Lord%20Mandelson%20of%20Foy%20and%20Hartlepool%20is%20a%20descendant%20of%20which%20Labour%20leader?

1
Lord Mandelson of Foy and Hartlepool is a
descendant of which Labour leader?

1. Harold Wilson
2. George Brown
3. Gordon Brown
4. Herbert Morrison
5. Clement Attlee

2
Herbert Morrison

• Not entirely obvious, I admit.
• Morrison was Mandelsons mothers father, and
  Labour leader of London County Council.

3
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)

4
Please select a TeamIf youre not here, wait.

1. Team A
2. Team B
3. Team C
4. Team D
5. Team E

5
Please select a Team If youre not here, wait.

1. Team F
2. Team G
3. Team H
4. Team I
5. Team J

6
Please select a Team If youre not here, you
should have been done already

1. Team K
2. Team L
3. Team M
4. Team N

7
Now for your questions

• Im going to show the individual slide please
  answer as individuals.
• Then Ill ask you to start conferring. Part way
  through this, Ill show the individual answers.
• Then Ill put up the group slide, and ask groups
  to vote using the group clickers only
• A group should vote on its own question

8
I What would be the biggest impact of removing
the ACK field from TCP?

1. Without ACKs 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.

9
G What would be the biggest impact of removing
the ACK field from TCP?

1. Without ACKs 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.

10
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.

11
I When a user performs a DNS query, they often
get back more information than they asked for.
Why?

1. Its faster because the DNS server doesnt have
   to work out which specific DNS record type the
   user needs.
2. It figures youll 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. Its useful for debugging purposes, in case there
   are errors resolving the address.
5. Networking enthusiasts find this information
   interesting, and it doesnt add much additional
   traffic by including it.

12
G When a user performs a DNS query, they often
get back more information than they asked for.
Why?

1. Its faster because the DNS server doesnt have
   to work out which specific DNS record type the
   user needs.
2. It figures youll 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. Its useful for debugging purposes, in case there
   are errors resolving the address.
5. Networking enthusiasts find this information
   interesting, and it doesnt add much additional
   traffic by including it.

13
When a user performs a DNS query, they often get
back more information than they asked for. Why?

1. Its faster because the DNS server doesnt 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 youll 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. Its 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 doesnt add much additional
   traffic by including it.- It wont add much
   additional traffic, but it isnt specifically
   there just to please networking enthusiasts.

14
I Which of these criteria need to be considered
when deciding if an autonomous system is needed?

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.

15
G Which of these criteria need to be considered
when deciding if an autonomous system is needed?

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.

16
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 ASs can be created.
• 3 is incorrect. It isnt relevant.

17
I What is the most important algorithm for
preventing packet timeouts occurring in TCP?

1. Fast recovery
2. Jacobson RTO calculation
3. Karns algorithm
4. Fast retransmit

18
G What is the most important algorithm for
preventing packet timeouts occurring in TCP?

1. Fast recovery
2. Jacobson RTO calculation
3. Karns algorithm
4. Fast retransmit

19
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.
• Karns algorithm is relevant to packet timeouts
  but does not directly impact them.

20
I Which of the following situations is the
trivial file transfer protocol (TFTP) best suited
for?

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

21
G Which of the following situations is the
trivial file transfer protocol (TFTP) best suited
for?

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

22
Answer

• Best 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 once
• Despite using UDP TFTP does support parallel
  access to a file by using multiple ports.
• Possibly Systems with limited memory
• TFTP can fit in read only memory and is
  therefore useful where resources are limited.
• Possibly Transferring small files quickly on a
  secured network
• There 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 LAN
• TFTP 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.

23
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?

1. Yes, it's always true - its even written in an
   RFC.
2. Yes, although this could signal reordered rather
   than lost segments, the consequences are not
   serious
3. No, although it's unlikely the ACKs could signal
   something else, such as reordering of segments,
   unnecessary retransmissions could potentially
   flood the network.
4. No, it's a completely flawed assumption that has
   yet to be removed from implementations of TCP.

24
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?

1. Yes, it's always true - its even written in an
   RFC.
2. Yes, although this could signal reordered rather
   than lost segments, the consequences are not
   serious
3. No, although it's unlikely the ACKs could signal
   something else, such as reordering of segments,
   unnecessary retransmissions could potentially
   flood the network.
4. No, it's a completely flawed assumption that has
   yet to be removed from implementations of TCP.

25
And the most likely answer isB

• 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.

26
I Which is the more suitable option out of the
following

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 cant handle
   multicasting, then the loss is acceptable

27
G Which is the more suitable option out of the
following

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 cant handle
   multicasting, then the loss is acceptable

28
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.

29
I Historically Nagles algorithm was developed
to reduce network congestion for applications
like rlogin and telnet. With today's networks why
would someone wish to disable it?

1. It increases network latency
2. It conflicts with other TCP congestion avoidance
   techniques
3. It is not relevant anymore with todays high
   bandwidth connections
4. A newer algorithm has been developed that is now
   used

30
G Historically Nagles algorithm was developed
to reduce network congestion for applications
like rlogin and telnet. With today's networks why
would someone wish to disable it?

1. It increases network latency
2. It conflicts with other TCP congestion avoidance
   techniques
3. It is not relevant anymore with todays high
   bandwidth connections
4. A newer algorithm has been developed that is now
   used

31
Answer more than one correct

• 1) Yes it can increase latency but only when the
  network is lightly loaded. For heavily loaded
  networks Nagles 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
  Nagles 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 Nagles algorithm have been
  suggested, like for example Minshalls algorithm.

32
I Given the current technology, should Delayed
ACKs be used?

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.

33
G Given the current technology, should Delayed
ACKs be used?

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.

34
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.