CLOUD COMPUTING

Protocol (definition)

A set of conventions governing the treatment and especially the formatting of data in an electronic communication system

Why split networking into layers?

Each layer handles one specific part of the problem and possibly the connection with the other layers

The 4 families of networking problems

Physical, topological/organisational, software, reliability

Examples of physical constraints

Attenuation, noise, bandwidth

Reliability problems in networking

Coping with data loss (detection and correction) and security

Number of layers in the OSI model

7

Number of layers in the TCP/IP model

4 (Network Access, Internet, Transport, Application)

Number of layers in the course model

5 (Physical, Data-Link, Network, Transport, Application)

OSI layers from 1 to 7

1 Physical, 2 Data Link, 3 Network, 4 Transport, 5 Session, 6 Presentation, 7 Application

Course model layers from 1 to 5

1 Physical, 2 Data-Link, 3 Network, 4 Transport, 5 Application

How are layers numbered?

From the bottom: layer 1 = Physical

Which OSI layers merge into the course Application layer?

Application, Presentation and Session (OSI 7, 6, 5)

TCP/IP layer that groups Data-Link and Physical

Network Access

TCP/IP name of the Network layer

Internet

Application layer: protocol or program?

The protocol (HTTP), not the program (Firefox)

Protocols at the Application layer

HTTP, DNS, DHCP, FTP, SMTP, POP, IMAP, SSH

Protocols at the Transport layer

TCP and UDP

Protocols at the Network layer

IPv4, IPv6, ICMPv4, ICMPv6

Protocols at the Data-Link layer

Ethernet, WLAN, SONET, SDH

Encapsulation

Moving down the stack, each layer adds its header to the data of the layer above

De-encapsulation

Moving up the stack, each layer removes its header and passes the payload to the right protocol above

Payload

The data a layer carries for the layer above (charge utile)

PDU at the Application layer

Data

PDU at the Transport layer

Segment (called a datagram in UDP)

PDU at the Network layer

Packet

PDU at the Data-Link layer

Frame (medium dependent, has a header and a trailer)

PDU at the Physical layer

Bits

Encapsulation chain for HTTP

HTTP > TCP > IP > Ethernet > bits

Encapsulation chain for DNS

DNS > UDP > IP > Ethernet > bits

1 byte = ? bits

8 bits

Client-server model: the 3 steps

Client sends a request; server processes it and sends an answer or an error; either side can end the connection

Who can end a client-server connection?

Either the client or the server

Common web server software

Apache and NGINX

Mail server equivalent of Apache

Postfix

Wireshark

A network analyser that captures traffic and shows every header

Port of HTTP

80

Port of HTTPS

443

Port of DNS

53

Port of SMTP / SMTPS

25 / 465

Port of POP / POPS

110 / 995

Port of IMAP / IMAPS

143 / 993

Port of SSH

22

FTP stands for

File Transfer Protocol

Secure alternative to telnet

SSH

DNS record A

Maps a name to an IPv4 address

DNS record AAAA

Maps a name to an IPv6 address

Domain name

A name associated with an IP address; it is rented, not really bought

Role of the Transport layer

Defines how data is transported to and from the application layer over the network

UDP stands for

User Datagram Protocol

TCP stands for

Transmission Control Protocol

Who chooses UDP or TCP?

Usually the application developer

What identifies the listening application?

The port number

Destination port

Standard: identifies the service (80, 443, 53…)

Source port

Ephemeral: random, chosen by the client OS at runtime, greater than 1023, different for each connection

Course example: browser to web server ports

Source 1240, destination 80; the answer is source 80, destination 1240

How does the server's answer use the ports?

It swaps source and destination ports

How does one PC run Firefox and Outlook at the same time?

Each connection has a different ephemeral source port (e.g. a' to 80 and a'' to 25)

http://IP_B:8080

Forces destination port 8080 instead of the default 80

UDP main characteristics

No connection, datagrams only sent and received, loss possible, wrong order possible

Size of the UDP header

8 bytes

Fields of the UDP header

Source port, destination port, length, checksum (16 bits each)

Checksum

Detects transmission errors; does not correct them

TCP main characteristics

Connection established; handles lost segments, re-ordering and acknowledgments

Size of the TCP header (no options)

20 bytes

TCP fields that UDP doesn't have

Sequence number, acknowledgement number, control bits, window, urgent

TCP three-way handshake

SYN, then SYN-ACK, then ACK

Closing a TCP connection

FIN and ACK from each side: 4 messages (FIN, ACK, FIN, ACK) or 3 (FIN, FIN+ACK, ACK)

Windowing

Number of segments sent before waiting for an ACK, adjusted dynamically to limit ACKs and boost performance

UDP vs TCP: which is faster?

UDP, because it is lighter

UDP: who handles connected behaviour if needed?

The application layer

Advantage of TCP for developers

Connected behaviour is fully handled by the protocol, so application code is simplified

Is TCP more secure than UDP?

No. TCP is more reliable, not more secure; neither encrypts

When is UDP still preferred?

Some loss is acceptable, the app handles loss/disorder, or the data fits in few datagrams

Transport used by web, mail, FTP, SSH

TCP

Transport used by video conferencing, VoIP, multicast, TFTP

UDP

Transport used by DNS and online gaming

UDP and TCP

Transport used by a VPN

UDP or TCP

Router

Device that interconnects networks; its job is to find a route

The two addresses of a device

A hardware address (MAC, layer 2) and a logical address (IP, layer 3)

MAC stands for

Media Access Control

Who sets the MAC address?

The network card manufacturer

Who sets the IP address?

The network administrator (or DHCP); it depends on the network and on the location

Scope of a MAC address

Unique on a given local (layer 2) network

Size of a MAC address

48 bits, shown as 12 hexadecimal digits

1 hexadecimal digit = ? bits

4 bits

First 6 hex digits of a MAC address

Usually identify the manufacturer

Can a MAC address be changed?

Not physically, but it can be reassigned through software

MAC address of a virtual machine

Set in a configuration file

One network card = ?

One MAC address

Basic security using MAC addresses

MAC address filtering

Broadcast MAC address

FF:FF:FF:FF:FF:FF

Multicast MAC prefixes

01:00:5E and 01:00:0A for IPv4, 33:33 for IPv6

Hex F = ? in decimal and binary

15 = 1111

Unicast

1 to 1 traffic (web browsing, mail)

Broadcast

1 to all traffic (printer service sharing)

Multicast

1 to many: devices interested in the content (IP TV streaming)

Anycast

A single IP shared by several devices in different locations

Routing decision

Is the target in my network or in another network? Decided with the routing table

A to B in the same network: destination MAC?

MAC B; the router is not involved

A to C in another network: destination MAC?

MAC of the router