Types DL_TPR and DL_FDDI: SNAP Processing

For media types DL_TPR and DL_FDDI, GLD implements minimal SNAP (Sub-Net Access Protocol) processing for any Stream bound to a SAP value greater than 255. SAP values in the range [0-255] are LLC SAP values and are carried naturally by the media packet format. SAP values greater than 255 require a SNAP header, subordinate to the LLC header, to carry the 16-bit Ethernet V2-style SAP value.

SNAP headers are carried under LLC headers with destination SAP 0xAA. For outgoing packets with SAP values greater than 255, GLD creates an LLC+SNAP header that always looks like:

AA AA 03 00 00 00 XX XX

where ``XX XX'' represents the 16-bit SAP, corresponding to the Ethernet V2 style ``type.'' This is the only class of SNAP header supported—non-zero OUI fields and LLC control fields other than 03 are considered to be LLC packets with SAP 0xAA. Clients wanting to use SNAP formats other than this one must use LLC and bind to SAP 0xAA.

Incoming packets are examined to ascertain whether they conform to the format shown above. Packets that conform to this format are matched to any Streams bound to the packet's 16-bit SNAP type, as well as being considered to match the LLC SNAP SAP 0xAA.

Packets received for any LLC SAP are passed up all Streams that are bound to an LLC SAP, as described for media type DL_ETHER.