Mediaproxy Module

Dan Pascu

Edited by

Dan Pascu


Table of Contents
1. User's Guide
1.1. Overview
1.2. Principle of operation
1.3. Types of SIP clients
1.4. Features
1.5. Dependencies
1.5.1. OpenSER Modules
1.5.2. External Libraries or Applications
1.6. Exported parameters
1.6.1. mediaproxy_socket (string)
1.6.2. sip_asymmetrics (string)
1.6.3. rtp_asymmetrics (string)
1.6.4. natping_interval (integer)
1.6.5. signaling_ip_avp (string)
1.6.6. domain_avp (string)
1.7. Exported Functions
1.7.1. client_nat_test(type)
1.7.2. fix_contact()
1.7.3. use_media_proxy()
1.7.4. end_media_session()
1.8. Comparison with the nathelper module
1.9. How to use sip_ping from the nathelper module
2. Developer's Guide
3. Frequently Asked Questions
List of Examples
1-1. Setting mediaproxy_socket parameter
1-2. Setting sip_asymmetrics parameter
1-3. Setting rtp_asymmetrics parameter
1-4. Setting natping_interval parameter
1-5. Setting signaling_ip_avp parameter
1-6. Setting domain_avp parameter
1-7. client_nat_test usage
1-8. fix_contact usage
1-9. use_media_proxy usage
1-10. Using mediaproxies from a specific domain
1-11. end_media_session usage
1-12. Pinging example configuration

Chapter 1. User's Guide

1.1. Overview

Mediaproxy is an OpenSER module that is designed to allow automatic NAT traversal for the majority of existing SIP clients. This means that there will be no need to configure anything in particular on the NAT box to allow these clients to work behind NAT when using the mediaproxy module.


1.2. Principle of operation

This NAT traversal solution operates by placing a mediaproxy server in the middle between 2 SIP user-agents. It mangles the SDP messages for both of them in a way that will make the parties talk with mediaproxy while they think they talk directly with each other.

To achieve this, mediaproxy is actually composed by 2 components:

  • the OpenSER mediaproxy module itself

  • an external proxy server called OpenSER MediaProxy (available from http://mediaproxy.ag-projects.com/ )

To avoid confusion in this document the mediaproxy module will be called 'module' or 'mediaproxy module', while the mediaproxy server will be called 'proxy server' from here on.

The proxy server can be run on the same machine as the module or on a remote host. Moreover it is possible for a single module to control multiple proxy servers running on multiple geographically distributed hosts. To find out more about the architecture of OpenSER MediaProxy please read the documentation that comes with it.

To be able to act as a proxy between the 2 talking parties, the machine(s) running the module/proxy server must have a public IP address.

The module will ask the proxy server to allocate as many sockets as there are media streams in the SDP body of the SIP INVITE/Ok messages. The proxy server will send back to the module the address and port(s) for them. Then the module will replace the original contact IP and RTP ports from the SDP messages with the ones provided by the proxy server. By doing this both clients will try to contact the proxy server instead of talking directly with each other. Once the clients contact the proxy server, it will record the addresses they came from and will know where to forward packets received from the other party This is needed because the address/port the NAT box will allocate for the leaving streams is not known before they actually leave the NAT box. However the address of the proxy server is always known (being a public one) so the 2 parties know where to connect and then after they did so, the proxy learns the addresses they came from and can forward packets between them.


1.3. Types of SIP clients

The SIP clients that will work transparently behind NAT when using the mediaproxy module are the so-called symmetric clients. The symmetric clients have the particularity that use the same port to send the data as the one they use to receive it. In other words, if they are for example configured to use port 5060 for SIP signaling, they will use the same port when sending data as well as when receiving it. This must be true for both the SIP signaling as well as the RTP streams for a client to work transparently with the mediaproxy module without any additional configuration on the NAT box.

This ability is important because the only way to get back to a client behind NAT is to send to the IP address and port the packet was received from. Once a packet is sent from the client behind NAT to the outside world, it opens a communication channel in the NAT box that is open in both directions for a while (it will timeout after a while after no more data is sent through it, but it can be kept active by sending data through it at certain regular time intervals). While this channel is open, any data sent to the public address and port that the NAT box assigned for the address and port the client behind NAT is sending from (and this mapping is guaranteed to be unique), will go back straight to the address and port the client has sent from. This is why is necessary for the clients to be symmetric. If they listen on the same port they sent from, the data sent back to the public address that the NAT box assigned to the leaving packets will actually reach the listening port of the client behind NAT.

Some SIP clients implement particular algorithms to detect if they are actually behind a NAT box and try to act smart by detecting the IP address of the NAT box (or simply allowing one to manually configure it), and then use this IP address in the SIP and SDP messages instead of their own private IP address. This situation can be confusing for a module that tries to perform transparent NAT traversal as it can wrongly mistake such a client that is behind NAT with a client that is actually in the public address space. However for the mediaproxy module it is not important if the clients apply or not this kind of behavior, as it is able to cope with both situations gracefully.

This doesn't mean that mediaproxy is not able to work with asymmetric clients behind NAT, but in their case special static forwarding routes need to be configured on the NAT box.

Mediaproxy has special support for asymmetric clients, can detect them and send the data to the ports they expect it to, however they can work behind NAT only if static routes are configured on the NAT box since there is no way of getting back to an address/port that has not previously opened a data channel in the NAT box by sending something out first. Nevertheless the support for asymmetric clients is important, because without it they won't be able to work even when they have public Internet addresses. Also this support allows one to use an asymmetric client behind NAT if he can configure the NAT box to forward the packets meant to that client.

The only requirement a symmetric SIP client must met to be able to work transparently behind NAT when using the mediaproxy module is to accept to be configured to use a so called outbound proxy and this proxy must be the one running with the mediaproxy module loaded.


1.4. Features

  • make symmetric clients work behind NAT transparently if they use the SIP server as the outbound SIP server.

  • handle all media streams specified in the SDP body. There is a limit of 64 RTP streams per session in the code now, but we hardly find this to be a limitation for the time being.

  • able to distribute RTP traffic load on multiple proxy servers running on multiple hosts.

  • able to specify which proxy server to use based on the SIP domain of the caller/destination (done by the proxy server's dispatcher module).

  • handle asymmetric clients properly. They can even work behind NAT if a proper port forwarding is done for them on the NAT box.


1.5. Dependencies

1.5.1. OpenSER Modules

The following modules must be loaded before this module:

  • domain module

  • usrloc module - this is required if pinging of contacts behind NAT is enabled. In this case the nat_bflag must also be defined on the ursloc module.


1.5.2. External Libraries or Applications

The following libraries or applications must be installed before running OpenSER with this module loaded:

  • None.


1.6. Exported parameters

1.6.1. mediaproxy_socket (string)

It is the path to the filesystem socket where the proxy server listens for commands from the module.

Default value is "/var/run/proxydispatcher.sock".

Example 1-1. Setting mediaproxy_socket parameter

...
modparam("mediaproxy", "mediaproxy_socket", "/var/run/proxydispatcher.sock")
...

1.6.2. sip_asymmetrics (string)

It is the path to a file that lists regular expressions that match 'User-Agent' or 'Server' fields from clients that are asymmetric regarding SIP signaling. Needed to detect when a client is asymmetric regarding SIP signaling. An example file is in the config/ subdirectory.

Default value is "/etc/openser/sip-asymmetric-clients".

Example 1-2. Setting sip_asymmetrics parameter

...
modparam("mediaproxy", "sip_asymmetrics", "/etc/openser/sip-asymmetrics-clients")
...

1.6.3. rtp_asymmetrics (string)

It is the path to a file that lists regular expressions that match 'User-Agent' or 'Server' fields from clients that are asymmetric regarding the RTP media. Needed to detect when a client is asymmetric regarding the RTP media. An example file is in the config/ subdirectory.

Default value is "/etc/openser/rtp-asymmetric-clients".

Example 1-3. Setting rtp_asymmetrics parameter

...
modparam("mediaproxy", "rtp_asymmetrics", "/etc/openser/rtp-asymmetrics-clients")
...

1.6.4. natping_interval (integer)

It holds an integer value representing how often the module will send packets to all registered clients that are behind NAT to keep their opened channels alive. Represents an interval in seconds.

Default value is 60.

Example 1-4. Setting natping_interval parameter

...
modparam("mediaproxy", "natping_interval", 20)
...

1.6.5. signaling_ip_avp (string)

Specification of the AVP which holds the IP address from where the SIP signaling originated. If this AVP is set it will be used to get the signaling IP address, else the source IP address from where the SIP message was received will be used. This AVP is meant to be used in cases where there are more than one proxy in the call setup path and the proxy that actually starts mediaproxy doesn't receive the SIP messages directly from the UA and it cannot determine the NAT IP address from where the signaling originated. In such a case attaching a SIP header at the first proxy and then copying that header's value into the signaling_ip_avp on the proxy that starts mediaproxy will allow it to get the correct NAT IP address from where the SIP signaling originated.

Default value is "$avp(s:signaling_ip)".

Example 1-5. Setting signaling_ip_avp parameter

...
modparam("mediaproxy", "signaling_ip_avp", "$avp(s:nat_ip)")
...

1.6.6. domain_avp (string)

Specification of the AVP which holds an optional application defined domain name, that specifies the domain to be used to select the mediaproxies for the current call. If a domain name is written to this AVP before calling use_media_proxy(), it will be used to select the mediaproxies for the call, overwriting the normal selection process based on the caller/callee domain.

Default value is "$avp(s:mediaproxy_domain)".

Example 1-6. Setting domain_avp parameter

...
modparam("mediaproxy", "domain_avp", "$avp(s:mediaproxy_domain)")
...

1.7. Exported Functions

1.7.1. client_nat_test(type)

Tests if the client is behind NAT or not. The types of tests are specified by the type parameter which represents a sum of the following numbers (add the values of the ones you wish to perform tests for):

  • 1 - tests if client has a private IP address (as defined by RFC1918) in the Contact field of the SIP message.

  • 2 - tests if client has contacted OpenSER from an address that is different from the one in the Via field.

  • 4 - tests if client has a private IP address (as defined by RFC1918) in the top Via field of the SIP message.

For example calling client_nat_test("3") in openser.cfg will perform first 2 tests listen above and return true as soon as one succeeds if both fail will return false.

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, FAILURE_ROUTE, BRANCH_ROUTE.

Example 1-7. client_nat_test usage

...
if (client_nat_test("3")) {
    .....
}
...

1.7.2. fix_contact()

Will replace the IP:Port in the Contact field of the SIP message with the ones the SIP message was received from. For clients that are asymmetric regarding SIP signaling (as determined from the sip_asymmetrics file) will preserve the port.

Usually called after an if (client_nat_test(type)) has succeded

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, BRANCH_ROUTE.

Example 1-8. fix_contact usage

...
if (client_nat_test("3")) {
    fix_contact();
}
...

1.7.3. use_media_proxy()

Will make a call to the proxy server and replace the IPs and ports in the SDP body with the ones returned by the proxy server for each media stream that the SDP message describes. This will force the media streams to be routed through the proxy server.

Called when you want to make the session go through a proxy server.

This function has the following return codes:

  • +1 - successfully modified message (true value)

  • -1 - error in processing message (false value)

  • -2 - missing SDP body, nothing to process (false value)

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, FAILURE_ROUTE, BRANCH_ROUTE.

Example 1-9. use_media_proxy usage

...
if (method==INVITE) {
    use_media_proxy();
}
...

Example 1-10. Using mediaproxies from a specific domain

...
if (method==INVITE) {
    $avp(s:mediaproxy_domain) = "example.com";
    use_media_proxy();
}
...

1.7.4. end_media_session()

Will call on the proxy server to end the media session for that call this is done at the end of the call to instruct the proxy server to free the resources allocated to that call as well as to save log information about the call.

Called when a session should end (BYE or CANCEL received).

This function can be used from REQUEST_ROUTE, ONREPLY_ROUTE, FAILURE_ROUTE, BRANCH_ROUTE.

Example 1-11. end_media_session usage

...
if (method==BYE) {
    end_media_session();
}
...

1.8. Comparison with the nathelper module

After reading all this you may wonder what this module can offer you that the nathelper module (a similar nat traversal solution) can't and why was necessary to develop this module.

While at surface they seem to offer about the same functionality, there are a few core differences that make them quite different.

The main and most notable difference is that mediaproxy offers a distributed environment, where the mediaproxy module can control multiple mediaproxy servers. The mediaproxy servers can be local or remote and they can be specified per domain or as defaults for domains that don't have their own mediaproxy servers defined. These mediaproxy servers can be arranged in load balancing and fallback schemes allowing the platform to scale up easily and also offer redundancy to keep the service running even if some of the mediaproxies go offline. Mediaproxy is able to detect the dead proxies and redistribute the calls among the other mediaproxies that are available. (More details about this can be found in the OpenSER MediaProxy documentation.)

Another important difference is that mediaproxy tries to move the complex logic of decision from the OpenSER configuration file to the module and the proxy servers themselves. This is why there are very few functions in this module that take any parameters. Instead, control is achieved by modifying resources outside of openser.cfg. This includes for example specifying the mediaproxy servers using DNS SRV records, or declaring asymmetric clients in external files that are automatically re-read as soon as they change. This allows OpenSER to run without interruption or restarts. If one wants to change OpenSER's behavior, instead of changing openser.cfg and restarting OpenSER, one will change these external resources and OpenSER will adapt it's behavior on the fly without any need for restart.

Another advantage of this is that openser.cfg becomes simpler and easier to maintain.


1.9. How to use sip_ping from the nathelper module

The nathelper module provides an option to ping with real SIP messages instead of just sending 4 zero bytes, which has the advantage that the communication is bidirectional and thus some NATs that only keep the connection open if there is traffic from the inside won't close the pinholes (the 4 zero byte ping doesn't have a reply from inside the NAT).

Example 1-12. Pinging example configuration

...
loadmodule "/lib/openser/modules/mediaproxy.so"
loadmodule "/lib/openser/modules/nathelper.so"

modparam("mediaproxy", "natping_interval", 0)

modparam("nathelper", "rtpproxy_disable", 1)
modparam("nathelper", "natping_interval", 30)
modparam("nathelper", "sipping_from", "sip:ping@sipserver.net")

modparam("registrar", "nat_flag", 6)
modparam("registrar", "sip_natping_flag", 2)
....
....
if (method=="REGISTER") {
    setflag(6);  # nat
    setflag(2);  # sip_ping
    if (!save("location")) {
        sl_reply_error();
    }
}
...

Chapter 2. Developer's Guide

The module does not provide any API to use in other OpenSER modules.


Chapter 3. Frequently Asked Questions

3.1. Where can I find more about OpenSER?
3.2. Where can I post a question about this module?
3.3. How can I report a bug?

3.1. Where can I find more about OpenSER?

3.2. Where can I post a question about this module?

First at all check if your question was already answered on one of our mailing lists:

E-mails regarding any stable OpenSER release should be sent to and e-mails regarding development versions should be sent to .

If you want to keep the mail private, send it to .

3.3. How can I report a bug?

Please follow the guidelines provided at: http://sourceforge.net/tracker/?group_id=139143.