Untitled Quiz

DNS Overview

• DNS (Domain Name System) translates domain names (e.g., www.example.com) into IP addresses (e.g., 192.168.1.1).
• This allows users to use memorable names instead of numerical IP addresses.
• A hierarchical system with different types of DNS servers working together: Root, TLD (Top-Level Domain), Authoritative, and Local.

Recursive Queries in DNS

• A recursive query puts the burden of name resolution on the contacted name server.
• This places a heavy load on the upper levels of the hierarchy.
• The contacted DNS server will query other servers until the answer is found.

DNS Caching

• DNS caching is used to improve performance and reduce the number of DNS messages on the internet.
• Caching improves performance by reducing delay.
• Caching saves bandwidth by reducing the amount of data transferred across the internet.
• Cache entries timeout after a specific time (TTL).
• TLD servers are typically cached in local name servers.
• Root name servers are not often visited due to caching.

DNS Records

• Resource records (RRs) contain name, address, and type of record.
• Type A: maps hostname to IP address of end device.
• Type NS: an authoritative name server for a domain.
• Type CNAME: maps alias hostname to canonical hostname.
• Type MX: indicates a mail server for an alias hostname.

DNS Protocol and Messages

• DNS uses query and reply messages, both with the same message form.
• Messages have identification (16-bit number) for matching queries and replies.
• Flags specify if recursion is desired or available, and whether the reply is authoritative.

DNS Messages Structure

• DNS messages have 12-byte headers.
• Identification: for query-reply matching.
• Questions: to be answered.
• Answers, Authority, Additional information: resource records providing the answer, for contact information about other servers, and additional helpful related data needed for the query.

Using nslookup

• nslookup is a command-line tool for querying DNS servers.
• It provides results showing domain names, addresses, and other relevant information (names of authoritative servers, addresses, etc).

Getting Your Information into the DNS

• Registering a domain name with a registrar.
• Providing the name, IP address of primary and secondary authoritative DNS servers.
• Using a registrar's function to put host name and A records into the relevant TLD server.

DNS Security

• DDoS Attacks target root and TLD servers with high traffic, often overwhelming them.
• Spoofing attacks intercept DNS queries and return false replies.
• DNS cache poisoning can occur where DNS servers receive bogus entries for a domain.

P2P Architecture

• No central server.
• Peers request service from other peers, and provide service in return.
• Self-scalable with new peers bringing new service, capacity, and demands.

File Distribution: Client-Server vs P2P

• Client-server distribution time increases linearly with the number of peers.
• P2P distribution time increases with the total upload capacity of all participating peers.

File Distribution: BitTorrent

• Tracker: tracks peers participating in the torrent.
• Peers in a torrent exchange file chunks and exchange portions of data in a tit-for-tat manner.

Video Streaming and CDNs

• Video traffic consumes a significant amount of Internet bandwidth.
• CDNs distribute video content over geographically distributed servers (caches) to improve performance and user experience.
• CDN placement strategies include "Enter Deep" and "Bring Home" locations.

Multimedia: Video

• Video is a sequence of images displayed at a constant rate (e.g., 24 images/second).
• Digital images are represented by an array of pixels.
• Coding techniques (spatial, temporal) reduce redundancy within and between images to decrease the number of bits needed to encode an image.
• CBR (constant bit rate) encoding maintains a fixed bit rate, while VBR (variable bit rate) adjusts the bit rate based on video content.

Streaming Stored Video

• In a simple stored video scenario, all clients receive the same encoding despite bandwidth variations.
• This can lead to a suboptimal experience.

Streaming Multimedia: DASH

• DASH (Dynamic Adaptive Streaming over HTTP) allows for adaptive encoding rates based on changing bandwidth conditions.
• Servers provide different chunks encoded at varying bit rates.
• Clients periodically measure server-to-client bandwidth and adjust playback to use more or less CPU-intensive or bandwidth-intensive encoding.
• Client-side intelligence manages buffer starvation and overflow.

Content Distribution Networks (CDNs)

• CDNs use multiple geographically distributed servers and cache content close to users to reduce latency, increase bandwidth availability, and distribute traffic load.
• CDN Placement Strategies involve choosing locations that are close to users (Enter Deep), and locations closer to ISPs (Bring Home).

CDN Services

• CDNs enhance regular hosting by reducing bandwidth consumption, minimizing latency, and scaling up for abnormal traffic loads.
• CDNs often store multiple copies of videos using a pull strategy where users request data, and store it locally.

Netflix Video Streaming Platform

• Netflix uses a combination of Amazon Cloud services and its own private CDN infrastructure.
• Netflix distributes various versions of videos in different formats suitable for different platforms and devices, with various bit rates for bandwidth optimization.
• Adaptive streaming (e.g. the DASH standard) is used for changing bandwidth and quality.

Socket Programming

• Socket programming is a way to build client-server applications that use sockets to communicate.
• Sockets provide a door between the process of a program running on an end-device and the transport protocol such as UDP or TCP.

Socket Programming with UDP

• UDP (User Datagram Protocol) is an unreliable datagram-based service.
• There is no connection or handshaking before sending client and server data.
• The sender explicitly appends the IP address and port number to each packet.
• UDP packets can be lost or arrive out of order.

Socket Programming with TCP

• TCP (Transmission Control Protocol) is a reliable byte stream-oriented service.
• A connection is required before data can be exchanged.