Re: [MPTCP] [PATCH 1/2] skbuff: Add shared control buffer

[Christoph Paasch <cpaasch at apple.com>]

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Hello,

On 08/11/17 - 12:41:31, Rao Shoaib wrote:
> 
> 
> On 11/07/2017 04:25 PM, Christoph Paasch wrote:
> > The way it is handled currently is that the mapping is decided during
> > mptcp_skb_entail(), which writes the mapping to the skb->cb.
> > 
> >  From that moment on it won't change anymore and, as you know, it will simply
> > get copied from the skb->cb to the TCP-header in mptcp_options_write().
> > 
> > 
> > Christoph
> Yes thanks I get it. And the receiver is coded that way. That is very
> implementation specific.
> 
> RFC 6824 Says:
> 
> 
>    A data sequence mapping does not need to be included in every MPTCP
>    packet, as long as the subflow sequence space in that packet is
>    covered by a mapping known at the receiver.  This can be used to
>    reduce overhead in cases where the mapping is known in advance; one
>    such case is when there is a single subflow between the hosts,
>    another is when segments of data are scheduled in larger than packet-
>    sized chunks.
> 
> So DSS mapping is not required in every packet.

Yes, and currently Linux supports that gracefully.

> There could also be a sub
> mapping for the segment itself that does not violates the original larger
> mapping or there can be a super mapping which extends the current mapping
> without violating it. Current code does not handle that, I will update the
> code.

The difficulty here is in the DSS checksum verification. When you have
different overlapping mappings, you will need to do quite some magic to
verify the checksums.


Christoph

> Thanks for pointing out this case as I had not tested partial ACK. Without
> partial ACK everything will work. Luckily the fix is very straight forward.
> 
> Regards,
> 
> Shoaib
> 
>