CCNA2_M8_TCIPIP_Error_Control_Messages (1130738)
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CCNA – Semester2Module 8TCP/IP and Error Control MessagesObjectives• TCP/IP error messages• TCP/IP control messagesOverview of TCP/IP ErrorMessageIP Limitation• IP is an unreliable, best effort deliverymechanism.• No built-in processes to ensure that data isdelivered in the event that problems exist withnetwork communication.• If an intermediary device such as a router fails,or if a destination device is disconnected fromthe network, data cannot be delivered.Internet Control Message Protocol• ICMP is the componentof the TCP/IP protocolstack that addressesthe basic limitation ofIP.• ICMP does notovercome theunreliability issues inIP.
Reliability must beprovided by upperlayer protocols if it isneeded.Error reporting and error correction•ICMP is an error reporting protocol for IP. When datagramdelivery errors occur, ICMP is used to report these errors backto the source of the datagram.ICMP message delivery• ICMP messages are encapsulated into datagrams in thesame way any other data is delivered using IP.• ICMP messages are transmitted in the same way as anyother data, they are subject to the same delivery failures.For this reason, errors created by ICMP messages donot generate their own ICMP messages.Network Communication• Network communication requires basicconditions:– sending and receiving devices must have the TCP/IPprotocol stack properly configured.– default gateway must also be configured if datagrams areto travel outside of the local network– intermediary devices must be in place to route thedatagram from the source device and its network to thedestination network.Unreachable NetworksUsing ping to test destination reachabilityDetecting excessively long routes• ICMP uses a time exceeded message to notify thesource device that the TTL of the datagram hasbeen exceeded.Echo messages• All ICMP message formats start with these same threefields:– Type: the type of ICMP message being sent.– Code: includes further information specific to the message type.– Checksum: verify the integrity of the data.ICMP Message TypeDestination unreachable message• Datagrams cannot always be forwarded to theirdestinations.
In these cases, ICMP delivers back to thesender a destination unreachable message indicating tothe sender that the datagram could not be properlyforwarded.Destination unreachable messageMiscellaneous error reporting• Devices that process datagrams may not be able toforward a datagram due to some type of error in theheader. In this case, an ICMP type 12 parameter problemmessage is sent to the source of the datagram.TCP/IP Suite Control MessagesIntroduction to control messages• Unlike error messages, control messages arenot the results of lost packets or errorconditions which occur during packettransmission.• Instead, they are used to inform hosts ofconditions such as network congestion or theexistence of a better gateway to a remotenetwork.ICMP Redirection• R1 forwards the packetand sends an ICMPredirect / change requestto Host H telling it to useR2 as the gateway toforward all futurerequests to network10.0.0.0/8.Redirect/Change Request MessageICMP Timestamp Request• The ICMP timestamp request message allows a host toask for the current time according to the remote host.The remote host uses an ICMP timestamp replymessage to respond to the request.Transit Time Estimation• Using the timestamps, the host can estimatetransit time across the networks.• The host that originated the timestamp requestcan also estimate the local time on the remotecomputer.• More robust protocols such as Network TimeProtocol (NTP) at the upper layers of the TCP/IPprotocol stack perform clock synchronization ina more reliable manner.Information requests and reply message• Allow a host to determine its network number.• Other protocols such as BOOTP and DHCP arenow used to allow hosts to obtain their networknumbers.Address mask requirements• If a host does not know the subnet mask, it maysend an address mask request to the localrouter.• When the router receives the request, it willrespond with an address mask reply.
Thisaddress mask reply will identify the correctsubnet mask.Router discovery message• The host, has not been manually configuredwith a default gateway, can learn of availablerouters through the process of router discovery.• This process begins with the host sending arouter solicitation message to all routers, usingthe multicast address 224.0.0.2 as thedestination address.Router discovery message• When a router that supports the discovery processreceives the router discovery message, a routeradvertisement is sent in return.Router solicitation message••A host generates an ICMP router solicitation message inresponse to a missing default gateway.A local router will respond with a router advertisementidentifying the default gateway for the local host.Congestion and flow control messages•••Packets dropped occur as there is too much congestionICMP source-quench ask senders to reduce the transmissionrateSender slowly increase the transmission rate as no othersource-quench messages are received.Summary•••••••ICMP error and control messagesICMP destination unreachableICMP echo messageICMP redirectICMP timestampICMP router discoveryICMP sourse-quenchCCNA2 – Module8.
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