Computer Networks

Network of networks

global communication network that interconnects millions of networks

Hosts, routers, communication links with TCP/IP

Information superhighway

infrastructure that provides services to distributed applications

Email, web P2P, games

Internet two types of service provision

Reliable and connection-oriented

Unreliable, not connection-oriented

Computer network

Digital communication network that allows its nodes to share resources and communicate

Host (PC, phone, servers) and network hardware (routers, switches)

Types of networks according to communication channels

Broadcast

Point-to-point

Broadcast network

Shared communication channel, can have multiple recipients. Small networks in general.

Point-to-point network

Dedicated communication channels for communication between two machines

Types of networks according to their length

Local Area Network (LAN) - (10m - several km)

Wide Are Network (WAN) - (>10km)

Circuit switching

When two nodes want to communicate and establish a terminal-to-terminal connection

Necessary resources are reserved along the path, reservation is maintained during the session (telephone networks)

Packet switching

No reservation of resources

Session messages use on-demand resources → May have to wait before they can use them

(Internet)

Packet-switched networks

Messages are divided, sent through links and routers, using store-and-forward transmission technique

For each link, the router has an output/input buffer which stores the packets to be sent to that link

Store-and-forward transmission technique

The router must receive the entire packet before it can transmit the first bit to the next destination

Diagram networks

Packet sending is based on the destination address

Virtual Circuit networks

Packet delivery is based on the virtual number

Types of delays in packet-switched networks

Processing: time required by the router to examine the header and determine where to follow

Queue: waiting time to be transmitted (in the output)

Transmission: time to transmit all bits to the link

Propagation: time required to propagate from beginning to the end of the link (next router)

Protocol

set of rules for routing and addressing packets of data so they can travel across networks and arrive at the correct destination

Network architecture

Set of protocols and layers that enable communication between computers

OSI reference model - Open System Interconnect, layers

Application

Presentation

Session

Transport

Network

Link

Physical

Physical OSI level

Transmit bits between physically connected entities

Encoding scheme for bit representation

Bit-level synchronization

Link OSI level

Set of bits, each having a beginning and an end

First software layer

Highly dependent on physical medium

Network OSI layer

A set of links to form the end-to-end link

Allows one system to communicate with another, calculating the route between them

Provides unique addresses

A level that exists in BOTH final and intermediate systems

Routing, fragmentation and assembly, error detection

Transport OSI layer

Creates a multiplexed end-to-end link with error and flow control

Multiple applications over the same connection, adding identifiers for each

Messages reach their destination regardless of situation

Transmission speed of the source is adapted to the receiver

Two very popular internet transport protocols

TCP - Connection-oriented and reliable, provides flow, error control and multiplexing

UDP - connectionless and unreliable, only provides multiplexing

Session OSI level

Not very common. Provides full-duplex service, urgent data transmission and session synchronization.

Manages two independent connections to create a full duplex service

Skips the message queue

Allows atomic data transfers

Presentation OSI level

Hides the differences in data representation between applications

Can encrypt and compress data

Application OSI level

Set of applications that use the network

Does not provide services to any other layer/level

Web (HTTP, HTTPS), email (SMTP, IMAP), DNS

TCP/IP protocol

allows computers of all sizes, different manufacturers, operating systems to communicate with each other

Physical layer + link = ?

Manage details of the communication medium

Application level = ?

Manages the details of each specific application

Hub

repeats each frame received by its input ports through the rest of the output ports

Switch

allows you to connect different devices to form a LAN

IP Class TO/A range

0.0.0.0 to 127.255.255.255

Network ID: 1 byte

Host ID: 3 bytes

IP Class B range

128.0.0.0 to 191.255.255.255

Network ID: 2 bytes

Host ID: 2 bytes

IP Class C range

192.0.0.0 to 223.255.255.255

Network ID: 3 bytes

Host ID: 1 byte

IP Class D range

224.0.0.0 to 239.255.255.255

Multicast addressing

Difference between Public and Private IPs?

One is to uniquely identify a device on the Internet, the other is exclusively for internal use

What does Network Address Translation (NAT) do?

Converts private IPs to public ones in order to communicate on the internet

Loopback

Intented to be an interface to which packets destined to the same machine are sent

Three types of IPv4 addresses

unicast - one IP address - single machine (interface)

broadcast - one IP address - all machines on a network

multicast - one IP address - one group of machines

Domain Name System (DNS)

turns names into ip addresses and vice versa so we wouldn’t need to memorize each site’s IP address

Stores information about machine names and IPs

Port numbers

used to let the computer know which application specifically to communicate with

16-bit number

servers use fixed and known ports - from 1-1023

http - 80, ftp - 21, smtp - 25

Client-server model

Client sends request to the server (requesting a service)

Server receives the requests, processes them and sends the response

Peer-to-peer model

Both ends provide a service and request services

Web’s components

Page: Base HTML file + objects (CSS)

Browser: User agent

Server: stores objects addressable via a URL

HTTP: allows communication between the server and browser

URI

identifier that allows access to a web source

Scheme: protocol used to access it

Authority: server name/IP

Path: to access the resource

HTTP

Hypertext Transfer Protocol

Uses TCP protocol, each message issued by client/server arrives at the other end without modification

Is a stateless protocol, the server does not store info about previous requests

HTTPS

HTTP over Transport Layer Security (TLS), guarantees confidentiality, integrity and authentication

HTTP: Persistent Connections without a pipeline

Client only sends a new request after it has received the previous response

HTTP: Persistent Connections with pipeline

Client makes a request as soon as it finds a reference to an object

HTTP: Cache

Reduces object retrieval delays and network traffic

But a copy of an object may become outdated

Email

Asynchronous means of communication, consists of:

Readers / user agents

Servers

SMTP

SMTP (Simple Mail Transfer Protocol)

Allows exchange of messages between mail servers

Establishes TCP connection with port 25

Uses messages in ASCII, if message has non-ASCII or binary characters - must be encoded (MIME)

MIME

Multipurpose Internet Mail Extensions

Allows you to send content other than ASCII text in email messages (languages with accents or non-latin languages etc)

POP3 (Post Office Protocol v3)

Simple email access protocol

Three-phase mode

Authorization: login and password

Transaction: Retrieve messages, mark for deletion/email statistics

Update: When the session ends, the server deletes marked messages

IMAP (Internet Mail Access Protocol)

Allows you to create and manage remote mailboxes

Associates each message with a mailbox

Provides commands to create mailboxes, move messages and search for them

Maintains user status information between sessions

DNS Client

Each machine has a client (resolver)

Whenever any application needs to find out an IP address, it passes the query to the client

The client sends the query to the server, it receives it and sends the response to the application

DNS Server

Receives queries from clients, finds out IP addresses and sends it to the clients

There is no single server that knows all names and IPs, they ask other servers, depending on the DNS namespace

DNS Recursive queries

The server does all the necessary work to return the complete answer

May involve multiple transactions with other servers

Not mandatory for servers to support this query

DNS Iterative queries

If the server has the answer, it returns it

If it doesn’t, it will return useful information but not make requests to other servers

The root and TLD servers are non-recursive (Iterative)