Subnetwork Access Protocol
（重定向自SNAP header）
The Subnetwork Access Protocol (SNAP) is an extension of the IEEE 802.2 Logical Link Control (LLC) to distinguish much more protocols of the higher layer than using of the 8-bit Service Access Point fields (LSAP) present in the IEEE 802.2 header.
The SNAP and LSAP fields are added to the packets at the transmitting node in order to allow the receiving node to pass each received frame to an appropriate device driver which understands given protocol.