OSI reference model

layered, abstract, idealized framework/description/model of networking.
organizes (description of) network architecture.
each layer provides services to layer above it and relies on services provided by lower layer.
different protocols at each layer perform functionality. suite of protocols organized in layers: protocol stack.
enables modular development of layers.
layering allows changing the implementation of layer, and without affecting other layers (theoretically).
sending: each lower layer encapsulates PDU passed down from layer above, in its "envelope" by segmenting (maybe) and adding header (and maybe trailer).
receiving: each upper layer decapsulates PDU passed up from layer below, stripping header and joining segments (if needed).
a layer (logically) communicates with its peer at same layer on other host, in its layer's protocol "language". Header provides the meta-information about the encapsulated payload.
analogy of general dictates message to colonel who writes letter then gives it down to captain who puts in addressed envelope, passes it down to sargeant who adds stamps who gives to corporal who puts in mailing envelope and adds barcode gives to airman who puts in pouch gives to runner who goes across field gives to receiving airman who removes from pouch and sees by barcode which corporal to give to who strips off mailing envelope passes it up to sargeant who sees the stamps are correct then gives to his captain who opens the envelope and sees which colonel to give it to who reads it to his general. (no segmentation in this analogy)

#	Name	use	PDU	protocols	"address"	(connectivity) devices / HW	implemented by
7	Application	app's gateway to network. supports network apps.	"data"/"message" + header	HTTP FTP Telnet DHCP DNS SNMP SMTP BGP UPnP ...	host name	client/server peer-to-peer, "gateway"	app program in user mode
6	Presentation	"translation of syntax" from app to network e.g. compression, encryption, file formats, coding (MIME, ASCII / EBCDIC, XML, JSON		e.g. TLS/SSL
5	Session	establishes, maintains, ends sessions. "dialog control" e.g. duplex; checkpointing / recovery
4	Transport	reliable end-to-end data transfer: error detection / correction, acknowledgement, flow control, segmentation	TCP: segment UDP: datagram	TCP (connecton-oriented: guaranteed delivery, incl. flow control, congestion control, segmentation), UDP (connectionless)	port		O.S. TCP/IP stack
3	Network	routing datagrams from one host to another. hierarchical logical addressing, fragmenting, routing (path determination in internetwork), delivery error reporting. QoS prioritization. Best effort, connectionless.	datagram / packet	IP, ICMP, NDP, routing	IP address; protocol field	router	O.S. TCP/IP stack
2	Data link	move packet from one node to the next. framing, media access control, physical addressing, transmission error detection	frame	Ethernet (802.3), WiFi (802.11), ARP, PPP, Frame Relay, CDP, LLDP	MAC; Type field	NIC, switch, bridge, WAP	NIC firmware & driver
1	Physical	move bits from one node to the next. medium, encoding, signaling voltages, data rates, transceiving	(encoded / modulated bits)			hub, repeater, media converter, "modem" (ISDN, T1, POTS, DSL, 802.11 PHY), cabling / connectors / topology	NIC electronics / transceiver

please do not throw sausage pizza away
layers 1 to 3: "lower" layers. network
layers 4 to 7: "upper" layers. host
"8th layer" problem: your users

TCP/IP model lumps application, presentation, session layers into an Application layer. Transport layer also called Host-to-Host layer. Network layer also called Internet(working) layer. TCP and IP work as a team, complementing each other. There is only one IP protocol and all Internet components that have a network layer component must run it. IP is the glue that binds the Internet together.

OSI layers and corresponding TCP/IP layers and protocols:

Encapsulation of PDUs: