Re: [Qemu-devel] [RFC PATCH 3/3] filter-rewriter: rewrite tcp packet to keep secondary connection

[Jason Wang <jasowang@redhat.com>]

On 2016年06月29日 14:13, Zhang Chen wrote:
>
>
> On 06/29/2016 09:55 AM, Jason Wang wrote:
>>
>>
>> On 2016年06月28日 14:33, Zhang Chen wrote:
>>>>>
>>>>>>
>>>>>>> primary guest response 
>>>>>>> pkt(seq=primary_seq+1,ack=client_seq+1+data_len,flag=ACK)
>>>>>>> secondary guest response 
>>>>>>> pkt(seq=secondary_seq+1,ack=client_seq+1+data_len,flag=ACK)
>>>>>>
>>>>>> Is ACK a must here?
>>>>>
>>>>> Yes.
>>>>>
>>>>
>>>> Looks not, e.g what happens if guest does not use piggybacking acks?
>>>>
>>>>
>>>
>>> If guest does not use piggybacking acks, it will send a independent 
>>> packet for ack.
>>> we will get this packet.
>>> like:
>>> pkt(seq=xxxx,ack=xxx,flag=ACK). 
>>
>> Right, so looks like if guest want to send some data too, it can send 
>> tcp packet without ACK set?
>
> NO, I tried it. the tcp packet always has ACK set except the SYN 
> packet and FIN packet.
> you can dump the packet to see it.
>
> Thanks
> Zhang Chen

Right, RFC said:

"   If the ACK control bit is set this field contains the value of the
     next sequence number the sender of the segment is expecting to
     receive.  Once a connection is established this is always sent.
"