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Seagull - H248/MEGACO protocol

H248/MEGACO protocol details

The implementation of H248/MEGACO in Seagull conforms to H248 standards, ASCII form (ASN.1 version not supported).

Note

Due to the nature of H248 (one scenario contains several H248 transactions with multiple transaction-ids), Seagull supports only client scenarios. Seagull supports server scenarios with the limitation of one H248 transaction per scenario.

Note

H248 implementation in Seagull can be entirely customized through the H248 XML dictionary - which should allow almost any H248 extensions to be supported.

Getting started with H248

First try

Note

As a generic rule, Seagull is packaged with default server and client scenario so that you can start to derive test scenarios from the one already embedded. In the case of H.248, the default client (client.xml) and server (server.xml) scenario are not H.248 compliant in the sense that the transaction-id is the same for all transactions during a scenario. As Seagull supports H248 "client" scenarios with multiple transaction-ids, and to provide an H248 compliant example, another scenario called "client_multi_tid.xml" is provided but cannot be executed against a Seagull server. The following section details this "client_multi_tid.xml" scenario.

So that you can get familiar with Seagull in the context of H248, here is an example that will launch the following H248 scenario:

     | Trans (Add) |
     |------------>|
     | Reply (Add) |
     |<------------|
     |TransRespAck |
     |------------>|
     | Trans (Play)|
     |------------>|
     | Trans (Sub) |
     |------------>|
     | Reply (Sub) |
     |<------------|
     |TransRespAck |
     |------------>|

You can also see the client_multi_tid XML scenario.

First try explained

Here is the commented version of the H248 client which, for the example, mixes both short and pretty H248 form:

Scenario Comments 
<?xml version="1.0" encoding="ISO-8859-1" ?>
<scenario>

  <counter>
    <counterdef name="transaction-counter" init="999">
    </counterdef>
  </counter>

  <traffic>

<!---------------## ADD ##------------------>
<!---------------Send ADD------------------->
    <send channel="channel-1">
      <action>
        <inc-counter name="transaction-counter"></inc-counter>
        <set-value name="transaction-id"
            format="$(transaction-counter)"></set-value>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554
                T=18571]] >
        <!-- body -->
        <![CDATA[C=${A=${M{TS{SI=iv,BF=off},
                ST=1{O{MO=sr,RV=off,RG=off},
                R{m=audio 49152 RTP/AVP 3 97 98 8 0 101
                  c=IN IP4 16.16.214.175
                  a=rtpmap:3 GSM/8000
                  a=rtpmap:97 iLBC/8000
                  a=rtpmap:98 iLBC/8000
                  a=fmtp:98 mode=20
                  a=rtpmap:8 PCMA/8000
                  a=rtpmap:0 PCMU/8000
                  a=rtpmap:101 telephone-event/8000
                  a=fmtp:101 0-11,16
                 }}}}}}]] >
      </message>
      <action>
        <!-- Store the transaction-id for following messages -->
        <store name="TID" entity="transaction-id">
          <regexp name="t-id"
              expr="[Tt][[:space:]]*[=][[:space:]]*([0-9]+)"
              nbexpr="2"
              subexpr="1"></regexp>
        </store>
      </action>
    </send>
<!-------------Receive ADD reply------------>
    <receive channel="channel-1">
      <message>
        <!-- header -->
        <![CDATA[MEGACO/1 [16.16.88.188\]:55555
                Reply=18571]] >
        <!-- body -->
        <![CDATA[Context=1{Add=T1{Media
                {TerminationState{
                 ServiceStates=inservice,Buffer=off},
                Stream=1{LocalControl{Mode=sendreceive,Reserve
                dValue=off,ReservedGroup=off},Local{
                  v=0
                  o=user 0 0 IN IP4 16.16.88.188
                  s=-
                  t=0 0
                  m=audio 3436 RTP/AVP 8 0 101
                  c=IN IP4 16.16.88.188
                  a=rtpmap:8 PCMA/8000/1
                  a=rtpmap:0 PCMU/8000/1
                  a=rtpmap:101 telephone-event/8000/1
                },
                Remote{
                  v=0
                  o=user 0 0 IN IP4 16.16.88.188
                  s=-
                  t=0 0
                  m=audio 49152 RTP/AVP 3 97 98 8 0 101
                  c=IN IP4 16.16.214.175
                  a=rtpmap:3 GSM/8000
                  a=rtpmap:97 iLBC/8000
                  a=rtpmap:98 iLBC/8000
                  a=fmtp:98 mode=20
                  a=rtpmap:8 PCMA/8000
                  a=rtpmap:0 PCMU/8000
                  a=rtpmap:101 telephone-event/8000
                  a=fmtp:101 0-11,16
                }}}}}}]] >
      </message>
      <action>
        <!-- Store the termination-id for following messages -->
        <store name="TERMID" entity="termination-id"></store>
        <!-- Store the context-id for following messages -->
        <store name="CONTID" entity="context-id">
          <regexp name="cont-id"
        expr="Context[[:space:]]*[=]([[:digit:]]*)[[:space:]]*[{]"
              nbexpr="2"
              subexpr="1"></regexp>
        </store>
      </action>
    </receive>
<!-------------Send Ack--------------------->
    <send channel="channel-1">
      <action>
        <restore name="TID" entity="transaction-id-in-body">
        </restore>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554 
            TransactionResponseAck]] >
        <!-- body -->
        <![CDATA[18571}]] >
     </message>
    </send>

<!--------------## MODIFY ##---------------->
<!------------Send MODIFY------------------->
    <send channel="channel-1">
      <action>
        <inc-counter name="transaction-counter"></inc-counter>
        <set-value name="transaction-id" 
          format="$(transaction-counter)"></set-value>
        <!-- Seagull's session ID was value of TID and 
              is now value of transaction-id -->
        <set-new-session-id name="TID" entity="transaction-id">
           </set-new-session-id>
        <restore name="TERMID" entity="termination-id"></restore>
        <restore name="CONTID" entity="context-id"></restore>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554
                Transaction=18572]] >
        <!-- body -->
        <![CDATA[C=1{MF=T1
                  {SG{aasb/play
         {an="sid=<file://home/ocmpadm/welcome.wav>"}
                   },
                   E=100{g/sc}}}}]] > 
      </message>
      <action>
        <!-- Store the transaction-id for following messages -->
        <store name="TID" entity="transaction-id">
          <regexp name="t-id"
          expr="Transaction*[=][[:space:]]*([0-9]+)"
          nbexpr="2"
          subexpr="1"></regexp>
        </store>
      </action>
    </send>
 <!----------Receive MODIFY response--------->
    <receive channel="channel-1">
      <message>
        <!-- header -->
        <![CDATA[MEGACO/1 [16.16.88.188\]:55555
                Reply=18572]] >
        <!-- body -->
        <![CDATA[Context=1{Modify=T1}}]] >
     </message>
    </receive>
 <!-------------Send MODIFY Ack------------->
    <send channel="channel-1">
      <action>
        <restore name="TID" entity="transaction-id-in-body">
        </restore>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554
                TransactionResponseAck]] >
        <!-- body -->
        <![CDATA[18572}]] > 
     </message>
    </send>

 <!--------------## SUBTRACT ##---------------->
 <!--------------Send SUBTRACT----------------->
    <send channel="channel-1">
      <action>
        <inc-counter name="transaction-counter"></inc-counter>
        <set-value name="transaction-id" 
          format="$(transaction-counter)"></set-value>
        <!-- Seagull's session ID was value of TID and 
             is now value of transaction-id -->
        <set-new-session-id name="TID" entity="transaction-id">
             </set-new-session-id>
        <restore name="CONTID" entity="context-id"></restore>
        <restore name="TERMID" entity="termination-id"></restore>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554
                 Transaction=18573]] >
        <!-- body -->
        <![CDATA[C=1{S=T1}}]] > 
      </message>
      <action>
      <!-- Store the transaction-id for following messages -->
        <store name="TID" entity="transaction-id">
          <regexp name="t-id"
              expr="Transaction*[=][[:space:]]*([0-9]+)"
              nbexpr="2"
              subexpr="1"></regexp>
        </store>
      </action>
    </send>
 <!-----------Receive SUBTRACT reply---------->
    <receive channel="channel-1">
      <message>
        <!-- header -->
        <![CDATA[MEGACO/1 [16.16.88.188\]:55555
                Reply=18573]] >
        <!-- body -->
        <![CDATA[Context=1{Subtract=T1}}]] >
     </message>
    </receive>
 <!---------------Send Ack-------------------->
    <send channel="channel-1">
      <action>
        <restore name="TID" entity="transaction-id-in-body">
        </restore>
      </action>
      <message>
        <!-- header -->
        <![CDATA[!/1 [16.16.88.188\]:55554
                TransactionResponseAck]] >
        <!-- body -->
        <![CDATA[18573}]] > 
     </message>
    </send>

  </traffic>
</scenario>

XML header

              
The transaction-counter counter is declared and 
will be incremented for each new transaction 
(several time per call)





Send the ADD


Increment the transaction counter
...and set the transaction-id field, as declared
in the dictionary with the transaction-counter value


H248 header. Note the "\]" instead of "]"

The value of the Transaction-Id (T=18571 here) is 
replaced by Seagull by the previous action.

H248 body. Note that the first "{" is omitted (see 
note below)












Now that the message is sent, we store the value of the
transaction ID (the counter is global to the entire traffic
and thus its value needs to be saved)







Receive the Reply on channel-1





Note that the content of the message is not checked by
Seagull. Content check can be done using the check-value
action (see core documentation)






























Termination-Id and Context-Id are stored from the 
message received so that they can be re-used later 
in the scenario









Send the TransactionResponseAck. For this message,
the transaction id is located in the body. This is
why you must use the 'restore' action on the 
'transaction-id-in-body' field. Note that fields
are described in the dictionary and correspond to regular
expressions.
 






Send the Modify




Increment the transaction-counter
Set the value of transaction-id field with the new value 
of the transaction counter. Make Seagull aware that the 
scenario execution that was identified with transaction-id "TID"
is now identified with the value of transaction-id field.
Re-use the termination-id and context-id stored earlier






The value of Transaction is replaced by the set-value action

The value of context and termination-id are replaced by the 
restore actions


 





Just after sending the message with the new transaction-id, store
this transaction-id for later use in the scenario



































Increment the transaction-counter, as done previously

Miscellaneous considerations

When creating the H248 scenario, several points must be considered:

  • The "set-new-session-id" action allows to change how Seagull identifies the execution of the current scenario. In the H248 case, it used before each new transaction, when the transaction-id needs to change. See Core documentation for explanations.
  • In a CDATA section, "]" must be escaped by using "\]" - this does not apply to "[".
  • The number of "{" and "}" in H248 messages is not equal: because of the way the dictionary is done, there must be one less "{" than the number of "}" (no starting "{" in the body, but ending "}" must be present.
  • In H248 scenarios, heading spaces and tabs are "eaten" by the scenario parser and are not sent on the network.