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Network Fundamentals

Network Protocols

Define rules and format of communication between network devices.

  • Can apply to hardware or software (e.g., directly connected devices and HTTPS)
  • Can support mechanisms for efficient and reliable communication (message acknowledgement, data compression, etc.)

Layered Models

Standard framework for interconnecting network protocols for end-to-end communication.

  • Protocols should be modular and independent.

Open Systems Interconnection (OSI)

Used to teach computer networking. Developed in 1970s.

TODO PlantUML

  1. Application
  2. Presentation
  3. Session
  4. Transport
  5. Network
  6. Data Link
  7. Physical

Transmission Control Protocol / Internal Protocol (TCP/IP)

More widely-used abstraction model.

  • Based on OSI.

TODO plantUML

  1. Application
  2. Transport
  3. Internet
  4. Host-Network

Breakdown of data units at each layer:

"Don't Smoke Pot From Bongs"

Data Unit TCP/IP Layer
Datagrams App
Segments Transport
Packets Network
Frames Data Link
Bits Physical

Data Encapsulation

Encapsulation: Application $\to$ Physical

  • Information is encapsulated with header (and sometimes trailer) data on the way down.
  • More info is added at each layer.

Decapsulation: Physical $\to$ Application

  • Happens on the way back up the protocol stack.

Example: Encapsulation

TODO Names
1 Data
2 TCP/UDP Header Data
3 IP Header TCP/UDP Header Data
4 Frame Header IP Header TCP/UDP Header Data Frame Trailer

Physical Layer

Transmission of bits/waveform/timing/et cetera.

Examples: coaxial, radio, fiber optic, et cetera.

Things to Consider:

  • Security
  • Bandwidth
  • Ease of Installation

Data Link Layer

Delivers data in a local network (or point-to-point).

  • Data Units: Frame

IEEE 802.3 (Ethernet)

  • 48-bit MAC address: Unique ID
    • 24-bits for manufacturer ID
    • 24-bits for device ID

IEEE 802.11 (Wi-Fi)

  • Data link layer devices (e.g., switches, bridges)

Network Layer

Delivers data between hosts, possibly on different networks.

  • Data Unit: Packet

Internet Protocol (IPv4)

  • 32-bit addresses
    • Usually written as four octes in dotted decimal (e.g., 172.16.1.250)
  • Network and host addresses

Note: Current standard is IPv6

Other Protocols

Address Resolution Protocol (ARP): Resolves MAC address to corresponding IP address.

Internet Control Message Protocol (ICMP): Allows hosts to communicate about network conditions.

  • e.g., ICMP Destination Unreachable message

Dynamic Host Configuration Protocol (DHCP): Dynamically assigns IP addresses.

Transport layer

Delivers data from process on one host to process on another.

  • e.g., web browser on laptop talking to web server at NASA.

Port Numbers: Used to get the data to the correct process on a host.

  • Well Known: < 1024
  • Registered and Dynamic: > 49151

Transmission Control Protocol (TCP)

Connection-oriented with reliable delivery, flow, and congestion control.

  • Sequence numbers (e.g., message 1/5), acknowledgement numbers (e.g., SIN/ACK)

Flags: SYN, ACK, FIN, PSH, URG

Most internet traffic uses TCP.

Universal Control Protocol (UDP)

Connectionless, less overhead than TCP.

  • Efficient for small transfers.
  • Unreliable; no flow control, congestion control, or order.

Basically, anything that buffers or can be lossy, is good for UDP.

  • Multicasting, streaming, tunneling, et cetera.

Ap pl Display status of network connectionsication Layer Protocols

Domain Name System (DNS)

Resolves domain names to IP addresses.

Hypertext Transfer Protocol (HTTP/s)

Web browser/server interaction.

Email

  • Simple Mail Transfer Protocol (SMTP)
  • Post Office Protocol (POP3) and IMAP 4

Secure Shell (SSH)

Remote accessing other computers.

File Transfer Protocol (FTP)

Sharing files between computers.

Network Address Translation

Basic NAT: Translates one IP into another.

  • e.g.,, mapping internal to external IP address.

Port Address Translation: Multiple hosts share one single public IP.

Can be static or dynamic (IP assigned to you).

Benefits:

  • Sharing IP addressses.
  • Securing devices behind NAT device (less visible points of entry)

Basic Networking Commands

Command Description
ipconfig Display network interface config
ping Send ICMP echo request
netstat Display status of network connections
nslookup DNS info

Layers and Devices