Basics of Networking

Brief History of Internet

Early computers were huge, expensive machines so much that an entire labs would only have single machine to serve their needs. The key requirement was to allow multiple people to access this system at same time, which lead to development of ideas to share processing power, storage, printers and terminals across connection. This lead to development of projects like ARPANET which was the early ancestor of the internet. At this point, multiple computers within a network could communicate with each other. But as more networks appeared, each used their own implementation for communication which proved as a hurdle to communicate across different networks. This required standardization in communication protocol which could be implemented independent of underlying hardware. This solution was provided by TCP/IP protocol which did two critical jobs:

• IP moves packets across different networks
• TCP ensures those packets arrive reliably and in the correct order

This marked as the starting point for development of internet which further evolved using key technologies like DNS, WWW, Browsers, etc.

Due to lack of standardized way of communication, computer vendors invented their own standards which caused huge problems when integrating different systems . OSI Model was an attempt to provide common standards which could be implemented by any hardware regardless of its vendor. Its idea was to break down networking in different layers, where each layer handled specific tasks. But by the time OSI protocols were ready, TCP/IP was already working in real networks and had huge adoption. Due to this reason OSI protocols never took off, however its conceptual model survived because it explains networking much clearly.

OSI Model

OSI (Open System Interconnection) model uses 7 layers of abstraction where each layer represents a networking component.

• Layer 7/Application Layer, handles protocol like application protocols like HTTP, FTP, gRPC. Usually these protocols are mostly implemented using standard libraries of respective programming languages, so that you won't have to go into implementation specific.
• Layer 6/Presentation Layer, handles serialization and deserialization of programming objects into byte strings as per the encoding provided. This may include encrypting/decrypting (SSL/TLS), formatting or encoding/decoding data.
• Layer 5/Session Layer, handles sessions/connections b/w application. Usually involves around opening/closing connections, keeping sessions active and synchronizing data.
• Layer 4/Transport Layer, handles end-to-end communication b/w two systems using protocols like TCP/UDP. It involves segmenting data into chunks (packets/datagrams), assigning them ports to allow sending/reading them by other processes (on same/different computer across network).
• Layer 3/Network Layer, handles routing packets across networks to destined party using IP address.
• Layer 2/Data Link, creates a reliable link between two devices on the same network using MAC address.

• Layer 1/Physical, moves raw bits (0s and 1s) across wired/wireless signals by translating/assembling digital signal into/from analog signals(like (1)) .

  1. like electric, light or radio waves.

For better understanding, follow below sequence diagrams for journey of POST request from both sender and receiver perspective.

sequenceDiagram
    participant L7 as Layer 7<br/>(Application)
    participant L6 as Layer 6<br/>(Presentation)
    participant L5 as Layer 5<br/>(Session)
    participant L4 as Layer 4<br/>(Transport)
    participant L3 as Layer 3<br/>(Network)
    participant L2 as Layer 2<br/>(Data Link)
    participant L1 as Layer 1<br/>(Physical)

    %%Sender
    Note over L7,L6: From Senders point of view
    L7->>L6: Send POST request<br/> with JSON payload
    L6->>L5: Serialize JSON <br/>into byte strings
    L5->>L4: Establish/Identify<br/> connection
    L4->>L3: Segment bytes into segments<br/>(add source & destination ports)
    L3->>L2: Encapsulate segments into IP packets<br/>(add source & destination IPs)
    L2->>L1: Encapsulate packet into frames<br/>(add source & destination MACs)
    Note over L1: -> Convert frames into analog<br/> signal and send over network

    %%Receiver
    Note over L1: <- Convert analog signals into digital bits
    L1->>L2: Assemble bits into frames
    L2->>L3: Extract IP packets from frames
    L3->>L4: Reassemble IP packets into TCP segments
    L4->>L5: Establish/identify<br/> connection
    L5->>L6: Deserialize bytes into<br/> JSON data
    L6->>L7: Trigger “request received” <br/>event for app
    Note over L7, L6: From Receivers point of view

Use mouse to pan and zoom

Basic network routing

When your data is transmitted over network, it goes through different devices which helps to rout it towards the fastest and appropriate direction. This redirection requires peeking information within our request like IP addresses, MAC addresses.

• The request is first send to Switch which allows you to communicate within your LAN. For this it needs to look up the MAC address of data (L2), to move data b/w right device on LAN. However, switch doesn't have any information about external networks.
• To communicate with external networks, you've to use a Router (also known as default gateway) which has routing information required to send the data towards appropriate network. This requires looking up the destination IP address (L3).
• From there, data travel through ISP's access network which aggregate multiple data frames (L2) before moving them to their routers where they can be forwarded across cities, countries or continents. Across these WAN, data just hops from one router to another.
• Finally, when data reaches the destination router, it moves the data to LAN switch which looks up the MAC address and forwards the request to correct host.

This is a brief overview of how multiple networking components interact to allow host to host communication over world. In following chapters, we'll dive deep into each of these components and understand their HOWS and WHYS.

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