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© 2002, Cisco Systems, Inc. All rights reserved.
Module 8: TCP/IP Suite Error
and Control Messages
Frank Mann CCAI-
2
8.1 Overview of TCP/IP Error Message
IP is limited because it is a best effort delivery
system. It has no mechanism to ensure that the
data is delivered, regardless of the problems it
may encounter on the network.
Data may fail to reach its destination for a variety
of reasons, such as hardware failure, improper
configuration or incorrect routing information.
To help identify these failures, IP uses the Internet
Control Message Protocol (ICMP) to notify the
sender of the data that there was an error in the
delivery process.
Host to Host Communication Success
Three conditions must be met in order for
host to host communication over an
internetwork to occur on IP networks:
• A default gateway must be properly configured
on the hosts and local networks.
• 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 the hosts must be in the
same network or subnet as their respective
gateway devices.
8.1.1 Internet Control Message
Protocol (ICMP)
Internet Control Message
Protocol (ICMP) is the
component of the TCP/IP
protocol stack that
addresses this basic
limitation of IP
ICMP does not overcome the
unreliability issues in IP
Two functions are performed
by the Internet Message
Control Protocol
• Error messages to Hosts
• Control Messages to Hosts
– Network Congestion
– Default Gateway Available
Reliability must be
provided by upper
layer protocols if it is
needed
8.1.2 Error reporting and error
correction
Assume
Workstation 1 is
sending a
datagram to
Workstation 6,
but interface
Fa0/0 on Router
C goes down
Router C then
utilizes ICMP to
send a message
back to
Workstation 1
indicating that
the datagram
could not be
delivered.
ICMP
merely
reports the
problem
ICMP error messages
ICMP does not correct the encountered
network problem.
ICMP just reports on the status of the
delivered packet to the sender. ICMP error
messages are returned to the sending
device when a packet is dropped by a
router.
Its function is not to propagate information
about network changes.
8.1.3 ICMP message delivery
ICMP 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.
• It is thus possible to have a datagram delivery error that is
never reported back to the sender of the data.
8.1.4 Unreachable networks
8.1.5 Using ping to test destination
reachability
The ICMP
protocol can be
used to test the
availability of a
particular
destination
8.1.6 Detecting excessively long
routes
This is also known as reaching its time-to-live (TTL),
because the TTL value typically matches the maximum
hop count defined by the routing protocol. A TTL value
is defined in each datagram
8.1.7 Echo messages
As with any type of
packet, ICMP messages
have special formats.
Each ICMP message
type shown has its own
unique characteristics,
but all ICMP message
formats start with these
same three fields:
• Type
• Code
• Checksum
ICMP Frame format
The type field (8 bits) indicates the type of ICMP
message being sent.
The code field includes further information specific
to the message type.
The checksum field, as in other types of packets,
is used to verify the integrity of the data.
8.1.8 Destination unreachable
message
The router sends an ICMP "Destination Unreachable" message back
to the source.
The router discards the original packet.
Code Values for Destination Unreachable
8.2 TCP/IP Suite Control Messages
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 to a
remote network.
Like all ICMP messages, ICMP control messages are
encapsulated within an IP datagram
8.2.1 Introduction to control messages
Multiple types of control
messages are used by
ICMP.
eg., source quench and
echo request
8.2.2 ICMP redirect/change requests
In some
circumstances,
a host connects
to a segment
that has two or
more directly
connected
routers.
In this case, 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
It requests that a host change
a route
Redirects
Default gateways only send ICMP
Redirects/Change Requests 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.
– router(config-if)# ip redirects
8.2.3 Clock synchronization and
transit time estimation
While ICMP timestamp request messages provide
a simple way to estimate time on a remote host
and total network transit time, this is not the best
way to obtain this information.
• ICMP timestamp request messages request
– the current time of the remote host
– gather information needed to estimate the transit
time across the network
Instead, more robust protocols such as Network
Time Protocol (NTP) at the upper layers of the
TCP/IP protocol stack perform clock
synchronization in a more reliable manner.
8.2.4 Information requests and reply
message formats
The ICMP information requests and reply
messages were originally intended to allow a
host to determine its network number. Figure
shows the format for an ICMP information
request and reply message.
Two type codes are available in this message. Type
15 signifies an information request message,
and type 16 identifies an information reply
message. 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.
8.2.5 Address mask requirements
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.
8.2.6 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
8.2.7 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.
8.2.8 Congestion and flow control
messages
ICMP source-quench
messages are used to
reduce the amount of
data lost. The sourcequench message asks
senders to reduce the
rate at which they are
transmitting packets.
In most cases, congestion will subside after a short
period of time, and the source will slowly increase
the transmission rate as long as no other sourcequench messages are received.
Congestion
Congestion occurs when multiple computers try to
access the same receiver or when traffic from a
high speed LAN reaches a slower WAN
connection. The effect on congestion on a network
is dropped packets that result in a loss of data.
In order to reduce data lost, ICMP messages must
be sent to the source of the congestion.
This type of ICMP message is called a sourcequench message.
• The source-quench message notifies the sender of the
congestion and asks the sender to reduce its rate. A
destination host or the Sender’s gateway may send the
message.
• This usually reduces the congestion. Then the rate of
transmission will slowly increase if no other sourcequench messages are received.
Module 8: TCP/IP Suite Error and Control
Messages
There are no labs in this module.