RFC 792

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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).

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.
  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.

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
     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.

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
     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.

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.
  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.

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
     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.

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.
  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.

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
     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.

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
     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.

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.
  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.

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

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.

Ссылки

Файл:RFC-792.pdf