1.1.d Explain IP operations

1.1.d [ii] IPv4 options, IPv6 extension headers

from: http://www.cisco.com/en/US/technologies/tk648/tk872/technologies_white_paper0900aecd8054d37d.html

The options field in the IPv4 header is used to convey additional information on the packet or on the way it should be processed. Routers, unless instructed otherwise, must process the options in the IPv4 header. The processing of most header options pushes the packet into the slow path leading to a forwarding performance hit.
IPv4 Options perform a very important role in the IP protocol operation therefore the capability had to be preserved in IPv6. On the other hand, the impact of IPv4 Options on performance was taken into consideration in the development of IPv6. The functionality of options is removed from the main header and implemented through a set of additional headers called extension headers. The main header remains fixed in size (40 bytes) while customized EHs are added as needed.
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Extension Headers

Zero or more extension headers can be present and are of varying lengths. A Next Header field in the IPv6 header indicates the next extension header. Within each extension header is a Next Header field that indicates the next extension header. The last extension header indicates the upper layer protocol (such as TCP, UDP, or ICMPv6) contained within the upper layer protocol data unit.

The IPv6 header and extension headers replace the existing IPv4 IP header with options. The new extension header format allows IPv6 to be augmented to support future needs and capabilities. Unlike options in the IPv4 header, IPv6 extension headers have no maximum size and can expand to accommodate all the extension data needed for IPv6 communication.

Table 5    Values of the Next Header Field

Value (in decimal) Header
0 Hop-by-Hop Options Header
6 TCP
17 UDP
41 Encapsulated IPv6 Header
43 Routing Header
44 Fragment Header
46 Resource ReSerVation Protocol
50 Encapsulating Security Payload
51 Authentication Header
58 ICMPv6
59 No next header
60 Destination Options Header

Comparing the IPv4 and IPv6 Headers

Table 6 shows the differences between the IPv4 and IPv6 header fields.

Table 6    IPv4 Header Fields and Corresponding IPv6 Equivalents

IPv4 Header Field IPv6 Header Field
Version Same field but with different version numbers.
Internet Header Length Removed in IPv6. IPv6 does not include a Header Length field because the IPv6 header is always a fixed size of 40 bytes. Each extension header is either a fixed size or indicates its own size.
Type of Service Replaced by the IPv6 Traffic Class field.
Total Length Replaced by the IPv6 Payload Length field, which only indicates the size of the payload.
Identification
Fragmentation Flags
Fragment Offset
Removed in IPv6. Fragmentation information is not included in the IPv6 header. It is contained in a Fragment extension header.
Time to Live Replaced by the IPv6 Hop Limit field.
Protocol Replaced by the IPv6 Next Header field.
Header Checksum Removed in IPv6. In IPv6, bit-level error detection for the entire IPv6 packet is performed by the link layer.
Source Address The field is the same except that IPv6 addresses are 128 bits in length.
Destination Address The field is the same except that IPv6 addresses are 128 bits in length.
Options Removed in IPv6. IPv4 options are replaced by IPv6 extension headers.