問(wèn)題

大部分人都知道tcp的keepalive. 假設(shè)讀者知道keepalive會(huì)如何觸發(fā). 這篇文章想討論keepalive觸發(fā)后, 對(duì)socket使用者的影響.

keepalive設(shè)置

修改/etc/sysctl.conf

ubuntu# vim /etc/sysctl.conf

ubuntu# sysctl -p

fs.file-max = 131072

net.ipv4.tcp_keepalive_time = 10

net.ipv4.tcp_keepalive_intvl = 5

net.ipv4.tcp_keepalive_probes = 3

驗(yàn)證

ubuntu# sysctl -a | grep keepalive

net.ipv4.tcp_keepalive_intvl = 5

net.ipv4.tcp_keepalive_probes = 3

net.ipv4.tcp_keepalive_time = 10

tcp_server.py

import socket

import sys

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

server_address = ('localhost', 22345)

sock.bind(server_address)

sock.listen(1)

connection, client_address = sock.accept()

while True:

data = connection.recv(1024)

print("data", data)

tcp_client.py

import socket

import sys

import time

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

server_address = ('localhost', 22345)

sock.connect(server_address)

time.sleep(999999999)

可以看到, 因?yàn)閠cp_client開(kāi)啟了SO_KEEPALIVE, 所以tcp_client主動(dòng)往tcp_server發(fā)起KEEPALIVE探測(cè).

若tcp_server開(kāi)啟SO_KEEPALIVE, 則是tcp_server往tcp_client發(fā)送KEEPALIVE探測(cè).

如果tcp_server/tcp_client都開(kāi)啟KEEPALIVE, 則會(huì)雙向探測(cè).

對(duì)應(yīng)用層socket api的影響

準(zhǔn)備工作

為了模擬keepalive生效的情況, 用docker模擬斷網(wǎng)線(xiàn)的情況.

準(zhǔn)備好安裝有docker, python, vim, tcpdump的ubuntu鏡像, 創(chuàng)建好docker 網(wǎng)絡(luò).

跑起來(lái), 修改heartbeat設(shè)置.

ubuntu# sudo docker run -it \

--volume=//home/enjolras/code_repo/python/keepalive_test://home/enjolras/code_repo/python/keepalive_test \

--detach=true \

--name=tcp_server \

--privileged=true \

--network=multi-host-network \

ubuntu_with_python

08f89dcff3547bb15c7aed975dfa5a0821e4d0246d6d812e02fd1470f3cef6c3

ubuntu# sudo docker run -it \

--volume=//home/enjolras/code_repo/python/keepalive_test://home/enjolras/code_repo/python/keepalive_test \

--detach=true \

--name=tcp_client \

--privileged=true \

--network=multi-host-network \

ubuntu_with_python

對(duì)阻塞式send/recv的影響

tcp_server

import socket

import sys

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

server_address = ('0.0.0.0', 22345)

sock.bind(server_address)

sock.listen(1)

connection, client_address = sock.accept()

connection.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

data = connection.recv(1024)

print("data", data)

tcp_client

import socket

import sys

import time

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

server_address = ('tcp_server', 22345)

sock.connect(server_address)

time.sleep(999999999)

send/recv會(huì)以異常/錯(cuò)誤碼方式得知 heartbeat 檢測(cè)到的鏈接斷開(kāi).

可以看到, tcp_server/tcp_client互發(fā)心跳.

root@0b3f1ee81446:/# tcpdump -i any port 22345

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes

12:29:34.491239 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [S], seq 2347845399, win 28200, options [mss 1410,sackOK,TS val 951128354 ecr 0,nop,wscale 7], length 0

12:29:34.491279 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [S.], seq 1169988006, ack 2347845400, win 27960, options [mss 1410,sackOK,TS val 2298965862 ecr 951128354,nop,wscale 7], length 0

12:29:34.491299 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951128354 ecr 2298965862], length 0

12:29:44.666952 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2298976038 ecr 951128354], length 0

12:29:44.666969 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951138530 ecr 2298965862], length 0

12:29:44.666978 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2298976038 ecr 951128354], length 0

12:29:44.666987 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951138530 ecr 2298976038], length 0

12:29:54.907019 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2298986278 ecr 951138530], length 0

12:29:54.907054 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951148770 ecr 2298976038], length 0

12:29:54.907059 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951148770 ecr 2298976038], length 0

12:29:54.907062 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2298986278 ecr 951138530], length 0

將tcp_server/tcp_client斷網(wǎng).

ubuntu# docker network disconnect multi-host-network tcp_client

可以看到tcp_server在連續(xù)3個(gè)探測(cè)包沒(méi)有回復(fù)后, 往tcp_client發(fā)了一個(gè)RST.

12:31:47.547010 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951261408 ecr 2299088676], length 0

12:31:47.547019 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2299098916 ecr 951251168], length 0

12:31:47.547061 IP tcp_client.multi-host-network.57130 > 0b3f1ee81446.22345: Flags [.], ack 1, win 221, options [nop,nop,TS val 951261408 ecr 2299098916], length 0

12:31:57.787226 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2299109156 ecr 951261408], length 0

12:32:02.906612 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2299114276 ecr 951261408], length 0

12:32:08.026829 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [.], ack 1, win 219, options [nop,nop,TS val 2299119396 ecr 951261408], length 0

12:32:13.146776 IP 0b3f1ee81446.22345 > tcp_client.multi-host-network.57130: Flags [R.], seq 1, ack 1, win 219, options [nop,nop,TS val 2299124516 ecr 951261408], length 0

可以看到, 在心跳機(jī)制檢測(cè)到socket狀態(tài)異常后, 會(huì)通過(guò)異常/錯(cuò)誤碼等方式通知調(diào)用者.

3f1ee81446:/home/enjolras/code_repo/python/keepalive_test# python tcp_serv

Traceback (most recent call last):

File "tcp_server.py", line 11, in

data = connection.recv(1024)

socket.error: [Errno 110] Connection timed out

對(duì)select的影響

tcp_server

import socket

import sys

import select

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)

server_address = ('0.0.0.0', 22345)

sock.bind(server_address)

sock.listen(1)

connection, client_address = sock.accept()

connection.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)

readable, writable, exeptional = select.select([connection], [], [])

print("readable", readable, writable, exeptional)

data = connection.recv(1024)

print("data", data)

對(duì)套接字select返回可讀事件.

3f1ee81446:/home/enjolras/code_repo/python/keepalive_test# python tcp_serv

('readable', [], [], [])

Traceback (most recent call last):

File "tcp_server.py", line 14, in

data = connection.recv(1024)

socket.error: [Errno 110] Connection timed out

對(duì)epoll的影響

不做實(shí)驗(yàn), 應(yīng)該和select一致.

結(jié)論

heartbeat檢測(cè)到tcp鏈接斷開(kāi)后, 會(huì)以可讀事件方式通知應(yīng)用層. 若無(wú)tcp heartbeat, 也無(wú)應(yīng)用層heartbeat, 應(yīng)用層無(wú)法得知鏈接的真實(shí)狀態(tài).

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