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   and superceded by the ISO standard.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

   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

      NSAP address.  The rules of segmentation defined for a DT PDU also
      apply to the echo-request PDU.

      The echo-request is switched through the network toward its
      destination.  Upon reaching the destination system, the PDU is
      processed according to normal processing rules.  At the end of the
      input processing, the echo-request PDU is delivered to the echo-
      request entity.

      The echo-request entity will build and dispatch the echo-reply
      PDU.  This is a new PDU.  Except as noted below, this second PDU
      is built using the normal construction procedures.  The
      destination address of the echo-reply PDU is taken from the source
      address of the echo-request PDU.  Most options present in the
      echo-request PDU are copied into the echo-reply PDU (see
      implementation notes for more information).

   2.2  The Echo Reply

      The entire echo-request PDU is included in the data portion of the
      echo-reply PDU.  This includes the echo-request PDU header as well
      as the any data that accompanies the echo-request PDU.  The entire
      echo-request PDU is included in the echo-reply so that fields such
      as the echo-request lifetime may be examined when the reply is
      received.  After the echo-reply PDU is built, it is transmitted
      toward the new destination (the original source of the echo-
      request).  The rules of segmentation defined for a DT PDU also
      apply to the echo-reply PDU.

      The echo-reply PDU is relayed through the network toward its
      destination.  Upon reaching its destination, it is processed by
      the PDU input function and delivered to the entity that created
      the echo-request.

3.  The Short Term Implementation Mechanism

   The short term implementation mechanism  will use an ISO 8473 normal
   data PDU as the echo-request and echo-reply PDU.  A special NSAP
   selector value will be used to identify the echo-request and insure
   that it reaches the echo-request entity.  This selector value is
   known as the echo-request selector.  In addition, an echo-reply
   selector is defined so that the echo-reply PDU may be identified at
   the destination system.  It is important to note that (except for the
   NSAP selector) the echo-request PDU and the echo-reply PDU are
   indistinguishable from a DT PDU.

   This approach has the advantage that it is simple and does not allow
   any code-path divergence.  In addition, this approach requires that

   only the systems which wish to generate an echo-reply PDU must
   change.  Systems that do not adhere to this memo will not generate an
   echo-reply PDU, but will still switch other echo-request and echo-
   reply PDUs.

   3.1  The Echo Request

      An echo-request is built using the normal DT PDU construction
      procedures.  All fields of the PDU header are assigned normal
      values (see implementation notes).  The address of the system to
      be pinged is inserted as the destination NSAP address.  The
      selector field of the destination NSAP address must contain the
      echo-request selector.  The selector field of the source NSAP
      address must contain the echo-reply selector.

   3.2  The Echo Reply

      Except as noted below (see implementation notes), an echo-reply is
      built using the normal DT PDU construction procedures.  The
      destination NSAP address is taken from the source address of the
      echo-request PDU.

   3.3  Use of NSAP Selectors

      The choice of echo-request and echo-reply NSAP selectors is a
      local matter.  However, to insure interoperability, and as an
      interim measure until use of the directory service becomes
      widespread, this memo will recommend the following default values
      (specified in decimal):

         Echo Request Selector - 30
         Echo Reply Selector - 31

4.  The Long Term Implementation Mechanism

   The long term implementation mechanism will define two new 8473 PDU
   types: ERQ (echo-request) and ERP (echo-reply).  With the exception
   of a new type code, these PDUs will be identical to the DT PDU in
   every respect.

   4.1  The Echo Request

      The type code for the ERQ PDU is decimal 30.

   4.2  The Echo Reply

      The type code for the ERP PDU is decimal 31.

5.  Implementation Notes

   The following notes are an integral part of memo.  It is important
   that implementors take heed of these points.

   5.1  Discarding PDUs

      The rules used for discarding a DT PDU (8473, sec 6.9 - sec 6.10)
      are applied when an echo-request or echo-reply is discarded.

   5.2  Error Report Flag

      The error report flag may be set on the echo-request PDU, the
      echo-reply PDU, or both.  If an echo-request is discarded, the
      associated ER PDU will be sent to the echo-request source address
      on the originating machine.  If an echo-reply is discarded, the
      associated ER PDU will be sent to the echo-reply source address.
      In general, this will be the address of the echo-request entity.
      It should be noted that the echo-request entity and the originator
      of the echo-request PDU are not required to process ER PDUs.

   5.3  Use of the Lifetime Field

      The lifetime field of the echo-request and echo-reply PDU should
      be set to the value normally used for a DT PDU.  Note: although
      this memo does not prohibit the generation of a PDU with a
      smaller-than-normal lifetime field, this memo explicitly does not
      attempt to define a mechanism for varying the lifetime field set
      in the echo-reply PDU.  This memo recommends that the normal DT
      lifetime value should be set in the echo-request and echo-reply
      PDU.

   5.4  Transfer of Options from the echo-request
        PDU to the echo-reply PDU

      With two exceptions, all options present in the echo-request
      header are copied directly into the echo-reply header.  The two
      exceptions are the record route option and the source route
      option.  A record route option present in an echo-request PDU is
      copied into the echo-reply PDU, but the routes recorded in the
      option are "erased" by resetting the second octet of the option to
      3.  This allows the entire record route option space to be used by
      the echo-reply PDU.  Note: the record route present on the echo-
      request is not lost because the echo-request PDU is wholly
      contained in the data part of the echo-reply PDU.

      The second exception concerns the source route option.  A source
      route option present on the echo-request PDU is not copied into

      the echo-reply PDU.

   5.5  Use of the Priority Option

      If the priority option is included, it will normally be set to
      value 0 (default priority).  This memo allows for priority values
      higher than 0 to be set in the echo-request or echo-reply header,
      but cautions against this practice.

   5.6  Use of the Source Route Option

      Use of the source route option in ISO 8473 may cause packets to
      loop until their lifetime expires.  For this reason, this memo
      recommends against the use of the source route option in either an
      echo-request or echo-reply PDU.  If the source route option is
      used to specify the route that the echo-request PDU takes toward
      its destination, this memo does not recommend the use of an
      automatically generated source route on the echo-reply PDU.

   5.7  Transmission of Multiple Echo Requests

      The echo function may be utilized by more than one process on any
      individual machine.  The mechanism by which multiple echo-requests
      and echo-replies are correlated between multiple processes on a
      single machine is a local matter and not defined by this memo.

Security Considerations

   Security issues are not addressed in this memo.

Author's Address

   Robert A. Hagens
   Computer Science Department
   1210 West Dayton Street
   Madison, WI  53706

   Phone: (608) 262-1204

   EMail:  hagens@CS.WISC.EDU