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

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Date:   Tue, 17 Apr 2018 23:33:26 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH RFC net-next 00/11] udp gso

On Tue, Apr 17, 2018 at 10:25 PM, Samudrala, Sridhar
<sridhar.samudrala@...el.com> wrote:
>
> On 4/17/2018 2:07 PM, Willem de Bruijn wrote:
>>
>> On Tue, Apr 17, 2018 at 4:48 PM, Sowmini Varadhan
>> <sowmini.varadhan@...cle.com> wrote:
>>>
>>> On (04/17/18 16:23), Willem de Bruijn wrote:
>>>>
>>>> Assuming IPv4 with an MTU of 1500 and the maximum segment
>>>> size of 1472, the receiver will see three datagrams with MSS of
>>>> 1472B, 528B and 512B.
>>>
>>> so the recvmsg will also pass up 1472, 526, 512, right?
>>
>> That's right.
>>
>>> If yes, how will the recvmsg differentiate between the case
>>> (2000 byte message followed by 512 byte message) and
>>> (1472 byte message, 526 byte message, then 512 byte message),
>>> in other words, how are UDP message boundary semantics preserved?
>>
>> They aren't. This is purely an optimization to amortize the cost of
>> repeated tx stack traversal. Unlike UFO, which would preserve the
>> boundaries of the original larger than MTU datagram.
>
>
> Doesn't this break UDP applications that expect message boundary
> preservation semantics? Is it possible to negotiate this feature?

A process has to explicitly request the feature with socket option
or cmsg UDP_SEGMENT. By setting that to gso size is signals
its intent to send multiple datagrams in one call.

Or were you responding to the hypothetical GRO example below?
Yes, that clearly would have to be limited to negotiated flows, not
unlike how foo-over-udp tunneling is detected. It is also not a serious
suggestion at this point.

>> A prime use case is bulk transfer of data. Think video streaming
>> with QUIC. It must send MTU sized or smaller packets, but has
>> no application-layer requirement to reconstruct large packets on
>> the peer.
>>
>> That said, for negotiated flows an inverse GRO feature could
>> conceivably be implemented to reduce rx stack traversal, too.
>> Though due to interleaving of packets on the wire, it aggregation
>> would be best effort, similar to TCP TSO and GRO using the
>> PSH bit as packetization signal.