NAME

icmp − Internet Control Message Protocol

SYNOPSIS

None; included automatically with inet(4F).

DESCRIPTION

The Internet Control Message Protocol (ICMP) is used by gateways and destination hosts that process datagrams to communicate errors in datagram-processing to source hosts.  The datagram level of Internet is discussed in ip(4P). ICMP uses the basic support of IP as if it were a higher level protocol; however, ICMP is actually an integral part of IP. ICMP messages are sent in several situations, such as 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 that a control message will be returned.  Some datagrams can  be undelivered without any report of their loss.  The higher level protocols that use IP must implement their own reliability mechanisms, if reliable communication is required. 

The ICMP messages typically report errors in the processing of datagrams; for fragmented datagrams, ICMP messages are sent only about errors in handling fragment zero of the datagram.  To avoid the infinite regression of messages about messages, no ICMP messages are sent about ICMP messages.  However, ICMP can be sent in response to ICMP messages (for example, ECHOREPLY).  There are eleven types of ICMP packets that can be received by the system.  They are defined in <netinet/ip_icmp.h>, which also defines the values of some additional codes specifying the cause of certain errors.

ICMP_ECHOREPLYecho reply
ICMP_UNREACHdest unreachable, codes:
ICMP_UNREACH_NETbad net
ICMP_UNREACH_HOSTbad host
ICMP_UNREACH_PROTOCOLbad protocol
ICMP_UNREACH_PORTbad port
ICMP_UNREACH_NEEDFRAGIP_DF caused drop
ICMP_UNREACH_SRCFAILsrc route failed
ICMP_SOURCEQUENCHpacket lost, slow down
ICMP_REDIRECTshorter route, codes:
ICMP_REDIRECT_NETfor network
ICMP_REDIRECT_HOSTfor host
ICMP_REDIRECT_TOSNETfor tos and net
ICMP_REDIRECT_TOSHOSTfor tos and host
ICMP_ECHOecho service
ICMP_TIMXCEEDtime exceeded, code:
ICMP_TIMXCEED_INTRANSttl==0 in transit
ICMP_TIMXCEED_REASSttl==0 in reass
ICMP_PARAMPROBip header bad
ICMP_TSTAMPtimestamp request
ICMP_TSTAMPREPLYtimestamp reply
ICMP_IREQinformation request
ICMP_IREQREPLYinformation reply

Arriving ECHO and TSTAMP packets cause the system to generate ECHOREPLY and TSTAMPREPLY packets.  IREQ packets are not yet processed by the system, and are discarded.   UNREACH, SOURCEQUENCH, TIMXCEED and PARAMPROB packets are processed internally by the protocols implemented in the system, or reflected to the user if a raw socket is being used; see ip(4P). REDIRECT, ECHOREPLY, TSTAMPREPLY and IREQREPLY are also reflected to users of raw sockets.  In addition, REDIRECT messages cause the kernel routing tables to be updated; see routing(4N).

SEE ALSO

inet(4F), ip(4P)
Internet Control Message Protocol, RFC792, J. Postel, USC-ISI

BUGS

IREQ messages are not processed properly: the address fields are not set. 

Messages that are source routed are not sent back using inverted source routes, but go back through the normal routing mechanisms. 

4BSD