ccna systematic

ccna systematic.Upon receipt of the SYN/ACK data segment, the source device responds with an ACK data segment to acknowledge receipt of the SYN/ACK data segment. In this ACK data segment, the acknowledgement field is set accordingly to the serial number of the target device and also the corresponding bit in the code bit field.The following is a simple example of three handshakes covering sequence number and acknowledgement number. The source device sends a SYN data segment with a sequence number of 1. 2. The target device responds with a SYN/ACK data segment with a sequence number of 10 and an acknowledgment number of 2. 3. The source device responds with an ACK data segment with a sequence number of 2 and an acknowledgment number of 11.When the exam point acknowledges that the data segment has been received, the target device returns a data segment with an acknowledgement number that is 1 greater than the received sequence number.In step 2 of this example, the acknowledgment number provided by the target device is 1 greater than the sequence number provided by the source device, which tells the source device that the next data segment the target device expects to receive is data segment 2. In step 3, the source device sends a second data segment with an acknowledgment number field containing acknowledgment number 11 (1 greater than the sequence number in the SYN/ACK data segment sent by the target device), indicating that the data segment sent by the target device has been received. The data segment sent by the target device has been received.8.4.3 TCP Flow Control and Windowing TechniquesTCP supports control of the rate at which data segments are transmitted so that a host does not send too many segments and cause another host's receive buffer to overflow.TCP uses a floating window mechanism to control traffic. For example, if the window size is 1, the host must wait for an acknowledgement after sending a segment and receive it before sending the next segment; if the window size is 20, the host must wait for an acknowledgement that it has received 20 segments after sending them, and send 20 more segments after receiving the acknowledgement. Windowing techniques are discussed in Chapter 2.TCP uses the positive acknowledgment and retransmission (PAR) mechanism to recover lost data segments. If no acknowledgement is received from the target device, the data segment is continually retransmitted (with a certain interval between retransmissions) until an acknowledgement is received. The acknowledgement contains a sequence number which indicates that all data segments prior to that sequence number have been received, which avoids multiple acknowledgements and retransmissions of acknowledgements.The larger the window of a session, the fewer acknowledgements need to be sent, and therefore the more efficient the session is. A window that is too small affects throughput because the host sends a small number of data segments, then must wait for an acknowledgement, receive an acknowledgement, then send a small number of data segments, then wait for another acknowledgement. The trick is to figure out the best window size to maximize efficiency based on the current state of the network and the current capabilities of the two hosts.One feature of TCP windowing technology is the ability to dynamically adjust the window size over the lifetime of the session. This is important because hosts may create more sessions with varying bandwidth requirements. When too many data segments are received from too many sessions to handle, the host can assume that these sessions are all using TCP, and thus reduce the window to reduce the number of data segments received. In addition, the network between the source and destination devices may become congested, resulting in the loss of data segments; to counter this problem, the window can be reduced and then increased when the network is no longer congested to take advantage of the excess bandwidth in the network between the source and destination devices.Exam Essentials Narrowing the window improves reliability, but reduces throughput.