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The Internet Protocol (IP) implements datagram fragmentation, breaking it into smaller pieces, so that packets may be formed that can pass through a link with a smaller maximum transmission unit (MTU) than the original datagram size. RFC 791 describes the procedure for IP fragmentation, and transmission and reassembly of datagrams.〔RFC 791, ''Internet Protocol'', Information Sciences Institute (September 1981)〕 RFC 815 describes a simplified reassembly algorithm.〔RFC 815, ''IP Datagram Reassembly Algorithms'', David D. Clark (July 1982)〕 The ''Identification'' field, and ''Fragment'' offset field along with ''Don't Fragment'' and ''More Fragment'' flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams. In a case where a router receives a protocol data unit (PDU) larger than the next hop's MTU, it has two options if the transport is IPv4: drop the PDU and send an Internet Control Message Protocol (ICMP) message which indicates the condition ''Packet too Big'', or fragment the IP packet and send it over the link with a smaller MTU. IPv6 hosts are required to determine the optimal Path MTU before sending packets; however, it is guaranteed that any IPv6 packet smaller than or equal to 1280 bytes must be deliverable without the need to use IPv6 fragmentation. If a receiving host receives a fragmented IP packet, it has to reassemble the datagram and pass it to the higher protocol layer. Reassembly is intended to happen in the receiving host but in practice it may be done by an intermediate router, for example, network address translation (NAT) ''may'' need to re-assemble fragments in order to translate data streams, description provided in RFC 2993.〔RFC 2993, ''Architectural Implications of NAT'' (November 2000)〕 IP fragmentation can cause excessive retransmissions when fragments encounter packet loss and reliable protocols such as TCP must retransmit all of the fragments in order to recover from the loss of a single fragment.〔(【引用サイトリンク】url=http://www.cs.binghamton.edu/~nael/classes/cs528/fragment.pdf )〕 Thus, senders typically use two approaches to decide the size of IP datagrams to send over the network. The first is for the sending host to send an IP datagram of size equal to the MTU of the first hop of the source destination pair. The second is to run the path MTU discovery algorithm,〔RFC 1191, ''Path MTU Discovery'' (November 1990)〕 described in RFC 1191, to determine the path MTU between two IP hosts, so that IP fragmentation can be avoided. == Impact of fragmentation on network forwarding == When a network has multiple parallel paths, technologies like LAG and CEF split traffic across the paths according to a hash algorithm. One goal of the algorithm is to ensure all packets of the same flow are sent out the same path to minimize unnecessary packet reordering. If the hash algorithm uses TCP/UDP port numbers, the packet fragments may be forwarded out on different paths since only the first fragment has layer-4 information. This can lead to non-initial fragments arriving before the initial fragment, which some hosts and security devices treat as an error condition, and drop. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IP fragmentation」の詳細全文を読む スポンサード リンク
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