Chapter 20 Network Layer: Internet Protocol Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

20-1 INTERNETWORKING In this section, we discuss internetworking, connecting networks together to make an internetwork or an internet. Need for Network Layer Internet as a Datagram Network Internet as a Connectionless Network Topics discussed in this section: (Study from Book)

Figure 20.1 Links between two hosts

Figure 20.2 Network layer in an internetwork

Figure 20.3 Network layer at the source, router, and destination

Figure 20.3 Network layer at the source, router, and destination (continued)

Switching at the network layer in the Internet uses the datagram approach to packet switching. Note

Communication at the network layer in the Internet is connectionless. Note

20-2 IPv4 The Internet Protocol version 4 (IPv4) is the delivery mechanism used by the TCP/IP protocols. IP Datagram ARP Topics discussed in this section: study from lecure notes

Figure 20.4 Position of IPv4 in TCP/IP protocol suite

Figure 20.5 IPv4 datagram format

IPv4 datagram fields Minimum Header length is 20 bytes without options. With options the maximum can go to 60 bytes Largest data that can be carried in the datagram is – 20 = Version field: will carry the version number which is 4 = (0100) 2 Header length: the length of the header in bytes after dividing it by 4. Min is 20/4 = 5 = (0101) 2 and the max is 60/4 = 15 = (1111 ) 2 Total length: total length of the packet: header + data. Max = bytes Identification, flags, and offset used for fragmentation and reassembly at the destination. Packet can be fragmented at any node between the source and the destination but reassembly is done ONLY at the destination node.

IPv4 datagram fields Time to Live: is used to prevent lost packets from circulating between routers forever. This field is set to certain value depending on the device operating system. Each router will decrement this field by one and check the value. If the value is zero the packet will be dropped. Protocol: contains a code for what is being carried in the data field. Refer to table (20.4) and Figure (20.8) Header checksum: used for checking if there is error in the header only. The checksum is recomputed at each router between the source and the destination.

Fragmentation The Internet Protocol (IP) implements datagram fragmentation, so that packets may be formed that can pass through a physical network with a smaller maximum transmission unit (MTU) than the original datagram size. The Identification field, and Fragment offset field along with Don't Fragment and More Fragment flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams. The value of MTU depends on the physical network protocol. In a case where a router receives a packet larger than the next hop's MTU, it has two options if the protocol is IPv4. Drop the PDU and send an Internet Control Message Protocol (ICMP) message which indicates the condition Packet too Big, or to fragment the IP packet and send it over the link with a smaller MTU.

Figure 20.8 Protocol field and encapsulated data

Table 20.4 Protocol values in Hex

Figure 20.9 Maximum transfer unit (MTU)

Table 20.5 MTUs for some networks

IP RouterSourceDestination Network Fragmentation and Reassembly Fragment at router Fragment at source Reassemble at destination  Fragmentation takes place at the sender and routers  Reassembly takes place at the receiver ONLY.

21.1 Address Resolution Protocol (ARP) At the network level hosts and routers are recognized by their IP address Packets must pass through physical networks to reach hosts and routers. At the physical network, hosts and routers are recognized by their MAC addresses which is local address. ARP is a network layer protocol that translates between Internet IP address and MAC sublayer (layer-2) address

Figure 21.1 ARP operation