anchao commented on pull request #1589:
URL: https://github.com/apache/incubator-nuttx/pull/1589#issuecomment-674139608
No, they are completely different problems, PHY notification does not change
the behavior of the TCP/IP stack. The stack does not have any interface to
determine whether the network is disconnected unexpectedly,
If the wireless network is disconnected after the application completes a
socket send(2), then the TCP stack will keep retrying fetch for the ACK, the
connection state is remain in TCP_ESTABLISHED
```
void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
int hsec)
{
...
if (conn->tcpstateflags == TCP_TIME_WAIT ||
conn->tcpstateflags == TCP_FIN_WAIT_2) // skip, state remain
TCP_ESTABLISHED
{
...
else if (conn->tcpstateflags != TCP_CLOSED)
{
...
if (conn->tx_unacked > 0) // still waiting for the ack but the
network is unreachable
{
...
if (conn->tcpstateflags == TCP_SYN_RCVD && // skip, state
remain TCP_ESTABLISHED
conn->nrtx >= TCP_MAXSYNRTX)
{
...
else if (
#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
conn->expired > 0 ||
#else
conn->nrtx >= TCP_MAXRTX ||
#endif
(conn->tcpstateflags == TCP_SYN_SENT && // skip, state
remain TCP_ESTABLISHED
conn->nrtx >= TCP_MAXSYNRTX)
)
{
....
```
In the process of waiting for ACK, network disconnection will lead to
tcp_timer keeps trying to get the ACK, If the application close(2) the TCP
connection, the close(2) call will blocked because the TCP stack does not know
that the network is unreachable
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