Wednesday, July 27, 2016

Ethernet II (DIX) vs 802.3 with LLC vs 802.3 with LLC/SNAP

In wireless header there is no field specified as Ether Type which means when an Ethernet frame is converted to wireless frame then some additional header (SNAP) is required to hold the ether type.

Lets look at different Ethernet frames first: (Cisco Link)

  • Ethernet II or DIX Frames:
    • Most popular
    • Ether Type is used for indicating upper layer protocol.
    • Exists prior to standardization work started by IEEE.
  • IEEE 802.3 Skeleton:
    • Treats ether type as length instead of protocol
    • Just a skeleton (as there is no field to indicate upper layer protocol)
    • Used to hold IEEE standardized Ethernet frames (discussed next).
  • IEEE 802.3 with 802.2 LLC
    • Define 3 bytes LLC header (DSAP, SSAP, Control) taken from HDLC.
    • DSAP & SSAP refers to Destination SAP & Source SAP.
    • DSAP and SSAP are just one byte field and hence not capable to hold upper protocol type (need 2 bytes for that). IEEE introduced SNAP header to remove this limitation. 
  • IEEE 802.3 with 802.2 LLC/SNAP
    • DSAP & SSAP of LLC header are fixed to 0xAA and control field is fixed to 0x03.
    • SNAP header is of 5 bytes (3 bytes for OUI & 2 bytes for specifying protocol)
    • In wireless frame, OUI is 00-00-00.
Note: Ether Type > 1536 (0x600) indicates the Ethernet II frame.