Re: Odd TCP problems with 2.2.x

From: Pete Wyckoff (
Date: Thu Mar 16 2000 - 21:21:13 EST said:
> It looks as if peace (my box) is not acking about 2/3 of the packets
> sent. I'm not Mr. TCP guru, so I'm not sure why that might be (or maybe I'm
> not reading the output correctly). The packets do appear to be complete, and
> show up with the -x option to tcpdump. Eventually transfers time out and
> (nc)ftp aborts.

Here's the starts of a couple of odd packets from ursula which may be

18:36:46.396656 >
  P 1:1449(1448) ack 1 win 32120 <eol>
  (DF) [tos 0x8] (ttl 44, id 256)
     4508 05dc 0100 4000 2c06 92cc 186c 0036
     cea3 ce02 0014 08e1 10b0 7605 49ef 13a1
     8018 7d78 a549 0000 0000 0000 0000 0000
     0000 0000 1495 702a 4a3d 4c85 f016 f8b4

18:36:49.395958 >
  P 1:1449(1448) ack 1 win 32120 <nop,nop,timestamp 88123447 9551512>
  (DF) [tos 0x8] (ttl 44, id 258)
     4508 05dc 0102 4000 2c06 92ca 186c 0036
     cea3 ce02 0014 08e1 10b0 7605 49ef 13a1
     8018 7d78 a41d 0000 0101 080a 0540 a837
     0091 be98 1495 702a 4a3d 4c85 f016 f8b4

The second is after ursula notices that it got no ack for the first.
They have the same data, but the first try at sending the segment did
not have the tcp timestamp options. Instead ursula (or somebody)
filled that area with a bunch of zeroes (option <eol>).

Ursula and peace did both negotiate sack and timestamp at the beginning
of the transfer, but then ursula doesn't send timestamp sometimes. When
it does, it gets them correct. Should it not be the case that a tcp
receiver should just accept the packet even if there's no timestamp?

Are you doing anything funky with filtering in your LAN? Can you also
take a look at /proc/net/snmp and see if the Tcp InErrs counter goes up
during such a session? I wonder if the checksum is off. My test ftp
to ursula did not exhibit the oddness you're seeing.

                -- Pete

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