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Computer Networks

Computer Networks. Marwan Al- Namari Week 6. TCP operation. TCP is a reliable, connection-oriented delivery service. The data is transmitted in segments.

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Computer Networks

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  1. Computer Networks Marwan Al-Namari Week 6

  2. TCP operation TCP is a reliable, connection-oriented delivery service. The data is transmitted in segments. Connection-oriented means that a connection must be established before hosts can exchange data. Reliability is achieved by assigning a sequence number to each segment transmitted. An acknowledgement is used to verify that the data was received by the other host. For each segment sent, the receiving host must return an acknowledgement (ACK) within a specified period for bytes received. If an ACK is not received, the data is retransmitted. TCP uses byte-stream communications, wherein data within the TCP segment is treated as a sequence of bytes with no record or field boundaries. Following table describes the key fields in the TCP header.

  3. Field Function Source Port TCP port of sending host. Destination Port TCP port of destination host. Sequence Number Sequence number of the first byte of data in the TCP segment. Acknowledgement Number Sequence number of the byte the sender expects to receive next from the other side of the connection. Window Current size of a TCP buffer on the host sending this TCP segment to store incoming segments. TCP Checksum Verifies the integrity of the TCP header and the TCP data. TCP header

  4. TCP 3-way handshake A TCP connection is initialized through a three-way handshake. The purpose of the three-way handshake is to synchronize the sequence number and acknowledgement numbers of both sides of the connection and exchange TCP Window sizes. The following steps outline the process:

  5. TCP handshake step 1 The client sends a TCP segment to the server with an initial Sequence Number for the connection and a Window size indicating the size of a buffer on the client to store incoming segments from the server.

  6. TCP handshake step 2 The server sends back a TCP segment containing its chosen initial Sequence Number, an acknowledgement of the client’s Sequence Number, and a Window size indicating the size of a buffer on the server to store incoming segments from the client.

  7. TCP handshake step 3 The client sends a TCP segment to the server containing an acknowledgement of the server’s Sequence Number. N.B. TCP uses a similar handshake process to end a connection. This guarantees that both hosts have finished transmitting and that all data was received.

  8. TCP Port Number Description 20 FTP (Data Channel) 21 FTP (Control Channel) 23 Telnet 80 HTTP used for the World Wide Web 139 NetBIOS session service Well Known TCP ports 1-1023 For a current list of TCP and UDP well known ports, see the Internet Assigned Numbers Authority (IANA)

  9. Netstat -n A useful TCP/IP diagnostic utility which shows the ports in use

  10. Active connections Active Connections Proto Local Address Foreign Address State TCP 195.44.210.137:1028 195.44.0.112:80 CLOSE_WAIT TCP 195.44.210.137:1029 195.44.0.112:80 CLOSE_WAIT TCP 195.44.210.137:1031 195.44.0.112:80 CLOSE_WAIT TCP 195.44.210.137:1032 195.44.0.112:80 CLOSE_WAIT TCP 195.44.210.137:1037 213.161.66.155:80 ESTABLISHED TCP 195.44.210.137:1040 213.86.246.80:80 TIME_WAIT TCP 195.44.210.137:1041 212.113.31.41:80 TIME_WAIT TCP 195.44.210.137:1042 212.113.31.41:80 SYN_SENT

  11. Netstat –a Will show the listening ports This is a good check to see if you have a Trojan programme running which opens a port as a ‘backdoor’ for hackers.

  12. Time_Wait When a TCP connection is closed, the socket pair is placed into a state known as TIME-WAIT This is so that a new connection does not use the same protocol, source IP address, destination IP address, source port, and destination port until enough time has passed to ensure that any segments that have been misrouted or delayed will not be delivered unexpectedly.

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