# TCP Transport

The tcp transport is an implementation of Salt's transport using raw tcp sockets. Since this isn't using a pre-defined messaging library we will describe the wire protocol, message semantics, etc. in this document.

The tcp transport is enabled by changing the `transport` setting to `tcp` on each Salt minion and Salt master.

    transport: tcp

Warning

We currently recommend that when using Syndics that all Masters and Minions use the same transport. We're investigating a report of an error when using mixed transport types at very heavy loads.

## Wire Protocol

This implementation over TCP focuses on flexibility over absolute efficiency. This means we are okay to spend a couple of bytes of wire space for flexibility in the future. That being said, the wire framing is quite efficient and looks like:

    msgpack({'head': SOMEHEADER, 'body': SOMEBODY})

Since msgpack is an iterably parsed serialization, we can simply write the serialized payload to the wire. Within that payload we have two items "head" and "body". Head contains header information (such as "message id"). The Body contains the actual message that we are sending. With this flexible wire protocol we can implement any message semantics that we'd like-- including multiplexed message passing on a single socket.

## TLS Support

New in version 2016.11.1.

The TCP transport allows for the master/minion communication to be optionally wrapped in a TLS connection. Enabling this is simple, the master and minion need to be using the tcp connection, then the ssl option is enabled. The ssl option is passed as a dict and corresponds to the options passed to the Python ssl.wrap_socket <https://docs.python.org/3/library/ssl.html#ssl.wrap_socket> function.

A simple setup looks like this, on the Salt Master add the ssl option to the master configuration file:

    ssl:
      keyfile: <path_to_keyfile>
      certfile: <path_to_certfile>
      ssl_version: PROTOCOL_TLSv1_2

The minimal ssl option in the minion configuration file looks like this:

    ssl: True
    # Versions below 2016.11.4:
    ssl: {}

Specific options can be sent to the minion also, as defined in the Python ssl.wrap_socket function.

Note

While setting the ssl_version is not required, we recommend it. Some older versions of python do not support the latest TLS protocol and if this is the case for your version of python we strongly recommend upgrading your version of Python.

## Crypto

The current implementation uses the same crypto as the `zeromq` transport.

## Publish Server and Client

For the publish server and client we send messages without "message ids" which the remote end interprets as a one-way send.

Note

As of today we send all publishes to all minions and rely on minion-side filtering.

## Request Server and Client

For the request server and client we send messages with a "message id". This "message id" allows us to multiplex messages across the socket.