Networking

• A few questions on the course

• A few questions about the course

The Chancellor of the University is

Alis

tair

Darlin

g

Lord

Pat

ten

Gly

nis B

reak

wel

l

Prin

ce P

hilip

Lord

Tugen

dhat

Lord

Hin

ton o

f Ban

ksid

e

11%

19% 19%

15%

4%

33%A. Alistair Darling

B. Lord Patten

C. Glynis Breakwell

D. Prince Philip

E. Lord Tugendhat

F. Lord Hinton of Bankside

That was the “warmup”: E

A. Darling is Chancellor of the ExchequerB. Lord Patten is Chancellor of OxfordC. Glynis Breakwell is the Vice-ChancellorD. Prince Philip is Chancellor of CambridgeE. Lord Tugendhat is the ChancellorF. Lord Hinton was the first Chancellor: the

mullets (stars) on the unicorn and wyvern supporting the coat of arms are a heraldic pun on his own arms

Questions on the course

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

How long is an Ethernet address?

Var

iabl

e le

ngth

16

byte

s

4 b

ytes

6 b

ytes

0%

72%

8%

20%

A. Variable length

B. 16 bytes

C. 4 bytes

D. 6 bytes

The answer is D (6 bytes)

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

Which of the following will DHCP not tell you

Your I

P addr

ess

A ro

uter’s

IP a

ddres

s

A n

ames

erve

r’s IP

a...

A n

ames

erve

r’s n

ame

The

subne

t mas

k

15%11%

15%

48%

11%

1. Your IP address

2. A router’s IP address

3. A nameserver’s IP address

4. A nameserver’s name

5. The subnet mask

D — the nameserver’s name

• It’s actually not useful

• And anyway you can find it out: how?

The reset (RST) bit is set in the last packet of a 4-way close

Tru

e

Fal

se

83%

17%

1. True

2. False

False

• RST is not a normal part of TCP operation: it is there to indicate some kind of error/ abnormal circumstance

After the 3-way open, MSS is

Neg

otia

ted

The

min

imum

of w

ha...

The

aver

age

of what

t..

Poss

ibly

diff

eren

t in t.

.

3%

48%

10%

38%

A. Negotiated

B. The minimum of what the two said

C. The average of what the two said

D. Possibly different in the two directions

D — possibly different

There is no negotiation, and it is perfectly possible for MSS to be different in both directions (e.g. one end is on a dial-up link)

Consider the following TCP Open scenario:

• P→Q: SYN (with MSS)

• Q→P: SYN (with MSS, and ACK)

• P→lost: ACK

What happens next?

P’s ACKof Q’s SYN is lost, and

The

conne

ctio

n han

gs

P e

ventu

ally

tim

es o

ut

P s

tarts

sen

ding d

ata

P re

sends

its S

YN

Q re

sends

its S

YN

P s

ends

a RST

0%3%

15%

65%

12%6%

A. The connection hangs

B. P eventually times out

C. P starts sending data

D. P resends its SYN

E. Q resends its SYN

F. P sends a RST

C (or possibly E)

A. Hanging would be a failure of TCPB. P thinks the connection’s open, so why time

out?C. Therefore can send data (which will

acknowledge the SYN as well)D. Why should P send SYN: it’s been ackedE. Q hasn’t has SYN acked, so will resend if C

doesn’t happen (e.g. SMTP, where the server first sends a 220)

F. Why should it send RST?

Which of these is not in the TCP header

Seq

uence

Num

ber

Optio

ns

Rec

eive

r Win

dow

Conge

stio

n W

indow

Urg

ent P

ointe

r

3%

14%9%

54%

20%

A. Sequence Number

B. Options

C. Receiver Window

D. Congestion Window

E. Urgent Pointer

D — Congestion Window

A. Sequence number is always there

B. Options, if present are in the header

C. Receiver Window is always there

D. Congestion Window is purely maintained by the sender, and is not passed to the other end (its effects may be felt, but that’s another matter)

E. Urgent pointer is always there

A receiver drops packets to indicate that its window is full

Tru

e

Fal

se

72%

28%

1. True

2. False

False

It may drop, and not acknowledge, the data (as in the persist timer examples), but it does always respond to the packet, even if the acknowledgement field is not incremented.

Network congestion can be indicated by

Tim

e out

Win

dow =

0

2 d

uplic

ate

ACKs

4 A

CKs of t

he s

ame

...

A a

nd D

A,B

and C

B a

nd C

12%

6%3%

15%

36%

21%

6%

A. Time outB. Window =0C. 2 duplicate ACKsD. 4 ACKs of the

same numberE. A and DF. A,B and CG. B and C

E (A and D) is right

A. Timeout is a good indicator of congestion (but the other end might have crashed …)

B. Not network congestion

C. Not 2 Duplicate ACKs

D. 4, with the same number, means 3 are duplicates, so this is congestion

The persist timer deals with which problem?

Host

s th

at g

o dow

n

Host

s th

at h

ave

no s.

..

Lost

SYN p

acke

ts

Lost

win

dow adve

rts

Lost

FIN

pac

kets

11%

31%

25%22%

11%

A. Hosts that go down

B. Hosts that have no space for more data

C. Lost SYN packets

D. Lost window adverts

E. Lost FIN packets

D: lost window advertsA. Hosts that go down: TCP times out

sendingB. Hosts with no space — there’s no cure

for this!C. Lost SYN packets, retransmission deals

with thisD. The response to the persist timer will give

us a new window size (which may still be same, if there is still no space)

E. Lost FIN packets, retransmission deals with this

“slow start” is somewhat of a misnomer

Tru

e

Fal

se

53%

47%

1. True

2. False

Possibly

It does indicate that we start slow (so far, so good), but we get exponentially faster, so most people probably wouldn’t call it slow

Sending `window scale’ means

I w

ill s

end s

cale

d wi..

.

I w

ill u

nderst

and

sca.

..

I w

ill s

end s

cale

d wi..

.

You c

an s

end

scal

ed...

B a

nd C

B a

nd D

5%8%

68%

8%5%5%

A. I will send scaled windows

B. I will understand scaled windows

C. I will send scaled windows if you will

D. You can send scaled windows if you want

E. B and CF. B and D

E is the best answer

A. You can’t scale unless the other end says it understands scaling

B. If you can send them, you must be able to understand them

C. Certainly correct

D. Some TCPs send window scale automatically even if the other end didn’t (of course, they can’t use it)

In FTP, the control channel

Sta

rts in

ASCII

or bin

a..

Sta

rts in

ASCII,

but c

..

Is a

lway

s in

ASCII

Use

s XDR

26%

6%

35%32%

A. Starts in ASCII or binary depending on the options

B. Starts in ASCII, but can be switched to binary

C. Is always in ASCII

D. Uses XDR

C – always ASCII

A. The other end doesn’t know what the options are, so this can’t be right (a general point about protocols!)

B. The IMAGE command changes the data channel only

C. Right

D. XDR is connected with NFS, not FTP

One of these statements is true

SM

TP const

ruct

s th

...

The

enve

lope

deter

m...

The

header

det

erm

in..

The

header

MUST

ag...

The

header

shou

ld a

...

3%

14%

31%

36%

17%

A. SMTP constructs the header from the envelope

B. The envelope determines who gets the mail

C. The header determines who gets the mail

D. The header MUST agree with the envelope on this

E. The header should agree with the envelope on this

B: envelope→sender

A. If anything, the sending agent (not SMTP) constructs the envelope from the header

B. CorrectC. FalseD. Not an RFC requirement: The header might

well not agreeE. It is impossible for C’s machine to check

whether C occurs on a mailing list administered by B, or even whether it is a mailing list

MIME is

A fe

ature

of e

-mai

l only

A fe

ature

of t

he W

eb ..

.

Wha

t Mar

cel M

arce

au...

A g

ener

al-p

urpose

e...

11%

86%

3%0%

1. A feature of e-mail only

2. A feature of the Web only

3. What Marcel Marceau did

4. A general-purpose encoding, first invented for mail

D — general purpose

• It did start out in the mail community, but there is nothing really mail-specific about it

• (Marcel Marceau did Mime: different capitalisation!)

NFS can be told to handle both binary and ASCII files

Tru

e

Fal

se

30%

70%

1. True

2. False

False

• NFS files are fundamentally binary, and the application has to deal with any differences (e.g. extra CR/LF in ASCII from Windows)

Now that we have NFS and the Web, FTP is obsolete

Tru

e

Fal

se

94%

6%

1. True

2. False

False

• It’s less common to type the ftp command, but we see (or at least use) ftp:// URLs.

• FTP is fundamentally different from NFS — FTP creates copies, whereas NFS allows multiple computers to access the same file.

UDP is used in many circumstances when TCP could be

used instead

Tru

e

Fal

se

28%

72%1. True

2. False

True

• NFS often uses UDP, where TCP would be as good, possibly better

• DNS uses UDP (which is simpler), and TCP could be used, though probably less efficient.

Therefore the internet could be run without UDP

Tru

e

Fal

se

62%

38%

1. True

2. False

False

• RIP and BOOTP/DHCP both rely on broadcast (or multicast for some RIP v2 installations) and therefore can’t use TCP

• Many uses of multicasting (internet radio etc.) would be impossible, which would cause bandwidth/load problems as well

It is impossible to build reliability on top of UDP

Tru

e

Fal

se

69%

31%

1. True

2. False

False

• TFTP manages to do it.

• TCP builds it on top of IP, and UDP is (by definition) no worse than IP, so clearly it’s possible (which doesn’t mean it’s desirable)

Web hosting firms need HTTP 1.0 (or later) because

It a

llow

s bin

ary

It a

llow

s M

IME

It a

llow

s th

e co

mple

..

It c

an s

end th

e le

ngth...

3%

29%

47%

21%

A. It allows binary

B. It allows MIME

C. It allows the complete URL in GET

D. It can send the length of the data file

C — complete URL

A. All HTTP allow binary (in the data)

B. MIME was in 0.9, and anyway isn’t very relevant

C. Without the complete URL, we wouldn’t know what page we were called on

D. Size is useful, but not necessary

Which of these is not part of the NFS complete suite

XDR

Portm

apper

TLS

Rem

ote p

roce

dure c

all

3%

22%

68%

8%

1. XDR

2. Portmapper

3. TLS

4. Remote procedure call

C — TLS

A. eXternal Data Representation is vital: times, lengths etc.

B. Portmapper to find RPC programs

C. TLS — Transport Layer Security. Not an intrinsic part.

D. Remote Procedure Call — the underlying paradigm

Recall the ISO 7-layer model

1. Physical

2. Link

3. Network

4. Transport

5. Session

6. Presentation

7. Application

Which layer is MIME?

Phys

ical

Lin

k

Net

wor

k

Tra

nspor

t

Ses

sion

Pre

senta

tion

Applic

atio

n

0% 0%3%

14%

56%

14%14%

1. Physical

2. Link

3. Network

4. Transport

5. Session

6. Presentation

7. Application

6 — Presentation

• Describes how the data is meant ot be interpreted at the other end.

Networking

• A few questions on the course

• A few questions about the course and the subject

There’s too much on the basics and not enough on the applications

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

5%

14%

24%27%

30%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I have bought/been given a copy of the book

Yes N

o

50%50%1. Yes

2. No