fixup chapter 2

1 files changed, 0 insertions, 554 deletions

diff --git a/NOTES.md b/NOTES.md
index 7721d32..e69de29 100644
--- a/NOTES.md
+++ b/NOTES.md
@@ -1,554 +0,0 @@
-# Chapter 1: Computer Networks and the Internet - Study Notes
-
-## 1.1 What Is the Internet?
-
-### Nuts-and-Bolts Description
-- **Hosts/End Systems**: Computing devices connected to the Internet (computers, smartphones, servers, IoT devices)
-  - ~18 billion devices in 2017, projected 28.5 billion by 2022
-- **Communication Links**: Physical media connecting devices
-  - Different types: coaxial cable, copper wire, optical fiber, radio spectrum
-  - **Transmission rate**: Measured in bits/second
-- **Packet Switches**: Forward packets between links
-  - **Routers**: Used in network core
-  - **Link-layer switches**: Used in access networks
-- **Packets**: Segments of data with added headers
-- **Route/Path**: Sequence of links and switches traversed by packet
-- **Internet Service Providers (ISPs)**: Networks of packet switches and links
-- **Protocols**: TCP/IP are the principal protocols
-- **Internet Standards**: Developed by IETF as RFCs (Requests for Comments)
-
-### Services Description
-- Internet as infrastructure providing services to distributed applications
-- **Socket Interface**: Rules for how programs request Internet to deliver data
-- Applications run on end systems, not in network core
-
-### What Is a Protocol?
-**Definition**: A protocol defines the format and order of messages exchanged between communicating entities, as well as the actions taken on transmission/receipt of messages
-
-## 1.2 The Network Edge
-
-### Access Networks
-
-#### Home Access
-1. **DSL (Digital Subscriber Line)**
-   - Uses existing telephone lines
-   - Asymmetric speeds (different up/down)
-   - Up to 24/52 Mbps down, 3.5/16 Mbps up
-   - Limited by distance from central office (5-10 miles)
-
-2. **Cable Internet**
-   - Uses cable TV infrastructure (HFC - Hybrid Fiber Coax)
-   - Shared broadcast medium
-   - DOCSIS standards: up to 1.2 Gbps down, 100 Mbps up
-
-3. **FTTH (Fiber to the Home)**
-   - Direct optical fiber to homes
-   - Can provide gigabit speeds
-   - PON (Passive Optical Networks) commonly used
-
-4. **5G Fixed Wireless**
-   - Wireless data transmission to home modem
-   - No physical cabling required
-
-#### Enterprise Access
-- **Ethernet**: Most prevalent in corporate/university networks
-  - 100 Mbps to 10+ Gbps speeds
-- **WiFi (802.11)**: Wireless LAN access
-  - Up to 100+ Mbps shared transmission rate
-
-#### Wide-Area Wireless
-- **4G/5G**: Cellular networks for mobile devices
-  - 4G: up to 60 Mbps real-world speeds
-  - Coverage within tens of kilometers of base station
-
-### Physical Media
-
-#### Guided Media
-- **Twisted-Pair Copper Wire**: Most common, up to 10 Gbps (Cat 6a)
-- **Coaxial Cable**: Used in cable TV/Internet systems
-- **Fiber Optics**: High bandwidth, low attenuation, immune to interference
-
-#### Unguided Media
-- **Terrestrial Radio**: Short-range, local area, wide area
-- **Satellite**: Geostationary (280ms delay) and LEO satellites
-
-## 1.3 The Network Core
-
-### Packet Switching
-- **Store-and-Forward Transmission**: Switch must receive entire packet before forwarding
-  - End-to-end delay = N × (L/R) for N links
-- **Queuing Delays**: Variable delays when packets wait in output buffers
-- **Packet Loss**: Occurs when buffers overflow
-- **Forwarding Tables**: Map destination addresses to outbound links
-- **Routing Protocols**: Automatically set forwarding tables
-
-### Circuit Switching
-- Resources reserved for duration of connection
-- **Multiplexing Methods**:
-  - FDM (Frequency-Division): Divides frequency spectrum
-  - TDM (Time-Division): Divides time into slots
-- Guaranteed constant rate but wastes resources during idle periods
-
-### Packet vs Circuit Switching
-- **Packet switching advantages**:
-  - Better resource sharing
-  - Simpler implementation
-  - More efficient for bursty data
-- **Circuit switching advantages**:
-  - Guaranteed resources
-  - Predictable performance
-
-### Network of Networks
-- Internet is interconnection of ISPs at different tiers:
-  - **Tier-1 ISPs**: Global backbone networks
-  - **Regional ISPs**: Connect access ISPs to Tier-1
-  - **Access ISPs**: Provide Internet access to end users
-- **IXPs (Internet Exchange Points)**: Where ISPs peer
-- **Content Provider Networks**: (e.g., Google, Amazon) bypass upper tiers when possible
-
-## 1.4 Delay, Loss, and Throughput
-
-### Types of Delay (d_nodal = d_proc + d_queue + d_trans + d_prop)
-
-1. **Processing Delay (d_proc)**: Time to examine packet header
-2. **Queuing Delay (d_queue)**: Time waiting in queue
-   - **Traffic Intensity = La/R** (arrival rate × packet size / transmission rate)
-   - Must be < 1 to avoid infinite delays
-3. **Transmission Delay (d_trans)**: L/R (packet size / transmission rate)
-4. **Propagation Delay (d_prop)**: d/s (distance / propagation speed)
-
-### Packet Loss
-- Occurs when queue capacity exceeded
-- Performance measured by delay and loss probability
-
-### Throughput
-- **Instantaneous throughput**: Rate at any instant
-- **Average throughput**: F/T (file size / transfer time)
-- Limited by **bottleneck link** (minimum rate along path)
-
-## 1.5 Protocol Layers
-
-### Five-Layer Internet Protocol Stack
-1. **Application Layer**: Network applications (HTTP, SMTP, FTP, DNS)
-   - Data unit: Message
-2. **Transport Layer**: Process-to-process data transfer (TCP, UDP)
-   - Data unit: Segment
-3. **Network Layer**: Routing datagrams (IP protocol)
-   - Data unit: Datagram
-4. **Link Layer**: Data transfer between neighboring network elements
-   - Data unit: Frame
-5. **Physical Layer**: Bits on the wire
-
-### Encapsulation
-- Each layer adds its own header information
-- Payload = packet from layer above
-
-## 1.6 Networks Under Attack
-
-### Types of Attacks
-1. **Malware**: Viruses, worms, spyware, botnets
-2. **Denial-of-Service (DoS)**:
-   - Vulnerability attacks
-   - Bandwidth flooding
-   - Connection flooding
-   - DDoS uses multiple sources
-3. **Packet Sniffing**: Passive interception of packets
-4. **IP Spoofing**: Fake source addresses
-
-## Key Performance Metrics
-- **Bandwidth-Delay Product**: R × d_prop (capacity of "pipe")
-- **Utilization**: Actual use vs available capacity
-- **Traffic Intensity**: Must be < 1 for stable operation
-
-## Important Formulas
-- Store-and-forward delay (N links): d = N × (L/R)
-- Total nodal delay: d_nodal = d_proc + d_queue + d_trans + d_prop
-- Transmission delay: d_trans = L/R
-- Propagation delay: d_prop = d/s
-- Traffic intensity: La/R (must be < 1)
-- Throughput = min(R1, R2, ..., RN) for path with N links
-
-## Key Concepts to Remember
-- Internet is a "network of networks"
-- Packet switching more efficient than circuit switching for bursty data
-- Protocols define communication rules between entities
-- Layering provides modularity and abstraction
-- Performance limited by delays, loss, and throughput constraints
-- Security was not originally built into Internet design
-
-# Chapter 2: Application Layer - Study Notes
-
-## 2.1 Principles of Network Applications
-
-### Network Application Architectures
-
-**Two Main Architectures:**
-
-1. **Client-Server Architecture**
-   - Always-on server with fixed, well-known IP address
-   - Clients communicate with server, not directly with each other
-   - Server often housed in data centers for scalability
-   - Examples: Web, FTP, Telnet, email
-
-2. **P2P (Peer-to-Peer) Architecture**
-   - Minimal reliance on dedicated servers
-   - Direct communication between intermittently connected hosts (peers)
-   - **Self-scalability**: Each peer adds both workload and service capacity
-   - Cost-effective (no significant server infrastructure needed)
-   - Challenges: security, performance, reliability
-
-### Processes Communicating
-
-- **Process**: A program running within an end system
-- **Socket**: Software interface between application layer and transport layer
-  - Analogy: Process = house, Socket = door
-- **Client Process**: Initiates communication
-- **Server Process**: Waits to be contacted
-
-### Addressing Processes
-
-To identify a receiving process, need:
-1. **IP Address**: 32-bit quantity identifying the host
-2. **Port Number**: Identifies the receiving process/socket in the host
-   - Well-known ports: HTTP (80), SMTP (25)
-
-### Transport Services Available to Applications
-
-**Four Dimensions of Service:**
-
-1. **Reliable Data Transfer**
-   - Guaranteed delivery without errors
-   - Important for: email, file transfer, Web documents
-
-2. **Throughput**
-   - Guaranteed available throughput at specified rate
-   - **Bandwidth-sensitive apps**: multimedia applications
-   - **Elastic apps**: email, file transfer, Web
-
-3. **Timing**
-   - Guarantees on delivery time (e.g., <100ms)
-   - Important for: real-time apps, games, telephony
-
-4. **Security**
-   - Encryption, data integrity, authentication
-
-### Transport Services Provided by Internet
-
-**TCP (Transmission Control Protocol)**
-- Connection-oriented service (handshaking)
-- Reliable data transfer
-- Full-duplex connection
-- Congestion control
-- NO throughput or timing guarantees
-
-**UDP (User Datagram Protocol)**
-- Connectionless (no handshaking)
-- Unreliable data transfer
-- No congestion control
-- Lightweight, minimal services
-
-**TLS (Transport Layer Security)**
-- Enhancement for TCP
-- Provides encryption, data integrity, authentication
-- Implemented in application layer
-
-## 2.2 The Web and HTTP
-
-### HTTP Overview
-
-- **HyperText Transfer Protocol**: Web's application-layer protocol
-- Client-server model: browsers (clients) and Web servers
-- Uses TCP as transport protocol
-- **Stateless protocol**: Server maintains no client information
-
-### HTTP Connections
-
-**Non-Persistent Connections (HTTP/1.0)**
-- Each request/response over separate TCP connection
-- At most one object sent over each connection
-- Response time: 2 RTTs + transmission time per object
-- Overhead of establishing multiple TCP connections
-
-**Persistent Connections (HTTP/1.1 default)**
-- Multiple objects sent over single TCP connection
-- Server leaves connection open after response
-- Pipelining: Back-to-back requests without waiting
-
-### HTTP Message Format
-
-**Request Message:**
-```
-GET /somedir/page.html HTTP/1.1
-Host: www.someschool.edu
-Connection: close
-User-agent: Mozilla/5.0
-Accept-language: fr
-```
-
-Components:
-- Request line: method, URL, version
-- Header lines: Host, User-agent, Accept-language, etc.
-- Entity body (empty for GET, used with POST)
-
-**Response Message:**
-```
-HTTP/1.1 200 OK
-Connection: close
-Date: Tue, 18 Aug 2015 15:44:04 GMT
-Server: Apache/2.2.3
-Last-Modified: Tue, 18 Aug 2015 15:11:03 GMT
-Content-Length: 6821
-Content-Type: text/html
-(data...)
-```
-
-Components:
-- Status line: version, status code, status message
-- Header lines
-- Entity body (the requested object)
-
-**Common Status Codes:**
-- 200 OK: Success
-- 301 Moved Permanently
-- 400 Bad Request
-- 404 Not Found
-- 505 HTTP Version Not Supported
-
-### Cookies
-
-**Four Components:**
-1. Cookie header in HTTP response
-2. Cookie header in HTTP request
-3. Cookie file on client
-4. Back-end database at Web site
-
-**Uses:** User identification, shopping carts, recommendations
-
-### Web Caching (Proxy Servers)
-
-**Benefits:**
-- Reduces response time for clients
-- Reduces traffic on institution's access link
-- Cost savings (bandwidth)
-
-**Conditional GET:**
-- Uses `If-Modified-Since:` header
-- Server responds with 304 Not Modified if unchanged
-- Reduces unnecessary data transfer
-
-### HTTP/2
-
-**Goals:**
-- Reduce latency through multiplexing
-- Request prioritization
-- Server push
-- Header compression
-
-**Key Features:**
-- **Framing**: Messages broken into small frames, interleaved
-- Binary protocol (more efficient than text)
-- Solves Head-of-Line (HOL) blocking problem
-- Single TCP connection for entire page
-
-## 2.3 Electronic Mail
-
-### Components
-
-1. **User Agents**: Mail clients (Outlook, Gmail app)
-2. **Mail Servers**: Store mailboxes, run SMTP
-3. **SMTP**: Simple Mail Transfer Protocol
-
-### SMTP
-
-- Uses TCP, port 25
-- Push protocol
-- Three phases: handshaking, transfer, closure
-- Commands: HELO, MAIL FROM, RCPT TO, DATA, QUIT
-- 7-bit ASCII restriction (legacy)
-- Uses persistent connections
-
-### Mail Access Protocols
-
-**Problem:** SMTP is push; need pull for retrieval
-
-**Solutions:**
-- **HTTP**: Web-based email (Gmail)
-- **IMAP**: Internet Mail Access Protocol
-  - Allows folder management on server
-  - More features than POP3
-
-## 2.4 DNS (Domain Name System)
-
-### Services Provided
-
-1. **Hostname to IP translation** (main service)
-2. **Host aliasing**: Canonical and alias names
-3. **Mail server aliasing**
-4. **Load distribution**: Rotate IP addresses for replicated servers
-
-### DNS Hierarchy
-
-1. **Root DNS servers** (~1000 instances of 13 servers)
-2. **Top-Level Domain (TLD) servers** (com, org, edu, country codes)
-3. **Authoritative DNS servers** (organization's own servers)
-4. **Local DNS servers** (ISP's default name servers)
-
-### DNS Operation
-
-**Iterative Queries** (typical):
-- Local DNS server queries on behalf of client
-- Gets referrals to other servers
-
-**Recursive Queries**:
-- DNS server obtains mapping on behalf of requester
-
-**DNS Caching**:
-- Servers cache mappings to improve performance
-- TTL (Time To Live) determines cache duration
-- Reduces load on root servers
-
-### DNS Records (Resource Records)
-
-Format: `(Name, Value, Type, TTL)`
-
-**Types:**
-- **Type=A**: Name is hostname, Value is IP address
-- **Type=NS**: Name is domain, Value is authoritative DNS server
-- **Type=CNAME**: Name is alias, Value is canonical name
-- **Type=MX**: Name is alias, Value is mail server name
-
-## 2.5 Peer-to-Peer File Distribution
-
-### P2P Scalability
-
-**Distribution Time Formulas:**
-
-Client-Server: `D_cs = max{NF/u_s, F/d_min}`
-- Linear increase with N peers
-
-P2P: `D_P2P = max{F/u_s, F/d_min, NF/(u_s + Σu_i)}`
-- Self-scaling: More peers = more capacity
-
-### BitTorrent
-
-**Key Concepts:**
-- **Torrent**: Collection of peers distributing a file
-- **Chunks**: Equal-size pieces (typically 256KB)
-- **Tracker**: Infrastructure node tracking peers
-
-**Mechanisms:**
-1. **Rarest First**: Request rarest chunks first
-2. **Tit-for-Tat**: Upload to peers providing best download rates
-   - Top 4 uploaders get priority (unchoked)
-   - 1 random peer (optimistically unchoked) every 30 seconds
-
-## 2.6 Video Streaming and CDNs
-
-### Video Characteristics
-- High bit rate (100 kbps to 4+ Mbps)
-- Can be compressed to different quality levels
-- Storage/bandwidth intensive
-
-### HTTP Streaming and DASH
-
-**Simple HTTP Streaming:**
-- Video stored as file with URL
-- Client buffers before playback
-
-**DASH (Dynamic Adaptive Streaming over HTTP):**
-- Multiple versions at different bit rates
-- Client requests chunks adaptively
-- Manifest file lists all versions
-- Client measures bandwidth, selects quality
-
-### Content Distribution Networks (CDNs)
-
-**Server Placement Strategies:**
-
-1. **Enter Deep** (Akamai approach)
-   - Servers in many access ISPs
-   - Get close to users
-   - Higher maintenance overhead
-
-2. **Bring Home** (Limelight approach)
-   - Large clusters at IXPs
-   - Lower maintenance
-   - Potentially higher latency
-
-**CDN Operation:**
-- DNS redirect to select CDN server
-- Cluster selection based on:
-  - Geographic proximity
-  - Real-time performance measurements
-  - Load balancing
-
-### Case Studies
-
-**Netflix:**
-- Uses Amazon cloud for website/processing
-- Own private CDN for video delivery
-- Push caching during off-peak hours
-- No DNS redirect needed
-
-**YouTube:**
-- Google's private CDN
-- Pull caching
-- DNS redirect for server selection
-- HTTP streaming (not adaptive)
-
-## 2.7 Socket Programming
-
-### UDP Socket Programming
-
-**Key Concepts:**
-- Connectionless
-- Must attach destination address to each packet
-- No guarantee of delivery
-
-**Python Functions:**
-- `socket(AF_INET, SOCK_DGRAM)`: Create UDP socket
-- `sendto()`: Send with destination address
-- `recvfrom()`: Receive data and source address
-- `bind()`: Assign port to socket
-
-### TCP Socket Programming
-
-**Key Concepts:**
-- Connection-oriented (three-way handshake)
-- Reliable, in-order delivery
-- Server has welcoming socket and connection sockets
-
-**Python Functions:**
-- `socket(AF_INET, SOCK_STREAM)`: Create TCP socket
-- `bind()`: Assign port to server socket
-- `listen()`: Server listens for connections
-- `accept()`: Create connection socket for client
-- `connect()`: Client initiates connection
-- `send()/recv()`: Data transfer
-
-## Key Formulas and Metrics
-
-- **RTT (Round-Trip Time)**: Time for small packet to travel from client to server and back
-- **HTTP Response Time (non-persistent)**: 2 RTTs per object + transmission time
-- **Traffic Intensity**: (arrival rate × packet size) / link rate (must be < 1)
-- **Cache Hit Rate**: Fraction of requests satisfied by cache
-
-## Important Port Numbers
-
-- HTTP: 80
-- HTTPS: 443
-- SMTP: 25
-- DNS: 53
-- IMAP: 143
-- POP3: 110
-
-## Key Takeaways
-
-1. Application layer protocols define message format, order, and actions
-2. Client-server is simpler but less scalable than P2P
-3. HTTP is stateless; cookies add state
-4. DNS is critical Internet infrastructure using hierarchy
-5. CDNs bring content closer to users
-6. DASH enables adaptive video streaming
-7. Socket programming allows custom network applications
-8. TCP provides reliability; UDP provides simplicity