The Inter-Domain Routing Protocol (IDRP) provides routing for OSI defined network environments, which is similar to BGP in the TCP/IP network. In an OSI network, there are End Systems, Intermediate Systems, Areas and Domains. End systems are user devices. Intermediate systems are routers. Routers are organized into local groups called "areas", and several areas are grouped together into a "domain". Inter-Domain Routing Protocol (IDRP) is designed to provide routing among domains. IDRP, working in conjunction with CLNP, ES-IS, and IS-IS, provides complete routing over the entire network.

A router that participates in IDRP is called a Boundary Intermediate System (BIS), which may belong to only one domain. IDRP governs the exchange of routing information between a pair of neighbors, either external or internal. IDRP is self-contained with respect to the exchange of information between external neighbors. Exchange of information between internal neighbors relies on additional support provided by intra-domain routing (unless internal neighbors share a common subnetwork).

To facilitate routing information aggregation/abstraction, IDRP allows grouping of a set of connected domains into a Routing Domain Confederation (RDC). A given domain may belong to more than one RDC. The ability to group domains in RDCs provides a simple, yet powerful mechanism for routing information aggregation and abstraction. It allows reduction of topological information by replacing a sequence of RDIs carried by the RD_PATH attribute with a single RDCI. It also allows reduction of the amount of information related to transit policies, and simplifies the route selection policies.

Each domain participating in IDRP is assigned a unique Routing Domain Identifier (RDI), which is basically an OSI network layer address. Each RDC is assigned a unique Routing Domain Confederation Identifier (RDCI). RDCIs are assigned out of the address space allocated for RDIs. RDCIs and RDIs are syntactically indistinguishable. It is expected that RDI and RDCI assignment and management would be part of the network layer assignment and management procedures.

Protocol Structure

ES-IS Protocol Data Unit contains the following:

The General Format of NSAP:

• IDP - Initial Domain Part
• AFI - Authority and Format Identifier, two-decimal-digit, 38 for decimal abstract syntax of the DSP or 39 for binary abstract syntax of the DSP
• IDI - Initial Domain Identifier, a three-decimal-digit country code, as defined in ISO 3166
• DSP - Domain Specific Part
• CDP - Country Domain Part, 2..4 octets
• CFI - Country Format Identifier, one digit
• CDI - Country Domain Identifier, 3 to 7 digits, fills CDP to an octet boundary
• CDSP - Country Domain Specific Part
• RDAA - Routing Domain and Area Address
• ID - System Identifier (1..8 octet)
• SEL - NSAP Selector

While RDIs and RDCIs need not be related to the set of addresses within the domains they depict, RDIs and RDCIs are assigned based on the NSAP prefixes assigned to domains. A subscriber RD should use the NSAP prefix assigned to it as its RDI. A multihomed RD should use one of the NSAP prefixes assigned to it as its RDI.

Related protocols: ES-IS, CLNP, IDRP, IS-IS, BGP

Sponsor Source: IDRP is defined in ISO (http://www.iso.org) 10747 and discussed by IETF (http://www.ietf.org).

Reference:
http://www.javvin.com/protocol/rfc1629.pdf: Guidelines for OSI NSAP Allocation in the Internet
http://www.acm.org/sigcomm/standards/iso_stds/IDRP/10747.TXT: Protocol for the Exchange of Inter-Domain Routing Information among Intermediate Systems to Support Forwarding of ISO 8473 PDUs