feat(server): implement echo core

AGENTS.md

@@ -57,7 +57,7 @@ Tokio must remain optional for `irtt-client`; default builds must remain runtime
 
 ### Server architecture (`irtt-server`)
 
-`irtt-server` is a first-class reusable server crate. It currently contains the deterministic OPEN/session core — packet admission, authentication policy, open negotiation, the bounded session table, open reply construction and client-initiated close. ECHO processing, server-initiated close and session lifetime remain unimplemented, as does the Tokio runtime; see `crates/irtt-server/AGENTS.md`. The rest of this section describes the boundary the crate is built to, including the parts not yet written.
+`irtt-server` is a first-class reusable server crate. It currently contains the deterministic OPEN/ECHO/session core — packet admission, authentication policy, open negotiation, the bounded session table, open reply construction, echo processing with its per-session receive state and timestamps, and client-initiated close. Rate limiting, session lifetime, server-initiated close, DSCP application and the server fill policy remain unimplemented, as does the Tokio runtime; see `crates/irtt-server/AGENTS.md`. The rest of this section describes the boundary the crate is built to, including the parts not yet written.
 
 - It shares wire semantics with the client through `irtt-proto` and follows the same clean-room boundary.
 - It must **not** depend on `irtt-client` merely to reuse protocol behavior.

crates/irtt-server/AGENTS.md

@@ -103,9 +103,43 @@ effective parameters must be safe for the server to run.
 - Zero is a valid maximum and is not a synonym for unlimited: it refuses every
   session, and no-test opens with it, since both are validated against the same
   effective session.
-- The echo slice must reuse this same configured maximum to admit inbound echo
-  request datagrams by their received length, before any receive state is
-  mutated.
+- Echo request admission reuses this same configured maximum, comparing it
+  against the received datagram length before any receive state is mutated.
+
+## Echo processing
+
+- An accepted echo owns per-session `ReceiveState` — count, window and the
+  previous accepted sequence number. Admission order is authentication, then
+  received datagram length against the configured maximum, then token, then
+  endpoint; a request failing any of them mutates nothing, in this session or
+  any other.
+- The count and window transitions follow the clean specification's Section 10
+  and the Section 1 vectors, including the parts that read as defects: a
+  duplicate advances the count and leaves the window alone, and a late,
+  reordered or far-gap request resets the window to `0x1` rather than setting a
+  historical bit, because the distance is unsigned. Do not replace this with a
+  conventional selective-acknowledgement bitmap. Reference the vectors rather
+  than restating the tables here.
+- Only the token and sequence number mean anything in a request. The tail is
+  opaque, and the request's length never sizes the reply — layout and length
+  come from the session's negotiated params through `irtt-proto` alone.
+- The transition is pure and is committed only after the reply has encoded, so
+  an internal encoding failure cannot leave a session claiming to have answered
+  a request it never did.
+- Clock sampling is a private injected seam, like token generation, not a
+  runtime abstraction: the receive instant is taken as soon as a datagram is
+  classified as an echo and the send instant just before the reply is built, so
+  they bracket the server's own handling. The monotonic origin belongs to the
+  clock source and is stable for its life.
+- For a single-clock midpoint, `irtt-rs` emits the one negotiated field. It does
+  not reproduce upstream 0.9.1's dual-field midpoint; `irtt-proto` still decodes
+  that form from a peer.
+- Reply payload bytes are zero, which is the initial safe fill policy: payload
+  carries no protocol meaning, and a server must never emit residue from another
+  request or client. Request payload bytes never reach a reply.
+- Rate limiting, idle expiry, maximum duration, server-initiated close, DSCP
+  socket application and the full `ServerFill` policy are later slices. Do not
+  add placeholder session fields for them.
 
 ## Resource policy
 
@@ -130,9 +164,10 @@ half-created session.
   maintain shadow encoders for well-formed wire data.
 - Hand-build packets only where the point is a payload a compliant encoder
   cannot produce — truncated varints, out-of-range enums, corrupted MACs.
-- Token generation is the only nondeterministic part; tests inject a scripted
-  source so identity, collisions and allocation failure are assertable. Keep
-  that seam private.
+- Token generation and clock sampling are the only nondeterministic parts; tests
+  inject a scripted source for each, so identity, collisions, allocation failure
+  and timestamp values are all assertable. Keep both seams private, and keep
+  timestamp tests free of sleeps, tolerances and real wall-clock assertions.
 - As the server gains behavior, prefer it over fake peers for normal compliant
   behavior in other crates' tests. Keep small raw/adversarial peers for
   intentionally malformed or non-compliant wire behavior.

crates/irtt-server/src/clock.rs

@@ -0,0 +1,189 @@
+//! Clock sampling for echo timestamps.
+//!
+//! An echo reply reports when the server received the request and when it sent
+//! the answer, so the core needs a clock. It takes one through this private
+//! seam rather than reading the system clock inline, so that timestamp behavior
+//! is deterministically testable without a runtime, sleeps or timing
+//! tolerances.
+//!
+//! This is the clock counterpart of [`TokenSource`](crate::token::TokenSource)
+//! and nothing more. It is **not** a runtime abstraction, not a transport
+//! boundary and not a pluggable product API: it is crate-private, the public
+//! constructor still takes only a [`ServerConfig`](crate::ServerConfig), and
+//! callers never pass timestamps into the core.
+
+use std::{
+    fmt,
+    time::{Duration, Instant, SystemTime, UNIX_EPOCH},
+};
+
+/// One instant, read from both clock domains together.
+///
+/// Both fields are signed nanoseconds, matching the wire encoding: `wall_ns`
+/// counts from the Unix epoch, and `mono_ns` from an origin the clock source
+/// owns. The specification lets a server pick any monotonic origin as long as
+/// it is stable for as long as any session it stamps may live, so `mono_ns` is
+/// deliberately not process uptime or anything else externally meaningful —
+/// only the difference between two samples from the same source has meaning.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct ClockSample {
+    pub(crate) wall_ns: i64,
+    pub(crate) mono_ns: i64,
+}
+
+impl ClockSample {
+    /// The arithmetic mean of two samples, taken separately per clock domain.
+    ///
+    /// This is what a midpoint timestamp is: the mean of one reply's receive
+    /// and send instants. The two domains never mix, and the mean goes through
+    /// `i128` because `a + b` can overflow `i64` while the mean itself never
+    /// can.
+    pub(crate) fn midpoint(self, other: Self) -> Self {
+        Self {
+            wall_ns: mean_ns(self.wall_ns, other.wall_ns),
+            mono_ns: mean_ns(self.mono_ns, other.mono_ns),
+        }
+    }
+}
+
+/// A source of clock samples.
+///
+/// Deliberately private, like [`TokenSource`](crate::token::TokenSource): it
+/// exists so tests can script the receive and send instants of a reply, not as
+/// an extension point. The production implementation is [`SystemClock`].
+///
+/// Sampling is infallible. A clock that could refuse would make an admitted
+/// echo fail for a reason that is neither the peer's fault nor recoverable, so
+/// the production source saturates rather than reporting an error.
+pub(crate) trait ClockSource: fmt::Debug + Send {
+    fn sample(&mut self) -> ClockSample;
+}
+
+/// Reads the wall clock from [`SystemTime`] and the monotonic clock from an
+/// [`Instant`] captured when the source was created.
+///
+/// That instant is the monotonic origin, so it is fixed for the life of the
+/// source and shared by every session the core using it holds — which is
+/// exactly the stability the specification asks for.
+#[derive(Debug)]
+pub(crate) struct SystemClock {
+    origin: Instant,
+}
+
+impl SystemClock {
+    pub(crate) fn new() -> Self {
+        Self {
+            origin: Instant::now(),
+        }
+    }
+}
+
+impl Default for SystemClock {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl ClockSource for SystemClock {
+    fn sample(&mut self) -> ClockSample {
+        ClockSample {
+            wall_ns: wall_ns(),
+            mono_ns: saturating_ns(self.origin.elapsed()),
+        }
+    }
+}
+
+/// Nanoseconds since the Unix epoch, negative before it.
+fn wall_ns() -> i64 {
+    match SystemTime::now().duration_since(UNIX_EPOCH) {
+        Ok(since_epoch) => saturating_ns(since_epoch),
+        Err(before_epoch) => saturating_ns(before_epoch.duration()).saturating_neg(),
+    }
+}
+
+/// A duration as nanoseconds, saturating rather than panicking.
+///
+/// [`Duration`] counts nanoseconds in `u128` and reaches far beyond `i64`,
+/// which the wire field is. A host clock set past the year 2262 is a local
+/// misconfiguration; it must not become a panic that a remote peer can reach by
+/// sending an echo request.
+fn saturating_ns(duration: Duration) -> i64 {
+    i64::try_from(duration.as_nanos()).unwrap_or(i64::MAX)
+}
+
+/// The mean of two nanosecond values, computed without overflowing.
+fn mean_ns(a: i64, b: i64) -> i64 {
+    let mean = (i128::from(a) + i128::from(b)) / 2;
+    // The mean of two `i64` values is always an `i64`, so this conversion
+    // cannot actually fail. Clamping rather than unwrapping keeps timestamp
+    // arithmetic free of any panic at all.
+    i64::try_from(mean).unwrap_or(if mean.is_negative() {
+        i64::MIN
+    } else {
+        i64::MAX
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Constructed values only: the production clock's real readings are not
+    /// assertable without a timing tolerance, and the behavioral echo tests use
+    /// a scripted source instead.
+    #[test]
+    fn a_midpoint_is_the_per_domain_mean_and_cannot_overflow() {
+        let receive = ClockSample {
+            wall_ns: 1_000,
+            mono_ns: 10_000,
+        };
+        let send = ClockSample {
+            wall_ns: 1_200,
+            mono_ns: 10_300,
+        };
+        assert_eq!(
+            receive.midpoint(send),
+            ClockSample {
+                wall_ns: 1_100,
+                mono_ns: 10_150,
+            }
+        );
+
+        // The sum overflows `i64`; the mean does not.
+        let high = ClockSample {
+            wall_ns: i64::MAX,
+            mono_ns: i64::MAX,
+        };
+        assert_eq!(
+            high.midpoint(high),
+            ClockSample {
+                wall_ns: i64::MAX,
+                mono_ns: i64::MAX,
+            }
+        );
+        let low = ClockSample {
+            wall_ns: i64::MIN,
+            mono_ns: i64::MIN,
+        };
+        assert_eq!(
+            low.midpoint(low),
+            ClockSample {
+                wall_ns: i64::MIN,
+                mono_ns: i64::MIN,
+            }
+        );
+        assert_eq!(
+            high.midpoint(low),
+            ClockSample {
+                wall_ns: 0,
+                mono_ns: 0,
+            }
+        );
+    }
+
+    #[test]
+    fn a_duration_beyond_the_wire_field_saturates_rather_than_panicking() {
+        assert_eq!(saturating_ns(Duration::from_nanos(1_500)), 1_500);
+        assert_eq!(saturating_ns(Duration::MAX), i64::MAX);
+    }
+}

crates/irtt-server/src/config.rs

@@ -71,8 +71,8 @@ impl ServerConfig {
     /// reduced to it during negotiation, and an open whose mandatory echo field
     /// block would not fit is silently refused, since the server could not emit
     /// a compliant reply for it. Authentication counts toward that field block.
-    /// The echo slice will additionally use this limit to admit inbound echo
-    /// request datagrams by their received length.
+    /// The same limit additionally admits inbound echo request datagrams by
+    /// their received length, before any receive state moves.
     ///
     /// The value is trusted local configuration, like
     /// [`with_max_sessions`](Self::with_max_sessions): an operator may

crates/irtt-server/src/core.rs

@@ -1,11 +1,13 @@
 use std::{collections::HashMap, net::SocketAddr};
 
 use irtt_proto::{
-    decode_request, echo_packet_len, encode_open_reply, verify_packet_hmac, Clock, DecodedParams,
-    DecodedRequestKind, OpenReply, Params, StampAt, FLAG_CLOSE, FLAG_OPEN, FLAG_REPLY,
+    decode_request, echo_packet_len, encode_echo_reply, encode_open_reply, verify_packet_hmac,
+    Clock, DecodedParams, DecodedRequestKind, EchoReply, OpenReply, Params, StampAt,
+    TimestampFields, FLAG_CLOSE, FLAG_OPEN, FLAG_REPLY,
 };
 
 use crate::{
+    clock::{ClockSample, ClockSource, SystemClock},
     config::ServerConfig,
     error::ServerError,
     negotiate::negotiate_params,
@@ -16,10 +18,12 @@ use crate::{
 /// The deterministic protocol and session engine of the server.
 ///
 /// `ServerCore` owns admission, negotiation, the session table and reply
-/// construction. It performs no I/O and keeps no clock: a caller hands it a
-/// source endpoint and the bytes of one received datagram, and gets back the
-/// bytes to send to that endpoint, or nothing. Everything it does is a pure
-/// function of its state and that input, except for drawing session tokens.
+/// construction. It performs no I/O: a caller hands it a source endpoint and
+/// the bytes of one received datagram, and gets back the bytes to send to that
+/// endpoint, or nothing. Everything it does is a pure function of its state and
+/// that input, except for drawing session tokens and sampling the clock an echo
+/// reply's timestamps come from — both of which are private injected seams, so
+/// the whole engine stays deterministically testable.
 ///
 /// The Tokio runtime slice will wrap it directly:
 ///
@@ -35,29 +39,45 @@ use crate::{
 ///
 /// # Scope
 ///
-/// This slice implements open handling, session creation and client-initiated
-/// close. Echo requests are structurally recognized and then deliberately
-/// ignored — no reply, no session mutation — until their own slice.
+/// This slice implements open handling, session creation, normal echo
+/// processing and client-initiated close. Rate limiting, session lifetime and
+/// expiry, server-initiated close, DSCP application and the server fill policy
+/// are separate slices, so every otherwise admissible echo is answered and no
+/// session ever ages out.
 #[derive(Debug)]
 pub struct ServerCore {
     config: ServerConfig,
     sessions: HashMap<u64, Session>,
     tokens: Box<dyn TokenSource>,
+    clock: Box<dyn ClockSource>,
 }
 
 impl ServerCore {
     /// Creates a server core drawing session tokens from the operating system's
-    /// random source.
+    /// random source and timestamps from the system clock.
     #[must_use]
     pub fn new(config: ServerConfig) -> Self {
         Self::with_token_source(config, Box::new(OsTokenSource))
     }
 
+    /// A core with a chosen token source and the production clock, which is
+    /// what tests that care about session identity but not about timestamp
+    /// values want.
     pub(crate) fn with_token_source(config: ServerConfig, tokens: Box<dyn TokenSource>) -> Self {
+        Self::with_sources(config, tokens, Box::new(SystemClock::new()))
+    }
+
+    /// A core with both nondeterministic sources chosen.
+    pub(crate) fn with_sources(
+        config: ServerConfig,
+        tokens: Box<dyn TokenSource>,
+        clock: Box<dyn ClockSource>,
+    ) -> Self {
         Self {
             config,
             sessions: HashMap::new(),
             tokens,
+            clock,
         }
     }
 
@@ -96,17 +116,19 @@ impl ServerCore {
     /// flag, disagrees with this server's authentication configuration, fails
     /// authentication, carries a malformed or out-of-range open parameter
     /// payload, negotiates to parameters the server could not execute, would
-    /// exceed the session bound, or is a request kind this slice does not
-    /// implement. None of that mutates any live session.
+    /// exceed the session bound, is longer than the configured maximum packet
+    /// length, or names no live session from the endpoint that opened it. None
+    /// of that mutates any live session.
     ///
     /// `Ok(None)` is not only a rejection, though. A close request that does
     /// release its session is answered with nothing as well, because protocol
     /// version 1 defines no acknowledgement for one.
     ///
-    /// Authentication is checked before open parameters are decoded. That
-    /// ordering is deliberate: a packet that fails authentication is discarded
-    /// without parsing untrusted parameter data, and no reply behavior may
-    /// reveal which stage rejected it.
+    /// Authentication is checked before open parameters are decoded and before
+    /// any session is looked up. That ordering is deliberate: a packet that
+    /// fails authentication is discarded without parsing untrusted parameter
+    /// data or consulting the session table, and no reply behavior may reveal
+    /// which stage rejected it.
     ///
     /// # Errors
     ///
@@ -122,6 +144,16 @@ impl ServerCore {
         let Ok(request) = decode_request(packet) else {
             return Ok(None);
         };
+        // An echo's arrival instant is sampled the moment the datagram is known
+        // to be one, so that everything the reply's receive timestamp should
+        // bracket — authentication, the session lookup, the receive-state
+        // transition and encoding — happens after it. A datagram rejected later
+        // has consumed one clock sample and nothing else; sampling is not
+        // protocol state, and it is a strictly better trade than looking a
+        // session up before authentication to discover whether it even wanted
+        // timestamps.
+        let kind = self.classify(&request.kind, packet.len());
+
         // The HMAC flag must agree with this server's configuration in both
         // directions of mismatch, and the MAC itself is verified separately.
         if request.hmac_present != self.config.hmac_key().is_some() {
@@ -133,22 +165,116 @@ impl ServerCore {
             }
         }
 
-        match request.kind {
-            DecodedRequestKind::Open { no_test, params } => self.handle_open(peer, no_test, params),
-            DecodedRequestKind::Close { token } => {
+        match kind {
+            ClassifiedKind::Open { no_test, params } => self.handle_open(peer, no_test, params),
+            ClassifiedKind::Close { token } => {
                 self.handle_close(peer, token);
                 Ok(None)
             }
-            // Not implemented in this slice. An echo must not advance receive
-            // state until the slice that implements it; emitting a placeholder
-            // reply now would be worse than silence. That slice must admit an
-            // echo by its received datagram length against
-            // `max_packet_length` before it touches any receive state; adding
-            // that branch now would only be dead code guarding nothing.
-            DecodedRequestKind::Echo { .. } => Ok(None),
+            ClassifiedKind::Echo(echo) => self.handle_echo(peer, echo),
         }
     }
 
+    /// Pairs a structurally decoded request with the clock sample an echo
+    /// needs, leaving every other kind untouched.
+    fn classify<'a>(
+        &mut self,
+        kind: &DecodedRequestKind<'a>,
+        datagram_len: usize,
+    ) -> ClassifiedKind<'a> {
+        match *kind {
+            DecodedRequestKind::Open { no_test, params } => {
+                ClassifiedKind::Open { no_test, params }
+            }
+            DecodedRequestKind::Close { token } => ClassifiedKind::Close { token },
+            // Everything after the sequence number is opaque, so the tail is
+            // read no further than its length.
+            DecodedRequestKind::Echo {
+                token, sequence, ..
+            } => ClassifiedKind::Echo(EchoRequest {
+                token,
+                sequence,
+                datagram_len,
+                received_at: self.clock.sample(),
+            }),
+        }
+    }
+
+    /// Answers an echo request that names one of this server's sessions.
+    ///
+    /// Admission runs before any state moves, and each step is a silent
+    /// discard: the datagram must not be longer than
+    /// [`max_packet_length`](ServerConfig::max_packet_length), its token must
+    /// name a live session, and it must have arrived from that session's own
+    /// endpoint. Authentication was already settled by the caller. A request
+    /// failing any of these leaves the receive state — of this session and of
+    /// every other — exactly where it was.
+    ///
+    /// Only the token and the sequence number mean anything. The request's own
+    /// length is judged against the configured maximum and then forgotten: it
+    /// does not have to equal the negotiated length, and it never sizes the
+    /// reply, which is laid out and sized from the session's negotiated
+    /// parameters alone.
+    ///
+    /// The next receive state is computed but not committed until the reply has
+    /// been encoded, so a reply this server failed to build cannot leave a
+    /// session claiming to have answered a request it never did.
+    fn handle_echo(
+        &mut self,
+        peer: SocketAddr,
+        request: EchoRequest,
+    ) -> Result<Option<Vec<u8>>, ServerError> {
+        // Resource policy, applied to the length actually received. This is
+        // `irtt-rs` policy rather than upstream-mechanism emulation: there is
+        // no interface-MTU effective length here, just the datagram the caller
+        // handed over. The comparison is strict — a request of exactly the
+        // maximum is served.
+        if request.datagram_len > self.config.max_packet_length() {
+            return Ok(None);
+        }
+        let Some(session) = self.sessions.get_mut(&request.token) else {
+            return Ok(None);
+        };
+        if !same_endpoint(session.peer(), peer) {
+            return Ok(None);
+        }
+
+        let next = session.receive_state().accepted(request.sequence);
+        // The departure side, sampled as late as the core can: after admission
+        // and the transition, immediately before the reply is built. There is
+        // no socket here, so this is the core's departure instant rather than a
+        // kernel transmission time; the runtime slice does not need to rewrite
+        // it.
+        let sent_at = self.clock.sample();
+        let params = session.params();
+        let reply = EchoReply {
+            // Reply only. Open must be clear or upstream clients abort, and
+            // Close belongs to the server-initiated close slice. The
+            // authentication flag and field are the encoder's, decided by the
+            // key it is given.
+            flags: FLAG_REPLY,
+            // Both are copied through unchanged: no token is allocated for an
+            // echo, and no sequence number is normalized or range-checked.
+            token: request.token,
+            sequence: request.sequence,
+            recv_count: params.received_stats.has_count().then_some(next.count),
+            recv_window: params.received_stats.has_window().then_some(next.window),
+            timestamps: timestamp_fields(params, request.received_at, sent_at),
+            // No payload bytes of our own: the encoder zero-fills the whole
+            // region between the field block and the negotiated length. That is
+            // the initial safe fill policy — payload bytes carry no protocol
+            // meaning, and zeroes cannot disclose residue from another request
+            // or another client. Request payload bytes never reach a reply.
+            payload: Vec::new(),
+        };
+        let packet = encode_echo_reply(&reply, params, self.config.hmac_key())
+            .map_err(|source| ServerError::ReplyEncoding { source })?;
+
+        // The last fallible step is behind us, so the transition can be made.
+        session.commit_receive_state(next);
+        Ok(Some(packet))
+    }
+
     fn handle_open(
         &mut self,
         peer: SocketAddr,
@@ -252,6 +378,74 @@ impl ServerCore {
     }
 }
 
+/// A structurally decoded request kind, with an echo's arrival instant already
+/// sampled.
+///
+/// This mirrors [`DecodedRequestKind`] rather than replacing it: the decoder
+/// classifies the packet, and this records what the core had to capture at that
+/// moment. Keeping the two apart is what lets the clock be read before
+/// authentication without any handler having to ask whether a sample exists.
+enum ClassifiedKind<'a> {
+    Open { no_test: bool, params: &'a [u8] },
+    Close { token: u64 },
+    Echo(EchoRequest),
+}
+
+/// The whole of what an echo request means to this server.
+///
+/// Everything after the sequence number is opaque, so nothing beyond these
+/// fields is carried forward. `datagram_len` is the length the datagram
+/// actually arrived with, which the configured maximum judges; it is not
+/// required to match the negotiated length, and it never sizes the reply.
+#[derive(Debug, Clone, Copy)]
+struct EchoRequest {
+    token: u64,
+    sequence: u32,
+    datagram_len: usize,
+    received_at: ClockSample,
+}
+
+/// Builds the timestamp fields a session negotiated, and only those.
+///
+/// [`StampAt`] selects which instants are reported and [`Clock`] selects which
+/// domains, exactly as the reply layout does, so a field this returns as `Some`
+/// is a field the encoder has a slot for. A midpoint is the per-domain mean of
+/// the two instants.
+///
+/// A single-clock midpoint therefore produces exactly one midpoint field. That
+/// is the conforming representation; upstream 0.9.1's habit of emitting both
+/// midpoint fields regardless of the negotiated clock is a defect this server
+/// deliberately does not reproduce. `irtt-proto` still *decodes* that form, so
+/// interoperating with a peer that sends it is unaffected.
+///
+/// [`Clock::Unspecified`] selects no domain and so produces no field. It cannot
+/// be reached with a non-none `StampAt` on an acknowledged session, because
+/// that combination is refused at open; nothing here needs to repair it.
+fn timestamp_fields(params: &Params, received: ClockSample, sent: ClockSample) -> TimestampFields {
+    let per_clock = |sample: ClockSample| {
+        (
+            params.clock.has_wall().then_some(sample.wall_ns),
+            params.clock.has_mono().then_some(sample.mono_ns),
+        )
+    };
+    let absent = (None, None);
+    let (recv, midpoint, send) = match params.stamp_at {
+        StampAt::None => (absent, absent, absent),
+        StampAt::Receive => (per_clock(received), absent, absent),
+        StampAt::Send => (absent, absent, per_clock(sent)),
+        StampAt::Both => (per_clock(received), absent, per_clock(sent)),
+        StampAt::Midpoint => (absent, per_clock(received.midpoint(sent)), absent),
+    };
+    TimestampFields {
+        recv_wall: recv.0,
+        recv_mono: recv.1,
+        midpoint_wall: midpoint.0,
+        midpoint_mono: midpoint.1,
+        send_wall: send.0,
+        send_mono: send.1,
+    }
+}
+
 /// Whether two source endpoints identify the same session endpoint.
 ///
 /// The address family, the IP address and the UDP source port are established
@@ -352,7 +546,7 @@ fn open_params_are_admissible(decoded: &DecodedParams) -> bool {
 /// open rather than acknowledged and then found unserviceable.
 ///
 /// The size comes from [`echo_packet_len`], the one packet-size authority, so
-/// this cannot drift from what the echo slice will actually encode. Its
+/// this cannot drift from what an echo reply actually encodes. Its
 /// unrepresentable-length error is a rejection here, not a [`ServerError`]: a
 /// pathological requested length is remote input, not an internal failure. The
 /// default policy makes it unreachable for an acknowledged session anyway, since

crates/irtt-server/src/lib.rs

@@ -7,7 +7,9 @@
 //!
 //! [`ServerCore`] is the deterministic protocol and session engine: packet
 //! admission, authentication policy, open negotiation, the session table,
-//! resource decisions and reply construction. It performs no I/O. Socket and
+//! echo receive state, resource decisions and reply construction. It performs
+//! no I/O; the clock its echo timestamps come from is a private injected seam,
+//! not a runtime abstraction, so the engine stays testable. Socket and
 //! runtime orchestration — the UDP listener, address handling, timers, expiry
 //! and shutdown — will live around it and is intentionally Tokio-native. There
 //! is no blocking or alternate-runtime counterpart and no transport
@@ -33,12 +35,14 @@
 //!
 //! # Current scope
 //!
-//! Open handling, session creation and client-initiated close. Echo
-//! processing, server-initiated close, session lifetime and expiry, and the
-//! Tokio runtime are separate slices; a structurally valid echo request is
-//! currently accepted by the admission boundary and then ignored.
+//! Open handling, session creation, normal echo processing and
+//! client-initiated close. Rate limiting, session lifetime and expiry,
+//! server-initiated close, DSCP application, the server fill policy and the
+//! Tokio runtime are separate slices: every otherwise admissible echo is
+//! answered, and a session lives until its client closes it.
 #![forbid(unsafe_code)]
 
+mod clock;
 mod config;
 mod core;
 mod error;

crates/irtt-server/src/session.rs

@@ -4,25 +4,29 @@ use irtt_proto::Params;
 
 /// A live session created by an accepted open request.
 ///
-/// This slice records only what identifies the session and what was negotiated
-/// for it. Echo receive state (counts, windows, sequence tracking) and lifetime
-/// state (activity, deadlines, expiry) belong to the slices that implement
-/// those behaviors, and are deliberately absent rather than present as
-/// placeholders.
+/// A session records what identifies it, what was negotiated for it, and what
+/// its accepted echo requests have added up to. Lifetime state (activity,
+/// deadlines, expiry) and rate state belong to the slices that implement those
+/// behaviors, and are deliberately absent rather than present as placeholders.
 ///
-/// Deliberately crate-private. The next slices are all but certain to add and
-/// restructure fields here, and nothing outside the crate needs a session yet;
-/// publishing internal session state before ECHO even exists would buy nothing
-/// and commit us to a shape we are about to change.
+/// Deliberately crate-private. The remaining slices are all but certain to add
+/// and restructure fields here, and nothing outside the crate needs a session;
+/// publishing internal session state would buy nothing and commit us to a shape
+/// we are about to change.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct Session {
     peer: SocketAddr,
     params: Params,
+    receive: ReceiveState,
 }
 
 impl Session {
     pub(crate) fn new(peer: SocketAddr, params: Params) -> Self {
-        Self { peer, params }
+        Self {
+            peer,
+            params,
+            receive: ReceiveState::default(),
+        }
     }
 
     /// The exact source endpoint the open request arrived from.
@@ -30,10 +34,10 @@ impl Session {
     /// A session is bound to this endpoint, not merely to its address: the
     /// address family, the address and the UDP source port all form part of its
     /// identity, as — by `irtt-rs` policy rather than observed behavior — does
-    /// the IPv6 scope identifier. Close processing requires both a token match
-    /// and an endpoint match before removing a session, and echo processing
-    /// will require the same; `same_endpoint` in `core` is where the comparison
-    /// and its reasoning live.
+    /// the IPv6 scope identifier. Close and echo processing both require a
+    /// token match *and* an endpoint match before touching the session;
+    /// `same_endpoint` in `core` is where the comparison and its reasoning
+    /// live.
     ///
     /// The address is stored exactly as received, including the flow label that
     /// identity ignores, so a reply goes back to the endpoint the request
@@ -42,13 +46,95 @@ impl Session {
         self.peer
     }
 
-    /// The negotiated parameters the server returned for this session and will
-    /// enforce for it.
-    //
-    // Also awaiting its production consumer: echo replies are built from the
-    // negotiated params.
-    #[allow(dead_code)]
+    /// The negotiated parameters the server returned for this session and
+    /// enforces for it.
+    ///
+    /// Echo replies are laid out and sized from these, never from the request
+    /// that provoked them.
     pub(crate) fn params(&self) -> &Params {
         &self.params
     }
+
+    /// What this session's accepted echo requests have added up to so far.
+    pub(crate) fn receive_state(&self) -> ReceiveState {
+        self.receive
+    }
+
+    /// Adopts the state computed for an accepted echo request.
+    ///
+    /// Kept separate from [`ReceiveState::accepted`] on purpose. The transition
+    /// is pure and this is the irreversible step, so echo handling runs every
+    /// fallible part — building and encoding the reply — before calling it, and
+    /// a reply that fails to encode leaves the session exactly as it was.
+    pub(crate) fn commit_receive_state(&mut self, next: ReceiveState) {
+        self.receive = next;
+    }
+}
+
+/// What a session's accepted echo requests have added up to.
+///
+/// The semantics are those of the clean server specification, Section 10, and
+/// the input → output vectors in `test-vectors/SERVER_BEHAVIORAL_VECTORS.md`
+/// Section 1. Both values are per session, start at zero, and move only for a
+/// request that reaches the reply stage: a datagram dropped for its size, its
+/// authentication, an unknown token or a foreign endpoint leaves them alone.
+///
+/// The state is maintained whatever the session negotiated. Which of the two
+/// values actually appears in a reply is a separate question, answered by the
+/// negotiated `ReceivedStats` when the reply is built.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub(crate) struct ReceiveState {
+    /// Accepted echo requests, including the one currently being answered.
+    pub(crate) count: u32,
+    /// The received window: bit 0 is the request being answered and bit *k* is
+    /// the request with sequence number (current − *k*), for 1 ≤ *k* ≤ 63.
+    pub(crate) window: u64,
+    /// The sequence number the previous accepted request carried, which the
+    /// next transition is measured against. `None` before the session's first.
+    last_sequence: Option<u32>,
+}
+
+impl ReceiveState {
+    /// The state after accepting a request carrying `sequence`.
+    ///
+    /// Pure, so the caller can compute the next state, build and encode a reply
+    /// from it, and only then commit it.
+    ///
+    /// The window transition is the observed one, and it is **not** a
+    /// selective-acknowledgement bitmap: history is shifted along by the
+    /// unsigned distance from the previous accepted sequence number, and a
+    /// distance it cannot represent throws all of it away. The consequence is
+    /// the distinctive part — because the distance is unsigned, a *late or
+    /// reordered* request produces a very large one, so it resets the window to
+    /// `0x1` instead of setting an old bit, and the history it discards is
+    /// never recovered. That late sequence number then becomes the reference
+    /// for the next transition like any other.
+    pub(crate) fn accepted(self, sequence: u32) -> Self {
+        let window = match self.last_sequence {
+            // The first accepted request of a session reports only itself,
+            // whatever its sequence number.
+            None => 0x1,
+            Some(last) => match sequence.wrapping_sub(last) {
+                // A duplicate of the most recent sequence number leaves the
+                // window unchanged, since bit 0 is already set for it.
+                0 => self.window | 1,
+                // In order, or a gap the window can still represent: shift the
+                // history along by the distance and record this request.
+                distance @ 1..=63 => (self.window << distance) | 1,
+                // A gap of 64 or more discards every earlier bit — and so does
+                // any late or reordered request, which lands here because the
+                // unsigned distance wrapped.
+                _ => 0x1,
+            },
+        };
+        Self {
+            // The wire field is 32 bits. No clean evidence reaches 2^32
+            // requests in one session, so the wrap is not verified behavior: it
+            // is simply what maintaining a fixed-width counter means, and it is
+            // chosen over a debug-build overflow panic a peer could drive.
+            count: self.count.wrapping_add(1),
+            window,
+            last_sequence: Some(sequence),
+        }
+    }
 }

crates/irtt-server/src/tests/admission.rs

@@ -0,0 +1,42 @@
+//! The structural admission boundary.
+//!
+//! A datagram that never decodes as a request is discarded before any request
+//! kind is chosen, so it can reach neither the session table nor an echo's
+//! receive state.
+
+use super::support::{client_params, core_for, open_request, open_session, peer, ScriptedTokens};
+
+const TOKEN_A: u64 = 0x0102_0304_0506_0708;
+
+#[test]
+fn structurally_invalid_datagrams_are_discarded() {
+    let mut core = core_for(None, ScriptedTokens::new([TOKEN_A]));
+    open_session(&mut core, peer(), None);
+    let session = core.session(TOKEN_A).cloned().expect("session must exist");
+
+    let mut bad_magic = open_request(&client_params(), None);
+    bad_magic[0] ^= 0x01;
+
+    let mut reserved_flag = open_request(&client_params(), None);
+    reserved_flag[3] |= 0x10;
+
+    let mut reply_flag = open_request(&client_params(), None);
+    reply_flag[3] |= irtt_proto::FLAG_REPLY;
+
+    for (name, packet) in [
+        ("empty datagram", Vec::new()),
+        ("datagram shorter than the header", vec![0x14, 0xa7, 0x5b]),
+        ("bad magic", bad_magic),
+        ("reserved flag bit", reserved_flag),
+        ("reply flag", reply_flag),
+    ] {
+        assert_eq!(
+            core.handle_datagram(peer(), &packet).unwrap(),
+            None,
+            "{name} must be discarded"
+        );
+    }
+
+    assert_eq!(core.session_count(), 1);
+    assert_eq!(core.session(TOKEN_A), Some(&session));
+}

crates/irtt-server/src/tests/close.rs

@@ -63,8 +63,8 @@ fn a_repeated_close_is_a_stale_token_and_changes_nothing() {
 
 #[test]
 fn an_echo_after_a_close_is_a_stale_token() {
-    // Echo processing does not exist yet; the point here is that the token is
-    // gone, so the request has no session to reach even once it does.
+    // The same request would have been answered a moment earlier: releasing the
+    // session takes its token with it, so there is nothing left to reach.
     for hmac_key in [None, Some(KEY)] {
         let mut core = core_for(hmac_key, ScriptedTokens::new([TOKEN_A]));
         let token = open_session(&mut core, peer(), hmac_key);

crates/irtt-server/src/tests/echo.rs

@@ -0,0 +1,670 @@
+//! Normal echo processing.
+//!
+//! An echo request that authenticates, fits the configured maximum, names a
+//! live session and arrives from that session's endpoint is answered from the
+//! session's negotiated parameters. Everything else is silence that leaves the
+//! receive state alone, which these tests prove from the wire — the count and
+//! window in the *next* accepted reply are what show that a rejected datagram
+//! changed nothing.
+//!
+//! The count and window vectors come from the clean
+//! `test-vectors/SERVER_BEHAVIORAL_VECTORS.md` Section 1 and are hard-coded
+//! here rather than recomputed, so that replacing the observed semantics with a
+//! conventional selective-acknowledgement bitmap fails these tests.
+
+use std::net::{SocketAddr, SocketAddrV6};
+
+use irtt_proto::{
+    echo_packet_len, verify_packet_hmac, Clock, Params, ReceivedStats, StampAt, TimestampFields,
+};
+
+use super::support::{
+    core_for, core_with_sources, core_with_tokens, echo_params, echo_request, expect_echo_reply,
+    open_negotiated, other_peer, peer, sample, ScriptedClock, ScriptedTokens, KEY, OTHER_KEY,
+};
+use crate::ServerConfig;
+
+const TOKEN_A: u64 = 0x0102_0304_0506_0708;
+const UNKNOWN_TOKEN: u64 = 0x2122_2324_2526_2728;
+
+/// Params selecting no optional field at all, so the echo layout is the
+/// mandatory field block and nothing else.
+fn bare_params() -> Params {
+    echo_params(
+        ReceivedStats::None,
+        StampAt::None,
+        Clock::Unspecified,
+        /* length */ 0,
+    )
+}
+
+/// Params reporting both statistics, which is what most vectors are stated in.
+fn counting_params(length: i64) -> Params {
+    echo_params(
+        ReceivedStats::Both,
+        StampAt::None,
+        Clock::Unspecified,
+        length,
+    )
+}
+
+/// Opens one session on a default core and returns the core, the token and the
+/// negotiated params.
+fn session_with(params: &Params) -> (crate::ServerCore, u64, Params) {
+    let mut core = core_with_tokens(ServerConfig::default(), ScriptedTokens::new([TOKEN_A]));
+    let (token, negotiated) = open_negotiated(&mut core, peer(), params, None);
+    (core, token, negotiated)
+}
+
+#[test]
+fn a_valid_echo_is_answered_from_the_negotiated_params() {
+    let (mut core, token, negotiated) = session_with(&bare_params());
+
+    let packet = core
+        .handle_datagram(peer(), &echo_request(token, 42, &negotiated, &[], None))
+        .unwrap()
+        .expect("an admissible echo must be answered");
+
+    let reply = expect_echo_reply(&packet, &negotiated, None);
+    assert_eq!(reply.token, token, "the session token is copied through");
+    assert_eq!(
+        reply.sequence, 42,
+        "the sequence number is copied unchanged"
+    );
+    assert_eq!(reply.recv_count, None, "no statistics were negotiated");
+    assert_eq!(reply.recv_window, None);
+    assert_eq!(
+        reply.timestamps,
+        TimestampFields::default(),
+        "no timestamps were negotiated, so none are emitted"
+    );
+    assert!(reply.payload.is_empty());
+    assert_eq!(packet.len(), echo_packet_len(false, &negotiated).unwrap());
+    assert_eq!(packet.len(), 16, "magic, flags, token and sequence only");
+
+    assert_eq!(
+        core.session_count(),
+        1,
+        "an echo neither creates nor releases a session"
+    );
+}
+
+#[test]
+fn a_reply_is_sized_by_the_session_not_by_the_request() {
+    // A request may be shorter or longer than the negotiated length, and its
+    // bytes past the sequence number are opaque either way.
+    let (mut core, token, negotiated) = session_with(&counting_params(64));
+    assert_eq!(negotiated.length, 64);
+
+    let short = Params {
+        length: 0,
+        ..negotiated.clone()
+    };
+    // Requesting no statistics moves this request's payload region up to where
+    // the *session's* reply keeps its count and window, so the opaque bytes
+    // land squarely on fields a server must not read from a request.
+    let long = Params {
+        length: 100,
+        received_stats: ReceivedStats::None,
+        ..negotiated.clone()
+    };
+
+    for (sequence, name, sizing, payload, expected) in [
+        (
+            0,
+            "a request shorter than the negotiated length",
+            &short,
+            &[][..],
+            (1, 0x1),
+        ),
+        (
+            1,
+            "a request longer than the negotiated length, with opaque bytes",
+            &long,
+            &[0xa5; 40][..],
+            (2, 0x3),
+        ),
+    ] {
+        let request = echo_request(token, sequence, sizing, payload, None);
+        assert_ne!(request.len(), 64, "{name} must not be the reply's length");
+
+        let packet = core
+            .handle_datagram(peer(), &request)
+            .unwrap()
+            .unwrap_or_else(|| panic!("{name} must be answered"));
+        let reply = expect_echo_reply(&packet, &negotiated, None);
+
+        assert_eq!(
+            packet.len(),
+            64,
+            "{name}: the reply is the negotiated length"
+        );
+        assert_eq!(reply.sequence, sequence);
+        assert_eq!(
+            (reply.recv_count, reply.recv_window),
+            (Some(expected.0), Some(expected.1)),
+            "{name}: the opaque tail must not influence the reply"
+        );
+    }
+}
+
+#[test]
+fn an_echo_over_the_configured_maximum_is_dropped_before_anything_advances() {
+    let config = ServerConfig::default().with_max_packet_length(64);
+    let mut core = core_with_tokens(config, ScriptedTokens::new([TOKEN_A]));
+    let (token, negotiated) = open_negotiated(&mut core, peer(), &counting_params(64), None);
+    assert_eq!(negotiated.length, 64);
+
+    let at_maximum = echo_request(token, 0, &negotiated, &[], None);
+    assert_eq!(at_maximum.len(), 64);
+    let over_maximum = echo_request(
+        token,
+        1,
+        &Params {
+            length: 65,
+            ..negotiated.clone()
+        },
+        &[],
+        None,
+    );
+    assert_eq!(over_maximum.len(), 65);
+
+    // The comparison is strict, so exactly the maximum is served.
+    let packet = core
+        .handle_datagram(peer(), &at_maximum)
+        .unwrap()
+        .expect("a request of exactly the maximum must be answered");
+    let reply = expect_echo_reply(&packet, &negotiated, None);
+    assert_eq!((reply.recv_count, reply.recv_window), (Some(1), Some(0x1)));
+
+    assert_eq!(
+        core.handle_datagram(peer(), &over_maximum).unwrap(),
+        None,
+        "one byte over the maximum must be answered with silence"
+    );
+
+    // The next admissible request is what proves the drop advanced nothing: had
+    // the oversized one counted, this would report 3 rather than 2.
+    let packet = core
+        .handle_datagram(peer(), &echo_request(token, 1, &negotiated, &[], None))
+        .unwrap()
+        .expect("an admissible echo after a dropped one must be answered");
+    let reply = expect_echo_reply(&packet, &negotiated, None);
+    assert_eq!((reply.recv_count, reply.recv_window), (Some(2), Some(0x3)));
+}
+
+#[test]
+fn an_echo_that_owns_no_session_here_is_answered_with_silence() {
+    let (mut core, token, negotiated) = session_with(&counting_params(0));
+
+    for (name, endpoint, request_token) in [
+        ("an unknown token", peer(), UNKNOWN_TOKEN),
+        ("a zero token", peer(), 0),
+        (
+            "a valid token from another source port",
+            other_peer(),
+            token,
+        ),
+    ] {
+        let request = echo_request(request_token, 0, &negotiated, &[], None);
+        assert_eq!(
+            core.handle_datagram(endpoint, &request).unwrap(),
+            None,
+            "{name} must be answered with silence"
+        );
+    }
+
+    // Count 1 and window 0x1 are the first-request values, so none of the three
+    // rejected datagrams touched this session's receive state.
+    let packet = core
+        .handle_datagram(peer(), &echo_request(token, 0, &negotiated, &[], None))
+        .unwrap()
+        .expect("the session's own echo must still be answered");
+    let reply = expect_echo_reply(&packet, &negotiated, None);
+    assert_eq!((reply.recv_count, reply.recv_window), (Some(1), Some(0x1)));
+    assert_eq!(core.session_count(), 1);
+}
+
+#[test]
+fn an_echo_resolves_ipv6_endpoints_by_the_same_identity_rule_as_a_close() {
+    // The comparator itself is pinned by the close tests; this is the echo path
+    // reaching the same one rather than growing a second rule.
+    let global = "2001:db8::1".parse().unwrap();
+    let link_local = "fe80::1".parse().unwrap();
+    let unlabeled = SocketAddr::V6(SocketAddrV6::new(global, 2112, 0, 0));
+    let labeled = SocketAddr::V6(SocketAddrV6::new(global, 2112, 0x000f_1234, 0));
+    let zone_three = SocketAddr::V6(SocketAddrV6::new(link_local, 2112, 0, 3));
+    let zone_four = SocketAddr::V6(SocketAddrV6::new(link_local, 2112, 0, 4));
+
+    for (bound, sender, answered) in [
+        // Flow information is routing metadata that identifies no endpoint.
+        (unlabeled, labeled, true),
+        (labeled, unlabeled, true),
+        // The zone is identity by `irtt-rs` policy, not by observed behavior.
+        (zone_three, zone_four, false),
+    ] {
+        let mut core = core_with_tokens(ServerConfig::default(), ScriptedTokens::new([TOKEN_A]));
+        let (token, negotiated) = open_negotiated(&mut core, bound, &counting_params(0), None);
+
+        let reply = core
+            .handle_datagram(sender, &echo_request(token, 0, &negotiated, &[], None))
+            .unwrap();
+
+        assert_eq!(
+            reply.is_some(),
+            answered,
+            "an echo from {sender} on a session opened from {bound}"
+        );
+    }
+}
+
+#[test]
+fn received_count_and_window_follow_the_observed_vectors() {
+    // Every row is `(request sequence, reply count, reply window)` exactly as
+    // measured in Section 1 of the behavioral vectors.
+    for (name, vectors) in [
+        (
+            "1.1 sequential, no loss",
+            &[
+                (0, 1, 0x1),
+                (1, 2, 0x3),
+                (2, 3, 0x7),
+                (3, 4, 0xf),
+                (4, 5, 0x1f),
+                (5, 6, 0x3f),
+            ][..],
+        ),
+        (
+            "1.2 one gap",
+            &[(0, 1, 0x1), (1, 2, 0x3), (3, 3, 0xd), (4, 4, 0x1b)][..],
+        ),
+        (
+            "1.3 multiple gaps",
+            &[(0, 1, 0x1), (3, 2, 0x9), (7, 3, 0x91), (8, 4, 0x123)][..],
+        ),
+        (
+            // Filling a gap late discards the history rather than setting the
+            // bit the gap left clear.
+            "1.4 later gap fill",
+            &[(0, 1, 0x1), (1, 2, 0x3), (5, 3, 0x31), (3, 4, 0x1)][..],
+        ),
+        (
+            "1.5 duplicate sequence number",
+            &[(0, 1, 0x1), (1, 2, 0x3), (1, 3, 0x3), (2, 4, 0x7)][..],
+        ),
+        (
+            // Row 5 rebuilds from the reset: bit 1 stays clear even though
+            // sequence 2 was received before the reordering.
+            "1.6 reordering",
+            &[
+                (0, 1, 0x1),
+                (1, 2, 0x3),
+                (2, 3, 0x7),
+                (1, 4, 0x1),
+                (3, 5, 0x5),
+            ][..],
+        ),
+        (
+            "1.7 strictly descending sequence numbers",
+            &[
+                (5, 1, 0x1),
+                (4, 2, 0x1),
+                (3, 3, 0x1),
+                (2, 4, 0x1),
+                (1, 5, 0x1),
+                (0, 6, 0x1),
+            ][..],
+        ),
+        (
+            "1.8 a gap of exactly 63",
+            &[(0, 1, 0x1), (63, 2, 0x8000_0000_0000_0001)][..],
+        ),
+        ("1.8 a gap of 64", &[(0, 1, 0x1), (64, 2, 0x1)][..]),
+        ("1.8 a gap of 65", &[(0, 1, 0x1), (65, 2, 0x1)][..]),
+        ("1.8 a gap of 1000", &[(0, 1, 0x1), (1000, 2, 0x1)][..]),
+        (
+            "1.9 a non-zero starting sequence number",
+            &[(100, 1, 0x1), (101, 2, 0x3), (102, 3, 0x7)][..],
+        ),
+        (
+            "1.10 32-bit sequence wraparound",
+            &[
+                (0xffff_fffd, 1, 0x1),
+                (0xffff_fffe, 2, 0x3),
+                (0xffff_ffff, 3, 0x7),
+                (0, 4, 0xf),
+                (1, 5, 0x1f),
+            ][..],
+        ),
+    ] {
+        let (mut core, token, negotiated) = session_with(&counting_params(0));
+
+        for &(sequence, count, window) in vectors {
+            let packet = core
+                .handle_datagram(
+                    peer(),
+                    &echo_request(token, sequence, &negotiated, &[], None),
+                )
+                .unwrap()
+                .unwrap_or_else(|| panic!("{name}: sequence {sequence} must be answered"));
+            let reply = expect_echo_reply(&packet, &negotiated, None);
+
+            assert_eq!(
+                reply.recv_count,
+                Some(count),
+                "{name}: count after sequence {sequence}"
+            );
+            assert_eq!(
+                reply.recv_window,
+                Some(window),
+                "{name}: window after sequence {sequence}"
+            );
+        }
+    }
+}
+
+#[test]
+fn only_the_negotiated_received_statistics_are_reported() {
+    // Field presence only; the transition itself is pinned by the vectors
+    // above, so two in-order requests are enough here.
+    for (stats, count, window) in [
+        (ReceivedStats::None, None, None),
+        (ReceivedStats::Count, Some(2), None),
+        (ReceivedStats::Window, None, Some(0x3)),
+        (ReceivedStats::Both, Some(2), Some(0x3)),
+    ] {
+        let params = echo_params(stats, StampAt::None, Clock::Unspecified, 0);
+        let (mut core, token, negotiated) = session_with(&params);
+
+        let mut reply = None;
+        for sequence in 0..2 {
+            let packet = core
+                .handle_datagram(
+                    peer(),
+                    &echo_request(token, sequence, &negotiated, &[], None),
+                )
+                .unwrap()
+                .unwrap_or_else(|| panic!("{stats:?}: sequence {sequence} must be answered"));
+            reply = Some(expect_echo_reply(&packet, &negotiated, None));
+        }
+
+        let reply = reply.expect("two requests were sent");
+        assert_eq!(reply.recv_count, count, "count presence for {stats:?}");
+        assert_eq!(reply.recv_window, window, "window presence for {stats:?}");
+    }
+}
+
+#[test]
+fn an_authenticated_session_answers_only_authenticated_echoes() {
+    let mut core = core_for(Some(KEY), ScriptedTokens::new([TOKEN_A]));
+    let (token, negotiated) = open_negotiated(&mut core, peer(), &counting_params(0), Some(KEY));
+
+    let packet = core
+        .handle_datagram(peer(), &echo_request(token, 3, &negotiated, &[], Some(KEY)))
+        .unwrap()
+        .expect("a correctly authenticated echo must be answered");
+    assert!(
+        verify_packet_hmac(KEY, &packet).is_ok(),
+        "the reply must carry a MAC that verifies with the server's key"
+    );
+    // `expect_echo_reply` pins the HMAC flag and verifies the MAC again while
+    // decoding, so a reply that authenticated but omitted the flag would fail
+    // here — the failure mode that silently breaks upstream clients.
+    let reply = expect_echo_reply(&packet, &negotiated, Some(KEY));
+    assert_eq!(reply.token, token);
+    assert_eq!(reply.sequence, 3);
+    assert_eq!((reply.recv_count, reply.recv_window), (Some(1), Some(0x1)));
+
+    let mut corrupted = echo_request(token, 4, &negotiated, &[], Some(KEY));
+    corrupted[5] ^= 0x80;
+    for (name, packet) in [
+        (
+            "an echo with no MAC field at all",
+            echo_request(token, 4, &negotiated, &[], None),
+        ),
+        (
+            "an echo signed with a different key",
+            echo_request(token, 4, &negotiated, &[], Some(OTHER_KEY)),
+        ),
+        ("an echo with one bit flipped in the MAC", corrupted),
+    ] {
+        assert_eq!(
+            core.handle_datagram(peer(), &packet).unwrap(),
+            None,
+            "{name} must be answered with silence"
+        );
+    }
+
+    let packet = core
+        .handle_datagram(peer(), &echo_request(token, 4, &negotiated, &[], Some(KEY)))
+        .unwrap()
+        .expect("an authenticated echo after failed ones must be answered");
+    let reply = expect_echo_reply(&packet, &negotiated, Some(KEY));
+    assert_eq!(
+        (reply.recv_count, reply.recv_window),
+        (Some(2), Some(0x3)),
+        "none of the packets that failed authentication may be counted"
+    );
+}
+
+#[test]
+fn only_the_negotiated_timestamps_are_emitted() {
+    let received = sample(1_000, 10_000);
+    let sent = sample(1_200, 10_300);
+
+    for (stamp_at, clock, expected) in [
+        (
+            StampAt::Send,
+            Clock::Wall,
+            TimestampFields {
+                send_wall: Some(1_200),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Send,
+            Clock::Monotonic,
+            TimestampFields {
+                send_mono: Some(10_300),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Send,
+            Clock::Both,
+            TimestampFields {
+                send_wall: Some(1_200),
+                send_mono: Some(10_300),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Receive,
+            Clock::Wall,
+            TimestampFields {
+                recv_wall: Some(1_000),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Receive,
+            Clock::Monotonic,
+            TimestampFields {
+                recv_mono: Some(10_000),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Receive,
+            Clock::Both,
+            TimestampFields {
+                recv_wall: Some(1_000),
+                recv_mono: Some(10_000),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Both,
+            Clock::Wall,
+            TimestampFields {
+                recv_wall: Some(1_000),
+                send_wall: Some(1_200),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Both,
+            Clock::Monotonic,
+            TimestampFields {
+                recv_mono: Some(10_000),
+                send_mono: Some(10_300),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Both,
+            Clock::Both,
+            TimestampFields {
+                recv_wall: Some(1_000),
+                recv_mono: Some(10_000),
+                send_wall: Some(1_200),
+                send_mono: Some(10_300),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Midpoint,
+            Clock::Wall,
+            TimestampFields {
+                midpoint_wall: Some(1_100),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Midpoint,
+            Clock::Monotonic,
+            TimestampFields {
+                midpoint_mono: Some(10_150),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            StampAt::Midpoint,
+            Clock::Both,
+            TimestampFields {
+                midpoint_wall: Some(1_100),
+                midpoint_mono: Some(10_150),
+                ..TimestampFields::default()
+            },
+        ),
+    ] {
+        let clock_source = ScriptedClock::new([received, sent]);
+        let mut core = core_with_sources(
+            ServerConfig::default(),
+            ScriptedTokens::new([TOKEN_A]),
+            clock_source.clone(),
+        );
+        let params = echo_params(ReceivedStats::None, stamp_at, clock, 0);
+        let (token, negotiated) = open_negotiated(&mut core, peer(), &params, None);
+        assert_eq!(
+            clock_source.remaining(),
+            2,
+            "an open must not consume a clock sample"
+        );
+
+        let packet = core
+            .handle_datagram(peer(), &echo_request(token, 0, &negotiated, &[], None))
+            .unwrap()
+            .unwrap_or_else(|| panic!("{stamp_at:?} on {clock:?} must be answered"));
+        let reply = expect_echo_reply(&packet, &negotiated, None);
+
+        // Equality over the whole struct, so an unselected field appearing is a
+        // failure as much as a selected one carrying the wrong instant.
+        assert_eq!(reply.timestamps, expected, "{stamp_at:?} on {clock:?}");
+        assert_eq!(
+            packet.len(),
+            echo_packet_len(false, &negotiated).unwrap(),
+            "{stamp_at:?} on {clock:?} must be the negotiated layout's length"
+        );
+        assert_eq!(
+            clock_source.remaining(),
+            0,
+            "{stamp_at:?} on {clock:?}: one receive and one send sample"
+        );
+    }
+}
+
+#[test]
+fn a_single_clock_midpoint_emits_exactly_one_field() {
+    // A guard against reproducing upstream 0.9.1's dual-field midpoint, which
+    // emits both midpoint fields whatever the negotiated clock. `irtt-proto`
+    // still decodes that form from a peer; this is about what this server
+    // sends.
+    for (clock, expected) in [
+        (
+            Clock::Wall,
+            TimestampFields {
+                midpoint_wall: Some(1_100),
+                ..TimestampFields::default()
+            },
+        ),
+        (
+            Clock::Monotonic,
+            TimestampFields {
+                midpoint_mono: Some(10_150),
+                ..TimestampFields::default()
+            },
+        ),
+    ] {
+        let mut core = core_with_sources(
+            ServerConfig::default(),
+            ScriptedTokens::new([TOKEN_A]),
+            ScriptedClock::new([sample(1_000, 10_000), sample(1_200, 10_300)]),
+        );
+        let params = echo_params(ReceivedStats::None, StampAt::Midpoint, clock, 0);
+        let (token, negotiated) = open_negotiated(&mut core, peer(), &params, None);
+
+        let packet = core
+            .handle_datagram(peer(), &echo_request(token, 0, &negotiated, &[], None))
+            .unwrap()
+            .unwrap_or_else(|| panic!("a midpoint echo on {clock:?} must be answered"));
+        let reply = expect_echo_reply(&packet, &negotiated, None);
+
+        assert_eq!(
+            packet.len(),
+            echo_packet_len(false, &negotiated).unwrap(),
+            "{clock:?}: the negotiated length, not the compatibility one"
+        );
+        assert_eq!(
+            packet.len(),
+            24,
+            "{clock:?}: one 8-byte midpoint field, not the upstream two"
+        );
+        assert_eq!(reply.timestamps, expected);
+    }
+}
+
+#[test]
+fn a_reply_payload_is_zero_filled_and_never_reflects_the_request() {
+    let params = echo_params(ReceivedStats::None, StampAt::None, Clock::Unspecified, 64);
+    let (mut core, token, negotiated) = session_with(&params);
+
+    let request = echo_request(token, 0, &negotiated, &[0xa5; 48], None);
+    assert!(
+        request[16..].iter().all(|byte| *byte == 0xa5),
+        "the request must carry non-zero payload bytes to reflect"
+    );
+
+    let packet = core
+        .handle_datagram(peer(), &request)
+        .unwrap()
+        .expect("an admissible echo must be answered");
+    let reply = expect_echo_reply(&packet, &negotiated, None);
+
+    assert_eq!(packet.len(), 64);
+    assert_eq!(reply.payload.len(), 48);
+    assert!(
+        reply.payload.iter().all(|byte| *byte == 0),
+        "the payload region must be zero-filled, never request or buffer residue"
+    );
+}

crates/irtt-server/src/tests/mod.rs

@@ -5,10 +5,15 @@
 //! production encoders wherever the wire form is well-formed, and by hand only
 //! where the point of the test is a payload a compliant encoder cannot produce.
 //!
-//! Tokens come from a scripted source so session identity, collisions and
-//! allocation failure are all assertable.
+//! The two nondeterministic parts are injected. Tokens come from a scripted
+//! source so session identity, collisions and allocation failure are all
+//! assertable; the clock an echo reply's timestamps come from is scripted the
+//! same way, so timestamp behavior is pinned exactly, with no sleeps, no
+//! runtime and no timing tolerances.
 
+mod admission;
 mod close;
+mod echo;
 mod hmac;
 mod no_test;
 mod open;
@@ -16,4 +21,3 @@ mod params;
 mod sessions;
 mod support;
 mod tokens;
-mod unsupported;

crates/irtt-server/src/tests/support.rs

@@ -5,11 +5,17 @@ use std::{
 };
 
 use irtt_proto::{
-    decode_open_reply, encode_request, varint, OpenReply, PacketLayout, Params, RequestToEncode,
-    FLAG_CLOSE, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY,
+    decode_echo_reply, decode_open_reply, encode_request, varint, EchoReply, OpenReply,
+    PacketLayout, Params, RequestToEncode, FLAG_CLOSE, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY,
 };
 
-use crate::{core::ServerCore, error::ServerError, token::TokenSource, ServerConfig};
+use crate::{
+    clock::{ClockSample, ClockSource},
+    core::ServerCore,
+    error::ServerError,
+    token::TokenSource,
+    ServerConfig,
+};
 
 pub(crate) const KEY: &[u8] = b"correctkey";
 pub(crate) const OTHER_KEY: &[u8] = b"wrongkey";
@@ -67,11 +73,63 @@ impl TokenSource for ScriptedTokens {
     }
 }
 
-/// A core with deterministic tokens, so session identity is assertable.
+/// A clock handing out a fixed script of samples, then failing loudly.
+///
+/// Running the script dry is a test bug, not a wrap-around: a test that asserts
+/// timestamps must know exactly which sample each field came from. The core
+/// takes one sample when it classifies a datagram as an echo, and a second when
+/// an admitted echo is about to be answered.
+#[derive(Debug, Clone)]
+pub(crate) struct ScriptedClock {
+    samples: Arc<Mutex<VecDeque<ClockSample>>>,
+}
+
+impl ScriptedClock {
+    pub(crate) fn new<I>(samples: I) -> Self
+    where
+        I: IntoIterator<Item = ClockSample>,
+    {
+        Self {
+            samples: Arc::new(Mutex::new(samples.into_iter().collect())),
+        }
+    }
+
+    /// How many scripted samples are still unused.
+    pub(crate) fn remaining(&self) -> usize {
+        self.samples.lock().unwrap().len()
+    }
+}
+
+impl ClockSource for ScriptedClock {
+    fn sample(&mut self) -> ClockSample {
+        self.samples
+            .lock()
+            .unwrap()
+            .pop_front()
+            .expect("the scripted clock ran out: script one sample per echo received, and one more per echo answered")
+    }
+}
+
+pub(crate) fn sample(wall_ns: i64, mono_ns: i64) -> ClockSample {
+    ClockSample { wall_ns, mono_ns }
+}
+
+/// A core with deterministic tokens, so session identity is assertable. Its
+/// clock is the production one, which is fine for everything that does not
+/// assert timestamp values.
 pub(crate) fn core_with_tokens(config: ServerConfig, tokens: ScriptedTokens) -> ServerCore {
     ServerCore::with_token_source(config, Box::new(tokens))
 }
 
+/// A core with both nondeterministic sources scripted, for timestamp tests.
+pub(crate) fn core_with_sources(
+    config: ServerConfig,
+    tokens: ScriptedTokens,
+    clock: ScriptedClock,
+) -> ServerCore {
+    ServerCore::with_sources(config, Box::new(tokens), Box::new(clock))
+}
+
 /// An otherwise default core that authenticates iff `hmac_key` is set.
 ///
 /// Most behavior has to hold identically with and without authentication, so
@@ -97,6 +155,80 @@ pub(crate) fn open_session(
     expect_normal_open_reply(&packet, hmac_key).token
 }
 
+/// Opens one session carrying `params` and returns its token together with the
+/// parameters the server negotiated for it.
+///
+/// Echo tests need both: the token addresses the session, and the negotiated
+/// params are what the reply is laid out from and what decoding it requires.
+pub(crate) fn open_negotiated(
+    core: &mut ServerCore,
+    endpoint: SocketAddr,
+    params: &Params,
+    hmac_key: Option<&[u8]>,
+) -> (u64, Params) {
+    let packet = core
+        .handle_datagram(endpoint, &open_request(params, hmac_key))
+        .unwrap()
+        .expect("the session-creating open must be answered");
+    let reply = expect_normal_open_reply(&packet, hmac_key);
+    (reply.token, reply.params)
+}
+
+/// Params requesting exactly the echo layout a test cares about, with the
+/// duration and interval a well-behaved client sends.
+pub(crate) fn echo_params(
+    received_stats: irtt_proto::ReceivedStats,
+    stamp_at: irtt_proto::StampAt,
+    clock: irtt_proto::Clock,
+    length: i64,
+) -> Params {
+    Params {
+        received_stats,
+        stamp_at,
+        clock,
+        length,
+        ..client_params()
+    }
+}
+
+/// Encodes an echo request. `params` supplies only the *request's* own sizing;
+/// it is deliberately a separate argument from the session's negotiated params
+/// so tests can send requests shorter and longer than the session negotiated.
+pub(crate) fn echo_request(
+    token: u64,
+    sequence: u32,
+    params: &Params,
+    payload: &[u8],
+    hmac_key: Option<&[u8]>,
+) -> Vec<u8> {
+    encode_request(
+        RequestToEncode::Echo {
+            token,
+            sequence,
+            params,
+            payload,
+        },
+        hmac_key,
+    )
+    .unwrap()
+}
+
+/// Decodes an echo reply the server produced and checks the flags every reply
+/// must carry. Decoding verifies the MAC against `hmac_key` on the way.
+pub(crate) fn expect_echo_reply(
+    packet: &[u8],
+    params: &Params,
+    hmac_key: Option<&[u8]>,
+) -> EchoReply {
+    let reply = decode_echo_reply(packet, params, hmac_key).expect("server echo reply must decode");
+    // Exact equality, so Open and Close are pinned clear as well: Open makes an
+    // upstream client abort, and Close belongs to the server-initiated close
+    // slice.
+    let expected = FLAG_REPLY | if hmac_key.is_some() { FLAG_HMAC } else { 0 };
+    assert_eq!(reply.flags, expected, "echo reply flags");
+    reply
+}
+
 /// Encodes a normal open request carrying `params`.
 pub(crate) fn open_request(params: &Params, hmac_key: Option<&[u8]>) -> Vec<u8> {
     encode_request(

crates/irtt-server/src/tests/unsupported.rs

@@ -1,74 +0,0 @@
-//! Requests this slice deliberately does not implement.
-//!
-//! Echo is recognized by the admission boundary and then ignored. These tests
-//! exist to keep the scope boundary honest: receive state must not appear by
-//! accident before the slice that owns it.
-
-use irtt_proto::{encode_request, RequestToEncode};
-
-use super::support::{
-    client_params, core_for, open_request, open_session, peer, ScriptedTokens, KEY,
-};
-
-const TOKEN_A: u64 = 0x0102_0304_0506_0708;
-
-fn open_one_session(hmac_key: Option<&[u8]>) -> crate::ServerCore {
-    let mut core = core_for(hmac_key, ScriptedTokens::new([TOKEN_A]));
-    open_session(&mut core, peer(), hmac_key);
-    core
-}
-
-#[test]
-fn a_valid_echo_is_neither_answered_nor_counted_yet() {
-    for hmac_key in [None, Some(KEY)] {
-        let mut core = open_one_session(hmac_key);
-        let session = core.session(TOKEN_A).cloned().expect("session must exist");
-
-        let echo = encode_request(
-            RequestToEncode::Echo {
-                token: TOKEN_A,
-                sequence: 0,
-                params: session.params(),
-                payload: &[],
-            },
-            hmac_key,
-        )
-        .unwrap();
-
-        assert_eq!(core.handle_datagram(peer(), &echo).unwrap(), None);
-        assert_eq!(core.session_count(), 1);
-        assert_eq!(core.session(TOKEN_A), Some(&session));
-    }
-}
-
-#[test]
-fn structurally_invalid_datagrams_are_discarded() {
-    let mut core = open_one_session(None);
-    let session = core.session(TOKEN_A).cloned().expect("session must exist");
-
-    let mut bad_magic = open_request(&client_params(), None);
-    bad_magic[0] ^= 0x01;
-
-    let mut reserved_flag = open_request(&client_params(), None);
-    reserved_flag[3] |= 0x10;
-
-    let mut reply_flag = open_request(&client_params(), None);
-    reply_flag[3] |= irtt_proto::FLAG_REPLY;
-
-    for (name, packet) in [
-        ("empty datagram", Vec::new()),
-        ("datagram shorter than the header", vec![0x14, 0xa7, 0x5b]),
-        ("bad magic", bad_magic),
-        ("reserved flag bit", reserved_flag),
-        ("reply flag", reply_flag),
-    ] {
-        assert_eq!(
-            core.handle_datagram(peer(), &packet).unwrap(),
-            None,
-            "{name} must be discarded"
-        );
-    }
-
-    assert_eq!(core.session_count(), 1);
-    assert_eq!(core.session(TOKEN_A), Some(&session));
-}