openethereum/util/network-devp2p/src/discovery.rs

// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of Parity Ethereum.

// Parity Ethereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Parity Ethereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Parity Ethereum.  If not, see <http://www.gnu.org/licenses/>.

use std::collections::{HashMap, HashSet, VecDeque};
use std::collections::hash_map::Entry;
use std::default::Default;
use std::net::SocketAddr;
use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};

use ethereum_types::{H256, H520};
use keccak_hash::keccak;
use log::{debug, trace, warn};
use lru_cache::LruCache;
use parity_bytes::Bytes;
use rlp::{Rlp, RlpStream};

use parity_crypto::publickey::{KeyPair, recover, Secret, sign};
use network::Error;
use network::IpFilter;

use crate::node_table::*;
use crate::PROTOCOL_VERSION;

const ADDRESS_BYTES_SIZE: usize = 32;						// Size of address type in bytes.
const ADDRESS_BITS: usize = 8 * ADDRESS_BYTES_SIZE;			// Denoted by n in [Kademlia].
const DISCOVERY_MAX_STEPS: u16 = 8;							// Max iterations of discovery. (discover)
const BUCKET_SIZE: usize = 16;		// Denoted by k in [Kademlia]. Number of nodes stored in each bucket.
const ALPHA: usize = 3;				// Denoted by \alpha in [Kademlia]. Number of concurrent FindNode requests.
pub const MAX_DATAGRAM_SIZE: usize = 1280;

const PACKET_PING: u8 = 1;
const PACKET_PONG: u8 = 2;
const PACKET_FIND_NODE: u8 = 3;
const PACKET_NEIGHBOURS: u8 = 4;

const PING_TIMEOUT: Duration = Duration::from_millis(500);
const FIND_NODE_TIMEOUT: Duration = Duration::from_secs(2);
const EXPIRY_TIME: Duration = Duration::from_secs(20);
const MAX_NODES_PING: usize = 32; // Max nodes to add/ping at once
const REQUEST_BACKOFF: [Duration; 4] = [
	Duration::from_secs(1),
	Duration::from_secs(4),
	Duration::from_secs(16),
	Duration::from_secs(64)
];

const NODE_LAST_SEEN_TIMEOUT: Duration = Duration::from_secs(24*60*60);

const OBSERVED_NODES_MAX_SIZE: usize = 10_000;

#[derive(Clone, Debug)]
pub struct NodeEntry {
	pub id: NodeId,
	pub endpoint: NodeEndpoint,
}

#[derive(Debug)]
pub struct BucketEntry {
	pub address: NodeEntry,
	pub id_hash: H256,
	pub last_seen: Instant,
	backoff_until: Instant,
	fail_count: usize,
}

impl BucketEntry {
	fn new(address: NodeEntry) -> Self {
		let now = Instant::now();
		BucketEntry {
			id_hash: keccak(address.id),
			address,
			last_seen: now,
			backoff_until: now,
			fail_count: 0,
		}
	}
}

struct FindNodeRequest {
	// Time when the request was sent
	sent_at: Instant,
	// Number of items sent by the node
	response_count: usize,
	// Whether the request have been answered yet
	answered: bool,
}

#[derive(Clone, Copy)]
enum PingReason {
	Default,
	FromDiscoveryRequest(NodeId, NodeValidity),
}

#[derive(Clone, Copy, PartialEq)]
enum NodeCategory {
	Bucket,
	Observed
}

#[derive(Clone, Copy, PartialEq)]
enum NodeValidity {
	Ourselves,
	ValidNode(NodeCategory),
	ExpiredNode(NodeCategory),
	UnknownNode
}

#[derive(Debug)]
enum BucketError {
	Ourselves,
	NotInTheBucket{node_entry: NodeEntry, bucket_distance: usize},
}

struct PingRequest {
	// Time when the request was sent
	sent_at: Instant,
	// The node to which the request was sent
	node: NodeEntry,
	// The hash sent in the Ping request
	echo_hash: H256,
	// The hash Parity used to respond with (until rev 01f825b0e1f1c4c420197b51fc801cbe89284b29)
	#[deprecated()]
	deprecated_echo_hash: H256,
	reason: PingReason
}

#[derive(Debug)]
pub struct NodeBucket {
	nodes: VecDeque<BucketEntry>, //sorted by last active
}

impl Default for NodeBucket {
	fn default() -> Self {
		NodeBucket::new()
	}
}

impl NodeBucket {
	fn new() -> Self {
		NodeBucket {
			nodes: VecDeque::new()
		}
	}
}

pub struct Datagram {
	pub payload: Bytes,
	pub address: SocketAddr,
}

pub struct Discovery<'a> {
	id: NodeId,
	id_hash: H256,
	secret: Secret,
	public_endpoint: NodeEndpoint,
	discovery_initiated: bool,
	discovery_round: Option<u16>,
	discovery_id: NodeId,
	discovery_nodes: HashSet<NodeId>,
	node_buckets: Vec<NodeBucket>,
	// Sometimes we don't want to add nodes to the NodeTable, but still want to
	// keep track of them to avoid excessive pinging (happens when an unknown node sends
	// a discovery request to us -- the node might be on a different net).
	other_observed_nodes: LruCache<NodeId, (NodeEndpoint, Instant)>,

	in_flight_pings: HashMap<NodeId, PingRequest>,
	in_flight_find_nodes: HashMap<NodeId, FindNodeRequest>,
	send_queue: VecDeque<Datagram>,
	check_timestamps: bool,
	adding_nodes: Vec<NodeEntry>,
	ip_filter: IpFilter,
	request_backoff: &'a [Duration],
}

pub struct TableUpdates {
	pub added: HashMap<NodeId, NodeEntry>,
	pub removed: HashSet<NodeId>,
}

impl<'a> Discovery<'a> {
	pub fn new(key: &KeyPair, public: NodeEndpoint, ip_filter: IpFilter) -> Discovery<'static> {
		Discovery {
			id: *key.public(),
			id_hash: keccak(key.public()),
			secret: key.secret().clone(),
			public_endpoint: public,
			discovery_initiated: false,
			discovery_round: None,
			discovery_id: NodeId::default(),
			discovery_nodes: HashSet::new(),
			node_buckets: (0..ADDRESS_BITS).map(|_| NodeBucket::new()).collect(),
			other_observed_nodes: LruCache::new(OBSERVED_NODES_MAX_SIZE),
			in_flight_pings: HashMap::new(),
			in_flight_find_nodes: HashMap::new(),
			send_queue: VecDeque::new(),
			check_timestamps: true,
			adding_nodes: Vec::new(),
			ip_filter,
			request_backoff: &REQUEST_BACKOFF,
		}
	}

	/// Add a new node to discovery table. Pings the node.
	pub fn add_node(&mut self, e: NodeEntry) {
		// If distance returns None, then we are trying to add ourself.
		let id_hash = keccak(e.id);
		if let Some(dist) = Discovery::distance(&self.id_hash, &id_hash) {
			if self.node_buckets[dist].nodes.iter().any(|n| n.id_hash == id_hash) {
				return;
			}
			self.try_ping(e, PingReason::Default);
		}
	}

	/// Add a list of nodes. Pings a few nodes each round
	pub fn add_node_list(&mut self, nodes: Vec<NodeEntry>) {
		for node in nodes {
			self.add_node(node);
		}
	}

	fn update_bucket_record(&mut self, e: NodeEntry) -> Result<(), BucketError> {
		let id_hash = keccak(e.id);
		let dist = match Discovery::distance(&self.id_hash, &id_hash) {
			Some(dist) => dist,
			None => {
				debug!(target: "discovery", "Attempted to update own entry: {:?}", e);
				return Err(BucketError::Ourselves);
			}
		};
		let bucket = &mut self.node_buckets[dist];
		bucket.nodes.iter_mut().find(|n| n.address.id == e.id)
			.map_or(Err(BucketError::NotInTheBucket{node_entry: e.clone(), bucket_distance: dist}.into()), |entry| {
				entry.address = e;
				entry.last_seen = Instant::now();
				entry.backoff_until = Instant::now();
				entry.fail_count = 0;
				Ok(())
			})
	}

	fn update_node(&mut self, e: NodeEntry) -> Option<TableUpdates> {
		trace!(target: "discovery", "Inserting {:?}", &e);

        match self.update_bucket_record(e) {
            Ok(()) => None,
            Err(BucketError::Ourselves) => None,
            Err(BucketError::NotInTheBucket{node_entry, bucket_distance}) => Some((node_entry, bucket_distance))
        }.and_then(|(node_entry, bucket_distance)| {
			trace!(target: "discovery", "Adding a new node {:?} into our bucket {}", &node_entry, bucket_distance);

            let mut added = HashMap::with_capacity(1);
            added.insert(node_entry.id, node_entry.clone());

			let node_to_ping = {
				let bucket = &mut self.node_buckets[bucket_distance];
				bucket.nodes.push_front(BucketEntry::new(node_entry.clone()));
				if bucket.nodes.len() > BUCKET_SIZE {
					select_bucket_ping(bucket.nodes.iter())
				} else {
					None
				}
			};
			if let Some(node) = node_to_ping {
				self.try_ping(node, PingReason::Default);
			};

            if node_entry.endpoint.is_valid_sync_node() {
				Some(TableUpdates { added, removed: HashSet::new() })
			} else {
				None
			}
        })
	}

	/// Starts the discovery process at round 0
	fn start(&mut self) {
		trace!(target: "discovery", "Starting discovery");
		self.discovery_round = Some(0);
		self.discovery_id.randomize(); //TODO: use cryptographic nonce
		self.discovery_nodes.clear();
	}

	/// Complete the discovery process
	fn stop(&mut self) {
		trace!(target: "discovery", "Completing discovery");
		self.discovery_round = None;
		self.discovery_nodes.clear();
	}

	fn update_new_nodes(&mut self) {
		while self.in_flight_pings.len() < MAX_NODES_PING {
			match self.adding_nodes.pop() {
				Some(next) => self.try_ping(next, PingReason::Default),
				None => break,
			}
		}
	}

	fn discover(&mut self) {
		let discovery_round = match self.discovery_round {
			Some(r) => r,
			None => return,
		};
		if discovery_round == DISCOVERY_MAX_STEPS {
			self.stop();
			return;
		}
		trace!(target: "discovery", "Starting round {:?}", self.discovery_round);
		let mut tried_count = 0;
		{
			let nearest = self.nearest_node_entries(&self.discovery_id).into_iter();
			let nearest = nearest.filter(|x| !self.discovery_nodes.contains(&x.id)).take(ALPHA).collect::<Vec<_>>();
			let target = self.discovery_id;
			for r in nearest {
				match self.send_find_node(&r, &target) {
					Ok(()) => {
						self.discovery_nodes.insert(r.id);
						tried_count += 1;
					},
					Err(e) => {
						warn!(target: "discovery", "Error sending node discovery packet for {:?}: {:?}", &r.endpoint, e);
					},
				};
			}
		}

		if tried_count == 0 {
			self.stop();
			return;
		}
		self.discovery_round = Some(discovery_round + 1);
	}

	/// The base 2 log of the distance between a and b using the XOR metric.
	fn distance(a: &H256, b: &H256) -> Option<usize> {
		for i in (0..ADDRESS_BYTES_SIZE).rev() {
			let byte_index = ADDRESS_BYTES_SIZE - i - 1;
			let d: u8 = a[byte_index] ^ b[byte_index];
			if d != 0 {
				let high_bit_index = 7 - d.leading_zeros() as usize;
				return Some(i * 8 + high_bit_index);
			}
		}
		None // a and b are equal, so log distance is -inf
	}

	fn try_ping(&mut self, node: NodeEntry, reason: PingReason) {
		if !self.is_allowed(&node) {
			trace!(target: "discovery", "Node {:?} not allowed", node);
			return;
		}
		if self.in_flight_pings.contains_key(&node.id) || self.in_flight_find_nodes.contains_key(&node.id) {
			trace!(target: "discovery", "Node {:?} in flight requests", node);
			return;
		}
		if self.adding_nodes.iter().any(|n| n.id == node.id) {
			trace!(target: "discovery", "Node {:?} in adding nodes", node);
			return;
		}

		if self.in_flight_pings.len() < MAX_NODES_PING {
			self.ping(&node, reason)
				.unwrap_or_else(|e| {
					warn!(target: "discovery", "Error sending Ping packet: {:?}", e);
				});
		} else {
			self.adding_nodes.push(node);
		}
	}

	fn ping(&mut self, node: &NodeEntry, reason: PingReason) -> Result<(), Error> {
		let mut rlp = RlpStream::new_list(4);
		rlp.append(&PROTOCOL_VERSION);
		self.public_endpoint.to_rlp_list(&mut rlp);
		node.endpoint.to_rlp_list(&mut rlp);
		append_expiration(&mut rlp);
		let old_parity_hash = keccak(rlp.as_raw());
		let hash = self.send_packet(PACKET_PING, &node.endpoint.udp_address(), &rlp.drain())?;

		self.in_flight_pings.insert(node.id, PingRequest {
			sent_at: Instant::now(),
			node: node.clone(),
			echo_hash: hash,
			deprecated_echo_hash: old_parity_hash,
			reason: reason
		});

		trace!(target: "discovery", "Sent Ping to {:?} ; node_id={:#x}", &node.endpoint, node.id);
		Ok(())
	}

	fn send_find_node(&mut self, node: &NodeEntry, target: &NodeId) -> Result<(), Error> {
		let mut rlp = RlpStream::new_list(2);
		rlp.append(target);
		append_expiration(&mut rlp);
		self.send_packet(PACKET_FIND_NODE, &node.endpoint.udp_address(), &rlp.drain())?;

		self.in_flight_find_nodes.insert(node.id, FindNodeRequest {
			sent_at: Instant::now(),
			response_count: 0,
			answered: false,
		});

		trace!(target: "discovery", "Sent FindNode to {:?}", &node.endpoint);
		Ok(())
	}

	fn send_packet(&mut self, packet_id: u8, address: &SocketAddr, payload: &[u8]) -> Result<H256, Error> {
		let packet = assemble_packet(packet_id, payload, &self.secret)?;
		let hash = H256::from_slice(&packet[0..32]);
		self.send_to(packet, address.clone());
		Ok(hash)
	}

	fn nearest_node_entries(&self, target: &NodeId) -> Vec<NodeEntry> {
		let target_hash = keccak(target);
		let target_distance = self.id_hash ^ target_hash;

		let mut ret = Vec::<NodeEntry>::with_capacity(BUCKET_SIZE);

		// Sort bucket entries by distance to target and append to end of result vector.
		let append_bucket = |results: &mut Vec<NodeEntry>, bucket: &NodeBucket| -> bool {
			let mut sorted_entries: Vec<&BucketEntry> = bucket.nodes.iter().collect();
			sorted_entries.sort_unstable_by_key(|entry| entry.id_hash ^ target_hash);

			let remaining_capacity = results.capacity() - results.len();
			let to_append = if remaining_capacity < sorted_entries.len() {
				&sorted_entries[0..remaining_capacity]
			} else {
				&sorted_entries
			};
			for entry in to_append.iter() {
				results.push(entry.address.clone());
			}
			results.len() == results.capacity()
		};

		// This algorithm leverages the structure of the routing table to efficiently find the
		// nearest entries to a target hash. First, we compute the XOR distance from this node to
		// the target. On a first pass, we iterate from the MSB of the distance, stopping at any
		// buckets where the distance bit is set, and skipping the buckets where it is unset. These
		// must be in order the nearest to the target. On a second pass, we traverse from LSB to
		// MSB, appending the buckets skipped on the first pass. The reason this works is that all
		// entries in bucket i have a common prefix of length exactly 32 - i - 1 with the ID of this
		// node.

		for i in 0..ADDRESS_BITS {
			if ((target_distance[i / 8] << (i % 8)) & 0x80) != 0 {
				let bucket = &self.node_buckets[ADDRESS_BITS - i - 1];
				if !bucket.nodes.is_empty() && append_bucket(&mut ret, bucket) {
					return ret;
				}
			}
		}
		for i in (0..ADDRESS_BITS).rev() {
			if ((target_distance[i / 8] << (i % 8)) & 0x80) == 0 {
				let bucket = &self.node_buckets[ADDRESS_BITS - i - 1];
				if !bucket.nodes.is_empty() && append_bucket(&mut ret, bucket) {
					return ret;
				}
			}
		}
		ret
	}

	fn send_to(&mut self, payload: Bytes, address: SocketAddr) {
		self.send_queue.push_back(Datagram { payload, address });
	}

	pub fn on_packet(&mut self, packet: &[u8], from: SocketAddr) -> Result<Option<TableUpdates>, Error> {
		// validate packet
		if packet.len() < 32 + 65 + 4 + 1 {
			return Err(Error::BadProtocol);
		}

		let hash_signed = keccak(&packet[32..]);
		if hash_signed[..] != packet[0..32] {
			return Err(Error::BadProtocol);
		}

		let signed = &packet[(32 + 65)..];
		let signature = H520::from_slice(&packet[32..(32 + 65)]);
		let node_id = recover(&signature.into(), &keccak(signed))?;
		let packet_id = signed[0];
		let rlp = Rlp::new(&signed[1..]);
		match packet_id {
			PACKET_PING => self.on_ping(&rlp, &node_id, &from, hash_signed.as_bytes()),
			PACKET_PONG => self.on_pong(&rlp, &node_id, &from),
			PACKET_FIND_NODE => self.on_find_node(&rlp, &node_id, &from),
			PACKET_NEIGHBOURS => self.on_neighbours(&rlp, &node_id, &from),
			_ => {
				debug!(target: "discovery", "Unknown UDP packet: {}", packet_id);
				Ok(None)
			}
		}
	}

	/// Validate that given timestamp is in within one second of now or in the future
	fn check_timestamp(&self, timestamp: u64) -> Result<(), Error> {
		let secs_since_epoch = SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs();
		if self.check_timestamps && timestamp < secs_since_epoch {
			debug!(target: "discovery", "Expired packet");
			return Err(Error::Expired);
		}
		Ok(())
	}

	fn is_allowed(&self, entry: &NodeEntry) -> bool {
		entry.endpoint.is_allowed(&self.ip_filter) && entry.id != self.id
	}

	fn on_ping(&mut self, rlp: &Rlp, node_id: &NodeId, from: &SocketAddr, echo_hash: &[u8]) -> Result<Option<TableUpdates>, Error> {
		trace!(target: "discovery", "Got Ping from {:?}", &from);
		let ping_from = if let Ok(node_endpoint) = NodeEndpoint::from_rlp(&rlp.at(1)?) {
			node_endpoint
		} else {
			let mut address = from.clone();
			// address here is the node's tcp port. If we are unable to get the `NodeEndpoint` from the `ping_from`
			// rlp field then this is most likely a BootNode, set the tcp port to 0 because it can not be used for syncing.
			address.set_port(0);
			NodeEndpoint {
				address,
				udp_port: from.port()
			}
		};
		let ping_to = NodeEndpoint::from_rlp(&rlp.at(2)?)?;
		let timestamp: u64 = rlp.val_at(3)?;
		self.check_timestamp(timestamp)?;
		let mut response = RlpStream::new_list(3);
		let pong_to = NodeEndpoint {
			address: from.clone(),
			udp_port: ping_from.udp_port
		};
		// Here the PONG's `To` field should be the node we are
		// sending the request to
		// WARNING: this field _should not be used_, but old Parity versions
		// use it in order to get the node's address.
		// So this is a temporary fix so that older Parity versions don't brake completely.
		ping_to.to_rlp_list(&mut response);
		// pong_to.to_rlp_list(&mut response);

		response.append(&echo_hash);
		append_expiration(&mut response);
		self.send_packet(PACKET_PONG, from, &response.drain())?;

		let entry = NodeEntry { id: *node_id, endpoint: pong_to.clone() };
		if !entry.endpoint.is_valid_discovery_node() {
			debug!(target: "discovery", "Got bad address: {:?}", entry);
		} else if !self.is_allowed(&entry) {
			debug!(target: "discovery", "Address not allowed: {:?}", entry);
		} else {
			self.add_node(entry.clone());
		}
		Ok(None)
	}

	fn on_pong(&mut self, rlp: &Rlp, node_id: &NodeId, from: &SocketAddr) -> Result<Option<TableUpdates>, Error> {
		trace!(target: "discovery", "Got Pong from {:?} ; node_id={:#x}", &from, node_id);
		let _pong_to = NodeEndpoint::from_rlp(&rlp.at(0)?)?;
		let echo_hash: H256 = rlp.val_at(1)?;
		let timestamp: u64 = rlp.val_at(2)?;
		self.check_timestamp(timestamp)?;

		let expected_node = match self.in_flight_pings.entry(*node_id) {
			Entry::Occupied(entry) => {
				let expected_node = {
					let request = entry.get();
					if request.echo_hash != echo_hash && request.deprecated_echo_hash != echo_hash {
						debug!(target: "discovery", "Got unexpected Pong from {:?} ; packet_hash={:#x} ; expected_hash={:#x}", &from, request.echo_hash, echo_hash);
						None
					} else {
						if request.deprecated_echo_hash == echo_hash {
							trace!(target: "discovery", "Got Pong from an old parity-ethereum version.");
						}
						Some((request.node.clone(), request.reason.clone()))
					}
				};

				if expected_node.is_some() {
					entry.remove();
				}
				expected_node
			},
			Entry::Vacant(_) => {
				None
			},
		};

		if let Some((node, ping_reason)) = expected_node {
			if let PingReason::FromDiscoveryRequest(target, validity) = ping_reason {
				self.respond_with_discovery(target, &node)?;
				// kirushik: I would prefer to probe the network id of the remote node here, and add it to the nodes list if it's on "our" net --
				// but `on_packet` happens synchronously, so doing the full TCP handshake ceremony here is a bad idea.
				// So instead we just LRU-caching most recently seen nodes to avoid unnecessary pinging
				match validity {
					NodeValidity::ValidNode(NodeCategory::Bucket) | NodeValidity::ExpiredNode(NodeCategory::Bucket) => {
						trace!(target: "discovery", "Updating node {:?} in our Kad buckets", &node);
						self.update_bucket_record(node).unwrap_or_else(|error| {
							debug!(target: "discovery", "Error occured when processing ping from a bucket node: {:?}", &error);
						});
					},
					NodeValidity::UnknownNode | NodeValidity::ExpiredNode(NodeCategory::Observed) | NodeValidity::ValidNode(NodeCategory::Observed)=> {
						trace!(target: "discovery", "Updating node {:?} in the list of other_observed_nodes", &node);
						self.other_observed_nodes.insert(node.id, (node.endpoint, Instant::now()));
					},
					NodeValidity::Ourselves => (),
				}
				Ok(None)
			} else {
				Ok(self.update_node(node))
			}
		} else {
			debug!(target: "discovery", "Got unexpected Pong from {:?} ; request not found", &from);
			Ok(None)
		}
	}

	fn on_find_node(&mut self, rlp: &Rlp, node_id: &NodeId, from: &SocketAddr) -> Result<Option<TableUpdates>, Error> {
		trace!(target: "discovery", "Got FindNode from {:?}", &from);
		let target: NodeId = rlp.val_at(0)?;
		let timestamp: u64 = rlp.val_at(1)?;
		self.check_timestamp(timestamp)?;

		let node = NodeEntry {
			id: node_id.clone(),
			endpoint: NodeEndpoint {
				address: *from,
				udp_port: from.port()
			}
		};

		match self.check_validity(&node) {
			NodeValidity::Ourselves => (), // It makes no sense to respond to the discovery request from ourselves
			NodeValidity::ValidNode(_) => self.respond_with_discovery(target, &node)?,
			// Make sure the request source is actually there and responds to pings before actually responding
			invalidity_reason => self.try_ping(node, PingReason::FromDiscoveryRequest(target, invalidity_reason))
		}
		Ok(None)
	}

	fn check_validity(&mut self, node: &NodeEntry) -> NodeValidity {
		let id_hash = keccak(node.id);
		let dist = match Discovery::distance(&self.id_hash, &id_hash) {
			Some(dist) => dist,
			None => {
				debug!(target: "discovery", "Got an incoming discovery request from self: {:?}", node);
				return NodeValidity::Ourselves;
			}
		};

		let bucket = &self.node_buckets[dist];
		if let Some(known_node) = bucket.nodes.iter().find(|n| n.address.id == node.id) {
			debug!(target: "discovery", "Found a known node in a bucket when processing discovery: {:?}/{:?}", known_node, node);
			match ((known_node.address.endpoint == node.endpoint), (known_node.last_seen.elapsed() < NODE_LAST_SEEN_TIMEOUT)) {
				(true, true) => NodeValidity::ValidNode(NodeCategory::Bucket),
				(true, false) => NodeValidity::ExpiredNode(NodeCategory::Bucket),
				_ => NodeValidity::UnknownNode
			}
		} else {
			self.other_observed_nodes.get_mut(&node.id).map_or(NodeValidity::UnknownNode, |(endpoint, observed_at)| {
				match ((node.endpoint==*endpoint), (observed_at.elapsed() < NODE_LAST_SEEN_TIMEOUT)) {
					(true, true) => NodeValidity::ValidNode(NodeCategory::Observed),
					(true, false) => NodeValidity::ExpiredNode(NodeCategory::Observed),
					_ => NodeValidity::UnknownNode
				}
			})
		}
	}

	fn respond_with_discovery(&mut self, target: NodeId, node: &NodeEntry) -> Result<(), Error> {
		let nearest = self.nearest_node_entries(&target);
		if nearest.is_empty() {
			return Ok(());
		}
		let mut packets = Discovery::prepare_neighbours_packets(&nearest);
		for p in packets.drain(..) {
			self.send_packet(PACKET_NEIGHBOURS, &node.endpoint.address, &p)?;
		}
		trace!(target: "discovery", "Sent {} Neighbours to {:?}", nearest.len(), &node.endpoint);
		Ok(())
	}

	fn prepare_neighbours_packets(nearest: &[NodeEntry]) -> Vec<Bytes> {
		let limit = (MAX_DATAGRAM_SIZE - 109) / 90;
		let chunks = nearest.chunks(limit);
		let packets = chunks.map(|c| {
			let mut rlp = RlpStream::new_list(2);
			rlp.begin_list(c.len());
			for n in c {
				rlp.begin_list(4);
				n.endpoint.to_rlp(&mut rlp);
				rlp.append(&n.id);
			}
			append_expiration(&mut rlp);
			rlp.out()
		});
		packets.collect()
	}

	fn on_neighbours(&mut self, rlp: &Rlp, node_id: &NodeId, from: &SocketAddr) -> Result<Option<TableUpdates>, Error> {
		let results_count = rlp.at(0)?.item_count()?;

		let is_expected = match self.in_flight_find_nodes.entry(*node_id) {
			Entry::Occupied(mut entry) => {
				let expected = {
					let request = entry.get_mut();
					// Mark the request as answered
					request.answered = true;
					if request.response_count + results_count <= BUCKET_SIZE {
						request.response_count += results_count;
						true
					} else {
						debug!(target: "discovery", "Got unexpected Neighbors from {:?} ; oversized packet ({} + {}) node_id={:#x}", &from, request.response_count, results_count, node_id);
						false
					}
				};
				if entry.get().response_count == BUCKET_SIZE {
					entry.remove();
				}
				expected
			}
			Entry::Vacant(_) => {
				debug!(target: "discovery", "Got unexpected Neighbors from {:?} ; couldn't find node_id={:#x}", &from, node_id);
				false
			},
		};

		if !is_expected {
			return Ok(None);
		}

		trace!(target: "discovery", "Got {} Neighbours from {:?}", results_count, &from);
		for r in rlp.at(0)?.iter() {
			let endpoint = NodeEndpoint::from_rlp(&r)?;
			if !endpoint.is_valid_discovery_node() {
				debug!(target: "discovery", "Bad address: {:?}", endpoint);
				continue;
			}
			let node_id: NodeId = r.val_at(3)?;
			if node_id == self.id {
				continue;
			}
			let entry = NodeEntry { id: node_id, endpoint };
			if !self.is_allowed(&entry) {
				debug!(target: "discovery", "Address not allowed: {:?}", entry);
				continue;
			}
			self.add_node(entry);
		}
		Ok(None)
	}

	fn check_expired(&mut self, time: Instant) {
		let mut nodes_to_expire = Vec::new();
		self.in_flight_pings.retain(|node_id, ping_request| {
			if time.duration_since(ping_request.sent_at) > PING_TIMEOUT {
				debug!(target: "discovery", "Removing expired PING request for node_id={:#x}", node_id);
				nodes_to_expire.push(*node_id);
				false
			} else {
				true
			}
		});
		self.in_flight_find_nodes.retain(|node_id, find_node_request| {
			if time.duration_since(find_node_request.sent_at) > FIND_NODE_TIMEOUT {
				if !find_node_request.answered {
					debug!(target: "discovery", "Removing expired FIND NODE request for node_id={:#x}", node_id);
					nodes_to_expire.push(*node_id);
				}
				false
			} else {
				true
			}
		});
		for node_id in nodes_to_expire {
			self.expire_node_request(node_id);
		}
	}

	fn expire_node_request(&mut self, node_id: NodeId) {
		// Attempt to remove from bucket if in one.
		let id_hash = keccak(&node_id);
		let dist = Discovery::distance(&self.id_hash, &id_hash)
			.expect("distance is None only if id hashes are equal; will never send request to self; qed");
		let bucket = &mut self.node_buckets[dist];
		if let Some(index) = bucket.nodes.iter().position(|n| n.id_hash == id_hash) {
			if bucket.nodes[index].fail_count < self.request_backoff.len() {
				let node = &mut bucket.nodes[index];
				node.backoff_until = Instant::now() + self.request_backoff[node.fail_count];
				node.fail_count += 1;
				trace!(
					target: "discovery",
					"Requests to node {:?} timed out {} consecutive time(s)",
					&node.address, node.fail_count
				);
			} else {
				let node = bucket.nodes.remove(index).expect("index was located in if condition");
				debug!(target: "discovery", "Removed expired node {:?}", &node.address);
			}
		}
	}

	pub fn round(&mut self) {
		self.check_expired(Instant::now());
		self.update_new_nodes();

		if self.discovery_round.is_some() {
			self.discover();
		// Start discovering if the first pings have been sent (or timed out)
		} else if self.in_flight_pings.len() == 0 && !self.discovery_initiated {
			self.discovery_initiated = true;
			self.refresh();
		}
	}

	pub fn refresh(&mut self) {
		if self.discovery_round.is_none() {
			self.start();
		}
	}

	pub fn any_sends_queued(&self) -> bool {
		!self.send_queue.is_empty()
	}

	pub fn dequeue_send(&mut self) -> Option<Datagram> {
		self.send_queue.pop_front()
	}

	pub fn requeue_send(&mut self, datagram: Datagram) {
		self.send_queue.push_front(datagram)
	}

	/// Add a list of known nodes to the table.
	#[cfg(test)]
	pub fn init_node_list(&mut self, nodes: Vec<NodeEntry>) {
		for n in nodes {
			if self.is_allowed(&n) {
				self.update_node(n);
			}
		}
	}
}

fn append_expiration(rlp: &mut RlpStream) {
	let expiry = SystemTime::now() + EXPIRY_TIME;
	let timestamp = expiry.duration_since(UNIX_EPOCH).unwrap_or_default().as_secs() as u32;
	rlp.append(&timestamp);
}

fn assemble_packet(packet_id: u8, bytes: &[u8], secret: &Secret) -> Result<Bytes, Error> {
	let mut packet = Bytes::with_capacity(bytes.len() + 32 + 65 + 1);
	packet.resize(32 + 65, 0); // Filled in below
	packet.push(packet_id);
	packet.extend_from_slice(bytes);

	let hash = keccak(&packet[(32 + 65)..]);
	let signature = match sign(secret, &hash) {
		Ok(s) => s,
		Err(e) => {
			warn!(target: "discovery", "Error signing UDP packet");
			return Err(Error::from(e));
		}
	};
	packet[32..(32 + 65)].copy_from_slice(&signature[..]);
	let signed_hash = keccak(&packet[32..]);
	packet[0..32].copy_from_slice(signed_hash.as_bytes());
	Ok(packet)
}

// Selects the next node in a bucket to ping. Chooses the eligible node least recently seen.
fn select_bucket_ping<'a, I>(nodes: I) -> Option<NodeEntry>
where
	I: Iterator<Item=&'a BucketEntry>
{
	let now = Instant::now();
	nodes
		.filter(|n| n.backoff_until < now)
		.min_by_key(|n| n.last_seen)
		.map(|n| n.address.clone())
}

#[cfg(test)]
mod tests {
	use std::net::{IpAddr, Ipv4Addr};
	use std::str::FromStr;

	use rustc_hex::FromHex;

	use parity_crypto::publickey::{Generator, Random};

	use crate::node_table::{Node, NodeEndpoint, NodeId};

	use super::*;

	#[test]
	fn find_node() {
		let mut nearest = Vec::new();
		let node = Node::from_str("enode://a979fb575495b8d6db44f750317d0f4622bf4c2aa3365d6af7c284339968eef29b69ad0dce72a4d8db5ebb4968de0e3bec910127f134779fbcb0cb6d3331163c@127.0.0.1:7770").unwrap();
		for _ in 0..1000 {
			nearest.push( NodeEntry { id: node.id.clone(), endpoint: node.endpoint.clone() });
		}

		let packets = Discovery::prepare_neighbours_packets(&nearest);
		assert_eq!(packets.len(), 77);
		for p in &packets[0..76] {
			assert!(p.len() > 1280/2);
			assert!(p.len() <= 1280);
		}
		assert!(packets.last().unwrap().len() > 0);
	}

	#[test]
	fn ping_queue() {
		let key = Random.generate().unwrap();
		let ep = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40445").unwrap(), udp_port: 40445 };
		let mut discovery = Discovery::new(&key, ep.clone(), IpFilter::default());

		for i in 1..(MAX_NODES_PING+1) {
			discovery.add_node(NodeEntry { id: NodeId::random(), endpoint: ep.clone() });
			assert_eq!(discovery.in_flight_pings.len(), i);
			assert_eq!(discovery.send_queue.len(), i);
			assert_eq!(discovery.adding_nodes.len(), 0);
		}
		for i in 1..20 {
			discovery.add_node(NodeEntry { id: NodeId::random(), endpoint: ep.clone() });
			assert_eq!(discovery.in_flight_pings.len(), MAX_NODES_PING);
			assert_eq!(discovery.send_queue.len(), MAX_NODES_PING);
			assert_eq!(discovery.adding_nodes.len(), i);
		}
	}

	#[test]
	fn discovery() {
		let mut discovery_handlers = (0..5).map(|i| {
			let key = Random.generate().unwrap();
			let ep = NodeEndpoint {
				address: SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 41000 + i),
				udp_port: 41000 + i,
			};
			Discovery::new(&key, ep, IpFilter::default())
		})
			.collect::<Vec<_>>();

		// Sort inversely by XOR distance to the 0 hash.
		discovery_handlers.sort_by(|a, b| b.id_hash.cmp(&a.id_hash));

		// Initialize the routing table of each with the next one in order.
		for i in 0 .. 5 {
			let node = NodeEntry {
				id: discovery_handlers[(i + 1) % 5].id,
				endpoint: discovery_handlers[(i + 1) % 5].public_endpoint.clone(),
			};
			discovery_handlers[i].update_node(node);
		}

		// After 4 discovery rounds, the first one should have learned about the rest.
		for _round in 0 .. 5 {
			discovery_handlers[0].round();

			let mut continue_loop = true;
			while continue_loop {
				continue_loop = false;

				// Process all queued messages.
				for i in 0 .. 20 {
					let src = discovery_handlers[i%5].public_endpoint.address.clone();
					while let Some(datagram) = discovery_handlers[i%5].dequeue_send() {
						let dest = discovery_handlers.iter_mut()
							.find(|disc| datagram.address == disc.public_endpoint.address)
							.unwrap();
						dest.on_packet(&datagram.payload, src).ok();

						continue_loop = true;
					}
				}
			}
		}

		let results = discovery_handlers[0].nearest_node_entries(&NodeId::zero());
		assert_eq!(results.len(), 4);
	}

	#[test]
	fn removes_expired() {
		let key = Random.generate().unwrap();
		let ep = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40446").unwrap(), udp_port: 40447 };
		let discovery = Discovery::new(&key, ep.clone(), IpFilter::default());

		let mut discovery = Discovery { request_backoff: &[], ..discovery };

		let total_bucket_nodes = |node_buckets: &Vec<NodeBucket>| -> usize {
			node_buckets.iter().map(|bucket| bucket.nodes.len()).sum()
		};

		let node_entries = (0..1200)
			.map(|_| NodeEntry { id: NodeId::random(), endpoint: ep.clone() })
			.collect::<Vec<_>>();

		discovery.init_node_list(node_entries.clone());
		assert_eq!(total_bucket_nodes(&discovery.node_buckets), 1200);

		// Requests have not expired yet.
		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now());
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert_eq!(removed, 0);

		// Expiring pings to bucket nodes removes them from bucket.
		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now() + PING_TIMEOUT);
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert!(removed > 0);
		assert_eq!(total_bucket_nodes(&discovery.node_buckets), 1200 - removed);

		for _ in 0..100 {
			discovery.add_node(NodeEntry { id: NodeId::random(), endpoint: ep.clone() });
		}
		assert!(discovery.in_flight_pings.len() > 0);

		// Expire pings to nodes that are not in buckets.
		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now() + PING_TIMEOUT);
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert_eq!(removed, 0);
		assert_eq!(discovery.in_flight_pings.len(), 0);

		let from = SocketAddr::from_str("99.99.99.99:40445").unwrap();

		// FIND_NODE times out because it doesn't receive k results.
		let key = Random.generate().unwrap();
		discovery.send_find_node(&node_entries[100], key.public()).unwrap();
		for payload in Discovery::prepare_neighbours_packets(&node_entries[101..116]) {
			let packet = assemble_packet(PACKET_NEIGHBOURS, &payload, &key.secret()).unwrap();
			discovery.on_packet(&packet, from.clone()).unwrap();
		}

		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now() + FIND_NODE_TIMEOUT);
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert!(removed > 0);

		// FIND_NODE does not time out because it receives k results.
		discovery.send_find_node(&node_entries[100], key.public()).unwrap();
		for payload in Discovery::prepare_neighbours_packets(&node_entries[101..117]) {
			let packet = assemble_packet(PACKET_NEIGHBOURS, &payload, &key.secret()).unwrap();
			discovery.on_packet(&packet, from.clone()).unwrap();
		}

		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now() + FIND_NODE_TIMEOUT);
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert_eq!(removed, 0);

		// Test bucket evictions with retries.
		let request_backoff = [Duration::new(0, 0); 2];
		let mut discovery = Discovery { request_backoff: &request_backoff, ..discovery };

		for _ in 0..2 {
			discovery.ping(&node_entries[101], PingReason::Default).unwrap();
			let num_nodes = total_bucket_nodes(&discovery.node_buckets);
			discovery.check_expired(Instant::now() + PING_TIMEOUT);
			let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
			assert_eq!(removed, 0);
		}

		discovery.ping(&node_entries[101], PingReason::Default).unwrap();
		let num_nodes = total_bucket_nodes(&discovery.node_buckets);
		discovery.check_expired(Instant::now() + PING_TIMEOUT);
		let removed = num_nodes - total_bucket_nodes(&discovery.node_buckets);
		assert_eq!(removed, 1);
	}

	#[test]
	fn find_nearest_saturated() {
		use super::*;

		let key = Random.generate().unwrap();
		let ep = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40447").unwrap(), udp_port: 40447 };
		let mut discovery = Discovery::new(&key, ep.clone(), IpFilter::default());

		for _ in 0..(16 + 10) {
			let entry = BucketEntry::new(NodeEntry { id: NodeId::zero(), endpoint: ep.clone() });
			discovery.node_buckets[0].nodes.push_back(entry);
		}
		let nearest = discovery.nearest_node_entries(&NodeId::zero());
		assert_eq!(nearest.len(), 16)
	}

	#[test]
	fn routing_table_insertions_lookups() {
		use super::*;
		let ep = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40448").unwrap(), udp_port: 40447 };
		let node_ids_hex: [&str; 32] = [
			"22536fa57acc12c4993295cbc26fef4550513496712b301ad2283d356c8108521244a362e64e6d907a0d0b4e65526699c5ae3cfebfc680505fe3b33d50672835",
			"22c482f42401546f8dd7ed6b1c0cad976da6630730f1116614579ccb084791a528ff2676bfe94434de80e5d7e479f1ea1d7737077da3bd5e69a0f3e5bf596091",
			"234c73e3a8f6835a7f9a9d2a896bff4908d66d21d5433a2c37d94f1fa9a6ca17d02388f31013ff87e3ad86506e76bd1006b9cac3815974a2b47c8d4f2124697e",
			"2a5aaf4e2046c521e890dc82313c6151a55078f045a7e3d259f168238d029271cdd9a0943468d45c1e36a34a8a6d4de4b0262e48d3c8cfdd4c2aab5df42926b9",
			"341d8c94d9670461186cfc1f66d4246cb12384940e9f621ec8d6c216b5d037cde5f7a41b70474ca36ced4a4f2fe91c9dc5a24a128414672661f78e8611d54bfd",
			"3d9fd01851f3ae1bfd06b48e89738f29f9a2b4dce3ab7864df4fccca55d1ac88044956ba47d0c4cb44a19924626a3a3aa5a4de8958365cb7385111ce7b929200",
			"406d5507a7fbc194a495800ae8cf408093336febc24d03d6c63756f522274ab02146ceb1b0213291a9a1544680503837519f88f1e8677d921de62c82935b4e6c",
			"4c537f00805f320616ee49c7bc36e1d7e52a04a782b0cc00fd3d6b77200b027cef5f875ed38f1167fef4b02d7bd49a661812301d9d680bb62297131204c035f9",
			"4fc8e3fdbdd7acad82b283ac52c121b805f3b15ffcaa6b2ca67b9e375aa88e978951ffa3d03ee13be99f0ee987db0bbfc6a7ca02b175e9123d79826025b4089d",
			"55b5042a6910bc908a0520966e8cbcc92ac299bdb7efbfbcf703df1506fa0f9b09c5eeb930080de848d2864cca71f885942852c51233db0ee46fe0447306d61f",
			"5d24f28b350c4c37fc4dad7f418e029992c9e4ac356bb3d9a1356ba1076339863c05044d7ceba233c65779401f8a3b38fe67b6a592c1be4834dc869f7bb932eb",
			"5f6edaf2f2ae3003f4b4ff90b8e71a717c832c71a634d96e77fe046f9a88adc8de5718ff3c47659aea4cead5376df5b731e1b6530e6b0999f56ad75d4dabd3f6",
			"6214c04211efe91abd23d65e2dc8e711b06d4fb13dcfd65b691dc51f58455b2145f9b38f523b72a45a12705a28d389308a34455720d774c9b805326df42b5a63",
			"69df92573ddbbce88b72a930843dbb70728b2a020e0cc4e8ba805dcf7f19297bfc5def4ca447e9e6ec66971be1815b8f49042720431f698b6a87a185d94fa6c8",
			"72ffc23de007cf8b6f4a117f7427b532d05861c314344ffa265175f57ee45dae041a710a4dc74124dba1dabdc0f52dfd21e3154d1d4285aab529810c6161d623",
			"80b567f279a9512f3a66ebd8f87a93acd4d50bf66f5eff6d04039c1f5838e37021e981539659b33e0644b243fc9671209a80cbef40d1bcf7c7117d353cb45532",
			"9009dc9e3bf50595f84271f46d4c7a5ad6971f7d2ffce1905bfc40a407d34fc5e2dcebd92746eadcd2c5fa4d5aaccb0e01b542d506b361851df3f19e6bc629a3",
			"95264f56e091efeba911003fd01eeb2c81f6fc4bb7b10c92e4c7bfaf460b7246d232e61ad8a223d74870981a84e15b2d5134c25d931cb860c6912b20a2d3ac01",
			"96013a472a9f7ff9c5c76b5ca958f14ee510d826703aa41d4c88eac51d30d14229b9f19f6e0469c37aaa6d2136a978a4aaa38ca766f48e53e569f84e44252962",
			"a513c988cf8480ad2992caa64e3fa059ce07efda260dfeefed78e1d41ea3f97844603b8a9737eb633086fd9ac2f201200cb656cda8a91bf6cc500d6039db6f53",
			"ab3311f38e3641c8b3b1fd36dd7f94b148166e267258e840d29d1859537c74f202bd3342359b3623f96c23fa662d1b65182a898bf20343744b37cb265182e500",
			"ac8f41dbd637891a08c9cf715c23577bdd431ba40231682a5a9ba7fd6cb6d66c04f63d6d65c7d9f8737e641e05fdbeede57138a174f0d55e7835575dd6cddd98",
			"accdad251888d53e4e18efee1e0d749d050216b14896efb657e9c7b1b78dab82a5b6fb3234017aa19a2f50475d73960f352d308b2e0e841cbebaf418362a4f21",
			"b138622208f74d2b8e8fc10bcd4cf3302685cd77d339280a939474b92be8b93e441c50709e25c82cc88a2a4207e9f2938912d60600226efe322b43c6ef5e7aef",
			"b4f64e1fa6a5cd6198b2515bde63fbdabaf7e7a31dbaf5369babbda4b8cd0bf5025ac4b7d2d6e6e3bc76c890df585d28d4815e464c8792ef677df9206864a12b",
			"c1136e08a27c93812ae2dd47201d9e81c82d1995001b88dba9eec700e1d3385dfaf7ae834226c3c90a138f1808cd10b5502f49ee774a2bc707f34bd7d160b7bd",
			"c203ae9b5d1953b0ac462e66338800ec26982e2af54bd444fc8978973191633d4f483e31b28233c07bb99f34d57c680fa5f8e093e64f13b235005b7ab6e2d594",
			"c2e1067c58a9948e773e0a3637d946e26d95762f89ec9d35e2ad84f770309d94168d4e112c78d62b60efc6216bc5d31475f24307b1b8e0fa8dcbb18a10cb85f5",
			"d60ecb1a89e0d5aeff14c9a95da9f5492eb15871c53563b86b7c5ddf0da74b4c29e682fdd22aae2290e0b16ef4b6d707ef55396ca98f755c95b689cf65ce5f80",
			"df5ad4ea6242929df86f2162d1cc62b0e0a6f0a03428a39dea98f6a689335b5ceaf1f0696c17b717b141aeb45a29108d95c3a7d2d1d0bb3441219504ae672917",
			"e1268f5dd9552a11989df9d4953bb388e7466711b2bd9882a3ed4d0767a21f046c53c20f9a18d66bae1d6a5544492857ddecb0b5b4818bd4557be252ddd66c71",
			"e626019dc0b50b9e254461f19d29e69a4669c5256134a6352c6c30d3bc55d201a5b43fc2e006556cfaf29765b683e807e03093798942826244e4ee9e47c75d3f",
		];
		let node_entries = node_ids_hex.iter()
			.map(|node_id_hex| NodeId::from_str(node_id_hex).unwrap())
			.map(|node_id| NodeEntry { id: node_id, endpoint: ep.clone() })
			.collect::<Vec<_>>();

		let secret_hex = "6c71d1b8930d29e6371be1081f2c909c64b46440a1716314c3c9df995cb3aed1";
		let key = Secret::from_str(secret_hex)
			.and_then(|secret| KeyPair::from_secret(secret))
			.unwrap();
		let mut discovery = Discovery::new(&key, ep.clone(), IpFilter::default());

		discovery.init_node_list(node_entries.clone());

		let expected_bucket_sizes = vec![
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
			0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 3, 7, 8, 12
		];
		let actual_bucket_sizes = discovery.node_buckets.iter()
			.map(|ref bucket| bucket.nodes.len())
			.collect::<Vec<_>>();
		assert_eq!(actual_bucket_sizes, expected_bucket_sizes);

		for entry in &node_entries {
			let nearest = discovery.nearest_node_entries(&entry.id);
			assert_eq!(nearest.len(), 16);
			assert_eq!(nearest[0].id, entry.id);

			let mut expected_ids: Vec<NodeId> = node_entries.iter().map(|entry| entry.id).collect();
			expected_ids.sort_unstable_by_key(|id| keccak(id) ^ keccak(entry.id));
			expected_ids.resize(BUCKET_SIZE, NodeId::default());

			let actual_ids: Vec<NodeId> = nearest.iter().map(|entry| entry.id).collect();
			assert_eq!(actual_ids, expected_ids);
		}
	}

	#[test]
	fn packets() {
		let key = Random.generate().unwrap();
		let ep = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40449").unwrap(), udp_port: 40449 };
		let mut discovery = Discovery::new(&key, ep.clone(), IpFilter::default());
		discovery.check_timestamps = false;
		let from = SocketAddr::from_str("99.99.99.99:40445").unwrap();

		let packet = "\
		e9614ccfd9fc3e74360018522d30e1419a143407ffcce748de3e22116b7e8dc92ff74788c0b6663a\
		aa3d67d641936511c8f8d6ad8698b820a7cf9e1be7155e9a241f556658c55428ec0563514365799a\
		4be2be5a685a80971ddcfa80cb422cdd0101ec04cb847f000001820cfa8215a8d790000000000000\
		000000000000000000018208ae820d058443b9a3550102\
		".from_hex().unwrap();
		let _ = discovery.on_packet(&packet, from.clone()).expect("packet to be ok");

		let packet = "\
		577be4349c4dd26768081f58de4c6f375a7a22f3f7adda654d1428637412c3d7fe917cadc56d4e5e\
		7ffae1dbe3efffb9849feb71b262de37977e7c7a44e677295680e9e38ab26bee2fcbae207fba3ff3\
		d74069a50b902a82c9903ed37cc993c50001f83e82022bd79020010db83c4d001500000000abcdef\
		12820cfa8215a8d79020010db885a308d313198a2e037073488208ae82823a8443b9a355c5010203\
		040531b9019afde696e582a78fa8d95ea13ce3297d4afb8ba6433e4154caa5ac6431af1b80ba7602\
		3fa4090c408f6b4bc3701562c031041d4702971d102c9ab7fa5eed4cd6bab8f7af956f7d565ee191\
		7084a95398b6a21eac920fe3dd1345ec0a7ef39367ee69ddf092cbfe5b93e5e568ebc491983c09c7\
		6d922dc3\
		".from_hex().unwrap();
		let _ = discovery.on_packet(&packet, from.clone()).expect("packet to be ok");

		let packet = "\
		09b2428d83348d27cdf7064ad9024f526cebc19e4958f0fdad87c15eb598dd61d08423e0bf66b206\
		9869e1724125f820d851c136684082774f870e614d95a2855d000f05d1648b2d5945470bc187c2d2\
		216fbe870f43ed0909009882e176a46b0102f846d79020010db885a308d313198a2e037073488208\
		ae82823aa0fbc914b16819237dcd8801d7e53f69e9719adecb3cc0e790c57e91ca4461c9548443b9\
		a355c6010203c2040506a0c969a58f6f9095004c0177a6b47f451530cab38966a25cca5cb58f0555
		42124e\
		".from_hex().unwrap();
		let _ = discovery.on_packet(&packet, from.clone()).expect("packet to be ok");

		let packet = "\
		c7c44041b9f7c7e41934417ebac9a8e1a4c6298f74553f2fcfdcae6ed6fe53163eb3d2b52e39fe91\
		831b8a927bf4fc222c3902202027e5e9eb812195f95d20061ef5cd31d502e47ecb61183f74a504fe\
		04c51e73df81f25c4d506b26db4517490103f84eb840ca634cae0d49acb401d8a4c6b6fe8c55b70d\
		115bf400769cc1400f3258cd31387574077f301b421bc84df7266c44e9e6d569fc56be0081290476\
		7bf5ccd1fc7f8443b9a35582999983999999280dc62cc8255c73471e0a61da0c89acdc0e035e260a\
		dd7fc0c04ad9ebf3919644c91cb247affc82b69bd2ca235c71eab8e49737c937a2c396\
		".from_hex().unwrap();
		let _ = discovery.on_packet(&packet, from.clone()).expect("packet to be ok");

		let packet = "\
		c679fc8fe0b8b12f06577f2e802d34f6fa257e6137a995f6f4cbfc9ee50ed3710faf6e66f932c4c8\
		d81d64343f429651328758b47d3dbc02c4042f0fff6946a50f4a49037a72bb550f3a7872363a83e1\
		b9ee6469856c24eb4ef80b7535bcf99c0004f9015bf90150f84d846321163782115c82115db84031\
		55e1427f85f10a5c9a7755877748041af1bcd8d474ec065eb33df57a97babf54bfd2103575fa8291\
		15d224c523596b401065a97f74010610fce76382c0bf32f84984010203040101b840312c55512422\
		cf9b8a4097e9a6ad79402e87a15ae909a4bfefa22398f03d20951933beea1e4dfa6f968212385e82\
		9f04c2d314fc2d4e255e0d3bc08792b069dbf8599020010db83c4d001500000000abcdef12820d05\
		820d05b84038643200b172dcfef857492156971f0e6aa2c538d8b74010f8e140811d53b98c765dd2\
		d96126051913f44582e8c199ad7c6d6819e9a56483f637feaac9448aacf8599020010db885a308d3\
		13198a2e037073488203e78203e8b8408dcab8618c3253b558d459da53bd8fa68935a719aff8b811\
		197101a4b2b47dd2d47295286fc00cc081bb542d760717d1bdd6bec2c37cd72eca367d6dd3b9df73\
		8443b9a355010203b525a138aa34383fec3d2719a0\
		".from_hex().unwrap();
		let _ = discovery.on_packet(&packet, from.clone()).expect("packet to be ok");
	}

	#[test]
	fn test_ping() {
		let key1 = Random.generate().unwrap();
		let key2 = Random.generate().unwrap();
		let key3 = Random.generate().unwrap();
		let ep1 = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40344").unwrap(), udp_port: 40344 };
		let ep2 = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40345").unwrap(), udp_port: 40345 };
		let ep3 = NodeEndpoint { address: SocketAddr::from_str("127.0.0.1:40346").unwrap(), udp_port: 40345 };
		let mut discovery1 = Discovery::new(&key1, ep1.clone(), IpFilter::default());
		let mut discovery2 = Discovery::new(&key2, ep2.clone(), IpFilter::default());

		discovery1.ping(&NodeEntry { id: discovery2.id, endpoint: ep2.clone() }, PingReason::Default).unwrap();
		let ping_data = discovery1.dequeue_send().unwrap();
		assert!(!discovery1.any_sends_queued());
		let data = &ping_data.payload[(32 + 65)..];
		assert_eq!(data[0], PACKET_PING);
		let rlp = Rlp::new(&data[1..]);
		assert_eq!(ep1, NodeEndpoint::from_rlp(&rlp.at(1).unwrap()).unwrap());
		assert_eq!(ep2, NodeEndpoint::from_rlp(&rlp.at(2).unwrap()).unwrap());

		// `discovery1` should be added to node table on ping received
		if let Some(_) = discovery2.on_packet(&ping_data.payload, ep1.address.clone()).unwrap() {
			panic!("Expected no changes to discovery2's table");
		}

		let pong_data = discovery2.dequeue_send().unwrap();
		let data = &pong_data.payload[(32 + 65)..];
		assert_eq!(data[0], PACKET_PONG);
		let rlp = Rlp::new(&data[1..]);
		assert_eq!(ping_data.payload[0..32], rlp.val_at::<Vec<u8>>(1).unwrap()[..]);

		// Create a pong packet with incorrect echo hash and assert that it is rejected.
		let mut incorrect_pong_rlp = RlpStream::new_list(3);
		ep1.to_rlp_list(&mut incorrect_pong_rlp);
		incorrect_pong_rlp.append(&H256::zero());
		append_expiration(&mut incorrect_pong_rlp);
		let incorrect_pong_data = assemble_packet(
			PACKET_PONG, &incorrect_pong_rlp.drain(), &discovery2.secret
		).unwrap();
		if let Some(_) = discovery1.on_packet(&incorrect_pong_data, ep2.address.clone()).unwrap() {
			panic!("Expected no changes to discovery1's table because pong hash is incorrect");
		}

		// Delivery of valid pong response should add to routing table.
		if let Some(table_updates) = discovery1.on_packet(&pong_data.payload, ep2.address.clone()).unwrap() {
			assert_eq!(table_updates.added.len(), 1);
			assert_eq!(table_updates.removed.len(), 0);
			assert!(table_updates.added.contains_key(&discovery2.id));
		} else {
			panic!("Expected discovery1 to be added to discovery1's table");
		}

		let ping_back = discovery2.dequeue_send().unwrap();
		assert!(!discovery2.any_sends_queued());
		let data = &ping_back.payload[(32 + 65)..];
		assert_eq!(data[0], PACKET_PING);
		let rlp = Rlp::new(&data[1..]);
		assert_eq!(ep2, NodeEndpoint::from_rlp(&rlp.at(1).unwrap()).unwrap());
		assert_eq!(ep1, NodeEndpoint::from_rlp(&rlp.at(2).unwrap()).unwrap());

		// Deliver an unexpected PONG message to discover1.
		let mut unexpected_pong_rlp = RlpStream::new_list(3);
		ep3.to_rlp_list(&mut unexpected_pong_rlp);
		unexpected_pong_rlp.append(&H256::zero());
		append_expiration(&mut unexpected_pong_rlp);
		let unexpected_pong = assemble_packet(
			PACKET_PONG, &unexpected_pong_rlp.drain(), key3.secret()
		).unwrap();
		if let Some(_) = discovery1.on_packet(&unexpected_pong, ep3.address.clone()).unwrap() {
			panic!("Expected no changes to discovery1's table for unexpected pong");
		}
	}
}