[Users] Question concerning nathelper and rtpproxy

Wolfgang Hottgenroth wolfgang.hottgenroth at verizonbusiness.com
Wed Jan 25 13:21:17 CET 2006


I'm trying to get nathelper and rtpproxy to work, following the guide
found on onsip.org, but unfortunately, it does not work correctly for.
Don't know why.

My setup is the following:

SIP-Phone ( -> Proxy1 ( -> Proxy2
( -> PSTN-GW (

Proxy1 supports authentication and handles the NAT travseral stuff.
Since in the current testbed, SIP-Phone is not actual behind a NAT
device I bypassed the test in the openser configuration to recognize
this phone as "nated".

What I see is, for a not nated phone, that the signaling goes the chain
as drawn above and the media stream goes from the phone to the ngw and
vice-versa, as I've expected. Everything is fine in this case.

For a nated phone, the signaling again goes through the chain, but the
media stream from the phone goes to the rtpproxy on Proxy1, which tries
to send it to Proxy2, where no rtpproxy is running, and thereby an "ICMP
port unreachable" is returned (which actual does prevent the rtpproxy
from continously sending the media stream).

Digging into the SIP and SDP is see the following:

SIP-Phone sends an INVITE to Proxy1, in the SDP it has "Connection
Information:" (IP of the phone)

Proxy1 replies with "100 Trying" and sends the INVITE to Proxy2, in the
SDP it has now "Connection Information:" (IP of Proxy1),
as I've expected.

Proxy2 replied with "100 Trying" and sends the INVITE to the ngw, in the
SDP it still has "Connection Information:".

Now the ngw sends "183 Session Progress" to Proxy2, which has in the SDP
"Connection Information:" (IP of the ngw).

Proxy2 sends "183 Session Progress" to Proxy1, still with "Connection
Information:" in the SDP.

Finally Proxy1 sends "183 Session Progress" to the phone, now with
"Connection Information:" (IP of Proxy1) in the SDP, as
I've expected, since the phone should send the media stream through the
rtpproxy running on Proxy1.

The phone sends the media stream as expected to Proxy1.

But, and this is the problem, the rtpproxy sends the media stream not to
the ngw, as I've expected looking into "SDP Connection Information"
received by this box, but to Proxy2, which never was mentioned in a
Connection Information.

When starting rtpproxy with -f, I saw the it was asked by
openser/nathelper to proxy between the phone and Proxy2, so the rtpproxy
seems to work correctly. But why asks the nathelper on Proxy1 to proxy
the media between the phone and Proxy2 and not, as I've expected between
the phone and the ngw?

My openser configuration is attached, if someone would be so nice to
have a look and send a hint for me.


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