Friday, 20 September 2013

WiFi: Layer-3 fragmentation vs 802.11 Fragmentation

We all know that fragmentation of packets happens at layer-3. 802.11 standard also specifies fragmentation of frames. Generally 802.11 fragmentation happens at network driver level. However there are few other trivia  worth of discussing.

While layer-3 decides to fragment packets based on MTU (Maximum Transmission Unit), in wireless (802.11/WiFi) the decision is taken based on a constant called, "Fragmentation Threshold".  This fragmentation threshold is generally configurable.

Command to set Threshold
On Linux, for layer-3, MTU can be configured by ifconfig and 802.11 fragmentation threshold can be configured using iwpriv.

Sequence Number
Please note that we refer to the sequence number from 802.11 header in the following.

When packets are fragmented at layer-3, each fragment is given as a separate packet to the wireless network driver and they are considered as different frames. Each frame gets a different sequence number.

In the case of 802.11 fragmentation, the frames are fragmented by the network driver and each fragment gets the same sequence number with different fragment number.

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