HTTP/3 与 QUIC：在 UDP 上重建世界

python3 -c '
import socket, threading, time
 
def server():
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.bind(("127.0.0.1", 9000))
    while True:
        data, addr = s.recvfrom(2048)
        # 原样回显，并报告对方地址（注意端口）
        s.sendto(b"echo: " + data + b" (from " + str(addr[1]).encode() + b")", addr)
 
threading.Thread(target=server, daemon=True).start()
time.sleep(0.2)
 
cli = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
t0 = time.time()
cli.sendto(b"hello-udp", ("127.0.0.1", 9000))   # 没有任何握手，直接发
print(cli.recvfrom(2048)[0].decode())
print("首包往返耗时:", round((time.time() - t0) * 1000, 2), "ms —— 0 次握手")
 
# 对无人监听的端口发 UDP：不报错，数据静默消失
cli.sendto(b"into the void", ("127.0.0.1", 9999))
print("发往无人端口: sendto 正常返回，包丢了也没人告诉你")
'

python3 -c '
import socket, threading, time, random
 
random.seed(7)
received = {}
 
def server():
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.bind(("127.0.0.1", 9001))
    while True:
        data, addr = s.recvfrom(2048)
        seq, payload = data.split(b"|", 1)
        # 模拟 30% 丢包：假装没收到，不回 ACK
        if random.random() < 0.3:
            print("  [网络] 丢弃了包", seq.decode())
            continue
        received[int(seq)] = payload
        s.sendto(b"ACK|" + seq, addr)     # 收到就确认
 
threading.Thread(target=server, daemon=True).start()
time.sleep(0.2)
 
cli = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
cli.settimeout(0.3)                        # 超时时间（真实协议动态估算 RTT）
 
chunks = [b"GET /index", b".html HTTP", b"/3-style!"]
for seq, chunk in enumerate(chunks):
    attempt = 0
    while True:
        attempt += 1
        cli.sendto(str(seq).encode() + b"|" + chunk, ("127.0.0.1", 9001))
        try:
            ack = cli.recvfrom(2048)[0]
            print("包", seq, "第", attempt, "次发送后收到", ack.decode())
            break
        except socket.timeout:
            print("包", seq, "超时，重传...")
 
time.sleep(0.2)
print("服务端按序重组:", b"".join(received[i] for i in sorted(received)).decode())
'

HTTP/2 over TCP:  流A包丢 → TCP 卡住整条字节流 → 流B流C全部罚站
HTTP/3 over QUIC: 流A包丢 → 只有流A等重传   → 流B流C照常交付

首次连接:  1-RTT 后即可发 HTTP 请求（内置 TLS 1.3，加密不可选）
再次连接:  0-RTT —— 第一个 UDP 包里就带着加密的 HTTP 请求
           （与 TLS 1.3 0-RTT 同款重放风险，只许幂等请求）

python3 -c '
import socket, threading, time
 
# 服务器：按"连接ID"（载荷前8字节）识别会话，而不是按来源地址
sessions = {}
 
def server():
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.bind(("127.0.0.1", 9002))
    while True:
        data, addr = s.recvfrom(2048)
        cid, msg = data[:8], data[8:]
        if cid not in sessions:
            sessions[cid] = {"count": 0}
        sessions[cid]["count"] += 1
        reply = ("会话 " + cid.decode() + " 第 " + str(sessions[cid]["count"]) +
                 " 个包，来自端口 " + str(addr[1])).encode()
        s.sendto(reply, addr)
 
threading.Thread(target=server, daemon=True).start()
time.sleep(0.2)
 
CID = b"cid-7f3a"
 
# 客户端用第一个 socket（端口A）发两个包
c1 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
c1.sendto(CID + b"hello", ("127.0.0.1", 9002))
print(c1.recvfrom(2048)[0].decode())
c1.sendto(CID + b"again", ("127.0.0.1", 9002))
print(c1.recvfrom(2048)[0].decode())
 
# 模拟切换网络：换一个全新 socket（源端口变了 = 四元组变了）
c2 = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
c2.sendto(CID + b"after-network-switch", ("127.0.0.1", 9002))
print(c2.recvfrom(2048)[0].decode(), "<- 端口变了，会话计数却连续！")
'