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Part III: Appendixes > Path MTU Discovery

C.5. Path MTU Discovery

Whenever a connection is about to be established, the three-way handshake opens negotiation, and the segment size used will be set to the minimum of either the smallest MTU of an outgoing interface or the Maximum Segment Size (MSS) announced by the peer. If the remote peer does not announce a MSS, usually the value 536 will be assumed. If path MTU discovery is active, all outgoing PDUs have the IP don't fragment (DF) option set.

If the ICMP error message "fragmentation needed" is received, this means that a router on the way to the destination needed to fragment the PDU, but was not allowed to do so. Therefore, the router discarded the PDU and sent back the ICMP error. Newer router implementations enclose the needed MSS in the error message. If the needed MSS is not included, the correct MSS must be determined by trial and error.


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