TCP Header and UDP Checksum Verification

Part A — TCP header

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data Offset | Reserved  |C|E|U|A|P|R|S|F|      Window         |
|     (4)      |    (4)    |W|C|R|C|S|Y|I|N|N|    (16 bits)      |
|              |           |R|E|G|K|H|T|N|N|                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |       Urgent Pointer          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Options (variable, 0–40 bytes)     |    Padding   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field description

Field	Bits	Purpose
Source / Destination Port	16 + 16	Identify the sending and receiving processes (sockets) for demultiplexing
Sequence Number	32	Number of the first data byte of this segment (SYN/FIN each consume one number)
Acknowledgment Number	32	Next byte the receiver expects — valid only when ACK = 1
Data Offset	4	Header length in 32-bit words: 5 words = 20 bytes min., 15 words = 60 bytes max.
Reserved	4	Must be zero (future use)
Flags	8	CWR, ECE (ECN), URG (urgent pointer valid), ACK (ack # valid), PSH (push to app now), RST (reset), SYN (open connection), FIN (close connection)
Window	16	Flow control — number of bytes the receiver can still accept (rwnd); sender may transmit min(cwnd, rwnd)
Checksum	16	One's-complement sum of a pseudo-header (src IP, dst IP, zero, protocol = 6, TCP length) + TCP header + data — detects corruption on any link
Urgent Pointer	16	Offset from the sequence number to the end of urgent data (valid if URG = 1)
Options	0–40	MSS, Window Scale, SACK permitted / selective ACK blocks, timestamps, NOP, End-of-Option

Fixed part = 20 bytes; with options up to 60 bytes. Ports give process-to-process delivery, sequence/ack numbers give byte-stream reliability, the window gives flow control, and the checksum (with pseudo-header) guarantees end-to-end integrity across IP.

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Part B — UDP checksum verification

Given

• Sum of all 16-bit words excluding the checksum = 00110010 00001101 = 0x320D
• Received checksum field = 01011101 11110010 = 0x5DF2

Test: the receiver adds all 16-bit words including the checksum field (folding any end-around carry). The segment is correct only if the result is 11111111 11111111 = 0xFFFF.

Computation

      00110010 00001101   (sum of all other fields)   0x320D
    + 01011101 11110010   (checksum field)            0x5DF2
    ------------------------------------------
      10001111 11111111                              0x8FFF

There is no carry out of bit 15, so no end-around carry is added, and

$$0x320D + 0x5DF2 = 0x8FFF eq 0xFFFF$$

Cross-check: the expected checksum is the one's complement of the sum of the other fields:
~00110010 00001101 = 11001101 11110010 = 0xCDF2, which is not the received value 0x5DF2.

Conclusion: the sum is not 0xFFFF ⇒ the segment is NOT correctly received (at least one bit has been corrupted).

What the receiver does

1. Silently discards the UDP segment. UDP is connectionless and unreliable: it sends no NAK, no error ICMP and no request for retransmission — recovery, if any, is left to the application.
2. The missing data must be re-sent by the application after its own timeout (or the transfer simply fails).
3. Had this been a TCP segment, the receiver would likewise discard it but would send no ACK; TCP's retransmission timer would then retransmit the segment, so the error would be repaired end-to-end.
4. (Note: in IPv4 the UDP checksum is optional and may be sent as 0 = "not computed"; in IPv6 it is mandatory. If the calculated checksum is 0 it is transmitted as all 1s.)