P2P Network

We instantiate a p2p network and call start(). This will begin running a single p2p network until stop() is called.

There are 3 session types:

  • InboundSession, concerned with incoming connections
  • OutboundSession, concerned with outgoing connections
  • SeedSession is a special session type which connects to seed nodes to populate the hosts pool, then finishes once synced.

Connections are made by either Acceptor or Connector for incoming or outgoing respectively. They have multiple transport types; see src/net/transport/ for the full list.

Connections are then wrapped in a Channel abstraction which allows protocols to be attached. See src/net/protocol/ and run fd protocol for custom application specific network protocols. Also see the follow tutorial:

Outbound Session

The outbound session is responsible to ensure the hosts pool is populated, either through currently connected nodes or using the seed session. It performs this algorithm:

  1. Start slots, and a sleeping peer discovery process

Then each slot performs this algorithm:

  1. If no addresses matching our filters are in the hosts pool then:
    1. Wakeup the peer discovery process. This does nothing if peer discovery is already active.
    2. Peer discovery tries first 2 times to poll the current network if there are connected nodes, otherwise it will do a seed server sync.
    3. Peer discovery then wakes any sleeping slots and goes back to sleep.


Design Considerations

  • Mitigate attacks to a reasonable degree. Ensuring complete coverage against attacks is infeasible and likely introduces significant latency into protocols.
  • Primarily target the p2p network running over anonymity networks like Tor, i2p or Nym. This means we cannot rely on node addresses being reliable. Even on the clearnet, attackers can easily obtain large numbers of proxy addresses.

The main attacks are:

  • Sybil attack. A malicious actor tries to subvert the network using sockpuppet nodes. For example false signalling using version messages on the p2p network.
  • Eclipse attack. Targets a single node, through a p2p MitM attack where the malicious actor controls all the traffic you see. For example they might send you a payment, then want to doublespend without you knowing about it.
  • Denial of Service. Usually happens when a node is overloaded by too much data being sent.

Common Mitigations

  • Backoff/falloff. This is the strategy implemented in Bitcoin. This can be bad when arbitrary limits are implemented since we slow down traffic for no reason.
  • Choking controller. BitTorrent no longer uses naive tit-for-tat, instead libtorrent implements an anti-leech seeding algo from the paper Improving BitTorrent: A Simple Approach, which is focused on distributing bandwidth to all peers. See also libtorrent/src/choker.cpp.
    • All p2p messages will have a score which represents workload for the node. There is a hard limit, and in general the choker will try to balance the scores between all available channels.
    • Opening the connection itself has a score with inbound connections assigned more cost than outgoing ones.
  • Smart ban. Malicious peers which violate protocols are hard banned. For example sending the wrong data for a chunk.
    • Add a method channel.ban() which immediately disconnects and blacklists the address.
  • uTP congestion control. BitTorrent implements a UDP protocol with its own congestion control. We could do such a similar strategy with the addition of removing ordering. This reduces protocol latency mitigating attacks. See libtorrent.org/utp.html for more info.
    • Maybe less important if we use alternative networks like Tor or i2p.
  • White, gray and black lists. See section 2.2 of Exploring the Monero P2P Network for details of this algorithm. This aids with network connectivity, avoiding netsplits which could make the network more susceptible to eclipse/sybil attacks (large scale MiTM).
    • For this we would need a function to connect to a host, send a ping, receive a pong and disconnect to test node connectivity.
  • Protocol-level reputation system. You have a keypair, which accrues more trust from the network. Nodes gossip trust metrics.
  • Reduce blast radius. The p2p subsystem should run on its own dedicated executor, separate from database lookups or other system operations.
  • fail2ban
  • Optimized blockchain database. Most databases are written for interleaved reads and writes as well as deletion. Blockchains follow a different pattern of infrequent writes being mostly append-only, and requiring weaker guarantees.

Protocol Suggestions

Core protocols should be modeled and analyzed with DoS protections added. Below are suggestions to start the investigation.

  • Do not forward orphan txs or blocks.
  • Drop all double spend txs.
    • Alternatively require a higher fee if we want to enable replace by fee.
    • Do not forward double spend txs.
  • Do not forward the same object (block, transaction .etc) to the same peer twice. Violation results in channel.ban().
  • Very low fee txs are rate limited.
  • Limit orphan txs.
  • Drop large or unusual orphan transactions to limit damage.
  • Consider a verification cache to prevent attacks that try to trigger re-verification of stored orphan txs. Also limit the size of the cache. See Fixed vulnerability explanation: Why the signature cache is a DoS protection.
  • Perform more expensive checks later in tx validation.
  • Nodes will only relay valid transactions with a certain fee amount. More expensive transactions require a higher fee.
  • Complex operations such as requesting data can be mitigated by tracking the number of requests from a peer.
    • Attackers may attempt flooding invs for invalid data. The peer responds with get data but that object doesn't exist using up precious bandwidth. Limit both the rate of invs to 2/s and size of items to 35.
    • Also loops can cause an issue if triggered by the network. This should be carefully analyzed and flattened if possible, otherwise they should be guarded against attack.

Customizable Policy

Apps should be able to configure:

  • Reject hosts, for example based off current overall resource utilization or the host addr.
  • Accounting abstraction for scoring connections.


TODO: research how this is handled on bittorrent. How do we lookup nodes in the swarm? Does the network maintain routing tables? Is this done through a DHT like Kademlia?

Swarming means more efficient downloading of data specific to a certain subset. A new p2p instance is spawned with a clean hosts table. This subnetwork is self contained.

An application is for example DarkIRC where everyday a new event graph is spawned. With swarming, you would connect to nodes maintaining this particular day's event graph.

The feature allows overlaying multiple different features in a single network such as tau, darkirc and so on. New networks require nodes to bootstrap, but with swarming, we reduce all these networks to a single bootstrap. The overlay network maintaining the routing tables is a kind of decentralized lilith which keeps track of all the swarms.

Possibly a post-mainnet feature depending on the scale of architectural changes or new code required in the net submodule.