RFC1393 - Traceroute Using an IP Option

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　　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

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.

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.

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.

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.

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.

4. References

[1] Postel, J., "Internet Control Message Protocol", STD 5, RFC792,
USC/Information Sciences Institute, September 1981.

[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1340,
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: gmalkin@Xylogics.COM

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