Internet Control Message Protocol
RFC 792
Document | Type |
RFC - Internet Standard
(September 1981; Errata)
Obsoletes RFC 777
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Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text pdf htmlized with errata bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 792 (Internet Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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). [Page 1] September 1981 RFC 792 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. [Page 2] September 1981 RFC 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 1981 RFC 792 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 |Show full document text