[Freeswitch-users] RTP flows and device doing a local redirection

Steven Ayre steveayre at gmail.com
Thu Sep 25 19:06:55 MSD 2014

> then FS seems to not be ready for it and reply ICMP unreachable.

This sounds odd. ICMP unreachable will be sent by the OS/firewall not FS.
It means nothing is listening on that UDP port (or it's blocked). What's
odd is the SDP FS sends in its INVITE packet will contain the IP and port
it's using. It finds that port by creating a socket using a random port
then checking what it's using. So before the INVITE is even sent the port
is open, so you shouldn't see ICMP unreachable from the OS.

Are they sending to the exact same IP and port you send in the SDP? Are you
running any stateful firewalls that might not have opened the port for the
RTP stream?

On 25 September 2014 14:59, Florent Krieg <flokrrr at gmail.com> wrote:

> Hi mates,
> We are encountering a problem with some of our customers in the following
> situation:
> * we are running Freeswitch with a public IP address, detecting far-end
> nat traversal when needed (customers behind NAT)
> * our customers interconnects their IPBXes to our FS platform (we do only
> SIP trunking with FS)
> * if the customer has its IPBX behind a NAT and sets a local redirect,
> there's no sound (and I completely understand why, I'm just trying to find
> a solution, if any...)
> So there are two distinct calls:
> PSTN_X         <===> FS <===> customer1 IPBX
> customer1 IPBX <===> FS <===> PSTN_Y (redirection)
> When PSTN_Y answers the call, FS sends 200 OK w/ SDP to customer1 IPBX.
> The IPBX then sends an empty RTP packet, I reckon to allow the remote
> platform (FS in this case) to be able to detect the IP/port couple where to
> send RTP packets.
> The problem I have is that on the 'caller' side, this single empty RTP
> packet is sent by the IPBX before sending the 200 OK (like, a few ms
> before) and then FS seems to not be ready for it and reply ICMP unreachable.
> To make this whole stuff almost-work, I've set 'rtp_auto_adjust_threshold'
> to 1, otherwise FS will wait for far more than 1 RTP packet to detect NAT.
> My idea is that the IPBX should send the 200 OK first and right after this
> empty RTP packet.
> The problem is I'm encountering the issue with two different constructors
> (on one of them we ended using STUN, but the other one has a terrible STUN
> implementation, hence I'm really looking for a solution).
> I know it's tricky (SIP, NAT, redirection and so on) but what do you think
> about this use case?
> Would you know a way to solve this trouble? If you have any clue please
> share!
> Thanks in advance
> Florent
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