Download ICMP

Module 8
TCP/IP Suite Error and Control Messages
IP
• Unreliable method for delivery of network
data
• Known as a best effort delivery mechanism
• No built-in processes to ensure that data is
delivered in the event that problems exist
with network communication
• Reliability must be provided by upper layer
protocols if it is needed
ICMP
• Error and Control reporting protocol for IP.
– Component of the TCP/IP protocol stack that
addresses the basic limitation of IP.
– Does not correct the encountered network problem; it
merely reports the problem.
– ICMP reports on the status of the delivered packet
(control messages) only to the source device (host).
– Error messages are returned to the sending device
(hosts) when a packet is dropped by the router.
– It does not propagate information about network
changes to routers!!!
• Since ICMP messages are transmitted in the
same way as any other data, they are subject to
the same delivery failures.
• This creates a scenario where error reports
could generate more error reports, causing
increased congestion on an already ailing
network.
• For this reason, errors created by ICMP
messages do not generate their own ICMP
messages.
• It is possible to have a datagram delivery error
that is never reported back to the sender of the
data.
• ICMP messages are encapsulated into
datagrams in the same way any other data is
delivered using IP
Contain type#,
code# and
checksum
Error in Header – Type 12
Parameter Problem
• ICMP message has header and data areas
• Complete ICMP message is treated as data in IP datagram
• Complete IP datagram is treated as data in physical network frame
ICMP Message
IP Header
Frame Header
IP Data
Frame Data
ICMP header follows IP header, and contains eight bytes
ICMP Message Format
• An ICMP message is used for many different
purposes (control and error report)
• Each type of ICMP message has its own format
• They all begin with the same three fields
– an 8-bit message TYPE that identifies the message
– an 8-bit CODE field that provides further information
about the message type
– a 16-bit CHECKSUM for the ICMP message, verify
the integrity of the data
Network communication
• Depends upon certain basic conditions being
met
– The sending and receiving devices must have the TCP/IP
protocol stack properly configured.
– This includes the installation of the TCP/IP protocol and proper
configuration of IP address and subnet mask.
– A default gateway must also be configured if datagrams are to
travel outside of the local network.
– Intermediary devices must be in place to route the datagram
from the source device and its network to the destination
network. Routers serve this function.
– A router also must have the TCP/IP protocol properly configured
on its interfaces, and it must use an appropriate routing protocol.
Long Routes
• The limitations of the routing protocol can result
in destinations being unreachable.
• For example, RIP has a limit on the distance
certain routing information is allowed to travel.
The hop limit of RIP is 15, which means that a
route will not be advertised further than 15 hops.
• Datagram Life – Each router decreases the TTL
value by 1 until it reaches zero. The datagram is
then discarded and a destination unreachable
message is sent to the source. The TTL value
typically matches the maximum hop count
defined by the routing protocol (not always).
Size 8
Bits!
Identified in ICMP message header
sent by
host –
TYPE 8
sent by
destination –
TYPE 0
Router will drop
the datagram –
discards the
original packet
(sent to host!)
TYPE 3
Control Messages
• Use to inform hosts of conditions such as
network congestion or the existence of a
better gateway to a remote network
• Encapsulated within an IP datagram
Obsolete!
Normally, Host B uses
single gateway to reach
networks not directly
connected
• Host H sends a packet to the
host on network 10.0.0.0/8
• Host H is not directly
connected to same LAN, so
forwards the packet to its
default gateway, R1
• Determines that the path to
the network is back out the
same interface the request to
forward the packet came from
• Forwards packet and sends
an ICMP redirect/change
request to Host H telling it to
use R2 as the gateway when
forwarding to 10.0.0.0/8
TYPE 5 - ICMP redirect or change, request the router to route
the packet out of the same interface on which it was received
1. The following debug
message shows router R1, as
in the above diagram, sending
a debug message to host H
(172.16.1.1).
Redirect sent by gateway–
172.16.1.100
R1#debug ip icmp
ICMP packet debugging is on
ICMP: redirect sent to 172.16.1.1 for dest 10.1.1.1, use gw 172.16.1.200
R1#
3. By default the router is configured
to send redirects. The interface
subcommand no ip redirects will
disable ICMP redirects
2. Router R1
(172.16.1.100) is sending a
redirect to host H
(172.16.1.1) to use router
R2 (172.16.1.200) as the
gateway to reach the
destination 10.1.1.1.
ICMP Timestamp Message Type
• ICMP timestamp request message (type 13)
– Allows a host to ask for the current time according to the remote
host
– Contains an originate timestamp (time on requesting host)
– Contains a receive timestamp (time the destination host received
the timestamp request)
– Transmit timestamp filled in just before the ICMP timestamp
reply is returned
• ICMP timestamp reply message (type 14)
– Reply from remote host
– Contains originate, receive and transmit timestamps
– Using these timestamps, the host can estimate transit time
across the network (transit – originate = transit time)
– Other upper layer protocols do a better job
ICMP Info Request and Reply
• Intended to allow a host to determine a
network number
• Type 15 – signifies an information request
message
• Type 16 – identifies an information reply
message
• Considered obsolete
• BOOTP (contains mac to ip address table)
and DHCP are now used
Address Mask Request
• Used if a host does not know the subnet mask
• Sent to the local router
– Maybe unicast or broadcast
• Will identify the correct subnet mask
• Type 17 – request
• Type 18 – reply
• Reply is sent by the router – contains the 32-bit
mask for the subnet from which the request was
received
Router Discovery Message
• Type 9
• Host not manually configured with a default
gateway
• Uses multicast address 224.0.0.2 as the
destination
• Some routers do not support
• When a router that supports the discovery
process receives the router discovery message,
a router advertisement is sent in return
ICMP Router Solicitation
• Type 10
• Generated by host in response to a
missing default gateway
• First step in router discovery process
ICMP Source-Quench
• Type 4
• Used when the destination computer is overwhelmed
with traffic or traffic from a high speed LAN to a slower
WAN connection
• Asks senders to reduce the rate at which they are
transmitting packets
• After congestion subsides, transmission rate is increased
• Most Cisco routers do not send by default (adds to
network congestion)
• Example: 4 computers using Internet over one 56K
bandwidth, host acting as the gateway can request that
the other hosts reduce transmission rates
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 messages are transmitted using the IP
protocol so their delivery is unreliable.
• ICMP packets have their own special header
information starting with a type field and a code
field.
True or False
1. ICMP can correct many network problems as they occur
2. ICMP error messages are returned to the sending
device when a packet is dropped by a router
3. The checksum part of the ICMP message header
indicates that the message is either an echo request or
an echo reply
4. An ICMP redirect occurs if the router routes the packet
out of the same interface on which it was received
5. RIP is used to notify an IP host that transmission errors
have occurred
6. An ICMP control message is sent to the destination if
the network is experiencing congestion
True or False
1. ICMP can correct many network problems as they occur
2. ICMP error messages are returned to the sending
device when a packet is dropped by a router
3. The checksum part of the ICMP message header
indicates that the message is either an echo request or
an echo reply
4. An ICMP redirect occurs if the router routes the packet
out of the same interface on which it was received
5. RIP is used to notify an IP host that transmission errors
have occurred
6. An ICMP control message is sent to the destination if
the network is experiencing congestion
False – 1, 3, 5, 6
True or False
1. A source squelch is an example of a ICMP control
messages
2. The purpose of an ICMP timestamp request message is
to gather information needed to estimate the transit time
across the network
3. Each router decreases the TTL value by one until it
reaches zero. The datagram is then discarded and a
destination unreachable message is sent to the source
4. If a router is unable to deliver a packet to its destination
the router discards the original packet
5. A host or gateway may send the ICMP source-quench
message
6. When an ICMP packet is not deliverable, an ICMP
packet is returned to the original sender
True or False
1. A source squelch is an example of a ICMP control
messages
2. The purpose of an ICMP timestamp request message is
to gather information needed to estimate the transit time
across the network
3. Each router decreases the TTL value by one until it
reaches zero. The datagram is then discarded and a
destination unreachable message is sent to the source
4. If a router is unable to deliver a packet to its destination
the router discards the original packet
5. A host or gateway may send the ICMP source-quench
message
6. When an ICMP packet is not deliverable, an ICMP
packet is returned to the original sender
False – 1, 6
True or False
1. When an ICMP packet is not deliverable, an ICMP
packet is returned to the nearest router in the path
2. When an ICMP packet is not deliverable, the packet is
dropped
3. When a router does not have the information necessary
to forward a datagram to the destination network the
router will return the datagram to the source
4. The ICMP address mask reply message is sent by a
host
True or False
1. When an ICMP packet is not deliverable, an ICMP
packet is returned to the nearest router in the path
2. When an ICMP packet is not deliverable, the packet is
dropped
3. When a router does not have the information necessary
to forward a datagram to the destination network the
router will return the datagram to the source
4. The ICMP address mask reply message is sent by a
host
False – 1, 3, 4
Fill In The Blank
• Two functions of ICMP is to provide
______ and ______ messages to hosts.
• An ICMP echo _______ is generated by a
host when the ping command is entered.
• _______ and _______ are used instead of
ICMP information request and reply
messages.
• _____ is used to notify an IP host that
transmission errors have occurred.
Fill In The Blank
• Two functions of ICMP is to provide error
and control messages to hosts.
• An ICMP echo request is generated by a
host when the ping command is entered.
• BOOTP and DHCP are used instead of
ICMP information request and reply
messages.
• ICMP is used to notify an IP host that
transmission errors have occurred.
Finally
• IP does not have the facilities to send error
messages.
• It uses ICMP to send, receive, and
process error and control messages.
• http://www.faqs.org/rfcs/rfc792.html