IP Fragmentation - DF Flag

IP Fragmentation Flags

• Don't Fragment (DF) Flag:

  • The lecture mentions setting the "Don't Fragment" flag within an IP packet.
  • The DF flag instructs routers along the path not to fragment the packet.
  • If a router encounters a packet with the DF flag set but needs to fragment it to forward it over a link with a smaller MTU (Maximum Transmission Unit), the router will drop the packet and typically send an ICMP "Fragmentation Needed and DF Set" message back to the source.

• Purpose of DF Flag:

  • Path MTU Discovery: The DF flag is commonly used to perform Path MTU Discovery (PMTUD). PMTUD is a technique where the source host attempts to determine the smallest MTU along the path to the destination.
  • Avoiding Fragmentation: Fragmentation can be resource-intensive for both routers and the destination host (which has to reassemble the fragments). Avoiding fragmentation can improve network performance.

• Flagging Packet:

  • The lecture references "flagging" a packet, which refers to setting specific flags, such as the DF flag, within the IP header.