RFC 1526 (rfc1526) - Page 2 of 8

Assignment of System Identifiers for TUBA/CLNP Hosts

RFC 1526              System Identifiers for TUBA         September 1993


1.  Background

   The general format of OSI network service access point (NSAP)
   addresses is illustrated in Figure 1.

          _______________________________________________
          |____IDP_____|_______________DSP______________|
          |__AFI_|_IDI_|_____HO-DSP______|___ID___|_SEL_|

                IDP     Initial Domain Part
                AFI     Authority and Format Identifier
                IDI     Initial Domain Identifier
                DSP     Domain Specific Part
                HO-DSP  High-order DSP
                ID      System Identifier
                SEL     NSAP Selector

                Figure 1: OSI NSAP Address Structure.

   The recommended encoding and allocation of NSAP addresses in the
   Internet is specified in RFC 1237. RFC 1237 makes the following
   statements regarding the system identifier (ID) field of the NSAPA:

  1.  the ID field may be from one to eight octets in length

  2.  the ID must have a single known length in any particular
      routing domain

  3.  the ID field must be unique within an area for ESs and
      level 1 ISs, and unique within the routing domain for level
      2 ISs.

  4.  the ID field is assumed to be flat

   RFC 1237 further indicates that, within a routing domain that
   conforms to the OSI intradomain routing protocol [3] the lower-order
   octets of the NSAP should be structured as the ID and SEL fields
   shown in Figure 1 to take full advantage of intradomain IS-IS
   routing. (End systems with addresses which do not conform may require
   additional manual configuration and be subject to inferior routing
   performance.)

   Both GOSIP Version 2 (under DFI-80h, see Figure 2a) and ANSI DCC NSAP
   addressing (Figure 2b) define a common DSP structure in which the
   system identifier is assumed to be a fixed length of 6 octets.






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