rfc0792-icmp-protocol.txt

一个rst守护程序

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.