8 Apr
2020
8 Apr
'20
4:14 p.m.
On Wed, Apr 08, 2020 at 04:54:48PM +0100, David Villasmil wrote:
I have a question on this, since 180 arrived before
the 200, shouldn’t it
go out before? I.e.: FIFO? Or is this not happening because there are
multiple processes and one may take the 180 and the other the 200 and they
are processes independently?
I guess setting the childs to only 1 should prove this theory.
In theory, it should all be FIFO, of course. In practice, there are
stochastic aspects to OS process scheduling and I/O scheduling which
make the ingress and egress of messages arriving at Kamailio
simultaneously or substantially simultaneously somewhat indeterminate.
And, of course, it depends on how complicated the processing of the
messages respectively is, either in computational terms, or external I/O
wait, or both.
To Steve Davies' point, that's just the nature of the multiprocess
architecture, together with the lack of a distributor thread.
-- Alex
--
Alex Balashov | Principal | Evariste Systems LLC
Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
8 Apr
8 Apr
4:30 p.m.
New subject: Kamailio propagates 180 and 200 OK OUT OF ORDER
Alex Balashov <abalashov(a)evaristesys.com> writes:
On Wed, Apr 08, 2020 at 04:54:48PM +0100, David
Villasmil wrote:
I don't see this as being about the nature of parallel processing.
Either the protocol specifications have ordering rules (in which case an
implementation which does not follow those is defective), or they do not
have ordering rules (in which case an implementation that is troubled by
lack of ordering is defective).
There is a messy middle ground of course, where significant misordering
is inefficient. An example is the transport of IP packets. TCP can
deal with misordered packets, but this ends up not so efficient. So
there is a notion that a system that causes frequent or gross
misordering is not as good as one that doesn't. Compare to ATM, where
cells may not be reordered at all -- this is a major burden on
implementers.
I am curious which situation we are discussing here. Given that SIP can
use UDP, which has no ordering guarantees, it would be surprising if an
endpoint can expect ordering of any messages.
I have a question on this, since 180 arrived
before the 200, shouldn’t it
go out before? I.e.: FIFO? Or is this not happening because there are
multiple processes and one may take the 180 and the other the 200 and they
are processes independently?
I guess setting the childs to only 1 should prove this theory.
In theory, it should all be FIFO, of course. In practice, there are
stochastic aspects to OS process scheduling and I/O scheduling which
make the ingress and egress of messages arriving at Kamailio
simultaneously or substantially simultaneously somewhat indeterminate.
4:38 p.m.
New subject: Kamailio propagates 180 and 200 OK OUT OF ORDER
On Wed, Apr 08, 2020 at 12:30:45PM -0400, Greg Troxel wrote:
I don't see this as being about the nature of
parallel processing.
It absolutely is -- when it comes to explaining why Kamailio behaves the
way it does.
It bears remark that it's not a common occurrence; if Kamailio could not
be relied upon to forward messages in a FIFO order on a widespread
basis, it wouldn't be very useful. This race occurs, in my experience,
in highly contended environments, e.g. a VM on a busy hypervisor.
Either the protocol specifications have ordering rules
(in which case
an implementation which does not follow those is defective), or they
do not have ordering rules (in which case an implementation that is
troubled by lack of ordering is defective).
And yet, SIP-as-specified is replete with race conditions.
One way to look at this: if you want guaranteed ordering
(network-introduced ordering issues not withstanding), use a single
child process. If you want "enhanced performance", use multiple child
processes, with the understanding that this entails a small amount of
risk as a trade-off. :-)
-- Alex
--
Alex Balashov | Principal | Evariste Systems LLC
Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
5:22 p.m.
New subject: Kamailio propagates 180 and 200 OK OUT OF ORDER
On 08.04.20 18:14, Alex Balashov wrote:
On Wed, Apr 08, 2020 at 04:54:48PM +0100, David
Villasmil wrote:
A distributor thread (or process) won't help here, if it distributes
traffic to other threads (processes) without waiting for them to finish,
which ends up to be serial processing. The distributor role for UDP
traffic is done by the kernel. For tcp/tls there is a distributor
process for connections.
Cheers,
Daniel
--
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
I have a question on this, since 180 arrived
before the 200, shouldn’t it
go out before? I.e.: FIFO? Or is this not happening because there are
multiple processes and one may take the 180 and the other the 200 and they
are processes independently?
I guess setting the childs to only 1 should prove this theory.
In theory, it
should all be FIFO, of course. In practice, there are
stochastic aspects to OS process scheduling and I/O scheduling which
make the ingress and egress of messages arriving at Kamailio
simultaneously or substantially simultaneously somewhat indeterminate.
And, of course, it depends on how complicated the processing of the
messages respectively is, either in computational terms, or external I/O
wait, or both.
To Steve Davies' point, that's just the nature of the multiprocess
architecture, together with the lack of a distributor thread.
5:24 p.m.
New subject: Kamailio propagates 180 and 200 OK OUT OF ORDER
On Wed, Apr 08, 2020 at 07:22:17PM +0200, Daniel-Constantin Mierla wrote:
A distributor thread (or process) won't help here,
if it distributes
traffic to other threads (processes) without waiting for them to
finish, which ends up to be serial processing. The distributor role
for UDP traffic is done by the kernel. For tcp/tls there is a
distributor process for connections.
I was imagining a distributor thread that is aware of SIP semantics, but
I agree it's a terrible idea. :-)
-- Alex
--
Alex Balashov | Principal | Evariste Systems LLC
Tel: +1-706-510-6800 / +1-800-250-5920 (toll-free)
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/
5:39 p.m.
New subject: Kamailio propagates 180 and 200 OK OUT OF ORDER
On 08.04.20 18:30, Greg Troxel wrote:
Alex Balashov <abalashov(a)evaristesys.com>
writes:
SIP is designed to deal (pretty well) with packets received not in the
same order as they were sent.
If it is a request with a lower CSeq than a previously received one, it
has to ignore it and send a 500 response.
The responses for the active transaction (matching the cseq of the
request that created the transactions), the response codes have a
priority "rank". 200ok is the highest priority in the way that if a 1xx
or [3456]xx is received after it, just discard it.
The ACK-reINVITE seems to be the one that creates the most of the
troubles, from my experience. A SIP proxy or a server cannot do much
here, because even it can process sequentially, the re-ordering can
still happen on the network (before or after it). The clients have the
mechanisms in the SIP specs to implement it somehow nice, but I haven't
seen it in reality. The receiver of the reINVITE before the ACK can
reject it with a Retry-After header. But I bet that the (most of) sender
will tear down the call if the reINVITE is not getting a 200ok. As I
remember now, I haven't seen it written explicitly in the RFCs (at least
those from the early years of SIP, may be in the new ones that update
some specs), but it is legit to reject a request and ask to be resent
later, without considering it a complete failure.
Cheers,
Daniel
--
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
On Wed, Apr 08, 2020 at 04:54:48PM +0100, David
Villasmil wrote:
I don't
see this as being about the nature of parallel processing.
Either the protocol specifications have ordering rules (in which case an
implementation which does not follow those is defective), or they do not
have ordering rules (in which case an implementation that is troubled by
lack of ordering is defective).
There is a messy middle ground of course, where significant misordering
is inefficient. An example is the transport of IP packets. TCP can
deal with misordered packets, but this ends up not so efficient. So
there is a notion that a system that causes frequent or gross
misordering is not as good as one that doesn't. Compare to ATM, where
cells may not be reordered at all -- this is a major burden on
implementers.
I am curious which situation we are discussing here. Given that SIP can
use UDP, which has no ordering guarantees, it would be surprising if an
endpoint can expect ordering of any messages. I have a question on this, since 180 arrived
before the 200, shouldn’t it
go out before? I.e.: FIFO? Or is this not happening because there are
multiple processes and one may take the 180 and the other the 200 and they
are processes independently?
I guess setting the childs to only 1 should prove this theory.
In theory, it
should all be FIFO, of course. In practice, there are
stochastic aspects to OS process scheduling and I/O scheduling which
make the ingress and egress of messages arriving at Kamailio
simultaneously or substantially simultaneously somewhat indeterminate.
1495
days inactive
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days old
5 comments
3 participants
participants (3)
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Alex Balashov -
Daniel-Constantin Mierla -
Greg Troxel