ccna syllabus pdf 2019

ccna syllabus pdf 2019.3. receivedSYN/ACK, the source device responds using the ACK data segment (which sets the ACK flag in the TCP header). This tells the target device that the source device received the SYN data segment it sent and that the session is now established. Exam Essentials TCP uses a three-step handshake to establish a reliable connection: SYN, SYN/ACK, and ACK. once the three-step handshake process is complete, data can be transmitted over the session. This service is called connection-oriented because it requires a connection to be established first. Don't forget that this service requires three handshakes before the device can start sending and receiving messages. Practical experienceAn attacker can use the three-handshake process to compromise a host: The attacker sends a large number of forged TCP SYN data segments to the victim. To perform the spoofing, the attacker modifies the source IP address to another address, which may or may not be valid. The host receiving these TCP SYNs considers them all new connection requests: adds the SYN to a local table, responds using TCPSYN/ACK, and waits for the third handshake - the ACK answer. The problem here is that the ACK answer for the forged session never arrives. Typically, after 30.60 seconds, the host will discover this problem and in turn remove the connection in a half-open state from its local table. However, this local table has a limited number of connections that can be stored, and after filling it up the host will reject new connection requests - either forged or legitimate. This is a problem with TCP that good firewalls and intrusion prevention/detection systems can deal with effectively.8.4.2 TCP Sequence Numbers and AcknowledgementsOne of the measures TCP takes to provide a reliable session between devices is the use of sequence numbers and acknowledgements. Each TCP data segment that is sentcontains a sequence number, which not only allows the target device to reorder data segments that arrive out of sequence, but also provides a way to verify that all data segments have been received. The target device uses an acknowledgement to respond to the source device, indicating that all data segments it sent were received.To provide a reliable session, TCP must go through a synchronization process - three handshakes. This process is described in more detail below covering sequence and acknowledgement numbers.The source device sends a synchronization data segment (with the SYN flag in the Code Bits (Codebits) field set). This data segment contains the initial sequence number and is referred to as the SYN data segment.2. receivedAfter the SYN data segment is received, the target device responds with its own data segment. This data segment contains the initial sequence number of the target device and the acknowledgement field contains the appropriate value indicating that the SYN data segment sent by the source device has been received. This data segment is referred to as the SYN/ACK data segment, and it sets the corresponding bit in the Code Bit field.