Question 1 With respect to IPv4 and IPv6: (II) There are some fields in IPv4 hea
ID: 3685302 • Letter: Q
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Question 1 With respect to IPv4 and IPv6: (II) There are some fields in IPv4 header that allow packet fragmentation and reassembly. Why (III) IPv6 does not support fragmentation. What happens when the packet is too large to be sent (IV) With respect to Table 4.2, page 337 of the reference book, while the link layer protocol which one can be processed faster by the routers? why? is that necessary? on its output link? supports MTU-1500 B, why the amount of data in the first and second fragments are 1480?Explanation / Answer
1)
ipv6 prcessed faster by the routers .
Because IPv6 contains simplified Header Structures leading to faster routing as compared to IPv4 .
The present IP uses a Datagram service to transfer packets of data between point to point using routers. The IPv4 packet header structure contains 20 bytes of data, such that it contains within the header, all possible options thereby forcing intermediate routers to check whether these options exist and if they do, process them before forwarding them. In the IPv4 packet header, these options have a certain maximum permitted size.
The IPv4 header has two main problems that are instrumental in slowing down throughput - each packet must be processed and checksum computed, and each router that processes a packet must process the option field. This can cause a gradual degradation in performance during the forwarding of the IPv4 packets.
When compared to IPv4, IPv6 has a much simpler packet header structure, which is essentially designed to minimize the time and efforts that go in to header processing. This has been achieved by moving the optional fields as well as the nonessential fields to the extension headers that are placed only after the IPv6 header. Consequently, the IPv6 headers are processed more efficiently at the intermediate routers without having to parse through headers or recompute network-layer checksums or even fragment and reassemble packets. This efficiency allows for reduced processing overhead for routers, making hardware less complex and allowing for packets to be processed much faster.
Another feature of the IPv6 header structure is that the extension header allows for more flexible protocol inclusions than what IPv4 does. In contrast, IPv6 extension headers have no such restriction on the maximum size. They can be expanded to accommodate whatever extension data is thought necessary for efficient IPv6 communication. In fact, a typical IPv6 packet contains no extension header and only if intermediate routers or the destination require some special handling, will the host sending the packets add one or more extension headers depending on the requirement. This new extension header makes IPv6 fully equipped to support any future need or capabilities.
2)
IP fragmentation and reassembly employs updating and using the values in the second 32 bits of the IPv4 packet header. An end system that accepts an IP packet (with a destination IP address that matches its own IP source address) will also reassemble any fragmented IP packets before these are passed to the next higher protocol layer.
The system stores all received fragments (i.e., IP packets with a more-fragments flag (MF) set to one, or where the fragment offset is non-zero), in one of a number of buffers (memory space). Packets with the same 16-bit Identification value are stored in the same buffer, at the offset specified by the fragment offset field specified in the packet header.
Packets which are incomplete remain stored in the buffer until either all fragments are received, OR a timer expires, indicating that the receiver does not expect to receive any more fragments. Completed packets are forwarded to the next higher protocol layer.
3)
Path MTU Discovery allows a sender to fragment/segment a long internet packet, rather than relying on routers to perform IP-level fragmentation. This is more efficient and more scalable. It is therefore the recommended method in the current Internet. This is also the only method supported in IPv6.
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