Internet Control Message Protocol is ...

Internet Control Message Protocol

The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is used by network devices, like routers, to send error messages indicating, for example, that a requested service isn't available or that a host or router could not be reached. ICMP can also be used to relay query messages. It is assigned protocol number 1.
ICMP differs from transport protocols such as TCP and UDP in that it isn't typically used to exchange data between systems, nor is it regularly employed by end-user network applications.
ICMP for Internet Protocol version 4 is also known as ICMPv4. IPv6 has a similar protocol, ICMPv6.
The Internet Control Message Protocol is part of the Internet Protocol Suite, as defined in RFC 792. ICMP messages are typically used for diagnostic or control purposes or generated in response to errors in IP operations. ICMP errors are directed to the source IP address of the originating packet.
For example, every device forwarding an IP datagram 1st decrements the time to live (TTL) field in the IP header by one. If the resulting TTL is 0, the packet is discarded and an ICMP Time To Live exceeded in transit message is sent to the datagram's source address.
Although ICMP messages are contained within standard IP packets, ICMP messages are usually processed as a special case, distinguished from normal IP processing, rather than processed as a normal sub-protocol of IP. In many cases, it is necessary to inspect the contents of the ICMP message and deliver the appropriate error message to the application that generated the original IP packet, the one that sent the packet that prompted the sending of the ICMP message.
Many commonly used network utilities are based on ICMP messages. The tracert, Pathping commands are implemented by transmitting UDP datagrams with specially set IP TTL header fields, and looking for ICMP Time to live exceeded in transit (above) and "Destination unreachable" messages generated in response. The related ping utility is implemented using the ICMP "Echo request" and "Echo reply" messages.
The ICMP header starts after the IPv4 header and is identified by protocol number '1'. All ICMP packets will have an 8-byte header and variable-sized data section. The 1st 4 bytes of the header will be consistent. The 1st byte is for the ICMP type. The 2nd byte is for the ICMP code. The 3rd and 4th bytes are a checksum of the entire ICMP message. The contents of the remaining 4 bytes of the header will vary based on the ICMP type and code.
ICMP error messages contain a data section that includes the entire IP header plus the 1st 8 bytes of data from the IP packet that caused the error message. The ICMP packet is then encapsulated in a new IP packet.
Source Quench requests that the sender decrease the rate of messages sent to a router or host. This message may be generated if a router or host does not have sufficient buffer space to process the request, or may occur if the router or host buffer is approaching its limit.
Data is sent at a very high speed from a host or from several hosts at the same time to a particular router on a network. Although a router has buffering capabilities, the buffering is limited to within a specified range. The router cannot queue any more data than the capacity of the limited buffering space. Thus if the queue gets filled up, incoming data is discarded until the queue is no longer full. But as no acknowledgement mechanism is present in the network layer, the client does not know whether the data has reached the destination successfully. Hence some remedial measures should be taken by the network layer to avoid these kind of situations. These measures are referred to as source quench. In a source quench mechanism, the router sees that the incoming data rate is much faster than the outgoing data rate, and sends an ICMP message to the clients, informing them that they should slow down their data transfer speeds or wait for a certain amount of time before attempting to send more data. When a client receives this message, it will automatically slow down the outgoing data rate or wait for a sufficient amount of time, which enables the router to empty the queue. Thus the source quench ICMP message acts as flow control in the network layer.
Since research suggested that ICMP Source Quench was an ineffective antidote for congestion, routers' creation of source quench messages was deprecated from 1995 by RFC1812. Furthermore, forwarding of and any kind of reaction to (flow control actions) source quench messages was deprecated from 2012 by RFC6633.
Redirect requests data packets be sent on an alternative route. ICMP Redirect is a mechanism for routers to convey routing information to hosts. The message informs a host to update its routing information. If a host tries to send data through a router (R1) and R1 sends the data on another router (R2) and a direct path from the host to R2 is available (that is, the host and R2 are on the same Ethernet segment), then R1 will send a redirect message to inform the host that the best route for the destination is via R2. The host should then send packets for the destination directly to R2. The router will still send the original datagram to the intended destination. However, if the datagram contains routing information, this message will not be sent even if a better route is available. RFC 1122 states that redirects should only be sent by gateways and should not be sent by Internet hosts.
Time Exceeded is generated by a gateway to inform the source of a discarded datagram due to the time to live field reaching zero. A time exceeded message may also be sent by a host if it fails to reassemble a fragmented datagram within its time limit.
Time exceeded messages are used by the traceroute utility to identify gateways on the path between two hosts.
Timestamp is used for time synchronization. It consists of the originating timestamp.
Timestamp Reply replies to a Timestamp message. It consists of the originating timestamp sent by the sender of the Timestamp as well as a receive timestamp and a transmit timestamp.
Address mask request is normally sent by a host to a router in order to obtain an appropriate subnet mask.
Recipients should reply to this message with an Address mask reply message.
ICMP Address Mask Request may be used as a part of reconnaissance attack to gather information on the target network, therefore ICMP Address Mask Reply is disabled by default on Cisco IOS.
Address mask reply is used to reply to an address mask request message with an appropriate subnet mask.
Destination unreachable is generated by the host or its inbound gateway] to inform the client that the destination is unreachable for some reason. A Destination Unreachable message may be generated as a result of a TCP, UDP or another ICMP transmission. Unreachable TCP ports notably respond with TCP RST rather than a Destination Unreachable type 3 as might be expected.
The error will not be generated if the original datagram has a multicast destination address. Reasons for this message may include: the physical connection to the host does not exist ; the indicated protocol or port isn't active; the data must be fragmented but the 'don't fragment' flag is on.

Related Sites for Internet Control Message Protocol

• Apply UniFi for Business read Internet Control Message Protocol
• ICMP, Internet Control Message Protocol - Network Sorcery Inc read Internet Control Message Protocol
• Internet Control Message Protocol â€"… read Internet Control Message Protocol
• Internet Control Message Protocol â€"… read Internet Control Message Protocol

Tweet