SST Module (SIP Session Timer)

Ron Winacott

SOMA Networks, Inc.

Edited by

Ron Winacott


Table of Contents

1. Admin Guide
1. Overview
2. How it works
3. Dependencies
3.1. Kamailio Modules
3.2. External Libraries or Applications
4. Parameters
4.1. enable_stats (integer)
4.2. min_se (integer)
4.3. timeout_avp (string)
4.4. reject_to_small (integer)
4.5. sst_flag (integer)
5. Functions
5.1. sstCheckMin(send_reply_flag)
6. Statistics
6.1. expired_sst
7. Installation and Running

List of Examples

1.1. Session timer call flow
1.2. Set enable_stats parameter
1.3. Set min_se parameter
1.4. Set timeout_avp parameter
1.5. Set reject_to_small parameter
1.6. Set sst_flag parameter
1.7. sstCheckMin usage

Chapter 1. Admin Guide

1. Overview

SIP session timers are used to make sure that a session (dialog) is still alive, even though there may have been a long time since the last in-dialog message. If the other end is not responding, the dialog will be hung up automatically. SIP session timers need to be supported by all end points for it to work. It's a SIP extension, standardized by the IETF.

The sst module provides a way to update the dialog expiry timer based on the SIP INVITE/200 OK Session-Expires header value. You can use the sst module in an Kamailio proxy to allow freeing of local resources of dead calls.

You can also use the sst module to validate the MIN_SE header value and reply to any request with a "422 - Session Timer Too Small" if the value is too small for your Kamailio configuration.

2. How it works

The sst module uses the dialog module to be notified of any new or updated dialogs. It will then look for and extract the session-expire: header value (if there is one) and override the dialog expire timer value for the current context dialog by setting the avp value.

You flag any call setup INVITE that you want to cause a timed session to be established. This will cause Kamailio to request the use of session timers if the UAC does not request it.

All of this happens with a properly configured dialog and sst module and setting the dialog flag and the sst flag at the time any INVITE sip message is seen. There is no kamailio.cfg script function call required to set the dialog expire timeout value. See the dialog module users guide for more information.

The sstCheckMin() script function can be used to verify that the Session-expires / MIN-SE header field values are not too small for your server. If the SST min_se parameter value is smaller than the messages Session-Expires / MIN-SE values, the test will return true. You can also configure the function to send the 422 error response for you.

The following was taken from the RFC as a call flow example:

Example 1.1. Session timer call flow

+-------+    +-------+       +-------+
| UAC-1 |    | PROXY |       | UAC-2 |
+-------+    +-------+       +-------+
    |(1) INVITE  |               |
    |SE: 50      |               |
    |----------->|               |
    |            |(2)sstCheckMin |
    |            |-----+         |
    |            |     |         |
    |            |<----+         |
    |(3) 422     |               |
    |MSE:1800    |               |
    |<-----------|               |
    |            |               |
    |(4)ACK      |               |
    |----------->|               |
    |            |               |
    |(5) INVITE  |               |
    |SE: 1800    |               |
    |MSE: 1800   |               |
    |----------->|               |
    |            |(6)sstCheckMin |
    |            |-----+         |
    |            |     |         |
    |            |<----+         |
    |            |(7)setflag     |
    |            |Dialog flag    |
    |            |Set expire     |
    |            |-----+         |
    |            |     |         |
    |            |<----+         |
    |            |               |
    |            |(8)INVITE      |
    |            |SE: 1800       |
    |            |MSE: 1800      |
    |            |-------------->|
    |            |               |
 ...

3. Dependencies

3.1. Kamailio Modules

The following modules must be loaded before this module:

  • dialog or dialog-ng - dialog module and its dependencies. (tm)

  • sl - stateless module.

3.2. External Libraries or Applications

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

  • None.

4. Parameters

4.1. enable_stats (integer)

If the statistics support should be enabled or not. Via statistic variables, the module provide information about the dialog processing. Set it to zero to disable or to non-zero to enable it.

Default value is 1 (enabled).

Example 1.2. Set enable_stats parameter

...
modparam("sst", "enable_stats", 0)
...

4.2. min_se (integer)

The value is used to set the proxies MIN-SE value and is used in the 422 error reply as the proxies MIN-SE: header value if the sstCheckMin() flag is set to true and the check fails.

If not set and sstCheckMin() is called with the send-reply flag set to true, the default 1800 seconds will be used as the compare and the MIN-SE: header value if the 422 reply is sent.

Default value is 1800 seconds.

Example 1.3. Set min_se parameter

...
modparam("sst", "min_se", 2400)
...

4.3. timeout_avp (string)

This parameter MUST be set to the same value as the dialog module parameter of the same name. If this parameter is NOT set, the sst module will not do anything!

This is how the sst module knows which avp in the dialog module it has to change with the new expire value.

Default value is NULL! it is not set by default.

Example 1.4. Set timeout_avp parameter

...
modparam("dialog", "timeout_avp", "$avp(i:10)")
# Set the sst modules timeout_avp to be the same value
modparam("sst", "timeout_avp", "$avp(i:10)")
...

4.4. reject_to_small (integer)

In the initial INVITE if the UAC has requested a Session-Expire: and it's value is smaller than our local policies Min-SE (see min_se above), then the proxy has the right to reject the call by replying to the message with a 422 Session Timer Too Small and state our local Min-SE: value. The INVITE is NOT forwarded on through the proxy.

If this flag is true, the SST module to reject the INVITE with a 422 response. If false, the INVITE is forwarded through the PROXY without any modifications.

Default value is 1 (true/on).

Example 1.5. Set reject_to_small parameter

...
modparam("sst", "reject_to_small", 0)
...

4.5. sst_flag (integer)

Keeping with Kamailio, the module will not do anything to any message unless instructed to do so via the kamailio.cfg script. You must set the sst_flag value in the setflag() call of the INVITE you want the sst module to process. But before you can do that, you need to tell the sst module which flag value you are assigning to sst.

In most cases whenever you set the dialog flag you will want to set the sst flag. If the dialog flag is not set and the sst flag is set, it will not have any effect.

This parameter must be set or the module will not load.

Default value is Not set!.

Example 1.6. Set sst_flag parameter

...
modparam("dialog", "dlg_flag", 5)
modparam("sst", "sst_flag", 6)
...
route {
  ...
  if (method=="INVITE") {
    setflag(5); # set the dialog flag
    setflag(6); # Set the sst flag
  }
  ...
}

5. Functions

5.1.  sstCheckMin(send_reply_flag)

Check the current Session-Expires / MIN-SE values against the sst_min_se parameter value. If the Session-Expires or MIN_SE header value is less than the modules minimum value, this function will return true.

If the function is called with the send_reply_flag set to true (1) and the requested Session-Expires / MIN-SE values are too small, a 422 reply will be sent for you. The 422 will carry a MIN-SE: header with the sst min_se parameter value set.

Meaning of the parameters is as follows:

  • min_allowed - The value to compare the MIN_SE header value to.

Example 1.7. sstCheckMin usage

...
modparam("dialog", "timeout_avp", "$avp(i:4242)")
modparam("dialog", "dlg_flag", 5)
...
modparam("sst", "sst_flag", 6)
modparam("sst", "timeout_avp", "$avp(i:4242)")
modparam("sst", "min_se", 2400) # Must be >= 90
...

route {
  if (method=="INVITE") {
	if (sstCheckMin("1")) {
		xlog("L_ERR", "422 Session Timer Too Small reply sent.\n");
		exit;
	}
	# track the session timers via the dialog module
	setflag(5);
	setflag(6);
  }
}

... or ...

route {
  if (method=="INVITE") {
	if (sstCheckMin("0")) {
		xlog("L_ERR", "Session Timer Too Small, dropping request\n");
		exit;
	}
	# track the session timers via the dialog module
	setflag(5);
	setflag(6);
  }
}
...

6. Statistics

6.1. expired_sst

Number of dialogs which got expired session timer.

7. Installation and Running

Just load the module and remember to set the timeout_avp value.