TCP 连接：握手与挥手

客户端                                服务器
CLOSED                                LISTEN
  │  ① SYN, seq=x                       │
  │ ───────────────────────────────────▶│  收到：知道"客户端能发"
SYN_SENT                            SYN_RCVD
  │  ② SYN+ACK, seq=y, ack=x+1          │
  │ ◀─────────────────────────────────── │  客户端收到：知道"双向都通"
  │  ③ ACK, ack=y+1                     │
  │ ───────────────────────────────────▶│  服务器收到：知道"服务器发的能被收到"
ESTABLISHED                        ESTABLISHED

python3 -c '
import socket, time
 
STATES = {"01": "ESTABLISHED", "06": "TIME_WAIT", "0A": "LISTEN",
          "08": "CLOSE_WAIT", "05": "FIN_WAIT2"}
 
def show(tag):
    # 9000 的十六进制是 2328，筛出本实验相关的行
    found = []
    for line in open("/proc/net/tcp"):
        p = line.split()
        if len(p) > 3 and ("2328" in p[1] or "2328" in p[2]):
            found.append(STATES.get(p[3], p[3]))
    print(tag, found)
 
srv = socket.socket()
srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
srv.bind(("127.0.0.1", 9000))
srv.listen(5)
show("listen 后:  ")
 
cli = socket.create_connection(("127.0.0.1", 9000))
conn, _ = srv.accept()
show("握手完成后:")
 
cli.close()          # 客户端主动关闭 -> 客户端一侧进入 TIME_WAIT
conn.close()
time.sleep(0.3)
show("双方关闭后:")
'

python3 -c '
import socket
 
# 1) 目标端口没人监听：SYN 被内核直接回 RST，立刻失败
try:
    socket.create_connection(("127.0.0.1", 9999), timeout=2)
except OSError as e:
    print("无人监听的端口:", e)
 
# 2) 服务器只 listen、从不 accept：客户端照样握手成功！
srv = socket.socket()
srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
srv.bind(("127.0.0.1", 9001))
srv.listen(5)
 
cli = socket.create_connection(("127.0.0.1", 9001), timeout=2)
cli.sendall(b"data lands in kernel buffer")
print("未 accept 也连接成功:", cli.getpeername())
cli.close()
'

主动方                                被动方
ESTABLISHED                        ESTABLISHED
  │  ① FIN  "我不再发数据了"            │
  │ ───────────────────────────────────▶│
FIN_WAIT_1                         CLOSE_WAIT
  │  ② ACK                              │
  │ ◀─────────────────────────────────── │   被动方可能还有数据要发
FIN_WAIT_2                             │   （②③ 之间可以隔很久）
  │  ③ FIN  "我也发完了"                 │
  │ ◀─────────────────────────────────── │
  │  ④ ACK                              │  LAST_ACK
  │ ───────────────────────────────────▶│
TIME_WAIT（等 2MSL）                  CLOSED
  │
CLOSED

python3 -c '
import socket, time
 
STATES = {"01": "ESTABLISHED", "06": "TIME_WAIT", "08": "CLOSE_WAIT", "0A": "LISTEN"}
 
def rows(port_hex):
    out = []
    for line in open("/proc/net/tcp"):
        p = line.split()
        # 跳过表头（首行无冒号），且本地/远端地址必须含冒号
        if len(p) < 4 or ":" not in p[1] or ":" not in p[2]:
            continue
        local_hex = p[1].split(":")[1]
        remote_hex = p[2].split(":")[1]
        if port_hex in (local_hex, remote_hex):
            local = int(local_hex, 16)
            out.append((local, STATES.get(p[3], p[3])))
    return out
 
# 实验：这次让服务器一侧主动关闭
srv = socket.socket()
srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
srv.bind(("127.0.0.1", 9002)); srv.listen(1)     # 9002 hex = 232A
cli = socket.create_connection(("127.0.0.1", 9002))
conn, _ = srv.accept()
 
conn.close()      # 服务器先关 -> 服务器侧(9002端口) TIME_WAIT
time.sleep(0.2)
for local, st in rows("232A"):
    who = "服务器侧" if local == 9002 else "客户端侧"
    print(who, "端口", local, "状态", st)
cli.close()
'