docs(server): describe single allowance as one instant

README.md

@@ -213,11 +213,12 @@ irtt-server \
 ```
 
 `--timestamp-allowance` takes `dual` (the default, honoring every requested
-placement), `single`, which provides at most one timestamp and answers a request
+placement), `single`, which provides at most one timestamp instant and answers a
-for both receive and send timestamps with the midpoint, or `none`, which provides
+request for both receive and send timestamps with the midpoint, or `none`, which
-no timestamps. The requested clock source is never changed. `--no-dscp`
+provides no timestamps. The requested clock source is never changed, so a single
-negotiates any requested DSCP to zero, so clients are told their echo replies
+instant is still reported once per requested clock. `--no-dscp` negotiates any
-will be unmarked, and they are sent unmarked.
+requested DSCP to zero, so clients are told their echo replies will be unmarked,
+and they are sent unmarked.
 
 To require authentication, pass the shared key both sides use:
 

crates/irtt-cli/src/cmd/server/args.rs

@@ -85,7 +85,7 @@ pub struct ServerArgs {
         long,
         value_name = "MODE",
         value_enum,
-        long_help = "How many timestamps this server will provide.\n\ndual: honor a request for send, receive, both or midpoint timestamps.\nsingle: provide at most one timestamp; a request for both is negotiated to midpoint.\nnone: provide no timestamps at all.\n\nThe requested clock source is never changed, only the timestamp placement. Omit this flag to honor every requested placement."
+        long_help = "How many timestamps this server will provide.\n\ndual: honor a request for send, receive, both or midpoint timestamps.\nsingle: provide at most one timestamp instant; a request for both is negotiated to midpoint.\nnone: provide no timestamps at all.\n\nThe requested clock source is never changed, only which instants are reported, so a single instant is still reported once per requested clock. Omit this flag to honor every requested placement."
     )]
     pub timestamp_allowance: Option<TimestampAllowanceArg>,
 
@@ -105,7 +105,7 @@ pub struct ServerArgs {
 pub enum TimestampAllowanceArg {
     /// Honor every requested timestamp placement.
     Dual,
-    /// Provide at most one timestamp; both becomes midpoint.
+    /// Provide at most one timestamp instant; both becomes midpoint.
     Single,
     /// Provide no timestamps.
     None,

crates/irtt-server/AGENTS.md

@@ -272,7 +272,9 @@ of open negotiation, and neither reaches the runtime.
   interesting row; the rest is in the clean spec's Section 11.4.
 - **`Clock` is never rewritten.** The evidence describes an allowance on timestamp
   *placement*, not on clock domains, and the echo layout already carries no
-  timestamp field once the placement is none. Rewriting the clock would answer
+  timestamp field once the placement is none. So `Single` bounds reported
+  *instants*, not field count: on `Clock::Both` a negotiated midpoint still emits
+  a wall and a monotonic field. Do not document it as one field. Rewriting the clock would answer
   with a session the client did not ask for.
 - **Restriction runs before the effective-session executability check**, and that
   ordering is load-bearing: it is what makes a request selecting timestamps with

crates/irtt-server/src/config.rs

@@ -69,9 +69,14 @@ pub enum TimestampAllowance {
     /// No timestamps at all: every request is negotiated to
     /// [`StampAt::None`](irtt_proto::StampAt::None).
     None,
-    /// At most one timestamp field. A request for both receive and send becomes
+    /// At most one timestamp *instant*. A request for both receive and send
-    /// [`StampAt::Midpoint`](irtt_proto::StampAt::Midpoint); every mode that is
+    /// becomes [`StampAt::Midpoint`](irtt_proto::StampAt::Midpoint); every
-    /// already single is left alone.
+    /// placement that already names a single instant is left alone.
+    ///
+    /// This restricts placement, not field count: the reply still carries one
+    /// field per negotiated clock domain, so a session on
+    /// [`Clock::Both`](irtt_proto::Clock::Both) receives both a wall and a
+    /// monotonic midpoint field.
     Single,
     /// Every requested placement is honored. This is the default, and the
     /// behavior of a server that configures nothing.
@@ -266,9 +271,12 @@ impl ServerConfig {
     /// not with whichever of the two the server felt like keeping.
     ///
     /// The requested [`Clock`](irtt_proto::Clock) is left exactly as it was in
-    /// every case. The allowance restricts placement, and the echo layout simply
+    /// every case. The allowance restricts which *instants* are reported, and
-    /// carries no timestamp field once the placement is
+    /// each reported instant still carries one field per negotiated clock domain
-    /// [`StampAt::None`](irtt_proto::StampAt::None).
+    /// — [`Single`] on [`Clock::Both`](irtt_proto::Clock::Both) reports a wall
+    /// and a monotonic midpoint. Only [`None`] removes timestamp fields
+    /// outright, and it does so because the echo layout carries none once the
+    /// placement is [`StampAt::None`](irtt_proto::StampAt::None).
     ///
     /// Restriction runs *before* the effective-session check, which is what
     /// makes a [`None`] allowance accept a request that selected timestamps

crates/irtt-server/src/lib.rs

@@ -70,9 +70,10 @@
 //! negotiates exactly what it always did.
 //!
 //! [`TimestampAllowance`] reduces the requested timestamp placement: `Single`
-//! provides at most one timestamp, answering a request for both instants with
+//! provides at most one timestamp instant, answering a request for both instants
-//! the midpoint, and `None` provides no timestamps at all. The requested clock
+//! with the midpoint, and `None` provides no timestamps at all. The requested
-//! is never rewritten. [`ServerConfig::with_dscp_allowed`] refuses to provide
+//! clock is never rewritten, so a single placement still reports one field per
+//! negotiated clock domain. [`ServerConfig::with_dscp_allowed`] refuses to provide
 //! traffic-class marking, negotiating any requested DSCP value to zero, after
 //! which the session's replies leave unmarked like any other zero-DSCP session.
 //!

crates/irtt-server/src/negotiate.rs

@@ -129,10 +129,13 @@ pub(crate) fn negotiate_params(mut requested: Params, config: &ServerConfig) ->
 ///
 /// The mapping is the observed one, from the clean specification's Section 11.4.
 /// Only one row substitutes rather than removes: under [`Single`], a request for
-/// both instants is answered with [`StampAt::Midpoint`] — one timestamp that
+/// both instants is answered with [`StampAt::Midpoint`] — one instant that still
-/// still describes both — and not with whichever of Send or Receive an
+/// describes both — and not with whichever of Send or Receive an implementation
-/// implementation happened to prefer. Every already-single placement passes
+/// happened to prefer. Every already-single placement passes through, and
-/// through, and [`None`] removes all of them.
+/// [`None`] removes all of them.
+///
+/// The allowance bounds reported *instants*, not fields: the clock is untouched,
+/// so a negotiated midpoint on `Clock::Both` still reports one field per domain.
 ///
 /// [`Single`]: TimestampAllowance::Single
 /// [`None`]: TimestampAllowance::None