UtMetadata

BEP-9 "Extension for Peers to Send Metadata Files" (ut_metadata). This is the pure message codec, ported from libtorrent's ut_metadata.cpp (src/ut_metadata.cpp, ut_metadata_peer_plugin::write_metadata_packet and on_extended).

The metadata that this protocol exchanges is the bencoded info dictionary of a torrent (the same bytes whose SHA-1 is the v1 info-hash). It is sliced into 16 KiB pieces and pulled from peers a piece at a time.

Wire format

An ut_metadata message is the payload of a BEP-10 extended message (id 20) carried under the extension id the remote peer advertised for ut_metadata in its extension handshake. That payload is:

<bencode dict>[<raw 16 KiB metadata piece>]

The bencode dict always has:

• msg_type: 0 = request, 1 = data, 2 = reject

• piece: the 0-based 16 KiB piece index

and, for a data message, also:

• total_size: the total length in bytes of the whole metadata (info) blob.

For a data message the raw bytes of that one piece are appended after the bencode dict (this is why a plain bdecode of the buffer stops at the end of the dict, because the trailing bytes are the payload). libtorrent recovers them with body.subspan(msg.data_section().size()), i.e. everything past the bencoded dict; parse does the same using BdecodeNode.dataSection.

This class is the codec only. The request scheduling and rate limiting that the C++ peer plugin adds lives elsewhere. MetadataTransfer is the small reassembly + verification helper.