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Hi,<br />
I am just trying to understand SIP and RTP so i have some doubts in the above explanation.<br />
As i read the RTP port is exchanged between SIP clients in the SDP body. So i guess these are also fixed before the RTP flow starts. So how are the ports for RTP stream chosen dynamically ?<br />
<br />
Thanks
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It is exchanged in the SIP stream, but is dynamic and negotiated. It is not dissimilar to FTP where the actual data stream is dynamic and created at the time of transfer, which is why most firewalls have FTP &quot;helper&quot; inspection rules to discover the dynamic port from the FTP control stream. You'll also see newer firewalls have SIP inspection that can dynamically open RTP ports based on inspected SIP traffic.
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Does the RTP port which is negotiated between the two SIP clients change in between flow of RTP stream ??
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It will not usually change during a single session. But it will likely change with every new session. The RTP stream only constructed as needed. So if you have a SIP trunk provider, each incoming or outgoing call creates a new RTP stream on a new port that lasts only as long as that call, as an example.
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How will the call be handled if suppose client1 ip changes during rtp flow. That is say he moves to a different network while talking.
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That depends on the client. Some (most) will simply disconnect. Those that handle network changes gracefully do so by subterfuge. Both the SIP traffic and RTP traffic us changing so new constructs are built. This usually involves a new RTP port as it is actually a new stream.
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Can you please let me know the exact steps that will happen in case the ip changes. How exactly will the client handle this network change.<br />
Does the client need to simply inform the other client that this is my new ip so send the rtp data to this ip instead of the old one.
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There is no standard for this scenario. Different applications (or clients) and servers handle it differently. I can't give exact steps because they vary wildly. &nbsp;Lynch handles it differently than asterisk. And asterisk handles it differently when talking to a Cisco endpoint than it does a 3Com endpoint. Or to one SIP trunk provider vs. another, or even dependent on an SBC in place. So many variables.
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HI,<br />
Thanks for the answers. &nbsp;I am writing a new question on lines with this discussion. Please help me with it.<br />
<a href="https://www.experts-exchange.com/questions/28574172/what-is-a-Media-proxy-and-how-does-it-solve-the-RTP-firewall-problem.html" rel="ugc">https://www.experts-exchange.com/questions/28574172/what-is-a-Media-proxy-and-how-does-it-solve-the-RTP-firewall-problem.html</a><br />
Thanks
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<div class="content wysiwyg-content">
Hi,<br />
I saw the following statement in a tutorial on SIP, RTP firewall n NAT. I am unable to understand it - '<br />
<br />
When there is a firewall in between the SIP signaling request response is allowed. But there is a problem during RTP. &nbsp;During RTP the data will be able to go through firewall to User agent but data from user agent won’t be able to reach the other side of the firewall as the firewall does not have information about port to which the User Agent is sending data.<br />
<br />
What i dont understand here is that <br />
1) Why the firewall needs to have prior information about the port to which the User Agent will send data ?<br />
why cant it allow data transfer to the port.<br />
2) If there is a problem with RTP transfer when there is a firewall, why is there no problem during SIP signalling.<br />
<br />
Also please let me know any good resources or tools to study SIP (session initiation protocol) and RTP, firewalls<br />
Thanks
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</div>


Hi,
I saw the following statement in a tutorial on SIP, RTP firewall n NAT. I am unable to understand it - '

When there is a firewall in between the SIP signaling request response is allowed. But there is a problem during RTP.  During RTP the data will be able to go through firewall to User agent but data from user agent won’t be able to reach the other side of the firewall as the firewall does not have information about port to which the User Agent is sending data.

What i dont understand here is that 
1) Why the firewall needs to have prior information about the port to which the User Agent will send data ?
why cant it allow data transfer to the port.
2) If there is a problem with RTP transfer when there is a firewall, why is there no problem during SIP signalling.

Also please let me know any good resources or tools to study SIP (session initiation protocol) and RTP, firewalls
Thanks

RTP Problem with Firewall

Networking software modules are interfaced with a framework implemented on the machine's operating system that implements the networking functionality of the operating system. The best known frameworks are the TCP/IP model and the OSI model. Systems typically do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a protocol family or protocol suite.[9] Some of the best known protocol suites include: IPX/SPX, X.25, AX.25, AppleTalk and TCP/IP. Other protocols indirectly related to networking include the hypertext transfer protocol (HTTP) and its related technologies, Dynamic Host Configuration Protocol (DHCP), Domain Name Server (DNS) and other Internet protocols.

Networking Protocols

IP telephony (Internet Protocol telephony) is a general term for the technologies that use the Internet Protocol's packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN). Using the Internet, calls travel as packets of data on shared lines, avoiding the tolls of the PSTN. The challenge in IP telephony is to deliver the voice, fax, or video packets in a dependable flow to the user.

IP Telephony

Networking is the process of connecting computing devices, peripherals and terminals together through a system that uses wiring, cabling or radio waves that enable their users to communicate, share information and interact over distances. Often associated are issues regarding operating systems, hardware and equipment, cloud and virtual networking, protocols, architecture, storage and management.