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Module 8 TCP/IP Suite Error and Control Messages IP • Unreliable method for delivery of network data • Known as a best effort delivery mechanism • No built-in processes to ensure that data is delivered in the event that problems exist with network communication • Reliability must be provided by upper layer protocols if it is needed ICMP • Error and Control reporting protocol for IP. – Component of the TCP/IP protocol stack that addresses the basic limitation of IP. – Does not correct the encountered network problem; it merely reports the problem. – ICMP reports on the status of the delivered packet (control messages) only to the source device (host). – Error messages are returned to the sending device (hosts) when a packet is dropped by the router. – It does not propagate information about network changes to routers!!! • Since ICMP messages are transmitted in the same way as any other data, they are subject to the same delivery failures. • This creates a scenario where error reports could generate more error reports, causing increased congestion on an already ailing network. • For this reason, errors created by ICMP messages do not generate their own ICMP messages. • It is possible to have a datagram delivery error that is never reported back to the sender of the data. • ICMP messages are encapsulated into datagrams in the same way any other data is delivered using IP Contain type#, code# and checksum Error in Header – Type 12 Parameter Problem • ICMP message has header and data areas • Complete ICMP message is treated as data in IP datagram • Complete IP datagram is treated as data in physical network frame ICMP Message IP Header Frame Header IP Data Frame Data ICMP header follows IP header, and contains eight bytes ICMP Message Format • An ICMP message is used for many different purposes (control and error report) • Each type of ICMP message has its own format • They all begin with the same three fields – an 8-bit message TYPE that identifies the message – an 8-bit CODE field that provides further information about the message type – a 16-bit CHECKSUM for the ICMP message, verify the integrity of the data Network communication • Depends upon certain basic conditions being met – The sending and receiving devices must have the TCP/IP protocol stack properly configured. – This includes the installation of the TCP/IP protocol and proper configuration of IP address and subnet mask. – A default gateway must also be configured if datagrams are to travel outside of the local network. – Intermediary devices must be in place to route the datagram from the source device and its network to the destination network. Routers serve this function. – A router also must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol. Long Routes • The limitations of the routing protocol can result in destinations being unreachable. • For example, RIP has a limit on the distance certain routing information is allowed to travel. The hop limit of RIP is 15, which means that a route will not be advertised further than 15 hops. • Datagram Life – Each router decreases the TTL value by 1 until it reaches zero. The datagram is then discarded and a destination unreachable message is sent to the source. The TTL value typically matches the maximum hop count defined by the routing protocol (not always). Size 8 Bits! Identified in ICMP message header sent by host – TYPE 8 sent by destination – TYPE 0 Router will drop the datagram – discards the original packet (sent to host!) TYPE 3 Control Messages • Use to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network • Encapsulated within an IP datagram Obsolete! Normally, Host B uses single gateway to reach networks not directly connected • Host H sends a packet to the host on network 10.0.0.0/8 • Host H is not directly connected to same LAN, so forwards the packet to its default gateway, R1 • Determines that the path to the network is back out the same interface the request to forward the packet came from • Forwards packet and sends an ICMP redirect/change request to Host H telling it to use R2 as the gateway when forwarding to 10.0.0.0/8 TYPE 5 - ICMP redirect or change, request the router to route the packet out of the same interface on which it was received 1. The following debug message shows router R1, as in the above diagram, sending a debug message to host H (172.16.1.1). Redirect sent by gateway– 172.16.1.100 R1#debug ip icmp ICMP packet debugging is on ICMP: redirect sent to 172.16.1.1 for dest 10.1.1.1, use gw 172.16.1.200 R1# 3. By default the router is configured to send redirects. The interface subcommand no ip redirects will disable ICMP redirects 2. Router R1 (172.16.1.100) is sending a redirect to host H (172.16.1.1) to use router R2 (172.16.1.200) as the gateway to reach the destination 10.1.1.1. ICMP Timestamp Message Type • ICMP timestamp request message (type 13) – Allows a host to ask for the current time according to the remote host – Contains an originate timestamp (time on requesting host) – Contains a receive timestamp (time the destination host received the timestamp request) – Transmit timestamp filled in just before the ICMP timestamp reply is returned • ICMP timestamp reply message (type 14) – Reply from remote host – Contains originate, receive and transmit timestamps – Using these timestamps, the host can estimate transit time across the network (transit – originate = transit time) – Other upper layer protocols do a better job ICMP Info Request and Reply • Intended to allow a host to determine a network number • Type 15 – signifies an information request message • Type 16 – identifies an information reply message • Considered obsolete • BOOTP (contains mac to ip address table) and DHCP are now used Address Mask Request • Used if a host does not know the subnet mask • Sent to the local router – Maybe unicast or broadcast • Will identify the correct subnet mask • Type 17 – request • Type 18 – reply • Reply is sent by the router – contains the 32-bit mask for the subnet from which the request was received Router Discovery Message • Type 9 • Host not manually configured with a default gateway • Uses multicast address 224.0.0.2 as the destination • Some routers do not support • When a router that supports the discovery process receives the router discovery message, a router advertisement is sent in return ICMP Router Solicitation • Type 10 • Generated by host in response to a missing default gateway • First step in router discovery process ICMP Source-Quench • Type 4 • Used when the destination computer is overwhelmed with traffic or traffic from a high speed LAN to a slower WAN connection • Asks senders to reduce the rate at which they are transmitting packets • After congestion subsides, transmission rate is increased • Most Cisco routers do not send by default (adds to network congestion) • Example: 4 computers using Internet over one 56K bandwidth, host acting as the gateway can request that the other hosts reduce transmission rates 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 messages are transmitted using the IP protocol so their delivery is unreliable. • ICMP packets have their own special header information starting with a type field and a code field. True or False 1. ICMP can correct many network problems as they occur 2. ICMP error messages are returned to the sending device when a packet is dropped by a router 3. The checksum part of the ICMP message header indicates that the message is either an echo request or an echo reply 4. An ICMP redirect occurs if the router routes the packet out of the same interface on which it was received 5. RIP is used to notify an IP host that transmission errors have occurred 6. An ICMP control message is sent to the destination if the network is experiencing congestion True or False 1. ICMP can correct many network problems as they occur 2. ICMP error messages are returned to the sending device when a packet is dropped by a router 3. The checksum part of the ICMP message header indicates that the message is either an echo request or an echo reply 4. An ICMP redirect occurs if the router routes the packet out of the same interface on which it was received 5. RIP is used to notify an IP host that transmission errors have occurred 6. An ICMP control message is sent to the destination if the network is experiencing congestion False – 1, 3, 5, 6 True or False 1. A source squelch is an example of a ICMP control messages 2. The purpose of an ICMP timestamp request message is to gather information needed to estimate the transit time across the network 3. Each router decreases the TTL value by one until it reaches zero. The datagram is then discarded and a destination unreachable message is sent to the source 4. If a router is unable to deliver a packet to its destination the router discards the original packet 5. A host or gateway may send the ICMP source-quench message 6. When an ICMP packet is not deliverable, an ICMP packet is returned to the original sender True or False 1. A source squelch is an example of a ICMP control messages 2. The purpose of an ICMP timestamp request message is to gather information needed to estimate the transit time across the network 3. Each router decreases the TTL value by one until it reaches zero. The datagram is then discarded and a destination unreachable message is sent to the source 4. If a router is unable to deliver a packet to its destination the router discards the original packet 5. A host or gateway may send the ICMP source-quench message 6. When an ICMP packet is not deliverable, an ICMP packet is returned to the original sender False – 1, 6 True or False 1. When an ICMP packet is not deliverable, an ICMP packet is returned to the nearest router in the path 2. When an ICMP packet is not deliverable, the packet is dropped 3. When a router does not have the information necessary to forward a datagram to the destination network the router will return the datagram to the source 4. The ICMP address mask reply message is sent by a host True or False 1. When an ICMP packet is not deliverable, an ICMP packet is returned to the nearest router in the path 2. When an ICMP packet is not deliverable, the packet is dropped 3. When a router does not have the information necessary to forward a datagram to the destination network the router will return the datagram to the source 4. The ICMP address mask reply message is sent by a host False – 1, 3, 4 Fill In The Blank • Two functions of ICMP is to provide ______ and ______ messages to hosts. • An ICMP echo _______ is generated by a host when the ping command is entered. • _______ and _______ are used instead of ICMP information request and reply messages. • _____ is used to notify an IP host that transmission errors have occurred. Fill In The Blank • Two functions of ICMP is to provide error and control messages to hosts. • An ICMP echo request is generated by a host when the ping command is entered. • BOOTP and DHCP are used instead of ICMP information request and reply messages. • ICMP is used to notify an IP host that transmission errors have occurred. Finally • IP does not have the facilities to send error messages. • It uses ICMP to send, receive, and process error and control messages. • http://www.faqs.org/rfcs/rfc792.html