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10.5 TCP Characteristics
9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/10/pages/5 1/3 TCP Characteristics Section 10: Understanding TCP/IP Transport Layer TCP is a core protocol in the TCP/IP protocol suite. Applications leverage the connectionoriented services of TCP to provide data reliability between hosts. TCP includes a number of important features that provide for reliable data transmission. TCP can be characterized as follows: TCP operates at the transport layer (OSI Layer 4) of the TCP/IP stack. TCP provides application access to the network layer (OSI Layer 3), where application data is routed from the source IP host to the destination IP host. TCP is connectionoriented and requires that network devices set up a connection to exchange data. The end systems synchronize with one another to manage packet flows and adapt to congestion in the network. TCP provides error checking by including a checksum in the IP datagram to verify that the TCP header information is not corrupt. A TCP connection is a pair of virtual circuits, one in each direction, so it operates in fullduplex mode. 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/10/pages/5 2/3 TCP segments are numbered and sequenced so that the destination can reorder segments and determine if data is missing. Upon receipt of one or more TCP segments, the receiver returns an acknowledgment to the sender to indicate that it received the segment. Acknowledgments form the basis of reliability within the TCP session. When the source receives acknowledgment, it knows that the data has been successfully delivered. If the source does not receive acknowledgment within a predetermined period, it retransmits that data to the destination. The source may also terminate the connection if it determines that the receiver is no longer on the connection. TCP provides recovery services in which the receiver can request retransmission of a segment. TCP provides mechanisms for flow control. Flow control assists the reliability of TCP transmission by adjusting the effective rate of data flow between the two services in the session. Reliable data delivery services are critical for applications such as file transfers, database services, transaction processing, and other missioncritical applications in which delivery of every packet must be guaranteed. TCP segments are sent by using IP packets. The TCP header follows the IP header and supplies information that is specific to the TCP protocol. Flow control, reliability, and other TCP characteristics are achieved by using fields in the TCP header. Each field has a specific function. The fields of the TCP header include the following: Source Port: Number of the calling port (16 bits) Destination Port: Number of the called port (16 bits) Sequence Number and Acknowledgment Number: Used for reliability and congestion avoidance (each 32 bits) Checksum: Calculated checksum of the header and fields that are used for error checking (16 bits) Data: ULP data (varies in size) As shown in the table below, the TCP protocol defines many header fields. In the context of this training, you only need to concern yourself with the five fields in the list below. If you would like to read more about the function of TCP header fields, please use your favorite search engine to locate to RFC 793 (Transmission Control Protocol). 9/12/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/10/pages/5 3/3 TCP Characteristics 16Bit Source Port 16Bit Destination Port 32Bit Sequence Number 32Bit Acknowledgment Number 4Bit Header Length Reserved Flags 16Bit Window Size 16Bit TCP Checksum 16Bit Urgent Pointer Options Data Up Next: UDP Characteristics