Transcript Module 8

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
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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
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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
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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
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Ping
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IP Address of destination device
Issues four echo requests
Receives four echo replies
Confirming IP connectivity between the two devices
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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
datagram back and forth
between them, thinking the
other should be the next hop
to the destination
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• 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
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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
verify
theand
integrity
of the
ICMPTo
echo
request
echo reply
data
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
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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
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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
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TCP/IP Suite Control Messages
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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
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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 Type 5 indicates redirect/change request
host may need to use a
redirect/change request
to inform the host of the
best path to a certain
network
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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
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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
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Information requests and reply message formats
Request
Message
Reply
Message
• 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.
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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
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Router Discovery Message
•
If host has no default
gateway
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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
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If sent to a router that does
not support the discovery
process
solicitation is unanswered
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If sent to a router that
supports the discovery
process
router advertisement returned
router advertisement
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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
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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
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