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.
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....
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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