darkfi/doc/src/arch/p2p-network.md

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:

• Understanding Protocols

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 $N$ 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.

Security

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.
  • See AnonRep: Towards Tracking-Resistant Anonymous Reputation
  • Also the discussion in Semaphore RLN, rate limiting nullifier for spam prevention in anonymous p2p setting
• 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.

Swarming

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.

Proposed update

Monero p2p recap

In Monero, each node maintains a peer list consisting of two parts, a whitelist and a greylist. White/ greylists contain a last_seen data field, a timestamp of the last time the peer was interacted with.

The lists are ordered chronologically according to last_seen, with the most recently seen peers at the top of the list. The whitelist max size is 1000. The greylist max size is 5000. If the number of peers in these lists reach this maximum, then the peers with the oldest last_seen fields are removed from the list.

Each time a node receives info about a set of peers, the info is inserted into its greylist. To discover peers, nodes exchange SYN messages to their neighbors. Upon receiving a SYN message, peers reply with a message with the top 250 peers from their whitelist. The requester inserts the received peer data into its greylist.

Nodes update their whitelist and greylist through a mechanism called "greylist housekeeping", which periodically pings randomly selected peers from its greylist. If a peer is responsive, then it is promoted to the whitelist with an updated last_seen field, otherwise it is removed from the greylist.

To handle idle connections, nodes check connections through the IDLE_HANDSHAKE protocol. Nodes iterate through their connections, send a SYN message, and update the last seen fields of the connection if they receive a response. Otherwise, they drop the associated connection and select another peer from the whitelist. The disconnected peer stays on the whitelist.

Nodes broadcast messages to connections choosen from their whitelist. If not enough peers from the whitelist are currently online, then a node will establish connections from its greylist. Nodes to which previous connections were established are classified as anchor nodes, and stored in the white list. Monero ensures that every node is connected to at least two anchor nodes.

Source

Scope of the update

• Refactor src/hosts.rs to store two Vec<(Url, u64)> instead of single Hashset<Url>, replacing the host list and its associated methods with a whitelist and a greylist. [STATUS: COMPLETE/ TESTING]

• Create a GreylistRefinery protocol in OutboundSession (renamed from Monero's "greylist housekeeping" for succinctness) that periodically selects random peers from its greylist and pings them. If a peer is responsive, update the last_seen field and add it to the whitelist, otherwise remove it from the greylist. [STATUS: COMPLETE/ TESTING]

• lilith currently checks connections on the host list using a method called periodic_purge that gets hosts from the host list, copies them to a local ring buffer, and periodically handshakes the connections. If the handshake fails, lilith removes the host from the hostlist. This protocol has been replaced by a method called whitelist_cleansing. Like the prior method, whitelist_cleansing pulls connections from the whitelist, copies them to a ring buffer and handshakes them periodically. If they respond, the last_seen is updated, otherwise nothing happens. This is loosely based on Monero's IDLE_HANDSHAKE protocol. [STATUS: COMPLETE/ REEVALUATE/ TESTING]

• ProtocolAddress: On receiving an address, append it to the greylist. On receiving get_addr, fetch an address from the whitelist. [STATUS: COMPLETE/ TESTING].

• SeedSyncSession: on receiving whitelisted peers, append them to greylist. [STATUS: INCOMPLETE/ FIXME]

• ProtocolSeed: Send our address to the seed node, and on receiving addresses, append them to the whitelist. [STATUS: INCOMPLETE/ FIXME]

• Create a new list in hosts.rs called anchorlist. OutboundSession first tries to connect to address in the anchorlist on start(). [STATUS: TODO]

• Potentially create a new Protocol to send the top 250 nodes from the whitelist (Monero SYN exchange). [STATUS: TODO/ EVALUATE]