Network Working Group                                           S. Hares
Request for Comments: 1575                                  Merit/NSFNET
Obsoletes: 1139                                             C. Wittbrodt
Category: Standards Track                    Stanford University/BARRNet
                                                           February 1994

An Echo Function for CLNP (ISO 8473)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo defines an echo function for the connection-less network
   layer protocol.  The mechanism that is mandated here is in the final
   process of being standardized by ISO as "Amendment X: Addition of an
   Echo function to ISO 8473" an integral part of Version 2 of ISO 8473.

   Section 1. Conventions .................................    2
   Section 2. Introduction ................................    2
   Section 3. The Generic Echo Function ...................    3
   Section 3.1 The Echo-Request ...........................    3
   Section 3.2 The Echo-Response ..........................    3
   Section 4. The Implementation Mechanism ................    4
   Section 4.1 The Echo-Request ...........................    4
   Section 4.2 The Echo-Response ..........................    4
   Section 5. Implementation Notes ........................    4
   Section 5.1 Discarding Packets .........................    4
   Section 5.2 Error Report Flag ..........................    4
   Section 5.3 Use of the Lifetime Field ..................    5
   Section 5.4 Echo-request function ......................    5
   Section 5.5 Echo-response function .....................    6
   Section 5.6 Use of the Priority Option .................    8
   Section 5.7 Use of the Source Route Option .............    8
   Section 5.8 Transmission of Multiple Echo-Requests .....    9
   Section 6. Security Considerations .....................    9
   Section 7. Authors' Addresses ..........................    9
   Section 8. References ..................................    9

1. Conventions

   The following language conventions are used in the items of
   specification in this document:

      o MUST, SHALL, or MANDATORY -- the item is an absolute
        requirement of the specification.

      o SHOULD or RECOMMENDED -- the item should generally be followed
        for all but exceptional circumstances.

      o MAY or OPTIONAL -- the item is truly optional and may be
        followed or ignored according to the needs of the implementor.

2. Introduction

   The OSI Connection-less network layer protocol (ISO 8473) defines a
   means for transmitting and relaying data and error protocol data
   units, (PDUs) or preferably, packets 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.  Hosts and routers which support the OSI network layer MUST be
   able to originate an echo packet as well as respond when an echo is
   received.

   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.

   The code-path consideration requires that the echo path through a
   system be 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.

   Previous drafts describing an echo function for CLNP offered two
   implementation alternatives (see RFC 1139).  Although backward
   compatibility is an important consideration whenever a change is made
   to a protocol, it is more important at this point that the echo
   mechanisms used on the Internet interoperate.  For this reason, this
   memo defines one implementation mechanism (consistent with one of the
   previous drafts).

3. The Generic Echo Function

   The following section describes 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 packet,
   perform some processing, and generate an echo-response packet.  The
   echo implementation may be thought of as an entity that coexists with
   the network layer.  Subsequent sections will detail the
   implementation mechanism.

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

3.1. The Echo-Request

   When a system decides to ping a remote system, an echo-request is
   built.  All fields of the packet 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 data (DT)
   packet also apply to the echo-request packet.

   The echo-request is switched through the network toward its
   destination.  (An echo packet must follow the same path as CLNP data
   packet with the same options in the CLNP header.)  Upon reaching the
   destination system, the packet is processed according to normal
   processing rules.  At the end of the input processing, the echo-
   request packet is delivered to the echo-request entity.

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

3.2. The Echo-Response

   The entire echo-request packet is included in the data portion of the
   echo-response packet.  This includes the echo-request packet header
   as well as any data that accompanies the echo-request packet.  The
   entire echo-request packet is included in the echo-response so that
   fields such as the echo-request lifetime may be examined when the
   response is received.  After the echo-response packet is built, it is
   transmitted toward the new destination (the original source of the

   echo-request).  The rules of segmentation defined for a data packet
   also apply to the echo-response packet.

   The echo-response packet is relayed through the network toward its
   destination. (A echo response packet must follow the same path as a
   CLNP data packet with the same options in the CLNP header.)  Upon
   reaching its destination, it is processed by the packet input
   function and delivered to the entity that created the echo-request.

4. The Implementation Mechanism

   The implementation mechanism defines two new 8473 packet types: ERQ
   (echo-request) and ERP (echo-response).  With the exception of a new
   type code, these packets will be identical to the date packet in
   every respect.

4.1. The Echo-Request

The type code for the echo-request packet is decimal 30.

4.2. The Echo-Response

The type code for the echo-response packet 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 Packets

   The rules used for discarding a data packet (ISO 8473, Section 6.9 -
   Section 6.10) are applied when an echo-request or echo-response is
   discarded.

5.2. Error Report Flag

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

5.3. Use of the Lifetime Field

   The lifetime field of the echo-request and echo-response packets
   should be set to the value normally used for a data packet.  Note:
   although this memo does not prohibit the generation of a packet 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-response packet.  This memo recommends the lifetime value
   that would under normal circumstances by used when sending a data
   packet.

5.4. Echo-request function

   This function is invoked by system management to obtain information
   about the dynamic state of the Network layer with respect to (a) the
   reachability of specific network-entities, and (b) the
   characteristics of the path or paths that can be created between
   network-entities through the operation of Network layer routing
   functions.  When invoked, the echo-request function causes an echo-
   request (ERQ) packet to be created.  The echo-request packet shall be
   constructed and processed by ISO 8473 network-entities in end systems
   and intermediate systems in exactly the same way as the data packet,
   with the following caveats:

      a) The information available to the packet composition function
      (ISO 8473) consists of current state, local information, and
      information supplied by system management.

      b) The source and destination address fields of the echo-request
      packet shall contain, respectively, a Network entity title (NET)
      of the originating network-entity and a Network entity title of
      the destination network-entity (which may be in either an end
      system or an intermediate system).  NOTE: A Network entity title
      is syntactically indistinguishable from an NSAP address.  The
      additional information in an NSAP address, if any, beyond that
      which is present in a Network entity title, is relevant only to
      the operation of the packet decomposition function in a
      destination end system, and therefore is not needed for the
      processing of an echo-request packet (from which no N-UNITDATA
      indication is ever produced).  The fact that the source and
      destination address fields of the echo-request packet contain NETs
      rather than NSAP addresses therefore does not affect the
      processing of an echo-request packet by any network-entity.

      c) When an echo-request packet has reached its destination, as
      determined by the Header processing (call HEADER FORMAT Analysis
      function in ISO 8473), the echo-response function shall handle
      this Network Protocol Data Units (NPDU) instead of the packet

      decomposition function.  In ISO 8473, the packet decomposition
      function is like a decomposing fish on the sea shore - it takes a
      packet down to its bare bones and processes it.

      Also, it is up to each individual system whether or not handling
      echo-request packets involves system management.  One example of
      involving system management is the reporting reception of the echo
      packets as some systems do with the ping packet.  Some systems
      find this of value if they are being pinged to death.

      d) The maximum length of the echo-request packet is equal to the
      maximum length of the echo-response packet minus the maximum
      length of the echo-response packet header.  This ensures that the
      entire echo-request packet can be contained within the data field
      of the echo-response packet (see ISO 8473).

      e) The data part of the echo-request packet may, as a local
      matter, contain zero or more octets with any values that fit
      within the echo-request packet. (see (d) above for maximum length
      of the echo-request packet). If the first octet of data is binary
      1000 0001, then an echo-response header is contained in the echo-
      request packet.  The existence of this header insures that a
      router can formulate a standard echo-response packet.

   Normally, the "more segmentation" flag in the encapsulated echo-
   response packet header shall be zero, and the segmentation portion of
   the encapsulated packet shall not be included.  The segmentation
   length in the echo-response packet header shall be zero.

   If the "more segmentation" flag is set in the encapsulated echo-
   response packet header, then a segmentation length shall be filled in
   and the segmentation part of the echo-response packet header will be
   present in the echo-response header.  This same segmentation function
   shall be present in the echo-response sent by the router.

   NOTE: However, this formulated echo-response is not required between
   any two systems.  With a common format for an echo-request packet
   used in an environment such as the Internet, the echo-response header
   may not be needed, and may in fact be unnecessary overhead.

5.5. Echo-response function

   This function is performed by a network-entity when it has received
   an echo-request packet that has reached its destination, as
   determined by the Header format analysis function (ISO 8473 clause
   6.3) that is, an echo-request packet which contains, in its
   destination address field, a Network entity title that identifies the
   network-entity.  When invoked, the echo-response function causes an

   echo-response (ERP) packet to be created.  The echo-response packet
   shall be constructed and processed by ISO 8473 network-entities in
   end systems and intermediate systems in exactly the same way as the
   data packet, with the following caveats:

      a) The information available to the packet composition function
      consists of current state, local information, and information
      contained in the corresponding echo-request packet.

      b) The source address field of the echo-response packet shall
      contain the value of the destination address field of the
      corresponding echo-request packet.  The destination address field
      of the echo-response packet shall contain the value of the source
      address field of the corresponding echo-request packet.

      c) The echo-request packet, in its entirety, shall be placed into
      the data part of the echo-response packet.  The data part of the
      echo-response packet shall contain only the corresponding echo-
      request packet.

      d) If the data part of the echo-request packet contains an echo-
      response header, the packet composition function may, but is not
      required to, use some or all of the information contained therein
      to select values for the fields of the echo-response packet
      header.  In this case, however, the value of the lifetime field
      contained in the echo-response packet header in the echo-request
      packet data part must be used as the value of the lifetime field
      in the echo-response packet.  The values of the segment length and
      checksum fields shall be computed by the network-entity regardless
      of the contents of those fields in the echo-response packet header
      in the data part of the echo-request packet.

      e) The options part of the echo-response packet may contain any
      (or none) of the options described in ISO 8473 (but see Section
      5.7 of this RFC).  The values for these options, if present, are
      determined by the network-entity as a local matter.  They may be,
      but are not required to be, either identical to or derived from
      the corresponding options in the echo-request packet and/or the
      echo-response packet header contained in the data part of the
      echo-request packet (if present).  The source routing option in
      the echo-response packet shall not be identical to (copied from)
      the source routing option in the echo-request packet header.  If
      the recording of route option in the echo-response packet is
      identical to (copied from) the recording of route option in the
      echo-request packet header, the second octet of the parameter
      value field shall be set to the value 3.

      f) It is a local matter whether or not the destination network-
      entity performs the lifetime control function on an echo-request
      packet before performing the echo-response function.  The
      destination network-entity shall make the same decision in this
      regard that it would make, as a local matter, for a data packet in
      accordance with ISO 8473.

5.6. Use of the Priority Option

      The 8473 priority function indicates the relative priority of
      packet.  0 is normal and 14 is the highest.  Packets with higher
      values will be transmitted before lower values.  The specific
      action upon receiving a 8473 packet with the priority field set is
      a "LOCAL MATTER".  These means, any two systems could do it
      differently.

      Hopefully, in the future, Internet routers will handle this as a
      priority queueing function.  Some implementors consider the
      priority queueing function to be a cap.  For example, if a router
      is congested, all those packets with priorities higher than 20,
      will be allowed through, and those with priority less than 20 will
      be dropped.

      In short, the basic function of priority has wide latitude in the
      ISO specification.  This wide latitude of implementation needs to
      be narrowed for implementations within a common network
      environment such as the Internet.  The 8473 priority function is
      rarely implemented in today's Internet.  The transmission of an
      echo-request packet with a priority set may provided unexcepted
      results until a more wide spread deployment of the priority
      feature in 8473 capable routers and end systems.

      However, if the priority function must be used it is the safest
      value may be the value 0 - which indicates Normal priority.  It
      most likely this value will follow the 8473 pathways.

      In the future, as the implementation of the priority function
      further Internet documents will need to deal with its expected
      use.

5.7. 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-response packets.  If the source route option
      is used to specify the route that the echo-request packet takes
      toward its destination, this memo does not recommend the use of an

automatically generated source route on the echo-response packet.

5.8. 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-responses are correlated between multiple processes on a
      single machine is a local matter and not defined by this memo.

6. Security Considerations

Security issues are not discussed in this memo.

7. Authors' Addresses

      Susan K. Hares
      MERIT/NSFNET
      Internet Engineering
      1075 Beal Avenue
      Ann Arbor, MI 48109-2112

      Phone: (313) 936-3000
      EMail: skh@merit.edu

      Cathy J. Wittbrodt
      Stanford University/BARRNet
      Networking Systems
      Pine Hall 115
      Stanford, CA 94305

      Phone: (415) 725-5481
      EMail: cjw@magnolia.Stanford.EDU

8. References

   [1] ISO/IEC.  Protocol for Providing the Connectionless-mode Network
       Service.  International Standard 8473, ISO/IEC JTC 1,
       Switzerland, 1986.

   [2] Hagens, R., "An Echo Function for ISO 8473", RFC 1139, IETF-OSI
       Working Group, January 1990.

   [3] ISO 8473-1993 Protocol for providing the connectionless-mode
       network service, edition 2 (IS under preparation).