Using path MTU discovery, a source host assumes that the path MTU is the MTU of the first hop (which is known). The host sends all datagrams on that path with the “don‘t fragment” (DF) bit set which restricts fragmentation. If any of the datagrams must be fragmented by an Extreme switch along the path, the Extreme switch discards the datagrams and returns an ICMP (Internet Control Message Protocol) Destination Unreachable message to the sending host, with a code meaning "fragmentation needed and DF set." When the source host receives the message (sometimes called a “Datagram Too Big” message), the source host reduces its assumed path MTU and retransmits the datagrams.
The source host sets the path MTU low enough that its datagrams can be delivered without fragmentation.
The source host does not set the DF bit in the datagram headers.
If it is willing to have datagrams fragmented, a source host can choose not to set the DF bit in datagram headers. Normally, the host continues to set DF in all datagrams, so that if the route changes and the new path MTU is lower, the host can perform path MTU discovery again.
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