Echo messages

Echo messages

8.1.7 This page will provide information about ICMP messages.

As with any type of packet, ICMP messages have special formats. Each ICMP message type shown in Figure has its own unique characteristics. All ICMP message formats start with the same three fields:

• Type

• Code

• Checksum

The type field indicates the type of ICMP message being sent. The code field includes further information specific to the message type. The checksum field, as in other types of packets, is used to verify the integrity of the data.

Figure shows the message format for the ICMP echo request and echo reply messages. The relevant type and code numbers are shown for each message type. The identifier and sequence number fields are unique to the echo request and echo reply messages. The identifier and sequence fields are used to match the echo replies to the corresponding echo request. The data field contains additional information that may be a part of the echo reply or echo request message.

The Interactive Media Activity will test the ability of students to place the ICMP message fields in the correct order.

The next page will explain why destination unreachable messages occur

Detecting excessively long routes

Detecting excessively long routes

8.1.6 This page will explain how excessively long routes are created.

Situations can occur in network communication where a datagram travels in a circle, never reaching its destination. This might occur if two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination. When there are several routers involved, a routing cycle is created. In a routing cycle, a router sends the datagram to the next hop router and thinks the next hop router will route the datagram to the correct destination. The next hop router then routes the datagram to the next router in the cycle. This can be caused by incorrect routing information.

The limitations of the routing protocol can result in unreachable destinations. The hop limit of RIP is 15, which means that networks that are greater than 15 hops will not be learned through RIP.

In either of these cases, an excessively long route exists. Whether the actual path includes a circular routing path or too many hops, the packet will eventually exceed the maximum hop count.

The next page will discuss ICMP messages

Detecting excessively long routes

Detecting excessively long routes

8.1.6 This page will explain how excessively long routes are created.

Situations can occur in network communication where a datagram travels in a circle, never reaching its destination. This might occur if two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination. When there are several routers involved, a routing cycle is created. In a routing cycle, a router sends the datagram to the next hop router and thinks the next hop router will route the datagram to the correct destination. The next hop router then routes the datagram to the next router in the cycle. This can be caused by incorrect routing information.

The limitations of the routing protocol can result in unreachable destinations. The hop limit of RIP is 15, which means that networks that are greater than 15 hops will not be learned through RIP.

In either of these cases, an excessively long route exists. Whether the actual path includes a circular routing path or too many hops, the packet will eventually exceed the maximum hop count.

The next page will discuss ICMP messages.

Use ping to test destination reachability

Use ping to test destination reachability

8.1.5 This page will explain how the ping command can be used to test the reachability of a network.

The ICMP protocol can be used to test the availability of a particular destination. Figure shows ICMP being used to issue an echo request message to the destination device. If the destination device receives the ICMP echo request, it formulates an echo reply message to send back to the source of the echo request. If the sender receives the echo reply, this confirms that the destination device can be reached using the IP protocol.


The echo request message is typically initiated with the ping command as shown in Figure . In this example, the command is used with the IP address of the destination device. The command can also be entered with the IP address of the destination device as shown in Figure . In these examples, the ping command issues four echo requests and receives four echo replies. This confirms IP connectivity between the two devices.


As seen in Figure , the echo reply includes a time-to-live (TTL) value. TTL is a field in the IP packet header used by IP to provide a limitation on packet forwarding. As each router processes the packet, it decreases the TTL value by one. When a router receives a packet with a TTL value of 1, it will decrement the TTL value to 0 and the packet cannot be forwarded. An ICMP message may be generated and sent back to the source machine, and the undeliverable packet is dropped.

The next page will discuss excessively long routes

Unreachable networks

Unreachable networks

8.1.4 This page will explain why some networks are unreachable.

Network communication depends on some basic conditions that must be met. First, the TCP/IP protocol must be properly configured for devices that send and receive data. This includes the installation of the TCP/IP protocol and proper configuration of an IP address and subnet mask. A default gateway must also be configured if datagrams are to travel outside of the local network. Second, intermediary devices must be in place to route the datagram from the source device and its network to the destination network. Routers perform this function. A router also must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol.

If these conditions are not met, then network communication cannot take place. For instance, the sending device may address the datagram to a non-existent IP address or to a destination device that is disconnected from its network. Routers can also 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. If a destination network is not accessible, it is said to be an unreachable network.

Figures and show a router that receives a packet that cannot be delivered. The packet is undeliverable because there is no known route to the destination. Because of this, the router sends an ICMP host unreachable message to the source.

The next page will teach students how to test network reachability

ICMP message delivery

ICMP message delivery

8.1.3 This page will describe the delivery method that is used by ICMP.

ICMP messages are encapsulated into datagrams in the same way any other data is delivered when IP is used. Figure displays the encapsulation of ICMP data within an IP datagram.

Since ICMP messages are transmitted in the same way as any other data, they are subject to the same delivery failures. This creates a scenario where error reports could generate more error reports and cause increased congestion on a network. For this reason, errors created by ICMP messages do not generate their own ICMP messages. Therefore, it is possible to have a datagram delivery error that is never reported back to the sender of the data.

The next page will discuss unreachable networks.

Error reporting and error correction

Error reporting and error correction

8.1.2 This page will explain how ICMP reports errors for IP. When datagram delivery errors occur, ICMP is used to report these errors back to the source of the datagram. Look at the example in Figure . Workstation 1 tries to send a datagram to Workstation 6, but interface Fa0/0 on Router C goes down. Router C uses ICMP to send a message back to Workstation 1. The message indicates that the datagram could not be delivered. ICMP does not correct any network problems that it encounters, it only reports them.

When Router C receives the datagram from Workstation 1, it knows only the source and destination IP addresses of the datagram. It does not know the exact path that the datagram took. Therefore, Router C can only notify Workstation 1 of the failure and no ICMP messages are sent to Router A and Router B. ICMP reports on the status of the delivered packet only to the source device. It does not send information about network changes to other routers.

The next page will explain how ICMP message delivery occurs.