rs_matter/

transport.rs

/*
 *
 *    Copyright (c) 2022-2026 Project CHIP Authors
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *        http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */

use core::fmt::{self, Display};
use core::future::Future;
use core::num::NonZeroU8;
use core::ops::{Deref, DerefMut};
use core::pin::pin;

use domain::base::name::ToLabelIter;

use embassy_futures::select::{select, select3, select4, Either};
use embassy_time::{Duration, Timer};

use rand_core::RngCore;

use crate::crypto::Crypto;
use crate::dm::clusters::basic_info::BasicInfoConfig;
use crate::dm::NodeId;
use crate::error::{Error, ErrorCode};
use crate::fabric::{MAX_FABRICS, MAX_GROUPS_PER_FABRIC};
use crate::fmt::Bytes;
use crate::sc::case::CaseInitiator;
use crate::sc::pase::PaseInitiator;
use crate::sc::{
    sc_write, OpCode, SCStatusCodes, SessionParameters, StatusReport, PROTO_ID_SECURE_CHANNEL,
};
use crate::tlv::TLVElement;
use crate::transport::network::mdns::{
    commissionable_instance_id, score_ip_address, BrowseExclude, CommissionableFilter,
    MdnsBrowseState, MdnsRemoteService, MdnsResolveState, ResolvedNode,
};
use crate::transport::network::{MatterRemoteService, NetworkMulticast};
use crate::utils::init::{init, Init};
use crate::utils::ipv6::compute_group_multicast_addr;
use crate::utils::select::Coalesce;
use crate::utils::storage::Vec;
use crate::utils::storage::{pooled::Buffers, ParseBuf, WriteBuf};
use crate::utils::sync::{IfMutex, IfMutexGuard, Notification, Signal};
use crate::{Matter, MATTER_PORT};

use exchange::{Exchange, ExchangeId, ExchangeState, MessageMeta, ResponderState, Role};
use network::{Address, IpAddr, Ipv6Addr, NetworkReceive, NetworkSend, SocketAddr, SocketAddrV6};
use packet::PacketHdr;
use proto_hdr::ProtoHdr;
use session::{Session, Sessions};

mod dedup;

pub mod exchange;
pub mod mrp;
pub mod network;
pub mod packet;
pub mod plain_hdr;
pub mod proto_hdr;
pub mod session;

pub const MATTER_SOCKET_BIND_ADDR: SocketAddr =
    SocketAddr::V6(SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, MATTER_PORT, 0, 0));

const MAX_GROUP_ADDRS: usize = MAX_FABRICS * MAX_GROUPS_PER_FABRIC;

const ACCEPT_TIMEOUT_MS: u64 = 1000;

#[cfg(feature = "large-buffers")]
pub(crate) const MAX_RX_BUF_SIZE: usize = network::MAX_RX_LARGE_PACKET_SIZE;
#[cfg(feature = "large-buffers")]
pub(crate) const MAX_TX_BUF_SIZE: usize = network::MAX_TX_LARGE_PACKET_SIZE;

#[cfg(not(feature = "large-buffers"))]
pub(crate) const MAX_RX_BUF_SIZE: usize = network::MAX_RX_PACKET_SIZE;
#[cfg(not(feature = "large-buffers"))]
pub(crate) const MAX_TX_BUF_SIZE: usize = network::MAX_TX_PACKET_SIZE;

/// The maximum application payload of an incoming (RX) packet, i.e. the RX buffer
/// minus the two packet headers (plain + protocol) and the trailing AEAD tag.
pub const MAX_RX_PAYLOAD_SIZE: usize =
    MAX_RX_BUF_SIZE - PacketHdr::HDR_RESERVE - PacketHdr::TAIL_RESERVE;

/// The maximum application payload of an outgoing (TX) packet, i.e. the TX buffer
/// minus the two packet headers (plain + protocol) and the trailing AEAD tag.
pub const MAX_TX_PAYLOAD_SIZE: usize =
    MAX_TX_BUF_SIZE - PacketHdr::HDR_RESERVE - PacketHdr::TAIL_RESERVE;

/// Represents the state of the transport layer of a `Matter` instance.
pub struct Transport {
    /// Buffer for an incoming (RX) packet.
    // TODO XXX FIXME: Needs multiple wakers for work-stealing executors
    rx: IfMutex<Packet<MAX_RX_BUF_SIZE>>,
    /// Buffer for an outgoing (TX) packet.
    // TODO XXX FIXME: Needs multiple wakers for work-stealing executors
    tx: IfMutex<Packet<MAX_TX_BUF_SIZE>>,
    /// List of currently joined group addresses, used for managing multicast group membership.
    group_addrs: IfMutex<Vec<Ipv6Addr, MAX_GROUP_ADDRS>>,
    /// Notification for when an exchange is dropped.
    exchange_dropped: Notification,
    /// A notification that the Matter mDNS services might have changed
    mdns_changed: Notification,
    /// The single in-flight mDNS resolve rendezvous, shared between [`Transport::resolve`]
    /// callers and the running mDNS responder.
    mdns_resolve: Signal<MdnsResolveState>,
    /// The single in-flight mDNS commissionable-browse rendezvous, shared between
    /// [`Transport::browse_commissionable`] callers and the running mDNS responder.
    mdns_browse: Signal<MdnsBrowseState>,
    /// A notification that a session had been removed
    session_removed: Notification,
    /// A notification that the groups have been modified
    groups_modified: Notification,
    /// Device SAI (Secure Association Identifier)
    device_sai: Option<u32>,
    /// Device SII (Secure Identity Identifier)
    device_sii: Option<u32>,
}

impl Transport {
    /// Create a new `Transport` with empty RX and TX buffers, and the given device SAI/SII.
    #[inline(always)]
    pub(crate) const fn new(dev_det: &BasicInfoConfig<'_>) -> Self {
        Self {
            rx: IfMutex::new(Packet::new()),
            tx: IfMutex::new(Packet::new()),
            group_addrs: IfMutex::new(Vec::new()),
            exchange_dropped: Notification::new(),
            mdns_changed: Notification::new(),
            mdns_resolve: Signal::new(MdnsResolveState::Idle),
            mdns_browse: Signal::new(MdnsBrowseState::Idle),
            session_removed: Notification::new(),
            groups_modified: Notification::new(),
            device_sai: dev_det.sai,
            device_sii: dev_det.sii,
        }
    }

    /// Initialize the transport state by initializing the RX and TX buffers, and setting up the exchange dropped notification.
    pub(crate) fn init<'m>(dev_det: &'m BasicInfoConfig<'m>) -> impl Init<Self> + 'm {
        init!(Self {
            rx <- IfMutex::init(Packet::init()),
            tx <- IfMutex::init(Packet::init()),
            group_addrs <- IfMutex::init(Vec::new()),
            exchange_dropped: Notification::new(),
            mdns_changed: Notification::new(),
            mdns_resolve: Signal::new(MdnsResolveState::Idle),
            mdns_browse: Signal::new(MdnsBrowseState::Idle),
            session_removed: Notification::new(),
            groups_modified: Notification::new(),
            device_sai: dev_det.sai,
            device_sii: dev_det.sii,
        })
    }

    /// Reset the transport state by clearing the RX buffer and the TX buffer
    /// NOTE: User should be careful _not_ to call this method while the transport layer and/or the built-in mDNS is running.
    pub fn reset(&self) -> Result<(), Error> {
        self.rx
            .try_lock()
            .map_err(|_| ErrorCode::InvalidState)?
            .buf
            .clear();
        self.tx
            .try_lock()
            .map_err(|_| ErrorCode::InvalidState)?
            .buf
            .clear();

        Ok(())
    }

    /// Return a reference to the transport RX buffer.
    ///
    /// Useful when external code (like i.e. a user-provided mDNS implementation)
    /// needs an RX buffer.
    pub fn rx_buffer(&self) -> PacketBufferExternalAccess<'_, MAX_RX_BUF_SIZE> {
        PacketBufferExternalAccess(&self.rx)
    }

    /// Return a reference to the transport TX buffer.
    ///
    /// Useful when external code (like i.e. a user-provided mDNS implementation)
    /// needs a TX buffer.
    pub fn tx_buffer(&self) -> PacketBufferExternalAccess<'_, MAX_TX_BUF_SIZE> {
        PacketBufferExternalAccess(&self.tx)
    }

    /// Notify that the Matter mDNS services _might_ have changed.
    pub(crate) fn notify_mdns_changed(&self) {
        self.mdns_changed.notify();
    }

    /// A hook for user code to wait for notification that the Matter mDNS services might have changed.
    ///
    /// Once this future resolves, user code is supposed to inspect the mDNS services for changes, and
    /// if there are changes, re-publish the changed mDNS services in an mDNS responder accordingly.
    pub fn wait_mdns(&self) -> impl Future<Output = ()> + '_ {
        self.mdns_changed.wait()
    }

    /// Notify that groups have changed (keys, key maps, or membership) and
    /// multicast registrations need updating.
    pub(crate) fn notify_groups_changed(&self) {
        self.groups_modified.notify();
    }

    /// Wait until the groups have changed (see [`Transport::notify_groups_changed`]).
    fn wait_groups_changed(&self) -> impl Future<Output = ()> + '_ {
        self.groups_modified.wait()
    }

    /// Notify that a session has been removed.
    pub(crate) fn notify_session_removed(&self) {
        self.session_removed.notify();
    }

    /// Wait until a session has been removed (see [`Transport::notify_session_removed`]).
    pub(crate) fn wait_session_removed(&self) -> impl Future<Output = ()> + '_ {
        self.session_removed.wait()
    }

    /// Resolve a Matter service instance's address over mDNS.
    ///
    /// Places a single resolve request into the shared rendezvous and waits up
    /// to `timeout_ms` for the running mDNS responder to fire the query and
    /// deposit a service answer. Multiple concurrent callers serialize: each waits for
    /// any in-flight resolve to finish before placing its own.
    ///
    /// Returns [`ErrorCode::NotFound`] if no service answer arrives within the timeout.
    /// If this future is dropped before completing, the rendezvous is reset so
    /// other callers can proceed.
    ///
    /// Requires a running mDNS responder (e.g. `BuiltinMdns::run`) to
    /// service the request; without one, every resolve will time out.
    async fn resolve(
        &self,
        service: MatterRemoteService,
        timeout_ms: u32,
    ) -> Result<ResolvedNode, Error> {
        // 1. Serialize with other resolvers and place the request.
        self.mdns_resolve
            .wait(|state| {
                if matches!(state, MdnsResolveState::Idle) {
                    *state = MdnsResolveState::Requested {
                        service: service.clone(),
                    };
                    Some(())
                } else {
                    None
                }
            })
            .await;

        // 2. Ensure the slot is released if this future is dropped (or times out).
        let mut guard = MdnsResolveGuard {
            signal: &self.mdns_resolve,
            armed: true,
        };

        // 3. Wait for the responder to deposit our answer, or time out.
        let mut wait = pin!(self.mdns_resolve.wait(|state| match state {
            MdnsResolveState::Resolved {
                ip,
                port,
                scope_id,
                sii,
                sai,
                sat,
            } => {
                let node = ResolvedNode {
                    addr: Self::scoped_socket_addr(*ip, *port, *scope_id),
                    sii: *sii,
                    sai: *sai,
                    sat: *sat,
                };
                *state = MdnsResolveState::Idle;
                Some(node)
            }
            _ => None,
        }));

        let mut timer = pin!(Timer::after(Duration::from_millis(timeout_ms as u64)));

        match select(&mut wait, &mut timer).await {
            Either::First(node) => {
                // The `wait` above already reset the slot to `Idle`.
                guard.armed = false;

                Ok(node)
            }
            Either::Second(_) => Err(ErrorCode::NotFound.into()),
        }
    }

    /// Responder-side: await the next pending mDNS resolve request, marking it
    /// in-flight.
    ///
    /// Part of the public responder contract: a third-party mDNS responder
    /// (living outside this crate) drives the resolve use case by awaiting a
    /// request here, issuing its own query, and depositing any answers via
    /// [`Transport::try_deposit_mdns_resolve`].
    pub async fn wait_mdns_resolve_request(&self) -> MatterRemoteService {
        self.mdns_resolve
            .wait(|state| match state {
                MdnsResolveState::Requested { service } => {
                    let service = service.clone();
                    *state = MdnsResolveState::InFlight {
                        service: service.clone(),
                    };
                    Some(service)
                }
                _ => None,
            })
            .await
    }

    /// Whether an operational resolve is currently in flight.
    ///
    /// Used by mDNS implementations to poll-drive their resolve
    /// loop only while a request is outstanding.
    #[allow(dead_code)]
    pub fn mdns_resolve_in_flight(&self) -> bool {
        self.mdns_resolve
            .modify(|state| (false, matches!(state, MdnsResolveState::InFlight { .. })))
    }

    /// Deposit a discovered [`MdnsRemoteService`] against an in-flight resolve
    /// request, transitioning it to `Resolved` if the answer's instance name
    /// matches. Best-effort: a non-matching, address-less, or absent request is
    /// a no-op.
    ///
    /// The single resolve-deposit entry point shared by the builtin parser, the
    /// OS-backed responders, and any third-party responder - all hand it an
    /// [`MdnsRemoteService`] (the builtin lazily over the packet, the others over
    /// their native records).
    pub fn try_deposit_mdns_resolve<'a, I, A, T>(&self, answer: &MdnsRemoteService<I, A, T>)
    where
        I: ToLabelIter,
        A: Iterator<Item = IpAddr> + Clone,
        T: Iterator<Item = (&'a str, &'a str)> + Clone,
    {
        let Some(ip) = answer.addrs.clone().max_by_key(score_ip_address) else {
            return;
        };
        let Some(port) = answer.port else {
            return;
        };
        let scope_id = answer.scope_id;

        let (sii, sai, sat) = answer.session_params();

        self.mdns_resolve.modify(|state| match state {
            MdnsResolveState::InFlight { service }
                if service.matches_instance(&answer.instance_name) =>
            {
                *state = MdnsResolveState::Resolved {
                    ip,
                    port,
                    scope_id,
                    sii,
                    sai,
                    sat,
                };
                (true, ())
            }
            // Already resolved (same in-flight target — the rendezvous is
            // single-slot) but not yet consumed: keep the better-scoring address
            // so a later IPv6 deposit can upgrade an earlier IPv4 one (e.g.
            // zeroconf surfaces one address per family at a time).
            //
            // Preserve any session params already resolved: a per-address deposit
            // may carry no TXT (e.g. zeroconf's `ServiceDiscovery.txt` is
            // optional and can arrive only on a different callback than the
            // address), in which case `sii`/`sai`/`sat` are `None` here and would
            // otherwise clobber the good values from the earlier deposit.
            MdnsResolveState::Resolved {
                ip: cur_ip,
                sii: cur_sii,
                sai: cur_sai,
                sat: cur_sat,
                ..
            } if score_ip_address(&ip) > score_ip_address(cur_ip) => {
                *state = MdnsResolveState::Resolved {
                    ip,
                    port,
                    scope_id,
                    sii: sii.or(*cur_sii),
                    sai: sai.or(*cur_sai),
                    sat: sat.or(*cur_sat),
                };
                (true, ())
            }
            _ => (false, ()),
        });
    }

    /// Browse the mDNS network for a **commissionable** node matching `filter`,
    /// returning the first match's address and commissionable instance id.
    ///
    /// Places a single browse request into the shared rendezvous and waits up to
    /// `timeout_ms` for the running mDNS responder to fire the browse query and
    /// deposit the first node whose advertisement matches **all** non-`None`
    /// fields of `filter` (see [`CommissionableFilter::matches`]) and whose
    /// commissionable instance id is **not** in `exclude`. Multiple concurrent
    /// callers serialize.
    ///
    /// `exclude` is how a caller steps to the **next** candidate when several
    /// nodes share a (short) discriminator: pass the ids already tried (PASE
    /// failed), and the next un-tried match is returned; repeat until
    /// [`ErrorCode::NotFound`] (exhausted). Pass `&[]` for the first attempt.
    /// At most [`MAX_BROWSE_EXCLUDE`](crate::transport::network::mdns) ids - more
    /// returns [`ErrorCode::ResourceExhausted`].
    ///
    /// Returns `(address, commissionable_instance_id)` - the address can be fed
    /// straight into [`Transport::initiate_pase`] to start PASE. Returns
    /// [`ErrorCode::NotFound`] on timeout; the rendezvous is reset if this future
    /// is dropped.
    ///
    /// Requires a running mDNS responder (e.g. `BuiltinMdns::run`).
    ///
    // TODO: A BLE/BTP equivalent is needed to discover wireless devices that
    // advertise commissionable over BLE rather than mDNS - future work.
    pub async fn browse_commissionable(
        &self,
        filter: &CommissionableFilter,
        exclude: &[u64],
        timeout_ms: u32,
    ) -> Result<(Address, u64), Error> {
        let mut exclude_vec = BrowseExclude::new();
        exclude_vec
            .extend_from_slice(exclude)
            .map_err(|_| ErrorCode::ResourceExhausted)?;

        // 1. Serialize with other browsers and place the request.
        self.mdns_browse
            .wait(|state| {
                if matches!(state, MdnsBrowseState::Idle) {
                    *state = MdnsBrowseState::Requested {
                        filter: filter.clone(),
                        exclude: exclude_vec.clone(),
                    };
                    Some(())
                } else {
                    None
                }
            })
            .await;

        // 2. Release the slot if this future is dropped (or times out).
        let mut guard = MdnsBrowseGuard {
            signal: &self.mdns_browse,
            armed: true,
        };

        // 3. Wait for the first matching, non-excluded commissionable node, or time out.
        let mut wait = pin!(self.mdns_browse.wait(|state| match state {
            MdnsBrowseState::Found {
                ip,
                port,
                scope_id,
                id,
            } => {
                let found = (
                    Address::Udp(Self::scoped_socket_addr(*ip, *port, *scope_id)),
                    *id,
                );
                *state = MdnsBrowseState::Idle;
                Some(found)
            }
            _ => None,
        }));

        let mut timer = pin!(Timer::after(Duration::from_millis(timeout_ms as u64)));

        match select(&mut wait, &mut timer).await {
            Either::First(found) => {
                guard.armed = false;

                Ok(found)
            }
            Either::Second(_) => Err(ErrorCode::NotFound.into()),
        }
    }

    /// Responder-side: await the next pending mDNS browse request, marking it
    /// in-flight. Returns the filter to query for (the exclude set is consulted
    /// later, at deposit time).
    ///
    /// Part of the public responder contract: a third-party mDNS responder
    /// (living outside this crate) drives the commissionable-browse use case by
    /// awaiting a request here, issuing its own query, and depositing matches via
    /// [`Transport::try_deposit_mdns_browse`].
    pub async fn wait_mdns_browse_request(&self) -> CommissionableFilter {
        self.mdns_browse
            .wait(|state| match state {
                MdnsBrowseState::Requested { filter, exclude } => {
                    let filter = filter.clone();
                    *state = MdnsBrowseState::InFlight {
                        filter: filter.clone(),
                        exclude: core::mem::take(exclude),
                    };

                    Some(filter)
                }
                _ => None,
            })
            .await
    }

    /// Whether a commissionable browse is currently in flight.
    ///
    /// Used by mDNS implementations to poll-drive their browse
    /// loop only while a request is outstanding.
    #[allow(dead_code)]
    pub fn mdns_browse_in_flight(&self) -> bool {
        self.mdns_browse
            .modify(|state| (false, matches!(state, MdnsBrowseState::InFlight { .. })))
    }

    /// Deposit a discovered [`MdnsRemoteService`] against an in-flight browse
    /// request, transitioning it to `Found` if the instance is not excluded and
    /// its TXT records match the filter. Best-effort: a non-matching, excluded,
    /// address-less, or absent request is a no-op.
    ///
    /// The single browse-deposit entry point shared by the builtin parser, the
    /// OS-backed responders, and any third-party responder - all hand it an
    /// [`MdnsRemoteService`].
    pub fn try_deposit_mdns_browse<'a, I, A, T>(&self, answer: &MdnsRemoteService<I, A, T>)
    where
        I: ToLabelIter,
        A: Iterator<Item = IpAddr> + Clone,
        T: Iterator<Item = (&'a str, &'a str)> + Clone,
    {
        let Some(id) = commissionable_instance_id(&answer.instance_name) else {
            return;
        };
        let Some(ip) = answer.addrs.clone().max_by_key(score_ip_address) else {
            return;
        };
        let Some(port) = answer.port else {
            return;
        };
        let scope_id = answer.scope_id;

        self.mdns_browse.modify(|state| match state {
            MdnsBrowseState::InFlight { filter, exclude }
                if !exclude.contains(&id) && filter.matches(answer) =>
            {
                *state = MdnsBrowseState::Found {
                    ip,
                    port,
                    scope_id,
                    id,
                };
                (true, ())
            }
            // Already matched this same instance but not yet consumed: keep the
            // better-scoring address. Backends that surface one address per
            // family at a time (e.g. zeroconf fires a callback per A/AAAA record)
            // can thus still yield the preferred IPv6 address even if the IPv4
            // one was deposited first.
            MdnsBrowseState::Found {
                ip: cur_ip,
                id: cur_id,
                ..
            } if *cur_id == id && score_ip_address(&ip) > score_ip_address(cur_ip) => {
                *state = MdnsBrowseState::Found {
                    ip,
                    port,
                    scope_id,
                    id,
                };
                (true, ())
            }
            _ => (false, ()),
        });
    }

    pub(crate) async fn accept_if<'a, F>(
        &self,
        matter: &'a Matter<'a>,
        mut f: F,
    ) -> Result<Exchange<'a>, Error>
    where
        F: FnMut(&Session, &ExchangeState, &Packet<MAX_RX_BUF_SIZE>) -> bool,
    {
        let exchange = self
            .rx
            .with(|packet| {
                matter.with_state(|state| {
                    let session = state
                        .sessions
                        .get_for_rx(&packet.peer, &packet.header.plain)?;
                    let exch_index = session.get_exch_for_rx(&packet.header.proto)?;

                    let matches = {
                        // `unwrap` is safe because the transport code is single threaded, and since we don't `await`
                        // after computing `exch_index` no code can remove the exchange from the session
                        let exch = unwrap!(session.exchanges[exch_index].as_ref());

                        matches!(exch.role, Role::Responder(ResponderState::AcceptPending))
                            && f(session, exch, packet)
                    };

                    if !matches {
                        return None;
                    }

                    // `unwrap` is safe because the transport code is single threaded, and since we don't `await`
                    // after computing `exch_index` no code can remove the exchange from the session
                    let exch = unwrap!(session.exchanges[exch_index].as_mut());

                    exch.role = Role::Responder(ResponderState::Owned);

                    let id = ExchangeId::new(session.id, exch_index);

                    debug!("Exchange {}: Accepted", id.display(session));

                    let exchange = Exchange::new(id, matter);

                    Some(exchange)
                })
            })
            .await;

        Ok(exchange)
    }

    /// The mDNS resolve timeout used when auto-establishing a session
    /// (CASE operational resolve, or PASE commissionable resolve).
    /// Single-shot for now (no backoff re-query); see follow-ups.
    const RESOLVE_TIMEOUT_MS: u32 = 5_000;

    /// Open an exchange over a CASE session to an already-commissioned node.
    ///
    /// If a CASE session for `(fabric_idx, peer_node_id)` already exists, an
    /// exchange is opened on it directly (the common case - session and peer
    /// address reused, no mDNS). Otherwise the peer's operational address is
    /// resolved over mDNS and a fresh CASE session is established (driving
    /// [`CaseInitiator`]) before the exchange is opened on it; the peer's
    /// MRP/session parameters advertised in the mDNS TXT records seed the
    /// session.
    pub(crate) async fn initiate<'a, C: Crypto>(
        &self,
        matter: &'a Matter<'a>,
        crypto: C,
        fabric_idx: NonZeroU8,
        peer_node_id: NodeId,
    ) -> Result<Exchange<'a>, Error> {
        // Reuse an existing CASE session if present.
        let existing = matter.with_state(|state| {
            Ok::<_, Error>(
                state
                    .sessions
                    .get_for_node(fabric_idx, peer_node_id)
                    .map(|s| s.id),
            )
        })?;

        if let Some(session_id) = existing {
            return self.initiate_for_session(matter, session_id);
        }

        // No CASE session: resolve the operational address and establish one.
        let compressed_fabric_id = matter.with_state(|state| {
            Ok::<_, Error>(state.fabrics.fabric(fabric_idx)?.compressed_fabric_id())
        })?;

        let service = MatterRemoteService::Operational {
            compressed_fabric_id,
            node_id: peer_node_id,
        };

        let resolved = self.resolve(service, Self::RESOLVE_TIMEOUT_MS).await?;

        // Establish CASE over a fresh, one-shot unsecured exchange to the
        // resolved address. On success a secure session keyed at
        // `(fabric_idx, peer_node_id)` is recorded in the stack.
        {
            let mut exchange = self
                .initiate_plaintext(matter, &crypto, Address::Udp(resolved.addr))
                .await?;

            CaseInitiator::initiate(&mut exchange, &crypto, fabric_idx, peer_node_id).await?;
        }

        // Seed the new CASE session's peer MRP/session params from the resolve
        // TXT (rs-matter does not yet exchange these in CASE Sigma1/2) and grab
        // its id for the exchange.
        let params = SessionParameters {
            sii: resolved.sii,
            sai: resolved.sai,
            sat: resolved.sat,
            ..Default::default()
        };

        let session_id = matter.with_state(|state| {
            let session = state
                .sessions
                .get_for_node(fabric_idx, peer_node_id)
                .ok_or(ErrorCode::NoSession)?;

            session.set_peer_session_params(&params);

            Ok::<_, Error>(session.id)
        })?;

        self.initiate_for_session(matter, session_id)
    }

    /// Open an exchange over a PASE session to a not-yet-commissioned node at the
    /// given peer address.
    ///
    /// If a PASE session **to that peer** already exists, an exchange is opened on
    /// it directly. Otherwise a new PASE session is established: a plaintext
    /// session is opened and the PASE protocol ([`PaseInitiator`]) is run with
    /// `passcode`, then an exchange is opened on the resulting PASE session.
    ///
    /// Reuse is keyed by peer address (not a single global PASE session), so a
    /// commissioner can drive several concurrent commissionings.
    ///
    /// Discovery of the address is out of scope (mDNS via
    /// [`Transport::browse_commissionable`], a BLE/BTP advertisement, etc.); this
    /// method is transport-agnostic and takes the already-known address.
    pub(crate) async fn initiate_pase<'a, C: Crypto>(
        &self,
        matter: &'a Matter<'a>,
        crypto: C,
        peer_addr: Address,
        passcode: u32,
    ) -> Result<Exchange<'a>, Error> {
        // Reuse an existing PASE session to this peer, if present.
        let existing = matter.with_state(|state| {
            Ok::<_, Error>(state.sessions.get_pase_for_addr(&peer_addr).map(|s| s.id))
        })?;

        if let Some(session_id) = existing {
            return self.initiate_for_session(matter, session_id);
        }

        // Establish a new PASE session to this peer.
        {
            let mut handshake = self.initiate_plaintext(matter, &crypto, peer_addr).await?;
            PaseInitiator::initiate(&mut handshake, &crypto, passcode).await?;
            // The PASE-establishment exchange is one-shot; drop it here so the
            // caller opens fresh exchanges on the new PASE session.
        }

        let session_id = matter.with_state(|state| {
            state
                .sessions
                .get_pase_for_addr(&peer_addr)
                .map(|s| s.id)
                .ok_or_else(|| Error::from(ErrorCode::NoSession))
        })?;

        self.initiate_for_session(matter, session_id)
    }

    pub(crate) fn initiate_for_session<'a>(
        &self,
        matter: &'a Matter<'a>,
        session_id: u32,
    ) -> Result<Exchange<'a>, Error> {
        matter.with_state(|state| {
            state
                .sessions
                .get(session_id)
                // Expired sessions are not allowed to initiate new exchanges
                .filter(|sess| !sess.is_expired())
                .ok_or(ErrorCode::NoSession)?;

            let exch_id = state.sessions.get_next_exch_id();

            // `unwrap` is safe because we know we have a session or else the early return from above would've triggered
            // The reason why we call `get_for_node` twice is to ensure that we don't waste an `exch_id` in case
            // we don't have a session in the first place
            let session = unwrap!(state.sessions.get(session_id));

            let exch_index = session
                .add_exch(exch_id, Role::Initiator(Default::default()))
                .ok_or(ErrorCode::NoSpaceExchanges)?;

            let id = ExchangeId::new(session.id, exch_index);

            debug!("Exchange {}: Initiated", id.display(session));

            Ok(Exchange::new(id, matter))
        })
    }

    /// Create a new initiator exchange on a new plaintext session to
    /// the given peer address, evicting an existing session and retrying once if
    /// there is no space.
    ///
    /// Low-level primitive used to carry the first handshake message of PASE
    /// ([`PaseInitiator`]) or CASE ([`CaseInitiator`]).
    async fn initiate_plaintext<'a, C: Crypto>(
        &self,
        matter: &'a Matter<'a>,
        crypto: C,
        peer_addr: Address,
    ) -> Result<Exchange<'a>, Error> {
        match self.try_initiate_plaintext(matter, &crypto, peer_addr) {
            Ok(exchange) => Ok(exchange),
            Err(e) if e.code() == ErrorCode::NoSpaceSessions => {
                matter
                    .transport_runner(&crypto)
                    .evict_some_session()
                    .await?;
                self.try_initiate_plaintext(matter, &crypto, peer_addr)
            }
            Err(e) => Err(e),
        }
    }

    /// Create a new plaintext session and initiate an exchange on it in one step.
    ///
    /// This is a convenience method that combines `create_plaintext_session()` and
    /// `initiate_for_session()`. Fails immediately if there is no space for a new session.
    ///
    /// For flows that need the session ID (e.g. to upgrade the session after PASE/CASE),
    /// use `create_plaintext_session()` + `initiate_for_session()` separately.
    fn try_initiate_plaintext<'a, C: Crypto>(
        &self,
        matter: &'a Matter<'a>,
        crypto: C,
        peer_addr: Address,
    ) -> Result<Exchange<'a>, Error> {
        let session_id = self.create_plaintext_session(matter, crypto, peer_addr)?;

        self.initiate_for_session(matter, session_id)
    }

    /// Create a new unsecured (plain-text) session to a given peer address.
    ///
    /// Returns the internal session ID that can be used with `initiate_for_session()`.
    ///
    /// This is the low-level building block for controller-initiated communication
    /// (e.g. PASE/CASE initiator flows), analogous to the SDK's
    /// `SessionManager::CreateUnauthenticatedSession()`.
    fn create_plaintext_session<C: Crypto>(
        &self,
        matter: &Matter<'_>,
        crypto: C,
        peer_addr: Address,
    ) -> Result<u32, Error> {
        matter.with_state(|state| {
            let mut rand = crypto.rand()?;

            let session =
                state
                    .sessions
                    .add(rand.next_u32(), false, peer_addr, None, matter.dev_det())?;

            // Generate ephemeral initiator node ID per spec:
            // "Randomly selected for each session by the initiator from the Operational Node ID range"
            // Operational Node ID range is 0x0000_0000_0000_0001 to 0xFFFF_FFEF_FFFF_FFFF
            // (Matter Core spec).
            const MAX_OPERATIONAL_NODE_ID: u64 = 0xFFFF_FFEF_FFFF_FFFF;
            let mut ephemeral_id = rand.next_u64();
            while ephemeral_id == 0 || ephemeral_id > MAX_OPERATIONAL_NODE_ID {
                ephemeral_id = rand.next_u64();
            }
            session.set_local_nodeid(ephemeral_id);

            let session_id = session.id;

            debug!(
                "Unsecured session {} created for peer {}",
                session_id, peer_addr
            );

            Ok(session_id)
        })
    }

    pub(crate) async fn get_if_rx<F>(&self, f: F) -> PacketAccess<'_, MAX_RX_BUF_SIZE>
    where
        F: Fn(&Packet<MAX_RX_BUF_SIZE>) -> bool,
    {
        Self::get_if(&self.rx, f).await
    }

    pub(crate) async fn get_if_tx<F>(&self, f: F) -> PacketAccess<'_, MAX_TX_BUF_SIZE>
    where
        F: Fn(&Packet<MAX_TX_BUF_SIZE>) -> bool,
    {
        Self::get_if(&self.tx, f).await
    }

    async fn get_if<'b, F, const N: usize>(
        packet_mutex: &'b IfMutex<Packet<N>>,
        f: F,
    ) -> PacketAccess<'b, N>
    where
        F: Fn(&Packet<N>) -> bool,
    {
        PacketAccess(packet_mutex.lock_if(f).await, false)
    }

    /// Build a `SocketAddr` from a resolved `(ip, port)`, attaching the IPv6
    /// `scope_id` (interface zone) so a **link-local** destination (`fe80::/10`)
    /// is routable.
    ///
    /// The scope is only meaningful — and only applied — for link-local IPv6;
    /// for any other address it is irrelevant and `SocketAddr::new` is used
    /// as-is.
    fn scoped_socket_addr(ip: IpAddr, port: u16, scope_id: u32) -> SocketAddr {
        match ip {
            IpAddr::V6(v6) if v6.is_unicast_link_local() => {
                SocketAddr::V6(SocketAddrV6::new(v6, port, 0, scope_id))
            }
            _ => SocketAddr::new(ip, port),
        }
    }
}

/// Resets the mDNS resolve rendezvous to `Idle` on drop, unless disarmed.
///
/// This guarantees that a dropped (cancelled or timed-out) [`Transport::resolve`]
/// future does not leave the single-slot rendezvous occupied for other callers.
struct MdnsResolveGuard<'a> {
    signal: &'a Signal<MdnsResolveState>,
    armed: bool,
}

impl Drop for MdnsResolveGuard<'_> {
    fn drop(&mut self) {
        if self.armed {
            self.signal.modify(|state| {
                if matches!(state, MdnsResolveState::Idle) {
                    (false, ())
                } else {
                    *state = MdnsResolveState::Idle;
                    (true, ())
                }
            });
        }
    }
}

/// Resets the mDNS browse rendezvous to `Idle` on drop, unless disarmed (the
/// browse analog of [`ResolveGuard`]).
struct MdnsBrowseGuard<'a> {
    signal: &'a Signal<MdnsBrowseState>,
    armed: bool,
}

impl Drop for MdnsBrowseGuard<'_> {
    fn drop(&mut self) {
        if self.armed {
            self.signal.modify(|state| {
                if matches!(state, MdnsBrowseState::Idle) {
                    (false, ())
                } else {
                    *state = MdnsBrowseState::Idle;
                    (true, ())
                }
            });
        }
    }
}

/// The Matter Transport Runner, responsible for running the network transport by processing incoming and ougoing packets
/// and thus also managing sessions and exchanges.
///
/// The transport runner is wrapping the whole Matter Object, because it needs access to various states, like
/// sessions, fabrics and the transport buffers / state itself.
pub struct TransportRunner<'a, C> {
    matter: &'a Matter<'a>,
    crypto: C,
}

impl<'a, C: Crypto> TransportRunner<'a, C> {
    /// Create a new `TransportRunner` instance with the given `Matter` instance and `Crypto` implementation.
    pub const fn new(matter: &'a Matter<'a>, crypto: C) -> Self {
        Self { matter, crypto }
    }

    /// Run the transport runner with the given network send, receive and multicast implementations.
    pub async fn run<S, R, M>(&mut self, send: S, recv: R, multicast: M) -> Result<(), Error>
    where
        S: NetworkSend,
        R: NetworkReceive,
        M: NetworkMulticast,
    {
        info!("Running Matter transport");

        // Do not remove this logging line or change its formatting.
        // C++ E2E tests rely on this log line to determine when the tested app is ready
        debug!("APP STATUS: Starting event loop");

        let mut joined = self.transport().group_addrs.lock().await;

        let send = IfMutex::new(send);

        let mut rx = pin!(self.process_rx(recv, &send));
        let mut tx = pin!(self.process_tx(&send));
        let mut orphaned = pin!(self.process_orphaned());
        let mut groups = pin!(self.process_groups(multicast, &mut joined));

        select4(&mut rx, &mut tx, &mut orphaned, &mut groups)
            .coalesce()
            .await
    }

    async fn process_groups<M>(
        &self,
        mut multicast: M,
        joined: &mut Vec<Ipv6Addr, MAX_GROUP_ADDRS>,
    ) -> Result<(), Error>
    where
        M: NetworkMulticast,
    {
        joined.clear();

        loop {
            let addr_op = self.matter.with_state(|state| {
                let group_addrs = || {
                    state.fabrics.iter().flat_map(|fabric| {
                        fabric.groups().iter().map(|group| {
                            compute_group_multicast_addr(fabric.fabric_id(), group.group_id)
                        })
                    })
                };

                if let Some(new_addr) = group_addrs().find(|addr| !joined.contains(addr)) {
                    Some((new_addr, true))
                } else {
                    joined
                        .iter()
                        .find(|addr| !group_addrs().any(|a| a == **addr))
                        .map(|&removed_addr| (removed_addr, false))
                }
            });

            match addr_op {
                Some((new_addr, true)) => {
                    match multicast.join(new_addr.into()).await {
                        Ok(_) => {
                            debug!("Joined multicast group: {}", new_addr);
                            // `joined` should be able to contain theoretical maximum number of multicast address
                            // So this unwrap should be safe
                            unwrap!(joined.push(new_addr));
                        }
                        Err(e) => error!(
                            "Joining multicast group {} failed with error: {}",
                            new_addr, e
                        ),
                    }
                }
                Some((removed_addr, false)) => match multicast.leave(removed_addr.into()).await {
                    Ok(_) => {
                        debug!("Left multicast group: {}", removed_addr);
                        let index = joined
                            .iter()
                            .position(|&addr| addr == removed_addr)
                            .unwrap();
                        joined.swap_remove(index);
                    }
                    Err(e) => error!(
                        "Leaving multicast group {} failed with error: {}",
                        removed_addr, e
                    ),
                },
                None => {
                    self.transport().wait_groups_changed().await;
                }
            }
        }
    }

    async fn process_tx<S>(&self, send: &IfMutex<S>) -> Result<(), Error>
    where
        S: NetworkSend,
    {
        loop {
            trace!("Waiting for outgoing packet");

            let mut tx = self
                .matter
                .transport
                .get_if_tx(|packet| !packet.buf.is_empty())
                .await;
            tx.clear_on_drop(true);

            if let TxPayloadState::NotEncoded { session_id } = tx.tx_info.payload_state {
                let encoded = self.matter.with_state(|state| {
                    if let Some(session) = state.sessions.get_for_tx(session_id) {
                        self.encode_packet(&mut tx, Some(session))?;

                        Ok::<_, Error>(true)
                    } else {
                        error!(
                            "TX packet has session ID {}, but no such session exists, dropping",
                            session_id
                        );

                        Ok(false)
                    }
                })?;

                if !encoded {
                    continue;
                }
            }

            Self::netw_send(send, tx.peer, &tx.buf[tx.payload_start..], false).await?;
        }
    }

    async fn process_rx<R, S>(&self, mut recv: R, send: &IfMutex<S>) -> Result<(), Error>
    where
        R: NetworkReceive,
        S: NetworkSend,
    {
        loop {
            trace!("Waiting for incoming packet");

            recv.wait_available().await?;

            let mut rx = self
                .matter
                .transport
                .get_if_rx(|packet| packet.buf.is_empty())
                .await;
            rx.clear_on_drop(true); // In case of error, or if the future is dropped

            // TODO: Resizing might be a bit expensive with large buffers
            // Resizing to `MAX_RX_BUF_SIZE` is always safe because the size of the `buf` heapless vec `MAX_RX_BUF_SIZE`
            unwrap!(rx.buf.resize_default(MAX_RX_BUF_SIZE));

            let (len, peer) = Self::netw_recv(&mut recv, &mut rx.buf).await?;

            rx.peer = peer;
            rx.buf.truncate(len);
            rx.payload_start = 0;

            match self.handle_rx_packet(&mut rx, send).await {
                Ok(true) => {
                    // Leave the packet in place for accepting by responders
                    rx.clear_on_drop(false);
                }
                Ok(false) => {
                    // Drop the packet, as no further processing is necessary
                }
                Err(e) => {
                    // Drop the packet and report the unexpected error
                    error!("UNEXPECTED RX ERROR: {:?}", e);
                }
            }
        }
    }

    async fn process_orphaned(&self) -> Result<(), Error> {
        let mut rx_accept_timeout = pin!(self.process_accept_timeout_rx());
        let mut rx_orphaned = pin!(self.process_orphaned_rx());
        let mut exch_dropped = pin!(self.process_dropped_exchanges());

        select3(&mut rx_accept_timeout, &mut rx_orphaned, &mut exch_dropped)
            .coalesce()
            .await
    }

    async fn process_accept_timeout_rx(&self) -> Result<(), Error> {
        loop {
            trace!("Waiting for accept timeout");

            let mut accept_timeout = pin!(self
                .matter
                .transport
                .rx
                .with(|packet| { self.handle_accept_timeout_rx_packet(packet).then_some(()) }));

            let mut timer = pin!(Timer::after(embassy_time::Duration::from_millis(50)));

            select(&mut accept_timeout, &mut timer).await;
        }
    }

    async fn process_orphaned_rx(&self) -> Result<(), Error> {
        loop {
            trace!("Waiting for orphaned RX packets");

            self.transport()
                .rx
                .with(|packet| self.handle_orphaned_rx_packet(packet).then_some(()))
                .await;
        }
    }

    async fn process_dropped_exchanges(&self) -> Result<(), Error> {
        loop {
            trace!("Waiting for dropped exchanges");

            let mut tx = self
                .matter
                .transport
                .get_if_tx(|packet| packet.buf.is_empty())
                .await;
            tx.clear_on_drop(true); // In case of error, or if the future is dropped

            let wait = match self.handle_dropped_exchange(&mut tx) {
                Ok(wait) => {
                    tx.clear_on_drop(false);
                    wait
                }
                Err(e) => {
                    error!("UNEXPECTED RX ERROR: {:?}", e);
                    false
                }
            };

            drop(tx);

            if wait {
                let mut timeout = pin!(Timer::after(embassy_time::Duration::from_millis(100)));
                let mut wait = pin!(self.transport().exchange_dropped.wait());

                select(&mut timeout, &mut wait).await;
            }
        }
    }

    async fn handle_rx_packet<const N: usize, S>(
        &self,
        packet: &mut Packet<N>,
        send: &IfMutex<S>,
    ) -> Result<bool, Error>
    where
        S: NetworkSend,
    {
        let result = self.decode_packet(packet);
        match result {
            Err(e) if matches!(e.code(), ErrorCode::Duplicate) => {
                if packet.header.plain.is_group_session() {
                    // Group messages are multicast and don't use MRP; silently discard duplicates
                    debug!(
                        "\n>>RCV {}\n      => Duplicate group message, discarding",
                        packet
                    );
                } else if !packet.peer.is_reliable()
                    && !MessageMeta::from(&packet.header.proto).is_standalone_ack()
                {
                    debug!("\n>>RCV {}\n      => Duplicate, sending ACK", packet);

                    self.matter.with_state(|state| {
                        // `unwrap` is safe because we know we have a session.
                        // If we didn't have a session, the error code would've been `NoSession`
                        //
                        // Also, since the transport code is single threaded, and since we don't `await`
                        // after decoding the packet, no code can the session
                        let session = unwrap!(state
                            .sessions
                            .get_for_rx(&packet.peer, &packet.header.plain));

                        let ack = packet.header.plain.ctr;

                        packet.header.proto.toggle_initiator();
                        packet.header.proto.set_ack(Some(ack));

                        self.write_packet(packet, Some(session), None, true, |_| {
                            Ok(Some(OpCode::MRPStandAloneAck.into()))
                        })
                    })?;

                    Self::netw_send(send, packet.peer, &packet.buf[packet.payload_start..], true)
                        .await?;
                } else {
                    debug!("\n>>RCV {}\n      => Duplicate, discarding", packet);
                }
            }
            Err(e) if matches!(e.code(), ErrorCode::NoSpaceSessions) => {
                if !packet.header.plain.is_encrypted()
                    && MessageMeta::from(&packet.header.proto).is_new_session()
                {
                    warn!(
                        "\n>>RCV {}\n      => No space for a new unencrypted session, sending Busy",
                        packet
                    );

                    let ack = packet.header.plain.ctr;

                    packet.header.proto.toggle_initiator();
                    packet.header.proto.set_ack(Some(ack));

                    self.write_packet(packet, None, None, true, |wb| {
                        sc_write(wb, SCStatusCodes::Busy, &[0xF4, 0x01])
                    })?;

                    Self::netw_send(send, packet.peer, &packet.buf[packet.payload_start..], true)
                        .await?;

                    if self.write_evict_some_session_packet(packet, true)? {
                        Self::netw_send(
                            send,
                            packet.peer,
                            &packet.buf[packet.payload_start..],
                            true,
                        )
                        .await?;
                    }
                } else {
                    error!(
                        "\n>>RCV {}\n      => No space for a new encrypted session, dropping",
                        packet
                    );
                }
            }
            Err(e) if matches!(e.code(), ErrorCode::NoSpaceExchanges) => {
                // TODO: Before closing the session, try to take other measures:
                // - For CASESigma1 & PBKDFParamRequest - send Busy instead
                // - For Interaction Model interactions that do need an ACK - send IM Busy,
                //   wait for ACK and retransmit without releasing the RX buffer, potentially
                //   blocking all other interactions

                error!(
                    "\n>>RCV {}\n      => No space for a new exchange, closing session",
                    packet
                );

                self.matter.with_state(|state| {
                    // `unwrap` is safe because we know we have a session.
                    // If we didn't have a session, the error code would've been `NoSession`
                    //
                    // Also, since the transport code is single threaded, and since we don't `await`
                    // after decoding the packet, no code can the session
                    let session_id = unwrap!(state
                        .sessions
                        .get_for_rx(&packet.peer, &packet.header.plain))
                    .id;

                    packet.header.proto.exch_id = state.sessions.get_next_exch_id();
                    packet.header.proto.set_initiator();

                    // See above why `unwrap` is safe
                    let mut session = unwrap!(state.sessions.remove(session_id));
                    self.transport().notify_session_removed();

                    self.write_packet(packet, Some(&mut session), None, true, |wb| {
                        sc_write(wb, SCStatusCodes::CloseSession, &[])
                    })
                })?;

                Self::netw_send(send, packet.peer, &packet.buf[packet.payload_start..], true)
                    .await?;
            }
            Err(e) if matches!(e.code(), ErrorCode::NoExchange) => {
                warn!(
                    "\n>>RCV {}\n      => No valid exchange found, dropping",
                    packet
                );
            }
            Err(e) if matches!(e.code(), ErrorCode::NoSession) => {
                // Per Matter Core spec, when a session-bearing
                // message arrives for which we have no matching secure session
                // (e.g. after a reboot has wiped the session table while the
                // peer still believes the old session is alive), we reply with
                // an unsecured `SessionNotFound` Status Report on the Secure
                // Channel protocol. This nudges the peer to drop its stale
                // session and re-establish CASE, instead of waiting for MRP
                // retries to exhaust on its side.
                warn!(
                    "\n>>RCV {}\n      => No valid session found, replying with SessionNotFound",
                    packet
                );

                // `write_packet` with `session = None` requires the incoming
                // header to look like an unsecured, non-reliable, source-tagged
                // packet (see its preconditions). Clear `sess_id` so it is not
                // considered encrypted, drop the reliable/ack flags since we
                // are not party to any exchange, and stamp a placeholder
                // `src_nodeid` (echoed as the response's `dst_nodeid` — UDP
                // peer addressing is what actually routes the reply).
                packet.header.plain.sess_id = 0;
                packet.header.plain.set_src_nodeid(Some(0));
                packet.header.proto.unset_reliable();
                packet.header.proto.set_ack(None);

                self.write_packet(packet, None, None, true, |wb| {
                    sc_write(wb, SCStatusCodes::SessionNotFound, &[])
                })?;

                Self::netw_send(send, packet.peer, &packet.buf[packet.payload_start..], true)
                    .await?;
            }
            Err(e) => {
                error!("\n>>RCV {}\n      => Error ({:?}), dropping", packet, e);
            }
            Ok(new_exchange) => {
                let meta = MessageMeta::from(&packet.header.proto);

                if meta.is_standalone_ack() {
                    // No need to propagate this further
                    debug!("\n>>RCV {}\n      => Standalone Ack, dropping", packet);
                } else if meta.is_sc_status()
                    && matches!(
                        Self::is_close_session(&mut packet.buf[packet.payload_start..]),
                        Ok(true)
                    )
                {
                    warn!(
                        "\n>>RCV {}\n      => Close session received, removing this session",
                        packet
                    );

                    self.matter.with_state(|state| {
                        if let Some(session_id) = state
                            .sessions
                            .get_for_rx(&packet.peer, &packet.header.plain)
                            .map(|sess| sess.id)
                        {
                            state.sessions.remove(session_id);
                            self.transport().notify_session_removed();
                        }
                    });
                } else {
                    debug!(
                        "\n>>RCV {}\n      => Processing{}",
                        packet,
                        if new_exchange { " (new exchange)" } else { "" }
                    );

                    #[cfg(feature = "debug-tlv-payload")]
                    debug!(
                        "{}",
                        Packet::<0>::display_payload(
                            &packet.header.proto,
                            &packet.buf[core::cmp::min(packet.payload_start, packet.buf.len())..]
                        )
                    );

                    #[cfg(not(feature = "debug-tlv-payload"))]
                    trace!(
                        "{}",
                        Packet::<0>::display_payload(
                            &packet.header.proto,
                            &packet.buf[core::cmp::min(packet.payload_start, packet.buf.len())..]
                        )
                    );

                    return Ok(true);
                }
            }
        }

        Ok(false)
    }

    fn handle_accept_timeout_rx_packet<const N: usize>(&self, packet: &mut Packet<N>) -> bool {
        if packet.buf.is_empty() {
            return false;
        }

        self.matter.with_state(|state| {
            let Some(session) = state
                .sessions
                .get_for_rx(&packet.peer, &packet.header.plain)
            else {
                return false;
            };

            let Some(exch_index) = session.get_exch_for_rx(&packet.header.proto) else {
                return false;
            };

            // `unwrap` is safe because we know we have a session and an exchange, or else the early returns from above would've triggered
            let exchange = unwrap!(session.exchanges[exch_index].as_mut());

            if !matches!(
                exchange.role,
                Role::Responder(ResponderState::AcceptPending)
            ) || !exchange.mrp.has_rx_timed_out(ACCEPT_TIMEOUT_MS)
            {
                return false;
            }

            warn!(
                "\n>>RCV {}\n => Accept timeout, marking exchange as dropped",
                packet
            );

            exchange.role = Role::Responder(ResponderState::Dropped);
            packet.buf.clear();
            self.transport().exchange_dropped.notify();

            true
        })
    }

    fn handle_orphaned_rx_packet<const N: usize>(&self, packet: &mut Packet<N>) -> bool {
        if packet.buf.is_empty() {
            return false;
        }

        self.matter.with_state(|state| {
            let Some(session) = state
                .sessions
                .get_for_rx(&packet.peer, &packet.header.plain)
            else {
                warn!("\n>>RCV {}\n => No session, dropping", packet);

                packet.buf.clear();
                return true;
            };

            let Some(exch_index) = session.get_exch_for_rx(&packet.header.proto) else {
                warn!("\n>>RCV {}\n => No exchange, dropping", packet);

                packet.buf.clear();
                return true;
            };

            // `unwrap` is safe because we know we have a session and an exchange, or else the early returns from above would've triggered
            let exchange = unwrap!(session.exchanges[exch_index].as_mut());

            if exchange.role.is_dropped_state() {
                warn!(
                    "\n>>RCV {}\n => Owned by orphaned dropped {}, dropping packet",
                    packet,
                    ExchangeId::new(session.id, exch_index)
                );

                packet.buf.clear();
                return true;
            }

            false
        })
    }

    fn handle_dropped_exchange<const N: usize>(
        &self,
        packet: &mut Packet<N>,
    ) -> Result<bool, Error> {
        self.matter.with_state(|state| {
            let exch = state
                .sessions
                .get_exch(|_, exch| exch.role.is_dropped_state() && exch.mrp.is_retrans_pending())
                .map(|(sess, exch_index)| (sess.id, exch_index, true))
                .or_else(|| {
                    state
                        .sessions
                        .get_exch(|_, exch| {
                            exch.role.is_dropped_state() && !exch.mrp.is_retrans_pending()
                        })
                        .map(|(sess, exch_index)| (sess.id, exch_index, false))
                });

            let Some((session_id, exch_index, close_session)) = exch else {
                return Ok(exch.is_none());
            };

            let exchange_id = ExchangeId::new(session_id, exch_index);

            if close_session {
                // Found a dropped exchange which has an incomplete (re)transmission
                // Close the whole session

                error!(
                    "Dropped exchange {}: Closing session because the exchange cannot be closed cleanly",
                    exchange_id.display(unwrap!(state.sessions.get(session_id))) // Session exists or else we wouldn't be here
                );

                self.write_evict_session_packet(packet, &mut state.sessions, session_id, false)?;
            } else {
                // Found a dropped exchange which has no outstanding (re)transmission
                // Send a standalone ACK if necessary and then close it

                // `unwrap` is safe because we know we have a session and an exchange, or else the early returns from above would've triggered
                let session = unwrap!(state.sessions.get(session_id));
                // Ditto
                let exchange = unwrap!(session.exchanges[exch_index].as_mut());

                if exchange.mrp.is_ack_pending() {
                    self.write_packet(
                        packet,
                        Some(session),
                        Some(exch_index),
                        false,
                        |_| Ok(Some(OpCode::MRPStandAloneAck.into())),
                    )?;
                }

                warn!("Dropped exchange {}: Closed", exchange_id.display(session));
                session.exchanges[exch_index] = None;
            }

            Ok(exch.is_none())
        })
    }

    pub(crate) async fn evict_some_session(&self) -> Result<(), Error> {
        let mut tx = self
            .matter
            .transport
            .get_if_tx(|packet| packet.buf.is_empty())
            .await;
        tx.clear_on_drop(true); // By default, if an error occurs

        let evicted = self.write_evict_some_session_packet(&mut tx, true)?;

        if evicted {
            // Send it
            tx.clear_on_drop(false);

            Ok(())
        } else {
            Err(ErrorCode::NoSpaceSessions.into())
        }
    }

    fn decode_packet<const N: usize>(&self, packet: &mut Packet<N>) -> Result<bool, Error> {
        self.matter.with_state(|state| {
            packet.header.reset();

            let mut pb = ParseBuf::new(&mut packet.buf[packet.payload_start..]);
            packet.header.plain.decode(&mut pb)?;

            let set_payload = |packet: &mut Packet<N>, (start, end)| {
                packet.payload_start = start;
                packet.buf.truncate(end);
            };

            if let Some(session) = state
                .sessions
                .get_for_rx(&packet.peer, &packet.header.plain)
            {
                // Found existing session: decode, indicate packet payload slice and process further

                let payload_range =
                    session.decode_remaining(&self.crypto, &mut packet.header, pb)?;
                set_payload(packet, payload_range);

                return session.post_recv(&packet.header);
            }

            // No existing session: we either have to create one, or return an error

            if !packet.header.plain.is_encrypted() {
                // Unencrypted packets can be decoded without a session, and we need to anyway do that
                // in order to determine (based on proto hdr data) whether to create a new session or not
                packet
                    .header
                    .decode_remaining(&self.crypto, None, 0, &mut pb)?;
                packet.header.proto.adjust_reliability(true, &packet.peer);

                let payload_range = pb.slice_range();
                set_payload(packet, payload_range);

                if MessageMeta::from(&packet.header.proto).is_new_session() {
                    // As per spec, new unencrypted sessions are only created for
                    // `PBKDFParamRequest` or `CASESigma1` unencrypted messages

                    let mut rand = self.crypto.rand()?;

                    let session = state.sessions.add(
                        rand.next_u32(),
                        false,
                        packet.peer,
                        packet.header.plain.get_src_nodeid(),
                        self.matter.dev_det(),
                    )?;

                    // Session created successfully: decode, indicate packet payload slice and process further
                    return session.post_recv(&packet.header);
                }
            } else if packet.header.plain.is_group_session() {
                // Group (multicast) message — derive keys on-the-fly and decrypt
                let (session, payload_range) = state.sessions.get_or_create_for_group_rx(
                    &self.crypto,
                    &state.fabrics,
                    packet,
                    self.matter.dev_det(),
                )?;
                set_payload(packet, payload_range);

                return session.post_recv(&packet.header);
            } else {
                // Encrypted unicast packet with no matching session — cannot be decoded
                set_payload(packet, (0, 0));
            }

            Err(ErrorCode::NoSession.into())
        })
    }

    fn encode_packet<const N: usize>(
        &self,
        packet: &mut Packet<N>,
        session: Option<&mut Session>,
    ) -> Result<(), Error> {
        assert!(matches!(
            packet.tx_info.payload_state,
            TxPayloadState::NotEncoded { .. }
        ));

        let payload_end = packet.buf.len();

        debug!(
            "\n<<SND {}\n      => {}",
            Packet::<0>::display(&packet.peer, &packet.header),
            if packet.tx_info.retransmission {
                "Re-sending"
            } else {
                "Sending"
            }
        );

        #[cfg(feature = "debug-tlv-payload")]
        debug!(
            "{}",
            Packet::<0>::display_payload(
                &packet.header.proto,
                &packet.buf[packet.payload_start..payload_end]
            )
        );

        #[cfg(not(feature = "debug-tlv-payload"))]
        trace!(
            "{}",
            Packet::<0>::display_payload(
                &packet.header.proto,
                &packet.buf[packet.payload_start..payload_end]
            )
        );

        unwrap!(packet.buf.resize_default(N));

        let mut wb = WriteBuf::new_with(&mut packet.buf, packet.payload_start, payload_end);
        if let Some(session) = session {
            session.encode(&self.crypto, &packet.header, &mut wb)?;
        } else {
            packet.header.encode(&self.crypto, None, 0, &mut wb)?;
        }

        let encoded_payload_start = wb.get_start();
        let encoded_payload_end = wb.get_tail();

        packet.payload_start = encoded_payload_start;
        packet.tx_info.payload_state = TxPayloadState::Encoded;
        packet.buf.truncate(encoded_payload_end);

        Ok(())
    }

    fn write_packet<const N: usize, F>(
        &self,
        packet: &mut Packet<N>,
        mut session: Option<&mut Session>,
        exchange_index: Option<usize>,
        encode: bool,
        payload_writer: F,
    ) -> Result<(), Error>
    where
        F: FnOnce(&mut WriteBuf) -> Result<Option<MessageMeta>, Error>,
    {
        // TODO: Resizing might be a bit expensive with large buffers
        // Resizing to `N` is always safe because it is a responsibility of the caller to ensure that N is <= `MAX_RX_BUF_SIZE`,
        // which is the size of `buf` heapless vec
        unwrap!(packet.buf.resize_default(N));

        let mut wb = WriteBuf::new_with(
            &mut packet.buf,
            PacketHdr::HDR_RESERVE,
            PacketHdr::HDR_RESERVE,
        );

        let Some(meta) = payload_writer(&mut wb)? else {
            packet.buf.clear();
            return Ok(());
        };

        let (start, end) = (wb.get_start(), wb.get_tail());

        packet.payload_start = start;
        packet.buf.truncate(end);

        meta.set_into(&mut packet.header.proto);

        if let Some(session) = &mut session {
            packet.header.plain = Default::default();

            let (peer, retransmission) = session.pre_send(
                exchange_index,
                &mut packet.header,
                self.transport().device_sai,
                self.transport().device_sii,
            )?;

            packet.peer = peer;
            packet.tx_info.retransmission = retransmission;
            packet.tx_info.payload_state = TxPayloadState::NotEncoded {
                session_id: session.id,
            };
        } else {
            if packet.header.plain.is_encrypted()
                || packet.header.plain.get_src_nodeid().is_none()
                || packet.header.proto.is_reliable()
            {
                // We can encode packets without a session only when they are unencrypted and do not need a retransmission
                Err(ErrorCode::NoSession)?;
            }

            let src_nodeid = packet.header.plain.get_src_nodeid();

            packet.header.plain = Default::default();

            packet.header.plain.sess_id = 0;
            packet.header.plain.ctr = 1;
            packet.header.plain.set_src_nodeid(None);
            packet.header.plain.set_dst_unicast_nodeid(src_nodeid);

            packet.header.proto.unset_initiator();
            packet.header.proto.adjust_reliability(false, &packet.peer);

            packet.tx_info.retransmission = false;
            packet.tx_info.payload_state = TxPayloadState::NotEncoded { session_id: 0 };
        }

        if encode {
            self.encode_packet(packet, session)?;
        }

        Ok(())
    }

    fn write_evict_some_session_packet<const N: usize>(
        &self,
        packet: &mut Packet<N>,
        encode: bool,
    ) -> Result<bool, Error> {
        self.matter.with_state(|state| {
            let id = state
                .sessions
                .get_session_for_eviction()
                .map(|sess| sess.id);
            if let Some(id) = id {
                self.write_evict_session_packet(packet, &mut state.sessions, id, encode)?;

                Ok(true)
            } else {
                error!("All sessions have active exchanges, cannot evict any session");

                Ok(false)
            }
        })
    }

    fn write_evict_session_packet<const N: usize>(
        &self,
        packet: &mut Packet<N>,
        sessions: &mut Sessions,
        id: u32,
        encode: bool,
    ) -> Result<(), Error> {
        packet.header.proto.exch_id = sessions.get_next_exch_id();
        packet.header.proto.set_initiator();

        // It is a responsibility of the caller to ensure that this method is called with a valid session ID
        let mut session = unwrap!(sessions.remove(id));
        self.transport().notify_session_removed();

        debug!(
            "Evicting session {} [SID:{:x},RSID:{:x}]",
            session.id,
            session.get_local_sess_id(),
            session.get_peer_sess_id()
        );

        self.write_packet(packet, Some(&mut session), None, encode, |wb| {
            sc_write(wb, SCStatusCodes::CloseSession, &[])
        })?;

        Ok(())
    }

    fn is_close_session(payload: &mut [u8]) -> Result<bool, Error> {
        let mut pb = ParseBuf::new(payload);
        let report = StatusReport::read(&mut pb)?;

        let close_session = report.proto_id == PROTO_ID_SECURE_CHANNEL as u32
            && report.proto_code == SCStatusCodes::CloseSession as u16;

        Ok(close_session)
    }

    async fn netw_recv<R>(mut recv: R, buf: &mut [u8]) -> Result<(usize, Address), Error>
    where
        R: NetworkReceive,
    {
        match recv.recv_from(buf).await {
            Ok((len, addr)) => {
                trace!("\n>>RCV {} {}B:\n     {}", addr, len, Bytes(&buf[..len]));

                Ok((len, addr))
            }
            Err(e) => {
                error!("FAILED network recv: {:?}", e);

                Err(e)
            }
        }
    }

    async fn netw_send<S>(
        send: &IfMutex<S>,
        peer: Address,
        data: &[u8],
        system: bool,
    ) -> Result<(), Error>
    where
        S: NetworkSend,
    {
        match send.lock().await.send_to(data, peer).await {
            Ok(_) => {
                trace!(
                    "\n<<SND {} {}B{}: {}",
                    peer,
                    data.len(),
                    if system { " (system)" } else { "" },
                    Bytes(data)
                );

                Ok(())
            }
            Err(e) => {
                error!(
                    "\n<<SND {} {}B{} !FAILED!: {:?}",
                    peer,
                    data.len(),
                    if system { " (system)" } else { "" },
                    e
                );

                // Do not return an error as that would unroll the main `rs-matter` loop
                // and sending errors are normal and can happen for various reasons
                // TODO: Provide the error as a feedback to the packet creator instead, in the mutex data
                Ok(())
            }
        }
    }

    #[inline(always)]
    const fn transport(&self) -> &Transport {
        self.matter.transport()
    }
}

#[derive(Copy, Clone, Default, PartialEq, Eq, Debug, Hash)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub(crate) enum TxPayloadState {
    #[default]
    Encoded,
    NotEncoded {
        session_id: u32,
    },
}

#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub(crate) struct TxInfo {
    pub(crate) retransmission: bool,
    pub(crate) payload_state: TxPayloadState,
}

impl TxInfo {
    pub const fn new() -> Self {
        Self {
            retransmission: false,
            payload_state: TxPayloadState::Encoded,
        }
    }
}

impl Default for TxInfo {
    fn default() -> Self {
        Self::new()
    }
}

// The internal representation of a packet in the transport layer.
// There are only two such packets - RX and TX.
//
// This type is only known and used by the `transport` and the `exchange` modules
pub(crate) struct Packet<const N: usize> {
    pub(crate) peer: Address,
    pub(crate) header: PacketHdr,
    pub(crate) buf: PacketBuffer<N>,
    pub(crate) payload_start: usize,
    pub(crate) tx_info: TxInfo,
}

impl<const N: usize> Packet<N> {
    #[inline(always)]
    pub(crate) const fn new() -> Self {
        Self {
            peer: Address::new(),
            header: PacketHdr::new(),
            buf: PacketBuffer::new(),
            payload_start: 0,
            tx_info: TxInfo::new(),
        }
    }

    pub(crate) fn init() -> impl Init<Self> {
        init!(Self {
            peer: Address::new(),
            header: PacketHdr::new(),
            buf <- PacketBuffer::init(),
            payload_start: 0,
            tx_info: TxInfo::new(),
        })
    }

    #[cfg(feature = "defmt")]
    pub fn display<'a>(
        peer: &'a Address,
        header: &'a PacketHdr,
    ) -> impl Display + defmt::Format + 'a {
        PacketInfo(peer, header)
    }

    #[cfg(not(feature = "defmt"))]
    pub fn display<'a>(peer: &'a Address, header: &'a PacketHdr) -> impl Display + 'a {
        PacketInfo(peer, header)
    }

    #[cfg(feature = "defmt")]
    pub fn display_payload<'a>(
        proto: &'a ProtoHdr,
        buf: &'a [u8],
    ) -> impl Display + defmt::Format + 'a {
        DetailedPacketInfo(proto, buf)
    }

    #[cfg(not(feature = "defmt"))]
    pub fn display_payload<'a>(proto: &'a ProtoHdr, buf: &'a [u8]) -> impl Display + 'a {
        DetailedPacketInfo(proto, buf)
    }

    fn fmt(f: &mut fmt::Formatter<'_>, peer: &Address, header: &PacketHdr) -> fmt::Result {
        write!(f, "{peer} {header}")?;

        if header.proto.is_decoded() {
            let meta = MessageMeta::from(&header.proto);

            write!(f, "\n      {meta}")?;
        }

        Ok(())
    }

    #[cfg(feature = "defmt")]
    fn format(f: defmt::Formatter<'_>, peer: &Address, header: &PacketHdr) {
        defmt::write!(f, "{} {}", peer, header);

        if header.proto.is_decoded() {
            let meta = MessageMeta::from(&header.proto);

            defmt::write!(f, "\n      {}", meta);
        }
    }

    fn fmt_payload(f: &mut fmt::Formatter<'_>, proto: &ProtoHdr, buf: &[u8]) -> fmt::Result {
        let meta = MessageMeta::from(proto);

        write!(f, "{meta}")?;

        if meta.is_tlv() {
            write!(
                f,
                "; TLV:\n----------------\n{}\n----------------\n",
                TLVElement::new(buf)
            )?;
        } else {
            write!(
                f,
                "; Payload:\n----------------\n{:02x?}\n----------------\n",
                buf
            )?;
        }

        Ok(())
    }

    #[cfg(feature = "defmt")]
    fn format_payload(f: defmt::Formatter<'_>, proto: &ProtoHdr, buf: &[u8]) {
        let meta = MessageMeta::from(proto);

        defmt::write!(f, "{}", meta);

        if meta.is_tlv() {
            defmt::write!(
                f,
                "; TLV:\n----------------\n{}\n----------------\n",
                TLVElement::new(buf)
            );
        } else {
            defmt::write!(
                f,
                "; Payload:\n----------------\n{}\n----------------\n",
                crate::fmt::Bytes(buf)
            );
        }
    }
}

impl<const N: usize> Display for Packet<N> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Self::fmt(f, &self.peer, &self.header)
    }
}

#[cfg(feature = "defmt")]
impl<const N: usize> defmt::Format for Packet<N> {
    fn format(&self, f: defmt::Formatter<'_>) {
        Self::format(f, &self.peer, &self.header)
    }
}

struct PacketInfo<'a>(&'a Address, &'a PacketHdr);

impl Display for PacketInfo<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Packet::<0>::fmt(f, self.0, self.1)
    }
}

#[cfg(feature = "defmt")]
impl defmt::Format for PacketInfo<'_> {
    fn format(&self, f: defmt::Formatter<'_>) {
        Packet::<0>::format(f, self.0, self.1)
    }
}

struct DetailedPacketInfo<'a>(&'a ProtoHdr, &'a [u8]);

impl Display for DetailedPacketInfo<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Packet::<0>::fmt_payload(f, self.0, self.1)
    }
}

#[cfg(feature = "defmt")]
impl defmt::Format for DetailedPacketInfo<'_> {
    fn format(&self, f: defmt::Formatter<'_>) {
        Packet::<0>::format_payload(f, self.0, self.1)
    }
}

// The buffer used inside the pair of RX and TX `Packet` instances.
//
// The payload is allocated inline. With `large-buffers` enabled the inner
// `Vec` is ~1 MiB, so prefer constructing the buffer in place via
// [`PacketBuffer::init`].
// Constructing one by value with `new()` works too, as long as the resulting
// `Matter` is not moved through a small stack.
//
// This type is only known and used by the `transport` and the `exchange` modules
pub(crate) struct PacketBuffer<const N: usize> {
    buffer: crate::utils::storage::Vec<u8, N>,
}

impl<const N: usize> PacketBuffer<N> {
    pub const fn new() -> Self {
        Self {
            buffer: crate::utils::storage::Vec::new(),
        }
    }

    pub fn init() -> impl Init<Self> {
        init!(Self {
            buffer <- crate::utils::storage::Vec::init(),
        })
    }

    pub fn buf_mut(&mut self) -> &mut crate::utils::storage::Vec<u8, N> {
        &mut self.buffer
    }

    pub fn buf_ref(&self) -> &crate::utils::storage::Vec<u8, N> {
        &self.buffer
    }
}

impl<const N: usize> Deref for PacketBuffer<N> {
    type Target = crate::utils::storage::Vec<u8, N>;

    fn deref(&self) -> &Self::Target {
        self.buf_ref()
    }
}

impl<const N: usize> DerefMut for PacketBuffer<N> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.buf_mut()
    }
}

// Represents the fact that either `Transport` or some `Exchange` instace has an exclusive access to the
// RX or TX packet of the transport layer.
//
// At any point in time, either the `Transport` singleton, or exactly one `Exchange` instance, or nobody
// holds a lock on the RX or TX packet. This is enforced by protecting the packets with an `IfMutex` asynchronous mutex.
//
// This type is only known and used by the `transport` and the `exchange` modules
pub(crate) struct PacketAccess<'a, const N: usize>(IfMutexGuard<'a, Packet<N>>, bool);

impl<const N: usize> PacketAccess<'_, N> {
    pub fn clear_on_drop(&mut self, clear: bool) {
        self.1 = clear;
    }
}

impl<const N: usize> Deref for PacketAccess<'_, N> {
    type Target = Packet<N>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<const N: usize> DerefMut for PacketAccess<'_, N> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

impl<const N: usize> Drop for PacketAccess<'_, N> {
    fn drop(&mut self) {
        if self.1 {
            self.buf.clear();
        }
    }
}

impl<const N: usize> Display for PacketAccess<'_, N> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.0.fmt(f)
    }
}

// Allows other code in `rs-matter` to (ab)use the packet buffers of the transport layer
// in case it needs temporary access to a `&mut [u8]`-shaped memory
//
// Used by the builtin mDNS responder, as well as by the QR code generator
pub struct PacketBufferExternalAccess<'a, const N: usize>(pub(crate) &'a IfMutex<Packet<N>>);

impl<const N: usize> Buffers<[u8]> for PacketBufferExternalAccess<'_, N> {
    type Buffer<'b>
        = ExternalPacketBuffer<'b, N>
    where
        Self: 'b;

    async fn get(&self) -> Option<ExternalPacketBuffer<'_, N>> {
        let mut packet = self.0.lock_if(|packet| packet.buf.is_empty()).await;

        // TODO: Resizing might be a bit expensive with large buffers
        // Resizing to `N` is always safe because the size of `buf` heapless vec is `N`
        unwrap!(packet.buf.resize_default(N));

        Some(ExternalPacketBuffer(packet))
    }

    fn get_immediate(&self) -> Option<Self::Buffer<'_>> {
        self.0
            .try_lock_if(|packet| packet.buf.is_empty())
            .ok()
            .map(|mut packet| {
                // TODO: Resizing might be a bit expensive with large buffers
                // Resizing to `N` is always safe because the size of `buf` heapless vec is `N`
                unwrap!(packet.buf.resize_default(N));

                ExternalPacketBuffer(packet)
            })
    }
}

// Wraps the RX or TX packet of the transport manager in something that looks like a `&mut [u8]` buffer.
pub struct ExternalPacketBuffer<'a, const N: usize>(IfMutexGuard<'a, Packet<N>>);

impl<const N: usize> Deref for ExternalPacketBuffer<'_, N> {
    type Target = [u8];

    fn deref(&self) -> &Self::Target {
        &self.0.buf
    }
}

impl<const N: usize> DerefMut for ExternalPacketBuffer<'_, N> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0.buf
    }
}

impl<const N: usize> Drop for ExternalPacketBuffer<'_, N> {
    fn drop(&mut self) {
        self.0.buf.clear();
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::crypto::test_only_crypto;
    use crate::dm::devices::test::{TEST_DEV_ATT, TEST_DEV_COMM, TEST_DEV_DET};

    fn test_matter() -> Matter<'static> {
        Matter::new(&TEST_DEV_DET, TEST_DEV_COMM, &TEST_DEV_ATT, 0)
    }

    #[test]
    fn test_create_unsecured_session_creates_plaintext_session() {
        let matter = test_matter();
        let crypto = test_only_crypto();
        let peer = Address::new();

        let session_id = matter
            .transport
            .create_plaintext_session(&matter, &crypto, peer)
            .unwrap();

        matter.with_state(|state| {
            let session = state.sessions.get(session_id).unwrap();

            assert_eq!(session.id, session_id);
            assert!(!session.is_encrypted());
            assert_eq!(session.get_peer_node_id(), None);
            assert_eq!(*session.get_session_mode(), session::SessionMode::PlainText);
        });
    }

    #[test]
    fn test_initiate_unsecured_now_creates_initiator_exchange() {
        let matter = test_matter();
        let crypto = test_only_crypto();
        let peer = Address::new();

        let exchange = matter
            .transport
            .try_initiate_plaintext(&matter, &crypto, peer)
            .unwrap();

        exchange
            .with_state(|state| {
                let sess = exchange.id().session(&mut state.sessions);
                let exch = exchange.id().exch(sess);

                assert!(matches!(exch.role, Role::Initiator(_)));
                assert_eq!(sess.id, exchange.id().session_id());
                Ok(())
            })
            .unwrap();
    }
}

#[cfg(test)]
mod resolve_tests {
    use core::net::{IpAddr, Ipv4Addr, SocketAddr};

    use futures_lite::future::{block_on, zip};

    use crate::error::ErrorCode;
    use crate::test::test_matter;
    use crate::transport::network::mdns::{DottedName, MdnsRemoteService};
    use crate::transport::network::MatterRemoteService;

    fn op_service() -> MatterRemoteService {
        MatterRemoteService::Operational {
            compressed_fabric_id: 0x1122,
            node_id: 0x3344,
        }
    }

    /// A resolver and the responder rendezvous on the single in-flight slot: the
    /// responder picks up the request and deposits an answer, which the resolver
    /// returns as the resolved `Address`.
    #[test]
    fn resolve_rendezvous_delivers_answer() {
        let matter = test_matter();
        let service = op_service();

        let resolved = block_on(async {
            let resolver = matter.transport().resolve(service.clone(), 5_000);

            let responder = async {
                let picked = matter.transport().wait_mdns_resolve_request().await;
                assert_eq!(picked, service);

                let mut name = heapless::String::<128>::new();
                service.instance_name(&mut name);

                let answer = MdnsRemoteService {
                    instance_name: DottedName(name.as_str()),
                    port: Some(1234),
                    addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 5))].into_iter(),
                    txt: [("SII", "300"), ("SAI", "4000"), ("SAT", "5000")].into_iter(),
                    scope_id: 0,
                };

                matter.transport().try_deposit_mdns_resolve(&answer);
            };

            let (node, ()) = zip(resolver, responder).await;
            node
        })
        .unwrap();

        assert_eq!(
            resolved.addr,
            SocketAddr::new(IpAddr::V4(Ipv4Addr::new(10, 0, 0, 5)), 1234)
        );
        // The peer's MRP/session params are carried out of the resolve TXT.
        assert_eq!(resolved.sii, Some(300));
        assert_eq!(resolved.sai, Some(4000));
        assert_eq!(resolved.sat, Some(5000));
    }

    /// Without a responder, a resolve times out and releases the slot (drop-clean),
    /// so a subsequent resolve also simply times out rather than hanging.
    #[test]
    fn resolve_times_out_and_releases_slot() {
        let matter = test_matter();

        let err = block_on(matter.transport().resolve(op_service(), 50)).unwrap_err();
        assert!(matches!(err.code(), ErrorCode::NotFound));

        let err = block_on(matter.transport().resolve(op_service(), 50)).unwrap_err();
        assert!(matches!(err.code(), ErrorCode::NotFound));
    }

    /// Depositing an answer with no in-flight request is a no-op (does not strand
    /// a phantom answer): a later resolve still times out.
    #[test]
    fn deposit_without_request_is_noop() {
        let matter = test_matter();

        let mut name = heapless::String::<128>::new();
        op_service().instance_name(&mut name);
        let answer = MdnsRemoteService {
            instance_name: DottedName(name.as_str()),
            port: Some(1234),
            addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 9))].into_iter(),
            txt: core::iter::empty::<(&str, &str)>(),
            scope_id: 0,
        };
        matter.transport().try_deposit_mdns_resolve(&answer);

        let err = block_on(matter.transport().resolve(op_service(), 50)).unwrap_err();
        assert!(matches!(err.code(), ErrorCode::NotFound));
    }
}

#[cfg(test)]
mod browse_tests {
    use core::net::{IpAddr, Ipv4Addr, SocketAddr};

    use futures_lite::future::{block_on, zip};

    use crate::error::ErrorCode;
    use crate::test::test_matter;
    use crate::transport::network::mdns::{CommissionableFilter, DottedName, MdnsRemoteService};
    use crate::transport::network::Address;

    /// A browser and the responder rendezvous: the responder picks up the request,
    /// deposits a matching commissionable answer, and the browser returns its
    /// address + commissionable instance id.
    #[test]
    fn browse_rendezvous_delivers_first_match() {
        let matter = test_matter();

        let filter = CommissionableFilter {
            discriminator: Some(0xA5A),
            vendor_id: Some(0xFFF1),
            ..Default::default()
        };

        let found = block_on(async {
            let browser = matter
                .transport()
                .browse_commissionable(&filter, &[], 5_000);

            let responder = async {
                let picked = matter.transport().wait_mdns_browse_request().await;
                assert_eq!(picked, filter);

                // A non-matching node (wrong vendor) must be ignored.
                let other = MdnsRemoteService {
                    instance_name: DottedName("0000000000000001._matterc._udp.local"),
                    port: Some(5540),
                    addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 1))].into_iter(),
                    txt: [("D", "2650"), ("VP", "9999+1"), ("CM", "1")].into_iter(),
                    scope_id: 0,
                };
                matter.transport().try_deposit_mdns_browse(&other);

                // The matching node (D=0xA5A=2650, VP vendor 0xFFF1=65521).
                let answer = MdnsRemoteService {
                    instance_name: DottedName("00000000ABCD1234._matterc._udp.local"),
                    port: Some(5541),
                    addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 7))].into_iter(),
                    txt: [("D", "2650"), ("VP", "65521+42"), ("CM", "1")].into_iter(),
                    scope_id: 0,
                };
                matter.transport().try_deposit_mdns_browse(&answer);
            };

            let (found, ()) = zip(browser, responder).await;
            found
        })
        .unwrap();

        assert_eq!(
            found,
            (
                Address::Udp(SocketAddr::new(
                    IpAddr::V4(Ipv4Addr::new(10, 0, 0, 7)),
                    5541
                )),
                0x00000000ABCD1234,
            )
        );
    }

    /// Without a responder, a browse times out and releases the slot (drop-clean).
    #[test]
    fn browse_times_out_and_releases_slot() {
        let matter = test_matter();
        let filter = CommissionableFilter {
            short_discriminator: Some(0xA),
            ..Default::default()
        };

        let err = block_on(matter.transport().browse_commissionable(&filter, &[], 50)).unwrap_err();
        assert!(matches!(err.code(), ErrorCode::NotFound));

        let err = block_on(matter.transport().browse_commissionable(&filter, &[], 50)).unwrap_err();
        assert!(matches!(err.code(), ErrorCode::NotFound));
    }

    /// `exclude` steps past an already-tried candidate: with two nodes sharing the
    /// (short) discriminator, excluding the first id yields the second.
    #[test]
    fn browse_exclude_steps_to_next_match() {
        let matter = test_matter();

        // Short discriminator 0xA = top 4 bits of 0xA12 (2578) and 0xAFF (2815).
        let filter = CommissionableFilter {
            short_discriminator: Some(0xA),
            ..Default::default()
        };

        // Both nodes match the filter; exclude the first id, expect the second.
        let found = block_on(async {
            let browser = matter
                .transport()
                .browse_commissionable(&filter, &[0x1111], 5_000);

            let responder = async {
                matter.transport().wait_mdns_browse_request().await;

                let node_a = MdnsRemoteService {
                    instance_name: DottedName("0000000000001111._matterc._udp.local"),
                    port: Some(5540),
                    addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 1))].into_iter(),
                    txt: [("D", "2578"), ("CM", "1")].into_iter(), // 0xA12, short 0xA
                    scope_id: 0,
                };
                // Excluded id -> ignored.
                matter.transport().try_deposit_mdns_browse(&node_a);

                let node_b = MdnsRemoteService {
                    instance_name: DottedName("0000000000002222._matterc._udp.local"),
                    port: Some(5541),
                    addrs: [IpAddr::V4(Ipv4Addr::new(10, 0, 0, 2))].into_iter(),
                    txt: [("D", "2815"), ("CM", "1")].into_iter(), // 0xAFF, short 0xA
                    scope_id: 0,
                };
                matter.transport().try_deposit_mdns_browse(&node_b);
            };

            let (found, ()) = zip(browser, responder).await;
            found
        })
        .unwrap();

        assert_eq!(
            found,
            (
                Address::Udp(SocketAddr::new(
                    IpAddr::V4(Ipv4Addr::new(10, 0, 0, 2)),
                    5541
                )),
                0x2222,
            )
        );
    }
}