[SIPForum-discussion] [Sip-implementors] Address change in "Via" field from private IP to public IP.

[Rudra Kant Kanth]

Hi Gaurav,

I think what you are seeing with your registration attempts to Skype is a
standard. With what I have read in you explanation, I am expecting that
when you see your public REGISTER message, you would find another via
header appended over there while yours private IP would have got stacked
down.

This new VIA IP would be your proxy or B2BUA, which could also have NAT
functionality. The idea is that your REGISTRAR is sitting over Internet and
there is no way for it to recognise your private IP. So, your B2BUA/ Nat
server provides its IP in the 1st VIA header so that all responses from the
Skype 1st hits this server and thereafter, your server can uniquely route
the response to your private IP.

I am also expecting the presence of Record-Route headers in the going out
requests and for all subsequent request a ROUTE header imbibed-into as
well. The afore-stated details should be aptly seen if you have a trace
running on your B2bUA/NAt server else you may not see all dialogs together
and the sniffer which you are taking would only explain how the end-client
sees tings.

Now, coming to your attempts to register to a PBX which doesn't let you get
the calls. I would like to get a little more details about it to understand
what is happening.

I hope the above explanation would have added value.

Regards,
Rudra


On Fri, Jun 14, 2013 at 12:28 PM, Srivastava, Gaurav <
Gaurav.Srivastava at inin.com> wrote:

> This is interesting Somesh,  maybe such a mechanism is involved after all.
> I wouldn't read much into the second REGISTER part as now I observe all the
> new REGISTERs going out with the public IP address.
>
> Thank you so much for highlighting that, I will check if STUN (or a
> similar mechanism) is in place for the client to be aware of the public IP.
>
> What if such a mechanism was not there , would I face problems ? The sip
> client (un aware of the public IP address) would send out REGISTER having
> my laptop's private IP in the contact & via headers - but I am sure some
> other device along the way would do the honours - I am just not sure which.
>
> I am actually not facing any issues with this. I am having trouble with a
> PBX that I have which does not seem to be "aware" & sends out a REGISTER
> with its private IP in contact & via. Calls do not land on my PBX
> subsequently from the outside, public networks.
>
> Thanks again!
>
> Gaurav Srivastava | Senior Systems Engineer
> phone | +61.449642846 | Gaurav.Srivastava at inin.com
>
> Interactive Intelligence Inc.
> Deliberately Innovative
> www.inin.com/au-nz
>
> -----Original Message-----
> From: Shanbhag, Somesh (NSN - IN/Bangalore) [mailto:
> somesh.shanbhag at nsn.com]
> Sent: Friday, 14 June 2013 4:51 PM
> To: Srivastava, Gaurav; SIP Implementors; discussion at sipforum.org
> Subject: RE: [Sip-implementors] Address change in "Via" field from private
> IP to public IP.
>
> Hello Gaurav,
>
> May be your sip client is learning the IP address (NAT) via some mechanism
> like STUN etc. But I am not sure, why it only does for second REGISTER. May
> be its designed that way that after learning, during re-REGISTRATIONs it
> will use the learnt address.
>
> One more interesting point - are you facing any issues with this? May be
> your organization NAT is SIP aware?
>
> Br,
> Somesh
>
> -----Original Message-----
> From: ext Srivastava, Gaurav [mailto:Gaurav.Srivastava at inin.com]
> Sent: Wednesday, June 12, 2013 8:00 AM
> To: SIP Implementors; discussion at sipforum.org
> Subject: [Sip-implementors] Address change in "Via" field from private IP
> to public IP.
>
> Hi,
>
> I am using a SIP soft phone (Zopier) that registers to Skype. The
> softphone sends out a REGISTER message to sip.skype.com for this.
>
> While going through the messaging, I noticed that initially the via field
> contains my private IP address.
>
> When the client re REGISTERs, the address changes from my private IP to
> the public IP which my organization has.
>
> Please note that I am running the packet capture on my laptop.
>
> I am trying to understand the change in IP address - please let me know if
> you have an understanding of this.
>
> Warm Regards,
> Gaurav Srivastava.
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