Monday, April 23, 2018

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Routers and Routing Basics - TCP/IP Suite Error and Control Messages

“IP” is for “Isn’t Perfect”…

• TCP/IP is regarded as a reliable form of data delivery.

– Reliable, connection-oriented services provided by TCP at Layer 4.

– IP at Layer 3 is an unreliable, best-effort delivery protocol.

• No mechanisms built in to IP to tell it if something goes wrong.

– Where service reliability is taken care of by TCP, error reporting and service control is handled by ICMP.


ICMP apologises for the delay…

• Internet Control Message Protocol (ICMP) reports delivery errors to the source of the undelivered datagram.

– ICMP can only report errors, not correct errors.

• When preparing to report back to the source device about an undelivered datagram, ICMP adds its own header and datato the datagram before sending.

– No ICMP errors are generated from undeliverable ICMP error messages, hence no recursive ICMP message generation.

Dude, where’s my network?

• Internetwork communication won’t take place unless:

– TCP/IP stack is installed and functioning on all hosts.

• IP addresses, subnet masks, and default gateway addresses properly configured.

– At least one functioning router is in place to route data between networks.

• What happens if:

– A host doesn’t have a properly configured TCP/IP stack? The host is unreachable.

– An interface on a router goes down? The attached network is unreachable.

– Too many routers are between source and destination? The hop count for the routing protocol in use may be exceeded.

You’ve been pinged!

• To test if a host is reachable, issue the ping command.

• Ping sends an ICMP echo requestto the specified destination host.

– By default, ping sends five ICMP echo requests. The number of ICMP echo requests and timeout values can be changed in IOS.

• Upon receipt of an ICMP echo request, the destination device sends back an ICMP echo reply.

ICMP Redirect Requests

• In addition to reporting on delivery errors, ICMP can inform hosts of changing network conditions.

• One of the more important control messages is ICMP redirect request.

– Issued where a better gateway than the default gateway exists for access to a certain specific network.

ICMP has it all under control

• Other ICMP control messages include:

– ICMP timestamp

• Synchronises system clocks of two hosts.

• Largely rendered obsolete by Network Time Protocol (NTP).

– ICMP information request/reply

• Allows a host to determine its network number.

• Largely rendered obsolete by DHCP, RARP, and BootP.

– ICMP address mask request/reply

• Allows a host to discover its subnet mask.

– ICMP router discovery/advertisement message

• Allows a host not configured with a default gateway to discover routers via multicasting on 224.0.0.2.

Relieving a Blocked Node

• Dropped packets are the result of the network becoming too congested.

– Too many hosts are trying to access the same network resource at the same time.

– Traffic from a high speed network is being routed onto a lower speed network.

• ICMP source quench messages request to senders of dropped packets that they reduce their rate of data transmission until congestion subsides.

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