Hi all, I would appreciate your insight in the following question I have received from my student. My answer follows but I am not sure I have not missed some important piece here. Q: Why was the meaning of the packet length parameter changed from total length to payload length when moving from IPv4 to IPv6? It looks to me like deliberately created fuel for confusion. My Answer: IPv4 header might vary between 20 to 60 octets, and in first 20 octets there are 11 fields which need to be looked at and possibly processed by routers along the path traversing internet. This unnecessary complexity leads to inefficient router's performance. By employing a simple header of fixed length with 8 mandatory fields, IPv6 routers enhance their performance. As we could see, many fields were either removed or embedded in the extension headers in IPv6. As the IPv6 header has a fixed length of 40 octets, the Header length field could be eliminated. Payload length is the length of the remainder of the IPv6 packet following the header, in octets (extension headers plus the transport-level PDU). IPv6 as opposed to IPv4 does not perform any checksum in the base header, again to allow for faster processing at intermediate nodes (routers). The routers only check what is needed to check: in case of IPv6 they are interested that the complete header is there (they know what length to expect - fixed 40 octets) and that the rest of the datagram is complete in terms of advertized length. Therefore instead of identifying the whole datagram length from which they would substract the 40 octets, they know immediately what the payload length is. This seems quite sensible; perhaps we could attribute this sort of discrepancy between versions rather to the IPv4 way of specifying the whole datagram length instead of the data (payload) length. IPv6 moved away from that for practical reasons - routers' efficiency. Rita Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=8336&t=8336 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
