2. What is fragmentation?
Splitting up things into smaller more
manageable entities for incorporating
ease in use and flexibility in transport.
Where could it possibly come in
Networks ?
3. Each n/w imposes some maximum
size on its packets, the reason being;
Hardware (frame size)
Operating System (buffers size 512 bytes)
Protocols (no. of bits in packet length field)
Compliance with some standard
To reduce error-induced retransmission to
some level
Prevent packet from choking channel
4. Problems
A large packet wants to travel through
a network whose maximum packet
size is small.
What if the original source packet is
too large to be handled by the
destination network?
Routing algorithms to prevent such
situation (not applicable)
5. Fragmentation in networks
Allow gateways to break up packets
into fragments, sending each packet
as a separate internet packet
Breaking up a thing is easy, but fixing
it back is not that easy
8. Transparent Fragmentation
Caused by a “small packet” network
transparent to any subsequent networks
through which the packet may pass on its
way to the destination
Gateways fragment oversized packets,
each addressed to the same exit gateway,
where they are recombined
The process carries on until destination is
reached
9. Problems
Exit gateway should know that it has
received all fragments to start
combination sequence
All packets must exit via the same
gateway, else data loss occurs
Overhead required to fragment and
reassemble at each gateway
10. Non transparent Fragmentation
Refrain from recombining fragments
at intermediate gateways
Fragments are considered as packets
and are passed through the gateways
Recombination occurs only at the
destination host
IP follows this methodology
11. Problems
Requires all hosts to do Reassembly
Increases overhead, larger the
packet, more the headers
An advantage being multiple exit
gateways can be employed increasing
the efficiency of the transmission
12. Schemes
Fragments should be numbered in a way
that aids recombination
Trees can be used
0 can be split as 0.1, 0.2, 0.3 …..
These can be again fragmented as 0.0.0,
0.0.1, 0.0.2, …….., 0.1.0, 0.1.1 etc.
Only if enough space is reserved in header
such terminology can be employed
Sufficient to reorder fragments
