Interpreting MGCP Debug Parameters

I manage a number of deployments that still feature ISDN extensively, so debug mgcp packet is something that I’m reading off IOS CLI on a fairly routine basis at the moment.

There are a lot of blog posts that cover reading MGCP debugs pretty well, but one thing that I struggled with for a while was being able to quickly interpret the  debug parameters and what they stood for in each respective MGCP message.  Often these are fairly intuitive, but sometimes a quick point of reference is needed.

I looked around on Cisco forums etc., and really there just isn’t anything that covers this well.  Eventually I resorted to the RFC, and was pleasantly unsurprised that all the parameters were well tabulated and easily reference-able.

I’ve included the list below for reference, but as always it is best to go straight to RFC 3435 to get this information!

 


3.2.2 Parameter Lines

Parameter lines are composed of a parameter name, which in most cases
   is composed of one or two characters, followed by a colon, optional
   white space(s) and the parameter value.  The parameters that can be
   present in commands are defined in the following table:


    ------------------------------------------------------------------
   |Parameter name        | Code |  Parameter value                   |
   |----------------------|------|------------------------------------|
   |BearerInformation     |   B  |  See description (3.2.2.1).        |
   |CallId                |   C  |  See description (3.2.2.2).        |
   |Capabilities          |   A  |  See description (3.2.2.3).        |
   |ConnectionId          |   I  |  See description (3.2.2.5).        |
   |ConnectionMode        |   M  |  See description (3.2.2.6).        |
   |ConnectionParameters  |   P  |  See description (3.2.2.7).        |
   |DetectEvents          |   T  |  See description (3.2.2.8).        |
   |DigitMap              |   D  |  A text encoding of a digit map.   |
   |EventStates           |   ES |  See description (3.2.2.9).        |
   |LocalConnectionOptions|   L  |  See description (3.2.2.10).       |
   |MaxMGCPDatagram       |   MD |  See description (3.2.2.11).       |
   |NotifiedEntity        |   N  |  An identifier, in RFC 821 format, |
   |                      |      |  composed of an arbitrary string   |
   |                      |      |  and of the domain name of the     |
   |                      |      |  requesting entity, possibly com-  |
   |                      |      |  pleted by a port number, as in:   |
   |                      |      |    Call-agent@ca.example.net:5234  |
   |                      |      |  See also Section 3.2.1.3.         |
   |ObservedEvents        |   O  |  See description (3.2.2.12).       |
   |PackageList           |   PL |  See description (3.2.2.13).       |
   |QuarantineHandling    |   Q  |  See description (3.2.2.14).       |
   |ReasonCode            |   E  |  A string with a 3 digit integer   |
   |                      |      |  optionally followed by a set of   |
   |                      |      |  arbitrary characters (3.2.2.15).  |
   |RequestedEvents       |   R  |  See description (3.2.2.16).       |
   |RequestedInfo         |   F  |  See description (3.2.2.17).       |
   |RequestIdentifier     |   X  |  See description (3.2.2.18).       |
   |ResponseAck           |   K  |  See description (3.2.2.19).       |
   |RestartDelay          |   RD |  A number of seconds, encoded as   |
   |                      |      |  a decimal number.                 |
   |RestartMethod         |   RM |  See description (3.2.2.20).       |
   |SecondConnectionId    |   I2 |  Connection Id.                    |
   |SecondEndpointId      |   Z2 |  Endpoint Id.                      |
   |SignalRequests        |   S  |  See description (3.2.2.21).       |
   |SpecificEndPointId    |   Z  |  An identifier, in RFC 821 format, |
   |                      |      |  composed of an arbitrary string,  |
   |                      |      |  followed by an "@" followed by    |
   |                      |      |  the domain name of the gateway to |
   |                      |      |  which this endpoint is attached.  |
   |                      |      |  See also Section 3.2.1.3.         |
   |RemoteConnection-     |   RC |  Session Description.              |
   |         Descriptor   |      |                                    |
   |LocalConnection-      |   LC |  Session Description.              |
   |         Descriptor   |      |                                    |
    ------------------------------------------------------------------

   The parameters are not necessarily present in all commands.  The
   following table provides the association between parameters and
   commands.  The letter M stands for mandatory, O for optional and F
   for forbidden.  Unless otherwise specified, a parameter MUST NOT be
   present more than once.

This table is great as I can simply look up the parameter code in my MGCP packet debug and do a quick interpretation.  I also find this very handy when reading off MGCP Connection ID’s and other call-specific tracing that can be cross-referenced in SDL!


 

For example, in a simple AUEP (Audit Endpoint), we see:

 

*Jul 17 11:14:18.332: MGCP Packet received from 192.168.123.213:2427—>
AUEP 31 S0/SU2/DS1-0/1@RTR1.uc.lab MGCP 0.1
F: X, A, I
<—

What has remote side requested information on?

 

Firstly, the side has requested information in a comma-separated list:

|RequestedInfo         |   F  |

 

The list contains the following:

|RequestIdentifier     |   X  |
|Capabilities          |   A  |
|ConnectionId          |   I  |

 

Simple stuff.

 

#dontcalltac

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