Networking

• A few questions on the course

• A few questions about the course

The Director of Studies is

Alw

yn B

arry

Jam

es D

aven

port

Ala

n Hay

es

Mic

key

Mouse

9%5%

77%

9%

A. Alwyn Barry

B. James Davenport

C. Alan Hayes

D. Mickey Mouse

That was the “warmup”

• I hope you got Alan Hayes:

• James Davenport is so last year!

• Alwyn Barry is just history

• (as far as this job is concerned!)

Questions on the course

(mostly factual, but there is more to the course than facts)

An Ethernet address is how long?

4 b

ytes

6 b

ytes

16

byte

s

Var

iabl

e le

ngth

13%9%

58%

20%

A. 4 bytes

B. 6 bytes

C. 16 bytes

D. Variable length

The answer is B (6 bytes)

If you didn’t get that, you’re pretty confused: re-read sections 2.2 and 3.2 of the book.

100 Mbps twisted pair length limit is

60

met

res

100

met

res

195

met

res

500

met

res

14% 16%12%

58%A. 60 metres

B. 100 metres

C. 195 metres

D. 500 metres

The correct answer is B: 100m

However, this is the sort of factual answer:

a) I don’t expect you to know, and would give you in an exam question;

b) You should always check in real life, especially if equipment installation depends on it;

c) As I showed in the Library, isn’t what it seems in practice

Internet and Ethernet

An in

tern

et is

impo

ss...

The

Inte

rnet

nee

ds Et..

.

Today

’s In

tern

et d

ep...

Tota

lly in

depen

dent .

..

2%

64%

23%

11%

A. An internet is impossible without Ethernet

B. The Internet needs Ethernet

C. Today’s Internet depends on Ethernet

D. Totally independent concepts

I hope that made you think

A. Certainly false

B. Was false in the beginning, and may well be false again.

C. True in practice today.

D. True, but not as helpful as C.

When it comes to END characters, PPP

Is b

ette

r than

SLIP

, as

..

Has

one,

whic

h can

’t ..

Allo

ws it

in th

e m

iddle

..

Allo

ws it

anyw

here

in...

21%

29%

14%

36%1. Is better than SLIP,

as it doesn’t use one

2. Has one, which can’t be used in IP

3. Allows it in the middle of IP packets

4. Allows it anywhere in IP packets

The correct answer is D:anywhere in IP packets

• Since PPP doesn’t have a length field, it must have some END marker

• IP can contain arbitrary binary data, so B and C can’t be right.

• If there is an END character in the IP packet, it is “escaped” by the sending PPP, and re-interpreted by the receiver

• See figure 2.2

When it comes to delivering packets, IP

Guar

ante

es to

del

iver

Guar

ante

es to

del

iver

...

Mak

es n

o guar

ante

es

2%

91%

7%

A. Guarantees to deliver

B. Guarantees to deliver or return an ICMP error

C. Makes no guarantees

The answer is C

• On Ethernets, you have no idea if the packet is received

• Congested routers may just drop packets

• No error is generated for header checksum errors

• No error is generated for broadcast or multicast packets

• Etc.

The system administrator decides on the routing mechanism

Tru

e

Fal

se

44%

56%1. True

2. False

False

• There is only one routing mechanism: see Stevens sections 9.1, 9.2.

• Who confused routing mechanism with policy?

The manager of an autonomous system in the Internet

Can

do w

hatev

er h

e l..

.

Must

imple

men

t the

r...

Must

sat

isfy

the

cons.

..

Dep

ends

on whe

ther

i...

30%

23%

40%

7%

A. Can do whatever he likes

B. Must implement the routing policy of whoever his AS is connected to

C. Must satisfy the constraints of whoever his AS is connected to

D. Depends on whether it’s stub/transit

A. True, but he may not stay connected to the Internet for very long.

B. It would be rare to be told what routing policy to implement.

C. Yes, such constraints must be satisfied.

D. In theory no, but in practice a transit AS will have many more constraints.

Networking

• A few questions on the course

• A few questions about the course

The textbook is a fantastic resource

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

14%

36%

0%

11%

39%A. Strongly Agree

B. Agree

C. Neutral

D. Disagree

E. Strongly Disagree

Actually, I use other sources

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

2%

14%18%

43%

23%

A. Strongly Agree

B. Agree

C. Neutral

D. Disagree

E. Strongly Disagree

My principal other source is

Bra

dford

’s b

ook

Far

ouzan’

s book

Oth

er b

ook

Wik

iped

ia

Oth

er o

n-line

non-b...

A m

ixtu

re

No o

ther

sourc

e

2%0% 0%

42%

12%12%

33%

A. Bradford’s bookB. Farouzan’s bookC. Other bookD. WikipediaE. Other on-line non-

book sourcesF. A mixtureG. No other source

If you had “other”

• Then you can tell us.• Text to 07624804921.

The lecturer uses the textbook far too much in lectures

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

28%

49%

0%5%

19%

A. Strongly Agree

B. Agree

C. Neutral

D. Disagree

E. Strongly Disagree

What about this technology

Rev

olutio

nises

pro

b...

Use

ful e

xtra

, but

sho.

..

OK 1

-2 ti

mes

/term

Nev

er a

gain

Abst

ain

66%

32%

0%0%2%

A. Revolutionises problem classes

B. Useful extra, but shouldn’t dominate

C. OK 1-2 times/term

D. Never again

E. Abstain