101 lines
3.7 KiB
Python
101 lines
3.7 KiB
Python
"""Stop-and-wait: a complete, working reliable protocol with a window of one. Given.
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Read this before you write gbn.py. It shows every piece your protocols need:
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splitting the data, the retransmission timer, what the receiver ACKs, and how
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the two sides agree the transfer is over (FIN, FINACK, and the receiver's
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linger). Its receiver is already a correct Go-Back-N receiver.
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ACKs here are cumulative: ACK n means "I have every packet before n; send me n".
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"""
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import socket
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import time
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import packet
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class Sender:
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def __init__(self, sock, peer, window=1, timeout=0.1, max_fin_tries=20):
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self.sock, self.peer = sock, peer
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self.timeout = timeout
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self.max_fin_tries = max_fin_tries
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self.stats = {"data_packets": 0, "transmissions": 0, "retransmissions": 0}
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def send(self, data):
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"""Deliver `data` reliably, then close. Returns the stats dict."""
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chunks = packet.split(data)
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self.stats["data_packets"] = len(chunks)
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for seq, chunk in enumerate(chunks):
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self._until_acked(packet.encode(packet.DATA, seq, chunk), seq + 1)
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self._close(len(chunks))
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return self.stats
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def _until_acked(self, raw, want):
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first = True
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while True:
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self.sock.sendto(raw, self.peer)
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self.stats["transmissions"] += 1
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if not first:
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self.stats["retransmissions"] += 1
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first = False
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deadline = time.monotonic() + self.timeout
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while time.monotonic() < deadline:
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self.sock.settimeout(max(0.001, deadline - time.monotonic()))
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try:
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got = packet.decode(self.sock.recvfrom(2048)[0])
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except socket.timeout:
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break
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if got and got[0] == packet.ACK and got[1] >= want:
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return
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def _close(self, count):
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fin = packet.encode(packet.FIN, count)
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for _ in range(self.max_fin_tries):
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self.sock.sendto(fin, self.peer)
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self.sock.settimeout(self.timeout)
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try:
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got = packet.decode(self.sock.recvfrom(2048)[0])
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except socket.timeout:
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continue
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if got and got[0] == packet.FINACK:
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return
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# every data packet was ACKed, so the data arrived; only the goodbye was lost
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class Receiver:
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def __init__(self, sock, linger=0.5):
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self.sock = sock
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self.linger = linger
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def receive(self):
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"""Return every byte the sender sent, in order."""
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expected, parts, peer = 0, [], None
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self.sock.settimeout(None)
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while True:
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raw, peer = self.sock.recvfrom(2048)
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got = packet.decode(raw)
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if got is None:
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continue # corrupted: say nothing
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kind, seq, payload = got
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if kind == packet.DATA:
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if seq == expected:
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parts.append(payload)
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expected += 1
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self.sock.sendto(packet.encode(packet.ACK, expected), peer)
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elif kind == packet.FIN and seq == expected:
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self._finish(peer)
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return b"".join(parts)
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def _finish(self, peer):
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"""Answer FIN, then keep answering repeats until the line goes quiet."""
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finack = packet.encode(packet.FINACK, 0)
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self.sock.sendto(finack, peer)
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self.sock.settimeout(self.linger)
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while True:
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try:
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raw, peer = self.sock.recvfrom(2048)
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except socket.timeout:
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return
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got = packet.decode(raw)
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if got and got[0] == packet.FIN:
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self.sock.sendto(finack, peer)
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