Transcript CCNA2 3.0-08 TCPIP Suite Error and Control Messages

Cisco 2 - Routers
Chapter 8
TCP/IP Error Message
Some of the conditions that must be met in order for host to host
communication over an internetwork is:
• a default gateway must be properly configured on the hosts and
local network
• intermediary devices, such as routers, must be present on the
network and have knowledge of how to reach the destination
network.
• the IP addresses of hosts must be in the same network or subnet
as their respective gateway devices.
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TCP/IP Error Message
IP is an unreliable method for delivery of network data. It is a best effort
delivery.
Nothing in IP basic design allows it to notify the sender that a data
transmission has failed.
Internet Control Message Protocol (ICMP) is component of TCP/IP that
addresses this basic limitation.
NOTE:
• ICMP does not overcome the unreliability issues in IP
• reliability must be provided by upper layer protocols, if it is needed.
ICMP is an error reporting protocol for IP.
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TCP/IP Error Message
If ICMP fails, it does not produce ICMP error messages.
If a route is down, such as a serial connection between routers so that
the destination network is unreachable, the router will send ICMP host
unreachable message to the source.
Hence when a router doesn’t have the information to a destination, it:
• sends ICMP message of designation unreachable
• drop the datagram
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Test Reachability
To test reachability , use the ping command.
Ping sends echo request message to the destination.
The destination responds with echo reply message.
The messages are indicated in the type field (8 bits) of the ICMP message.
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Excessively Long Routes
To keep ICMP from route an infinite number of times it has a Time-To-Live
(TTL) timer.
The TTL value is defined by the routing protocol – RIP, hop count 15.
As the ICMP goes through each router the TTL is decremented, and when
it reaches zero, the datagram is discarded.
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Echo Message
All ICMP messages format start with these fields:
• type
• code
• checksum
For ICMP echo request & echo reply, there are two unique fields:
• identifier
• sequence
These two fields are used to match the echo request with the echo
reply.
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Unreachable Message
If a packet can not reach it’s destination, then ICMP sends a destination
unreachable message. The code value in the ICMP datagram is 0.
Other reasons for destination unreachable are:
• packet fragmentation from Token Ring to Ethernet
• FTP or Web services are unavailable
Some examples of code values:
0 = net unreachable
1 = host unreachable
2 = protocol unreachable
3 = port unreachable
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Miscellaneous Error Reporting
If there is an error in the header of a datagram, so it can’t be delivered to
it’s destination, ICMP has a type value of 12; code value of 0, and a pointer
field indicating the octet of the datagram that produced the error.
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ICMP Control Messages
Since IP has no built-in method to provide information or control
messages why a datagram can’t be delivered, ICMP performs these
functions.
Unlike error messages, control messages are not the results of lost
packets or error conditions which occur during packet transmission.
Instead, they are used to inform hosts of conditions such as network
congestion or the existence of a better gateway.
Some examples of control messages are:
0 = echo reply
3 = destination unreachable
5 = redirect/change request
8 = echo request
11 = time exceeded
12 = parameter problem
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Control Messages
Redirect/Change Request
Redirect/Change request can only be initiated by a gateway. The gateway
is the ethernet port address that a host uses to get to the network.
In some cases the host has a ‘hard’ coded gateway.
RouterA
RouterB
¾
M a c intos h SE
Gateway of the host is to RouterB. But the best route is to RouterA.
RouterB will send a ICMP redirect to the host. That is, router will send the packet
out the same interface that it was received on.
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Control Messages
Redirect/Change Request
To disable redirect messages on a router interface, do:
RouterA(config-if)# no ip redirects
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Control Messages
ICMP Time Stamp
The TCP/IP protocol suite allows systems to connect over vast distances.
Each individual networks might provide it’s own synchronization. If host
are using software that requires time synchronization can cause troubles.
ICMP timestamp request can be used. There is a timestamp request &
reply.
This ICMP message contain the originate, receive and transmit
timestamps to calculate the correct timing. This is used to request the
current time of the remote host, and gather information needed to
estimate the transit time across the network.
However, the Network Time Protocol (NTP) at the upper layers of the
TCP/IP is a more reliable manner.
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Control Message
Request & Reply
ICMP request & reply messages are considered obsolete.
Protocols such as BOOTP and DHCP now provide the service of
providing a network number.
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Control Message
Address Mask Request
This ICMP message is replied to a host by a router.
The reply contains the 32-bit mask for the subnet from which the
request was received.
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Control Message RouterDiscovery Message
If a host is missing a default gateway, it can generate an ICMP router
solicitation message.
Message is sent via multicast. A local router will respond with the gateway
address.
The router discovery process begins when a host requires a default
gateway IP address.
The router advertisement & solicitation messages are used during the
ICMP router discovery process.
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Congestion & Flow Control Messages
If a host encounters congestion because of difference in bandwidth, an
ICMP source-quench message can be sent to ask senders to reduce the
rate at which they are transmitting traffic.
Hence, the source-quench message
• a host can send the message
• a gateway may send the message
• message tells the sending device to slow down
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