As part of a testing framework I am creating, I am trying to convert a list of (IP) packets into a tcp stream, enabling easy mocking.

I have been trying to modify the loopback example in smoltcp
like so, to get an initial POC:

fn main() {
    env_logger::init();

    let mut device = Loopback::new(Medium::Ip);

    let mut iface = Interface::new(
        Config::new(smoltcp::wire::HardwareAddress::Ip),
        &mut device,
        Instant::now(),
    );
    iface.update_ip_addrs(|ip_addrs| {
        ip_addrs
            .push(IpCidr::new(IpAddress::v4(127, 0, 0, 1), 8))
            .unwrap();
    });

    let server_socket = tcp::Socket::new(
        tcp::SocketBuffer::new(ManagedSlice::Owned(vec![0; 1024])),
        tcp::SocketBuffer::new(ManagedSlice::Owned(vec![0; 1024])),
    );

    let client_packets = vec![
        hex::decode("450000340000400040063cc27f0000017f000001fde804d277368c50000000008002040070af00000204ffd70303000402000000").unwrap(), // syn
    ];

    let client_socket = raw::Socket::new(
        smoltcp::wire::IpVersion::Ipv4,
        smoltcp::wire::IpProtocol::Tcp,
        raw::PacketBuffer::new(
            ManagedSlice::Owned(vec![raw::PacketMetadata::EMPTY; 1024]),
            ManagedSlice::Owned(vec![0; 1024]),
        ),
        raw::PacketBuffer::new(
            ManagedSlice::Owned(vec![raw::PacketMetadata::EMPTY; 1024]),
            ManagedSlice::Owned(vec![0; 1024]),
        ),
    );

    let mut sockets: [_; 2] = Default::default();
    let mut sockets = SocketSet::new(&mut sockets[..]);
    let server_handle = sockets.add(server_socket);
    let client_handle = sockets.add(client_socket);

    iface.poll(Instant::now(), &mut device, &mut sockets);
    let server_socket = sockets.get_mut::<tcp::Socket>(server_handle);
    debug!("server listening");
    server_socket.listen(1234).unwrap();

    let client_socket = sockets.get_mut::<raw::Socket>(client_handle);
    debug!("client connecting");
    client_socket
        .send_slice(client_packets[0].as_slice())
        .unwrap(); // syn, for connect

    iface.poll(Instant::now(), &mut device, &mut sockets);
}

The problem is that the logs show that a RST packet from the client to the server is sent after the synack (after adding logs for the packets transmitted):

server listening
client connecting
127.0.0.1:65000 => 127.0.0.1:1234 (SYN)
127.0.0.1:1234 => 127.0.0.1:65000 (SYN ACK)
127.0.0.1:65000 => 127.0.0.1:1234 (RST)

So – is this something I’m missing?