The seven layers of the Open Systems Interconnection (OSI) reference model:
| Application | Network applications such as terminal emulation and file transfer HTTP, FTP, DNS, SMTP |
| Presentation | Formatting of data and encryption, compression ASCII, HTML, JPEG, TIFF |
| Session | Establishment and maintenance of sessions NetBIOS |
| Transport | Provision of reliable and unreliable end-to-end delivery. Host to host, Segments. SSL, TCP, UDP, SPX, NetBEUI |
| Network | Packet delivery, including routing. Packets, logical (IP) addressing Router, IP, IPSEC |
| Data-Link | Framing of units of information and error checking. Physical (MAC, hardware) addressing, checksum Bridge, Switch, Ethernet, Token-Ring, FDDI, ATM, ARP, PPP |
| Physical | Transmission of bits on the physical hardware. Frames, electrical signals Repeater, wiring, hub |
The OSI model is easily used as a checklist. It is a reference model. Step through the OSI model to review if you have attended to all aspects of interconnection.
The OSI model is not easily used in reverse. Given a product or technology, where does it fit in the OSI model? Is it data-link? Is it network? Is it data-link and network? Is it Session? No, its probably not Session.
Take, for example, SSL. There are persons who insist it is a Transport layer protocol, while others (scilnet.fortlewis.edu, Optimizing Network Performance with Content Switching: Server, Firewall and Cache Load Balancing) suggest it resides higher in the stack.
Consider Multi-Protocol Label Switching (MPLS). When people refer to it as a “Layer 2.5 protocol,” they indicate that the OSI reference model is of limited use as a taxonomy. MPLS directly maps to an intermediate step between the Data-Link layer and the Network Layer of the IP protocol stack, and these implementations correspond to the layers 2 and 3 of the (protocol agnostic) OSI model.
You do not develop products to map to a layer in the OSI model. You develop products to fill a need, to address a problem, to fit a niche.
Use the OSI model as a checklist and don’t try to force products into its layers. Don’t worry about it.
For example, walk through the seven layers of the Open Systems Interconnection (OSI) reference model and try to think of the types of problems which may arise:
| Application | Name resolution issues with DNS or NetBIOS names Network application issues System application issues HTTP, FTP, SMTP application issues SMB signing issues Man-in-the-middle attacks |
| Presentation | |
| Session | |
| Transport | Retransmission problems Packet fragmentation Port issues TCP Windowing issues |
| Network | IP addressing problems Duplicate IP addresses Routing problems Protocol errors ICMP errors or ICMP filtering External attacks |
| Data-Link | Improperly configured network addresses ARP table and ARP cache issues Speed / duplex mismatch (e.g., “Auto”? Don’t you require “Full”?) Wireless radio interference Excessive hardware errors (e.g., “chatter”) |
| Physical | Power issues Hardware failure Connector issues Cabling issues |
Try to think of products to address these problems:
| Application | Network simulators Traffic generators Protocol analyzers |
| Presentation | |
| Session | |
| Transport | Network simulators Traffic generators Protocol analyzers Netflow Network probes Traffic analyzers |
| Network | Netflow Network probes Traffic analyzers |
| Data-Link | Netflow Network probes Network connectivity tools |
| Physical | Cable testers Network connectivity tools |
