5: TCP (Telnet) Established

This is the third of a series of 9 packets corresponding to the setup and clear-down of a TCP connection. In this case the connection was made to the telnet (terminal) port of the remote computer. The computer accepts the connection and produces a login prompt to the client. Instead of proceeding to login to the server, the client then decides to terminate the connection.

In the TCP header, the ACK bit is set to complete the synchronisation exchange at the start of a TCP connection. The ACK bit confirms that the connection has been accepted, and is sent in response to the TCP Connection Acceptance. Once received, the connection is established and may transfer data.


Hexadecimal Dump of the Packet

           0: 00e0 f726 3fe9 0800 2086 354b 0800 4500    ..÷&?... .5K..E.
          16: 0028 08b9 4000 ff06 999a 8b85 d96e 8b85    .(..@........n..
          32: e902 9005 0017 7214 f115 9431 1028 5010    ......r....1.(P.
          48: 2238 1c80 0000                             "8....


Decode of the Packet

ETHER:  ----- Ether Header -----
ETHER:
ETHER:  Packet 5 arrived at 17:37:23.94
ETHER:  Packet size = 54 bytes
ETHER:  Destination = 0:e0:f7:26:3f:e9, CISCO Router
ETHER:  Source      = 8:0:20:86:35:4b, Sun
ETHER:  Ethertype = 0800 (IP)
ETHER:
IP:   ----- IP Header -----
IP:
IP:   Version = 4
IP:   Header length = 20 bytes
IP:   Type of service = 0x00 (normal)
IP:   Total length = 40 bytes
IP:   Identification = 2233
IP:   Flags = 0x4
IP:         .1.. .... = do not fragment
IP:         ..0. .... = last fragment
IP:   Fragment offset = 0 bytes
IP:   Time to live = 255 seconds/hops
IP:   Protocol = 6 (TCP)
IP:   Header checksum = 999a
IP:   Source address = 139.133.217.110, client
IP:   Destination address = 139.133.233.2, server.abdn.ac.uk
IP:   No options
IP:
TCP:  ----- TCP Header -----
TCP:
TCP:  Source port = 36869
TCP:  Destination port = 23 (TELNET)
TCP:  Sequence number = 1913975061
TCP:  Acknowledgement number = 2486243368
TCP:  Data offset = 20 bytes
TCP:  Flags = 0x10
TCP:        ..0. .... = No urgent pointer
TCP:        ...1 .... = Acknowledgement
TCP:        .... 0... = No push
TCP:        .... .0.. = No reset
TCP:        .... ..0. = No Syn
TCP:        .... ...0 = No Fin
TCP:  Window = 8760
TCP:  Checksum = 0x1c80
TCP:  Urgent pointer = 0
TCP:  No options


SYN-ACK exchange at the start of a TCP session.

The packet header shown is the last (third) packet of the sequence containing the ACK.


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