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Subject: Re: ip_output()/if_output() behaviour
From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de>
In-Reply-To: <CAJ-Vmo=Jsf=7uXxwJ=Md5KLFpvSYAcaaNrq+bHsw75nfSG_ZaQ@mail.gmail.com>
Date: Fri, 29 Nov 2013 10:42:38 +0100
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To: Adrian Chadd <adrian@freebsd.org>
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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.

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