ip_output()/if_output() behaviour

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Fri Nov 29 14:06:11 UTC 2013

On Nov 29, 2013, at 12:44 PM, Julian Elischer <julian at freebsd.org> wrote:

> On 11/29/13, 5:42 PM, Michael Tuexen wrote:
>> On Nov 29, 2013, at 3:54 AM, Adrian Chadd <adrian at freebsd.org> wrote:
>>> On 28 November 2013 12:35, Michael Tuexen
>>> <Michael.Tuexen at 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:

           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
>> Best regards
>> Michael
>>>> Before digging further, I would like to know what the intended
>>>> behaviour of ip_output() is.
>>> -adrian
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