Gossip data dissemination protocol

Hyperledger Fabric optimizes blockchain network performance, security and scalability by dividing workload across transaction execution (endorsing and committing) peers and transaction ordering nodes. This decoupling of network operations requires a secure, reliable and scalable data dissemination protocol to ensure data integrity and consistency. To meet these requirements, Hyperledger Fabric implements a gossip data dissemination protocol.

Gossip protocol

Peers leverage gossip to broadcast ledger and channel data in a scalable fashion. Gossip messaging is continuous, and each peer on a channel is constantly receiving current and consistent ledger data, from multiple peers. Each gossiped message is signed, thereby allowing Byzantine participants sending faked messages to be easily identified and the distribution of the message(s) to unwanted targets to be prevented. Peers affected by delays, network partitions or other causations resulting in missed blocks, will eventually be synced up to the current ledger state by contacting peers in possession of these missing blocks.

The gossip-based data dissemination protocol performs three primary functions on a Hyperledger Fabric network:

  1. Manages peer discovery and channel membership, by continually identifying available member peers, and eventually detecting peers that have gone offline.
  2. Disseminates ledger data across all peers on a channel. Any peer with data that is out of sync with the rest of the channel identifies the missing blocks and syncs itself by copying the correct data.
  3. Bring newly connected peers up to speed by allowing peer-to-peer state transfer update of ledger data.

Gossip-based broadcasting operates by peers receiving messages from other peers on the channel, and then forwarding these messages to a number of randomly-selected peers on the channel, where this number is a configurable constant. Peers can also exercise a pull mechanism, rather than waiting for delivery of a message. This cycle repeats, with the result of channel membership, ledger and state information continually being kept current and in sync. For dissemination of new blocks, the leader peer on the channel pulls the data from the ordering service and initiates gossip dissemination to peers.

Leader election

The leader election mechanism is used to elect one peer per each organization which will maintain connection with ordering service and initiate distribution of newly arrived blocks across peers of its own organization. Leveraging leader election provides system with ability to efficiently utilize bandwidth of the ordering service. There are two possible operation modes for leader election module:

  1. Static - system administrator manually configures one peer in the organization to be the leader, e.g. one to maintain open connection with the ordering service.
  2. Dynamic - peers execute a leader election procedure to select one peer in an organization to become leader, pull blocks from the ordering service, and disseminate blocks to the other peers in the organization..

Static leader election

Using static leader election allows to manually define a set of leader peers within the organization, it’s possible to define a single node to be a leader or all available peers, it should be taken into account that - making too many peers to connect to the ordering service might lead to inefficient bandwidth utilization. To enable static leader election mode, configure the following parameters within the section of core.yaml:

    # Gossip related configuration
        useLeaderElection: false
        orgLeader: true

Alternatively these parameters could be configured and overrided with environmental variables:

  1. Following configuration will keep peer in stand-by mode, i.e. peer will not try to become a leader:
  1. Setting CORE_PEER_GOSSIP_USELEADERELECTION and CORE_PEER_GOSSIP_USELEADERELECTION with true value is ambiguous and will lead to an error.
  2. In static configuration organization admin is responsible to provide high availability of the leader node in case for failure or crashes.

Dynamic leader election

Dynamic leader election enables organization peers to elect one peer which will connect to the ordering service and pull out new blocks. Leader is elected for set of peers for each organization independently.

Elected leader is responsible to send the heartbeat messages to the rest of the peers as an evidence of liveness. If one or more peers won’t get heartbeats updates during period of time, they will initiate a new round of leader election procedure, eventually selecting a new leader. In case of a network partition in the worst case there will be more than one active leader for organization thus to guarantee resiliency and availability allowing the organization’s peers to continue making progress. After the network partition is healed one of the leaders will relinquish its leadership, therefore in steady state and in no presence of network partitions for each organization there will be only one active leader connecting to the ordering service.

Following configuration controls frequency of the leader heartbeat messages:

    # Gossip related configuration
            leaderAliveThreshold: 10s

In order to enable dynamic leader election, the following parameters need to be configured within core.yaml:

    # Gossip related configuration
        useLeaderElection: true
        orgLeader: false

Alternatively these parameters could be configured and overrided with environmental variables:


Gossip messaging

Online peers indicate their availability by continually broadcasting “alive” messages, with each containing the public key infrastructure (PKI) ID and the signature of the sender over the message. Peers maintain channel membership by collecting these alive messages; if no peer receives an alive message from a specific peer, this “dead” peer is eventually purged from channel membership. Because “alive” messages are cryptographically signed, malicious peers can never impersonate other peers, as they lack a signing key authorized by a root certificate authority (CA).

In addition to the automatic forwarding of received messages, a state reconciliation process synchronizes world state across peers on each channel. Each peer continually pulls blocks from other peers on the channel, in order to repair its own state if discrepancies are identified. Because fixed connectivity is not required to maintain gossip-based data dissemination, the process reliably provides data consistency and integrity to the shared ledger, including tolerance for node crashes.

Because channels are segregated, peers on one channel cannot message or share information on any other channel. Though any peer can belong to multiple channels, partitioned messaging prevents blocks from being disseminated to peers that are not in the channel by applying message routing policies based on peers’ channel subscriptions.

1. Security of point-to-point messages are handled by the peer TLS layer, and do not require signatures. Peers are authenticated by their certificates, which are assigned by a CA. Although TLS certs are also used, it is the peer certificates that are authenticated in the gossip layer. Ledger blocks are signed by the ordering service, and then delivered to the leader peers on a channel. 2. Authentication is governed by the membership service provider for the peer. When the peer connects to the channel for the first time, the TLS session binds with the membership identity. This essentially authenticates each peer to the connecting peer, with respect to membership in the network and channel.