Lord Mandelson of Foy and Hartlepool is a descendant of which

Labour leader?

Har

old W

ilson

Geo

rge

Brow

n

Gord

on B

row

n

Her

bert M

orris.

..

Cle

men

t Attl

ee

30%

11%

24%

16%19%

1. Harold Wilson

2. George Brown

3. Gordon Brown

4. Herbert Morrison

5. Clement Attlee

Herbert Morrison

• Not entirely obvious, I admit.

• Morrison was Mandelson’s mother’s father, and Labour leader of London County Council.

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

25%

19%

13%

25%

19%

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.

1 2 3 4 5

14%

21%

29%

14%

21%

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

23%

38%

15%

23%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: What would be the biggest impact of removing the ACK field from

TCP?

1 2 3 4

34%

5%2%

59%1. Without ACK’s reliability benefits, TCP would become obsolete.

2. Reliability would always need to be implemented at application level.

3. Fewer small packets would result in a faster Internet.

4. Lack of “self clocking” would result in a slower Internet.

G: What would be the biggest impact of removing the ACK field

from TCP?

1 2 3 4

7%0%0%

93%1. Without ACK’s reliability

benefits, TCP would become obsolete.

2. Reliability would always need to be implemented at application level.

3. Fewer small packets would result in a faster Internet.

4. Lack of “self clocking” would result in a slower Internet.

What would be the biggest impact of removing the ACK field from

TCP?2. Reliability would always need to be implemented at

application level.

• By losing the option to use TCP for reliability, the feature would need to be implemented at application level (as in UDP).

• “Guaranteed Delivery” would become less common as it would be down to the developer to implement it.

• The quality of alternative reliability systems would vary significantly.

• Lack of protocol would make inter-application communication difficult.

I: When a user performs a DNS query, they often get back more information than they asked for. Why?

1 2 3 4 5

15%

41%

0%

24%

20%

1. It’s faster because the DNS server doesn’t have to work out which specific DNS record type the user needs.

2. It figures you’ll ask subsequent questions based on the same information so it gives you the answers in advance.

3. It reduces the likelihood of DNS spoofing, because we can check that the additional information backs up the information we did want.

4. It’s useful for debugging purposes, in case there are errors resolving the address.

5. Networking enthusiasts find this information interesting, and it doesn’t add much additional traffic by including it.

G: When a user performs a DNS query, they often get back more information than they asked for. Why?

1 2 3 4 5

0%

88%

0%0%

13%

1. It’s faster because the DNS server doesn’t have to work out which specific DNS record type the user needs.

2. It figures you’ll ask subsequent questions based on the same information so it gives you the answers in advance.

3. It reduces the likelihood of DNS spoofing, because we can check that the additional information backs up the information we did want.

4. It’s useful for debugging purposes, in case there are errors resolving the address.

5. Networking enthusiasts find this information interesting, and it doesn’t add much additional traffic by including it.

When a user performs a DNS query, they often get back more information than they asked for. Why?

1. It’s faster because the DNS server doesn’t have to work out which specific DNS record type the user needs.- The user has already told the DNS server what record it needs.

2. It figures you’ll ask subsequent questions based on the same information so it gives you the answers in advance.

3. It reduces the likelihood of DNS spoofing, because we can check that the additional information backs up the information we did want.- The whole DNS record could still be spoofed.

4. It’s useful for debugging purposes, in case there are errors resolving the address.- Debugging is not its primary purpose.

5. Networking enthusiasts find this information interesting, and it doesn’t add much additional traffic by including it.- It won’t add much additional traffic, but it isn’t specifically there just to please networking enthusiasts.

I: Which of these criteria need to be considered when deciding if an autonomous

system is needed?

1 2 3 4

33%

23%

15%

30%

1. You manage a collection of networks

2. You have a different routing policy to your border gateway peers.

3. You need to send ICMP redirects.

4. You need to exchange external routing information.

G: Which of these criteria need to be considered when deciding if an autonomous

system is needed?

1 2 3 4

40%

27%

0%

33%

1. You manage a collection of networks

2. You have a different routing policy to your border gateway peers.

3. You need to send ICMP redirects.

4. You need to exchange external routing information.

Answer Slide

• The most appropriate criteria is 2. This is because an AS is only required in this circumstance.

• 1 & 4 can also be considered. There are scenarios when AS’s can be created.

• 3 is incorrect. It isn’t relevant.

I: What is the most important algorithm for preventing packet timeouts occurring in TCP?

1 2 3 4

16%

35%

30%

19%

1. Fast recovery

2. Jacobson RTO calculation

3. Karn’s algorithm

4. Fast retransmit

G: What is the most important algorithm for preventing packet timeouts occurring in TCP?

1 2 3 4

7%

33%

47%

13%

1. Fast recovery

2. Jacobson RTO calculation

3. Karn’s algorithm

4. Fast retransmit

Answer

• Fast recovery and fast retransmit work together to ensure lost packets are dealt with more quickly than allowing the default timeout to occur.

• Jacobson RTO calculation is used to calculate the default timeout but this does not directly prevent them occuring.

• Karn’s algorithm is relevant to packet timeouts but does not directly impact them.

I: Which of the following situations is the trivial file transfer protocol (TFTP) best

suited for?

1 2 3 4 5

35%

9%7%

19%

30%

1. Transferring small files quickly on a secured network

2. Transferring files securely over a LAN

3. Bootstrapping a networked diskless system

4. Transferring a file to multiple clients at once

5. Use on systems with limited memory

G: Which of the following situations is the trivial file transfer protocol (TFTP)

best suited for?

1 2 3 4 5

44%

6%

13%

0%

38%

1. Transferring small files quickly on a secured network

2. Transferring files securely over a LAN

3. Bootstrapping a networked diskless system

4. Transferring a file to multiple clients at once

5. Use on systems with limited memory

AnswerBest: Bootstrapping a networked diskless system

The main use of TFTP is for booting multiple networked devices such as thin clients.

Possibly: Transferring a file to multiple clients at onceDespite using UDP TFTP does support parallel access to a file by using multiple ports.

Possibly: Systems with limited memoryTFTP can fit in read only memory and is therefore useful where resources are limited.

Possibly: Transferring small files quickly on a secured networkThere is no real speed increase using TFTP other than the fact it is lightweight, and the timeouts are lousy.

Incorrect: Transferring files securely over a LANTFTP has no security features and therefore should only be used over a network where the security is carefully controlled, or via another secure method such as a VPN.

I: The triggering of fast retransmission of data is based on a  key assumption: "if three or more duplicate ACKs are received in  a row, it is a strong indication that a segment has been lost” (Stevens, TCP/IP Illustrated, Volume 1: The Protocols, 2003).

Is this assumption safe?

A. B. C. D.

7% 7%

40%45%A. Yes, it's always true - its even

written in an RFC.

B. Yes, although this could signal reordered rather than lost segments, the consequences are not serious

C. No, although it's unlikely the ACKs could signal something else, such as reordering of segments, unnecessary retransmissions could potentially flood the network.

D. No, it's a completely flawed assumption that has yet to be removed from implementations of TCP.

G: The triggering of fast retransmission of data is based on a  key assumption: "if three or more duplicate ACKs are received in  a row, it is a strong indication that a segment has been lost” (Stevens, TCP/IP Illustrated, Volume 1: The Protocols, 2003).

Is this assumption safe?

A. B. C. D.

7%0%

21%

71%A. Yes, it's always true - its even written in an RFC.

B. Yes, although this could signal reordered rather than lost segments, the consequences are not serious

C. No, although it's unlikely the ACKs could signal something else, such as reordering of segments, unnecessary retransmissions could potentially flood the network.

D. No, it's a completely flawed assumption that has yet to be removed from implementations of TCP.

And the most likely answer is…B

• A duplicate ACK may be a sign of segments getting reordered or a packet being lost. In the case of reordering however, it is unlikely that more than two would be received.

• Even if a packet was mistakenly retransmitted it would not be processed more than once - the receiver would send out a duplicate ACK thereafter.

I: Which is the more suitable option out of the following:

1 2 3 4

51%

12%

27%

10%

1. All Multicasting should be handled at the Network layer

2. All Multicasting should be handled at the Application layer

3. Either is fine and both layers should be able to handle multicasting

4. Either is fine and if it can’t handle multicasting, then the loss is acceptable

G: Which is the more suitable option out of the following:

1 2 3 4

71%

6%12%12%

1. All Multicasting should be handled at the Network layer

2. All Multicasting should be handled at the Application layer

3. Either is fine and both layers should be able to handle multicasting

4. Either is fine and if it can’t handle multicasting, then the loss is acceptable

No right answer!

• Any one of them can be justified.• Currently performed in Network layer, but

does not have to be.• Just as justifiable to use Application Layer,

which is used for Mobile Devices.• Can argue that separation is important, but

can also argue that depends on context.• Forcing all network layer components to

handle it would be expensive though.

I: Historically Nagle’s algorithm was developed to reduce network congestion for applications like

rlogin and telnet. With today's networks why would someone wish to disable it?

It in

crea

ses

n...

It c

onfli

cts

w...

It is

not r

ele.

..

A n

ewer

alg

ori...

21%

10%

26%

43%1. It increases network latency

2. It conflicts with other TCP congestion avoidance techniques

3. It is not relevant anymore with today’s high bandwidth connections

4. A newer algorithm has been developed that is now used

G: Historically Nagle’s algorithm was developed to reduce network congestion for applications like

rlogin and telnet. With today's networks why would someone wish to disable it?

It in

crea

ses

n...

It c

onfli

cts

w...

It is

not r

ele.

..

A n

ewer

alg

ori...

7%13%

7%

73%1. It increases network latency

2. It conflicts with other TCP congestion avoidance techniques

3. It is not relevant anymore with today’s high bandwidth connections

4. A newer algorithm has been developed that is now used

Answer – more than one correct• 1) Yes it can increase latency but only when the network is lightly loaded.

For heavily loaded networks Nagle’s algorithm is relevant.

• 2) Yes, Nagle’ algorithm can cause extended waiting when used in combination with the delayed ACK algorithm

• 3) No, bandwidth does not play a part. It depends on the load of the network (see point 1) and the type of data being sent. Small packets which require a fast response time (interactive such as mouse movements in x-windows) are unsuitable for Nagle’s algorithm. An unloaded network can often lead to many delayed ACKs not being sent until the timer fires, again useless for highly interactive applications.

• 4) No, no newer algorithm has been widely accepted. However, newer propositions and enhancements to Nagle’s algorithm have been suggested, like for example Minshall’s algorithm.

I: Given the current technology, should Delayed ACKs be used?

1 2 3 4 5

10%

57%

19%

7%7%

1. No, because advances in technology have rendered them obsolete.

2. Yes, because they reduce the number of segments sent.

3. Yes, because it says so in RFC2581.

4. Yes, but only on WANs.

5. No, because they can disturb round-trip timing and packet ‘clocking’ algorithms.

G: Given the current technology, should Delayed ACKs be used?

1 2 3 4 5

0%

100%

0%0%0%

1. No, because advances in technology have rendered them obsolete.

2. Yes, because they reduce the number of segments sent.

3. Yes, because it says so in RFC2581.

4. Yes, but only on WANs.

5. No, because they can disturb round-trip timing and packet ‘clocking’ algorithms.

Given the current technology, should Delayed ACKs be used?

1. Wrong. WANs can still suffer congestion.

2. Delayed ACKs can reduce the number of segments sent over a network.

3. Wrong. RFC2581 only states that an acknowledgment must be sent within 500ms, thus making a delay possible, not enforcing it.

4. Delayed ACKs can reduce congestion on WANs by reducing the number of segments sent.

5. Wrong. Only seriously delayed ACKs will cause trouble like this. RFC2581 says that ACKs should be sent within 500ms so this should not be a problem.