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Cheatography

Network+ OSI Model 2/22 Cheat Sheet by

CompTIA Network+ certification course sum up. Oriented for cybersecurity later courses.

Layers and Wording

7
Applic­ation
Data
6
Presen­tation
Data
5
Session
Data
4
Transport
Segmen­ts/­Dat­agram
3
Network
Packets
2
Data-Link
Frames
1
Phyiscal
Bits

1. Physical

Function
transm­ission of bits across network
Repres­ent­ation
Electrical voltage on wires -> 1 or 0
- NRZ
0 volt = 0, +/-5 volts = 1
- Transition Modulation
during a clock cycle, no change = 0, change = 1
Topology
see cheatsheet 1/20
Async comm.
use of start bits and stop bits to indicate when transm. occurs
Sync comm.
use of a reference clock to coordinate transm.
Broadband bandwith
divides bw into separate channels (ex Cable TV)
Baseband bandwith
uses different freqs on a cable & a ref clock to coordinate transm.
Baseband Multix­plexing
TDM, StatTDM, FDM are ways to allocate time slots and freqs over channels
ex
cables, radio freqs, devices (hubs, WAP, conver­ters, ...)

2. Data-Link

MAC
48-bit -> Network Interface Card (NIC) / 1st 24bits : manufa­cturer, 2nd 24bits : unique device value
LLC
Logical Link Control
 
Reliable transm­ission of data
 
Segmen­tation & addressing
 
Flow and Error control (checksum)
Syncro
Isochr­onous, Syncronous or Asyncr­onous
Devices
NIC, bridges, switches

3. Network

Function
forwards traffic with logical adress
Logical adress
IPv4, IPv6, IPX, AppleTalk
Packet switching
divides data into packets and forward
Circuit switching
dedicated comm link
Message switching
divides data into storable messages which can be stored and forwarded later
Routers
routing table based on IP adress, static or dynamic route
 
protocol RIP, OSPF, EIGRP
Flow control
regulates data flow/speed
Packet reordering
thanks to numbering and sequen­cing, packets can be sent across multiple routes
ICMP
Internet Control Message Protocol
 
send error msg & ops info about an IP, uses ping and traceroute
ex:
routers, multilayer switches, IPv4, IPv6, ICMP
 

4. Transport

TCP
Transm­ission Control Protocol
connection oriented
reliable, resend lost segments, acknowlege (3-way handshake)
 
UDP
User Datagram Protocol
connec­tio­nless
ureliable, no retran­smi­ssion, faster due to low overhead
Windowing
adjust amount of data, based on retran­smi­ssion reception quantity
Buffering
router allocates memory to store segments
 
buffer overflow = segments dropped
ex:
TCP, UDP, WAN accele­rators, load balancers, firewalls

5. Session

Function
setup a conver­sation
Setting up
check creden­tials, session id, services needed and who starts
Mainta­ining
transfer data, reesta­blish connec­tion, acknow­ledge data receptĀ°
Tearing Down
on mutual agreement or other party discon­necting
ex:
H.323/264 (voice­/video stream­ing), NetBIOS (file exchange)

6. Presen­tation

Function
format data for readab­ility, encrypt and secure data
Data Formating
for compat­ibility purposes, readab­ility (ASCII, JPG, etc)
 
compre­ssion, conversion
Encryption
scrambles data, provide confid­ent­iality (TLS)
ex:
progra­mmation languages, text formats, pict extent­ions, protocols like TLS, SSL

7. Applic­ation

Function
interface user <-> computer
App. Services
File transfer, sharing, email, remote access, NW mgmt, cl/srv processes
Advert­isement
service initiating a service offer to a NW
ex :
POP3, IMAP, SMTP / HTTP-S / DNS / FTP-S / Telnet, SSH / SNMP

Encaps­ulation & Decaps­ulation

Function
"­env­elo­ppi­ng" data with headers
PDU
Protocol Data Unit. ex: L3PDU-­>Packet
Flags
SYN
initiates synchr­oni­zation of connection
ACK
acknow­ledges during 3-WSH and packet reception
FIN
initates termin­ation of connection
RST
when client or server receives a non expected packet
PSH
gives priority to data (for sender)
URG
gives priority to data (for recipient)
MAC
phyiscal adress of a NIC
EtherType
identify the protocol used (IPv4/v6)
from L7 to L1
at L4
+TCP/UDP header (source & dest ports)
at L3
+IP header (source & dest adresses)
at L2
+MAC+LLC ---- +FCS
at L1
transmit L2 in bits (0 and 1)
       
 

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