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The TCP/IP Reference Model

Layer 4 Application Combines Application, Session and Presentation Layers of the OSI model.
- aka the Process Layer:
- Emphasize is on maximum flexibility for software developers.
- Handles all high level protocols:
-- FTP, HTTP, SMTP [ TCP examples ]
-- TFTP, SNMP, DHCP, DNS, BOOTP [ UDP examples ]
-- Data representation, encoding and dialog control.
-- Ensures data is properly packaged for the next layer.
Layer 3 Transport aka the host-to-host layer.
- TCP - Transmission Control Protocol:
-- Connection-oriented protocol.
--- supports dialogues between source and destination.
--- packages application information into segments.
--- provides reliable full-duplex transmission.
--- supports flow control that exchanges packets.
--- windowing, acknowledgements.
--- retransmission - resends anything not received.
- UDP - User Datagram Protocol:
-- Connectionless protocol that exchanges datagrams.
--- does not provide flow control.
--- does not support windowing, acknowledgments or guaranteed delivery.
--- error processing and retransmission must be handled by other protocols.
--- unreliable.
Layer 2 Internet Purpose is to send source packets from any network on the internetwork and to have them arrive at the destination independent of the path and networks they took to get there.
- IP (Internet Protocol) governs the Internet layer. (All packets go through IP!)
- Best path determination and packet switching occur at this layer.
Layer 1 Network Access Combines the Data-Link and Physical Layers of the OSI model.
Concerned with all issues that an IP packet requires to cross a physical link from one device to another directly connected device.
-Networks include:
-- WAN - Wide Area Network
-- LAN - Local Area Network
-- Internet
Important Note: Regardless of which application requests network services, and regardless of which transport protocol is used, there is only one network protocol: Internet Protocol (IP) which serves as a universal protocol that allows any computer, anywhere, to communicate at any time.
The TCP/IP suite in relation to the OSI model is illustrated here:
Click on the poster for a larger view.

TCP/IP in relation to the OSI model