Practical TCP/IP and Ethernet Networking- P24: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | Internet layer protocols 97 IPv6 extensions IPv6 includes an improved option mechanism over IPv4. Instead of placing extra options bytes within the main header IPv6 options are placed in separate extension headers that are located between the IPv6 header and the transport layer header in a packet. Most IPv6 extension headers are not examined or processed by routers along a packet s path until it arrives at its final destination. This leads to a major improvement in router performance for packets containing options. In IPv4 the presence of any options requires the router to examine all options. IPv6 extension headers can be of arbitrary length and the total amount of options carried in a packet is not limited to 40 bytes as with IPv4. They are also not carried within the main header as with IPv4 but are only used when needed and are carried behind the main header. This feature plus the manner in which they are processed permits IPv6 options to be used for functions which were not practical in IPv4. Good examples of this are the IPv6 authentication and security encapsulation options. In order to improve the performance when handling subsequent option headers and the transport protocol which follows IPv6 options are always an integer multiple of 8 octets long in order to retain this alignment for subsequent headers. The IPv6 extension headers currently defined are Routing header for extended routing similar to the IPv4 loose source route . Fragment header for fragmentation and reassembly . Authentication header for integrity and authentication . Encrypted security payload for confidentiality . Hop-by-hop options header for special options that require hop-by-hop processing . Destination options header for optional information to be examined by the destination node . Figure Carrying IPv6 extension headers IPv6 addresses IPv6 addresses are 128 bits long and are identifiers for individual interfaces or sets of interfaces. IPv6 Addresses of all types are
