TCP, UDP and ports · Computer Networks

Why you need it

The intuition

Definition

Port

A number from 0 to 65535 that picks one program on a machine. A server listens on a port; a client connects to address:port.

TCP and UDP

TCP: connection, ordering, retransmission, flow control. UDP: single datagrams, no guarantees, less delay.

Examples

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arrived = [(3, "lo w"), (1, "hel"), (4, "orld"), (3, "lo w"), (2, "")]
received = {}
for seq, data in arrived:
    if seq in received:
        print("duplicate", seq, "ignored")
        continue
    received[seq] = data
message = "".join(received[s] for s in sorted(received))
print(message)

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got = {1, 2, 4, 5}
expected = 1
while expected in got:
    expected += 1
print("ACK", expected)

Common mistakes

Quick quiz

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handshake = ["SYN", "SYN-ACK", "ACK"]
for i, msg in enumerate(handshake, 1):
    sender = "client" if msg != "SYN-ACK" else "server"
    print(i, sender, msg)

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

1. PredictReassemble

   Type exactly what this program prints.

2. ModifyName the service

   service(port) should name well-known ports. Add HTTPS (443), SSH (22) and PostgreSQL (5432, return "postgres"), and return "unknown" for anything else instead of crashing.

3. WriteCumulative ACK

   Write next_ack(received) that takes a list of sequence numbers that arrived (in any order, maybe with duplicates, starting from 1) and returns the next number the receiver should ask for: the smallest positive number missing.

Terms in this lesson