Computer Networks

A computer network is a set of computers sharing resources
Communications are commonly established using cables or radio waves
Network nodes are identified by a network address (ex: IP address)
- That can be accompanied by a more memorable hostname
The Internet is a global system of interconnected computer networks, with:
- Local scopes, with private network addresses (ex: LANs, RFC 1918)
- A global scope, with public networks addresses managed by the IANA
  - Made accessible by ISPs

Collisions: will occur when devices both:
- Can only handle one operation at any given time
- Are queried operations while another is in progress
Collision domain: link in which collisions can happen, resolved with:
- CSMA/CD with Ethernet (cables)
- CSMA/CA with Wi-Fi (radio)
Duplex system: two connected parties can bilaterally communicate
- Half: a device can either send or receive at any given time
- Full: a device can both send and receive at the same time
Flooding: received data is forwarded through all but the receiving port
Routing: process of selecting network traffic paths
- Delivery schemes can be defined by messages source to target nodes:
  - Unicast: one-to-one
  - Broadcast: one-to-all
  - Multicast: one-to-many, many-to-many
  - Anycast: one-to-one-of-many

Networking hardware can be grouped by roles, locations
Core devices interconnect other devices:
- Hubs: half duplex, flood all received frames, almost unused today
- Switches:
  - Provide connectivity to hosts within the same LAN
  - Maintain a FIB (Forwarding Information Base)
    - Maps MAC addresses to physical ports
    - Aka:
      - A CAM table (Content Addressable Memory)
      - A MAC address table (Media Access Control)
- Routers:
  - Originally called gateways
  - Provide connectivity between LANs (Local Area Networks)
  - Can be used to send data over the Internet
  - Maintain a routing table
- Wireless Access Points: half duplex
Border devices generally sit between networks:
- Firewalls
  - Permit or deny inbound or outbound traffic by setting rules
  - Can be hardware (network fw) or software (host based fw)
Hosts, End Systems, Data Terminal Equipment (DTE)
- Network Interface Controller (NIC): can connect a computer
- Modem (MOdulator DEModulator)
  - Converts data to a format adapted to transmission mediums:
    - DSL (Digital Subscriber Line): over telephone lines
    - CATV (CAble TV): over CATV lines
  - Takes advantage of existing lines
  - Can be separate devices, or integrated into home routers

1950s: the SAGE computer network is built for the US military
- Coordinates radar sites to produce wide airspace imagery
- Uses the Bell 101 modem, 1st commercial modem (AT&T, release: 1958)
  - Transmit over telephone lines at 110 bits/s
1960s
- Development of the concept of packet switching for computer networking
- 1969: the ARPANET goes online:
  - A packet switching network between labs and universities
1970s
- 1973: The CYCLADES allows internetworking (connecting $n$ networks)
- 1974: RFC 675 contains the 1st: use of the term Internet and TCP specs
1980s: RFC 799: Internet Name Domains
- 1983: P. Mockapetris creates the DNS (rfcs: 882, 883, 973, 1034, 1035)
- 1984: 1st Unix name server: BIND (Berkeley Internet Name Domain)
1983-01-01: ARPANET adopts TCP/IP, becoming a subnet of the Internet
1989: 1st Internet public access offered by ISPs in the US and Aus
1991: The World Wide Web (www.) introduction helps global adoption

k-128/00 - Computer Networks.md