[ovs-discuss] SKB is modified 42 bytes in ovs module！help！

[wenxu]

>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" <jesse at nicira.com> wrote:
>On Mon, Apr 20, 2015 at 3:00 AM, frankzfz <frankzfz at 163.com> 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.
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