Date: Fri, 29 Nov 2013 15:06:07 +0100 From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> To: Julian Elischer <julian@freebsd.org> Cc: "freebsd-net@freebsd.org list" <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org> Subject: Re: ip_output()/if_output() behaviour Message-ID: <8C291076-5F03-4406-B689-A3185E6DD313@lurchi.franken.de> In-Reply-To: <52987E27.10503@freebsd.org> References: <BF7B04F7-0D45-4708-99A8-8BE030109CEC@lurchi.franken.de> <CAJ-Vmo=Jsf=7uXxwJ=Md5KLFpvSYAcaaNrq%2BbHsw75nfSG_ZaQ@mail.gmail.com> <B7E3AA58-172A-4D20-B625-95C4712D46E7@lurchi.franken.de> <52987E27.10503@freebsd.org>
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On Nov 29, 2013, at 12:44 PM, Julian Elischer <julian@freebsd.org> wrote:
> On 11/29/13, 5:42 PM, Michael Tuexen wrote:
>> On Nov 29, 2013, at 3:54 AM, Adrian Chadd <adrian@freebsd.org> wrote:
>>
>>> On 28 November 2013 12:35, Michael Tuexen
>>> <Michael.Tuexen@lurchi.franken.de> wrote:
>>>> Dear all,
>>>>
>>>> I'm investigating a problem and need to understand the behaviour
>>>> of ip_output(). Is it correct that if ip_output() returns an
>>>> non-zero error, the corresponding packet was never sent?
>>>> In the SCTP stack we assume this, but it seems that at least
>>>> the em and the igb driver might return an error from
>>>> igb_mq_start_locked(), for example, but have accepted the packet.
>>> Which error(s) ?
>> ENOBUFS, but does it matter? What is the correct reaction to
>> ip_output() returning an error? The SCTP stack assumes that the
>> packet was not put on the wire. With the current version of the
>> igb driver we are wrong. igb_mq_start() might return an error,
>> even if the packets was enqueued successfully (in case
>> igb_mq_start_locked() fails).
>>
>> But the SCTP stacks assumes in general that if ip_output() returns
>> an error, the packet didn't make it out.
> From my memory it's always been the case that you really have little
> idea if the packet makes it out onto the wire or not.
> In the past it's been the case that an error indicates that it probably DIDN'T make it out, but
> the converse is not true.. NO error is not an indication of success.
> I'm surprised that you could get an error when it was broadcast however.. that is counter
> to the last 30 years of behaviour.
ifnet(9) says:
if_transmit()
Transmit a packet on an interface or queue it if the interface is
in use. This function will return ENOBUFS if the devices software
and hardware queues are both full. ...
So I guess returning ENOBUFS when the packet was queued is wrong...
Any comments?
Best regards
Michael
>
>
>>
>> Best regards
>> Michael
>>>> Before digging further, I would like to know what the intended
>>>> behaviour of ip_output() is.
>>>
>>> -adrian
>>>
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>
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