crates/irtt-server/AGENTS.md

@@ -83,13 +83,42 @@ effective parameters must be safe for the server to run.
   the open reply and stored verbatim in the session. It needs no clamping to be
   executable: `irtt-proto` floors actual echo packet sizing at the required
   protocol field block, so a negative length behaves as zero does.
+- A session's echo datagram must fit `ServerConfig::max_packet_length`, whose
+  default is 65,507 bytes. That is `irtt-rs` resource policy, not an
+  interoperability requirement: it is the largest IPv4 UDP payload, a
+  conservative cross-family ceiling that bounds one session's echo buffer to
+  roughly 64 KiB and that no normal client exceeds. It is **not** an MTU, not an
+  upstream default, and says nothing about what a path will carry. Upstream's
+  unlimited default is not a compatibility target.
+- A requested positive `Length` above the configured maximum is reduced to it
+  during negotiation, so the reply is honest about what the server will emit.
+  Zero and negative values are left exactly as requested.
+- Capping the parameter is not sufficient. After negotiation, the mandatory echo
+  field block must itself fit the configured maximum or the open is silently
+  refused — statistics and timestamp fields, and authentication's 16 bytes, can
+  exceed a small maximum on their own. Ask `irtt_proto::echo_packet_len` with
+  this server's actual HMAC mode rather than growing a second packet-size rule
+  here; an unrepresentable length from it is a request rejection, not a
+  `ServerError`.
+- 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.
 
 ## Resource policy
 
 Total session and resource state must stay bounded. A single unauthenticated
-datagram creates a session and opens are never deduplicated, so an unbounded
-table is a liability. Upstream's observed absence of session and per-peer bounds
-is explicitly **not** a compatibility target and must not be reproduced.
+datagram creates a session and opens are never deduplicated, so neither an
+unbounded table nor a remotely chosen packet buffer is defensible. Upstream's
+observed absence of session and per-peer bounds, and its unlimited default
+packet length, are explicitly **not** compatibility targets and must not be
+reproduced.
+
+Configured bounds are trusted local policy: an operator may deliberately set an
+absurd `max_sessions` or `max_packet_length`, and that is their choice. The
+defaults are what must stay bounded, and no bound gets an unlimited setting.
 
 Keep fallible preparation before irreversible state mutation: a reply is
 prepared before a session is inserted, so an internal failure cannot leave a

crates/irtt-server/src/config.rs

@@ -8,17 +8,36 @@
 /// server is not expected to reproduce.
 pub const DEFAULT_MAX_SESSIONS: usize = 1024;
 
+/// Default bound on the executable echo datagram size of one session, in bytes.
+///
+/// This is `irtt-rs` resource policy, not an interoperability requirement and
+/// not a protocol constant. The server specification records upstream defaulting
+/// to *unlimited*, which leaves a remotely negotiated allocation unbounded; that
+/// is explicitly not a compatibility target.
+///
+/// 65,507 is the largest payload an IPv4 UDP datagram can carry, so it is the
+/// conservative ceiling above which no negotiated length could be emitted as one
+/// datagram on any address family. It bounds a session's echo buffer to roughly
+/// 64 KiB, and it is already the maximum a normal `irtt-rs` client will ask for,
+/// so ordinary traffic never meets it.
+///
+/// It is **not** an MTU and promises nothing about what a path will carry. Path
+/// and interface policy is a separate, later concern; this value exists to keep
+/// allocation bounded, not to predict fragmentation.
+pub const DEFAULT_MAX_PACKET_LENGTH: usize = 65_507;
+
 /// Configuration for a [`ServerCore`](crate::ServerCore).
 ///
 /// Fields are private and set through consuming builder methods, so later
 /// slices can add configuration without breaking construction. Only the
-/// settings this OPEN/session slice actually uses exist yet; parameter
-/// restriction knobs, lifetime policy and rate limits arrive with the slices
-/// that implement them.
+/// settings this OPEN/session slice actually uses exist yet; the remaining
+/// parameter restriction knobs, lifetime policy and rate limits arrive with the
+/// slices that implement them.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ServerConfig {
     hmac_key: Option<Vec<u8>>,
     max_sessions: usize,
+    max_packet_length: usize,
 }
 
 impl ServerConfig {
@@ -45,6 +64,32 @@ impl ServerConfig {
         self
     }
 
+    /// Sets the maximum echo datagram size a session may negotiate, in bytes.
+    ///
+    /// This governs the *executable* echo datagram of a session, not the size of
+    /// the open exchange: a requested positive Length above this value is
+    /// 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 value is trusted local configuration, like
+    /// [`with_max_sessions`](Self::with_max_sessions): an operator may
+    /// deliberately raise it far past what any path carries, and that is their
+    /// resource choice to make. What must stay bounded is the *default*, which
+    /// is why there is deliberately no unlimited setting.
+    ///
+    /// Zero is accepted and is not a synonym for unlimited. No session can
+    /// describe an echo packet smaller than its mandatory field block, so a zero
+    /// maximum silently refuses every session-creating open — and every no-test
+    /// open too, since both are validated against the same effective session.
+    #[must_use]
+    pub fn with_max_packet_length(mut self, max_packet_length: usize) -> Self {
+        self.max_packet_length = max_packet_length;
+        self
+    }
+
     /// The configured HMAC key, if this server authenticates.
     #[must_use]
     pub fn hmac_key(&self) -> Option<&[u8]> {
@@ -56,14 +101,22 @@ impl ServerConfig {
     pub fn max_sessions(&self) -> usize {
         self.max_sessions
     }
+
+    /// The maximum echo datagram size a session may negotiate, in bytes.
+    #[must_use]
+    pub fn max_packet_length(&self) -> usize {
+        self.max_packet_length
+    }
 }
 
 impl Default for ServerConfig {
-    /// An unauthenticated server bounded to [`DEFAULT_MAX_SESSIONS`] sessions.
+    /// An unauthenticated server bounded to [`DEFAULT_MAX_SESSIONS`] sessions,
+    /// each negotiating at most [`DEFAULT_MAX_PACKET_LENGTH`] bytes per echo.
     fn default() -> Self {
         Self {
             hmac_key: None,
             max_sessions: DEFAULT_MAX_SESSIONS,
+            max_packet_length: DEFAULT_MAX_PACKET_LENGTH,
         }
     }
 }

crates/irtt-server/src/core.rs

@@ -1,7 +1,7 @@
 use std::{collections::HashMap, net::SocketAddr};
 
 use irtt_proto::{
-    decode_request, encode_open_reply, verify_packet_hmac, Clock, DecodedParams,
+    decode_request, echo_packet_len, encode_open_reply, verify_packet_hmac, Clock, DecodedParams,
     DecodedRequestKind, OpenReply, Params, StampAt, FLAG_CLOSE, FLAG_OPEN, FLAG_REPLY,
 };
 
@@ -141,7 +141,10 @@ impl ServerCore {
             }
             // 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.
+            // 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),
         }
     }
@@ -164,7 +167,7 @@ impl ServerCore {
         let params = negotiate_params(decoded.params, &self.config);
         // Deliberately after negotiation, and before either open path: what has
         // to be executable is the effective session, not the request.
-        if !negotiated_params_are_admissible(&params) {
+        if !negotiated_params_are_admissible(&params, &self.config) {
             return Ok(None);
         }
 
@@ -324,10 +327,12 @@ fn open_params_are_admissible(decoded: &DecodedParams) -> bool {
 /// exists to tell a client what the session would be, so it must not report a
 /// session that could not exist.
 ///
-/// One rule so far: **selecting timestamps requires a specified clock.** A
-/// non-none `stamp_at` with [`Clock::Unspecified`] asks for timestamp fields
-/// from no clock at all, which lays out no timestamp field and leaves the
-/// session with a request the server cannot honor.
+/// Two rules so far.
+///
+/// **Selecting timestamps requires a specified clock.** A non-none `stamp_at`
+/// with [`Clock::Unspecified`] asks for timestamp fields from no clock at all,
+/// which lays out no timestamp field and leaves the session with a request the
+/// server cannot honor.
 ///
 /// [`Clock::Unspecified`] means the Clock tag was absent — an explicit zero is
 /// already out of range for the decoder — so this cannot be reached by a
@@ -337,10 +342,32 @@ fn open_params_are_admissible(decoded: &DecodedParams) -> bool {
 /// `stamp_at` to none would silently drop the measurement it did ask for.
 /// Neither is the server's to decide.
 ///
+/// **The session's echo datagram must fit
+/// [`max_packet_length`](ServerConfig::max_packet_length).** Negotiation already
+/// reduced an oversized Length, but that only bounds what the *parameter* asks
+/// for: the mandatory field block grows with the received statistics, the
+/// timestamps and — by 16 bytes — authentication, and can exceed the configured
+/// maximum on its own. A session whose smallest compliant reply is already too
+/// large is one the server could never answer an echo for, so it is refused at
+/// 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
+/// 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
+/// negotiation has already reduced the length to a representable maximum.
+///
 /// The check is deliberately *after* negotiation so that a later policy slice
 /// restricting timestamps can restrict `stamp_at` to [`StampAt::None`], at
 /// which point an absent clock is safe and this predicate holds without
 /// changing.
-fn negotiated_params_are_admissible(params: &Params) -> bool {
-    params.stamp_at == StampAt::None || params.clock != Clock::Unspecified
+///
+/// [`ServerError`]: crate::ServerError
+fn negotiated_params_are_admissible(params: &Params, config: &ServerConfig) -> bool {
+    if params.stamp_at != StampAt::None && params.clock == Clock::Unspecified {
+        return false;
+    }
+    echo_packet_len(config.hmac_key().is_some(), params)
+        .is_ok_and(|len| len <= config.max_packet_length())
 }

crates/irtt-server/src/lib.rs

@@ -23,11 +23,13 @@
 //!
 //! # Resource policy
 //!
-//! Total session state is bounded (see [`DEFAULT_MAX_SESSIONS`]). A single
-//! unauthenticated datagram creates a session and opens are never deduplicated,
-//! so an unbounded table is a liability rather than a compatibility feature.
-//! Upstream's observed lack of session bounds is explicitly not a compatibility
-//! target.
+//! Total session state is bounded (see [`DEFAULT_MAX_SESSIONS`]), and so is the
+//! echo datagram a single session can negotiate (see
+//! [`DEFAULT_MAX_PACKET_LENGTH`]). A single unauthenticated datagram creates a
+//! session and opens are never deduplicated, so neither an unbounded table nor a
+//! remotely chosen buffer size is a compatibility feature. Upstream's observed
+//! lack of session bounds, and its unlimited default packet length, are
+//! explicitly not compatibility targets.
 //!
 //! # Current scope
 //!
@@ -47,6 +49,6 @@ mod token;
 #[cfg(test)]
 mod tests;
 
-pub use config::{ServerConfig, DEFAULT_MAX_SESSIONS};
+pub use config::{ServerConfig, DEFAULT_MAX_PACKET_LENGTH, DEFAULT_MAX_SESSIONS};
 pub use core::ServerCore;
 pub use error::ServerError;

crates/irtt-server/src/negotiate.rs

@@ -5,26 +5,46 @@ use crate::config::ServerConfig;
 /// Produces the parameters the server returns in an open reply and will enforce
 /// for the session.
 ///
-/// This slice applies the minimum the specification requires: the requested
-/// known parameters, with the protocol version rewritten to
-/// [`PROTOCOL_VERSION`]. A server must return version 1 so that version-1
-/// clients accept the session; any requested value, including an absent,
-/// zero, negative or future one, is accepted and answered with 1. Detecting a
-/// version mismatch is therefore client-side only.
+/// Two restrictions apply so far.
 ///
-/// Nothing else is restricted yet. A server *may* reduce or replace requested
-/// parameters, and later policy slices will add the maximum test duration
-/// clamp, minimum interval, idle-timeout interval cap, maximum packet length,
-/// timestamp allowance, DSCP policy and server-fill allow-list. This function
-/// is where they belong, which is why the configuration is threaded through
-/// even though no knob reads it yet.
+/// **Protocol version is rewritten to [`PROTOCOL_VERSION`].** A server must
+/// return version 1 so that version-1 clients accept the session; any requested
+/// value, including an absent, zero, negative or future one, is accepted and
+/// answered with 1. Detecting a version mismatch is therefore client-side only.
 ///
-/// Negotiation rewriting nothing but the version does not mean every open is
-/// answered: the core validates what this function returns and silently
-/// discards an open whose effective parameters it could not execute. That is a
-/// separate step on purpose — this function decides what the session *would*
-/// be, and the core decides whether that session can exist.
-pub(crate) fn negotiate_params(mut requested: Params, _config: &ServerConfig) -> Params {
+/// **A positive Length above the configured maximum is reduced to it.** That
+/// bounds the echo datagram — and so the buffer — a remote peer can make this
+/// server negotiate, which is why it happens here rather than at send time: the
+/// value returned in the open reply has to be the one the server will actually
+/// honor. A negative Length is left exactly as requested and a zero stays zero;
+/// neither asks for space beyond the mandatory field block, which `irtt-proto`
+/// floors echo sizing at, so there is nothing to reduce and rewriting them would
+/// answer with a session the client did not ask for.
+///
+/// Those two are the whole of the current policy. A server *may* reduce or
+/// replace requested parameters, and later policy slices will add the maximum
+/// test duration clamp,
+/// minimum interval, idle-timeout interval cap, timestamp allowance, DSCP policy
+/// and server-fill allow-list.
+///
+/// Negotiation is not by itself an acknowledgement: the core validates what this
+/// function returns and silently discards an open whose effective parameters it
+/// could not execute. That is a separate step on purpose — this function decides
+/// what the session *would* be, and the core decides whether that session can
+/// exist. Capping Length here is not enough on its own, because the mandatory
+/// echo field block can exceed the configured maximum by itself.
+pub(crate) fn negotiate_params(mut requested: Params, config: &ServerConfig) -> Params {
     requested.protocol_version = PROTOCOL_VERSION;
+
+    // A configured maximum wider than `i64` cannot restrict anything: the wire
+    // field cannot carry more than `i64::MAX` in the first place. Saturating
+    // there is therefore exact, not an approximation — unlike converting the
+    // requested length the other way, which would turn a value this platform
+    // cannot represent into a plausible buffer size.
+    let max_length = i64::try_from(config.max_packet_length()).unwrap_or(i64::MAX);
+    if requested.length > max_length {
+        requested.length = max_length;
+    }
+
     requested
 }

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

@@ -98,6 +98,51 @@ fn a_no_test_open_selecting_timestamps_without_a_clock_is_silently_refused() {
     assert_eq!(tokens.remaining(), 1, "no token may be drawn");
 }
 
+#[test]
+fn a_no_test_open_reports_the_reduced_packet_length() {
+    // No-test answers what the session *would* be, so it has to report the same
+    // reduced length a session-creating open would enforce — while still
+    // creating nothing and drawing no token.
+    let tokens = ScriptedTokens::new([TOKEN_A]);
+    let mut core = core_with_tokens(
+        ServerConfig::default().with_max_packet_length(64),
+        tokens.clone(),
+    );
+    let requested = Params {
+        length: 1000,
+        ..client_params()
+    };
+
+    let packet = core
+        .handle_datagram(peer(), &no_test_request(&requested, None))
+        .unwrap()
+        .expect("a reducible length must still be answered");
+
+    assert_eq!(expect_no_test_reply(&packet, None).params.length, 64);
+    assert_eq!(core.session_count(), 0);
+    assert_eq!(tokens.remaining(), 1, "no-test must not draw a token");
+}
+
+#[test]
+fn a_no_test_open_is_refused_when_its_echo_field_block_exceeds_the_maximum() {
+    // A maximum below the mandatory echo block describes a session that could
+    // not exist at all, so no-test must not report one. Both open forms run the
+    // same effective-parameter check before no-test branches off.
+    let tokens = ScriptedTokens::new([TOKEN_A]);
+    let mut core = core_with_tokens(
+        ServerConfig::default().with_max_packet_length(0),
+        tokens.clone(),
+    );
+
+    assert_eq!(
+        core.handle_datagram(peer(), &no_test_request(&client_params(), None))
+            .expect("an unservable packet size is not a server error"),
+        None
+    );
+    assert_eq!(core.session_count(), 0);
+    assert_eq!(tokens.remaining(), 1, "no token may be drawn");
+}
+
 #[test]
 fn a_no_test_open_is_still_answered_when_no_session_could_be_created() {
     // No-test consumes no capacity, so it must keep working exactly where a

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

@@ -2,9 +2,9 @@ use irtt_proto::{Clock, Params, ReceivedStats, ServerFill, StampAt};
 
 use super::support::{
     client_params, core_with_tokens, expect_normal_open_reply, open_request,
-    open_request_with_raw_params, param_int, peer, ScriptedTokens,
+    open_request_with_raw_params, param_int, peer, ScriptedTokens, KEY,
 };
-use crate::ServerConfig;
+use crate::{ServerConfig, DEFAULT_MAX_PACKET_LENGTH};
 
 const TOKEN_A: u64 = 0x7896_b6ab_8771_5213;
 
@@ -186,10 +186,13 @@ fn selecting_timestamps_without_a_clock_is_silently_refused() {
 }
 
 #[test]
-fn negotiation_restricts_nothing_but_the_protocol_version() {
-    // These are the values a later restriction-policy slice will deliberately
-    // start changing. Pinning them now makes that a visible decision rather
-    // than an accident.
+fn current_non_length_parameters_survive_negotiation_unchanged() {
+    // Duration, interval, statistics, timestamps, DSCP and server fill are the
+    // values a later restriction-policy slice will deliberately start changing.
+    // Pinning them now makes that a visible decision rather than an accident.
+    // Version and an oversized packet length are already restricted, and have
+    // their own tests; the length here is deliberately under the default
+    // maximum so that this test says nothing about the packet-length policy.
     //
     // This is about what *negotiation* rewrites, which is a separate question
     // from whether the core will acknowledge the result: an open whose
@@ -199,7 +202,7 @@ fn negotiation_restricts_nothing_but_the_protocol_version() {
         protocol_version: 2,
         duration_ns: 3_600_000_000_000,
         interval_ns: 1,
-        length: 65_535,
+        length: 1472,
         received_stats: ReceivedStats::Window,
         stamp_at: StampAt::Midpoint,
         clock: Clock::Monotonic,
@@ -225,16 +228,145 @@ fn negotiation_restricts_nothing_but_the_protocol_version() {
     );
 }
 
+#[test]
+fn a_positive_length_above_the_configured_maximum_is_reduced_to_it() {
+    // The reduced value is what the client is told and what the session holds,
+    // because it is what the server will actually emit. Both have to agree: an
+    // upstream client treats a reply shorter than the negotiated length as
+    // fatal, so an honest reduction at open is the only serviceable answer.
+    let tokens = ScriptedTokens::new([TOKEN_A]);
+    let mut core = core_with_tokens(
+        ServerConfig::default().with_max_packet_length(64),
+        tokens.clone(),
+    );
+    let requested = Params {
+        length: 1000,
+        ..client_params()
+    };
+
+    let packet = core
+        .handle_datagram(peer(), &open_request(&requested, None))
+        .unwrap()
+        .expect("a reducible length must still be answered");
+
+    let reply = expect_normal_open_reply(&packet, None);
+    assert_eq!(reply.params.length, 64);
+    assert_eq!(core.session_count(), 1);
+    assert_eq!(tokens.remaining(), 0, "one token was consumed");
+    let session = core.session(TOKEN_A).expect("the session must be live");
+    assert_eq!(
+        session.params().length,
+        64,
+        "the session must enforce the reduced length"
+    );
+}
+
+#[test]
+fn the_default_maximum_reduces_an_absurd_requested_length() {
+    // The largest length the wire can carry is representable as a `usize` on a
+    // 64-bit target, so packet sizing would happily have turned it into an
+    // allocation; the configured maximum is what stops it. Restriction happens
+    // during negotiation, before any sizing, so this holds on a 32-bit target
+    // too — the request never reaches a conversion that could fail.
+    let mut core = core_with_tokens(ServerConfig::default(), ScriptedTokens::new([TOKEN_A]));
+    let requested = Params {
+        length: i64::MAX,
+        ..client_params()
+    };
+
+    let packet = core
+        .handle_datagram(peer(), &open_request(&requested, None))
+        .unwrap()
+        .expect("an absurd length is reduced, not refused");
+
+    let reply = expect_normal_open_reply(&packet, None);
+    assert_eq!(reply.params.length, DEFAULT_MAX_PACKET_LENGTH as i64);
+    let session = core.session(TOKEN_A).expect("the session must be live");
+    assert_eq!(session.params().length, DEFAULT_MAX_PACKET_LENGTH as i64);
+}
+
+#[test]
+fn an_open_whose_echo_field_block_exceeds_the_maximum_is_silently_refused() {
+    // Length itself satisfies the policy — zero asks for nothing beyond the
+    // mandatory fields — but the received-statistics fields alone make the
+    // smallest compliant echo reply larger than 20 bytes. Capping the Length
+    // parameter is therefore not enough on its own: this session could never be
+    // answered, so it is never acknowledged.
+    let requested = Params {
+        protocol_version: 1,
+        length: 0,
+        received_stats: ReceivedStats::Both,
+        stamp_at: StampAt::None,
+        clock: Clock::Unspecified,
+        ..Params::default()
+    };
+    let tokens = ScriptedTokens::new([TOKEN_A]);
+    let mut core = core_with_tokens(
+        ServerConfig::default().with_max_packet_length(20),
+        tokens.clone(),
+    );
+
+    assert_eq!(
+        core.handle_datagram(peer(), &open_request(&requested, None))
+            .expect("an unservable packet size is not a server error"),
+        None
+    );
+    assert_eq!(core.session_count(), 0);
+    assert_eq!(tokens.remaining(), 1, "no token may be drawn");
+}
+
+#[test]
+fn authentication_counts_toward_the_executable_echo_size() {
+    // An authentication field adds 16 bytes to the mandatory echo block, so a
+    // maximum between the two floors admits the same minimal session on an
+    // unauthenticated server and refuses it on an authenticated one. The sizes
+    // themselves come from `irtt_proto`; what this pins is that the server asks
+    // about its own authentication mode rather than assuming the smaller floor.
+    let minimal = Params {
+        protocol_version: 1,
+        ..Params::default()
+    };
+
+    let tokens = ScriptedTokens::new([TOKEN_A]);
+    let mut authenticated = core_with_tokens(
+        ServerConfig::default()
+            .with_hmac_key(KEY)
+            .with_max_packet_length(31),
+        tokens.clone(),
+    );
+    assert_eq!(
+        authenticated
+            .handle_datagram(peer(), &open_request(&minimal, Some(KEY)))
+            .expect("an unservable packet size is not a server error"),
+        None
+    );
+    assert_eq!(authenticated.session_count(), 0);
+    assert_eq!(tokens.remaining(), 1, "no token may be drawn");
+
+    let mut unauthenticated = core_with_tokens(
+        ServerConfig::default().with_max_packet_length(31),
+        ScriptedTokens::new([TOKEN_A]),
+    );
+    let packet = unauthenticated
+        .handle_datagram(peer(), &open_request(&minimal, None))
+        .unwrap()
+        .expect("the same session fits without an authentication field");
+    expect_normal_open_reply(&packet, None);
+    assert_eq!(unauthenticated.session_count(), 1);
+}
+
 #[test]
 fn negative_and_out_of_range_length_and_dscp_survive_negotiation() {
-    // Length and DSCP are accepted as decoded in this slice, including values a
-    // socket could never carry. Restricting a DSCP the server cannot apply is a
-    // later policy decision, not an admission rule.
+    // These lengths are all at or below the default maximum, and DSCP is
+    // accepted as decoded in this slice, including values a socket could never
+    // carry. Restricting a DSCP the server cannot apply is a later policy
+    // decision, not an admission rule.
     //
     // A negative length in particular stays negative in both the reply and the
-    // stored session. It does not need clamping here to be executable: echo
-    // packet sizing in `irtt-proto` floors it at the required field block, so
-    // the session this creates can be run as negotiated.
+    // stored session; the packet-length policy reduces oversized *positive*
+    // lengths and touches nothing else. It does not need clamping here to be
+    // executable: echo packet sizing in `irtt-proto` floors it at the required
+    // field block, so the session this creates can be run as negotiated.
     for (length, dscp) in [(-1, -1), (0, 0), (1472, 184), (-4096, 512)] {
         let requested = Params {
             protocol_version: 1,