loader: tftp: Don't let tftp timeout

[manu]

Summary

When we load a kernel or module we open/close it a few times.
Since we're using the same port number each time and that we requested
the same file the ACK that we send are valid on the server side and the
server send us the file multiple times.
This makes tftp loading time very inconsistant due to the UDP "flood" that
we have to process.

MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG

Test Plan

Test done on one of my machine, rebooting between each tests :
x time-base
+ time-tftp-timeout
+-----------------------------------------------------------------------------------------------------------------------------+
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|+                             x xx    x   x x        x                              x  x                                    x|
|A                        |_________________M_____________A______________________________|                                    |
+-----------------------------------------------------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x  10           104           193           116         128.9     29.856881
+  10            75            75            75            75             0
Difference at 95.0% confidence
	-53.9 +/- 19.8367
	-41.8154% +/- 8.95418%
	(Student's t, pooled s = 21.112)

[tsoome]

stand/libsa/tftp.c

600–601

There is a small nit, however....

When we did read whole file:

192.168.150.227 -> beastie TFTP Read "/boot/loader.conf" (octet)

beastie -> 192.168.150.227 TFTP OACK: blksize: 1428 tsize: 15

192.168.150.227 -> beastie TFTP Ack block 0

beastie -> 192.168.150.227 TFTP Data block 1 (15 bytes) (last block)

192.168.150.227 -> beastie TFTP Ack block 1
192.168.150.227 -> beastie TFTP Error: not defined

beastie -> 192.168.150.227 ICMP Destination unreachable (UDP port 37446 unreachable)

That is, tftpd did close the session after ACK, we did send Error 0 and we will get ICMP packet. So the clean way would be to send Error 0 only when we havent acked last packet yet...

[manu]

Only send the last ack if we didn't do it prior.
This was the case for small file that fits in one block.