Re: Interlan drops a byte?


Steven D. Miller (steve@umiacs.UMD.EDU)
Wed, 24 Aug 88 07:23:31 EDT


   The problem here is not in your Interlan card; I've had the same problem
with my Sun-3/60. I did a packet spy once, and it seems that under certain
circumstances, expo seems to send the FIN with a sequence number that is one
too low.

   The spy I made of the problem is enclosed below, so that others more
knowledgeable in the ways of TCP may check my reasoning. I think expo is
running vanilla SunOS 3.4 TCP, but I'm by no means certain of that...

        -Steve

Spoken: Steve Miller Domain: steve@mimsy.umd.edu UUCP: uunet!mimsy!steve
Phone: +1-301-454-1808 USPS: UMIACS, Univ. of Maryland, College Park, MD 20742

pktnum 2483, timestamp 577459160 sec 410000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 228, id 95c5, off 0,
    ttl 18, protocol 6, sum e, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d52972,108d4e01
    data off 5, flags=10<ACK> window 1000, sum 45a2, urgent 0
    TCP data length 512 (0x200) bytes

pktnum 2484, timestamp 577459160 sec 410000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6f9, off 0,
    ttl 1e, protocol 6, sum dad9, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d52f72
    data off 5, flags=10<ACK> window 2000, sum f076, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2485, timestamp 577459160 sec 530000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 228, id 95c6, off 0,
    ttl 18, protocol 6, sum d, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d52b72,108d4e01
    data off 5, flags=10<ACK> window 1000, sum 81b0, urgent 0
    TCP data length 512 (0x200) bytes

pktnum 2486, timestamp 577459160 sec 530000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6fa, off 0,
    ttl 1e, protocol 6, sum dad8, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d52f72
    data off 5, flags=10<ACK> window 2000, sum f076, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2487, timestamp 577459160 sec 810000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 228, id 95c7, off 0,
    ttl 18, protocol 6, sum c, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d52d72,108d4e01
    data off 5, flags=10<ACK> window 1000, sum 96d3, urgent 0
    TCP data length 512 (0x200) bytes

[expo sent 512 bytes after seq 1d52d72 ]

pktnum 2488, timestamp 577459160 sec 810000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6fb, off 0,
    ttl 1e, protocol 6, sum dad7, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d52f72
    data off 5, flags=10<ACK> window 2000, sum f076, urgent 0
    TCP data length 0 (0x0) bytes

[fnord acks that]

pktnum 2489, timestamp 577459160 sec 930000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 1b8, id 95c8, off 0,
    ttl 18, protocol 6, sum 7b, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d52f71,108d4e01
    data off 5, flags=19<FIN,PUSH,ACK> window 1000, sum 2a8, urgent 0
    TCP data length 400 (0x190) bytes

[expo, whose sequence number was 1d52f72, now sends a FIN with the sequence
number one too low.]

pktnum 2490, timestamp 577459160 sec 930000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6fc, off 0,
    ttl 1e, protocol 6, sum dad6, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d53102
    data off 5, flags=10<ACK> window 1e71, sum f075, urgent 0
    TCP data length 0 (0x0) bytes

[fnord acks that]

pktnum 2491, timestamp 577459161 sec 70000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6fe, off 0,
    ttl 1e, protocol 6, sum dad4, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d53102
    data off 5, flags=11<FIN,ACK> window 2000, sum eee5, urgent 0
    TCP data length 0 (0x0) bytes

[fnord sends its fin, with seq # 108d4e02]

pktnum 2492, timestamp 577459161 sec 230000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 28, id 95cb, off 0,
    ttl 18, protocol 6, sum 208, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d53102,108d4e01
    data off 5, flags=11<FIN,ACK> window 1000, sum fee5, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2493, timestamp 577459161 sec 250000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b6ff, off 0,
    ttl 1e, protocol 6, sum dad3, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e01,1d53103
    data off 5, flags=11<FIN,ACK> window 2000, sum eee4, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2494, timestamp 577459161 sec 270000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 28, id 95cc, off 0,
    ttl 18, protocol 6, sum 207, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d53102,108d4e02
    data off 5, flags=11<FIN,ACK> window 1000, sum fee4, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2495, timestamp 577459161 sec 410000 usec, len 54
Ethernet level: dst host 08:00:20:00:6f:74,
    src host 02:07:01:00:8a:17, type 800
IP header: version 4, header len 5,
    service 0, len 28, id 95cd, off 0,
    ttl 18, protocol 6, sum 206, src 121e00d4,
    dst 80087803
TCP header: source port 14, dst port 477, <seq,ack> 1d53103,108d4e02
    data off 5, flags=10<ACK> window 1000, sum fee4, urgent 0
    TCP data length 0 (0x0) bytes

pktnum 2496, timestamp 577459161 sec 410000 usec, len 54
Ethernet level: dst host 02:07:01:00:8a:17,
    src host 08:00:20:00:6f:74, type 800
IP header: version 4, header len 5,
    service 0, len 28, id b700, off 0,
    ttl 1e, protocol 6, sum dad2, src 80087803,
    dst 121e00d4
TCP header: source port 477, dst port 14, <seq,ack> 108d4e02,0
    data off 5, flags=4<RST> window 0, sum 41c9, urgent 0
    TCP data length 0 (0x0) bytes



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