We are offering Internet on VSATs, the VSAT backbone supports TCP spoofing. All the traffic from the remote VSATs land on F1/0 interface of 7206 router 12.2(7)c IOS, F1/1 has a connection to ISP backbone.
We are noticing that most of the traffic is contributed by TCP ACKs. Please suggest a way to reduce the TCP ACKs.
SITHUB#sh tcp stat
Rcvd: 29997 Total, 0 no port
0 checksum error, 0 bad offset, 0 too short
5993 packets (50171 bytes) in sequence
1 dup packets (1 bytes)
2 partially dup packets (2 bytes)
0 out-of-order packets (0 bytes)
0 packets (0 bytes) with data after window
0 packets after close
0 window probe packets, 0 window update packets
0 dup ack packets, 0 ack packets with unsend data
6782 ack packets (1003566 bytes)
Sent: 9004 Total, 0 urgent packets
307 control packets (including 0 retransmitted)
8447 data packets (1003564 bytes)
23 data packets (4515 bytes) retransmitted
0 data packets (0 bytes) fastretransmitted
226 ack only packets (169 delayed)
0 window probe packets, 1 window update packets
1 Connections initiated, 1 connections accepted, 2 connections established
The round trip delay is more than 1 sec ( 2 satellite hops to access internet).
Whenever a remote VSAT tries to access any URL, the server sends 2 full segments of data and requests for an ACK. The client at the remote VSAT send backs the ack. If the Window size agreed between server and client is 64K ( Win2K window size default) with 1500 MTU, then after 40 frames an ack is supposed to be received , but why after 2 segments an ACK is requested ?.
The received-ACKs from the remote VSATs are more ( seen at the central router). There are no much retransmissions.
No I have not tried tuning any timeout thresholds.
Thanks Rais. You are right the VSAT links are Asymmetric the send is very low compared to receive ( typically for http application). There are few duplicate packets but are very low.
I was mentioning of the default window size if somebody uses Win2k then it would be 64K ( we cannot change the window sizes to 1 Gb, because this requires a field visit ). Can we do someting at the router end ?.
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