IP Packet Delivery, Forwarding, and Routing Overview

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

2. Introduction • 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

3. 6.1 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

4. Direct 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

5. Direct versus Indirect Delivery (cont’d) • Direct delivery

6. Direct versus Indirect Delivery (cont’d) • 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

7. Direct versus Indirect Delivery (cont’d) • Indirect delivery

8. 6.3 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

9. Forwarding 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

10. Forwarding Techniques (cont’d)

11. Forwarding Techniques (cont’d) • Network-Specific Method • Having only one entry to define the address of network itself

12. Forwarding Techniques (cont’d) • 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

13. Routing methods (cont’d)

14. 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)

15. Forwarding with Classful Addressing • Forwarding without Subnetting

16. Example 1 • Figure 6.8 shows an imaginary part of the Internet. Show the routing tables for router R1.

17. Example 1 - Solution

18. 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

19. Example 2 - 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.

20. Example 3 • Router R1 in Figure 6.8 receives a packet with destination address167.24.160.5. 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 is00000000 00000000 00000000 00001010or 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.

21. Forwarding with Subnetting

22. Example 4 • Figure 6.11 shows a router connected to four subnets.

23. Example 5 • The router in Figure 6.11 receives a packet with destination address145.14.32.78. Show how the packet is forwarded. • Solution The mask is/18.After applying the mask, the subnet address is145.14.0.0. The packet is delivered to ARP with the next-hop address145.14.32.78and the outgoing interfacem0.

24. Example 6 • A host in network 145.14.0.0 in Figure 6.11 has a packet to send to the host with address7.22.67.91. Show how the packet is routed. • Solution The router receives the packet and applies the mask (/18). The network address is7.22.64.0. 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.

25. Forwarding 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

26. Example 7 • Make a routing table for router R1 using the configuration in Figure 6.13.

27. Example 7 - Solution • SolutionTable 6.1 shows the corresponding table Table 6.1 Routing table for router R1 in Figure 6.13

28. Example 8 • Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 180.70.65.140.

29. Example 8 - Solution • 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.

30. Example 9 • Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address201.4.22.35.

31. Example 9 - Solution • SolutionThe 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.

32. Example 10 • Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address18.24.32.78. • SolutionThis 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 address180.70.65.200and 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.

33. 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 Table 6.2. Can we draw its topology? • Table 6.2 Routing table for Example 11

34. Example 11 - Solution

35. Address Aggregation • Figure 6.15 Address aggregation

36. Longest Mask Matching • The routing table is sorted from the longest mask to the shortest mask.

37. Hierarchical Routing • To solve the problem of gigantic routing tables, creating a sense of the routing tables • Routing table can decrease in size

38. Example 12 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.

39. Example 12

40. 6.3 Routing - Static 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

41. Routing Module

42. Routing Table • Routing table • In classless addressing, routing table has a minimum of four columns. - Some routers have even more columns Flags U (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.

43. 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

44. Example 13 • 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.

45. 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

46. Example 13 (cont’d) 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.

47. Example 13 (cont’d) Ifconfig command gives us the IP address and the physical address (hardware) address of the interface

48. 6.4 Structure of a Router

49. Components • Input port • Output port

50. 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