crates/irtt-cli/tests/client_multi_target.rs

@@ -778,7 +778,7 @@ fn start_synchronized_peer_close_server(
         );
         assert_eq!(
             packet.len(),
-            echo_packet_len(false, &params).unwrap(),
+            echo_packet_len(false, &params),
             "expected a complete echo probe before synchronized peer-close reply"
         );
         let seq = u32::from_le_bytes(packet[12..16].try_into().unwrap());
@@ -829,7 +829,7 @@ fn start_gated_peer_close_server(
         }
         assert_eq!(
             packet.len(),
-            echo_packet_len(false, &params).unwrap(),
+            echo_packet_len(false, &params),
             "expected a complete echo probe before peer-close reply"
         );
         let seq = u32::from_le_bytes(packet[12..16].try_into().unwrap());

crates/irtt-client/src/async_client.rs

@@ -246,7 +246,7 @@ impl AsyncClient {
             machine,
             schedule: None,
             remote,
-            recv_buffer: vec![0_u8; recv_buffer_size(false, None)?],
+            recv_buffer: vec![0_u8; recv_buffer_size(false, None)],
             applied_traffic_class: None,
             prepared_open: Some(prepared_open),
             prepared_probe: None,
@@ -820,12 +820,7 @@ impl AsyncClient {
                 return Err(Box::new(PreparedAsyncOpenFailure { primary, machine }));
             }
         };
-        let recv_buffer_len = match recv_buffer_size(self.machine.has_hmac(), Some(negotiated)) {
-            Ok(size) => size,
-            Err(primary) => {
-                return Err(Box::new(PreparedAsyncOpenFailure { primary, machine }));
-            }
-        };
+        let recv_buffer_len = recv_buffer_size(self.machine.has_hmac(), Some(negotiated));
         let negotiated_traffic_class = match u8::try_from(negotiated.params.dscp) {
             Ok(traffic_class) => traffic_class,
             Err(_) => {

crates/irtt-client/src/client.rs

@@ -131,7 +131,7 @@ impl Client {
             schedule: None,
             socket,
             remote,
-            recv_buffer: vec![0_u8; recv_buffer_size(false, None)?],
+            recv_buffer: vec![0_u8; recv_buffer_size(false, None)],
             applied_traffic_class: None,
             #[cfg(test)]
             test_hooks: ClientTestHooks::default(),
@@ -530,10 +530,7 @@ impl Client {
             Ok(schedule) => schedule,
             Err(primary) => return Err(Box::new(PreparedClientOpenFailure { primary, machine })),
         };
-        let recv_buffer_len = match recv_buffer_size(self.runtime.has_hmac(), Some(negotiated)) {
-            Ok(size) => size,
-            Err(primary) => return Err(Box::new(PreparedClientOpenFailure { primary, machine })),
-        };
+        let recv_buffer_len = recv_buffer_size(self.runtime.has_hmac(), Some(negotiated));
         #[cfg(test)]
         let recv_buffer_len = self
             .test_hooks

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

@@ -1,8 +1,8 @@
 use super::*;
+pub(crate) use irtt_proto::echo_packet_len;
 use irtt_proto::{
-    echo_packet_len as checked_echo_packet_len, encode_echo_reply, encode_open_reply,
-    flags::FLAG_OPEN, flags::FLAG_REPLY, layout::PacketLayout, Clock, EchoReply, OpenReply,
-    ReceivedStats, StampAt,
+    encode_echo_reply, encode_open_reply, flags::FLAG_OPEN, flags::FLAG_REPLY,
+    layout::PacketLayout, Clock, EchoReply, OpenReply, ReceivedStats, StampAt,
 };
 use std::{
     net::UdpSocket,
@@ -72,11 +72,6 @@ pub(super) fn open_reply(
     .unwrap()
 }
 
-pub(crate) fn echo_packet_len(hmac: bool, params: &Params) -> usize {
-    checked_echo_packet_len(hmac, params)
-        .expect("test params must have a non-negative packet length")
-}
-
 pub(super) fn echo_reply_packet(
     token: u64,
     seq: u32,

crates/irtt-client/src/session/machine.rs

@@ -235,7 +235,6 @@ impl SessionMachine {
                 | irtt_proto::ProtoError::VarintOverflow
                 | irtt_proto::ProtoError::InvalidUtf8
                 | irtt_proto::ProtoError::InvalidEnum { .. }
-                | irtt_proto::ProtoError::NegativePacketLength { .. }
                 | irtt_proto::ProtoError::ParameterLengthTooLarge { .. }
                 | irtt_proto::ProtoError::MalformedParams),
             ) => Err(ClientError::Protocol(error)),
@@ -824,16 +823,13 @@ impl SessionMachine {
     }
 }
 
-pub(crate) fn recv_buffer_size(
-    has_hmac: bool,
-    negotiated: Option<&NegotiatedParams>,
-) -> Result<usize, ClientError> {
-    Ok(match negotiated {
-        Some(negotiated) => echo_packet_len(has_hmac, &negotiated.params)?
+pub(crate) fn recv_buffer_size(has_hmac: bool, negotiated: Option<&NegotiatedParams>) -> usize {
+    match negotiated {
+        Some(negotiated) => echo_packet_len(has_hmac, &negotiated.params)
             .saturating_add(1)
             .max(MIN_RECV_BUFFER_SIZE),
         None => MIN_RECV_BUFFER_SIZE,
-    })
+    }
 }
 
 pub(crate) fn params_from_config(config: &ClientConfig) -> Result<Params, ClientError> {

crates/irtt-client/tests/support/mod.rs

@@ -12,12 +12,13 @@ use std::{
 
 use irtt_client::{Client, ClientConfig, ClientEvent, NegotiatedParams, OpenOutcome, SocketConfig};
 use irtt_proto::{
-    compute_hmac_in_place, decode_request, echo_packet_len, encode_echo_reply, encode_open_reply,
-    flags, verify_packet_hmac, Clock, DecodedRequest, DecodedRequestKind, EchoReply, OpenReply,
+    compute_hmac_in_place, decode_request, encode_echo_reply, encode_open_reply, flags,
+    verify_packet_hmac, Clock, DecodedRequest, DecodedRequestKind, EchoReply, OpenReply,
     PacketLayout, Params, ProtoError, ReceivedStats, ServerFill, StampAt, TimestampFields,
     HMAC_SIZE, MAGIC, PROTOCOL_VERSION,
 };
 
+pub(super) use irtt_proto::echo_packet_len as test_echo_packet_len;
 pub use real_irtt::RealIrtServer;
 
 const HMAC_OFFSET: usize = 4;
@@ -533,11 +534,6 @@ fn open_reply(flags: u8, token: u64, params: &Params, hmac_key: Option<&[u8]>) -
     .unwrap()
 }
 
-pub(super) fn test_echo_packet_len(hmac: bool, params: &Params) -> usize {
-    echo_packet_len(hmac, params)
-        .expect("managed runner test params must have a non-negative packet length")
-}
-
 fn echo_reply_packet(
     token: u64,
     seq: u32,

crates/irtt-proto/src/echo.rs

@@ -66,7 +66,7 @@ pub fn encode_echo_reply(
     hmac_key: Option<&[u8]>,
 ) -> Result<Vec<u8>> {
     let layout = PacketLayout::echo(hmac_key.is_some(), params);
-    let len = echo_packet_len(hmac_key.is_some(), params)?;
+    let len = echo_packet_len(hmac_key.is_some(), params);
     let payload_offset = layout.header_len();
     let available_payload_len = len.saturating_sub(payload_offset);
     if reply.payload.len() > available_payload_len {
@@ -186,7 +186,7 @@ fn validate_echo_length(packet: &[u8], params: &Params, layout: PacketLayout) ->
             actual: packet.len(),
         });
     }
-    let expected_len = echo_packet_len(layout.hmac, params)?;
+    let expected_len = echo_packet_len(layout.hmac, params);
     if packet.len() == expected_len {
         return Ok(false);
     }
@@ -396,7 +396,7 @@ mod tests {
         };
         params.received_stats = ReceivedStats::Both;
         let layout = PacketLayout::echo(true, &params);
-        let mut packet = Vec::with_capacity(echo_packet_len(true, &params).unwrap());
+        let mut packet = Vec::with_capacity(echo_packet_len(true, &params));
         write_header(&mut packet, FLAG_REPLY | FLAG_HMAC);
         packet.extend_from_slice(&[0; HMAC_SIZE]);
         packet.extend_from_slice(&0x7896_b6ab_8771_5213u64.to_le_bytes());

crates/irtt-proto/src/error.rs

@@ -30,8 +30,6 @@ pub enum ProtoError {
     InvalidUtf8,
     #[error("invalid enum value {value} for {name}")]
     InvalidEnum { name: &'static str, value: i64 },
-    #[error("packet length must be non-negative, got {length}")]
-    NegativePacketLength { length: i64 },
     #[error("parameter {tag} length is too large: {length} bytes")]
     ParameterLengthTooLarge { tag: u64, length: u64 },
     #[error("open reply has zero token without close flag")]

crates/irtt-proto/src/layout.rs

@@ -1,7 +1,7 @@
 use crate::{
     params::{Params, StampAt},
-    ProtoError, Result, HEADER_SIZE, HMAC_SIZE, RECV_COUNT_SIZE, RECV_WINDOW_SIZE, SEQ_SIZE,
-    TIMESTAMP_SIZE, TOKEN_SIZE,
+    HEADER_SIZE, HMAC_SIZE, RECV_COUNT_SIZE, RECV_WINDOW_SIZE, SEQ_SIZE, TIMESTAMP_SIZE,
+    TOKEN_SIZE,
 };
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -111,13 +111,23 @@ pub fn echo_header_len(hmac: bool, params: &Params) -> usize {
     PacketLayout::echo(hmac, params).header_len()
 }
 
-pub fn echo_packet_len(hmac: bool, params: &Params) -> Result<usize> {
-    let header_len = echo_header_len(hmac, params);
-    let requested =
-        usize::try_from(params.length).map_err(|_| ProtoError::NegativePacketLength {
-            length: params.length,
-        })?;
-    Ok(header_len.max(requested))
+/// The datagram length of an ECHO request or reply under `params`.
+///
+/// The negotiated length is a floor a peer asks for, never a ceiling: a packet
+/// can never be shorter than the field block its negotiated layout requires, so
+/// the result is `max(header, requested)`.
+///
+/// A negative negotiated length is a legitimate input rather than an encoder
+/// error. It is accepted during open and echoed back unchanged, so a session
+/// can genuinely carry one, and there is no datagram shorter than none — it
+/// therefore requests no space beyond the mandatory field block, exactly as
+/// zero does.
+pub fn echo_packet_len(hmac: bool, params: &Params) -> usize {
+    // Saturating rather than wrapping: on a target whose `usize` is narrower
+    // than the requested length, the largest representable packet is the
+    // closest honest answer, and the encoder's own limits reject it from there.
+    let requested = usize::try_from(params.length.max(0)).unwrap_or(usize::MAX);
+    echo_header_len(hmac, params).max(requested)
 }
 
 #[cfg(test)]
@@ -278,32 +288,18 @@ mod tests {
     }
 
     #[test]
-    fn negotiated_length_never_smaller_than_header() {
+    fn negotiated_length_is_floored_at_the_required_field_block() {
+        // The 60-byte header of this layout is the floor. A negative negotiated
+        // length is not an error: it asks for nothing beyond the mandatory
+        // fields, exactly as zero does.
         let mut p = params(ReceivedStats::Both, StampAt::Both, Clock::Both);
-        p.length = 20;
-        assert_eq!(echo_packet_len(false, &p).unwrap(), 60);
-        p.length = 92;
-        assert_eq!(echo_packet_len(false, &p).unwrap(), 92);
-    }
-
-    #[test]
-    fn checked_packet_len_rejects_negative_requested_length() {
-        let mut p = params(ReceivedStats::Both, StampAt::Both, Clock::Both);
-        p.length = -1;
-
-        assert_eq!(
-            echo_packet_len(false, &p),
-            Err(ProtoError::NegativePacketLength { length: -1 })
-        );
-    }
-
-    #[test]
-    fn checked_packet_len_preserves_non_negative_header_floor() {
-        let mut p = params(ReceivedStats::Both, StampAt::Both, Clock::Both);
-        p.length = 0;
-        assert_eq!(echo_packet_len(false, &p), Ok(60));
-
-        p.length = 92;
-        assert_eq!(echo_packet_len(false, &p), Ok(92));
+        for (length, expected) in [(-4096, 60), (-1, 60), (0, 60), (20, 60), (92, 92)] {
+            p.length = length;
+            assert_eq!(
+                echo_packet_len(false, &p),
+                expected,
+                "unexpected packet length for negotiated length {length}"
+            );
+        }
     }
 }

crates/irtt-proto/src/request.rs

@@ -84,8 +84,8 @@ pub enum DecodedRequestKind<'a> {
 /// # Errors
 ///
 /// Returns [`ProtoError::PayloadTooLarge`] when an ECHO payload does not fit
-/// the negotiated length, and [`ProtoError::NegativePacketLength`] when the
-/// negotiated length is negative.
+/// the negotiated length. A negative negotiated length is not an error: see
+/// [`echo_packet_len`], which floors it at the mandatory field block.
 pub fn encode_request(request: RequestToEncode<'_>, hmac_key: Option<&[u8]>) -> Result<Vec<u8>> {
     match request {
         RequestToEncode::Open { params, no_test } => encode_open(params, no_test, hmac_key),
@@ -205,7 +205,7 @@ fn encode_echo(
     hmac_key: Option<&[u8]>,
 ) -> Result<Vec<u8>> {
     let layout = PacketLayout::echo(hmac_key.is_some(), params);
-    let len = echo_packet_len(hmac_key.is_some(), params)?;
+    let len = echo_packet_len(hmac_key.is_some(), params);
     let payload_offset = layout.header_len();
     let available_payload_len = len.saturating_sub(payload_offset);
     if payload.len() > available_payload_len {

crates/irtt-proto/tests/directional_codecs.rs

@@ -305,6 +305,32 @@ fn echo_payload_is_copied_zero_filled_and_bounded_in_both_directions() {
     );
 }
 
+#[test]
+fn a_negative_negotiated_length_still_encodes_in_both_directions() {
+    // A server accepts a negative negotiated length during open and returns it
+    // unchanged, so a live session can hold one and must stay executable. There
+    // is no datagram shorter than none, so both directions fall back to the
+    // mandatory field block — header (4) + token (8) + sequence (4) — exactly
+    // as a zero length would.
+    let params = Params {
+        length: -4096,
+        ..Params::default()
+    };
+    assert_eq!(echo_packet_len(false, &params), 16);
+
+    let request = encode_request(echo_request(&params, &[]), None)
+        .expect("a negative negotiated length must still encode a request");
+    assert_eq!(request.len(), 16);
+
+    let reply = encode_echo_reply(&echo_reply(&params, FLAG_REPLY), &params, None)
+        .expect("a negative negotiated length must still encode a reply");
+    assert_eq!(reply.len(), 16);
+    assert_eq!(
+        decode_echo_reply(&reply, &params, None).unwrap().token,
+        TOKEN
+    );
+}
+
 // ---------- Upstream midpoint ECHO-reply compatibility ----------
 //
 // Verified upstream irtt 0.9.1 behavior: for single-clock
@@ -346,7 +372,7 @@ fn midpoint_params(clock: Clock, payload_space: usize) -> Params {
 
 fn midpoint_compat_len(params: &Params, hmac: bool) -> usize {
     let compat_header = PacketLayout::echo(hmac, params).header_len() + 8;
-    compat_header.max(echo_packet_len(hmac, params).unwrap())
+    compat_header.max(echo_packet_len(hmac, params))
 }
 
 /// Hand-builds upstream's dual-midpoint ECHO reply: both midpoint fields on
@@ -399,7 +425,7 @@ fn midpoint_dual_field_reply_is_accepted_wherever_length_identifies_it() {
     for (payload_space, expected_extra) in [(0, 8), (1, 7), (4, 4), (7, 1)] {
         for clock in [Clock::Wall, Clock::Monotonic] {
             let params = midpoint_params(clock, payload_space);
-            let normal_len = echo_packet_len(false, &params).unwrap();
+            let normal_len = echo_packet_len(false, &params);
             let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 3, (111, 222), None);
             assert_eq!(
                 packet.len(),
@@ -438,7 +464,7 @@ fn midpoint_dual_field_reply_of_equal_length_is_accepted_but_not_correctable() {
     for payload_space in [8, 64] {
         for clock in [Clock::Wall, Clock::Monotonic] {
             let params = midpoint_params(clock, payload_space);
-            let normal_len = echo_packet_len(false, &params).unwrap();
+            let normal_len = echo_packet_len(false, &params);
             let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 4, (111, 222), None);
             assert_eq!(
                 packet.len(),
@@ -491,7 +517,7 @@ fn midpoint_dual_field_reply_authenticates_over_full_packet() {
 fn only_the_exact_compat_length_is_accepted_as_an_extension() {
     let params = midpoint_params(Clock::Wall, 0);
     let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 1, (10, 20), None);
-    let expected_len = echo_packet_len(false, &params).unwrap();
+    let expected_len = echo_packet_len(false, &params);
     assert_eq!(packet.len(), expected_len + 8);
 
     // Every other length around the compatibility size stays rejected,

crates/irtt-proto/tests/request_codec.rs

@@ -450,21 +450,34 @@ fn encoded_requests_decode_back_to_their_sender_side_identity() {
 }
 
 #[test]
-fn echo_encoding_rejects_a_negative_negotiated_length() {
+fn echo_encoding_floors_a_negative_negotiated_length_at_the_field_block() {
+    // A negative negotiated length is accepted during open and echoed back
+    // unchanged, so a real session can carry one. It must therefore stay
+    // encodable: it asks for no space beyond the mandatory fields.
     let params = Params {
         length: -1,
         ..Params::default()
     };
+    let packet = encode_request(
+        RequestToEncode::Echo {
+            token: TOKEN,
+            sequence: SEQUENCE,
+            params: &params,
+            payload: &[],
+        },
+        None,
+    )
+    .expect("a negative negotiated length must still encode");
+
+    // header (4) + token (8) + sequence (4).
+    assert_eq!(packet.len(), 16);
+    let request = decode_request(&packet).unwrap();
     assert_eq!(
-        encode_request(
-            RequestToEncode::Echo {
-                token: TOKEN,
-                sequence: SEQUENCE,
-                params: &params,
-                payload: &[],
-            },
-            None,
-        ),
-        Err(ProtoError::NegativePacketLength { length: -1 })
+        request.kind,
+        DecodedRequestKind::Echo {
+            token: TOKEN,
+            sequence: SEQUENCE,
+            tail: &[],
+        }
     );
 }