Network Working Group                                          G. Malkin
Request for Comments: 1393                                Xylogics, Inc.
                                                           January 1993


                    Traceroute Using an IP Option

Status of this Memo

  This memo defines an Experimental Protocol for the Internet
  community.  Discussion and suggestions for improvement are requested.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" for the standardization state and status of this protocol.
  Distribution of this memo is unlimited.

Abstract

  Traceroute serves as a valuable network debugging tool.  The way in
  which it is currently implemented has the advantage of being
  automatically supported by all of the routers.  It's two problems are
  the number of packets it generates and the amount of time it takes to
  run.

  This document specifies a new IP option and ICMP message type which
  duplicates the functionality of the existing traceroute method while
  generating fewer packets and completing in a shorter time.

Table of Contents

  1.  Traceroute Today  . . . . . . . . . . . . . . . . . . . . . 2
  2.  Traceroute Tomorrow . . . . . . . . . . . . . . . . . . . . 2
  2.1 Basic Algorithm . . . . . . . . . . . . . . . . . . . . . . 2
  2.2 IP Traceroute option format . . . . . . . . . . . . . . . . 3
  2.3 ICMP Traceroute message format  . . . . . . . . . . . . . . 4
  3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . 5
  3.1 Hop Counts  . . . . . . . . . . . . . . . . . . . . . . . . 5
  3.2 Destination Node Operation  . . . . . . . . . . . . . . . . 6
  3.3 Router Operation  . . . . . . . . . . . . . . . . . . . . . 6
  4.  References  . . . . . . . . . . . . . . . . . . . . . . . . 7
  5.  Security Considerations . . . . . . . . . . . . . . . . . . 7
  6.  Author's Address  . . . . . . . . . . . . . . . . . . . . . 7










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1.  Traceroute Today

  The existing traceroute operates by sending out a packet with a Time
  To Live (TTL) of 1.  The first hop then sends back an ICMP [1] error
  message indicating that the packet could not be forwarded because the
  TTL expired.  The packet is then resent with a TTL of 2, and the
  second hop returns the TTL expired.  This process continues until the
  destination is reached.  The purpose behind this is to record the
  source of each ICMP TTL exceeded message to provide a trace of the
  path the packet took to reach the destination.

  The advantage of this algorithm, is that every router already has the
  ability to send TTL exceeded messages.  No special code is required.
  The disadvantages are the number of packets generated (2n, where n is
  the number of hops), the time it takes to duplicate all the nearer
  hops with each successive packet, and the fact that the path may
  change during this process.  Also, this algorithm does not trace the
  return path, which may differ from the outbound path.

2.  Traceroute Tomorrow

  The proposed traceroute would use a different algorithm to achieve
  the same goal, namely, to trace the path to a host.  Because the new
  traceroute uses an ICMP message designed for the purpose, additional
  information, unavailable to the original traceroute user, can be made
  available.

2.1 Basic Algorithm

  A new IP Traceroute option will be defined.  The presence of this
  option in an ICMP Echo (or any other) packet, hereinafter referred to
  as the Outbound Packet, will cause a router to send the newly defined
  ICMP Traceroute message to the originator of the Outbound Packet.  In
  this way, the path of the Outbound Packet will be logged by the
  originator with only n+1 (instead of 2n) packets.  This algorithm
  does not suffer from a changing path and allows the response to the
  Outbound Packet, hereinafter refered to as the Return Packet, to be
  traced (provided the Outbound Packet's destination preserves the IP
  Traceroute option in the Return Packet).

  The disadvantage of this method is that the traceroute function will
  have to be put into the routers.  To counter this disadvantage,
  however, is the fact that this mechanism may be easily ported to a
  new IP version.







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RFC 1393                       Traceroute                   January 1993


2.2 IP Traceroute option format

   0               8              16              24
  +-+-+-+-+-+-+-+-+---------------+---------------+---------------+
  |F| C |  Number |    Length     |          ID Number            |
  +-+-+-+-+-+-+-+-+---------------+---------------+---------------+
  |      Outbound Hop Count       |       Return Hop Count        |
  +---------------+---------------+---------------+---------------+
  |                     Originator IP Address                     |
  +---------------+---------------+---------------+---------------+

  F (copy to fragments)

     0 (do not copy to fragments)

  C (class)

     2 (Debugging & Measurement)

  Number

     18 (F+C+Number = 82)

  ID Number

     An arbitrary number used by the originator of the Outbound Packet
     to identify the ICMP Traceroute messages.  It is NOT related to
     the ID number in the IP header.

  Originator IP Address

     The IP address of the originator of the Outbound Packet.  This is
     needed so the routers know where to send the ICMP Traceroute
     message for Return Packets.  It is also needed for Outbound
     Packets which have a Source Route option.

  Outbound Hop Count (OHC)

     The number of routers through which the Outbound Packet has
     passed.  This field is not incremented by the Outbound Packet's
     destination.

  Return Hop Count (RHC)

     The number of routers through which the Return Packet has passed.
     This field is not incremented by the Return Packet's destination.





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RFC 1393                       Traceroute                   January 1993


2.3  ICMP Traceroute message format

   0               8              16              24
  +---------------+---------------+---------------+---------------+
  |     Type      |     Code      |           Checksum            |
  +---------------+---------------+---------------+---------------+
  |           ID Number           |            unused             |
  +---------------+---------------+---------------+---------------+
  |      Outbound Hop Count       |       Return Hop Count        |
  +---------------+---------------+---------------+---------------+
  |                       Output Link Speed                       |
  +---------------+---------------+---------------+---------------+
  |                        Output Link MTU                        |
  +---------------+---------------+---------------+---------------+

  Type

     30

  Code

     0 - Outbound Packet successfully forwarded
     1 - No route for Outbound Packet; packet discarded

  Checksum

     The 16 bit one's complement of the one's complement sum of all 16
     bit words in the header.  For computing the checksum, the checksum
     field should be zero.

  ID Number

     The ID Number as copied from the IP Traceroute option of the
     packet which caused this Traceroute message to be sent.  This is
     NOT related to the ID number in the IP header.

  Outbound Hop Count

     The Outbound Hop Count as copied from the IP Traceroute option of
     the packet which caused this Traceroute message to be sent.

  Return Hop Count

     The Return Hop Count as copied from the IP Traceroute option of
     the packet which caused this Traceroute message to be sent.






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RFC 1393                       Traceroute                   January 1993


  Output Link Speed

     The speed, in OCTETS per second, of the link over which the
     Outbound/Return Packet will be sent.  Since it will not be long
     before network speeds exceed 4.3Gb/s, and since some machines deal
     poorly with fields longer than 32 bits, octets per second was
     chosen over bits per second.  If this value cannot be determined,
     the field should be set to zero.

  Output Link MTU

     The MTU, in bytes, of the link over which the Outbound/Return
     Packet will be sent.  MTU refers to the data portion (includes IP
     header; excludes datalink header/trailer) of the packet.  If this
     value cannot be determined, the field should be set to zero.


3.  Protocol

  The Outbound Packet which is used to carry the IP Traceroute option
  should use no special Type Of Service (TOS) or Precedence, unless the
  purpose is to trace the path of packets with special TOS or
  Precedence values.

  The TTL of the Outbound Packet should be set to the default value
  specified in "Assigned Numbers" [2].

3.1 Hop Counts

  The hop counts ultimately provide information on the length of the
  outbound and return paths to the destination.  They also provide a
  means of determining whether or not any ICMP Traceroute messages have
  been lost.  For example, if a Traceroute message with an OHC of 4 is
  followed by a message with an OHC of 6, then the the message with an
  OHC of 5 was lost.  This is why simply counting Traceroute messages
  is not sufficient for determining path length.

  The originator of the Outbound Packet should set the OHC to zero and
  the RHC to 0xFFFF.  0xFFFF is a special value which indicates to
  routers that the packet is an Outbound Packet rather than a Return
  Packet (which begins with an RHC of zero).

  It is important to note that the Traceroute hop counts are NOT
  related to the IP TTL.  A hop count should only be incremented when
  an ICMP Traceroute message is sent.






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RFC 1393                       Traceroute                   January 1993


3.2 Destination Node Operation

  When a node receives an Outbound Packet with an IP Traceroute option,
  the Return Packet, if such is required (e.g., ICMP Echo
  Request/Reply), should also carry that option.  The values in the ID
  Number, OHC, and Originator Address fields should be copied into the
  Return Packet.  The value of the RHC field should be set to zero.

  The destination should NOT increment any hop counts or send any ICMP
  Traceroute messages.

3.3 Router Operation

  When a router forwards a packet with an IP Traceroute option, it
  should send an ICMP Traceroute message to the host in the Originator
  IP Address field of the option.  If the value of the RHC field is
  0xFFFF then the packet is an Outbound Packet and the OHC should be
  incremented; otherwise, the RHC field should be incremented.  The
  Traceroute message should reflect the incremented hop count.  The
  Output Link Speed field should be set to the speed, in OCTETS per
  second, of the link over which the Outbound/Return Packet will be
  sent (e.g., 1,250,000 for an Ethernet) or zero if the output link
  speed cannot be determined.  The Output Link MTU field should be set
  to the MTU of the link over which the Outbound/Return Packet will be
  sent or zero if the MTU cannot be determined.

  The Outbound/Return Packet should be forwarded as though the
  Traceroute option did not exist; that is, it should take the same
  path to the destination as an optionless packet.

  The ICMP Traceroute message should have the same TOS and Precedence
  values as the Outbound/Return Packet.  The TTL should be set to the
  default defined in "Assigned Numbers".

  The ICMP Traceroute message should not carry the IP Traceroute
  option.

  If the Outbound Packet cannot be forwarded, the ICMP Traceroute
  message should have a Code value of 1.  If the Return Packet cannot
  be forwarded because there is no route, then there is no need to send
  a Traceroute message since it could not be forwarded either.










Malkin                                                          [Page 6]

RFC 1393                       Traceroute                   January 1993


4.  References

  [1] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
      USC/Information Sciences Institute, September 1981.

  [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
      USC/Information Sciences Institute, July 1992.


5.  Security Considerations

  Security issues are not discussed in this memo.


6.  Author's Address

  Gary Scott Malkin
  Xylogics, Inc.
  53 Third Avenue
  Burlington, MA 01803

  Phone:  (617) 272-8140
  EMail:  [email protected]




























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