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+# BUS Protocol
+
+The {{i:*BUS* protocol}}{{hi:*BUS*}} provides for building mesh networks where
+every peer is connected to every other peer.
+In this protocol, each message sent by a node is sent to every one of
+its directly connected peers.
+
+> [!TIP]
+> Messages are only sent to directly connected peers.
+> This means that in the event that a peer is connected indirectly, it will not
+> receive messages.
+> When using this protocol to build mesh networks, it
+> is therefore important that a _fully-connected_ mesh network be constructed.
+
+All message delivery in this pattern is {{i:best-effort}}, which means that
+peers may not receive messages.
+Furthermore, delivery may occur to some,
+all, or none of the directly connected peers.
+(Messages are not delivered when peer nodes are unable to receive.)
+Hence, send operations will never block; instead if the
+message cannot be delivered for any reason it is discarded.
+
+> [!TIP]
+> In order to minimize the likelihood of message loss, this protocol
+> should not be used for high throughput communications.
+> Furthermore, the more traffic _in aggregate_ that occurs across the topology,
+> the more likely that message loss is to occur.
+
+## Socket Operations
+
+The [`nng_bus0_open`][nng_bus_open] functions create a bus socket.
+This socket may be used to send and receive messages.
+Sending messages will attempt to deliver to each directly connected peer.
+
+## Protocol Versions
+
+Only version 0 of this protocol is supported.
+(At the time of writing, no other versions of this protocol have been defined.)
+
+## Protocol Options
+
+The _BUS_ protocol has no protocol-specific options.
+
+## Protocol Headers
+
+When using a _BUS_ socket in [raw mode][raw], received messages will
+contain the incoming [pipe][pipe] ID as the sole element in the header.
+If a message containing such a header is sent using a raw _BUS_ socket, then,
+the message will be delivered to all connected pipes _except_ the one
+identified in the header.
+This behavior is intended for use with [device][device]
+configurations consisting of just a single socket.
+Such configurations are useful in the creation of rebroadcasters, and this
+capability prevents a message from being routed back to its source.
+If no header is present, then a message is sent to all connected pipes.
+
+When using normal (cooked mode) _BUS_ sockets, no message headers are present.
+
+[nng_bus_open]: TODO.md
+[device]: TODO.md
+[pipe]: TODO.md
+[raw]: TODO.md