Lectures notes for 2010 - KTH

Fragmentation
• If an IP datagram is larger than the MTU of the link layer, it must be
divided into several pieces ⇒ fragmentation
• Fragmentation may occur multiple times
• as a fragment might need to go across a link with an even smaller MTU!
• Both hosts and routers may fragment
• However, only destination host reassemble!
– as fragments only need to come together at the final host - thus the fragments can take
different paths though the network
– also reassembly requires waiting for the other fragments - so doing this earlier in the network
could add unnecessary delay
• Each fragment is routed separately (i.e., as independent datagram)
• TCP uses either 576 byte MTU or path MTU discovery
Maguire Fragmentation 3: 7 of 33
maguire@kth.se 2010.03.21 Internetworking/Internetteknik