ut_metadata Torrent Metadata Block Count Guide

The ut_metadata extension (specified in BitTorrent Enhancement Proposal 9, or BEP 9) allows peers to share .torrent metadata over magnet links by dividing the metadata into standardized chunks. The exact number of blocks a metadata file is split into depends on the total size of its info dictionary, as the protocol enforces a fixed chunk size of 16 KiB (16,384 bytes) per block.

The Standard Block Size

Under BEP 9, metadata is not transferred as a single payload. Instead, the torrent’s raw bencoded info dictionary is partitioned into discrete pieces:

• Fixed Block Size: 16,384 bytes (16 KiB)
• Final Block: Any remaining bytes smaller than 16 KiB

The total block count is calculated using the formula:

\[\text{Total Blocks} = \lceil \frac{\text{Total Metadata Size in Bytes}}{16384} \rceil\]

Typical Block Counts

The size of a torrent’s metadata depends primarily on the number of files in the transfer and the total number of piece hashes contained in the info dictionary.

• Small Torrents (1 Block): Torrents containing a single file or a small number of pieces often have metadata sizes under 16 KiB, requiring only 1 block.
• Average Torrents (2 to 8 Blocks): Most typical multi-file torrents or movies with standard piece sizes have metadata ranging between 30 KiB and 120 KiB, requiring between 2 and 8 blocks.
• Large Torrents (10 to 64+ Blocks): Torrents with tens of thousands of individual files or extremely large piece tables (several hundred gigabytes or terabytes of total data with small piece sizes) can have metadata exceeding 1 MiB, resulting in 64 or more blocks.

How Blocks Are Transferred

When a client initiates a metadata transfer via ut_metadata:

1. Handshake & Size Exchange: The peers negotiate the extension and advertise the metadata_size in bytes.
2. Block Indexing: The receiving client calculates the number of blocks and requests them individually by their zero-based index (e.g., piece 0, piece 1, up to piece N-1).
3. Reassembly & Verification: Once all blocks are received, the client concatenates them in order and verifies the complete info dictionary against the 20-byte SHA-1 info-hash specified in the magnet link. If the hash matches, the metadata is complete and the regular file download begins.