Review of IPv4 Routing

Dr Ram P Rustagi, PESITrprustagi@pes.edu

Jun 06, 2013

http://ise.pesit.pes.edu/Workshops/CDoT/

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IPv4 Overview

• Resources

2

Network service modelQ: What service model for “channel” transporting datagrams from sender to receiver?

•Can transport layer rely on n/w layer?

•Will the packets be in order?

•Will the time gap between two pkts be maintained?

•Will network provide any congestion information?

•Will network provide any time gurantees?

•will network provide any BW guarantees?

Network Layer4-17

1

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Datagram forwarding table

IP destination address in arriving packet’s header

routing algorithm

local forwarding table

dest address output link

address-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses, so rather than list individual destination addresslist range of addresses(aggregate table entries)

src: Kurose & Ross

Review of IPv4

• General IP Networking Overview– Network Configuration– IPv4 Addressing * 1981 IP was standardized * Dotted Decimal Notation * Classful addressing - wastage

IP Addressing Classful addressing:

first byte value determines the class Class A (unicast): first bit is 0.

• value of first byte 0 - 126

Class B (unicast): first 2 bits have value 10• value of first byte: 128 - 191

Class C (unicast): first 3 bits have value 110• value of first byte: 191 - 223

Class D (Multicast): First 4 bits have value: 1110

• value of first byte: 224 - 239

Class E (Experimental) : First 4 bits have value: 1111

• value of first byte: 240 - 255

Large part of address space is wasted

Growth of the Internet

• In 1985 (RFC 950) subnetting concept was introduced.

• In 1987 VLSM (RFC 1009)• It uses longest prefix match• It allows Route aggregation

Subnet Address HierarchyExtended-Network-Prefix

Growth of the Internet

• By 1992 IETF identified the exponential growth of the internet.

• Three serious problems - Exhaustion of Class B address space - Rapid growth of Backbone Routing tables - Exhaustion of 32 bit IPv4 address space

• In 1993 CIDR was officially documented - Supernetting - Extensively supports Route aggregation

• IETF started the IPng working group for IPv6

IP Addressing IPv4 addresses are unique and universal

exceptions ? IPv4 address is 32 bit long

total available addresses: 4,294,967,296 Uses Dotted Decimal Notation (DDN)

example: 119.82.126.182 Exercise:

Find the error in following addresses• 119.082.126.182

• 119.82.126.182.80• 119.82.126.282• 119.01010010.126.82

Subnets Identified by subnet masks Defines one network A router is needed to connect two networks Masks for classful addresses

Class A: 255.0.0.0 or /8 Class B: 255.255.0.0 or /16 Class C: 255.255.255.0 or /24

Glassful addressing obsolete now replaced with classless addressing (CIDR)

RFCs RFC 1518: Architecture for IP addr allocation

with CIDR RFC 1466: Guidelines for IP addr space

management RFC 917: Internet subnets

Subnets Few terms to understand

network portion and host portion network number

• apply subnet mask to IP address (bitwise AND)

Broadcast address• set all bits to 1 in host portion

network mask• set all bits to 0 in host portion

first available address in the block• value of host portion = 1

last available address in the block• value of host portion = 2n-2

Subnets Exercise 1

a block of addresses is granted to a small organization. one of the address is 119.82.126.182/27. Find out the following:

• the network number• subnet mask• broadcast address• first available address and • last available address

Exercise 2: repeat the above exercise for address

• 192.168.100.200/18

Address Types

Unicast Multicast Broadcast Anycast

Forwarding Table Needs at least 4 entries in forwarding table

Network Address Network Mask Next Hop Address Interface

Forwarding table principles Each routers makes its decision independently Different routers may have different information Tells how to reach destination but not how to get back

Effect of Forwarding Table principles Packets are forwarded on hop by hop basis Packets from A to B go via path X but return via path Y

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.2

223.1.7.1

223.1.8.2223.1.8.1

223.1.9.1

223.1.9.2R1

R3R2

Rest of Internet

Exercise: Build the routing table for all the routers

Note: Take subnet mask for all network as /24

Q: What should be routing table for PCs?

Forwarding Table

e1e2

e3e2

e1e3

e3e2

e1

src: Kurose & Ross

Network Layer4-53

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23

Organization 2

.

..

..

.

hierarchical addressing allows efficient advertisement of routing information:

src: Kurose & Ross

Route Aggregation

200.23.16.0/23

11001000 00010111 00010000 00000000

200.23.18.0/23

11001000 00010111 00010010 00000000

200.23.20.0/23

11001000 00010111 00010100 00000000

200.23.22.0/23

11001000 00010111 00010110 00000000

200.23.24.0/23

11001000 00010111 00011000 00000000

200.23.26.0/23

11001000 00010111 00011010 00000000

200.23.28.0/23

11001000 00010111 00011100 00000000

200.23.30.0/23

11001000 00010111 00011110 00000000

Exercise: Summary Route

Summary Route ?

Design the subnetting/routing

N/w given: 192.168.0.0/24-Each LAN has 10 hosts-serial link n/w needs two addresses-LAN of R3-R7-R6 needs 3 addresses

Answer: subnetting exercise

• 9 LANs of 10 hosts• 1 LAN of 3 addresses• 8 Serial links of 2 addresses

Using Routing Table For a given packet

take the destination address repeat the following for each entry in routing

table apply the netmask match the computed n/w number with routing

table entry if matches

• forward the packet to next-hop on listed interface• exit

else • continue to next entry

when no match found (assuming 0.0.0.0/0 not defined)

• drop the packet

Network Layer4-55

IP addressing: the last word...

Q: how does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned Names and Numbers http://www.icann.org/

allocates addresses manages DNS assigns domain names, resolves disputes

ARP - Address Resolution Protocol Packet delivery to a host requires two

addresses• Logical address - IP Address• Physical address - MAC address

Need to find mapping from logical to physical

• ARP is used - RFC 826

Fig Src: Forouzan - Data Communication and Networking, SIE

ARP - 4 cases

Src: Forouzan

ARP ARP Request and Reply

ARP Request is broadcast ARP Reply is Unicast

Other forms of ARP Proxy ARP (RFC 1027) Reverse ARP (RFC 903) Gratuitous ARP

Proxy ARP Router (Proxy ARP Server) replied to all requests Used when

splitting a network w/o changing hosts netmask Taking care of statically configured m/c Mobile IP

Src: Forouzan

Reverse ARP Reverse ARP (RARP)

RFC 903 Used for diskless stations Organization does not have enough IP

Address Target as MAC Bcast does not cross the

router Needs one RARP server for each subnet

BOOTP Improvement over RARP Has a relay agent to forward across network has static mapping of MAC to IP

• manageability issues

DHCP - replaces BOOTP

Gratuitous ARP Ref: http://wiki.wireshark.org/Gratuitous_ARP Gratuitous ARP Request

both src and dstn IP is set to that of m/c dstn MAC is broadcast i.e. ff:ff:ff:ff:ff:ff

Ordinarily, no reply will occur normally•if a m/c exists, it may respond

Gratuitous ARP Reply a reply to which no request has occurred

Gratuitous ARP

Why Gratuitous ARP help detect IP conflicts

•if a m/c receives G-ARP req which is its own, implies IP conflict

helps in updating other m/cs ARP tables•used in clustering solutions, when IP is

moved helps inform the switch to update its port

table each time an i/f comes up (after down),

sends G-ARP Practice: use send_arp to perform gratuitous

arp

Network Setup

• We are not using the backbone network - better delete the address 172.16.3.x/21

– These are backbone addresses (College network)• with default gateway of 172.16.3.1

• Should we not use commands like ifconfig, route, arp etc.– These are deprecated commands, use commands from

iproute2 pkg.• ip addr [options]• ip route [options]• ip neigh [options]

Utilities/Tools

• wireshark/tcpdump• nc• ping• arping

Wireshark • It is a network packet analyser• In late 1997 Gerald Combs started writing Ethereal

for his use• 1998 version 0.2.0 was released• later several patches, plug-ins were added • In 2006 it is renamed as wireshark• Users - Administrators - Security Engineers - Developers - Ordinary users

Wireshark

Main window: Main menu bar The main toolbar The filter toolbar (Display filter) The packet list pane The packet details pane The packet byte pane The status bar

Wireshark capture

Utilities/Tools

• Go to capture option• Specify the interface• Specify the capture filter (otherwise you will get too

much traffic)• Examples of good capture filters

– for capturing a traffic for a given network or networks• net 172.16.1.0/24 or 172.16.2.0/24

– for capturing specific source and/or destination• src 172.16.1.201 and dst 172.16.3.201

– for capture specific protocol or applications or TCP/UDP port no• http• port 80 or port 8080

Utilities/Tools

• Difference between the capture filter and the display filter– Capture filters are used for capturing only relevant

packets– display filters are used for displaying selected packets

from what is captured.• these may be used to analyze a subset of packets

e.g. – packets on a TCP Connection– looking at only TCP SYN/RST packets etc

• We can save captured data and analyse• To check the connectivity we can use ping

command with –c N option

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Utilities/Tools

• netcat (nc) – to send packets on the network– supports both udp and tcp– can act as a client and server as well– data is to be specified by the user

• can be piped from other programs (or files)– example of connecting using UDP– example of connecting using TCP

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Summary

• Growth of internet• IPv4 addressing• IPv4 Routing• Building IPv4 network• Analysis

Thank You