IP Header IHL Field (What is the maximum IP header length?)

kfarrington · ‎04-25-2003

Hello peeps :))

The standard IP header length is 20 bytes. The "Routing TCP/IP Volume 1" book indicates on page 33 that with options set within the IP header, the sizes can be variable from 20 bytes "upto a maximum size of 24 bytes."

So I wanted to see an IP packet with options in the header to verify this fact, so I did a record route ping. Here is what I found in the packet. I assume that there is an explication to this and it has something to do with EIP (extended IP formats which I know nothing about)

Can anyone help?

Internet Protocol, Src Addr: 10.1.1.1 (10.1.1.1), Dst Addr: 10.1.1.253 (10.1.1.253)

Version: 4

Header length: 60 bytes

Full frame below:-

Frame 245 (114 on wire, 114 captured)

Arrival Time: Apr 25, 2003 19:56:58.631923000

Time delta from previous packet: 0.000092000 seconds

Time relative to first packet: 50.166327000 seconds

Frame Number: 245

Packet Length: 114 bytes

Capture Length: 114 bytes

Ethernet II

Destination: 00:10:f6:a8:18:00 (Cisco_a8:18:00)

Source: 00:d0:ba:e2:21:41 (Cisco_e2:21:41)

Type: IP (0x0800)

Internet Protocol, Src Addr: 10.1.1.1 (10.1.1.1), Dst Addr: 10.1.1.253 (10.1.1.253)

Version: 4

Header length: 60 bytes

Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)

0000 00.. = Differentiated Services Codepoint: Default (0x00)

.... ..0. = ECN-Capable Transport (ECT): 0

.... ...0 = ECN-CE: 0

Total Length: 100

Identification: 0x0004

Flags: 0x00

.0.. = Don't fragment: Not set

..0. = More fragments: Not set

Fragment offset: 0

Time to live: 255

Protocol: ICMP (0x01)

Header checksum: 0x8963 (correct)

Source: 10.1.1.1 (10.1.1.1)

Destination: 10.1.1.253 (10.1.1.253)

Options: (40 bytes)

Record route (39 bytes)

Pointer: 8

10.1.1.1

- <- (current)

-

EOL

Internet Control Message Protocol

Type: 8 (Echo (ping) request)

Code: 0

Checksum: 0x4c00 (correct)

Identifier: 0x1b93

Sequence number: 16:cf

Data (32 bytes)

0000 00 00 00 00 00 11 6b e8 ab cd ab cd ab cd ab cd ......k.........

0010 ab cd ab cd ab cd ab cd ab cd ab cd ab cd ab cd ................

fbarreras · ‎04-27-2003

IHL field is 4 bits long and indicates number of 32 bit words. With 4 bits you can count up to 15 decimal and therefore can have up to 15 32-bit words. If one byte is eight bits long, each 32-bit word has 4 bytes. With 15 total words you have a maximum of 60 bytes that can be indicates with a 4 bit IHL.