C-sharp uses the HTTP protocol to call the interface, and it is common for TCP to send SYN unsuccessfully with the first handshake. what is the possible reason?

topic description

Restsharp.dll is used under the IIS site to call the remote interface through the HTTP protocol, and it is often found that the call fails. System.Net returns a TCP error, code 10060.

by installing the Wireshark grab package, it is found that the first TCP handshake of Client connecting to the server failed to send SYN, resulting in multiple retransmissions, but still failed, and there were no subsequent HTTP requests.

wireshark grabs the package as shown in the figure:

< H2 > try a solution < / H2 >

what I think is that the network request failed, simply because the client failed to issue the network request successfully, or the server failed to respond. First of all, it is suspected that the client failed to send the request, so the other two tests are as follows:

the same function on another client, the use of the third-party API interface is normal, and the error does not occur. (I think this shows that the client code is correct)
write a separate interface calling program and run it on the server. The same problem is also found. It is the first time that TCP failed to send SYN.

questions

so in this case, is it caused by the client environment, or is it caused by the server"s failure to respond? Ask experienced friends for advice, thank you!