System Design - Network

December 20, 2025 | networking, system-design

Review of some computer networking concepts and architecture.

Network protocols#

  • Defines a communication pattern
    • How one or more system can communicate with each other
  • Set of rules that define the format and sequence of messages
    • How they should be sent, received, structured.
  • Ensures correct interpretation between different systems
  • Different protocol implementation, with different standards and rules.

OSI model#

Conceptual model of how these different protocols relate to each other. Works like a map of where each functionality of these protocols lie.

  • Open System Interconnection

  • Standardizes the functionality of these protocols

  • 7 abstraction layers

    Software engineers generally work with Application, Presentation and Session layers.

    1. Application: HTTP, HTTPS, FTP, gRPC, Websockets, etc.
    2. Presentation: SSL/TLS, MIME, JPEG, GIF, etc.
    3. Session: RPC, SCP, SOCKS,
    4. Transport: TCP/UDP, etc.
    5. Network: IPv4, IPv6, etc.
    6. Data link: Point-to-Point Protocol, Mac Address
    7. Physical: Wires, WiFi, Binary, etc.

Flow of communication:

Request: 1..7 Response: 7..1

IP, IPv4 and IPv6 Protocol#

  • Operates in the Network layer of the OSI model
  • Heart of data communication of the Internet.
  • Allows devices to connect in an internal or external network
  • Defines unique IP addresses for each device in the network
  • IPv4 and IPv6 are the most common examples

IPv4 (Internet Protocol Version 4)#

  • Oldest version and most used protocol
  • 32-bit address format – allows 4.3 billion addresses of IPs
  • Lack of IPv4 IPs available
  • Private IP allocation
  • NAT (Network Address Translation)

Most used protocol at the moment.

NAT (Network Address Translation)#

It’s a strategy/pattern

  • Proxy between networks (private -> public)

  • Allows multiple devices with private IPs to access the network through an unique IP

IPv6 (Internet Protocol Version 6)#

  • Developed to resolve the problem of lack of IP addresses
  • 128-bit format – Trilion of IP addresses
  • IPsec (Internet Protocol Security)
  • Packet processing in routers and offers a better security

Dual Stack#

Not all device network implementation works with IPv6. Dual stack allows IPv4 and IPv6 to live with each other and communicate with each other.

  • Communication between networks that use IPv4 and IPv6
  • Transition mechanism
  • Requires a transition mechanism
  • Direct addressing is not compatible
  • Can communicate with IPv4 and IPv6
  • Chooses between the appropriate protocol based on the destiny.

TCP and UDP#

Transport protocol.

UDP (User Datagram Protocol)#

  • Belongs to the Transport layer of the OSI model
  • Smaller packets called Datagrams
  • Unreliable data transmission between hosts in the network
    • We may loose packets during transmission
    • No need to ACK whether we received the packet
  • Fire and forget: no delivery guarantee

TCP (Transmission Control Protocol)#

  • Belongs to the Transport layer of the OSI model
  • Protocol based on connection
  • Opens, maintains, verifies and closes the connection
  • Data sent to the host are reliable and ordered.
  • Establishes a connection BEFORE any transmission of data between the hosts
  • Three-way handshake
    • SYN, SYN-ACK and ACK

SSL/TLS#

TLS (Transport Layer Security)#

  • Successor of SSL
  • Privacy and integrity of data
  • Handshake during the session
  • Public and private keys are exchanged
  • Unique key per session
  • Encrypts the transmitted data
  • Belongs to the Presentation layer of the OSI model

DNS (Domain Name Service)#

  • Domain name system
  • Translates domain names into IPs
    • Swaps friendly names to IPs
  • Telephone list of the network
  • Finds the proper IP for a particular site or service
  • Recursive

Root Servers#

  • Base hierarchy of the DNS
  • 13 sets of the root DNS

TLD (Top-Level Domain)#

  • Find top level domain for .com

Authoritative#

  • Responsible for knowing all the details of a domain

Example#

DNS calls when we type seds.nl:

dig +trace seds.nl
; <<>> DiG 9.20.17 <<>> +trace seds.nl
;; global options: +cmd
.			513157	IN	NS	a.root-servers.net.
.			513157	IN	NS	b.root-servers.net.
.			513157	IN	NS	c.root-servers.net.
.			513157	IN	NS	d.root-servers.net.
.			513157	IN	NS	e.root-servers.net.
.			513157	IN	NS	f.root-servers.net.
.			513157	IN	NS	g.root-servers.net.
.			513157	IN	NS	h.root-servers.net.
.			513157	IN	NS	i.root-servers.net.
.			513157	IN	NS	j.root-servers.net.
.			513157	IN	NS	k.root-servers.net.
.			513157	IN	NS	l.root-servers.net.
.			513157	IN	NS	m.root-servers.net.
.			513157	IN	RRSIG	NS 8 0 518400 20260102050000 20251220040000 61809 . Fq0uact6cCRSrn/ej8lKyE5IjrOU/uljNMBx5tcVxhKb91S+MbroWVLy g35UAVS9lfS8xt5FwXE6r20603Kqz94A/8CSd++26L9Z2W0Sk31lSAPq 2xY5hhDJLLOMd/IG/xy617TYnf2wwN5F+qPsdwIqiajEwydmer2YdznL hu9PdR7kyAjUOwmn4RebH5GfQtJwmujsEhgynNhhjLTSy16kw9zg3XnR LLFDId8nocYZda8KDDAv0rYOXEmHHyRLCVp8sm7eWyOOLzdWRUfFsQ7A kOibT1HWwOW236W7TEGa931ACQpFfULRkTE3xJQuZulzLF+RoUJmHZRT Igj7RA==
;; Received 525 bytes from 192.168.50.1#53(192.168.50.1) in 14 ms

;; UDP setup with 2001:503:ba3e::2:30#53(2001:503:ba3e::2:30) for seds.nl failed: network unreachable.
;; no servers could be reached
;; UDP setup with 2001:503:ba3e::2:30#53(2001:503:ba3e::2:30) for seds.nl failed: network unreachable.
;; no servers could be reached
;; UDP setup with 2001:503:ba3e::2:30#53(2001:503:ba3e::2:30) for seds.nl failed: network unreachable.
;; UDP setup with 2001:500:2::c#53(2001:500:2::c) for seds.nl failed: network unreachable.
nl.			172800	IN	NS	ns1.dns.nl.
nl.			172800	IN	NS	ns3.dns.nl.
nl.			172800	IN	NS	ns4.dns.nl.
nl.			86400	IN	DS	17153 13 2 C5DFDDC91E7532562A35F3C2CD30823894BE08F20101F1ABF45C8AB9 739F3F49
nl.			86400	IN	RRSIG	DS 8 1 86400 20260102050000 20251220040000 61809 . Q/uCDGUgvNI/rBKEPpL5rJLUuE1LXpGIL+MIt9IBX6BvjemxCuHJ+w0J +a+6FRutGC14+iOd0LDrZwvHAy+xP7HPbcEAKsluJwXY1wHPk3aPk9UD pH4HEhfi7aU1nFvQtDkCaKhuHfdIxvLRXoERKAiqK2/RqmvSqtQIZAfY J9KHc09QzVJcDhFDtAxm5aD6q72eCzDM6NbplzODkHaYTiwFARkgB1XF F1blw1e9oW2TN4viAYF1Uuq0+n+awu/JBxWcMNDezKr6PSkG45OjxiB2 WyBWnuTSWNGW5+xmumyYVlo5JbkgJRPEYMr/EuxaZh12tgHr8Xw5dezQ ATv3pA==
;; Received 561 bytes from 192.58.128.30#53(j.root-servers.net) in 16 ms

;; UDP setup with 2001:678:20::24#53(2001:678:20::24) for seds.nl failed: network unreachable.
;; UDP setup with 2620:10a:80ac::200#53(2620:10a:80ac::200) for seds.nl failed: network unreachable.
seds.nl.		3600	IN	NS	dave.ns.cloudflare.com.
seds.nl.		3600	IN	NS	diva.ns.cloudflare.com.
seds.nl.		3600	IN	DS	2371 13 2 3C8B70CBFBDBE8E6605CE6205274B5955A929D84C4A8BD49088CD835 8CF5335B
seds.nl.		3600	IN	RRSIG	DS 13 2 3600 20260101220704 20251219070720 37807 nl. ubYajyZrOJccCp2GK+VAmbvf6RzSIFQXVLr37JeYOKB/DXfnWVKYgcen IrSYn/86cUDY0WPXw8L0pcKJPROcTg==
;; Received 237 bytes from 185.159.199.200#53(ns4.dns.nl) in 20 ms

;; UDP setup with 2606:4700:58::adf5:3b6d#53(2606:4700:58::adf5:3b6d) for seds.nl failed: network unreachable.
seds.nl.		120	IN	A	63.176.8.218
seds.nl.		120	IN	A	35.157.26.135
seds.nl.		120	IN	RRSIG	A 13 2 120 20251221151738 20251219131738 34505 seds.nl. Ha58AyZ9+CuXk9KyN4n50xk9e7Qcgjp3i41dd3IXdfbGzCv82QS7lnyM 3EmIp+OO5uNDsvFt6rRBXXu6T3YW8Q==
;; Received 171 bytes from 172.64.33.109#53(dave.ns.cloudflare.com) in 18 ms

DHCP (Dynamic Host Configuration Protocol)#

  • Eases IP attribution in small networks (home, bars, etc)
  • Dynamic configuration of hosts
  • Designates automatically an IP address for a particular host
  • Reduces address conflict
  • DORA – 4 main steps
    • Discovery
    • Offer
    • Request
    • Acknowledgment

HTTP/1, HTTP/2 and HTTP/3 Protocols#

  • Layer 7 of the OSI model
  • Built on top of TCP/IP
  • Request/Response
  • Client/Server
  • Synchronous protocols
  • Body, headers, cookies, status code, etc.
  • HTTP/1.x, HTTP/2 and HTTP/3

HTTP/1#

  • Discontinued
  • Persistent connection
    • Eliminates the need of a new connection in every request
  • Pipeline
    • Resolves Head-of-Line Blocking
  • Allows sending numerous request without waiting for the previous response

HTTP/2#

  • Released in 2015
  • Format, prioritizes and transport data
  • Request priority
  • Multiplexing
    • Multiple request and responses within the same TCP connection
  • Server push

HTTP/3 (QUIC)#

  • Started by Google
  • New updates to the protocol
  • Substitutes TCP for UDP
  • Latency, security and efficiency when transfering data
  • Reduces latency and handshake
  • Improves multiplexing
  • Streaming

Response time#

Big difference between latency and response time.

Latency             Processing time     Response latency
========================================================
###############-------------------------################
========================================================

The sum of latency + processing time is the response time.

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