On 04/18/2018 11:12 AM, Alexander Duyck wrote:

On Wed, Apr 18, 2018 at 10:28 AM, David Miller [off-list ref] wrote:

quoted

From: Sowmini Varadhan <redacted>
Date: Wed, 18 Apr 2018 08:31:03 -0400

quoted

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.

My only concern with the patch set is verifying what mitigations are
in case so that we aren't trying to set an MSS size that results in a
frame larger than MTU. I'm still digging through the code and trying
to grok it, but I figured I might just put the question out there to
may my reviewing easier.

Also any plans for HW offload support for this? I vaguely recall that
the igb and ixgbe parts had support for something like this in
hardware. I would have to double check to see what exactly is
supported.

Alex,

If by HW support you meant UFO (UDP Fragmentation Offload), then I have
dig into that last year using ixgbe. And I found that Intel 10G HW does
break large UDP packets into MTU size however it does not generate
*true* IP fragments. Instead, when large (> MTU) size UDP packet is
given to NIC, HW generates unique UDP packets with distinct IP
fragments. This makes it impossible for receiving station to reassemble
them into one UDP packet.

I am not sure about igb!

-Tushar

Thanks.

- Alex