Add data protocol.

Author: BenediktBurger

data_protocol.md

@@ -0,0 +1,58 @@
+# Data protocol
+
+The data protocol transmits messages from `publishers` to `subscribers` via a `proxy server` in a broadcasting fashion.
+
+## Transport layer
+
+The transport layer ensures that a message arrives at its destination.
+
+### Socket configuration
+
+A `proxy server` is the transmitting station, it shall offer an XSUBSCRIBER and an XPUBLISHER socket.
+The two sockets shall be connected, e.g. via the `zmq.proxy_server` method.
+Each of both sockets shall be bound to its own address.
+
+A `Publisher` is a Component, which sends data messages via the data protocol.
+It shall have a PUBLISHER socket connecting to the proxy server's XSUBSCRIBER socket.
+
+A `Subscriber` is a Component, which wants to reveice data messages via the data protocol.
+It shall have a SUBSCRIBER socket connecting to the proxy server's XPUBLISHER socket.
+It should subscribe to the {ref}`Topics <data_protocol.md#topic>` it wants to receive.
+
+:::{note}
+Subscribing to a topic in zmq means to subscribe to all topics which **start** with the given topic name!
+:::
+
+#### Recommended configuration
+
+It is recommended to have a single program with two proxy servers.
+The first one transfers any data and binds its XSUBSCRIBER to port number 11100 and binds its XPUBLISHER to 11099.
+The second one transfers log entries and binds to 11098 and 11097, respectively.
+
+
+## Message format
+
+A Data Protocol Message consists in three or more frames ([#62](https://github.com/pymeasure/leco-protocol/issues/62)):
+1. {ref}`data_protocol.md#topic`
+2. {ref}`data_protocol.md#header`
+3. One or more data frames, the first one is the {ref}`data_protocol.md#content`
+
+### Topic
+
+The topic is the full name of the sending Component. ([#60](https://github.com/pymeasure/leco-protocol/issues/60))
+
+### Header
+
+Similar to the {ref}`control protocol header <control_protocol.md#message-composition>`, the header consists in
+1. UUIDv7
+2. a one byte `message_type` (`0` not defined, `1` JSON, `>127` user defined)
+
+### Content
+
+#### Log message content
+
+For log messages, the content is a JSON encoded list of:
+- `record.asctime`: Timestamp formatted as `'%Y-%m-%d %H:%M:%S'`
+- `record.levelname`: Logger level name
+- `record.name`: Logger name
+- `record.text` (including traceback)

index.rst

@@ -16,6 +16,7 @@
    messages
    network-structure
    control_protocol
+   data_protocol
    glossary
    Hello_world
    appendix