Internet Control Message Protocol

The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be reached. ICMP can also be used to relay query messages.^[1] It is assigned protocol number 1.^[2]

ICMP^[3] 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 Internet Protocol version 4 (IPv4) is also known as ICMPv4. IPv6 has a similar protocol, ICMPv6.

Technical details

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 (as specified in RFC 1122). ICMP errors are directed to the source IP address of the originating packet.^[1]

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 To Live exceeded in transit message is sent to the datagram's source address.

Although ICMP messages are contained within standard IP datagrams, 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 prompted the sending of the ICMP message.

Many commonly used network utilities are based on ICMP messages. The tracert (traceroute), 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.

ICMP segment structure

Header

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 first 4 bytes of the header will be consistent. The first byte is for the ICMP type. The second byte is for the ICMP code. The third and fourth 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.^[1]

ICMP error messages contain a data section that includes the entire IP header plus the first 8 bytes of data from the IP datagram that caused the error message. The ICMP datagram is then encapsulated in a new IP datagram.^[1]

• Type – ICMP type as specified below.
• Code – Subtype to the given type.
• Checksum – Error checking data. Calculated from the ICMP header+data, with value 0 for this field. The checksum algorithm is specified in RFC 1071.
• Rest of Header – Four byte field. Will vary based on the ICMP type and code.

Control messages

Source quench

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.

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

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 alternate 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 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. RFC1122 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, 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

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

Timestamp is used for time synchronization. It consists of the originating timestamp.

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

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.

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.

Address mask request

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.^[6]

Address mask reply

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

Destination unreachable is generated by the host or its inbound gateway]^[3] 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 (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the 'don't fragment' flag is on.

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.

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

• ICMP tunnel
• ICMP hole punching
• ICMPv6
• IRDP
• PMTU blackhole
• Pathping
• Path MTU Discovery
• Ping
• Smurf attack
• TCP

References

External links

• RFCs
  • RFC 792, Internet Control Message Protocol
  • RFC 950
  • 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 Router
  • RFC 1812, Requirements for IP Version 4 Routers
• IANA ICMP parameters
• IANA protocol numbers
• Explanation of ICMP Redirect Behavior
• Linux Kernel Networking (network_overview) by Rami Rosen