The OSI model.

OSI is Open Systems Interconnection.

OSI was created by ISO.

ISO is International Organization for Standardization.
ISO is an international organization which makes standards (more than 20,000).
A product with ISO label should ensure quality.
ISO website is:

Definition: OSI is a standard that splits in seven virtual layers the communication between two systems interconnected through network(s).
These systems interconnected can be: computers, servers, tablets and smartphones.

The seven OSI layers are:
Layer 1: physical.
Layer 2: data link.
Layer 3: network.
Layer 4: transport.
Layer 5: session.
Layer 6: presentation.
Layer 7: application.

Global view of OSI layers:

1- OSI physical layer.
-The information are called BITS (BInary digiTS, 0 and 1).
-Here are: repeaters, hubs, modems, network cards, coaxial cables, twisted pairs, optical fiber, Bluetooth, Wi-Fi, Ethernet (cards and cables),…

2- OSI data link layer.
-The information are called frames or data frames.
-Here are: switches, Wi-Fi, Ethernet (protocol), MAC, VLANs (ISL, dot1Q, DTP), ATM, Frame Relay, MPLS, HDLC, PPP, Token Ring, CSMA/CD, CDP,
LLDP, HSRP, STP, EtherChannel (LACP, PAGP)…

3- OSI network layer.
-The information are called packets or datagrams.
-Here are: routers, NAT, firewalls (also in layers 4 and 7), IP (IPv4 and IPv6), IPsec, IPX, ARP, RARP, ICMP, IS-IS, RIP, OSPF, BGP, Syslog…

4- OSI transport layer.
-The information are called segments or datagrams.
-Here are: TCP, UDP, SSL and TLS, DCCP, RSVP, RTP, SCTP, gateway, (web) proxy server (also in layer 7), firewall (also in layers 3 and 7),…

5- OSI session layer.
-The information are called data
-Here are: NetBIOS, RPC, RTP, RTCP,…

6- OSI presentation layer.
-The information are called data
-Here are: XDR, LPP, NDR, SSL and TLS,…

7- OSI application layer.
-The information are called data or applications
-Here are: DNS, DHCP, SMTP, POP3, IMAP, SSH, Telnet, LDAP, IRC, RDP, HTTP, HTTPS, FTP, FTPS, NTP, SNMP,…, (web) proxy server, firewall.

The OSI layers with details:

1- OSI physical layer
In OSI physical layer, there are only physical devices:
repeaters, hubs, modems, network cards, coaxial cables, twisted pairs, optical fiber, Bluetooth, Wi-Fi, Ethernet (cards and cables),…

2- OSI data link layer
Here are: switches, Wi-Fi, Ethernet (protocol), MAC, VLANs (ISL, dot1Q, DTP), ATM, Frame Relay, MPLS, HDLC, PPP, Token Ring, CSMA/CD, CDP,
LLDP, HSRP, Spanning-Tree Protocol (STP), EtherChannel (LACP, PAGP)…

Data Link layer is between physical layer and network layer:
This layer receives bits from the physical layer, gathers it into frames, then converts it into packets for transmission to the network layer;
and, on the other way:
this layer receives packets from the network layer, converts it into frames, then converts it into bits for transmission to the physical layer.

Data link layer is divided into two sublayers: MAC and LLC.
LLC is Logical Link Control.
MAC is Media Access Control.

The roles of LLC sub-layer are:
-Manages the frames (to upper & lower layers)
-Controls the flow of data
-Controls the errors.

The roles of MAC sub-layer are:
-Controls the access to the physical layer
-Carries the physical address of each device from the physical layer to network layer.

Error control in LLC:
-it detects if errors occurred during the transmission of the bits across the wire
-it uses a calculated value called the CRC (Cyclic Redundancy Check) added into the frame’s trailer
-the receiving computer recalculates the CRC and compares it to the one sent with the data:
*if the two values are equal, the data arrived without errors
*otherwise, the message needs to be retransmitted.

There are: ethernet card, ethernet cable and ethernet protocol (used in LAN and MAN).
Ethernet is both in layer 1 and layer 2 of OSI, because:
-ethernet card is in layer 1 (physical device)
-ethernet cable is in layer 1 (physical device)
-ethernet protocol is in layer 2.

In IT networking, a frame (also called a data frame) is an organized structure of the data.
Each frame contains a header, a payload and a trailer.
The structure of a frame depends on the type of network.
Three examples of frames:
-ethernet frame
-point to point frame
-fiber channel frame.

WiFi is both in layer 1 and layer 2 because:
-the WiFi devices (router, checkpoint...) are in layer 1
-the WiFi security protocols (WPA3, WPA2, WPA, WEP) are in layer 2.

3- OSI network layer
Here are: routers, NAT, firewalls (also in layers 4 and 7), IP (IPv4 and IPv6), IPsec, IPX, ARP, RARP, ICMP, IS-IS, RIP, OSPF, BGP, Syslog…

-In OSI network layer, the information are in packets or in datagrams:
->packets, when the network connection is reliable: if transported with TCP (Transport Control Protocol)
->datagrams, when the network connection is unreliable: if transported with UDP (User Datagram Protocol).

-The OSI network layer roles are:
*Provides the logical addresses (the IP addresses) to computers; and
*Determines a path on the network(s) for transferring the packets (or datagrams) through routers from the source computer to the destination computer.
Then the packets are handed to the transport layer for transferring.

-The network layer on internet is based on either packet-switching or virtual-circuit.
->Packet-switching: each packet is broken in several, sent and transferred in different paths through the network(s) and recomposed in one packet at the end.
A routing decision is made on every packet.
->Virtual-circuit: looks like circuit-switching.
A virtual path is set up, then all packets will follow the same fixed path until the destination computer (ex: VPN).

-The network layer on internet provides a best-effort service.
’’Best-effort delivery’’ describes a network service in which the network does not provide any guarantees that data is delivered.


IP is Internet Protocol.

IP is the most important network protocol.

IP is used not only on internet but also in LANs and WANs.

IP is used for transferring data.

IP offers a best-effort service of delivering datagrams between hosts, which may be lost, delayed, corrupted, or duplicated.

The maximum size of an IP packet is of 65515 bytes
-the maximum size of an IP packet encapsulated on ethernet is of 1500 bytes
-the maximum size of an IP packet encapsulated on ATM is of 53 bytes
-the maximum size of an IP packet encapsulated on frame relay is of 4096 bytes.

IP is very popular and very used so that we can say:
-''Everything over IP'', because IP can transport: voice, video, photos, data, text…
-''IP over everything'', because IP can be transported by ATM, Frame Relay, ethernet, optical fiber, twisted pair, satellite…

There are IP version 4 (IPv4) and IP version 6 (IPv6).
IP version 5 was experimental.

RFC 791 describes IPv4 and RFC 2460 describes IPv6.

On IPv4: the header has 20 bytes, with 13 fields
On IPv6: the header has 40 bytes, with 7 fields only (so the routers process packets faster than with IPv4).

Information about IP are available by clicking on the following red link: Internet Protocol

-IPsec is Internet Protocol Security
-It’s a protocol of OSI network layer
-IPsec is compatible with IPv4 and IPv6
-The goal of IPsec is to secure internet connection between hosts. IPsec is used in VPN networks
-Each IP packet is encapsulated in IPsec packet.

-ICMP is Internet Control Message Protocol
-Internet is monitored closely by the routers: when something unexpected occurs during packet processing at a router, the event is reported to the sender by an ICMP message
-Each ICMP message is encapsulated in an IP packet
-ICMP is also used to test internet links
-ICMP messages are sometimes called PING
-For IP version 6, there is ICMP v6 protocol.

ARP is Address Resolution Protocol
Definition: ARP protocol resolves IP address to MAC address

Some protocols for routing IP packets:
-BGP (Border Gateway Protocol). Routers use BGP to exchange information on internet.
-RIP (Routing Information Protocol)
-OSPF (Open Shortest Path First)
-IS-IS (Intermediate System to Intermediate System)
-RIP, OSPF and IS-IS are protocols that calculate (with some algorithms) the shortest path on networks for routing IP packets
OSPF protocol is better because it calculates the shortest and fastest path.

4- OSI transport layer
The roles of OSI transport layer are:
-transports the information (segments with TCP or datagrams with UDP) between the sending and receiving computers, by following the path determined by the network layer;
-controls the data flow (with TCP only, but not with UDP);
-controls the errors (with TCP only, but not with UDP).

The protocols of OSI transport layer are:

-TCP is Transmission Control Protocol
-UDP is User Datagram Protocol

Comparison between TCP and UDP:
-with TCP, the transport is more reliable but slower;
-with UDP, the transport is faster but less secure.

TCP is a connection-oriented
because it establishes a connection between the two computers before any data is sent, and it guarantees that data will arrive in the proper sequence.

UDP is a connectionless protocol,
because it sends data without first ensuring that the destination computer is available and ready to receive the data.
UDP does not provide error recovery and flow control.
UDP header needs less bytes than TCP header.
UDP is used for applications such VoIP, video, and gaming that do not need error recovery and flow control. Moreover, DNS and DHCP use UDP.

-SSL is Secure Socket Layer.
-TLS is Transport Layer Security.
TLS is the new version of SSL. SSL and TLS are security protocols.
Definition: SSL and TLS are cryptographic protocols to secure the data in computer network communication.
Example: SSL and TLS are used to secure the data transfer on VPN, on emails, and on websites for credit card payment.
SSL and TLS ensure:
-the authentication on server(s);
-the confidentiality of data;
-the integrity of data transferred.

-DCCP is Datagram Congestion Control Protocol (this protocol makes reliable connections)

-RSVP is Resource Reservation Protocol (this protocol books resources in a network)

-RTP is Real-time Transport Protocol (this protocol is used in VoIP and videoconference)

-SCTP is Stream Control Transmission Protocol (this protocol has almost the same role than TCP)


-Web proxy server (also in layer 7)

-Firewall (also in layers 3 and 7)

5- OSI session layer
The roles of OSI session layer are:
-creates a session between the two systems interconnected as computers (authentication with login and password);
-manages the connection and the communication between these two systems interconnected;
-terminates the session when the communication between the two systems interconnected is completed.

The protocols of OSI session layer are:

-RPC is Remote Procedure Call.

-RTP is Real-time Transport Protocol.

-RTCP is Real-time Transport Control Protocol.

-NetBIOS is Network Basic Input/Output System.
NetBIOS provides services allowing applications on separate computers to communicate over a LAN. NetBIOS is a service, but not a protocol.
How it works? Call; listen; send data; receive data; close the session.

6- OSI presentation layer
The roles of OSI presentation layer are:
-conversion of the data coming from application layer or from session layer;
-compression or decompression of the data;
-and sometimes encryption or decryption of the data (for security reasons, with protocols SSL and TLS).

The protocols of OSI presentation layer are:

-XDR is eXternal Data Representation.

-LPP is Lightweight Presentation Protocol.

-NDR is Network Data Representation.

-SSL is Secure Socket Layer and TLS is Transport Layer Security (also and seen in layer 4).

7- OSI application layer
In this layer, there are the applications used by computers and some protocols too.

Example of applications in OSI application layer:

-Emails (Microsoft Outlook, Gmail or Lotus Notes)

-Web pages (Google Chrome, Firefox or Internet Explorer)

-Microsoft Office software (Word, Excel, PowerPoint, Access).

The protocols of the application layer are:

-SMTP is Simple Mail Transfer Protocol.

-POP3 is Post Office Protocol 3.

-IMAP is Internet Message Access Protocol.

-HTTP is HyperText Transfer Protocol.

-HTTPS is HyperText Transfer Protocol Secure (it is HTTP over SSL or TLS).

-IRC is Internet Relay Chat.

-FTP is File Transfer Protocol.

-FTPS is FTP Secure.

-Telnet is Telecommunication Network. It’s a client/server communication protocol, but without security.

-SSH is Secure SHell. It’s a security protocol. SSH can replace Telnet because it secures the communications.

-NTP is Network Time Protocol.It is a clock synchronization protocol used by computers, servers and routers.

-SNMP is Simple Network Management Protocol. It is used by administrators to monitor network devices.

-RDP is Remote Desktop Protocol. Windows: start-> run-> mstsc (Microsoft Terminal Services).

-LDAP is Lightweight Directory Access Protocol. Example: the Active Directory of Windows uses LDAP.

-DHCP is Dynamic Host Configuration Protocol: a DHCP server delivers automatically a unique IP address to each host.

-DNS is Domain Name System: a DNS server resolves a domain name (or a host name) to an IP address.

There is another model than OSI: it is TCP/IP model.
Comparison between OSI model and TCP/IP model:
-OSI model has 7 layers: physical; data link; network; transport; session; presentation; application.
-TCP/IP model has 5 layers only: physical; data link; network; transport; applications.

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