[enhancement] Provide a `PayloadField` class that eases the definition of packets with trailer data after the payload

[alxroyer-thales]

This happens quite often when protocols include an integrity field at the end of the packet.

Example of proposal:

class A(Packet):
    def extract_padding(self, s):
        return b'', s

class B(Packet):
    def extract_padding(self, s):
        return b'', s

class M(Packet):
    fields_desc = [
        ByteField("type", default=0),
        PayloadField("pld"),
        XShortField("crc", default=0),
    ]
bind_layers(M, A, type=0x0A)
bind_layers(M, B, type=0x0B)

The same as we do for regular payloads.

[alxroyer-thales]

Successfully implemented in a Scapy wrapper.

If of interest, I may provide a PR to propose the integration of it directly in Scapy implementation.

[gpotter2]

For the crc usage, we already have FCSField or TrailerField.

[alxroyer-thales]

Ok, thanks for the info.
That was surely a duplicate feature of FCSField or TrailerField.

Are these documented in https://scapy.readthedocs.io/? I went through the Adding new protocols guidelines, but I was not aware of them.

[gpotter2]

No, you're right that's a bummer.