IP Fragment Reassembly Bug Identified and Corrected (99932)

CAUSE

Many wide-area networks such as the Internet have links that don't support full-sized Ethernet or token ring frames. Routers that send packets over these links are forced to break them up into fragments at the IP layer. Each fragment of a given packet is labeled with a fragment ID# so that they can be reassembled into one packet by the IP layer of the destination machine.

LAN Manager TCP/IP machines reassemble packets into control blocks (CBs). Once the first fragment of a packet is received, a control block is allocated for reassembling that packet. If all the fragments composing that packet are not received within a time-out period, the fragments in the CB should be discarded, and the CB returned to the free list for future use. LAN Manager versions up to and including 2.2 had problems:

• allocating CBs at system initialization
• timing out CBs
• sending ICMP "time exceeded" messages back to the originating host