netdev - Re: [PATCH] udp: Allow to defer reception until connect() happened

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Date:   Thu, 29 Nov 2018 14:47:03 -0800
From:   Christoph Paasch <cpaasch@...le.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     jri.ietf@...il.com, netdev <netdev@...r.kernel.org>,
        Ian Swett <ianswett@...gle.com>, lhedstrom@...le.com
Subject: Re: [PATCH] udp: Allow to defer reception until connect() happened

Hello,

On 28/11/18 - 19:15:12, Eric Dumazet wrote:
> On Wed, Nov 28, 2018 at 7:09 PM Jana Iyengar <jri.ietf@...il.com> wrote:
> >
> >
> > On Wed, Nov 28, 2018 at 6:19 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>
> >> On Wed, Nov 28, 2018 at 5:57 PM Christoph Paasch <cpaasch@...le.com> wrote:
> >> >
> >> > There are use-cases where a host wants to use a UDP socket with a
> >> > specific 4-tuple. The way to do this is to bind() and then connect() the
> >> > socket. However, after the bind(), the socket starts receiving data even
> >> > if it does not match the intended 4-tuple. That is because after the
> >> > bind() UDP-socket will match in the lookup for all incoming UDP-traffic
> >> > that has the specific IP/port.
> >> >
> >> > This patch prevents any incoming traffic until the connect() system-call
> >> > is called whenever the app sets the UDP socket-option
> >> > UDP_WAIT_FOR_CONNECT.
> >>
> >> Please do not add something that could mislead applications writers to
> >> think UDP stack can scale.
> >
> >
> >> UDP stack does not have a full hash on 4-tuples, it means that
> >> incoming traffic on a 'shared port' has
> >> to scan a list of XXX sockets to find the best match ...
> >
> >
> >> Also you add another cache line miss in UDP lookup to access
> >> udp_sk()->wait_for_connect.

Fair enough. We could add the socket later to the hash (see below).

> >>
> >> recvfrom() can be used to filter whatever frame that came before the connect()
> >
> >
> > I don't think I understand that argument -- connect() is supported for UDP sockets, and UDP sockets are being used for serving QUIC traffic. Are you suggesting that connect() never be used?
> 
> If the source port is not shared, Christoph patch is not needed.
> 
> If it is shared, then a whole can of worm is opened.
> 
> Trying to hack UDP stack while it is not fully 4-tuple ready is not
> going to fly.

Indeed, the UDP-stack is not fully 4-tuple ready.


What are your thoughts on getting it there?

Should be doable by simply using a similar approach as TCP, no? Any caveats
you see with that?

Then, when one sets the "wait-for-connect"-flag we would add the socket to
the hash-table only at connect()-time also addressing the cache-line miss
you mentioned above.


Thanks,
Christoph