Internet Control Message Protocol (ICMP) and

Super-Net & Sub-Net Address Extension

Tarun Bhalla*, Dhiraj Kapila

**,Anchal Nayyar

***

* Assistant Professor, Anand College of Engineering & Management, Kapurthala ** Assistant Professor, DAV Institute of Engineering & Technology, Jalandhar

***Lecturer, Doaba Group of Colleges, Rahon

Abstract: This paper specifies the concept and the

message format of the Internet Control Message

Protocol (ICMP) to enable hosts attached to multicast

or broadcast networks to discover the IP addresses of

their neighboring routers. Also, we represent the super-

net addressing and subnet addressing with its

implementation of subnets with mask representation.

We are also discussed the motivation, purpose,

restrictions and the encapsulation of ICMP.

Keywords:-Introduction, ICMP message format,

Motivation, Purpose, Restrictions and Encapsulation of

ICMP, super-net and subnet addressing, Implementation

of subnets with masks representation, conclusion.

1. Introduction

ICMP is used for error messages, and other messages

intended for the TCP/IP software itself, rather than any

particular user program. For example, if you attempt to

connect to a host, your system may get back an ICMP

message saying "host unreachable". ICMP can also be

used to find out some information about the network.

ICMP is similar to UDP, in that it handles messages that

fit in one datagram. However it is even simpler than

UDP. It doesn't even have port numbers in its header.

Since all ICMP messages are interpreted by the network

software itself, no port numbers are needed to say

where a ICMP message is supposed to go. [1]

Before a host can send IP datagrams beyond its directly-

attached subnet, it must discover the address of at least

one operational router on that subnet. Typically, this is

accomplished by reading a list of one or more router

addresses from a (possibly remote) configuration file at

startup time. On multicast links, some hosts also

discover router addresses by listening to routing

protocol traffic. Both of these methods have serious

drawbacks: configuration files must be maintained

manually -- a significant administrative burden – and

are unable to track dynamic changes in router

availability; eavesdropping on routing traffic requires

that hosts recognize the particular routing protocols in

use, which vary from subnet to subnet and which are

subject to change at any time. This document specifies

an alternative router discovery method using a pair of

ICMP messages, for use on multicast links. It eliminates

the need for manual configuration of router addresses

and is independent of any specific routing protocol. [2] 2. ICMP Message Format

ICMP Messages

The Common ICMP header

Each ICMP message has its own format, they

all begin with the same three fields

TYPE (8-bit): identifies the message

CODE (8-bit): provides further information

about the message type

CHECKSUM (16-bit):

In addition, ICMP messages that report errors

always include the header and the first 64 data

bits of the datagram causing the problem.[3]

Fig: ICMP Encapsulation [3]

Fig: ICMP message format [3]

Echo request and reply message

Used to test reachability

An echo request can also contain optional data

(the content does not matter)

An echo reply always returns exactly the same

data as was received in the request

Sent by ping program [4]

Host Unreachable

When a router cannot forward or deliver an IP

datagram, it sends a destination unreachable

message back to the original source

The CODE field specifies details

0: network unreachable

1: host unreachable

2: protocol unreachable

3: port unreachable

4: fragmentation needed and DF

(don’t fragment) set

5:source route failed

Source Quench

To deal with congestion and datagram flow

control

When routers are overrun with traffic, it is

called congestion.

A machine uses ICMP source quench

messages to report congestion to the original

source

There is no ICMP message to reverse the effect

of a source quench. Usually the host gradually

increases the rate when no further source

quench requests are received.[4]

Route Redirect

Routers exchange routing information

periodically to accommodate network changes

and keep their routes up-to-date.

A general rule:-

Routers are assumed to know correct routes;

hosts begin with minimal routing information

and learn new routes from routers.

When a router detects a host using a non-

optimal route, it sends the host an ICMP

redirect message, requesting that the host

change its rout.

Limited to interactions between a router and a

host on a directly connected network.

Time Exceeded

The sending operating system (or application)

sets the time to live field in the IP header to a

value that represents the maximum time the

datagram is allowed to travel on the Internet.

The field value is decreased at each point that

the Internet header (IP Header) is being

processed.

The field value is measured in units of

seconds.

The datagram must be discarded if this field

value equals zero - before reaching its

destination.[4]

Parameter Problem

ICMP Parameter Problem message is sent

when a router (must generate this message) or

a host (should generate this message) process a

datagram and finds a problem with the IP

header parameters.

It is only sent if the error caused the datagram

to be discarded.

Timestamp Request and Timestamp Reply

The ICMP Time Stamp Request and Reply

allows a node to query another for the current

time.

This allows a sender to determine the amount

of latency that a particular network is

experiencing.

The sender initializes the identifier (used to

identify Timestamp requests aimed at different

destination hosts) and sequence number (if

multiple Timestamp requests are sent to the

same destination host), sets the originate time

stamp and sends it to the recipient.

The receiving host fills in the receive and

transmit time stamps, change the type of the

message to time stamp reply and returns it to

the recipient. [4]

Information Request and Reply

The ICMP Information Request/Reply pair

was intended to support self-configuring

systems such as diskless workstations at boot

time, to allow them to discover their network

address.

The sender (a host) fills in the request with the

Destination IP address in the IP Header set to

zero (meaning this network).

The request may be sent with both Source IP

Address and Destination IP Address set to

zero.

The sender initializes the identifier and the

sequence number, both used to match the

replies with the requests, and sends out the

request.[4]

Address Mask Request and Reply

The ICMP Address Mask Request (and Reply)

is intended for diskless systems to obtain its

subnet mask in use on the local network at

bootstrap time.

Address Mask request is also used when a

node wants to know the address mask of an

interface. The reply (if any) contains the mask

of that interface.

Once a host has obtained an IP address, it

could than send an Address Mask request

message to the broadcast address of the

network they reside on (255.255.255.255).[4]

Motivation

IP may fail to deliver datagrams because

the destination is not available

the time-to-live counter expires

routers become congested

We need to let the sender know what has

happened

ICMP is a required part of IP

Purpose

ICMP allows routers (and hosts) to send error

or control messages to other routers or hosts

ICMP provides communication between the

Internet Protocol software on one machine and

the Internet Protocol software on another

Restrictions

ICMP messages are not generated for errors

that result from datagrams carrying ICMP error

messages. Why? ICMP is only sent to the original source. Why?

ICMP Encapsulation

ICMP is encapsulated in an IP packet, but is

considered part of the IP or Internet layer.[3]

3. Super-net Addressing A super-net is an Internet Protocol (IP) network that is

formed from the combination of two or more networks

(or subnets) with a common Classless Inter-Domain

Routing (CIDR) routing prefix. The new routing prefix

for the combined network aggregates the prefixes of the

constituent networks. It must not contain other prefixes

of networks that do not lie in the same routing path. The

process of forming a super-net is often called super-

netting, route aggregation, or route summarization.

Step 1:

A method by which an organization may use a block of

class C addresses rather than a Class B address. The

block: large enough to provide an individual class C

address to every possible network, likely to be

connected to the Internet. [5]

Routing: Instead of one entry per organization, this may

require multiple entries.

Classless Inter Domain routing (CIDR):

CIDR collapses a block of contiguous (class C)

addresses into a single entry (network address, count)

where

- Network address: the smallest address in the block

- Count: the total number of network addresses in the

block.

Thus (211.15.136.0, 8) can be used to specify 8

addresses from

EXAMPLE: 211.15.136.0 to 211.15.143.0

In practice CIDR does not restrict itself to only class C

addresses. [5]

The only requirement is that count should be 2n.

The bit mask is used to specify the total network part of

the 32 bit IP address of the lowest net address. Since

136 is 10001000, the mask will have 16+5 bit set to 1.

ie., the mask will be 255.255.248.0 in the given

example

Super netting requires unconventional router software

for all internal routers of the organization to understand

the Range of addresses.

Particularly suitable for ISPs where for each ISP's own

Routers, the routing table contains the address of each

subscriber. But for other ISPs, the table has one entry

each for each of the other ISP. [5]

Step 2:

The block may be written as (211.15.136.0, 8) OR

211.15.136.0, 2048

Where 2048 is the block of host addresses OR

211.15.136.0, 255.255.248.0 where 255.255.248.0 is

the mask.

This reduces the number of entries from 8 (for standard

class C mask of 255.255.255.0) to only 1 with the new

mask. [5]

INPUT OUTPUT

Any address b/w

211.15.136.0 to

211.15.143.0

211.15.136.0

211.15.135.0 211.15.128.0

211.15.144.0 211.15.144.0

NEED OF ISPs:-

Steps 3:

GENERALIZATION: CIDR

- No need to restrict to class C (32 – m)

- The block of (host) addresses = 2

Where m is the number of leading bits in the

address, which specify the network part of the

address. [5]

- Uses a bit mask to identify the size of the block

CIDR Notation / Slash notation: CIDR block may be

represented by the (lowest) addresses and the no of bits

which are 1 in the mask) Thus for the example

211.15.136.0/21 defines the block in the above

example.

If this were to be generalized as x. y. z. w/m where

1<= m <= 32, one gets blocks of addresses of a large

no. of sizes [5]

CIDR

NOTATION

MASK Block of

addresses

/1 128.0.0.0 2,147,484,448

/2 192.0.0.0 1,073,742,224

.

.

/8 255.0.0.0 16,777,216

/9 255.128.0.0 8,388,608

.

.

/15 255.254.0.0 131,072

/16 255.255.0.0 65,536

/17 255.255.128.0 32,768

.

/23

255.255.254.0

512

/24 255.255.255.0 256

/25 255.255.255.128 128

.

.

.

/29 255.255.255.248 8

/30 255.255.255.252 4

/31 255.255.255.254 2

/32 255.255.255.255 1

4. SUBNET Addressing

A subnet-work, or subnet, is a logically visible

subdivision of an IP network. All computers that belong

to a subnet are addressed with a common, identical,

most-significant bit-group in their IP address. This

results in the logical division of an IP address into two

fields, a network or routing prefix and the rest field. The

rest field is a specific identifier for the computer or the

network interface.

Some basic points about subnet addressing as follows:

In the original IP addressing scheme, each

physical network is assigned a unique network

address.

Individual sites can have the freedom to

modify addresses and routes as long as the

modifications remain invisible to other sites.

Conceptual interpretation of a 32-bit IP

address[6]

Hierarchical addressing, hierarchical routing.

E.g., telephone system

Subdivides the host suffix into a pair of fields

for physical network and host.

Allows an organization to use a single network

prefix for multiple physical networks.[6]

Interpreted only by routers and hosts at the

site; treated like normal address elsewhere

Both physical networks share prefix 128.10.

Router R uses third octet of address to choose

physical net

Address Mask: Each physical network is

assigned 32-bit address mask (also called subnet

mask)[6]

Flexibility in Subnet Address Assignment

To allow maximum flexibility in choosing how to

partition subnet address, the TCP/IP subnet standard

permits subnet interpretation to be chosen

independently for each physical network. Once a subnet

partition has been selected, all machines on that

network must honor it.[6]

5. Implementation of Subnet with marks

representation

A site using subnet addressing must choose a 32-

bit subnet mask for each network.

Example:-

11111111 11111111 11111111 00000000

11111111 11111111 00011000 01000000

Subnet mask representation

Dotted decimal notation

e.g. 255.255.255.0

{<network number>, <subnet number>, <host

number>}

e.g.) { 128.10, 1, 0 } [6]

CONCLUSION

We have concluded that ICMP is widely used in the

internet for message transmission. It is a connectionless

and non-reliable protocol. We have discussed message

format of ICMP and its details which give brief

information how the ICMP (internet control message

protocol) works. A super-net is an Internet Protocol (IP)

network that is formed from the combination of two or

more networks (or subnets) with a common Classless

Inter-Domain Routing (CIDR) routing prefix. A subnet

work, or subnet, is a logically visible subdivision of

an IP network. All computers that belong to a subnet are

addressed with a common, identical, most-significant

bit-group in their IP address.

REFERENCES

[1]

http://www.doc.ic.ac.uk/~ih/doc/pc_conn/tcpip/intro/int

ro4.html

[2]

http://www.javvin.com/protocol/rfc1256.pdf

[3]

http://www.cis.syr.edu/~wedu/Teaching/cis758/Lecture

Notes/ICMP.pdf

[4]

http://althing.cs.dartmouth.edu/local/ICMP_Scanning_v

3.0.pdf

[5]

http://web2.uwindsor.ca/courses/cs/aggarwal/cs60375/n

otes/chap10.doc

[6]

http://mmlab.snu.ac.kr/courses/2006_computer_networ

ks/prev/lecture2004/5_subnettin

Tarun Bhalla received his B.Tech

degree in Computer Science from Punjab

Technical University. He is currently

working as Assistant Professor in Anand

College of Engineering and Management,

Kapurthala. His research interest area

includes Database, Network Security Mobile

Computing and adhoc network.

Dhiraj Kapila received his B.Tech

degree in Computer Science from DAV

Institute of Engg & Technoly. He is

working as Assistant Professor in

DAVIET, Jaladhar. His research areas

include image processing, computer

networks and Network Security.

Anchal Nayyar received her B.Tech

and degree in Computer Science from

Punjab Technical University. She is

working as a Lecturer in Doaba Group

of Colleges Rahon. Her research

interest area includes Mobile

Computing and adhoc network.