SNAP: SubNetwork Access Protocol Overview (RFC 1042)

SNAP: SubNetwork Access Protocol

The SubNetwork Access Protocol (SNAP) is an a standard for the transmission of IP datagrams over IEEE 802 networks. In other words, IP datagrams could be sent on IEEE 802 networks encapsulated within the 802.2 LLC and SNAP data link layers and the 802.3, 802.4 or 802.5 physical network layers.

SNAP is included in an extension of the Logic Link Control (LLC IEEE 802.2) header and is used for encapsulating IP datagrams and ARP requests and replies on IEEE 802 networks. The SNAP header follows the LLC header and contains an organization code indicating that the following 16 bits specify the EtherType code. Normally, all communication is performed using 802.2 type 1 communication. Consenting systems on the same IEEE 802 network may use 802.2 type 2 communication after verifying that it is supported by both nodes. This is accomplished using the 802.2 XID mechanism. However, type 1 communication is the recommended method at this time and must be supported by all implementations.

The mapping of 32-bit Internet addresses to 16-bit or 48-bit IEEE 802 addresses is done via the dynamic discovery procedure of the Address Resolution Protocol (ARP). The IEEE 802 networks may have 16-bit or 48-bit physical addresses. SNAP allows the use of either size of address within a given IEEE 802 network.

With SNAP, the transmission of IP datagrams does not depend on the transmission rate of the under layer LAN technologies (various types of Ethernet, or Token Ring), which may have different very different transmission rate from 1 to 20 Mbps.

The Subnetwork Access Protocol is detailed in IETF RFC 1042.

Protocol Structure - SNAP: SubNetwork Access Protocol

Logic Link Control Layer (LLC) Header:

8

16

24 or 32bits

Variable

DSAP

SSAP

Control

LLC information

For details of the LLC header, please see the LLC page.

SNAP header:

24	40bits
Organization code	EtherType

When SNAP is present the DSAP and SSAP fields within the LLC header contain the value 170 (decimal) each and the Control field is set to 3 (unnumbered information).

Organization code - Set to 0.

EtherType - Specifies which protocol is encapsulated within the IEEE 802 network: IP = 2048, ARP = 2054.

Related Protocols
IEEE 802.2 , 802.3 , 802.4, 802.5 , IP , ARP

Sponsor Source
SNAP is defined by IEEE (http://www.ieee.org ) and IETF (http://www.ietf.org ).

Reference

http://standards.ieee.org/getieee802/download/802.2-1998.pdf : IEEE 802.2 specification.
http://www.javvin.com/protocol/rfc1042.pdf : A Standard for the Transmission of IP Datagrams over IEEE 802 Networks.