HPFOCUS

Understanding the TCP/IP Transport Layer

Transport Layer

Reliable vs. Best-Effort Comparison

The purpose of the transport layer is to hide the network requirements from the application layer.
Connection-oriented transport provides reliable transport; connectionless transport provides best-effort transport.

UDP is a protocol that operates at the transport layer and provides applications with access to the network layer without the overhead of the reliability mechanisms of TCP. UDP is a connectionless, best-effort delivery protocol.

UDP Characteristics

- Operates at transport layer of OSI and TCP/IP models
- Provides applications with access to the network layer without the overhead of reliability mechanisms
- Is a connectionless protocol
- Provides limited error checking
- Provides best-effort delivery
- Has no data-recovery features

UDP Header
TCP is a protocol that operates at the transport layer and provides applications with access to the network layer. TCP is connection-oriented, provides error checking, delivers data reliably, operates in full-duplex mode, and provides some data recovery functions.


TCP Characteristics

- Transport layer of the TCP/IP stack
- Access to the network layer for applications
- Connection-oriented protocol
- Full-duplex mode operation
- Error checking
- Sequencing of data packets
- Acknowledgement of receipt
- Data-recovery features

TCP Header

TCP/IP supports a number of applications, including FTP (supports bidirectional binary and ASCII file transfers), TFTP (transfers configuration files and Cisco IOS images), and Telnet (provides capability to remotely access another computer).
IP uses a protocol number in the datagram header to identify which protocol to use for a particular datagram.
Port numbers are used to map Layer 4 to an application.
Flow control avoids the problem of a transmitting host overflowing the buffers in the receiving host and slowing network performance.
TCP provides sequencing of segments with a forward reference acknowledgment. When a single segment is sent, receipt is acknowledged and the next segment is then sent.
The TCP window size decreases the transmission rate to a level at which congestion and data loss do not occur. The TCP window size allows a specified number of unacknowledged segments to be sent.

A fixed window is a window with an unchanging size that can accommodate a specific flow of segments.
A TCP sliding window is a window that can change size dynamically to accommodate the flow of segments.
TCP provides the sequencing of segments by providing sequence numbers and acknowledgment numbers in TCP headers.