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

[Gary Allen]

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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