(Chapters: 1 & 2) Introduction to Computer Networks A Top Down Approach 8th Edition

Internet

a computer network that interconnects billions of computing devices throughout the world.

Host/End System

devices connected to the internet.

Types of Physical Media

1. Coaxial cable

2. Twisted Pair(TP) - Copper Wire

3. Optical Fiber

4. Radio Spectrum

Transmission Rate

The speed of a link measured in bits/second.

Packets

bundling data into smaller chunks, resulting packages of information.

Packet Switch

takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links.

Most Prominent Packet Switches

1. Routers

2. Link-Layer Switches

Link-Layer Switches

generally used in access networks.

Router Switches

used in the network core.

Route/Path

the sequence of communication links and packet switches traversed by a packet from the sending end-system to the receiving end-system.

Internet Service Providers

(ISPs) in itself is a network of packet switches and communication links.

ISP Network Access Types:

1. cable modem/DSL

2. high-speed local area network access

3. mobile wireless access.

Protocols

defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

Two Most Important Protocols

1. Transmission Control Protocol (TCP)

2. Internet Protocol (IP)

Internet Protocol (IP)

specifies the format of the packets that are sent and received among routers and end systems.

The Internet's Principal Protocols

TCP/IP

Distributed Applications

applications that involve multiple end systems that exchange data with each other.

Socket Interface

specifies how a program running on one end systems asks the internet infrastructure to deliver data to a specific destination program running on another end system.

2 Elements of Hosts

1. Clients

2. Servers

Access Network

the network that physically connects an end system to the first router (also known as the "edge router") on a path from the end system to any other distant end system.

Edge Router

the first router.

Prevalent Broadband Residential Access Types:

1.DSL - Digital Subscriber Line

2.Cable

Residential Phone Line Encoded Frequencies

1. High speed down stream: 50 kHz to 1 MHz

2. Medium speed down stream: 4 kHz to 50 kHz

3. Ordinary two way: 0 to kHz

FTTH (fiber to the home)

provides home users with Internet access via fiber-optic cable

Optical Networks

Active (AONs)

Passive(PONs)

Optical Network Terminator(ONT)

which is connected by dedicated optical fiber to a neighborhood splitter.

Frequency Division Multiplexing (FDM)

different channels transmitted in different frequency bands.

Hybrid Fiber Coax (HFC)

asymmetric: up to 40 Mbps - 1.2 Gbs download and 30 - 100 Mbps upload

Digital Subscriber Line (DSL)

use existing telephone line to central office.

Wireless Local Area Networks (WLANs)

a computer network in a limited geographical area that uses wireless transmission for communication

Bit

binary digit which propagates between transmitter/receiver pairs.

Physical Link

what lies between transmitter and receiver.

Guided Media

signals travel in solid media routes: copper, fiber, coax

Unguided Media

signals travel without route or better said:(freely), e.g. radio

Radio Link Types:

-Terrestrial Microwave

-Wireless LAN (Wi-Fi)

-Wide Area (Cellular)

-Satellite

Packet Switching

the host break application layer messages into packets.

Transmission Delay

takes L/R seconds to transmit L-bit packet into link at R bps

Store and Forward

entire packet must arrive at router before it can be transmitted on next link..

End-end delay

2L/R (above), assuming zero propagation delay

Packet Queuing and Loss

if arrival rate (in bps) to link exceeds transmission rate (bps) of link for a period of time, packets will queue and while buffering too long, can be lost.

Routing

(Global Action) determining the source destination paths taken by packets.

Forwarding

(Local Action) move arriving packets from router's input link to appropriate router output link.

Circuit Switching

a dedicated connection is formed between two points and the connection remains active for the duration of the transmission

Frequency Division Multiplexing (FDM)

optical electromagnetic frequencies divided into (narrow) frequency bands. Each call allocated its own band, transmission max rate is same as narrow band (horizontal long lines)

Time Division Multiplexing(TDM)

time is divided into slots, each call allocated periodic slot(s), can transmit at maximum rate of wider frequency only during its time slots.

Packet Switching vs. Circuit Switching

1. Circuit switching allows only 10 users, packet switching allows 35 users but chances of 10 users at same time is less than 0.0004%

2. Packet switching is good for resource sharing and also is simpler.

Packet Loss and Delay

-Packets queue and wait for turn

-arrival rate to link(temporarily) exceeds output link capacity, packets are then dropped and lost

Packet Delay 4 Sources:

1. Nodal Processing

2. Queueing delay

3. Transmission Delay

4. Propagation Delay

Notation (R, L, a)

R: link bandwidth

L: packet length (bits)

a: average packet arrival time

Queueing Delay Possibilities:

1.La/R ~0

2.La/R > 1

3.La/R -> 1

1. average delay

2. Infinite Delay

3. Large Delay

traceroute

provides information such as delay measurement from source to router along end-end internet path to destination.

[send 3 packets, router will return them, sender measures time to receive back]

Throughput

rate (bits/time unit) at which bits are being sent from sender to receiver.

Instantaneous = rate at given point in time

average = rate over longer period of time

bottleneck link

link on end-end path that constrains end-end throughput.

Malware

software that is intended to damage or disable computers and computer systems.

[Virus & Worm]

Virus

an infection that self replicates by executing objects

Worm

self replicating infection by passively receiving objects that gets itself executed.

Spyware

can record keystrokes, websites visited, upload info to a collection site.

Denial of Service(DoS)

attackers make resources (server bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic.

DoS steps

1. Target selected

2. break into hosts around the network

3. send packets to target from hosts the attacker has seized control over.

Packet Sniffing

promiscuous network interface reads/records all packets passing by

IP Spoofing

sending packets with false IP addresses linked.

Internet Protocol Stack layers

1. Application

2. Transport

3. Network

4. Link

5. Physical

Application layer

supporting network applications (FTP, SMTP, HTTP)

Transport Layer

process-process data transfer

TCP, UDP

Network

routing of datagrams from source to destination (IP)

Link

data transfer between neighboring network elements (Ethernet, WiFi)

Physical

bits within the wire.

Transmission Control Protocol (TCP)

provides reliable, ordered, and error-checked delivery of a stream of packets on the internet. TCP is tightly linked with IP and usually seen as TCP/IP in writing.

User Datagram Protocol (UDP)

An alternative to TCP that achieves higher transmission speeds at the cost of reliability

Server

always-on host, permanent IP address

Client

a contact that communicates with the server, dynamic IP addresses (HTTP, IMAP, FTP)

Peer-Peer (PP) Architecture

-no always-on server

-peers request service from other peers, service is then provided in return.

-peers are intermittently connected and change IP addresses

Process

a program running within a host.

Inter-Process Communication

within the same host, 2 processes are communicating.

Client Process

process that initiates communication

Server Process

process that awaits to be contacted

Socket

A network connection consisting of a port number combined with a computer's IP address.

HTTP

(Hypertext Transfer Protocol) is the web's application layer protocol

HTTP Client & Server

client: browser that requests and receives objects

server: sends objects in response to requests

HTTP Connection Type

TCP

Round Trip Time (RTT)

time for a small packet to travel from client to server and back.

HTTP Response time

1. One RTT to initiate TCP connection

2. One RTT for HTTP request and first few bytes of HTTP response to return

3. object/file transmission time

HTTP Message Type

1. Request [ASCII]

2. Response

HTTP Request Messages Methods

1. Post, Get, Put, Head

HTTP Response Codes

-200 OK

-301 Move Permanently

-400 Bad Request

-404 Not Found

-505 HTTP Version Not Supported

Cookies

permit sites to learn a lot about you on their site.

Web Caching

A technique in which Web pages are stored locally, either on a host or network, and then delivered to requesters more quickly than if they had been obtained from the original source.

Email Components

1. user agents

2. mail servers

3. Simple Mail Transfer Protocol(SMTP)

User Agent

mail reader

Mail Servers

mailbox containing incoming messages for the user

HTTP vs SMTP

HTTP: pull

SMTP: push

Simple Mail Transfer Protocol(SMTP)

delivery and storage of e-mail messages to receiver's server

Internet Mail Access Protocol(IMAP)

messages are stored on server, IMAP retrieves messages, deletes messages, and folders messages

SMTP vs IMAP

SMTP pushes IMAP pops

Domain Name System(DNS)

a distributed database implemented in hierarchy of many name servers

Application-Layer Protocol

hosts, name servers communicate to resolve names(address/name translation)

DNS services

-hostname to IP address translation

-host aliasing

-mail server aliasing

-load distribution

Top-Level Domain (TLD) Servers

response for .com, .org, .net, etc.

Local DNS name servers

-does not strictly belong to a hierarchy

-when host makes DNS query, query is sent to its local DNS server

Resource Records (RR)

-type=A

-type=NS

-type=CNAME

-type=MX