#define CTL_NET_ETHERNET_PDU_SIZE      1500

CTL_NET_ETHERNET_PDU_SIZE defines the number of bytes of payload data (the network PDU) in an Ethernet II frame.

In general, when dealing with Ethernet MAC drivers, we have:

  1. A 16-bit padding short, 2 bytes. (Required to align TCP headers)
  2. Destination MAC, 6 bytes.
  3. Source MAC, 6 bytes.
  4. Ethernet Type/Frame Size, 2 bytes. (16 bits, including padding)
  5. Payload of 1,500 bytes.
  6. FCS, 4 bytes.

Excluding the padding, 1518 bytes. Including the padding, 1520 bytes, which is divisible by four.