Network Working Group J. Postel
Request for Comments: 777 ISI
April 1981
Updates: IENs 109, 128
Updates: RFC 760
Internet Control Message Protocol
Introduction
The Internet Protocol (IP) [1] is used for host-to-host datagram
service in a system of interconnected networks called the
Catenet [2]. The network connecting devices are called Gateways.
These gateways communicate between themselves for control purposes
via a Gateway to Gateway Protocol (GGP) [3,4]. Occasionally a
gateway or destination host will communicate with a source host, for
example, to report an error in datagram processing. For such
purposes this protocol, the Internet Control Message Protocol (ICMP),
is used. ICMP, uses the basic support of IP as if it were a higher
level protocol, however, ICMP is actually an integral part of IP, and
must be implemented by every IP module.
ICMP messages are sent in several situations: for example, when a
datagram cannot reach its destination, when the gateway does not have
the buffering capacity to forward a datagram, and when the gateway
can direct the host to send traffic on a shorter route.
The Internet Protocol is not designed to be absolutely reliable. The
purpose of these control messages is to provide feedback about
problems in the communication environment, not to make IP reliable.
There are still no guarantees that a datagram will be delivered or a
control message will be returned. Some datagrams may still be
undelivered without any report of their loss. The higher level
protocols that use IP must implement their own reliability procedures
if reliable communication is required.
The ICMP messages typically report errors in the processing of
datagrams, to avoid the infinite regress of messages about messages
etc., no ICMP messages are sent about ICMP messages.
Message Formats
ICMP messages are sent using the basic IP header. The first octet of
the data portion of the datagram is a ICMP type field; the value of
this field determines the format of the remaining data. Any field
labeled "unused" is reserved for later extensions and must be zero
when sent, but receivers should not check these fields. Unless
otherwise noted under the individual format descriptions, the values
of the internet header fields are as follows:
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Version
4
IHL
Internet header length in 32-bit words.
Type of Service
0
Total Length
Length of internet header and data in octets.
Identification, Flags, Fragment Offset
Used in fragmentation, see [1].
Time to Live
Time to live in seconds; as this field is decremented at each
machine in which the datagram is processed, the value in this
field should be at least as great as the number of gateways which
this datagram will traverse.
Protocol
ICMP = 1
Header 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. This checksum may be replaced in the
future.
Source Address
The address of the gateway or host that composes the ICMP message.
Unless otherwise noted, this can be any of a gateway's addresses.
Destination Address
The address of the gateway or host to which the message should be
sent.
The source network and address from the original datagram's data.
ICMP Fields:
Type
3
Code
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If, according to the information in the gateway's routing tables,
the network specified in the internet destination field of a
datagram is unreachable, e.g., the distance to the network is
infinity, the gateway sends a destination unreachable message to
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the internet source host of the datagram. In addition, in some
networks, the gateway may be able to determine if the internet
destination host is unreachable. Gateways in these networks may
send destination unreachable messages to the source host when the
destination host is unreachable.
If, in the destination host, the IP module cannot deliver the
datagram because the indicated protocol module or process port is
not active, the destination host may send a destination
unreachable message to the source host.
Another case is when a datagram must be fragmented to be forwarded
by a gateway yet the Don't Fragment flag is on. In this case the
gateway must discard the datagram and return a destination
unreachable message.
The source network and address from the original datagram's data.
ICMP Fields:
Type
11
Code
0 = time to live exceeded in transit;
1 = fragment reassembly time exceeded.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If the gateway processing a datagram finds the time to live field
is zero it must discard the datagram. The gateway may also notify
the source host via the time exceeded message.
If a host reassembling a fragmented datagram cannot complete the
reassembly due to missing fragments within its time limit it
discards the datagram, and it may send a time exceeded message.
The source network and address from the original datagram's data.
ICMP Fields:
Type
12
Code
0 = problem with option.
Parameter
If code = 0, IP option type.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
If the gateway or host processing a datagram finds a problem with
the header parameters such that it cannot complete processing the
datagram it must discard the datagram. One potential source of
such a problem is an option that is not implemented, or incorrect
arguments in an option. The gateway or host may also notify the
source host via the parameter problem message.
The source network and address of the original datagram's data.
ICMP Fields:
Type
4
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
Description
A gateway may discard internet datagrams if it does not have the
buffer space needed to queue the datagrams for output to the next
network on the route to the destination network. If a gateway
discards a datagram, it may send a source quench message to the
internet source host of the datagram. A destination host may also
send a source quench message if datagrams arrive too fast to be
processed. The source quench message is a request to the host to
cut back the rate at which it is sending traffic to the internet
destination. The gateway may send a source quench message for
every message that it discards. On receipt of a source quench
message, the source host should cut back the rate at which it is
sending traffic to the specified destination until it no longer
receives source quench messages from the gateway. The source host
can then gradually increase the rate at which it sends traffic to
the destination until it again receives source quench messages.
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The gateway or host may send the source quench message when it
approaches its capacity limit rather than waiting until the
capacity is exceeded. This means that the data datagram which
triggered the source quench message may be delivered.
The source network and address of the original datagram's data.
ICMP Fields:
Type
5
Code
0 = Redirect datagrams for the Network.
1 = Redirect datagrams for the Host.
2 = Redirect datagrams for the Type of Service and Network.
3 = Redirect datagrams for the Type of Service and Host.
Gateway Internet Address
Address of the gateway to which traffic for the network specified
in the internet destination network field of the original
datagram's data should be sent.
Internet Header + 64 bits of Data Datagram
The internet header plus the first 64 bits of the original
datagram's data. This data is used by the host to match the
message to the appropriate process. If a higher level protocol
uses port numbers, they are assumed to be in the first 64 data
bits of the original datagram's data.
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Description
The gateway sends a redirect message to a host in the following
situation. A gateway, G1, receives an internet datagram from a
host on a network to which the gateway is attached. The gateway,
G1, checks its routing table and obtains the address of the next
gateway, G2, on the route to the datagram's internet destination
network, X. If G2 and the host identified by the internet source
address of the datagram are on the same network, a redirect
message is sent to the host. The redirect message advises the
host to send its traffic for network X directly to gateway G2 as
this is a shorter path to the destination. The gateway forwards
the original datagram's data to its internet destination.
The address of the source in an echo message will be the
destination of the echo reply message. To form an echo reply
message, the source and destination addresses are simply reversed.
IP Fields:
Type
8 for echo message;
0 for echo reply message.
Description
The data received in the echo message must be returned in the echo
reply message.
The address of the source in a timestamp message will be the
destination of the timestamp reply message. To form a timestamp
reply message, the source and destination addresses are simply
reversed.
IP Fields:
Type
13 for timestamp message;
14 for timestamp reply message.
Description
The data received (a timestamp) in the message is returned in the
reply together with an additional timestamp. The timestamp is 32
bits of milliseconds since midnight UT. One use of these
timestamps is described by Mills [5].
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Summary of Message Types
0 Echo Reply
3 Destination Unreachable
4 Source Quench
5 Redirect
8 Echo
11 Time Exceeded
12 Parameter Problem
13 Timestamp
14 Timestamp Reply
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References
[1] Postel, J., ed., "DOD Standard Internet Protocol", IEN 128,
RFC 760, USC/Information Sciences Institute, NTIS ADA079730,
January 1980. Appears in: Computer Communication Review,
Special Interest Group on Data Communications, ACM, V.10, N.4,
October 1980.
[2] Cerf, V., "The Catenet Model for Internetworking," Information
Processing Techniques Office, Defense Advanced Research
Projects Agency, IEN 48, July 1978.
[3] Strazisar, V., "Gateway Routing: An Implementation
Specification", IEN 30, Bolt Beranek and Newman, April 1979.
[4] Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek
and Newman, August 1979.
[5] Mills, D., "DCNET Internet Clock Service," RFC 778, COMSAT
Laboratories, April 1981.
