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Tcpmssd

May 26, 2018

Utility to correct requested TCP receive segment size

tcpmssd was written by Ruslan Ermilov ru@FreeBSD.org based on work done by Patrick Bihan-Faou patrick@mindstep.com.

tcpmssd is a divert4 program that adjusts outgoing TCP data so that the requested segment size is not greater than the amount allowed by the interface MTU.

This is necessary in many setups to avoid problems caused by routers that drop ICMP Datagram Too Big messages. Without these messages, the originating machine sends data, it passes the rogue router then hits a machine that has an MTU that is not big enough for the data. Because the IP Don’t Fragment option is set, this machine sends an ICMP Datagram Too Big message back to the originator and drops the packet. The rogue router drops the ICMP message and the originator never gets to discover that it must reduce the fragment size or drop the IP Don’t Fragment option from its outgoing data.