Question

There is possible to use very big frames with some ethernet cards. One case is 9k frames or jumbo frame and other case is super jumbo frame (as i know, up to 64k).

What is format of frame used for such huge packets?

As I know, for normal frames two formats used widely:

Preamble    Start_byte  dest_mac    src_mac     Ethertype/length    Payload     CRC     Interframe gap
10101010x7  10101011    6 bytes     6 bytes     2 bytes         46–1500 bytes   4 bytes 12 bytes

In one case, the ethertype is used for length, and in second – for packet type. If this field is < 0x0600 (decimal 1536 bytes), this is a length; if >= 0x0600 – it is a type.

So, it looks impossible to store 9000 in this field as length. How length of jumbo and super jumbo frames is stored?

Answer 1

The format used for jumbos is the same. Despite this description, the Ethertype field is not normally used to store a length. Normally in the Ethertype field, you will not see a length; you will see a type. Types are specified by IANA here:

https://www.iana.org/assignments/ieee-802-numbers

Usually you’ll see one of the following types from the table:

Ethertype         Exp. Ethernet   Description           References
----------------  --------------  --------------------  ----------
  2054   0806        -      -     ARP                       [IANA]
  2048   0800        513   1001   Internet IP (IPv4)        [IANA]
         86DD                     IPv6                      [IANA]

There are two reasons this works:

• The hardware sending the packet doesn’t depend on the Layer 2 length field to know the Layer 1 length.
• Some Layer 3 packets such as ARP have a known size (at least, for a known combination of hardware/protocol address length, such as Ethernet/IP where it is normally used). IPv4/IPv6 packets have a length field in their own header.