-28 Bytes Later
Author: Stu Feeser
When calculating ICMP payload size for MTU testing, you are asked to subtract 28 bytes. For instance, if you want to test MTU transmission of a 9000 byte jumbo frame, you perform the following math:
9000 - 28 = 8972.
So where does the 28 bytes come from?
The answer can be found in RFC 792, but the following diagram clearly shows where that 28 bytes originates:
---------------- +-----------------------------------------------------------------------------------------------+
^ ^ | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
| | +-----------------------------------------------------------------------------------------------+
| | | | | | |
| | | IP Version | Hdr Len|th Type of Service | Total Length |
| | | | | | |
| | +--------------+--------+--------------------------------+--------------------------------------+
| | | | FLAGS | |
| | | Identification +--------+ Offset |
| | | |DF|MF|RB| |
| V +-----------------------+-----------------------------------------------------------------------+
| IP Header | | | |
| 20 Bytes | Time To Live | Protocol | Header Checksum |
| | | | 1=ICMP (ping) | |
| | +-----------------------+-----------------------+-----------------------------------------------+
| | | |
+ | | Source IP Address |
9000 | | |
Bytes | +-----------------------------------------------------------------------------------------------+
+ | | |
| | | Destination IP Address |
| v | |
| ------ +-----------------------+------------------------+----------------------------------------------+
| ^ | | | |
| | | Type | Code | Checksum |
| ICMP | |8=ping req, 0=ping reply| |
| Header +-----------------------------------------------------------------------------------------------+
| 8 Bytes | |
| | | Unused |
| V | |
| ------ +-----------------------------------------------------------------------------------------------+
| | | |
| 8972 | Data size must be calculated & determined by you! |
| BYTES | Note that the IP header is 20 bytes and the IMCP header is 8 bytes |
v | | 8972 Bytes |
---------------- +-----------------------------------------------------------------------------------------------+