DLSW peer drops on a Frame Relay link (T1) and tcp segment size messages.

[brooks.donald]

We have a dlsw connection executing on a 2612 router (IOS 12.2(10b)) connected via Frame Relay T1 to a 7206 router (IOS 12.1(19)). The dlsw peer drops after receiving numerous messages :

(Jul 13 18:20:04.786 edt: TCP0: bad seg from 172.28.24.1 -- outside window: seq 2795121948 ack 307888035 rcvnxt 2795127740 rcvwnd 20480 len 1448).

We opened a case on this problem and the TAC had us issue a ip tcp mss 1460 parameter in our router. We continue to see the messages. We had the Frame Relay provider check the netwqork and all seems well. We have not received any errors on this connection. Why would we receive these messages and why would this drop our peer? Please help.

[dixho]

The error message simple means that there is a TCP retransmission. One of the example is as follows. Image DLSw_A sends a TCP segment to DLSw_B. DLSw_B receives the TCP segment and sends a TCP acknowledgement to DLSw_A. If the acknowledgemnt is dropped in the network, DLSw_A will retransmit the TCP segment because DLSw_A thinks that DLSw_B does not receive the TCP segment. When DLSw_B receive the retransmitted segment, it finds out that it has already acknowledged this segment. From the message, you find the value of rcvnxt (2795127740) is greater than seq (2795121948).

The debug message cannot tell why the DLSw peer drops. Please take a sniffer trace on the frame relay circuit.

[brooks.donald]

Thanks so much for the feedback. This was the same conclusion we came up with but thought I would check and see if we were missing something.

Thanks again.