Packet Header Analysis

Following are the easy to analyze ICMP, TCP and UDP packet headers along with short description.
All header Drawings by- "Matt Baxter" (


ICMP Message Types :- ICMP message type-code/name format
0 Echo Reply
3 Destination Unreachable
4 Source Quench
5 Redirect
8 Echo
11 Time Exceeded
12 Parameter Problem
13 Timestamp
14 Timestamp Reply
15 Information Request
16 Information Reply

Checksum :- The checksum is the 16-bit ones's complement of the one's complement sum of the ICMP message starting with the ICMP Type.

RFC 792 :- Please refer to RFC 792 for Internet Control Message Protocol (ICMP) Specification.


TCP Flags :- C E U A P R S F
C 0x80 Reduced (CWR)
E 0x40 ECN Echo (ECE)
U 0x20 Urgent
A 0x10 Ack
P 0x08 Push
R 0x04 Reset
S 0x02 Syn
F 0x01 Fin
TCP Options :-
0 End of Options List
1 No Operation (NOP, Pad)
2 Maximum Segment Size
3 Window Scale
4 Selective ACK ok
8 Timestamp

Checksum :- Checksum of entire TCP segment and pseudo header (parts of IP header)

Offset :- Number of 32-bit words in TCP header, minimum value of 5. Multiply by 4 to get byte count.

RFC 793 :- Please refer to RFC 793 for Transmission Control Protocol (TCP) Specification.


Source Port :- an optional field, when meaningful, it indicates the port of the sending process, and may be assumed to be the port to which a reply should be addressed in the absence of any other information. If not used, a value of zero is inserted.

Destination Port :- Destination Port has a meaning within the context of a particular internet destination address.

Length :- Length is the length in octets of this user datagram including this header and the data. (This means the minimum value of the length is eight.)

Checksum :- Checksum is the 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with zero octets at the end (if necessary) to make a multiple of two octets.

RFC 768 :- Please refer to RFC 768 for User Datagram Protocol (UDP) Specification.