[SIPForum-discussion] From field in SIP header

Nittin Dutt Nittin.Dutt at csr.com
Fri Nov 30 10:59:22 UTC 2007

I think you may be facing classic SIP-NAT issue. If you are behind a
NAT/Firewall and using the private IP address (192.168.x.x) then you
must be either using STUN in your SIP client or your SIP Server must
have SBC(Session border controller) functionality.

I think what's happening is that since you have initiated the SIP call
your firewall/NAT has a pin hole opened and you receive response because
of that. And that's too because you SIP proxy figure out that your
private address in Via is not routable so it sends it to source address
(where pin hole is opened). Since RTP flows directly between clients,
there is no way that remote (called party) can route the RTP to private
IP you published in your SDP. An SBC fixes all these issues for you. Try
VoipTalk Server, I think they have an SBC in place.


-Nittin Dutt



From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Fredrik Sandedal
Sent: 30 November 2007 09:25
To: Rohin Koul; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] From field in SIP header




Thanks for your answer.


Do you mean that the IP references in the SDP should change when the
packets pass the firewall?

In the trace from the WAN-side of the firewall all IP references in the
SDP are to the local IP address. Not to the WAN address of the firewall.




Fredrik Sandedal, Enera


From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Rohin Koul
Sent: Friday, November 30, 2007 8:59 AM
To: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] >From field in SIP header



I guess your firewall is behaving correctly... if you are not recieving
RTP check the SDP being exchanged.






On 30/11/2007, Fredrik Sandedal <fredrik.sandedal at enera.se> wrote: 



I have a question regarding the "From" field in the SIP header.


I send an INVITE request from within our LAN and through a SIP
supporting firewall to a SIP termination service.

The firewall changes IP-address of the "Contact" field from
<>  to the firewall's WAN IP but the "From" field
still has the LAN IP-address.



From: <sip:02073367100 at>;tag=....

Contact: <sip:02073367100 at>


WAN (after passing firewall)

Via: SIP/2.0/UDP 88.131....:5060;branch=...

From: <sip:02073367100 at>;tag=....

Contact: <sip:02073367100 at 88.131....:5060>


Is this a correct behavior? The reason I'm asking is because I do not
receive any RTP traffic from the SIP termination service. 


Kind regards,

Fredrik Sandedal, Enera


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