🚀 go-pugleaf

I have a personal interest in this discussion on the terminology of
LAN interconnection, since I have just written a paper on a project
in which I was involved, interconnecting transparently Ethernets
by means of a high-speed, general-purpose backbone network. I have
included the (more or less) relevant parts of the paper below:

		     ------------------------------

     There are several approaches by which communications can  be  extended
over  several  networks.  They  fit  into  four  main categories: gateways,
relays, bridges, and repeaters.

      A gateway translates  between  protocols.  Gateways  can  operate  at
different  levels  in  the  ISO  model.  A  gateway that operates below the
Application layer is also known as a "protocol  converter".  Gateways  are,
naturally, specific to the protocols for which they are designed.

     A relay forwards a packet towards its final destination.  This  is  an
operation  carried  out  in  the  Network  layer  and  is  a characteristic
mechanism  of  store-and-forward  networks.   It  allows  mixing  different
communications technologies, but imposes an overall Network level protocol.

    A bridge  is  assumed  to  provide  a  means  of  propagating  a  basic
information frame from the source to the destination network, in such a way
that the information content is unchanged.  This operation  is  constrained
by  the  OSI  model  to  being  performed in the lower sublayer of the Link
layer, whence the complete  term  "Medium  Access  Control  (or  MAC)-level
Bridge".

    This approach to intercommunication  allows  any  mix  of  higher-level
protocols  to operate within the resulting bridged area network in the same
way as within a single local area network.

    In addition, a bridge can carry  out  "filtering"  on  the  destination
address.  This  entails  forwarding  to  each segment only the frames whose
destination corresponds to an address on that segment.

    A repeater amplifies or regenerates the physical signals on a  link  or
bus-type  network.  It  is  used  to  overcome  distance limitations due to
signal degradation.  It should, however, be noted that  the  term  buffered
repeater is generally used to describe a simple MAC-level bridge.

MAC-level Bridge Service Definition
===================================

    This service definition is based on work being carried out within  IEEE
project  802  and  expected  to  be  incorporated  into  ISO^8880  and  the
long-awaited ISO^8802/1.

    The term station is used to represent a (generally intelligent)  device
connected to a LAN.

    The service characteristics are examined with their implication on  the
required characteristics of the bridge:-

topology independence:
bridges are invisible to the upper part of level^2 (LLC),  and  all  higher
layers.   They   are   not  explicitly  addressed,  except  for  management
functions;

MAC transparency:
the  bridge  should  not  need  any  modification  to  the   existing   MAC
specifications  (ISO^8802/3,4,5).  The  assignment  of LAN addresses should
not be affected by the presence of the bridge;

minimal increase in errors:
bridges should not significantly increase the incidence of  errors  between
communicating  stations.  It  should  be  noted,  however, that the general
bridge  service  definition  does  not  rule  out  loss,   duplication   or
misordering of frames;

sufficient data length:
the bridges should be able to forward at  least  the  minimum  useful  data
unit.  The minimum acceptable size is set by IEEE to 263^bytes;

limited transit delay:
the resultant  bridged  area  network  should  not  exhibit  too  large  an
end-to-end transit delay.  This upper limit is set by IEEE at 5^seconds.

Potential disadvantages
=======================

    The main problems that can be encountered in a bridged area network fit
roughly into the following categories:

increased delay: the bridges impose a store-and-forward delay;

congestion: congestion in the bridge can lead to several symptoms  such  as
lost frames, irregular traffic, etc.;

increased  security  risks:  remote  access  allows  unauthorised  use   of
connected segments;

degradation of some service characteristics: the overall service relies  on
the individual services available along the traversed path.  This can lead,
for example, to reductions in the maximum frame length compared  with  that
available on each of the end segments;

loss of local features: one typical such  example  is  for  the  ISO^8802/5
token  ring, where the MAC-level protocol provides the transmitting station
with the information as to whether the receiving station  on  the  ring  is
present  and  whether  the  data  was  copied  ("R"  and" "C" bits).  These
features can obviously  not  be  given  end-to-end  significance  across  a
bridged area network;

management complexity: analysis of performance and diagnosis of  end-to-end
problems become even more complicated than in a single network.

Implications of bridges
=======================

   In order to be able to provide the services needed by the bridge itself,
the LAN interface must have a certain minimal functionality.

    For reception, it must be capable of receiving all frames travelling on
the  LAN  ("promiscuous"  mode),  in order to allow the software to select,
from the total traffic, the frames to be forwarded.

    For emission, it must be able to insert any acceptable value  for  both
the  source and destination addresses, in order to allow the original frame
to be  recreated  identically  on  the  destination  segment.  It  is  also
desirable  for  the emitter to be able to specify the value of the checksum
to be included with the frame.

			  ------------------------

I hope that this contributes to the discussion. An effort has been made to
match the terminology to that emerging from the standards organisations,
insofar as it is relevant to the actual, practical application.