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Internet Engineering (Adel Nadjaran Toosi) Slide 1

Internet Control MessageProtocol (ICMP)

Adel Nadjaran Toosi

http://NadjaranToosi.mshdiau.ac.irEmail: NadjaranToosi@mshdiau.ac.ir

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Internet Engineering (Adel Nadjaran Toosi) Slide 2

Internet Control Message Protocol (ICMP)

for Supervisory Messages

Host Unreachable

Error Message

Router

EchoRequestEcho

Response

ICMP Message IP Header

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Internet Engineering (Adel Nadjaran Toosi) Slide 4

Abstract Function

Error reporting

Reachability testing

Congestion control

Route change notification

Performance measuring

Subnet addressing

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Internet Engineering (Adel Nadjaran Toosi) Slide 5

ICMP Format 15 different types.

Some types use a code to further specify thecondition.

IP header ICMP message

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

(content depends on type and code)

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Internet Engineering (Adel Nadjaran Toosi) Slide 6

ICMP MessageType

Type Field: Identifies message type

Type ICMP Message Type

0 Echo Reply

3 Destination Unreachable

4 Source Quench

5 Redirect (change a route)

8 Echo Request

11 Time Exceeded for a Datagram

12 Parameter Problem on a Datagram

13 Timestamp Request

14 Timestamp Reply

17 Address Mask Request

18 Address Mask Reply

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Internet Engineering (Adel Nadjaran Toosi) Slide 7

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Internet Engineering (Adel Nadjaran Toosi) Slide 8

Echo Request and Reply Message Format

Used when ping is invoked

Identifier and Sequence# used for the sender to matchreplies to request.

Optional Data: variable length field containing data to be

returned to the sender.

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Internet Engineering (Adel Nadjaran Toosi) Slide 10

Congestion and Datagram Flow Control

A machine uses ICMP source quench messages

to report congestion to the original source. A source quench message is a request for the

source to reduce its current rate of datagram

transmission. No ICMP message to reverse source quench

message. Sources after receiving a quenchmessage reduce their transmission rate and thengradually increase their rate provided noadditional quench messages are received.

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Internet Engineering (Adel Nadjaran Toosi) Slide 11

Route Change Request from Routers

Routers are assumed to know the correct routes.

Host begin with minimal routing information andlearn new routes from routers.

When a router detects a host using a nonoptimal

route, it sends the host a redirect message,

requesting that the host change the route.

Redirect messages limited to interactions between

a router and a directly connected network.

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Internet Engineering (Adel Nadjaran Toosi) Slide 12

ICMP Redirect Message

Redirect message Format

Router Internet Address address of the router that the host is to use to reach the destination

Code Value Meaning

0 Redirect datagrams for the Net(obsolete)

1 Redirect datagrams for the Host

2 Redirect datagrams for the Type of Service andNet

3 Redirect datagrams for the Type of Service andHost

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Internet Engineering (Adel Nadjaran Toosi) Slide 13

Detecting Circular or Excessively Long

Routes

Cycle can be produced

ICMP time exceeded message sent When Time-to-live counter value = 0, the datagram is

discarded.

Timeout occurs while waiting for fragments, thefragments are discarded.

Message Format

Code = 0 means time-to-live count exceeded Code = 1 means fragment reassembly time exceeded

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Internet Engineering (Adel Nadjaran Toosi) Slide 14

Parameter Problems

When a router encounters a problem with a

datagram not covered by other ICMP errormessages such as an incorrect datagram header, a

parameter problem message is sent to the source.

Message Format Code= 1 is used to report that a required option is missing (i.e.,

security option in military community)

Pointer Field: identify the octet field in the datagram that caused

the problem. Not used when Code = 1

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Internet Engineering (Adel Nadjaran Toosi) Slide 15

Clock Synchronization and Transit Time

Estimation

Clocks differ widely in a distributed systemsenvironment.

ICMP message can be used to obtain time informationfrom other machines.

ICMP timestamp request message

ICMP timestamp reply message Type = 13 (request), 14(reply)

Identifier and Sequence Number to match request with replies

Originate timestamp: timestamp of sender (T1)

Receive timestamp: timestamp upon receipt at receivers end (T2) Transmit timestamp: timestamp immediately before the reply is sent

(T3)

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Internet Engineering (Adel Nadjaran Toosi) Slide 16

Clock Synchronization and Transit Time

Estimation

SenderSender

T1T1T2T2

T3T3

T4T4

RTT = (T4T1)(T3T2)

= T2(T1 + (RTT / 2) )

ReceiverReceiver

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Internet Engineering (Adel Nadjaran Toosi) Slide 17

THANK YOU