OSI Layer Notes

I watched a series of YouTube videos on the OSI model to shore up my fluency on the details of computer networking and its different layers of abstraction.

Overview:

• OSI and TCP IP Models - Best Explanation
• The OSI Model Demystified

1. Physical layer
   • hardware
   • CAT-5 cable
   • “95% of networking problems”
2. Data-link layer
   • https://www.youtube.com/watch?v=vj3ut2uGCgs
   • https://www.youtube.com/watch?v=_gmnOBujm4Q
   • MAC address = media access control
     • every hardware device that has a NIC (network interface card) has a MAC
     • 48-bit address assigned to a NIC, hexadecimal characters (4-bits)
     • 1st half (24-bits or 6 characters) vendor code
     • 2nd half (24-bits or 6 characters) “unique value” for that particular card
   • LLC = link layer controls
     • acknowledges receipt of messages
     • flow control (occurs at other OSI layers too)
       • limit amt of data from sender
     • error control
       • allows receiver to notify sender when frame not received or checksum fails
     • Synchronizing transmissions (agreeing about clock)
       • isochronous (shared clock), lowest overhead
       • async
       • synchronous
   • Switches, bridges, NICs, VLANs
   • Protocols
     • Ethernet
     • CDP (Cisco discovery protocol)
     • Frame Relay, superced by MPLS
     • LLDP (link layer discovery protocol)
     • ARP = address resolution protocol, Wikipedia
       • IP to MAC address mapping
3. Network layer
   • https://www.youtube.com/watch?v=_d2zkAFytPk
   • https://www.youtube.com/watch?v=mp1l-x5AXi4
   • forwarding traffic based on addresses
   • IP v4/v6 addresses, logical addresses
   • switching
     • packet switching (routing “packets”)
     • circuit switching (dedicated link btwn 2 parties, e.g. telephony)
     • message switching (distinct from packets, but may be retained like emails)
   • route discovery and connection
     • routers maintain the routing table
     • manual or dynamically configured based on routing protocol
   • connection services
     • similar to layer 2, flow/error control, packet ordering
   • bandwidth usage
   • multiplexing strategy
   • Protocols:
     • ICMP
     • IPsec
     • RIP: old, do not need to know
     • OSPF
     • EIGRP
4. Transport layer
   • https://www.youtube.com/watch?v=DsQcX-7n6fY
   • https://www.youtube.com/watch?v=YeB9RuFElVg
   • how large a block of data to transfer
     • windowing
       • dynamical segment size based on # of repeat successes
     • buffering
   • buffering/flushing IO via socket
   • WAN accelerators
   • load balancers
   • firewalls (TCP/IP)
   • Protocols:
     • TCP: 3-step handshake (SYN, ACK, SYN/ACK), no dropped packets
       • usage: reliable
     • UDP: no re-transmission of dropped packets, no guaranteed delivery order, no acknowldegments
       • usage: speed, audio/video streaming
5. Session layer
   • https://www.youtube.com/watch?v=aPcku3orRmI
   • https://www.youtube.com/watch?v=OgXB2HJgXZY
   • connection state btwn client/server
   • setup
     • verify user’s credentials
     • create a unique session ID
     • coordinate sequence of who sends first
   • maintaining
     • data transfer
     • re-establish disconnected session
     • acknowledge receipt of data
   • teardown
     • mutual agreement or disconnected
   • Protocols:
     • H.323
     • NetBIOS
6. Presentation layer (OS)
   • https://www.youtube.com/watch?v=AtITX-U2mL4
   • https://www.youtube.com/watch?v=htw7DxtNlFs
   • data formatting, e.g.
     • HTML
     • character formatting (ASCII, UTF)
     • file formats (JPEG, PNG)
   • encryption
     • SSL
7. Application layer (user interface–e.g. web browser)
   • application services
   • service discovery
   • Protocols:
     • HTTP
     • SMTP
     • TELNET