    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).

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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

        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

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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

          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

          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

        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

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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

          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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    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

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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

        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.

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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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    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.

                                                                   [Page 21]