[Freeswitch-users] Bypass media and NAT Traversal with Aastra

Phil Quesinberry philq at qsystemsengineering.com
Tue Jul 24 21:25:45 MSD 2012


Is this a bug?  (just kidding)

As the number of extensions we serve increases, we're trying to avoid
saturating our local bandwidth by proxying all of the media between
endpoints.  This seems to work fine with the Linksys devices but not with
Aastra in all scenarios.  The Aastras are configured with the NAT IP field
populated appropriately and Rport (RFC 3581) enabled.

The FS server is behind a NAT firewall with SIP and RTP port ranges
forwarded to it.  There are endpoints here on the same network as the
switch, and there are remote endpoints behind their own NAT firewalls.
Everything works fine in Proxy Media mode but in bypass mode we see the
following behavior:

Scenario 1:
Remote endpoint places a call to PSTN, FS negotiates bypass media between
remote endpoint and PSTN gateway.  Call succeeds.

Scenario 2:
Remote endpoint places a call to another remote endpoint on same LAN.  Call

Scenario 3:
Local endpoint places a call to PSTN gateway.  Call succeeds.

Scenario 4:
Remote endpoint places a call to local endpoint (or vice versa).  Call fails
with no audio with Aastra devices on both ends, but if one of the endpoints
is a LinkSys, the call succeeds.

I realize that I should be able to solve this problem with a local SBC like
OpenSIPS/RTPproxy, but in the meantime I was hoping that there might be a
setting that I was overlooking with the Aastra.  When I look at the SIP
traffic for the call, FS appears to be negotiating the addresses/ports
properly so I suspect that this problem is related to Aastra's inability to
deal with symmetric NAT.  The Linksys devices seem to be able to deal with
it with FS' help, presumably by doing some UDP hole-punching.

Phil Quesinberry
Q Systems Engineering, Inc.
Electronic Controls and Embedded Systems Development
(410) 969-8002

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