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

Background In chapter 19 we studied how to assign addresses to nodes in a network. Addresses assigned to nodes are logical addresses called IP addresses. This chapter is about the IP i.e. the Internet protocol used at the network layer. NOTE: Kindly do not confuse IP address with IP protocol. These are two different things.

Difference between IP (protocol) and IP address: IP address is the logical name of the computer. IP protocol is a set of rules to govern communication on the network layer.

Data link Vs. Network Layer Data link layer provides hop to hop delivery. Network layer provides host to host delivery. If the transmission is within a network we use only physical and data link layer. If the transmission is outside the network we use network layer+data link+physical layer.

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

Figure 20.1 Links between two hosts

Figure 20.2 Network layer in an internetwork

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

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

Topics discussed in this section: 20-2 IPv4 The Internet Protocol version 4 (IPv4) is the delivery mechanism used by the TCP/IP protocols. Topics discussed in this section: Datagram Fragmentation Checksum Options

Figure 20.4 Position of IPv4 in TCP/IP protocol suite

Figure 20.5 IPv4 datagram format

IPv4 Datagram Format IPv4 Packet is called datagram. A datagram is of variable length. Consists of two parts: Header + Data Header’s length is 20 to 60 bytes. Header contains information essential for routing and delivering Data. It is customary in TCP/IP to show the header in 4-byte sections.

Header Fields (1) VERSION (VER) Header Length (HLEN) 4 bit in length Defines the version of IP (either IPv6 or IPv4) Header Length (HLEN) Defines the length of the header. Its value falls between 20 to 60 bytes

Header Fields (2) Services the IETF has changed the interpretation and name of this 8-bit field. It was previously called as service type, now called differentiated services. I will explain “service type”. “differentiated services” is your homework.

Service Type(1) First 3 bits are Precedence bits. Next 4 bits are called Type of Service (TOS) bits, and the last bit is not used. Precedence: Value ranges from 000 to 111. Defines priority of the datagram Used in situations of Network Congestion Router discards datagrams of low precedence in case of congestion.

Figure 20.6 Service type or differentiated services

Service Type(2) TOS bits 4 bit in length Out of 4 only a single bit can be 1 at a time, thus we have 5 different types of services. Bit patterns and their interpretations are shown below.

Table 20.2 Default types of service

Total Length This field defines the total length of the Datagram (header + Data) Value lies between 20 to 65536 bytes.

Time to Live A datagram has a limited lifetime in its travel through an internet. It holds a timestamp which is decremented on each visit of a router. The datagram is discarded when the value of this field becomes zero. The purpose is prevent datagram from monopolizing the network and causing congestion.

Protocol 8-bit length It defines the higher level protocol that uses the services of the IPv4 Layer. It defines the higher level protocol to which the IPv4 datagram is delivered.

Figure 20.8 Protocol field and encapsulated data

Table 20.3 protocol values

Checksum An error detection mechanism Performed only with header fields Detects error in header part of datagram only.

Source/ Destination Address Source Address 32 bit field Defines the IPv4 address of the source Remains unchanged during travel from source to destination. Destination Address Defines the IPv4 address of the destination

Fragmentation Why Fragmentation is Required? A datagram can travel through different networks whose Protocols are defined by the data link and Physical Layer. We know that at the data link layer we deal with Frames. For different network Protocols at data link layer we have different formats and sizes of frames. Now we also know that the Packet from network layer called datagram (Header + data) act completely as data for the data link Frame.

Figure 20.9 Maximum transfer unit (MTU)

Continued Different Data link layer Protocols e.g. X.25, Frame Relay, Ethernet etc have different frame formats in which there is a field that limits the size of the Data in the frame called Maximum Transfer Unit. Thus in many cases (datagram traveling from LAN to WAN) it is required to fragment the datagram according to the MTU of the underlying network.

Table 20.5 MTUs for some networks

Fields Related To Fragmentation Identification 16-bit field Each datagram is assigned a unique number When the datagram is fragmented the same identification number is copied to all the fragments. Flags 3 bit field 1st bit is reserved

Continued….. 2nd bit is Do not Fragment 3rd bit is More Fragment if the value of this field is 1 the machine must not fragment the datagram. If it cannot pass the datagram though any available physical network, it discards the datagram and sends and ICMP error message to the source host. If the value is 0, this means that whenever required the datagram can be fragmented according to the requirement of the physical network it is travelling. 3rd bit is More Fragment If its value is 1, it means this is not the last fragment more fragments have to come. If its value is 0, it means this is the last fragment or the only fragment.

Figure 20.10 Flags used in fragmentation

Continued….. Fragmentation Offset 13 bit Field Shows the relative position of the fragment in the whole datagram. Offset is measured in units of 8 bytes.

Figure 20.11 Fragmentation example

Figure 20.12 Detailed fragmentation example

Example 20.5 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.

Example 20.6 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).

Example 20.7 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.

Example 20.8 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.

OPTIONS Options field can be used for network testing and debugging.

Topics discussed in this section: 20-3 IPv6 The network layer protocol in the TCP/IP protocol suite is currently IPv4. Although IPv4 is well designed, data communication has evolved since the inception of IPv4 in the 1970s. IPv4 has some deficiencies that make it unsuitable for the fast-growing Internet. Topics discussed in this section: Deficiencies of IPv4 Advantages of IPv6 Packet Format Extension Headers

Deficiencies of IPv4 Despite all short-term solution the problem of address Depletion still persists in IPv4. Demand of Real time audio and video Fast growing Mobile IP, IP telephony, IP- capable mobile telephony services IPv4 do not have any security measures i.e. Encryption and Authentication

Advantages of IPv6 Larger Address Space Better Header Format 128 bits address Better Header Format Options separated from base header and made part of data. Improves Routing New Options To allow additional functionalities Support For More Security

Continued…. Allowance for extension Support For Resource Allocation IPv6 is designed to handle future extensions Support For Resource Allocation The field “flow label” provides support for Resource allocation for special applications like real time audio and video. Support for more Security Encryption and authentication provides Confidentiality and Integrity.

Packet Format Each Packet is composed of: Mandatory Base Header (40 bytes) Payload (65535 bytes) Consists of optional extension header + data

Figure 20.15 IPv6 datagram header and payload

Base Header There are 8 fields: Version Priority Flow Label 4-bit in length defines the version of IP, here value is ‘6’. Priority 4-bit defines the priority of the packet with respect to traffic congestion. Flow Label 24-bit designed to provide special handling for a particular flow of data.

Source Address/ Destination Address Payload Length 2 byte defines the length of the payload. Next Header 8-bit Either optional extension header or header of another protocol e.g. TCP, UDP Just like the protocol field in IPv4. Hop Limit 8-bit Same as TTL field in IPv4 Source Address/ Destination Address 16 bytes both

Figure 20.16 Format of an IPv6 datagram

Table 20.6 Next header codes for IPv6

Table 20.9 Comparison between IPv4 and IPv6 packet headers

Topics discussed in this section: 20-4 TRANSITION FROM IPv4 TO IPv6 Because of the huge number of systems on the Internet, the transition from IPv4 to IPv6 cannot happen suddenly. It takes a considerable amount of time before every system in the Internet can move from IPv4 to IPv6. The transition must be smooth to prevent any problems between IPv4 and IPv6 systems. Topics discussed in this section: Dual Stack Tunneling Header Translation

Figure 20.18 Three transition strategies

Dual Stack All hosts before complete migration from IPv6 to IPv4 must have a dual stack of protocols. A station must run IPv4 and IPv6 simultaneously. To determine which version a destination host is using, the source host queries the DNS. If the DNS returns an IPv4 address, the source then send IPv4 packets. If the DNS returns an IPv6 address, the source then send IPv6 packets.

Figure 20.19 Dual stack

Tunneling It is a mechanism used when both sender and receiver hosts use IPv6 but in between a region falls that uses IPv4. To pass through this region the IPv6 packet is first encapsulated in IPv4 Header and after coming out this header is removed. The field ‘protocol’ in IPv4 has value 41 when the data it contains is an IPv6 Packet.

Figure 20.20 Tunneling strategy

Header Translation In this case the header of IPv6 is completely changed in IPv4 header.

Figure 20.21 Header translation strategy

Table 20.11 Header translation