Why does V2GTP20_ReadHeader expect me to know the payload ID?

[rmeister]

Hi there,

I have a quick question behind the intention of

int V2GTP20_ReadHeader(const uint8_t* stream_data, uint32_t* stream_payload_length, uint16_t v2gtp20_payload_id);

Why does this signature expect the caller to know the payload id? When receiving data, you have no clue what type of payload you will receive. Probing until V2GTP20_ReadHeader returns without error seems a little cumbersome, doesn't it?

Cheers,
Richard

[barsnick]

Hi Richard,
this is indeed an omission. We need to either change this function or add a corresponding one for returning the actual payload ID.

[rmeister]

Hi barsnick, thank you for the clarification.

If changing the signature to accept a pointer, would you like to change the implementation of V2GTP_ReadHeader to

int V2GTP_ReadHeader(const uint8_t* stream_data, uint32_t* stream_payload_length)
{
    uint16_t payload_id;
    int result = V2GTP20_ReadHeader(stream_data, stream_payload_length, &payload_id);
    if (payload_id != V2GTP20_SAP_PAYLOAD_ID)
    {
        return V2GTP_ERROR__PAYLOAD_ID_DOES_NOT_MATCH;
    }
    return result;
}

?
This should ensure backward compatibility.

If you don't mind I would open a PR.

[barsnick]

We're not sure about the backward compatibility yet. Perhaps we should add another function for returning the ID, as some code already has this function in use.

I don't quite see the point of your code example though: What's with all the other valid payload IDs?

[rmeister]

Right, I forgot that for the old TP header definition there are different payload types too. Seems we're not using the library method for the TP header of SDP packets. Otherwise it would have already caused an error, as V2GTP_ReadHeader uses the fixed payload type for SAP packets.

Then it would probably make sense to remove V2GTP20_ReadHeader and add the pointer argument for the payload type to V2GTP_ReadHeader ? The header structure is the same after all, there are just more types in ISO 15118-20.