 IP protocol has no built-in mechanism to
notify the original host.
 The IP protocol also lacks a mechanism for
host and management queries.
 (ICMPv4) has been designed to compensate
for the above two deficiencies.
 It is a companion to the IP protocol. ICMP
itself is a network-layer protocol.

 messages are not passed directly to the data-
link layer as would be expected.
 the messages are first encapsulated inside IP
datagrams before going to the lower layer.
 When an IP datagram encapsulates an ICMP
message, the value of the protocol field in
the IP datagram is set to 1 to indicate that
the IP payroll is an
ICMP message.

 ICMP messages are divided into two broad
categories:
 error-reporting messages: report problems that a
router or a host(destination) may encounter when it
processes an IP packet.
 query messages: messages, which help a host or a
network manager get specific information from a
router or another host.

 An ICMP message has an 8-byte header and a variable-size data
section.
 Type, defines the type of the message.
 Code field specifies the reason for the particular message type.
 The last common field is the Checksum field.

 In error messages the data section carries
information for finding the original packet
that had the error.
 In query messages, the data section carries
extra information based on the type of
query.

 The main responsibilities of ICMP is to report some
errors that may occur during the processing of the IP
datagram.
 Error messages are always sent to the original source
because the only information available in the
datagram about the route is the source and
destination IP addresses.
 Note that all error messages contain a data section
that includes the IP header of the original datagram
plus the first 8 bytes of data in that datagram.

ICMP follows some rules in reporting messages.
 First, no error message will be generated for a
datagram having a multicast address or special
address.
 Second, no ICMP error message will be generated in
response to a datagram carrying an ICMP error
message.
 Third, no ICMP error message will be generated for
a fragmented datagram that is not the first
fragment.

ICMP forms an error packet:
 Destination Unreachable: This message uses different codes (0
to 15) to define the type of error message and the reason why a
datagram has not reached its final destination.
This may happen, for example, when we use the HTTP protocol to
access a web page, but the server is down. The message
“destination host is not reachable” is created and sent back to
the source.

 Source Quench: informs the sender that the network has
encountered congestion and the datagram has been dropped;
The source needs to slow down sending more datagrams. In other
words, ICMP adds a kind of congestion control mechanism to
the IP protocol by using this type of message.
 Parameter Problem: sent when either there is a problem in the
header of a datagram (code 0) or some options are missing or
cannot be interpreted (code 1).

 Redirection Message: used when the source uses a wrong
router to send out its message.
The router redirects the message to the appropriate router, but
informs the source that it needs to change its default router in
the future.
The IP address of the default router is sent in the message.

 query messages in ICMP can be used independently
without relation to an IP datagram.
 needs to be encapsulated in a datagram, as a
carrier.
 used to probe or test the liveliness of hosts or
routers in the Internet,
 find the one-way or the round-trip time for an IP
datagram between two devices, or even find out
whether the clocks in two devices are synchronized.

 query messages come in pairs: request and reply.
 The echo request (type 8)
 The echo reply (type 0)
 pair of messages are used by a host or a router to
test the liveliness of another host or router.
 A host or router sends an echo request message to
another host or router; if the latter is alive, it
responds with an echo reply message.

 The timestamp request (type 13) and the timestamp reply
(type 14) pair of messages are used to find the round-trip time
between two devices or to check whether the clocks in two
devices are synchronized.
 The timestamp request message sends a 32-bit number, which
defines the time the message is sent.
 The timestamp reply resends that number, but also includes
two new 32-bit numbers representing the time the request
was received and the time the response was sent.
 If all timestamps represent Universal time, the sender can
calculate the one-way and round-trip time.

Three pairs of messages are declared obsolete by IETF:
1. Information request and replay messages are not used
today because their duties are done by the Address
Resolution Protocol (ARP.
2. Address mask request and reply messages are not used
today because their duties are done by the Dynamic Host
Configuration Protocol (DHCP).
3. Router solicitation and advertisement messages are not
used today because their duties are done by the Dynamic
Host Configuration Protocol (DHCP).

There are several tools that can be used in the Internet
for debugging, We introduce two tools that use ICMP:
 Ping: We can use the ping program to find if a host is alive and
responding. We use ping here to see how it uses ICMP packets.
The source host sends ICMP echo-request messages; the
destination, if alive, responds with ICMP echo-reply messages.
The ping program sets the identifier field in the echo-request and
echo-reply message and starts the sequence number from 0; this
number is incremented by 1 each time a new message is sent.

 The traceroute program in UNIX or tracert in Windows
can be used to trace the path of a packet from a
source to the destination.
 It can find the IP addresses of all the routers that are
visited along the path.
 The program is usually set to check for the maximum
of 30 hops (routers) to be visited.
 The number of hops in the Internet is normally less
than this.

 The traceroute program gets help from two error-reporting
messages: time-exceeded and destination-unreachable.
 The traceroute is an application layer program, but only the
client program is needed, because, as we can see, the client
program never reaches the application layer in the destination
host.
 The traceroute application program is encapsulated in a UDP
user datagram, but traceroute intentionally uses a port number
that is not available at the destination.

 If there are n routers in the path, the traceroute
program sends (n + 1) messages.
 The first n messages are discarded by the n
routers, one by each router;
 the last message is discarded by the destination
host.
 The traceroute client program uses the (n + 1)
ICMP error-reporting messages received to find
the path between the routers.

 The first traceroute message is sent with time-to-live (TTL)
value set to 1; the message is discarded at the first router and
a time-exceeded ICMP error message is sent, from which the
traceroute program can find the IP address of the first router
and the router name
 The second traceroute message is sent with TTL set to 2,
which can find the IP address and the name of the second
router.
 Similarly, the third message can find the information about
router 3. The fourth message, however, reaches the
destination host.

 This host is also dropped, but for another reason. The
destination host cannot find the port number specified in the
UDP user datagram. This time ICMP sends a different message,
the destination-unreachable message with code 3 to show the
port number is not found.
 After receiving this different ICMP message, the traceroute
program knows that the final destination is reached.
 It uses the information in the received message to find the IP
address and the name of the final destination.
 also sets a timer to find the round-trip time for each router and
the destination. Most traceroute programs send three messages
to each device.

Ch 19 Network-layer protocols - section 2

 The tracert program in windows behaves
differently.
 The tracert messages are encapsulated directly
in IP datagrams.
 The tracert, like traceroute, sends echo-
request messages.
 However, when the last echo request reaches
the destination host, an echo replay message is
issued.

 In ICMP the checksum is calculated over the entire
message (header and data).

Ch 19 Network-layer protocols - section 2

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Ch 19 Network-layer protocols - section 2