 |
|
2. Principles of Operation
Here we discuss the basic concepts behind the operation of a Usenet news system.
2.1. Newsgroups and articles
A Usenet news article sits in a file or in some other on-disk data structure on the disks of a Usenet server, and its contents look like this:
Xref: news.starcomsoftware.com starcom.tech.misc:211 starcom.tech.security:452 Newsgroups: starcom.tech.misc,starcom.tech.security Path: news.starcomsoftware.com!purva!shuvam From: Shuvam <shuvam@starcomsoftware.com> Subject: "You just throw up your hands and reboot" (fwd) Content-Type: TEXT/PLAIN; charset=US-ASCII Distribution: starcom Organization: Starcom Software Pvt Ltd, India Message-ID: <Pine.LNX.4.31.0107022153490.30462-100000@starcomsoftware.com> Mime-Version: 1.0 Date: Mon, 2 Jul 2001 16:27:57 GMT Interesting quote, and interesting article. Incidentally, comp.risks may be an interesting newsgroup to follow. We must be receiving the feed for this group on our server, since we receive all groups under comp.*, unless specifically cancelled. Check it out sometime. comp.risks tracks risks in the use of computer technology, including issues in protecting ourselves from failures of such stuff. Shuvam > Date: Thu, 14 Jun 2001 08:11:00 -0400 > From: "Chris Norloff" <cnorloff@norloff.com> > Subject: NYSE: "Throw up your hands and reboot" > > When the New York Stock Exchange computer systems crashed for 85 > minutes (8 Jun 2001), Andrew Brooks, chief of equity trading at > Baltimore mutual fund giant T. Rowe Price, was quoted as saying "Hey, > we're all subject to the vagaries of technology. It happens on your > own PC at home. You just throw up your hands and reboot." > > http://www.washingtonpost.com/ac3/ContentServer?articleid=A42885-2001Jun8&pagename=article > > Chris Norloff > > > This is from -- > > From: risko@csl.sri.com (RISKS List Owner) > Newsgroups: comp.risks > Subject: Risks Digest 21.48 > Date: Mon, 18 Jun 2001 19:14:57 +0000 (UTC) > Organization: University of California, Berkeley > > RISKS-LIST: Risks-Forum Digest Monday 19 June 2001 > Volume 21 : Issue 48 > > FORUM ON RISKS TO THE PUBLIC IN COMPUTERS AND RELATED SYSTEMS (comp.risks) > ACM Committee on Computers and Public Policy, > Peter G. Neumann, moderator > > This issue is archived at <URL:http://catless.ncl.ac.uk/Risks/21.48.html> > and by anonymous ftp at ftp.sri.com, cd risks . > |
A Usenet article's header is very interesting if you want to learn about the functioning of the Usenet. The From, Subject, and Date headers are familiar to anyone who has used email. The Message-ID header contains a unique ID for each message, and is present in each email message, though not many non-technical email users know about it. The Content-Type and Mime-Version headers are used for MIME encoding of articles, attaching files and other attachments, and so on, just like in email messages.
The Organisation header is an informational header which is supposed to carry some information identifying the organisation to which the author of the article belongs. What remains now are the Newsgroups, Xref, Path and Distributions headers. These are special to Usenet articles and are very important.
The Newsgroups header specifies which newsgroups this article should belong to. The Distributions header, sadly under-utilised in today's globalised Internet world, allows the author of an article to specify how far the article will be re-transmitted. The author of an article, working in conjunction with well-configured networks of Usenet servers, can control the ``radius'' of replication of his article, thus posting an article of local significance into a newsgroup but setting the Distribution header to some suitable setting, e.g. local or starcom, to prevent the article from being relayed to servers outside the specified domain.
The Xref header specifies the precise article number of this article in each of the newsgroups in which it is inserted, for the current server. When an article is copied from one server to another as part of a newsfeed, the receiving server throws away the old Xref header and inserts its own, with its own article numbers. This indicates an interesting feature of the Usenet system: each article in a Usenet server has a unique number (an integer) for each newsgroup it is a part of. Our sample above has been added to two newsgroups on our server, and has the article numbers 211 and 452 in those groups. Therefore, any Usenet client software can query our server and ask for article number 211 in the newsgroup starcom.tech.misc and get this article. Asking for article number 452 in starcom.tech.security will fetch the article too. On another server, the numbers may be very different.
The Path specifies the list of machines through which this article has travelled before it has reached the current server. UUCP-style syntax is used for this string. The current example indicates that a user called shuvam first wrote this article and posted it onto a computer which calls itself purva, and this computer then transferred this article by a newsfeed to news.starcomsoftware.com. The Path header is critical for breaking loops in newsfeeds, and will be discussed in detail later.
Our sample article will sit in the two newsgroups listed above forever, unless expired. The Usenet software on a server is usually configured to expire articles based on certain conditions, e.g. after it's older than a certain number of days. The C-News software we use allows expiry control based on the newsgroup hierarchy and the type of newsgroup, i.e. moderated or unmoderated. Against each class of newsgroups, it allows the administrator to specify a number of days after which the article will be expired. It is possible for an article to control its own expiry, by carrying an Expires header specifying a date and time. Unless overriden in the Usenet server software, the article will be expired only after its explicit expiry time is reached.
2.2. Of readers and servers
Computers which access Usenet articles are broadly of two classes: the readers and the servers. A Usenet server carries a repository of articles, manages them, handles newsfeeds, and offers its repository to authorised readers to read. A Usenet reader is merely a computer with the appropriate software to allow a user to access a software, fetch articles, post new articles, and keep track of which articles it has read in each newsgroup. In terms of functionality, Usenet reading software is less interesting to a Usenet administrator than a Usenet server software. However, in terms of lines of code, the Usenet reader software can often be much larger than Usenet server software, primarily because of the complexities of modern GUI code.
Most modern computers almost exclusively access Usenet servers using the NNTP (Network News Transfer Protocol) for reading and posting. This protocol can also be used for inter-server communication, but those aspects will be discussed later. The NNTP protocol, like any other well-designed TCP-based Internet protocol, carries ASCII commands and responses terminated with CR-LF, and comprises a sequence of commands, somewhat reminiscent of the POP3 protocol for email. Using NNTP, a Usenet reader program connects to a Usenet server, asks for a list of active newsgroups, and receives this (often huge) list. It then sets the ``current newsgroup'' to one of these, depending on what the user wants to browse through. Having done this, it gets the meta-data of all current articles in the group, including the author, subject line, date, and size of each article, and displays an index of articles to the user.
The user then scans through this list, selects an article, and asks the reader to fetch it. The reader gives the article number of this article to the server, and fetches the full article for the user to read through. Once the user finishes his NNTP session, he exits, and the reader program closes the NNTP socket. It then (usually) updates a local file in the user's home area, keeping track of which news articles the user has read. These articles are typically not shown to the user next time, thus allowing the user to progress rapidly to new articles in each session. The reader software is helped along in this endeavour by the Xref header, using which it knows all the different identities by which a single article is identified in the server. Thus, if you read the sample article given above by accessing starcom.tech.misc, you'll never be shown this article again when you access starcom.tech.misc or starcom.tech.security; your reader software will do this by tracking the Xref header and mapping article numbers.
When a user posts an article, he first composes his message using the user interface of his reader software. When he finally gives the command to send the article, the reader software contacts the Usenet server using the pre-existing NNTP connection and sends the article to it. The article carries a Newsgroups header with the list of newsgroups to post to, often a Distribution header with a distribution specification, and other headers like From, Subject etc. These headers are used by the server software to do the right thing. Special and rare headers like Expires and Approved are acted upon when present. The server assigns a new article number to the article for each newsgroup it is posted to, and creates a new Xref header for the article.
Transfer of articles between servers is done in various ways, and is discussed in quite a bit of detail in Section XXX titled ``Newsfeeds'' below.
2.3. Newsfeeds
2.3.1. Fundamental concepts
When we try to analyse newsfeeds in real life, we begin to see that, for most sites, traffic flow is not symmetrical in both directions. We usually find that one server will feed the bulk of the world's articles to one or more secondary servers every day, and receive a few articles written by the users of those secondary servers in exchange. Thus, we usually find that articles flow down from the stem to the branches to the leaves of the worldwide Usenet server network, and not exactly in a totally balanced mesh flow pattern. Therefore, we use the term ``upstream server'' to refer to the server from which we receive the bulk of our daily dose of articles, and ``downstream server'' to refer to those servers which receive the bulk dose of articles from us.
Newsfeeds relay articles from one server to their ``next door neighbour'' servers, metaphorically speaking. Therefore, articles move around the globe, not by a massive number of single-hop transfers from the originating server to every other server in the world, but in a sequence of hops, like passing the baton in a relay race. This increases the latency time for an article to reach a remote tertiary server after, say, ten hops, but it allows tighter control of what gets relayed at every hop, and helps in redundancy, decentralisation of server loads, and conservation of network bandwidth. In this respect, Usenet newsfeeds are more complex than HTTP data flows, which typically use single-hop techniques.
Each Usenet news server therefore has to worry about newsfeeds each time it receives an article, either by a fresh post or from an incoming newsfeed. When the Usenet server digests this article and files it away in its repository, it simultaneously looks through its database to see which other server it should feed the article to. In order to do this, it carries out a sequence of checks, described below.
Each server knows which other servers are its ``next door neighbours;'' this information is kept in its newsfeed configuration information. Against each of its ``next door neighbours,'' there will be a list of newsgroups which it wants, and a list of distributions. The new article's list of newsgroups will be matched against the newsgroup list of the ``next door neighbour'' to see whether there's even a single common newsgroup which makes it necessary to feed the article to it. If there's a matching newsgroup, and the server's distribution list matches the article's distribution, then the article is marked for feeding to this neighbour.
When the neighbour receives the article as part of the feed, it performs some sanity checks of its own. The first check it performs is on the Newsgroups header of the new article. If none of the newsgroups listed there are part of the active newsgroups list of this server, then the article can be rejected. An article rejected thus may even be queued for outgoing feeds to other servers, but will not be digested for incorporation into the local article repository.
The next check performed is against the Path header of the incoming article. If this header lists the name of the current Usenet server anywhere, it indicates that it has already passed through this server at least once before, and is now re-appearing here erroneously because of a newsfeed loop. Such loops are quite often configured into newsfeed topologies for redundancy: ``I'll get the articles from Server X if not Server Y, and may the first one in win.'' The Usenet server software automatically detects a duplicate feed of an article and rejects it.
The next check is against what is called the server's history database. Every Usenet server has a history database, which is a list of the message IDs of all current articles in the local repository. Oftentimes the history database also carries the message IDs of all messages recently expired. If the incoming article's message ID matches any of the entries in the database, then again it is rejected without being filed in the local repository. This is a second loop detection method. Sometimes, the mere checking of the article's Path header does not detection of all potential problems, because the problem may be a re-insertion instead of a loop. A re-insertion happens when the same incoming batch of news articles is re-fed into the local server, perhaps after recovering the system's data from tapes after a system crash. In such cases, there's no newsfeed loop, but there's still the risk that one article may be digested into the local server twice. The history database prevents this.
All these simple checks are very effective, and work across server and software types, as per the Internet standards. Together, they allow robust and fail-safe Usenet article flow across the world.
2.3.2. Types of newsfeeds
This section explains the basics of newsfeeds, without getting into details of software and configuration files.
2.3.2.1. Queued feeds
This is the commonest method of sending articles from one server to another, and is followed whenever large volumes of articles are to be transferred per day. This approach needs a one-time modification to the upstream server's configuration for each outgoing feed, to define a new queue.
In essence all queued feeds work in the following way. When the sending server receives an article, it processes it for inclusion into its local repository, and also checks through all its outgoing feed definitions to see whether the article needs to be queued for any of the feeds. If yes, it is added to a queue file for each outgoing feed. The precise details of the queue file can change depending on the software implementation, but the basic processes remain the same. A queue file is a list of queued articles, but does not contain the article contents. Typical queue files are ASCII text files with one line per article giving the path to a copy of the article in the local spool area.
Later, a separate process picks up each queue file and creates one or more batches for each outgoing feed. A batch is a large file containing multiple Usenet news articles. Once the batches are created, various transport mechanisms can be used to move the files from sending server to receiving server. You can even use scripted FTP. You only need to ensure that the batch is picked up from the upstream server and somehow copied into a designated incoming batch directory in the downstream server.
UUCP has traditionally been the mechanism of choice for batch movement, because it predates the Internet and wide availability of fast packet-switched data networks. Today, with TCP/IP everywhere, UUCP once again emerges as the most logical choice of batch movement, because it too has moved with the times: it can work over TCP.
NNTP is the de facto mechanism of choice for moving queued newsfeeds for carrier-class Usenet servers on the Internet, and unfortunately, for a lot of other Usenet servers as well. The reason why we find this choice unfortunate is discussed in Section 12.1> below. But in NNTP feeds, an intermediate step of building batches out of queue files can be eliminated --- this is both its strength and its weakness.
In the case of queued NNTP feeds, articles get added to queue files as described above. An NNTP transmit process periodically wakes up, picks up a queue file, and makes an NNTP connection to the downstream server. It then begins a processing loop where, for each queued article, it uses the NNTP IHAVE command to inform the downstream server of the article's message~ID. The downstream server checks its local repository to see whether it already has the message. If not, it responds with a SENDME response. The transmitting server then pumps out the article contents in plaintext form. When all articles in the queue have been thus processed, the sending server closes the connection. If the NNTP connection breaks in between due to any reason, the sending server truncates the queue file and retains only those articles which are yet to be transmitted, thus minimising repeat transmissions.
> A queued NNTP feed works with the sending server making an NNTP connection to the receiving server. This implies that the receiving server must have an IP address which is known to the sending server or can be looked up in the DNS. If the receiving server connects to the Internet periodically using a dialup connection and works with a dynamically assigned IP address, this can get tricky. UUCP feeds suffer no such problems because the sending server for the newsfeed can be the UUCP server, i.e. passive. The receiving server for the feed can be the UUCP master, i.e. the active party. So the receiving server can then initiate the UUCP connection and connect to the sending server. Thus, if even one of the two parties has a static IP address, UUCP queued feeds can work fine.
Thus, NNTP feeds can be sent out a little faster than the batched transmission processes used for UUCP and other older methods, because no batches need to be constructed. However, NNTP is often used in newsfeeds where it is not necessary and it results in colossal waste of bandwidth. Before we study efficiency issues of NNTP versus batched feeds, we will cover another way feeds can be organised using NNTP: the pull feeds.
2.3.2.2. Pull feeds
This method of transferring a set of articles works only over NNTP, and requires absolutely no configuration on the transmitting, or upstream, server. In fact, the upstream server cannot even easily detect that the downstream server is pulling out a feed --- it appears to be just a heavy and thorough newsreader, that's all.
This pull feed works by the downstream server pulling out articles i one by one, just like any NNTP newsreader, using the NNTP ARTICLE command with the Message-ID as parameter. The interesting detail is how it gets the message~IDs to begin with. For this, it uses an NNTP command, specially designed for pull feeds, called NEWNEWS. This command takes a hierarchy and a date,
NEWNEWS comp 15081997 |
This command is sent by the downstream server over NNTP to the upstream server, and in effect asks the upstream server to list out all news articles which are newer than 15 August 1997 in the comp hierarchy. The upstream server responds with a (often huge) list of message~IDs, one per line, ending with a period on a line by itself.
The pulling server then compares each newly received message~ID with its own article database and makes a (possibly shorter) list of all articles which it does not have, thus eliminating duplicate fetches. That done, it begins fetching articles one by one, using the NNTP ARTICLE command as mentioned above.
In addition, there is another NNTP command, NEWGROUPS, which allows the NNTP client --- i.e. the downstream server in this case --- to ask its upstream server what were the new newsgroups created since a given date. This allows the downstream server to add the new groups to its active file.
The NEWNEWS based approach is usually one of the most inefficient methods of pulling out a large Usenet feed. By inefficiency, here we refer to the CPU loads and RAM utilisation on the upstream server, not on bandwidth usage. This inefficiency is because most Usenet news servers do not keep their article databases indexed by hierarchy and date; CNews certainly does not. This means that a NEWNEWS command issued to an upstream server will put that server into a sequential search of its article database, to see which articles fit into the hierarchy given and are newer than the given date.
If pull feeds were to become the most common way of sending out articles, then all upstream servers would badly need an efficient way of sorting their article databases to allow each NEWNEWS command to rapidly generate its list of matching articles. A slow upstream server today might take minutes to begin responding to a NEWNEWS command, and the downstream server may time out and close its NNTP connection in the meanwhile. We have often seen this happening, till we tweak timeouts.
There are basic efficiency issues of bandwidth utilisation involved in NNTP for news feeds, which are applicable for both queued and pull feeds. But the problem with NEWNEWS is unique to pull feeds, and relates to server loads, not bandwidth wastage.
2.4. Control messages
The Usenet is a massive dispersed collection of servers which operate almost without any supervision, provided they have adequate disk space and do not suffer disk corruption due to power failures, etc. (It is indeed surprising how self-managing a good Usenet server is, provided these two pre-requisites are met.) These servers are each under the control of human administrators, but it is preferable that certain routine actions be performed across all these servers remotely from one location, without the manual intervention of these humans.
One common need for centralised operations is the creation of new groups in the standard eight hierarchies. The Usenet follows a fairly formal process which asks for votes from readers worldwide before deciding on the restructuring of its newsgroups list, including merging of low-volume groups, splitting of high-volume groups into many specialised groups, creating new groups, and even deleting groups. Once the voting process for a change concludes and the change action is to be carried out, it would be extremely tedious to send email to the hundreds of thousands of Usenet administrators and hope that they make the changes right, and answer their doubts if they get confused. It would be much better to have an automatic way to make the changes across all servers, of course with proper authorisation.
The solution to this does not lie in giving some central authority the ability to run an OS-level command of his choice on all the world's Usenet servers, because OS commands differ from OS to OS, and because few Usenet administrators would trust a stranger from another part of the world with OS level access. Therefore, the solution lay in defining a small set of common Usenet maintenance actions, and permitting only these actions to be triggered on all servers through the passing of special command messages, called control messages.
Control messages look like ordinary Usenet articles, more or less. They have an extra header line, with its value in a specific format, but they usually carry body text which looks like a normal human-written article. Here is a control message (a spurious one at that, but it'll do for now):
Xref: news.starcomsoftware.com control:814217
Path: news.starcomsoftware.com!linux594.dn.net!news.dn.hoopoo.com!
feed-out.newsfeeds.com!newsfeeds.com!feed.newsfeeds.com!
newsfeeds.com!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!
newsfeed.icl.net!newsfeed.skycache.com!Cidera!newsfeed.gamma.ru!
Gamma.RU!carrier.kiev.ua!goblin.nadrabank.kiev.ua!not-for-mail
From: tale@uunet.uu.net (David C Lawrence)
Newsgroups: news.groups,humanities.hipcrime
Subject: cmsg newgroup humanities.hipcrime
Control: newgroup humanities.hipcrime
Date: Sun, 18 Feb 2001 11:50:28 GMT
Organization: The Cabal
Lines: 20
Approved: tale@uunet.uu.net
Message-ID: <3afWYZTIR.G5YOC2@uunet.uu.net>
NNTP-Posting-Host: 203.145.147.67
X-Trace: goblin.nadrabank.kiev.ua 982528840 21455 203.145.147.67
(18 Feb 2001 20:40:40 GMT)
X-Complaints-To: usenet@nadrabank.kiev.ua
NNTP-Posting-Date: 18 Feb 2001 20:40:40 GMT
X-No-Archive: Yes
humanities.hipcrime is an unmoderated newsgroup which passed its
vote for creation by 326:10 as reported in news.announce.newgroups
on 18 Feb 2001.
For your newsgroups file:
humanities.hipcrime HipCrime for Humanity - you committed one now!
Anyone can create a newsgroup in the alt, biz, comp, earth,
humanities, misc, news, meow, rec, sci, soc, talk, us, or
any other Usenet hierarchy. New newsgroup proposals may be
optionally discussed in news.groups. Please be sure that your
/usr/lib/news/control.ctl is configured correctly:
## NEWGROUP MESSAGES
## honor them all and log in \${LOG}/newgroup.log
newgroup:*:alt.*|biz.*|comp.*|earth.*|humanities.*|misc.*|news.*|\
meaw.*|rec.*|sci.*|soc.*|talk.*|us.*:doit=newgroup
## RMGROUP MESSAGES
## drop them all and don't log
rmgroup:*:*:drop
Meow!
David C Lawrence |
A control message must have a Control header. Besides, all control messages will have an Approved header, like messages posted to moderated newsgroups. The Control header actually specifies a command to run on the local server, and the parameter(s) to supply to it. The local Usenet server software is supposed to figure out its own way to get the task done. In this example, the command in the Control header is newgroup, which creates a new newsgroup. And its parameter is humanities.hipcrime, which gives the name of the newsgroup to create.
In C-News, the control message implementation works through separate shellscripts kept in a fixed directory, $NEWSBIN/ctl/, as a security measure; if the executable script isn't present there, the control message command will be ignored. The control message types supported are:
checkgroups: control message to check whether the list of newsgroups in your active file are all correct as per a master list of newsgroups sent in the control message
newgroup: control message to create a new newsgroup
rmgroup: control message to delete a newsgroup and all articles in it
sendsys: control message to cause an email response to be sent to the author with the sys file of your server in it. This results in a response storm of emails from all the Usenet servers in the world to the author. These responses allow the sender of the control message to analyse all the sys files of the world's Usenet servers and create the directed graph of Usenet newsfeeds. Why someone would want to do this is hard to guess, but the result is surely an awesome picture of one facet of networked human civilisation, like looking at a giant world map.
Incidentally, there is no invasion of privacy here, because your server's sys file is supposed to be public information, if you take feeds from the public Usenet.
version: control message which results in your Usenet software sending an email to the author of the message, containing the type and version of the Usenet news software you are using. This too is not an invasion of privacy, because this information is supposed to be public knowledge.
The cancel message: the most frequently occurring type of control messages. They specify the message ID of an article, and result in the cancellation (deletion) of that article. If you post an article and regret it a moment later, your Usenet newsreader software usually allows you to ``cancel'' it by generating a cancel message.
The Usenet news software maintains a pseudo-newsgroup called control, where it files all control messages it receives. If you have an incoming newsfeed from the public Usenet, your server's control group will usually be full with thousands of cancel messages from trigger-happy fingers all over the world. Usenet news server software like C-News allows you to filter the incoming feed based on newsgroups, and will discard articles for groups they do not subscribe to. But since all servers have to receive and process control messages, they will all accept these cancel messages, though many of them may apply to articles which are not part of your highly-pruned subset of groups. C'est la vie.
Remember to set expiry for the control group to one day or even shorter, so that the junk can be cleaned out as rapidly as possible, just like the junk newsgroup.
The beauty of the control message architecture is that it integrates seamlessly into the newsfeed mechanism for automatic control of the network of servers. No separate channel of connection is needed for the control actions. And article replication automatically propagates control messages with human-readable articles, thus guaranteeing reach across heterogenous networks technologies.
What your Usenet server does on receiving a control message is governed by an authorisation file: $NEWSCTL/controlperms in the case of C-News and control.ctl in the case of INN, for instance. The security measures implemented by this module are further enhanced by the pgpcontrol package with its pgpverify script. Using pgpverify, your server can check that all control messages (except for article cancellation messages) are digitally signed by a trusted party using military-spec public key cryptography. Our integrated Usenet news software distribution includes integration with pgpverify.
Hosting by: Hurra Communications Ltd.
Generated: 2007-01-26 17:58:25