ICMP redirect/change reque

ICMP redirect/change request
8.2.2 This page will introduce the ICMP redirect request, which is a common ICMP control message. This type of message can only be initiated by a gateway, which is a term commonly used to describe a router. All hosts that communicate with multiple IP networks must be configured with a default gateway. This default gateway is the address of a router port connected to the same network as the host. Figure displays a host connected to a router that has access to the Internet. After Host B is configured with the IP address of FastEthernet 0/0 as its default gateway, it uses that IP address to reach any network that is not directly connected. Normally, Host B is connected to a single gateway. However, a host may be connected to a segment that has two or more directly connected routers. In this case, the default gateway of the host may need to use a redirect/change request to inform the host of the best path to a certain network. 
Figure shows a network where ICMP redirects would be used. Host B sends a packet to Host C on network 10.0.0.0/8. Since Host B is not directly connected to the same network, it forwards the packet to its default gateway, Router A. Router A finds the correct route to network 10.0.0.0/8 by looking into its route table. It determines that the path to the network is back out the same interface the request to forward the packet came from. It forwards the packet and sends an ICMP redirect/change request to Host B. The request instructs Host B to use Router B as the gateway to forward all future requests to network 10.0.0.0/8.
Default gateways only send ICMP redirect/change request messages if the following conditions are met: 

• The interface on which the packet comes into the router is the same interface on which the packet gets routed out.
• The subnet/network of the source IP address is the same subnet/network of the next-hop IP address of the routed packet.
• The datagram is not source-routed.
• The route for the redirect is not another ICMP redirect or a default route.
• The router is configured to send redirects. By default, Cisco routers send ICMP redirects. The interface subcommand no ip redirects will disable ICMP redirects.

The ICMP redirect/change request uses the format shown in Figure . It has an ICMP type code of 5. In addition, it has a code value of 0, 1, 2, or 3.
The Router Internet Address field in the ICMP redirect is the IP address that should be used as the default gateway for a particular network. In the example in Figure , the ICMP redirect sent from Router A to Host B would have a Router Internet Address field value of 172.16.1.200, which is the IP address of E0 on Router B.
The next page will describe clock synchronization.

TCP/IP Suite Control Messages / Introduction to control messages

Introduction to control messages 
8.2.1 This page will provide an overview of TCP/IP control messages.
ICMP is an important part of the TCP/IP protocol suite. All IP implementations must include ICMP support. The reasons for this are simple. Since IP does not guarantee delivery, it cannot inform hosts when errors occur. Second, IP has no built-in method to provide informational or control messages to hosts.
Unlike error messages, control messages are not the results of lost packets or error conditions that occur during packet transmission. Instead, they are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network. ICMP uses the basic IP header to travel through multiple networks.
Multiple types of control messages are used by ICMP. Some of the most common are shown in Figure . Many of these are discussed in this lesson.
The next page will describe ICMP redirect requests.

Miscellaneous error reporting

Miscellaneous error reporting
8.1.9 This page will explain what a parameter problem message is and why it occurs.
Devices that process datagrams may not be able to forward a datagram due to an error in the header parameter. This error does not relate to the state of the destination host or network but still prevents the datagram from being processed and delivered, and because of that, the datagram is discarded. In this case, an ICMP type 12 parameter problem message is sent to the source of the datagram. Figure shows the parameter problem message header.
The parameter problem message includes the pointer field in the header. When the code value is 0, the pointer field indicates the octet of the datagram that produced the error.
This page concludes this lesson. The next lesson will describe TCP/IP suite control messages. The first page will provide an overview of control messages.

Destination unreachable message

Destination unreachable message 
8.1.8 This page will explain what a destination unreachable message is and why it occurs.
Datagrams cannot always be forwarded to their destinations. Hardware failures, improper protocol configuration, down interfaces, and incorrect routing information are some of the factors that prevent successful delivery. In these cases, ICMP sends the sender a destination unreachable message, which indicates that the datagram could not be forwarded. 
Figure shows an ICMP destination unreachable message header. The value of 3 in the type field indicates it is a destination unreachable message. The code value indicates the reason the packet could not be delivered. Figure has a code value of 0, which indicates that the network was unreachable. Figure shows the meaning for each possible code value in a destination unreachable message.
A destination unreachable message may also be sent when packet fragmentation is required to forward a packet. Fragmentation is usually necessary when a datagram is forwarded from a Token Ring network to an Ethernet network. If the datagram does not allow fragmentation, the packet cannot be forwarded, so a destination unreachable message will be sent. Destination unreachable messages may also be generated if IP-related services such as FTP or Web services are unavailable. To effectively troubleshoot an IP network, it is necessary to understand the various causes of ICMP destination unreachable messages.
The next page introduces parameter problem messages.

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.