Meissi-jian opened a new pull request, #15118:
URL: https://github.com/apache/nuttx/pull/15118
when tcp retransmission only double conn->timer in Karn(tcp_timer.c L602),
after retransmission in Jacobson M is is now rtt test will become a negative
value.
```
signed char m;
m = conn->rto - conn->timer; // M is now rtt test
/* This is taken directly from VJs original code in his paper */
m = m - (conn->sa >> 3);
conn->sa += m; //conn->sa is a negative value
if (m < 0)
{
m = -m;
}
m = m - (conn->sv >> 2);
conn->sv += m;
conn->rto = (conn->sa >> 3) + conn->sv; //rto
```
For example,we lost one ack packet, we will set conn->timer = 6 by
backoff，conn->rto still 3. After retransmission we will Do RTT estimation, then
will get
```
conn->sa = 253
conn->rto = 46
```
Then if any packets lost it will wait for a long time before triggering a
retransmission.
## Summary
- The issue of abnormal increase in RTO (retransmission timeout) after TCP
retransmission has been fixed.
- In the Karn algorithm, when TCP retransmits, only conn->timer is doubled,
which may cause the estimated RTT value to become negative.
- The update logic of conn->rto and conn->timer has been corrected to ensure
that they remain consistent after retransmission.
- This issue was reproduced by discarding ACK packets in tcp_input and the
fix was verified by debugging conn->rto after the Jacobson algorithm.
## Impact
Impacts the implementation of tcp timer, fixing tcp timer abnoramlly large
after do backoff.
## Testing
Test method on reproduce.
We can reproduce this issue by adding drop ACK packets in tcp_input, and
debug conn->rto after Jacobson.
```
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);
}
```
test before patch with debug version:
```
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 ###
```
test by the patch:
```
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 ###
```
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