TCP sockets: a loopback echo server

The concept

TCP provides a reliable, ordered, byte-stream connection between two processes. A server creates a socket, binds it to an address and port, and listens. A client connects, and the server accepts the connection and gets a new socket for that client. Both sides then send and receive bytes until one of them closes.

The program runs both ends in one process using the loopback address 127.0.0.1, so no traffic leaves the machine. Asking the operating system to bind port 0 picks a free port automatically; getsockname() reports which one. The server thread uppercases what it receives and sends it back, and the client prints the reply.

TCP is a stream, not a sequence of messages. One send can arrive split across several recv calls, and two sends can arrive combined. Real protocols therefore add framing, such as a length prefix or a delimiter, and loop until a full message has been read. The single small message here arrives in one piece, but code should not depend on that.

Code example

import socket
import threading

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(("127.0.0.1", 0))
server.listen(1)
port = server.getsockname()[1]


def serve():
    connection, _ = server.accept()
    with connection:
        data = connection.recv(1024)
        connection.sendall(data.upper())


thread = threading.Thread(target=serve)
thread.start()
with socket.create_connection(("127.0.0.1", port)) as client:
    client.sendall(b"hello tcp")
    reply = client.recv(1024)
thread.join()
server.close()
print(port > 0)
print(reply.decode())

True
HELLO TCP

Follow the code

Open the walkthrough to inspect the example one step at a time.

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True
HELLO TCP

Behind the scenes

Cost and common mistakes

Setup is O(1). Transferring n bytes takes O(n) time; TCP handles retransmission and ordering for you.

• recv(n) returns up to n bytes, not necessarily a whole message. Loop until you have read a complete, framed message.
• Sockets carry bytes. Encode text with .encode() and decode received data with .decode().
• Close sockets, or use with blocks, to release the port and file descriptor. A server that never closes leaks resources.

Try it yourself

Why does the program bind to port 0 instead of a fixed port such as 8080?

Further reading

• Python socket programming HOWTO (external)
• RFC 9293: Transmission Control Protocol (TCP) (external)
• Computer Networking: A Top-Down Approach (Kurose and Ross) (external)