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Linux 2.4.22 loopback question

To: netdev@xxxxxxxxxxx
Subject: Linux 2.4.22 loopback question
From: Cheng Jin <chengjin@xxxxxxxxxxxxxx>
Date: Wed, 5 May 2004 14:53:41 -0700 (PDT)
Sender: netdev-bounce@xxxxxxxxxxx
Hi,

I am running into a strange problem with timestamping packets on loopback 
connections.

I calculate the RTT for each outgoing TCP packet by timestamping each skb 
after tcp_transmit_skb returns successfully, and timestamping the 
corresponding ack in tcp_ack (actually netif_rx timestamps incoming skbs 
so I use that whenever it is available).  However, I have noticed 
that sometimes, the timestamp for an ack comes before the timestamp for 
the trasnmitted skb that is being acked.

I have instrumented the code in net/core/dev.c, and it seems that indeed
the corresponding ack would sometimes be generated by the receiver, 
received and timestamped by netif_rx before tcp_transmit_skb returns.

I have a dual Xeon 2.66Ghz machine with Redhat 7.3 base install + 2.4.22
kernel (SMP enabled) from kernel.org.  The network driver is e1000 without 
NAPI.

Any ideas on why this happens?

Many thanks,

Cheng


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