CS计算机代考程序代写 algorithm Microsoft Word – quiz5key.docx

Microsoft Word – quiz5key.docx

Quiz 5 CMPT 471 SOLUTION

1. [1 point auto graded] Different Networks can have different MTU values. The MTU value
determines the maximum (and most common) size of an Ethernet payload within the
network.
TRUE
The MTU can be set by the network manager and may be different for different types of
hardware. So different networks can have different values of MTU.
The MTU is the maximum Ethernet frame payload size. The payload of the Ethernet frame
is the length of the IP datagram. Most IP datagrams will have a length equal to the
maximum MTU size. Some datagrams may be shorter, usually when a datagram is sent
partially full to deliver the end of a block of data from an application.

2. [1 point auto graded] A pseudo header is used to calculate the check sums for TCP and UDP.
The pseudo header includes the ethernet addresses of the sending host and the receiving
host
FALSE
When a checksum is calculated the pseudo header the TCP or UDP header and the data are
all included in the calculation, so the first sentence is true.
The pseudo header is calculated in the transport header and include the IP source and
destination addresses, the TCP or UDP header contain the source and destination port
addresses. No Ethernet addresses are included.

3. [1 point auto graded] Sliding windows flow control is usually more efficient than stop and
wait flow control
TRUE
The purpose of introducing the sliding window is the enable the sending of multiple packets
(more data) before an ACK is required. This means more data can be delivered before an
ACK must be received.

4. [1 point auto graded] In IPv6 router announcements can be used to inform all hosts on a
network of
1) The IP addresses of the routers
2) If the network is using stateful or stateless address allocation
3) The network prefix
TRUE

5. [3 points] Fill in the blanks in the following paragraph. Each blank may contain a single word
or a short phrase.

Which will be more efficient, transmitting a single long packet, or transmitting a series of
short packets containing the same amount of data. We will assume that the additional
overhead for the headers of the extra short packets is negligible. Since the probability that a
packet contains an error ______ 1 _______ as the length of the packet increases, a longer
packet will be ____2_____ likely to contain an error. A longer packet also takes longer to
_____3_____ . So we have two reasons why a series of shorter packets would be more
efficient. Therefore, to increase the efficiency of flow control it makes more sense to send
multiple short packets instead of increasing the length of the single packet. Instead of
requiring an ACK of each packet before sending the next packet we will require an ACK of a
_____4____ of ___5____packets before _____6_____ an ACK.

1 increases
2 more
3 transmit (or send)
4 block (or group)
5 N
6 requiring
½ point per word/phrase,
any word phrase conveying the same ideas may be accepted

6. [3 points] A number of different problems can result in IPv6 ICMP messages. Explain 3

different conditions that could cause a destination unreachable message to be
generated. Give a 1-3 sentence explanation of each condition.

Port unreachable: The message is generated when an IP datagram is received and the port
address for the datagram does not belong to any process running on the receiving host.

Address unreachable: Cannot reach the host from the final hop router that is directly
connected to the network the host should be part of.

No route to destination: Routing table of router does not have any entry that includes the
destination address

Also allowed
Administratively prohibited, source address failed, error in source routing
1 point per IPv6 ICMP message with explanation, ½ for stating correct
message ½ for explanation of what causes the message to be generated

7. [5 points] Consider the state diagram illustrating closing a TCP connection shown
below. You should consider the case of a client server connection between client A
and server B. For the two paths through this diagram that do not include a FIN with a
piggyback ACK explain what is happening as the connection is closing.

In your explanation of what happens while the connection is being closed include
A. A mention of all FIN or ACK IP datagrams sent or received by A
B. A mention of all FIN or ACK IP datagrams sent or received by B
C. A mention of all state transitions made A
D. A mention of all state transitions made by B

Pay special attention to the order in which the actions happen.

Path 1
1. Client sends a SYN and moves into FINWAIT
2. Server receives the SYN, sends an ACK of the FIN, and moves into CLOSEWAIT
3. Server completes remaining transfers, sends FIN and moves into LASTACK
4. Client receives the ACK of FIN and moves into FINWAIT2
5. Client receives FIN, sends ACK of FIN, and moves into TIMEWAIT
6. Server receives ACK of FIN, and moves into CLOSED
7. Timer for TIMEWAIT expires and client moves into CLOSED

Path2
8. Client sends a SYN and moves into FINWAIT

Server receives the SYN, sends an ACK of the FIN and moves into CLOSEWAIT
Server completes remaining transfers, sends FIN and moves into LASTACK

9. Client receives the FIN, sends ACK, and moves into CLOSING
10. Client receives ACK of the FIN and moves into TIMEWAIT

Server receives ACK of FIN, and moves into CLOSED
Timer for TIMEWAIT expires and client moves into CLOSED

1 point for contents of each numbered step,
UnNumbered steps are the same in both paths

8. Fill in the blanks in the following paragraph discussing MTU discovery. Each blank my contain
one word or one short phrase.

When IPv6 host (not a router) creates a IP datagram it will by default create one with a length
equal to the ____1_____ for the network. That datagram will be sent to the
____2____ router, A. Router A will send it to the next hop router, B. Router B will send it
the ___3___ router C, and so on. When a router tries to forward the packet into the next
network there is a possibility that the MTU of the next network, Y, is smaller that the MTU
of the network the packet arrived through, X.

In the case that the packet is too large to fit through the network Y. A ICMP packet too big
message is generated and sent to _____4______. The packet too big message will contain
the _____5____ of the network, Y. The ______6_____ flag is set on all IP datagrams sent by
the source in IPv6. This prevents the datagram from being _____7_____ at an intermediate
router. Instead the datagram will be _____8______.

1 MTU
2 Next hop
3 destination
4 The source
5 MTU
6 Don’t fragment
7 Fragmented
8 Fragmented at the source or resent in smaller datagrams that will pass

through the network with the smaller MTU
½ point per word/phrase,
any word phrase conveying the same ideas may be accepted

9. Explain how the traceroute algorithm works, step by step using the following
example. The order of your steps is important. Your step by step explanation should
indicate

A. what packets are sent and received
B. What messages if any the receipt of each packet produces and why those

messages are produced
C. What information about the route is being sent/received in each message when

applicable.

EXAMPLE:

The route taken is from host X through router A then routerB to host C.

 First 3 frames with the IP ttl set to 1 are sent by host X
 When these frames arrive at the first hop router A the ttl in the IP header of

each frames is decremented.
 Since this causes the ttl to becomes zero each frame is dropped
 Router B then sends an ICMP lifetime exceeded message to host X for each

frame that was dropped..
 Each lifetime exceeded message has, as the source address, the address of

router A, so the host X now has determined the first hop router A’s address
and can report it to the user.

 Next the host X sends three frames with a ttl of 2
 When these frames arrive at B the ttl is decremented to 1
 The frames are forwarded to host C
 When host C receives the three frames they are dropped
 The frames are dropped because host C discovers there is no process with

the destination port address in transport layer header of the frames.
 Host C sends an ICMP destination unreachable (port unreachable) message

back to the source host X.in response to each received frame.
 When host X receives the destination unreachable message. It knows the

source IP address is the destination address it sent to so it has received the
entire route of the frames to the destination. .

½ point for the idea contained in each bullet point