Delayed Acknowledgement side effect

Hi,

?Sporadically, an unidirectional test decides to slow down on TCP/IP. I have the packet traces around that period:
"
780779:26:06 PM 1/21/2014631.0230520large_buffer.exe10.0.0.210.0.0.1TCPTCP:Flags=…A…, SrcPort=5432, DstPort=49273, PayloadLen=0, Seq=2001370213, Ack=129742246, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780789:26:06 PM 1/21/2014631.0230967large_buffer.exe10.0.0.110.0.0.2TCPTCP:[Continuation to #76429]Flags=…A…, SrcPort=49273, DstPort=5432, PayloadLen=65482, Seq=129742246 - 129807728, Ack=2001370213, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780799:26:06 PM 1/21/2014631.2230417large_buffer.exe10.0.0.210.0.0.1TCPTCP:Flags=…A…, SrcPort=5432, DstPort=49273, PayloadLen=0, Seq=2001370213, Ack=129807728, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780809:26:06 PM 1/21/2014631.2230870large_buffer.exe10.0.0.110.0.0.2TCPTCP:[Continuation to #76429]Flags=…A…, SrcPort=49273, DstPort=5432, PayloadLen=65482, Seq=129807728 - 129873210, Ack=2001370213, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780819:26:06 PM 1/21/2014631.4230115large_buffer.exe10.0.0.210.0.0.1TCPTCP:Flags=…A…, SrcPort=5432, DstPort=49273, PayloadLen=0, Seq=2001370213, Ack=129873210, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780829:26:06 PM 1/21/2014631.4230362large_buffer.exe10.0.0.110.0.0.2TCPTCP:[Continuation to #76429]Flags=…A…, SrcPort=49273, DstPort=5432, PayloadLen=65482, Seq=129873210 - 129938692, Ack=2001370213, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780839:26:07 PM 1/21/2014631.6229867large_buffer.exe10.0.0.210.0.0.1TCPTCP:Flags=…A…, SrcPort=5432, DstPort=49273, PayloadLen=0, Seq=2001370213, Ack=129938692, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780849:26:07 PM 1/21/2014631.6230132large_buffer.exe10.0.0.110.0.0.2TCPTCP:[Continuation to #76429]Flags=…A…, SrcPort=49273, DstPort=5432, PayloadLen=65482, Seq=129938692 - 130004174, Ack=2001370213, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780859:26:07 PM 1/21/2014631.8230488large_buffer.exe10.0.0.210.0.0.1TCPTCP:Flags=…A…, SrcPort=5432, DstPort=49273, PayloadLen=0, Seq=2001370213, Ack=130004174, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
780869:26:07 PM 1/21/2014631.8231004large_buffer.exe10.0.0.110.0.0.2TCPTCP:[Continuation to #76429]Flags=…A…, SrcPort=49273, DstPort=5432, PayloadLen=65482, Seq=130004174 - 130069656, Ack=2001370213, Win=65482 (scale factor 0x0) = 65482{TCP:84, IPv4:1}
"
? Why is the RSS going down to 65482 and the send() operation is just as big, when there is no other traffic on the interface? Is it possible that the interface (NDIS) driver reports an invalid throughput and consequently the TCP/IP scales down?

? During normal operation, every 2 packets are acknowledged. In the above case, the ACK follows after 200 ms from the send().

? Thank you for taking the time to look into this issue.

Calin