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Figure 1710 example of three way handshakes 9

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Figure: 17.10 Example of Three-Way HandShakes 9. Describe the TCP Header Format. (20marks) Solution TCP uses only a single type of protocol data unit called a TCP segment. One header must serve to perform al protocol mechanisms with a minimum length of 20 octets. The fields are as follows: ± Source port (16 bits): source TCP user. ± Destination port (16 bits): destination TCP user. ± Sequence number (32 bits): sequence number of the first data octet in this segment except when SYN flag is set. If SYN is set, it is the initial sequence number (ISN) and the first data octet is ISN+1. ± Acknowledgment number (32 bits): a piggybacked acknowledgement. Contains the sequence number of the next data octet that TCP entity expects to receive. ± Data offset (4 bits): number of 32-bit words in the header. ± Reserved (6 bits): reserved for future use. ± Flags (6bits): URG : urgent pointer field significant ACK : acknowledgment field significant PSH : push function RST : reset the connection SYN : synchronize the sequence numbers FLN : no more data from sender ± Window (16bits): flow control credit allocation, in octets. Contains the number of data octets beginning with the one indicated in the acknowledgement field that the sender is willing to accept. ± Checksum (16 bits): the ones complements of the sum modulo 216 -1 of all the 16 –bit words in the segment plus a pseudo header. (Or)
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± Checksum (16 bits): this fields contains the result of a cyclic redundancy check (CRC) computation performed by the transmitting system, and is used by the receiving system to detect errors in the UDP header, data and the part of IP header. ± Urgent Pointer (16 bits): points to the last octet in a sequence of urgent data. This allows the receiver to know how much urgent data are coming. ± Options (Variable): an example is the option that specifies the maximum segment size that will be accepted. The sequence number and acknowledgement number are bound to octets rather than to entire segments. The checksum field applies to the entire segment plus a pseudo header prefixed to the header at the time of calculation. Figure 17.11: TCP Header 10. Write down the TCP mechanisms. (20 marks) Solution TCP Mechanisms TCP mechanisms can be categorized as connection establishment, data transfer and connection termination. Connection Establishment Connection establishment in TCP uses a three-way handshake. When the SYN flag is set, the segment is essentially a request for connection and functions. To initiate a connection, an entity sends a SYN, SN = X, where X is the initial sequence number. The receiver responds with SYN, SN = Y, AN = X + 1 by setting both SYN and ACK flags. Acknowledgement indicates that the receiver is now expecting to receive a segment beginning with data octet X + 1, acknowledging SYN, which occupied SN = X. The initiator responds with AN = Y +1.
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Figure 1710 Example of Three Way HandShakes 9 Describe the...

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