1 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, KNU 2 Objectives Upon completion you will be able to: oUnderstand the different types of delivery and the connection oUnderstand forwarding techniques in classful addressing oUnderstand forwarding techniques in classless addressing oUnderstand how a routing table works oUnderstand the structure of a router 3 Introduction o Delivery Meaning the physical forwarding of the packets Delivery of packet to its final destination Connectionless and connection-oriented services Direct and indirect delivery oRouting Related to finding the route (next hop) for a datagram 4 6.1 Delivery oConnection Types Connection-oriented service l Using same path l 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 l Dealing with each packet independently l Packets may not travel the same path to their destination oIP is : Connectionless protocol 5 Direct versus Indirect Delivery oTwo methods delivering a packet to its final destination Direct Indirect oDirect 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 l Extracting the network address of the destination packet (setting the hostid part to all 0s) l Then, comparing the addresses of the network to which it is connected 6 Direct versus Indirect Delivery (contd) oDirect delivery 7 Direct versus Indirect Delivery (contd) oIndirect 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 l Mapping between the IP address of next router and the physical address of the next router 8 Direct versus Indirect Delivery (contd) oIndirect delivery 9 6.3 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 l Next-Hop Routing l Network-Specific Routing l Host-Specific Routing l Default Routing Forwarding (cont.) oNext-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. Forwarding (cont.) oNetwork-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 Forwarding (cont.) oHost-specific routing RT has the destination addresses Forwarding (cont.) oDefault routing To simplify routing 14 Forwarding with Classful Addressing oRouting Table Destination address l Destination host address l Using network-specific forwarding and not the rarely-used host-specific forwarding Next hop address l In Indirect delivery Interface number l Outgoing port 15 Simplified forwarding module in classful address without subnetting 16 Example 1 o Figure 6.8 shows an imaginary part of the Internet. Show the routing tables for router R1. 17 Example 1 - Solution 18 Example 2 oRouter R1 in Figure 6.8 receives a packet with destination address Show how the packet is forwarded. oSolution 19 Example 2 - Solution oThe 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. 20 Example 3 oRouter R1 in Figure 6.8 receives a packet with destination address Show how the packet is forwarded o 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. 21 Forwarding with Subnetting 22 Example 4 o Figure 6.11 shows a router connected to four subnets. 23 Example 5 oThe router in Figure 6.11 receives a packet with destination address Show how the packet is forwarded. o 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. 24 Example 6 o A host in network in Figure 6.11 has a packet to send to the host with address Show how the packet is routed. o 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. 25 Forwarding with Classless Addressing o 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 o Make a routing table for router R1 using the configuration in Figure 6.13. 27 Example 7 - Solution o Solution Table 6.1 shows the corresponding table Table 6.1 Routing table for router R1 in Figure 6.13 28 Example 8 o Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 29 Example 8 - Solution Solution o 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. 30 Example 9 o Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address 31 Example 9 - Solution o Solution o 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. 32 Example 10 o Show the forwarding process if a packet arrives at R1 in Figure 6.13 with the destination address o 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. 33 Example 11 o 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 o Table 6.2 Routing table for Example 11 34 Example 11 - Solution 35 Address Aggregation o Figure 6.15 Address aggregation 36 Longest Mask Matching o The routing table is sorted from the longest mask to the shortest mask. 37 Hierarchical Routing o To solve the problem of gigantic routing tables, creating a sense of the routing tables o Routing table can decrease in size 38 CLASSLESS ADDRESSING : CIDR oISSUES Routing Table Size l 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 l 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. In classless addressing, there is no self-contained information in the destination address to facilitate routing table searching. oModern routers are all based on classless addressing. They all include the mask in the routing table 39 Example 12 As an example of hierarchical routing, let us consider Figure A regional ISP is granted addresses starting from 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. 40 Example 12 Routing Routing deals with the issues of creating and maintaining routing tables. o Routing deals with the issues of creating and maintaining routing tables. The topics discussed in this section include: o The topics discussed in this section include: Static Versus Dynamic Routing Tables Routing Table Routing - Static versus Dynamic Routing oStatic 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 oDynamic 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 Routing Module oRouting Table oReceive an IP packet from the IP processing module oRouting module consults the routing table to find the best route for the packets oAfter the route is found, the packet is sent along with the next- hop address to the fragmentation module Routing Table oRouting Table omask l A l B l C odestination address Routing Table (cont.) onext-hop address oflag U(Up) : G(Gateway) : H(Host-Specific) : D(Added by redirection) : ICMP M(Modified by redirection) : ICMP oreference count ouse ointerface Routing Table (cont.) oRouting 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 47 Example 13 oOne 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. 48 Example 13 (contd) $ netstat -rn Kernel IP routing table Destination Gateway Mask Flags Iface U eth U lo UG eth0. Loopback interface 49 Example 13 (contd) 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: Bcast: Mask: From the above information, we can deduce the configuration of the server as shown in Figure 6.19. 50 Example 13 (contd) Ifconfig command gives us the IP address and the physical address (hardware) address of the interface 51 We represent a router as a black box that accepts incoming packets from one of the input ports (interfaces), uses a routing table to find the departing output port, and sends the packet from this output port. The topics discussed in this section include: Components 6.4 Structure of a Router 52 Components oA router has a four components : input ports, output ports, the routing processor and the switching fabric 53 Components (contd) o Input port o Output port 54 Components (contd) o 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 55 Switching fabrics o Crossbar switch 56 Switching Fabrics o Banyan switch o log2 (n) stages with n/2 microswitches 57 Switching Fabrics (contd) o Examples of routing in a banyan switch 58 Switching Fabrics (contd) o Possibility of internal collision even when two packets are not heading for the same output port in banyan switch solving the problem by sorting the arriving packets based on their destination port Trap module: preventing duplicate packets (packets with the same output destination) from passing to the banyan switch simultaneously 59 Switching Fabrics (contd) o Batcher-banyan switch