[9fans] Re: IP_DF bit and broken connections

[Fco.J.Ballesteros <nemo@plan9.escet.urjc.es>]

This is what we found regarding broken connections when
going through some routers. The setting is

	Plan 9 <-⋯ether⋯-> adsl router <--> Linux.

Linux always sets the DF bit in outgoing packets.
Plan 9 doesnt, but setting it does not change things (we tested that).

Some routers eat packets with size 1500 (eg.  adsl from telefonica in
spain) without fragmenting or sending icmps. Too bad.

With icmp (sized 1500), Linux sends two fragments despite the mtu
being big enough.  We get both fragments in Plan 9 but since they came
with the DF bit set, the reply is dropped by the if in ip.c.
Perhaps once reassembled, the DF bit could be cleared; I don't know.

With tcp, Linux has its mtu at 1500 and negotiates its mss of 1460.
The same does Plan 9, but despite this, Linux sends packets of at most
1444 bytes when going through the router and Plan 9 sends up to 1500
(which IMHO is legitimate). Thus, the linux packets get through and our
1500 packets don't get through. (The connection gets hanged so that
Plan 9 retransmits the poor 1500 packet once and again).

The workaround is simply to adjust the mtu not to reach 1500. Doing so
either on Linux or on Plan 9 lets things work (because the negotiated
mss is smaller).

I don't know how linux discovers that it shouldn't send 1500 bytes.
There are no icmp packets, and the negotiated mss is the same (as I
said before).