[asterisk-users] Why are 4 ports used for a single call?

Danny Nicholas danny at debsinc.com
Fri Jan 14 14:33:45 CST 2011


Hurray for Microsoft Outlook (for creating this whole top-post thread).
Just my .02;  The other two ports must have been a remnant of another
channel;  as for the 4 ports - I think that the 4 port requirement is
probably for "niceties" like conferencing and transfers.

 

  _____  

From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Bruce B
Sent: Friday, January 14, 2011 2:15 PM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [asterisk-users] Why are 4 ports used for a single call?

 

Thanks guys. I am not sure whether that call was asymmetric or not but I saw
4 ports open. It could be that the other two ports were remnant of another
channel even though I doubt it. 

 

Now, when I tried again, it is only 2 ports that is opened like you
mentioned, even RTP port, and RTP port +1. So, does Asterisk usually use the
symmetric method or is the asymmetric method used as well by some media
servers? 

 

The reason why I am asking is because there are many many online responses
that there is 4 ports needed per call and make sure you keep enough ports
open, blah blah...

 

Thanks again

On Fri, Jan 14, 2011 at 2:57 PM, Gary Allen <solstars1 at gmail.com> wrote:

RTP always uses a random even numbered port, then RTCP will use the next
port, which will always be odd numbered.  Symmetric RTP only needs two
ports, while asymmetric RTP uses four.

http://www.armware.dk/RFC/rfc/rfc4961.html




On Fri, Jan 14, 2011 at 12:44 PM, Bruce B <bruceb444 at gmail.com> wrote:

I mean part of RTP RFC?

 

On Fri, Jan 14, 2011 at 2:41 PM, Bruce B <bruceb444 at gmail.com> wrote:

Hi Everyone,

 

I am just tweaking a pfSense router and learning lots about NAT etc....I
noticed that each call uses four UDP port for RTP. Here is an example of
port for a call I made:

 

10200

10201

10504

10505

 

Seems like they are random in pair. I have a restriction of 10000-11000 in
my rtp.conf so that makes sense. But why use 4 ports per call? is that part
of SIP RFC?

 

Thanks

 

 

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