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The Gossip Service
The Gossip Service acts as a gateway to nodes in the control plane. Fullnodes use the service to ensure information is available to all other nodes in a cluster. The service broadcasts information using a gossip protocol.
Gossip Overview
Nodes continuously share signed data objects among themselves in order to manage a cluster. For example, they share their contact information, ledger height, and votes.
Every tenth of a second, each 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 cluster.
Gossip runs on a well-known UDP/IP port or a port in a well-known range. Once a cluster 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.
Gossip Service Interface
Push Message
A node sends a push message to tells the cluster 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:
-
Duplication: if the message has been seen before, the node responds with
PushMessagePruneand drops the message -
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 afterPUSH_MSG_TIMEOUT * 5ms)c. Retransmits the messages to its own push peers
-
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 cluster 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.