Learn Networking - Layers

Layer #/Name: 0 / [Slang] Purpose: When we refer to a “Layer 0 Problem”, it means that the administrator has made an error (usually an error in syntax or logic) rather than an error in the equipment or data transfer. This is by no means a validly established layer in the OSI model, but many people use this as slang or irony. It may also be referred to as a PICNIC (Problem in Chair, Not in Computer). Corruption Occurs: N/A Common Protocols: N/A

Layer #/Name: 1 / Physical Purpose: The physical layer is anything that can be physically perceived. This includes wires, cables, and radio frequencies. Data is expressed digitally as either small electrical pulses in cables or in radio waves. A pulse is either on or off (1 or 0 respectively) and the receiver captures these bits, groups them into bytes, and processes them. Corruption Occurs: Heat, Fluorescent Fights, Electromagnetic Fields Common Protocols: Bluetooth, Telephone Lines, Ethernet, Radio Waves etc...

Layer #/Name: 2 / Data-Link Purpose: The Data-Link layer is comprised of 2 sublayers: LLC (Logical Link Control) and MAC (Media Access Control). It is in charge of the encapsulation and transfer of data. Essentially all data in networks use MAC addresses as destinations, not IP addresses, which is precisely why attacks such as ARP spoofing exist.
LLC - The LLC sub-layer is in charge of establishing logical links between other devices on the network. This is the “top” layer and acts as an intermediary sub-layer between Network and MAC.
MAC - The MAC sub-layer is in charge encapsulating and transferring data to the physical NIC.
Corruption Occurs: Rarely. Subject to Layer 1 errors. Common Protocols: PPP, PPTP, L2TP, STP, VTP, Ethernet, WiFi, etc..

Layer #/Name: 3 / Network
Purpose: The Network layer is in charge of data transfer through interconnected networks. This is where the IP address lies. Header information includes the source IP address, but is not port specific. Corruption Occurs: Device errors, routing table errors, L1.
Common Protocols: IPv4/IPv6, IPSec, OSPF, IGRP/EIGRP

Layer #/Name: 4 / Transport Purpose: Layer 4 is in charge of quickly and reliably transferring data. This is the meat and bones of the internet and protocols, from here on, will use the first 4 layers. It is port specific and very reliable. TCP is capable of flow control, data recovery, and error detection. Corruption Occurs: Packet corruption, L1. Common Protocols: “Big 3” are: TCP, UDP, and SCTP

Layer #/Name: 5 / Session Purpose: This is my least favorite layer. It is in charge of establishing, managing, and terminating sessions between applications across a network. Corruption Occurs: Below Layers Common Protocols: SMB, SOCKS, NFS, etc...

Layer #/Name: 6 / Presentation Purpose: This layer is in charge of encoding/decoding data usually to/from the application layer. Corruption Occurs: RAM/Disk errors, Corruption Common Protocols: ASCII, MP3, JPG, PNG, GIF, MP4, AVI, MOV, etc...

Layer #/Name: 7 / Application Purpose: This is the most commonly used layer in the internet. It is any communication between applications (IE: a browser and a server). It uses one of the layer 4 protocols to transfer data and usually has some sort of encoded or interpreted syntax or data. Corruption Occurs: Attacks/modification, Corruption, L4 and below. Common Protocols: HTTP, SSH, FTP, SMTP, POP, RADIUS, DNS, NTP, etc.

-H

Houga@entropy.cat