Transport Layer Protocols Questions

import socket
import time

# Packet format: b'D' or b'A' + seq byte (b'0' or b'1') + payload (for data only)

def send_reliable(sock: socket.socket, dest: tuple, payload: bytes, timeout: float = 1.0):
    seq = 0
    data_pkt = b'D' + bytes([seq]) + payload

    sock.settimeout(timeout)
    while True:
        try:
            sock.sendto(data_pkt, dest)
            # wait for ACK
            ack, _ = sock.recvfrom(1024)
            if len(ack) >= 2 and ack[0:1] == b'A' and ack[1] == seq:
                # success, flip seq for next send (if caller uses same function repeatedly)
                return
        except socket.timeout:
            # timeout -> retransmit
            continue

def receive_reliable(sock: socket.socket):
    expected = 0
    while True:
        pkt, addr = sock.recvfrom(65535)
        if len(pkt) < 2:
            continue
        ptype = pkt[0:1]
        seq = pkt[1]
        if ptype == b'D':
            # data packet
            payload = pkt[2:]
            if seq == expected:
                # deliver payload
                # send ACK and flip expected
                ack = b'A' + bytes([seq])
                sock.sendto(ack, addr)
                expected ^= 1
                return payload, addr
            else:
                # duplicate: re-ack but don't deliver
                ack = b'A' + bytes([seq])
                sock.sendto(ack, addr)
                # continue waiting for expected packet
        # ignore other packet types