Networking layers summary

OSI: Open Systems Interconnection created by ISO

Acronyms:
All People Seem To Need Data Processing (7 to 1)

Please Do Not Touch Samy's Pet Alligator (1 to 7)

Layers:

1. Physical (Binary):  Cable, Radio frequency, voltages, pins, electric signals
2. Data (Frame): MAC (Media Access Control) to id device, Logical Link Control (LLC). Eg: PPP, HDLC, ATM, Frame Relay, SLIP and Ethernet. Ethernet with exponential backoff (exponential time + random time to retry if network congestion). Switches operate at this layer to group computers & reduce congestion.
3. Network (Packet): IP at this layer. An IP Packet has the IP header & payload. Packet forwarding, routing (to decide the best path), routers with ICMP (Internet Control Message Protocol), BGP (Border Gateway Protocol). Network hops counted. High hops indicate issues. Hop limits set. traceroute can help figure out the route.
4. Transport (Segment): TCP, UDP, SPX on top of IP. UDP: User Datagram Protocol: simple header with port & checksum, no mechanism to retry. TCP: sequence number of packets in headers with ACKs (that doubles the network traffic) to ensure receipt.
5. Session (Data): Session, timeouts. Eg: NFS, SQL, RDBMS, ASP, SIP.
6. Presentation (Data): Eg: jpg, gif, ascii, ansi, utf8.
7. Application (Data): Eg: http, https, smtp, snmp, ftp, dns, browsers, Skype, Outlook. DNS: Tree structure with Top Level Domains (eg: .com), Second Level Domains (eg: google.com) & Sub-Domain of Parent (eg: drive.google.com), distributed across many trusted servers.