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4 Internal form and its encodings

This version of the stack is compliant with:

4.1 Internal form of messages

We use the same internal form for both the binary and text encoding. Our internal form of Megaco/H.248 messages is heavily influenced by the internal format used by ASN.1 encoders/decoders:

In order to fully understand the internal form you must get hold on a ASN.1 specification for the Megaco/H.248 protocol, and apply the rules above. Please, see the documentation of the ASN.1 compiler in Erlang/OTP for more details of the semantics in mapping between ASN.1 and the corresponding internal form.

Observe that the 'TerminationId' record is not used in the internal form. It has been replaced with a megaco_term_id record (defined in "megaco/include/megaco.hrl").

4.2 The different encodings

The Megaco/H.248 standard defines both a plain text encoding and a binary encoding (ASN.1 BER) and we have implemented encoders and decoders for both. We do in fact supply five different encoding/decoding modules.

In the text encoding, implementors have the choice of using a mix of short and long keywords. It is also possible to add white spaces to improve readability. We use the term compact for text messages with the shortest possible keywords and no optional white spaces, and the term pretty for a well indented text format using long keywords and an indentation style like the text examples in the Megaco/H.248 specification).

Here follows an example of a text message to give a feeling of the difference between the pretty and compact versions of text messages. First the pretty, well indented version with long keywords:

   MEGACO/1 []
   Transaction = 9998 {
           Context = - {
                   ServiceChange = ROOT {
                           Services {
                                   Method = Restart,
                                   ServiceChangeAddress = 55555,
                                   Profile = ResGW/1,
                                   Reason = "901 Cold Boot"

Then the compact version without indentation and with short keywords:

   !/1 []
   T=9998{C=-{SC=ROOT{SV{MT=RS,AD=55555,PF=ResGW/1,RE="901 Cold Boot"}}}}    

And the programmers view of the same message. First a list of ActionRequest records are constructed and then it is sent with one of the send functions in the API:

  Prof = #'ServiceChangeProfile'{profileName = "resgw", version = 1},
  Parm = #'ServiceChangeParm'{serviceChangeMethod  = restart,
                              serviceChangeAddress = {portNumber, 55555},
                              serviceChangeReason  = "901 Cold Boot",
                              serviceChangeProfile = Prof},
  Req = #'ServiceChangeRequest'{terminationID = [?megaco_root_termination_id],
                                serviceChangeParms = Parm},
  Actions = [#'ActionRequest'{contextId = ?megaco_null_context_id,
                              commandRequests = {serviceChangeReq, Req}}],
  megaco:call(ConnHandle, Actions, Config).    

And finally a print-out of the entire internal form:

    {ip4Address,{'IP4Address', [124,124,124,222], asn1_NOVALUE}},
                {megaco_term_id, false, ["root"]}],
                 {portNumber, 55555},
                 {'ServiceChangeProfile', "resgw", version = 1},
                 "901 MG Cold Boot",

The following encoding modules are provided:

4.3 Configuration of Erlang distribution encoding module

The encoding_config of the megaco_erl_dist_encoder module may be one of these:

4.4 Configuration of text encoding module(s)

When using text encoding(s), there is actually two different configs controlling what software to use:

The Flex scanner is a Megaco scanner written as a linked in driver (in C). There are two ways to get this working:

4.5 Configuration of binary encoding module(s)

When using binary encoding, the structure of the termination id's needs to be specified.

4.6 Handling megaco versions

Since the version 3 implemented, in this version of the Megaco application, is preliminary, it is necessary to have a way to handle different version 3 implementations. For this reason the encoding config option {version3, version3()} has been introduced. This option, if present, has to be first in the encoding config list. Version 1 and 2 codec's ignore this option, if found.

version3() -> prev3a | prev3b | prev3c | v3    

There are two ways to handle the different megaco encoding versions. Either using dynamic version detection (only valid for for incoming messages) or by explicit version setting in the connection info.

For incomming messages:

For outgoing messages:

4.7 Encoder callback functions

The encoder callback interface is defined by the megaco_encoder behaviour, see megaco_encoder.

megaco 3.7
Copyright © 1991-2007 Ericsson AB