How Does Usenet Handle News?Next: C-News
Up: Netnews
Previous: What is Usenet Anyway?How Does Usenet Handle News?
Today, Usenet has grown to enormous proportions. Sites that carry
the whole of netnews usually transfer something like a paltry sixty megabytes a day. Of course this requires much more than pushing around files. So let's
take a look at the way most systems handle Usenet news.News is distributed through the net by various transports. The
historical medium used to be UUCP, but today the main traffic is carried
by Internet sites. The routing algorithm used is called flooding:
Each site maintains a number of links (news feeds) to other sites.
Any article generated or received by the local news system is forwarded
to them, unless it has already been seen at that site, in which case it
is discarded. A site may find out about all other sites the article has
already traversed by looking at the Path: header field. This
header contains a list of all systems the article has been forwarded by
in bang path notation.To distinguish articles and recognize duplicates, Usenet articles have
to carry a message id (specified in the Message-Id: header
field), which combines the posting site's name and a serial number into
``<serial@site>''. For each article processed, the
news system logs this id into a history file against which all
newly arrived articles are checked.The flow between any two sites may be limited by two criteria: for one,
an article is assigned a distribution (in the Distribution:
header field) which may be used to confine it to a certain group of
sites. On the other hand, the newsgroups exchanged may be limited by
both the sending or receiving system. The set of newsgroups and
distributions allowed for transmission to a site are usually kept in the
sys file.The sheer number of articles usually requires that improvements be made
to the above scheme. On UUCP networks, the natural thing to do is to
collect articles over a period of time, and combine them into a single
file, which is compressed and sent to the remote site. This is called
batching.An alternative technique is the ihave/sendme protocol that
prevents duplicate articles from being transferred in the first place,
thus saving net bandwidth. Instead of putting all articles in batch
files and sending them along, only the message ids of articles are
combined into a giant ``ihave'' message and sent to the remote site. It
reads this message, compares it to its history file, and returns the
list of articles it wants in a ``sendme'' message. Only these articles
are then sent.Of course, ihave/sendme only makes sense if it involves two big sites
that receive news from several independent feeds each, and who poll each
other often enough for an efficient flow of news.Sites that are on the Internet generally rely on TCP/IP-based software
that uses the Network News Transfer Protocol, NNTP. It transfers news between feeds and provides Usenet access to single
users on remote hosts.NNTP knows three different ways to transfer news. One is a real-time
version of ihave/sendme, also referred to as pushing news. The
second technique is called pulling news, in which the client
requests a list of articles in a given newsgroup or hierarchy that have
arrived at the server's site after a specified date, and chooses those
it cannot find in its history file. The third mode is for interactive
newsreading, and allows you or your newsreader to retrieve articles from
specified newsgroups, as well as post articles with incomplete header
information.At each site, news are kept in a directory hierarchy below /var/spool/news,
each article in a separate file, and each newsgroup in a separate
directory. The directory name is made up of the newsgroup name, with
the components being the path components. Thus,
comp.os.linux.misc articles are kept in
/var/spool/news/comp/os/linux/misc. The articles in a newsgroup are
assigned numbers in the order they arrive. This number serves as the
file's name. The range of numbers of articles currently online is kept
in a file called active, which at the same time serves as a list
of newsgroups known at your site.Since disk space is a finite resource, one has to start throwing away articles after some time. This is
called expiring. Usually, articles from certain groups and
hierarchies are expired at a fixed number of days after they arrive.
This may be overridden by the poster by specifying a date of expiration
in the Expires: field of the article header.Next: C-News
Up: Netnews
Previous: What is Usenet Anyway?Andrew AndersonThu Mar 7 23:22:06 EST 1996t
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