ccna study guide pdf 2019

ccna study guide pdf 2019.device sends a list of T sequence numbers indicating which PDUs were not received, and the source device retransmits only those PDUs.However, most reliable protocols use _a simpler and less efficient approach: the target device sends the sequence number of the first PDU not received, and the source device retransmits that PDU and all subsequent PDUs.The larger the window, the more PDUs the source device can send at once, and thus the more efficient The larger the window, the more PDUs the source device can send at once, and therefore the more efficient it may be, but if the missing PDU is at the front of the entire batch of PDUs, it will result in a large number of unnecessary retransmissions.8.1.3 MultiplexingMultiplexing refers to the ability of a host to open multiple sessions with one or more other hosts at the same time. A session occurs when a connection is opened by sending one or more PDUs and receiving an answer from the target device (usually, but not always). A session may or may not be reliable; it may or may not also involve flow control. To support multiplexing, the transport layer protocol must be able to distinguish each session with each target host. Some protocols assign a number to the session, and this number, called a session number, is used to uniquely identify the session.TCP/IP uses a more sophisticated mechanism to achieve the same function.8.1.4 SegmentationSegmentation refers to the division of data into smaller, transport layer-identifiable PDUso In TCP/IP, the transport layer encapsulates application layer data into data segments to be sent to the target device. The target device is responsible for extracting the data in these data segments and forwarding it to the correct application. In the data segments, information must be included to help the target device with this forwarding process, such as which application should be given the encapsulated data for processing.Authentication Target 8.028.2 Transport Layer ProtocolsTCP/IP uses two transport layer protocols, TCP and UDP.The next two subsections discuss these protocols in depth and describe their characteristics and the segmentation methods used, including the layout of the data segment headers.8.2.1 Transmission Control ProtocolTCP uses a reliable transport system to transmit Layer 4 data segments to the destination device, similar to using the U.S. Postal System for Registered Mail, Priority Mail, or Next Day Delivery services. For registered mail, for example, the recipient must sign to certify that the letter has been delivered.TCP provides a similar safeguard: it can determine whether the target device has received the data segment that was sent. In the postal service, if a registered letter is lost, it will be the responsibility of the letter writer to resend it; but in TCP, the application does not have to worry about which data was received and which was not - TCP will take care of the entire tracking and retransmission of the lost data.