netdev - Re: [RFC PATCH net-next 0/3] sctp: add GSO support

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Message-Id: <928E1272-9D07-4495-B811-FDD018E1E179@lurchi.franken.de>
Date:	Fri, 29 Jan 2016 13:25:51 +0100
From:	Michael Tuexen <Michael.Tuexen@...chi.franken.de>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Neil Horman <nhorman@...driver.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	David Miller <davem@...emloft.net>,
	"brouer@...hat.com" <brouer@...hat.com>,
	"alexander.duyck@...il.com" <alexander.duyck@...il.com>,
	"alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
	"borkmann@...earbox.net" <borkmann@...earbox.net>,
	"marek@...udflare.com" <marek@...udflare.com>,
	"hannes@...essinduktion.org" <hannes@...essinduktion.org>,
	"fw@...len.de" <fw@...len.de>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"john.r.fastabend@...el.com" <john.r.fastabend@...el.com>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: Re: [RFC PATCH net-next 0/3] sctp: add GSO support


> On 29 Jan 2016, at 12:26, Marcelo Ricardo Leitner <marcelo.leitner@...il.com> wrote:
> 
> On Fri, Jan 29, 2016 at 11:57:46AM +0100, Michael Tuexen wrote:
>>> On 29 Jan 2016, at 02:18, Marcelo Ricardo Leitner <marcelo.leitner@...il.com> wrote:
>>> 
>>> On Fri, Jan 29, 2016 at 12:36:05AM +0100, Michael Tuexen wrote:
>>>> 
>>>>> On 28 Jan 2016, at 22:03, Marcelo Ricardo Leitner <marcelo.leitner@...il.com> wrote:
>>>>> 
>>>>> On Thu, Jan 28, 2016 at 06:54:06PM +0100, Michael Tuexen wrote:
>>>>>>> On 28 Jan 2016, at 14:51, David Laight <David.Laight@...LAB.COM> wrote:
>>>>>>> 
>>>>>>> From: Marcelo Ricardo Leitner
>>>>>>>> Sent: 27 January 2016 17:07
>>>>>>>> This patchset is merely a RFC for the moment. There are some
>>>>>>>> controversial points that I'd like to discuss before actually proposing
>>>>>>>> the patches.
>>>>>>> 
>>>>>>> You also need to look at how a 'user' can actually get SCTP to
>>>>>>> merge data chunks in the first place.
>>>>>>> 
>>>>>>> With Nagle disabled (and it probably has to be since the data flow
>>>>>>> is unlikely to be 'command-response' or 'unidirectional bulk')
>>>>>>> it is currently almost impossible to get more than one chunk
>>>>>>> into an ethernet frame.
>>>>>>> 
>>>>>>> Support for MSG_MORE would help.
>>>>>> What about adding support for the explicit EOR mode as specified in
>>>>>> https://tools.ietf.org/html/rfc6458#section-8.1.26
>>>>> 
>>>>> Seizing the moment to clarify my understanding on that. :)
>>>>> Such multiple calls to send system calls will result in a single data
>>>>> chunk. Is that so? That's what I get from that text and also from this
>>>> No. It results in a single user message. This means you can send
>>>> a user message larger than the send buffer size. How the user message
>>>> is fragmented in DATA chunks is transparent to the upper layer.
>>>> 
>>>> Does this make things clearer?
>>> 
>>> I think so, yes. So it allows delaying setting the Ending fragment bit
>>> until the application set SCTP_EOR. All the rest before this stays as
>>> before: first send() will generate a chunk with Beginning bit set and
>>> may generate some other middle-fragments (no B nor E bit set) if
>>> necessary, second to N-1 call to send will generate only middle
>>> fragments, while the last send, with SCTP_EOF, will then set the Ending
>>> fragment in the last one. Right?
>> Yes. But there are no restrictions on the user data provided in send()
>> calls and DATA chunks. So you can
>> send(100000 byte, no SCTP_EOR)
>> resulting in one DATA chunk with the B bit, several with no B and no E bit.
>> send(100000 byte, no SCTP_EOR)
>> resulting in several chunks with no B and no E bit.
>> send(100000 byte, SCTP_EOR)
>> resulting in several chunks with no B and no E bit and one (the last) chunk
>> with the E bit.
>> 
>> On the other hand you can do
>> send(1 byte, no SCTP_EOR)
>> resulting in a single DATA chunk with the E bit set.
>> send(1 byte, no SCTP_EOR)
>> send(1 byte, no SCTP_EOR)
>> send(1 byte, no SCTP_EOR)
>> send(1 byte, no SCTP_EOR)
>> send(1 byte, no SCTP_EOR)
>> All resulting in a single DATA chunk with 5 bytes user data and no B or E bit.
>> (For example if Nagle is enabled and only after the last send call the SACK arrives).
>> send(1 byte, SCTP_EOR)
>> results in a single DATA chunk with the E bist set.
> 
> Cool, thanks Michael. It will be quite fun to mix this with MSG_MORE
> logic, I think :)
Don't know. In FreeBSD we do support SCTP_EOR, but not MSG_MORE, which seems
to be Linux specific.

Best regards
Michael
> 
> Best regards,
> Marcelo
> 
>