netdev - Re: [PATCH RFC net-next 00/11] udp gso

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Date:   Wed, 18 Apr 2018 13:28:08 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     sowmini.varadhan@...cle.com
Cc:     willemdebruijn.kernel@...il.com, sridhar.samudrala@...el.com,
        netdev@...r.kernel.org, willemb@...gle.com
Subject: Re: [PATCH RFC net-next 00/11] udp gso

From: Sowmini Varadhan <sowmini.varadhan@...cle.com>
Date: Wed, 18 Apr 2018 08:31:03 -0400

> However, I share Sridhar's concerns about the very fundamental change
> to UDP message boundary semantics here.  There is actually no such thing
> as a "segment" in udp, so in general this feature makes me a little
> uneasy.  Well behaved udp applications should already be sending mtu
> sized datagrams. And the not-so-well-behaved ones are probably relying
> on IP fragmentation/reassembly to take care of datagram boundary semantics
> for them?
> 
> As Sridhar points out, the feature is not really "negotiated" - one side
> unilaterally sets the option. If the receiver is a classic/POSIX UDP
> implementation, it will have no way of knowing that message boundaries
> have been re-adjusted at the sender.  

There are no "semantics".

What ends up on the wire is the same before the kernel/app changes as
afterwards.

The only difference is that instead of the application doing N - 1
sendmsg() calls with mtu sized writes, it's giving everything all at
once and asking the kernel to segment.

It even gives the application control over the size of the packets,
which I think is completely prudent in this situation.