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" (www.fatpipe.org/~mjb/Drawings/)

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.

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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.

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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.

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