Re: ASN.1-based dissector decoding by port number vs switch/case using 1st octet

[Vincent Randal <vtrandal () gmail com>]

The protocol does not exist yet.

Neither. I am helping develop this protocol for IEEE 1451.0. I do not
represent the IEEE. I am simply volunteering (as others) in one of the
working groups (IEEE 1451.0).

Why on earth did I choose to use ASN.1? Because I was asked to provide some
form of IDL for the messages, and I found esnacc and omiidl in Linux as a
means for translating ASN.1 to IDL and syntax checking it. So then I
decided it was worth it to convert our message descriptions to ASN.1
because I could translate them to IDL (per esnacc) and generate a dissector
(ASN.1-based) with Wireshark build step "make asn1"

On Tue, Jun 22, 2021 at 7:44 PM Guy Harris <gharris () sonic net> wrote:

> On Jun 22, 2021, at 6:33 PM, Vincent Randal <vtrandal () gmail com> wrote:
>
> > We are using PER per the foo example (Simple ASN.1-based dissector).
> Wow, I never about all these different encodings.
> > Maybe we should be using something other than PER? We think we like PER
> because the dissected values agree with what we can see in the raw UDP data.
>
> You should be using whatever encoding the protocol is using.
The protocol does not exist yet. I am helping develop this protocol.

Is this a protocol for which you have an ASN.1 specification plus an
> indication of the encoding being used, or is this something you're
> reverse-engineering?
Neither? I am helping develop this protocol for IEEE 1451.0. I do not
represent the IEEE. I am simply volunteering (as others) in one of the
working groups (IEEE 1451.0).

Why on earth did I choose to use ASN.1? Because I was asked to provide some
form of IDL for the messages. I found esnacc and omiidl in Linux as a means
for translating ASN.1 to IDL and syntax checking it.

So then I decided it was worth it to write our message descriptions in
ASN.1 because I could translate them to IDL (per esnacc) and generate
dissectors (ASN.1-based) with Wireshark [build step "make asn1"]. And it
seems to be working (insofar as we have tested the entire process on
several messages) except we currently have
one-port-per-message-per-dissector (over 100 new folders in
./epan/dissectors/asn1). We are wasting a lot of UDP ports on a single
protocol (again, still under development).

Anders' suggestion to use CHOICE in ASN.1 looks like a good suggestion for
decoding the first octet. Then we can have a single UDP port handle all
messages (or one port for Network Services and another port for Transducer
Services).


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