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From: "Samudrala, Sridhar" <sridhar.samudrala@intel.com>
Subject: Re: [PATCH RFC net-next 00/11] udp gso
Date: Tue, 17 Apr 2018 19:25:14 -0700
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Cc: Network Development <netdev@vger.kernel.org>,
        Willem de Bruijn <willemb@google.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Sowmini Varadhan <sowmini.varadhan@oracle.com>
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On 4/17/2018 2:07 PM, Willem de Bruijn wrote:
> On Tue, Apr 17, 2018 at 4:48 PM, Sowmini Varadhan
> <sowmini.varadhan@oracle.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 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.