tcp timer异常导致重传时间异常变大 · Pull Request !1 · openvela/nuttx

概要：tcp发生重传之后，tcp重传时间异常
1. 在TCP连接发生重传之后，tcp backoff 算法更新了conn->timer
2. 在重传恢复之后，Jacobson / Karels 算法，其中RTTest M = conn->rto - conn->timer为负，导致conn->sa为负值并翻转，从而使conn->rto异常变大
测试：
在tcp_input拦截ack包，制造丢包，并打印Jacobson / Karels 算法后的conn->rto
```
diff --git a/net/tcp/tcp_input.c b/net/tcp/tcp_input.c
index 70b99f345b..b812ef3ff5 100644
--- a/net/tcp/tcp_input.c
+++ b/net/tcp/tcp_input.c
@@ -61,6 +61,7 @@
 #include "devif/devif.h"
 #include "utils/utils.h"
 #include "tcp/tcp.h"
@@ -742,6 +743,13 @@ static void tcp_input(FAR struct net_driver_s *dev, uint8_t domain,
   /* Demultiplex this segment. First check any active connections. */
 
   conn = tcp_active(dev, tcp);
+  if (((tcp->flags & TCP_CTL) == (TCP_SYN | TCP_ACK)) && conn->drop_count < 2)
+    {
+      conn->drop_count++;
+      printf("WARNING: ************* Drop first sync ack packet \n");
+      goto drop;
+    }
+
   if (conn)
     {
       /* We found an active connection.. Check for the subsequent SYN
@@ -1135,6 +1143,8 @@ found:
           m = m - (conn->sv >> 2);
           conn->sv += m;
           conn->rto = (conn->sa >> 3) + conn->sv;
+          printf("WARNING:### conn=%p conn->rto=%d m=%d conn->timer=%d conn->sv=%d conn->sa=%d ###\n",
+                 conn, (int)conn->rto, (int)m,(int)conn->timer,(int)conn->sv,(int)conn->sa);
         }
```
合入patch前测试结果：
```
nsh> iperf -c 10.192.41.28 -t 5 -i 1
     IP: 192.168.31.92
 mode=tcp-client sip=192.168.31.92:5001,dip=10.192.41.28:5001, interval=1, time=5
*****tcp_timer 0
WARNING: ************* Drop first sync ack packet 
WARNING: ************* Drop first sync ack packet 
*****tcp_timer 3
WARNING:### conn=0x3fc8d8a0 conn->rto=46 m=-1 conn->timer=6 conn->sv=15 conn->sa=253 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=70 m=28 conn->timer=46 conn->sv=43 conn->sa=222 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=84 m=17 conn->timer=70 conn->sv=60 conn->sa=195 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=90 m=9 conn->timer=84 conn->sv=69 conn->sa=171 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=91 m=4 conn->timer=90 conn->sv=73 conn->sa=150 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=89 m=0 conn->timer=91 conn->sv=73 conn->sa=132 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=85 m=-2 conn->timer=89 conn->sv=71 conn->sa=116 ###
WARNING:### conn=0x3fc8d8a0 conn->rto=80 m=-3 conn->timer=85 conn->sv=68 conn->sa=102 ###
```
合入patch后测试结果
```
nsh> iperf -c 10.192.41.28 -t 10 -i 1
     IP: 192.168.31.92
 mode=tcp-client sip=192.168.31.92:5001,dip=10.192.41.28:5001, interval=1, time=10
*****tcp_timer 0
WARNING: ************* Drop first sync ack packet 
WARNING: ************* Drop first sync ack packet 
*****tcp_timer 3
WARNING:### conn=0x3fc8e010 conn->rto=12 m=-4 conn->timer=6 conn->sv=12 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=9 m=-3 conn->timer=12 conn->sv=9 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=7 m=-2 conn->timer=9 conn->sv=7 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=6 m=-1 conn->timer=7 conn->sv=6 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=5 m=-1 conn->timer=6 conn->sv=5 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=4 m=-1 conn->timer=5 conn->sv=4 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=-1 conn->timer=4 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
WARNING:### conn=0x3fc8e010 conn->rto=3 m=0 conn->timer=3 conn->sv=3 conn->sa=0 ###
```

类型	指派人员	状态
审查	openvela-robot	进行中 (0/1人)
测试	openvela-robot	进行中 (0/1人)

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