net: tcp retransmissions happens to fast

From: Dawid Stawiarski
Date: Tue Aug 06 2013 - 07:16:04 EST


during tracking problem with NFS shares, we've found something weard happening to TCP connection.
We're using Ubuntu 12.04 with kernel 3.8.0-27-generic. We've managed to capture network traffic on client side, when the problem starts. After sending the pcap to linux-nfs group, we were sugested to send here (the thead on linux-nfs: ).
Dump of traffic attached shows that:
0. operation SETATTR before problem starts works as charm (attached for reference)
1. retransmission happens very fast after sending 3 packets for WRITE operation
1a. linux is not using jumbo frame (3 packets of ~1K size instead of one)
2. linux ignores the ACK received after retransmission (actually it's cumulative ACK for the third packet)

netstats shows that data is in queue and the retransmissions are happening (however things start to work normally after NEW connection is established):
tcp 0 132902 ESTABLISHED - on (43.21/3/0)


Attachment: n28-part.pcap
Description: application/vnd.tcpdump.pcap