cnap 1 08 icmp protocol

8/3/2019 Cnap 1 08 ICMP Protocol

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Module 8

TCP/IP Suite Error

and

Control Messages

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Objectives

Describe ICMP

Describe the ICMP message format

Identify ICMP error message types

Identify potential causes of specific ICMP errormessages

Describe ICMP control messages

Identify a variety of ICMP control messages used innetworks today

Determine the causes for ICMP control messages

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Table of Content

TCP/IP Suite Control Messages2

Overview of TCP/IP Error Message1

4

OVERVIEW

OF

TCP/IP ERROR MESSAGES


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Internet Control Message Protocol (ICMP)

Transport

Network

IP

TCP UDP

ICMPARP RARP

Destination unreachableEcho (Ping)Others

ICMP is the component of the TCP/IP protocol stack

that addresses this basic limitation of IP.

Used to report these errors back to the source of thedatagram.

An error reporting protocol for IP.6

Error reporting and error correction

Workstation 6

CA

Workstation 1

Fa0/0

ICMP

Destinationunreachable

IP

ICMP reports on the status of the delivered packet

only to the source device. It does not propagate information about network

changes to routers.

Does not correct the encountered network problem

C

7

Encapsulation of an ICMP in an IP packet

Datagram Header ICMP Header ICMP DataFrame Header

Option Data

Option Header

Code

8

Checksum

16

Type

310

8

Destination unreachable

CA

To Z

Send DataTo Z

I don not knowHow to get to Z!

Send ICMP

Data network

Destinationunreachable

An ICMP destination unreachable message is send if:

Host or port unreachable.

Network unreachable.


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Destination unreachable message

Internet Header + First 64 bits of datagram

Code(0-12)

8

Checksum

16

Unused (must be zero)

Type(3)

310

The value of 3 in the type field indicates it is adestination unreachable message.

The code value indicates the reason the packetcould not be delivered.

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Code values for destination unreachable message

Host unreachable for type of device12

Network unreachable for type of device11

Communication with destination network administratively prohibited10

Communication with destination network administratively prohibited9

Source Host Isolated8Destination host unknown7

Destination network unknown6

Source route failed.5

Fragmentation needed and DF set.4

Port unreachable.3

Protocol unreachable.2

Host unreachable.1

Net unreachable.0

DescriptionCode

1

1

Using ping to test destination reachability

ACA

Is Breachable

Yes, I amhere.

B

ICMP echo reply

ICMP echo request

1

2

ICMP echo messages

Sequence numberIdentifier

Option Data

Code (0)

8

Checksum

16

Type (0 or 8)

310

The value of 0 in the type field indicates it is theecho request.

The value of 8 in the type field indicates it is theecho reply.


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Miscellaneous error reporting

Unused ( Must be zero)Pointer


Code (0-2)

8

Checksum

16

Type (12)

310

Parameter problem.

When the code value is 0, the pointer field indicatesthe octet of the datagram that produced the error.

14

Detecting excessively long routes

When the TTL of the datagram value reaches zero, thepacket is discarded.

ICMP uses a time exceeded message to notify thesource device that the TTL of the datagram has beenexceeded

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5

TCP/IP SUITE CONTROL MESSAGES

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6

ICMP Type of Control messages

Address Mask Reply18

Address Mask Request17

Information Reply16

Information Request15

Timestamp reply.14

Timestamp.13

Parameter problem.12

Time exceeded.11

Router Selection10

Routers advertisment9

Echo Request8

Redirect / Change request5

Source quench.4

Destination unreachable.3

Echo reply.0

DescriptionType

Unlike error messages, controlmessages are used to informhosts of conditions such asnetwork congestion or theexistence of a better gateway to a

remote network.

Like all ICMP messages, ICMPcontrol messages areencapsulated within an IPdatagram.

ICMP uses IP datagrams in orderto traverse multiple networks.


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ICMP redirect/change requests

Router A

172.16.1.100 172.16.1.200E0 E0

172.16.1.1/24

Default GW:172.16.1.100

10.0.0.1/8C

B

Router B

Router A sends an ICMP redirect/change request toHost B telling it to use Router B as the gateway toforward all future requests to network 10.0.0.0/8.

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Conditions to send ICMP redirect/change request

Default gateways only send ICMP redirect/change requestmessages if the following conditions are met:

The interface on which the packet comes into the router isthe same interface on which the packet gets routed out.

The subnet/network of the source IP address is the samesubnet/network of the next-hop IP address of the routedpacket.

The datagram is not source-routed.

The route for the redirect is not another ICMP redirect or adefault route.

The router is configured to send redirects. (By default,Cisco routers send ICMP redirects. The interfacesubcommand no ip redirects will disable ICMP redirects.)

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The ICMP redirect/change request message


Router Internet address

Code (0-3)

8

Checksum

16

Type (5)

310

The Router Internet Address field in the ICMPredirect is the IP address that should be used as thedefault gateway for a particular network.

2

0

Clock synchronization and transit time estimation

Transit Timestamp


Receive Timestamp

Originate Timestamp

Code (0)

8

Checksum

16

Type (13 or 14)

310

Allows a host to ask for the current time accordingto the remote host.

More robust protocols such as Network TimeProtocol (NTP) at the upper layers of the TCP/IPprotocol stack perform clock synchronization in a

more reliable manner.


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Information requests and reply message formats


Code (0)

8

Checksum

16

Type (15 or 16)

310

Originally intended to allow a host to determine itsnetwork number, is considered obsolete.

Other protocols such as BOOTP and Dynamic HostConfiguration Protocol (DHCP) are now used to allowhosts to obtain their network numbers.

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Address mask requirements

Address Mask


Code (0)

8

Checksum

16

Type (17 or 18)

310

Subnet mask is crucial in identifying network,subnet, and host bits in an IP address.

If a host does not know the subnet mask, it maysend an address mask request to the local router orbroadcast.

When the router receives the request, it will respondwith an address mask reply. This address mask replywill identify the correct subnet mask.

2

3

Router discovery message

Preferences Level 2

Router address 2Preferences Level 1

Router address 1

LifetimeAddress entry

sizeNumber ofaddresses

Code (0)

8

Checksum

16

Type (9)

310

Hosts use router discovery message to learn of availablerouters (gateway).

Using the multicast address 224.0.0.2 as the destinationaddress. May also be broadcast.

If router that does not support the discovery process, the

solicitation will go unanswered. 24

Router solicitation message

Reversed

Code (0)

8

Checksum

16

Type (10)

310

A host generates an ICMP router solicitation message in

response to a missing default gateway.This message is sent via multicast and it is the first step inthe router discovery process.

A local router will respond with a router advertisementidentifying the default gateway for the local host.


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Congestion and flow control messages

High speedLAN

Slow

link

Network congestion isexperienced at the WAN link

ICMP source-quench messages are used to reducethe amount of data lost.

The source-quench message asks senders to reducethe rate at which they are transmitting packets.

Most Cisco routers do not send source-quenchmessages by default

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Summary

IP uses Internet Control Message Protocol (ICMP) to notifythe sender of data that there was an error in the deliveryprocess.

ICMP messages are transmitted using the IP protocol sotheir delivery is unreliable.

ICMP echo request and echo reply messages allow thenetwork administrator to test IP connectivity to aid in thetroubleshooting process.

cnap 1 08 icmp protocol

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