[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] TCP Segment offloading + DF
On Tue, Nov 13, 2012 at 03:22:16PM +0100, Killian De Volder wrote: > Hello, > > I don't know if TCO is supported for XEN_NETDEV_FRONTED in kernel 3.1.6, > however I found an issue. > > I had to track down an issue today: > Packet from a local machine didn't get routed to the internet. > After long searching I found the issue: > > The TCP-Segmentation-offloading assembles fragments with Do Not Fragment set. > This introduces issues when the packet is to be rerouted trough a router. > > Situation: > > DOM0 > |->Bridge 1 - MTU 1500 > | |-> PHY - eth0 > | |-> VIF - Dom1 > | \-> VIF - Dom2 (router) > | > \->Bridge 2 - MTU 1500 > |-> PHY - eth1 > \-> VIF - Dom2 (router) > > Dom 1 <-> VIF <--(bridge 1)--> VIF <-> dom2 <-> VIF <--(bridge 2)--> > > Practical example: > Dom1 generate 2 packet to be routed to bridge 2 from bridge 1. > Packet 1: 1300 bytes DF (TCP) > Packet 2: 400 bytes DF (TCP) > > The TCP-offloading throws them together and passes 1 packet: 1700 bytes DF > (TCP). > Smart thing to do, reduces load etc. > > HOWEVER > Then it arrives at dom2, it looks at the packets, sees a 1700 byte packet, > and sees the DF. > Dom2 would put it on the bridge 2 network, fragmented, but it's not allowed, > so instead it drops the packet. > > I'm not sure if it's "working as designed" or if this is an unfortunate side > effect. That sounds like an unfortunate side effect. Is there a reason that the bridge decides it is not allowed? Is that b/c of its MTU and the 'DF' bit? Or is the netback on the bridge the one that can't handle DF packets and hence the bridge drops it? > > Feedback welcome, > (even "You are an idiot, of course it's going to fail !" :) ) > Killian De Volder > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@xxxxxxxxxxxxx > http://lists.xen.org/xen-devel > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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