Computer Networks

WAN (Wide Area Network)

A network spanning large geographic distances connecting multiple LANs.

Scalability in WANs

WANs must grow while maintaining performance across long distances.

Packet Switch

A small computer system in a WAN that stores and forwards packets.

Traditional WAN Architecture

Uses packet switches connected by data circuits to interconnect remote sites.

Router vs Switch Separation

Switches handle local delivery; routers handle inter-site delivery.

Forming a WAN

Sites are connected by leased lines, microwave, or satellite links.

Store-and-Forward Paradigm

Packet switches buffer packets before forwarding them.

WAN Address Format

A two-part hierarchical address: packet switch ID + computer ID.

Hierarchical Address Example

Address [2,6] means computer 6 connected to switch 2.

Next-Hop Forwarding

Router chooses only the next hop toward destination, not full path.

Forwarding (Routing) Table

Maps destinations to next-hop routers.

Advantages of Next-Hop Forwarding

Smaller tables, faster lookup, simpler management.

Source Independence

Routing decision depends only on destination, not source.

Dynamic Routing Updates

Routers automatically update paths when topology changes.

Routing Software

Computes shortest paths using graph algorithms.

Default Route

A fallback route used when no specific match exists.

Static Routing

Routes manually configured and unchanged unless edited.

Dynamic Routing

Routes update automatically via routing protocols.

Distributed Route Computation

Each router computes its own routing table.

Link-State Routing (LSR)

Routers flood link info and compute full shortest-path trees.

Shortest Path First (SPF)

The algorithmic approach used by LSR.

Dijkstra’s Algorithm

Computes shortest paths from a source to all nodes.

Distance Vector Routing (DVR)

Routers exchange distance vectors with neighbors.

DVR Update Message

"I can reach X at cost Y."

LSR vs DVR

LSR uses full maps; DVR uses neighbor exchanges.

Weights in Routing Graph

Numerical link costs representing performance attributes.

Infinity (∞) in Routing

A value representing unreachable nodes.

Primary vs Backup Paths

Links assigned lower weights are preferred; higher weights are backups.

Routing Problems (LSR)

Inconsistent updates can cause incorrect paths.

Routing Loops (DVR)

Routers repeatedly forward packets to each other due to incorrect info.

Backwash Problem

Router receives its own outdated info indirectly and accepts it.

Split Horizon

Prevents loops by not advertising a route back to its source.

Hysteresis

Delays route changes to avoid rapid oscillation.

Internetworking

Connecting multiple networks so they operate as one.

Need for Internetworking

No single network technology is optimal for all uses.

Universal Service

Any computer can communicate with any other computer.

Multiple Network Problem

Early networks were isolated "islands" requiring internetworking.

Router

Device that forwards packets between networks.

Internet Architecture

Many networks interconnected via routers.

Redundancy in Internet

Extra routers and links prevent failures.

Internet vs internet vs intranet

internet = any interconnected networks; Internet = global network; intranet = private internal internet.

Virtual Network Concept

Software makes many networks appear as one unified network.

TCP/IP Role

The protocol suite enabling global internetworking.

TCP/IP Layers

Physical → Link → IP → Transport → Application layers.

Host Computer

Any device connected to the network running applications.

Router Layers Used

Routers use physical, link, internet, and partial transport layers.

Router Architecture

Router with multiple interfaces forwarding IP packets.

Routing Table

List of destinations and next hops.

Static vs Dynamic Routing

Static is fixed; dynamic adapts to changes.

CIDR (Classless Inter-Domain Routing)

Removes rigid class-based addressing; allows variable prefix lengths.

CIDR Address Format

/.

Supernetting

Aggregating contiguous networks into a larger prefix.

Longest Prefix Match

Router picks route with the most specific matching prefix.

IPv4 Address Exhaustion

The depletion of the 32-bit IPv4 address space.

IPv6 Motivation

Need for larger addressing, simpler headers, security, and mobility.

IPv6 Address Size

128-bit address space.

IPv6 Address Notation

Eight groups of hexadecimal 16-bit blocks separated by colons.

Zero Compression in IPv6

"::" replaces consecutive zero groups.

IPv6 Header Improvements

Simpler fixed header; extension headers used for options.

Transition to IPv6

Dual-stack, tunneling, and translation mechanisms.

IP Responsibilities

Routing, addressing, fragmentation, and best-effort delivery.

Best-Effort Delivery

Packets may be lost, duplicated, delayed, or corrupted.

IP Datagram Format

Contains addresses, TTL, checksum, protocol field, etc.

TTL (Time To Live)

Limits datagram lifetime to prevent loops.

Protocol Field

Identifies upper-layer protocol (TCP, UDP, ICMP).

Fragmentation

Splits datagrams exceeding MTU into smaller fragments.

Reassembly

Only the destination host reassembles fragments.

Header Checksum

Verifies IP header integrity only.

ARP (Address Resolution Protocol)

Maps IPv4 addresses to MAC addresses.

ICMP (Internet Control Message Protocol)

Sends error and diagnostic messages.

Subnetting

Dividing a network prefix into smaller logical subnets.

Multicast

Sending data to multiple receivers efficiently.

Unicast vs Broadcast vs Multicast

One-to-one, one-to-all, one-to-many.

Multicast Address

Special addresses representing receiver groups.

IGMP (Internet Group Management Protocol)

Manages host membership in multicast groups.

Multicast Routing Challenge

Avoid unnecessary duplication; split traffic only when needed.

Reverse Path Forwarding (RPF)

Forwards multicast only if it arrives on shortest path from source.

Pruning

Removing unnecessary multicast branches.

Flood-and-Prune

Initial flooding, then removing unused paths.

UDP (User Datagram Protocol)

A lightweight, unreliable, connectionless transport protocol.

End-to-End Transport

Allows applications—not just hosts—to act as endpoints.

UDP Characteristics

Connectionless, best-effort, message-oriented.

Message-Oriented Interface

Application sends messages exactly as delivered; boundaries preserved.

UDP Advantages

Low overhead, flexible interaction patterns (1:1, 1:many, many:many).

UDP Disadvantages

No reliability; messages may be lost, duplicated, or corrupted.

Applications Suited to UDP

Audio/video streaming, VoIP, DNS, gaming.

Applications Unsuitable for UDP

E-commerce, banking, file transfer.

Protocol Port Numbers

Standard identifiers used to identify applications across systems.

UDP Header

Contains source port, destination port, length, checksum.

UDP Checksum

Optional; uses ones-complement arithmetic with pseudo-header.

Pseudo Header

Includes IP info such as source, destination, protocol, and UDP length.

UDP Encapsulation

UDP datagram carried inside an IP datagram.

TCP (Transmission Control Protocol)

Reliable, connection-oriented, byte-stream transport layer protocol.

Connection-Oriented

TCP requires a connection to be established before data exchange.

Reliable Delivery

Ensures data is delivered correctly or fails gracefully.

Byte Stream Service

Applications send bytes without regard to boundaries.

Flow Control

Prevents sender from overwhelming receiver.

Congestion Control

Prevents overloading the network using algorithms like slow start.

Three-Way Handshake

SYN → SYN/ACK → ACK connection establishment.

Four-Way Teardown

FIN exchanges to close each direction of the connection.