TCP/IPv6 Configuration and Management Manual (G06.24+)

NonStop TCP/IPv6 Protocols and Configuration Files
HP NonStop TCP/IPv6 Configuration and Management Manual—524523-008
D-6
Subnetwork Access Protocol (SNAP)
that effectively hides physical addresses and allows the IP to use 32-bit Internet
addresses. Figure D-1 shows ARP’s relationship to the Ethernet and Internet protocols.
The caller (host A) broadcasts a special packet addressed to the receiver’s Internet
address (host B). All nodes receive the packet; however, only host B replies, and its
reply includes its physical address. When host A receives this reply, it dynamically
binds the Internet-to-physical address and caches it for subsequent use.
For more 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 local area networks
(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 D-2
on page D-7, the SNAP is part of the TCP/IP process and has
an interface driver built into it.
Figure D-1. Address Resolution Protocol
Ethernet 48-Bit
Address
Internet 32-Bit
Address
ARP Translation
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