1 Version 3.1 modified by Brierley Module 8 TCP/IP Suite Error and Control Messages

2 Version 3.1 modified by Brierley Internet Protocol (IP) IP is an unreliable method for delivery of network data. It is known as a best effort delivery mechanism. It has no built-in processes to ensure that data is delivered. Nothing in its basic design allows IP to notify the sender that a data transmission has failed.

3 Version 3.1 modified by Brierley Internet Control Message Protocol (ICMP) ICMP is the component of the TCP/IP protocol stack that addresses the basic limitation of IP. –ICMP does not overcome the unreliability issues in IP. –Reliability is provided by the upper layer protocols if it is needed. It is an error reporting protocol for IP. When datagram delivery errors occur, ICMP is used to report these errors back to the source of the datagram.

4 Version 3.1 modified by Brierley Internet Control Message Protocol (ICMP) Does not correct the encountered network problem; it merely reports the problem. 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. For this reason, it is possible to have a datagram delivery error that is never reported back to the sender

5 Version 3.1 modified by Brierley ICMP Messages Two types of ICMP messages: –Error messages - are the results of lost packets or error conditions which occur during packet transmission –Control messages- are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network.

6 Version 3.1 modified by Brierley Network Communication Depends upon certain basic conditions being met. First, the sending and receiving devices must have the TCP/IP protocol stack. –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. (continued)

7 Version 3.1 modified by Brierley Network Communication Depends upon certain basic conditions being met Second, intermediary devices (routers) must be in place to route the datagram from the source device through its network to the destination network. A router also must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol.

8 Version 3.1 modified by Brierley Destination Unreachable If a destination network is not accessible, it is said to be an unreachable network. Possible causes of failure: –the sending device may be addressing the datagram to a non-existent IP address –the default gateway may not be configured correctly –the destination device may be disconnected from its network –the routers interface is down –the router does not have adequate information necessary to find the destination network.

9 Version 3.1 modified by Brierley Destination Unreachable If a host or network is unreachable: –A reply will be sent by the router to the sending host  Destination Unreachable –The packet will be dropped –No further ICMP messages will be sent as a result of this transaction Trash Data Packet

10 Version 3.1 modified by Brierley ICMP Echo Request (PING) Can be used to test the availability of a particular destination. An ICMP echo request message ( ping ) is sent to the destination device. –If received - the ICMP echo request formulates an echo reply message back to the source. –If the sender receives the echo reply, this confirms the destination device can be reached via the IP protocol.

11 Version 3.1 modified by Brierley ICMP Echo Request (PING)

12 Version 3.1 modified by Brierley ICMP Echo Request (PING)

13 Version 3.1 modified by Brierley ICMP Echo Request (PING)

14 Version 3.1 modified by Brierley ICMP Echo Request (PING)

15 Version 3.1 modified by Brierley ICMP Echo Request (PING)

16 Version 3.1 modified by Brierley ICMP Request / Reply Message ICMP messages have special formats. All ICMP message formats start with these same three fields: –Type - indicates the type of ICMP message being sent –Code - indicates the reason the packet could not be delivered –Checksum - used to verify the integrity of the data Sequence Number DataIdentifierChecksumTypeCode

17 Version 3.1 modified by Brierley ICMP Message Types

18 Version 3.1 modified by Brierley ICMP Message Types Common ICMP messages: –Echo - used by the ping utility to test the connection between two devices –Echo reply - reply to a ping –Destination unreachable - a variety of reasons cause a destination to be unreachable (unavailable port, unknown network, etc.) –Source quench - tells the data source to reduce its transmission rate –Redirect - informs the source of a better route to the destination –Time exceeded - sent when a packet's time-to-live (TTL) reaches zero

19 Version 3.1 modified by Brierley ICMP Codes

20 Version 3.1 modified by Brierley Destination Unreachable Message Datagrams cannot always be forwarded to their destinations. If ICMP delivers back to the sender a destination unreachable message the problem could be: –hardware failures –improper protocol configuration –down interfaces –incorrect routing information –packet fragmentation is required in order to forward a packet –IP related services such as FTP or Web services are unavailable (application layer processes)

21 Version 3.1 modified by Brierley ICMP redirect/change request messages 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.)

22 Version 3.1 modified by Brierley ICMP redirect/change request messages Host B sends a packet to Host C on network /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 /8 by looking into its route table. Host B Host C Router ARouter B (continued)

23 Version 3.1 modified by Brierley ICMP redirect/change request messages It determines that the path to the network is back through the same interface that the request to forward the packet came from. It forwards the packet and sends an ICMP redirect/change request to Host B telling it to use Router B as the gateway to forward all future requests to network /8. Host B Host C Router ARouter B

24 Version 3.1 modified by Brierley ICMP Timestamp Message Hosts on different networks who are trying to communicate using software that requires time synchronization can sometimes encounter problems. The timestamp request message allows a host to ask for the current time according to the remote host. Remote hosts use the timestamp reply message to respond to the request. 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 transmit time.

25 Version 3.1 modified by Brierley ICMP Information Requests and Reply Messages The ICMP information requests and reply messages were originally intended to allow a host to determine its network number. It is now considered obsolete. Other protocols such as BOOTP and Dynamic Host Configuration Protocol (DHCP) are now used to allow hosts to obtain their network numbers.

26 Version 3.1 modified by Brierley ICMP Address Request 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. This address mask reply will identify the correct subnet mask from which the request was received.

27 Version 3.1 modified by Brierley ICMP Router Discovery This process begins with the host sending a router solicitation message to all routers, using the multicast address as the destination address. When a router that supports the discovery process receives the router discovery message, a router advertisement is sent in return.

28 Version 3.1 modified by Brierley ICMP Source-Quench Messages ICMP source-quench messages are used to reduce the amount of data lost due to congestion on the network. The source-quench message asks senders to reduce the rate at which they are transmitting packets. ICMP source-quench messages can be sent by the host or router (gateway).

29 Version 3.1 modified by Brierley Module 8 TCP/IP Suite Error and Control Messages End