## Bus Protocol (_BUS_)

The ((_BUS_ protocol))(((potocol, _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 ((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.
If a 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 xref:../sock/nng_bus_open.adoc[`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.footnote:[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 xref:../sock/raw.adoc[raw mode], received messages will contain the incoming xref:../pipe/index.adoc[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 xref:../sock/nng_device.adoc[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.
