Hi,
I just analysed the SIP traffic a bit more in my scenario. The invite is sent from a UA behind a NAT with private address 192.168.0.5 and public address 84.151.207.89. When OpenSBC receives teh INVITE it detects the source IP address as 84.151.207.89 which is correct but the transport layer in the SIP stack in OpenSBC sends the 100 Trying to 192.168.0.5. According to RFC3261 section 18.2.1 the 100 Trying must follow the soruce IP address. Here from the spec
{noformat}+When the server transport receives a request over any transport, it
MUST examine the value of the "sent-by" parameter in the top Via
header field value. If the host portion of the "sent-by" parameter
contains a domain name, or if it contains an IP address that differs
from the packet source address, the server MUST add a "received"
parameter to that Via header field value. This parameter MUST
contain the source address from which the packet was received. This
is to assist the server transport layer in sending the response,
*since it must be sent to the source IP address from which the request
came.*+{noformat}
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