CCNA 2 Week 8 TCP/IP Suite Error Control Messages

Copyright © 2005 University of Bolton Best Effort May remember from CCNA 1 that IP (layer 3) is unreliable and operated on Best Effort basis Data delivery may fail for many reasons –hardware failure, configuration/routing table errors IP uses ICMP to notify sender of delivery problems

Copyright © 2005 University of Bolton Error Notification Nothing in the basic design of IP notifies the sender that a data transmission has failed ICMP is the component of the TCP/IP protocol stack that addresses this basic limitation of IP ICMP doesn't overcome the unreliability issues Reliability is still provided by upper layer protocols

Copyright © 2005 University of Bolton Error Reporting When errors detected they are reported back to source Reports sent to source IP address only Does not notify routers along the way

Copyright © 2005 University of Bolton ICMP Encapsulation ICMP messages are encapsulated at layer 3 as packets and layer 2 frames Sent by same mechanisms as regular IP packets Subject to delivery failures too To prevent escalating numbers of error messages, failure to delivery ICMP is not notified

Copyright © 2005 University of Bolton Error Conditions TCP/IP must be properly configured at each end –Including IP address and subnet mask. A default gateway must be set if data will go outside the LAN Properly configured routers must be in place Failures may include destination device offline, intermediate links down Router may send "Destination Unreachable" ICMP packet

Copyright © 2005 University of Bolton Ping Used to test whether a remote device is reachable Sends ICMP echo request Triggers ICMP echo reply Packet includes a TTL value –Decremented each time it passes though a router –If it reaches zero, packet is dropped –Unreachable response may be sent

Copyright © 2005 University of Bolton ICMP Message Types Each ICMP packet type has a different format All types contain –Type –Code –Checksum Echo requests include Identifier and Sequence Number for example 0Echo Reply 3Destination Unreachable 4Source Quench 5Redirect/Change Request 8Echo Request 9Router Advertisement 10Router Selection 11Time Exceeded 12Parameter Problem 13Timestamp Request 14Timestamp Reply 15Information Request 16Information Reply 17Address Mask Request 18Address Mask Response

Copyright © 2005 University of Bolton Destination Unreachable

Copyright © 2005 University of Bolton Parameter Problem Errors in packet headers may prevent delivery ICMP type 12 error generated When code=0 pointer indicates problem octet in datagram

Copyright © 2005 University of Bolton Control Messages ICMP also allows devices to send control messages at layer 3 Can inform devices about congestion Can identify alternative gateways

Copyright © 2005 University of Bolton ICMP Redirect Request All hosts configured with a default gateway (router) to forward packets to other networks Where more than one router is on a segment, may be better to direct packets via other router for some destinations Router will send ICMP redirect (5) message if outbound interface is same as source interface

Copyright © 2005 University of Bolton Timestamps The ICMP timestamp request (13) message allows a host to check the time at the remote host. The remote host uses an ICMP timestamp reply (14) message to respond to the request Times are expressed in milliseconds past midnight GMT

Copyright © 2005 University of Bolton Information Request Info Req (type 15) and response (type 16) can be used to configure network settings on a host This protocol is obsolete Alternatives: –DHCP –Bootp –RARP

Copyright © 2005 University of Bolton

Address Mask Request Devices can learn their netmask from local router if not already configured Request (Type 17) and Response (Type 18) If address of router is unknown, request may be sent as broadcast

Copyright © 2005 University of Bolton Router Solicitation ICMP Type 9 Sent when default gateway not configured Multicast to Router responds with router advertisement which identifies default gateway (Type 10)

Copyright © 2005 University of Bolton Source Quench If a host is receiving too much traffic, can send ICMP source quench (4) Asks sender to reduce sending rate Cisco routers don't send by default because SQ packets can increase congestion May be appropriate in small networks