ICMP

Question 1

What is the purpose of the Internet Control Message Protocol (ICMP)?

Answer 1

To compensate for the deficiencies of the IP protocol regarding error reporting and management queries

ICMP is a companion protocol to the IP protocol, designed to provide feedback about issues in the communication environment.

Question 2

What are the two broad categories of ICMP messages?

Answer 2

• Error-reporting messages
• Query messages

Question 3

What is the format of an ICMP message?

Answer 3

An 8-byte header and a variable-size data section

The first 4 bytes of the header are common to all ICMP messages.

Question 4

What does the ‘type’ field in an ICMP message define?

Answer 4

The type of the message

Question 5

What does the ‘code’ field in an ICMP message specify?

Answer 5

The reason for the particular message type

Question 6

How does ICMP handle error reporting?

Answer 6

It reports problems encountered by a router or host when processing an IP packet

ICMP does not correct errors; it simply reports them.

Question 7

What information is included in the data section of an error message?

Answer 7

• The IP header of the original datagram
• The first 8 bytes of data in that datagram

Question 8

What does a destination-unreachable message signify?

Answer 8

A router or host cannot route or deliver a datagram, leading to its discard

The message is sent back to the source host that initiated the datagram.

Question 9

What does Code 0 in a destination-unreachable message indicate?

Answer 9

The network is unreachable, possibly due to hardware failure

Question 10

What does Code 3 in a destination-unreachable message indicate?

Answer 10

The port is unreachable, meaning the application program is not running

Question 11

What is the purpose of the source-quench message in ICMP?

Answer 11

To inform the source that a datagram has been discarded due to congestion and to warn it to slow down the sending process

Question 12

True or False: The source-quench message informs the source when congestion has been relieved.

Answer 12

False

Question 13

What happens when a router discards a datagram due to congestion?

Answer 13

It sends a source-quench message to the sender of the datagram

Question 14

Which ICMP messages can only be created by the destination host?

Answer 14

• Code 2
• Code 3

Question 15

What is the limitation of source-quench messages in many-to-one communication?

Answer 15

The router or destination host may not know which source is responsible for the congestion

Question 16

Fill in the blank: ICMP does not correct errors; it simply ______ them.

Answer 16

[reports]

Question 17

What does the IP protocol lack that ICMP compensates for?

Answer 17

Error-reporting and management query mechanisms

Question 18

What is the significance of the protocol field value ‘1’ in an IP datagram?

Answer 18

It indicates that the IP data is an ICMP message

Question 19

What is the role of the data section in query messages?

Answer 19

It carries extra information based on the type of query

Question 20

What does Code 4 in a destination-unreachable message indicate?

Answer 20

Fragmentation is required, but the DF field has been set

Question 21

What does Code 6 in a destination-unreachable message indicate?

Answer 21

The destination network is unknown

Question 22

What does Code 12 in a destination-unreachable message indicate?

Answer 22

The host is unreachable for the specified type of service

Question 23

What is a source-quench message?

Answer 23

A message that informs the source that a datagram has been discarded due to congestion in a router or the destination host

It requires the source to slow down the sending of datagrams until the congestion is relieved.

Question 24

How many source-quench messages are sent for discarded datagrams?

Answer 24

One source-quench message is sent for each datagram that is discarded due to congestion

This helps to notify the source of the issue.

Question 25

What generates a time-exceeded message?

Answer 25

A time-exceeded message is generated when either the time-to-live value reaches 0 or when not all fragments of a datagram arrive within a certain time limit ## Footnote In both cases, the router must send this message to the original source.

Question 26

What are the two codes for time-exceeded messages?

Answer 26

* Code 0: Datagram is discarded due to a time-to-live field value of zero * Code 1: Arrived fragments of a datagram are discarded because some fragments have not arrived within the time limit

Question 27

What causes a parameter problem message?

Answer 27

Any ambiguity in the header part of a datagram can create issues as it travels through the Internet ## Footnote If a router or destination host discovers an ambiguous or missing value in any field, it discards the datagram and sends a parameter-problem message back to the source.

Question 28

What are the codes for a parameter problem message?

Answer 28

* Code 0: There is an error or ambiguity in one of the header fields * Code 1: The required part of an option is missing

Question 29

What is the purpose of a redirection message?

Answer 29

To update the routing table of a host when it sends a datagram to the wrong router ## Footnote The router that receives the datagram will forward it to the correct router and send a redirection message to the host.

Question 30

What types of routes can a redirection message indicate?

Answer 30

* Code 0: Redirection for a network-specific route * Code 1: Redirection for a host-specific route * Code 2: Redirection for a network-specific route based on a specified type of service * Code 3: Redirection for a host-specific route based on a specified type of service

Question 31

What two pairs of messages are used for query purposes in ICMP?

Answer 31

* Echo request and reply * Timestamp request and reply

Question 32

What is the purpose of echo-request and echo-reply messages?

Answer 32

Designed for diagnostic purposes to identify network problems ## Footnote They help determine if there is communication at the IP level.

Question 33

What is the significance of the receipt of an echo-reply message?

Answer 33

It is proof that the IP protocols in the sender and receiver are communicating ## Footnote It also indicates that the intermediate routers are processing and forwarding IP datagrams.

Question 34

What can timestamp-request and timestamp-reply messages be used for?

Answer 34

To determine round-trip time and synchronize clocks in two machines ## Footnote The original timestamp field is filled with the clock at departure time.

Question 35

What are the three timestamp fields in a timestamp message?

Answer 35

* Original timestamp field * Receive timestamp field * Transmit timestamp field

Question 36

What is a key point about ICMP error messages?

Answer 36

No ICMP error message will be generated in response to specific types of datagrams ## Footnote These include datagrams carrying ICMP error messages, fragmented datagrams that are not the first fragment, and datagrams with multicast addresses.

Question 37

Which ICMP messages have been deprecated?

Answer 37

* Information request and reply messages * Address mask request and reply messages * Router solicitation and advertisement messages

Question 38

What are common debugging tools mentioned in TCP/IP?

Answer 38

* Ping * Traceroute