Consider a network that uses Ethernet and IPv4. Assume that IPv4 headers do not use any options field. Each Ethernet frame can carry a maximum of 1500 bytes in its data field. A UDP segment is transmitted. The payload (data) in the UDP segment is 7488 bytes. Which ONE of the following choices has the CORRECT total number of fragments transmitted and the size of the last fragment including IPv4 header?

The correct option is 6 fragments, 116 bytes.

Here is the step-by-step explanation of the solution:

1. Determine the Total Size of the UDP Segment:
A UDP segment consists of a UDP header and the UDP payload (data). The standard UDP header size is 8 bytes.
Given that the UDP payload is 7488 bytes, we can calculate the total size of the UDP segment (which acts as the IP payload before fragmentation):

$\text{Total UDP Segment Size}=\text{UDP Payload}+\text{UDP Header}$

$\text{Total UDP Segment Size}=7488\text{bytes}+8\text{bytes}=7496\text{bytes}$

2. Determine the Maximum IP Payload per Fragment:
The Maximum Transmission Unit (MTU) of Ethernet is 1500 bytes, which is the maximum size of the data field in an Ethernet frame. This means each IP packet (fragment) can be at most 1500 bytes in size, including its IPv4 header.
Since the IPv4 header has no options, its size is fixed at 20 bytes.
Therefore, the maximum amount of IP payload (UDP data) that each fragment can carry is:

$\text{Max IP Payload per Fragment}=\text{MTU}-\text{IPv4 Header Size}$

$\text{Max IP Payload per Fragment}=1500\text{bytes}-20\text{bytes}=1480\text{bytes}$

3. Check for Fragment Offset Alignment:
In IPv4, the Fragment Offset field in the IP header is measured in units of 8-byte blocks. Consequently, the payload size of all intermediate fragments must be a multiple of 8 bytes.
We check if 1480 is a multiple of 8:

$\frac{1480}{8}=185$

Since 1480 is divisible by 8, each of the intermediate fragments can carry the maximum possible payload of exactly 1480 bytes.

4. Calculate the Number of Fragments and Last Fragment Size:
We divide the total UDP segment size by the payload capacity per fragment to find the number of fragments:

$\text{Number of intermediate fragments}=\lfloor \frac{7496}{1480}\rfloor =5$

The total payload carried by these 5 fragments is:

$5\times 1480\text{bytes}=7400\text{bytes}$

The remaining payload for the last (6th) fragment is:

$7496\text{bytes}-7400\text{bytes}=96\text{bytes}$

Therefore, a total of 6 fragments are transmitted.

5. Calculate the Total Size of the Last Fragment:
The last fragment contains the remaining 96 bytes of IP payload and the 20-byte IPv4 header:

$\text{Last Fragment Size}=\text{Remaining Payload}+\text{IPv4 Header Size}$

$\text{Last Fragment Size}=96\text{bytes}+20\text{bytes}=116\text{bytes}$