Retransmission happen when using loopback

[strobecat]

I'm writing some unit test for my personal project, so running into the issue that some unusual situation like retransmission, duplicated ack happened.

Code to trigger that problem

use log::{debug, error, info};

use smoltcp::iface::{Config, Interface, SocketSet};
use smoltcp::phy::{Loopback, Medium, PcapMode, PcapWriter};
use smoltcp::socket::tcp;
use smoltcp::time::{Duration, Instant};
use smoltcp::wire::{IpAddress, IpCidr};
use std::fs::File;
use std::thread::sleep;

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

    let loopback = Loopback::new(Medium::Ip);
    let mut device = PcapWriter::new(
        loopback,
        File::create_new("dump.pcap").unwrap(),
        PcapMode::TxOnly,
    );

    // Create interface
    let config = Config::new(smoltcp::wire::HardwareAddress::Ip);

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

    // Create sockets
    let server_socket = tcp::Socket::new(
        tcp::SocketBuffer::new(vec![0u8; 65535]),
        tcp::SocketBuffer::new(vec![0u8; 65535]),
    );

    let client_socket = tcp::Socket::new(
        tcp::SocketBuffer::new(vec![0u8; 65535]),
        tcp::SocketBuffer::new(vec![0u8; 65535]),
    );

    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);

    let mut sent = false;
    let mut received = 0usize;
    loop {
        iface.poll(Instant::now(), &mut device, &mut sockets);

        let socket = sockets.get_mut::<tcp::Socket>(server_handle);
        if !socket.is_active() && !socket.is_listening() {
            socket.listen(1234).unwrap();
        }
        if socket.can_recv() {
            match socket.recv(|buffer| (buffer.len(), buffer.len())) {
                Ok(size) => {
                    received += size;
                }
                Err(e) => error!("{:?}", e),
            }

            debug!(
                "got {}/{} bytes, left {}",
                received,
                65535,
                socket.recv_queue()
            );
            if received == 65535 {
                socket.close();
            }
        }

        let socket = sockets.get_mut::<tcp::Socket>(client_handle);
        if !socket.is_active() {
            socket
                .connect(iface.context(), (IpAddress::v4(127, 0, 0, 1), 1234), 65000)
                .unwrap();
        }
        if !sent && socket.can_send() {
            debug!("sending");
            socket.send_slice(&vec![0u8; 65535]).unwrap();
            sent = true;
        }

        match iface.poll_delay(Instant::now(), &sockets) {
            Some(Duration::ZERO) => debug!("resuming"),
            Some(delay) => {
                debug!("sleeping for {}", delay);
                sleep(delay.into());
            }
            None => {
                if sent {
                    break;
                }
            }
        }
    }

    info!("done")
}

Once run, it yields these log:

[2025-07-16T16:37:49Z DEBUG test1] resuming
[2025-07-16T16:37:49Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:50Z DEBUG smoltcp::socket::tcp] retransmitting at t+0.700s��������������������������������[2025-07-16T16:37:50Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:50Z DEBUG smoltcp::socket::tcp] expecting an ACK
[2025-07-16T16:37:50Z DEBUG smoltcp::socket::tcp] retransmitting at t+0.700s
[2025-07-16T16:37:50Z DEBUG test1] sending
[2025-07-16T16:37:50Z DEBUG test1] resuming
[2025-07-16T16:37:50Z DEBUG smoltcp::socket::tcp] received a keep-alive or window probe packet, will send an ACK
[2025-07-16T16:37:50Z DEBUG smoltcp::socket::tcp] sending sACK option with current assembler ranges
[2025-07-16T16:37:50Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:51Z DEBUG smoltcp::socket::tcp] retransmitting at t+0.700s
[2025-07-16T16:37:51Z DEBUG test1] got 65495/65535 bytes, left 0
[2025-07-16T16:37:51Z DEBUG test1] resuming
[2025-07-16T16:37:51Z DEBUG smoltcp::socket::tcp] segment not in receive window (2000063569..2000129064 not intersecting 2000129064..2000194599), will send challenge ACK
[2025-07-16T16:37:51Z DEBUG smoltcp::socket::tcp] sending sACK option with current assembler ranges
[2025-07-16T16:37:51Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:52Z DEBUG smoltcp::socket::tcp] retransmitting at t+0.700s
[2025-07-16T16:37:52Z DEBUG test1] got 65535/65535 bytes, left 0
[2025-07-16T16:37:52Z DEBUG test1] resuming
[2025-07-16T16:37:52Z DEBUG smoltcp::socket::tcp] segment not in receive window (2000129064..2000129104 not intersecting 2000129104..2000194639), will send challenge ACK
[2025-07-16T16:37:52Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:53Z DEBUG smoltcp::socket::tcp] retransmitting at t+0.700s
[2025-07-16T16:37:53Z DEBUG test1] sleeping for 0.699s
[2025-07-16T16:37:53Z DEBUG smoltcp::socket::tcp] received a keep-alive or window probe packet, will send an ACK
[2025-07-16T16:37:53Z DEBUG smoltcp::socket::tcp] sending sACK option with current assembler ranges
[2025-07-16T16:37:53Z INFO  test1] done

Why this is happening? Am I doing something wrong?