Echo function for ISO 8473
RFC 1139

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Network Working Group                             IETF-OSI Working Group
Request for Comments: 1139                                     R. Hagens
                                                            January 1990

                     An Echo Function for ISO 8473

Status of this Memo

   This memo defines an echo function for the connection-less network
   layer protocol.  This memo is not intended to compete with an ISO
   standard.  This is a Proposed Elective Standard for the Internet.
   Distribution of this memo is unlimited.

Abstract

   This memo defines an echo function for the connection-less network
   layer protocol.  Two mechanisms are introduced that may be used to
   implement the echo function.  The first mechanism is recommended as
   an interim solution for the Internet community.  The second mechanism
   will be progressed to the ANSI X3S3.3 working group for consideration
   as a work item.

   When an ISO standard is adopted that provides functionality similar
   to that described by this memo, then this memo will become obsolete
   and superceded by the ISO standard.

1.  Introduction

   The OSI Connection-less network layer protocol (ISO 8473) defines a
   means for transmitting and relaying data and error report PDUs
   through an OSI internet.  Unfortunately, the world that these packets
   travel through is imperfect.  Gateways and links may fail.  This memo
   defines an echo function to be used in the debugging and testing of
   the OSI network layer.

   Network management protocols can be used to determine the state of a
   gateway or link.  However, since these protocols themselves utilize a
   protocol that may experience packet loss, it cannot be guaranteed
   that the network management applications can be utilized.  A simple
   mechanism in the network layer is required so that systems can be
   probed to determine if the lowest levels of the networking software
   are operating correctly.  This mechanism is not intended to compete
   with or replace network management; rather it should be viewed as an
   addition to the facilities offered by network management.

   There are three important issues to consider when defining an echo
   extension to ISO 8473: complexity, code-path divergence, and backward

IETF-OSI Working Group                                          [Page 1]
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