数据通信基础：第20章 网络层协议

1、数据通信基础第20章 (网络层协议) Network Layer Protocols20120305Chapter 20Network LayerProtocols:ARP, IPv4, ICMPv4 20120305Figure 20.1 Protocols at network layer20120305Address Resolution Protocol (ARP)Problem if a host wants to contact a known destination IP address, what MAC address should it put in the Etherne

2、t frame ?Solution it should use ARP to broadcast a message to all hosts on the network asking for the MAC address of the given IP address(it must be a broadcast why?)20120305Figure 20.2 ARP operationIP address of Host B20120305Figure 20.3 ARP packetEthernet = type 1IPv4 is type 080016These two lengt

3、hs define the size of the rest of the packet16bitsNote: 4 byte boundaries are used on this type of diagram20120305Figure 20.4 Encapsulation of ARP packetNote that the ARP packet is the data for an Ethernet frame this is EncapsulationARP/RARPtype20120305Figure 20.5 Four cases using ARPAll the above r

4、equire ARP to discover the MAC address to be inserted into the Ethernet frame20120305An ARP request is broadcast it must be, since it does not know the MAC address yet the broadcast MAC address is 11111111111116 (48 bits)An ARP reply is unicast it replies to the address that made the requestNote:201

5、20305Example 1A host with IP address 130.23.3.20 and physical address B23455102210 has a packet to send to another host with IP address 130.23.43.25 and physical address A46EF45983AB. The two hosts are on the same Ethernet network. Show the ARP request and reply packets encapsulated in Ethernet fram

6、es. SolutionFigure 20.6 shows the ARP request and reply packets. Note that the ARP data field in this case is 28 bytes, and that the individual addresses do not fit in the 4-byte boundary. That is why we do not show the regular 4-byte boundaries for these addresses. Note that we use hexadecimal for

7、every field except the IP addresses. 20120305Figure 20.6 Example 1Do not conveniently fit into 4 byte boundaryA request does not fill this part in, since it is unknownThis is the required MAC address2012030520.2 IPv4 protocolDatagram the equivalent of an Ethernet frame is called a datagram or a pack

8、et at layer 3Fragmentation the process of breaking up large datagrams so that they can be transferred over a given network link20120305Figure 20.7 IP datagram20120305VER IP version, currently 4HLEN header length expressed in 4 byte blocks note that options make the header length variableDS defines q

9、uality of service (QoS)Total length field defines the total length of the datagram including the headerTime to live (TTL) decremented after each hop avoids packets circulating endlesslyProtocol being carried e.g. TCP, UDP, ICMP, etc.20120305Figure 20.8 Multiplexing20120305Figure 20.9 Example of chec

10、ksum calculationNOTE:Only the HEADER has any kind of error detection IP is NOT responsible for checking or correcting dataAt the next hop, the Sum is recalculated and added to the received checksum. If the answer is not zero, the packet is discarded.The checksum changes at each router, because the T

11、TL changes each time.20120305Figure 20.10 MTUThe data link layer will have a maximum frame size (this varies depending on layer 2 protocol). This affects the maximum data payload, or MTU maximum transfer unit.The diagram above illustrates E N C A P S U L A T I O N - the layer 3 datagram becomes the

12、layer 2 data20120305Figure 20.11 Fragmentation exampleWhen a packet is fragmented at layer 3, each fragments offset needs to be stored within the fragmented packet so that it can be reconstructed later.The fragmented packet also needs to record which original packet it belonged to hence the need for

13、 the Identification fieldThe flags field contains bits which indicate that there are more fragments to come, or that fragmentation is not allowed2012030520.3 ICMPInternet Control Message ProtocolTypes of Messages: Error reporting QueryEncapsulation20120305ICMP always reports error messages to the or

14、iginal source.Cannot routeLimited Flow controlTTL down to zeroHeader errorHost is using wrong default routerThere is no formal flow control or congestion control mechanism in IP. Source-Quench simple reports the problem to the source20120305Figure 20.14 Query messagesUsed in ping and tracertTo deter

15、mine round trip timeTo determine subnet maskHosts use solicitation messages to ask where routers are.Routers use advertisement to say where they areICMP Query messages always expect a reply2012030520.4 IPv6IPv6 AddressesCategories of AddressesIPv6 Packet FormatFragmentationICMPv6Transition20120305Fi

16、gure 20.15 IPv6 addressHexadecimal Colon Notation=20120305Figure 20.16 Abbreviated address20120305Figure 20.17 Abbreviated address with consecutive zerosOnly one string of zeros can be abbreviated.20120305Figure 20.18 CIDR address20120305Categories of IPv6 AddressesUnicast AddressSpecific computerAnycast Address (NEW TYPE)Any computer in a groupMulticast AddressAll computers in a group20120305Figure 20.19 Format of an IPv6 datagram20120305Figure 20.20 Comparison of network layers in version 4 and version 6A