5.3 Fragmentation
IP datagrams are wrapped into link frames. The link protocols only enable data transmission within their frames up to a certain maximum limit. The maximum data size that can be inserted into one link frame is called Maximum Transfer Unit (MTU).
|
Link Protocol |
MTU |
|---|---|
|
Ethernet II |
1500 |
|
Ethernet 802.3 SNAP |
1492 |
|
Frame Relay |
1600 |
|
FDDI |
4478 |
It is obvious from looking at the previous table that most link protocols have MTU in ones of kilobytes. On lines connecting remote locations, we also sometimes encounter an MTU smaller than 1 KB. The total field length of an IP datagram, however, is 16 bits long; so theoretically, it is possible to create an IP datagram up to 64 KB long.
But what happens when an IP datagram on route from the sender to the recipient encounters a router (see Router 2 in Figure 5.14), from which the line leading in the direction of the recipient has an MTU less than the size of our IP datagram?
Figure 5.14: MTU between the router and the recipient
The...
