dcsctp 0.1.12

// Copyright 2026 The dcSCTP 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
//
//     https://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 crate::api::ErrorKind;
use crate::api::Message;
use crate::api::SendError;
use crate::api::SendOptions;
use crate::api::SocketEvent;
use crate::api::SocketTime;
use crate::packet::SkippedStream;
use crate::packet::chunk::Chunk;
use crate::packet::chunk_validators::clean_sack;
use crate::packet::data::Data;
use crate::packet::error_causes::ErrorCause;
use crate::packet::error_chunk::ErrorChunk;
use crate::packet::no_user_data_error_cause::NoUserDataErrorCause;
use crate::packet::sack_chunk::SackChunk;
use crate::socket::context::Context;
use crate::socket::shutdown::maybe_send_shutdown;
use crate::socket::shutdown::maybe_send_shutdown_ack;
use crate::socket::state::State;
use crate::tx::retransmission_queue::HandleSackResult;
use crate::types::Tsn;

pub(crate) fn handle_data(
    state: &mut State,
    ctx: &mut Context,
    now: SocketTime,
    tsn: Tsn,
    data: Data,
) {
    if data.payload.is_empty() {
        ctx.events.borrow_mut().add(SocketEvent::OnError(
            ErrorKind::ProtocolViolation,
            "Received DATA chunk with no user data".into(),
        ));
        if let Some(tcb) = state.tcb_mut() {
            ctx.events.borrow_mut().add(SocketEvent::SendPacket(
                tcb.new_packet()
                    .add(&Chunk::Error(ErrorChunk {
                        error_causes: vec![ErrorCause::NoUserData(NoUserDataErrorCause { tsn })],
                    }))
                    .build(),
            ));
            ctx.tx_packets_count += 1;
        }
        return;
    }
    let Some(tcb) = state.tcb_mut() else {
        ctx.events.borrow_mut().add(SocketEvent::OnError(
            ErrorKind::NotConnected,
            "Received unexpected commands on socket that is not connected".into(),
        ));
        return;
    };
    if tcb.reassembly_queue.is_full() {
        // If the reassembly queue is full, there is nothing that can be done. The specification
        // only allows dropping gap-ack-blocks, and that's not likely to help as the socket has
        // been trying to fill gaps since the watermark was reached.
        return;
    }
    if tcb.reassembly_queue.is_above_watermark() {
        // TODO: Implement
        return;
    }
    if !tcb.data_tracker.is_tsn_valid(tsn) {
        // TODO: Implement
        return;
    }
    if tcb.data_tracker.observe(now, tsn, false) {
        tcb.reassembly_queue.add(tsn, data);
    }
}

pub(crate) fn handle_sack(state: &mut State, ctx: &mut Context, now: SocketTime, sack: SackChunk) {
    let Some(tcb) = state.tcb_mut() else {
        ctx.events.borrow_mut().add(SocketEvent::OnError(ErrorKind::NotConnected, "No TCB".into()));
        return;
    };

    let sack = clean_sack(sack);
    match tcb.retransmission_queue.handle_sack(now, &sack) {
        HandleSackResult::Invalid => {
            log::debug!("Dropping out-of-order SACK with TSN {}", sack.cumulative_tsn_ack);
            return;
        }
        HandleSackResult::Valid { rtt, reset_error_counter } => {
            if let Some(rtt) = rtt {
                tcb.rto.observe_rto(rtt);
                tcb.retransmission_queue.update_rto(tcb.rto.rto());
                tcb.data_tracker.update_rto(tcb.rto.rto());
            }
            if reset_error_counter {
                ctx.tx_error_counter.reset();
            }
        }
    }

    match state {
        State::ShutdownPending(_) => maybe_send_shutdown(state, ctx, now),
        State::ShutdownReceived(_) => maybe_send_shutdown_ack(state, ctx, now),
        _ => (),
    }

    // Receiving an ACK may make the socket go into fast recovery mode. From
    // <https://datatracker.ietf.org/doc/html/rfc9260#section-7.2.4>:
    //
    //   If not in Fast Recovery, determine how many of the earliest (i.e., lowest TSN) DATA
    //   chunks marked for retransmission will fit into a single packet, subject to constraint
    //   of the PMTU of the destination transport address to which the packet is being sent.
    //   Call this value K. Retransmit those K DATA chunks in a single packet. When a Fast
    //   Retransmit is being performed, the sender SHOULD ignore the value of cwnd and SHOULD
    //   NOT delay retransmission for this single packet.
    maybe_send_fast_retransmit(state, ctx, now);

    // Receiving an ACK will decrease outstanding bytes (maybe now below cwnd?) or indicate
    // packet loss that may result in sending FORWARD-TSN.
    ctx.send_buffered_packets(state, now);
}

pub(crate) fn handle_forward_tsn(
    state: &mut State,
    now: SocketTime,
    new_cumulative_tsn: Tsn,
    skipped_streams: Vec<SkippedStream>,
) {
    if let Some(tcb) = state.tcb_mut() {
        if tcb.data_tracker.handle_forward_tsn(now, new_cumulative_tsn) {
            tcb.reassembly_queue.handle_forward_tsn(new_cumulative_tsn, skipped_streams);
        }
    }
}

pub(crate) fn maybe_send_sack(state: &mut State, ctx: &mut Context, now: SocketTime) {
    if let Some(tcb) = state.tcb_mut() {
        tcb.data_tracker.observe_packet_end(now);
        if tcb.data_tracker.should_send_ack(now, false) {
            let mut b = tcb.new_packet();
            let rwnd = tcb.reassembly_queue.remaining_bytes();
            b.add(&Chunk::Sack(tcb.data_tracker.create_selective_ack(rwnd as u32)));
            ctx.send_buffered_packets_with(state, now, &mut b);
        }
    }
}

fn maybe_send_fast_retransmit(state: &mut State, ctx: &mut Context, now: SocketTime) {
    let tcb = state.tcb_mut().unwrap();
    if !tcb.retransmission_queue.has_data_to_be_fast_retransmitted() {
        return;
    }

    let mut builder = tcb.new_packet();

    let chunks =
        tcb.retransmission_queue.get_chunks_for_fast_retransmit(now, builder.bytes_remaining());
    for (tsn, data) in chunks {
        builder.add(&tcb.make_data_chunk(tsn, data));
    }

    debug_assert!(!builder.is_empty());
    ctx.events.borrow_mut().add(SocketEvent::SendPacket(builder.build()));
    ctx.tx_packets_count += 1;
}

pub(crate) fn validate_send(
    state: &mut State,
    ctx: &mut Context,
    message: &Message,
    send_options: &SendOptions,
) -> Result<(), SendError> {
    let lifecycle_id = &send_options.lifecycle_id;
    let add_error_events = |kind, msg: &str| {
        if let Some(id) = lifecycle_id {
            ctx.events.borrow_mut().add(SocketEvent::OnLifecycleEnd(id.clone()));
        }
        ctx.events.borrow_mut().add(SocketEvent::OnError(kind, msg.to_string()));
    };

    if message.payload.is_empty() {
        add_error_events(ErrorKind::ProtocolViolation, "Unable to send empty message");
        return Err(SendError::EmptyPayload);
    }
    let payload_len = message.payload.len();
    if payload_len > ctx.options.max_message_size {
        add_error_events(ErrorKind::ProtocolViolation, "Unable to send too large message");
        return Err(SendError::MessageTooLarge {
            len: payload_len,
            limit: ctx.options.max_message_size,
        });
    }
    if matches!(
        state,
        State::ShutdownPending(_)
            | State::ShutdownSent(_)
            | State::ShutdownReceived(_)
            | State::ShutdownAckSent(_)
    ) {
        add_error_events(
            ErrorKind::WrongSequence,
            "Unable to send message as the socket is shutting down",
        );
        return Err(SendError::ShuttingDown);
    }
    if ctx.send_queue.total_buffered_amount() >= ctx.options.max_send_buffer_size
        || ctx.send_queue.buffered_amount(message.stream_id)
            >= ctx.options.per_stream_send_queue_limit
    {
        add_error_events(
            ErrorKind::ResourceExhaustion,
            "Unable to send message as the send queue is full",
        );
        return Err(SendError::ResourceExhaustion);
    }
    Ok(())
}