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NNTP Client-Server Communication Process: News Posting and Access
(Page 3 of 4)
News Access Methods
There are several different ways
that the newsreader can access messages in a group, depending on how
it is programmed and on what the user of the software wants.
Full Newsgroup Retrieval
This is the brute force
technique: the client simply requests that the server send it all the
messages in the group. This is done by issuing the ARTICLE command
to select the first current message in the group, using the first article
number returned by the GROUP command. This sets the server's
internal pointer for the session to point to the first article, so it
can be retrieved. The NEXT command is then used to advance the
pointer to the next message, and the ARTICLE command to retrieve
it. This continues until the entire group has been read. Figure 313
illustrates the process.
Figure 313: NNTP Full Newsgroup Retrieval Process
There are many ways that an NNTP client can access and read Usenet messages on a server. One common method is to retrieve the entire contents of a newsgroup. In this example, the client uses the GROUP command to select the newsgroup comp.protocols.tcp-ip for reading; the server responds with a 211 group selected reply, which includes important statistics about the group. The client uses the ARTICLE command with the number of the first article in the group, 177, to read it from the server. The server then sends the message line by line, ending it with a single period on a line. The client uses the NEXT command to tell the server to advance its internal article pointer to the next message, which often will not be the next consecutive number after the one just read; here it is 179. The client can then read that message by sending the ARTICLE command by itself; since no parameters are given the server returns the current message (#179).
The retrieved messages
are stored by the newsreader and available for instant access by the
user. This method is most suitable for relatively small newsgroups and/or
users with fast Internet connections.
Newsgroup Header Retrieval
Since downloading an entire newsgroup
is time-consuming, many newsreaders compromise by downloading the headers
of all messages instead of the full message. The process is the same
as for full newsgroup retrieval, but the HEAD command is used
to retrieve just an article's headers instead of the ARTICLE
command. This takes less time than retrieving each message in its entirety.
The XHDR command extension
can also be used, if the server supports it, to more efficiently retrieve
only a subset of the headers for the messages, such as the subject line
Individual Article Retrieval
It is also possible to retrieve a
single message from a group, using the ARTICLE command and specifying
the article's message identifier.
Key Concept: While NNTP is best known for its role in inter-server propagation, it is also used by Usenet clients to write and read articles. A number of different commands provides flexibility in how articles can be read by a client device. A client can retrieve an entire newsgroup, only a set of newsgroup headers, or individual articles. Other commands also support various administrative functions.
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Version 3.0 - Version Date: September 20, 2005
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