netstat命令大家肯定已经很熟悉了，但是在2001年的时候netstat 1.42版本之后就没更新了，之后取代的工具是ss命令，是iproute2 package的一员。
rpm -ql iproute | grep ss
/usr/sbin/ss
netstat的替代工具是nstat，当然netstat的大部分功能ss也可以替代
ss可以显示跟netstat类似的信息，但是速度却比netstat快很多，netstat是基于/proc/net/tcp获取 TCP socket 的相关统计信息，用strace跟踪一下netstat查询tcp的连接，会看到他open的是/proc/net/tcp的信息。ss快的秘密就在于它利用的是TCP协议的tcp_diag模块，而且是从内核直接读取信息，当内核不支持 tcp_diag 内核模块时，会回退到 /proc/net/tcp 模式。
/proc/net/snmp 存放的是系统启动以来的累加值，netstat -s 读取它
/proc/net/tcp 是存放目前活跃的tcp连接的统计值，连接断开统计值清空， ss -it 读取它
ss 查看Buffer窗口
ss参数说明权威参考
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The entire print format of ss -m is given in the source:
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–memory/-m ： 展示buffer窗口的大小
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tb指可分配的发送buffer大小，不够还可以动态调整（应用没有写死的话），w[The memory allocated for sending packet (which has not been sent to layer 3)]已经预分配好了的size，t[the memory used for sending packet (which has been sent to layer 3)] , 似乎 w总是等于大于t？
example:
sysctl -a | grep mem
对172.16.210.17和172.16.160.1之间的带宽限速50MB后观察(带宽限制后，发送buffer就很容易被撑满了）：
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推荐 -m -i 一起查看状态，比如 rcv_space 表示buffer达到过的最大水位
rcv_space is the high water mark of the rate of the local application reading from the receive buffer during any RTT. This is used internally within the kernel to adjust sk_rcvbuf.
ss 查看拥塞窗口、RTO
//rto的定义，不让修改，每个ip的rt都不一样，必须通过rtt计算所得, HZ 一般是1秒
#define TCP_RTO_MAX ((unsigned)(120*HZ))
#define TCP_RTO_MIN ((unsigned)(HZ/5)) //在rt很小的环境中计算下来RTO基本等于TCP_RTO_MIN
下面看到的rto和rtt单位都是毫秒，一般rto最小为200ms、最大为120秒
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RTO计算算法
RTO的计算依赖于RTT值，或者说一系列RTT值。rto=f(rtt)
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RTTVAR表示的是平滑过的平均偏差，SRTT表示的平滑过的RTT。这两个值的具体含义会在后面介绍
具体实现的时候进一步的解释。
以上是计算一个初始RTO值的过程，当连续出现RTO超时后，
RTO值会用一个叫做指数回避的策略进行调整，下面来具体介绍。
从系统cache中查看 tcp_metrics item
$sudo ip tcp_metrics show | grep 100.118.58.7
100.118.58.7 age 1457674.290sec tw_ts 3195267888/5752641sec ago rtt 1000us rttvar 1000us ssthresh 361 cwnd 40 ----这两个值对传输性能很重要
192.168.1.100 age 1051050.859sec ssthresh 4 cwnd 2 rtt 4805us rttvar 4805us source 192.168.0.174 ---这条记录有问题，缓存的ssthresh 4 cwnd 2都太小，传输速度一定慢
清除 tcp_metrics, sudo ip tcp_metrics flush all
关闭 tcp_metrics 功能，net.ipv4.tcp_no_metrics_save = 1
sudo ip tcp_metrics delete 100.118.58.7
每个连接的ssthresh默认是个无穷大的值，但是内核会cache对端ip上次的ssthresh（大部分时候两个ip之间的拥塞窗口大小不会变），这样大概率到达ssthresh之后就基本拥塞了，然后进入cwnd的慢增长阶段。
ss 过滤地址和端口号，类似tcpdump的用法
过滤目标端口是80的或者源端口是1723的连接，dst后面要跟空格然后加“：”：
# ss -ant dst :80 or src :1723
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 3 *:1723 *:*
TIME-WAIT 0 0 172.31.23.95:37269 111.161.68.235:80
TIME-WAIT 0 0 172.31.23.95:37263 111.161.68.235:80
TIME-WAIT 0 0 172.31.23.95:37267
or：
ss -ant dport = :80 or sport = :1723
地址筛选，目标地址是111.161.68.235的连接
ss -ant dst 111.161.68.235
端口大小筛选，源端口大于1024的端口：
ss sport gt 1024
How Do I Compare Local and/or Remote Port To A Number?
Use the following syntax:
## Compares remote port to a number ##
ss dport OP PORT
## Compares local port to a number ##
sport OP PORT
Where OP can be one of the following:
<= or le : Less than or equal to port
>= or ge : Greater than or equal to port
== or eq : Equal to port
!= or ne : Not equal to port
< or gt : Less than to port
> or lt : Greater than to port
Note: le, gt, eq, ne etc. are use in unix shell and are accepted as well.
###################################################################################
### Do not forget to escape special characters when typing them in command line ###
###################################################################################
ss sport = :http
ss dport = :http
ss dport \> :1024
ss sport \> :1024
ss sport \< :32000
ss sport eq :22
ss dport != :22
ss state connected sport = :http
ss \( sport = :http or sport = :https \)
ss -o state fin-wait-1 \( sport = :http or sport = :https \) dst 192.168.1/24
ss 查看 timer 状态
ss -atonp
按连接状态过滤
Display All Established HTTP Connections
ss -o state established '( dport = :http or sport = :http )'
List all the TCP sockets in state -FIN-WAIT-1 for our httpd to network 202.54.1/24 and look at their timers:
ss -o state fin-wait-1 ‘( sport = :http or sport = :https )’ dst 202.54.1/24
Filter Sockets Using TCP States
ss -4 state FILTER-NAME-HERE
Where FILTER-NAME-HERE can be any one of the following,
established
syn-sent
syn-recv
fin-wait-1
fin-wait-2
time-wait
closed
close-wait
last-ack
listen
closing
all : All of the above states
connected : All the states except for listen and closed
synchronized : All the connected states except for syn-sent
bucket : Show states, which are maintained as minisockets, i.e. time-wait and syn-recv.
big : Opposite to bucket state.
ss分析重传的包数量
通过抓取ss命令，可以分析出来重传的包数量，然后将重传的流的数量和重传的包的数量按照对端IP:port的维度分段聚合，参考命令：
ss -itn |grep -v "Address:Port" | xargs -L 1 | grep retrans | awk '{gsub("retrans:.*/", "",$21); print $5, $21}' | awk '{arr[$1]+=$2} END {for (i in arr) {print i,arr[i]}}' | sort -rnk 2
xargs -L 1 每一行处理一次，但是这个行如果是空格、tab结尾，那么会被认为是连续行，跟下一行合并
高版本Linux内核的话，可以用systemtap或者bcc来获取每个连接的重传包以及发生重传的阶段
当前和最大全连接队列确认
$ss -lt
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 127.0.0.1:10248 *:*
LISTEN 0 128 *:2376 *:*
LISTEN 0 128 127.0.0.1:10249 *:*
LISTEN 0 128 *:7337 *:*
LISTEN 0 128 *:10250 *:*
LISTEN 0 128 11.163.187.44:7946 *:*
LISTEN 0 128 127.0.0.1:55631 *:*
LISTEN 0 128 *:10256 *:*
LISTEN 0 10 *:6640 *:*
LISTEN 0 128 127.0.0.1:vmware-fdm *:*
LISTEN 0 128 11.163.187.44:vmware-fdm *:*
LISTEN 0 128 *:ssh *:*
LISTEN 0 10 127.0.0.1:15772 *:*
LISTEN 0 10 127.0.0.1:15776 *:*
LISTEN 0 10 127.0.0.1:19777 *:*
LISTEN 0 10 11.163.187.44:15778 *:*
LISTEN 0 128 *:tr-rsrb-p2 *:*
ss -s
统计所有连接的状态
nstat
nstat -z -t 1 类似 netstat -s (ss –info 展示rto、拥塞算法等更详细信息； netstat -ant -o 展示keepalive是否)
netstat参考
knetstat
最后给出的一个工具，knetstat（需要单独安装），也可以查看tcp的状态下的各种参数，需要单独安装
example(3306是本地server，4192是后端MySQL）：
Recv-Q Send-Q Local Address Foreign Address Stat Diag Options
0 0 0.0.0.0:3306 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 0.0.0.0:3406 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 127.0.0.1:8182 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 10.0.186.73:8182 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 0.0.0.0:22 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 0.0.0.0:8188 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 0 127.0.0.1:15778 0.0.0.0:* LSTN SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=0,TCP_NODELAY=0,TCP_FASTOPEN=0,TCP_DEFER_ACCEPT=0
0 138 10.0.186.73:51756 10.0.160.1:4192 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 0 10.0.186.73:3306 10.0.186.70:37428 ESTB SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVBUF=32768,SO_SNDBUF=65536,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 138 10.0.186.73:51476 10.0.160.1:4192 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 0 10.0.186.73:3306 10.0.186.70:37304 ESTB SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVBUF=32768,SO_SNDBUF=65536,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 0 10.0.186.73:51842 10.0.160.1:4192 ESTB SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
44 0 10.0.186.73:3306 10.0.186.70:36238 ESTB SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVBUF=32768,SO_SNDBUF=65536,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
44 0 10.0.186.73:3306 10.0.186.70:36160 ESTB SO_REUSEADDR=1,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVBUF=32768,SO_SNDBUF=65536,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 0 10.0.186.73:19030 10.0.171.188:8000 TIMW
3306对应的client上：
Recv-Q Send-Q Local Address Foreign Address Stat Diag Options
0 44 10.0.186.70:42428 10.0.186.73:3306 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVTIMEO=31536000000ms,SO_SNDTIMEO=31536000000ms,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 44 10.0.186.70:42298 10.0.186.73:3306 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVTIMEO=31536000000ms,SO_SNDTIMEO=31536000000ms,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 44 10.0.186.70:42296 10.0.186.73:3306 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVTIMEO=31536000000ms,SO_SNDTIMEO=31536000000ms,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
0 44 10.0.186.70:42322 10.0.186.73:3306 ESTB ># SO_REUSEADDR=0,SO_REUSEPORT=0,SO_KEEPALIVE=1,SO_RCVTIMEO=31536000000ms,SO_SNDTIMEO=31536000000ms,TCP_NODELAY=1,TCP_DEFER_ACCEPT=0
Diag列的说明
Indicator Meaning
>| The sender window (i.e. the window advertised by the remote endpoint) is 0. No data can be sent to the peer.
>|< The receiver window (i.e. the window advertised by the local endpoint) is 0. No data can be received from the peer.
>
># There are unacknowledged packets and the last ACK was received more than one second ago. This may be an indication that there are network problems or that the peer crashed.
参考文章
https://www.cyberciti.biz/tips/linux-investigate-sockets-network-connections.html
http://perthcharles.github.io/2015/11/10/wiki-netstat-proc/
源代码：https://github.com/sivasankariit/iproute2/blob/master/misc/ss.c
https://github.com/veithen/knetstat/tree/master
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