Table of Contents Link to heading
- Internet Control Message Protocol (ICMP)
- ICMP Message Encapsulation
- Error Reporting and Error Correction
- Reliability of ICMP
- Unreachable Networks
- Testing Destination Reachability
- ICMP Message Format
- Miscellaneous Error Reporting
Internet Control Message Protocol (ICMP) Link to heading
ICMP Message Encapsulation Link to heading
The encapsulation of an ICMP packet within the IP packet datagram data area is as follows:
| Frame header | IP datagram header | ICMP header | ICMP data |
| Frame header | IP datagram header | IP datagram data area | |
| Frame header | Frame data area | ||
Frame header can be from a LAN protocol (e.g. Ethernet) or a WAN protocol (e.g. HDLC).
Error Reporting and Error Correction Link to heading
Reliability of ICMP Link to heading
This creates a scenario in which error reports could generate even more error reports, causing increased congestion on an already ailing network.
For this reason, errors created by ICMP messages do not generate their own ICMP messages; therefore, it is possible for a datagram delivery error to occur but never be reported back to the sender of the data.
Unreachable Networks Link to heading
Network communication depends on certain basic conditions:
- The TCP/IP protocol stack must be properly configured in the sending and receiving devices. This includes the installation of TCP/IP and proper configuration of the IP address and the subnet mask. A default gateway also must be configured if datagrams are to travel outside the local network.
- Intermediary devices must be in place to route the datagram from the source device and its network to the destination network. Routers serve this function.
- A router must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol or static routes.
If these conditions are not met, network communication cannot take place. For example, the sending device might address the datagram to a nonexistent IP address or to a destination device that is disconnected from its network. Routers also can be points of failure if a connecting interface is down or if the router does not have the information necessary to find the destination network
Destination unreachable messages can include the following:
Network Unreachable Link to heading
Host Unreachable Link to heading
Protocol Unreachable Link to heading
Port Unreachable Link to heading
Testing Destination Reachability Link to heading
ping Command
Link to heading
This command is a very basic testing mechanism.
Cisco ping Return Codes Link to heading
| Code | Meaning | Possible Cause(s) |
|---|---|---|
| ! | Each exclamation point indicates receipt of an ICMP echo reply. | The ping completed successfully. |
| . | Each period indicates that the network server timed out while waiting for a reply. | This message can indicate many problems: - ping was blocked by an access list or firewall. - A router along the path did not have a route to the destination and did not send an ICMP destination unreachable message. - A physical connectivity problem occurred somewhere along the path. |
| U | An ICMP unreachable message was received. | A router along the path did not have a route to the destination address. |
| C | An ICMP source quench message was received. | A device along the path (possibly the destination) might be receiving too much traffic; check input queues. |
| & | An ICMP time exceeded message was received. | A routing loop might have occurred. |
ICMP Message Format Link to heading
Each ICMP message type has its own unique characteristics. However, all ICMP message formats start with the following three fields:
Type Link to heading
| ICMP | Message Types |
|---|---|
| 0 | Echo reply |
| 3 | Destination unreachable |
| 4 | Source quench |
| 5 | Redirect/change request |
| 8 | Echo request |
| 9 | Router advertisement |
| 10 | Router selection |
| 11 | Time exceeded |
| 12 | Parameter problem |
| 13 | Timestamp request |
| 14 | Timestamp reply |
| 15 | Information request |
| 16 | Information reply |
| 17 | Address mask request |
| 18 | Address mask reply |
Code Link to heading
| Code Value | Description |
|---|---|
| 0 | Network unreachable |
| 1 | Host unreachable |
| 2 | Protocol unreachable |
| 3 | Port unreachable |
| 4 | Fragmentation needed and DF set |
| 5 | Source route failed |
| 6 | Destination network unknown |
| 7 | Destination host unknown |
| 8 | Source host isolated |
| 9 | Communication with data network administratively prohibited |
| 10 | Communication with data host administratively prohibited |
| 11 | Network unreachable for type of service |
| 12 | Host unreachable for type of service |
Checksum Link to heading
Miscellaneous Error Reporting Link to heading
Devices that process datagrams might not be capable of forwarding a datagram because of some type of error in the header. This error does not relate to the state of the destination host or network, but it still prevents the datagram from being processed and delivered. In this case, an ICMP Type 12 parameter problem message is sent to the source of the datagram.