maxfwd Module

Bogdan-Andrei Iancu

   Voice Sistem SRL

Edited by

Bogdan-Andrei Iancu

   Copyright © 2003 FhG FOKUS
     __________________________________________________________________

   Table of Contents

   1. Admin Guide

        1. Overview
        2. Dependencies

              2.1. Kamailio Modules
              2.2. External Libraries or Applications

        3. Parameters

              3.1. max_limit (integer)

        4. Functions

              4.1. maxfwd_process(max_value)
              4.2. mf_process_maxfwd_header(max_value)
              4.3. process_maxfwd(max_value)
              4.4. is_maxfwd_lt(max_value)
              4.5. maxfwd_at_least(max_value)
              4.6. mf_lowlimit(max_value)

   List of Examples

   1.1. Set max_limit parameter
   1.2. maxfwd_process usage
   1.3. is_maxfwd_lt usage

Chapter 1. Admin Guide

   Table of Contents

   1. Overview
   2. Dependencies

        2.1. Kamailio Modules
        2.2. External Libraries or Applications

   3. Parameters

        3.1. max_limit (integer)

   4. Functions

        4.1. maxfwd_process(max_value)
        4.2. mf_process_maxfwd_header(max_value)
        4.3. process_maxfwd(max_value)
        4.4. is_maxfwd_lt(max_value)
        4.5. maxfwd_at_least(max_value)
        4.6. mf_lowlimit(max_value)

1. Overview

   The module implements all of the operations that handle the Max-Forward
   SIP header field, like adding it (if not present) or decrementing and
   checking the value of the existent one.

   The SIP Max-Forward header is used to prevent loops in a SIP network.
   Every server that process and forward a SIP request lowers the
   Max-Forward value with one. When the value reaches zero, the request is
   not forwarded and an error response is sent to the UAC.

2. Dependencies

   2.1. Kamailio Modules
   2.2. External Libraries or Applications

2.1. Kamailio Modules

   The following modules must be loaded before this module:
     * No dependencies on other Kamailio modules.

2.2. External Libraries or Applications

   The following libraries or applications must be installed before
   running Kamailio with this module loaded:
     * None.

3. Parameters

   3.1. max_limit (integer)

3.1. max_limit (integer)

   Set an upper limit for the max-forward value in the outgoing requests.
   If the header is present, the decremented value is not allowed to
   exceed this max_limits - if it does, the header value will by decreased
   to "max_limit".

   Note: This check is done when calling the maxfwd_process() function.

   The range of values stretches from 1 to 256, which is the maximum
   MAX-FORWARDS value allowed by RFC 3261. The value can be changed at
   runtime.

   Default value is "16".

   Example 1.1. Set max_limit parameter
...
modparam("maxfwd", "max_limit", 32)
...

4. Functions

   4.1. maxfwd_process(max_value)
   4.2. mf_process_maxfwd_header(max_value)
   4.3. process_maxfwd(max_value)
   4.4. is_maxfwd_lt(max_value)
   4.5. maxfwd_at_least(max_value)
   4.6. mf_lowlimit(max_value)

4.1. maxfwd_process(max_value)

   If no Max-Forward header is present in the received request, a header
   will be added having the original value equal with "max_value". If a
   Max-Forward header is already present, its value will be decremented
   (if not 0). The parameter can be a variable.

   Return codes:
     * 2 (true) - header was not found and a new header was succesfully
       added.
     * 1 (true) - header was found and its value was successfully
       decremented (had a non-0 value).
     * -1 (false) - the header was found and its value is 0 (cannot be
       decremented).
     * -2 (false) - error during processing.

   The return code may be extensivly tested via script variable "retcode"
   (or "$?").

   Meaning of the parameters is as follows:
     * max_value - Value to be added if there is no Max-Forwards header
       field in the message.

   This function can be used from REQUEST_ROUTE.

   Example 1.2. maxfwd_process usage
...
# initial sanity checks -- messages with
# max_forwards==0, or excessively long requests
if (!maxfwd_process("10") && $retcode==-1) {
        sl_send_reply("483","Too Many Hops");
        exit;
};
...

4.2. mf_process_maxfwd_header(max_value)

   Same as maxfwd_process(max_value).

4.3. process_maxfwd(max_value)

   Same as maxfwd_process(max_value).

4.4. is_maxfwd_lt(max_value)

   Checks if the Max-Forward header value is less then the "max_value"
   parameter value. It considers also the value of the new inserted header
   (if locally added). The parameter can be a variable.

   Retuning codes:
     * 1 (true) - header was found or set and its value is stricly less
       than "max_value".
     * -1 (false) - the header was found or set and its value is greater
       or equal to "max_value".
     * -2 (false) - header was not found or not set.
     * -3 (false) - error during processing.

   The return code may be extensivly tested via script variable "retcode"
   (or "$?").

   Meaning of the parameters is as follows:
     * max_value - value to check the Max-Forward.value against (as less
       than).

   Example 1.3. is_maxfwd_lt usage
...
# next hop is a gateway, so make no sense to
# forward if MF is 0 (after decrement)
if ( is_maxfwd_lt("1") ) {
        sl_send_reply("483","Too Many Hops");
        exit;
};
...

4.5. maxfwd_at_least(max_value)

   Same as is_maxfwd_lt(max_value).

4.6. mf_lowlimit(max_value)

   Same as is_maxfwd_lt(max_value).
