1-1

Foundation

Objectives:1.1 What’s the Internet?1.2 Network edge1.3 Network core1.4 Network access and physical media1.5 Internet structure and ISPs1.6 Delay & loss in packet-switched

networks1.7 Protocol layers, service model

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What’s the Internet

millions of connected computing devices: hosts = end systems

running network apps communication links

fiber, copper, radio, satellite

transmission rate = bandwidth

routers: forward packets (chunks of data)

local ISP

companynetwork

regional ISP

router workstation

servermobile

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What’s the Internet protocols control sending,

receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP

Internet: “network of networks” a collection of individual

networks public Internet versus private

intranet Internet standards

RFC: Request for comments IETF: Internet Engineering

Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile

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What’s the Internet: a service view

communication infrastructure enables distributed applications: Web, email, games, e-

commerce, file sharing

communication services provided to apps: Connectionless-oriented

unreliable service connection-oriented

reliable service

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What’s a protocol?a human protocol and a computer network protocol:

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponseGet http://www.kfupm.edu.sa/downloads

<file>time

protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

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What’s a protocol? What does a protocol tell us?

Syntax Semantics Actions

A network that provides many services needs many protocols

For example, consider a file transfer protocol Packet transfer is one step in the execution of a

reliable file transfer protocol This form of dependency is called layering That is, reliable file transfer is layered above

packet transfer protocol

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Network structure:

network edge: applications and hosts

network core: routers network of networks

access networks, physical media: communication links

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The network edge: end systems (hosts):

run application programs e.g. Web, email at “edge of network”

client/server model client host requests, receives

service from always-on server e.g. Web browser/server;

email client/server

peer-peer model: minimal (or no) use of

dedicated servers e.g. Gnutella, KaZaA

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Network edge: connection-oriented service

Goal: data transfer between end systems

A connection-oriented service requires Establishing a connection Maintaining the

connection Releasing the connection

TCP - Transmission Control Protocol Internet’s connection-

oriented service

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements

and retransmissions

flow control: sender won’t overwhelm

receiver

congestion control: senders “slow down

sending rate” when network congested

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Network edge: connectionless service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: connectionless unreliable data

transfer no flow control no congestion

control

App’s using TCP: HTTP (Web), FTP (file

transfer), Telnet, SMTP

App’s using UDP: streaming media,

teleconferencing, DNS, Internet telephony

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A Taxonomy of Communication Networks

So far we have looked at only the topology of the Internet: a mesh of computers interconnected

The fundamental question: how is data (the bits) transferred through a communication network?

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Broadcast vs. Switched Communication Networks

Broadcast networks Nodes share a common channel; information

transmitted by a node is received by all other nodes in the network

Examples: TV, radio

Switched networks Information is transmitted to a small sub-set (usually

only one) of the nodes

communication networks

switchednetworks

broadcastnetworks

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The Network Core

mesh of interconnected routers

In a switched network, how is data transferred through the net? circuit switching packet switching

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Network Core: Circuit Switching

dedicated circuit per call: telephone net

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required

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Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls

resource piece idle if not used by owning call (no sharing)

dividing link bandwidth into “pieces” frequency division time division

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Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

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Network Core: Packet Switching data sent thru net in

discrete “chunks”

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed

resource contention: aggregate resource

demand can exceed amount available

Congestion can happen Packets queue, wait

for link use

store and forward Packets move one hop

at a time Node receives complete

packet before forwarding

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Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame.

A

B

C10 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

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Packet switching versus circuit switching

Great for bursty data resource sharing simpler, no call setup

Excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem

Packet switching is

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

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Packet-switching: store-and-forward

Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps

Entire packet must arrive at router before it can be transmitted on next link: store and forward

delay = 3L/R (assuming zero propagation delay)

R R RL

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How do loss and delay occur?packets queue in router buffers packet arrival rate to link exceeds output link

capacity packets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

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Four sources of packet delay

1. nodal processing: check bit errors determine output link

A

B

propagation

transmission

nodalprocessing queueing

2. queueing time waiting at output

link for transmission depends on congestion

level of router

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Delay in packet-switched networks3. Transmission delay: R=link bandwidth

(bps) L=packet length (bits) time to send bits into

link = L/R

4. Propagation delay: d = length of physical

link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

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Nodal delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R, significant for low-speed links

dprop = propagation delay a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd

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Packet loss

queue (aka buffer) preceding link in buffer has finite capacity

when packet arrives to full queue, packet is dropped (aka lost)

lost packet may be retransmitted by previous node by source end system or not retransmitted at all

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Queueing delay (revisited)

R=link bandwidth (bps) L=packet length (bits) a=average packet

arrival rate

traffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can

be serviced, average delay infinite!

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Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity,

Sprint, AT&T), national/international coverage 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)

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Internet structure: network of networks

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

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Internet structure: network of networks

“Tier-3” ISPs and local ISPs 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

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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

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Internet structure: network of networks

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

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

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Why layering?

Dealing with complex systems: explicit structure allows identification,

relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance, updating of system change of implementation of layer’s service

transparent to rest of system e.g., change in gate procedure doesn’t

affect rest of system

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Internet protocol stack application: supporting network

applications FTP, SMTP, STTP

transport: host-host data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

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messagesegment

datagram

frame

sourceapplicatio

ntransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHnHl M

HtHn M

HtHnHl M HtHnHl M

router

switch

Encapsulation

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Foundation: Summary

Material Covered Internet overview what’s a protocol? network edge, core,

access network packet-switching

versus circuit-switching Internet/ISP structure performance: loss, delay layering and service

models history (which you will be

reading on your own)

You now have: context, overview,

“feel” of networking more depth, detail

to follow!