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CCNA 2 v3.1 Module 8

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TCP/IP Suite Error and

Control Messages

CCNA 2 Module 8

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• IP

Uses best effort delivery of messages

No mechanism to ensure that the data is delivered

Data may fail to reach its destination due to

Hardware failure - router fails, destination device disconnected

Improper configuration

Incorrect routing information

No mechanism for sending error and control messages

it uses the ICMP to send and receive error and control messages to hosts on a network

• Internet Control Message Protocol (ICMP)

Notifies the sender that there was an error in the delivery

Used in network troubleshooting

444

ICMP

• Error reporting protocol for IP

• It reports these errors back to the source of the datagram

• ICMP messages are encapsulated into datagrams

• The following is the format of an ICMP message

555

Unreachable Networks• Network communication depends upon

Sending and receiving devices must have TCP/IP properly configured

TCP/IP protocol installed

IP address and subnet mask configured

If communicating outside LAN there must be a default gateway

Routers

Must have the TCP/IP protocol properly configured on its interfaces

Use an appropriate routing protocol

• Omission of the above conditions results in

Datagram sent to a non-existent IP address

Destination device that is disconnected from its network

Routers interface is down

Router has no known route to the destination

• If any of the above occur the router sends an ICMP host unreachable message to the source

666

Ping Tests Destination Reachability

•ICMP issues an echo request to the destination device

•If destination receives ICMP request, it formulates an echo reply

•Destination sends echo reply to source of the echo request

•If sender receives echo reply, this confirms destination reachable

777

Ping

888

Issues four echo requests

Receives four echo replies

Confirming IP connectivity between the two devices

IP Address of destination device

999

Detecting Excessively Long Routes• Faulty routing can cause a datagram to

Travels in a circle

Never reach its destination

• Faulty routes occur if

two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination

• How to prevent this

RIP hop count is a maximum 15

TTL – Time To Live – maximum hop count

TTL reduced by 1 each time it goes through a router

TTL of 0 – packet discarded and ICMP time exceeded message

101010

ICMP message type • All ICMP message formats start

with these same three fields:

Type

Type of ICMP message

Code

More information specific to the message type

Checksum

To verify the integrity of the data

ICMP echo request and echo reply

Identifier and Sequence Number are unique to the echo request / reply messages. They are used to match the echo replies to the corresponding echo request

111111

Destination unreachable message

• Reasons delivery is not possible:

Hardware failures

improper protocol configuration

down interfaces

incorrect routing information

packet fragmentation is required but not allowed (4)

services such as FTP or Web are unavailable (2)

•ICMP delivers back to the sender a destination unreachable message

121212

Miscellaneous error reporting

• ICMP type 12 parameter

Sent when datagrams have some type of error in the header and therefore, cannot be forwarded

• This ICMP message includes the pointer field in the header

• When the code value is 0

The pointer field indicates the octet of the datagram that produced the error

131313

TCP/IP Suite Control Messages

141414

ICMP Control Message

• IP cannot provide informational or control messages to hosts

• ICMP provides informational or control messages to hosts

• Error messages result from

Lost packets during packet transmission

Error conditions which occur during packet transmission

• Control messages are used to inform hosts of

Conditions such as network congestion

The existence of a better gateway to a remote network

• ICMP control messages are encapsulated within an IP datagram

151515

ICMP redirect/change requests

• This type of message can only be initiated by a gateway

• When a host connects to a segment that has two or more directly connected routers, the default gateway of the host may need to use a redirect/change request to inform the host of the best path to a certain network

Type 5 indicates redirect/change request

161616

ICMP redirect/change requests

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

• The interface on which the packet comes into the router is the same interface on which the packet gets routed out

ie subnet/network of the source IP address is the same subnet/network of the next-hop

• The datagram is not source-routed

• The route for the redirect is not another ICMP redirect or a default route

• The router is configured to send redirects

By default, Cisco routers send ICMP redirects

Router(config-if)no ip redirects will disable ICMP redirects

171717

Clock synchronization & transit time estimation

• Hosts on different networks using software that requires time synchronization can sometimes encounter problems.

• The ICMP timestamp message type is designed to help alleviate this problem.

• ICMP timestamp request message (13) allows a host to ask for the current time according to the remote host.

• Remote host uses an ICMP timestamp reply message (14) to respond to the request

• Provides a simple way to estimate time on a remote host and total network transit time

• Network Time Protocol (NTP) at the upper layers of the TCP/IP protocol stack perform clock synchronization in a more reliable manner

181818

Information requests and reply message formats

• ICMP information requests and reply messages were intended to allow a host to determine its network number

• This ICMP message type is considered obsolete

Other protocols such as BOOTP and DHCP are now used to allow hosts to obtain their network numbers. 

RequestMessage

ReplyMessage

191919

Address mask requirements

• Used by hosts to identify subnet mask when the network is subnetted and the host doesn’t know its subnet mask

• Request is sent directly to the router if the routers address is known, otherwise the request is broadcast

202020

Router Discovery Message

• If host has no default gateway

• Host sends a router solicitation message to all routers, using

multicast address 224.0.0.2 as the destination address

broadcast to include routers that may not be configured for multicasting

• If sent to a router that does not support the discovery process

solicitation is unanswered

• If sent to a router that supports the discovery process

router advertisement returned router advertisement

212121

Router solicitation message

• 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 in the router discovery process.

• A local router will respond with a router advertisement identifying the default gateway for the local host

222222

Congestion and flow control messages

• Multiple access to the same destination at the same time

• High speed LAN traffic reaches slower speed WAN connection

• If there is too much congestion on the network packets are dropped

• ICMP source-quench messages

are used to reduce the amount of data lost

Asks senders to reduce the rate at which they are transmitting packets

• If no other source-quench messages are received

The source will slowly increase the transmission rate

• Most Cisco routers do not send source-quench messages by default