Abstract

To reduce the size of tracker responses and to reduce memory and computational requirements in trackers, trackers may return peers as a packed string rather than as a bencoded list.

Overview

In BitTorrent as described in BEP 3 [1], peers wishing to transfer a file contact a central tracker. This tracker returns a list of peers that are currently transferring the file. The list of peers is implemented as a list of bencoded dicts. Each dict in the list contains three fields: peer id, ip, and port. The peer id is 20 bytes plus 3 bytes bencoding overhead. The ip is a string containing a domain name or an IP address, and an integer port number. The ip is variable length, but since in its longest form it is a domain name it cannot exceed 255 bytes [2] plus 4 bytes bencoding overhead. Bencoded integers are also variable length but since it is a port number, it cannot be more than 7 bytes including bencoding overhead. Thus,

::

total peer list length in bytes < n * ( 23 + 259 + 7 )

It is common now to use a packed format where each peer is represented using only 6 bytes. The first 4 bytes contain the 32-bit ipv4 address. The remaining two bytes contain the port number. Both address and port use network-byte order.

The packed format uses the same peers key in the bencoded tracker response, but the value is a bencoded string rather than a bencoded list.

The peer id does not appear in the packed format. The format has been in use for years and the lack of a peer id has posed no problems.

This packed format is supported by BitTorrent mainline, Azureus, libtorrent, uTorrent, and probably most other clients.

References

Copyright

This document has been placed in the public domain.