Transcription
Chapter 18Introduction to Network Layer
Communication at the network layer18.2
Packetizing- encapsulating the payload in a network-layerpacket at the source and decapsulating thepayload from the network-layer packet at thedestination.- In other words, one duty of the network layer isto carry a payload from the source to thedestination without changing it or using it.- Similar to the service of a carrier such as thepostal office18.3
Routing and ForwardingOther duties of the network layer are- routing- forwarding18.4
Forwarding processForwardingvalueSend the packetout of interface 2B18.5DataBData
PACKET SWITCHINGA router is a switch that creates a connection betweenan input port and an output port (or a set of outputports18.6
Datagram Approach- The network layer was designed to provide aconnectionless service in which the network-layerprotocol treats each packet independently, witheach packet having no relationship to any otherpacket.- The idea was that the network layer is onlyresponsible for delivery of packets from thesource to the destination.- In this approach, the packets in a message mayor may not travel the same path to theirdestination.18.7
A connectionless packet-switched network18.8
Virtual-Circuit Approach- There is a relationship between all packetsbelonging to a message.- Before all datagrams in a message can be sent, avirtual connection should be set up to define thepath for the datagrams.- After connection setup, the datagrams can allfollow the same path.- In this type of service, not only must the packetcontain the source and destination addresses, itmust also contain a flow label, a virtual circuitidentifier that defines the virtual path the packetshould follow.18.9
A virtual-circuit packet-switched network18.10
Forwarding process in a router when used in a virtualcircuit network18.11
Sending request packet in a virtual-circuit networkA to BA to BA to B18.12A to B
Sending acknowledgments in a virtual-circuit network18.13
Flow of one packet in an established virtual circuit18.14
NETWORK-LAYER PERFORMANCEThe performance of a network can be measured interms of delay, throughput, and packet loss.Congestion control is an issue that can improve theperformance.18.15
DelayThe delays in a network can be divided into fourtypes:- transmission delay- propagation delay- processing delay- queuing delay18.16
Throughput- is defined as the number of bits passing throughthe point in a second, which is actually thetransmission rate of data at that point.- In a path from source to destination, a packetmay pass through several links (networks), eachwith a different transmission rate.- How can we determine the throughput of thewhole path?18.17
Throughput in a path with three links in a series18.18
A path through the Internet backbone18.19
Effect of throughput in shared links18.20
Packet Loss- Another issue that severely affects theperformance of communication is the number ofpackets lost during transmission.- The effect of packet loss on the Internet networklayer is that the packet needs to be resent, whichin turn may create overflow and cause morepacket loss.18.21
IPv4 ADDRESSES- The identifier used in the IP layer of the TCP/IPprotocol suite to identify the connection of eachdevice to the Internet.- An IPv4 address is a 32-bit address that uniquelyand universally defines the connection of a host ora router to the Internet.- The IP address is the address of the connection,not the host or the router.18.22
Three different notations in IPv4 addressing18.23
Hierarchy in addressing18.24
Classful Addressing- The whole address space was divided into fiveclasses (class A, B, C, D, and E)- This scheme is referred to as classful addressing.18.25
Occupation of the address space in classful addressing18.26
Network address18.27
Example 18.5An organization is granted a block of addresses with thebeginning address 14.24.74.0/24. The organization needs tohave 3 subblocks of addresses to use in its three subnets: onesubblock of 10 addresses, one subblock of 60 addresses, andone subblock of 120 addresses. Design the subblocks.SolutionThere are 232– 24 256 addresses in this block. The firstaddress is 14.24.74.0/24; the last address is 14.24.74.255/24.To satisfy the third requirement, we assign addresses tosubblocks, starting with the largest and ending with thesmallest one.18.28
Example 18.5 (continued)a. The number of addresses in the largest subblock, whichrequires 120 addresses, is not a power of 2. We allocate 128addresses. The subnet mask for this subnet can be found asn1 32 log2 128 25. The first address in this block is14.24.74.0/25; 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. Weallocate 64 addresses. The subnet mask for this subnet canbe found as n2 32 log2 64 26. The first address in thisblock is 14.24.74.128/26; the last address is14.24.74.191/26.18.29
Example 18.5 (continued)c. The number of addresses in the largest subblock, whichrequires 120 addresses, is not a power of 2. We allocate 128addresses. The subnet mask for this subnet can be found asn1 32 log2 128 25. The first address in this block is14.24.74.0/25; the last address is 14.24.74.127/25.If we add all addresses in the previous subblocks, the resultis 208 addresses, which means 48 addresses are left inreserve. The first address in this range is 14.24.74.208. Thelast address is 14.24.74.255. We don’t know about the prefixlength yet.18.30
Solution to Example 4.518.31
Example 18.6Figure 18.24 shows how four small blocks of addresses areassigned to four organizations by an ISP. The ISP combinesthese four blocks into one single block and advertises thelarger block to the rest of the world. Any packet destined forthis larger block should be sent to this ISP. It is theresponsibility of the ISP to forward the packet to theappropriate organization. This is similar to routing we canfind in a postal network. All packages coming from outsidea country are sent first to the capital and then distributed tothe corresponding destination.18.32
Figure 18.24: Example of address aggregation18.33
DHCPAfter a block of addresses are assigned to anorganization, the network administration canmanually assign addresses to the individual hosts orrouters. However, address assignment in anorganization can be done automatically using theDynamic Host Configuration Protocol (DHCP).DHCP is an application-layer program, using theclient-server paradigm, that actually helps TCP/IP atthe network layer.18.34
Operation of DHCP18.35
NAT- A technology that can provide the mappingbetween the private and universal addresses, andat the same time support virtual private networks- allows a site to use a set of private addresses forinternal communication and a set of globalInternet addresses (at least one) forcommunication with the rest of the world.18.36
NAT18.37
Address translation18.38
Translation18.39
Five-column translation table18.40
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.
