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Network Protocols

Chapter 6 (TCP/IP Suite Book): IP Forwarding

Copyright © Lopamudra Roychoudhuri

Packet Delivery

IP at Network layer supervises

delivery IP is a Connectionless protocol

IP treats each packet independently

Packets from the same message may or may

not travel the same path to their destination

Decision about each packet is made

individually by each intermediate router 2

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Direct delivery

Indirect Delivery

Same network/subnet/supernet address

IP Packet – Direct Delivery

IF destination IP address is on the same

network/subnet/supernet, then IP uses direct delivery to send the data packet

directly to the destination without going

through a router.

Sender extracts destination network address

and compares with the networks to which it is

connected

Sender uses destination IP address to find

physical address using Address Resolution

Protocol (ARP) (ARP converts the IP address to

the physical address.)

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Direct Delivery cont. The Direct Delivery method looks up

the layer 2 address (i.e. Ethernet address) of the destination in an ARP Table, or by ARP Request, and places this address in the frame header.

Packet will be delivered directly to destination by the layer 2 network.

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IP Packet – Indirect Delivery

IF destination address is on different

network/subnet/supernet, then

IP uses indirect delivery by sending the data

packet directly to a router that is on the local

subnet

Packet goes from router to router until it reaches

final destination

Sender uses destination IP address and a

routing table to find the IP address of the next

router

Sender uses ARP to find the physical address

of the next router

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Indirect Delivery The Indirect Delivery method looks up the

layer 2 address (i.e. Ethernet address) of the local router (the Default Gateway) in the ARP Table and places this address in the frame header. IP address of the local router was provided to this

IP host by network manager during host configuration.

Packet will be delivered directly to the router by the layer 2 network.

Router will then decide how to forward the packet to the destination subnet. 7

Routing Tables Both Hosts and Routers need some

type of Routing Table that tells them what to do for Indirect Delivery.

Routing Tables store Destination Addresses (can be network,

subnet or host addresses) Routing Information for each address

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Figure 5.20 Network addresses

Direct or Indirect? Example:

My IP address is 140.192.68.29, Mask = 255.255.248.0

I’m sending data to address 140.192.65.118 Should I use Direct or Indirect delivery?

Answer: 140.192.68.29 AND 255.255.248.0 =

140.192.64.0 140.192.65.118 AND 255.255.248.0 =

140.192.64.0 Both addresses are on subnet 140.192.64.0. Use

Direct Delivery!!

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Direct or Indirect? Example:

My IP address is 140.192.68.29, Mask = 255.255.248.0

I’m sending data to address 140.192.98.26 Should I use Direct or Indirect delivery?

Answer: 140.192.68.29 AND 255.255.248.0 =

140.192.64.0 140.192.98.26 AND 255.255.248.0 =

140.192.96.0 Addresses are on different subnets. Use Indirect

Delivery!! This packet must be sent through the default router.

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Forwarding Techniques

Forwarding – placing the packet in its route to its destination

Source Routing Routing Table stores entire path to destination

Next-Hop Routing Table stores only address of the next router – not

the entire path Network-Specific

One routing table entry for each network or subnet address Host-Specific

One routing table entry for each host address Default

A default route entry specifies where to send all packets that are not included in other table entries 12

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Figure 6.3 Source Routing vs. Next-hop method

Next-hop

Source routing

vs.

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Figure 6.4 Network-specific method

N2 R1

Destination Next Hop

Network-specificrouting table for host S

ABCD

DestinationR1R1R1R1

Next Hop

Host-specificrouting table for host S

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Figure 6.5 Host-specific routing

R2

Host B

R3

Host A

R1

N1

N2 N3

Routing table for host A

R3R1R3

......

Destination Next Hop

Host BN2N3......

The administratorwants to have more control:All packets arriving B should go thru R3

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Figure 6.6 Default routing

R1

Host A N1

Rest of the Internet

Defaultrouter R2

N2Routing table for host A

Destination Next Hop

......N2

Default

......R1

R2

Default: designated bynetwork address 0.0.0.0

Routing Implementations Many IP Hosts just use Default Routing

All Indirect deliveries just go to one router Most IP routers use

Mainly Network-Specific rather than Host-Specific routing (to save routing table space)

However, Host-Specific table entries are permitted for special cases.

Mainly Next-Hop rather than Source Routing (to simplify routing table and updates)

A default route so that they don’t need to have a routing table entry for every possible network in the Internet

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Static vs. Dynamic Tables

Static Routing Table Routing Table is manually entered and updated

by Network Administrator Dynamic Routing Table

Routing Table is dynamically updated by means of the exchange of Router Table Update messages between adjacent routers.

Example: RIP, OSPF, IGRP, EIGRP, and BGP

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Configuration for routing, Example 1

R1 Routing table entries

• R1 receives a packet with dest address 192.16.7.14. How will the packet be forwarded?

• Next R1 receives a packet with dest address 167.24.160.5. How will the packet be forwarded?

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Simplified Forwarding in Classful Address with Subnetting

Subnetting happens inside an organization

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Example Configuration – Example 6.4

Example 6.4: points to note

The site address is 145.14.0.0/16 (a class B address). Every packet with destination address in the range 145.14.0.0 to 145.14.255.255 is delivered to the interface m4 and distributed to the final destination subnet by the router.

Second, we have used the address x.y.z.t/n for the interface m4 because we do not know to which network this router is connected.

Third, the table has a default entry for packets that are to be sent out of the site.

The router is configured to apply the subnet mask /18 to any destination address.

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Simplified forwarding module in classless address

We need mask in the table to determine the netid of a classless address

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Make a routing table for router R1 using the configuration (Fig. 6.13).

Example 6.7

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Routing table for router R1 in Figure 6.13

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Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 180.70.65.140.

Example 6.8

SolutionThe router performs the following steps:

1. The first mask (/26) is applied to the destination address. The result is 180.70.65.128, which does not match the corresponding network address.

2. The second mask (/25) is applied to the destination address. The result is 180.70.65.128, which matches the corresponding network address. The next-hop address (the destination address of the packet in this case) and the interface number m0 are passed to ARP for further processing.

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Example 6.9

Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 201.4.22.35.1. The first mask (/26) is applied to the destination address. The result is 201.4.22.0, which does not match the corresponding network address (row 1).

2. The second mask (/25) is applied to the destination address. The result is 201.4.22.0, which does not match the corresponding network address (row 2).

3. The third mask (/24) is applied to the destination address. The result is 201.4.22.0, which matches the corresponding network address. The destination address of the package and the interface number m3 are passed to ARP.

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Routing module and routing table

Common Fields in routing table

Router Up

G = Gateway,meaning destination in another network

# of users using this route

# of packets

H

H = Host-specific

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U Flag

U The route is up. Destination in the same network If U flag is set. It is a network

address.

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G Flag

G The route is to a gateway (router) means the route uses a gateway.

The G flag is important because it differentiates between an indirect route and a direct route.

If this flag is not set, the destination is directly connected. If this flag is set, the destination is indirectly connected.

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H Flag

Indicates this is a route to a specific host. If the H flag is set, specifies that the

destination address is a complete host address.

If this flag is not set, the route is to a network, and the destination is a network address: a net ID, or a combination of a net ID and a subnet ID. 

This flag signifies that the destination address in the entry is a host address or a network address

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Summary

Description Flags

Using a route, destination in the same network, it is a network address.

U

G Flag is not set, the destination is directly connected. G flag is set, the destination is indirectly connected.

G

If this flag is not set, the route is to a network, and the destination is a network address.If this flag is set, the route is to a host, and the destination is a host address.

H

Using a route, destination in another network, it is a network address.

UG

Using a route, the destination is a host, it is on a different network.

UGH

Using a route, the destination is a host, it is on the same network.

UH

Typical Router Table Fields

Mask: Each router table entry has its own mask (differentiates host-specific from network-specific entries)

Destination: This is matched against the address in the packet

Next Hop Address: Next hop router if Destination matches

Physical Port (Interface): Router port to send packet out if Destination matches

Distance: Distance to destination (used to compare different routes)

Flags: Flags that specify information about status of this routing table entry 33

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One utility that can be used to find the contents of a routing table for a host or router is netstat in UNIX, Windows, or LINUX.

The following shows the listing of the contents of the default server. The options:

r - we are interested in the routing table

n - we are looking for numeric addresses.

Note: this is a routing table for a host, not a router.

Although we discussed the routing table for a router throughout the chapter, a host also needs a routing table.

Example

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$ netstat -rnKernel IP routing table

Destination Gateway Mask Flags Iface

153.18.16.0 0.0.0.0 255.255.240.0 U eth0

127.0.0.0 0.0.0.0 255.0.0.0 U lo

0.0.0.0 153.18.31.254 0.0.0.0 UG eth0

Example (continued)

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More information about the IP address and physical address of the server can be found using the ifconfig command on the given interface (eth0).

Example (continued)

$ ifconfig eth0

eth0 Link encap:Ethernet HWaddr 00:B0:D0:DF:09:5D

inet addr:153.18.17.11 Bcast:153.18.31.255 Mask:255.255.240.0

....

From the above information, we can deduce the configuration of the server as shown in next Figure.

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Configuration of the server for Example

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Another example: Routing table for R1 on the next slide

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Routing table for R1 in the previous slide

Mask Dest. Next Hop Flags R.C. U. I.

255.0.0.0 111.0.0.0 --- U 0 0 m0

255.255.255.224

193.14.5.160

--- U 0 0 m2

255.255.255.224

193.14.5.192

--- U 0 0 m1

255.255.255.255

194.17.21.16

111.20.18.14

UGH 0 0 m0

255.255.255.0 192.16.7.0 111.15.17.32

UG 0 0 m0

255.255.255.0 194.17.21.0

111.20.18.14

UG 0 0 m0

0.0.0.0 0.0.0.0 111.30.31.18

UG 0 0 m0

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Example 1

Router R1 receives 500 packets for destination 192.16.7.14; the algorithm applies the masks row by row to the destination address until a match (with the value in the second column) is found:

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Solution

Direct delivery

192.16.7.14 & 255.0.0.0 192.0.0.0 no match

192.16.7.14 & 255.255.255.224 192.16.7.0 no match

192.16.7.14 & 255.255.255.224 192.16.7.0 no match

Host-specific

192.16.7.14 & 255.255.255.255 192.16.7.14 no match

Network-specific

192.16.7.14 & 255.255.255.0 192.16.7.0 match

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Example 2

Router R1 receives 100 packets for destination 193.14.5.176; the algorithm applies the masks row by row to the destination address until a match is found:

Direct delivery

193.14.5.176 & 255.0.0.0 193.0.0.0 no match

193.14.5.176 & 255.255.255.224 193.14.5.160 match

Solution

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Example 3

Router R1 receives 20 packets for destination 200.34.12.34; the algorithm applies the masks row by row to the destination address until a match is found:

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Solution

Direct delivery

200.34.12.34 & 255.0.0.0 200.0.0.0 no match

200.34.12.34 & 255.255.255.224 200.34.12.32 no match

200.34.12.34 & 255.255.255.224 200.34.12.32 no match

Host-specific

200.34.12.34 & 255.255.255.255 200.34.12.34 no match

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Solution

Network-specific

200.34.12.34 & 255.255.255.0 200.34.12.0 no match

200.34.12.34 & 255.255.255.0 200.34.12.0 no match

Default

200.34.12.34 & 0.0.0.0 0.0.0.0. match

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Address aggregation In classless addressing, number of routing table

entries will increase.This is called address aggregation because the blocks of addresses for four organizations are aggregated into one larger block.

Figure 6.16 Longest mask matching

To other nws

To other nws

To the rest of the Internet

IPaddr of m2 of R3

IPaddr of m3 of R1

(R2, IPaddr of m0 )

(R2, IPaddr of m1)

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Longest Mask Matching: In R2 routing table, 1 should be matched before 2. Why?

IPaddr of m2 of R3

Suppose a packet arrives for organization 4 with destination address 140.24.7.200 at R2

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As an example of hierarchical routing, let us consider next Figure. A regional ISP is granted 16384 addresses starting from 120.14.64.0. The regional ISP has decided to divide this block into four subblocks, each with 4096 addresses. Three of these subblocks are assigned to three local ISPs, the second subblock is reserved for future use.

• Note that the mask for each block is /20 because the original block with mask /18 is divided into 4 blocks.

Example 6.12

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Hierarchical routing with ISPs

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The first local ISP has divided its assigned subblock into 8 smaller blocks and assigned each to a small ISP. Each small ISP provides services to 128 households (H001 to H128), each using four addresses. Note that the mask for each small ISP is now /23 because the block is further divided into 8 blocks. Each household has a mask of /30, because a household has only 4 addresses (232−30 is 4).

The second local ISP has divided its block into 4 blocks and has assigned the addresses to 4 large organizations (LOrg01 to LOrg04). Note that each large organization has 1024 addresses and the mask is /22.

Example 6.12 (Continued)

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The third local ISP has divided its block into 16 blocks and assigned each block to a small organization (SOrg01 to SOrg15). Each small organization has 256 addresses and the mask is /24.

There is a sense of hierarchy in this configuration. All routers in the Internet send a packet with destination address 120.14.64.0 to 120.14.127.255 to the regional ISP. The regional ISP sends every packet with destination address 120.14.64.0 to 120.14.79.255 to Local ISP1. Local ISP1 sends every packet with destination address 120.14.64.0 to 120.14.64.3 to H001.

Example 6.12 (Continued)