Xen devel - Nov 2012 - TCP Segment offloading + DF

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.

Feedback welcome,
(even "You are an idiot, of course it''s  going to fail !" :)
)
Killian De Volder

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
> 
> 
> 
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