Rendezvous
What is Rendezvous?
A rendezvous protocol is a routing protocol that enables nodes and resources in a P2P network to discover each other. Rendezvous is used as a common location (point) to route between two routes.
Rendezvous points are typically nodes that are well-connected and stable in a network and can handle large amounts of traffic and data. They serve as a hub for nodes to discover.
Rendezvous is not decentralized
It is important to note that Rendezvous is not decentralized but rather federated. While this has its use cases, it also introduces a single point of failure into the network. This can be contrasted with fully decentralized solutions like DHT and Gossipsub. DHT (Distributed Hash Table) and Gossipsub are decentralized alternatives to Rendezvous.
DHT is a distributed network protocol used to store and retrieve data in a P2P network efficiently. It is like a hash table mapping keys to values, allowing for fast lookups and efficient data distribution across the network.
Gossipsub, on the other hand, is a pub-sub (publish-subscribe) protocol that is used to distribute messages and data across a network. It uses a gossip-based mechanism to propagate messages throughout the network, allowing fast and efficient distribution without relying on a central control point.
Rendezvous in libp2p
The libp2p rendezvous protocol can be used for different use cases. E.g. it can be used during bootstrap to discover circuit relays that provide connectivity for browser nodes. Generally, a peer can use known rendezvous points to find peers that provide network services. Rendezvous is also used throughout the lifetime of an application for peer discovery by registering and polling rendezvous points. In an application-specific setting, rendezvous points can be used to progressively discover peers that can answer specific queries or host shards of content.
The libp2p rendezvous protocol allows peers to connect to a rendezvous point and
register their presence by sending a REGISTER message in one or more
namespaces. Any node implementing the rendezvous protocol can act as a
rendezvous point, and any peer can connect to a rendezvous point. However, only
peers initiating a registration can register themselves at a rendezvous point.
By registering with a rendezvous point, peers allow for their discovery by other peers who query the rendezvous point. The query may:
- provide namespace(s), such as
test-app; - optionally provide a maximum number of peers to return;
- can include a cookie that is obtained from the response to a previous query,
thus the current query only contain registrations that weren’t part of the
previous response.
This simplifies discovery as it reduces the overhead of queried peers and allows for the pagination of query responses.
There is a default peer registration lifetime of 2 hours. Peers can optionally specify the
lifetime using a TTL parameter in the REGISTER message, with an upper bound of 72 hours.
The rendezvous protocol runs over libp2p streams using the protocol ID /rendezvous/1.0.0.
Rendezvous and publish-subscribe
For effective discovery, rendezvous can be combined with libp2p publish/subscribe. At a basic level, rendezvous can bootstrap pubsub by discovering peers subscribed to a topic. The rendezvous would be responsible for publishing packets, subscribing, or unsubscribing from packet shapes.
Pubsub can also be used as a mechanism for building rendezvous services, where a number of rendezvous points can federate using pubsub for internal distribution while still providing a simple interface to clients.