ch07.pdf

Because learning changes everything. ® Chapter 07 Network Layer: Data Transfer Data Communications and Networking, With TCP/IP protocol suite Sixth Edition Behrouz A. Forouzan © 2022 McGraw Hill, LLC. All...

Because learning changes everything. ® Chapter 07 Network Layer: Data Transfer Data Communications and Networking, With TCP/IP protocol suite Sixth Edition Behrouz A. Forouzan © 2022 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill, LLC.  Chapter 7: Outline 7.1 Services 7.2 Packet Switching 7.3 Performance 7.4 Internet Protocol V4 7.5 Internet Protocol V6 7.6 Transition from V4 To V6 © McGraw Hill, LLC 2  Figure 7.1 Communication at the network layer Access the text alternative for slide images. © McGraw Hill, LLC 3  7-1 SERVICES We briefly discuss the services provided at the network layer. © McGraw Hill, LLC 4  7.1.1 Packetizing The first duty of the network layer is definitely packetizing: encapsulating the payload in a network-layer packet at the source and decapsulating the payload from the network-layer packet at the destination. In other words, one duty of the network layer is to carry a payload from the source to the destination without changing it or using it. The network layer is doing the service of a carrier such as the postal office, which is responsible for delivery of packages from a sender to a receiver without changing or using the contents. © McGraw Hill, LLC 5  7.1.2 Routing Other duties of the network layer, which are as important as the first, are routing and forwarding, which are directly related to each other. © McGraw Hill, LLC 6  7.1.3 Error Control Although error control can be implemented in the network layer, the designers of the network layer in the Internet ignored this issue for the data being carried by the network layer. One reason for this decision is the fact that the packet in the network layer may be fragmented at each router, which makes error checking at this layer inefficient. © McGraw Hill, LLC 7  7.1.4 Flow Control Flow control regulates the amount of data a source can send without overwhelming the receiver. If the upper layer at the source computer produces data faster than the upper layer at the destination computer can consume it, the receiver will be overwhelmed with data. To control the flow of data, the receiver needs to send some feedback to the sender to inform the latter that it is overwhelmed with data. © McGraw Hill, LLC 8  7.1.5 Congestion Control Another issue in a network-layer protocol is congestion control. Congestion in the network layer is a situation in which too many datagrams are present in an area of the Internet. Congestion may occur if the number of datagrams sent by source computers is beyond the capacity of the network or routers. © McGraw Hill, LLC 9  7.1.6 Quality of Service As the Internet has allowed new applications such as multimedia communication (in particular real-time communication of audio and video), the quality of service (QoS) of the communication has become more and more important. The Internet has thrived by providing better quality of service to support these applications. However, to keep the network layer untouched, these provisions are mostly implemented in the upper layer. © McGraw Hill, LLC 10  7.1.7 Security Another issue related to communication at the network layer is security. Security was not a concern when the Internet was originally designed because it was used by a small number of users at universities for research activities; other people had no access to the Internet. The network layer was designed with no security provision. Today, however, security is a big concern. To provide security for a connectionless network layer, we need to have another virtual level that changes the connectionless service to a connection-oriented service. © McGraw Hill, LLC 11  7-2 PACKET SWITCHING From the discussion of routing and forwarding in the previous section, we infer that a kind of switching occurs at the network layer. A router, in fact, is a switch that creates a connection between an input port and an output port (or a set of output ports), just as an electrical switch connects the input to the output to let electricity flow. © McGraw Hill, LLC 12  7.2.1 Datagram Approach When the Internet started, to make it simple, the network layer was designed to provide a connectionless service in which the network- layer protocol treats each packet independently, with each packet having no relationship to any other packet. The idea was that the network layer is only responsible for delivery of packets from the source to the destination. In this approach, the packets in a message may or may not travel the same path to their destination. © McGraw Hill, LLC 13  7.2.2 Virtual-Circuit Approach In a connection-oriented service (also called virtual-circuit approach), there is a relationship between all packets belonging to a message. Before all datagrams in a message can be sent, a virtual connection should be set up to define the path for the datagrams. After connection setup, the datagrams can all follow the same path. In this type of service, not only must the packet contain the source and destination addresses, it must also contain a flow label, a virtual circuit identifier that defines the virtual path the packet should follow. © McGraw Hill, LLC 14  7-3 PERFORMANCE The upper-layer protocols that use the service of the network layer expect to receive an ideal service, but the network layer is not perfect. The performance of a network can be measured in terms of delay, throughput, and packet loss. Congestion control is an issue that can improve the performance. © McGraw Hill, LLC 15  7.3.1 Delay All of us expect instantaneous response from a network, but a packet, from its source to its destination, encounters delays. The delays in a network can be divided into four types: transmission delay, propagation delay, processing delay, and queuing delay. Let us first discuss each of these delay types and then show how to calculate a packet delay from the source to the destination. © McGraw Hill, LLC 16  Transmission Delay 1 A source host or a router cannot send a packet instantaneously. A sender needs to put the bits in a packet on the line one by one. If the first bit of the packet is put on the line at time t1 and the last bit is put on the line at time t2, transmission delay of the packet is (t2 - t1). Definitely, the transmission delay is longer for a longer packet and shorter if the sender can transmit faster. In other words, the transmission delay is. Delay tr   Packet length   Transmission rate  © McGraw Hill, LLC 17  Propagation Delay Propagation delay is the time it takes for a bit to travel from point A to point B in the transmission media. The propagation delay for a packet-switched network depends on the propagation delay of each network (LAN or WAN). The propagation delay depends on the propagation speed of the media, which is 3 ´ 108 meters/second in a vacuum and normally much less in a wired medium; it also depends on the distance of the link. In other words, propagation delay is. Delay pg   Distance   Propagation speed  © McGraw Hill, LLC 18  Processing Delay The processing delay is the time required for a router or a destination host to receive a packet from its input port, remove the header, perform an error detection procedure, and deliver the packet to the output port (in the case of a router) or deliver the packet to the upper-layer protocol (in the case of the destination host). The processing delay may be different for each packet, but normally is calculated as an average. Delay pr  Time required to process a packet © McGraw Hill, LLC 19  Queueing delay: Let the packet is received by the destination, the packet will not be processed by the destination immediately. It has to wait in a queue in something called a buffer. So the amount of time it waits in queue before being processed is called queueing delay. In general, we can’t calculate queueing delay because we don’t have any formula for that. Delayqu  The time a packet waits in queues © McGraw Hill, LLC 20  Transmission Delay 2 A source host or a router cannot send a packet instantaneously. A sender needs to put the bits in a packet on the line one by one. If the first bit of the packet is put on the line at time t1 and the last bit is put on the line at time t2, transmission delay of the packet is (t2 - t1). Definitely, the transmission delay is longer for a longer packet and shorter if the sender can transmit faster. In other words, the transmission delay is: Delay tr   Packet length   Transmission rate  © McGraw Hill, LLC 21  Total Delay Assuming equal delays for the sender, routers, and receiver, the total delay (source-to-destination delay) a packet encounters can be calculated if we know the number of routers, n, in the whole path. Ttotal = Tt + Tp + Tq + Tpro © McGraw Hill, LLC 22  7.3.2 Throughput Throughput at any point in a network is defined as the number of bits passing through the point in a second, which is actually the transmission rate of data at that point. In a path from source to destination, a packet may pass through several links (networks), each with a different transmission rate. How, then, can we determine the throughput of the whole path? To see the situation, assume that we have three links, each with a different transmission rate, as shown in Figure 7.2. © McGraw Hill, LLC 23  Figure 7.2 Throughput in a path with three links in a series Access the text alternative for slide images. © McGraw Hill, LLC 24  Figure 7.3 A path through the Internet backbone Access the text alternative for slide images. © McGraw Hill, LLC 25  7.3.3 Packet Loss Another issue that severely affects the performance of communication is the number of packets lost during transmission. When a router receives a packet while processing another packet, the received packet needs to be stored in the input buffer waiting for its turn. A router, however, has an input buffer with a limited size. A time may come when the buffer is full and the next packet needs to be dropped. The effect of packet loss on the Internet network layer is that the packet needs to be resent, which in turn may create overflow and cause more packet loss. © McGraw Hill, LLC 26  7.3.4 Congestion Control Congestion control is a mechanism for improving performance. Although congestion at the network layer is not explicitly addressed in the Internet model, the study of congestion at this layer may help us to better understand the cause of congestion at the transport layer and find possible remedies to be used at the network layer. Congestion at the network layer is related to two issues, throughput and delay, which we discussed in the previous section. © McGraw Hill, LLC 27  7-4 INTERNET PROTOCOL VERSION 4 The network layer in the Internet has gone through several versions, but only two versions have survived: IP Version 4 (IPv4) and IP Version 6 (IPv6). Although IPv4 is almost depleted, we discuss it because there are still some areas that use this version and also because it is the foundation for IPv6. © McGraw Hill, LLC 28  7.4.1 IPv4 Addressing The identifier used in the IP layer of the TCP/IP protocol suite to identify the connection of each device to the Internet is called the Internet address or IP address. An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a host or a router to the Internet. The IP address is the address of the connection, not the host or the router, because if the device is moved to another network, the IP address may be changed. © McGraw Hill, LLC 29  Address Space 1 A protocol like IPv4 that defines addresses has an address space. An address space is the total number of addresses used by the protocol. If a protocol uses b bits to define an address, the address space is 2b because each bit can have two different values (0 or 1). IPv4 uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more than four billion). If there were no restrictions, more than 4 billion devices could be connected to the Internet. © McGraw Hill, LLC 30  Notation There are three common notations to show an IPv4 address: binary notation (base 2), dotted-decimal notation (base 256), and hexadecimal notation (base 16). © McGraw Hill, LLC 31  Figure 7.4 Three different notations in IPv4 addressing Access the text alternative for slide images. © McGraw Hill, LLC 32  Hierarchy in Addressing In any communication network that involves delivery, such as a telephone network or a postal network, the addressing system is hierarchical. A 32-bit IPv4 address is also hierarchical but divided only into two parts. The first part of the address, called the prefix, defines the network; the second part of the address, called the suffix, defines the node. © McGraw Hill, LLC 33  Figure 7.5 Hierarchy in addressing Access the text alternative for slide images. © McGraw Hill, LLC 34  Classful Addressing When the Internet started, an IPv4 address was designed with a fixed-length prefix, but to accommodate both small and large networks, three fixed-length prefixes were designed instead of one (n = 8, n = 16, and n = 24). The whole address space was divided into five classes (class A, B, C, D, and E), as shown in Figure 7.6. This scheme is referred to as classful addressing. Although classful addressing belongs to the past, it helps us to understand classless addressing, discussed later. © McGraw Hill, LLC 35  Figure 7.6 Occupation of the address space in classful addressing Access the text alternative for slide images. © McGraw Hill, LLC 36  Classful addressing © McGraw Hill, LLC 37  Classless Addressing With the growth of the Internet, it was clear that a larger address space was needed as a long-term solution. The larger address space, however, requires that the length of IP addresses also be increased, which means the format of the IP packets needs to be changed. Although the long-range solution has already been devised and is called IPv6, a short-term solution was also devised to use the same address space but to change the distribution of addresses to provide a fair share to each organization. The short- term solution still uses IPv4 addresses, but it is called classless addressing. © McGraw Hill, LLC 38 19.39 © McGraw Hill, LLC  Figure 7.7 Variable-length blocks in classless addressing Access the text alternative for slide images. © McGraw Hill, LLC 40  Figure 7.8 Slash notation (CIDR) Access the text alternative for slide images. © McGraw Hill, LLC 41 19.42 Note In IPv4 addressing, a block of addresses can be defined as x.y.z.t /n in which x.y.z.t defines one of the addresses and the /n defines the mask. © McGraw Hill, LLC 19.43 Note The first address in the block can be found by setting the rightmost 32 − n bits to 0s. © McGraw Hill, LLC 19.44 Note The last address in the block can be found by setting the rightmost 32 − n bits to 1s. © McGraw Hill, LLC 19.45 Note The number of addresses in the block can be found by using the formula 232−n. © McGraw Hill, LLC  Example 7.1 A classless address is given as 167.199.170.82/27. We can find the above three pieces of information as follows. The number of addresses in the network is 232n  25  32 addresses. The first address can be found by keeping the first 27 bits and changing the rest of the bits to 0s. Address: 167.199.170.82/27 10100111 11000111 10101010 01010010 First address: 167.199.170.64/27 10100111 11000111 10101010 01000000 The last address can be found by keeping the first 27 bits and changing the rest of the bits to 1s. Address: 167.199.170.82/27 10100111 11000111 10101010 01011111 Last address: 167.199.170.95/27 10100111 11000111 10101010 01011111 © McGraw Hill, LLC 46 19.47 Example 7.2 Another way to find the first address, the last address, and the number of addresses is to represent the mask as a 32- bit binary (or 8-digit hexadecimal) number. This is particularly useful when we are writing a program to find these pieces of information. For example for the block 205.16.37.39/28, the mask is represented as 11111111 11111111 11111111 11110000 (twenty-eight 1s and four 0s). Find a. The first address b. The last address c. The number of addresses. © McGraw Hill, LLC 19.48 Solution a. The first address can be found by ANDing the given addresses with the mask. ANDing here is done bit by bit. The result of ANDing 2 bits is 1 if both bits are 1s; the result is 0 otherwise. © McGraw Hill, LLC 19.49 b. The last address can be found by ORing the given addresses with the complement of the mask. ORing here is done bit by bit. The result of ORing 2 bits is 0 if both bits are 0s; the result is 1 otherwise. The complement of a number is found by changing each 1 to 0 and each 0 to 1. © McGraw Hill, LLC 19.50 c. The number of addresses can be found by complementing the mask, interpreting it as a decimal number, and adding 1 to it. © McGraw Hill, LLC  Example 7.3 In classless addressing, an address cannot per se define the block the address belongs to. For example, the address 230.8.24.56 can belong to many blocks. Some of them are shown below with the value of the prefix associated with that block. 1110 0110.0000 1000. 0001 1000. 0011 1000 Prefix length:16 → Block: 230.8.0.0 to 230.8.255.255 Prefix length:20 → Block: 230.8.16.0 to 230.8.31.255 Prefix length:26 → Block: 230.8.24.0 to 230.8.24.63 Prefix length:27 → Block: 230.8.24.32 to 230.8.24.63 Prefix length:29 → Block: 230.8.24.56 to 230.8.24.63 Prefix length:31 → Block: 230.8.24.56 to 230.8.24.57 © McGraw Hill, LLC 51  Figure 7.9 Network address Access the text alternative for slide images. © McGraw Hill, LLC 52  Example 7.4 An ISP has requested a block of 1000 addresses. Since 1000 is not a power of 2, 1024 addresses are granted. The prefix length is calculated as n  32  log 2 1024  22. An available block, 18.14.12.0/22, is granted to the ISP. It can be seen that the first address in decimal is 302,910,464, which is divisible by 1024. © McGraw Hill, LLC 53  Example 7.5 (1) An organization is granted a block of addresses with the beginning address 14.24.74.0/24. The organization needs to have 3 subblocks of addresses to use in its three subnets: one subblock of 10 addresses, one subblock of 60 addresses, and one subblock of 120 addresses. Design the subblocks. Solution There are 232–24  256 addresses in this block. The first address is 14.24.74.0/24; the last address is 14.24.74.255/24. To satisfy the third requirement, we assign addresses to subblocks, starting with the largest and ending with the smallest one. © McGraw Hill, LLC 54  Example 7.5 (2) a. The number of addresses in the largest subblock, which requires 120 addresses, is not a power of 2. We allocate 128 addresses. The subnet mask for this subnet can be found as n1  32  log 2 128  25. The first address in this block is 14.24.74.0/25; the last address is 14.24.74.127/25. b. The number of addresses in the second largest subblock, which requires 60 addresses, is not a power of 2 either. We allocate 64 addresses. The subnet mask for this subnet can be found as n2  32  log 2 64  26. The first address in this block is 14.24.74.128/26; the last address is 14.24.74.191/26. © McGraw Hill, LLC 55  Example 7.5 (3) c. The number of addresses in the smallest subblock, which requires 10 addresses, is not a power of 2. We allocate 16 addresses. The subnet mask for this subnet can be found as n = 32 – log2 16 = 28 The first address in this block is 14.24.74.192/28; the last address is 14.24.192.207/28. If we add all addresses in the previous subblocks, the result is 208 addresses, which means 48 addresses are left in reserve. The first address in this range is 14.24.74.208. The last address is 14.24.74.255. We don’t know about the prefix length yet. Figure 4.36 shows the configuration of blocks. We have shown the first address in each block. © McGraw Hill, LLC 56  Figure 7.10 Solution to Example 4.5 14.24.74.192/28 Access the text alternative for slide images. © McGraw Hill, LLC 57  Example 7.6 Figure 7.11 shows how four small blocks of addresses are assigned to four organizations by an ISP. The ISP combines these four blocks into one single block and advertises the larger block to the rest of the world. Any packet destined for this larger block should be sent to this ISP. It is the responsibility of the ISP to forward the packet to the appropriate organization. This is similar to routing we can find in a postal network. All packages coming from outside a country are sent first to the capital and then distributed to the corresponding destination. © McGraw Hill, LLC 58  Figure 7.11 Example of address aggregation Access the text alternative for slide images. © McGraw Hill, LLC 59  7.4.2 Four Related Protocols The network layer in version 4 can be thought of as one main protocol and three auxiliary ones. The main protocol, IPv4, is responsible for packetizing, forwarding, and delivery of a packet. The ICMPv4 helps IPv4 to handle some errors that may occur in delivery. The IGMP is used to help IPv4 in multicasting. ARP is used in address mapping. © McGraw Hill, LLC 60  Figure 7.12 Position of IP and other network-layer protocols in TCP/IP protocol suite Access the text alternative for slide images. © McGraw Hill, LLC 61  Datagram Format Packets used by the IP are called datagrams. Figure 7.13 shows the IPv4 datagram format. A datagram is a variable-length packet consisting of two parts: header and payload (data). The header is 20 to 60 bytes in length and contains information essential to routing and delivery. It is customary in TCP/IP to show the header in 4-byte sections. © McGraw Hill, LLC 62  Figure 7.13 IP datagram Access the text alternative for slide images. © McGraw Hill, LLC 63  Figure 7.14 Multiplexing and demultiplexing using the value of the protocol field Access the text alternative for slide images. © McGraw Hill, LLC 64  Example 7.7 An IPv4 packet has arrived with the first 8 bits as  01000010 2. The receiver discards the packet. Why? Solution There is an error in this packet. The 4 leftmost bits (0100)2 show the version, which is correct. The next 4 bits (0010)2 show an invalid header length (2 * 4 = 8). The minimum number of bytes in the header must be 20. The packet has been corrupted in transmission. © McGraw Hill, LLC 65  Example 7.8 In an IPv4 packet, the value of HLEN is 1000 2. How many bytes of options are being carried by this packet? Solution The HLEN value is 8, which means the total number of bytes in the header is 8 * 4, or 32 bytes. The first 20 bytes are the base header, the next 12 bytes are the options. © McGraw Hill, LLC 66  Example 7.9 In an IPv4 packet, the value of HLEN is 5, and the value of the total length field is  0028 16. How many bytes of data are being carried by this packet? Solution The HLEN value is 5, which means the total number of bytes in the header is 5 * 4, or 20 bytes (no options). The total length is  0028 16 or 40 bytes, which means the packet is carrying 20 bytes of data (40 - 20). © McGraw Hill, LLC 67  Example 7.10 An IPv4 packet has arrived with the first few hexadecimal digits as shown  45000028000100000102 16 How many hops can this packet travel before being dropped? The data belong to what upper-layer protocol? Solution To find the time-to-live field, we skip 8 bytes (16 hexadecimal digits). The time-to-live field is the ninth byte, which is  0116. This means the packet can travel only one hop. The protocol field is the next byte  02 16 , which means that the protocol is IGMP. It is not note that ICMP, IGMP is a network layer protocol © McGraw Hill, LLC 68  Fragmentation A datagram can travel through different networks. Each router decapsulates the IP datagram from the frame it receives, processes it, and then encapsulates it in another frame. The format and size of the received frame depend on the protocol used by the physical network through which the frame has just traveled. The format and size of the sent frame depend on the protocol used by the physical network through which the frame is going to travel. For example, if a router connects a LAN to a WAN, it receives a frame in the LAN format and sends a frame in the WAN format. © McGraw Hill, LLC 69  Maximum Transfer Unit Each link-layer protocol has its own frame format. One of the features of each format is the maximum size of the payload that can be encapsulated in a frame, total size of the datagram must be less than the maximum size (see Figure 7.16). © McGraw Hill, LLC 70  Figure 7.16 Maximum transfer unit (MTU) Access the text alternative for slide images. © McGraw Hill, LLC 71  Fields Related to Fragmentation We mentioned before that three fields in an IP datagram are related to fragmentation: identification, flags, and fragmentation offset. The 16-bit identification field identifies a datagram originating from the source host. The 3-bit flags field defines three flags. The 13-bit fragmentation offset field shows the relative position of this fragment with respect to the whole datagram. © McGraw Hill, LLC 72  Figure 7.17 Fragmentation example: MTU 1400 bytes 4000/1400=2.857: so we will have 3 fragments Access the text alternative for slide images. © McGraw Hill, LLC 73  Figure 7.18 Detailed fragmentation example MTU=1400 MTU=1400 MTU=800 MTU=800 4020: the 20 for header Note 175*8=1400 Note 175+175=350 Note:4000-(1400+1400)=1200 800/8 =100 Access the text alternative for slide images. © McGraw Hill, LLC 74  Example 7.12 A packet has arrived with an M bit value of 0. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented? Solution If the M bit is 0, it means that there are no more fragments; the fragment is the last one. However, we cannot say if the original packet was fragmented or not. A non-fragmented packet is considered the last fragment. © McGraw Hill, LLC 75  Example 7.13 A packet has arrived with an M bit value of 1. Is this the first fragment, the last fragment, or a middle fragment? Do we know if the packet was fragmented? Solution If the M bit is 1, it means that there is at least one more fragment. This fragment can be the first one or a middle one, but not the last one. We don’t know if it is the first one or a middle one; we need more information (the value of the fragmentation offset). © McGraw Hill, LLC 76  Example 7.14 A packet has arrived with an M bit value of 1 and a fragmentation offset value of 0. Is this the first fragment, the last fragment, or a middle fragment? Solution Because the M bit is 1, it is either the first fragment or a middle one. Because the offset value is 0, it is the first fragment. © McGraw Hill, LLC 77  Example 7.15 A packet has arrived in which the offset value is 100. What is the number of the first byte? Do we know the number of the last byte? Solution To find the number of the first byte, we multiply the offset value by 8. This means that the first byte number is 800. We cannot determine the number of the last byte unless we know the length of the data. © McGraw Hill, LLC 78  Example 7.16 A packet has arrived in which the offset value is 100, the value of HLEN is 5, and the value of the total length field is 100. What are the numbers of the first byte and the last byte? Solution The first byte number is 100 * 8 = 800. The total length is 100 bytes, and the header length is 20 bytes (5 * 4), which means that there are 80 bytes in this datagram. If the first byte number is 800, the last byte number must be 879. Note: The Total Length=MTU (80 i.e. payload)+Header( 20, no option) © McGraw Hill, LLC 79  7.4.3 Options The header of the IPv4 datagram is made of two parts: a fixed part and a variable part. The fixed part is 20 bytes long and was discussed in the previous section. The variable part comprises the options that can be a maximum of 40 bytes (in multiples of 4 bytes) to preserve the boundary of the header. The variable part comprises the options, which can be a maximum of 40 bytes. Options, as the name implies, are not required for a datagram. They can be used for network testing and debugging. Although options are not a required part of the IP header, option processing is required of the IP software. © McGraw Hill, LLC 80  Single-Byte Options There are two single-byte options. No Operation A no-operation option is a 1-byte option used as a filler between options. End of Option An end-of-option option is a 1-byte option used for padding at the end of the option field. It, however, can only be used as the last option. © McGraw Hill, LLC 81  Security of IPv4 Datagrams We give a brief idea about the security issues in IP protocol. Packet Sniffing An intruder may intercept an IP packet and make a copy of it. Packet Modification The attacker intercepts the packet, changes its contents, and sends the new packet to the receiver.. IP Spoofing An attacker can masquerade as somebody else and create an IP packet that carries the source address of another computer. © McGraw Hill, LLC 82  IPSec The IP packets today can be protected from the previously mentioned attacks using a protocol called IPSec (IP Security). This protocol, which is used in conjunction with the IP protocol, creates a connection-oriented service between two entities in which they can exchange IP packets without worrying about the three attacks discussed above. We will discuss IPSec in detail in Chapter 13. © McGraw Hill, LLC 83  7.4.4 ICMPv4 The IPv4 has no error-reporting or error-correcting mechanism. The IP protocol also lacks a mechanism for host and management queries. The Internet Control Message Protocol version 4 (ICMPv4) has been designed to compensate for the above two deficiencies. © McGraw Hill, LLC 84  Messages ICMP messages are divided into two broad categories: error- reporting messages and query messages. The error-reporting messages report problems that a router or a host (destination) may encounter when it processes an IP packet. The query messages, which occur in pairs, help a host or a network manager get specific information from a router or another host. For example, nodes can discover their neighbors. Also, hosts can discover and learn about routers on their network and routers can help a node redirect its messages. © McGraw Hill, LLC 85  Figure 7.19 General format of ICMP messages Access the text alternative for slide images. © McGraw Hill, LLC 86  Figure 7.20 Contents of data field for error messages Access the text alternative for slide images. © McGraw Hill, LLC 87  Deprecated Messages Three pairs of messages are declared obsolete by IETF: 1. Information request and replay messages 2. Address mask request and reply messages 3. Router solicitation and advertisement messages © McGraw Hill, LLC 88  Debugging Tools There are several tools that can be used in the Internet for debugging. We can determine the viability of a host or router. We can trace the route of a packet. We introduce two tools that use ICMP for debugging: ping and traceroute. © McGraw Hill, LLC 89  Ping We can use the ping program to find if a host is alive and is responding. © McGraw Hill, LLC 90  Example 7.17 The following shows how we send a ping message to the auniversity.edu site. $ ping auniversity.edu PING auniversity.edu (152.181.8.3) 56 (84) bytes of data. ttl=62 time=1.91 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=0 ttl=62 time=2.04 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=1 ttl=62 time=1.90 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=2 ttl=62 time=1.90 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=3 ttl=62 time=1.97 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=4 ttl=62 time=1.93 ms 64 bytes from auniversity.edu (152.181.8.3): icmp_seq=5 ttl=62 time=2.00 ms --- auniversity.edu statistics --- 6 packets transmitted, 6 received, 0% packet loss rtt min/avg/max = 1.90/1.95/2.04 ms © McGraw Hill, LLC 91  Traceroute or Tracert The traceroute program in UNIX or tracert in Windows can be used to trace the path of a packet from a source to the destination. It can find the IP addresses of all the routers that are visited along the path. The program is usually set to check for the maximum of 30 hops (routers) to be visited. The number of hops in the Internet is normally less than this. Since these two programs behave different in Unix and Windows, we explain them separately. © McGraw Hill, LLC 92  Figure 7.21 Example of traceroute program Access the text alternative for slide images. © McGraw Hill, LLC 93  7.4.6 Forwarding of IP Packets We discussed the concept of forwarding at the network layer earlier in this chapter. In this section, we extend the concept to include the role of IP addresses in forwarding. As we discussed before, forwarding means to place the packet in its route to its destination. Since the Internet today is made of a combination of links (networks), forwarding means to deliver the packet to the next hop (which can be the final destination or the intermediate connecting device). Although the IP protocol was originally designed as a connectionless protocol, today the tendency is to change it to connection-oriented protocol. We discuss both cases. © McGraw Hill, LLC 94  Forwarding Based on Destination Address We first discuss forwarding based on the destination address. This is a traditional approach, which is prevalent today. In this case, forwarding requires a host or a router to have a forwarding 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 next hop to deliver the packet to. © McGraw Hill, LLC 95  Figure 7.32 Simplified forwarding module in classless address Access the text alternative for slide images. © McGraw Hill, LLC 96  Example 7.19 Make a forwarding table for router R1 using the configuration in Figure 7.33. © McGraw Hill, LLC 97  Network Next hop Interface address/mask Figure 7.33 Configuration for Example 7.19— 180.70.65.192/26 m2 180.70.65.128/25 — m0 201.4.22.0/24 — m3 201.4.16.0/22 — m1 Default 180.70.65.200 m2 10110100.01000110.01000001.11000000 10110100.01000110.01000001.10000000 11001001.00000100.00010110.00000000 11001001.00000100.00010000.00000000 Access the text alternative for slide images. © McGraw Hill, LLC 98  Table 7.3 Forwarding table for router R1 Network address/mask Next hop Interface 180.70.65.192/26 — m2 180.70.65.128/25 — m0 201.4.22.0/24 — m3 201.4.16.0/22 — m1 Default 180.70.65.200 m2 © McGraw Hill, LLC 99 22.100 Example 22.1 Make a routing table for router R1, using the configuration in Figure 22.6. Solution Table 22.1 shows the corresponding table. © McGraw Hill, LLC 22.101 Figure 22.6 Configuration for Example 22.1 © McGraw Hill, LLC 22.102 Table 22.1 Routing table for router R1 in Figure 22.6 © McGraw Hill, LLC 22.103 Example 22.2 Show the forwarding process if a packet arrives at R1 in Figure 22.6 with the destination address 180.70.65.140. Solution The 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 and the interface number m0 are passed to ARP for further processing. © McGraw Hill, LLC 22.104 Example 22.3 Show the forwarding process if a packet arrives at R1 in Figure 22.6 with the destination address 201.4.22.35. Solution The 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. 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). © McGraw Hill, LLC 22.105 Example 22.4 Show the forwarding process if a packet arrives at R1 in Figure 22.6 with the destination address 18.24.32.78. Solution This time all masks are applied, one by one, 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 someplace else in the Internet. © McGraw Hill, LLC  Address Aggregation When we use classful addressing, there is only one entry in the forwarding table for each site outside the organization. The entry defines the site even if that site is subnetted. When a packet arrives at the router, the router checks the corresponding entry and forwards the packet accordingly. When we use classless addressing, it is likely that the number of forwarding table entries will increase. To alleviate the problem, the idea of address aggregation was designed. In Figure 7.34 we have two routers. © McGraw Hill, LLC 106  Figure 7.34 Address aggregation 10001100.00011000.00000111.00000000 Access the text alternative for slide images. © McGraw Hill, LLC 107  Figure 7.35 Longest mask addressing Access the text alternative for slide images. © McGraw Hill, LLC 108  Example 7.22 As an example of hierarchical routing, let us consider. A regional ISP is granted 16,384 addresses starting from 120.14.64.0. The regional ISP has decided to divide this block into 4 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. © McGraw Hill, LLC 109  Figure 7.36 Hierarchical routing with ISPs Access the text alternative for slide images. © McGraw Hill, LLC 110  Forwarding Table Search Algorithm In classless addressing, there is no network information in the destination address. The simplest, but not the most efficient, search method is called the longest prefix match (as we discussed before). The forwarding table can be divided into buckets, one for each prefix. The router first tries the longest prefix. If the destination address is found in this bucket, the search is complete. If the address is not found, the next prefix is searched, and so on. It is obvious that this type of search takes a long time. © McGraw Hill, LLC 111  Example 7.23 Figure 7.37 shows a simple example of searching in a forwarding table using the longest mask algorithm. Although there are some more efficient algorithms today, the principle is the same. © McGraw Hill, LLC 112  Figure 7.37 Example 7.23 Access the text alternative for slide images. © McGraw Hill, LLC 113  7.5 NEXT GENERATION IP (IPv6) The address depletion of IPv4 and other shortcoming of this protocol prompted a new version of IP protocol in the early 1990s, which is called Internet Protocol version 6 (IPv6) or IP new generation (Ipng). © McGraw Hill, LLC 114  7.5.1 IPv6 Addressing The main reason for migration from IPv4 to IPv6 was the small size of the address space of IPv4. An IPv6 address is 128 bytes or 16 bytes, four times the address length in IPv4. © McGraw Hill, LLC 115  Representation An IPv6 address is 128 bits or 16 bytes long; four times the address length of IPv4. Binary (128 bits) 11111110111101101011 … 1111111100000000 Colon hexadecimal FEF6:BA98:7654:3210:ADEF:BBFF:2922:FF00 © McGraw Hill, LLC 116  Address Space 2 The address space of IPv6 contains 2128 addresses. This address space is 296 times the IPv4 address—definitely no address depletion—as shown, the size of the space is 340, 282, 366, 920, 938, 463, 374, 607, 431, 768, 211, 456 © McGraw Hill, LLC 117  Address Space Allocation Like the address space of IPv4, the address space of IPv6 is divided into several blocks of varying size and each block is allocated for a special purpose. Most of the blocks are still unassigned and have been set aside for future use. Table 7.5 shows only the assigned blocks. In this table, the last column shows the fraction each block occupies in the whole address space. © McGraw Hill, LLC 118  Table 7.5 Prefixes for assigned IPv6 addresses Block prefix CIDR Block assignment Fraction 0000 0000 0000::/8 Special addresses 1/256 001 2000::/3 Global unicast 1/8 1111 110 FC00::/7 Unique local unicast 1/128 1111 1110 10 FE80::/10 Link local addresses 1/1024 1111 1111 FF00::/8 Multicast addresses 1/256 © McGraw Hill, LLC 119  Figure 7.41 Global unicast address Access the text alternative for slide images. © McGraw Hill, LLC 120  Figure 7.44 Special addresses Access the text alternative for slide images. © McGraw Hill, LLC 121  Other Assigned Blocks IPv6 uses two large blocks for private addressing and one large block for multicasting, as shown in Figure 7.45. © McGraw Hill, LLC 122  Figure 7.45 Unique local unicast block Access the text alternative for slide images. © McGraw Hill, LLC 123  Autoconfiguration One of the interesting features of IPv6 addressing is the auto- configuration of hosts. As we discussed in IPv4, the host and routers are originally configured manually by the network manager. However, the Dynamic Host Configuration Protocol, DHCP, can be used to allocate an IPv4 address to a host that joins the network. In IPv6, DHCP protocol can still be used to allocate an IPv6 address to a host, but a host can also configure itself. © McGraw Hill, LLC 124 Because learning changes everything. ® www.mheducation.com © 2022 McGraw Hill, LLC. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw Hill, LLC.