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.

Practice

1. Which of the following is NOT a field in the UDP datagram header?

easy

A. Sequence Number

B. Destination Port

C. Source Port

D. Checksum

Solution

1. Step 1: Recall UDP header fields

   The UDP header contains Source Port, Destination Port, Length, and Checksum fields.

2. Step 2: Identify the incorrect field

   Sequence Number is a field in TCP, not UDP.

3. Final Answer:

   Sequence Number -> Option A

4. Quick Check:

   UDP header fields exclude Sequence Number [OK]

Hint: Remember UDP header has no sequence number, unlike TCP [OK]

Common Mistakes:

• Confusing UDP with TCP header fields
• Thinking UDP has sequence numbers
• Mixing checksum with sequence number

2. What is the total size in bytes of the UDP header?

easy

A. 8 bytes

B. 4 bytes

C. 12 bytes

D. 16 bytes

Solution

1. Step 1: Recall UDP header size

   The UDP header consists of four fields, each 2 bytes long: Source Port, Destination Port, Length, and Checksum.

2. Step 2: Calculate total size

   4 fields x 2 bytes each = 8 bytes total.

3. Final Answer:

   8 bytes -> Option A

4. Quick Check:

   4 fields x 2 bytes = 8 bytes [OK]

Hint: UDP header always 8 bytes, fixed size [OK]

Common Mistakes:

• Confusing with IP or TCP header size
• Assuming variable header size
• Counting data length as header size

3. Given a UDP datagram with Source Port = 1234, Destination Port = 80, Length = 20, and Checksum = 0x1A2B, which field indicates the size of the entire UDP datagram including header and data?

medium

A. Source Port

B. Checksum

C. Destination Port

D. Length

Solution

1. Step 1: Understand UDP Length field meaning

   The Length field specifies the total length of the UDP datagram, including header and data, in bytes.

2. Step 2: Identify the field indicating size

   Among the given fields, only Length represents the size of the entire UDP datagram.

3. Final Answer:

   Length -> Option D

4. Quick Check:

   Length field = total UDP datagram size [OK]

Hint: Length field shows total UDP datagram size [OK]

Common Mistakes:

• Confusing port numbers with size
• Thinking checksum indicates size
• Ignoring header size in length

4. A programmer wrote code to parse a UDP datagram but forgot to read the Checksum field. What is the likely consequence?

medium

A. The datagram will be rejected by the receiver

B. The length field will be misinterpreted

C. The datagram may be accepted without error checking

D. The source port will be incorrect

Solution

1. Step 1: Understand role of Checksum in UDP

   The Checksum is used for error checking but is optional in IPv4; ignoring it means no error verification.

2. Step 2: Consequence of ignoring Checksum

   If the Checksum is not read or verified, the datagram may still be accepted but without error checking, risking corrupted data.

3. Final Answer:

   The datagram may be accepted without error checking -> Option C

4. Quick Check:

   Ignoring checksum skips error check but datagram accepted [OK]

Hint: Ignoring checksum skips error check, datagram still accepted [OK]

Common Mistakes:

• Assuming datagram is rejected
• Confusing checksum with port fields
• Thinking length is affected

5. You need to design a simple application that sends small messages quickly without guaranteed delivery. Which UDP datagram feature makes it suitable for this?

hard

A. Automatic retransmission of lost packets

B. Fixed 8-byte header with no connection setup

C. Sequence numbers for ordered delivery

D. Built-in encryption for security

Solution

1. Step 1: Identify UDP features for fast, simple messaging

   UDP uses a fixed 8-byte header and does not require connection setup, enabling quick transmission.

2. Step 2: Eliminate incorrect features

   UDP does not provide sequence numbers, retransmission, or encryption by default.

3. Final Answer:

   Fixed 8-byte header with no connection setup -> Option B

4. Quick Check:

   UDP's simple header and no handshake enable fast messaging [OK]

Hint: UDP's simple header and no handshake enable fast messaging [OK]

Common Mistakes:

• Confusing UDP with TCP features
• Expecting retransmission or ordering
• Assuming built-in security