solana/book/src/ncp.md

# The Network Control Plane

The Network Control Plane (NCP) is the gateway to the control plane. Any node
may join the control plane. It is when nodes need to ensure information is
available to *all* nodes in a Solana cluster. The control plane is implemented
using a gossip protocol.

## Gossip Overview

Nodes continuously share signed data objects among themselves in order to find
each other, structure the network data plane, prevent censoring, etc.

The basic mechanism is a 10Hz loop wherein every node sends a push message
and/or a pull message.  Push and pull messages may elicit responses, and push
messages may be forwarded on to others in the network.

Gossip runs on a well-known UDP/IP port or a port in a well-known range.  Once
a network is bootstrapped, nodes advertise to each other where to find their
gossip endpoint (a socket address).

## Gossip Records

Records shared over gossip are arbitrary, but signed and versioned (with a
timestamp) as needed to make sense to the node receiving them. If a node
recieves two records from the same source, it it updates its own copy with the
record with the most recent timestamp.

## NCP Interface

### Push Message

A node sends a push message to tells the network it has information to share.
Nodes send push messages to `PUSH_FANOUT` push peers.

Upon receiving a push message, a node examines the message for:

1. Duplication: if the message has been seen before, the node responds with
   `PushMessagePrune` and drops the message

2. New information: if the message is new the node

   a. Stores the new information and updates its version

   b. Stores the message in `pushed_once` (used for detecting duplicates,
purged after `PUSH_MSG_TIMEOUT * 5` ms)

   c. Retransmits the messages to its own push peers

3. Expiration: nodes drop push messages that are older than `PUSH_MSG_TIMEOUT`

### Push Peers, Prune Message

A nodes selects its push peers at random from the active set of known peers.
The node keeps this selection for a relatively long time.  When a prune message
is received, the node drops the push peer that sent the prune.  Prune is an
indication that there is another, faster path to that node than direct push.

The set of push peers is kept fresh by rotating a new node into the set every
`PUSH_MSG_TIMEOUT/2` milliseconds.

### Pull Message

A node sends a pull message to ask the network if there is any new information.
A pull message is sent to a single peer at random and comprises a Bloom filter
that represents things it already has.  A node receiving a pull message
iterates over its values and constructs a pull response of things that miss the
filter and would fit in a message.

A node constructs the pull Bloom filter by iterating over the values it
currently has.