Transcript CCNA 1 Module 11 TCP/IP Transport and Application Layers

CCNA 2 Module 8
TCP/IP Suite Error and
Control Messages
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Objectives
• TCP/IP error messages
• TCP/IP control messages
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Internet Control Message Protocol
(ICMP)
• IP is limited because it is a best effort
delivery system. It has no mechanism to
ensure whether the data is delivered.
• IP uses the Internet Control Message
Protocol (ICMP) to notify the sender of the
data that there was an error in the delivery
process.
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Internet Control Message Protocol
(ICMP) Continued
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Error Reporting and Error Correction
• ICMP is an error-reporting protocol for IP.
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ICMP Message Delivery
• ICMP messages are encapsulated as data
in datagrams in the same way any other
data is delivered using IP.
• Errors created by ICMP messages do not
generate their own ICMP messages in
order to prevent the congestion.
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Unreachable Networks
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Ping
• It’s used to confirms
that the destination
device can be
reached via the IP
protocol
• Traffic generated
by ping command
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Detecting Excessively Long Routes
• Whether the actual path includes too many hops or a circular
routing path exists, the packet will eventually reach the end of
its life, known as its time-to-live (TTL).
• As each router
processes the
datagram, it
decreases the TTL
value by one.
• When the TTL
reaches zero, the
packet is discarded;
and time exceeded
message is send to
notify the source
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ICMP Message
• Each ICMP message type has its own unique
characteristics, but all ICMP message formats start
with these same three fields
• Type - indicates the type of ICMP message
being sent
• Code - includes further information specific to
message type
• Checksum - used to verify the integrity of the
data
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ICMP Message Types
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Code Values for Destination
Unreachable Messages
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ICMP Echo Request and Echo Reply
Message
• The identifier and sequence number fields are
unique to the echo request and echo reply messages.
The identifier and sequence fields are used to match
the echo replies to the corresponding echo request.
• The data field contains additional information that
may be a part of the echo reply or echo request
message
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TCP/IP Suite Control Messages
• 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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ICMP Redirect/Change Requests
• 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.
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Clock Synchronization and Transit Time
Estimation
• The ICMP timestamp request message allows a host to ask for
the current time according to the remote host.
• The remote host uses an ICMP timestamp reply message to
respond to the request.
• The ICMP timestamp request contains an originate timestamp,
which is the time on the requesting host just before the
timestamp request is sent.
• The receive timestamp is the time that the destination host
receives the ICMP timestamp request.
• The transmit timestamp is filled in just before the ICMP
timestamp reply is returned.
• Originate, receive and transmit timestamps are computed in
numbers milliseconds elapsed since midnight Universal Time
(UT).
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Clock Synchronization and Transit Time
Estimation Continued
• 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 transit time.
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Information Requests or Replies Message
• The ICMP information requests and reply
messages were originally intended to allow
a host to determine its network number.
• This particular ICMP message type is
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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Address Mask Requirements
• When a network administrator uses the process of
subnetting to divide a major IP address into multiple
subnets, a new subnet mask is created.
• If a host does not know the subnet mask, it may send
an address mask request to the local router. This
request may be sent directly to the router or may be
broadcast. When the router receives the request, it
will respond with an address mask reply.
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Router Discovery Message
• When a host on the network boots, and the host has
not been manually configured with a default gateway,
it can learn of available routers through the process
of 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 or using broadcast address 255.255.255.255.
• When a router that supports the discovery process
receives the router discovery message , a router
advertisement is sent in return.
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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.
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Congestion and Flow Control
Messages
ICMP source-quench messages are
used to reduce the amount of data lost
from congestion.
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Summary
•
IP is a best-effort delivery method that uses ICMP messages to alert the
sender that the data did not reach its destination.
•
ICMP echo request and echo reply messages allow the network
administrator to test IP connectivity to aid in the troubleshooting
process.
•
ICMP packets have their own special header information starting with a
type field and a code field.
•
The functions of ICMP control messages:
• ICMP redirect/change request messages
• ICMP clock synchronization and transit time estimation messages
• ICMP information request and reply messages
• ICMP address mask request and reply messages
• ICMP router discovery message
• ICMP router solicitation message
• ICMP congestion and flow control messages
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