Chapter: Computer Networks : Network Layer


The network layer protocol in the TCPIIP protocol suite is currently IPv4 (Internetworking Protocol, version 4).



The network layer protocol in the TCPIIP protocol suite is currently IPv4 (Internetworking Protocol, version 4). IPv4 provides the host-to-host communication between systems in the Internet. 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.


Despite all short-term solutions, such as subnetting, classless addressing, and NAT, address depletion is still a long-term problem in the Internet. The Internet must accommodate real-time audio and video transmission. This type of transmission requires minimum delay strategies and reservation of resources not provided in the IPv4 design.


1. Advantages


The next-generation IP, or IPv6, has some advantages over IPv4 that can be summarized as follows:


1.        Larger address space

2.        Better header format

3.        New options

4.        Allowance for extension

5.        Support for resource allocation

6.        Support for more security


2. Packet Format


Each packet is composed of a mandatory base header followed by the payload. The payload consists of two parts: optional extension headers and data from an upper layer. The base header occupies 40 bytes, whereas the extension headers and data from the upper layer contain up to 65,535 bytes of information.


a. Base Header

These fields are as follows:


·           Version: This 4-bit field defines the version number of the IP. For IPv6, the value is 6.


·           Priority: The 4-bit priority field defines the priority of the packet with respect totraffic congestion.


·           Flow label: The flow label is a 3-byte (24-bit) field that is designed to provide specialhandling for a particular flow of data.


·           Payload length: The 2-byte payload length field defines the length of the IP datagramexcluding the base header.


·           Next header: The next header is an 8-bit field defining the header that follows thebase header in the datagram. The next header is either one of the optional extension headers used by IP or the header of an encapsulated packet such as UDP or TCP. Each extension header also contains this field.


·           Hop limit: This 8-bit hop limit field serves the same purpose as the TIL field in IPv4.


·           Source address: The source address field is a 16-byte (128-bit) Internet address thatidentifies the original source of the datagram.


·           Destination address. The destination address field is a 16-byte (128-bit) Internetaddress that usually identifies the final destination of the datagram. However, if source routing is used, this field contains the address of the next router.


·           Priority: The priority field of the IPv6 packet defines the priority of each packet withrespect to other packets from the same source. For example, if one of two consecutive datagrams must be discarded due to congestion, the datagram with the lower packet priority will be discarded. IPv6 divides traffic into two broad categories: congestion-controlled and noncongestion-controlled.

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