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.
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.
Table of Contents
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
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.
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
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
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
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
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
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
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-
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
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
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
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
1075 Beal Avenue
Ann Arbor, MI 48109-2112
Phone: (313) 936-3000
Cathy J. Wittbrodt
Pine Hall 115
Stanford, CA 94305
Phone: (415) 725-5481
 ISO/IEC. Protocol for Providing the Connectionless-mode Network
Service. International Standard 8473, ISO/IEC JTC 1,
 Hagens, R., "An Echo Function for ISO 8473", RFC 1139, IETF-OSI
Working Group, January 1990.
 ISO 8473-1993 Protocol for providing the connectionless-mode
network service, edition 2 (IS under preparation).