Administration Version 6.2.1
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TCP/IP Protocol Stack

As shown in the following diagram, the TCP/IP protocol stack contains four layers:

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This document covers the following topics:


Physical Layer

At the bottom of the stack is the physical layer, which deals with the actual transmission of data over physical media such as serial lines, Ethernet, token rings, FDDI rings, and hyperchannels. Messages can also be sent and received over other, non-physical access methods such as VTAM/SNA.

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Internet Protocol (IP) Layer

Above the physical layer is the Internet Protocol (IP) layer, which deals with the routing of packets from one computer to another. The IP layer

However, the IP layer does not keep track of a packet after it is sent, nor does it guarantee that the packet will be delivered.

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Transport Layer

Above the IP layer is the transport layer, which contains the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

Transmission Control Protocol (TCP)

The Transmission Control Protocol (TCP) guarantees that data sent by higher levels is delivered in order and without corruption. To accomplish this level of service, the TCP implementation on one computer establishes a session or connection with the TCP implementation on another computer. This process is referred to as Connection Oriented Transport Service (COTS).

After a session is established, data is sent and received as a stream of contiguous bytes; each byte can be referenced by an exact sequence number. When data is received by the remote TCP, it sends an acknowledgment back to the local TCP advising it of the sequence number of the last byte of data received. If an acknowledgment is not received, or if an acknowledgment for previously sent data is received twice, the local TCP retransmits the data until it is all acknowledged. The remote TCP discards any bytes that are received more than once.

All data sent and received by TCP is validated for corruption using checksums. Whenever a checksum is incorrect, the bad data is discarded by TCP, and the correct data is retransmitted until it is accurately received.

User Datagram Protocol (UDP)

Unlike the TCP, the User Datagram Protocol (UDP) transmits and receives data in packets (datagrams), and delivery is not guaranteed. The contents of the data can be sent with or without a checksum. The use of checksums varies widely from one implementation to another.

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Applications Layer

Above the transport layer is the applications layer, which contains both general applications and function libraries for use by applications.

Some general applications that run over TCP include

Interface with TCP and UDP

Function libraries provide routines to simplify the interface between applications and TCP/UDP of the Transport layer:

Ports

The interface that exists between an application and TCP is referred to as a port. Ports are classified as server ports and client ports:

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