TCP、UDP 与端口 · 计算机网络

为什么需要它

直觉理解

定义

端口 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.

0 到 65535 之间的一个数，用来选中机器上的某个程序。服务器在某个端口上监听；客户端连接 地址:端口。

TCP 与 UDP

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

TCP：有连接、保证顺序、会重传、有流量控制。UDP：单个数据报，不做保证，延迟更低。

示例

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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)

常见错误

小测验

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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)

动手练习

3 道练习，分三层：先预测程序的输出，再按新需求改程序，最后从零写一个。代码在浏览器里运行，提交马上判题。

1. 预测输出重组

   准确写出这段程序的输出。

2. 改需求说出服务名

   service(port) 应该说出常见端口的服务名。加上 HTTPS（443）、SSH（22）和 PostgreSQL（5432，返回 "postgres"），其他端口返回 "unknown"，而不是崩溃。

3. 从零写累积确认

   写函数 next_ack(received)：参数是已到达的序列号列表（顺序任意，可能有重复，从 1 开始），返回接收方应该请求的下一个编号：缺失的最小正整数。

本课术语