Knowledge Checks

Does UDP preserve application-layer message boundaries?

True or False: On the sending side, the UDP sender will take each application-layer chunk of data written into a UDP socket and send it in a distinct UDP datagram. And then on the receiving side, UDP will deliver a segment’s payload into the appropriate socket, preserving the application-defined message boundary.

True

False

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1/10

UDP header fields.

Which of the fields below are in a UDP segment header? [Hint: note the use of the word "header" in this question statement.]

Source port number

Internet checksum

Destination port number

Data (payload)

Sequence number

Upper layer protocol

Source IP address

Length (of UDP header plus payload)

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2/10

UDP segment length field.

Why is the UDP header length field needed?

Because this field is needed in TCP as well.

Because the payload section can be of variable length, and this lets UDP know where the segment ends.

To make the header and even number of bytes

(a) and (b) above

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3/10

Internet checksum and UDP.

Over what set of bytes is the checksum field in the UDP header computed over?

The entire UDP segment, except the checksum field itself, and the IP sender and receive address fields

Just the UDP header but not the payload.

The entire UDP segment, except the checksum field itself.

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4/10

What is a checksum?

Which of the following statements are true about a checksum? Hint: more than one statement is true.

The receiver of a packet with a checksum will add up the received bytes, just as the sender did, and compare this locally-computed checksum with the checksum value in the packet header. If these two values are the same then the receiver knowsthat all of the bits in the received packet are correct, i.e., that no bits have been changed during transmission from sender to receiver.

The sender-computed checksum value is often included in a checksum field within a packet header.

A checksum is computed at a sender by considering each byte within a packet as a number, and then adding these numbers (each number representing a bytes) together to compute a sum (which is known as a checksum).

The receiver of a packet with a checksum field will add up the received bytes, just as the sender did, and compare this locally-computed checksum with the checksum value in the packet header. If these two values are different then the receiverknows that one of the bits in the received packet has been changed during transmission from sender to receiver.

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5/10

Computing the Internet Checksum (1).

Compute the Internet checksum value for these two 16-bit words: 11110101 11010011 and 10110011 01000100

[Note: you can find more problems like this one here.]

01010110 11101000

01010110 11100111

01011110 11000101

01101110 11010101

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6/10

Computing the Internet Checksum (2).

Compute the Internet checksum value for these two 16-bit words: 01000001 11000100 and 00100000 00101011

[Note: you can find more problems like this one here.]

10011110 00001111

10011110 00010000

01101110 11010101

10011110 00010001

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7/10

UDP Checksum: how good is it?

True or False: When computing the Internet checksum for two numbers, a single flipped bit (i.e., in just one of the two numbers) will always result in a changed checksum.

False

True

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8/10

UDP Checksum: how good is it?

True or False: When computing the Internet checksum for two numbers, a single flipped bit in each of the two numbers will always result in a changed checksum.

False

True

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9/10

IP addresses and port numbers in a UDP segment sent in reply.

Suppose a UDP segment (A in the figure below) arrives at a host with an IP address of 128.119.40.186. The source port in the UDP segment is 4829 and the destination port is 3546. The IP address of the sending host is 60.54.75.24.

Now consider the UDP datagram (and the IP datagram that will encapsulate it) sent in reply by the application on host 128.119.40.186 to the original sender host, labeled B in the figure above. Complete the sentences below ...

What are the source and destination port numbers and IP addresses? (Enter the integer port number or the 4-part dotted decimal IP address, included the period)

The source port number of the UDP segment (B) sent in reply is:

The source IP address of the IP datagram containing the UDP segment (B) sent in reply is:

The destination port number of the UDP segment (B) sent in reply is:

The destination IP address of the IP datagram containing the UDP segment (B) sent in reply is:

[Note: you can find more problems like this one here.]

Question List:

The source port number of the UDP segment (B) sent in reply is:

The source IP address of the IP datagram containing the UDP segment (B) sent in reply is:

The destination port number of the UDP segment (B) sent in reply is:

The destination IP address of the IP datagram containing the UDP segment (B) sent in reply is:

Answer List:

24
128.119.40.186
60.54.75.24
4829
80
3546
10.0.0.1

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10/10

Knowledge checks

Does UDP preserve application-layer message boundaries?

UDP header fields.

UDP segment length field.

Internet checksum and UDP.

What is a checksum?

Computing the Internet Checksum (1).

Computing the Internet Checksum (2).

UDP Checksum: how good is it?

UDP Checksum: how good is it?

IP addresses and port numbers in a UDP segment sent in reply.

Question List:

Answer List: