The second created new packet is then processed. The packet header field is identical to that of the original packet including the same value of the packet ID, the total length field, the more-fragments flag MF and the fragment offset field in the original packet.
The packet header field is updated with a new offset field, by adding the number of payload bytes sent in the first fragment. If this new packet is larger than the allowed link MTU, the packet is again fragmented. Any packet that has a more fragments MF flag set, must have an integral multiple of 8 bytes. The final fragment, which does not have this flag set, may have an arbitrary number of bytes.
IP Router fragmentation is not recommended in the modern Internet, and this feature was not carried-forward when the next generation Internet Protocol IPv6 was specified. This is more efficient and more scalable. It is therefore the recommended method in the current Internet. The processes of fragmentation and reassembly involve a number of IP header fields being set in the fragments. These header field values also change either because they normally would at every hop, or as a by-product of fragmentation:.
Enough of the theory, lets work through an example of a byte packet that must be routed through an interface with an MTU of bytes the packet size all hosts should be prepared to accept to comply with the IP RFC. The following fragments are created with the sizes and fields set as follows:.
We can confirm these fragments contain the bytes of data from the original packet with two different calculations the first being the most efficient :. Packets that must be fragmented but have the DF bit set are discarded. This is directly related to fragmentation and worth covering briefly so you have a full understanding the relationship between the two. Note that PMTUD operates independently in each direction along the path between two hosts if both hosts support it and have it enabled.
These paths and the MTU of the networks they comprise of may be different. PMTUD Helps to avoid issues with fragmentation but of course has issues of its own, the most common being that ICMP messages are blocked by firewalls, routers or other network devices somewhere on the path between the router generating the message s and the host they are destined for.
This is referred to as a black-hole connection. Fragmentation with IPv6 operates in a fundamentally different way to to that of IPv4, although most of the header fields remain and have the same purpose. In contrast with IPv These header field values also change in the standard header compared to the original unfragmented packet:.
UDP Is often used for real-time applications such as voice and video so fragmentation and reassembly are highly undesirable as they may introduce delay and jitter problems in addition to the numerous other issues fragmentation can cause. Being a connectionless protocol it is unable to use the MSS mechanism to even attempt to avoid fragmentation. Applications relying on UDP can only deal with this at the application layer.
A number of protocols such as the real-time transport protocol RTP and Session Initiation Protocol SIP can be used to establish a session state and help minimise or avoid the issues caused by IP fragmentation. RTP Actually has its own fragmentation mechanism.
A common simplistic approach is to just use a small packet size. All IPv4 hosts including routers should be capable of accepting byte packets for instance. To express this in table form as we did earlier:. With IPv4 things are more complicated; an initial timeout of 15 seconds is suggested when the first fragment is received. The TTL field value of each subsequent fragment received is then used to reset the timer value, as long as it is higher than the current value.
This allows for a gap of up to 4. Should the timer expire, all fragments are discarded. Attention reader! It is done by the network layer at the destination side and is usually done at routers. Source side does not require fragmentation due to wise good segmentation by transport layer i. Receiver identifies the frame with the identification 16 bits field in the IP header. Each fragment of a frame has the same identification number.
Receiver identifies the sequence of frames using the fragment offset 13 bits field in the IP header Overhead at the network layer is present due to the extra header introduced due to fragmentation. Fields in IP header for fragmentation — Identification 16 bits — use to identify fragments of the same frame. Fragment offset 13 bits — use to identify the sequence of fragments in the frame. It generally indicates a number of data bytes preceding or ahead of the fragment.
Reassembly of Fragments — It takes place only at the destination and not at routers since packets take an independent path datagram packet switching , so all may not meet at a router and hence a need of fragmentation may arise again. The fragments may arrive out of order also.
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