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

[Bruce B]

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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