RFC 792: Internet Control Message Protocol (original) (raw)

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INTERNET STANDARD
Updated by: 950, 4884, 6633, 6918 Errata Exist

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](#ref-1 ""Internet Protocol - DARPA Internet Program Protocol Specification,"")] is used for host-to-host datagram service in a system of interconnected networks called the Catenet [[2](#ref-2 ""The Catenet Model for Internetworking,"")]. The network connecting devices are called Gateways. These gateways communicate between themselves for control purposes via a Gateway to Gateway Protocol (GGP) [[3](#ref-3 ""Gateway Routing: An Implementation Specification""),[4](#ref-4 ""How to Build a Gateway"")]. 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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RFC 792

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

IHL

  Internet header length in 32-bit words.

Type of Service

Total Length

  Length of internet header and data in octets.

Identification, Flags, Fragment Offset

  Used in fragmentation, see [[1](#ref-1 ""Internet Protocol - DARPA Internet Program Protocol Specification,"")].

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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September 1981 RFC 792

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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RFC 792

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

IP Fields:

Destination Address

  The source network and address from the original datagram's data.

ICMP Fields:

Type

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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RFC 792

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

IP Fields:

Destination Address

  The source network and address from the original datagram's data.

ICMP Fields:

Type

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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RFC 792

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

IP Fields:

Destination Address

  The source network and address from the original datagram's data.

ICMP Fields:

Type

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.

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September 1981 RFC 792

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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RFC 792

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

IP Fields:

Destination Address

  The source network and address of the original datagram's data.

ICMP Fields:

Type

Code

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

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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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RFC 792

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

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

  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.


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RFC 792

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

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

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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RFC 792

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

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](#ref-5 ""DCNET Internet Clock Service,"")].

  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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RFC 792

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

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

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,
  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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RFC 792

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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September 1981 RFC 792

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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