# Reduced seen-by lines.

     | Document: FSC-0093
     | Version:  001
     | Date:     13 September, 1996
     | Title:    Reduced seen-by lines
     | Author:   Frank Ellermann, 2:240/5815.1

      Reduced seen-by lines
      Frank Ellermann, 2:240/5815.1

      Abstract
      --------
      A way to save great amounts (estimated 10 %) of echo mail traffic by
      reducing "seen by" informations, compatible with existing echo mail
      tossers conforming to FTS-0004.

      Definitions
      -----------
      A thorough understanding of FTS-0004 is required, the reader should
      be familiar with PATH and SEEN-BY lines in echo mail, illegal and
      legal echo mail distribution topologies, i.e. dup-rings, as well
      as with some pre-requisite knowledge of zones, 4D and 2D addresses,
      and the "sticky" 2D notation in PATH and SEEN-BY lines.

      PATH: path lines as specified in FTS-0004
      FSB:  full seen-by informations as specified in FTS-0004
      TSB:  tiny seen-by informations as mentioned in FTS-0004, see below
      RSB:  reduced seen-by informations specified below
      dupe: multiple arrival of the same echo mail (e.g. different paths)

      Examples of echo mail distribution topologies
      ---------------------------------------------
      In all examples a) to d) echo mail entered at system 1 is sent to
      systems 2 and 3 with FSB 1 2 3. Therefore system 2 (3) knows, that
      system 3 (2) already got this mail, topology a) is perfectly legal.

      a) 1 - 3  b) 1 - 3  c) 1 - 3  d) 1 - 2
         | /       |   |     | / |     | X |
         2         2 - 4     2 - 4     2 - 4

      In the exanmples b) and c) both systems 2 and 3 forward all mails
      from system 1 to system 4, these topologies contain a dup-ring and
      are therefore illegal following FTS-0004.

      The examples a) and d) show fully connected polygons with three or
      four nodes. In example d) a mail entered at system 1 is sent to
      systems 2, 3, and 4 with FSB 1 2 3 4. The topologies a) and d) are
      perfectly legal, there are no dupes caused by distribution.

      In example b) each mail entered at system 1 reaching system 4 via
      system 2 carries FSB 1 2 3 4, therefore system 4 will not forward
      such mails to 3. Using TSB at system 2 the same mails would carry
      TSB 2 4, therefore system 4 would forward them to 3 as dupes.

      Note that illegal topologies as in example b) and c) cause dupes
      with either FSB or TSB. The real problem with TSB is example b),
      as it allows for loop mails on the dup-ring 1 - 2 - 3 - 4 - ...
      and vice versa, if no additional checks for dupes are employed.

      With RSB (specified below) systems contained in the PATH are not
      stripped from the seen-by informations, therefore RSB avoid loop
      mail much like FSB.

      FSB algorithm
      -------------
      1) add own system to the PATH.
      2) all area links not contained in the FSB qualify as recipients.
      3) add own address(es) to the FSB set if not already contained.
      4) add recipients to FSB, sort FSB, forward mail to recipients.

      TSB algorithm
      -------------
      1) add own system to the PATH.
      2) all area links not contained in the TSB qualify as recipients.
      3) strip old TSB and start new TSB with own address(es).
      4) add recipients to TSB, sort TSB and forward mail to recipients.

      RSB algorithm
      -------------
      1) add own system to the PATH.
      2) all area links not contained in the RSB qualify as recipients.
      3) strip RSB addresses not matching an address in the PATH, then
         add own address(es) to the RSB set if not already contained.
      4) add recipients to RSB, sort RSB and forward mail to recipients.

      PATH considerations
      -------------------
      There are 2 problems with the PATH kludge as specified in FTS-0004:

      First like in the FSB the addresses in the PATH are 2D, and having
      the same 2D address in different zones is possible. Therefore zone
      gates are required to use the TSB algorithm. Unfortunately the PATH
      is forwarded without regarding zone gating, therefore detection of
      loop mail based solely on the PATH could be erroneous.

      Further FTS-0004 (written 1989) expects future echo mail tossers to
      implement PATH support, but doesn't require this support from old
      implementations. Strictly spoken the PATH is still only an option.

      In some areas of FidoNet (e.g. in zone 2) at least all non-terminal
      nodes are required to fully support the PATH line, therefore this
      problem will probably not show up in praxis. Of course any tosser
      implementing the RSB feature is required to fully support the PATH.

      Summary
      -------
      To show the benfits of RSB compared with FSB assume the following:

      An echo mail travels from node to echo hub, host, major star, echo
      host, hub, and finally arrives at a recipient. Each routing system
      has 10 links, i.e. FSB at the recipient contain 51 addresses, about
      400 characters, but RSB only 15 addresses in about 150 characters.

      Therefore in an echo mail with 2500 characters about 10 % of its
      size can be reduced using RSB in favour of FSB. If this estimation
      is applicable on world wide FidoNet echo mail traffic, RSB can save
      us an immense amount of costs.

      This document can be adopted by the FTSC as FTS, in this case it
      has to be regarded as an addition to FTS-0004 with the extension,
      that all non-terminal nodes are required to support PATH lines as
      specified in FTS-0004.

      For additional informations (e.g. aspects of zone gating) feel free
      to send mails to Frank Ellermann 2:240/5815 or leo@bfispc.hanse.de

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