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SubjectSlow PPP - caused by duplicate packets?
My PPP link has been suffering the dreaded slowness, particularaly if I talk to
the Solaris box which is the main server at the remote end. My ppp server is a
NetBSD/sun3 machine; the remote modem is on a cisco terminal server from which
I rlogin to the netbsd box. I use an mtu of 1500.

Looking at the modem traffic while using 'more' on a large file with a 80x60
xterm, I noticed that packets seemed to be still coming down the link after I
had got to the end of one page, but before a started the next one. It would
only happen for a second or so after the end of the page.

So I grabbed tcpdump and had a look at the packets, and there seem to be
duplicate packets on my link! At the end of this mail, you will find the
tcpdump output for one screenful of text. staff is the remote server machine,
running Solaris 2.5. Fuzzbox is the local Linux box, running 1.3.79.

There are two large packets being sent from staff to fuzzbox. The first one has
a size of 1451, and covers data sequnce numbers 229386484:229387935. Fuzzbox
sends back an ack for it. Staff then sends the same packet again, and fuzzbox
then sends back two more acks. Staff then send the next packet, of length 1186
covering data sequnce numbers 229387935:229389121. Fuzzbox acks this. staff
sends it again; fuzzbox aks for it twice. staff sends it yet again, fuzzbox
again acks for it twice, and then the link goes idle. Of course me screen gets
updated with the second packet the first time it is sent; the modem activity
after the screen redraw is presumably the duplicate packets at the end of the
message.

I don't know a great deal about tcp, but I am sure this shouldn't be happening!

David

fuzzbox:~ # /usr/local/sbin/tcpdump -S -i ppp0
tcpdump: listening on ppp0
12:05:38.477293 fuzzbox.1023 > staff.login: P 1267692566:1267692567(1) ack
229386462 win 29417 (DF) [tos 0x10]
12:05:38.747298 staff.login > fuzzbox.1023: P 229386462:229386484(22) ack
1267692567 win 8760 (DF)
12:05:39.057303 fuzzbox.1023 > staff.login: . ack 229386484 win 29395 [tos
0x10]
12:05:41.087341 staff.login > fuzzbox.1023: P 229386484:229387935(1451) ack
1267692567 win 8760 (DF)
12:05:41.387346 fuzzbox.1023 > staff.login: . ack 229387935 win 29404 [tos
0x10]
12:05:42.627369 staff.login > fuzzbox.1023: . 229386484:229387935(1451) ack
1267692567 win 8760 (DF)
12:05:42.627369 fuzzbox.1023 > staff.login: . ack 229387935 win 29404 [tos
0x10]
12:05:42.927375 fuzzbox.1023 > staff.login: . ack 229387935 win 29404 [tos
0x10]
12:05:43.867392 staff.login > fuzzbox.1023: P 229387935:229389121(1186) ack
1267692567 win 8760 (DF)
12:05:44.167397 fuzzbox.1023 > staff.login: . ack 229389121 win 29678 [tos
0x10]
12:05:45.147416 staff.login > fuzzbox.1023: P 229387935:229389121(1186) ack
1267692567 win 8760 (DF)
12:05:45.147416 fuzzbox.1023 > staff.login: . ack 229389121 win 29678 [tos
0x10]
12:05:45.447421 fuzzbox.1023 > staff.login: . ack 229389121 win 29678 [tos
0x10]
12:05:46.417439 staff.login > fuzzbox.1023: P 229387935:229389121(1186) ack
1267692567 win 8760 (DF)
12:05:46.417439 fuzzbox.1023 > staff.login: . ack 229389121 win 29678 [tos
0x10]
12:05:46.717445 fuzzbox.1023 > staff.login: . ack 229389121 win 29678 [tos
0x10]
16 packets received by filter
0 packets dropped by kernel
fuzzbox:~ #




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