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CCNA 2 Module 8 TCP/IP Suite Error and

Control Messages

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Objectives

• TCP/IP error messages

• TCP/IP control messages

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• IP is limited because it is a best effort delivery system. It has no mechanism to ensure whether the data is delivered.

• IP uses the Internet Control Message Protocol (ICMP) to notify the sender of the data that there was an error in the delivery process.

Internet Control Message Protocol (ICMP)

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Internet Control Message Protocol (ICMP) Continued

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Error Reporting and Error Correction

• ICMP is an error-reporting protocol for IP.

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ICMP Message Delivery

• ICMP messages are encapsulated as data in datagrams in the same way any other data is delivered using IP.

• Errors created by ICMP messages do not generate their own ICMP messages in order to prevent the congestion.

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Unreachable Networks

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Ping

• Traffic generated by ping command

• It’s used to confirms that the destination device can be reached via the IP protocol

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Detecting Excessively Long Routes

• Whether the actual path includes too many hops or a circular routing path exists, the packet will eventually reach the end of its life, known as its time-to-live (TTL).

• As each router processes the datagram, it decreases the TTL value by one.

• When the TTL reaches zero, the packet is discarded; and time exceeded message is send to notify the source

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ICMP Message

• Each ICMP message type has its own unique characteristics, but all ICMP message formats start with these same three fields

• Type - indicates the type of ICMP message being sent

• Code - includes further information specific to message type

• Checksum - used to verify the integrity of the data

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ICMP Message Types

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Code Values for Destination Unreachable Messages

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ICMP Echo Request and Echo Reply Message

• 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

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TCP/IP Suite Control Messages

• Control messages are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network.

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ICMP Redirect/Change Requests

• 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.

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Clock Synchronization and Transit Time Estimation

• The ICMP timestamp request message allows a host to ask for the current time according to the remote host.

• The remote host uses an ICMP timestamp reply message to respond to the request.

• The ICMP timestamp request contains an originate timestamp, which is the time on the requesting host just before the timestamp request is sent.

• The receive timestamp is the time that the destination host receives the ICMP timestamp request.

• The transmit timestamp is filled in just before the ICMP timestamp reply is returned.

• Originate, receive and transmit timestamps are computed in numbers milliseconds elapsed since midnight Universal Time (UT).

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Clock Synchronization and Transit Time Estimation Continued

• All ICMP timestamp reply messages contain the originate, receive and transmit timestamps. Using these three timestamps, the host can estimate transit time across the network by subtracting the originate time from the transit time.

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Information Requests or Replies Message

• The ICMP information requests and reply messages were originally intended to allow a host to determine its network number.

• 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.

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Address Mask Requirements

• When a network administrator uses the process of subnetting to divide a major IP address into multiple subnets, a new subnet mask is created.

• If a host does not know the subnet mask, it may send an address mask request to the local router. This request may be sent directly to the router or may be broadcast. When the router receives the request, it will respond with an address mask reply.

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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.

• This process begins with the host sending a router solicitation message to all routers, using the multicast address 224.0.0.2 as the destination address or using broadcast address 255.255.255.255.

• When a router that supports the discovery process receives the router discovery message , a router advertisement is sent in return.

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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.

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Congestion and Flow Control Messages

ICMP source-quench messages are used to reduce the amount of data lost from congestion.

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Summary

• IP is a best-effort delivery method that uses ICMP messages to alert the sender that the data did not reach its destination.

• ICMP echo request and echo reply messages allow the network administrator to test IP connectivity to aid in the troubleshooting process.

• ICMP packets have their own special header information starting with a type field and a code field.

• The functions of ICMP control messages:

• ICMP redirect/change request messages

• ICMP clock synchronization and transit time estimation messages

• ICMP information request and reply messages

• ICMP address mask request and reply messages

• ICMP router discovery message

• ICMP router solicitation message

• ICMP congestion and flow control messages