shifted down by three bits) are the stream ID this message pertains to:
header = readUvarint()
flag = header & 0x07
id = header >> 3
The maximum header length is 9 bytes (per the unsigned-varint spec). With 9 continuation bits and 3 message flag bits the maximum stream ID is 60 bits (maximum value of 2^60 - 1).
| NewStream | 0 |
| MessageReceiver | 1 |
| MessageInitiator | 2 |
| CloseReceiver | 3 |
| CloseInitiator | 4 |
| ResetReceiver | 5 |
| ResetInitiator | 6 |
The data segment is length prefixed by another unsigned varint. This results in one message looking like:
| header | length | data |
| uvarint | uvarint | 'length' bytes |
Mplex operates over a reliable ordered pipe between two peers, such as a TCP socket, or a unix pipe.
To open a new stream, first allocate a new stream ID. Then, send a message with the flag set to NewStream, the ID set to the newly allocated stream ID, and the data of the message set to the name of the stream.
Stream names are purely for debugging purposes and are not otherwise considered by the protocol. An empty string may also be used for the stream name, and they may also be repeated (using the same stream name for every stream is valid). Reusing a stream ID after closing a stream may result in undefined behaviour.
The party that opens a stream is called the stream initiator. Both parties can open a substream with the same ID, therefore this distinction is used to identify whether each message concerns the channel opened locally or remotely.
To write data to a stream, one must send a message with the flag MessageReceiver (1) or MessageInitiator (2) (depending on whether or not the writer is the one initiating the stream). The data field should contain the data you wish to write to the stream, up to 1MiB per message.
Mplex supports half-closed streams. Closing a stream closes it for writing and closes the remote end for reading but allows writing in the other direction.
To close a stream, send a message with a zero length body and a CloseReceiver (3) or CloseInitiator (4) flag (depending on whether or not the closer is the one initiaing the stream). Writing to a stream after it has been closed is a protocol violation. Reading from a remote-closed stream should return all data sent before closing the stream and then EOF thereafter.
To immediately close a stream for both reading and writing, use reset. This should generally only be used on error; during normal operation, both sides should close instead.
To reset a stream, send a message with a zero length body and a ResetReceiver (5) or ResetInitiator (6) flag. Reset must immediately close both ends of the stream for both reading and writing. Writing to a stream after it has been reset is a protocol violation. Since reset is generally sent when an error happens, all future reads from a reset stream should return an error (not EOF).
If a stream is being actively written to, the reader must take care to keep up with inbound data. Due to the lack of back pressure at the protocol level, the implementation must handle slow readers by doing one or both of:
- Blocking the entire connection until the offending stream is read.
- Resetting the offending stream.
For example, the go-mplex implementation blocks for a short period of time and then resets the stream if necessary.