[asterisk-dev] Re: [Fwd: [svn-commits] oej: branch oej/iaxtrunkfix-1.2 r44779

[Benny Amorsen]

>>>>> "RA" == Rich Adamson <radamson at routers.com> writes:

First of all, I totally don't get what the original patch is about.

What is the point of trying to make your packets smaller than the path
MTU? If you're trying to transmit information which has something to
do with VoIP, you generally want it transmitted as fast as possible.
Using path MTU sized packets accomplished that. Anything else just
goes slower.

RA> Path mtu discovery will not provide the required performance mtu
RA> sizing with the single exception of when an asterisk box is
RA> directly attached to a dsl modem (or other transmission link).
RA> That means absolutely no other box sharing the link. The patch is
RA> oriented around moving rtp packets out of the local ethernet
RA> interface, and does not consider the impact from the "next" box in
RA> the transmission path.

The patch won't provide the required performance MTU either.

RA> This might be a poor example, but I'll try it anyway.

RA> Example: Asterisk box with patch applied. Another linux box on the
RA> same local segment providing dns, simple web, and maybe email. Dsl
RA> modem with 512k bandwidth.

RA> The path mtu discovery for the general linux box will likely
RA> return a 1500 bytes, and will attempt to send all data via the dsl
RA> modem in packets of 1500 byte size.

RA> The asterisk box (with patch applied) will attempt to impact the
RA> mtu size of the packets sent only by the asterisk box, sizing
RA> those packets to something less then 1500 byte packets. (That will
RA> only occur when using iax trunking only.)

But that just slows down the link at a time when throughput is
important. Of course if the IAX packets are larger than 1500 bytes,
that should be fixed -- path MTU discovery will help in that case.
Also, if someone runs their DSL link at a 512 byte MTU, IAX packets
should be sized at 512 bytes max. Again, path MTU discovery does the
right thing.

RA> When a number of large packets from the general linux machine
RA> queues up at the dsl modem (or cisco router if you want that
RA> instead), as soon as one large packet begins to flow across the
RA> dsl link, the link is 100% utilized until that packet has been
RA> sent, and that elapsed time is much larger then is wanted for
RA> consistent rtp packet flows (resulting in jitter). In effect, it
RA> turns out to be iax packet starvation.

RA> I'm not saying the patch is bad, however it will have very little
RA> impact with the exception of large installations where many iax
RA> conversations are occurring across a singe path.

It seems to me that the impact in the best case is nonexistent, and
worst case it makes behaviour worse.


/Benny