UDP datagram structure in Computer Networks - Step-by-Step Execution

Concept Flow - UDP datagram structure

Start UDP Datagram

↓

Read Source Port (16 bits)

↓

Read Destination Port (16 bits)

↓

Read Length (16 bits)

↓

Read Checksum (16 bits)

↓

Read Data (Payload)

↓

End of Datagram

The UDP datagram is read step-by-step from the header fields to the data payload, starting with source port and ending with the data.

Execution Sample

Computer Networks

Source Port: 1234
Destination Port: 80
Length: 20
Checksum: 0x1a2b
Data: "Hello, UDP!!"

This shows a UDP datagram with ports, length, checksum, and a small data payload.

Analysis Table

Step	Field	Bits Read	Value	Description
1	Source Port	16	1234	Port number of sender
2	Destination Port	16	80	Port number of receiver
3	Length	16	20	Total length of UDP datagram in bytes
4	Checksum	16	0x1a2b	Error-checking value
5	Data	Variable	"Hello, UDP!!"	Payload data sent
6	End	0	-	Reached end of UDP datagram

💡 All header fields and data read; datagram fully processed

State Tracker

Field	Value
Source Port	1234
Destination Port	80
Length	20
Checksum	0x1a2b
Data	"Hello, UDP!!"

Key Insights - 3 Insights

Why is the Length field important in the UDP datagram?

The Length field tells how many bytes the entire UDP datagram has, including header and data. This is shown in step 3 of the execution_table.

What happens if the Checksum is incorrect?

If the Checksum is wrong, the datagram is considered corrupted and discarded. This is why the Checksum field (step 4) is critical for error detection.

Is the Data field fixed size?

No, the Data field size varies. The Length field minus header size (8 bytes) tells how big the Data is. This is implied in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of the Destination Port at step 2?

A 80

B 1234

C 20

D 0x1a2b

💡 Hint

Check the 'Value' column for step 2 in the execution_table.

At which step does the UDP datagram processing end according to the execution_table?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look for the row where 'Field' is 'End' in the execution_table.

If the Length field was 28 instead of 20, how would the Data field size change?

A Data size would be 28 bytes

B Data size would be 8 bytes

C Data size would be 20 bytes

D Data size would be 20 bytes plus header

💡 Hint

Remember UDP header is 8 bytes; Data size = Length - 8 (see key_moments about Length and Data).

Concept Snapshot

UDP datagram has 4 header fields each 16 bits: Source Port, Destination Port, Length, Checksum.
Length includes header + data size.
Data payload follows header and varies in size.
Checksum helps detect errors.
Total header size is always 8 bytes.

Full Transcript

A UDP datagram starts with a 16-bit Source Port, followed by a 16-bit Destination Port. Next is the 16-bit Length field indicating the total datagram size including header and data. Then comes the 16-bit Checksum for error checking. Finally, the variable-sized Data payload is read. The header is always 8 bytes. The Length field minus 8 gives the data size. The Checksum ensures data integrity. Processing ends after reading all these fields and the data.