Transport Layer Honolulu Community College Cisco Academy Training Center Semester 1 Version 2.1.1

Functions  Flow Control  Reliability

Protocols  T ransmission C ontrol P rotocol  U ser D atagram P rotocol

TCP  Connection-Oriented  Reliable

UDP  Connectionless  Unreliable

Port Numbers  TCP and UDP communicate with the upper layers through ‘ports’ which are distinguished from each other by ‘port numbers’. Port numbers less than 255 are for public applications. Port numbers 255 to 1023 are assigned to companies which use them in their applications. Port numbers 1024 and higher are unregulated.

Port Numbers

A Closer Look at TCP  In TCP communication, the source port is usually numbered greater than  The destination port depends on the application (telnet uses 23).  So you see that the source port distinguishes between different conversations.

Three-Way Handshake  This well-known process is used to synchronize a TCP connection.  Sequence numbers are exchanged between the source and destination.  This allows for lost data to be later recovered through retransmission.

Windowing

Sequence Numbers  Each TCP segment is numbered. When they arrive at the destination, they are reassembled according to the numbering.  If a sequence number is missing, the segment is retransmitted.  If a segment does not arrive in a certain time period, then the segment is retransmitted.

TCP Segment Format

More on UDP

More on UDP (cont.)  No windowing or acknowledgements.  Relies on applications to provide reliability.  Designed for applications which don’t need to reassemble sequences of segments.  UDP is used by TFTP, SNMP, NFS, and DNS.

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