President Obama was born in which U.S. state

Illin

ois

Tex

as

New

York

Haw

aii

Tric

k Q

uest

ion.

..

Ken

ya

30%

9%

13%13%

35%

0%

1. Illinois

2. Texas

3. New York

4. Hawaii

5. Trick Question: Washington DC

6. Kenya

Hawaii (4) is right

• He was Senator for Illinois

• George W Bush was Texas

• His father was from Kenya, but had he been born there he would probably not have been eligible (article 2.1.5 of U.S. constitution)

• DC is indeed not a state, but counts as U.S. for the purpose

That was the warmup

• Now to check clickers against groups

• IF your group name is shown, please press the associated number on each individual clicker AND the group clicker

• ELSE wait for next slide(s)

Please select a Team:If you’re not here, wait.

Tea

m A

Tea

m B

Tea

m C

Tea

m D

Tea

m E

0% 0%

22%

33%

44%1. Team A

2. Team B

3. Team C

4. Team D

5. Team E

Please select a Team: If you’re not here, wait.

Tea

m F

Tea

m G

Tea

m H

Tea

m I

Tea

m J

0%

33%

0%

50%

17%

1. Team F

2. Team G

3. Team H

4. Team I

5. Team J

Please select a Team If you’re not here, you should have

been done already

1 2 3 4

40%

0%0%

60%

1. Team K

2. Team L

3. Team M

4. Team N

Now for your questions

• I’m going to show the individual slide — please answer as individuals.

• Then I’ll ask you to start conferring. Part way through this, I’ll show the individual answers.

• Then I’ll put up the group slide, and ask groups to vote using the group clickers only

• A group should vote on its own question

I: Which of the following scenarios would benefit most from disabling TCP slow start?

1 2 3 4 5

4%

50%

8%8%

29%

1. Many TCP connections between two nodes on the same physical LAN with one node connected via a physical link consisting of a wireless ethernet card.

2. Many TCP connections between two nodes connected over a WAN. Again one node is connected via a physical link consisting of a wireless ethernet card.

3. Many TCP connections between two nodes on the same physical LAN

4. Using VOIP over a LAN.

5. When transferring a large file over the internet.

G :Which of the following scenarios would benefit most from disabling TCP slow start?

1 2 3 4 5

0%

75%

0%0%

25%

1. Many TCP connections between two nodes on the same physical LAN with one node connected via a physical link consisting of a wireless ethernet card.

2. Many TCP connections between two nodes connected over a WAN. Again one node is connected via a physical link consisting of a wireless ethernet card.

3. Many TCP connections between two nodes on the same physical LAN

4. Using VOIP over a LAN.

5. When transferring a large file over the internet.

Justification of answers

We think *answer 1* is the scenario that would benefit *most *from having TCP slow start *disabled*. Therefore answer 1 is the "correct" answer.For 1,2,3 - The fact that 'many' TCP connections are made makes little difference. Any overhead (CPU/memory) required for many slow start algorithms running simultaneously is negligible.

Slow start is about protecting networks

I: With regard to Silly Window Syndrome (SWS), Nagle’s

Algorithm:

Solve

s the pro

...

Only so

lves ce...

Solve

s SW

S whe...

Is appro

priate

...

5%

25%

40%

30%

1. Solves the problem of SWS.

2. Only solves certain cases of SWS.

3. Solves SWS when a receiving TCP is serving an application that consumes data too slowly.

4. Is appropriate when Clark’s solution or Delayed Ack won’t work

G: With regard to Silly Window Syndrome (SWS), Nagle’s

Algorithm:

Solve

s the pro

...

Only so

lves ce...

Solve

s SW

S whe...

Is appro

priate

...

0%

43%

29%29%

1. Solves the problem of SWS.

2. Only solves certain cases of SWS.

3. Solves SWS when a receiving TCP is serving an application that consumes data too slowly.

4. Is appropriate when Clark’s solution or Delayed A won’t work.

Answers• The answer : 2!

1. False: It only solves half the problem; it solves it on the sender side, by preventing the sender from sending tinygrams, but it doesn’t stop the receiver from advertising tiny windows.

2. True: It doesn’t solve the problem of the application on the receiving side reading data too slowly.

3. False: Clark’s solution and delayed acknowledgements deal with this side of the problem.

4. True, but a worse answer than 2: Clark’s solution and Delayed Acknowledgements are actually solving the other half of the problem, and both techniques are required to prevent SWS from occuring.

I: Which of these is the most important factor in the reliability of a secondary DNS server?

Bei

ng loca

ted ..

.

The

primar

y se

...

Runni

ng o

n the.

..

Bei

ng conne

cte.

..

8%

71%

4%

17%

1. Being located in a different country to the primary

2. The primary server is polled for changes frequently

3. Running on the same hardware as the primary machine

4. Being connected to the net via different routes to the primary and other secondary servers

G: Which of these is the most important factor in the reliability of a secondary DNS server?

Bei

ng loca

ted ..

.

The

primar

y se

...

Runni

ng o

n the.

..

Bei

ng conne

cte.

..

13%

88%

0%0%

1. Being located in a different country to the primary

2. The primary server is polled for changes frequently

3. Running on the same hardware as the primary machine

4. Being connected to the net via different routes to the primary and other secondary servers

Answer

1. Correct; RFC 2182 describes “best current practice” as having at least one secondary server on a different international backbone to the primary server.

2. Correct; Polling frequently (normally around every 3 hours) ensures data is accurate and up to date.

3. Wrong4. Correct; This is (arguably) the best answer, and

can be implied by 1, but may not be. Two countries can use the same pipe to the rest of the world, having a primary in one and a secondary in the other wouldn’t be ideal.

Protocol-Independent Multicast:How to do multicast routing

• PIM is actually a variety of protocols, • PIM Sparse Mode (PIM-SM) explicitly builds unidirectional shared trees

rooted at a rendezvous point (RP) per group, and optionally creates shortest-path trees per source. PIM-SM generally scales fairly well for wide-area usage. See the PIM Internet Standard RFC 4601

• PIM Dense Mode (PIM-DM) uses dense multicast routing. It implicitly builds shortest-path trees by flooding multicast traffic domain wide, and then pruning back branches of the tree where no receivers are present. PIM-DM generally has poor scaling properties.[1]

• Bidirectional PIM explicitly builds shared bi-directional trees. It never builds a shortest path tree, so may have longer end-to-end delays than PIM-SM, but scales well because it needs no source-specific state. See Bidirectional PIM Internet Standard RFC 5015

• PIM source-specific multicast (PIM-SSM) builds trees that are rooted in just one source, offering a more secure and scalable model for a limited amount of applications (mostly broadcasting of content).

I: In what way is Internet Group Management Protocol inefficient?

IGM

P has

no g

u...

IGM

P is n

ot a ..

.

Multi

cast

use

...

It is

in fa

ct ..

.

17% 17%

0%

65%1. IGMP has no guarantee

that a report is delivered

2. IGMP is not a transport protocol, but instead instantiates PIM connections

3. Multicast use in WANs causes lots of problems

4. It is in fact efficient

G: In what way is Internet Group Management Protocol inefficient?

IGM

P has

no g

u...

IGM

P is n

ot a ..

.

Multi

cast

use

...

It is

in fa

ct ..

.

13% 13%

0%

75%1. IGMP has no guarantee

that a report is delivered

2. IGMP is not a transport protocol, but instead instantiates PIM connections

3. Multicast use in WANs causes lots of problems

4. It is in fact efficient

Answer Slide

All answers are true to an extent, but (3) is “best”.1) Although can be a problem, is generally solved due to

answer 4).2) It creates a group before it implements Protocol

Independent Multicast. It would be more efficient to use this all the time.

3) Not fully solved so problems may arise when used.4) Has efficiency methods For 1) IGMP sends

membership query messages repeatedly until an acknowledgment is received. Also has scheduling so not to send the same report twice.

DNS server records (SRV)

• MX records are specific to SMTP, and assume you know to use TCP port 25

• You (or your browser) have to guess to stick www on the front of a web address, and use TCP port 80

• Instead, why not look up a generic service record, e.g. _smtp._tcp.bath.ac.uk

I: MX Records serve a similar purpose for SMTP as SRV Records serve for other protocols.

Which of the following statements do you most agree with?

It is

necessa

r...

MX Reco

rds s

ho...

MX Reco

rds s

ho...

SMTP its

elf is..

.

17% 17%

50%

17%

1. It is necessary to have both MX and SRV Records

2. MX Records should be phased out and replaced by the more generic SRV Records

3. MX Records should remain because it would be too difficult to replace as their use in email is so entrenched them

4. SMTP itself is a flawed protocol and should be replaced altogether

G: MX Records serve a similar purpose for SMTP as SRV Records serve for other protocols.

Which of the following statements do you most agree with?

It is

necessa

r...

MX Reco

rds s

ho...

MX Reco

rds s

ho...

SMTP its

elf is..

.

0%

14%

86%

0%

1. It is necessary to have both MX and SRV Records

2. MX Records should be phased out and replaced by the more generic SRV Records

3. MX Records should remain because it would be too difficult to replace them their use in email is so entrenched

4. SMTP itself is a flawed protocol and should be replaced altogether

Answers

• 1. False. The data stored in SRV Records is a superset of the data in MX Records*, thus it is possible for SRV Records to serve the same purpose.

2. Arguably True. For the same reasons as 3, it’s debatable whether the impact on old software would cause a major problem. Otherwise, as mentioned, SMTP would be trivial to change.

3. Arguably True. It would be trivial to change the SMTP protocol to use SRV Records, but old software would still be looking for MX Records.

• 4. Debatable. This answer is meant to be thought provoking. Why is SMTP one of the few protocols that requires its own record?

* SRV has Priority, Weight, Port and Target. MX has Priority and Target

Networking

• A few questions on the course

• A few questions about the course, and this teaching methodology

I: “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

26%

57%

4%4%9%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I: Although good for general feed-back 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

26%

39%

4%

13%17%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I: Compared with individually answering, group answering is

1 2 3 4 5

35%

30%

9%

4%

22%

1. Much more educational

2. Somewhat more educational

3. Neutral

4. Rather less educational

5. Much less educational

I: My involvement in designing a question has improved my

understanding of networking

Stro

ngly A

gree

Agre

e

Neu

tral

Dis

agre

e

Stro

ngly D

isag

ree

45%

23%18%

0%

14%

1. Strongly Agree

2. Agree

3. Neutral

4. Disagree

5. Strongly Disagree

I: Working in groups to design these questions was

Ver

y be

nefic

ia...

Ben

efic

ial

Neu

tral

A s

ad n

eces

sit..

.

Dis

astro

us

17%

25%

13%

29%

17%

1. Very beneficial

2. Beneficial

3. Neutral

4. A sad necessity

5. Disastrous

I: Working in groups to answer other groups’ questions was

Ver

y be

nefic

ia...

Ben

efic

ial

Neu

tral

A s

ad n

eces

sit..

.

Dis

astro

us

17%

43%

4%

17%17%

1. Very beneficial

2. Beneficial

3. Neutral

4. A sad necessity

5. Disastrous