By RFC org•September 5, 2025•

rfc

rfc792

Network Working Group J. Postel

RFC 792 - Internet Control Message Protocol (ICMP)

Request for Comments: 792 ISI

September 1981
Updates: RFCs 777, 760 Updates: IENs 109, 128

Internet Control Message Protocol

DARPA INTERNET PROGRAM

PROTOCOL SPECIFICATION

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. Also ICMP

messages are only sent about errors in handling fragment zero of
fragemented datagrams. (Fragment zero has the fragment offeset equal zero).
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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 use these fields (except to include them in the checksum). Unless otherwise noted under the individual format descriptions, the values of the internet header fields are as follows: 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.
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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.
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Destination Unreachable Message

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields

Destination Address
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; 5 = source route failed. Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future. Internet Header + 64 bits of Data Datagram

The internet header plus the first 64 bits of the original
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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 may send a destination unreachable message to 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 may return a destination
unreachable message. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
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Time Exceeded Message

IP Fields

Destination Address
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. Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future. 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
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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.

If fragment zero is not available then no time exceeded need be
sent at all. Code 0 may be received from a gateway. Code 1 may be received from a host.
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Parameter Problem Message

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields

Destination Address
The source network and address from the original datagram’s data. ICMP Fields: Type 12 Code 0 = pointer indicates the error. Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future. Pointer If code = 0, identifies the octet where an error was detected. Internet Header + 64 bits of Data Datagram

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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 with incorrect arguments in an option. The

gateway or host may also notify the source host via the parameter
problem message. This message is only sent if the error caused the datagram to be discarded. The pointer identifies the octet of the original datagram’s header

where the error was detected (it may be in the middle of an
option). For example, 1 indicates something is wrong with the Type of Service, and (if there are options present) 20 indicates something is wrong with the type code of the first option. Code 0 may be received from a gateway or a host.
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Source Quench Message

IP Fields

Destination Address
The source network and address of the original datagram’s data. ICMP Fields: Type 4 Code 0 Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future. Internet Header + 64 bits of Data Datagram

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
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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.

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. Code 0 may be received from a gateway or a host.
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Redirect Message

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gateway Internet Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internet Header + 64 bits of Original Data Datagram |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields

Destination Address
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. Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future.

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.
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Internet Header + 64 bits of Data Datagram

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.

For datagrams with the IP source route options and the gateway
address in the destination address field, a redirect message is

not sent even if there is a better route to the ultimate
destination than the next address in the source route. Codes 0, 1, 2, and 3 may be received from a gateway.
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Echo or Echo Reply Message

IP Fields

Addresses

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, the type code changed to 0, and the checksum recomputed. IP Fields: Type 8 for echo message; 0 for echo reply message. Code 0 Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. If the total length is odd, the received data is padded with one octet of zeros for computing the checksum. This checksum may be replaced in the future.

Identifier
If code = 0, an identifier to aid in matching echos and replies, may be zero.

Sequence Number
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If code = 0, a sequence number to aid in matching echos and replies, may be zero.

Description

The data received in the echo message must be returned in the echo
reply message.

The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the echo requests. For example, the identifier might be used like a port in TCP or UDP to identify a session, and the sequence number might be incremented on each echo request sent. The echoer returns these same values in the echo reply. Code 0 may be received from a gateway or a host.
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Timestamp or Timestamp Reply Message

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originate Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transmit Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields

Addresses

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, the type code changed to 14, and the checksum recomputed. IP Fields: Type 13 for timestamp message; 14 for timestamp reply message. Code 0 Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future.

Identifier
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If code = 0, an identifier to aid in matching timestamp and replies, may be zero.

Sequence Number
If code = 0, a sequence number to aid in matching timestamp and replies, may be zero.

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].

The Originate Timestamp is the time the sender last touched the
message before sending it, the Receive Timestamp is the time the echoer first touched it on receipt, and the Transmit Timestamp is the time the echoer last touched the message on sending it.

If the time is not available in miliseconds or cannot be provided

with respect to midnight UT then any time can be inserted in a

timestamp provided the high order bit of the timestamp is also set
to indicate this non-standard value.

The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the requests. For example,

the identifier might be used like a port in TCP or UDP to identify
a session, and the sequence number might be incremented on each request sent. The destination returns these same values in the reply. Code 0 may be received from a gateway or a host.
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Information Request or Information Reply Message

IP Fields

Addresses

The address of the source in a information request message will be
the destination of the information reply message. To form a information reply message, the source and destination addresses are simply reversed, the type code changed to 16, and the checksum recomputed. IP Fields: Type 15 for information request message; 16 for information reply message. Code 0 Checksum The checksum is the 16-bit ones’s complement of the one’s complement sum of the ICMP message starting with the ICMP Type. For computing the checksum , the checksum field should be zero. This checksum may be replaced in the future.

Identifier
If code = 0, an identifier to aid in matching request and replies, may be zero.

Sequence Number
If code = 0, a sequence number to aid in matching request and replies, may be zero.
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Description

This message may be sent with the source network in the IP header
source and destination address fields zero (which means "this" network). The replying IP module should send the reply with the addresses fully specified. This message is a way for a host to find out the number of the network it is on.

The identifier and sequence number may be used by the echo sender
to aid in matching the replies with the requests. For example,

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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 15 Information Request 16 Information Reply
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References

[1] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
Protocol Specification," RFC 791, USC/Information Sciences Institute, September 1981.
[2] Cerf, V., "The Catenet Model for Internetworking," IEN 48,
Information Processing Techniques Office, Defense Advanced Research Projects Agency, 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.
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