is a model that describes seven layers that computer systems use to communicate over a network
it was the first standard model for network communications, adopted by all major computer and telecommunication companies in the early 1980s

Network Layers

Layer	Packets - Protocol Data Unit (PDU)	Description
1 - Physical Layer	bits	delivers an unstructured stream of bits across a link of some sort Link to original
2 - Data Link Layer	frames	delivers a piece of information across a single link. It organizes the physical layer’s bits into packets and controls who on a shared link gets each packet Link to original
3 - Network Layer	datagrams IP packets	computes paths across an interconnected mesh of links and packet switches, and forwards packets over multiple links from source to destination and routing through intermediate routers Link to original
4 - Transport Layer	segments TCP packets UDP datagrams	establishes a reliable communication stream between a pair of systems across an unreliable network by putting sequence numbers in packets, holding packets at the destination until they can be delivered in order, and retransmitting lost packets Link to original
5 - Session Layer	data TLS records	adds extra function to the reliable pairwise communication provided by the transport layer most network architectures do not have nor need the functionality in this layer, and it is not of concern to security Link to original
6 - Presentation Layer	data	encodes application data into a canonical (system-independent) format and decodes it into system-dependent format at the receiving end Link to original
7 - Application Layer	data	is where the applications use the network reside (e.g. web surfing, file transfer, and email) Link to original

Inter-Layer Communication