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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.
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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.
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