How to decode GSM-TCAP messages using asn1c generated code

Question

I am using the c code generated by asn1c from the TCAP protocol specification (i.e., the corresponding ASN1 files). I can successfully encode TCAP packets by the generated code. However, trying to "decode" related byte streams fails.

A sample code is as follows.

// A real byte stream of a TCAP message:
unsigned char packet_bytes[] = {
  0x62, 0x43, 0x48, 0x04, 0x00, 0x18, 0x02, 0x78,
  0x6b, 0x1a, 0x28, 0x18, 0x06, 0x07, 0x00, 0x11,
  0x86, 0x05, 0x01, 0x01, 0x01, 0xa0, 0x0d, 0x60,
  0x0b, 0xa1, 0x09, 0x06, 0x07, 0x04, 0x00, 0x00,
  0x01, 0x00, 0x14, 0x03, 0x6c, 0x1f, 0xa1, 0x1d,
  0x02, 0x01, 0x00, 0x02, 0x01, 0x2d, 0x30, 0x15,
  0x80, 0x07, 0x91, 0x64, 0x21, 0x92, 0x05, 0x31,
  0x74, 0x81, 0x01, 0x01, 0x82, 0x07, 0x91, 0x64,
  0x21, 0x00, 0x00, 0x90, 0x02
};
// Initializing ...
TCAP_TCMessage_t _pdu, *pdu = &_pdu;
memset(pdu, 0, sizeof(*pdu));    

// Decoding:
asn_dec_rval_t dec_ret = ber_decode(NULL, &asn_DEF_TCAP_TCMessage, (void **) &pdu, packet_bytes, sizeof(packet_bytes));

While the message type ("Begin", in this case), is correctly detected, but other paramters are not parsed.

Using other encoding rules, i.e., aper_decode() and uper_decode(), also fails. I would be thankful if anyone can describe how to use the auto-generated c code for decoding (parsing) a byte string of TCAP messages.

Abolfazl Diyanat · Accepted Answer · 2019-08-26T12:16:40.183

@Vasil, thank you very much for your answer.

Which asn1c are you using (git commit id) and where do you get it from as there are quite a log of forks out there?

I use the mouse07410's branch.

How do you know that Begin is correctly detected?

From the field present of the pdu variable that is evaluated by ber_decode (you can see the pdu type in the sample code). From the "Wireshark" output for this byte stream, I know that the correct type of the message is Begin.

You could try compiling with -DASN_EMIT_DEBUG=1 in CFLAGS (or -DEMIT_ASN_DEBUG=1 depending on the asn1c version you are using) to get some more debug messages.

Thanks for providing the hint; it was helpful.

The problem was related to the asn1 files I was using. I used osmocom asn1 files and compiled them by

ASN=../asn
asn1c $ASN/DialoguePDUs.asn $ASN/tcap.asn $ASN/UnidialoguePDUs.asn

in which, DialoguePortion is defined as follows (note that the first definition is commented):

--DialoguePortion ::= [APPLICATION 11] EXPLICIT EXTERNAL

-- WS adaptation
DialoguePortion ::= [APPLICATION  11] IMPLICIT DialogueOC
DialogueOC ::= OCTET STRING

To be able to decode TCAP messages, one needs to use the former definition (as is in the standard), i.e., DialoguePortion should be defined as

DialoguePortion ::= [APPLICATION 11] EXPLICIT EXTERNAL

When using this latter definition in the asn1 file, and recompiling the asn1 files, the problem solved.

P.S.: This question is also related to my problem.

Thanks for describing the fix. Please report this to osmocom as well. — Vasil Velichkov, Aug 26 '19 at 15:45
Post a message to openbsc mailing list https://lists.osmocom.org/mailman/listinfo/openbsc or open a new issue in their bug tracker https://osmocom.org/issues/new (not quite sure in which project). — Vasil Velichkov, Aug 27 '19 at 15:15

score 0 · Answer 2 · answered Aug 19 '19 at 15:20

I am using the c code generated by asn1c from the TCAP protocol specification

Which asn1c are you using (git commit id) and where do you get it from as there are quite a log of forks out there?

While the message type ("Begin", in this case), is correctly detected, but other paramters are not parsed.

How do you know that Begin is correctly detected?

Using other encoding rules, i.e., aper_decode() and uper_decode(), also fails.

There is no point in trying other encodings as they are not binary compatible.

I would be thankful if anyone can describe how to use the auto-generated c code for decoding (parsing) a byte string of TCAP messages.

You are using it correctly and probably there is a bug somewhere in the BER decoder.

You could try compiling with -DASN_EMIT_DEBUG=1 in CFLAGS (or -DEMIT_ASN_DEBUG=1 depending on the asn1c version you are using) to get some more debug messages.

Thank you very much for your answer. I finally solve the problem, and add my solution. — Abolfazl Diyanat, Aug 26 '19 at 11:56

How to decode GSM-TCAP messages using asn1c generated code

2 Answers2