feat(project): Establish initial IRTT workspace and protocol definitions

crates/irtt-cli/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "irtt-cli"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]

crates/irtt-cli/src/main.rs

@@ -0,0 +1 @@
+fn main() {}

crates/irtt-client/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "irtt-client"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]

crates/irtt-client/src/lib.rs

@@ -0,0 +1,4 @@
+//! Client crate skeleton.
+//!
+//! Networking and session runtime are intentionally out of scope for the
+//! current milestone.

crates/irtt-proto/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "irtt-proto"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+hmac = "0.12"
+md-5 = "0.10"
+subtle = "2"
+thiserror = "1"

crates/irtt-proto/src/close.rs

@@ -0,0 +1,46 @@
+use crate::{
+    flags::{FLAG_CLOSE, FLAG_HMAC},
+    hmac, write_header, Result, HMAC_SIZE,
+};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CloseRequest {
+    pub token: u64,
+}
+
+pub fn encode_close_request(request: &CloseRequest, hmac_key: Option<&[u8]>) -> Result<Vec<u8>> {
+    let mut flags = FLAG_CLOSE;
+    if hmac_key.is_some() {
+        flags |= FLAG_HMAC;
+    }
+
+    let mut out = Vec::new();
+    write_header(&mut out, flags);
+    if hmac_key.is_some() {
+        out.extend_from_slice(&[0; HMAC_SIZE]);
+    }
+    out.extend_from_slice(&request.token.to_le_bytes());
+    if let Some(key) = hmac_key {
+        hmac::compute_hmac_in_place(key, &mut out, hmac::hmac_offset())?;
+    }
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn close_request_sizes() {
+        let request = CloseRequest {
+            token: 0x7896_b6ab_8771_5213,
+        };
+        assert_eq!(encode_close_request(&request, None).unwrap().len(), 12);
+        assert_eq!(
+            encode_close_request(&request, Some(b"testkey"))
+                .unwrap()
+                .len(),
+            28
+        );
+    }
+}

crates/irtt-proto/src/echo.rs

@@ -0,0 +1,204 @@
+use crate::{
+    flags::{has, FLAG_CLOSE, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY},
+    hmac,
+    layout::{echo_packet_len, PacketLayout},
+    open::{check_hmac_presence, reject},
+    params::Params,
+    validate_header, write_header, ProtoError, Result, HEADER_SIZE, HMAC_SIZE, RECV_COUNT_SIZE,
+    RECV_WINDOW_SIZE, SEQ_SIZE, TIMESTAMP_SIZE, TOKEN_SIZE,
+};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct EchoRequest {
+    pub token: u64,
+    pub sequence: u32,
+    pub params: Params,
+    pub payload: Vec<u8>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct EchoReply {
+    pub flags: u8,
+    pub token: u64,
+    pub sequence: u32,
+    pub recv_count: Option<u32>,
+    pub recv_window: Option<u64>,
+    pub timestamps: TimestampFields,
+    pub payload: Vec<u8>,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct TimestampFields {
+    pub recv_wall: Option<i64>,
+    pub recv_mono: Option<i64>,
+    pub midpoint_wall: Option<i64>,
+    pub midpoint_mono: Option<i64>,
+    pub send_wall: Option<i64>,
+    pub send_mono: Option<i64>,
+}
+
+pub fn encode_echo_request(request: &EchoRequest, hmac_key: Option<&[u8]>) -> Result<Vec<u8>> {
+    let mut flags = 0;
+    if hmac_key.is_some() {
+        flags |= FLAG_HMAC;
+    }
+    let layout = PacketLayout::echo(hmac_key.is_some(), &request.params);
+    let len = echo_packet_len(hmac_key.is_some(), &request.params);
+    let mut out = Vec::with_capacity(len);
+    write_header(&mut out, flags);
+    if hmac_key.is_some() {
+        out.extend_from_slice(&[0; HMAC_SIZE]);
+    }
+    out.extend_from_slice(&request.token.to_le_bytes());
+    out.extend_from_slice(&request.sequence.to_le_bytes());
+    push_zeroed_layout_tail(layout, &mut out);
+    out.resize(len, 0);
+    let payload_offset = layout.header_len();
+    let payload_len = out
+        .len()
+        .saturating_sub(payload_offset)
+        .min(request.payload.len());
+    out[payload_offset..payload_offset + payload_len]
+        .copy_from_slice(&request.payload[..payload_len]);
+
+    if let Some(key) = hmac_key {
+        hmac::compute_hmac_in_place(key, &mut out, hmac::hmac_offset())?;
+    }
+    Ok(out)
+}
+
+pub fn decode_echo_reply(
+    packet: &[u8],
+    params: &Params,
+    hmac_key: Option<&[u8]>,
+) -> Result<EchoReply> {
+    let flags = validate_header(packet)?;
+    reject(flags, FLAG_OPEN)?;
+    crate::open::require(flags, FLAG_REPLY)?;
+    check_hmac_presence(flags, hmac_key)?;
+    if let Some(key) = hmac_key {
+        hmac::verify_hmac(key, packet, hmac::hmac_offset())?;
+    }
+
+    let layout = PacketLayout::echo(has(flags, FLAG_HMAC), params);
+    let header_len = layout.header_len();
+    if packet.len() < header_len {
+        return Err(ProtoError::PacketTooShort {
+            needed: header_len,
+            actual: packet.len(),
+        });
+    }
+
+    let mut pos = HEADER_SIZE;
+    if layout.hmac {
+        pos += HMAC_SIZE;
+    }
+    let token = read_u64(packet, &mut pos);
+    let sequence = read_u32(packet, &mut pos);
+    let recv_count = layout.recv_count.then(|| read_u32(packet, &mut pos));
+    let recv_window = layout.recv_window.then(|| read_u64(packet, &mut pos));
+    let timestamps = TimestampFields {
+        recv_wall: layout.recv_wall.then(|| read_i64(packet, &mut pos)),
+        recv_mono: layout.recv_mono.then(|| read_i64(packet, &mut pos)),
+        midpoint_wall: layout.midpoint_wall.then(|| read_i64(packet, &mut pos)),
+        midpoint_mono: layout.midpoint_mono.then(|| read_i64(packet, &mut pos)),
+        send_wall: layout.send_wall.then(|| read_i64(packet, &mut pos)),
+        send_mono: layout.send_mono.then(|| read_i64(packet, &mut pos)),
+    };
+
+    Ok(EchoReply {
+        flags,
+        token,
+        sequence,
+        recv_count,
+        recv_window,
+        timestamps,
+        payload: packet[header_len..].to_vec(),
+    })
+}
+
+fn push_zeroed_layout_tail(layout: PacketLayout, out: &mut Vec<u8>) {
+    if layout.recv_count {
+        out.extend_from_slice(&[0; RECV_COUNT_SIZE]);
+    }
+    if layout.recv_window {
+        out.extend_from_slice(&[0; RECV_WINDOW_SIZE]);
+    }
+    for _ in 0..layout.timestamp_count() {
+        out.extend_from_slice(&[0; TIMESTAMP_SIZE]);
+    }
+}
+
+fn read_u32(packet: &[u8], pos: &mut usize) -> u32 {
+    let value = u32::from_le_bytes(packet[*pos..*pos + SEQ_SIZE].try_into().unwrap());
+    *pos += SEQ_SIZE;
+    value
+}
+
+fn read_u64(packet: &[u8], pos: &mut usize) -> u64 {
+    let value = u64::from_le_bytes(packet[*pos..*pos + TOKEN_SIZE].try_into().unwrap());
+    *pos += TOKEN_SIZE;
+    value
+}
+
+fn read_i64(packet: &[u8], pos: &mut usize) -> i64 {
+    let value = i64::from_le_bytes(packet[*pos..*pos + TIMESTAMP_SIZE].try_into().unwrap());
+    *pos += TIMESTAMP_SIZE;
+    value
+}
+
+#[allow(dead_code)]
+fn _assert_close_flag_imported(_: u8) {
+    let _ = FLAG_CLOSE;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::params::{Clock, ReceivedStats, StampAt};
+
+    fn default_params() -> Params {
+        Params {
+            received_stats: ReceivedStats::Both,
+            stamp_at: StampAt::Both,
+            clock: Clock::Both,
+            ..Params::default()
+        }
+    }
+
+    #[test]
+    fn echo_request_encodes_default_placeholders() {
+        let packet = encode_echo_request(
+            &EchoRequest {
+                token: 0x7896_b6ab_8771_5213,
+                sequence: 0,
+                params: default_params(),
+                payload: Vec::new(),
+            },
+            None,
+        )
+        .unwrap();
+        assert_eq!(packet.len(), 60);
+        assert_eq!(&packet[..4], &[0x14, 0xa7, 0x5b, 0x00]);
+        assert_eq!(&packet[4..12], &0x7896_b6ab_8771_5213u64.to_le_bytes());
+        assert!(packet[12..].iter().all(|byte| *byte == 0));
+    }
+
+    #[test]
+    fn echo_reply_decodes_default_fields() {
+        let packet = [
+            0x14, 0xa7, 0x5b, 0x02, 0x13, 0x52, 0x71, 0x87, 0xab, 0xb6, 0x96, 0x78, 0x02, 0x00,
+            0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x07, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0xb8, 0x1a, 0x33, 0x0c, 0x86, 0x6d, 0xaa, 0x18, 0xde, 0x26, 0x35, 0x95, 0x00, 0x00,
+            0x00, 0x00, 0x80, 0x4d, 0x33, 0x0c, 0x86, 0x6d, 0xaa, 0x18, 0xb2, 0x57, 0x35, 0x95,
+            0x00, 0x00, 0x00, 0x00,
+        ];
+        let reply = decode_echo_reply(&packet, &default_params(), None).unwrap();
+        assert_eq!(reply.token, 0x7896_b6ab_8771_5213);
+        assert_eq!(reply.sequence, 2);
+        assert_eq!(reply.recv_count, Some(3));
+        assert_eq!(reply.recv_window, Some(7));
+        assert!(reply.timestamps.recv_wall.is_some());
+        assert!(reply.timestamps.midpoint_wall.is_none());
+    }
+}

crates/irtt-proto/src/error.rs

@@ -0,0 +1,35 @@
+use thiserror::Error;
+
+pub type Result<T> = std::result::Result<T, ProtoError>;
+
+#[derive(Debug, Error, PartialEq, Eq)]
+pub enum ProtoError {
+    #[error("packet is too short: needed {needed} bytes, got {actual}")]
+    PacketTooShort { needed: usize, actual: usize },
+    #[error("bad magic bytes")]
+    BadMagic,
+    #[error("reserved flag bits are set: 0x{0:02x}")]
+    ReservedFlags(u8),
+    #[error("required flag 0x{0:02x} is missing")]
+    MissingFlag(u8),
+    #[error("unexpected flag 0x{0:02x} is set")]
+    UnexpectedFlag(u8),
+    #[error("HMAC flag/key mismatch")]
+    HmacPresenceMismatch,
+    #[error("HMAC verification failed")]
+    BadHmac,
+    #[error("invalid HMAC field offset")]
+    InvalidHmacOffset,
+    #[error("varint is truncated")]
+    TruncatedVarint,
+    #[error("varint is too long")]
+    VarintOverflow,
+    #[error("string value is not valid UTF-8")]
+    InvalidUtf8,
+    #[error("invalid enum value {value} for {name}")]
+    InvalidEnum { name: &'static str, value: i64 },
+    #[error("open reply has zero token without close flag")]
+    ZeroToken,
+    #[error("trailing or malformed parameter payload")]
+    MalformedParams,
+}

crates/irtt-proto/src/flags.rs

@@ -0,0 +1,20 @@
+use crate::{error::Result, ProtoError};
+
+pub const FLAG_OPEN: u8 = 0x01;
+pub const FLAG_REPLY: u8 = 0x02;
+pub const FLAG_CLOSE: u8 = 0x04;
+pub const FLAG_HMAC: u8 = 0x08;
+
+pub const RESERVED_FLAGS_MASK: u8 = 0xF0;
+
+pub fn validate_flags(flags: u8) -> Result<()> {
+    let reserved = flags & RESERVED_FLAGS_MASK;
+    if reserved != 0 {
+        return Err(ProtoError::ReservedFlags(reserved));
+    }
+    Ok(())
+}
+
+pub fn has(flags: u8, flag: u8) -> bool {
+    flags & flag != 0
+}

crates/irtt-proto/src/hmac.rs

@@ -0,0 +1,71 @@
+use hmac::{Hmac, Mac};
+use md5::Md5;
+use subtle::ConstantTimeEq;
+
+use crate::{flags::FLAG_HMAC, ProtoError, Result, HEADER_SIZE, HMAC_SIZE};
+
+type HmacMd5 = Hmac<Md5>;
+
+pub fn compute_hmac(key: &[u8], packet: &[u8], hmac_offset: usize) -> Result<[u8; HMAC_SIZE]> {
+    if packet.len().saturating_sub(hmac_offset) < HMAC_SIZE {
+        return Err(ProtoError::InvalidHmacOffset);
+    }
+    let mut copy = packet.to_vec();
+    copy[hmac_offset..hmac_offset + HMAC_SIZE].fill(0);
+    let mut mac = HmacMd5::new_from_slice(key).expect("HMAC accepts keys of any size");
+    mac.update(&copy);
+    let bytes = mac.finalize().into_bytes();
+    let mut out = [0u8; HMAC_SIZE];
+    out.copy_from_slice(&bytes);
+    Ok(out)
+}
+
+pub fn compute_hmac_in_place(key: &[u8], packet: &mut [u8], hmac_offset: usize) -> Result<()> {
+    if packet.len().saturating_sub(hmac_offset) < HMAC_SIZE {
+        return Err(ProtoError::InvalidHmacOffset);
+    }
+    packet[3] |= FLAG_HMAC;
+    packet[hmac_offset..hmac_offset + HMAC_SIZE].fill(0);
+    let digest = compute_hmac(key, packet, hmac_offset)?;
+    packet[hmac_offset..hmac_offset + HMAC_SIZE].copy_from_slice(&digest);
+    Ok(())
+}
+
+pub fn verify_hmac(key: &[u8], packet: &[u8], hmac_offset: usize) -> Result<()> {
+    if packet.len().saturating_sub(hmac_offset) < HMAC_SIZE {
+        return Err(ProtoError::InvalidHmacOffset);
+    }
+    let expected = compute_hmac(key, packet, hmac_offset)?;
+    let actual = &packet[hmac_offset..hmac_offset + HMAC_SIZE];
+    if expected.as_slice().ct_eq(actual).into() {
+        Ok(())
+    } else {
+        Err(ProtoError::BadHmac)
+    }
+}
+
+pub(crate) fn hmac_offset() -> usize {
+    HEADER_SIZE
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{flags::FLAG_OPEN, MAGIC};
+
+    #[test]
+    fn compute_and_verify_hmac() {
+        let mut packet = Vec::new();
+        packet.extend_from_slice(&MAGIC);
+        packet.push(FLAG_OPEN | FLAG_HMAC);
+        packet.extend_from_slice(&[0; HMAC_SIZE]);
+        packet.extend_from_slice(&[1, 2, 3, 4]);
+
+        compute_hmac_in_place(b"testkey", &mut packet, hmac_offset()).unwrap();
+        verify_hmac(b"testkey", &packet, hmac_offset()).unwrap();
+        assert_eq!(
+            verify_hmac(b"wrong", &packet, hmac_offset()),
+            Err(ProtoError::BadHmac)
+        );
+    }
+}

crates/irtt-proto/src/layout.rs

@@ -0,0 +1,191 @@
+use crate::{
+    params::{Clock, Params, StampAt},
+    HEADER_SIZE, HMAC_SIZE, RECV_COUNT_SIZE, RECV_WINDOW_SIZE, SEQ_SIZE, TIMESTAMP_SIZE,
+    TOKEN_SIZE,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct PacketLayout {
+    pub hmac: bool,
+    pub token: bool,
+    pub sequence: bool,
+    pub recv_count: bool,
+    pub recv_window: bool,
+    pub recv_wall: bool,
+    pub recv_mono: bool,
+    pub midpoint_wall: bool,
+    pub midpoint_mono: bool,
+    pub send_wall: bool,
+    pub send_mono: bool,
+}
+
+impl PacketLayout {
+    pub fn open_request(hmac: bool) -> Self {
+        Self {
+            hmac,
+            token: false,
+            sequence: false,
+            recv_count: false,
+            recv_window: false,
+            recv_wall: false,
+            recv_mono: false,
+            midpoint_wall: false,
+            midpoint_mono: false,
+            send_wall: false,
+            send_mono: false,
+        }
+    }
+
+    pub fn open_reply(hmac: bool) -> Self {
+        Self {
+            token: true,
+            ..Self::open_request(hmac)
+        }
+    }
+
+    pub fn echo(hmac: bool, params: &Params) -> Self {
+        let clock = params.clock;
+        Self {
+            hmac,
+            token: true,
+            sequence: true,
+            recv_count: params.received_stats.has_count(),
+            recv_window: params.received_stats.has_window(),
+            recv_wall: matches!(params.stamp_at, StampAt::Receive | StampAt::Both)
+                && clock.has_wall(),
+            recv_mono: matches!(params.stamp_at, StampAt::Receive | StampAt::Both)
+                && clock.has_mono(),
+            midpoint_wall: matches!(params.stamp_at, StampAt::Midpoint) && clock.has_wall(),
+            midpoint_mono: matches!(params.stamp_at, StampAt::Midpoint) && clock.has_mono(),
+            send_wall: matches!(params.stamp_at, StampAt::Send | StampAt::Both) && clock.has_wall(),
+            send_mono: matches!(params.stamp_at, StampAt::Send | StampAt::Both) && clock.has_mono(),
+        }
+    }
+
+    pub fn close_request(hmac: bool) -> Self {
+        Self {
+            hmac,
+            token: true,
+            sequence: false,
+            recv_count: false,
+            recv_window: false,
+            recv_wall: false,
+            recv_mono: false,
+            midpoint_wall: false,
+            midpoint_mono: false,
+            send_wall: false,
+            send_mono: false,
+        }
+    }
+
+    pub fn header_len(self) -> usize {
+        HEADER_SIZE
+            + if self.hmac { HMAC_SIZE } else { 0 }
+            + if self.token { TOKEN_SIZE } else { 0 }
+            + if self.sequence { SEQ_SIZE } else { 0 }
+            + if self.recv_count { RECV_COUNT_SIZE } else { 0 }
+            + if self.recv_window {
+                RECV_WINDOW_SIZE
+            } else {
+                0
+            }
+            + self.timestamp_count() * TIMESTAMP_SIZE
+    }
+
+    pub fn timestamp_count(self) -> usize {
+        [
+            self.recv_wall,
+            self.recv_mono,
+            self.midpoint_wall,
+            self.midpoint_mono,
+            self.send_wall,
+            self.send_mono,
+        ]
+        .into_iter()
+        .filter(|present| *present)
+        .count()
+    }
+}
+
+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) -> usize {
+    let header_len = echo_header_len(hmac, params);
+    let requested = usize::try_from(params.length).unwrap_or(0);
+    header_len.max(requested)
+}
+
+#[allow(dead_code)]
+fn _assert_clock_is_used(_: Clock) {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::params::{ReceivedStats, StampAt};
+
+    fn params(stats: ReceivedStats, stamp_at: StampAt, clock: Clock) -> Params {
+        Params {
+            received_stats: stats,
+            stamp_at,
+            clock,
+            ..Params::default()
+        }
+    }
+
+    #[test]
+    fn verified_layout_lengths() {
+        assert_eq!(
+            echo_header_len(
+                false,
+                &params(ReceivedStats::None, StampAt::None, Clock::None)
+            ),
+            16
+        );
+        assert_eq!(
+            echo_header_len(
+                false,
+                &params(ReceivedStats::Count, StampAt::Send, Clock::Wall)
+            ),
+            28
+        );
+        assert_eq!(
+            echo_header_len(
+                false,
+                &params(ReceivedStats::Window, StampAt::Receive, Clock::Monotonic)
+            ),
+            32
+        );
+        assert_eq!(
+            echo_header_len(
+                false,
+                &params(ReceivedStats::Both, StampAt::Midpoint, Clock::Both)
+            ),
+            44
+        );
+        assert_eq!(
+            echo_header_len(
+                false,
+                &params(ReceivedStats::Both, StampAt::Both, Clock::Both)
+            ),
+            60
+        );
+        assert_eq!(
+            echo_header_len(
+                true,
+                &params(ReceivedStats::Both, StampAt::Both, Clock::Both)
+            ),
+            76
+        );
+    }
+
+    #[test]
+    fn negotiated_length_never_smaller_than_header() {
+        let mut p = params(ReceivedStats::Both, StampAt::Both, Clock::Both);
+        p.length = 20;
+        assert_eq!(echo_packet_len(false, &p), 60);
+        p.length = 92;
+        assert_eq!(echo_packet_len(false, &p), 92);
+    }
+}

crates/irtt-proto/src/lib.rs

@@ -0,0 +1,52 @@
+#![forbid(unsafe_code)]
+
+pub mod close;
+pub mod echo;
+pub mod error;
+pub mod flags;
+pub mod hmac;
+pub mod layout;
+pub mod open;
+pub mod params;
+pub mod varint;
+
+pub use close::{encode_close_request, CloseRequest};
+pub use echo::{decode_echo_reply, encode_echo_request, EchoReply, EchoRequest, TimestampFields};
+pub use error::{ProtoError, Result};
+pub use flags::*;
+pub use hmac::{compute_hmac, compute_hmac_in_place, verify_hmac};
+pub use layout::{echo_header_len, echo_packet_len, PacketLayout};
+pub use open::{decode_open_reply, encode_open_request, OpenReply, OpenRequest};
+pub use params::{Clock, Params, ReceivedStats, ServerFill, StampAt};
+
+pub const MAGIC: [u8; 3] = [0x14, 0xA7, 0x5B];
+pub const PROTOCOL_VERSION: i64 = 1;
+
+pub const HMAC_SIZE: usize = 16;
+pub const TOKEN_SIZE: usize = 8;
+pub const SEQ_SIZE: usize = 4;
+pub const RECV_COUNT_SIZE: usize = 4;
+pub const RECV_WINDOW_SIZE: usize = 8;
+pub const TIMESTAMP_SIZE: usize = 8;
+
+pub(crate) const HEADER_SIZE: usize = 4;
+
+pub(crate) fn write_header(out: &mut Vec<u8>, flags: u8) {
+    out.extend_from_slice(&MAGIC);
+    out.push(flags);
+}
+
+pub(crate) fn validate_header(packet: &[u8]) -> Result<u8> {
+    if packet.len() < HEADER_SIZE {
+        return Err(ProtoError::PacketTooShort {
+            needed: HEADER_SIZE,
+            actual: packet.len(),
+        });
+    }
+    if packet[..3] != MAGIC {
+        return Err(ProtoError::BadMagic);
+    }
+    let flags = packet[3];
+    flags::validate_flags(flags)?;
+    Ok(flags)
+}

crates/irtt-proto/src/open.rs

@@ -0,0 +1,143 @@
+use crate::{
+    flags::{has, FLAG_CLOSE, FLAG_HMAC, FLAG_OPEN, FLAG_REPLY},
+    hmac,
+    params::Params,
+    validate_header, write_header, ProtoError, Result, HEADER_SIZE, HMAC_SIZE, TOKEN_SIZE,
+};
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct OpenRequest {
+    pub params: Params,
+    pub close: bool,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct OpenReply {
+    pub flags: u8,
+    pub token: u64,
+    pub params: Params,
+}
+
+pub fn encode_open_request(request: &OpenRequest, hmac_key: Option<&[u8]>) -> Result<Vec<u8>> {
+    let mut flags = FLAG_OPEN;
+    if request.close {
+        flags |= FLAG_CLOSE;
+    }
+    if hmac_key.is_some() {
+        flags |= FLAG_HMAC;
+    }
+
+    let mut out = Vec::new();
+    write_header(&mut out, flags);
+    if hmac_key.is_some() {
+        out.extend_from_slice(&[0; HMAC_SIZE]);
+    }
+    out.extend_from_slice(&request.params.encode());
+
+    if let Some(key) = hmac_key {
+        hmac::compute_hmac_in_place(key, &mut out, hmac::hmac_offset())?;
+    }
+    Ok(out)
+}
+
+pub fn decode_open_reply(packet: &[u8], hmac_key: Option<&[u8]>) -> Result<OpenReply> {
+    let flags = validate_header(packet)?;
+    require(flags, FLAG_OPEN)?;
+    require(flags, FLAG_REPLY)?;
+    check_hmac_presence(flags, hmac_key)?;
+    if let Some(key) = hmac_key {
+        hmac::verify_hmac(key, packet, hmac::hmac_offset())?;
+    }
+
+    let mut pos = HEADER_SIZE;
+    if has(flags, FLAG_HMAC) {
+        pos += HMAC_SIZE;
+    }
+    let needed = pos + TOKEN_SIZE;
+    if packet.len() < needed {
+        return Err(ProtoError::PacketTooShort {
+            needed,
+            actual: packet.len(),
+        });
+    }
+    let token = u64::from_le_bytes(packet[pos..pos + TOKEN_SIZE].try_into().unwrap());
+    pos += TOKEN_SIZE;
+    if token == 0 && !has(flags, FLAG_CLOSE) {
+        return Err(ProtoError::ZeroToken);
+    }
+
+    Ok(OpenReply {
+        flags,
+        token,
+        params: Params::decode(&packet[pos..])?,
+    })
+}
+
+pub(crate) fn require(flags: u8, flag: u8) -> Result<()> {
+    if has(flags, flag) {
+        Ok(())
+    } else {
+        Err(ProtoError::MissingFlag(flag))
+    }
+}
+
+pub(crate) fn reject(flags: u8, flag: u8) -> Result<()> {
+    if has(flags, flag) {
+        Err(ProtoError::UnexpectedFlag(flag))
+    } else {
+        Ok(())
+    }
+}
+
+pub(crate) fn check_hmac_presence(flags: u8, hmac_key: Option<&[u8]>) -> Result<()> {
+    if has(flags, FLAG_HMAC) == hmac_key.is_some() {
+        Ok(())
+    } else {
+        Err(ProtoError::HmacPresenceMismatch)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::params::{Clock, ReceivedStats, StampAt};
+
+    fn default_params() -> Params {
+        Params {
+            protocol_version: 1,
+            duration_ns: 3_000_000_000,
+            interval_ns: 1_000_000_000,
+            received_stats: ReceivedStats::Both,
+            stamp_at: StampAt::Both,
+            clock: Clock::Both,
+            ..Params::default()
+        }
+    }
+
+    #[test]
+    fn open_request_has_no_token() {
+        let packet = encode_open_request(
+            &OpenRequest {
+                params: default_params(),
+                close: false,
+            },
+            None,
+        )
+        .unwrap();
+
+        assert_eq!(packet.len(), 24);
+        assert_eq!(&packet[..4], &[0x14, 0xa7, 0x5b, 0x01]);
+        assert_eq!(&packet[4..], &default_params().encode());
+    }
+
+    #[test]
+    fn open_reply_decodes_token() {
+        let mut packet = vec![0x14, 0xa7, 0x5b, 0x03];
+        packet.extend_from_slice(&0x7896_b6ab_8771_5213u64.to_le_bytes());
+        packet.extend_from_slice(&default_params().encode());
+
+        let reply = decode_open_reply(&packet, None).unwrap();
+        assert_eq!(reply.token, 0x7896_b6ab_8771_5213);
+        assert_eq!(reply.params, default_params());
+    }
+}

crates/irtt-proto/src/params.rs

@@ -0,0 +1,235 @@
+use crate::{varint, ProtoError, Result, PROTOCOL_VERSION};
+
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct Params {
+    pub protocol_version: i64,
+    pub duration_ns: i64,
+    pub interval_ns: i64,
+    pub length: i64,
+    pub received_stats: ReceivedStats,
+    pub stamp_at: StampAt,
+    pub clock: Clock,
+    pub dscp: i64,
+    pub server_fill: Option<ServerFill>,
+}
+
+impl Params {
+    pub fn with_protocol_defaults() -> Self {
+        Self {
+            protocol_version: PROTOCOL_VERSION,
+            ..Self::default()
+        }
+    }
+
+    pub fn encode(&self) -> Vec<u8> {
+        let mut out = Vec::new();
+        push_int(1, self.protocol_version, &mut out);
+        push_int(2, self.duration_ns, &mut out);
+        push_int(3, self.interval_ns, &mut out);
+        push_int(4, self.length, &mut out);
+        push_int(5, self.received_stats as i64, &mut out);
+        push_int(6, self.stamp_at as i64, &mut out);
+        push_int(7, self.clock as i64, &mut out);
+        push_int(8, self.dscp, &mut out);
+        if let Some(fill) = &self.server_fill {
+            varint::encode_uvarint(9, &mut out);
+            varint::encode_uvarint(fill.value.len() as u64, &mut out);
+            out.extend_from_slice(fill.value.as_bytes());
+        }
+        out
+    }
+
+    pub fn decode(input: &[u8]) -> Result<Self> {
+        let mut params = Self::default();
+        let mut pos = 0;
+        while pos < input.len() {
+            let (tag, used) = varint::decode_uvarint(&input[pos..])?;
+            pos += used;
+            match tag {
+                1 => params.protocol_version = read_int(input, &mut pos)?,
+                2 => params.duration_ns = read_int(input, &mut pos)?,
+                3 => params.interval_ns = read_int(input, &mut pos)?,
+                4 => params.length = read_int(input, &mut pos)?,
+                5 => params.received_stats = ReceivedStats::try_from(read_int(input, &mut pos)?)?,
+                6 => params.stamp_at = StampAt::try_from(read_int(input, &mut pos)?)?,
+                7 => params.clock = Clock::try_from(read_int(input, &mut pos)?)?,
+                8 => params.dscp = read_int(input, &mut pos)?,
+                9 => {
+                    let (len, used) = varint::decode_uvarint(&input[pos..])?;
+                    pos += used;
+                    let len = len as usize;
+                    if input.len().saturating_sub(pos) < len {
+                        return Err(ProtoError::MalformedParams);
+                    }
+                    let value = std::str::from_utf8(&input[pos..pos + len])
+                        .map_err(|_| ProtoError::InvalidUtf8)?
+                        .to_owned();
+                    pos += len;
+                    params.server_fill = Some(ServerFill { value });
+                }
+                _ => {
+                    let (_, used) = varint::decode_uvarint(&input[pos..])?;
+                    pos += used;
+                }
+            }
+        }
+        Ok(params)
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ServerFill {
+    pub value: String,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[repr(i64)]
+pub enum ReceivedStats {
+    #[default]
+    None = 0,
+    Count = 1,
+    Window = 2,
+    Both = 3,
+}
+
+impl ReceivedStats {
+    pub fn has_count(self) -> bool {
+        matches!(self, Self::Count | Self::Both)
+    }
+
+    pub fn has_window(self) -> bool {
+        matches!(self, Self::Window | Self::Both)
+    }
+}
+
+impl TryFrom<i64> for ReceivedStats {
+    type Error = ProtoError;
+
+    fn try_from(value: i64) -> Result<Self> {
+        match value {
+            0 => Ok(Self::None),
+            1 => Ok(Self::Count),
+            2 => Ok(Self::Window),
+            3 => Ok(Self::Both),
+            _ => Err(ProtoError::InvalidEnum {
+                name: "ReceivedStats",
+                value,
+            }),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[repr(i64)]
+pub enum StampAt {
+    #[default]
+    None = 0,
+    Send = 1,
+    Receive = 2,
+    Both = 3,
+    Midpoint = 4,
+}
+
+impl TryFrom<i64> for StampAt {
+    type Error = ProtoError;
+
+    fn try_from(value: i64) -> Result<Self> {
+        match value {
+            0 => Ok(Self::None),
+            1 => Ok(Self::Send),
+            2 => Ok(Self::Receive),
+            3 => Ok(Self::Both),
+            4 => Ok(Self::Midpoint),
+            _ => Err(ProtoError::InvalidEnum {
+                name: "StampAt",
+                value,
+            }),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[repr(i64)]
+pub enum Clock {
+    #[default]
+    None = 0,
+    Wall = 1,
+    Monotonic = 2,
+    Both = 3,
+}
+
+impl Clock {
+    pub fn has_wall(self) -> bool {
+        matches!(self, Self::Wall | Self::Both)
+    }
+
+    pub fn has_mono(self) -> bool {
+        matches!(self, Self::Monotonic | Self::Both)
+    }
+}
+
+impl TryFrom<i64> for Clock {
+    type Error = ProtoError;
+
+    fn try_from(value: i64) -> Result<Self> {
+        match value {
+            0 => Ok(Self::None),
+            1 => Ok(Self::Wall),
+            2 => Ok(Self::Monotonic),
+            3 => Ok(Self::Both),
+            _ => Err(ProtoError::InvalidEnum {
+                name: "Clock",
+                value,
+            }),
+        }
+    }
+}
+
+fn push_int(tag: u64, value: i64, out: &mut Vec<u8>) {
+    if value == 0 {
+        return;
+    }
+    varint::encode_uvarint(tag, out);
+    varint::encode_varint(value, out);
+}
+
+fn read_int(input: &[u8], pos: &mut usize) -> Result<i64> {
+    let (value, used) = varint::decode_varint(&input[*pos..])?;
+    *pos += used;
+    Ok(value)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn params_round_trip() {
+        let params = Params {
+            protocol_version: 1,
+            duration_ns: 3_000_000_000,
+            interval_ns: 1_000_000_000,
+            length: 1472,
+            received_stats: ReceivedStats::Both,
+            stamp_at: StampAt::Both,
+            clock: Clock::Both,
+            dscp: 184,
+            server_fill: Some(ServerFill {
+                value: "pattern:abc".to_owned(),
+            }),
+        };
+        assert_eq!(Params::decode(&params.encode()), Ok(params));
+    }
+
+    #[test]
+    fn unknown_tags_are_ignored() {
+        let mut encoded = Vec::new();
+        varint::encode_uvarint(99, &mut encoded);
+        varint::encode_varint(123, &mut encoded);
+        varint::encode_uvarint(1, &mut encoded);
+        varint::encode_varint(1, &mut encoded);
+
+        let params = Params::decode(&encoded).unwrap();
+        assert_eq!(params.protocol_version, 1);
+    }
+}

crates/irtt-proto/src/varint.rs

@@ -0,0 +1,115 @@
+use crate::{ProtoError, Result};
+
+pub fn encode_uvarint(mut value: u64, out: &mut Vec<u8>) {
+    while value >= 0x80 {
+        out.push((value as u8) | 0x80);
+        value >>= 7;
+    }
+    out.push(value as u8);
+}
+
+pub fn decode_uvarint(input: &[u8]) -> Result<(u64, usize)> {
+    let mut value = 0u64;
+    for (idx, byte) in input.iter().copied().enumerate() {
+        if idx == 10 {
+            return Err(ProtoError::VarintOverflow);
+        }
+        let low = u64::from(byte & 0x7f);
+        let shift = idx * 7;
+        if shift == 63 && low > 1 {
+            return Err(ProtoError::VarintOverflow);
+        }
+        value |= low << shift;
+        if byte < 0x80 {
+            return Ok((value, idx + 1));
+        }
+    }
+    Err(ProtoError::TruncatedVarint)
+}
+
+pub fn zigzag_encode(value: i64) -> u64 {
+    ((value << 1) ^ (value >> 63)) as u64
+}
+
+pub fn zigzag_decode(value: u64) -> i64 {
+    ((value >> 1) as i64) ^ (-((value & 1) as i64))
+}
+
+pub fn encode_varint(value: i64, out: &mut Vec<u8>) {
+    encode_uvarint(zigzag_encode(value), out);
+}
+
+pub fn decode_varint(input: &[u8]) -> Result<(i64, usize)> {
+    let (value, used) = decode_uvarint(input)?;
+    Ok((zigzag_decode(value), used))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn uvarint_round_trips() {
+        for value in [
+            0,
+            1,
+            2,
+            127,
+            128,
+            255,
+            16_384,
+            u32::MAX as u64,
+            i64::MAX as u64,
+            u64::MAX,
+        ] {
+            let mut encoded = Vec::new();
+            encode_uvarint(value, &mut encoded);
+            assert_eq!(decode_uvarint(&encoded), Ok((value, encoded.len())));
+        }
+    }
+
+    #[test]
+    fn signed_varint_round_trips() {
+        for value in [
+            i64::MIN,
+            -9_223_372_036,
+            -2,
+            -1,
+            0,
+            1,
+            2,
+            1_000_000_000,
+            3_000_000_000,
+            i64::MAX,
+        ] {
+            let mut encoded = Vec::new();
+            encode_varint(value, &mut encoded);
+            assert_eq!(decode_varint(&encoded), Ok((value, encoded.len())));
+        }
+    }
+
+    #[test]
+    fn verified_byte_examples() {
+        let cases: &[(i64, &[u8])] = &[
+            (1, &[0x02]),
+            (3_000_000_000, &[0x80, 0xf8, 0x82, 0xad, 0x16]),
+            (1_000_000_000, &[0x80, 0xa8, 0xd6, 0xb9, 0x07]),
+            (1472, &[0x80, 0x17]),
+            (3, &[0x06]),
+            (184, &[0xf0, 0x02]),
+        ];
+        for (value, expected) in cases {
+            let mut encoded = Vec::new();
+            encode_varint(*value, &mut encoded);
+            assert_eq!(&encoded, expected);
+            assert_eq!(decode_varint(expected), Ok((*value, expected.len())));
+        }
+
+        let mut tag = Vec::new();
+        encode_uvarint(9, &mut tag);
+        assert_eq!(tag, [0x09]);
+        tag.clear();
+        encode_uvarint(24, &mut tag);
+        assert_eq!(tag, [0x18]);
+    }
+}

crates/irtt-proto/tests/vectors.rs

@@ -0,0 +1,261 @@
+use irtt_proto::{
+    decode_echo_reply, decode_open_reply, encode_close_request, encode_echo_request,
+    encode_open_request, verify_hmac, Clock, CloseRequest, EchoRequest, OpenRequest, Params,
+    ReceivedStats, StampAt,
+};
+
+fn hex(input: &str) -> Vec<u8> {
+    input
+        .split_whitespace()
+        .map(|byte| u8::from_str_radix(byte, 16).unwrap())
+        .collect()
+}
+
+fn default_params_3s() -> Params {
+    Params {
+        protocol_version: 1,
+        duration_ns: 3_000_000_000,
+        interval_ns: 1_000_000_000,
+        received_stats: ReceivedStats::Both,
+        stamp_at: StampAt::Both,
+        clock: Clock::Both,
+        ..Params::default()
+    }
+}
+
+fn default_params_60s() -> Params {
+    Params {
+        duration_ns: 60_000_000_000,
+        ..default_params_3s()
+    }
+}
+
+fn hmac_params_2s() -> Params {
+    Params {
+        duration_ns: 2_000_000_000,
+        ..default_params_3s()
+    }
+}
+
+#[test]
+fn vector_1_open_request_no_hmac() {
+    let expected = hex("14 a7 5b 01 01 02 02 80 f8 82 ad 16 03 80 a8 d6
+         b9 07 05 06 06 06 07 06");
+    let packet = encode_open_request(
+        &OpenRequest {
+            params: default_params_3s(),
+            close: false,
+        },
+        None,
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_2_open_reply_no_hmac() {
+    let packet = hex("14 a7 5b 03 13 52 71 87 ab b6 96 78 01 02 02 80
+         f8 82 ad 16 03 80 a8 d6 b9 07 05 06 06 06 07 06");
+    let reply = decode_open_reply(&packet, None).unwrap();
+    assert_eq!(reply.token, 0x7896_b6ab_8771_5213);
+    assert_eq!(reply.params, default_params_3s());
+}
+
+#[test]
+fn vector_3_echo_request_no_hmac() {
+    let expected = hex("14 a7 5b 00 13 52 71 87 ab b6 96 78 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00");
+    let packet = encode_echo_request(
+        &EchoRequest {
+            token: 0x7896_b6ab_8771_5213,
+            sequence: 0,
+            params: default_params_3s(),
+            payload: Vec::new(),
+        },
+        None,
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_4_echo_reply_no_hmac() {
+    let packet = hex("14 a7 5b 02 13 52 71 87 ab b6 96 78 02 00 00 00
+         03 00 00 00 07 00 00 00 00 00 00 00 b8 1a 33 0c
+         86 6d aa 18 de 26 35 95 00 00 00 00 80 4d 33 0c
+         86 6d aa 18 b2 57 35 95 00 00 00 00");
+    let reply = decode_echo_reply(&packet, &default_params_3s(), None).unwrap();
+    assert_eq!(reply.sequence, 2);
+    assert_eq!(reply.recv_count, Some(3));
+    assert_eq!(reply.recv_window, Some(7));
+}
+
+#[test]
+fn vector_5_close_request_no_hmac() {
+    let expected = hex("14 a7 5b 04 13 52 71 87 ab b6 96 78");
+    let packet = encode_close_request(
+        &CloseRequest {
+            token: 0x7896_b6ab_8771_5213,
+        },
+        None,
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_6_hmac_open_request() {
+    let expected = hex("14 a7 5b 09 ff 90 16 a7 aa 53 78 16 9e c3 a2 d5
+         54 dc 30 36 01 02 02 80 d0 ac f3 0e 03 80 a8 d6
+         b9 07 05 06 06 06 07 06");
+    let packet = encode_open_request(
+        &OpenRequest {
+            params: hmac_params_2s(),
+            close: false,
+        },
+        Some(b"testkey"),
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+    verify_hmac(b"testkey", &packet, 4).unwrap();
+}
+
+#[test]
+fn vector_7_hmac_echo_request() {
+    let expected = hex("14 a7 5b 08 d9 87 4f 82 b3 13 10 31 59 ad 2c 8f
+         6b da ef ff 59 ca 3f 5d eb e9 87 43 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00");
+    let packet = encode_echo_request(
+        &EchoRequest {
+            token: 0x4387_e9eb_5d3f_ca59,
+            sequence: 0,
+            params: default_params_3s(),
+            payload: Vec::new(),
+        },
+        Some(b"testkey"),
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_8_hmac_close_request() {
+    let expected = hex("14 a7 5b 0c f5 cd 0f a9 de 9d 7d 66 8d 91 a5 32
+         48 0e 42 e0 59 ca 3f 5d eb e9 87 43");
+    let packet = encode_close_request(
+        &CloseRequest {
+            token: 0x4387_e9eb_5d3f_ca59,
+        },
+        Some(b"testkey"),
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_9_no_test_open_close_request() {
+    let expected = hex("14 a7 5b 05 01 02 02 80 e0 ba 84 bf 03 03 80 a8
+         d6 b9 07 05 06 06 06 07 06");
+    let packet = encode_open_request(
+        &OpenRequest {
+            params: default_params_60s(),
+            close: true,
+        },
+        None,
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_10_no_test_open_close_reply() {
+    let packet = hex("14 a7 5b 07 00 00 00 00 00 00 00 00 01 02 02 80
+         e0 ba 84 bf 03 03 80 a8 d6 b9 07 05 06 06 06 07
+         06");
+    let reply = decode_open_reply(&packet, None).unwrap();
+    assert_eq!(reply.token, 0);
+    assert_eq!(reply.params, default_params_60s());
+}
+
+#[test]
+fn vector_11_minimal_echo_packet() {
+    let expected = hex("14 a7 5b 00 4e 15 61 5c a2 6f 31 a0 00 00 00 00");
+    let packet = encode_echo_request(
+        &EchoRequest {
+            token: 0xa031_6fa2_5c61_154e,
+            sequence: 0,
+            params: Params::default(),
+            payload: Vec::new(),
+        },
+        None,
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn vector_12_midpoint_reply() {
+    let packet = hex("14 a7 5b 02 91 a4 1e 7a f0 14 f6 62 00 00 00 00
+         01 00 00 00 01 00 00 00 00 00 00 00 f7 0e 2c 93
+         a2 6d aa 18 5a 5f 14 47 03 00 00 00");
+    let params = Params {
+        received_stats: ReceivedStats::Both,
+        stamp_at: StampAt::Midpoint,
+        clock: Clock::Both,
+        ..Params::default()
+    };
+    let reply = decode_echo_reply(&packet, &params, None).unwrap();
+    assert_eq!(reply.recv_count, Some(1));
+    assert_eq!(reply.recv_window, Some(1));
+    assert!(reply.timestamps.midpoint_wall.is_some());
+    assert!(reply.timestamps.recv_wall.is_none());
+    assert!(reply.timestamps.send_wall.is_none());
+}
+
+#[test]
+fn spec_18_1_hmac_echo_request_fixture() {
+    let expected = hex("14 a7 5b 08 e7 03 41 e7 d4 08 cf 69 41 f3 f4 78
+         5a 56 0c 4c ea 3e b3 22 a7 c9 6b 88 bb a1 e2 6f
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
+         00 00 00 00 00 00 00 00 00 00 00 00 ff fe fd fc
+         ff fe fd fc ff fe fd fc ff fe fd fc");
+    let params = Params {
+        length: 92,
+        received_stats: ReceivedStats::Both,
+        stamp_at: StampAt::Both,
+        clock: Clock::Both,
+        ..Params::default()
+    };
+    let packet = encode_echo_request(
+        &EchoRequest {
+            token: 0x886b_c9a7_22b3_3eea,
+            sequence: 0x6fe2_a1bb,
+            params,
+            payload: [0xff, 0xfe, 0xfd, 0xfc].repeat(4),
+        },
+        Some(&[0x3c, 0x68, 0x1d, 0x39, 0x41, 0x1d, 0x72, 0x43]),
+    )
+    .unwrap();
+    assert_eq!(packet, expected);
+}
+
+#[test]
+fn spec_18_2_hmac_echo_reply_fixture() {
+    let packet = hex("14 a7 5b 08 d2 98 a3 4a 6a 13 41 02 68 b2 67 a8
+         d6 7e 28 25 c3 cb 6f 76 b6 ce 66 e6 06 07 3b 1d
+         19 9f bc a3 9b 3b f8 86 e5 39 e9 d7 a1 75 2a ee
+         3d f1 5a 52 9a c6 4a 3e 22 62 55 2d 69 3f 29 46
+         d4 05 97 58 66 50 2c dd 2f f1 1d 46 ff fe fd fc
+         ff fe fd fc ff fe fd fc ff fe fd fc");
+    verify_hmac(
+        &[0xda, 0xb3, 0xe9, 0x04, 0xa6, 0x87, 0x92, 0x49],
+        &packet,
+        4,
+    )
+    .unwrap();
+}

crates/irtt-stats/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "irtt-stats"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]

crates/irtt-stats/src/lib.rs

@@ -0,0 +1,4 @@
+//! Statistics crate skeleton.
+//!
+//! Statistics aggregation is intentionally out of scope for the current
+//! milestone.