TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-25
Subnetwork Access Protocol (SNAP)
For more detailed technical information on the ARP, refer to RFC 826 (DDN Protocol
Handbook, Volume 3, DDN Network Information Center, December, 1985, pp. 3-615
through 3-624). Also, refer to the book
TCP/IP Illustrated by W. Richard Stevens,
Prentice Hall, 1994.
Subnetwork Access Protocol (SNAP)
The Subnetwork Access Protocol (SNAP) defines an interface between the IP layer
and the Logical Link Control (LLC) layer. This interface is required to run the TCP/IP
protocol suite over IEEE 802.3 (Ethernet) LANs.
The IEEE 802.2 LLC type 1 is used for accessing the LAN. The interface is
accomplished by an extension of the LLC header that contains a predefined Service
Access Point (SAP) for use in the Source SAP (SSAP) and Destination SAP (DSAP)
fields of the LLC header.
The IEEE has defined one data-link layer standard (IEEE 802.2) and several
physical-layer standards (IEEE 802.3, 802.4, and 802.5) for LANs. The physical-layer
standards specify the physical and Media Access Control (MAC) sublayer
requirements for several different LANs, including IEEE 802.3 CSMA/CD LANs.
As shown in Figure B-15
, the SNAP is part of the TCP/IP process and has an interface
driver built into it.