TCP/IP Tutorial and Technical Overview - IBM Redbooks

Therefore, for all practical purposes, the Ethernet physical layer and the IEEE
802.3 physical layer are compatible. However, the Ethernet data link layer and
the IEEE 802.3/802.2 data link layer are incompatible.
The 802.2 Logical Link Control (LLC) layer above IEEE 802.3 uses a concept
known as link service access point (LSAP), which uses a 3-byte header, where
DSAP and SSAP stand for destination and source service access point,
respectively. Numbers for these fields are assigned by an IEEE committee (see
Figure 2-2).
Figure 2-2 ARP: IEEE 802.2 LSAP header
Due to a growing number of applications using IEEE 802 as lower protocol
layers, an extension was made to the IEEE 802.2 protocol in the form of the
Subnetwork Access Protocol (SNAP) (see Figure 2-3). It is an extension to the
LSAP header in Figure 2-2, and its use is indicated by the value 170 in both the
SSAP and DSAP fields of the LSAP frame Figure 2-3.
Figure 2-3 ARP: IEEE 802.2 SNAP header
In the evolution of TCP/IP, standards were established that describe the
encapsulation of IP and ARP frames on these networks:
► Introduced in 1984, RFC 894 – Standard for the Transmission of IP
Datagrams over Ethernet Networks specifies only the use of Ethernet type of
networks. The values assigned to the type field are:
– 2048 (hex 0800), for IP datagrams
– 2054 (hex 0806), for ARP datagrams
Chapter 2. Network interfaces 31