registering two protocol plugin sharing a same port

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@Guy Harris Both protocols do have the first Byte as protocol identification, can't it be used for dissection between the protocols packets having the same port.

It can but you need to use a "pre-dissector" which you register as a dissector for the full port range and let it invoke one of the two real dissectors depending on the conditions (if the port is 3100 and the first byte identifies one protocol, call the dissector for that protocol, otherwise call the dissector for the other protocol). Or you can merge the code of the two dissectors if that makes more sense. A single dissector plugin may register multiple protocol names.

@sindy How does the pre-dissector actually works, is there any example in the Wireshark i can look for?

"pre-dissector" is not an official name, that's why I've put it into quotation marks. It is just a piece of code with the formal structure of a dissector which registers itself as a dissector to the tcp table, but its executive part (the dissector function) doesn't really handle the data, it just calls one or the other dissector depending on the conditions. As said, you may instead tell the XYZ dissector to invoke the ABC dissector and pass to it the whole tvb to do the real job if it finds out that the port is 3100 and the first byte in the tvb identifies protocol ABC. The ABC need not be registered to the dissection table indexed by TCP port numbers, it is enough that it is registered as such and the XYZ knows its handle.