Service Unreachable

The Internet Control Messaging Protocol (ICMP) is a critical network layer protocol within the TCP/IP suite, enabling error reporting and diagnostics for IPv4 and IPv6. An Internet Protocol (IP) is unreliable because it does not provide messages to be sent in the event of specific errors. The Internet Control Message Protocol (ICMP) services send messages that include information about the network’s status rather than application data. It plays a key role in troubleshooting with tools like ping and traceroute, essential skills for CCNA/CCNP certification. Below is the simulator for the ping command.

Internet Control Messaging Protocol (ICMP) messages offer feedback on network issues, such as packet processing errors, without enhancing IP’s reliability, serving as a diagnostic tool. The Internet Control Messaging Protocol (ICMP) is often not required or allowed within a network due to security reasons. Security risks include Internet Control Messaging Protocol flood attacks; mitigate with access control lists (ACLs) or rate limiting, a critical topic.

Internet Control Messaging Protocol messages are distinct IP datagrams, separate from TCP and UDP. Ping uses ICMP Echo Request and Reply messages to test host reachability and measure round-trip time. It uses Internet Control Messaging Protocol (ICMP) to inspect remote hosts for reaction and measure the overall round-trip time of the investigated messages.

Internet Control Messaging Protocol supports both IPv4 (ICMPv4) and IPv6 (ICMPv6), with ICMPv6 offering enhanced functionality like Neighbor Discovery.

• The remote Host Confirmation message
• Destination Unreachable
• Service Unreachable
• Router Time exceeded Exceeded Message
• Route redirection

Comparison Table between ICMPv4 and ICMPv6

Remote Host Confirmation

We can use an Internet Control Messaging Protocol (ICMP)Echo Message to find out the availability and connectivity of the remote host. To find the availability and connectivity of the remote host, the local host sends an Internet Control Messaging Protocol (ICMP) Echo Request to the remote host. If the remote host is available and connected to the network and the configuration is correct, it responds with an Echo Reply. The Internet Control Messaging Protocol (ICMP) Echo messages are the basis of the ping utility.

Destination or Service Unreachable

When a gateway receives an Internet Control Messaging Protocol (ICMP) echo message destined for other hosts, and the destined host is not available, or there is some problem with that host and the gateway cannot send the echo message further, it replies with Destination Unreachable message to alert the source host that the destination host or service is unreachable. The message will contain a code that indicates why the packet could not be delivered. ICMPv6 has a similar code with a little difference for Destination Unreachable messages. Some Destination Unreachable codes for ICMPv4 are the following:

• 0 – Net unreachable
• 1 – Host unreachable
• 2 – Protocol unreachable
• 3 – Port unreachable

Example Packet Capture (Wireshark): ICMP Echo Request (Type 8, Code 0) and Reply (Type 0, Code 0).

Router Time Exceeded Message

The router uses a time-exceeded message to show that a packet cannot be forwarded because the packet’s Time To Live (TTL) field was decremented to 0. If a router receives a packet with the TTL field in the IPv4 packet to zero, it rejects the packet and sends a Time Exceeded message to the source host.

ICMPv6 also sends a Time Exceeded message if the router fails to forward an IPv6 packet expiration. IPv6 uses a hop limit field instead of the Time To Live (TTL) field to determine if the packet has expired.

TTL Exceeded Debugging

The “Time Exceeded” ICMP message is generated when a router discards a packet due to its Time To Live (TTL) or Hop Limit reaching zero. This is a common scenario in troubleshooting routing loops or misconfigured paths. For CCNA and CCNP students, understanding how to debug this issue is crucial.

Using “debug ip icmp” on Cisco IOS

To investigate TTL-exceeded messages, use the Cisco IOS command debug ip icmp on a router. This command displays real-time ICMP messages, including Time Exceeded notifications, helping you identify the source of the issue.

Router> enable

Router# configure terminal

Router(config)# interface GigabitEthernet0/0 Router(config-if)# ip address 192.168.1.1 255.255.255.0 Router(config-if)# no shutdown

Router(config)# debug ip icmp

Router(config)# end

Router# show ip route

Router# show ip cef

Router# show ip icmp statistics

Router# ping 192.168.1.10

Route Redirect

Route Redirect is an ICMP message sent by a router to inform the sender of a better next hop for a destination, optimizing routing paths.