Computer Networks

Tue, 15 Apr 2025 00:00:00 +0000

Chapter 1. Introduction

1.1 Evolution of Data Networks

Consider commercial viability, line installation cost (very high), and relatively cheaper equipment.
Evaluate whether new technologies can reuse existing infrastructure.
- Success: xDSL leveraged telephone lines for broadband.
- Failure: 1970s video phones—insufficient demand relative to infrastructure cost.
Modems convert digital ↔ analogue signals so legacy phone lines can transport data.

1.2 Development Path

Telcos gradually expanded data services while keeping the telephone backbone.
Phone number ↔ IP mapping allowed voice-over-IP on regular lines, though with lower audio quality.

1.3 Internet & ISPs

ISPs (e.g., KT, SKT) provide network access.
Hierarchical topology: local ISPs → backbone ISPs → global backbones connected via peering/transit.

Chapter 19. Host-to-Host Delivery

2.1 OSI Seven Layers (Quick Review)

Physical: signal transmission.
Data Link: MAC, error detection, hop-to-hop delivery.
Network: routing, addressing, multicast, ARP.
Transport: TCP/UDP for end-to-end reliability.
Session
Presentation
Application: HTTP, DNS, etc. (Labs typically focus on layers 4–7.)

2.2 Network Layer Roles

Internetworking: connects heterogeneous networks.
Addressing: assigns globally unique identifiers.
Routing: chooses optimal paths.
Packetizing: encapsulates upper-layer data.
Fragmenting: splits packets to respect MTU limits.

2.3 Switching

Circuit switching: dedicated path (telephone network).
Packet switching: best-effort, store-and-forward (Internet).

2.4 Datagram Service

Connectionless: each packet is routed independently.
Packets may arrive out of order; TCP reorders and handles flow control.

Chapter 19. IPv4 Addressing and Subnets

3.1 Address Classes

32-bit addresses must be unique and universally routable.
Classful ranges:
- Class A: 0xxxxxxx (0–127), NetID 7 bits, HostID 24 bits.
- Class B: 10xxxxxx (128–191), NetID 14 bits, HostID 16 bits.
- Class C: 110xxxxx (192–223), NetID 21 bits, HostID 8 bits.
- Class D: 1110xxxx (224–239), multicast.
- Class E: 1111xxxx (240–255), reserved for experiments.
Special addresses: -.0.0.0 (network), -.0.0.1 (gateway), -.255.255.255 (broadcast).

3.2 Subnetting

Partition large networks into manageable subnets to improve organisation and security.
Use bitwise AND: IP & subnet mask = subnet address.
CIDR enables flexible, classless allocation.

3.3 DHCP

Dynamically assigns IP configuration. Sequence: Discover → Offer → Request → ACK.
Mitigates address exhaustion; well-suited for mobile devices.
Combine with NAT to hide private addresses.

3.4 NAT

Translates private ↔ public addresses so many hosts share one public IP.
PAT (port address translation) differentiates sessions.
External access requires port forwarding or static public IPs.

3.5 IP Datagram

TTL decrements at each router; packet discarded at zero to avoid loops.
Fragmentation splits packets exceeding link MTU; reassembly happens at the final destination.

Chapter 20. Network Layer Protocols

4.1 ARP

Resolves IP addresses to MAC addresses within the same subnet.
Request sent via broadcast; reply delivered as unicast. Results cached.

4.2 IP

Minimum 20-byte header with version, header length, TTL, protocol, checksum, etc.
Protocol field multiplexes upper-layer protocols (TCP=6, UDP=17, OSPF=89, …).

4.3 Fragmentation & MTU

Routers split packets when downstream MTU is smaller.
DF (Don’t Fragment) flag forbids splitting, prompting ICMP errors instead.

4.4 ICMP

Reports network-layer errors and diagnostic information.
Error messages: Destination Unreachable, (deprecated) Source Quench, Time Exceeded, Parameter Problem, Redirect.
Query messages: Echo (ping), Timestamp, etc.
ICMP only reports problems—flow and congestion control live elsewhere.

4.5 IPv6 Overview

128-bit address space, simplified header, richer QoS support.
Address notation uses 16-bit hex blocks with :: compression.
Transition strategies:
1. Dual Stack (run IPv4 & IPv6 simultaneously).
2. Tunnelling (encapsulate IPv6 in IPv4).
3. Header Translation (convert headers; may lose info like flow labels).

Chapter 21. Unicast & Multicast Routing

5.1 Routing Types

Unicast: one-to-one (default Internet traffic).
Multicast: one-to-many (group members only).
Broadcast: one-to-all (limited in IPv4).
Anycast: one-to-nearest (e.g., DNS root replicas).

5.2 Overlay Networks

Logical networks built atop the physical Internet (e.g., CDNs).
Reduce load when delivering identical media streams.
New services must weigh overlay benefits against cost.

5.3 Unicast Protocols

RIP: distance vector, hop-count metric—small networks.
OSPF: link-state—large enterprise networks.
BGP: path vector—inter-AS routing on the global Internet.

5.4 Multicast Routing

Hosts join/leave groups via IGMP.
Routers build multicast trees to forward only along necessary links.
Protocols include PIM (protocol-independent multicast) variants.

Chapter 22. Security Fundamentals

6.1 HTTP Security

Basic auth: Base64 username/password—susceptible to eavesdropping.
Digest auth: adds hashing yet still vulnerable (no server authentication, MITM).
Real-world solution: HTTPS via SSL/TLS to encrypt the transport layer.

6.2 Secure Email

Requirements: confidentiality, authentication, integrity, non-repudiation.
Encryption approaches:
- Symmetric keys (fast but key distribution is hard).
- Public/private keys (PGP combines both).
Strong passphrases should be long, unpredictable, yet memorable.

6.3 Public-Key Validation

Certificate Authorities (CA): trusted third parties issue certificates binding public keys to identities. Certificates include serials, subject/issuer info, validity, signatures.
PGP Web of Trust: decentralised—users vouch for one another; keys have validity and trust scores.

6.4 Transport-Layer Security

SSL/TLS: negotiate session keys, encrypt data, authenticate endpoints. HTTPS = HTTP over TLS.
Drawback: handshake incurs public-key operations and latency.

6.5 Network-Layer Security (IPsec)

Provides tunnelling, encryption, and integrity between routers or hosts.
Protocols: ESP (encryption + authentication) and AH (authentication + integrity only).
Modes: transport (end-to-end) and tunnel (network-to-network).

6.6 VPN

Emulates a private network over the public Internet.
Comparison:
- Leased line: secure but expensive.
- Hybrid: mix of private lines and Internet.
- VPN: IPsec tunnel mode balances cost and security.

7. Summary & Study Tips

Analyse cost vs. benefit when reusing existing infrastructure versus deploying new tech.
Practise subnetting and CIDR to design addressing plans.
Compare routing protocols (RIP/OSPF/BGP) and match them to network scale.
Security spans multiple layers: application (HTTPS) → transport (TLS) → network (IPsec).
Use packet-analyser tools (Wireshark) to observe ARP, ICMP, DHCP, TLS handshakes and solidify understanding.

ComputerNetworks | Portfolio

Computer Networks

Chapter 1. Introduction

1.1 Evolution of Data Networks

1.2 Development Path

1.3 Internet & ISPs

Chapter 19. Host-to-Host Delivery

2.1 OSI Seven Layers (Quick Review)

2.2 Network Layer Roles

2.3 Switching

2.4 Datagram Service

Chapter 19. IPv4 Addressing and Subnets

3.1 Address Classes

3.2 Subnetting

3.3 DHCP

3.4 NAT

3.5 IP Datagram

Chapter 20. Network Layer Protocols

4.1 ARP

4.2 IP

4.3 Fragmentation & MTU

4.4 ICMP

4.5 IPv6 Overview

Chapter 21. Unicast & Multicast Routing

5.1 Routing Types

5.2 Overlay Networks

5.3 Unicast Protocols

5.4 Multicast Routing

Chapter 22. Security Fundamentals

6.1 HTTP Security

6.2 Secure Email

6.3 Public-Key Validation

6.4 Transport-Layer Security

6.5 Network-Layer Security (IPsec)

6.6 VPN

7. Summary & Study Tips