[OpenAFS-devel] nojumbo on server doesn't disable jumbograms on clients

[Thomas J. Moore]

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While tracing a one-way performance problem with our AFS server (MTU 1500), I 
found that on the client side (MTU 8000), every few packets it tried to send a 
jumbogram, overriding the negotiated MTU of 1500.  Somebody along the way seems 
to not fragment these packets, so they just get dropped.  1/2 second later, 
normal packets resume, but only for a few packets before it happens again. 
Needless to say, this causes a serious performance degradation, even though 
transfers eventually succeed.  To fix this, rather than fixing the router and 
encountering the problem again the future to some other random client, I wanted 
to just disable jumbograms.  It turns out that the (mostly undocumented) 
-nojumbo option on the servers only works for the server, but not the client. 
Since I didn't see an easy way to turn off jumbo packets on the client, and 
didn't want to force all users to patch their client source, I made the 
following patch to force the advertised max jumbogram size to 1 when the 
-nojumbo option is specified (note: may not apply, since my email client 
probably screws up whitespace, but it's only one line, and you get the point):

diff -ruN src/rx/rx_user.c src/rx/rx_user.c
--- src/rx/rx_user.c    2005-04-03 14:18:56.000000000 -0400
+++ src/rx/rx_user.c    2006-03-31 08:30:36.000000000 -0500
@@ -689,6 +689,7 @@
   {
       rx_maxReceiveSize = OLD_MAX_PACKET_SIZE;
       rxi_nSendFrags = rxi_nRecvFrags = 1;
+    rxi_nDgramPackets = 1;
   }

   /* Override max MTU.  If rx_SetNoJumbo is called, it must be


This causes the client to disable jumbograms as well when the -nojumbo option is 
given to the server, and tranfers are fast again.  Actually, I'm not sure what 
setting the maxReceiveSize to OLD_MAX_PACKET_SIZE has to do with jumbograms, but 
I left it in anyway.  I hope it doesn't force the packet size to 1500 if/when we 
change the MTU on the AFS servers to 8000.

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