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

[David White]

Each media stream will use two, one for RTP and one for RTCP.  In your case
10200/10201 are an RTP/RTCP pair, and 10504/10505 are another pair.  RTP is
always and even numbered port, and RTCP is always RTP port + 1.  Yes, it's
in the RFC for RTP.

 

The fact that you have two pairs means that two media streams are being
negotiated, perhaps one for audio and one for video?  Your phone config or
wireshark captures will tell you for certain.

 

Of course, I'm assuming that those ports are for one endpoint (phone).  If
one pair is for caller and one pair is for callee, then this is a normal
"simplest" scenario, one pair for each side.   You didn't specify "whose"
ports they were.

 

 

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 11:45 AM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [asterisk-users] Why are 4 ports used for a single call?

 

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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