Differentiate between transparent and nontransparent fragmentation?

A number of problems are encountered because of the size of the data packets. There is no ability in the data link layer by means of which it could handle these problems and so the bridges also don’t work here. 

The Ethernet also experiences a number of problems because of the following:

Ø  Different way in which the maximum packet size is defined.

Ø  Maximum packet size that can be handled by a router.

Ø  The maximum length slot that are used for transmission

Ø  Errors due to the packet length

Ø  Standards

The data packets can be fragmented in two ways namely:

1. Transparent and
2. Non – transparent

Both these ways can be followed based on a network by network basis. We can also say that no such end – to – end agreement exists based up on which it can be decided which process is to be used.

Transparent Fragmentation: 

- This type of fragmentation is followed when a packet is split in to smaller fragments by a router.

- These fragments are sent to the next router which does just the opposite i.e., it reassembles the fragments and combine them to form original packet. 

- Here, the next network does not come to know whether any fragmentation has taken place. 

- Transparency is maintained between the small packet networks when compared to the other subsequent networks.

- For example, transparent fragmentation is used by the ATM networks by means of some special hardware. 

- There are some issues with this type of fragmentation. 

- It puts some burden on the performance of the network since all the fragments have to be transmitted through the same gateway. 

- Also, sometimes the repeated fragmentation and reassembling has to be done for small packet network in series. 

- Whenever an over-sized packet reaches a router, it is broken up in to small fragments. 

- These fragments are transported to the next exit router. 

- The fragments are assembled by this exit router which then forwards them to the next router.

- Awareness regarding this fragmentation is not maintained for the subsequent networks. 

- For a single packet fragmentation is done many times before the destination is finally reached. 

- This of course consumes a lot of time because the repeated fragmentation and assembling has to be carried out. 

- Sometimes, it also presents the reason of corrupting the packet’s integrity.

Non-Transparent Fragmentation: 

- In this type, the packet is split in to fragments by one router. 

- But the difference is that these fragments are not reassembled until the fragments reach their destination. 

- They remain split till then. 

- Since in this type of fragmentation the fragments are assembled only at the destination host, the fragments can be routed independent of each other. 

- This type of fragmentation also experiences some problems such as header has to be carried by each of the fragments till they reach their destination. 

- Numbering has to be done for all the fragments so that no problem is experienced in reconstructing the data stream.

Whichever type of fragmentation we use, one thing has to be made sure which is that later we should be able to form the original packets using the fragments. This insists on having some type of labeling for the fragments. 

Segmentation is another name for the fragmentation. A packet is injected in to the data link layer by the IP layer but it is not responsible for reliable transmission of the packets. Some maximum value on the size of the packets is imposed by each layer for their reasons. For a large packet that travels through the network for which the MTU is small, fragmentation is very much needed. 

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.