INTERNET-DRAFT                               Charles H. Lindsey
Usenet Format Working Group                  University of Manchester
                                             July 2001

8.8. Duties of a Gateway

8.8.  Duties of a Gateway
   A Gateway transforms an article into the native message format of
   another medium, or translates the messages of another medium into
   news articles. Encapsulation of a news article into a message of MIME
   type application/news-transmission, or the subsequent undoing of that
   encapsulation, is not gatewaying, since it involves no transformation
   of the article.

   There are two basic types of gateway, the Outgoing Gateway that
   transforms a news article into a different type of message, and the
   Incoming Gateway that transforms a message from another medium into a
   news article and injects it into a Netnews system. These are handled
   separately below.

   The primary dictat for a gateway is:

        Above all, prevent loops.

   Transformation of an article into another medium stands a very high
   chance of discarding or interfering with the protection inherent in
   the news system against duplicate articles. The most common problem
   caused by gateways is "spews," gateway loops that cause previously
   posted articles to be reinjected repeatedly into Usenet. To prevent
   this, a gateway MUST take precautions against loops, as detailed
   below.

   If bidirectional gatewaying (both an incoming and an outgoing
   gateway) is being set up between Netnews and some other medium, the
   incoming and outgoing gateways SHOULD be coordinated to avoid
   reinjection of gated articles. Circular gatewaying (gatewaying a
   message into another medium and then back into Netnews) SHOULD NOT be
   done; encapsulation of the article SHOULD be used instead where this
   is necessary.

   A second general principal of gatewaying is that the transformations
   applied to the message SHOULD be as minimal as possible while still
   accomplishing the gatewaying. Every change made by a gateway
   potentially breaks a property of one of the media or loses
   information, and therefore only those transformations made necessary
   by the differences between the media should be applied.

   It is worth noting that safe bidirectional gatewaying between a
   mailing list and a newsgroup is far easier if the newsgroup is
   moderated. Posts to the moderated group and submissions to the
   mailing list can then go through a single point that does the
   necessary gatewaying and then sends the message out to both the
   newsgroup and the mailing list at the same time, eliminating most of
   the possibility of loops. Bidirectional gatewaying between a mailing
   list and an unmoderated newsgroup, in contrast, is difficult to do
   correctly and is far more fragile.

   Newsgroups intended to be bidirectionally gated to a mailing list
   SHOULD therefore be moderated where possible, even if the moderator
   is a simple gateway and injecting agent that correctly handles
   crossposting to other moderated groups and otherwise passes all
   traffic.

Diffs to previous draft

--- {draft-04}	Wed Jul 11 21:56:17 2001
+++ {draft-05}	Wed Jul 11 21:56:18 2001
@@ -14,7 +14,6 @@
 
    The primary dictat for a gateway is:
 
-
         Above all, prevent loops.
 
    Transformation of an article into another medium stands a very high