LAP Manager 802.2 Protocol

The Institute of Electrical and Electronics Engineers (IEEE) has defined a

series of communications protocols for use on a variety of networks. At the

physical level, these protocols include the 802.3 CSMA/CD protocol, the

802.4 token bus protocol, and the 802.5 token ring protocol. At the data link

level, you access these protocols through another IEEE protocol, the 802.2

protocol. The AppleTalk LAP Manager includes two routines that allow you to

attach and detach protocol handlers for 802.2 Type 1 data packets: the

L802Attach and L802Detach routines. You can write an application that reads

802.2 Type1 data packets and use the L802Attach routine to install your

application as a client of The LAP Manager. The ANSI/IEEE standards for the

802 protocols are published by the IEEE.

The first 14 bytes of a packet sent or received by The .ENET Driver

constitute the header. The first 12 bytes consist of the destination and source

data-link addresses, such as the Ethernet hardware addresses. If the value of

the last two (2 ) bytes in the header is greater than 1500, then

The .ENET Driver treats that field as an Ethernet protocol type

discriminator. See The .ENET Driver for more information on Ethernet

protocols. If the value of the last 2 bytes in the header is less than or equal to

1500, then the packet is an 802.3 protocol packet and this field indicates the

length of the 802.3 data. The .ENET Driver passes all 802.3 packets to

The LAP Manager gets the 802.3 packet from The .ENET Driver. The

first 3 bytes of the 802.3 data constitute the header for the 802.2 protocol.

The first byte of the 802.2 header is known as the destination service access

point (DSAP). If the DSAP field is equal to 0x0AA, then the first 5 bytes of the

802.2 data constitute a protocol discriminator known as the subnetwork access

protocol (SNAP) type. If the SNAP type field is 0x000000080F3, indicating

the AppleTalk Address Resolution Protocol (AARP), then the next 4 bytes of

the 802.2 data constitute a third type field, the AARP packet type. AARP is

discussed in Inside AppleTalk, second edition.

When you call the L802Attach routine, you provide a pointer to your protocol

handler, the reference number of The .ENET Driver, and a pointer to a

string containing one or more type fields. The type fields indicate the DSAP

value and any other protocol type fields (such as the SNAP type and the AARP

type). The LAP Manager delivers to your protocol handler any 802.2 data

packets that have the protocol type you specify.

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