1. © 2002, Cisco Systems, Inc. All rights reserved.

2. 2 Frank Mann CCAI- CCNA Module 8: TCP/IP Suite Error and Control Messages

3. 8.1 Overview of TCP/IP Error Message IP is limited because it is a best effort delivery system. It has no mechanism to ensure that the data is delivered, regardless of the problems it may encounter on the network. Data may fail to reach its destination for a variety of reasons, such as hardware failure, improper configuration or incorrect routing information. To help identify these failures, IP uses the Internet Control Message Protocol (ICMP) to notify the sender of the data that there was an error in the delivery process.

4. Host to Host Communication Success Three conditions must be met in order for host to host communication over an internetwork to occur on IP networks: A default gateway must be properly configured on the hosts and local networks. Intermediary devices, such as routers, must be present on the network and have knowledge of how to reach the destination network. The IP addresses of the hosts must be in the same network or subnet as their respective gateway devices.

5. 8.1.1 Internet Control Message Protocol (ICMP) Internet Control Message Protocol (ICMP) is the component of the TCP/IP protocol stack that addresses this basic limitation of IP ICMP does not overcome the unreliability issues in IP Two functions are performed by the Internet Message Control Protocol Error messages to Hosts Control Messages to Hosts –Network Congestion –Default Gateway Available Reliability must be provided by upper layer protocols if it is needed

6. 8.1.2 Error reporting and error correction Assume Workstation 1 is sending a datagram to Workstation 6, but interface Fa0/0 on Router C goes down Router C then utilizes ICMP to send a message back to Workstation 1 indicating that the datagram could not be delivered. ICMP merely reports the problem

7. ICMP error messages ICMP does not correct the encountered network problem. ICMP just reports on the status of the delivered packet to the sender. ICMP error messages are returned to the sending device when a packet is dropped by a router. Its function is not to propagate information about network changes.

8. 8.1.3 ICMP message delivery ICMP messages are encapsulated into datagrams in the same way any other data is delivered using IP Errors created by ICMP messages do not generate their own ICMP messages. It is thus possible to have a datagram delivery error that is never reported back to the sender of the data.

9. 8.1.4 Unreachable networks

10. 8.1.5 Using ping to test destination reachability The ICMP protocol can be used to test the availability of a particular destination

11. 8.1.6 Detecting excessively long routes This is also known as reaching its time-to-live (TTL), because the TTL value typically matches the maximum hop count defined by the routing protocol. A TTL value is defined in each datagram

12. 8.1.7 Echo messages As with any type of packet, ICMP messages have special formats. Each ICMP message type shown has its own unique characteristics, but all ICMP message formats start with these same three fields: Type Code Checksum

13. ICMP Frame format The type field (8 bits) 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.

14. 8.1.8 Destination unreachable message The router sends an ICMP "Destination Unreachable" message back to the source. The router discards the original packet.

15. Code Values for Destination Unreachable

16. 8.2 TCP/IP Suite Control Messages Unlike error messages, control messages are not the results of lost packets or error conditions which 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. Like all ICMP messages, ICMP control messages are encapsulated within an IP datagram

17. 8.2.1 Introduction to control messages Multiple types of control messages are used by ICMP. eg., source quench and echo request

18. 8.2.2 ICMP redirect/change requests 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 In some circumstances, a host connects to a segment that has two or more directly connected routers. It requests that a host change a route

19. Redirects Default gateways only send ICMP Redirects/Change Requests 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. –router(config-if)# ip redirects

20. 8.2.3 Clock synchronization and transit time estimation While ICMP timestamp request messages provide a simple way to estimate time on a remote host and total network transit time, this is not the best way to obtain this information. ICMP timestamp request messages request –the current time of the remote host –gather information needed to estimate the transit time across the network Instead, more robust protocols such as Network Time Protocol (NTP) at the upper layers of the TCP/IP protocol stack perform clock synchronization in a more reliable manner.

21. 8.2.4 Information requests and reply message formats The ICMP information requests and reply messages were originally intended to allow a host to determine its network number. Figure shows the format for an ICMP information request and reply message. Two type codes are available in this message. Type 15 signifies an information request message, and type 16 identifies an information reply message. This particular ICMP message type is considered obsolete. Other protocols such as BOOTP and Dynamic Host Configuration Protocol (DHCP) are now used to allow hosts to obtain their network numbers.

22. 8.2.5 Address mask requirements If a host does not know the subnet mask, it may send an address mask request to the local router. If the address of the router is known, this request may be sent directly to the router. Otherwise, the request will be broadcast. When the router receives the request, it will respond with an address mask reply.

23. 8.2.6 Router discovery message When a host on the network boots, and the host has not been manually configured with a default gateway, it can learn of available routers through the process of router discovery

24. 8.2.7 Router solicitation message A host generates an ICMP router solicitation message in response to a missing default gateway. This message is sent via multicast and it is the first step in the router discovery process. A local router will respond with a router advertisement identifying the default gateway for the local host.

25. 8.2.8 Congestion and flow control messages In most cases, congestion will subside after a short period of time, and the source will slowly increase the transmission rate as long as no other source- quench messages are received. ICMP source-quench messages are used to reduce the amount of data lost. The source- quench message asks senders to reduce the rate at which they are transmitting packets.

26. Congestion Congestion occurs when multiple computers try to access the same receiver or when traffic from a high speed LAN reaches a slower WAN connection. The effect on congestion on a network is dropped packets that result in a loss of data. In order to reduce data lost, ICMP messages must be sent to the source of the congestion. This type of ICMP message is called a source- quench message. The source-quench message notifies the sender of the congestion and asks the sender to reduce its rate. A destination host or the Sender’s gateway may send the message. This usually reduces the congestion. Then the rate of transmission will slowly increase if no other source- quench messages are received.

27. Module 8: TCP/IP Suite Error and Control Messages There are no labs in this module.