Sunday, February 19, 2012


Intermediate System to Intermediate System (IS-IS; pronounced as “i-sys”) is a hierarchical link-state routing protocol for the Open System Interconnection (OSI) protocol suite. The OSI suite uses ConnectionLess Network Service (CLNS) to describe a connectionless data delivery service to the transport later; whereas ConnectionLess Network Protocol (CLNP) defines the actual network layer protocol. CLNS and CLNP which are used for unreliable connectionless data delivery is similar and equivalent to Internet Protocol (IP) in TCP/IP suite. IS-IS uses CLNS addresses extensively for its operations, eg: identifying routers and building the link-state database (LSDB).
Note: ISO makes a fine semantic distinction between the service offered to higher layers (CLNS) and the protocol used to implement it (CLNP). There is not such distinction in the TCP/IP world.

IS-IS is the most popular and stable IP routing protocol in the Internet Service Provider industry. IS-IS is robust in some of the world’s largest internetworks due to its simplicity and stability.

ISO has developed a suite of routing protocols – End System to Intermediate System (ES-IS), Intermediate System to Intermediate System (IS-IS), and Inter-Domain Routing Protocol (IDRP) for the OSI protocol suite. Note: Cisco IOS does not support IDRP.

What is the difference between ISO and OSI? International Organization for Standardization (ISO) is the largest standards organization in the world that develops standards for networking; whereas Open System Interconnection (OSI) Reference Model represents an international standardization program that facilitates multivendor networking equipment interoperability. In the networking world, the ISO is best known for its OSI Reference Model.

ES-IS and IS-IS were developed by Digital Equipment Corporation (DEC) for DECnet Phase V. DEC designed the DECnet protocol stack as part of its Digital Network Architecture (DNA). DECnet supports both connection-oriented and connectionless OSI network layers. DECnet Phase V is equivalent to ISO CLNS. DECnet Phase V implements full OSI routing, which includes support for ES-IS and IS-IS. IS-IS was submitted by DEC to the ISO as the routing protocol for OSI.

DECnet addresses are not associated with the physical networks to which the nodes are connected. DECnet locates hosts using area.node address pairs. The value of an area ranges from 1 to 63 (inclusive), and a node address between 1 and 1023 (inclusive). Therefore, each area can have 1023 nodes; and approximately 65000 nodes can be addressed in a single DECnet network. Areas can span across many routers, and a single cable can support many areas. Therefore, if a node has multiple network interfaces, it uses the same area.node address for all interfaces.

DECnet Locates Hosts using Area.Node Address Pairs

Cisco IOS supports packet forwarding and routing for ISO CLNS on networks with different data link layer technologies, including Ethernet, Token Ring, FDDI, and serial. CLNS routing can be implemented on serial interfaces with HDLC, PPP, Link Access Procedure, Balanced (LAPB), X.25, SMDS, or Frame Relay encapsulation.

The Cisco CLNS implementation is also compliant with the Government OSI Profile (GOSIP) Version 2. GOSIP is a specification that profiles networking products for procurement by the Federal Governments of the United States. This specification was first published as FIPS (Federal Information Processing Standard) 146-1 in 1990, and required OSI protocols to be used. FIPS 146-2 which was published on 1995 removed the procurement requirement for the OSI protocols, allows the procurement of products that implement ISO, ITU-T, or IETF standards. As the interest in OSI implementations declined, subsequent deployments of networking services for the civilian (non-military) government agencies are predominantly based on the TCP/IP suite.

Routers are being referred to as intermediate systems (ISs); while non-routing hosts are being referred to as end systems (ESs) in the OSI specification and terminology. IS-IS is a dynamic routing protocol for the OSI suite that allows the communication between routers to exchange and distribute routing information for routing CLNP packets in the ISO CLNS environment

Cisco IOS fully support the following ISO and American National Standards Institute (ANSI) standards as part of its CLNS support:
i) ISO 8473 – ISO ConnectionLess Network Protocol (CLNP)
ii) ISO 8348/AD2 – Network Service Access Point (NSAP) addresses
iii) ISO 9542 – End System to Intermediate System Routeing Information Exchange Protocol
iv) ISO 10589 – IS-IS Intra-domain Routeing Information Exchange Protocol
Note: The spelling of Routeing is not a typographic error but the British spelling adopted by the ISO committee.

The CLNP specification defines the format of packets. A data packet is generally referred to as Protocol Data Unit (PDU).

The Q3-Stack is a standardized management protocol which provides routing facilities (Layer 3) within a Telco Data Communication Network (DCN). The Q3 management traffic can be carried in a timeslot n of a 2Mbps signal. Below describes the general layer architecture of the Q3-Stack:

Application Provides 2 application specific services – CMISE for management and FTAM for file transfer (similar to FTP). FTAM uses services provided by ACSE (Application Control Service Element) while CMISE uses both ACSE and ROSE (Remote Operations Service Element).
Presentation Provides data representation and conversion, eg: ASN.1 encoding and BER decoding. Standards used in this layer are:
- ISO 8822 – Presentation Service Definition
- ISO 8823 – OSI Presentation Protocol (PRES)
- ISO 8825 – Specification of Basic Encoding Rules (BER) for Abstract
Syntax Notation (ASN)
Session Provides the management (opening and closing) of sessions. When a connection loss it will try to recover the connection; when a connection is idle for a long period, it may close it and reopen it for next use. These processes occur transparently to the higher layers. Standards used in this layer are:
- ISO 8326 – Session Service Definition
- ISO 8327 – OSI Session Protocol (SES)
Transport Provides flow control, error detection and correction at the packet level, segmentation, and reassembly of packets. ISO has defined different classes of transport protocols. Standards used in this layer are:
- ISO 8072 – Transport Service Definition
- ISO 8073 – Connection-Oriented Transport Protocol (COTP)
- ISO 8602 – ConnectionLess Transport Protocol (CLTP)
Network Provides routing and delivery of packets between nodes in a network. Standards used in this layer are:
- ISO 8348 – Network Service Definition. Defines the connection-mode (CONS) and connectionless-mode (CLNS) services as the network layer for the interface specification between the network and transport layers.
- ISO 8473-1 defines the ConnectionLess Network Service (CLNS).
- ISO 8473-2 defines CLNS on ISO 8802 subnetworks.
- ISO 8473-3 defines CLNS on X.25 subnetworks.
- ISO 8473-4 defines CLNS on subnetworks provide OSI data link service.
- ISO 8473-5 defines CLNS on ISDN circuit switched B channels.
- ISO 9542 defines the ES-IS protocol for CLNS.
- ISO 10589 defines the IS-IS Intra-domain (Level 1) for CLNS.
- ISO 10747 defines the IS-IS Inter-domain (Level 2) for CLNS.
- ISO 8208 defines the X.25 Packet Layer Protocol for DTE.
- ISO 8878 defines how to use X.25 to provide the OSI Connection-Mode Network Service (CONS).
Data Link Provides error detection and correction at the bit level when transmitting data over a network media; and identifies the upper network layer protocols.
Physical Provides the bit transfer over the physical medium. The X.25 protocol suite references X.21 and X.21bis as the standards for the physical layer; however, 2M TS n or G.703 may also be used. The most frequently used standard for LAN is Ethernet, which may be used for both OSI and TCP/IP.

OSI Management Stack

The OSI network layer can be divided into 2 main parts:
i) Basic – Forwarding packets from a node to another node based on the information in the local forwarding database. This part is specified by the CLNP protocol.
ii) Advanced – Automatically creating and updating the local forwarding database used by the first part. This part is specified by the ES-IS and IS-IS protocols.

OSI and TCP/IP Network Layer Protocols

Many sources claim that IS-IS runs on top of CLNP, but it is not the case! The identification of a L3 OSI protocol is based on the L2 info (DSAP = 0xFE) and the 1st byte of the L3 header – Network Layer Protocol Identifier (NLPID). IS-IS is therefore a separate network layer protocol and does not rely upon CLNP for datagram delivery; whereas IP routing protocols encapsulate their packets into IP, TCP, or UDP datagrams.

No comments:

Post a Comment