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  and superceded by the ISO standard.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



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  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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     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



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  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.




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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



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     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:  [email protected]











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