parsing BEP9(ut_metadata) data message

Question

BEP9(ut_metadata) data message has a total_size field and I'm having trouble seeing what it means.

The message is in this format:

<4-byte length prefix><20><1-byte extended msg id><bencode part><piece data>

I'm confused about two things:

1. Does length-prefix include <piece data> part?
2. What does total_size field in bencode_part specify?

It looks to me like we don't need total_size, unless I'm misunderstanding something.

Here is currently how I'm parsing it:

• Length prefix includes <piece data> so I know exactly how big this message is. I'm reading that many bytes.
• After parsing <bencode_part>, I'm just consuming rest of the bytes and consider this the data.

Can anyone help me understanding this format?

Thanks.

Answer 1

1. yes
2. the total length of the info section of the .torrent file. The metadata may be transferred in multiple chunks if they are too big for a single message.