18-WAN Technologies and Dynamic routing

18-WAN Technologies and Dynamic routing Dr. John P. Abraham Professor UTPA

Traditional WAN architecture • Packet switches (specialized computers) were used to direct packets. See fig 18.1 • WAN existed before LAN. When LAN was introduced the packet switching was divided into local and outside. Layer two was assigned LAN and layer 3 WAN. • Many WANs used leased circuits. • WANs were formed by interconnecting packet switches

Store and forward Paradigm • Packet switch buffers packets in memory to compensate for the speed of the routes. • The store operation occurs as the packet arrive • The forward operation occurs once a packet has arrived and waiting in memory. The processor examines the packet, determines its destination and sends the packet over the appropriate interface. • Structure of a router: 4 components • Input ports, output ports, routing processor and switching fabric. The routing processor has other components, such as the buffer, boot ROM, nvram, etc.

Addressing • Hierarchical addressing has two parts, site and the computer at that site. The first part of the address actually identifies a router (not exactly, we will discuss this later). IP addresses are not permanent. • Once the packet arrives at the router, then it queries all computers to find out which machine has a particular IP address. The machine with that address responds with its MAC address and the packet converted to frame is sent to that address. MAC addresses are permanent to a machine.

Next-Hop routing • When a packet arrives, the routing processor must choose an outgoing path. If it is destined for a local computer, the router sends to that computer using its MAC address. Otherwise, it is forwarded to one of the output ports on the router that leads to the destination. • To do this the router examines the destination address and extracts the network portion of the address. If the extracted address matches its own the packets are kept otherwise forwarded. • The router does not need to know the complete routing information, rather just next-hop. Think of baggage transport in the airlines. • Forwarding has nothing to do with the source (source independence)

Dynamic routing updates in wan • Each router must have a forwarding table. The table must guarantee: • Universal communication – a valid next-hop route for each possible destination. • Optimal routes. The route must be shortest path, either in hops, time, or using other factors. • If two paths exist, one become unavailable (hardware failure, routes should be tested), forwarding table should be changed. So the table must be dynamic.

A network

Explanation of the figure • Nodes in the graph are given a label (address of the router). Graph can be used to discover next hop, and shortest path. • To reach node 4 from node 1, the next hops are given 1,3, and 3,4. Alternatively, we can discover next hop as 1,3, 3,2 and 2,4. We can actually prepare a table of next hop for each router to reach all the others.

Default routes In case of router one, next hop is 1,3 regardless where you are going. So instead of listing 1,3 three times, why don’t we have one default entry as 1,3. So, we can have next hops for the ones we know, and all the repeated ones should have a default route. Default route is optional.

Forwarding table Static routing. All routes are entered into the router and it does not change. The routes are stored therefore, upon reboot it will retain these routes. In case of a down line, the router can’t send any more packets to that destination. Dynamic routing. Initial routing table is keyed in. Then in learns all the other routes. Most routers today use dynamic table.

Shortest Path computation in a graph Dijkstra’s agorithm Next hop table is constructed during the computation of shortest path In this algorithm, the edges in a graph can assume any non-negative value. i.e. Weight.

Routing problems Link State Routing (shortest path first) – routers periodically send messages across the network that carry status of the link (up, down), these messages can be used to build graphs. If packets are lost then two routers can disagree about the shortest path. Distance Vector routing Each link is assigned a weight. The distance the destination is the total of all weights. It also sends periodic messages, but adds weight like “I can reach destination x, and its current distance from me is Y”. DVR can create a loop, by one saying it can’t reach, and the neighbor saying it can reach through the one just reported it can’t reach. DVR employs split horizon to prevent this loop (router does not send information back to the one reported it can’t reach).

A MORE DETAILED LOOK AT ROUTING

Connection vs connectionless • In connection oriented, the network layer first makes a connection and all packets are sent over that connection. No need to calculate route for each packet • In connectionless each packet is treated independently, route is determined independently. Packets may arrive at different times. TCP/IP Protocol Suite

Figure 6.1Direct delivery TCP/IP Protocol Suite

Direct Delivery • The final sender and Receiver connected to the same network. • Destination IP address is mapped to destination MAC address. • In indirect delivery, the sender uses the destination IP address and the routing table to find the IP address of the next router. This called routing or forwarding. TCP/IP Protocol Suite

Figure 6.2Indirect delivery TCP/IP Protocol Suite

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 TCP/IP Protocol Suite

Route based vs. next hop • Top part of next slide shows table based on route. • Bottom part shows table based on next hop. TCP/IP Protocol Suite

Figure 6.3Next-hop method TCP/IP Protocol Suite

Network –specific routing • All hosts connected to the same network will only have one entry. • Therefore will have a smaller table • See next slide TCP/IP Protocol Suite

Figure 6.4Network-specific method TCP/IP Protocol Suite

Host Specific Routing • Every host is listed • Good for security measures • Administrator may not want packets go through any other route. • See next slide TCP/IP Protocol Suite

Figure 6.5Host-specific routing TCP/IP Protocol Suite

Figure 6.6Default routing For all hosts or networks not listed TCP/IP Protocol Suite

Forwarding with classfull addressing • Most routers in the global internet are not involved with routing. • Forwarding is based on class and network address (using network specific routing) • Packetextract distination addressfindclass extract network addressSearch tableARP for the next router TCP/IP Protocol Suite

Figure 6.7Simplified forwarding module in classful address without subnetting TCP/IP Protocol Suite

Example 1 Figure 6.8 shows an imaginary part of the Internet. Show the routing tables for router R1. See Next Slide TCP/IP Protocol Suite

Figure 6.8Configuration for routing, Example 1 TCP/IP Protocol Suite

Example 1 (Continued) Solution Figure 6.9 shows the three tables 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 Chapter 7. See Next Slide TCP/IP Protocol Suite

Figure 6.9Tables for Example 1 TCP/IP Protocol Suite

Example 2 Router R1 in Figure 6.8 receives a packet with destination address 192.16.7.14. Show how the packet is forwarded. SolutionThe 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. TCP/IP Protocol Suite

Example 3 Router R1 in Figure 6.8 receives a packet with destination address 167.24.160.5. Show how the packet is forwarded. SolutionThe 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. TCP/IP Protocol Suite

Figure 6.10Simplified forwarding module in classful address with subnetting TCP/IP Protocol Suite

Example 4 Figure 6.11 shows a router connected to four subnets. See Next Slide TCP/IP Protocol Suite

Example 4 (Continued) Note several points. First, the site address is 145.14.0.0/16 (a class B address). Every packet with destination address in the range 145.14.0.0 to 145.14.255.255 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. TCP/IP Protocol Suite

Figure 6.11Configuration for Example 4 TCP/IP Protocol Suite

Example 5 The router in Figure 6.11 receives a packet with destination address 145.14.32.78. Show how the packet is forwarded. SolutionThe 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. TCP/IP Protocol Suite

Example 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. SolutionThe 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. TCP/IP Protocol Suite

Note: In classful addressing we can have a routing table with three columns; in classless addressing, we need at least four columns. TCP/IP Protocol Suite

Figure 6.12Simplified forwarding module in classless address TCP/IP Protocol Suite

Example 7 Make a routing table for router R1 using the configuration in Figure 6.13. See Next Slide SolutionTable 6.1 shows the corresponding table. See the table after the figure. TCP/IP Protocol Suite

Figure 6.13Configuration for Example 7 TCP/IP Protocol Suite

Table 6.1 Routing table for router R1 in Figure 6.13 TCP/IP Protocol Suite

Example 8 Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 180.70.65.140. 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. See Next Slide TCP/IP Protocol Suite

Example 8 (Continued) 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. TCP/IP Protocol Suite

Example 9 Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 201.4.22.35. SolutionThe router performs the following steps: See Next Slide TCP/IP Protocol Suite

Example 9 (Continued) 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. TCP/IP Protocol Suite

Example 10 Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 18.24.32.78. 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 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. TCP/IP Protocol Suite

18-WAN Technologies and Dynamic routing