bittorrent vs HTTP

Here's a comparison between the use of bittorrent vs HTTP for data transfers.

Basic Facts

Bittorrent is a way for a client to figure out what peers that have a piece of a particular file so that it can get pieces from those machines instead of getting the entire thing from a single (central) place.

Transfer Speed

With HTTP a file is presented in a single stream by the HTTP server. The transfer is then typically done the fastest way the remote server, the network and the client manage. After the initial "request", HTTP reaches max speed pretty quickly. A typical such transfer is limited by the narrowest network condition along the way, or by the server being overloaded by requests from many clients. Some network operators do funny things, so a client can actually get higher transfer performance by connecting multiple times and transferring parts of the data in several simultaneous connections.

Bittorrent on the other hand requests "random" parts of the file from N number of peers and it can typically download several parts at once from different sources. A client will also soon start to hand out parts of the file that it has already downloaded, to other peers that want to download that same file. Bittorrent downloads tend to start slow, and increase in speed during the transfer and slow down again towards the end when it only gets a few remaining pieces to get.

Streaming

Things that must be sent and received in a specific order, such as a live audio or video stream, is problematic to distribute via bittorrent given the current design.

While clients (probably) could ask for chunks in a chronological order and then get the data in a sequential manner that is not how clients typically work, and it would greaty defeat some of the primary purposes of bittorrent as it will not at all distribute the load as evenly among the peers as the "random access" method does.

Uplink

Due to the nature of these protocols. A bittorrent transfer to you is very likely to reach you over other users' uplinks. Your client also hands out data to other users over your Internet connection. With bittorrent you're getting each individual part limited by the uplink speed of that user who has that part. Your client also hands out data so you use your uplink at lot (as much as you allow it basically).

With HTTP, all the uplink is used for basically, is sending TCP ACKs to the server. That's only a small fraction of the bandwidth you use in the downlink.

Firewalls

Bittorrent needs to go out and connect to multiple remote machines. For your client to be able to hand out stuff properly it also needs to allow remote clients to connect back to you (even on multiple inbound ports) and that requires that you have a firewall setup that allows this.

HTTP is a traditional client so it only connects to a server, which firewalls tend to be setup to allow with no extra fuzz needed.

Related, is the fact that bittorrent uses a lot more TCP connections in any typical use case which might be troublesome for OSes or intermediate routers etc with fixed limits.

Bittorrent typically runs on separate TCP ports (6881-6999) which are easily blocked by firewall admins.

Redundancy

A bittorrent client can survive that the primary server (tracker) goes away after it has gotten the primary torrent off it and found one or more peers to request file parts from, while a HTTP solution is depending on the server to stick around and to keep feeding the TCP stream. Bittorrent also has techniques to work in "trackerless" mode (DHT and Mainline DHT being two) which makes it even more robust.

The HTTP server can in fact go silent for a long time and the connection will then just continue again when the server re-appears, but both HTTP and bittorrent use TCP so that's pretty much the same for both protocols. It's just that even if one connection stalls with bittorrent, chances are the others still strive.

Server Load

One of the primary purposes of bittorrent is to ease the burden to provide data for download, and as such it is certainly way better than HTTP as you can distribute a particular file to a very large amount of clients with only a fraction of the load and bandwidth requirements on the server/tracker.

Encryption

HTTP has no encryption but the standard way is to do HTTP over TLS, called HTTPS. That provides what is generally accepted a secure data connection.

bittorrent offers a protocol extension called Message Stream Encryption (MPE) which provides a "message stream obfuscation by the application of fairly simple cryptographic techniques, not a full blown transport level security mechanism like SSL" (quote referece).

Protocol Standards

HTTP is an established and formalized protocol standard by the IETF.

Bittorrent is a protocol designed by the company named Bittorrent and there is a large amount of variations and extensions in implementations and clients.

Further Reading

The wikipedia article on bittorrent is very detailed and a good read.

Thanks

Feedback and improvements by: Austin Appel, Dave Chapman