[swarm] Configurable and "infinite" scores for external addresses. (#1842)

* Add "infinite" scores for external addresses.

Extend address scores with an infinite cardinal, permitting
addresses to be retained "forever" or until explicitly removed.

Expose (external) address scores on the API.

* Update swarm/src/registry.rs

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* Fix compilation.

* Update CHANGELOG

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

misc/core-derive/src/lib.rs

@@ -450,8 +450,8 @@ fn build_struct(ast: &DeriveInput, data_struct: &DataStruct) -> TokenStream {
                             event: #wrapped_event,
                         });
                     }
-                    std::task::Poll::Ready(#network_behaviour_action::ReportObservedAddr { address }) => {
-                        return std::task::Poll::Ready(#network_behaviour_action::ReportObservedAddr { address });
+                    std::task::Poll::Ready(#network_behaviour_action::ReportObservedAddr { address, score }) => {
+                        return std::task::Poll::Ready(#network_behaviour_action::ReportObservedAddr { address, score });
                     }
                     std::task::Poll::Pending => break,
                 }

protocols/gossipsub/src/behaviour.rs

@@ -1412,8 +1412,8 @@ impl NetworkBehaviour for Gossipsub {
                 NetworkBehaviourAction::DialPeer { peer_id, condition } => {
                     NetworkBehaviourAction::DialPeer { peer_id, condition }
                 }
-                NetworkBehaviourAction::ReportObservedAddr { address } => {
-                    NetworkBehaviourAction::ReportObservedAddr { address }
+                NetworkBehaviourAction::ReportObservedAddr { address, score } => {
+                    NetworkBehaviourAction::ReportObservedAddr { address, score }
                 }
             });
         }

protocols/identify/src/identify.rs

@@ -30,6 +30,7 @@ use libp2p_core::{
     upgrade::{ReadOneError, UpgradeError}
 };
 use libp2p_swarm::{
+    AddressScore,
     NegotiatedSubstream,
     NetworkBehaviour,
     NetworkBehaviourAction,
@@ -47,6 +48,10 @@ use std::{
 
 /// Network behaviour that automatically identifies nodes periodically, returns information
 /// about them, and answers identify queries from other nodes.
+///
+/// All external addresses of the local node supposedly observed by remotes
+/// are reported via [`NetworkBehaviourAction::ReportObservedAddr`] with a
+/// [score](AddressScore) of `1`.
 pub struct Identify {
     /// Protocol version to send back to remotes.
     protocol_version: String,
@@ -143,6 +148,7 @@ impl NetworkBehaviour for Identify {
                 self.events.push_back(
                     NetworkBehaviourAction::ReportObservedAddr {
                         address: remote.observed_addr,
+                        score: AddressScore::Finite(1),
                     });
             }
             IdentifyHandlerEvent::Identify(sender) => {
@@ -187,7 +193,7 @@ impl NetworkBehaviour for Identify {
                 .map(|p| String::from_utf8_lossy(&p).to_string())
                 .collect();
 
-            let mut listen_addrs: Vec<_> = params.external_addresses().collect();
+            let mut listen_addrs: Vec<_> = params.external_addresses().map(|r| r.addr).collect();
             listen_addrs.extend(params.listened_addresses());
 
             let mut sending = 0;

protocols/kad/src/behaviour.rs

@@ -1046,7 +1046,7 @@ where
                 phase: AddProviderPhase::GetClosestPeers
             } => {
                 let provider_id = params.local_peer_id().clone();
-                let external_addresses = params.external_addresses().collect();
+                let external_addresses = params.external_addresses().map(|r| r.addr).collect();
                 let inner = QueryInner::new(QueryInfo::AddProvider {
                     context,
                     key,

protocols/request-response/src/throttled.rs

@@ -553,8 +553,8 @@ where
                     NetworkBehaviourAction::DialPeer { peer_id, condition },
                 | NetworkBehaviourAction::NotifyHandler { peer_id, handler, event } =>
                     NetworkBehaviourAction::NotifyHandler { peer_id, handler, event },
-                | NetworkBehaviourAction::ReportObservedAddr { address } =>
-                    NetworkBehaviourAction::ReportObservedAddr { address }
+                | NetworkBehaviourAction::ReportObservedAddr { address, score } =>
+                    NetworkBehaviourAction::ReportObservedAddr { address, score }
             };
 
             return Poll::Ready(event)

swarm/CHANGELOG.md

@@ -2,6 +2,11 @@
 
 - Update `libp2p-core`.
 
+- Expose configurable scores for external addresses, as well as
+  the ability to remove them and to add addresses that are
+  retained "forever" (or until explicitly removed).
+  [PR 1842](https://github.com/libp2p/rust-libp2p/pull/1842).
+
 # 0.24.0 [2020-11-09]
 
 - Update dependencies.

swarm/src/behaviour.rs

@@ -18,6 +18,7 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+use crate::{AddressScore, AddressRecord};
 use crate::protocols_handler::{IntoProtocolsHandler, ProtocolsHandler};
 use libp2p_core::{ConnectedPoint, Multiaddr, PeerId, connection::{ConnectionId, ListenerId}};
 use std::{error, task::Context, task::Poll};
@@ -182,7 +183,7 @@ pub trait PollParameters {
     /// Iterator returned by [`listened_addresses`](PollParameters::listened_addresses).
     type ListenedAddressesIter: ExactSizeIterator<Item = Multiaddr>;
     /// Iterator returned by [`external_addresses`](PollParameters::external_addresses).
-    type ExternalAddressesIter: ExactSizeIterator<Item = Multiaddr>;
+    type ExternalAddressesIter: ExactSizeIterator<Item = AddressRecord>;
 
     /// Returns the list of protocol the behaviour supports when a remote negotiates a protocol on
     /// an inbound substream.
@@ -269,8 +270,9 @@ pub enum NetworkBehaviourAction<TInEvent, TOutEvent> {
         event: TInEvent,
     },
 
-    /// Informs the `Swarm` about a multi-address observed by a remote for
-    /// the local node.
+    /// Informs the `Swarm` about an address observed by a remote for
+    /// the local node by which the local node is supposedly publicly
+    /// reachable.
     ///
     /// It is advisable to issue `ReportObservedAddr` actions at a fixed frequency
     /// per node. This way address information will be more accurate over time
@@ -278,6 +280,10 @@ pub enum NetworkBehaviourAction<TInEvent, TOutEvent> {
     ReportObservedAddr {
         /// The observed address of the local node.
         address: Multiaddr,
+        /// The score to associate with this observation, i.e.
+        /// an indicator for the trusworthiness of this address
+        /// relative to other observed addresses.
+        score: AddressScore,
     },
 }
 
@@ -297,8 +303,8 @@ impl<TInEvent, TOutEvent> NetworkBehaviourAction<TInEvent, TOutEvent> {
                     handler,
                     event: f(event)
                 },
-            NetworkBehaviourAction::ReportObservedAddr { address } =>
-                NetworkBehaviourAction::ReportObservedAddr { address }
+            NetworkBehaviourAction::ReportObservedAddr { address, score } =>
+                NetworkBehaviourAction::ReportObservedAddr { address, score }
         }
     }
 
@@ -313,8 +319,8 @@ impl<TInEvent, TOutEvent> NetworkBehaviourAction<TInEvent, TOutEvent> {
                 NetworkBehaviourAction::DialPeer { peer_id, condition },
             NetworkBehaviourAction::NotifyHandler { peer_id, handler, event } =>
                 NetworkBehaviourAction::NotifyHandler { peer_id, handler, event },
-            NetworkBehaviourAction::ReportObservedAddr { address } =>
-                NetworkBehaviourAction::ReportObservedAddr { address }
+            NetworkBehaviourAction::ReportObservedAddr { address, score } =>
+                NetworkBehaviourAction::ReportObservedAddr { address, score }
         }
     }
 }

swarm/src/lib.rs

@@ -82,6 +82,7 @@ pub use protocols_handler::{
     OneShotHandlerConfig,
     SubstreamProtocol
 };
+pub use registry::{AddressScore, AddressRecord, AddAddressResult};
 
 use protocols_handler::{
     NodeHandlerWrapperBuilder,
@@ -397,34 +398,44 @@ where TBehaviour: NetworkBehaviour<ProtocolsHandler = THandler>,
         me.network.listen_addrs()
     }
 
-    /// Returns an iterator that produces the list of addresses that other nodes can use to reach
-    /// us.
-    pub fn external_addresses(me: &Self) -> impl Iterator<Item = &Multiaddr> {
-        me.external_addrs.iter()
-    }
-
     /// Returns the peer ID of the swarm passed as parameter.
     pub fn local_peer_id(me: &Self) -> &PeerId {
         &me.network.local_peer_id()
     }
 
-    /// Adds an external address.
-    ///
-    /// An external address is an address we are listening on but that accounts for things such as
-    /// NAT traversal.
-    pub fn add_external_address(me: &mut Self, addr: Multiaddr) {
-        me.external_addrs.add(addr)
+    /// Returns an iterator for [`AddressRecord`]s of external addresses
+    /// of the local node, in decreasing order of their current
+    /// [score](AddressScore).
+    pub fn external_addresses(me: &Self) -> impl Iterator<Item = &AddressRecord> {
+        me.external_addrs.iter()
     }
 
-    /// Returns the connection info for an arbitrary connection with the peer, or `None`
-    /// if there is no connection to that peer.
-    // TODO: should take &self instead of &mut self, but the API in network requires &mut
-    pub fn connection_info(me: &mut Self, peer_id: &PeerId) -> Option<PeerId> {
-        if let Some(mut n) = me.network.peer(peer_id.clone()).into_connected() {
-            Some(n.some_connection().info().clone())
-        } else {
-            None
-        }
+    /// Adds an external address record for the local node.
+    ///
+    /// An external address is an address of the local node known to
+    /// be (likely) reachable for other nodes, possibly taking into
+    /// account NAT. The external addresses of the local node may be
+    /// shared with other nodes by the `NetworkBehaviour`.
+    ///
+    /// The associated score determines both the position of the address
+    /// in the list of external addresses (which can determine the
+    /// order in which addresses are used to connect to) as well as
+    /// how long the address is retained in the list, depending on
+    /// how frequently it is reported by the `NetworkBehaviour` via
+    /// [`NetworkBehaviourAction::ReportObservedAddr`] or explicitly
+    /// through this method.
+    pub fn add_external_address(me: &mut Self, a: Multiaddr, s: AddressScore) -> AddAddressResult {
+        me.external_addrs.add(a, s)
+    }
+
+    /// Removes an external address of the local node, regardless of
+    /// its current score. See [`ExpandedSwarm::add_external_address`]
+    /// for details.
+    ///
+    /// Returns `true` if the address existed and was removed, `false`
+    /// otherwise.
+    pub fn remove_external_address(me: &mut Self, addr: &Multiaddr) -> bool {
+        me.external_addrs.remove(addr)
     }
 
     /// Bans a peer by its peer ID.
@@ -732,12 +743,12 @@ where TBehaviour: NetworkBehaviour<ProtocolsHandler = THandler>,
                         }
                     }
                 },
-                Poll::Ready(NetworkBehaviourAction::ReportObservedAddr { address }) => {
+                Poll::Ready(NetworkBehaviourAction::ReportObservedAddr { address, score }) => {
                     for addr in this.network.address_translation(&address) {
-                        if this.external_addrs.iter().all(|a| *a != addr) {
+                        if this.external_addrs.iter().all(|a| &a.addr != &addr) {
                             this.behaviour.inject_new_external_addr(&addr);
                         }
-                        this.external_addrs.add(addr);
+                        this.external_addrs.add(addr, score);
                     }
                 },
             }

swarm/src/registry.rs

@@ -20,29 +20,143 @@
 
 use libp2p_core::Multiaddr;
 use smallvec::SmallVec;
-use std::{collections::VecDeque, num::NonZeroUsize};
+use std::{collections::VecDeque, cmp::Ordering, num::NonZeroUsize};
+use std::ops::{Add, Sub};
 
-/// Hold a ranked collection of [`Multiaddr`] values.
+/// A ranked collection of [`Multiaddr`] values.
+///
+/// Every address has an associated [score](`AddressScore`) and iterating
+/// over the addresses will return them in order from highest to lowest score.
+///
+/// In addition to the currently held addresses and their score, the collection
+/// keeps track of a limited history of the most-recently added addresses.
+/// This history determines how address scores are reduced over time as old
+/// scores expire in the context of new addresses being added:
+///
+///   * An address's score is increased by a given amount whenever it is
+///     [(re-)added](Addresses::add) to the collection.
+///   * An address's score is decreased by the same amount used when it
+///     was added when the least-recently seen addition is (as per the
+///     limited history) for this address in the context of [`Addresses::add`].
+///   * If an address's score reaches 0 in the context of [`Addresses::add`],
+///     it is removed from the collection.
 ///
-/// Every address has an associated score and iterating over addresses will return them
-/// in order from highest to lowest. When reaching the limit, addresses with the lowest
-/// score will be dropped first.
 #[derive(Debug, Clone)]
 pub struct Addresses {
-    /// The ranked sequence of addresses.
-    registry: SmallVec<[Record; 8]>,
-    /// Number of historical reports. Similar to `reports.capacity()`.
+    /// The ranked sequence of addresses, from highest to lowest score.
+    ///
+    /// By design, the number of finitely scored addresses stored here is
+    /// never larger (but may be smaller) than the number of historic `reports`
+    /// at any time.
+    registry: SmallVec<[AddressRecord; 8]>,
+    /// The configured limit of the `reports` history of added addresses,
+    /// and thus also of the size of the `registry` w.r.t. finitely scored
+    /// addresses.
     limit: NonZeroUsize,
-    /// Queue of last reports. Every new report is added to the queue. If the queue reaches its
-    /// capacity, we also pop the first element.
-    reports: VecDeque<Multiaddr>,
+    /// The limited history of added addresses. If the queue reaches the `limit`,
+    /// the first record, i.e. the least-recently added, is removed in the
+    /// context of [`Addresses::add`] and the corresponding record in the
+    /// `registry` has its score reduced accordingly.
+    reports: VecDeque<Report>,
 }
 
-// An address record associates a score to a Multiaddr.
+/// An record in a prioritised list of addresses.
 #[derive(Clone, Debug, PartialEq, Eq)]
-struct Record {
+#[non_exhaustive]
+pub struct AddressRecord {
+    pub addr: Multiaddr,
+    pub score: AddressScore,
+}
+
+/// A report tracked for a finitely scored address.
+#[derive(Debug, Clone)]
+struct Report {
+    addr: Multiaddr,
     score: u32,
-    addr: Multiaddr
+}
+
+impl AddressRecord {
+    fn new(addr: Multiaddr, score: AddressScore) -> Self {
+        AddressRecord {
+            addr, score,
+        }
+    }
+}
+
+/// The "score" of an address w.r.t. an ordered collection of addresses.
+///
+/// A score is a measure of the trusworthyness of a particular
+/// observation of an address. The same address may be repeatedly
+/// reported with the same or differing scores.
+#[derive(PartialEq, Eq, Debug, Clone, Copy, Hash)]
+pub enum AddressScore {
+    /// The score is "infinite", i.e. an address with this score is never
+    /// purged from the associated address records and remains sorted at
+    /// the beginning (possibly with other `Infinite`ly scored addresses).
+    Infinite,
+    /// The score is finite, i.e. an address with this score has
+    /// its score increased and decreased as per the frequency of
+    /// reports (i.e. additions) of the same address relative to
+    /// the reports of other addresses.
+    Finite(u32),
+}
+
+impl AddressScore {
+    fn is_zero(&self) -> bool {
+        &AddressScore::Finite(0) == self
+    }
+}
+
+impl PartialOrd for AddressScore {
+    fn partial_cmp(&self, other: &AddressScore) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for AddressScore {
+    fn cmp(&self, other: &AddressScore) -> Ordering {
+        // Semantics of cardinal numbers with a single infinite cardinal.
+        match (self, other) {
+            (AddressScore::Infinite, AddressScore::Infinite) =>
+                Ordering::Equal,
+            (AddressScore::Infinite, AddressScore::Finite(_)) =>
+                Ordering::Greater,
+            (AddressScore::Finite(_), AddressScore::Infinite) =>
+                Ordering::Less,
+            (AddressScore::Finite(a), AddressScore::Finite(b)) =>
+                a.cmp(b),
+        }
+    }
+}
+
+impl Add for AddressScore {
+    type Output = AddressScore;
+
+    fn add(self, rhs: AddressScore) -> Self::Output {
+        // Semantics of cardinal numbers with a single infinite cardinal.
+        match (self, rhs) {
+            (AddressScore::Infinite, AddressScore::Infinite) =>
+                AddressScore::Infinite,
+            (AddressScore::Infinite, AddressScore::Finite(_)) =>
+                AddressScore::Infinite,
+            (AddressScore::Finite(_), AddressScore::Infinite) =>
+                AddressScore::Infinite,
+            (AddressScore::Finite(a), AddressScore::Finite(b)) =>
+                AddressScore::Finite(a.saturating_add(b))
+        }
+    }
+}
+
+impl Sub<u32> for AddressScore {
+    type Output = AddressScore;
+
+    fn sub(self, rhs: u32) -> Self::Output {
+        // Semantics of cardinal numbers with a single infinite cardinal.
+        match self {
+            AddressScore::Infinite => AddressScore::Infinite,
+            AddressScore::Finite(score) => AddressScore::Finite(score.saturating_sub(rhs))
+        }
+    }
 }
 
 impl Default for Addresses {
@@ -51,8 +165,16 @@ impl Default for Addresses {
     }
 }
 
+/// The result of adding an address to an ordered list of
+/// addresses with associated scores.
+pub enum AddAddressResult {
+    Inserted,
+    Updated,
+}
+
 impl Addresses {
-    /// Create a new address collection of bounded length.
+    /// Create a new ranked address collection with the given size limit
+    /// for [finitely scored](AddressScore::Finite) addresses.
     pub fn new(limit: NonZeroUsize) -> Self {
         Addresses {
             registry: SmallVec::new(),
@@ -63,40 +185,61 @@ impl Addresses {
 
     /// Add a [`Multiaddr`] to the collection.
     ///
-    /// Adding an existing address is interpreted as additional
-    /// confirmation and thus increases its score.
-    pub fn add(&mut self, a: Multiaddr) {
-
-        let oldest = if self.reports.len() == self.limit.get() {
-            self.reports.pop_front()
-        } else {
-            None
-        };
-
-        if let Some(oldest) = oldest {
-            if let Some(in_registry) = self.registry.iter_mut().find(|r| r.addr == oldest) {
-                in_registry.score = in_registry.score.saturating_sub(1);
+    /// If the given address already exists in the collection,
+    /// the given score is added to the current score of the address.
+    ///
+    /// If the collection has already observed the configured
+    /// number of address additions, the least-recently added address
+    /// as per this limited history has its score reduced by the amount
+    /// used in this prior report, with removal from the collection
+    /// occurring when the score drops to 0.
+    pub fn add(&mut self, addr: Multiaddr, score: AddressScore) -> AddAddressResult {
+        // If enough reports (i.e. address additions) occurred, reduce
+        // the score of the least-recently added address.
+        if self.reports.len() == self.limit.get() {
+            let old_report = self.reports.pop_front().expect("len = limit > 0");
+            // If the address is still in the collection, decrease its score.
+            if let Some(record) = self.registry.iter_mut().find(|r| r.addr == old_report.addr) {
+                record.score = record.score - old_report.score;
                 isort(&mut self.registry);
             }
         }
 
         // Remove addresses that have a score of 0.
-        while self.registry.last().map(|e| e.score == 0).unwrap_or(false) {
+        while self.registry.last().map(|e| e.score.is_zero()).unwrap_or(false) {
             self.registry.pop();
         }
 
-        self.reports.push_back(a.clone());
+        // If the address score is finite, remember this report.
+        if let AddressScore::Finite(score) = score {
+            self.reports.push_back(Report { addr: addr.clone(), score });
+        }
 
+        // If the address is already in the collection, increase its score.
         for r in &mut self.registry {
-            if r.addr == a {
-                r.score = r.score.saturating_add(1);
+            if r.addr == addr {
+                r.score = r.score + score;
                 isort(&mut self.registry);
-                return
+                return AddAddressResult::Updated
             }
         }
 
-        let r = Record { score: 1, addr: a };
-        self.registry.push(r)
+        // It is a new record.
+        self.registry.push(AddressRecord::new(addr, score));
+        AddAddressResult::Inserted
+    }
+
+    /// Explicitly remove an address from the collection.
+    ///
+    /// Returns `true` if the address existed in the collection
+    /// and was thus removed, false otherwise.
+    pub fn remove(&mut self, addr: &Multiaddr) -> bool {
+        if let Some(pos) = self.registry.iter().position(|r| &r.addr == addr) {
+            self.registry.remove(pos);
+            true
+        } else {
+            false
+        }
     }
 
     /// Return an iterator over all [`Multiaddr`] values.
@@ -117,12 +260,12 @@ impl Addresses {
 /// An iterator over [`Multiaddr`] values.
 #[derive(Clone)]
 pub struct AddressIter<'a> {
-    items: &'a [Record],
+    items: &'a [AddressRecord],
     offset: usize
 }
 
 impl<'a> Iterator for AddressIter<'a> {
-    type Item = &'a Multiaddr;
+    type Item = &'a AddressRecord;
 
     fn next(&mut self) -> Option<Self::Item> {
         if self.offset == self.items.len() {
@@ -130,7 +273,7 @@ impl<'a> Iterator for AddressIter<'a> {
         }
         let item = &self.items[self.offset];
         self.offset += 1;
-        Some(&item.addr)
+        Some(&item)
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
@@ -144,15 +287,15 @@ impl<'a> ExactSizeIterator for AddressIter<'a> {}
 /// An iterator over [`Multiaddr`] values.
 #[derive(Clone)]
 pub struct AddressIntoIter {
-    items: SmallVec<[Record; 8]>,
+    items: SmallVec<[AddressRecord; 8]>,
 }
 
 impl Iterator for AddressIntoIter {
-    type Item = Multiaddr;
+    type Item = AddressRecord;
 
     fn next(&mut self) -> Option<Self::Item> {
         if !self.items.is_empty() {
-            Some(self.items.remove(0).addr)
+            Some(self.items.remove(0))
         } else {
             None
         }
@@ -167,7 +310,7 @@ impl Iterator for AddressIntoIter {
 impl ExactSizeIterator for AddressIntoIter {}
 
 // Reverse insertion sort.
-fn isort(xs: &mut [Record]) {
+fn isort(xs: &mut [AddressRecord]) {
     for i in 1 .. xs.len() {
         for j in (1 ..= i).rev() {
             if xs[j].score <= xs[j - 1].score {
@@ -181,16 +324,34 @@ fn isort(xs: &mut [Record]) {
 #[cfg(test)]
 mod tests {
     use libp2p_core::multiaddr::{Multiaddr, Protocol};
-    use quickcheck::{Arbitrary, Gen, QuickCheck};
+    use quickcheck::*;
     use rand::Rng;
-    use std::num::NonZeroUsize;
-    use super::{isort, Addresses, Record};
+    use std::num::{NonZeroUsize, NonZeroU8};
+    use super::*;
+
+    impl Arbitrary for AddressScore {
+        fn arbitrary<G: Gen>(g: &mut G) -> AddressScore {
+            if g.gen_range(0, 10) == 0 { // ~10% "Infinitely" scored addresses
+                AddressScore::Infinite
+            } else {
+                AddressScore::Finite(g.gen())
+            }
+        }
+    }
+
+    impl Arbitrary for AddressRecord {
+        fn arbitrary<G: Gen>(g: &mut G) -> Self {
+            let addr = Protocol::Tcp(g.gen::<u16>() % 256).into();
+            let score = AddressScore::arbitrary(g);
+            AddressRecord::new(addr, score)
+        }
+    }
 
     #[test]
     fn isort_sorts() {
-        fn property(xs: Vec<u32>) -> bool {
+        fn property(xs: Vec<AddressScore>) {
             let mut xs = xs.into_iter()
-                .map(|s| Record { score: s, addr: Multiaddr::empty() })
+                .map(|score| AddressRecord::new(Multiaddr::empty(), score))
                 .collect::<Vec<_>>();
 
             isort(&mut xs);
@@ -198,61 +359,98 @@ mod tests {
             for i in 1 .. xs.len() {
                 assert!(xs[i - 1].score >= xs[i].score)
             }
-
-            true
         }
-        QuickCheck::new().quickcheck(property as fn(Vec<u32>) -> bool)
+
+        quickcheck(property as fn(_));
     }
 
     #[test]
-    fn old_reports_disappear() {
-        let mut addresses = Addresses::default();
+    fn score_retention() {
+        fn prop(first: AddressRecord, other: AddressRecord) -> TestResult {
+            if first.addr == other.addr {
+                return TestResult::discard()
+            }
 
-        // Add an address a single time.
-        let single: Multiaddr = "/tcp/2108".parse().unwrap();
-        addresses.add(single.clone());
-        assert!(addresses.iter().find(|a| **a == single).is_some());
+            let mut addresses = Addresses::default();
 
-        // Then fill `addresses` with random stuff.
-        let other: Multiaddr = "/tcp/120".parse().unwrap();
-        for _ in 0 .. 2000 {
-            addresses.add(other.clone());
+            // Add the first address.
+            addresses.add(first.addr.clone(), first.score);
+            assert!(addresses.iter().any(|a| &a.addr == &first.addr));
+
+            // Add another address so often that the initial report of
+            // the first address may be purged and, since it was the
+            // only report, the address removed.
+            for _ in 0 .. addresses.limit.get() + 1 {
+                addresses.add(other.addr.clone(), other.score);
+            }
+
+            let exists = addresses.iter().any(|a| &a.addr == &first.addr);
+
+            match (first.score, other.score) {
+                // Only finite scores push out other finite scores.
+                (AddressScore::Finite(_), AddressScore::Finite(_)) => assert!(!exists),
+                _ => assert!(exists),
+            }
+
+            TestResult::passed()
         }
 
-        // Check that `single` disappeared from the list.
-        assert!(addresses.iter().find(|a| **a == single).is_none());
+        quickcheck(prop as fn(_,_) -> _);
     }
 
     #[test]
-    fn record_score_equals_last_n_reports() {
-        #[derive(PartialEq, Eq, Clone, Hash, Debug)]
-        struct Ma(Multiaddr);
+    fn finitely_scored_address_limit() {
+        fn prop(reports: Vec<AddressRecord>, limit: NonZeroU8) {
+            let mut addresses = Addresses::new(limit.into());
 
-        impl Arbitrary for Ma {
-            fn arbitrary<G: Gen>(g: &mut G) -> Self {
-                Ma(Protocol::Tcp(g.gen::<u16>() % 16).into())
+            // Add all reports.
+            for r in reports {
+                addresses.add(r.addr, r.score);
             }
+
+            // Count the finitely scored addresses.
+            let num_finite = addresses.iter().filter(|r| match r {
+                AddressRecord { score: AddressScore::Finite(_), .. } => true,
+                _ => false,
+            }).count();
+
+            // Check against the limit.
+            assert!(num_finite <= limit.get() as usize);
         }
 
-        fn property(xs: Vec<Ma>, n: u8) -> bool {
-            let n = std::cmp::max(n, 1);
-            let mut addresses = Addresses::new(NonZeroUsize::new(usize::from(n)).unwrap());
-            for Ma(a) in &xs {
-                addresses.add(a.clone())
+        quickcheck(prop as fn(_,_));
+    }
+
+    #[test]
+    fn record_score_sum() {
+        fn prop(records: Vec<AddressRecord>) -> bool {
+            // Make sure the address collection can hold all reports.
+            let n = std::cmp::max(records.len(), 1);
+            let mut addresses = Addresses::new(NonZeroUsize::new(n).unwrap());
+
+            // Add all address reports to the collection.
+            for r in records.iter() {
+                addresses.add(r.addr.clone(), r.score.clone());
             }
+
+            // Check that each address in the registry has the expected score.
             for r in &addresses.registry {
-                let count = xs.iter()
-                    .rev()
-                    .take(usize::from(n))
-                    .filter(|Ma(x)| x == &r.addr)
-                    .count();
-                if r.score as usize != count {
+                let expected_score = records.iter().fold(
+                    None::<AddressScore>, |sum, rec|
+                        if &rec.addr == &r.addr {
+                            sum.map_or(Some(rec.score), |s| Some(s + rec.score))
+                        } else {
+                            sum
+                        });
+
+                if Some(r.score) != expected_score {
                     return false
                 }
             }
+
             true
         }
 
-        QuickCheck::new().quickcheck(property as fn(Vec<Ma>, u8) -> bool)
+        quickcheck(prop as fn(_) -> _)
     }
 }