Interesting Traces - Hung STCP VOS 14.6.0 server/Connection Backlog exceeded

The following trace was taken using packet_monitor on a VOS 14.6.0 system running the STCP stack. It shows what it looks like when connecting to a hung server application. In this case the server is listening on port 2080 and has a listen backlog of 5. The server is hung so it is no longer making calls to accept.

What you are seeing is that the first 6 connection attempts are ignored. The STCP stack will add the connection attempts to the backlog queue but will not respond to them until they are accepted by the server. If the server never accepts them the client will eventually time the connection attempt out, i.e. will stop sending SYN request packets. If the server later becomes unhung and issues an accept, the accept will complete without error but the SYN-ACK packet that is sent back after the accept is done gets a reset response so any atempt to use the newly acepted socket will generate an error (this is not in the trace)

The 7th connection attempt exceeds the backlog queue and is responded to immediately with a reset.

I have color coded the packets of each connection attempt so you can keep them straight.

>system>stcp>command_library>packet_monitor -numeric -time_stamp -verbose -pkt_h
+dr -filter -host 172.16.1.103
                dir                                                 icmp type
+        tcp
hh:mm:ss.ttt dir   len proto source             destination         src port  ds
+t port  type
 8:18:16.474 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f07, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  306e, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2443 to 172.16.1.203.2080
    seq  29cc9baa, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 55ba,  Urg-> 0000

 8:18:19.689 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f1f, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  3056, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2443 to 172.16.1.203.2080
    seq  29cc9baa, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 55ba,  Urg-> 0000

 8:18:21.909 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f3a, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  303b, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2444 to 172.16.1.203.2080
    seq  29e089e7, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 6768,  Urg-> 0000

 8:18:25.159 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f56, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  301f, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2444 to 172.16.1.203.2080
    seq  29e089e7, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 6768,  Urg-> 0000

 8:18:25.711 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f59, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  301c, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2447 to 172.16.1.203.2080
    seq  29ef9ca0, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 549d,  Urg-> 0000

 8:18:26.252 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f5d, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  3018, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2443 to 172.16.1.203.2080
    seq  29cc9baa, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 55ba,  Urg-> 0000

 8:18:28.986 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f68, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  300d, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2447 to 172.16.1.203.2080
    seq  29ef9ca0, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 549d,  Urg-> 0000

 8:18:29.596 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f71, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  3004, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2448 to 172.16.1.203.2080
    seq  29fdefc6, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 0168,  Urg-> 0000

 8:18:31.720 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f7d, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2ff8, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2444 to 172.16.1.203.2080
    seq  29e089e7, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 6768,  Urg-> 0000

 8:18:32.814 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f88, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fed, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2448 to 172.16.1.203.2080
    seq  29fdefc6, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 0168,  Urg-> 0000

 8:18:33.566 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f92, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fe3, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2451 to 172.16.1.203.2080
    seq  2a0e2a3b, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum c6df,  Urg-> 0000

 8:18:35.549 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6f9e, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fd7, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2447 to 172.16.1.203.2080
    seq  29ef9ca0, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 549d,  Urg-> 0000

 8:18:36.752 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fa3, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fd2, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2451 to 172.16.1.203.2080
    seq  2a0e2a3b, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum c6df,  Urg-> 0000

 8:18:37.573 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fbd, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fb8, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2452 to 172.16.1.203.2080
    seq  2a1cf10f, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum fffb,  Urg-> 0000

 8:18:39.377 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fc9, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fac, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2448 to 172.16.1.203.2080
    seq  29fdefc6, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum 0168,  Urg-> 0000

 8:18:40.799 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fd4, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2fa1, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2452 to 172.16.1.203.2080
    seq  2a1cf10f, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum fffb,  Urg-> 0000

 8:18:41.503 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fec, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2f89, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2455 to 172.16.1.203.2080
    seq  2a2d4aa4, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum a653,  Urg-> 0000

 8:18:41.503 Xmit Ether Dst 00:03:47:dd:a7:52  Src 00:10:32:10:03:df Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   28, ID 4084, Flg/Frg    0, TTL 3c,  Prtl  6
          Cksum  e2f9, Src ac1001cb, Dst ac100167
TCP from 172.16.1.203.2080 to 172.16.1.103.2455
    seq         0, ack 2a2d4aa5, window n.a., 0. data bytes, flags Rst Ack.
    X/Off 05, Flags 14, Cksum cdf4,  Urg-> 0000

 8:18:42.002 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6fef, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2f86, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2455 to 172.16.1.203.2080
    seq  2a2d4aa4, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum a653,  Urg-> 0000

 8:18:42.003 Xmit Ether Dst 00:03:47:dd:a7:52  Src 00:10:32:10:03:df Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   28, ID 4085, Flg/Frg    0, TTL 3c,  Prtl  6
          Cksum  e2f8, Src ac1001cb, Dst ac100167
TCP from 172.16.1.203.2080 to 172.16.1.103.2455
    seq         0, ack 2a2d4aa5, window n.a., 0. data bytes, flags Rst Ack.
    X/Off 05, Flags 14, Cksum cdf4,  Urg-> 0000

 8:18:42.549 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6ff1, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2f84, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2455 to 172.16.1.203.2080
    seq  2a2d4aa4, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum a653,  Urg-> 0000

 8:18:42.550 Xmit Ether Dst 00:03:47:dd:a7:52  Src 00:10:32:10:03:df Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   28, ID 4086, Flg/Frg    0, TTL 3c,  Prtl  6
          Cksum  e2f7, Src ac1001cb, Dst ac100167
TCP from 172.16.1.203.2080 to 172.16.1.103.2455
    seq         0, ack 2a2d4aa5, window n.a., 0. data bytes, flags Rst Ack.
    X/Off 05, Flags 14, Cksum cdf4,  Urg-> 0000

 8:18:43.315 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 6ff7, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2f7e, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2451 to 172.16.1.203.2080
    seq  2a0e2a3b, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum c6df,  Urg-> 0000

 8:18:47.361 Rcvd Ether Dst 00:10:32:10:03:df  Src 00:03:47:dd:a7:52 Type 0800
+(IP)
IP   Ver/HL 45, ToS  0, Len   30, ID 7010, Flg/Frg 4000, TTL 80,  Prtl  6
          Cksum  2f65, Src ac100167, Dst ac1001cb
TCP from 172.16.1.103.2452 to 172.16.1.203.2080
    seq  2a1cf10f, ack     n.a., window faf0, 8. data bytes, flags Syn.
    X/Off 07, Flags 02, Cksum fffb,  Urg-> 0000

This page was last modified on 03-06-16

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