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Network Working Group J. PostelRequest for Comments: 777 ISI April 1981Updates: IENs 109, 128Updates: RFC 760 Internet Control Message ProtocolIntroduction 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.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: [Page 1]

                                                              April 1981RFC 777   Version      4   IHL      Internet header length in 32-bit words.   Type of Service      0   Total Length      Length of internet header and data in octets.   Identification, Flags, Fragment Offset      Used in fragmentation, see [1].   Time to Live      Time to live in seconds; as this field is decremented at each      machine in which the datagram is processed, the value in this      field should be at least as great as the number of gateways which      this datagram will traverse.   Protocol      ICMP = 1   Header Checksum      The 16 bit one's complement of the one's complement sum of all 16      bit words in the header.  For computing the checksum, the checksum      field should be zero.  This checksum may be replaced in the      future.   Source Address      The address of the gateway or host that composes the ICMP message.      Unless otherwise noted, this can be any of a gateway's addresses.   Destination Address      The address of the gateway or host to which the message should be      sent.[Page 2]

April 1981RFC 777Destination 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      3   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                                                                [Page 3]

                                                              April 1981RFC 777      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.[Page 4]

April 1981RFC 777Time 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 | 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. 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. [Page 5]

April 1981RFC 777Parameter 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 | Parameter | 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 = 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.[Page 6]

April 1981RFC 777Source 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 4 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. [Page 7]

                                                              April 1981RFC 777      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.[Page 8]

April 1981RFC 777Redirect 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 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. 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. [Page 9]

                                                              April 1981RFC 777   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.[Page 10]

April 1981RFC 777Echo 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.                                                               [Page 11]

                                                              April 1981RFC 777Timestamp 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].[Page 12]

April 1981RFC 777Summary 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                                                               [Page 13]

                                                              April 1981RFC 777References   [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.[Page 14]

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