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.
Additional information:
OVS version:2.3.0
Linux kernel version:Linux2.6.32
Hope to get your help! Thanks.
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