> ## 4 Internal form and its encodings

> This version of the stack is compliant with Megaco/H.248 version 1 (RFC3015) updated to Implementors Guide version 6 (IGv6).

> Please observe that some incompatible features is introduced by IGv6.

> ### 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:

> - "SEQUENCE OF" is represented as a list.
> - "CHOICE" is represented as a tagged tuple with size 2.
> - "SEQUENCE" is represented as a record, defined in "megaco/include/megaco_message_v1.hrl".
> - "OPTIONAL" is represented as an ordinary field in a record which defaults to 'asn1_NOVALUE', meaning that the field has no value.
> - "OCTET STRING" is represented as a list of unsigned integers.
> - "ENUMERATED" is represented as a single atom.
> - "BIT STRING" is represented as a list of atoms.
> - "BOOLEAN" is represented as the atom 'true' or 'false'.
> - "INTEGER" is represented as an integer.
> - "IA5String" is represented as a list of integers, where each integer is the ASCII value of the corresponding character.
> - "NULL" is represented as the atom 'NULL'.

> 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 [124.124.124.222]
>        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 [124.124.124.222]
>        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:

>       {'MegacoMessage',
>        asn1_NOVALUE,
>        {'Message',
>         1,
>         {ip4Address,{'IP4Address', [124,124,124,222], asn1_NOVALUE}},
>         {transactions,
>          [
>           {transactionRequest,
>            {'TransactionRequest',
>              9998,
>              [{'ActionRequest',
>                0,
>                asn1_NOVALUE,
>                asn1_NOVALUE,
>                [
>                 {'CommandRequest',
>                  {serviceChangeReq,
>                   {'ServiceChangeRequest',
>                    [
>                     {megaco_term_id, false, ["root"]}],
>                     {'ServiceChangeParm',
>                      restart,
>                      {portNumber, 55555},
>                      asn1_NOVALUE,
>                      {'ServiceChangeProfile', "resgw", 1},
>                      "901 MG Cold Boot",
>                      asn1_NOVALUE,
>                      asn1_NOVALUE,
>                      asn1_NOVALUE
>                     }
>                   }
>                  },
>                  asn1_NOVALUE,
>                  asn1_NOVALUE
>                 }
>                ]
>               }
>              ]
>            }
>           }
>          ]
>         }
>        }
>       }

> The following encoding modules are supported:

> - megaco_pretty_text_encoder - encodes messages into pretty text format, decodes both pretty as well as compact text.
> - megaco_compact_text_encoder - encodes messages into compact text format, decodes both pretty as well as compact text.
> - megaco_binary_encoder - encode/decode ASN.1 BER messages. This encoder implements the fastest of the BER encoders/decoders. Recommended binary codec.
> - megaco_ber_encoder - encode/decode ASN.1 BER messages.
> - megaco_ber_bin_encoder - encode/decode ASN.1 BER messages. This encoder uses ASN.1 ber_bin which has been optimized using the bit syntax.
> - megaco_per_encoder - encode/decode ASN.1 PER messages. N.B. that this format is not included in the Megaco standard.
> - megaco_per_bin_encoder - encode/decode ASN.1 PER messages. N.B. that this format is not included in the Megaco standard. This encoder uses ASN.1 per_bin which has been optimized using the bit syntax.
> - megaco_erl_dist_encoder - encodes messages into Erlangs distribution format. It is rather verbose but encoding and decoding is blinding fast. N.B. that this format is not included in the Megaco standard.

> ### 4.3 Configuration of Erlang distribution encoding module

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

> - `[]` - Encodes the messages to the standard distribution format. It is rather verbose but encoding and decoding is blinding fast.
> - `[compressed]` - Encodes the messages to a compressed form of the standard distribution format. It is less verbose, but the encoding and decoding will on the other hand be slower.

> ### 4.4 Configuration of text encoding module(s)

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

> - `[]` - An empty list indicates that the erlang codec should be used.
> - `[{flex, port()}]` - Use the flex scanner when decoding.

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

> - Let the Megaco stack start the flex scanner (load the driver).
>   To make this happen the megaco stack has to be configured:
>   - Add the `{scanner, flex}` directive to an Erlang system config file for the megaco app. This will make the Megaco stack to initiate the default `megaco_receive_handle` with the encoding_config set to the `[{flex, port()}]`.
>   - When retrieving the `megaco_receive_handle`, retain the encoding_config.
>   The benefit of this is that Megaco handles the starting, holding and the supervision of the driver and port.
> - The Megaco client (user) starts the flex scanner (load the driver).
>   When starting the flex scanner a port to the linked in driver is created. This port has to be owned by a process. This process must not die. If it does the port will also terminate. Therefor:
>   - Create a permanent process. Make sure this process is supervised (so that if it does die, this will be noticed).
>   - Let this process start the flex scanner by calling the `megaco_flex_scanner:start()` function.
>   - Retrieve the `port()` and when initiating the `megaco_receive_handle`, set the encoding_config to [{flex, port()}].
>   - Pass the `receive_handle` to the transport module.

> ### 4.5 Configuration of binary encoding module(s)

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

> - `[native]` - skips the transformation phase, i.e. the decoded message(s) will not be transformed into our internal form.
> - `[integer()]` - A list containing the size (the number of bits) of each level. Example: `[3,8,5,8]`.
> - `integer()` - Number of one byte (8 bits) levels. N.B. This is currently converted into the previous config. Example: `3` (`[8,8,8]`).

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