RFC 777: Internet Control Message Protocol (original) (raw)

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](#ref-1 ""DOD Standard Internet Protocol"")] 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.

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

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 ""DOD Standard Internet Protocol"")].

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.

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April 1981 RFC 777

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 | 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

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.

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April 1981 RFC 777

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.

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.


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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

IP Fields:

Destination Address

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

ICMP Fields:

Type

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.

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April 1981 RFC 777

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 | 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

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.

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April 1981 RFC 777

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 | unused | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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.

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.

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April 1981 RFC 777

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 | unused | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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.

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.


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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 | unused | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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.

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

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April 1981 RFC 777

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.

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