TCP 拥塞控制限制发送端向容量有限、可能被共享的网络注入数据。拥塞窗口约束未确认数据量，采用 pacing 时还会控制发送在时间上的分布。接收端通告窗口施加的是另一类流量控制限制。这些机制相互作用，却描述不同约束，不能笼统合并成一个“发送速度设置”，否则很难判断真正限制当前连接的是哪一部分。
坏掉的样子 两端不需要选择完全相同的拥塞控制算法。每端负责控制自己发送的流量，因此 Reno 客户端能够与 CUBIC 服务器通信。TCP 建立过程协商传输选项，没有要求双方发送算法名称一致。把不同控制器名称诊断成协议不匹配，会分散注意力，掩盖真正的路由、缓冲或应用故障。算法选择需要按方向和具体套接字理解。
算法是否可用，也是需要核对的运行时事实。配置字符串写了某个控制器，不证明内核支持它或允许调用进程选择它。Linux 的 TCP_CONGESTION 按套接字设置，受可用和允许算法约束。应检查调用结果，并回读实际名称，不能默认请求标签已生效。选择失败与选择成功后性能不合适，属于不同问题，需要不同处理。
丢失恢复不存在适用于所有情况的统一曲线。重传超时、快速恢复、显式拥塞通知和基于模型的估计，会因控制器不同而产生不同响应。丢失可能来自拥塞，也可能来自其他原因，发送端仍须按算法规则作出反应。增加缓冲并不能自动修复持续丢包或等待确认的连接，也不能替代对具体恢复状态和触发信号的检查。
快速验证 将脚本保存为 check-congestion-control.py，准备 Docker 后运行 python3 check-congestion-control.py。一次性环境使用 Python 3.11.16、iproute2 6.1.0 和 nftables 1.0.6，在 Linux 6.12.76 上验证。NET_ADMIN 与 SYS_ADMIN 支持嵌套命名空间。新建 veth 两个方向出口各增加十毫秒延迟，不修改宿主接口或生产流量：
from pathlib import Path from tempfile import TemporaryDirectory import json,subprocess,uuid recipe='FROM python@sha256:528257d48c1da0dcecc2e725d1ae34498d60c965f1241e39cd6a85a8859bdf84\nRUN apt-get update && apt-get install -y --no-install-recommends iproute2=6.1.0-3 iputils-ping=3:20221126-1+deb12u1 nftables=1.0.6-2+deb12u2 tcpdump=4.99.3-1 traceroute=1:2.1.2-1 procps=2:4.0.2-3 && rm -rf /var/lib/apt/lists/*\nCMD ["python3"]\n' program=r""" from contextlib import ExitStack,contextmanager from pathlib import Path from tempfile import TemporaryDirectory import json,os,select,signal,subprocess,time def run(*args,check=True,input=None,timeout=8): result=subprocess.run(args,input=input,capture_output=True,text=True,timeout=timeout) if check:assert result.returncode==0,(args,result.returncode,result.stdout,result.stderr) return result def ns(name,*args,**kwargs):return run('ip','netns','exec',name,*args,**kwargs) def ip(name,*args,**kwargs):return ns(name,'ip',*args,**kwargs) def stop(process): if process.poll() is None: process.terminate() try:process.wait(timeout=2) except subprocess.TimeoutExpired:process.kill();process.wait(timeout=2) def line(process,timeout=3): assert select.select([process.stdout],[],[],timeout)[0],'child response deadline exceeded' value=process.stdout.readline();assert value,('child exited without a response',process.stderr.read()) return value def spawn(stack,command): process=subprocess.Popen(command,stdin=subprocess.PIPE,stdout=subprocess.PIPE,stderr=subprocess.PIPE,text=True) stack.callback(stop,process);return process @contextmanager def lab(names): with TemporaryDirectory(prefix='net-path-') as directory,ExitStack() as stack: root=Path(directory);proc=root/'proc';proc.mkdir();run('mount','-t','proc','proc',str(proc)) stack.callback(run,'umount',str(proc)) for name in names: run('ip','netns','add',name);stack.callback(run,'ip','netns','delete',name,check=False) ip(name,'link','set','lo','up') def setting(name,key,value): assert name in names path=proc/'sys'/key.replace('.','/') return ns(name,'python3','-c',f'from pathlib import Path; p=Path({str(path)!r}); p.write_text({str(value)!r}); print(p.read_text().strip())').stdout.strip() yield root,stack,setting def wire(left,left_if,right,right_if,left_addresses,right_addresses): run('ip','link','add',left_if,'type','veth','peer','name',right_if) for name,interface,addresses in [(left,left_if,left_addresses),(right,right_if,right_addresses)]: run('ip','link','set',interface,'netns',name) for address in addresses:ip(name,'address','add',address,'dev',interface,*(['nodad'] if ':' in address else [])) ip(name,'link','set',interface,'up') def nft(name,script):return ns(name,'nft','-f','-',input=script) def counters(name): data=json.loads(ns(name,'nft','-j','list','ruleset').stdout);found={} for entry in data['nftables']: rule=entry.get('rule',{}) if 'comment' in rule: for expression in rule['expr']: if 'counter' in expression:found[rule['comment']]=expression['counter']['packets'] return found info_code=r''' import struct def tcp_info(stream): raw=stream.getsockopt(socket.IPPROTO_TCP,socket.TCP_INFO,104);assert len(raw)==104 offsets={'snd_mss':16,'unacked':24,'rtt_us':68,'snd_ssthresh':76,'snd_cwnd':80,'total_retrans':100} return {'ca_state':raw[1],**{key:struct.unpack_from('=I',raw,offset)[0] for key,offset in offsets.items()}} ''' with lab(['client','target']) as (root,stack,setting): wire('client','c0','target','t0',['192.0.2.1/30'],['192.0.2.2/30']) available=ns('client','sysctl','-n','net.ipv4.tcp_available_congestion_control').stdout.split();assert {'reno','cubic'}<=set(available),available for name,interface in [('client','c0'),('target','t0')]: ns(name,'tc','qdisc','add','dev',interface,'root','netem','delay','10ms','limit','10000') server_code=r''' import socket,json,hashlib listener=socket.socket();listener.setsockopt(socket.IPPROTO_TCP,socket.TCP_CONGESTION,b'cubic') listener.bind(('192.0.2.2',18080));listener.listen();print('listening',flush=True) with listener.accept()[0] as connection: connection.settimeout(8) print(json.dumps({'algorithm':connection.getsockopt(socket.IPPROTO_TCP,socket.TCP_CONGESTION,16).split(b'\0')[0].decode()}),flush=True) for size in [65536,16384]: data=b'' while len(data)<size: chunk=connection.recv(size-len(data));assert chunk;data+=chunk assert data==b'x'*size connection.sendall((hashlib.sha256(data).hexdigest()+'\n').encode()) ''' service=spawn(stack,['ip','netns','exec','target','python3','-u','-c',server_code]);assert line(service).strip()=='listening' client_code=info_code+r''' import socket,json,sys,time,hashlib with socket.socket() as unsupported: try:unsupported.setsockopt(socket.IPPROTO_TCP,socket.TCP_CONGESTION,b'xbcatm_absent');raise AssertionError('unexpected available algorithm') except OSError as error:selection_failure={'errno':error.errno,'type':type(error).__name__} assert selection_failure['errno']==2,selection_failure stream=socket.socket();stream.settimeout(8);stream.setsockopt(socket.IPPROTO_TCP,socket.TCP_CONGESTION,b'reno') stream.bind(('192.0.2.1',40600));stream.connect(('192.0.2.2',18080)) with stream.makefile('rb') as incoming: stream.sendall(b'x'*65536);assert incoming.readline().decode().strip()==hashlib.sha256(b'x'*65536).hexdigest() baseline=tcp_info(stream);assert baseline['snd_cwnd']>1 algorithm=stream.getsockopt(socket.IPPROTO_TCP,socket.TCP_CONGESTION,16).split(b'\0')[0].decode() print(json.dumps({'baseline':baseline,'algorithm':algorithm,'unavailable_algorithm':selection_failure}),flush=True) assert sys.stdin.readline().strip()=='drop';stream.sendall(b'x'*16384) deadline=time.monotonic()+4;samples=[] while time.monotonic()<deadline: state=tcp_info(stream);samples.append(state) if state['total_retrans']>baseline['total_retrans'] and state['snd_cwnd']==1 and state['ca_state']==4:break time.sleep(.01) else:raise AssertionError(('no timeout recovery observed',samples)) print(json.dumps({'during_blackout':state}),flush=True) assert sys.stdin.readline().strip()=='restore' assert incoming.readline().decode().strip()==hashlib.sha256(b'x'*16384).hexdigest() print(json.dumps({'after_restore':tcp_info(stream),'verified_bytes':65536+16384}),flush=True) stream.close() ''' client=spawn(stack,['ip','netns','exec','client','python3','-u','-c',client_code]) baseline=json.loads(line(client));server_result=json.loads(line(service));assert baseline['algorithm']=='reno' and server_result['algorithm']=='cubic' nft('target','''table ip blackout { chain input { type filter hook input priority 0; policy accept; tcp sport 40600 tcp dport 18080 ip length > 60 counter drop comment "data_dropped" } } ''') client.stdin.write('drop\n');client.stdin.flush();loss=json.loads(line(client,5)) count=counters('target');assert count['data_dropped']>0 nft('target','delete table ip blackout');client.stdin.write('restore\n');client.stdin.flush() recovery=json.loads(line(client,8));client.wait(timeout=2);service.wait(timeout=2);assert client.returncode==service.returncode==0 print(json.dumps({'available':available,'client':baseline,'server':server_result,'loss':loss,'drop_counter':count,'recovery':recovery})) """ name='xbcatm-tcpflow-'+uuid.uuid4().hex[:10] def docker(*args,**kwargs):return subprocess.run(['docker',*args],capture_output=True,text=True,timeout=300,**kwargs) try: with TemporaryDirectory(prefix=name+'-') as directory: root=Path(directory);(root/'Dockerfile').write_text(recipe) build=docker('build','-t',name,str(root));assert build.returncode==0,build.stderr result=docker('run','--rm','-i','--name',name,'--network','none','--cap-add','NET_ADMIN','--cap-add','SYS_ADMIN', '--memory','256m','--pids-limit','128',name,'python3','-',input=program) assert result.returncode==0,result.stderr print(json.dumps(json.loads(result.stdout),sort_keys=True)) finally: docker('rm','-f',name);docker('image','rm',name) 脚本首先验证 Reno 与 CUBIC 可用。刻意选择一个不存在的控制器会失败，返回 ENOENT，错误号二，Python 表现为 FileNotFoundError。这个异常指控制器选择不可用，不是应用数据文件丢失。真实客户端明确选择 Reno，服务器监听套接字选择 CUBIC，接受后的套接字报告继承了该选择，分别验证配置意图和最终生效状态。
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