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Thursday, September 19, 2013

What is fragmentation?

- The fragmentation technique is implemented in the IP (internet protocol) for breaking down the datagrams into smaller pieces. 
- This is done so that it becomes easy for the data packets to be passed through the link with a datagram size smaller than that of the original MTU or the maximum transmission unit. 
- The procedure for the IP fragmentation along with the procedures for reassembling and transmitting the datagrams is given in the RFC 791. 
- For determining the optimal MTU path, the IPv6 hosts are needed so that the packets can be sent. 
- If in case the PDU i.e., the protocol data unit received by the router is larger than the MTU of the next hop, then there are two options are available if IPv4 transport is being used:
Ø Dropping the PDU and sending an ICMP (internet control message protocol) message indicating that the condition packet is quite big.
Ø  Fragmenting the IP packet and then transmitting it over the link whose MTU is smaller. Any IPv6 packet with a size less than or equal to 1280 bytes can be delivered without having the need for using the IPv6 fragmentation.

- If a fragmented IP packet is received by the recipient host, its job is to reassemble the datagram and then send it over to the protocols at the higher layers. 
- The purpose of reassembling is expected to take place at the recipient’s host side but for some practical reasons it might be done by some intermediate router. 
- For example, the fragments might be reassembled by the NAT (network address translation) for translating the data streams. 
- Excessive re-transmission can result as a consequence of the IP fragmentation whenever packet loss might be encountered by the fragments. 
It is required for all the reliable protocols (example, TCP) for re-transmitting the fragments in their correct order for recovering from the single fragment loss. 
Thus, typically two approaches are used by the senders for determining datagrams of what size should be transmitted over the network:
  1. First approach: The sender must transmit an IP datagram of size as same as that of the first hop’s MTU.
  2. Second approach: Running the path MTU discovery algorithm.

- Fragmentation does leave an impact on the network forwarding. 
- When there are multiple parallel paths for the internet router the traffic is split by the technologies such as the CEF and LAG throughout the links via some hash algorithms. 
- The major goal of this algorithm is to make sure that all the packets with the same flow are transmitted out on the same path for the minimization of the not so required packet reordering. 
- If the TCP or UDP port numbers are used by the hash algorithm, the fragmented packets might be forwarded through different paths. 
- This is so because the layer 4 information is contained only in the first fragment of the packet. 
- As a result of this, usually the initial fragment arrives after the non-initial fragments. 
- This condition is often treated as an error by most of the security devices in the hosts.  
- Therefore, they drop these packets.
- The fragmentation mechanism differs in IPv4 and IPv6. 
- In the former, the fragmentation is performed by the router. 
- On the other hand, in IPv6 fragments that are larger than MTU are dropped by the routers.
- Also, in both the cases there is a variation in the header format. 
- Since fragmentation is carried out using analogous fields, therefore the algorithm can be used again and again for the purpose of fragmentation and reassembling. 
- A best effort should be made by the IPv4 hosts for reassembling the datagram fragments. 

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