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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)

NNTP Overview and General Operation 1 2 3 4 NNTP Client-Server Communication Process: News Posting and Access

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.

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.

NNTP Overview and General Operation 1 2 3 4 NNTP Client-Server Communication Process: News Posting and Access