Date: Sat, 30 Nov 2013 00:14:21 +0800 From: Julian Elischer <julian@freebsd.org> To: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> Cc: "freebsd-net@freebsd.org list" <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org> Subject: Re: ip_output()/if_output() behaviour Message-ID: <5298BD5D.3020203@freebsd.org> In-Reply-To: <8C291076-5F03-4406-B689-A3185E6DD313@lurchi.franken.de> 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> <8C291076-5F03-4406-B689-A3185E6DD313@lurchi.franken.de>
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On 11/29/13, 10:06 PM, Michael Tuexen wrote: > 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... I think it is. ENOBUFS means "I couldn't proceed due to no buffers" not "I used up the last one on this operation".
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