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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  Usenet (Network News) and the TCP/IP Network News Transfer Protocol (NNTP)
                     9  TCP/IP Network News Transfer Protocol (NNTP)

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NNTP Overview and General Operation
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NNTP Client-Server Communication Process: News Posting and Access
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NNTP Inter-Server Communication Process: News Article Propagation
(Page 3 of 4)

Article Propagation Using the "Push" Model

Here’s how the push model works. When the administrators of an NNTP server establish a service relationship with an upstream Usenet service provider, they furnish the provider with a list of newsgroups that the downstream server wants to carry. Whenever a new article arrives at the upstream server within that list of groups, it is automatically sent to the downstream site. This saves the downstream server from constantly having to ask “has anything arrived?”

In the classical NNTP protocol as defined in RFC 977, the exchange of articles is based on the push model, and performed using the IHAVE command. Suppose three new messages arrive at the largenews server. It would establish an NNTP connection to mediumnews and use IHAVE to provide the message IDs of each of the three new messages, one at a time. The mediumnews server would respond to each one indicating whether it already had that message or not. If not, largenews would send it the message. An example article transaction using the push model of propagation is illustrated in Figure 312.


Figure 312: NNTP Article Propagation Using The “Push” Model

This example shows how Usenet articles are moved between servers using the conventional “push” model of propagation. Here, the device acting as an NNTP client (which recall may in fact be an NNTP server) has two messages available to offer to the server. It sends the IHAVE command specifying the message ID of the first message, but the server already has that message so it sends a 435 “do not send” reply. The client then issues an IHAVE with the second message ID; the server wants this one, so it sends a 335 reply; the client sends the Usenet message, ending with a single period on a line by itself. The server indicates that it received the message and the client, done with its transactions, quits the session.

 


The main advantage of this technique is that it ensures that a server is not sent a duplicate copy of a message that it already has. The problem with it in modern Usenet is that it is slow, because the server must respond to the IHAVE command before the message or the next command can be sent by the client.


Previous Topic/Section
NNTP Overview and General Operation
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Pages in Current Topic/Section
12
3
4
Next Page
NNTP Client-Server Communication Process: News Posting and Access
Next Topic/Section

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