b. There is a SNAP header to follow
When used as the Destination Service Access Point (DSAP) and Source
Service Access Point (SSAP), the bit pattern 0xAA is used to indicate
the presence of a SNAP, or Sub-Network Access Protocol, header. This
is used to transfer certain data for which there is no specific SAP
value assigned within an 802.2 frame.
Reference: Miller, LAN Protocol Handbook, Chapter 1.