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Embed code for: 24.6 Comparison of IPv4 and IPv6 Headers
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24.6 Comparison of IPv4 and IPv6 Headers
Comparison of IPv4 and IPv6 Headers Section 24: Introducing Basic IPv6 The IPv6 header design departs significantly from the IPv4 header in several ways. The IPv4 header contains 12 fields. Following the main header is an Options field (shown in yellow) and a data portion, which is usually the transport layer segment. The basic IPv4 header has a size of 20 octets, but it may include IP options that increase the size of the IP header. Of the 12 IPv4 header fields, six are removed in IPv6; these fields are shown in green and blue in the figure. The main reasons for removing these fields in IPv6 are as follows: The Internet Header Length (Hd Len) field was removed because it is no longer required. Unlike the variablelength IPv4 header, the IPv6 header is fixed at 40 bytes. Fragmentation is processed differently in IPv6 and does not need the Flags field in the basic IPv4 header. In IPv6, routers no longer process fragmentation. IPv6 hosts are responsible for path MTU discovery. If the host needs to send data that exceed the path MTU, the host is responsible for fragmentation. The related Flags field option appears in the Fragmentation Extension Header in IPv6. This header is attached to only a packet that is fragmented. The Header Checksum field at the IP layer was removed because most data link layer technologies already perform checksum and error control. This change forces formerly optional upperlayer checksums (such as UDP) to become mandatory. The Options field is not present in IPv6 and any additional services are processed by a chain of extension headers. Examples of extension headers include Fragmentation, AH, and ESP. Most other fields were either unchanged or changed only slightly. The IPv6 header has 40 octets, instead of 20 octets as in IPv4. The IPv6 header has fewer fields, and the header is aligned on 64bit boundaries to enable fast processing by current and nextgeneration processors. The Source and Destination IP fields are four times larger than in IPv4. The Source and Destination IP fields are the most important headers to understand. The IPv6 header contains eight fields: 1. Version: This 4bit field contains the number 6, instead of the number 4 as in IPv4. 2. Traffic Class: This 8bit field is similar to the ToS field in IPv4. The source node uses this field to mark the priority of outbound packets. 3. Flow Label: This new field has a length of 20 bits and is used to mark individual traffic flows with unique values. Routers are expected to apply an identical QoS treatment to each packet in a flow. 4. Payload Length: This field is like the Total Length field for IPv4, but because the IPv6 base header is a fixed size, this field describes the length of the payload only, not of the entire packet. 5. Next Header: The value of this field determines the type of information that follows the basic IPv6 header. 6. Hop Limit: This field specifies the maximum number of hops that an IP packet can take. This field begins at 255 and is decremented by each IPv6 router along the path to the destination. An IPv6 packet can pass through a maximum of 254 hops before it is deleted. The hop limit is designed to prevent packets from circulating forever if there was a routing error. In normal routing, this limit should never be reached. 7. Source Address: This field of 16 octets, or 128 bits, identifies the source of the packet. 8. Destination Address: This field of 16 octets, or 128 bits, identifies the destination of the packet. Following these eight fields are the extension headers, if any. The number of extension headers is not fixed, so the total length of the extension header chain is variable. For further exploration of IPv6 header fields and their functions, use your favorite search engine to locate RFC 2460, Internet Protocol, Version 6 (IPv6) Specification. Up Next: Internet Control Message Protocol version 6