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IP Header for BER Message [message #1793403]

Tue, 07 August 2018 14:42

Jason Laule

Messages: 1
Registered: August 2018

Junior Member

Hi all,

We have an existing testing framework in place that handles GCP messaging for our H.248 testing. The existing is using the external decoding of:

external function enc_PDU_GCP(in MegacoMessage pdu) return octetstring
 with { extension "prototype(convert) encode(BER:BER_ENCODE_DER)" };
external function dec_PDU_GCP(in octetstring stream) return MegacoMessage
 with { extension "prototype(convert) decode(BER:BER_ACCEPT_ALL)" };

For some new functionality, we are needing to be able to verify the dscp related information in the IP Header of the messages that contain the H.248. What we are trying to figure out is how to go about getting the decoded information from RAW to get the IP header and still retain the current decoding of the BER. Any help with this is greatly appreciated.

Regards,
Jason

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Re: IP Header for BER Message [message #1793490 is a reply to message #1793403]

Thu, 09 August 2018 06:45

Elemer Lelik

Messages: 1120
Registered: January 2015

Senior Member

Hi Jason,

Protocols are embedded one into another as Matrioshka dolls:

index.php/fa/33608/0/

The IP payload is usually a transport layer protocol , TCP or UDP, which carries a higher layer protocol, in your case GCP.

In TTCN-3 you have to do decoding layer by layer: assuming that you start from IP ( that is your test port receives structured IP messages) , first you will have to decode the TCP ( or UDP ) layer; now decoding TCP also assumes that you want to handle the TCP protocol from that layer which is not practical; for UDP this is not a problem.

From the structured UDP (or TCP ) message obtained you can extract the GCP payload and decode it with the BER codec.

This leads to the following architecture:

some IP test port that handles structured IP messages ;
here , a combination of the LANL2 test port and the IP protocol module (with teh associated RAW codec) can be used ( see product list on https://github.com/eclipse/titan.core)

in the next step , the TCP/UDP messages can be (binary RAW) decoded/encoded with the TCP/UDP protocol module

Finally , the extracted GCP payload is transferred to the BER codec

On every protocol level, the headers can be analyzed and verified as needed.

To bridge the gap between distant protocol layers its practical to use the translation port feature that has been described in several posts.

Now I understand you have an existing framework. The exact answer to your question depends on how this is structured already. If you have access to IP messages than just simply decode them along the way using the IP protocol module.

I hope this helps

Best regards
Elemer