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[lwip-devel] [patch #5834] Don't fragment flag


From: David Empson
Subject: [lwip-devel] [patch #5834] Don't fragment flag
Date: Mon, 02 Apr 2007 23:12:25 +0000
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3

Follow-up Comment #15, patch #5834 (project lwip):

It doesn't seem to me that associating the state of the IP_DF flag with
IP_REASSEMBLY is a reasonable choice. They apply to opposite directions of
the communication dialogue.

If a host is sending frames with DF set, it is an instruction to routers
along the way not to fragment the packet, so the other host will not receive
fragmented IP packets. This makes sense if the other host doesn't support
reassembly.

The receiver will generally pay no attention to the state of this flag
(except possibly in a case like replying to a ping).

DF flags in the transmit direction therefore convey little or no information
to the other host that they should also be using DF in the opposite
direction.

It only makes sense to associate it with IP_REASSEMBLY if both ends of the
communication are running LWIP with the same options.

To do it correctly, each host needs prior knowledge of whether the other host
supports fragmentation, so I think this should be a configurable option for
each UDP or TCP connection, possibly with a default setting which is
configurable in lwipopts.h or similar.

The default should be to allow fragmentation, as in most cases LWIP is
probably being used to talk to a "full" TCP/IP implementation.

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