Протокол передачи команд и сообщений об ошибках icm

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This article is about the support protocol for IPv4 (ICMPv4). For ICMPv6, see ICMPv6.

Communication protocol
A general header for ICMPv4
Purpose	Auxiliary protocol for IPv4[1]
Developer(s)	DARPA
Introduction	1981
OSI layer	Network layer
RFC(s)	RFC 792

The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when communicating with another IP address, for example, an error is indicated when a requested service is not available or that a host or router could not be reached.^[2] ICMP differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications (with the exception of some diagnostic tools like ping and traceroute).

ICMP for IPv4 is defined in RFC 792. A separate ICMPv6, defined by RFC 4443, is used with IPv6.

Technical details[edit]

ICMP 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 (as specified in RFC 1122). ICMP errors are directed to the source IP address of the originating packet.^[2]

For example, every device (such as an intermediate router) forwarding an IP datagram first 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 exceeded in transit message is sent to the datagram’s source address.

Many commonly used network utilities are based on ICMP messages. The traceroute command can be implemented by transmitting IP datagrams with specially set IP TTL header fields, and looking for ICMP time exceeded in transit and Destination unreachable messages generated in response. The related ping utility is implemented using the ICMP echo request and echo reply messages.

ICMP uses the basic support of IP as if it were a higher-level protocol, however, ICMP is actually an integral part of IP. Although ICMP messages are contained within standard IP packets, ICMP messages are usually processed as a special case, distinguished from normal IP processing. In many cases, it is necessary to inspect the contents of the ICMP message and deliver the appropriate error message to the application responsible for transmitting the IP packet that prompted the ICMP message to be sent.

ICMP is a network-layer protocol, this makes it layer 3 protocol by the 7 layer OSI model. Based on the 4 layer TCP/IP model, ICMP is an internet-layer protocol, which makes it layer 2 protocol (internet standard RFC 1122 TCP/IP model with 4 layers) or layer 3 protocol based on modern 5 layer TCP/IP protocol definitions (by Kozierok, Comer, Tanenbaum, Forouzan, Kurose, Stallings).^{[citation needed]} Forouzan and Kurose use network-layer instead of internet-layer in their TCP/IP model definition. These differences between models often lead to pointless and endless debates.^{[according to whom?]}

There is no TCP or UDP port number associated with ICMP packets as these numbers are associated with the transport layer above.^[3]

Datagram structure[edit]

The ICMP packet is encapsulated in an IPv4 packet.^[2] The packet consists of header and data sections.

[edit]

The ICMP header starts after the IPv4 header and is identified by IP protocol number ‘1’.^[4] All ICMP packets have an 8-byte header and variable-sized data section. The first 4 bytes of the header have fixed format, while the last 4 bytes depend on the type/code of that ICMP packet.^[2]

Type

ICMP type, see § Control messages.

Code

ICMP subtype, see § Control messages.

Checksum

Internet checksum (RFC 1071) for error checking, calculated from the ICMP header and data with value 0 substituted for this field.

Rest of header

Four-byte field, contents vary based on the ICMP type and code.

Data[edit]

ICMP error messages contain a data section that includes a copy of the entire IPv4 header, plus at least the first eight bytes of data from the IPv4 packet that caused the error message. The length of ICMP error messages should not exceed 576 bytes.^[5] 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 eight bytes of the original datagram’s data.^[6]

The variable size of the ICMP packet data section has been exploited. In the «Ping of death», large or fragmented ICMP packets are used for denial-of-service attacks. ICMP data can also be used to create covert channels for communication. These channels are known as ICMP tunnels.

Control messages[edit]

Control messages are identified by the value in the type field. The code field gives additional context information for the message. Some control messages have been deprecated since the protocol was first introduced.

Source quench[edit]

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 automatically slows down the outgoing data rate or waits 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 (and unfair) antidote for congestion»,^[11] routers’ creation of source quench messages was deprecated in 1995 by RFC 1812. Furthermore, forwarding of and any kind of reaction to (flow control actions) source quench messages was deprecated from 2012 by RFC 6633.

Where:

Type must be set to 4

Code must be set to 0

IP header and additional data is used by the sender to match the reply with the associated request

Redirect[edit]

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 (to send packets on an alternative route). 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 subnetwork), 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 change its route information and send packets for that destination directly to R2. The router will still send the original datagram to the intended destination.^[12] 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.

Where:

Type must be set to 5.

Code specifies the reason for the redirection, and may be one of the following:

Code	Description
0	Redirect for Network
1	Redirect for Host
2	Redirect for Type of Service and Network
3	Redirect for Type of Service and Host

IP address is the 32-bit address of the gateway to which the redirection should be sent.

IP header and additional data is included to allow the host to match the reply with the request that caused the redirection reply.

Time exceeded[edit]

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.

Where:

Type must be set to 11

Code specifies the reason for the time exceeded message, include the following:

Code	Description
0	Time-to-live exceeded in transit.
1	Fragment reassembly time exceeded.

IP header and first 64 bits of the original payload are used by the source host to match the time exceeded message to the discarded datagram. For higher-level protocols such as UDP and TCP the 64-bit payload will include the source and destination ports of the discarded packet.

Timestamp[edit]

Timestamp is used for time synchronization. The originating timestamp is set to the time (in milliseconds since midnight) the sender last touched the packet. The receive and transmit timestamps are not used.

Where:

Type must be set to 13

Code must be set to 0

Identifier and Sequence Number can be used by the client to match the timestamp reply with the timestamp request.

Originate timestamp is the number of milliseconds since midnight Universal Time (UT). If a UT reference is not available the most-significant bit can be set to indicate a non-standard time value.

Timestamp reply[edit]

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 indicating when the Timestamp was received and a transmit timestamp indicating when the Timestamp reply was sent.

Where:

Type must be set to 14

Code must be set to 0

Identifier and Sequence number can be used by the client to match the reply with the request that caused the reply.

Originate timestamp is the time the sender last touched the message before sending it.

Receive timestamp is the time the echoer first touched it on receipt.

Transmit timestamp is the time the echoer last touched the message on sending it.

All timestamps are in units of milliseconds since midnight UT. If the time is not available in milliseconds 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 use of Timestamp and Timestamp Reply messages to synchronize the clocks of Internet nodes has largely been replaced by the UDP-based Network Time Protocol and the Precision Time Protocol.^[13]

Address mask request[edit]

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.

Where:

Type must be set to 17

Code must be set to 0

Address mask can be set to 0

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.^[14]

Address mask reply[edit]

Address mask reply is used to reply to an address mask request message with an appropriate subnet mask.

Where:

Type must be set to 18

Code must be set to 0

Address mask should be set to the subnet mask

Destination unreachable[edit]

Destination unreachable is generated by the host or its inbound gateway^[6] to inform the client that the destination is unreachable for some reason. Reasons for this message may include: the physical connection to the host does not exist (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the ‘don’t fragment’ flag is on. Unreachable TCP ports notably respond with TCP RST rather than a destination unreachable type 3 as might be expected. Destination unreachable is never reported for IP multicast transmissions.

Where:

Type field (bits 0–7) must be set to 3

Code field (bits 8–15) is used to specify the type of error, and can be any of the following:

Code	Description
0	Network unreachable error.
1	Host unreachable error.
2	Protocol unreachable error (the designated transport protocol is not supported).
3	Port unreachable error (the designated protocol is unable to inform the host of the incoming message).
4	The datagram is too big. Packet fragmentation is required but the ‘don’t fragment’ (DF) flag is on.
5	Source route failed error.
6	Destination network unknown error.
7	Destination host unknown error.
8	Source host isolated error.
9	The destination network is administratively prohibited.
10	The destination host is administratively prohibited.
11	The network is unreachable for Type Of Service.
12	The host is unreachable for Type Of Service.
13	Communication administratively prohibited (administrative filtering prevents packet from being forwarded).
14	Host precedence violation (indicates the requested precedence is not permitted for the combination of host or network and port).
15	Precedence cutoff in effect (precedence of datagram is below the level set by the network administrators).

Next-hop MTU field (bits 48–63) contains the MTU of the next-hop network if a code 4 error occurs.^[15]

IP header and additional data is included to allow the client to match the reply with the request that caused the destination unreachable reply.

See also[edit]

References[edit]

Sources[edit]

RFCs[edit]

• RFC 792, Internet Control Message Protocol
• RFC 950, Internet Standard Subnetting Procedure
• RFC 1016, Something a Host Could Do with Source Quench: The Source Quench Introduced Delay (SQuID)
• RFC 1122, Requirements for Internet Hosts – Communication Layers
• RFC 1716, Towards Requirements for IP Routers
• RFC 1812, Requirements for IP Version 4 Routers
• RFC 4884, Extended ICMP to Support Multi-Part Messages

External links[edit]

• IANA ICMP parameters
• IANA protocol numbers
• Explanation of ICMP Redirect Behavior at the Wayback Machine (archived 2015-01-10)

Протокол Internet Control Message Protocol (ICMP) является важным протоколом сетевого уровня в компьютерных сетях. Он обеспечивает стандартизированный механизм передачи сетевыми устройствами важной информации, такой как подключение и состояние сети. Все устройства, подключенные к сети, включая маршрутизаторы и конечные устройства, могут обрабатывать сообщения ICMP. ICMP адаптирован для работы как с IPv4, так и с IPv6.

Подробнее о компьютерных сетях »

Далее мы обсудим некоторые распространенные варианты использования ICMP.

Отчеты об ошибках

Сообщения об ошибках ICMP информируют о сетевых ошибках – например, о недоступных местах назначения, тайм-аутах или проблемах фрагментации. Сообщения особенно важны для протокола пользовательских дейтаграмм (UDP), который использует модель коммуникации без подключения.

UDP не обеспечивает надежную упорядоченную доставку пакетов. При отправке пакета UDP существует вероятность, что пакет может быть утерян или доставлен с ошибками, например с ошибками контрольной суммы. Если это происходит, получатель отправляет сообщения об ошибках ICMP отправителю, чтобы уведомить его о проблеме.

Диагностика

ICMP можно использовать для диагностики сети. Чаще всего он используется для команд ping и traceroute.

Команда ping проверяет доступность сетевых устройств, отправляя пакеты эхо-запроса ICMP на целевое устройство. Если устройство доступно, оно возвращает эхо-ответ ICMP. Благодаря этому надежно проверяется задержка в сети и обеспечивается доступность устройства.

Команда traceroute отслеживает путь пакетов от источника до места назначения. Для этого команда отправляет сообщения эхо-запроса и эхо-ответа в указанное место назначения.

Эхо-запросы содержат значение времени жизни (TTL), которое уменьшается на единицу при каждом прохождении пакета через маршрутизатор. Когда пакет достигает маршрутизатора с нулевым значением TTL, маршрутизатор отправляет сообщение ICMP обратно источнику.

В сообщении содержится информация о маршруте, пройденном пакетом. С помощью команды Traceroute можно узнать точный путь пакета, чтобы получить представление о производительности сети.

сетевая безопасность;

ICMP можно использовать для обнаружения несанкционированного сетевого трафика и разрешать только законный трафик в сети. Брандмауэры используют ICMP для разрешения или блокировки определенных типов трафика. Сетевые администраторы также используют инструменты мониторинга ICMP для отслеживания состояния и подключения сетевых устройств, а также обнаружения неизвестных устройств.

Вы также можете использовать протокол для выявления необычных схем трафика, которые могут указывать на несанкционированную активность.

Internet Control Message Protocol (ICMP) is a network layer protocol used to diagnose communication errors by performing an error control mechanism. Since IP does not have an inbuilt mechanism for sending error and control messages. It depends on Internet Control Message Protocol(ICMP) to provide error control. 

ICMP is used for reporting errors and management queries. It is a supporting protocol and is used by network devices like routers for sending error messages and operations information. For example, the requested service is not available or a host or router could not be reached.

Uses of ICMP 

ICMP is used for error reporting if two devices connect over the internet and some error occurs, So, the router sends an ICMP error message to the source informing about the error. For Example, whenever a device sends any message which is large enough for the receiver, in that case, the receiver will drop the message and reply back ICMP message to the source.

Another important use of ICMP protocol is used to perform network diagnosis by making use of traceroute and ping utility. We will discuss them one by one.

Traceroute: Traceroute utility is used to know the route between two devices connected over the internet. It routes the journey from one router to another, and a traceroute is performed to check network issues before data transfer. 

Ping: Ping is a simple kind of traceroute known as the echo-request message, it is used to measure the time taken by data to reach the destination and return to the source, these replies are known as echo-replies messages.

How Does ICMP Work?

ICMP is the primary and important protocol of the IP suite, but ICMP isn’t associated with any transport layer protocol (TCP or UDP) as it doesn’t need to establish a connection with the destination device before sending any message as it is a connectionless protocol.

The working of ICMP is just contrasting with TCP, as TCP is a connection-oriented protocol whereas ICMP is a connectionless protocol. Whenever a connection is established before the message sending, both devices must be ready through a TCP Handshake.

ICMP packets are transmitted in the form of datagrams that contain an IP header with ICMP data. ICMP datagram is similar to a packet, which is an independent data entity. 

ICMP Packet Format

ICMP header comes after IPv4 and IPv6 packet header. 

In the ICMP packet format, the first 32 bits of the packet contain three fields:

Type (8-bit): The initial 8-bit of the packet is for message type, it provides a brief description of the message so that receiving network would know what kind of message it is receiving and how to respond to it. Some common message types are as follows:

• Type 0 – Echo reply
• Type 3 – Destination unreachable
• Type 5 – Redirect Message
• Type 8 – Echo Request
• Type 11 – Time Exceeded
• Type 12 – Parameter problem

Code (8-bit): Code is the next 8 bits of the ICMP packet format, this field carries some additional information about the error message and type.

Checksum (16-bit): Last 16 bits are for the checksum field in the ICMP packet header. The checksum is used to check the number of bits of the complete message and enable the ICMP tool to ensure that complete data is delivered.

The next 32 bits of the ICMP Header are Extended Header which has the work of pointing out the problem in IP Message. Byte locations are identified by the pointer which causes the problem message and receiving device looks here for pointing to the problem.

The last part of the ICMP packet is Data or Payload of variable length. The bytes included in IPv4 are 576 bytes and in IPv6, 1280 bytes.

ICMP in DDoS Attacks

In Distributed DOS (DDoS) attacks, attackers provide so much extra traffic to the target, so that it cannot provide service to users. There are so many ways through which an attacker executes these attacks, which are described below.

Ping of Death Attack

Whenever an attacker sends a ping, whose size is greater than the maximum allowable size, oversized packets are broken into smaller parts. When the sender re-assembles it, the size exceeds the limit which causes a buffer overflow and makes the machine freeze. This is simply called a Ping of Death Attack. Newer devices have protection from this attack, but older devices did not have protection from this attack.

ICMP Flood Attack

Whenever the sender sends so many pings that the device on whom the target is done is unable to handle the echo request. This type of attack is called an ICMP Flood Attack. This attack is also called a ping flood attack. It stops the target computer’s resources and causes a denial of service for the target computer.

Smurf Attack

Smurf Attack is a type of attack in which the attacker sends an ICMP packet with a spoofed source IP address. These type of attacks generally works on older devices like the ping of death attack.

Types of ICMP Messages

Source Quench Message

A source quench message is a request to decrease the traffic rate for messages sent to the host destination) or we can say when receiving host detects that the rate of sending packets (traffic rate) to it is too fast it sends the source quench message to the source to slow the pace down so that no packet can be lost. 

ICMP will take the source IP from the discarded packet and inform the source by sending a source quench message. The source will reduce the speed of transmission so that router will be free from congestion.  

When the congestion router is far away from the source the ICMP will send a hop-by-hop source quench message so that every router will reduce the speed of transmission.

Parameter Problem

Whenever packets come to the router then the calculated header checksum should be equal to the received header checksum then only the packet is accepted by the router. 

If there is a mismatch packet will be dropped by the router. 

ICMP will take the source IP from the discarded packet and inform the source by sending a parameter problem message. 

Time Exceeded Message

When some fragments are lost in a network then the holding fragment by the router will be dropped then ICMP will take the source IP from the discarded packet and informs the source, of discarded datagram due to the time to live field reaching zero, by sending the time exceeded message.  

Destination Un-reachable

The destination is unreachable and is generated by the host or its inbound gateway to inform the client that the destination is unreachable for some reason. 

There is no necessary condition that only the router gives the ICMP error message time the destination host sends an ICMP error message when any type of failure (link failure, hardware failure, port failure, etc) happens in the network. 

Redirection Message

Redirect requests data packets are sent on an alternate route. The message informs a host to update its routing information (to send packets on an alternate route). 

Example: If the host tries to send data through a router R1 and R1 sends data on a router R2 and there is a direct way from the host to R2. Then R1 will send a redirect message to inform the host that there is the best way to the destination directly through R2 available. The host then sends data packets for the destination directly to R2. 
The router R2 will send the original datagram to the intended destination. 
But if the datagram contains routing information then this message will not be sent even if a better route is available as redirects should only be sent by gateways and should not be sent by Internet hosts. 

Whenever a packet is forwarded in the wrong direction later it is re-directed in a current direction then ICMP will send a re-directed message.

For more, you can refer to Types of ICMP (Internet Control Message Protocol) Messages.

FAQs

1. What is ICMP used for?

Internet Control Message Protocol (ICMP) is used for error reporting. Error Reporting by ICMP works by sending messages to the sender from the receiver in the case when data is not received.

2. Is ICMP the same as ping?

ICMP and ping are two different things, but they are somehow related. ICMP is a protocol that manages the messages between the devices and Ping is produced using ICMP.

3. How does ICMP ping work?

ICMP ping is a way to check whether there is a connection established between two devices on the internet. We can check packet loss or any delay that happens within the network with the help of ICMP ping.