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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model
      9  Key OSI Reference Model Concepts

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Protocols: Horizontal (Corresponding Layer) Communication
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Indirect Device Connection and Message Routing
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Data Encapsulation, Protocol Data Units (PDUs) and Service Data Units (SDUs)
(Page 2 of 3)

Data Encapsulation in TCP/IP

The “N-1, N-2” stuff makes this seem more difficult than it really is, so let’s use a real-world (simplified) example instead. The Transmission Control Protocol (TCP) operates at layer 4 of the OSI model. It transmits messages called segments that contain data encapsulated from higher-layer protocols. The layer below TCP is the Internet Protocol (IP) at layer 3. It receives data from TCP and encapsulates it for transmission.

So, in the formal language of the OSI Reference Model, TCP segments are created as layer 4 PDUs. When passed to IP, they are treated as layer 3 SDUs. The IP software packages these SDUs into messages called IP packets or IP datagrams, which are layer 3 PDUs. These are in turn passed down to a layer 2 protocol, say Ethernet, which treats IP datagrams as layer 2 SDUs, and packages them into layer 2 PDUs (Ethernet frames) which are sent on layer 1. (Actually, in some technologies further encapsulation even occurs at layer one prior to transmission.)

On the receiving device, the process of encapsulation is reversed. The Ethernet software inspects the layer 2 PDU (Ethernet frame) and removes from it the layer 2 SDU (IP datagram) which it passes up to IP as a layer 3 PDU. The IP layer removes the layer 3 SDU (TCP segment) and passes it to TCP as a layer 4 PDU. TCP in turn continues the process, going back up the protocol layer stack. The complete process is illustrated in Figure 16.


Figure 16: OSI Reference Model PDU and SDU Encapsulation

This example shows in more detail how OSI PDUs and SDUs are created and encapsulated. A TCP segment (layer 4 PDU) becomes a layer 3 SDU, which is encapsulated into a layer 3 PDU through the addition of an IP header. This becomes the payload of an Ethernet frame, which is a layer 2 PDU containing an Ethernet header, layer 2 SDU (the IP datagram) and Ethernet footer. The receiving device extracts the IP datagram from the Ethernet header and passes it to layer 3; the IP software extracts the TCP segment and passes it up to the TCP software.

 


This whole matter of passing data up and down the protocol stack, encapsulation and so on may seem needlessly complex. It also may appear to be rather inefficient; why send a message with so many headers and footer? However, the notion of data encapsulation is critical to creating modular, flexible networks.


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Indirect Device Connection and Message Routing
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