defragmentation and fragment cache

defragmentation and fragment cache

quoted from professional linux kernel architecture by Wolfgang Mauerer, page 772-773.

1.The kernel manages the fragments of an originally composite packet in a separate cache known as a fragment cache. In the cache, fragments that belong together are held in a separate wait queue until ALL fragments are present.

2. The fragment cache uses a timer mechanism to remove fragments from the cache. When it expires, fragments in the cache are deleted if not all fragments have arrived by then.

3. The kernel recognizes that a packet is fragmented either by the set fragment bit or by a non-zero value in the offset field. A zero value in the offset field field indicated that this fragment is the LAST in the packet.