RFC977:[Previous][Up to Table of Contents] [Next]
|RFC977 1. Introduction|
For many years, the ARPA-Internet community has supported the distribution of bulletins, information, and data in a timely fashion to thousands of participants. We collectively refer to such items of information as "news". Such news provides for the rapid dissemination of items of interest such as software bug fixes, new product reviews, technical tips, and programming pointers, as well as rapid-fire discussions of matters of concern to the working computer professional. News is very popular among its readers. There are popularly two methods of distributing such news: the Internet method of direct mailing, and the USENET news system. RFC977 1.1. Internet Mailing Lists The Internet community distributes news by the use of mailing lists. These are lists of subscriber's mailbox addresses and remailing sublists of all intended recipients. These mailing lists operate by remailing a copy of the information to be distributed to each subscriber on the mailing list. Such remailing is inefficient when a mailing list grows beyond a dozen or so people, since sending a separate copy to each of the subscribers occupies large quantities of network bandwidth, CPU resources, and significant amounts of disk storage at the destination host. There is also a significant problem in maintenance of the list itself: as subscribers move from one job to another; as new subscribers join and old ones leave; and as hosts come in and out of service. RFC977 1.2. The USENET News System Clearly, a worthwhile reduction of the amount of these resources used can be achieved if articles are stored in a central database on the receiving host instead of in each subscriber's mailbox. The USENET news system provides a method of doing just this. There is a central repository of the news articles in one place (customarily a spool directory of some sort), and a set of programs that allow a subscriber to select those items he wishes to read. Indexing, cross-referencing, and expiration of aged messages are also provided. RFC977 1.3. Central Storage of News For clusters of hosts connected together by fast local area networks (such as Ethernet), it makes even more sense to consolidate news distribution onto one (or a very few) hosts, and to allow access to these news articles using a server and client model. Subscribers may then request only the articles they wish to see, without having to wastefully duplicate the storage of a copy of each item on each host. RFC977 1.4. A Central News Server A way to achieve these economies is to have a central computer system that can provide news service to the other systems on the local area network. Such a server would manage the collection of news articles and index files, with each person who desires to read news bulletins doing so over the LAN. For a large cluster of computer systems, the savings in total disk space is clearly worthwhile. Also, this allows workstations with limited disk storage space to participate in the news without incoming items consuming oppressive amounts of the workstation's disk storage. We have heard rumors of somewhat successful attempts to provide centralized news service using IBIS and other shared or distributed file systems. While it is possible that such a distributed file system implementation might work well with a group of similar computers running nearly identical operating systems, such a scheme is not general enough to offer service to a wide range of client systems, especially when many diverse operating systems may be in use among a group of clients. There are few (if any) shared or networked file systems that can offer the generality of service that stream connections using Internet TCP provide, particularly when a wide range of host hardware and operating systems are considered. NNTP specifies a protocol for the distribution, inquiry, retrieval, and posting of news articles using a reliable stream (such as TCP) server-client model. NNTP is designed so that news articles need only be stored on one (presumably central) host, and subscribers on other hosts attached to the LAN may read news articles using stream connections to the news host. NNTP is modelled upon the news article specifications in RFC 850, which describes the USENET news system. However, NNTP makes few demands upon the structure, content, or storage of news articles, and thus we believe it easily can be adapted to other non-USENET news systems. Typically, the NNTP server runs as a background process on one host, and would accept connections from other hosts on the LAN. This works well when there are a number of small computer systems (such as workstations, with only one or at most a few users each), and a large central server. RFC977 1.5. Intermediate News Servers For clusters of machines with many users (as might be the case in a university or large industrial environment), an intermediate server might be used. This intermediate or "slave" server runs on each computer system, and is responsible for mediating news reading requests and performing local caching of recently-retrieved news articles. Typically, a client attempting to obtain news service would first attempt to connect to the news service port on the local machine. If this attempt were unsuccessful, indicating a failed server, an installation might choose to either deny news access, or to permit connection to the central "master" news server. For workstations or other small systems, direct connection to the master server would probably be the normal manner of operation. This specification does not cover the operation of slave NNTP servers. We merely suggest that slave servers are a logical addition to NNTP server usage which would enhance operation on large local area networks. RFC977 1.6. News Distribution NNTP has commands which provide a straightforward method of exchanging articles between cooperating hosts. Hosts which are well connected on a local area or other fast network and who wish to actually obtain copies of news articles for local storage might well find NNTP to be a more efficient way to distribute news than more traditional transfer methods (such as UUCP). In the traditional method of distributing news articles, news is propagated from host to host by flooding - that is, each host will send all its new news articles on to each host that it feeds. These hosts will then in turn send these new articles on to other hosts that they feed. Clearly, sending articles that a host already has obtained a copy of from another feed (many hosts that receive news are redundantly fed) again is a waste of time and communications resources, but for transport mechanisms that are single-transaction based rather than interactive (such as UUCP in the UNIX-world <1>), distribution time is diminished by sending all articles and having the receiving host simply discard the duplicates. This is an especially true when communications sessions are limited to once a day. Using NNTP, hosts exchanging news articles have an interactive mechanism for deciding which articles are to be transmitted. A host desiring new news, or which has new news to send, will typically contact one or more of its neighbors using NNTP. First it will inquire if any new news groups have been created on the serving host by means of the NEWGROUPS command. If so, and those are appropriate or desired (as established by local site-dependent rules), those new newsgroups can be created. The client host will then inquire as to which new articles have arrived in all or some of the newsgroups that it desires to receive, using the NEWNEWS command. It will receive a list of new articles from the server, and can request transmission of those articles that it desires and does not already have. Finally, the client can advise the server of those new articles which the client has recently received. The server will indicate those articles that it has already obtained copies of, and which articles should be sent to add to its collection. In this manner, only those articles which are not duplicates and which are desired are transferred.
|[Source:"RFC977"] [Last Changed:February 1986]|
[Copyright: 1986 Brian Kantor, Phil Lapsley]