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The DF flag set in IPv4 packet header means _________.
Damaged frame
Don't fragment
Don't forward
Done forwarding
Compliments of Scott Hogg, NetworkWorld: http://www.networkworld.com/article/2224654/cisco-subnet/mtu-size-issues.html. Fragmentation
IPv4 routers fragment on behalf of the source node that is sending a larger packet. Routers can fragment IPv4 packets unless the Do-Not-Fragment (DF) bit is set to 1 in the IPv4 header. If the DF bit is set to 0 (the default), the router splits the packet that is too large to fit into the outgoing interface and send the two packets toward the destination. When the destination receives the two fragments, then the destination's protocol stack must perform reassembly of the fragments before processing the Protocol Data Unit (PDU). The danger is when an application sends its packets with DF=1 and does not pay attention to the ICMP “packet too big” messages and does not perform PMTUD.
All IPv6 networks must support an MTU size of 1280 bytes or greater (RFC 2460). This is because IPv6 routers do not fragment IPv6 packets on behalf of the source. IPv6 routers drop the packet and send back an ICMPv6 Type 4 packet (size exceeded) to the source indicating the proper MTU size. It then falls on the shoulders of the source to perform the fragmentation itself and cache the new reduced MTU size for that destination so future packets use the correct MTU size.
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