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

[Christoph Paasch <cpaasch at apple.com>]

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On 07/11/17 - 15:23:42, Rao Shoaib wrote:
> 
> 
> On 11/07/2017 01:15 PM, Christoph Paasch wrote:
> > hese are meta-level retransmissions that are being sent on a different
> > subflow and/or on the same subflow but with new TCP sequence numbers and a
> > new DSS-mapping. These indeed end up going through mptcp_skb_entail().
> > 
> > The retransmissions I mean are the TCP-level retransmissions (aka.,
> > fast-retransmits, tail-loss-probe, RTO,...). They don't go through
> > mptcp_skb_entail again.
> > 
> > I will take a look at the trace in the other mail.
> > 
> > 
> > Christoph
> RTO will go through this code.
> Partial ACK and fast-retransmit etc are fine if they are transmitted with
> the same mapping (or else DSS will fail as well) In fact it is required. On
> the receiver an adjustment is made for the TCP flow's seq number.  See
> mptcp_detect_mapping() it requires that the (partial)  skb has the exact
> same mapping as if it was transmitted as part of the original skb and than
> look at mptcp_validate_mapping() and mptcp_prepare_skb() that adjust the
> data sequence number based on the tcp sequence number of the packet.
> 
> If not than can you explain how the current mechanism works and what happens
> to the DSS settings in case of retransmission. Where/How does it gets
> updated because mptcp_options_write() only does a copy with the updated ACK.

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

> 
> When I have some more time I will verify by hacking the kernel. Sorry I can
> not upload the complete kernel as it is a mess right now, I changed a lot of
> things, that is why it has taken this long to even get ssh working. I will
> try to get you just the patch for this fix.
> 
> Shoaib