moqtap_client/draft11/

connection.rs

use bytes::{Buf, Bytes, BytesMut};

use crate::draft11::endpoint::{Endpoint, EndpointError};
use crate::draft11::event::{ClientEvent, Direction, FetchObject, StreamKind, SubgroupObject};
use crate::draft11::observer::ConnectionObserver;
use crate::draft11::session::request_id::Role;
use crate::draft11::session::setup;
use crate::forwarding_preference::ObjectForwardingPreference;
use crate::track_locations::{EndOfTrackForm, ObjectLocation, ObjectRole, TrackObjects};
use crate::transport::{RecvStream, SendStream, Transport, TransportError};
use moqtap_codec::dispatch::{
    AnyControlMessage, AnyDatagramHeader, AnyFetchHeader, AnySubgroupHeader,
};
use moqtap_codec::draft11::data_stream::{FetchObjectHeader, ObjectHeader, SubgroupGaps};
use moqtap_codec::draft11::message::ControlMessage;
use moqtap_codec::error::CodecError;
use moqtap_codec::kvp::KeyValuePair;
use moqtap_codec::types::*;
use moqtap_codec::varint::VarInt;
use moqtap_codec::version::DraftVersion;

/// MoQT ALPN identifier (used by raw QUIC transport).
pub const MOQT_ALPN: &[u8] = b"moq-00";

/// Errors from the draft-11 connection layer.
#[derive(Debug, thiserror::Error)]
pub enum ConnectionError {
    /// Endpoint state machine error.
    #[error("endpoint error: {0}")]
    Endpoint(#[from] EndpointError),
    /// Wire codec error.
    #[error("codec error: {0}")]
    Codec(#[from] CodecError),
    /// Transport-level error.
    #[error("transport error: {0}")]
    Transport(#[from] TransportError),
    /// Variable-length integer decoding error.
    #[error("varint error: {0}")]
    VarInt(#[from] moqtap_codec::varint::VarIntError),
    /// Control stream was not opened.
    #[error("control stream not open")]
    NoControlStream,
    /// Stream ended before a complete message was read.
    #[error("unexpected end of stream")]
    UnexpectedEnd,
    /// Stream was finished by the peer.
    #[error("stream finished")]
    StreamFinished,
    /// Invalid server address string.
    #[error("invalid server address: {0}")]
    InvalidAddress(String),
    /// TLS configuration error.
    #[error("TLS config error: {0}")]
    TlsConfig(String),
    /// Data stream used out of order: an object before its header, or an
    /// Object ID that does not advance on the last one written.
    #[error("data stream state error: {0}")]
    DataStreamState(&'static str),
    /// A control message this build decoded for draft-11 and then could not
    /// narrow to draft-11's own message type.
    ///
    /// Unreachable, and that is not the same as harmless. `read_control`
    /// decodes with this connection's own draft, so the `AnyControlMessage` it
    /// hands back can only carry this draft's variant — but the narrowing arm
    /// is compiled in every configuration anyway, under
    /// `#[allow(unreachable_patterns)]` rather than a `cfg` naming the other
    /// drafts, because such a list has to be edited in every draft
    /// module whenever a draft is added, and a copy that omits one leaves the
    /// match non-exhaustive.
    ///
    /// Spelled as `CodecError::UnknownMessageType(0)` it would not stay inert:
    /// every draft's
    /// [`codec_session_error_code`](Connection::codec_session_error_code)
    /// answers that variant `Some(PROTOCOL_VIOLATION)`. So the day the
    /// narrowing did fail, this build's own defect would reach a caller as *the
    /// peer sent a control message type this draft does not assign, and the
    /// session must be closed with a Protocol Violation* — carrying `0x00` as
    /// the codepoint that proved it. A conformance report reading that
    /// publishes a named, well-evidenced accusation against a relay for
    /// something no relay did.
    ///
    /// A variant of its own is what stops that.
    /// [`draft_specific_cause`](Connection::draft_specific_cause) answers it
    /// [`LocalRefusal`], the facade turns that into [`ErrorCause::Facade`], and
    /// nothing downstream can read a rule out of a cause that says nothing
    /// reached the wire. What is pinned is the consequence rather than the
    /// unreachability: nothing pins the arm's reachability, which is exactly
    /// why the consequence must not be an accusation.
    ///
    /// [`LocalRefusal`]: crate::above_codec_rules::DraftSpecificCause::LocalRefusal
    /// [`ErrorCause::Facade`]: crate::dispatch::ErrorCause::Facade
    #[error(
        "a control message decoded for draft-11 did not narrow to draft-11: a defect in this build, and evidence about nothing the peer did"
    )]
    ControlMessageNarrowing,
    /// A datagram whose Type announces extension headers arrived with an
    /// Extension Headers Length of 0.
    ///
    /// draft-11 Section 9.2: "If an endpoint receives a datagram with Type 0x01
    /// and Extension Headers Length is 0, it MUST close the session with
    /// Protocol Violation." draft-11 Section 9.3: "If an endpoint receives a
    /// datagram with Type 0x03 and Extension Headers Length is 0, it MUST close
    /// the session with Protocol Violation."
    ///
    /// The codec's header decoder refuses such a datagram as a bare
    /// `InvalidField`. [`Connection::recv_datagram`] names it here, and
    /// [`Connection::close_for_data_stream`] closes the session over it.
    #[error(
        "the datagram for object {object_id} announces extension headers and has an Extension Headers Length of 0"
    )]
    EmptyDatagramExtensions {
        /// The Object ID the datagram named.
        object_id: u64,
    },
}

/// [`ConnectionError::EmptyDatagramExtensions`] for a datagram the header
/// decoder refused as `InvalidField`, when its empty extension block was the
/// fault.
///
/// Types 0x01 and 0x03 are the two draft-11 Section 9.2 and draft-11 Section 9.3
/// name, and both carry an Object ID and a Publisher Priority.
fn empty_datagram_extensions(data: &[u8]) -> Option<ConnectionError> {
    use crate::datagram_prefix::{empty_extension_block, Prefix};
    empty_extension_block(data, |datagram_type, _| {
        matches!(datagram_type, 0x01 | 0x03).then_some(Prefix { object_id: true, priority: true })
    })
    .map(|object_id| ConnectionError::EmptyDatagramExtensions { object_id })
}

impl From<crate::transport::DialError> for ConnectionError {
    /// Maps a dial failure onto the variants this error already has, so a
    /// caller matches `InvalidAddress` or `TlsConfig`.
    ///
    /// A `LocalSocket` failure maps to `InvalidAddress`, which the facade reads
    /// as this side's failure
    /// ([`ErrorCause::Facade`](crate::dispatch::ErrorCause::Facade)). The dial
    /// phase stays on [`DialError::phase`](crate::transport::DialError::phase).
    fn from(e: crate::transport::DialError) -> Self {
        match e {
            // Two variants, one arm, deliberately — see above.
            crate::transport::DialError::InvalidAddress(s)
            | crate::transport::DialError::LocalSocket(s) => ConnectionError::InvalidAddress(s),
            crate::transport::DialError::TlsConfig(s) => ConnectionError::TlsConfig(s),
            crate::transport::DialError::Transport(e) => ConnectionError::Transport(e),
        }
    }
}

/// Transport type for the connection.
#[derive(Debug, Clone)]
pub enum TransportType {
    /// Raw QUIC via quinn. The `addr` field should be `host:port`.
    Quic,
    /// WebTransport via wtransport. The `url` field is the WebTransport URL.
    WebTransport {
        /// The WebTransport endpoint URL (e.g., `https://host:port/path`).
        url: String,
    },
}

/// Configuration for a draft-11 MoQT client connection.
pub struct ClientConfig {
    /// Additional draft versions to offer in CLIENT_SETUP (draft-11 is always
    /// offered first).
    pub additional_versions: Vec<DraftVersion>,
    /// The transport type (QUIC or WebTransport).
    pub transport: TransportType,
    /// Whether to skip TLS certificate verification (for testing).
    pub skip_cert_verification: bool,
    /// Custom CA certificates to trust (DER-encoded).
    pub ca_certs: Vec<Vec<u8>>,
    /// Setup parameters to include in CLIENT_SETUP (e.g., auth tokens).
    pub setup_parameters: Vec<moqtap_codec::kvp::KeyValuePair>,
}

impl ClientConfig {
    /// Returns the MoQT version varints for the CLIENT_SETUP message.
    /// Draft-11 first, then any additional versions.
    pub fn supported_versions(&self) -> Vec<VarInt> {
        let mut versions = vec![DraftVersion::Draft11.version_varint()];
        for v in &self.additional_versions {
            let varint = v.version_varint();
            if !versions.contains(&varint) {
                versions.push(varint);
            }
        }
        versions
    }

    /// Returns the ALPN protocol identifiers for the transport.
    pub fn alpn(&self) -> Vec<Vec<u8>> {
        match &self.transport {
            TransportType::Quic => vec![DraftVersion::Draft11.quic_alpn().to_vec()],
            TransportType::WebTransport { .. } => vec![b"h3".to_vec()],
        }
    }
}

/// A framed writer for a send stream. Handles MoQT length-prefixed framing.
pub struct FramedSendStream {
    inner: SendStream,
    /// What the subgroup header opened on this stream settled, once one has
    /// been written.
    ///
    /// `None` until a subgroup header has been written, which is what makes an
    /// object sent before its header answerable rather than unframed bytes.
    subgroup_objects: Option<SubgroupStreamState>,
}

/// What a subgroup header fixes for every object written after it.
///
/// Two facts, and neither is recoverable from an object on its own: the
/// Object ID it has to advance past, and whether the stream's type puts an
/// extension block on each object. The second has to be remembered rather than
/// guessed: a header type whose Extensions Present column reads Yes puts an
/// Extension Headers Length on every object of the subgroup, so assuming
/// "absent" would write a stream a reader could not follow.
#[derive(Debug, Clone, Copy)]
struct SubgroupStreamState {
    /// The last Object ID written, or `None` before the first object.
    previous_object_id: Option<u64>,
    /// Whether each object writes an Extension Headers Length field.
    carries_extension_block: bool,
}

impl FramedSendStream {
    /// Create a new framed send stream.
    pub fn new(inner: SendStream) -> Self {
        Self { inner, subgroup_objects: None }
    }

    /// Get the transport-level stream ID.
    pub fn stream_id(&self) -> u64 {
        self.inner.stream_id()
    }

    /// Write a control message to the stream with type+length framing.
    /// Returns the raw bytes that were written (for event capture).
    pub async fn write_control(
        &mut self,
        msg: &AnyControlMessage,
    ) -> Result<Vec<u8>, ConnectionError> {
        let mut buf = Vec::new();
        msg.encode(&mut buf)?;
        self.inner.write_all(&buf).await?;
        Ok(buf)
    }

    /// Write a subgroup stream header. Also opens the Object ID bookkeeping
    /// [`FramedSendStream::write_subgroup_object`] holds the stream to.
    ///
    /// The header is refused, and nothing is written, if its fields disagree
    /// with its own stream type. That check has to happen here rather than at
    /// the first object: the type is what every object after it is framed
    /// against, so a header that went out saying the wrong thing cannot be
    /// taken back.
    pub async fn write_subgroup_header(
        &mut self,
        header: &AnySubgroupHeader,
    ) -> Result<(), ConnectionError> {
        let mut buf = Vec::new();
        header.encode_stream_checked(&mut buf)?;
        self.inner.write_all(&buf).await?;
        // The framing is read off the header now, while the header is in hand.
        // An object arriving later cannot say whether the stream carries an
        // extension block, and writing one either way is not a recoverable
        // mistake: the reader takes the next field along as the missing length
        // and misframes every object after it.
        self.subgroup_objects = Some(SubgroupStreamState {
            previous_object_id: None,
            carries_extension_block: header.carries_extension_block(),
        });
        Ok(())
    }

    /// Write a fetch response header.
    pub async fn write_fetch_header(
        &mut self,
        header: &AnyFetchHeader,
    ) -> Result<(), ConnectionError> {
        let mut buf = Vec::new();
        header.encode_stream(&mut buf);
        self.inner.write_all(&buf).await?;
        Ok(())
    }

    /// Append a draft-11 subgroup object (header + payload) to the stream.
    ///
    /// Section 9.4.2: "A publisher MUST NOT send an Object on a stream if its
    /// Object ID is less than a previously sent Object ID within a given group
    /// in that stream." A subgroup stream carries one group, so the Object IDs
    /// written here are exactly the ones that sentence compares, and the
    /// comparison needs the object before - which no per-header check can see.
    /// The state advances only once the object has been written, so declining to
    /// write an object leaves the next one measured against the last one kept.
    ///
    /// An equal Object ID is refused as well as a smaller one. The draft's own
    /// sentence forbids only "less than", but an Object ID names an Object
    /// within a Group: writing one twice on a stream describes the same Object
    /// with two different payloads, and a reader has no way to choose. The
    /// dispatch-level writer in the codec draws the line in the same place, and
    /// two writers that disagreed about it would be worse than either answer.
    ///
    /// # Errors
    ///
    /// [`ConnectionError::DataStreamState`] if no subgroup header has been
    /// written yet, or if `object` does not advance past the last one written.
    pub async fn write_subgroup_object(
        &mut self,
        object: &SubgroupObject,
    ) -> Result<(), ConnectionError> {
        let state = self
            .subgroup_objects
            .as_mut()
            .ok_or(ConnectionError::DataStreamState("subgroup header not written yet"))?;
        let object_id = object.header.object_id.into_inner();
        if matches!(state.previous_object_id, Some(prev) if object_id <= prev) {
            return Err(ConnectionError::DataStreamState(
                "object id does not advance on the last one written to this stream",
            ));
        }
        // The declared length comes from the payload rather than from the
        // caller's field: a header that disagrees with the bytes beside it
        // desynchronises every object after it on the stream, and nothing
        // downstream can recover.
        let mut header = object.header.clone();
        header.payload_length = VarInt::from_usize(object.payload.len());
        let mut buf = Vec::new();
        header.encode_checked_with_extensions(state.carries_extension_block, &mut buf)?;
        buf.extend_from_slice(&object.payload);
        self.inner.write_all(&buf).await?;
        state.previous_object_id = Some(object_id);
        Ok(())
    }

    /// Append a draft-11 fetch object (header + payload) to the stream.
    pub async fn write_fetch_object(
        &mut self,
        object: &FetchObject,
    ) -> Result<(), ConnectionError> {
        // The declared length comes from the payload rather than from the
        // caller's field: a header that disagrees with the bytes beside it
        // desynchronises every object after it on the stream, and nothing
        // downstream can recover.
        let mut header = object.header.clone();
        header.payload_length = VarInt::from_usize(object.payload.len());
        let mut buf = Vec::new();
        header.encode_checked(&mut buf)?;
        buf.extend_from_slice(&object.payload);
        self.inner.write_all(&buf).await?;
        Ok(())
    }

    /// Finish the stream (send FIN).
    pub async fn finish(&mut self) -> Result<(), ConnectionError> {
        self.inner.finish()?;
        Ok(())
    }
}

/// What an Object Status makes of an object, for Section 9.1.1.1's table.
///
/// `None` is Normal and not "no status": a datagram carrying a payload has no
/// status field at all, and an object with a payload is an ordinary object.
fn object_role(status: Option<u64>) -> ObjectRole {
    match status {
        None | Some(0x0) => ObjectRole::Produced,
        // 0x4, end of Track. This draft folded drafts 08 through 10's second
        // end-of-track status into this one and kept the looser of the two
        // conditions, so the Group ID may equal the largest group seen.
        Some(0x4) => ObjectRole::EndsTrack(Some(EndOfTrackForm::LastGroup)),
        // Every other status is a statement about objects rather than one of
        // them. Two of them name an Object ID one past the largest on purpose,
        // so counting one as produced would refuse the end-of-track object the
        // draft goes on to define.
        _ => ObjectRole::Neither,
    }
}

/// A framed reader for a recv stream. Handles MoQT varint-length decoding.
pub struct FramedRecvStream {
    inner: RecvStream,
    buf: BytesMut,
    /// The record this stream's objects are measured against, and the Group ID
    /// its header named.
    ///
    /// One group for the whole stream: a subgroup header names it once and no
    /// object header repeats it. `None` on a stream that was never given one —
    /// a stream for an alias no live binding names, and every stream built
    /// outside [`Connection::accept_subgroup_stream`] — and on such a stream
    /// the objects are read without being measured against the track at all.
    tracking: Option<(TrackObjects, u64)>,
    /// Whether the subgroup stream this reader is on carries an extension block
    /// on every object.
    ///
    /// The stream's Type says so and nothing in an object header repeats it, so
    /// a reader that does not remember the Type cannot parse the objects at all:
    /// on an extensions-bearing stream it reads the Extension Headers Length as
    /// the Object Payload Length and every field after it is nonsense.
    ///
    /// Set by [`FramedRecvStream::read_subgroup_header`] and read by
    /// [`FramedRecvStream::read_subgroup_object`]. `false` until a subgroup
    /// header has been read, which is the only order those two may be called
    /// in.
    subgroup_has_extensions: bool,
    /// The Prior Group ID Gap values this subgroup stream's Objects carried.
    /// Set by [`FramedRecvStream::read_subgroup_header`].
    subgroup_gaps: Option<SubgroupGaps>,
    /// The Prior gap conditions this fetch stream's Objects show. Reset by
    /// [`FramedRecvStream::read_fetch_header`].
    fetch_gaps: moqtap_codec::draft11::data_stream::FetchGaps,
}

impl FramedRecvStream {
    /// Create a new framed receive stream.
    pub fn new(inner: RecvStream) -> Self {
        Self {
            inner,
            buf: BytesMut::with_capacity(4096),
            subgroup_has_extensions: false,
            subgroup_gaps: None,
            tracking: None,
            fetch_gaps: moqtap_codec::draft11::data_stream::FetchGaps::new(),
        }
    }

    /// Measure this stream's objects against `objects`, all of them in `group`.
    ///
    /// Called by [`Connection::accept_subgroup_stream`] once the header has been
    /// read, which is the only point at which both the track and the group are
    /// known.
    fn measure_objects_against(&mut self, objects: TrackObjects, group: u64) {
        self.tracking = Some((objects, group));
    }

    /// Record or judge one object this stream carried.
    ///
    /// The whole of Section 9.1.1.1's rule that a single header cannot settle: the
    /// object's Group ID is the stream's, its Object ID is its own, and what
    /// they are measured against is everything the track has carried on any
    /// stream.
    fn note_subgroup_object(&self, header: &ObjectHeader) -> Result<(), ConnectionError> {
        let Some((objects, group)) = &self.tracking else { return Ok(()) };
        let at = ObjectLocation { group: *group, object: header.object_id.into_inner() };
        objects.note(at, object_role(Some(header.object_status as u64))).map_err(|placement| {
            ConnectionError::Endpoint(EndpointError::EndOfTrackOutOfPlace {
                alias: objects.alias(),
                group: at.group,
                object: at.object,
                placement,
            })
        })
    }

    /// Get the transport-level stream ID.
    pub fn stream_id(&self) -> u64 {
        self.inner.stream_id()
    }

    /// Read more data from the stream into the internal buffer.
    async fn fill(&mut self) -> Result<bool, ConnectionError> {
        let mut tmp = [0u8; 4096];
        match self.inner.read(&mut tmp).await {
            Ok(Some(n)) => {
                self.buf.extend_from_slice(&tmp[..n]);
                Ok(true)
            }
            Ok(None) => Ok(false),
            Err(e) => Err(ConnectionError::Transport(e)),
        }
    }

    /// Ensure at least `n` bytes are available in the buffer.
    async fn ensure(&mut self, n: usize) -> Result<(), ConnectionError> {
        while self.buf.len() < n {
            if !self.fill().await? {
                return Err(ConnectionError::UnexpectedEnd);
            }
        }
        Ok(())
    }

    /// Read a control message from the stream.
    ///
    /// When `capture_raw` is true, the returned tuple includes a clone of the
    /// framed wire bytes (for observer emission). When false, the second
    /// element is `None` and the payload clone is skipped.
    pub async fn read_control(
        &mut self,
        capture_raw: bool,
    ) -> Result<(AnyControlMessage, Option<Vec<u8>>), ConnectionError> {
        // Read type ID varint
        self.ensure(1).await?;
        let type_len = varint_len(self.buf[0]);
        self.ensure(type_len).await?;

        let mut cursor = &self.buf[..type_len];
        let _type_id = VarInt::decode(&mut cursor)?;

        // Draft-11 uses a fixed 16-bit length after the type id.
        let len_field_size = 2;
        self.ensure(type_len + len_field_size).await?;
        let payload_len = u16::from_be_bytes([self.buf[type_len], self.buf[type_len + 1]]) as usize;

        // Read full payload
        let total = type_len + len_field_size + payload_len;
        self.ensure(total).await?;

        // Capture raw bytes only if requested (observer attached).
        let raw = capture_raw.then(|| self.buf[..total].to_vec());

        // Now decode the whole message using the draft-11 dispatcher
        let mut frame = &self.buf[..total];
        let msg = AnyControlMessage::decode(DraftVersion::Draft11, &mut frame)?;
        self.buf.advance(total);
        Ok((msg, raw))
    }

    /// Read a subgroup stream header.
    pub async fn read_subgroup_header(&mut self) -> Result<AnySubgroupHeader, ConnectionError> {
        self.ensure(1).await?;
        loop {
            let mut cursor = &self.buf[..];
            match AnySubgroupHeader::decode_stream(DraftVersion::Draft11, &mut cursor) {
                Ok(header) => {
                    let consumed = self.buf.len() - cursor.remaining();
                    self.buf.advance(consumed);
                    // Clippy would rather see these two arms as an `if let`, and rustc rejects
                    // that in a single-draft build, where the pattern is irrefutable. Only a
                    // `match` satisfies both.
                    #[allow(clippy::single_match)]
                    match header {
                        AnySubgroupHeader::Draft11(ref d) => {
                            self.subgroup_has_extensions = d.stream_type.has_extensions();
                            self.subgroup_gaps = Some(SubgroupGaps::new(d));
                        }
                        // Only this draft's header sets the flag. With draft 11 the only enabled
                        // draft `AnySubgroupHeader` has a single variant, the arm above is
                        // exhaustive and this one unreachable. Compiled in every configuration with
                        // the lint allowed, rather than gated on a `cfg` naming every other
                        // draft: such a list has to be edited in every draft module whenever a
                        // draft is added, and a copy that omits one leaves this match
                        // non-exhaustive.
                        #[allow(unreachable_patterns)]
                        _ => {}
                    }
                    return Ok(header);
                }
                Err(e) if e.is_incomplete() => {
                    if !self.fill().await? {
                        return Err(ConnectionError::UnexpectedEnd);
                    }
                }
                Err(e) => return Err(ConnectionError::Codec(e)),
            }
        }
    }

    /// Read a fetch response header.
    pub async fn read_fetch_header(&mut self) -> Result<AnyFetchHeader, ConnectionError> {
        self.ensure(1).await?;
        loop {
            let mut cursor = &self.buf[..];
            match AnyFetchHeader::decode_stream(DraftVersion::Draft11, &mut cursor) {
                Ok(header) => {
                    let consumed = self.buf.len() - cursor.remaining();
                    self.buf.advance(consumed);
                    self.fetch_gaps = moqtap_codec::draft11::data_stream::FetchGaps::new();
                    return Ok(header);
                }
                Err(e) if e.is_incomplete() => {
                    if !self.fill().await? {
                        return Err(ConnectionError::UnexpectedEnd);
                    }
                }
                Err(e) => return Err(ConnectionError::Codec(e)),
            }
        }
    }

    /// Read the next draft-11 subgroup object (header + payload).
    ///
    /// Whether an object carries an extension block is a property of the
    /// stream's Type, not of the object, so this reads it from the header
    /// [`FramedRecvStream::read_subgroup_header`] recorded rather than assuming
    /// either answer. Assuming "no extensions" is not a conservative default: on
    /// a stream whose Type announces them the Extension Headers Length is read
    /// as the Object Payload Length, and every object on the stream comes back
    /// wrong instead of being refused.
    ///
    /// It is also what puts the rule binding extension headers to Object Status
    /// within reach on a subgroup stream. A reader that never reads the block
    /// cannot notice that a non-existent Object carried one.
    ///
    /// A Prior Group ID Gap larger than the stream's Group ID, or gaps that
    /// leave no value common to every Object on the stream that carried one,
    /// are returned as [`CodecError::MalformedTrack`] (draft-11 Section 10.1).
    pub async fn read_subgroup_object(&mut self) -> Result<SubgroupObject, ConnectionError> {
        loop {
            let mut cursor = &self.buf[..];
            match ObjectHeader::decode_with_extensions(self.subgroup_has_extensions, &mut cursor) {
                Ok(header) => {
                    let header_consumed = self.buf.len() - cursor.remaining();
                    let payload_len = header.payload_length.into_inner() as usize;
                    let total = header_consumed + payload_len;
                    if self.buf.len() < total {
                        if !self.fill().await? {
                            return Err(ConnectionError::UnexpectedEnd);
                        }
                        continue;
                    }
                    if let Some(gaps) = &mut self.subgroup_gaps {
                        gaps.check(&header).map_err(ConnectionError::Codec)?;
                    }
                    let payload = self.buf[header_consumed..total].to_vec();
                    self.buf.advance(total);
                    self.note_subgroup_object(&header)?;
                    return Ok(SubgroupObject { header, payload });
                }
                Err(e) if e.is_incomplete() => {
                    if !self.fill().await? {
                        return Err(ConnectionError::UnexpectedEnd);
                    }
                }
                Err(e) => return Err(ConnectionError::Codec(e)),
            }
        }
    }

    /// Read the next draft-11 fetch object (header + payload).
    ///
    /// A Prior Group ID Gap fault the stream shows (draft-11 Section 10.1) is
    /// returned as [`CodecError::MalformedTrack`], and an extension block that
    /// does not parse as [`CodecError::MalformedObjectProperties`].
    pub async fn read_fetch_object(&mut self) -> Result<FetchObject, ConnectionError> {
        loop {
            let mut cursor = &self.buf[..];
            match FetchObjectHeader::decode(&mut cursor) {
                Ok(header) => {
                    let header_consumed = self.buf.len() - cursor.remaining();
                    let payload_len = header.payload_length.into_inner() as usize;
                    let total = header_consumed + payload_len;
                    if self.buf.len() < total {
                        if !self.fill().await? {
                            return Err(ConnectionError::UnexpectedEnd);
                        }
                        continue;
                    }
                    let payload = self.buf[header_consumed..total].to_vec();
                    self.buf.advance(total);
                    self.fetch_gaps
                        .check(
                            header.group_id.into_inner(),
                            header.object_id.into_inner(),
                            &header.extensions,
                        )
                        .map_err(ConnectionError::Codec)?;
                    return Ok(FetchObject { header, payload });
                }
                Err(e) if e.is_incomplete() => {
                    if !self.fill().await? {
                        return Err(ConnectionError::UnexpectedEnd);
                    }
                }
                Err(e) => return Err(ConnectionError::Codec(e)),
            }
        }
    }
}

/// A live draft-11 MoQT connection over QUIC or WebTransport.
pub struct Connection {
    transport: Transport,
    endpoint: Endpoint,
    control_send: Option<FramedSendStream>,
    control_recv: Option<FramedRecvStream>,
    observer: Option<Box<dyn ConnectionObserver>>,
    /// Setup events buffered during `connect()` and replayed when an
    /// observer attaches via `set_observer` — without this, an observer
    /// attached after `connect` returns would never see the handshake.
    pending_events: Vec<ClientEvent>,
    /// The server's half of the setup handshake, kept whole.
    ///
    /// The endpoint acts on the parameters it recognises and retains none of
    /// them, and which parameters a server sends — in what order, with what
    /// values — is the sharpest thing a session says about the implementation
    /// behind it.
    server_setup: AnyControlMessage,
    /// The framed wire bytes of [`Self::server_setup`].
    server_setup_raw: Option<Vec<u8>>,
}

impl Connection {
    /// Connect to a draft-11 MoQT server as a client.
    pub async fn connect(addr: &str, config: ClientConfig) -> Result<Self, ConnectionError> {
        // PATH is for native QUIC only, and the transport is known here and
        // nowhere further in. Refusing before dialling means a session that
        // the server would close on sight is never opened.
        setup::validate_client_path_transport(
            &config.setup_parameters,
            matches!(config.transport, TransportType::WebTransport { .. }),
        )
        .map_err(EndpointError::from)?;

        let transport = match &config.transport {
            TransportType::Quic => Self::connect_quic(addr, &config).await?,
            TransportType::WebTransport { url } => {
                let url = url.clone();
                Self::connect_webtransport(&url, &config).await?
            }
        };

        Self::adopt(transport, config).await
    }

    /// Run the MoQT setup handshake over a transport somebody else established.
    ///
    /// For choosing the draft from what the server selected: dial once through
    /// [`crate::transport::dial_quic`] offering every ALPN, then bring the
    /// connection to the module its answer names. [`Self::connect`] cannot do
    /// this — it derives its single ALPN from the draft it was given.
    ///
    /// `config.draft` must match this module. The transport is adopted as
    /// given; nothing here re-checks the ALPN it was negotiated with.
    pub async fn adopt(
        transport: Transport,
        config: ClientConfig,
    ) -> Result<Self, ConnectionError> {
        Self::adopt_offering(transport, config, None).await
    }

    /// [`Self::adopt`], offering exactly `versions` in CLIENT_SETUP.
    ///
    /// `None` offers what `config` implies, which is what [`Self::adopt`]
    /// passes. `Some` replaces the list outright, and takes raw varints rather
    /// than [`DraftVersion`]s because the reason to reach for this is to offer
    /// a version no draft assigns — which an enum of drafts cannot name.
    ///
    /// A server MUST answer with a version the client offered and MUST
    /// otherwise close the session; from draft-11 the code for that is
    /// `VERSION_NEGOTIATION_FAILED` (0x15). How a relay spells the refusal is
    /// a conformance measurement, and offering a version deliberately outside
    /// the negotiable set is the only way to ask for it.
    pub async fn adopt_offering(
        transport: Transport,
        config: ClientConfig,
        versions: Option<Vec<VarInt>>,
    ) -> Result<Self, ConnectionError> {
        // PATH is for native QUIC only, and the transport is known here and
        // nowhere further in. Refusing before dialling means a session that
        // the server would close on sight is never opened.
        setup::validate_client_path_transport(
            &config.setup_parameters,
            matches!(config.transport, TransportType::WebTransport { .. }),
        )
        .map_err(EndpointError::from)?;

        // Open bidirectional control stream
        let (send, recv) = transport.open_bi().await?;
        let mut control_send = FramedSendStream::new(send);
        let mut control_recv = FramedRecvStream::new(recv);

        // Perform setup handshake
        let mut endpoint = Endpoint::new(Role::Client);
        endpoint.connect()?;
        let setup_msg = endpoint.send_client_setup(
            versions.unwrap_or_else(|| config.supported_versions()),
            config.setup_parameters.clone(),
        )?;
        let any_setup = AnyControlMessage::Draft11(setup_msg);
        let raw_setup = control_send.write_control(&any_setup).await?;

        let (server_setup, raw_server_setup) = control_recv.read_control(true).await?;
        match &server_setup {
            AnyControlMessage::Draft11(ControlMessage::ServerSetup(ref ss)) => {
                endpoint.receive_server_setup(ss)?;
            }
            _ => {
                return Err(ConnectionError::Endpoint(EndpointError::NotActive));
            }
        }

        let mut pending_events = Vec::with_capacity(3);
        pending_events.push(ClientEvent::ControlMessage {
            direction: Direction::Send,
            message: any_setup,
            raw: Some(raw_setup),
        });
        pending_events.push(ClientEvent::ControlMessage {
            direction: Direction::Receive,
            message: server_setup.clone(),
            raw: raw_server_setup.clone(),
        });
        if let Some(v) = endpoint.negotiated_version() {
            pending_events.push(ClientEvent::SetupComplete { negotiated_version: v.into_inner() });
        }

        Ok(Self {
            transport,
            endpoint,
            control_send: Some(control_send),
            control_recv: Some(control_recv),
            observer: None,
            pending_events,
            server_setup,
            server_setup_raw: raw_server_setup,
        })
    }

    /// Establish a raw QUIC connection.
    ///
    /// Offers this draft's ALPN alone; [`crate::transport::dial_quic`] holds the
    /// TLS and endpoint setup.
    async fn connect_quic(addr: &str, config: &ClientConfig) -> Result<Transport, ConnectionError> {
        let (transport, _negotiated) = crate::transport::dial_quic(
            addr,
            &crate::transport::QuicDialOptions {
                skip_cert_verification: config.skip_cert_verification,
                ca_certs: config.ca_certs.clone(),
                ..crate::transport::QuicDialOptions::new(config.alpn())
            },
        )
        .await?;
        Ok(transport)
    }

    /// Establish a WebTransport connection.
    ///
    /// [`crate::transport::dial_webtransport`] holds the TLS and endpoint
    /// setup, exactly as `connect_quic` above defers its own. That is not
    /// only deduplication: both dials hand the same `QuicDialOptions` to the
    /// same config constructor in `transport::quic`, so the bundled roots and
    /// `config.ca_certs` are what each of them trusts and one relay gets one
    /// verdict whichever transport carries it. Settling trust at this call site
    /// instead — from `wtransport`'s own builder settings, or from a second
    /// config of this draft's own — puts the decision in two places, and a
    /// caller's private CA then reaches only the dials whose call site
    /// installed it.
    #[cfg(feature = "webtransport")]
    async fn connect_webtransport(
        url: &str,
        config: &ClientConfig,
    ) -> Result<Transport, ConnectionError> {
        Ok(crate::transport::dial_webtransport(
            url,
            &crate::transport::QuicDialOptions {
                skip_cert_verification: config.skip_cert_verification,
                ca_certs: config.ca_certs.clone(),
                ..crate::transport::QuicDialOptions::new(config.alpn())
            },
        )
        .await?)
    }

    /// Stub for when the webtransport feature is not enabled.
    #[cfg(not(feature = "webtransport"))]
    async fn connect_webtransport(
        _url: &str,
        _config: &ClientConfig,
    ) -> Result<Transport, ConnectionError> {
        Err(ConnectionError::Transport(TransportError::Connect(
            "webtransport feature not enabled".into(),
        )))
    }

    // ── Observer ───────────────────────────────────────────────

    /// Attach an observer. Buffered handshake events from `connect()` are
    /// flushed in arrival order before this returns.
    pub fn set_observer(&mut self, observer: Box<dyn ConnectionObserver>) {
        self.observer = Some(observer);
        for event in self.pending_events.drain(..) {
            if let Some(ref obs) = self.observer {
                obs.on_event_owned(event);
            }
        }
    }

    /// Remove the observer.
    pub fn clear_observer(&mut self) {
        self.observer = None;
    }

    /// Emit an event to the observer, if one is attached.
    fn emit(&self, event: ClientEvent) {
        if let Some(ref obs) = self.observer {
            obs.on_event_owned(event);
        }
    }

    // ── Control message I/O ─────────────────────────────────

    /// Send a control message on the control stream.
    pub async fn send_control(&mut self, msg: &ControlMessage) -> Result<(), ConnectionError> {
        let any = AnyControlMessage::Draft11(msg.clone());
        let send = self.control_send.as_mut().ok_or(ConnectionError::NoControlStream)?;
        let raw = send.write_control(&any).await?;
        self.emit(ClientEvent::ControlMessage {
            direction: Direction::Send,
            message: any,
            raw: Some(raw),
        });
        Ok(())
    }

    /// Read the next control message from the control stream.
    pub async fn recv_control(&mut self) -> Result<ControlMessage, ConnectionError> {
        let recv = self.control_recv.as_mut().ok_or(ConnectionError::NoControlStream)?;
        let capture_raw = self.observer.is_some();
        let (any, raw) = match recv.read_control(capture_raw).await {
            Ok(v) => v,
            Err(e) => return Err(self.close_for_codec(e)),
        };
        if capture_raw {
            self.emit(ClientEvent::ControlMessage {
                direction: Direction::Receive,
                message: any.clone(),
                raw,
            });
        }
        match any {
            AnyControlMessage::Draft11(msg) => Ok(msg),
            // `AnyControlMessage` carries one variant per enabled draft feature. With draft 11 the
            // only one enabled the arm above is exhaustive and this rejection arm unreachable.
            // Compiled in every configuration with the lint allowed, rather than gated on a `cfg`
            // naming the other drafts: such a list has to be edited in every draft
            // module whenever a draft is added, and a copy that omits one leaves this match
            // non-exhaustive.
            #[allow(unreachable_patterns)]
            _ => Err(ConnectionError::ControlMessageNarrowing),
        }
    }

    /// Read and dispatch the next incoming control message through the endpoint
    /// state machine. Returns the decoded message for inspection.
    pub async fn recv_and_dispatch(&mut self) -> Result<ControlMessage, ConnectionError> {
        let msg = self.recv_control().await?;
        self.endpoint.receive_message(msg.clone()).map_err(|e| self.close_if_session_fatal(e))?;

        if let ControlMessage::GoAway(ref ga) = msg {
            self.emit(ClientEvent::Draining { new_session_uri: ga.new_session_uri.clone() });
        }

        Ok(msg)
    }

    // ── Subscribe flow ──────────────────────────────────────
    /// The lowest Track Alias no live subscription on this connection holds.
    ///
    /// This draft makes the Track Alias the subscriber's to choose, so
    /// [`Self::subscribe`] takes one and every later-era caller has nothing to
    /// pass. See
    /// [`Endpoint::next_free_track_alias`](crate::draft11::endpoint::Endpoint::next_free_track_alias)
    /// for why the value is read off the endpoint rather than asked of the
    /// caller, and why picking one here stays correct alongside a caller that
    /// picks its own.
    pub fn next_free_track_alias(&self) -> VarInt {
        self.endpoint.next_free_track_alias()
    }

    /// Send a SUBSCRIBE and return the allocated request ID.
    #[allow(clippy::too_many_arguments)]
    pub async fn subscribe(
        &mut self,
        track_alias: VarInt,
        track_namespace: TrackNamespace,
        track_name: Vec<u8>,
        subscriber_priority: u8,
        group_order: GroupOrder,
        filter_type: VarInt,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.subscribe(
            track_alias,
            track_namespace,
            track_name,
            subscriber_priority,
            group_order,
            filter_type,
        )?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send a SUBSCRIBE for a range of the track and return the allocated ID.
    ///
    /// The Filter Type comes from the arguments, so the message cannot name a
    /// filter whose fields it does not carry.
    #[allow(clippy::too_many_arguments)]
    pub async fn subscribe_range(
        &mut self,
        track_alias: VarInt,
        track_namespace: TrackNamespace,
        track_name: Vec<u8>,
        subscriber_priority: u8,
        group_order: GroupOrder,
        start_location: Location,
        end_group: Option<VarInt>,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.subscribe_range(
            track_alias,
            track_namespace,
            track_name,
            subscriber_priority,
            group_order,
            start_location,
            end_group,
        )?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send an UNSUBSCRIBE for the given request ID.
    pub async fn unsubscribe(&mut self, request_id: VarInt) -> Result<(), ConnectionError> {
        let msg = self.endpoint.unsubscribe(request_id)?;
        self.send_control(&msg).await
    }

    /// Accept a subscription the peer opened, sending SUBSCRIBE_OK.
    pub async fn subscribe_ok(
        &mut self,
        request_id: VarInt,
        expires: VarInt,
        group_order: GroupOrder,
        parameters: Vec<KeyValuePair>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_subscribe_ok(request_id, expires, group_order, parameters)?;
        self.send_control(&msg).await
    }

    /// Reject a subscription the peer opened, sending SUBSCRIBE_ERROR.
    ///
    /// The Track Alias travels back with the refusal: under the 'Retry Track
    /// Alias' code it is the alias the peer should try again with, and under
    /// any other code it is ignored.
    pub async fn subscribe_error(
        &mut self,
        request_id: VarInt,
        error_code: VarInt,
        reason_phrase: Vec<u8>,
        track_alias: VarInt,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_subscribe_error(
            request_id,
            error_code,
            reason_phrase,
            track_alias,
        )?;
        self.send_control(&msg).await
    }

    /// End a subscription this endpoint accepted, sending SUBSCRIBE_DONE.
    pub async fn subscribe_done(
        &mut self,
        request_id: VarInt,
        status_code: VarInt,
        reason_phrase: Vec<u8>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_subscribe_done(request_id, status_code, reason_phrase)?;
        self.send_control(&msg).await
    }

    // ── Fetch flow ──────────────────────────────────────────

    /// Send a FETCH and return the allocated request ID.
    #[allow(clippy::too_many_arguments)]
    pub async fn fetch(
        &mut self,
        track_namespace: TrackNamespace,
        track_name: Vec<u8>,
        subscriber_priority: u8,
        group_order: GroupOrder,
        start_group: VarInt,
        start_object: VarInt,
        end_group: VarInt,
        end_object: VarInt,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.fetch(
            track_namespace,
            track_name,
            subscriber_priority,
            group_order,
            start_group,
            start_object,
            end_group,
            end_object,
        )?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send a Relative Joining Fetch and return the allocated request ID.
    ///
    /// `joining_start` counts groups back from the live edge of the
    /// subscription it names. Section 8.13.1 computes the range from "the
    /// Preceding Group Offset", which is this field under the name it carried
    /// one draft ago; the field list calls it Joining Start and says that for
    /// a Relative Joining Fetch "a value of 0 indicates the Fetch starts at
    /// the beginning of the Current Group". For the form that names the group
    /// outright, see
    /// [`absolute_joining_fetch`](Self::absolute_joining_fetch).
    ///
    /// The subscription is named by `joining_subscribe_id`, which is the
    /// field's name in this draft even though the draft describes it as "The
    /// Request ID of the existing subscription to be joined".
    pub async fn joining_fetch(
        &mut self,
        subscriber_priority: u8,
        group_order: GroupOrder,
        joining_subscribe_id: VarInt,
        joining_start: VarInt,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.joining_fetch(
            subscriber_priority,
            group_order,
            joining_subscribe_id,
            joining_start,
        )?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send an Absolute Joining Fetch and return the allocated request ID.
    ///
    /// `joining_start` is the group to begin at. Section 8.13 has an Absolute
    /// Joining Fetch "Identical to a Relative Joining Fetch except that the
    /// Start Group is determined by an absolute Group value rather than a
    /// relative offset to the subscription", so a subscriber that knows the
    /// group it wants can ask for it without first being told a Largest Group
    /// to count back from.
    ///
    /// Two calls rather than one taking a Fetch Type, because these are the
    /// only two the joining form admits. The endpoint is named the same way,
    /// so no call between an application and the wire has a Fetch Type in it
    /// to be given a wrong one.
    pub async fn absolute_joining_fetch(
        &mut self,
        subscriber_priority: u8,
        group_order: GroupOrder,
        joining_subscribe_id: VarInt,
        joining_start: VarInt,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.absolute_joining_fetch(
            subscriber_priority,
            group_order,
            joining_subscribe_id,
            joining_start,
        )?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send a FETCH_CANCEL for the given request ID.
    pub async fn fetch_cancel(&mut self, request_id: VarInt) -> Result<(), ConnectionError> {
        let msg = self.endpoint.fetch_cancel(request_id)?;
        self.send_control(&msg).await
    }

    /// Accept a fetch the peer opened, sending FETCH_OK.
    ///
    /// The endpoint refuses a Joining Fetch naming a subscription this session
    /// cannot join and refuses a second answer to one FETCH, so nothing is
    /// written on the wire when it does either.
    pub async fn fetch_ok(
        &mut self,
        request_id: VarInt,
        group_order: GroupOrder,
        end_of_track: u8,
        end_location: Location,
        parameters: Vec<KeyValuePair>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_fetch_ok(
            request_id,
            group_order,
            end_of_track,
            end_location,
            parameters,
        )?;
        self.send_control(&msg).await
    }

    /// Refuse a fetch the peer opened, sending FETCH_ERROR.
    ///
    /// The endpoint refuses a second answer to one FETCH, and refuses a
    /// Joining Fetch's refusal under any code but the one the draft names for
    /// it, so nothing is written on the wire when it does either.
    pub async fn fetch_error(
        &mut self,
        request_id: VarInt,
        error_code: VarInt,
        reason_phrase: Vec<u8>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_fetch_error(request_id, error_code, reason_phrase)?;
        self.send_control(&msg).await
    }

    // ── Namespace flows ─────────────────────────────────────

    /// Send a SUBSCRIBE_ANNOUNCES. Returns the allocated request ID.
    pub async fn subscribe_announces(
        &mut self,
        track_namespace_prefix: TrackNamespace,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.subscribe_announces(track_namespace_prefix)?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Accept a namespace subscription the peer made, sending SUBSCRIBE_ANNOUNCES_OK.
    ///
    /// The endpoint refuses a second answer to one SUBSCRIBE_ANNOUNCES, so nothing is
    /// written on the wire when it does.
    pub async fn subscribe_announces_ok(
        &mut self,
        request_id: VarInt,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_subscribe_announces_ok(request_id)?;
        self.send_control(&msg).await
    }

    /// Refuse a namespace subscription the peer made, sending SUBSCRIBE_ANNOUNCES_ERROR.
    ///
    /// The other half of the same sentence: one answer, and this is the other
    /// one it can be.
    pub async fn subscribe_announces_error(
        &mut self,
        request_id: VarInt,
        error_code: VarInt,
        reason_phrase: Vec<u8>,
    ) -> Result<(), ConnectionError> {
        let msg =
            self.endpoint.send_subscribe_announces_error(request_id, error_code, reason_phrase)?;
        self.send_control(&msg).await
    }

    /// Send an ANNOUNCE. Returns the allocated request ID.
    pub async fn announce(
        &mut self,
        track_namespace: TrackNamespace,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.announce(track_namespace)?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Send an UNANNOUNCE.
    pub async fn unannounce(
        &mut self,
        track_namespace: TrackNamespace,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.unannounce(track_namespace)?;
        self.send_control(&msg).await
    }

    /// Accept an announcement the peer made, sending ANNOUNCE_OK.
    ///
    /// The endpoint refuses a second answer to one ANNOUNCE, so nothing is
    /// written on the wire when it does.
    pub async fn announce_ok(&mut self, request_id: VarInt) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_announce_ok(request_id)?;
        self.send_control(&msg).await
    }

    /// Refuse an announcement the peer made, sending ANNOUNCE_ERROR.
    ///
    /// The other half of the same sentence: one answer, and this is the other
    /// one it can be.
    pub async fn announce_error(
        &mut self,
        request_id: VarInt,
        error_code: VarInt,
        reason_phrase: Vec<u8>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_announce_error(request_id, error_code, reason_phrase)?;
        self.send_control(&msg).await
    }

    /// Revoke an acceptance, sending ANNOUNCE_CANCEL.
    ///
    /// The endpoint refuses one for an announcement it never accepted, so
    /// nothing is written on the wire when it does.
    pub async fn announce_cancel(
        &mut self,
        track_namespace: TrackNamespace,
        error_code: VarInt,
        reason_phrase: Vec<u8>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.announce_cancel(track_namespace, error_code, reason_phrase)?;
        self.send_control(&msg).await
    }
    // ── Track Status flow ────────────────────────────────────

    /// Send a TRACK_STATUS_REQUEST. Returns the allocated request ID.
    pub async fn track_status_request(
        &mut self,
        track_namespace: TrackNamespace,
        track_name: Vec<u8>,
    ) -> Result<VarInt, ConnectionError> {
        let (req_id, msg) = self.endpoint.track_status_request(track_namespace, track_name)?;
        self.send_control(&msg).await?;
        Ok(req_id)
    }

    /// Answer a TRACK_STATUS_REQUEST the peer sent, sending TRACK_STATUS.
    ///
    /// The endpoint refuses a second answer to one request, so nothing is
    /// written on the wire when it does.
    pub async fn track_status(
        &mut self,
        request_id: VarInt,
        status_code: VarInt,
        largest_location: Location,
        parameters: Vec<KeyValuePair>,
    ) -> Result<(), ConnectionError> {
        let msg = self.endpoint.send_track_status(
            request_id,
            status_code,
            largest_location,
            parameters,
        )?;
        self.send_control(&msg).await
    }

    // ── Data streams ────────────────────────────────────────

    /// Open a new unidirectional stream for sending subgroup data.
    pub async fn open_subgroup_stream(
        &self,
        header: &AnySubgroupHeader,
    ) -> Result<FramedSendStream, ConnectionError> {
        // Before the stream is opened: the Original Publisher is who the rule
        // binds, so a header that would mix this track's framing is refused
        // here rather than written and answered by the peer.
        self.endpoint.note_object_forwarding_preference(
            header.track_alias(),
            ObjectForwardingPreference::Subgroup,
        )?;
        let send = self.transport.open_uni().await?;
        let mut framed = FramedSendStream::new(send);
        let sid = framed.stream_id();
        framed.write_subgroup_header(header).await?;
        self.emit(ClientEvent::StreamOpened {
            direction: Direction::Send,
            stream_kind: StreamKind::Subgroup,
            stream_id: sid,
        });
        self.emit(ClientEvent::DataStreamHeader {
            stream_id: sid,
            direction: Direction::Send,
            header: header.clone(),
        });
        Ok(framed)
    }

    /// Open a new unidirectional stream for sending a FETCH's objects.
    ///
    /// The objects answering a FETCH do not go on the request's own stream:
    /// they go on a unidirectional stream of their own, which opens with a
    /// FETCH_HEADER naming the request they belong to. This writes that header
    /// and hands back the stream, the same way
    /// [`open_subgroup_stream`](Self::open_subgroup_stream) does for a
    /// subgroup.
    ///
    /// The caller owns the stream that comes back. Nothing here remembers
    /// which request it belongs to, so an endpoint serving several fetches at
    /// once keeps its own map from Request ID to stream.
    pub async fn open_fetch_stream(
        &self,
        header: &AnyFetchHeader,
    ) -> Result<FramedSendStream, ConnectionError> {
        let send = self.transport.open_uni().await?;
        let mut framed = FramedSendStream::new(send);
        let sid = framed.stream_id();
        framed.write_fetch_header(header).await?;
        self.emit(ClientEvent::StreamOpened {
            direction: Direction::Send,
            stream_kind: StreamKind::Fetch,
            stream_id: sid,
        });
        Ok(framed)
    }

    /// Accept the next unidirectional stream and read its fetch header.
    ///
    /// [`accept_subgroup_stream`](Self::accept_subgroup_stream)'s twin. The two
    /// are separate because the header decides how every object after it is
    /// framed, so a caller has to know which it is expecting before the first
    /// byte is read.
    ///
    /// Objects come off the returned stream with
    /// [`FramedRecvStream::read_fetch_object`].
    pub async fn accept_fetch_stream(
        &self,
    ) -> Result<(AnyFetchHeader, FramedRecvStream), ConnectionError> {
        let recv = self.transport.accept_uni().await?;
        let mut framed = FramedRecvStream::new(recv);
        let sid = framed.stream_id();
        let header = framed.read_fetch_header().await?;
        self.emit(ClientEvent::StreamOpened {
            direction: Direction::Receive,
            stream_kind: StreamKind::Fetch,
            stream_id: sid,
        });
        self.emit(ClientEvent::FetchStreamHeader {
            stream_id: sid,
            direction: Direction::Receive,
            header: header.clone(),
        });
        // A fetch header goes out as `FetchStreamHeader`; `DataStreamHeader`
        // carries an `AnySubgroupHeader` and cannot express one. What
        // `accept_subgroup_stream` does beyond this - the forwarding-preference
        // note, the object measurement - is about a subgroup and has no
        // counterpart on a fetch stream.
        Ok((header, framed))
    }

    /// Accept an incoming unidirectional data stream and read its subgroup
    /// header.
    pub async fn accept_subgroup_stream(
        &self,
    ) -> Result<(AnySubgroupHeader, FramedRecvStream), ConnectionError> {
        let recv = self.transport.accept_uni().await?;
        let mut framed = FramedRecvStream::new(recv);
        let sid = framed.stream_id();
        let header = framed.read_subgroup_header().await?;
        self.emit(ClientEvent::StreamOpened {
            direction: Direction::Receive,
            stream_kind: StreamKind::Subgroup,
            stream_id: sid,
        });
        self.emit(ClientEvent::DataStreamHeader {
            stream_id: sid,
            direction: Direction::Receive,
            header: header.clone(),
        });
        // Every object on a subgroup stream has the Subgroup preference, so
        // the header settles the track's framing before a single object is
        // read.
        self.endpoint.note_object_forwarding_preference(
            header.track_alias(),
            ObjectForwardingPreference::Subgroup,
        )?;
        // The track is resolved here and not inside the stream: it takes the
        // endpoint's alias table, which a stream handle has no way back to.
        if let Some(objects) = self.endpoint.track_objects(header.track_alias()) {
            framed.measure_objects_against(objects, header.group_id());
        }
        Ok((header, framed))
    }

    /// Send an object via datagram.
    ///
    /// The header goes through `AnyDatagramHeader::encode`, which refuses a
    /// header whose Object Status the framing it names cannot carry. Such a
    /// header errors here and nothing is sent, rather than going out as an
    /// ordinary payload datagram with the status quietly dropped.
    pub fn send_datagram(
        &self,
        header: &AnyDatagramHeader,
        payload: &[u8],
    ) -> Result<(), ConnectionError> {
        // Before anything is encoded, for the reason `open_subgroup_stream`
        // gives.
        self.endpoint.note_object_forwarding_preference(
            header.meta().track_alias,
            ObjectForwardingPreference::Datagram,
        )?;
        let mut buf = Vec::new();
        header.encode(&mut buf)?;
        buf.extend_from_slice(payload);
        self.emit(ClientEvent::DatagramReceived {
            direction: Direction::Send,
            header: header.clone(),
            payload_len: payload.len(),
        });
        self.transport.send_datagram(bytes::Bytes::from(buf))?;
        Ok(())
    }

    /// Receive a datagram and decode it whole.
    ///
    /// An extension block announced with a length of 0 is returned as
    /// [`ConnectionError::EmptyDatagramExtensions`], which
    /// [`Connection::close_for_data_stream`] closes the session over.
    ///
    /// The codec's `Datagram::decode_object` then reads the whole datagram, and its
    /// refusals are returned as [`ConnectionError::Codec`]: an extension block that
    /// does not parse, a payload after a status, and a Prior Group ID Gap larger
    /// than the Group ID, which draft-11 Section 10.1 calls a malformed track. This
    /// draft has no Malformed Tracks section and names no answer, so nothing is
    /// withdrawn and the session stays open.
    pub async fn recv_datagram(&self) -> Result<(AnyDatagramHeader, Bytes), ConnectionError> {
        let data = self.transport.recv_datagram().await?;
        let mut cursor = &data[..];
        let header = match AnyDatagramHeader::decode(DraftVersion::Draft11, &mut cursor) {
            Ok(header) => header,
            Err(CodecError::InvalidField) => {
                return Err(empty_datagram_extensions(&data)
                    .unwrap_or(ConnectionError::Codec(CodecError::InvalidField)));
            }
            Err(err) => return Err(err.into()),
        };
        let consumed = data.len() - cursor.len();
        let payload = data.slice(consumed..);
        self.emit(ClientEvent::DatagramReceived {
            direction: Direction::Receive,
            header: header.clone(),
            payload_len: payload.len(),
        });
        moqtap_codec::draft11::data_stream::Datagram::decode_object(&mut &data[..])?;
        // A datagram is the other framing, and it settles the track's just as a
        // subgroup header does.
        self.endpoint.note_object_forwarding_preference(
            header.meta().track_alias,
            ObjectForwardingPreference::Datagram,
        )?;
        // A datagram is a whole object, so the connection can measure it
        // without help from the caller.
        let meta = header.meta();
        self.endpoint.note_received_object(
            meta.track_alias,
            ObjectLocation { group: meta.group_id, object: meta.object_id },
            object_role(meta.status),
        )?;
        Ok((header, payload))
    }

    // ── Accessors ───────────────────────────────────────────

    /// Access the underlying endpoint state machine.
    pub fn endpoint(&self) -> &Endpoint {
        &self.endpoint
    }

    /// Mutable access to the endpoint state machine.
    pub fn endpoint_mut(&mut self) -> &mut Endpoint {
        &mut self.endpoint
    }

    /// The SETUP message the server answered the handshake with.
    ///
    /// `SERVER_SETUP` through draft-16, the server's half of the unified
    /// `SETUP` from draft-17. [`AnyControlMessage::fields`] renders it under
    /// this draft's own parameter names, in the order they arrived.
    pub fn server_setup(&self) -> &AnyControlMessage {
        &self.server_setup
    }

    /// The framed wire bytes of [`Self::server_setup`], as they arrived.
    ///
    /// Kept beside the decoded form because the encoding is evidence the
    /// decoding discards: two relays sending the same parameter can still
    /// disagree on how wide a varint they wrote it in.
    pub fn server_setup_raw(&self) -> Option<&[u8]> {
        self.server_setup_raw.as_deref()
    }

    /// Get the negotiated MoQT version.
    pub fn negotiated_version(&self) -> Option<VarInt> {
        self.endpoint.negotiated_version()
    }

    /// Which of this draft's *own* `ConnectionError` variants this error is,
    /// and which kind of thing it says.
    ///
    /// The ten every draft carries answer `None` here: [`AnyConnectionError`]
    /// classifies those itself. Draft-11 adds two:
    /// [`ConnectionError::ControlMessageNarrowing`], a defect in this build, and
    /// [`ConnectionError::EmptyDatagramExtensions`], a decoder refusal the draft
    /// answers with a close.
    ///
    /// Matched exhaustively, with no wildcard arm and deliberately so: a
    /// variant added to this draft's error type has to arrive here as a compile
    /// error, beside the doc comment that quotes the sentence it enforces,
    /// rather than as a silent [`ErrorCause::Unclassified`] in the facade.
    ///
    /// [`AnyConnectionError`]: crate::dispatch::AnyConnectionError
    /// [`ErrorCause::Unclassified`]: crate::dispatch::ErrorCause::Unclassified
    pub fn draft_specific_cause(
        err: &ConnectionError,
    ) -> Option<crate::above_codec_rules::DraftSpecificCause> {
        match err {
            ConnectionError::Endpoint(_)
            | ConnectionError::Codec(_)
            | ConnectionError::Transport(_)
            | ConnectionError::VarInt(_)
            | ConnectionError::NoControlStream
            | ConnectionError::UnexpectedEnd
            | ConnectionError::StreamFinished
            | ConnectionError::InvalidAddress(_)
            | ConnectionError::TlsConfig(_)
            | ConnectionError::DataStreamState(_) => None,
            // This build decoding a message and then failing to narrow it to
            // its own draft. Nothing reached the wire and no peer is
            // implicated, which is the whole reason it is not
            // `ConnectionError::Codec`: under that name it would carry
            // `Some(PROTOCOL_VIOLATION)` out of `codec_session_error_code` and
            // publish a relay for this build's defect. See the variant's own
            // doc.
            ConnectionError::ControlMessageNarrowing => {
                Some(crate::above_codec_rules::DraftSpecificCause::LocalRefusal)
            }
            // The codec refuses the datagram as a bare `InvalidField`, and
            // `recv_datagram` names the rule, so `close_for_data_stream` reads
            // the close here. See the variant's own doc.
            ConnectionError::EmptyDatagramExtensions { .. } => {
                Some(crate::above_codec_rules::DraftSpecificCause::Codec {
                    error: CodecError::InvalidField,
                    close: Some(
                        moqtap_codec::draft11::error_codes::SessionErrorCode::ProtocolViolation
                            .as_u64(),
                    ),
                })
            }
        }
    }

    /// The code to close the session with when a message could not be decoded
    /// because the peer broke a rule draft-11 answers with a close.
    ///
    /// Every variant listed here comes from a sentence in this draft that names
    /// the consequence, and the list is per draft: answering a bound this draft
    /// does not state would close a session over traffic a conforming peer may
    /// send.
    ///
    ///   - Reason Phrase, maximum 1024 bytes: "If an endpoint receives a length
    ///     exceeding the maximum, it MUST close the session with a Protocol
    ///     Violation."
    ///   - GOAWAY New Session URI, maximum 8,192 bytes, with the same sentence.
    ///     Drafts 11 through 19 state it; 07 through 10 state no maximum for
    ///     the field at all.
    ///   - Key-Value-Pair value, maximum 2^16-1 bytes, with the same sentence.
    ///   - Track Namespace tuple size: "If an endpoint receives a Track
    ///     Namespace tuple with an N of 0 or more than 32, it MUST close the
    ///     session with a Protocol Violation." Note the lower bound - an empty
    ///     tuple is refused here, where drafts 17 and later permit one.
    ///   - Full Track Name, maximum 4,096 bytes, "computed as the sum of the
    ///     lengths of each Track Namespace tuple field and the Track Name
    ///     length field". This draft bounds the pair and not the namespace
    ///     alone; draft-16 widened it.
    ///   - Duplicate parameters, a SHOULD rather than a MUST: "Receivers SHOULD
    ///     check that there are no unauthorized duplicate parameters and close
    ///     the session as a 'Protocol Violation' if found." The rule is
    ///     asymmetric here - "Receivers MUST allow duplicates of unknown
    ///     parameters", and one known type is granted repeats - and the codec
    ///     reports only the repeats this draft actually forbids.
    ///   - Unknown control message type: "An endpoint that receives an unknown
    ///     message type MUST close the session."
    ///   - Extension headers on an Object whose status is Object Does Not
    ///     Exist, Section 9.1.1.2. That one arrives on a data stream or a
    ///     datagram, so [`Connection::close_for_data_stream`] is what carries
    ///     it.
    ///
    /// **Not** the zero-length Track Namespace Field, the delta-encoded
    /// parameter type overflow, or the Object ID delta wrap. Those enter the
    /// specification at drafts 16 and 18, and this draft states none of them.
    /// **Not** the 2^16-1 control message length either. This draft states the
    /// limit - "the total length of a control message is limited to 2^16-1" -
    /// and states no consequence for exceeding it, so an oversized message is
    /// refused by the decoder and stops there.
    ///
    /// **Not** [`CodecError::UnexpectedEnd`], which reports no rule at all: the
    /// reader raises it whenever a message is still arriving, and
    /// `read_control` loops on it. Closing over it would end a session on an
    /// ordinary short read.
    ///
    /// **Not** the unknown Message Parameter rule. Drafts 16 through 19 require
    /// a close for a Message Parameter whose type the negotiated version does
    /// not define. This draft states the opposite and states it about the same
    /// parameters: "Receivers MUST allow duplicates of unknown parameters",
    /// which presumes an unknown parameter arrives and is carried. Refusing one
    /// here would close a session over an extension this draft leaves room for.
    ///
    /// `None` for everything else, including [`CodecError::InvalidField`]. That
    /// variant is shared by a dozen unrelated malformations, only some of which
    /// the draft answers with a close, so a session cannot be ended on it
    /// without ending sessions the draft does not ask to be ended. Splitting it
    /// is the way to bring the rest of those rules under this function;
    /// widening the match is not - the extension-header rule above is in this
    /// table because it has a variant of its own.
    pub fn codec_session_error_code(
        err: &CodecError,
    ) -> Option<moqtap_codec::draft11::error_codes::SessionErrorCode> {
        use moqtap_codec::draft11::error_codes::SessionErrorCode;
        use moqtap_codec::kvp::KvpError;
        match err {
            // The declared Length disagreeing with the fields, which every
            // draft answers with a close. Drafts 07 through 10 name no code for
            // it, so it takes the one their other unnamed rules take.
            CodecError::ControlMessageLengthMismatch { .. } => {
                Some(SessionErrorCode::ProtocolViolation)
            }
            CodecError::ReasonPhraseTooLong
            | CodecError::GoAwayUriTooLong
            | CodecError::InvalidNamespaceTupleSize(_)
            | CodecError::TrackNameTooLong
            | CodecError::DuplicateParameter(_)
            | CodecError::UnknownMessageType(_)
            | CodecError::ExtensionsOnNonExistentObject(_)
            | CodecError::Kvp(KvpError::ValueTooLong(_)) => {
                Some(SessionErrorCode::ProtocolViolation)
            }
            // An unknown data-plane type, Section 9: "An endpoint that
            // receives an unknown stream or datagram type MUST close the
            // session." One sentence covering two tables, which is why both
            // variants sit here.
            // A Content Exists field that is neither zero nor one,
            // Section 8.8: "Any other value is a protocol error and
            // MUST terminate the session with a Protocol Violation".
            CodecError::InvalidContentExists(_) => Some(SessionErrorCode::ProtocolViolation),
            // A Forward field that is neither zero nor one, Section 8.7 and
            // Section 8.10: "Any other value is a protocol error and MUST
            // terminate the session with a Protocol Violation". Draft-11 is
            // where the field arrives, and it carries only these two sites;
            // PUBLISH and PUBLISH_OK add two more from draft-12.
            CodecError::InvalidForward(_) => Some(SessionErrorCode::ProtocolViolation),
            CodecError::UnknownStreamType(_) | CodecError::UnknownDatagramType(_) => {
                Some(SessionErrorCode::ProtocolViolation)
            }
            // A key-value pair whose value is not the serialization its own
            // Type defines, Section 1.3.2: "If a receiver understands a Type,
            // and the following Value or Length/Value does not match the
            // serialization defined by that Type, the receiver MUST terminate the
            // session with error code 'Key-Value Formatting Error'."
            //
            // This draft states the general rule alone. The Authorization Token
            // structure it defines gets its own sentence only from draft-12 on,
            // and a token that cannot be decoded is answered here either way.
            //
            // The one rule in this table that names a code other than Protocol
            // Violation.
            CodecError::KeyValueFormatting { .. } => {
                Some(SessionErrorCode::KeyValueFormattingError)
            }
            // Everything this draft does not answer, named rather than swept up
            // by a wildcard. The arm is exhaustive deliberately: a new
            // `CodecError` variant will not compile until it has been placed on
            // one side or the other, on this draft, which is the decision a `_`
            // arm makes silently and invisibly in every draft module at once.
            //
            // Adding one variant to `CodecError` produces an `E0004` in every
            // draft module that matches it exhaustively, each naming the
            // variant that has nowhere to go. That is the whole mechanism.
            //
            // The nesting stops at `VarInt`, whose variants report how the bytes
            // ran out rather than a rule an endpoint states, so there is nothing
            // in it for a draft to answer. `Kvp` is spelled out because it does
            // carry one.
            // Not `ParameterValueOutOfRange`: no parameter this draft defines
            // restricts its value's range. Forwarding is a message field here
            // and is answered above.
            CodecError::ParameterValueOutOfRange { .. }
            | CodecError::UnexpectedEnd
            | CodecError::MessageTooLong(_)
            | CodecError::VarInt(_)
            | CodecError::InvalidField
            | CodecError::EmptyNamespaceField
            | CodecError::InvalidRange(..)
            | CodecError::ParameterLengthMismatch(_)
            | CodecError::EndOfTrackObjectId(_)
            | CodecError::KeyDeltaOverflow(..)
            // Not `TrackPropertyValueOutOfRange`: this draft has neither
            // namespace the variant is about. Draft-16 opens an extension header
            // registry with value rules of its own, and draft-17 renames it to
            // the Track Property registry. Before that, everything with a
            // restricted range is either a message field or a Message Parameter.
            | CodecError::TrackPropertyValueOutOfRange { .. }
            | CodecError::ParametersOutOfOrder(..)
            | CodecError::ObjectIdOverflow(..)
            | CodecError::InvalidRequiredRequestIdDelta(..)
            | CodecError::InvalidStreamTypeValue { .. }
            | CodecError::InvalidDatagramTypeValue { .. }
            | CodecError::UnknownMessageParameter(_)
            // Not `ParameterOutOfScope`: this draft states the scope rule and
            // answers it the other way. Section 8.2.1 Version Specific Parameters: "Each
            // version-specific parameter definition indicates the message types in which it can
            // appear. If it appears in some other type of message, it MUST be
            // ignored." The codec carries such a parameter on this draft and never
            // raises the variant, so this arm records a rule this draft has and
            // does not close over, not one it is missing. Draft-17 is where the
            // second sentence becomes a close.
            | CodecError::ParameterOutOfScope { .. }
            // A Filter Type outside the set this draft assigns. Section 8.7
            // states the rule and stops there: "A filter type other than the
            // above MUST be treated as error." No code, no close, and no
            // sentence elsewhere in the draft that turns an error into one — so
            // the message is refused and the session stays open.
            // Draft-14 Section 9.7 is where the same sentence gained "MUST be
            // close the session with PROTOCOL_VIOLATION", and it is answered
            // there.
            //
            // The assigned set is not the same on every draft either: 07 and 08
            // assign 0x1 as Latest Group, 09 and 10 withdraw it, and 11 and
            // later reinstate it as Next Group Start. The decoder holds each
            // draft to its own list; this arm only decides what a refusal does
            // to the session.
            //
            // The two rules below belong to the parameter form of the filter,
            // which arrives at draft-15. This draft carries the Filter Type as a
            // field of SUBSCRIBE, so there is no parameter for either to be
            // about.
            | CodecError::InvalidFilterType(_)
            | CodecError::SubscriptionFilterMalformed { .. }
            | CodecError::FilterEndGroupOverflow { .. }
            // A Fetch Type outside the set this draft assigns. Section 8.13
            // states the rule and stops there: "A Fetch Type other than 0x1,
            // 0x2 or 0x3 MUST be treated as an error", naming no code and no
            // close, exactly as this draft's Filter Type sentence does.
            // Draft-14 Section 9.16 is where both gained "MUST be close the
            // session with a PROTOCOL_VIOLATION", and it answers them there.
            | CodecError::InvalidFetchType(_)
            // The object payload rule, Section 9.1.1.1: "Any object with a status
            // code other than zero MUST have an empty payload." A MUST on the
            // sender with no receiver action named anywhere — the "SHOULD be
            // treated as a protocol error" in the same paragraph belongs to the
            // sentence before it, which is about a status value this draft does
            // not assign — so an object carrying a payload it may not is refused
            // and the session stays open.
            | CodecError::PayloadNotPermitted { .. }
            | CodecError::MalformedObjectProperties { .. }
            // draft-11 Section 10.1 calls a track with a Prior Group ID Gap
            // fault malformed and names no close, so the session stays open.
            | CodecError::MalformedTrack { .. }
            // Raised only by the draft-23 codec.
            | CodecError::ControlMessageExceedsLimit { .. }
            | CodecError::MissingErrorDetail { .. }
            | CodecError::MalformedErrorDetail { .. }
            | CodecError::TokenAliasInSetup { .. }
            | CodecError::EmptyTrackName
            | CodecError::MalformedAuthToken { .. }
            | CodecError::TrackPropertiesNotPermitted
            | CodecError::DeltaDecodingOverflow { .. }
            | CodecError::MalformedObject { .. }
            | CodecError::UnknownObjectStatus(_)
            | CodecError::UnsupportedDraft(_)
            | CodecError::Kvp(
                KvpError::MissingLength | KvpError::UnexpectedEnd | KvpError::VarInt(_),
            ) => None,
        }
    }

    /// Close the session on the wire when a decode failure is one draft-11
    /// answers with a close, and hand the error back unchanged.
    /// Without it every bound the decoder enforces would stop at *this endpoint
    /// refused the frame* while the peer, which is the one that broke the rule,
    /// saw a session that was still open and went on sending. "MUST close the
    /// session" is a statement about the wire.
    fn close_for_codec(&self, err: ConnectionError) -> ConnectionError {
        if let ConnectionError::Codec(inner) = &err {
            if let Some(code) = Self::codec_session_error_code(inner) {
                // QUIC application error codes are 62-bit; every code in this
                // registry is far below `u32::MAX`, and saturating rather than
                // truncating means a future code that is not could never be
                // reported as a different, assigned one.
                let wire_code = u32::try_from(code.as_u64()).unwrap_or(u32::MAX);
                self.close(wire_code, inner.to_string().as_bytes());
            }
        }
        err
    }

    /// Close the session on the wire when the endpoint says a violation is
    /// fatal to it, and hand the error back unchanged.
    ///
    /// [`EndpointError::session_error_code`] answers `Some` for exactly the
    /// errors this draft ends the session over, and the endpoint has already
    /// moved its own state machine to Closed by the time this runs. Without
    /// this step that move is purely internal: the local endpoint refuses to
    /// start anything new while the peer, which is the one that broke the
    /// rule, sees a session that is still open and goes on sending. A rule
    /// that names a session termination code is a statement about the wire,
    /// so it takes a CONNECTION_CLOSE to satisfy it.
    ///
    /// The reason phrase is the error's own `Display` text, which names the
    /// rule rather than repeating the numeric code the close already carries.
    ///
    /// Errors that answer `None` are recoverable and nothing is sent.
    fn close_for(&self, err: &EndpointError) {
        if let Some(code) = err.session_error_code() {
            // QUIC application error codes are 62-bit; every code in this
            // registry is far below `u32::MAX`, and saturating rather than
            // truncating means a future code that is not could never be
            // reported as a different, assigned one.
            let wire_code = u32::try_from(code.as_u64()).unwrap_or(u32::MAX);
            self.close(wire_code, err.to_string().as_bytes());
        }
    }

    /// [`close_for`](Self::close_for), then the error unchanged, for the
    /// common case where the endpoint's error is also what the caller returns.
    fn close_if_session_fatal(&self, err: EndpointError) -> ConnectionError {
        self.close_for(&err);
        ConnectionError::Endpoint(err)
    }

    /// Close the session over a rule broken on a data stream, reporting whether
    /// it did.
    ///
    /// A data stream cannot close for itself the way `recv_control` does:
    /// [`Connection::accept_subgroup_stream`] hands the caller a
    /// [`FramedRecvStream`] holding no connection, so the reader that finds the
    /// violation is not the object that can act on it. Keeping it a separate
    /// call is deliberate as well - a permissive caller, one reproducing a
    /// capture, can read a violating stream and report it without tearing the
    /// session down.
    ///
    /// The rule this draft answers here is extension headers on an Object whose
    /// status is Object Does Not Exist, which reaches subgroup streams, fetch
    /// streams and status datagrams alike. It shares `codec_session_error_code`
    /// with the control path, so a rule is answered with one code whichever
    /// stream carried it.
    ///
    /// Not every rule that reaches here is the decoder's. A track whose objects
    /// mix forwarding preferences is the endpoint's to notice — it takes the
    /// alias table to know which track an object belongs to — and it arrives on
    /// exactly these streams. Both kinds are asked for a code the same way, and
    /// a rule with no code is declined rather than guessed at.
    pub fn close_for_data_stream(&self, err: &ConnectionError) -> bool {
        match err {
            ConnectionError::Codec(inner) => {
                let Some(code) = Self::codec_session_error_code(inner) else { return false };
                let wire_code = u32::try_from(code.as_u64()).unwrap_or(u32::MAX);
                self.close(wire_code, inner.to_string().as_bytes());
                true
            }
            // A rule the endpoint raises rather than the decoder. The two
            // reach their codes through different tables and mean the same
            // thing here: `Some` is a rule this draft ends the session over.
            ConnectionError::Endpoint(inner) => {
                let Some(code) = inner.session_error_code() else { return false };
                let wire_code = u32::try_from(code.as_u64()).unwrap_or(u32::MAX);
                self.close(wire_code, inner.to_string().as_bytes());
                true
            }
            // Not a `Codec` failure: the codec refuses the datagram as a bare
            // `InvalidField`, which `recv_datagram` names, so the code comes
            // from `draft_specific_cause`.
            ConnectionError::EmptyDatagramExtensions { .. } => {
                let Some(crate::above_codec_rules::DraftSpecificCause::Codec {
                    close: Some(code),
                    ..
                }) = Self::draft_specific_cause(err)
                else {
                    return false;
                };
                self.close(u32::try_from(code).unwrap_or(u32::MAX), err.to_string().as_bytes());
                true
            }
            _ => false,
        }
    }

    /// Close the connection.
    pub fn close(&self, code: u32, reason: &[u8]) {
        self.emit(ClientEvent::Closed { code, reason: reason.to_vec() });
        self.transport.close(code, reason);
    }
}

/// Determine the encoded length of a varint from its first byte.
fn varint_len(first_byte: u8) -> usize {
    1 << (first_byte >> 6)
}

#[cfg(test)]
mod tests {
    use super::*;

    /// This build failing to narrow a message it decoded is never a finding
    /// about the peer.
    ///
    /// The arm that raises `ControlMessageNarrowing` is unreachable — this
    /// draft's decoder can only hand back this draft's variant — and nothing
    /// pins that. What is pinned here is the half that matters.
    /// `CodecError::UnknownMessageType(0)` is what the arm must not raise:
    /// `codec_session_error_code` answers it `Some(PROTOCOL_VIOLATION)` on
    /// every draft in range, so the day the narrowing failed a conformance
    /// probe would publish a relay for sending a control message type this
    /// draft does not assign — with `0x00` attached as the codepoint that
    /// proved it, which is an accusation better evidenced than any real one
    /// this build makes. The section stating that rule is numbered differently
    /// on every draft, and the point does not turn on the number.
    #[test]
    fn a_message_this_build_could_not_narrow_names_nobody() {
        use crate::dispatch::{AnyConnectionError, ErrorCause};

        let err: AnyConnectionError = ConnectionError::ControlMessageNarrowing.into();
        assert!(err.is_local(), "a narrowing this build could not do is this build's");
        assert_eq!(
            err.cause(),
            &ErrorCause::Facade,
            "nothing reached the wire, so there is no rule and no close code to read"
        );
    }

    #[test]
    fn client_config_supported_versions_default() {
        let config = ClientConfig {
            additional_versions: Vec::new(),
            transport: TransportType::Quic,
            skip_cert_verification: false,
            ca_certs: Vec::new(),
            setup_parameters: Vec::new(),
        };
        let versions = config.supported_versions();
        assert_eq!(versions.len(), 1);
        assert_eq!(versions[0].into_inner(), 0xff000000 + 11);
    }

    #[test]
    fn client_config_alpn_quic() {
        let config = ClientConfig {
            additional_versions: Vec::new(),
            transport: TransportType::Quic,
            skip_cert_verification: false,
            ca_certs: Vec::new(),
            setup_parameters: Vec::new(),
        };
        assert_eq!(config.alpn(), vec![DraftVersion::Draft11.quic_alpn().to_vec()]);
    }

    #[test]
    fn moqt_alpn_value() {
        assert_eq!(MOQT_ALPN, b"moq-00");
    }
}