RFC 1139 (rfc1139) - Page 2 of 6

Echo function for ISO 8473

Alternative Format: Original Text Document


RFC 1139             An Echo Function for ISO 8473          January 1990


   compatibility.  The complexity of the echo facility must be kept low.
   If it is not, then there is a good chance that the facility will not
   be universally provided.  The code-path consideration requires that
   the echo path through a system is identical (or very close) to the
   path used by normal data.  An echo path must succeed and fail in
   unison with the normal data path or else it will not provide a useful
   diagnostic tool.

   Backward compatibility is an important consideration whenever a
   change is made to a protocol.  For this reason, this memo defines two
   implementation mechanisms: the short term approach and the long term
   approach.  The short term approach will produce echo packets that are
   indistinguishable from normal data ISO 8473 PDUs.  These echo packets
   may be switched through ISO 8473 routers that do not implement the
   echo function.  The short term approach will be adopted as an
   Elective Internet Standard because it is backward compatible with ISO
   8473.  However, due to its nature, the short term approach will never
   be incorporated into future versions of ISO 8473.

   The long term approach will produce echo packets that are not
   compatible with the existing standard.  However, the long term
   approach may be acceptable by ISO as an addendum to ISO 8473.  In
   this event, backward compatibility will no longer be an issue.  At
   that juncture, the short term approach defined by this memo will be
   obsolete and superseded by the ISO addendum.

2.  The Generic Echo Function

   The following section will describe the echo function in a generic
   fashion.  This memo defines an echo-request entity.  The function of
   the echo-request entity is to accept an incoming echo-request PDU,
   perform some processing, and generate an echo-reply PDU.  Depending
   on the echo implementation, the echo-request entity may be thought of
   as an entity that exists above the network layer, or as an entity
   that co-exists with the network layer.  Subsequent sections will
   detail the short and long term implementation mechanisms.

   For the purposes of this memo, the term "ping" shall be used to mean
   the act of transmitting an echo-request PDU to a remote system (with
   the expectation that an echo-reply PDU will be sent back to the
   transmitter).

   2.1  The Echo Request

      When a system decides to ping a remote system, an echo-request is
      built.  All fields of the PDU header are assigned normal values
      (see implementation specific sections for more information).  The
      address of the system to be pinged is inserted as the destination



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