Network Working Group                                          J. PostelRequest for Comments:  792                                           ISI                                                          September 1981Updates:  RFCs 777, 760Updates:  IENs 109, 128                   INTERNET CONTROL MESSAGE PROTOCOL                         DARPA INTERNET PROGRAM                         PROTOCOL SPECIFICATIONIntroduction   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).                                                                [Page 1]                                                          September 1981RFC 792Message 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.[Page 2]                                                                September 1981RFC 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.                                                                [Page 3]                                                          September 1981RFC 792Destination 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[Page 4]                                                                September 1981RFC 792      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.                                                                [Page 5]                                                          September 1981RFC 792Time 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[Page 6]                                                                September 1981RFC 792      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.                                                                [Page 7]                                                          September 1981RFC 792Parameter 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.[Page 8]                                                                September 1981RFC 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.                                                                [Page 9]                                                          September 1981RFC 792Source 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[Page 10]                                                               September 1981RFC 792      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.                                                               [Page 11]                                                          September 1981RFC 792Redirect 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.[Page 12]                                                               September 1981RFC 792   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.                                                               [Page 13]                                                          September 1981RFC 792Echo 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[Page 14]                                                               September 1981RFC 792      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.                                                               [Page 15]                                                          September 1981RFC 792Timestamp 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[Page 16]                                                               September 1981RFC 792      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.                                                               [Page 17]                                                          September 1981RFC 792Information 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.[Page 18]                                                               September 1981RFC 792   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.                                                               [Page 19]                                                          September 1981RFC 792Summary 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[Page 20]                                                               September 1981RFC 792References   [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.