Computer Networks

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)

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

2.2 Network Layer Roles

1. Internetworking: connects heterogeneous networks.
2. Addressing: assigns globally unique identifiers.
3. Routing: chooses optimal paths.
4. Packetizing: encapsulates upper-layer data.
5. 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.