1 Kyung Hee University Chapter 6 Delivery Forwarding, and Routing of IP Packets.

11Kyung Hee University

Chapter 6Chapter 6Delivery Delivery

Forwarding, and Forwarding, and Routing of IP Routing of IP

PacketsPackets


IntroductionIntroduction

Delivery

Meaning the physical forwarding of the packets

Connectionless and connection-oriented services

Direct and indirect delivery

Routing

Related to finding the route (next hop) for a datagram


6.1 Delivery6.1 Delivery

Connection Types

Connection-oriented service

Using same path The decision about the route of a sequence of packets with

the same source and destination addresses can be made only once, when the connection is established

Connectionless service

Dealing with each packet independently Packets may not travel the same path to their destination

IP is :

Connectionless protocol


Direct versus Indirect DeliveryDirect versus Indirect Delivery

Two methods delivering a packet to its final destination

Direct

Indirect

Direct delivery

The final destination of the packet is a host to the same physical network as the deliverer or the delivery is between the last router and the destination host

Decision making whether delivery is direct or not Extracting the network address of the destination packet

(setting the hostid part to all 0s) Then, comparing the addresses of the network to which it is

connected


Direct versus Indirect Delivery (cont’d)Direct versus Indirect Delivery (cont’d)

Direct delivery



Indirect delivery

The destination host is not on the same network as the deliverer

The packet goes from router to router until finding the final destination

Using ARP to find the next physical address

Mapping between the IP address of next router and the physical address of the next router



Indirect delivery


6.3 Forwarding6.3 Forwarding

Forwarding means to place the packet in its route to its

destination. So, it requires a host or a router a routing

table.

Routing table

Used to find the route to the final destination


Forwarding TechniquesForwarding Techniques

Next-hop Method

A technique to reduce the contents of a routing table

The routing table holds only the address of the next hop instead of holding information about the complete route

The entries of a routing table must be consistent with each other


Forwarding Techniques (cont’d)Forwarding Techniques (cont’d)



Network-Specific Method

Having only one entry to define the address of network itself



Host-Specific Method

Destination host addresses is given in the routing table

The efficiency is sacrificed for the advantages :

Giving to administrator more control over routing

Ex) if the administrator wants all packets arriving for host B delivered to router R3 instead of R1, one single entry in the routing table of host A can explicitly define the route


Routing methods (cont’d)Routing methods (cont’d)


Routing methods (cont’d)Routing methods (cont’d) Default Method

Instead of listing all networks in the entire Internet, host A can just have one entry called the default (network address 0.0.0.0)


Forwarding with Classful AddressingForwarding with Classful Addressing

Forwarding without Subnetting


Example 1Example 1

Figure 6.8 shows an imaginary part of the Internet.

Show the routing tables for router R1.


Example 1 - SolutionExample 1 - Solution


Example 2 Example 2

Router R1 in Figure 6.8 receives a packet with

destination address 192.16.7.14. Show how the packet

is forwarded.

Solution



The destination address in binary is 11000000 00010000

00000111 00001110. A copy of the address is shifted 28

bits to the right. The result is 00000000 00000000

00000000 00001100 or 12. The destination network is

class C. The network address is extracted by masking

off the leftmost 24 bits of the destination address; the

result is 192.16.7.0. The table for Class C is searched.

The network address is found in the first row. The next-

hop address 111.15.17.32. and the interface m0 are

passed to ARP.


Example 3Example 3

Router R1 in Figure 6.8 receives a packet with destination

address 167.24.160.5. Show how the packet is forwarded

Solution

The destination address in binary is 10100111 00011000 10100000 00000101. A copy of the address is shifted 28 bits to the right. The result is 00000000 00000000 00000000 00001010 or 10. The class is B. The network address can be found by masking off 16 bits of the destination address, the result is 167.24.0.0. The table for Class B is searched. No matching network address is found. The packet needs to be forwarded to the default router (the network is somewhere else in the Internet). The next-hop address 111.30.31.18 and the interface number m0 are passed to ARP.


Forwarding with SubnettingForwarding with Subnetting


Example 4Example 4

Figure 6.11 shows a router connected to four subnets.


Example 5Example 5

The router in Figure 6.11 receives a packet with

destination address 145.14.32.78. Show how the packet

is forwarded.

Solution

The mask is /18. After applying the mask, the subnet address is 145.14.0.0. The packet is delivered to ARP with the next-hop address 145.14.32.78 and the outgoing interface m0.


Example 6Example 6

A host in network 145.14.0.0 in Figure 6.11 has a

packet to send to the host with address 7.22.67.91.

Show how the packet is routed.

Solution

The router receives the packet and applies the mask (/18). The network address is 7.22.64.0. The table is searched and the address is not found. The router uses the address of the default router (not shown in figure) and sends the packet to that router.


Forwarding with Classless AddressingForwarding with Classless Addressing

In classful addressing we can have a routing table with

three columns; in classless addressing, we need at

least four columns.

Figure 6.12 Simplified forwarding module in classless address


Example 7Example 7

Make a routing table for router R1 using the

configuration in Figure 6.13.



Solution

Table 6.1 shows the corresponding table

Table 6.1 Routing table for router R1 in Figure 6.13Table 6.1 Routing table for router R1 in Figure 6.13


Example 8Example 8

Show the forwarding process if a packet arrives at R1

in Figure 6.13 with the destination address

180.70.65.140.



SolutionSolution

The 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.


Example 9Example 9


in Figure 6.13 with the destination address 201.4.22.35.



SolutionSolution

The router performs the following steps:

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.


Example 10Example 10


in Figure 6.13 with the destination address 18.24.32.78.

Solution

This time all masks are applied to the destination

address, but no matching network address is found.

When it reaches the end of the table, the module gives

the next-hop address 180.70.65.200 and interface

number m2 to ARP. This is probably an outgoing

package that needs to be sent, via the default router, to

some place else in the Internet.



Now let us give a different type of example. Can we

find the configuration of a router, if we know only its

routing table? The routing table for router R1 is given in

Table 6.2. Can we draw its topology?

Table 6.2 Routing table for Example 11Table 6.2 Routing table for Example 11




Address AggregationAddress Aggregation

Figure 6.15 Address aggregation


Longest Mask MatchingLongest Mask Matching The routing table is sorted from the longest mask to

the shortest mask.


Hierarchical RoutingHierarchical Routing

To solve the problem of gigantic routing tables,

creating a sense of the routing tables

Routing table can decrease in size



As an example of hierarchical routing, let us consider Figure 6.17. A regional ISP is granted 16,384 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.




6.3 Routing - 6.3 Routing - Static versus Dynamic RoutingStatic versus Dynamic Routing

Static routing table

Containing information entered manually

Cannot update automatically when there is a change in the internet

Used in small internet that does not change very much, or in an experimental internet for troubleshooting

Dynamic routing table

is updated periodically using one of the dynamic routing protocols such RIP, OSPF, or BGP (see Chap. 14)

Updating the routing table corresponding to shutdown of a router or breaking of a link


Routing ModuleRouting Module


Routing Table Routing Table Routing table

In classless addressing, routing table has a minimum of four columns.

- Some routers have even more columns

FlagsU (up) : The router is up and running. G (gateway) : The destination is in another network.H : Host-specific address. D : Added by redirection.M : Modified by redirection.


Routing Table (cont’d)Routing Table (cont’d)

Flags U (Up) : indicating the router’s running G (Gateway) : meaning that the destination is another network H (Host-specific) : indicating that the entry in the destination is a

host-specific address D (Added by redirection) : indicating that routing information for

this destination has been added to the host routing table by a redirection message from ICMP

M (Modified by redirection) : indicating that routing information for this destination has been modified by a redirection message from ICMP

Reference count : giving the number of users that are using this route at any moment

Use : showing the number of packets transmitted through this router for the corresponding destination

Interface : showing the name of the interface



One utility that can be used to find the contents of a

routing table for a host or router is netstat in UNIX or

LINUX. The following shows the listing of the contents

of the default server. We have used two options, r and

n. The option r indicates that we are interested in the

routing table and the option n indicates that we are

looking for numeric addresses. Note that 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 13 (cont’d)Example 13 (cont’d)

$ 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.

Loopback interface



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

$ 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 Figure 6.19.



Ifconfig command gives us the IP address and the physical address (hardware) address of the interface


6.4 Structure of a Router6.4 Structure of a Router


ComponentsComponents

Input port

Output port


Components (cont’d)Components (cont’d)

Routing Processor

performing the functions of the network layer

destination address is used to find the address of the next hop and output port number : table lookup


Switching fabricsSwitching fabrics

Crossbar switch


Switching FabricsSwitching Fabrics

Banyan switch

log2 (n) stages with n/2 microswitches


Switching Fabrics (cont’d)Switching Fabrics (cont’d)

Examples of routing in a banyan switch



Possibility of internal collision even when two packets

are not heading for the same output port in banyan

switch

solving the problem by sorting the arriving packets based on their destination port

Trap module: preventing duplicate packets (packets with the same output destination) from passing to the banyan switch simultaneously



Batcher-banyan switch


Summary(1)Summary(1)

In a connection-oriented service, the local network layer protocol first makes a connection with the network layer protocol at the remote site before sending a packet.

In a connectionless service, the network layer protocol treats each packet independently, with each packet having no relationship to any other packet. The packets in a message may or may not travel the same path to their destination. The IP protocol is a connectionless protocol.

The delivery of a packet is called direct if the deliverer(host or router) and the destination are on the same network.

The delivery of a packet is called indirect if the deliverer(host or router) and the destination are on different networks.

In the next-hop method, instead of a complete list of the stops the packet must make, only the address of the next hop is listed in the routing table.

In the network-specific method, all hosts on a network share one entry in the routing table.

In the host-specific method, the full IP address of a host is given in the routing table.


Summary(2)Summary(2)

In the default method, a router is assigned to receive all packets with no match in the routing table.

The routing table for classful forwarding can have three columns.

The routing table for classless addressing needs at least four columns.

The number of columns in a routing table is vendor dependent.

Address aggregation simplifies the forwarding process in classless addressing.

Longest mask matching is required in classless addressing.

Classless addressing requires hierarchical and geographical routing to prevent immense routing tables.

Search algorithms for classful addressing are not efficient for classless addressing.

A static routing table's entries are updated manually by an administrator.

A dynamic routing table's entries are updated automatically by a routing protocol.

A router is normally made of four components: input ports, output ports, the routing processor, and the switching fabric.

1 Kyung Hee University Chapter 6 Delivery Forwarding, and Routing of IP Packets.

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