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Packet Delivery and Forwarding in Computer Networks

This chapter covers the next-hop methods for packet forwarding, the basic fields in routing entries, and the process of IP packet forwarding. It also discusses forwarding techniques, routing tables, and next-hop methods.

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Packet Delivery and Forwarding in Computer Networks

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  1. Delivery and Forwarding Chapter 18 COMP 3270 Computer Networks Computing Science Thompson Rivers University

  2. Learning Objectives • List the three next-hop methods for packet forwarding. • List the three next-hop methods for packet forwarding. • List the four most basic fields in routing entries. • Explain step-by-step how a router forwards an IP packet to the next router (or host). You need to use, in the explanation, the terms: destination address, routing table, interface, address of next hop, ARP.

  3. 1. DELIVERY The network layer supervises the handling of the packets by the underlying physical networks. We define this handling as the delivery of a packet.

  4. Internetwork: a network of different physical networks Internetworking: communications over an internetwork ☺ The Internet? LAN f3 ☺ How to deliver packets from A to D and from D to A? Which route? ☺ Can we use the data link layer?

  5. Each data link handles its own network interface only; There is no interaction between them. -> We can not use the data link layer for internetworking. Host-to-host delivery ☺ How to decide the next hop for the traffic from A to D? LAN f3 Data link layer; Physical layer

  6. S1 is connected to S3 through f3. It decides the next hop (the IP address of S3 and the interface f3) f1 f3 f1 f2 LAN f3

  7. Datagram approach There is no specific route established from the source to the destination. Packets could arrive in a different order; Reordering does not occur in the network layer. ☺ Then what layer?

  8. Communication at the network layer in the Internet is • unreliableconnectionless. • Packets could be lost; Packets could arrive in a different order. ☺ Does IP include an error checking routine? ☺ Yes; then?

  9. 2. IP PACKET FORWARDING IP packet forwarding means to place the packet in its route to its destination. Forwarding 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, i.e., ???. Topics discussed in this section: • Forwarding Techniques • Forwarding Process • Routing Table

  10. Forwarding Techniques Routers: Route method versus next-hop method Do we have to keep this?

  11. Next-hop method Next-hop methods and their priorities: • Source routing methods • Host specific method • Network specific method • Default method Packet header options Related to classless addressing and netmask

  12. Source routing methods A routing path from the source to the destination is included in an option field in the IP packet header - SSR (Strict Source Routing): all routers on a path … - LSR (Loose Source Routing): some routers on a path … ☺ Any danger? Most routers in the public realm blocks SSR and LSR. SSR Y Z X LSR

  13. Next-hop method: Host-specific v.s. network-specific method ☺ How to distinguish host-specific entries and network specific entries in a routing table? Netmask: 32bits for host specific entries, or less bits for … Note. A host address is a special case of network address. However, it is not still easy to keep all the routing information for the whole Internet, due to the routing traffic overhead. An ordinary router in an AS (Autonomous System) keeps the routing information only for the AS. ☺ What if a packet is supposed to go to a destination in another AS?

  14. Just zero, i.e., 0.0.0.0/0 -> if all other entries do not match with the given destination address, then the entry of 0..0/0 will be matched. Default method Or also called, default gateway Default router Default router: Border gateway: It knows the whole topology of the Internet, using BGP (Border Gateway Protocol). Now transferred to the destination AS through other ASes. In the destination AS, host specific or network specific methods are used. AS AS will be discussed more when routing is discussed.

  15. Next-hop methods and their priorities: • Source routing methods • Host specific method • Network specific method • Default method • ☺ How do routers support the above priorities? • Using the length of netmasks Packet header options Related to netmask

  16. Forwarding Process Simplified forwarding module in classless address

  17. Routing Table In classless addressing, we need at least four columns in a routing table. They are

  18. ☺ When the source routing option is not included, what are the next hops for the following destinations? - 193.14.5.68 - 192.16.7.1 - 192.16.7.3 - 193.14.99.10 IP address; Not MAC address It will be matched for the last because of the “longest prefix match first” policy. Longest prefix match first Destination address in the packet & mask, and then compare the result to the destination address • Time consuming process • Need to use good algorithm for searching • Any good idea?

  19. Example ☺ Where is 180.70.65.0/25? ☺ Aren’t they strange? m3 eth1 198.161.22.24/24 R2 eth0 198.161.22.254/24 ☺ Can you make routing tables for routers R1 and R2? (One routing entry for each network to which the router is connected, and one default routing entry, and possibly more.)

  20. Routing table for router R1 0.0.0.0 /0 128: 1000 0000 ☺ Are they okay? 192: 1100 0000 180.70.65.128/25 is 180.70.65.128/26 because of the LPMF policy.

  21. (Very important) Show the forwarding process if a packet arrives at R1 with the destination addresses 180.70.65.140. 180.70.65.221, 201.4.22.35, 201.4.32.78, 201.4.17.34 ??? 0.0.0.0 /0 128: 1000 0000 192: 1100 0000

  22. Routing table for router R2 ? How many routing entries?

  23. Address aggregation (or also called supernetting)

  24. point-to-point -> addresses are not necessary. 0 -> 0000 0000 64 -> 0100 0000 128 -> 1000 0000 192 -> 1100 0000

  25. From the same address block 140.24.7.0/24, But different locations How many routing entries? Rest of the Internet 0000 0000 0100 0000 1000 0000 1100 0000 How many routing entries?

  26. Example: As an example of hierarchical routing: A regional ISP is granted 16,384 addresses starting from 120.14.64.0/18. The regional ISP has decided to divide this block into four subblocks, each with 4096 addresses. 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, each using four addresses. (8 -> 2x ?) The second local ISP has divided its block into 4 blocks and has assigned the addresses to four large organizations. (4 -> 2x ?) The third local ISP has divided its block into 16 blocks and assigned each block to a small organization. Each small organization has 256 addresses, and the mask is /24.(16 -> 2x ?) 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.

  27. 0100 0000 ... 0100 1110 0100 0000 0101 0000 0110 0000 0111 0000 local1 0110 0000 ... 0110 1100 regional local2 0100 0000 local3 0111 0000 ... 0111 1111

  28. Can you make routing tables for the routers in the previous slide?

  29. Related utilities: • netstat • route • ifconfig

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