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# System prepended metadata

title: 網概 hw3

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# 網概 hw3

## Q1 : Select the first UDP segment in your trace. Look at the details of this packet in Wireshark. How many fields there are in the UDP header?

![image](https://hackmd.io/_uploads/H1WxO_9-A.png)
There are four fields in the header:
1. Source Port
2. Destination Port
3. Length
4. Checksum

## Question 2: By consulting the displayed information in Wireshark’s packet content field for this packet, what is the length (in bytes) of each of the UDP header fields? 
![image](https://hackmd.io/_uploads/rkHuOO5ZC.png)

There are 8 bytes / 4 fields = 2 bytes per field

## Question 3: The value in the Length field is the length of what? Verify your claim with your captured UDP packet.
![image](https://hackmd.io/_uploads/HJOycd9W0.png)
The length 44 is equal to header length 8 + data length 36 = 44

## Question 4: What is the maximum number of bytes that can be included in a UDP payload? 
The maximum number of bytes of a package is 65535 bytes, therefore the maximum length of payload can carry is 65535 – 8(udp header) - 20(ip header) = 65507 bytes

## Question 5: What is the largest possible source port number? 
the largest possible source port number is $2^{16}-1=65535$

## What is the protocol number for UDP? Give your answer in decimal notation. To answer this question, you’ll need to look into the Protocol field of the IP datagram containing this UDP segment. 
![image](https://hackmd.io/_uploads/BJ-bROqZC.png)

the protocol number for UDP is 17