Cisco FTDs and TCP Retransmissions

[IamSamSaul]

Hi Team, 

I have Cisco FTD installed which is routing traffic between two subnets on "inside" interface. When we are copying files between these two subnets through SMB2 protocol, I can see that it's going very slow. When I did a capture, I came to know that there were a lot of TCP Retransmissions in my packet capture. I have configured a Pre-Filter Policy, Trust the traffic in ACP and TCP Bypass but nothing is improving the situation. 

I was wondering if someone can have tips on how to tweak or have a logical explanation to this behavior. 

Any help or suggestion will be highly appreciated. 

Thanks & Regards,

Sam

[MHM Cisco World]

https://www.cisco.com/c/en/us/td/docs/security/firepower/623/configuration/guide/fpmc-config-guide-v623/interfaces_for_firepower_threat_defense.html#ID-2076-00000035

Do you try adjust the MSS of TCP in FTD ?

[Peter Koltl]

Give some metrics. 'Slow' and 'a lot of' is not data. What is the throughput in bit/s ? Watch I/O graphs in Wireshark. What is the ratio of retransmitted packets to all TCP packets? How many retransmissions per second?