Subject: [Isis-wg] SNAP & NSAP

From: "Manav Bhatia" <[email-masked]>
Date: Thu, 9 Aug 2001 10:05:41 +0530

I have a small doubt . .

(i) NSAP is an OSI equivalent of an IP address in a TCP/IP network.
Is SNAP address equivalent of the ethernet address in the data link layer if
its a 802.2 ntwrk ?

Rgds,
Manav

"C is not a big language, and is not well served by a big book." -- K&R2

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From: Micah Bartell <[email-masked]>
Date: Thu, 09 Aug 2001 10:53:19 -0500

SNAP?  That refers to a data link encapsulation for Ethernet, it is an 
extension of the 802.2 protocol.

Are you sure you are not referring to SNPA, which is Subnetwork Point of 
Attachment?  This is the same as the MAC address.

/mpb

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From: mike shand <[email-masked]>
Date: Tue, 28 Aug 2001 10:34:10 +0100

>(i) NSAP is an OSI equivalent of an IP address in a TCP/IP network.

Almost. An NSAP is a Network Service Access Point, which has an address, 
the NSAP address, which is broadly equivalent to an IP address. But note 
that an NSAP address is NOT tied to an interface in the same way that an IP 
address is.

>Is SNAP address equivalent of the ethernet address in the data link layer if

No. SNAP, stands for Sub-network Access Protocol. 8802-2 (LLC) defines 
various LLC classes. Class 1, which provides a datagram service, has one 
byte DSAP and SSAP fields, which identify the higher layer (3) protocol. 
For example FE identifies ISO connectionless network layer protocols.

But SAP addresses are only assigned to standardized protocols, so there is 
another frame format which is used for non-standardized protocols. This is 
SNAP. It is assigned its own SAP of AA, but then the frame format diverges 
from LLC1, since it has a 5 byte Protocol ID field, which is used to 
identifier the higher layer protocol. The first 3 bytes are taken from a 24 
bit IEEE block identifier, and the remaining 2 bytes are assigned by the 
owner of that block. This allows proprietary protocols to be distinguished 
by a vendor.

So although the prot ID field of SNAP LOOKS a bit like an ethernet address, 
it is in fact a protocol identifier (in the same sense that the SAP 
addresses are).

But aren't we getting somewhat off topic with this spate of OSI tutorials?

         Mike