fix(iroh): Keep track of non-best-addr path validation and switch to them if best addr is soon outdated

Author: matheus23

iroh/src/magicsock/node_map/best_addr.rs

@@ -6,7 +6,51 @@ use n0_future::time::{Duration, Instant};
 use tracing::{debug, info};
 
 /// How long we trust a UDP address as the exclusive path (without using relay) without having heard a Pong reply.
-const TRUST_UDP_ADDR_DURATION: Duration = Duration::from_millis(6500);
+pub(super) const TRUST_UDP_ADDR_DURATION: Duration = Duration::from_millis(6500);
+
+/// The grace period at which we consider switching away from our best addr
+/// to another address that we've received data on.
+///
+/// The trusted address lifecycle goes as follows:
+/// - A UDP DICSO pong is received, this validates that the path is for sure valid.
+/// - The disco path that seems to have the lowest latency is the path we use to send on.
+/// - We trust this path as a path to send on for at least TRUST_UDP_ADDR_DURATION.
+/// - This time is extended every time we receive a UDP DISCO pong on the address.
+/// - This time is *also* extended every time we receive *application* payloads on this
+///   address (i.e. QUIC datagrams).
+/// - If our best address becomes outdated (TRUST_UDP_ADDR_DURATION expires) without
+///   another pong or payload data, then we'll start sending over the relay, too!
+///   (we switch to ConnectionType::Mixed)
+///
+/// However, we might not get any UDP DISCO pongs because they're UDP packets and get
+/// lost under e.g. high load.
+///
+/// This is usually fine, because we also receive on the best addr, and that extends its
+/// "trust period" just as well.
+///
+/// However, if *additionally* we send on a different address than the one we receive on,
+/// then this extension doesn't happen.
+///
+/// To fix this, we also apply the same path validation logic to non-best addresses.
+/// I.e. we keep track of when they last received a pong, at which point we consider them
+/// validated, and then extend this validation period when we receive application data
+/// while they're valid (or when we receive another pong).
+///
+/// Now, when our best address becomes outdated, we need to switch to another valid path.
+///
+/// We could switch to another path once the best address becomes outdated, but then we'd
+/// already start sending on the relay for a couple of iterations!
+///
+/// So instead, we switch to another path when it looks like the best address becomes
+/// outdated.
+/// Not just any path, but the path that we're currently receiving from for this node.
+///
+/// Since we might not be receiving constantly from the remote side (e.g. if it's a
+/// one-sided transfer), we need to take care to do consider switching early enough.
+///
+/// So this duration is chosen as at least 1 keep alive interval (1s default in iroh atm)
+/// + at maximum 400ms of latency spike.
+const TRUST_UDP_ADDR_SOON_OUTDATED: Duration = Duration::from_millis(1400);
 
 #[derive(Debug, Default)]
 pub(super) struct BestAddr(Option<BestAddrInner>);
@@ -38,12 +82,12 @@ pub(super) enum Source {
 }
 
 impl Source {
-    fn trust_until(&self, from: Instant) -> Instant {
+    fn trust_duration(&self) -> Duration {
         match self {
-            Source::ReceivedPong => from + TRUST_UDP_ADDR_DURATION,
+            Source::ReceivedPong => TRUST_UDP_ADDR_DURATION,
             // TODO: Fix time
-            Source::BestCandidate => from + Duration::from_secs(60 * 60),
-            Source::Udp => from + TRUST_UDP_ADDR_DURATION,
+            Source::BestCandidate => Duration::from_secs(60 * 60),
+            Source::Udp => TRUST_UDP_ADDR_DURATION,
         }
     }
 }
@@ -115,30 +159,48 @@ impl BestAddr {
         latency: Duration,
         source: Source,
         confirmed_at: Instant,
+        now: Instant,
     ) {
         match self.0.as_mut() {
             None => {
                 self.insert(addr, latency, source, confirmed_at);
             }
             Some(state) => {
                 let candidate = AddrLatency { addr, latency };
-                if !state.is_trusted(confirmed_at) || candidate.is_better_than(&state.addr) {
+                if !state.is_trusted(now) || candidate.is_better_than(&state.addr) {
                     self.insert(addr, latency, source, confirmed_at);
                 } else if state.addr.addr == addr {
                     state.confirmed_at = confirmed_at;
-                    state.trust_until = Some(source.trust_until(confirmed_at));
+                    state.trust_until = Some(confirmed_at + source.trust_duration());
                 }
             }
         }
     }
 
-    /// Reset the expiry, if the passed in addr matches the currently used one.
-    #[cfg(not(wasm_browser))]
-    pub fn reconfirm_if_used(&mut self, addr: SocketAddr, source: Source, confirmed_at: Instant) {
-        if let Some(state) = self.0.as_mut() {
-            if state.addr.addr == addr {
-                state.confirmed_at = confirmed_at;
-                state.trust_until = Some(source.trust_until(confirmed_at));
+    pub fn insert_if_soon_outdated_or_reconfirm(
+        &mut self,
+        addr: SocketAddr,
+        latency: Duration,
+        source: Source,
+        confirmed_at: Instant,
+        now: Instant,
+    ) {
+        match self.0.as_mut() {
+            None => {
+                self.insert(addr, latency, source, confirmed_at);
+            }
+            Some(state) => {
+                // If the current best addr will soon be outdated
+                // and the given candidate will be trusted for longer
+                if !state.is_trusted(now + TRUST_UDP_ADDR_SOON_OUTDATED)
+                    && state.confirmed_at < confirmed_at
+                {
+                    println!("best addr will soon not be trusted, let's switch");
+                    self.insert(addr, latency, source, confirmed_at);
+                } else if state.addr.addr == addr {
+                    state.confirmed_at = confirmed_at;
+                    state.trust_until = Some(confirmed_at + source.trust_duration());
+                }
             }
         }
     }
@@ -150,7 +212,7 @@ impl BestAddr {
         source: Source,
         confirmed_at: Instant,
     ) {
-        let trust_until = source.trust_until(confirmed_at);
+        let trust_until = confirmed_at + source.trust_duration();
 
         if self
             .0

iroh/src/magicsock/node_map/node_state.rs

@@ -978,6 +978,7 @@ impl NodeState {
                         latency,
                         best_addr::Source::ReceivedPong,
                         now,
+                        now,
                     );
                 }
 
@@ -1064,18 +1065,26 @@ impl NodeState {
             debug_assert!(false, "node map inconsistency by_ip_port <-> direct addr");
             return;
         };
-        state.last_payload_msg = Some(now);
+        state.receive_payload(now);
         self.last_used = Some(now);
+        if let Some((latency, confirmed_at)) = state.valid_best_addr_candidate(now) {
         self.udp_paths
             .best_addr
-            .reconfirm_if_used(addr.into(), BestAddrSource::Udp, now);
+                .insert_if_soon_outdated_or_reconfirm(
+                    addr.into(),
+                    latency,
+                    BestAddrSource::Udp,
+                    confirmed_at,
+                    now,
+                );
+        }
     }
 
     pub(super) fn receive_relay(&mut self, url: &RelayUrl, src: NodeId, now: Instant) {
         match self.relay_url.as_mut() {
             Some((current_home, state)) if current_home == url => {
                 // We received on the expected url. update state.
-                state.last_payload_msg = Some(now);
+                state.receive_payload(now);
             }
             Some((_current_home, _state)) => {
                 // we have a different url. we only update on ping, not on receive_relay.

iroh/src/magicsock/node_map/path_state.rs

@@ -14,7 +14,10 @@ use super::{
     node_state::{ControlMsg, PongReply, SESSION_ACTIVE_TIMEOUT},
     IpPort, PingRole, Source,
 };
-use crate::{disco::SendAddr, magicsock::HEARTBEAT_INTERVAL};
+use crate::{
+    disco::SendAddr,
+    magicsock::{node_map::best_addr::TRUST_UDP_ADDR_DURATION, HEARTBEAT_INTERVAL},
+};
 
 /// The minimum time between pings to an endpoint.
 ///
@@ -53,6 +56,9 @@ pub(super) struct PathState {
     ///
     /// This excludes DISCO messages.
     pub(super) last_payload_msg: Option<Instant>,
+    /// Whether the last payload msg was within [`TRUST_UDP_ADDR_DURATION`] of either
+    /// the last trusted payload message or a recent pong.
+    pub(super) last_payload_trusted: bool,
     /// Sources is a map of [`Source`]s to [`Instant`]s, keeping track of all the ways we have
     /// learned about this path
     ///
@@ -73,6 +79,7 @@ impl PathState {
             call_me_maybe_time: None,
             recent_pong: None,
             last_payload_msg: None,
+            last_payload_trusted: false,
             sources,
         }
     }
@@ -100,6 +107,7 @@ impl PathState {
             call_me_maybe_time: None,
             recent_pong: None,
             last_payload_msg: Some(now),
+            last_payload_trusted: false,
             sources,
         }
     }
@@ -129,6 +137,26 @@ impl PathState {
             }
         }
         self.recent_pong = Some(r);
+
+        if let Some(last_payload_msg) = self.last_payload_msg {
+            self.last_payload_trusted = self.within_trusted_pong_duration(last_payload_msg);
+        }
+    }
+
+    fn within_trusted_pong_duration(&self, instant: Instant) -> bool {
+        if let Some(pong) = &self.recent_pong {
+            return pong.pong_at <= instant && instant < pong.pong_at + TRUST_UDP_ADDR_DURATION;
+        }
+
+        false
+    }
+
+    pub(super) fn receive_payload(&mut self, now: Instant) {
+        self.last_payload_msg = Some(now);
+
+        if self.within_trusted_pong_duration(now) {
+            self.last_payload_trusted = true;
+        }
     }
 
     #[cfg(test)]
@@ -141,6 +169,7 @@ impl PathState {
             call_me_maybe_time: None,
             recent_pong: Some(r),
             last_payload_msg: None,
+            last_payload_trusted: false,
             sources: HashMap::new(),
         }
     }
@@ -187,6 +216,21 @@ impl PathState {
             .copied()
     }
 
+    fn last_confirmed_at(&self) -> Option<Instant> {
+        let last_trusted_payload_msg = if self.last_payload_trusted {
+            self.last_payload_msg
+        } else {
+            None
+        };
+
+        self.recent_pong
+            .as_ref()
+            .map(|pong| pong.pong_at)
+            .into_iter()
+            .chain(last_trusted_payload_msg)
+            .max()
+    }
+
     /// The last control or DISCO message **about** this path.
     ///
     /// This is the most recent instant among:
@@ -222,6 +266,18 @@ impl PathState {
         self.recent_pong.as_ref().map(|p| p.latency)
     }
 
+    /// Returns the latency and confirmed_at time if this path would make for a valid best addr `now`
+    pub(crate) fn valid_best_addr_candidate(&self, now: Instant) -> Option<(Duration, Instant)> {
+        let latency = self.recent_pong.as_ref()?.latency;
+        let confirmed_at = self.last_confirmed_at()?;
+
+        if confirmed_at <= now && now < confirmed_at + TRUST_UDP_ADDR_DURATION {
+            return Some((latency, confirmed_at));
+        }
+
+        None
+    }
+
     pub(super) fn needs_ping(&self, now: &Instant) -> bool {
         match self.last_ping {
             None => true,

iroh/src/magicsock/node_map/udp_paths.rs

@@ -98,7 +98,7 @@ impl NodeUdpPaths {
     /// it should be `&self` and not `&mut self`.  This is only possible once the state from
     /// [`NodeUdpPaths`] is no longer modified from outside.
     pub(super) fn send_addr(&mut self, now: Instant, have_ipv6: bool) -> UdpSendAddr {
-        self.assign_best_addr_from_candidates_if_empty();
+        self.assign_best_addr_from_candidates_if_empty(now);
         match self.best_addr.state(now) {
             best_addr::State::Valid(addr) => UdpSendAddr::Valid(addr.addr),
             best_addr::State::Outdated(addr) => UdpSendAddr::Outdated(addr.addr),
@@ -139,7 +139,7 @@ impl NodeUdpPaths {
     /// If somehow we end up in a state where we failed to set a best_addr, while we do have
     /// valid candidates, this will chose a candidate and set best_addr again.  Most likely
     /// this is a bug elsewhere though.
-    fn assign_best_addr_from_candidates_if_empty(&mut self) {
+    fn assign_best_addr_from_candidates_if_empty(&mut self, now: Instant) {
         if !self.best_addr.is_empty() {
             return;
         }
@@ -173,6 +173,7 @@ impl NodeUdpPaths {
                     pong.latency,
                     best_addr::Source::BestCandidate,
                     pong.pong_at,
+                    now,
                 )
             }
         }