How are new articles passed from one Usenet news server to another? Describe all the connections and commands that might be used. Discuss problems with the methods described.


(Note: This reply contains considerably more information than was required for full score on the exam.)

There are several ways to do this.

Method (1) is that the recieving server connects to the server with news and uses the NEWNEWS command to get a list of new articles. It then checks if it already has some of these articles, and gets the rest with a series of GROUP and ARTICLE command.

Method (2) is that the receiving server connects and issues a LIST command. Based on knowledge of which articles the receiving server has already received, it can then use a series of GROUP and ARTICLE commands to get the articles it does not yet have.

Method (3) is for the server with news to connect to the receiving server and use the IHAVE command to list all new articles which have arrived, and the receiving server responds to each IHAVE command with indication whether it wants that article or not.

Method (4) is to send all news in a batch operation of UUCP over TCP, or even to use mail for this purpose.

Method (5) is to allow remote execution (rnews) from one server to another.

Method (1) is rarely used, because it is often akward for servers, and many servers have disabled this command.

Since a news server may receive articles from more than one other server, it is important to reject multiple copies of the same article. To avoid sending data not needed, the receiving server will, with method (2) often first use the HEAD command to look at the article header and check if the server already has a message with this Message-ID, before downloading the full text. A non-standard, but commonly used command XOVER will get the Message-IDs in a faster way.

All methods have a problem in that to transfer a large amount of articles, many interactions back and forward are needed. To avoid delay time, pipelining (streaming) is necessary. The IHAVE method can get a new document available faster on the receiving server, since the server with news can initiate a connection immediately when a new article has arrived. The IHAVE method tends to cause smaller downloads more often, which is an advantage, since large downloads can take a long time. A disadvantage with IHAVE is that the sending server has to keep a list of which newsgroups are wanted by each receiving server.

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