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

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Date:   Wed, 18 Apr 2018 08:31:03 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH RFC net-next 00/11] udp gso

I went through the patch set and the code looks fine- it extends existing
infra for TCP/GSO to UDP.

One thing that was not clear to me about the API: shouldn't UDP_SEGMENT
just be automatically determined in the stack from the pmtu? Whats
the motivation for the socket option for this? also AIUI this can be
either a per-socket or a per-packet option?

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.  

One thought to recover from this: use the infra being proposed in
  https://tools.ietf.org/html/draft-touch-tsvwg-udp-options-09
to include a new UDP TLV option that tracks datagram# (similar to IP ID)
to help the receiver reassemble the UDP datagram and pass it up with
the POSIX-conformant UDP message boundary. I realize that this is also
not a perfect solution: as you point out, there are risks from
packet re-ordering/drops- you may well end up just reinventing IP
frag/re-assembly when you are done (with just the slight improvement
that each "fragment" has a full UDP header, so it has a better shot
at ECMP and RSS).

--Sowmini