The TCP/IP Guide - OSI Reference Model Layer Summary

To assist you in quickly comparing the layers of the OSI Reference Model, and understanding where they are different and how they relate to each other, I have created the summary chart shown in Table 18. It shows each layer's name and number, describes its key responsibilities, talks about what type of data is generally handled at each layer, and also what the scope of each layer is in approximate terms. I also show some of the more common protocols that are associated with each layer.

I will say here that “standard disclaimers apply”. I don't want to repeat all of the caveats I mentioned elsewhere in discussions of the OSI model, but know that they apply here as well. Namely: the layers aren't always hard-fast, I haven't listed every single protocol here, some may really fit into more than one layer, and so on. In particular, note that many of the technologies listed as being in the data link layer are there because that is the layer where their primary functionality resides. In reality, most of these technologies include components in other layers, especially the physical layer.

**Table 18: OSI Reference Model Layer Summary**
Group	#	Layer Name	Key Responsibilities	Data Type Handled	Scope	Common Protocols and Technologies
Lower Layers	1	Physical	Encoding and Signaling; Physical Data Transmission; Hardware Specifications; Topology and Design	Bits	Electrical or light signals sent between local devices	(Physical layers of most of the technologies listed for the data link layer)
	2	Data Link	Logical Link Control; Media Access Control; Data Framing; Addressing; Error Detection and Handling; Defining Requirements of Physical Layer	Frames	Low-level data messages between local devices	IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI and CDDI; IEEE 802.11 (WLAN, Wi-Fi); HomePNA; HomeRF; ATM; SLIP and PPP
	3	Network	Logical Addressing; Routing; Datagram Encapsulation; Fragmentation and Reassembly; Error Handling and Diagnostics	Datagrams / Packets	Messages between local or remote devices	IP; IPv6; IP NAT; IPsec; Mobile IP; ICMP; IPX; DLC; PLP; Routing protocols such as RIP and BGP
	4	Transport	Process-Level Addressing; Multiplexing/Demultiplexing; Connections; Segmentation and Reassembly; Acknowledgments and Retransmissions; Flow Control	Datagrams / Segments	Communication between software processes	TCP and UDP; SPX; NetBEUI/NBF
Upper Layers	5	Session	Session Establishment, Management and Termination	Sessions	Sessions between local or remote devices	NetBIOS, Sockets, Named Pipes, RPC
	6	Presentation	Data Translation; Compression and Encryption	Encoded User Data	Application data representations	SSL; Shells and Redirectors; MIME
	7	Application	User Application Services	User Data	Application data	DNS; NFS; BOOTP; DHCP; SNMP; RMON; FTP; TFTP; SMTP; POP3; IMAP; NNTP; HTTP; Telnet

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