The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in This Chapter?(29.3.2)
• ICMP Messages—Although IP is only a best-effort protocol, the TCP/IP suite does provide for error messages and informational messages when communicating with another IP device. These messages are sent using the services of ICMP. The purpose of these messages is to provide feedback about issues related to the processing of IP packets under certain conditions, not to make IP reliable. ICMP is available for both IPv4 and IPv6. ICMPv4 is the messaging protocol for IPv4. ICMPv6 provides these same services for IPv6 but includes additional functionality.
An ICMP Echo message can be used to test the reachability of a host on an IP network. The local host sends an ICMP Echo Request to a host. If the host is available, the destination host responds with an Echo Reply.
When a host or gateway receives a packet that it cannot deliver, it can use an ICMP Destination Unreachable message to notify the source that the destination or service is unreachable. The message includes a code that indicates why the packet could not be delivered.
An ICMPv4 Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the TTL field of the packet was decremented to 0. If a router receives a packet and decrements the TTL field in the IPv4 packet to 0, it discards the packet and sends a Time Exceeded message to the source host.
ICMPv6 also sends a Time Exceeded message if the router cannot forward an IPv6 packet because the packet has expired. The informational and error messages found in ICMPv6 are very similar to the control and error messages implemented by ICMPv4. However, ICMPv6 includes four new protocols as part of the Neighbor Discovery Protocol, as follows:
• RS message
• RA message
• NS message
• NA message
• Ping and Traceroute Tests—To test connectivity to another host on a network, an Echo Request is sent to the host address using the ping command. If the host at the specified address receives the Echo Request, it responds with an Echo Reply. As each Echo Reply is received, ping provides feedback on the time between when the request was sent and when the reply was received. This can provide a measure of network performance. Ping has a timeout value for the reply. If a reply is not received within the timeout, ping provides a message indicating that a response was not received.
Types of connectivity tests performed with ping include the following:
• Pinging the local loopback—Ping can be used to test the internal configuration of IPv4 or IPv6 on the local host. To perform this test, ping the local loopback address.
• Pinging the default gateway—This is generally done by pinging the IP address of the default gateway of the host. A successful ping to the default gateway indicates that both the host and the router interface serving as the default gateway are operational on the local network.
• Pinging the remote host—A successful ping across the internetwork confirms communication on the local network, the operation of the router serving as the default gateway, and the operation of all other routers that might be in the path between the local network and the network of the remote host.
The traceroute (tracert) utility is used to generate a list of hops that were successfully reached along the path. This list can provide important verification and troubleshooting information. If the data reaches the destination, the trace lists the interface of every router in the path between the hosts. If the data fails at some hop along the way, the address of the last router that responded to the trace provides an indication of where the problem or security restrictions are found.
The round-trip time is the time a packet takes to reach the remote host and for the response from the host to return. An asterisk (*) is used to indicate a lost or unreplied packet. Traceroute makes use of a function of the TTL field in IPv4 and the Hop Limit field in IPv6 in the Layer 3 headers, along with the ICMP Time Exceeded message.
There’s not much point in setting up a network if you don’t test it to make sure it is operating properly. Diego needs to ensure that his network is working and that it connects to the network at headquarters and to the Internet. This chapter provided some of the most common troubleshooting tools used by network administrators the world over. You can even use these tools on your home network. Try it and see!