Part 1: Introduction - INET: Internet Network Architectures · What’s the Internet: “Nuts and...

1

Part 1: Introduction

Goal:

r Review of how the Internet works

r Overview

m Get context

m Get overview, “feel” of the Internet

r Application layer protocols and addressing

r Network layer / Routing

r Link layer / Example

2

What’s the Internet: “Nuts and bolts” view

r Millions of connected computing devices: hosts, end-systems

m PC’s workstations, servers

m PDA’s, phones, toasters

running network apps

r Communication linksm Fiber, copper, radio, satellite

r Routers: forward packets (chunks) of data through network

local ISP

company / accessnetwork

regional ISP

routerworkstation

servermobile

3

Example access net: Home network

Typical home network components:

r ADSL or cable modem

r router/firewall

r Ethernet

r Wireless access points

wirelessaccess point

wirelesslaptops

router/firewall

cablemodem

to/fromcable

headend

Ethernet(switched)

4

What’s the Internet: “Nuts and bolts” view

r Protocols: control sending, receiving of messages

m E.g., TCP, IP, HTTP, FTP, PPP

r Internet: “network of networks”

m Loosely hierarchical

m Public Internet versus private intranet

r Internet standards

m RFC: Request for comments

m IETF: Internet Engineering Task Force

local ISP

companynetwork

regional ISP

routerworkstation

servermobile

5

What’s the Internet: A service view

r Communication infrastructure enables distributed applications:

m WWW, email, games, e-commerce, database, voting,

m More?

r Communication services provided:

m Connectionless

m Connection-oriented

r Cyberspace [Gibson]:“a consensual hallucination

experienced daily by billions of operators, in every nation, ...."

6

Principles of the Internet

r Edge vs. core (end-systems vs. routers)

m Dumb network

m Intelligence at the end-systems

r Different communication paradigms

m Connection oriented vs. connection less

m Packet vs. circuit switching

r Layered system

r Network of collaborating networks

7

A closer look at network structure

r Network edge:applications and hosts

r Network core:m Routers

m Network of networks

r Access networks, physical media:Communication links

8

The network edge

r End systems (hosts):m Run application programs

m E.g., WWW, email

m At “edge of network”

r Client/server modelm Client host requests, receives

service from server

m E.g., WWW client (browser)/ server; email client/server

r Peer-2-peer model:m Host interaction symmetric

m E.g.: File sharing

9

Network edge: Connection-oriented service

Goal: data transfer between end systems

r Handshake: setup (prepare for) data transfer ahead of time

m Hello, hello back human protocol

m Set up “state” in two communicating hosts

r TCP – Transmission Control Protocol

m Internet’s connection-oriented service

TCP service [RFC 793]

r Reliable, in-order byte-stream data transfer

m Loss: acknowledgements and retransmissions

r Flow control:m Sender won’t overwhelm

receiver

r Congestion control:m Senders “slow down sending

rate” when network congested

10

Network edge: Connectionless service

Goal: Data transfer between end systemsm Same as before!

r UDP – User Datagram Protocol [RFC 768]: Internet’s connectionless service

mUnreliable data transfer

mNo flow control

mNo congestion control

11

The network core

r Mesh of interconnected routers

r The fundamental question:How is data transferred through net?

m Circuit switching:Dedicated circuit per call: telephone net

m Packet switching: Data sent through net in discrete “chunks”

12

Network core: Circuit switching

End-end resources reserved for “call”

r Link bandwidth, switch capacity

r Dedicated resources: no sharing

r Circuit-like (guaranteed) performance

r Call setup required

13

Network core: Packet switching

Each end-end data stream divided into packets

r Users’ A, B packets share network resources

r Each packet uses full link bandwidth

r Resources used as needed

14

Network core: Packet switching

Packet-switching versus circuit switching: Human restaurant analogy

A

B

C1 GbsEthernet

100 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

15

Network core: Packet switching

Resource contention:

r Aggregate resource demand can exceed amount available

r Congestion: Packets queue, wait for link use

r Store and forward: Packets move one hop at a time

mTransmit over link

mWait turn at next link

16

Delay in packet-switched networks

Packets experience delay on end-to-end path

r Four sources of delay at each hop

r Nodal processing m Check bit errors

m Determine output link

r Queueingm Time waiting at output

link for transmission

m Depends on congestion level at router

A

B

propagation

transmission

nodalprocessing queueing

17

Delay in packet-switched networks

Transmission delay:

r R = link bandwidth (bps)

r L = packet length (bits)

r Time to send bits into link = L/R

Propagation delay:

r d = length of physical link

r s = propagation speed in medium (~2x108 m/sec)

r Propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are verydifferent quantities!

18

Queueing delay

r R = link bandwidth (bps)

r L = packet length (bits)

r A = average packet arrival rate

Traffic intensity = La/R

r La/R ~ 0: average queueing delay small

r La/R -> 1: delays become large

r La/R > 1: more “work” arriving than can be serviced => average delay infinite!

19

Packet switching vs. circuit switching

r Great for bursty data

m Resource sharing

m No call setup

r Excessive congestion: Packet delay and loss

m Protocols needed for reliable data transfer,congestion control

r Key question: How to provide circuit-like behavior?

m Bandwidth guarantees needed for audio/video apps

Is packet switching a “slam dunk winner?”

20

Packet-switched networks: Routing

r Goal: Move packets among routers from source to destination

r Datagram network:m Destination address determines next hop

m Routes may change during session

m Analogy: Drive towards dst. and ask for directions

r Virtual circuit network:m Fixed path determined at call setup time, remains fixed through call

m Each packet carries tag (virtual circuit ID);Tag determines next hop

m Routers maintain per-call state

m Analogy: Fixed driving instructions

21

What’s a protocol?

Human protocols:

r “What’s the time?”

r “I have a question!”

… introductions

… specific msgs sent

… specific actions taken when msgs received, or other events

Network protocols:

r machines rather than humans

r all communication activity in Internet governed by protocols

Protocols define format, order of msgs sent and received among network entities, and actions taken

on msg transmission, receipt

22

Protocol “layers”

Networks are complex!

r Many “pieces”:

m Hosts

m Routers

m Links of various media

m Applications

m Protocols

m Hardware, software

Question:

Is there any hope of organizing structure of

network?

Or at least in our discussion of networks?

23

Why layering?

Dealing with complex systems:

r Explicit structure allows identification, relationship of complex system’s pieces

m Layered reference model for discussion

r Modularization eases maintenance, updating of system

m Change of implementation of layer’s service transparent to rest of system

m E.g., change in gate procedure does not affect rest of system

r Layering considered harmful?

24

Protocol “Layers”

Networks are complex!

r many “pieces”:

m hosts

m routers

m links of various media

m applications

m protocols

m hardware, software

Question:

Is there any hope of organizingstructure of network?

Or at least our discussion of networks?

25

Why layering?

Dealing with complex systems:

r Explicit structure allows identification, relationship of complex system’s pieces

m Layered reference model for discussion

r Modularization eases maintenance, updating of system

m Change of implementation of layer’s service transparent to rest of system

m E.g., change in gate procedure doesn’t affect rest of system

r Can layering be considered harmful?

26

Internet protocol stack

r Application: Supporting network applications

r Transport: Host-host data transfer

r Network: Uniform format of packets, routing of datagrams from source to destination

r Link: Data transfer between neighboring network elements

r Physical: Bits “on the wire”

application

transport

network

link

physical

27

Layering: Logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

Each layer:

r Distributed

r “Entities”implement layer functions at each node

r Entities perform actions, exchange messages with peers

28

Layering: Logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

E.g., transport

r Take data from application

r Add addressing, reliability check info to form “datagram”

r Send datagram to peer

r Wait for peer to ack receipt

r Analogy: post office

data

transport

transport

ack

29

Layering: Physical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

30

Protocol layering and data

Each layer takes data from above

r Adds header information to create new data unit

r Passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnH l

M

M

M

M

Ht

HtHn

HtHnH l

message

segment

datagram

frame

31

Internet protocol stack

r Application: Supporting network applications

r Transport: Host-host data transfer

r Network: Uniform format of packets, routing of datagrams from source to destination

r Link: Data transfer between neighboring network elements

r Physical: Bits “on the wire”

application

transport

network

link

physical

32

What makes the Internet so sexy

r Applications can be deployed by anybody that is connected to the Internet(Fundamentally different to the Telephone world)

r Multi-service network: Everything over the Internet• Every application protocol over IP

• IP over any network technology

33

TCP/IP Protocol structure

Transport(Hosts)

Network

Link

Application(Processes)

IP

TCP UDP

Telnet FTP

DNSHTTP SMTP

FDDI ATM

Tokenring Ethernet

34

Internet structure: Network of networks

r Roughly hierarchical

r At center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage

m Treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

35

Internet structure: Network of networks

r “Tier-2” ISPs: smaller (often regional) ISPs

m Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

•Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet• tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

36

Internet structure: Network of networks

r “Tier-3” ISPs and local ISPs

m Last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISP

localISP

localISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

37

Example Tier-1 ISP: Sprint

38

Internet structure: Network of networks

r A packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISP

localISP

localISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Try traceroute!

39

Internet philosophy

r Interconnect networks

m Accross multiple different technologies

r Requirements on these networks

m Minimal assumption about network capabilities => some data should be transmitted from time to time

⇒ „Best Effort“ paradigma

r High survivability

m Mesh

m Intermediate nodes do not maintain per host state

=> „End-to-end“ paradigma

r Layering with simple application interface

m Sockets

40

Internet design philosophy

In order of importance:

m Connect existing networks

m Survivability

m Support multiple types of service

m Must accommodate a variety of networks

m Allow distributed management

m Allow host attachment with a low level of effort

m Be cost effective

m Allow resource accountability

41

Internet terminology

r internetm Collection of packet switched networks interconnected by routers running the IP/TCP protocols

r The „Internet“m Public Internet (in contrast to Intranets)

r End system == host attached to network

r Router = gateway = intermediate systemm Routes packets between its attached networks

r Firewall (device placed at network boundaries)m Restricts packet flows to improve security

42

Internet terminology (2.)r Name

m Identifies an object (e.g., an end system)

r Addressm Identifies where an object is located (here IP address)

r Name to address translation (and vice versa)m Links names and addresses

• mail.zrz.TU-Berlin.DE � 130.149.4.15

r Interfacem Network attachment (≥ 1 interfaces = multihomed)

r Routem How to get to an objects location

r Network prefix m Set of addresses