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Chapter 6 Delivery Forwarding, and Routing of IP Packets

Chapter 6 Delivery Forwarding, and Routing of IP Packets. Mi-Jung Choi Dept. of Computer Science and Engineering mjchoi@postech.ac.kr. Objectives. Upon completion you will be able to: Understand the different types of delivery and the connection

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Chapter 6 Delivery Forwarding, and Routing of IP Packets

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  1. Chapter 6 Delivery Forwarding, and Routing of IP Packets Mi-Jung Choi Dept. of Computer Science and Engineering mjchoi@postech.ac.kr

  2. Objectives Upon completion you will be able to: • Understand the different types of delivery and the connection • Understand forwarding techniques in classful addressing • Understand forwarding techniques in classless addressing • Understand how a routing table works • Understand the structure of a router

  3. Introduction • Delivery • Meaning the physical forwarding of the packets • Delivery of packet to its final destination • Connectionless and connection-oriented services • Direct and indirect delivery • Routing • Related to finding the route (next hop) for a datagram

  4. 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

  5. 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

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

  7. 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

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

  9. 6.2 Forwarding • Forwarding means to place the packet in its route to its destination. Forwarding requires a host or a router to have a routing table. • The topics discussed in this section include: • Forwarding Techniques • Forwarding with Classful Addressing • Forwarding with Classless Addressing • Combination • Routing Techniques • Routing techniques can make the size of the routing table manageable and handle issues such as security • Next-Hop Routing • Network-Specific Routing • Host-Specific Routing • Default Routing

  10. Forwarding (cont.) • Next-hop routing • To reduce the contents of a routing table • The RT holds only the address of the next hop instead of holing information about the complete route.

  11. Forwarding (cont.) • Network-specific routing • To reduce the routing table and simplify the searching process • RT holds only one entry to define the address of the network itself in stead of an entry for every host

  12. Forwarding (cont.) • Host-specific routing • RT has the destination addresses

  13. Forwarding (cont.) • Default routing • To simplify routing

  14. Forwarding with Classful Addressing • Routing Table • Destination address • Destination host address • Using network-specific forwarding and not the rarely-used host-specific forwarding • Next hop address • In Indirect delivery • Interface number • Outgoing port

  15. Simplified forwarding module in classful address 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.

  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 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.

  39. Example 12

  40. Example 12 (Continued) • 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.

  41. Example 12 (Continued) • 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.

  42. CLASSLESS ADDRESSING : CIDR • ISSUES • Routing Table Size • Aggregation routing • Hierarchical Routing • To solve the problem of gigantic routing problem • Geographical Routing • To decrease the size of the routing table : divide the entire address space into a few large space • Routing Table Search Algorithms • In classful addressing, each address has self-contained information that facilitates routing table searching. • In classless addressing, there is no self-contained information in the destination address to facilitate routing table searching. • Modern routers are all based on classless addressing. They all include the mask in the routing table

  43. 6.3 Routing • Routing deals with the issues of creating and maintaining routing tables. • The topics discussed in this section include: • Static Versus Dynamic Routing Tables • Routing Table

  44. 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 • Updating the routing table corresponding to shutdown of a router or breaking of a link • For big internet such as Internet

  45. 6.3 Routing Module • Routing Table • Receive an IP packet from the IP processing module • Routing module consults the routing table to find the best route for the packets • After the route is found, the packet is sent along with the next-hop address to the fragmentation module

  46. 6.3 Routing Table • Routing Table • mask • 목적지 주소에 적용될 마스크 • 호스트 지정 디폴트 라우팅인 경우 255.255.255.255 • 서브네트워크화 되지 않은 경우는 클래스의 디폴트 마스크 • 클래스 A 255.0.0.0 • 클래스 B 255.255.0.0 • 클래스 C 255.255.255.0 • destination address • 목적지 호스트 주소 또는 목적지 네트워크 주소

  47. 6.3 Routing Table (cont.) • next-hop address • 패킷이 전달되는 다음 홉 라우터 주소 • flag • U(Up) : 라우터의 동작 유무 • G(Gateway) : 목적지가 다른 네트워크에 있음을 나타냄 • H(Host-Specific) : 목적지 필드내의 엔트리가 호스트 지정 주소 • D(Added by redirection) : 라우팅 정보가 ICMP의 방향 재지정 메시지에 의해 라우팅 테이블에 추가 • M(Modified by redirection) : 목적지에 대한 라우팅 정보가 ICMP의 방향 재지정 메시지에 의해 수정 • reference count • 현재 시간에 이 경로를 사용하는 사용자의 수 • use • 라우터로부터 해당하는 목적지로 전달된 패킷의 수 • interface • 인터페이스 이름

  48. 6.3 Routing Table (cont.) • Routing Module 1. For each entry in the routing table 1. Apply the mask to packet destination address 2. If (the result matches the value in the destination field) 1. If (the G flag is absent) 1. Use packet destination address as next hop address 2. Send packet to fragmentation module with next hop address 3. Return 2. If no match is found, send an ICMP error message 3. Return

  49. 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.

  50. 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

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