ICMP ( Internet Control Message Protocol )

Address Mask Request and Reply
ICMP can be used by a program to read the Address Mask (the subnet mask) value from another machine. You'll very rarely see this one on a network.

Checksum
Checksum is a simple error-detection scheme in which each transmitted message is accompanied by a numerical value based on the number of set bits in the message. The receiving station then applies the same formula to the message and checks to make sure the accompanying numerical value is the same. If not, the receiver can assume that the message has been garbled.

Congestion
Congestion is a signaling technique used by data transmission systems in order to indicate the status of network congestion. Devices that are communicating data across a network rely on congestion notification to determine when to send or delay the transmission of data packets.

Forward congestion notification indicates to upstream data switching devices that data is being transmitted through congested channels and some of the data or packets may be discarded. Backward congestion notification indicates to downstream devices that data is going through congested channels.

Destination Unreachable
This message is generated by a router to inform the source host that the destination address is unreachable. 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.

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.

Echo and Echo Reply
Used to implement the PING command. When an ICMP Echo is received it is responded to with an Echo Reply message. 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

Gateway
A network device used to translate between two different protocols. Used to interconnect two networks that use incompatible protocols. It is a node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.

In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.

It is also a computer system located on earth that switches data signals and voice signals between satellites and terrestrial networks and an earlier term for router, though now obsolete in this sense as router is commonly used.

Itrace
Itrace (ICMP Traceback) is often useful to learn the path that packets take through the Internet. This is especially important for dealing with certain denial-of-service attacks, where the source IP is forged. There areother uses as well, including path characterization and detection of asymmetric routes. There are existing tools, such as traceroute, but these generally provide the forward path, not the reverse.

We propose an ICMP Traceback message to help solve this problem. When forwarding packets, routers can, with a low probability,generate a Traceback message that is sent along to the destination. With enough Traceback messages from enough routers along the path, the traffic source and path can be determined.

MIB
MIB (Management Information Base) is database of objects that can be monitored by a network management system. Both SNMP and RMON use standardized MIB formats that allows any SNMP and RMON tools to monitor any device defined by a MIB.

Parameter
Characteristic, means defining the characteristics of something. In general, parameters are used to customize a program. For example, filenames, page lengths, and font specifications could all be considered parameters.

In programming, the term parameter is synonymous with argument, a value that is passed to a routine.

Parameter Problem
This message is generated as a response for any error not specifically covered by another ICMP message.

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). Code 0 may be received from a gateway or a host.

Ping
A utility to determine whether a specific IP address is accessible. It works by sending a packet to the specified address and waiting for a reply. PING is used primarily to troubleshoot Internet connections. There are many freeware and shareware Ping utilities available for personal computers.

It is often believed that "Ping" is an abbreviation for Packet Internet Groper, but Ping's author has stated that the names comes from the sound that a sonar makes.

Redirect
In operating system shells, redirection refers to directing input and output to files and devices other than the default I/O devices. By default, input generally comes from the keyboard or mouse, and output goes to the display monitor. With a redirection operator, you can override these defaults so that a command or program takes input from some other device and sends output to a different device. ICMP Redirect is a mechanism for routers to convey routing information to hosts.

Router
A device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP network. Routers are located at gateways, the places where two or more networks connect.

Routers use headers and forwarding tables to determine the best path for forwarding the packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts.

SNMP
SNMP (Simple Network Management Protocol) is a set of protocols for managing complex networks. The first versions of SNMP were developed in the early 80s. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices, called agents, store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters.

Source Quench
This message is a request to decrease the traffic rate of data messages sent to an internet destination.

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. Code 0 may be received from a gateway or a host.

TCP
TCP (Transmission Control Protocol) is one of the main protocols in TCP/IP networks. TCP is one of the main protocols in TCP/IP networks. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

TTL
TTL (Time to Live) is a field in the Internet Protocol (IP) that specifies how many more hops a packet can travel before being discarded or returned.

Time Exceeded
This data is used by the host to match the message to the appropriate process. 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.

Timeout
An interrupt signal generated by a program or device that has waited a certain length of time for some input but has not received it. Many programs perform time-outs so that the program does not sit idle waiting for input that may never come. For example, automatic bank-teller machines perform a time-out if you do not enter your password quickly enough.

Timestamp
A router has received a frame that has had its Time To Live field decremented to one. The router now decrements the field to zero and discards the frame.

TraceRoute
A utility that traces a packet from your computer to an Internet host, showing how many hops the packet requires to reach the host and how long each hop takes. If you're visiting a Web site and pages are appearing slowly, you can use traceroute to figure out where the longest delays are occurring.
The original traceroute is a UNIX utility, but nearly all platforms have something similar. Windows includes a traceroute utility called tracert. In Windows, you can run tracert by selecting Start->Run¡­, and then entering tracert followed by the domain name of the host.

For example: tracert www.pcwebopedia.com

Traceroute utilities work by sending packets with low time-to-live (TTL) fields. The TTL value specifies how many hops the packet is allowed before it is returned. When a packet can't reach its destination because the TTL value is too low, the last host returns the packet and identifies itself. By sending a series of packets and incrementing the TTL value with each successive packet, traceroute finds out who all the intermediary hosts are.

UDP
UDP (User Datagram Protocol) is a connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams over an IP network. It's used primarily for broadcasting messages over a network.

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