Transcript CCNA2 3.1-08 TCPIP Suite Error and Control Messages

Module 8
TCP/IP Suite Error and
Control Messages
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Internet Protocol (IP)
• IP is an unreliable method for delivery of network data.
• It is known as a best effort delivery mechanism.
• It has no built-in processes to ensure that data is delivered.
• Nothing in its basic design allows IP to notify the sender that a
data transmission has failed.
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Internet Control Message Protocol (ICMP)
• ICMP is the component of the TCP/IP protocol stack that
addresses the basic limitation of IP.
– ICMP does not overcome the unreliability issues in IP.
– Reliability is provided by the upper layer protocols if it is needed.
• It is an error reporting protocol for IP.
• When datagram delivery errors occur, ICMP is used to report
these errors back to the source of the datagram.
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Internet Control Message Protocol (ICMP)
• Does not correct the encountered network problem; it merely
reports the problem.
• Messages are encapsulated into datagrams in the same way
any other data is delivered using IP.
• Errors created by ICMP messages do not generate their own
ICMP messages.
• For this reason, it is possible to have a datagram delivery error
that is never reported back to the sender
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ICMP Messages
• Two types of ICMP messages:
– Error messages - are the results of lost packets or error
conditions which occur during packet transmission
– Control messages- are used to inform hosts of conditions
such as network congestion or the existence of a better
gateway to a remote network.
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Network Communication
Depends upon certain basic conditions being met.
• First, the sending and receiving devices must have the TCP/IP
protocol stack.
– proper configuration of an IP address and subnet mask
– A default gateway must also be configured if datagrams are
to travel outside of the local network.
(continued)
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Network Communication
Depends upon certain basic conditions being met
• Second, intermediary devices (routers) must be in place to
route the datagram from the source device through its network
to the destination network. A router also must have the TCP/IP
protocol properly configured on its interfaces, and it must use
an appropriate routing protocol.
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Destination Unreachable
• If a destination network is not accessible, it is said to be an
unreachable network.
• Possible causes of failure:
– the sending device may be addressing the datagram to a
non-existent IP address
– the default gateway may not be configured correctly
– the destination device may be disconnected from its network
– the routers interface is down
– the router does not have adequate information necessary to
find the destination network.
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Destination Unreachable
• If a host or network is unreachable:
– A reply will be sent by the router to the sending host  Destination
Unreachable
– The packet will be dropped
– No further ICMP messages will be sent as a result of this
transaction
Trash
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ICMP Echo Request (PING)
• Can be used to test the availability of a particular destination.
• An ICMP echo request message (ping) is sent to the
destination device.
– If received - the ICMP echo request formulates an echo reply
message back to the source.
– If the sender receives the echo reply, this confirms the destination
device can be reached via the IP protocol.
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ICMP Echo Request (PING)
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ICMP Echo Request (PING)
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ICMP Echo Request (PING)
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ICMP Echo Request (PING)
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ICMP Echo Request (PING)
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ICMP Request / Reply Message
• ICMP messages have special formats.
Type
Code
Checksum
Identifier
Sequence
Number
Data
• All ICMP message formats start with these same three fields:
– Type - indicates the type of ICMP message being sent
– Code - indicates the reason the packet could not be delivered
– Checksum - used to verify the integrity of the data
0
1
2 3 4 5
Type (8 bits)
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Code (8 bits)
IMCP Header Checksum (16 bits)
Identifier (16 bits)
Sequence Number (16 bits)
Optional Data (variable length)
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ICMP Message Types
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ICMP Message Types
• Common ICMP messages:
– Echo - used by the ping utility to test the connection between two
devices
– Echo reply - reply to a ping
– Destination unreachable - a variety of reasons cause a destination
to be unreachable (unavailable port, unknown network, etc.)
– Source quench - tells the data source to reduce its transmission
rate
– Redirect - informs the source of a better route to the destination
– Time exceeded - sent when a packet's time-to-live (TTL) reaches
zero
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ICMP Codes
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Destination Unreachable Message
Datagrams cannot always be forwarded to their destinations.
• If ICMP delivers back to the sender a destination unreachable
message the problem could be:
–
–
–
–
–
–
hardware failures
improper protocol configuration
down interfaces
incorrect routing information
packet fragmentation is required in order to forward a packet
IP related services such as FTP or Web services are unavailable
(application layer processes)
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ICMP redirect/change request messages
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.
– The subnet/network of the source IP address is the same
subnet/network of the next-hop IP address of the routed packet.
– 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. The interface subcommand
no ip redirects will disable ICMP redirects.)
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ICMP redirect/change request messages
• Host B sends a packet to
Host C on network
10.0.0.0/8.
• Since Host B is not
directly connected to the
same network, it
forwards the packet to its
default gateway, Router
A.
• Router A finds the
correct route to network
10.0.0.0/8 by looking into
its route table.
Host C
Router A
Router B
Host B
(continued)
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ICMP redirect/change request messages
•
It determines that the path
to the network is back
through the same interface
that the request to forward
the packet came from.
•
It forwards the packet and
sends an ICMP
redirect/change request to
Host B telling it to use
Router B as the gateway to
forward all future requests
to network 10.0.0.0/8.
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Host C
Router A
Router B
Host B
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ICMP Timestamp Message
Hosts on different networks who are trying to communicate using
software that requires time synchronization can sometimes
encounter problems.
• The timestamp request message allows a host to ask for the
current time according to the remote host.
• Remote hosts use the timestamp reply message to respond to
the request.
• All ICMP timestamp reply messages contain the originate,
receive, and transmit timestamps.
• Using these three timestamps, the host can estimate transit
time across the network by subtracting the originate time from
the transmit time.
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ICMP Information Requests
and Reply Messages
• The ICMP information requests and reply messages were
originally intended to allow a host to determine its network
number.
• It is now considered obsolete.
• Other protocols such as BOOTP and Dynamic Host
Configuration Protocol (DHCP) are now used to allow hosts to
obtain their network numbers.
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ICMP Address Request
• If a host does not know the subnet mask, it may send an
address mask request to the local router.
• If the address of the router is known, this request may be sent
directly to the router.
• Otherwise, the request will be broadcast.
• When the router receives the request, it will respond with an
address mask reply.
• This address mask reply will identify the correct subnet mask
from which the request was received.
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ICMP Router Discovery
This process begins with the host sending a router solicitation
message to all routers, using the multicast address 224.0.0.2
as the destination address.
• When a router that supports the discovery process receives the
router discovery message, a router advertisement is sent in
return.
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ICMP Source-Quench Messages
• ICMP source-quench messages are used to reduce the amount
of data lost due to congestion on the network.
• The source-quench message asks senders to reduce the rate
at which they are transmitting packets.
• ICMP source-quench messages can be sent by the host or
router (gateway).
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Module 8
TCP/IP Suite Error and
Control Messages
End
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