CMPE 150: Introduction to Computer Networks

Apr 19, 2023 UCSC CMPE150 1

CMPE 150: Introduction to

Computer Networks

J.J. Garcia-Luna-AcevesEmail: [email protected]: 94153 Office: E2, Room 317CCRG Lab: E2, Room 315http://www.cse.ucsc.edu/research/ccrg/CMPE150/FALL2004


CMPE 150: Introduction to Computer NetworksLECTURE 1:

Introduction and Background


Outline What is a computer network? Brief history and outlook of the Internet. What are communication protocols and how

do we go about studying them? Architectural structure of the Internet. Issues of interest with transmission media as

a black box. More about networks and links.


What Is a Computer Network? A communication network is a set of nodes

connected by links and able to communicate with one another.

A computer network is a communication network in which nodes are computers.

The purpose of the network is to serve users, which can be humans or processes.

Network links can be point-to-point or multipoint and implemented with several transmission media.

Information exchanged can be represented in multiple media (audio, text, video, images, etc.)

Services provided to users can vary widely.


Why Learn about Computer Networks?

Computer networks started as a means for Distributed processing Communicating among people (electronic mail,

conferencing) Increasing system reliability

The “web” and affordable hardware have changed this!

We are evolving into Internet-based enterprises, Internet-based home services, and an Internet society

The network will be everywhere… Computers will be used in almost everything we build

(including sensors, appliances, books, newspapers) These computers need to be interconnectedNETWORKING = COMPUTING


Why Learn about Computer Networks?

Industry and research are wide open to innovation!

Today’s protocols are oriented to support “host-to-host” communication and assume a client-server model for services and an “open door” policy for the Internet community.

The continuing success of the Internet requires: Person-to-person communication (voice and other media over the

Internet) Client-to-content services. Security in the services, the infrastructure, and the clients of the

Internet

Innovation required includes: Mechanisms to “look-up” content, rather than addresses. Protocols aimed at the new types of communication and services Protocols that adhere to new principles of design. Protocol stacks for nomadic users.


Evolution of Computer Networks1876: Telephone by A. Graham Bell1890s: Electromagnetic telephone switches1897: Cathode Ray Tube by K.F. Braun1940s: Computers, error detection and retransmission1960: RS-232 physical layer interface (the “serial port”) and modems1960s: T-1 carrier system for telephone transmission (1.5Mbps)1961: The Compatible Time Sharing System1962: Paul Baran at RAND proposes packet switching1965: Automatic equalization by Bob Lucky and others1968: Carterfone FCC decision that led to AT&T divestiture in 1984.1969: DARPA funds project on packet switching1970s: Computerized switches; work on ISDN starts


Evolution of Computer Networks1970s: ARPANET starts (UCLA, Utah, SRI, UCSB); its technology evolved into today’s Internet1970s: ALOHA system at U. of Hawaii; first protocol for multiple access channels; leads to Ethernet1970s: GUI, mouse, hypertext by Doug Engelbart at SRI1974: “A Protocol for Packet Network Interconnection,” V. Cerf and R. Kahn, IEEE Trans. Comm (May).1980s: OSI (open system interconnection) reference model1982: TCP/IP is deployed in ARPANET/MILNET1984: Host table evolves into DNS in ARPANET1984: AT&T breaks up1986: NSFNET is created; becomes Internet backbone1992: WWW by Tim Berners-Lee (CERN) is released; gives a GUI to the Internet1990s: Caches and proxies helping clients access content


Evolution of Computer Networks

1970s: CCITT publishes standards for public data networks

(X.25 standards)1980s: Token ring LANs, FDDI emerge; do not replace Ethernet1990s: ATM evolves; does not replace IP1990s: Internet: From 4 to 30M+ wired, published nodes in two decades1990s: SONET (synchronous optical network) and

SDH (synchronous digital hierarchy) evolve1990s: Cellular phones, laptops, palmtops become

popular1999: Gigabit Ethernet starts, simplicity wins again.

Apr 19, 2023 UCSC CMPE150 10

Evolution of Computer NetworksWhat will happen in the 2000s?: Ad-hoc wireless networks; self-configuring nets Networked sensors and appliances System-area networks (“the network is [in] the

computer”) Network-based computing: grid computing (“the computer--processing and storage--is in the network”) Internet-to-go; deeply networked systems IP voice, IP devices Content routing: ISPs start to be CDNs, allow clients

to obtain content based on its name from the best location

…. Networking = Computing

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What Do We Study?

We will use the Internet as our running example.

The Internet has computer hardware, software, operating systems, transmission technology, services defined over it... What is its glue?

Communication protocols implemented in software or hardware transform otherwise isolated machines into a society of computers.

Protocols specify how processes in different machines can interact to provide a given service.

Distributed algorithms are the essence of what we study.

Apr 19, 2023 UCSC CMPE150 12

Communication Protocols A set of rules governing the interaction

of concurrent processes in a system. A protocol has five parts:

The service it provides. The assumptions about the environment where it

executes, including the services it enjoys. The vocabulary of messages used to implement it The format of each message in the vocabulary. The procedure rules (algorithms) guarding the

consistency of message exchanges and the integrity of the service provided.

Apr 19, 2023 UCSC CMPE150 13

What Do We Study Regarding Protocols?

What is a good protocol design? Judging by their survival, Ethernet and IP are

good; token ring protocols are not very good What are good and bad aspects in a

protocol? TCP adapts to congestion, but it inherently

assumes that the Internet sends packets in order. Use representative protocols to go over

these issues. Discuss new directions in computer

communication.

Apr 19, 2023 UCSC CMPE150 14

What Do We Study Regarding Protocols?

We will take a quick look at the principles of computer communication.

Our principles are: The description of a protocol has no ambiguity. A protocol does what it is supposed to do, all the

time. A protocol does not leave any communicating party

waiting forever for something to happen. A protocol makes efficient use of available

resources. A protocol enables the use of resources fairly or

according to a predefined contract. As with most engineering topics, simplicity is

important.

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Principles of Computer Communication

Protocol specification: The description of the protocol is complete and accurate.

Safety: A protocol does what it is supposed to do, all the time.

Liveness: A protocol does not leave any deadlocks.

Efficiency: A protocol makes efficient use of available resources.

Fairness: Fair or contractual use of resources

Simplicity is desirable, but not necessary.

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In The Beginning There Was ARPANET

UCSB

SRI UTAH

UCLA

IMP

IMP

IMP

IMP

12/1/69

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The Beginnings of Protocol Layering

Routing within ARPANET is transparent to hosts attaching to the ARPANET

HOST

Host-IMPIMP-IMP

HOST

Host-Host

IMP

IMP

IMP

IMP

applicationapplication

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Characteristics of ARPANET Architecture

Backbone-based: A finite number of identifiers suffices to name the IMPs

that constitute the backbone. The backbone provides a common packet-delivery service

to the backbone users. Host-centric:

The users of the backbone are host computers. The identifiers used to route packets to destinations are

the names of the attachment points of the hosts; these are unique identifiers throughout the network.

A centralized host table can be used to map text-based host names to the numeric names.

Apr 19, 2023 UCSC CMPE150 19

Internet Model

Problem: Need to maintain the details of how

packets are forwarded within and across heterogeneous networks transparent to the users (hosts) of the interconnect.

Approach: Use a common end-to-end protocol spoken

by all gateways interconnecting networks, and also by the hosts talking to gateways.

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Internet Model

G

GG G

NET

NET NET

NET A

3

B

A@3

Users of the interconnect are hosts. A single address space is used to draw numeric names for

networks (“3”) and hosts (“A@3”). An end-to-end protocol (the Internet Protocol or IP) is used for

delivering all user and control data, with a common definition of services.

A table is used to map people-friendly names to numeric names.

IPIP

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Internet Model

How data are forwarded within each network is transparent to IP, and IP is transparent to

each network

G

GG

G

R

R

R R

R

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Internet

NSFNET: Routers inside networks also use IP.IP in every router; no need for network gateways.

Names of routers are “router@net”

R

R

R

R R

RR

RR R

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ARPANET Growth

(a) December 1969, (b) July 1970, (c) March 1971, (d) April 1972, (e) September 1972.

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NSFNET Topology

The NSFNET backbone in 1988.

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Internet Elements

R

RR

R

R

R

R R

R

Routers/bridges/switches: Devices used to interconnect hosts and to forward data from source to destination host.

Hosts: The computers running user applications (clients, servers, proxies).

Networks: Aggregations of hosts and routers.

Links between devices.

Apr 19, 2023 UCSC CMPE150 26

What Is The Internet

protocols Internet: “network of

networks” loosely hierarchical public Internet versus

private intranet Internet standards

RFC: Request for comments

IETF: Internet Engineering Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile

Apr 19, 2023 UCSC CMPE150 27

Internet Example

Your client computer (client) and attached router have to be configured.

Host and router communicate with each other through a LAN or point-to-point link (PPP, CSMA/CD, 802.11, etc.).

Data is broken into “packets,” which are to be routed from client to server and from server to client over a large number of links, computers dedicated to routing, and networks (IP, ARP, ICMP, OSPF, RIP, BGP).

Host obtains the IP address of remote server from a name resolver (UDP, DNS) .

Client starts an end-to-end reliable connection with remote server (TCP).

Client and server start exchanging messages (HTTP).

What does it take to get a page from the web?

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Internet Example

R

RR

R R

RR

RR R

PP

One of many processes at one of many hosts in one of many networks communicates with another process, which is one of many processes at one of many hosts attached to one of many

networks.

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Layering Model Purpose is to divide and conquer complex software

and hardware needed to implement services Partition services and functions needed in system

into layers Each layer of service is provided by peer protocol

entities Communication can be point-to-point or multipoint

Layer-N Protocol Entity

Layer-N Protocol Entity

Layer-(N - 1)Protocol Entity

Layer-(N - 1) Protocol Entity

(virtual communication)

Layer N packets

interface

NODE A NODE B

protocol

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PHYSICAL

The OSI Architecture Proposed by the International Standards Organization Specifies the functions at each layer, not the protocols

that implement them

LINK

End-user services (e.g., mail, file transfer)

Formatting, encryption, compression of data

Setup and management of end-to-end dialogue

End-to-end delivery of messages to processes

End-to-end transmissions of packets in net

Transmissions of packets over a link

Transmission of bits over physical media

Web access

TCP

IP

PPP, CSMA/CD

SONET

NETWORK

TRANSPORT

SESSION

PRESENTATION

APPLICATION

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Protocol Description Specify the service to be provided by the

protocol Specify assumptions about environment Specify vocabulary (messages) needed in

the protocol Specify the algorithms used to process

and exchange information in the protocol Specify format of messages in vocabulary

We do this only in some cases Our specifications are informal!

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Protocol Correctness

A protocol must be safe and live Safety:

Protocol provides the desired service all the time

Liveness: Protocol has no deadlocks (no process

waits forever for an event to occur) Proving one may depend on the other

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Protocol Performance Average delay

Time between transmission of an information bit and reception of the bit at the receiver

Throughput or capacity Number of information bits sent divided by the

time between transmission of first bit and delivery of the last bit

Bottlenecks Computations will make strong assumptions;

in most cases, results of analytical model provide only a rough approximation

Most effective for comparative analysis

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Basic Network Services

S D

Shared network resources

All data flow along same path

1,2 1,2

Connection-oriented service: Reliable data transfer: In-order delivery, no

duplicates or missing data. Flow control: Do not congest the receiver(s). Congestion control: Do not congest the network(s).

Apr 19, 2023 UCSC CMPE150 35


Connection-oriented service: Reliable data transfer: In-order delivery, no

duplicates or missing data. Flow control: Do not congest the receiver(s). Congestion control: Do not congest the network(s).

S D


1

1 1

1

2 2 2 Data may take different paths to destination

1,2 1,2

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Connectionless service: No delivery guarantees needed from the

network. Any connection-oriented service to application

is provided by end-to-end protocol.

S D


1,2,3 2,1,2

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Switching Methods

Allocation of shared network resources to the transport of user data.

Circuit switching and packet switching are the two main types we will consider.

S

D


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Circuit Switching

Portion of physical resource is assigned to a single connection.

Delay and signaling overhead in establishing and ending connections.

S D

call accept

call request

DATA

termination ackcall termination

Apr 19, 2023 UCSC CMPE150 39

Multiplexing in Circuit Switching

Share a given communication channel among multiple connections.

Frequency division multiplexing (FDM): Frequency spectrum of a link is partitioned into

multiple bands, and each band is assigned to zero or one connection at any given time.

Time division multiplexing (TDM): Time is divided into frames of fixed duration, and

each frame is divided into a fixed number of time slots. A connection is assigned to a time slot, and occupies the same time slot for multiple frames.

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FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

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Message Switching

Message from sender is sent on a store-and-forward basis.

Message has a header used for forwarding.Resources shared among different calls.

S D

message

Apr 19, 2023 UCSC CMPE150 42

Statistical Multiplexing

Share the same communication channel among multiple connections without fixed allocations of the resource to those connections.

S1 D1

S2

D2

m2 m1

m2

m1

m2 m1m1m2 m2 m1

m2

m1

Link is shared based on the statistics of each connection or flow.

Limitation: Entire message must be received at a switch before it can be

forwarded

Apr 19, 2023 UCSC CMPE150 43

Packet Switching

Message from sender is broken into packets.

Each packet consists of a header and a payload.Header information is used to forward packet to destination.

S D

packet 1

packet 4

packet 2

packet 3

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Packet Switching

A packet switch stores each packet it receives and determines how to forward it based on the header

information in the packet.

S D

store

forward

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Resources are shared among connections Packets from the same connection can be processed

concurrentlyConnection setup delay can be avoided using datagrams

S DPacket Switching

packet 1

packet 4

packet 2

packet 3

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S D

Packet Switching vs Message Switching

Processing message as packets instore-and-forward mode saves time if propagation

delays are small!

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Packet Switching Information is organized into packets A packet consists of a header and a payload Header specifies the control information

needed to transport the packet from origin to destination

Packets are forwarded from source to destination using routing tables

There are two basic approaches to packet switching: datagrams virtual circuits

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Datagrams

Routing table specifies next hop to each destination Packets are forwarded based on the routing table Each packet is routed independently

To b go to 2 nextTo d go to 3 nextTo e go to 2 nextTo 4 go to 3 next….

d

b5

4

6

a

c3

1

2

7

e

a->b

a->b

a->b

c->d

c->d

a->e

a->e

a->e

a->e

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Virtual CircuitsVC1

VC2

VC3

Virtual circuits are established and terminated much like circuits in circuit switching.

Statistical multiplexing using packets, rather than FDM or TDM is used to share links among connections.

d

b5

4

6

a

c3

1

2

7

e

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Virtual Circuits

Routing table specifies the next hop of an established VC. Packet header specifies VC to be used for the packet. All packets of the same VC are routed the same way.

For VC1 use 2For VC2 use 3For VC3 use 3...

VC1

VC2 d

b5

4

6

a

c3

1

2

7

e

VC3

Apr 19, 2023 UCSC CMPE150 51

Virtual Circuits

Relative or global VC names can be used. Relative VC names require “label swapping.”

For VC5 in, use 4 and label as VC2For VC4 in, use 5 and label as VC3...

VC1

VC2 d

b5

4

6

a

c3

1

2

7

e

VC3

VC5

VC4

Apr 19, 2023 UCSC CMPE150 52

Packet Switching versus Circuit Switching

1 Mbit link Each user:

250 kbps when “active” active 10% of time

Circuit-switching: 4 users

Packet switching: With 10 users, the

probability of having more than 4 active users is 0.0016!

The average number of users active on the link is one.

Packet switching allows more users to use network!

1 Mbps link

Look at the probability of having up to 10 active users.

Look at the arithmetic average.

N users

Apr 19, 2023 UCSC CMPE150 53


Works great in the average!

However, more than 4 users may be active at the same time.

In that case, packets are queued at the switch, and congestion occurs.

Queuing delays are important.

N users

1 Mbps link

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By Contrast: FDM and TDM

FDM

frequency

time

TDM

frequency

time

2 active users

Example:

Apr 19, 2023 UCSC CMPE150 55


Great for bursty data Resource sharing Simpler, no call setup (with datagrams!)

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

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

Bandwidth guarantees needed for audio/video applications.

Answer: Still a research problem.

Apr 19, 2023 UCSC CMPE150 56

Transmission Media

We consider the physical layer as a “black box”

We are interested in the characteristics and services provided by the transmission media that impact the link layer and higher layers.

Parameters: Bandwidth Delay or latency: average and variance Storage capacity (bandwidth-delay product) Reliability and security Order of delivery Type of sharing or access

Apr 19, 2023 UCSC CMPE150 57

Bandwidth We can communicate information over transmission

media using energy, by varying some physical property (e.g., voltage or current).

Problem: What the sender transmits is not exactly what the receiver obtains from the communication link.

Reasons: Link incurs some energy loss and delays, and there are other interfering sources.

sender receiver

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Bandwidth We think of the bandwidth of a network or link as

the number of information bits that can be transmitted over it in a certain period of time (e.g., bits per second).

The bandwidth of a link is really the frequency range tolerated by the channel without major attenuation.

Telephone line is 3000 Hz (300Hz to 3300 Hz) Available bandwidth depends on the rate at which

channel can change stored energy. We can model waveforms as sums of sine waves of

different frequencies. Channel attenuates and delays each frequency

component differently, causing distortion.

Apr 19, 2023 UCSC CMPE150 59

Bandwidth Signals are run through a low-pass filter, and a signal can have V

discrete levels. Maximum data rate of a noiseless channel of bandwidth B when

V discrete levels are used is Data Rate = 2B log2 V bps (Nyquist, 1924).

Keep increasing V to achieve higher data rates with same B ?

Regardless of V, the maximum data rate (capacity) of a noisy channel with bandwidth B and signal-to-noise ratio S/N is

C = B log2 (1+S/N) bps (Shannon, 1948) S is the average signal power and N is the average noise power

Example: For a telephone line, B is 3000 Hz, with a typical S/N ratio of 1000, so C is 30Kbps or so

We can achieve higher capacity only by increasing S/N!

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Sources of Packet Delay

A Bt1

t2

What contributes to the delay from the time the first information bit is sent by the source (t1) to

the time when the last information bit is obtained by the receiver?

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Sources of Packet Delay

A Btransmission time of packet

over each link

propagation delayof each link

nodalprocessing

nodalprocessing

queueing delay

queueing delay

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Sources of Packet Delay Nodal processing:

Checking for bit errors. Determining output link.

Queueing delay: Time waiting at output link for transmission. Depends on congestion level of router.

Transmission delay: Time to send bits into link: L/R, where

R = link bandwidth (bps) and L = packet length (bits) Propagation delay:

Time for each bit to traverse a link: d/s, where

d = length of physical link and

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

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Packet Delay

Packet delay is the time elapsed between the instant when the sender transmits the first bit of a obtains the last bit of the packet.

Packet delay = Processing delay + Propagation delay + Transmission delay + Queuing delay

Propagation delay = Distance / Speed of lightSpeed of light = 3x108 meters/sec in the

vacuum ~ 2x108 meters/sec in fiber We can reduce processing, transmit, and queue

components using higher link speeds and faster processors, but we cannot reduce the speed of light!Long distances mean long latency!

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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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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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Packet Loss

Queue (i.e., the buffer) of each outgoing link has finite capacity.

When packet arrives to a full queue, packet is dropped (lost).

Lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all.

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Bandwidth-Delay Product The amount of data “stored” in the link. Think of a link as a pipe; the latency is the length of

the pipe and the bandwidth is its diameter. The BD product gives the volume of the pipe. Example: A channel of 50 ms latency and just 45

Mbps bandwidth can hold 2.25 million bits (the same as the memory of a PC of early 80s!).

We are moving to Gigabit networks... Big bandwidth and big distances require:

Big aggregation and big memories at hosts New reliable transmission algorithms Migration from client-server to client-content models.

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Bandwidth-Delay Product

Links stretching long distances have large storage capacity.

Problem: How do we provide feedback to senders? TCP was originally designed for such applications

as telnet and ftp over paths with small BDP.

S R

Low-speed link

high-speed link

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Other Parameters

Reliability: We will assume that information is transmitted correctly across a link or network with a given likelihood.

Security: We will likely not cover this aspect in much detail :(

Order of delivery: We will assume FIFO and non-FIFO delivery of packets or messages, depending on the protocol and transmission media.

Access: We will consider point-to-point and broadcast links.

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Internet Structure: Network of Networks

Roughly hierarchical At its 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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Tier-1 ISP Example: Sprint US Backbone Network

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Internet Structure Each packet passes through many networks! Limiting factor: Speed of light!

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

Apr 19, 2023 UCSC CMPE150 73

Internet Structure Why we need user-to-content rather

than client-server approach.

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

Content comes from nearest outlet to client.Content needs to be routed to nearest outlets.

proxy

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END

Apr 19, 2023 UCSC CMPE150 75

Internet Delays and Routes What do “real” Internet delay and loss look like? Traceroute program: provides delay

measurement from source to router along end-end Internet path towards destination.

For all i: sends three packets that will reach router i on path

towards destination router i will return packets to sender sender times interval between transmission and

reply.

3 probes

3 probes

3 probes

Apr 19, 2023 UCSC CMPE150 76

1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms17 * * *18 * * *19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms

traceroute: gaia.cs.umass.edu to www.eurecom.frThree delay measements from gaia.cs.umass.edu to cs-gw.cs.umass.edu

* means no reponse (probe lost, router not replying)

trans-oceaniclink

Internet Delays and Routes

Documents

CMPE 150: Introduction to Computer Networks