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#bittorrent
BitTorrent protocol extensions
If we look at the settings in all three clients, we will easily find something all three BitTorrent client I investigated has some must-haves:
!
All have the following options (all can be enabled or disabled):
- DHT support,
- peer-exchange or PEX,
- Local Peer Discovery
- Encryption
Additionally, outside of the BitTorrent protocol itself:
- NAT PMP (Port Mapping Protocol),
- UPnP (Universal Plug and Play),
DHT
In our case DHT needs to be always on unless we want to support centralised trackers. We already discuss some differences in the DHT usage in How BitTorrent-Codex integration may look like?.
Peer-Exchange or PEX
Peer Exchange (PEX) provides an alternative peer discovery mechanism for swarms once peers have bootstrapped via other mechanisms such as DHT or Tracker announces.
It provides a more up-to-date view of the swarm than most other sources and also reduces the need to query other sources frequently.
We do not seem to have anything like this in Codex... We just use BitSwap.
See BEP11 - Peer Exchange (PEX).
Local Service Discovery
Another addition to the BitTorrent protocol that Codex does not seem to have. Local Service Discovery (LSD) or Local Peer Discovery as it is named by sometimes, provides a SSDP-like (http over udp-multicast) mechanism to announce the presence in specific swarms to local neighbours. See BEP14 - Local Service Discovery.
NAT PMP (Port Mapping Protocol) and Universal Plug and Play (UPnP)
NAT Port Mapping Protocol (NAT-PMP) is a network protocol for establishing network address translation (NAT) settings and port forwarding configurations automatically without user effort.
Clients refer to this feature as Port Mapping (qBittorrent) or explicitly as NAT PMP/UPnP.
This does not seem to be part of the BitTorrent protocol, but rather an extra client feature.
libtorrent-rasterbar provides this feature through:
enum portmap_transport
enum portmap_protocol
The first one allows to choose between natpmp or upnp, while the second between tcp/udp.
I do not believe we implement support for that in our client. Do we?
uTorrent transport protocol (uTP)
What is not shown on the picture above and what is also supported by all BitTorrent clients is BEP29 - uTorrent transport protocol.
A transport protocol with delay based congestion control. See separate article. The motivation for uTP is for BitTorrent clients to not disrupt internet connections, while still utilizing the unused bandwidth fully.
We do not have this in Codex as far as I know.
Superseeding
The superseeding (BEP16) is a feature designed to help a torrent initiator with limited bandwidth to "pump up" a large torrent, reducing the amount of data it needs to upload in order to spawn new seeds in the torrent.
Simply speaking, the node has ability to "focus" on the content it really wants to get out, without loosing bandwidth for multiple smaller seeds. Super-seed mode is NOT recommended for general use.