Networking

• A few questions on the course

• A few questions about the course

The Head of Department is

Pet

er J

ohnso

n

Chris

Jen

nison

Gly

nis B

reak

wel

l

Phil

Will

is

Kev

in E

dge

44%

8%11%

31%

6%

A. Peter Johnson

B. Chris Jennison

C. Glynis Breakwell

D. Phil Willis

E. Kevin Edge

That was the “warmup”

• I hope you got Phil Willis:

• Peter Johnson is so last last year!

• Chris is Phil’s boss, Kevin is Chris’s boss, and Glynis is Kevin’s boss.

Questions on the course

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

An Ethernet address is how long?

Var

iabl

e le

ngth

16

byte

s

6 b

ytes

4 b

ytes

3%

18%

43%38%

A. Variable length

B. 16 bytes

C. 6 bytes

D. 4 bytes

The answer is C (6 bytes)

If you didn’t get that, you’re pretty confused: After last time, all I can suggest is that you really re-read sections 2.2 and 3.2 of the book.

Which of these is not a well-known routing protocol?

Routin

g Info

rmat

ion ..

.

Open

Shorte

st P

ath F

irst

Route

r Dis

cove

ry P

r...

Bord

er G

atew

ay P

ro...

8% 5%

55%

33%

A. Routing Information Protocol

B. Open Shortest Path First

C. Router Discovery Protocol

D. Border Gateway Protocol

C — Router Discovery Protocol

A. RIP is a well-known protocol

B. OSPF is a well-known protocol

C. RDP, as far as I know, doesn’t exist: Router Discovery Messages (chapter 9) say who routers are.

D. BGP is a well-known protocol

There are thee kinds of autonomous system: which of

these is not one:

Stu

b

Confe

dera

tion

Tra

nsit

Multi

hom

ed

2%

16%16%

66%

A. Stub

B. Confederation

C. Transit

D. Multihomed

B (Confederation) is not a kind of autonomous system

If you got this wrong, I suggest section 10.7 of the book, and also

the updating notes for that

Which one of these statements about IP fragmentation is true

A fr

agm

ente

d pac

ket..

.

Fra

gmen

ts a

re p

ut to.

..

Fra

gmen

tatio

n is

opt..

.

Fra

gmen

tatio

n is

wh..

It is

vita

l that

frag

men

..

2%11%

7%7%

73%

A. A fragmented packet can never be refragmented

B. Fragments are put together as soon as possible

C. Fragmentation is optional for the sender

D. Fragmentation is what happened to officers in Vietnam

E. It is vital that fragments arrive in the right order

C (optional) is the right answer

A. fragmented packets can be re-fragmented

B. The receiver (or its firewall) puts fragments together, not intermediate hosts

C. True: set the DF flagD. That’s not IP fragmentationE. Fragments can arrive in any order: the

offset field tells how to put them back

Which one of these is true

255

.255

.255

.255

is c

all..

Bro

adca

stin

g is h

ow ..

.

Giv

en th

e ch

oice,

mul..

.

Multi

cast

ing o

nly w

or..

20%16%

56%

9%

A. 255.255.255.255 is called limited broad-cast because it’s only used on startup

B. Broadcasting is how internet radio works

C. Given the choice, multicast is generally better than broadcast

D. Multicasting only works on Ethernets

The answer was C: multicast is generally better than broadcast

A. It’s called limited because routers don’t forward it, so it’s limited to that net

B. Internet radio etc. work on multicasting

C. Yes – doesn’t load those who aren’t interested

D. No: multicasting is very important on wide-area networks (e.g. radio)

Between A and PTR records

The

DNS is a

lway

s c.

..

Consi

sten

t exc

ept f

or...

Mig

ht wel

l be

inco

nsi...

Consi

sten

cy is

mea

n...

Consi

sten

cy is

unde.

..

14%

27%

20%18%

20%

A. The DNS is always consistent

B. Consistent except for deliberate fraud

C. Might well be inconsistent

D. Consistency is meaningless

E. Consistency is undecidable

A “thinking” question: C or E

A. Certainly not true

B. Fraud can certainly work by adding inconsistencies, but it’s not the only way

C. DNS maintainers can easily screw up!

D. An abstract definition is possible

E. But it requires testing infinitely many possible cases. Also the DNS is distributed, so untestable.

The DNS …

Alw

ays

uses

UDP

Alw

ays

uses

TCP

Norm

ally

use

s UDP

Ques

tions

are

in U

D..

20% 23%

43%

14%

A. Always uses UDP

B. Always uses TCP

C. Normally uses UDP

D. Questions are in UDP, but replies can be in either depending on length.

C — normally UDP

• Both can be used

• UDP is more common for most transactions

• Hence C is the answer

• A UDP question always gets a UDP answer, possibly marked TC (truncated). It can be re-asked in TCP if necessary

Which one of these statements about TFTP is true

TFTP w

as in

vente

d b...

TFTP, l

ike

UDP, is

unr...

TFTP is

obso

lete

TFTP u

ses

UDP for s

...

16%

56%

9%

19%

A. TFTP was invented before TCP, so uses UDP

B. TFTP, like UDP, is unreliable

C. TFTP is obsolete

D. TFTP uses UDP for simplicity

D — simplicity

A. No – TCP is RFC 793 but TFTP is RFC 1350. Anyway date isn’t the reason

B. TFTP numbers packets, and asks for repeats, so is reliable

C. TFTP is in daily use in the Library

D. Simplicity is the reason: UDP is simple, unchanging and fits in a boot PROM.

The TCP header is

Exa

ctly

20

bytes

long

At l

east

20

bytes

long

At m

ost 2

0 by

tes

long

Exa

ctly

60

bytes

long

Bet

wee

n 20

and 6

0 b...

34%

14%

36%

2%

14%

A. Exactly 20 bytes long

B. At least 20 bytes long

C. At most 20 bytes long

D. Exactly 60 bytes long

E. Between 20 and 60 bytes long

E (between 20 and 60)

A. 20 is the minimum

B. Correct, but not the whole truth

C. Wrong

D. 60 is the maximum, but it’s nearly always less

E. Correct

The ACK bit in the TCP header is set

All

the

time

Only

when

ther

e’s

ne...

On e

very

pac

ket e

xce.

.

On e

very

pac

ket e

xcep

...

0%

16%

59%

25%

A. All the timeB. Only when there’s

new data being acknowledged

C. On every packet except the first

D. On every packet except the first in each direction

C — every packet except first

A. Can’t be set on first packet — there’s nothing to acknowledge

B. The data might not seem new to us, but our previous acknowledgement might have been lost

C. Correct

D. No — the first packet from B to A has to acknowledge A’s SYN

A TCP segment with SYN must have a byte of data, so that it can

be acknowledged

Tru

e

Fal

se

42%

58%

1. True

2. False

False

The SYN bit itself is counted as if it were a byte of data. It is this “pretend” byte that is acknowledged.

The initiator of a TCP connection must be the one to close it.

Tru

e

Fal

se

93%

7%

1. True

2. False

False

In a typical client-server situation, it is often the client (which is the one who starts the connection) who finishes it, but this is not an invariable rule. HTTP is a counter-example (at least in HTTP 0.9/1.0)

Networking

• A few questions on the course

• A few questions about the course (starting with some comments from the survey some of you kindly did)

Good fun, something different and its very good to be interactive since we do not get much class participation, except

if we want to ask questions

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

54%

39%

2%0%5%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

Although good for general feedback how we are doing, it’s not so good

for passing course material on.

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

11%

34%

5%

27%

23%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I wish we’d had more networking earlier in the degree course

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

23%

40%

19%

12%

7%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I wish this Audience Response System had been used earlier

(i.e. in previous years)

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

62%

26%

2%0%

10%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree