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Subjectunnecessary retransmit from network stack
Date
i wrote a prototype for a nic driver for my theses some time
ago. when looking at the performance of my nic driver i could
not beleave how slow it is. i started to log tcp sequence
numbers and saw that the network stack asks my driver to
transmit the same tcp paket multiple times within less then
2ms. how can that happen? below you see what the drivers sees
when an ssh connection is set up on a PII 400MHz 2.4.7 kernel.

why does net_dev.hard_start_xmit get called multiple times
with the same tcp packet?


TxMit: transmit function of the nic driver
TxIrq: packet transmission reported by nic
RxIrq: packet reception reported by nic

cpu clock-tick | event | Tcp SeqNo | Tcp AckNo
---------------|-------|-----------|----------
0x186dce194bc8 | TxMit | 909902971 | 0
0x186dce1beb05 | TxIrq | 909902971 | 0
0x186dce1ef8f8 | RxIrq | 897283491 | 909902972
0x186dce1f8b03 | TxMit | 909902972 | 897283492
0x186dce220317 | TxIrq | 909902972 | 897283492
0x186dce3002e8 | RxIrq | 897283492 | 909902972
0x186dce308560 | TxMit | 909902972 | 897283515 <- dublicate
0x186dce330bab | TxIrq | 909902972 | 897283515
0x186dce3c09bf | TxMit | 909902972 | 897283515 <- dublicate
0x186dce3e9b6f | TxIrq | 909902972 | 897283515
0x186dce424310 | RxIrq | 897283515 | 909902994
0x186dce47bc44 | RxIrq | 897283515 | 909902994
0x186dcf094683 | TxMit | 909902994 | 897283791
0x186dcf0bcafb | TxIrq | 909902994 | 897283791


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