Subnetwork Access Protocol (SNAP)
The TCP/IP software provides protocol support down to the ISO network layer. Following this layer is the data link layer, which can be separated into two sublayers. These are the Logical Link Control (LLC) and the Media Access Control (MAC) layers.
The IEEE 802.2 standard defines the LLC sublayer, and the MAC sublayer is defined in IEEE 802.3, IEEE 802.4, and IEEE 802.5.
The format of an IEEE 802.2 LLC header with the SNAP header is shown in Figure 1.
Figure 1. SNAP header
The values of the fields in the LLC header when a SNAP
header is used are specified in RFC 1042 - Standard for Transmission
of IP Datagrams over IEEE 802 Networks. The values specified are:
- Field
- Value
- DSAP
- X'AA'
- SSAP
- X'AA'
- CONT
- X'03' Specifies unnumbered information (UI)
- P_id
- X'00 00 00'
- Type
-
X'8006' — ARP
X'8035' — RARP
X'86dd' — IPv6