Basic Concepts In Computer Networking - USI Informatics · Basic Concepts u The Internet uses...

Basic Concepts

In Computer Networking

Antonio Carzaniga

Faculty of InformaticsUniversity of Lugano

March 11, 2005

Outline

u What is the Internet?

u Types of network

u Types of service

u Protocols

u The Internet protocol stack

What is the Internet?

Internet

end systemor host

End Systems

u end system or host ( )

End Systems

u end system or host ( )I a computer

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)I a temperature sensor

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)I a temperature sensorI a cell phone

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)I a temperature sensorI a cell phoneI a PDA

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)I a temperature sensorI a cell phoneI a PDAI . . .I a carI a television setI a picture frameI a toasterI . . .

End Systems

u end system or host ( )I a computerI a server (well, that would also be a computer)I a camera (a.k.a., webcam)I a temperature sensorI a cell phoneI a PDAI . . .I a carI a television setI a picture frameI a toasterI . . .I a toilet seat?I a toothpick?I . . .

What is “Inside” the Internet?

local-areanetwork

packet switch

local-areanetwork

packet switch

Basic Concepts

u The Internet uses packet switching

Basic Concepts

u Packet switch: a link-layer switch or a router

Basic Concepts

u Route: sequence of switches that a packet goes through (a.k.a.path)

Basic Concepts

u Route: sequence of switches that a packet goes through (a.k.a.path)

u Protocol: control the sending and receiving of information to andfrom end systems and packet switches

Packet Switching

u The Internet is a packet-switched network

Packet Switching

u Information is transmitted in packets

Packet Switching

u Switches operate on an individual packet

Packet Switching

u A switch (router) receives packets and forwards them along toother switches or to end systems

Packet Switching

u A switch (router) receives packets and forwards them along toother switches or to end systems

u Every forwarding decision is done on the basis of the informationcontained in the packet

Circuit Switching

u The telephone network is a typical circuit-switched network

Circuit Switching

u Communication requires a connection setup phase in which thenetwork reserves all the necessary resources for that connection(links, buffers, switches, etc.)

Circuit Switching

u After a successful setup, the communicating systems areconnected by a set of links that are dedicated to their connectionfor the entire duration of their conversation

Circuit Switching

u After a successful setup, the communicating systems areconnected by a set of links that are dedicated to their connectionfor the entire duration of their conversation

u When the conversation ends, the network tears down theconnection, freeing the corresponding resources (links, buffers,etc.) for other connections

Circuit vs. Packet Switching

u Circuit switching requires an expensive setup phase. On theother hand, once the connection is established, circuit switchingis much more efficient in both space overhead and processing

u Packet switching does not incur any setup cost. However, italways incurs a significant processing cost for forwarding, as wellas space overhead, because every packet must beself-contained

u Circuit switching admits a straightforward implementation ofquality-of-service guarantees, because network resources arereserved at connection setup time

u Guaranteeing any quality of service with packet switching is verydifficult

Circuit vs. Packet Switching (2)

u Circuit switching allows only a limited sharing of communicationresources. In other words, circuit switching is an inefficient wayto use the network

Circuit vs. Packet Switching (2)

u Circuit switching allows only a limited sharing of communicationresources. In other words, circuit switching is an inefficient wayto use the network

u Packet switching achieves a much better utilization of networkresources, because it is designed specifically to share links(buffers, etc.)

Virtual Circuits

u Idea: combine the advantages of circuit switching and packetswitching

Virtual Circuits

u The service model is connection oriented, and therefore requiresa connection setup phase

Virtual Circuits

u The connection does not create a physical circuit, but rather a“virtual circuit”

Virtual Circuits

u Information is sent in packets, so links can be shared moreeffectively

Virtual Circuits

u Information is sent in packets, so links can be shared moreeffectively

u Packets carry a virtual circuit identifiers, which allow very fastprocessing (forwarding) and incur a space low-overhead

Taxonomy of Networks

communicationnetwork

circuitswitching

packetswitching

timedivision

multiplexing

frequencydivision

multiplexing

datagramnetwork

virtualcircuit

Service Perspective

local-areanetwork

packet switch

Service Perspective

local-areanetwork

packet switch

u What kind of service does the Internet offer to end systems?

Type of Service

u Two end systems can communicate through the Internet, butexactly what kind of communication service is that of theInternet?

Type of Service

u Connection-oriented reliableI virtual duplex communication channel (A ↔ B)—conceptually

similar to a telephone serviceI information is transmitted “reliably” and in order

Type of Service

u Connection-oriented reliableI virtual duplex communication channel (A ↔ B)—conceptually

similar to a telephone serviceI information is transmitted “reliably” and in order

u Connectionless unreliableI the network accepts “datagrams” for delivery—this is conceptually

similar to the postal serviceI “unreliable” service (a.k.a. “best effort”)

Type of Service (2)

u How reliable is a “reliable” service?

Type of Service (2)

u The term “reliable” means that information will eventually reachits destination if a route is viable within a certain amount of time.

Type of Service (2)

u The term “reliable” means that information will eventually reachits destination if a route is viable within a certain amount of time.

u The network makes absolutely no guarantees on latency (i.e.,the time it takes to transmit some information from a source to adestination)

Communication Protocols

u End systems as well as packet switches run protocols. What is aprotocol?

u End systems as well as packet switches run protocols. What is aprotocol?E.g., let’s consider a phone call: Alice calls Bob

Alice Bobhello

hello, this is Alice

Alice Bobhello

Alice, what’s up?

Alice Bobhello

Alice, what’s up?

bla, bla. . .

Alice Bobhello

Alice, what’s up?

bla, bla. . .

okay, bye

Alice Bobhello

Alice, what’s up?

bla, bla. . .

okay, bye

u Phases of the protocolI handshake establishes the identity and/or the context for a

conversationI conversation free-form exchangeI closing terminates the conversation

u Phases of the protocolI handshake establishes the identity and/or the context for a

conversationI conversation free-form exchangeI closing terminates the conversation

u This protocol assumes a connection-oriented mediumu The protocol involves two parties (Alice and Bob)u . . .

Communication Protocols (2)

u Another example: air traffic control

I . . . United 971, turn left heading 2-7-0

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbye

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800I . . . United 971, climb and maintain flight level 3-7-0

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800I . . . United 971, climb and maintain flight level 3-7-0I flight level 3-7-0, United 971

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800I . . . United 971, climb and maintain flight level 3-7-0I flight level 3-7-0, United 971I . . . Delta 800, go ahead

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800I . . . United 971, climb and maintain flight level 3-7-0I flight level 3-7-0, United 971I . . . Delta 800, go aheadI requesting flight level 3-5-0, Delta 800

I . . . United 971, turn left heading 2-7-0I left to 2-7-0, United 971I . . . Alitalia 631, contact Malpensa approach at 119.20I . . .I . . . Alitalia 631, contact Malpensa approach at 119.20I 1-1-9 point 2-0, Alitalia 631, goodbyeI . . . Center, request, Delta 800I . . . United 971, climb and maintain flight level 3-7-0I flight level 3-7-0, United 971I . . . Delta 800, go aheadI requesting flight level 3-5-0, Delta 800I Delta 800, unable at the moment

u A connectionless protocol

u Multi-party communication

u Medium access control (MAC) protocol

u Interleaved communication

u Acknowledgements

u Timeout and retransmission

u Acknowledgements

u Timeout and retransmission

u “Master” role

u Let’s revisit the phone-call protocol

Alice Bob

Alice Bobhello

Alice who?

Alice Bobhello

Alice who?

sorry, wrong number

u Another run of the phone-call protocol

Alice Bob

Alice Bobhello

hello?

Alice Bobhello

hello?

anybody there?

Communication Protocols: Principles

u A protocol is a lot like a programI in fact, it is a distributed program, where different processes can

send messages to each other

u It is an executable specification

u It must be unambiguous

u It must be completeI i.e., it must include actions and/or responses for all possible

situations and all possible messages

u It must be completeI i.e., it must include actions and/or responses for all possible

situations and all possible messages

u A network protocol must also define all the necessary messageformats

u Another protocol: deciding where to go for dinner

Alice Bobhow about pizza?

we did that already

okay, you decide

we did that already

okay, you decide

uhm. . . er. . .

we did that already

okay, you decide

uhm. . . er. . .

we did that already

okay, you decide

uhm. . . er. . .

let’s just do pizza

Protocol Layering

u Alice calls Bob to decide where to go for dinner

Protocol Layering

Alice Bobhello

Protocol Layering

Alice Bobhello

Protocol Layering

Alice Bobhello

Alice, what’s up?

Protocol Layering

Alice Bobhello

Alice, what’s up?

what to do for dinner?. . .

Protocol Layering

Alice Bobhello

Alice, what’s up?

okay, bye

Protocol Layering

Alice Bobhello

Alice, what’s up?

okay, bye

Protocol Layering

phone call protocol

Protocol Layering

phone call protocol

dinner protocol

Protocol Layering

phone call protocol

dinner protocol

call setup

Protocol Layering

phone call protocol

dinner protocol

call setup

voice over IP

Protocol Layering

phone call protocol

dinner protocol

call setup

voice over IP

Internet Protocol Stack

application

transport

application

transport

network

application

transport

network

application

transport

network

physical

Internet Protocol Stack (2)

u Application (e.g., HTTP, SMTP, and DNS)I application functionalitiesI application messages

u Transport (e.g., TCP and UDP)I application multiplexing, reliable transfer (TCP), congestion

control (TCP)I datagrams (UDP) or segments (TCP)

u Network (IP)I end to end datagram, best-effort service, routing, fragmentationI packets (IP)

u Link (e.g., Ethernet and PPP)I point-to-point or local broadcast communicationI frames (or packets)

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