ISO Connectionless Network Service

The International Organization for Standardization (ISO), currently known as the International Telecommunications Union ( ITU ), laid the foundation for standardization of computer networking by defining the seven-layer OSI reference model (see Figure 2-1). The OSI reference model, also known as the OSI stack, is specified in ISO 7498. (The ISO standards documents cited in this chapter can be found at www.itu.org/.)

Figure 2-1. The OSI reference model.

The OSI reference model is a significant contribution to the foundations and subsequent evolution of data communications and information technology. It provides the architectural framework for developing open standards that allow flexible interconnectivity and interoperability between communications devices from different manufacturers. Although the OSI reference model does not specify the internal details of any communications protocol or system, it provides general guidance regarding design and architecture of such systems. Each of the seven layers in the OSI reference model defines a single service capability and provides a premise for grouping related functional elements into functional layers, thus simplifying and facilitating protocol design. Each functional layer defines specific services provided to the adjacent higher layer. For example, the network layer provides services for the transport layer (see Figure 2-1), whereas the transport layer provides data transport services to the higher layers, thus helping transport user data between communicating devices. Furthermore, data transport services can be either connection-oriented or connectionless. Connection-oriented services require prior setup of a connection along a specific path between communicating nodes before data can be transmitted between them, whereas connectionless services do not.

Originally, only connection-oriented communication services (CONS) were specified by the network service definition component in the OSI reference model. CONS was defined by two specifications: X.25 Packet-Level Protocol for Data Terminal Equipment (ISO 8208) and Network Service Definition (ISO 8348).

Later amendments to the network services specification, which appeared in "Network Service Definition, Amendment 1," defined the capabilities for enabling connectionless communication between network devices, referred to as Connectionless Network Services (CLNS). Unlike CONS, CLNS does not require a predefined and presetup of the end-to-end path for forwarding data packets between two communicating devices. Instead, it provides a datagram service in which each data packet is forwarded independently by routers along the currently known best path between the source and destination. The connectionless datagram service defined by CLNS is supported by the following ISO protocols (see Figure 2-2):

Figure 2-2. Diagram of ISO protocols and specifications.

• ISO 8473 ” Connectionless Network Protocol (CLNP) for providing the CLNS

• ISO 9542 ” End System-to-Intermediate System (ES-IS) routing exchange protocol for use in conjunction with the protocol for providing the CLNS

• ISO 10589 ” Intermediate System-to-Intermediate System (IS-IS) intradomain routing exchange protocol for use in conjunction with the protocol for providing the connectionless-mode service

CLNP, ES-IS, and IS-IS are specified as separate network layer protocols, coexisting at Layer 3 of the OSI reference model. They are differentiated by the value of the Initial Protocol Identifier (IPI) field in the first octet of their encoded protocol formats, as follows :

CLNP:	10000001 (0x81)
ES-IS:	10000010 (0x82)
IS-IS:	10000011 (0x83)

CLNP

The Connectionless Network Protocol is similar to the Internet Protocol (IP), but specified for providing network services for ISO transport protocols rather than the transport protocols in the TCP/IP suite. Like IP, CLNP is defined to rely minimally on the underlying data link layer, which makes it virtually independent of the underlying physical medium. The physical medium can be either point-to-point (as is the case with most wide-area network [WAN] connectivity) or broadcast (as in local-area network [LAN] connectivity). Unlike IP, however, which is the only network layer protocol of TCP/IP and ultimately encapsulates all higher layer protocols, including routing and user applications, CLNP coexists at the network layer, ES-IS and ES-IS, all of which are defined to support the ISO CLNS environment. That is, CLNP, ES-IS, and IS-IS are all network layer protocols and are encapsulated independently in data-link frames . The ISO network layer protocol family is identified at the data-link layer by protocol type 0xFEFE.

This book does not delve into CLNP further, except to the extent that it pertains to the subject matter at hand: the IS-IS routing protocol.

ES-IS

The End System-to-Intermediate System (ES-IS) routing exchange protocol automates information exchange and facilitates adjacency discovery between ISO end systems and routers connected to the same network segment or link. The routers transmit intermediate system hello (ISH) messages and the hosts transmit end system hello (ESH) messages as part of the ES-IS protocol. The hellos, which are transmitted between directly connected nodes, convey network and data-link addresses of the communicating nodes. The hellos are also referred to as configuration information. The end systems forward packets to nonconnected devices through the routers.

Another type of packet used within the ES-IS protocol is called r oute redirection ( RD ) . A route redirection packet is sent by a router to an end system to inform it of a better path to reach a specific destination of interest. The function of ISO RDs is similar to those of Internet Control Message Protocol (ICMP) redirects used in IP environments. Basically, the operation of the ES-IS protocol between routers and end systems in the ISO environment can be related to the combined operation of the ICMP, Address Resolution Protocol (ARP), and Dynamic Host Configuration Protocol (DHCP) within the IP framework.

ES-IS is not relevant to IP workstations and servers that are involved in processing and transfer of only IP datagrams. For such IP devices, IP ARP provides the necessary network-to-data-link address resolution that might be needed to locate routers and other directly connected hosts. IP hosts also usually use static default routes for their default gateway, even though it is not uncommon for some advanced servers to support simple IP routing protocols, such as the Routing Information Protocol (RIP). Some modern IP servers even support the more sophisticated Open Shortest Path First (OSPF) Protocol for IP routing.

Some aspects of the operation of the IS-IS protocol are dependent on functions provided by the ES-IS protocol; therefore, even in situations where IS-IS is used for routing only IP on Cisco routers, the ES-IS protocol is needed to provide background support. For example, IS-IS point-to-point adjacency formation is preceded by the exchange of ES-IS-related ISHs between the two neighbor routers.

The IS-IS Routing Protocol

Specified in ISO 10589, the IS-IS routing protocol was intended to provide a way to dynamically exchange routing information between routers running CLNP in the ISO CLNS environment. CLNP was designed to use a hop-by-hop route-selection mechanism to move data within a network. The IS-IS protocol was specified to automate the best path calculation and selection process.

The design goals of IS-IS included the following:

• Function as an intradomain routing protocol

• Present a global view of the network for optimal routing decisions

• Provide fast convergence in case of failures

• Provide network stability

• Efficiently use network resources, such as router memory, CPU cycles, and network bandwidth

To achieve these goals, IS-IS was designed as a link-state routing protocol and optimized for use within a single network domain; therefore, it provides Interior Gateway Protocol functionality. IS-IS supports a two-level routing (level 1 and level 2 routing) designed to scale routing over large domains. It also uses the Dijkstra shortest path first (SPF) algorithm to optimize route calculation, path selection, and to achieve fast convergence.

Integrated IS-IS

The Internet Engineering Task Force (IETF) RFC 1195, "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments," specifies the version of the IS-IS protocol, commonly known as Integrated IS-IS or Dual IS-IS. Integrated IS-IS adapts the original IS-IS protocol that was specified for CLNS environments, for also routing IP. It is interesting to note that Integrated IS-IS is one of few protocols that provides an integrated framework for concurrent processing of more than one network layer protocol; in this case, IP and CLNP. Other routing protocols, such as the OSPF, usually support routing for only one type Layer 3 protocol. OSPF deals only with routing for IP. Integrated IS-IS can be used for routing in CLNP-only or IP-only networks, as well as in dual environments, which have both CLNP and IP traffic.

This book focuses on the use of Integrated IS-IS in IP networks and specifically on the service provider networks that make up the Internet. Even though not inherently designed for routing IP, the successful use of IS-IS for IP routing on the Internet led to development of many proprietary features outside of RFC 1195 to improve usability and to provide flexibility and scalability. The IETF has recently reopened the IS-IS Working Group to explore possibilities of standardizing some of the vendor-specific features of Integrated IS-IS and developing new standards to meet the requirements of emerging applications, such as Multiprotocol Label Switching (MPLS) traffic engineering. Many recently standardized capabilities of IS-IS are documented as RFCs, whereas others are still under review in the IETF standards track. Also, a second version of ISO 10589, which would include most of these new capabilities, is still undergoing review (at the time of writing) and should be published soon. IETF RFCs and draft publications that are relevant to the subject matter of this book are mentioned where necessary.