No SYN-ACK Packet from server

I have two servers and I used my own embedded system with LwIP to do connection to these server.

My embedded system with LwIP is the client and I have server1 and server2. I connected to server1 and end the connection before connecting to server2.

Further breakdown on the flow:

1. Client creates New Socket with server1
2. Client sent DNS packet to obtain server1's ip address; received ACK from AP
3. Client send TCP SYN packet;
4. Server1 send TCP SYN-ACK and perform some data transmission
5. Client ends connection with server1 by sending TCP RST packet; and close socket
6. Client creates New Socket with server2
7. Client sent DNS packet to obtain server2's ip address; received ACK from AP
8. Client send TCP SYN packet to server2
9. Server2 send TCP SYN-ACK and perform some data transmission
10. Client ends connection with server2 by sending TCP RST packet; and close socket

However, sometimes server2 did not response to client's SYN Packet which is in Step 9. Its only happens sometime. I checked several forum like:

[1] Why would a server not send a SYN/ACK packet in response to a SYN packet

[2] Server not sending a SYN/ACK packet in response to a SYN packet

My code does not enable window scaling. I cannot check the server as its a private server, so I am not very sure if it was dropped. My environment is quite noisy and busy with many routers plus communication devices. This problem only happens in noisy environment but not in a cleaner environment.

What can I do as a client to fix this problem?

Somes ideas on why sometimes your server doesn't answer with SYN-ACK :

• Application on the server is stopped when the SYN packet is sent : This can be a crash and then manage to work again with a auto restart mecanism of the program. When the server app is stopped or crashed, the tcp listening socket is closed, so the OS doesn't answer.
• You server has some troubles to find your client back. The can be some routing issue or asymetric routing (this break firewall).
• If the Client and server are on the same network, this can be some issue with Layer 2 issue like Spanning Tree protocol or ARP.
• Packet loss in the network : You SYN or SYN ACK packet is dropped.
• The server is overwhelm and sometime cannot answer you SYN packet.

There can be a large number of thing that can make server doesn't answer SYN packet.

What I will do :

• Doing some tcpdump on the server to verify the network : Does the server receive the SYN packet or there is an issue in the network beforehand ? Does the server generate and send a SYN ACK packet ? And work from there.
• If you don't have access to the server, doing the same thing on the closest router/firewall.
• Contact someone that has access on the server.