ICMP : Internet Control Message Protocol

Introduction

ICMP is often considered part of the IP layer. It communicates error messages and other

conditions that require attention. ICMP messages are usually acted on by

either IP layer or the higher layer protocol.

IP header (20 byte) ICMP message

IP datagram

ICMP Message

The first 4 bytes have the same format for all messages, but the remainder differs from one message to the next.

(contents depends on type and code)

8-bit type 8-bit code 16-bit checksum

0 7 8 15 16 31

ICMP Message Types

type Description

0 Echo Reply Echo Query

3 Destination Unreachable Error

4 Source Quench Error

5 Redirect Error

8 Echo Request Echo Query

9 Router Advertisement Query

10 Router Solicitation Query

11 Time Exceeded Error

12 Parameter Problem Query

13 Timestamp Request Query

14 Timestamp Reply Query

17 Address Mask Request Query

18 Address Mask Reply Reply

code Description

0 Network Unreachable

1 Host Unreachable

2 Protocol Unreachable

3 Port Unreachable

4 Fragmentation Needed and DF set

5 Source Route Failed

6 Destination Network Unknown

7 Destination Host Unknown

8 Source Host Isolated

9 Network Administratively Prohibited

10 Destination Host Administratively Prohibited

11 Network Unreachable For TOS

12 Host Unreachable For TOS

13 Communication Administratively Prohibited

14 Host Precedence Violation

15 Precedence Cutoff in Effect

ICMP Echo Request and Reply ICMP echo is a message to send and receive

the specified echo data packet between two TCP/IP nodes.

0 7 8 15 16 31

Echo data (variable length)

Type(0 or 8) Code(0)

identifier

checksum

sequence number

ICMP Addr Mask Request and Reply The ICMP Address mask request is intended

for a diskless system to obtain its subnet mask at bootstrap time.

0 7 8 15 16 31

32-bit subnet mask

Type(17 or 18) Code(0)

identifier

checksum

sequence number

ICMP Addr Mask

We can write a simple program that issues an ICMP address mask request and prints all replies.

% icmpaddrmask 140.252.13.63Received mask = ffffffe0, from 140.252.13.33Received mask = ffffffe0, from 140.252.13.35Received mask = ffff0000, from 140.252.13.34

ICMP Timestamp Request and Reply The ICMP timestamp request allows a

system to query another for the current time. The recommended value to be returned is the

number of milliseconds since midnight, Coordinated Universal Time (UTC).

0 7 8 15 16 31

32-bit originate timestamp

Type(13 or 14) Code(0)identifier

checksumsequence number

32-bit receive timestamp32-bit transmit timestamp

ICMP Timestamp Request and Reply The requestor fills in the originate timestamp

and sends the request. The replying system fills in the receive

timestamp when it receives the request. The transmit timestamp when it send the reply.

0 7 8 15 16 31

32-bit originate timestamp

Type(13 or 14) Code(0)identifier

checksumsequence number

32-bit receive timestamp32-bit transmit timestamp

ICMP Unreachable Message

It is an error message sent back to the transmission origin when the packet cannot be sent to service.

It is likely to reply from a machine on the way of routing (router), and it might reply from the machine (target host).

0 7 8 15 16 31

Unused (must be 0)Type(3) Code(0-15) checksum

IP header + first 8 bytes of original IP datagram data

ICMP Message Types

code Description

0 Network Unreachable

1 Host Unreachable

2 Protocol Unreachable

3 Port Unreachable

4 Fragmentation Needed and DF set

5 Source Route Failed

6 Destination Network Unknown

7 Destination Host Unknown

8 Source Host Isolated

9 Network Administratively Prohibited

10 Destination Host Administratively Prohibited

11 Network Unreachable For TOS

12 Host Unreachable For TOS

13 Communication Administratively Prohibited

14 Host Precedence Violation

15 Precedence Cutoff in Effect

Exercises

If your system provides the netstat command, use it to see what types of ICMP messages are received and sent.

Ping Program

Introduction

The name “ping” is taken from the sonar operation to locate objects.

The program sends an ICMP echo request message to a host, expecting an ICMP echo reply to be returned.

Ping also measures the round-trip time to the host, giving us some indication of how “far away” that host is.

(1)

(2)

(3)

Ping Options

-

options description

-t Ping the specified host until stopped.

To see statistics and continue - type Control-Break;

To stop - type Control-C.

-a Resolve addresses to hostnames.

-n count Number of echo requests to send.

-l size Send buffer size

-f Set Don't Fragment flag in packet.

-i TTL Time To Live.

-v TOS Type Of Service.

-r count Record route for count hops.

-s count Timestamp for count hops.

-j host-list Loose source route along host-list.

-k host-list Strict source route along host-list.

-w timeout Timeout in milliseconds to wait for each reply.

Ping

HOST A192.168.1.1

HOST B192.168.1.2

Please return the sent data.

I return the data as it is.

- Reply from 192.168.1.2 : bytes = 32 time < 10ms TTL = 128

- Ping 192.168.1.2

Format of ICMP message

We call the ping program that sends the echo requests the client, and the host being pinged the server.

0 7 8 15 16 31

Optional data

Type(0 or 8) Code(0)

identifier

checksum

sequence number

From the result

The network communication with the object host must be normal. Own host's network setting and operation. The object host's network setting and operation. Setting and operation of network/router.

Network performance The throughput of the network can guess from the

display of statistics.

Case of failure

The routing to Internet Protocol address cannot be done.

C:\>ping 192.168.10.1

Pinging 192.168.10.1 with 32 bytes of data:

Reply from 203.100.100.49: Destination net unreachable.Reply from 203.100.100.49: Destination net unreachable.Reply from 203.100.100.49: Destination net unreachable.:

Case of failure

Loop by routing setting mistake.C:\>ping 150.224.XXX.XXX

Pinging 150.224.XXX.XXX with 32 bytes of data:

Reply from 130.XXX.76.17: TTL expired in transit.Reply from 130.XXX.76.17: TTL expired in transit.Reply from 130.XXX.76.17: TTL expired in transit.Reply from 130.XXX.76.17: TTL expired in transit.

Ping statistics for 150.224.XXX.XXX:    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),Approximate round trip times in milli-seconds:    Minimum = 0ms, Maximum =  0ms, Average =  0ms

Case of failure

Failure of host name resolution.

The host's stop.

C:\>ping www.unknown.netUnknown host www.unknown.net.

C:\>ping host1.example.net

Pinging host1.example.net [211.XXX.XXX.190] with 32 bytes of data:

Request timed out.Request timed out.:

Summary

The ping program is the basic connectivity test between two systems running TCP/IP. It uses the ICMP echo request and echo reply

messages and does not use a transport layer. Ping also let us examine and use the IP

record route option.

Exercises

Calculate the RTT if the SLIP link between bsdi and slip is set to 9600 bits/sec. Assume the default of 56 bytes of data.

IP

ICMP

ICMP

ICMP

Echo Request

Echo Reply

Normal Access

Destination unreachable Because of router Couldn’t find Host X, router send a Error Message

HOST A

HOST B HOST Y

HOST X

ICMP Message Types

When an ICMP error message is sent, the message always contains the IP header and the first 8 bytes of the IP datagram that caused the ICMP error to be generated.

This lets the receiving ICMP module associate the message with one particular protocol and one particular user process.

ICMP Message Types

An ICMP error message is never generated in response to An ICMP error message A datagram destined to an IP broadcast address

or an IP multicast address. A datagram sent as a link-layer broadcast. A fragment other than the first. A datagram whose source address does not

define a single host.