RFC 792 INTERNET STANDARD NETWORK LAYER · DIAGNOSTICS
ICMP: Internet Control Message Protocol
The error-reporting and diagnostic protocol that keeps the Internet running
RFC EDITOR
Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, DOI 10.17487/RFC0792, September 1981, <https://www.rfc-editor.org/info/rfc792>.
BIBTEX
@misc{rfc792,
series = {Request for Comments},
number = 792,
howpublished = {RFC 792},
publisher = {RFC Editor},
doi = {10.17487/RFC0792},
url = {https://www.rfc-editor.org/info/rfc792},
author = {J. Postel},
title = {{Internet Control Message Protocol}},
pagetotal = 21,
year = 1981,
month = sep,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Network Working Group J. Postel
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).
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.
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.
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
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.
Time Exceeded 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
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
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.
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
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 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.
Source Quench 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 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
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.
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.
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.
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
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.
Echo or Echo 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+-+-+-
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
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.
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
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.
Information Request or Information 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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,
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.
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
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.
AI ELI5
On this page
PROBLEMIP is connectionless, so a gateway or host that drops or cannot deliver a datagram has no way to tell the sender why.
MECHANISMThe gateway or host sends an ICMP message, carried in an IP datagram with protocol number 1, back to the source. It carries a Type, a Code, a checksum and, for errors, the original IP header plus the first 64 bits of its data.
WHAT IT DOES NOT DOICMP reports problems but does not make IP reliable. It still underpins ping, traceroute and path MTU discovery.
§1Introduction
The Internet Control Message Protocol (ICMP) is used by network devices to send error messages and operational information. It is an integral part of the Internet Protocol suite, operating at the network layer alongside IP.
ICMP messages are encapsulated inside IP datagrams (protocol number 1). Though ICMP uses IP for delivery, it is considered part of the IP layer, not a transport protocol.
ICMP serves two primary purposes: error reporting (e.g., destination unreachable) and diagnostics (e.g., ping and traceroute).
ICMP is how the network says "oops". If a package cannot get through, a helper sends a short note back to the sender.
§2Message Types
ICMP messages are identified by a Type field and further refined by a Code field:
0Echo Reply — Response to a ping request
3Destination Unreachable — Packet could not be delivered
5Redirect — Better route available
8Echo Request — Ping request
11Time Exceeded — TTL expired in transit
12Parameter Problem — Bad IP header
Each note starts with a number that says what kind of note it is, and a second number with more detail.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 1 byte | Identifies the message: 0 Echo Reply, 3 Destination Unreachable, 4 Source Quench, 5 Redirect, 8 Echo, 11 Time Exceeded, 12 Parameter Problem, 13/14 Timestamp, 15/16 Information. | |
CODE | Code | 1 byte | Refines the Type, for example the reason a destination was unreachable. | |
CHECKSUM | Checksum | 2 bytes | 16-bit one's complement of the one's complement sum of the ICMP message starting with the Type, computed with the field as zero. | |
REST | Type-specific | 4 bytes | Depends on the type. Unused (must be zero) in Destination Unreachable and Time Exceeded, which are followed by the original Internet Header + 64 bits of data. |
§3Echo Request/Reply
| Field | Name | Size | Echo · type 8 | Echo Reply · type 0 | Description |
|---|---|---|---|---|---|
ECHO-TYPE | Type | 1 byte | 8 | 0 | 8 for echo message, 0 for echo reply message. |
ECHO-CODE | Code | 1 byte | 0 | 0 | Always 0. |
ECHO-CHECKSUM | Checksum | 2 bytes | One's complement checksum of the ICMP message, recomputed for the reply. | ||
ECHO-ID | Identifier | 2 bytes | Aids in matching echos and replies. May be zero. Like a port: it can identify a session. | ||
ECHO-SEQ | Sequence Number | 2 bytes | Aids in matching echos and replies. May be zero. Can be incremented on each echo request sent. |
The Echo Request (Type 8) and Echo Reply (Type 0) messages are the basis of the ping utility, one of the most widely used network diagnostic tools.
Ping Example
$ ping 198.51.100.1 PING 198.51.100.1: 56 data bytes 64 bytes from 198.51.100.1: icmp_seq=0 ttl=64 time=1.234 ms 64 bytes from 198.51.100.1: icmp_seq=1 ttl=64 time=1.156 ms 64 bytes from 198.51.100.1: icmp_seq=2 ttl=64 time=1.089 ms
Ping sends a little "are you there?" with some data in it. The other computer sends the same data right back, which proves it is awake.
Source hostDestination host
- Echo Request (type 8)Carries an identifier, sequence number and data.
- Echo Reply (type 0)Addresses are reversed, the type becomes 0, the checksum is recomputed and the data is returned unchanged.
§4Error Reporting
When a router or destination host encounters a problem delivering a packet, it generates an ICMP error message and sends it back to the source.
Destination Unreachable (Type 3)
Sent when a packet cannot reach its destination. Code values distinguish: 0=Network Unreachable, 1=Host Unreachable, 3=Port Unreachable, 4=Fragmentation Needed but DF set.
Time Exceeded (Type 11)
Sent when a packet's TTL reaches zero (Code 0) or when fragment reassembly times out (Code 1). This is the basis of the traceroute utility.
Redirect (Type 5)
Sent by a router to inform a host that a better next-hop router exists for a particular destination.
ICMP error messages must never be generated in response to other ICMP error messages, to prevent infinite loops. RFC 1122 adds that they must not be sent in response to broadcast or multicast datagrams.
When a package cannot be delivered, runs out of hops, or could take a better road, the helper tells the sender. It never sends a note about a note, to avoid endless loops.
Source hostGateway
- Datagram, TTL expiresThe gateway finds the time to live is zero and must discard the datagram.
- Time Exceeded (type 11, code 0)The reply carries the original IP header plus 64 bits of data, so the source can match it.
§5Usage
ICMP is essential to several widely-used network diagnostic tools:
Tool
ICMP Types Used
Purpose
ping
Type 8 / Type 0
Test reachability and measure RTT
traceroute
Type 11 / Type 3
Map the path packets take
Path MTU Discovery
Type 3, Code 4
Find largest packet size for a path
Many firewalls filter ICMP traffic for security reasons. While blocking unnecessary types is reasonable, blocking all ICMP can break Path MTU Discovery and make debugging difficult.
Ping and traceroute are just clever uses of these notes, and path MTU discovery uses them to find out how big a package can be.