The peer-to-peer layer

This document explains the inner workings of the peer-to-peer layer of the Tezos shell. This part is in charge of establishing and maintaining network connections with other nodes (gossip).

The P2P layer is instantiated by the node. It is parametrized by the type of messages that are exchanged over the network (to allow different P2P protocol versions/extensions), and the type of metadata associated to each peer. The latter is useful to compute a score for each peer that reflects the level of trust we have in it. Different policies can be used when communicating with peers with different score values.

The P2P layer is comprised of a pool of connections, a set of operations on those connections, and a set of workers following the worker pattern pervasively used in the code base.

The P2P layer is packaged in tezos-p2p, which has documentation for all modules.

General operation

I/O Scheduling

The P2P layer uses a scheduling mechanism in order to control its bandwidth usage as well as implementing different policies (e.g. read/write quotas) to different peers. For now, each peer is granted a fair share of the global allocated bandwidth, but it is planned for the individual allocated bandwidth to each peer to be a function of the peer’s score.

Encryption

The connection between each peer is encrypted using NaCl authenticated-encryption API. This is done to provide an additional level of security and tamper-proof guarantees in the communication between peers.

Message queues

On top of basic I/O scheduling, two finite-size typed message queues are used to store incoming (resp. outgoing) messages for each peer. This further restricts the speed at which communication is possible with a peer; when a queue is full, it is not possible to read (resp. write) an additional message. The high-level P2p_socket.connection type by the P2P layer is basically a UNIX socket upgraded with I/O scheduling, peer metadata, cryptographic keys and two messages queues operated by dedicated workers which operate on those queues.

Pool of connections

All the above modules are used in P2p_pool, which constitutes the core of the P2P layer, together with the worker processes described below. It comprises various tables of connections as well as methods to query them, also connections are extended with another message queue where lower level messages (like responses to ping) are filtered out and only application-level messages are kept.

The main entry point of the P2P layer is in module P2p. See below for a description of workers acting onto the P2P layer.

Welcome worker

The welcome worker is responsible for accepting incoming connections and register them into the pool of connections managed by the P2P layer. It basically runs the accept(2) syscall and call P2p_pool.accept so that it is made aware of an incoming connection. From there, the pool will decide how this new connection must be handled.

{Black, While, Grey}lists

The welcome worker takes care of filtering all incoming connections using two static lists of addresses handled either by tezos-admin-client and a system table that is handled automatically by the p2p layer. The node admin can block or whitelist individual ip addresses, while the p2p layer is in charge of temporarily banning ip addresses and peers who misbehave. The delay to remove an ip address from the greylist table is defined by the configuration variable greylist_timeout, while peers that are greylisted are periodically removed. The node admin can also flush greylist tables with the tezos-admin-client.

Maintenance worker

The maintenance worker is in charge of establishing an appropriate number of connections with other nodes in order to guarantee a realistic view of the state of the blockchain. It is created with a set of targets to reach regarding the desired amount of peers it needs to keep an active connection to.

At the pool level, the minimum (resp. maximum) acceptable number of connections is defined.

At the maintenance worker level, two other sets of thresholds are defined: target (min and max) and threshold (min and max).

Given these bounds, the maintenance worker:

  • Will be triggered every two minutes, when asked by the shell, or when the minimum or maximum number of acceptable connections is reached, whichever happens first.
  • Will perform the following actions when triggered: if the number of connections is above max_threshold, it will kill connections randomly until it reaches max_target connections. If the number of connections is below min_threshold, it will attempt to connect to peers until it reaches at least min_target connections (and never more than max_target connections).

The maintenance worker is also in charge of periodically run the greylists GC functions to unban ip addresses from the greylist.