Some conditions that must be met for host to host communication over an internetwork:

1
TCP/IP Error Message

• Some conditions that must be met for host to host
  communication over an internetwork
• a default gateway must be properly configured on
  the hosts and local network
• 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 hosts must be in the same
  network or subnet as their respective gateway
  devices

2
TCP/IP Error Message

• IP is an unreliable method for delivery of
  network data. It is a best effort delivery.
• Nothing in IP basic design allows it to notify
  the sender that a data transmission has failed.
• Internet Control Message Protocol (ICMP) is a
  component of TCP/IP that addresses this basic
  limitation.
• NOTE
• ICMP does not overcome the unreliability issues
  in IP
• reliability must be provided by upper layer
  protocols, if it is needed.
• ICMP is an error reporting protocol for IP.

3
TCP/IP Error Message

• If ICMP fails, it does not produce ICMP error
  messages.
• If a route is down, such as a serial connection
  between routers so that the destination network
  is unreachable, the router will send ICMP host
  unreachable message to the source.
• Hence when a router doesnt have the information
  to a destination, it
• sends ICMP message of designation unreachable to
  the source
• drops the datagram
• No further ICMP messages will be sent as a
  result of this transaction

4
ICMP Echo Request (PING)
5
Test Reachability
Use the PING command to test reachability PING
sends echo request message to the destination The
destination responds with echo reply message The
messages are indicated in the type field (8 bits)
of the ICMP message.
6
ICMP Echo Request (PING)
7
Excessively Long Routes
To keep ICMP from routing an infinite number of
times it has a Time-To-Live (TTL) timer. The TTL
value is defined by the routing protocol RIP,
hop count 15. As the ICMP goes through each
router the TTL is decremented, and when it
reaches zero, the datagram is discarded.
8
ICMP Echo Request (PING)
9
ICMP Echo Request (PING)
10
Echo Message

• All ICMP messages format start with these fields
• type
• code
• checksum
• For ICMP echo request and echo reply, there are
  two unique fields
• identifier
• sequence
• These two fields are used to match the echo
  request with the echo reply.

11
ICMP Request/Reply Message

• ICMP messages have special formats.
• 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

12
Unreachable Message

• If a packet can not reach its destination, then
  ICMP sends a destination unreachable message. The
  code value in the ICMP datagram is 0.
• Other reasons for destination unreachable
• packet fragmentation from Token Ring to Ethernet
• FTP or Web services are unavailable
• non-existent IP address
• default gateway may not be configured correctly
• destination device may be disconnected from its
  network
• routers interface is down

Some examples of code values 0 network
unreachable 1 host unreachable 2 protocol
unreachable 3 port unreachable
13
ICMP Codes
14
Miscellaneous Error Reporting
If there is an error in the header of a datagram,
to be delivered to its destination, ICMP has a
type value of 12 code value of 0, and a pointer
field indicating the octet of the datagram that
produced the error.
15
ICMP Control Messages
Since IP has no built-in method to provide
information or control messages why a datagram
cant be delivered, ICMP performs these
functions. 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.
Some examples of control messages are 0 echo
reply 3 destination unreachable 5
redirect/change request 8 echo request 11
time exceeded 12 parameter problem
16
Control Messages / Redirect Change Request

• Redirect/Change request can only be initiated by
  a gateway.
• The gateway is the ethernet port address that a
  host uses to get to the network.
• In some cases the host has a hard coded gateway.

• 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 B
Router A
Host B
(continued)
17
Control Messages / Redirect Change Request

• 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.

Host C
Router A
Router B
Host B
18
Control Messages / Redirect Change Request
To disable redirect messages on a router
interface RouterA(config-if) no ip redirects
19
Control Messages ICMP Time Stamp
The TCP/IP protocol suite allows systems to
connect over vast distances. Each individual
networks might provide its own synchronization.
ICMP timestamp request can be used. There is a
timestamp request reply. This ICMP message
contain the originate, receive and transmit
timestamps to calculate the correct timing. This
is used to request the current time of the remote
host, and gather information needed to estimate
the transit time across the network. However,
the Network Time Protocol (NTP) at the upper
layers of the TCP/IP is a more reliable manner.
20
Control MessageRequest and Reply
ICMP request reply messages are considered
obsolete. Protocols such as BOOTP and DHCP now
provide the service of providing a network number.
21
Control MessageAddress Mask Request
This ICMP message is replied to a host by a
router. The reply contains the 32-bit mask for
the subnet from which the request was received.
22
Control MessageRouter Discovery Message
If a host is missing a default gateway, it can
generate an ICMP router solicitation
message. Message is sent via multicast. A local
router will respond with the gateway address. The
router discovery process begins when a host
requires a default gateway IP address. The router
advertisement solicitation messages are used
during the ICMP router discovery process.
23
Congestion and Flow Control Messages

• If a host encounters congestion because of
  difference in bandwidth, an ICMP source-quench
  message can be sent to ask senders to reduce the
  rate at which they are transmitting traffic.
• Hence, for the source-quench message
• a host can send the message
• a gateway may send the message
• message tells the sending device to slow down