Open Network Addressing Previous Screen Howard Berkowitz Payoff Problems in Addressing Rank Among the Chief Reasons That Networked Components Fail to Interoperate
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52-10-20 Open Network Addressing Previous screen Howard Berkowitz Payoff Problems in addressing rank among the chief reasons that networked components fail to interoperate. The latest network addressing schemes can scale up to accommodate extremely large open internetworks; these schemes can coexist with older addressing methods that were suitable only for smaller networks. Introduction Proprietary networking architectures (e.g., System Network Architecture and NetWare) were designed to suit single organizations. The Novell identifier assigned to a workstation in one company can therefore duplicate that of a workstation in another company, and as business requirements continue to encourage internal and external networking among organizations, the duplication problem becomes much more significant. Proprietary address schemes also limit users' abilities to select alternative vendors' solutions when those alternatives use different yet incompatible addressing schemes. Basics of Naming and Addressing Names and addresses are the attributes of network objects that enable them to be found and identified by other network objects. A name can exist at different locations; an address is a specific logical or physical place at which names can be located. The systems that accomplish internetworking make use of addresses, not names. Names, on the other hand, are more easily understood by the human users of networks, so there must be a provision for translating names for the use of automated network elements. A subscriber's initial business relationship to a telephone company, for example, is based on a human name, which is then associated with a unique account number. The account number is then associated with one or more software-defined unique telephone numbers, and these telephone numbers are then associated with real wires at real locations. The business office deals with names, which may not be unique, and logical telephone number assignments. Plant personnel, however, deal with wires and other physical facilities that map to the telephone number. Most people accept that it would be impractical to label wires in a wire closet with the subscriber's name; a mapping to a telephone number would be all that is reasonably expected. Unfortunately, many data network users assume that name and address registration is a simple process in which an address is associated with a hardware address. Realistically, it is at least a two-step process, where a name is associated with a network layer address, and a network address is associated with a hardware address. Mapping Names to Addresses The names visible to users map to upper layer addresses. In X.400 this is done precisely, but not simply. The name Howard Berkowitz, for example, might map to an X.400 messaging address of C CA, ADMD DATAPAC, Private Management Domain PSC, SN BERKOWITZ. More complex mappings are possible, such as expansion of the name “Washington Office” to several individuals' X.400 addresses. Name-to-address mappings may involve organization policy decisions: If Mary Jones is promoted and transferred, should business messages addressed to her be routed to her at her new location, or to her Previous screen successor in her old job? People now put X.400 addresses on business cards, but X.400 may not become more popular until better directory tools free users from the details of addresses. If a relatively simple Internet name of hcb@ world.std.com is too complex for many people, an X.400 address with fields for country, administrative management domain, PRMD, organization, organization unit, subunit, and personal name can be overwhelming. X.400 addresses meet real technical needs; the problem is to improve the toosl for using these addresses rather than changing the address structure. Below the routing functions performed by the messaging service, which are geared to delivering the message to the end-user, are packet-oriented network layer routers and switches. Either a directory service or manually created table is necessary to map addresses between the upper layers and lower layers. If only a single address applies to upper and lower layers, a user's electronic mail address would have to change whenever that user moved to a new desk or used a laptop to read mail while traveling. Addressing conventions need to be more complex than, for example, the relatively simple rules that apply to human names. Humans can use context to decide with which of several people with identical names they wish to speak. Current computers, however, need to be told explicitly that “Mary Smith, the CEO,” is different from“Mary Smith in Accounting.” The most common and practical means of ensuring name and address uniqueness is to manage them in a hierarchy. A precedent for this exists in international telephony. An organization having international authority assigns country codes and then delegates the next level of addresses to national organizations. National organizations in turn assign area codes and delegate actual telephone number assignment to a subordinate organization. The scope in which an address authority can assign unique addresses is called a domain. A complete real-world address consists of segments that correspond to nested domains and subdomains; in practice, a real-world globally unique address usually contains a domain identifier as well as the addresses specific to those domains. The problem is not so much the theoretical maximum of network addresses as the number of unusable addresses created by an addressing scheme. If the country identifier forms the top level of the address, enough addresses have to be assigned to meet the needs of each country. If an addressing scheme supplies a country like the US with enough addresses and the same number of addresses are reserved for a country with much more modest requirements, many addresses will be wasted. In the current Internet addressing plan, the basic quanta of address assignment are blocks of approximately 65,000 or 250addresses. The former is too large for most organizations to fill; the latter is too small for many requirements. Mobile Addressing Considerations An evolving, and not completely solved, addressing problem becomes more apparent as cellular telephony and other mobile technologies become more common. As a car phone moves, it travels through a series of cells, or small radio service areas that connect the phone to the telephone network. Each cell has a number, and, while a cellular phone is in a cell, the phone uses a specific frequency between itself and the cellular telephone switch. Cellular switches are designed to keep track of the changing relationship between the persistently assigned telephone number and the transient cell number/frequency pair. While conventional network layer protocols can deal with an address (e.g., a LAN MAC address) below the network layer, these protocols do not assume this hardware address will be Previous screen dynamic. Application-Level Addressing If real networked applications are to work, system administrators must deal with both application-level and network-level naming and addressing. For example, if an organization wanted to be addressable for Internet electronic messaging, it would need both a domain name(e.g., whitehouse.gov) and an IP address for that domain. Users sometimes incorrectly assume that directory or other management tools can generate one given the other. The main global addressing need is for globally unique messaging addresses. The two main global addressing systems for messaging are those of the Open Systems Interconnection X.400 standard and the Internet's Domain Naming System. Both schemes are hierarchical, and the two can interwork by using a gateway to convert addresses from one scheme to the other. High-level Internet Domain Naming Service addresses are obtained from the Internet'sNetwork Information Center; subordinate addresses are managed by the user or user organization. Address assignment for X.400 is more complex. The International Telephone and Telegraph Consultative Committee(CCITT) authorizes certain organizations to act as X.400 national naming authorities, which can be national telecommunications monopolies, other national addressing authorities, or carriers offering public X.400 services. They are assigned address components at the Administrative Management Domain (ADMD) level. They, as national authorities, or ADMD owners, can in turn assign Private Management Domain (PRMD) address components. PRMD addresses directly assigned to user organizations by a national telecommunications monopoly may duplicate PRMD assigned by other Administrations. This does not create a practical problem, because an ADMD is not necessary if communication is directly betweenPRMD not subordinate to ADMDs. For example, if airline A connects its X.400-based reservation system directly to that of airline B, there is no need for ADMDs as long as both airlines' PRMD are assigned by the national body, or are assigned by the national bodies of different countries. Such directory services as X.500 accept either name or high-level address arguments and return the appropriate network address. They can also do pure high-level name-to-address translation, distribution list expansion, and mappings based on local policies. Between Application and Network Both Open System Interconnection (OSI) and Internet have a concept of something on the network to which datagram are routed. In Internet protocol (IP), this is the Internet address. In Open Systems Interconnection this is technically the Network Entity Title, but the term Network Service Access Point address is commonly, if not quite precisely, used. To link to an actual transport or higher protocol in an IP host, a supplemental protocol identifier further qualifies the IP address; the combination of IP address and protocol identifier uniquely identifies a user of IP. In OSI, the network service access point (NSAP) address actually is composed of the network entity title (NET) and a selector field. If the selector field has a value of zero, the network service access point (NSAP) address identifies the target of routing(i.e., the specific host interface).