BEP:19
Title:WebSeed - HTTP/FTP Seeding (GetRight style)
Version: dc3ab912db1bac367a592537c43fb2e758be20b8
Last-Modified:Tue Feb 26 01:05:04 2008 +0000
Author: Michael Burford <michael @at@ getright.com>
Status: Draft
Type:Standards Track
Content-Type:text/x-rst
Created:21-Feb-2008
Post-History:

Abstract

Using HTTP or FTP servers as seeds for BitTorrent downloads.

Rationale

Many websites that list a BitTorrent download also provide a HTTP or FTP URL for the same file. The files are identical. A WebSeeding BitTorrent client can download from either source, putting all the parts together into one complete file.

The HTTP or FTP server acts as a permanently unchoked seed.

Advantages

There is always an unchoked seed where anybody can get started.

It does not break or change anything for clients that do not recognize the addition to the metadata.

Clients that do not support HTTP/FTP Seeding would still get the benefit from peers sharing pieces originally from an HTTP/FTP server.

The metadata does not have to be changed. Download Manager tools (such as GetRight) commonly can add multiple HTTP/FTP mirror URLs for a file, usually by the user action of clicking multiple links on a web page and recognizing the same file name.

Everybody is quite familiar with HTTP/FTP servers and their protocols.

It has already been implemented in GetRight, the Mainline client, uTorrent, Azureus, libTorrent, and likely others.

Changes are only needed in BitTorrent clients. No changes to trackers, HTTP, or FTP servers are needed. No scripts are needed on the HTTP or FTP server.

As this is already supported in many very common clients (notably the Mainline client itself) seeding for a BitTorrent download could be done entirely with a company or individual's existing HTTP/FTP servers.

Metadata Extension

In the main area of the metadata file and not part of the "info" section, will be a new key, "url-list". This key will refer to a one or more URLs, and will contain a list of web addresses where torrent data can be retrieved. This key may be safely ignored if the client is not capable of using it.

For example (on multiple lines for readability):
d 8:announce27:http://tracker.com/announce 8:url-list26:http://mirror.com/file.exe 4:info...

If the "url-list" URL ends in a slash, "/" the client must add the "name" from the torrent to make the full URL. This allows .torrent generators to treat this field same for single file and multi-file torrents.

Multi-File Torrents

BitTorrent clients normally use the "name" from the torrent info section to make a folder, then use the "path/file" items from the info section within that folder. For the case of Multi-File torrents, the "url-list" must be a root folder where a client could add the same "name" and "path/file" to create the URL for the request.

For example:

... 8:url-list22:http://mirror.com/pub/ 4:infod5:filesld6:lengthi949e4:pathl10:Readme.txte e4:name7:michael

A client would use all that to build a url: http://mirror.com/pub/michael/Readme.txt

Client Implementation Overview

A client should ignore any protocols it does not know from the url-list. A client may choose to implement HTTP but not FTP, or vice-versa.

HTTP/FTP are "streaming" type protocols, and do not have BitTorrent's concept of blocks. For HTTP you can use byte-ranges to resume anywhere or download specific ranges you specify, but with FTP you can only say where to start the download.

I wanted to have many sequential blocks ("gaps") in the data downloaded from BitTorrent peers so a HTTP/FTP connection would have big spaces to fill in. You could use the byte-ranges with HTTP to request individual blocks--but each request will show in the server's logs, and somebody is going to think it is a denial of service attack on them if their shows 100's of connections in their log from the same IP.

In GetRight's implementation, I made a couple changes to the usual "rarest first" piece-selection method to better allow "gaps" to develop between pieces. That way there are longer spaces in the file for HTTP and FTP threads to fill. They can start at the beginning of a gap and download until they get to the end.

Client Implementation Notes

Changes to the standard BitTorrent algorithms to optimize the ability of HTTP and FTP connections to fill in many sequential blocks in the file.

Definition of Gaps

Gaps are spaces of multiple pieces in a row that the client does not have.

Given a bitfield "YYnnnnYnnY" where Y is pieces it has and n are ones it does not have, there are two "gaps", one of 4 pieces, and another of 2.

Piece Selection

The major change is to the Piece Selection.

If everything else is more-or-less equivalent, it is better to pick a piece to do from the gap of 2 when requesting a piece from a BitTorrent peer.

In any gap, it is best to fill in from the end (ie, the highest piece number first).

So given bitfield "YYnnnnYnnY" if the rareness of all the n pieces is similar it is better to select one of the # pieces "YYnnnnY##Y" and the best would be to select piece $ "YYnnnnYn$Y"

These maximize the contiguous pieces for HTTP/FTP connections. This allows a HTTP/FTP connection to start at the beginning of a large gap and download the most data before it gets to a piece the client already has.

Changes to Piece Selection

Change the "rarest first" piece selection to a "pretty rare with biggest distance from another completed piece".

Pretty Rare With Biggest Gap

When scanning for the rarest piece, if the distance from another completed piece is less than that for the current rarest piece, it must be "rare-X". Or if the distance is greater than the current piece's, it can be rare+X to be picked as the next piece. (For no better reason than it seemed to make sense at the time and scale, X is the square root of the number of peers minus 1.)

So if 3 peers had the current rarest piece, the normal algorithm would pick a piece where 2 peers had it. The changed algorithm would require that only 1 peer has the piece if that piece's distance from a complete piece was less than the gap for the current rarest piece.

If the gap is bigger and the piece is the same "rareness" or the usual "rare-1" that piece is selected. (So if the gap is bigger, a piece with either 2 or 3 peers would be chosen.)

So given "YYnnn1Yn2Y", unless 1 is a lot more rare than 2, it's better to pick piece 2.

Pseudo-Code logic:

X = sqrt(Peers) - 1;
Gap = 0;
CurGap = 0;
CurRarest = MaxPieces+1;
for (i=0; i<MaxPieces; i++) {
    if (IDoNotHavePiece(i)) {
        Gap++;
        if (PeerHasPiece(i)) {
            PieceRareness = NumberOfPeersWithThePiece();
            if (PieceRareness<(CurRarest-X) ||
                (PieceRareness<=(CurRarest+X) && Gap>CurGap)) {
                CurRarest = PieceRareness;
                CurGap = Gap;
                NextPiece = i;
            }
        }
    } else {
        Gap = 0;
    }
}

Fill In The Gaps

If a file is more than 50% complete, it uses a different method for piece selection randomly. (With over 50%, you should have a large number of pieces that other peers will want to download.)

Every few pieces (in GetRight it is randomly 1 in 10), it will pick the piece with the smallest gap from a complete piece, ignoring all rareness. For the bitfield "YYnnnnYnnY" it would select piece # "YYnnnnYn#Y". This helps fill in small gaps.

Clients can choose whether to do this step or not, and if implementing could use another percent of file completion.

Pseudo-Code logic:

Gap = 0;
Piece = -1;
CurGap = MaxPieces+1;
for (i=0; i<MaxPieces; i++) {
    if (IDoNotHavePiece(i)) {
        Gap++;
        if (PeerHasPiece(i)) {
            Piece = i;
        }
    } else {----
        if (Gap<CurGap && Gap>0 && Piece!=-1) {
            CurGap = Gap;
            NextPiece = Piece;
        }
        Gap = 0;
        Piece = -1;
    }
}

HTTP and FTP Optimizations

No changes are needed to the HTTP/FTP protocols or servers.

If the client knows the HTTP/FTP download is part of a BitTorrent download, when the very first connection is made it is better to start the HTTP/FTP download somewhere randomly in the file. This way it is more likely the first HTTP pieces it gets will be useful for sharing to the BitTorrent peers.

If a BitTorrent download is already progressing when starting a HTTP/FTP connection, the HTTP/FTP should start at the beginning of the biggest gap. Given a bitfield "YYnnnnYnnY" it should start at #: "YY#nnnYnnY"

If it successfully downloads a piece from a HTTP/FTP server, but the SHA checksum does not match, the connection must be closed and that URL should be discarded.

A client does not need to discard a HTTP or FTP server URL if it gets a "busy" reply.

Multi-File Torrents

Additional selection algorithms will be needed when using HTTP/FTP servers for a multi-file torrent.

A client may select BitTorrent pieces to optimize so entire large files can be downloaded from the HTTP/FTP servers.

For torrents containing small files, several HTTP/FTP transfers may be needed for one Piece. In this case, it may make more sense to do those using BitTorrent. For example, if there were 100 1KB files, assuming even the worst case of 32KB Pieces, it would take 100 HTTP/FTP transfers to do the files, but only 4 BitTorrent piece requests.

Giving BitTorrent piece selection a higher priority for smaller files, and HTTP/FTP a higher priority for larger files would work well.

Another Possible Client Implementation

If a client only supported HTTP and not FTP, it could take advantage of HTTP's byte-range requests, but request more than one piece at a time.

Blocks of pieces could treated as a single set and a single byte-range request to the HTTP server. This would reduce the number of HTTP connections, and might work well for a client.

Pieces could be treated as blocks of 10, 50, or 100. If I had done this way, I might have chosen "Pieces Per Block" to be MaxPieces/20. So requesting about 5% of the file at a time.

More On Other Protocols

HTTP and FTP are listed in this BEP as the seeding protocols, but a client could use any other protocol that allows downloading data. HTTPS, FTPS, or SFTP are obvious, but not likely supported by many clients (GetRight can do HTTPS and FTPS, but not SFTP).

I am sure RTSP and MMS are possible as well. Potentially even Usenet's NNTP protocol could be used.

Other protocols may have additional issues, such as not allowing a download to start anywhere other than the beginning of the file.

A client may choose to implement only one of the HTTP or FTP protocols but not both.

Acknowledgements

Thanks to Arvid Norberg (libtorrent.sf.net author) for helping clarify the Multi-File torrent parts. And of course to Bram Cohen for creating this whole BitTorrent protocol.

References