Practical Industrial Data Networks:Design, Installation and ...

16.3.2 UDPTCP/IP overview 271data with a sequence number of ISN+1. The ISN is a pseudo random number with valuesbetween 0 and 2 32 .The acknowledgments number (32 bits) is used to verify correct receipt of thetransmitted data. The receiver checks the incoming data and if the verification is positive,acknowledges it by placing the number of the next byte expected in the acknowledgmentsnumber field and setting the ACK flag. The sender, when transmitting, sets a timer and ifacknowledgments are not received within a specific time, an error is assumed and the datais retransmitted.Data offset (4 bits) is the number of 32-bit words in the TCP header. This indicateswhere the data begins. It is necessary since the header can contain options and thus doesnot have a fixed length.Six flags control the connection and data transfer. They are:• URG: Urgent pointer field significant• ACK: Acknowledgments field significant• PSH: Push function• RST: Reset the connection• SYN: Synchronize sequence numbers• FIN: No more data from senderThe window field (16 bits) provides flow control. Whenever a host sends anacknowledgment to the other party in the bi-directional communication, it also sends awindow advertisement by placing a number in the window field. The window sizeindicates the number of bytes, starting with the one in the acknowledgments field, that thehost is able accept.The checksum field (16 bits) is used for error controlThe urgent pointer field (16 bits) is used in conjunction with the URG flag and allows forthe insertion of a block of ‘urgent’ data in the beginning of a particular segment. Thepointer points to the first byte of the non-urgent data following the urgent data.User datagram protocol (UDP) is a ‘connectionless’ protocol and does not require aconnection to be established between two machines prior to data transmission. It istherefore said to be ‘unreliable’ – the word ‘unreliable’ used here as opposed to ‘reliable’in the case of TCP and should not be interpreted against its everyday context.Sending a UDP datagram involves very little overhead in that there are nosynchronization parameters, no priority options, no sequence numbers, no timers, and noretransmission of packets. The header is small, the protocol is streamlined functionally.The only major drawback is that delivery is not guaranteed. UDP is therefore used forcommunications that involve broadcasts, for general network announcements, or for realtimedata.The UDP headerThe UDP header is significantly smaller than the TCP header and only contains fourfields.