a)
Fragmentation and reassembly incurs overhead at the router. IPv6 moves this burden to the end nodes and requires that they perform MTU discovery to determine the maximum datagram size they can send. It stands to reason that the end nodes are better suited for the task because they have less data to process. Effectively, the routers have enough on their plates; it's makes sense to force the nodes to deal with it and allow the routers to simply drop something that exceeds their MTU threshold.
Ideally, the end result would be that routers can handle a larger load under IPv6 (all things being equal) than they did under IPv4 because there is no fragmentation/reassembly that they have to worry about. That processor power can be dedicated to routing traffic.
b) In IPv4, the IP header is protected by a header checksum, and higher-layer protocols generally also have a checksum. The checksum algorithm for the IPv4 header, Internet Control Message Protocol (ICMP), ICMPv6, TCP, and UDP is the same one’s complement addition, except that in IPv4, UDP packets may forego checksumming and simply set the checksum field to zero. In IPv6, this practice is no longer allowed: UDP packets must have a valid checksum.
The TCP, UDP, and ICMPv6 checksums are computed over a “pseudoheader” and the TCP, UDP, or ICMPv6 header, and user data, respectively. The pseudoheader consists of the source and destination addresses, the upper-layer packet length, and the protocol number. Including this information in the checksum calculation ensures that TCP, UDP, or ICMPv6 do not process packets that were delivered incorrectly, for instance, because of a bit error in the IP header.
IPv6 no longer has a header checksum to protect the IP header, meaning that when a packet header is corrupted by transmission errors, the packet is very likely to be delivered incorrectly. However, higher-layer protocols should be able to detect these problems, so they are not fatal. Also, lower layers almost always employ a Cyclic Redundancy Check (CRC) to detect errors.
