uncertainty that determines if experimentation makes sense from the point of view of meeting user needs.
normal phone numbers. When the network sees the 800 number, it sends a message across the SS7 network to query the 800 database; the result is a phone number the network knows how to route. This is an example of a centralized structure used to provide a name resolution protocol to trans- late the 800 number into an area code and telephone number. This man- agement structure is centralized because a central authority manages both the physical 800-number servers and the data on the server. This is not the only way to structure such a protocol, as seen in (b), which illustrates the distributed structure of the Domain Name Server service in the Internet. Its management structure is distributed because both the DNS servers and the data on the DNS server are managed by the local organization. For exam- ple, if a host in the bu.edu domain seeks the IP address for a host in some other domain under the .com prefix, then the DNS sends a series of mes- sages, eventually reaching a DNS server managed within the domain of the requested address.
Figure 2.1 Distributed compared to centralized management structure of network-based services.
800 Server
Controlled by a third party
phone switch SS7
(a)
800 # Routing
(b)
phone switch
root server
com server
Other
harvard server Harvard
eecs server IP
DNS
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Many services with centralized management, such as the 800-number lookup, provide a carrier revenue opportunity. This is unlike the DNS ser- vice where a revenue model does not make business sense. Centralized management lends itself to billing for services; for example, the centralized architecture of the 800-number lookup makes it easy to track use because 800-number lookup requests go to a few centralized servers that are man- aged by a central authority. Logging their use is easy because the informa- tion about who is doing what is centralized. The distributed nature of the DNS architecture would make billing much harder. Most low-level DNS servers responsible for local IP addresses are managed by the local organi- zation. The information about usage is very distributed, which adds com- plexity to billing. DNS caching creates more billing headaches because it distributes usage information even more. Centrally managed services, such as 800-number lookup, lend themselves to easy billing of the service, but services with a distributed management structure, such as DNS, make it difficult to know what users are doing, which creates billing problems.
The DNS is a service with a distributed management structure that is jus- tifiable even in the absence of market uncertainty. Because organizations value control over the data that defines their internal address space, dis- tributed management of DNS is desirable for many organizations. DNS is a distributed database used to resolve a name to IP address, a service sim- ilar to the 800 lookup service, but with a different management structure.
This underscores that services with similar functions may be architec- turally dissimilar.
The different ways that email can be stored and routed are a good illus- tration of how both centralized and distributed management structures can provide similar services. Chapter 8 will show that initially Internet email was completely end-2-end. Later, a partially centralized and par- tially distributed architecture for Internet email used POP or IMAP2 to fetch email messages from a remote server; the retrieved emails were then managed by the local system. New Web-based email systems forgo the POP/IMAP interface and access a central database of email via a Web interface. This (central) management of messages occurs on the email server, not on the local system.
Different networks, such as the PSTN and the Internet, have different infrastructures that tend to promote different styles of management, but each network allows some services to be provided in either a centralized or a distributed structure. For example, as discussed in Chapter 9, there are different ways to provide basic and advanced voice services in the PSTN.
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2IMAP, which came after POP, is more centralized because it allows the remote server user to view the message’s subject while the message is still managed by the IMAP server.
Users might choose a PBX at the user’s site (distributed) or a Centrex service provided within the Central Office (CO) of the local phone company (cen- tralized). Internet email may have either a distributed or a centralized management structure. A user running Sendmail on a desktop or using POP to communicate to an email server is a more distributed situation than a user employing a Web-based email system such as Hotmail or Yahoo!
mail. Web-based email is more centralized for two reasons. First, its servers are managed by a centralized organization. Second, the central server, not the user desktop, manages email messages. Again, these examples illus- trate how networks with a centralized infrastructure are able to provide services managed with a distributed architecture and how networks with a distributed infrastructure, such as the Internet, are able to provide services with a centralized structure.
The scope of management relative to the organizational affiliation of the users is one attribute of a management structure that helps determine how centralized the structure is. In the preceding 800-number example, a central authority manages the server for all users, regardless of where the user (or his or her organization) is or where the called 800 number resides. In con- trast, the likely manager of the DNS server responsible for the low-level IP address-to-name resolution is the organization that manages the host of the requested IP address. In the first example, the manager of the server has nothing to do with either user. In the DNS example, however, the manager of the first server queried is in the domain of the requesting host, while the management of the server that finally responds with the requested data is in the domain of the host with the destination address. In email, the situation is similar; ISP email is managed by the ISP. Its users are paying members of the ISP community. Web-based email, though, is managed by a central authority (for example, the management of Hotmail); its users don’t belong to any common ISP. The important point is this: There are no distributed organizations managing the service. Scope of management is one way to classify the management structure of a network-based service.
Another attribute of a management structure is the style in which it man- ages its data. In the case of the 800 service, a central manager is responsible for the data regardless of where the caller or called party resides. In con- trast to this is the management of the DNS (Figure 2.1(b)), where data is most likely to be managed in the same domain as the targeted host. ISP email is similar because, in most cases, management of email messages is on the local machine where the email is read. In contrast, with Web-based email, the central server, not the user’s machine, manages the messages.
These examples show how the style of data management can help classify the management as either distributed or centralized.
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