>What does it mean that the packet is modified? What specifically happens?
The VM sends a packet with 1500 bytes and the ovs will encapsulate the packet with gre tunnel and the packet length will larger than 1500. So the packet will be fragment to 2 packets. On the receiver side we find the last 42 bytes(user data) of the second packet was tampered by linux os. I think it's a matter with ovs and reassembly in linux kernel. if we set the mtu of VM to 1458, the problem is gone. At 2015-04-21 03:44:57, "Jesse Gross" <[email protected]> wrote: >On Mon, Apr 20, 2015 at 3:00 AM, frankzfz <[email protected]> wrote: >> >> Dear： >> Test scenario: two KVM guests sitting in different hosts communicate to each >> other with a GRE tunnel. >> >> >> All interface MTU is default 1500 Bytes, from guest point of view, its skb >> gso_size could be as bigger as 1448 Bytes, however after guest skb goes >> through GRE encapuslation, individual segments length of a gso packet could >> exceed physical NIC MTU 1500, which will be possible modified at recevier >> side.So Sender have to fragment the IP datagrams that are larger than the >> interface MTU . >> >> The packets is modified 42 bytes in ovs module at receiver side.42 bytes >> is equal to GRE encapsulation header（Ethernet header（14） + IP Header （20）+ >> GRE Header（8））. >> >> If interface MTU is modified 1458 Bytes, after guest skb goes through GRE >> encapsulation, IP fragmentation will not occur at sender side.SKb is good at >> receiver side. This mistake didn't happen at receiver side. >> >> Why SKB is modified 42 bytes at receiver side through OVS modules for >> fragmentation packets. > >What does it mean that the packet is modified? What specifically happens? > >However, as a general rule, Linux does not support fragmentation of >tunnel packets and OVS inherits this behavior. >_______________________________________________ >discuss mailing list >[email protected] >http://openvswitch.org/mailman/listinfo/discuss
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