1. Behrouz A. Forouzan TCP/IP Protocol Suite, 3rd Ed.
Delivery, Forwarding and Routing of Packets

2. Basic Concepts
Delivery refers to the way a packet is handled by the underlying Networks under the control of the network layer.
Forwarding refers to the way a packet is delivered to the next station.
Routing refers to the way routing tables are created to help in forwarding.

3. Delivery
Delivery of a packet in the network layer is accomplished using either a connection-oriented service or a connectionless service.
IP is a connectionless protocol.
The delivery of a packet to its final destination is accomplished using two different methods:
Direct delivery
Indirect delivery.

4. Direct Delivery
In a direct delivery, the final destination of the packet is a host connected to the same physical network.
The sender can easily determine if the delivery is direct.
It can extract the network address of the destination.
Compare it with the addresses of the networks to which its connected.
If a match is found, the delivery is direct.

5. Direct Delivery

6. Indirect Delivery
If the destination host is not on the same network, the packet is delivered indirectly.
In an indirectly delivery, the packet goes from router to router until it reaches the one connected to the same physical network.
In an indirect delivery, the sender uses the destination IP address and a routing table to find the IP address of the next router.

7. Indirect Delivery

8. Forwarding
Forwarding means to place the packet in its route to its destination.
Requires a host or a router to have a routing table.
When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination.
This simple solution is inefficient today in an internetwork such as the Internet because the number of entries needed in the routing table would make table lookups inefficient.

9. Forwarding Techniques
Next-hop method
To reduce the contents of a routing table.
In this technique, the routing table holds only the address of the next hop instead of information about the complete route.
Network-specific method
To reduce the routing table and simplify the searching process.
Instead of having an entry of every destination host connected to same physical network, we have only one entry that defines the address of the destination network itself.

10. Next-hop method

11. Network-specific method

12. Forwarding Techniques
Host-specific method
In the host-specific method, the destination host address is given in the routing table.
More control over the routing
Here efficiency is sacrificed for other advantages: Although it is not efficient to put the host address in the routing table, there are occasions in which the administrator wants to have more control over routing
Host-specific routing is used for purposes such as checking for route or providing security measure.

13. Host-specific method

14. Forwarding Techniques
Default method
Host A is connected to a network with two routers.
Router R1 routes the packets to hosts Connected to network N2. However, for the rest of the Internet, router R2 is used.
Instead of listing all networks in the entire Internet, host A can just have one entry called the default (normally defined as network address ).

15. Default method

16. Forwarding with Classful Addressing
In classful addressing, most of the routers in the global Internet are not involved in subnetting.
Subnetting happens inside the organization.
A typical forwarding module in this case can be designed using three tables, One for each unicast class (A, B, C).
If the router supports multicasting, another table can be added to handle class D addresses.

17. Forwarding with Classful Addressing
Each routing table has a minimum of three columns:
The network address of the destination network tell us where the destination host is located.
Network specific forwarding is used.
The next hop address tell us to which router the packet must be delivered for an indirect delivery.
The column is empty for a direct delivery.
The interface number defines the outgoing port from which the packet is sent out.

18. Simplified forwarding module

19. Forwarding Module The forwarding module follows these steps:
The destination address of the packet is extracted.
A copy of destination address is used to find the class of the address.
Shifting the copy of the address 28 bits to the right.
The result is s four bit number between 0 and 15.
0 to 7, the class is A.
8 to 11, the class is B.
12 or 13, the class is C.
14, the class is D.
15, the class is E.

20. Forwarding Module
The result of step 2 for class A, B or C and the destination address are used to extract te network address.
The class of address and the network address are used to find next hop information.
The class determine the table to be searched.
The module searches this table for the network address.
If a math is found, the next hop address and the interface number of the output port are extracted from the table.
If no match is found, the default is used.
The ARP module uses the next hop address and the interface number to find the physical address of the next router.

21. Example 1
Show the routing tables for router R1

22. Solution The following three tables are used by router R1.
Note that some entries in the next-hop address column are empty because in these cases, the destination is in the same network to which the router is connected (direct delivery).
In these cases, the next-hop address used by ARP is simply the destination address of the packet as we will see in the next Chapter.

23. Tables for Example 1

24. Example 2
Router R1 in the previous figure receives a packet with destination address Show how the packet is forwarded.
Solution The destination address in binary is A copy of the address is shifted 28 bits to the right. The result is 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
The table for Class C is searched.
The network address is found in the first row. The next-hop address and the interface m0 are passed to ARP.

25. Example 3
Router R1 receives a packet with destination address Show how the packet is forwarded.
Solution The destination address in binary is A copy of the address is shifted 28 bits to the right. The result is or 10. The class is B. The network address can be found by masking off 16 bits of the destination address, the result is 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 and the interface number m0 are passed to ARP.

26. Forwarding with subnetting
In classful addressing, subnetting happens inside the organization.
If the organization is using variable-length subnetting, we need several tables; otherwise, we need only one table.

27. Simplified forwarding module for fixed-length subnetting

28. Simplified forwarding module for fixed-length subnetting

29. Example 4
The figure below shows a router connected to four subnets. Note several points:
First, the site address is /16 (a class B address). Every packet with destination address in the range to 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 mask /18 to any destination address.

30. Example 4 (cont.)

31. Example 5
The router in previous figure receives a packet with destination address Show how the packet is forwarded.
Solution The mask is /18. After applying the mask, the subnet address is The packet is delivered to ARP with the next-hop address and the outgoing interface m0.

32. Example 6
A host in network in previous figure has a packet to send to the host with address Show how the packet is routed.
Solution The router receives the packet and applies the mask (/18). The network address is 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.

33. Forwarding with classless addressing
In classless addressing, the whole address space is one entity; there are no classes.
This means that forwarding requires one row of information for each block involved.
The table needs to be searched based on the network address (first address in the block).
Unfortunately, the destination address in the packet gives no clue about the network address (as it does in classful addressing).
To solve the problem, we need to include the mask (/n) in the table; we need to have an extra column that includes the mask for the corresponding block.

34. Forwarding with classless addressing
In classless addressing we can have a routing table with three columns; in classless addressing, we need at least four columns.

35. Example 7
Make a routing table for router R1 using the configuration in the figure.

36. Solution

37. Example 8
Show the forwarding process if a packet arrives at R1 in the previous figure with the destination address
Solution The router performs the following steps:
1. The first mask (/26) is applied to the destination address. The result is , which does not match the corresponding network address.
2. The second mask (/25) is applied to the destination address. The result is , 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.

38. Example 9
Show the forwarding process if a packet arrives at R1 with the destination address
Solution The router performs the following steps:
1. The first mask (/26) is applied to the destination address. The result is , which does not match the corresponding network address (row 1).
2. The second mask (/25) is applied to the destination address. The result is , which does not match the corresponding network address (row 2).
3. The third mask (/24) is applied to the destination address. The result is , which matches the corresponding network address. The destination address of the package and the interface number m3 are passed to ARP.

39. Example 10
Show the forwarding process if a packet arrives at R1 with the destination address
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 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.

40. Example 11
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 the table. Can we draw its topology.

41. Solution Router R1 has three interfaces:
m0, m1, and m2.
Three networks directly connected to router R1.
Two networks indirectly connected to R1.
There must be at least three other routers involved (see next-hop column).
These routers are connected by looking at their IP addresses. So we can put them at their appropriate place.
The default router, is connected to the rest of the Internet.

42. Solution (cont.)
But there is some missing information. We do not know if network is directly connected to router R2 or through a point-to-point network (WAN) and another router. We do not know if network is connected to router R3 directly or through a point-to-point network (WAN) and another router. Point-to-point networks normally do not have an entry in the routing table because no hosts are connected to them.

43. Solution (cont.)

44. Address Aggregation
When we use classful addressing, there is only one entry in the routing table for each site outside the organization.
The entry defines the site even if that site is subnetted. When a packet arrives at the router, the router checks the corresponding entry and forwards the packet accordingly.
When we use classless addressing, it is likely that the number of routing table entries will increase. This is because the intent of classless addressing is to divide up the whole address space into manageable blocks.

45. Address Aggregation
The increased size of the table results in an increase in the amount of time needed to search the table.
To alleviate the problem, the idea of address aggregation was designed.

46. Address Aggregation
In the figure below we have two routers. R1 is connected to networks of four organizations that each use 64 addresses.
R2 is somewhere far from R1. R1 has a longer routing table because each packet must be correctly routed to the appropriate organization.
R2, on the other hand, can have a very small routing table.

47. Address Aggregation
For R2, any packet with destination to is sent out from interface m0 regardless of the organization number.
This is called address aggregation because the blocks of addresses for four organizations are aggregated into one larger block.
R2 would have a longer routing table if each organization had addresses that could not be aggregated into one block.

48. Address Aggregation

49. Longest Mask Matching

50. Routing
Routing deals with the issues of creating and maintaining routing tables.
A host or router has a routing table with an entry for each destination or a combination of destinations, to route IP packets.
The routing table can be either static or dynamic.

51. Static Routing Table The information in table is entered manually.
The administrator enters the route for each destination into the table.
When there is a change in the internet, the table can not be update automatically.
The table must be manually altered by th administrator.
A static routing table can be used in a small internet that does not change very often.

52. Dynamic Routing Table
A dynamic routing table is updated periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP.
Whenever there is a change in the Internet, such as a shutdown of a router or breaking of a link, te dynamic routing protocols update all of the tables in the routers automatically.

53. Routing Table Mask. Network address. Next hop address.
This field defines the mask applied for the entry.
Network address.
Defines the network address to which the packet is finally delivered.
Next hop address.
Defines the address of the next hop router.

54. Routing Table Interface. Flags. Shows the name of the interface.
Flags are on/off switches that signify either presence or absence.
U (up) – indicates the router is up and running.
G (gateway) – means that the destination is in another network (indirect delivery).
H (host specific) – indicates that the entry in the network address field is a host-specific address.

55. Routing Table Reference count Use
D (added by redirection) – indicates that routing information for this destination has been added to the host routing table by a redirection message from ICMP.
M (modified by redirection) – indicates that the routing information for this destination has been modified by a redirection message from ICMP.
Reference count
Gives the number of users that are using this route at the moment.
Use
Shows the number of packets transmitted through this router for the corresponding destination.