netdev - Re: how to search for the best route from userspace in netlink?

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Message-ID: <CAA93jw5KLS2be7ZhaiCOM3Jz-TsmQBY=z7iF0Oq6QU6=mQH8pA@mail.gmail.com>
Date:   Tue, 3 Sep 2019 02:29:20 -0700
From:   Dave Taht <dave.taht@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: how to search for the best route from userspace in netlink?

On Mon, Sep 2, 2019 at 8:13 PM David Ahern <dsahern@...il.com> wrote:
>
> On 9/2/19 4:07 PM, Dave Taht wrote:
> > Windows has the "RtmGetMostSpecificDestination" call:
> > https://docs.microsoft.com/en-us/windows/win32/rras/search-for-the-best-route
> >
> > In particular, I wanted to search for the best route, AND pick up the
> > PMTU from that (if it existed)
> > for older UDP applications like dnssec[1] and newer ones like QUIC[2].
>
> RTM_GETROUTE with data for the route lookup. See iproute2 code as an
> example.

Yes. I really didn't describe my thinking very well. It's coping with
pmtu better
in the case of a more increasingly udp'd and tunneled internet. tcp
(being kernel based)
will do the probe and cache that attribute of the path, udp does not.
A udp based app with root privs could be setting it after figuring it
out,  a userspace one cannot.

for more detail from server-al sides of that philosophical debate,
please see the links I posted
originally.

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740